10-K

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes ☐ No ☒

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Indicate by check mark whether the issuer (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes ☒ No ☐

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes ☒ No ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Indicate by check mark whether the registrant has filed a report on and attestation to its management’s assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report. ☐

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Indicate by check mark whether any of those error corrections are restatements that required a recovery analysis of incentive-based compensation received by any of the registrant’s executive oﬃcers during the relevant recovery period pursuant to §240.10D-1(b). ☐

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes ☐ No ☒

The aggregate market value of the voting and non-voting common equity held by non-affiliates of the Registrant on June 30, 2022 (the last business day of the Registrant’s second fiscal quarter), based upon the closing price of $5.21 of the Registrant’s common stock as reported on The Nasdaq Global Market, was approximately $250.7 million.

The number of shares of the registrant’s common stock outstanding as of March 27, 2023, was 60,160,551.

Portions of the registrant’s definitive proxy statement to be filed for its 2023 Annual Meeting of Stockholders are incorporated by reference into Part III hereof. Such proxy statement will be filed with the Securities and Exchange Commission within 120 days of the end of the fiscal year covered by this Annual Report on Form 10-K.

This Annual Report on Form 10-K, or Annual Report, contains forward-looking statements within the meaning of Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act, and section 27A of the Securities Act of 1933, as amended, or the Securities Act. All statements contained in this Annual Report other than statements of historical fact, including statements regarding our future results of operations and financial position, business strategy, market size for our product candidates, potential future milestone and royalty payments, the value of the our holdings of Vaxcyte common stock, potential growth opportunities, nonclinical and clinical development activities, efficacy and safety profile of our product candidates, our ability to maintain and recognize the benefits of certain designations received by product candidates, our ability to successfully leverage Fast Track designation, the timing and results of nonclinical studies and clinical trials, collaboration with third parties, the expected impact of the COVID-19 pandemic on our operations, and the receipt and timing of potential regulatory designations, approvals and commercialization of product candidates, are forward-looking statements. The words “believe,” “may,” “will,” “potentially,” “estimate,” “continue,” “anticipate,” “predict,” “target,” “intend,” “could,” “would,” “should,” “project,” “plan,” “expect,” and similar expressions that convey uncertainty of future events or outcomes are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words.

These forward-looking statements are subject to a number of risks, uncertainties and assumptions, including those described in Item 1A, “Risk Factors” and elsewhere in this Annual Report. Moreover, we operate in a very competitive and rapidly changing environment, and new risks emerge from time to time. It is not possible for our management to predict all risks, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements we may make. In light of these risks, uncertainties, and assumptions, the forward-looking events and circumstances discussed in this Annual Report may not occur and actual results could differ materially and adversely from those anticipated or implied in the forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee that the future results, levels of activity, performance or events and circumstances reflected in the forward-looking statements will be achieved or occur. We undertake no obligation to update publicly any forward-looking statements for any reason after the date of this report to conform these statements to actual results or to changes in our expectations, except as required by law. You should read this Annual Report with the understanding that our actual future results, levels of activity, performance and events and circumstances may be materially different from what we expect.

Except where the context otherwise requires, in this Annual Report on Form 10-K, “we,” “us,” “our” and the “Company” refer to Sutro Biopharma, Inc.

This Annual Report on Form 10-K includes trademarks, service marks and trade names owned by us or other companies. All trademarks, service marks and trade names included in this Annual Report on Form 10-K are the property of their respective owners. We do not intend our use or display of other companies’ trade names, trademarks or service marks to imply a relationship with, or endorsement or sponsorship of us by, these other companies.

Our business is subject to a number of risks and uncertainties, including those highlighted in the section titled “Risk Factors”. Some of these risks include:

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We will need substantial additional funds to advance development of our product candidates and failure to obtain sufficient funding, may force us to delay, limit or terminate our product development programs, commercialization efforts or other operations. Continued unfavorable conditions in the capital markets may pose difficulties to us in accessing additional capital on reasonable, or even any, terms to continue our product and platform development or other operations. In addition, market volatility, high levels of inflation and interest rate fluctuations may increase our cost of capital or otherwise restrict our access to potential sources of future liquidity.

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Our product candidates are in early stages of development and may fail, be impacted by competitive products or suffer delays that materially and adversely affect their commercial viability. Our business is dependent on the success of our product candidates based on our proprietary XpressCF® and XpressCF+® platforms and, in particular, our most advanced product candidate, STRO-002, or luveltamab tazevibulin or luvelta, and other product candidates.

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Security breaches, cyber-attacks, loss of data, and other disruptions at our facilities or at our third party CROs, CMOs, or other vendors could compromise sensitive information related to our business or other personal information or prevent us from accessing critical information and expose us to liability, which could adversely affect our business, reputation, results of operations, financial condition and prospects.

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Our failure to comply with privacy and data protection laws or to adequately secure the personal information we hold could result in significant liability or reputational harm and, in turn, a material adverse effect on our client base, member base and revenue.

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We currently manufacture a portion of our product candidates internally and also rely on third-party manufacturing and supply partners to provide us with components of our product candidates and materials used for the manufacture of the product candidates. Our inability to manufacture sufficient quantities of our product candidates or such materials, or the loss of our third-party suppliers, or our or their failure to comply with applicable regulatory requirements or to supply sufficient quantities at acceptable quality levels or prices, or at all, would materially and adversely affect our business.

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We face competition from entities that have developed or may develop product candidates for cancer, including companies developing novel treatments and technology platforms. If these companies develop technologies or product candidates more rapidly than we do or their technologies are more effective, our ability to develop and successfully commercialize product candidates may be adversely affected.

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Our collaborators may fail to abide by the terms of the agreements with us, which would require us to seek to enforce our agreements in accordance with the dispute resolution procedures set forth therein. These procedures may require us to engage in litigation or arbitration to enforce our rights, which can be expensive, time-consuming, and distracting to our management and Board of Directors and that may ultimately end up being unsuccessful.

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If we are unable to develop, obtain regulatory approval for or commercialize our product candidates, or experience significant delays in doing so, our business will be materially harmed. Changes in regulatory policy may render our strategies for obtaining regulatory approval less effective or completely ineffective, preventing us from obtaining regulatory approval for our product candidates on time or at all.

We are a clinical-stage oncology company developing site-specific and novel-format antibody drug conjugates, or ADCs, enabled by our proprietary integrated cell-free protein synthesis platform, XpressCF®, and our site-specific conjugation platform, XpressCF+®. We aim to design and develop therapeutics using the most relevant and potent modalities, including ADCs, bispecific ADCs, immunostimulatory ADCs, or iADCs, dual conjugate ADCs, or ADC2s, and cytokine derivatives. Our molecules are directed primarily against clinically validated targets where the current standard of care is suboptimal. We believe that our platform allows us to accelerate the discovery and development of potential first-in-class and/or best-in-class molecules by enabling the rapid and systematic evaluation of protein structure-activity relationships to create optimized homogeneous product candidates. Our mission is to transform the lives of patients by creating medicines with improved therapeutic profiles for areas of unmet need.

Our most advanced product candidate is STRO-002, or luveltamab tazevibulin, or luvelta, an ADC directed against folate receptor-alpha, or FolRα, for patients with FolRα-expressing cancers, including ovarian cancer. In 2019, we began enrolling patients in a Phase 1 trial of luvelta that focused on ovarian and endometrial cancers. The dose escalation portion of the luvelta Phase 1 trial has been completed and the dose expansion portion of the trial to assess the efficacy, safety and tolerability of luvelta is ongoing. In January 2023 we reported preliminary final results from the dose-expansion cohort. The data from the dose-escalation and dose expansion cohorts suggested that luvelta exhibited a manageable safety profile together with promising preliminary efficacy data in the tested patient population, as discussed in detail below. In August 2021, luvelta was granted Fast Track designation by the U.S. Food and Drug Administration, or FDA, for the treatment of patients with platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer who have received one to three prior lines of systemic therapy. In mid-2022, we discussed with the FDA appropriate trial designs for a registration-directed trial of luvelta to potentially support accelerated approval. We expect to begin a registration-directed trial of luvelta for platinum-resistant ovarian cancer in the first half of 2023.

In addition, we have been offering compassionate use of luvelta to treat pediatric patients with relapsed/refractory CBFA2T3-GLIS2, or CBF/GLIS, acute myeloid leukemia, or AML, commonly known as RAM phenotype AML. Compassionate use data showed anti-leukemic activity of luvelta in pediatric patients with relapsed/refractory CBF/GLIS AML and was presented at the 64th American Society of Hematology Annual Meeting and Exposition (ASH 2022) in December 2022. The data showed that luvelta was well tolerated as a monotherapy agent and in combination with standard cancer therapies. Luvelta was granted Orphan Drug Designation by the FDA in December 2022 in this pediatric patient population.

Our next most advanced product candidate is STRO-001, an ADC directed against CD74, for patients with B-cell malignancies, such as multiple myeloma and non-Hodgkin lymphoma, or NHL. We have an ongoing Phase 1 trial of STRO-001 for the treatment of multiple myeloma and NHL. STRO-001 has been granted Orphan Drug Designation by the FDA for the treatment of multiple myeloma. Our most recent data update was provided in December 2020, as discussed in more detail below. Based on such reported data, STRO-001 has been generally well-tolerated. We have completed dose escalation in the STRO-001 Phase 1 trial following identification of the maximum tolerated dose. In October 2021, we granted BioNova Pharmaceuticals Limited, or BioNova, an option to exclusively license the right to develop and commercialize STRO-001 in Greater China, or the BioNova Option Agreement. In February 2023, BioNova announced that the first patient had been dosed in the Phase 1 clinical trial of STRO-001.

We also have a preclinical product candidate - STRO-003, which is a single homogeneous ADC directed against an anti-receptor tyrosine kinase-like orphan receptor 1, or ROR1, which we intend to develop for the treatment of solid tumors. Preparations are underway for IND-enabling studies for STRO-003, which we expect will be completed in the first quarter of 2024.

Enabled through our proprietary XpressCF® and XpressCF+® platforms, we have entered into multi-target, product-focused collaborations with leading pharmaceutical and biotechnology companies in the field of oncology, including an immunostimulatory antibody-drug conjugates collaboration with Astellas Pharma Inc., or Astellas, a cytokine derivatives collaboration with Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, or Merck; a B Cell Maturation Antigen, or BCMA, ADC collaboration with Celgene Corporation, or Celgene, a

wholly owned subsidiary of Bristol Myers Squibb Company, New York, NY, or BMS; a MUC1-EGFR ADC collaboration with Merck KGaA, Darmstadt Germany (operating in the United States and Canada under the name “EMD Serono”), or EMD Serono. Our XpressCF® and XpressCF+® platforms have also supported Vaxcyte, Inc., or Vaxcyte, focused on discovery and development of vaccines for the treatment and prophylaxis of infectious disease. In the fourth quarter of 2022, Vaxcyte announced positive topline data from a Phase 1/2 clinical proof-of-concept study of its lead product candidate, VAX-24, its 24-valent pneumococcal conjugate vaccine candidate, under investigation for the prevention of invasive pneumococcal disease in adults aged 18-64. Also in the fourth quarter of 2022, we entered into an agreement with Vaxcyte, granting it an option to access expanded rights to develop and manufacture cell-free extract for use in development and manufacture of its vaccine products, among certain other rights.

We believe our XpressCF® platform is the first and only current Good Manufacturing Practices, or cGMP, compliant and scalable cell-free protein synthesis technology that has resulted in multiple product candidates in clinical development. We believe key advantages of our cell-free protein synthesis platform over conventional biologic drug discovery and development include:

We plan to leverage these capabilities to accelerate the discovery and development of potential first-in-class and best-in-class molecules.

The benefits of our XpressCF® and XpressCF+® platforms have resulted in collaborations with leaders in the field of oncology, including Astellas, Merck, BMS and EMD Serono. In 2022, we entered into a License and Collaboration Agreement with Astellas, for the development of immunostimulatory antibody-drug conjugates for up to three biological targets. Our collaboration with Merck resulted in MK-1484, a selective IL-2 agonist that Merck is developing as a monotherapy and in combination with pembrolizumab for the treatment of solid tumors. We announced the dosing of the first patient with MK-1484 in a Phase I study in the third quarter of 2022. Our BMS collaboration yielded CC-99712, a novel ADC therapeutic directed against BCMA. BMS is studying CC-99712 in a Phase 1 trial initiated in 2019, both as a monotherapy and in combination with a gamma secretase inhibitor, in patients with relapsed and refractory multiple myeloma. CC-99712 has been granted orphan drug designation for the treatment of relapsed and refractory multiple myeloma. Finally, our collaboration with EMD Serono yielded a novel bispecific ADC product candidate targeting EGFR and MUC1, known as M1231, for which an IND submission was filed in 2020. Recently, EMD Serono decided to close the Phase 1a trial of M1231 in patients with solid tumors and not initiate a previously planned expansion study. EMD Serono stated that the decision was based on strategic portfolio considerations. Through December 31, 2022, we have received an aggregate of approximately $621 million in payments from all of our collaborations, which includes approximately $54 million in investments in our stock. We intend to selectively enter into additional collaborations with partners who are seeking efficient and effective drug discovery, preclinical development and manufacturing capabilities for the creation of novel therapeutics.

As discussed above, we are developing luvelta for the treatment of ovarian and endometrial cancers. In addition to the development discussed above, an expansion cohort assessing the effects of administration of prophylactic pegfilgrastim in combination with luvelta opened for enrollment in the second quarter of 2022; interim results from this cohort were presented in January 2023.

Other studies ongoing for luvelta include a trial assessing the combination of luvelta with bevacizumab for treatment of ovarian cancer and an expansion cohort for FolRα-selected endometrial cancer that opened for enrollment in the fourth quarter of 2021. Additionally, luvelta was provided to pediatric patients with CBF/GLIS AML on a compassionate use basis. Translational work is also ongoing to support an investigational new drug, or IND, application for the initiation of a non-small cell lung cancer study, for which submission is planned in 2023.

In December 2021, we entered into the Tasly License Agreement, as amended in April 2022, to develop and commercialize luvelta in the Greater China territory. We believe that our collaboration with Tasly extends the opportunity to realize the potential value of luvelta through clinical development and commercialization in Greater China.

We have also been internally developing STRO-001, which we believe has the potential to be a first-in-class and best-in-class ADC directed against CD74, an antigen that is highly expressed in many B cell malignancies. We have completed enrollment for STRO-001 dose escalation in a Phase 1 trial for multiple myeloma and NHL. STRO-001 has been generally well-tolerated and no ocular toxicity signals have been observed, with no patients receiving prophylactic corticosteroid eye drops. The maximum tolerated dose of STRO-001 has been identified.

In October 2021, we entered into the BioNova Option Agreement. We believe that our collaboration with BioNova extends the opportunity to realize the potential value of STRO-001 through clinical development and commercialization in Greater China. In February 2023, BioNova announced that the first patient had been dosed in the Phase 1 clinical trial of STRO-001 in which BioNova will conduct further dose optimization.

We are also developing STRO-003, which we believe has the potential to be a first-in-class and best-in-class ADC targeting ROR1. Preclinical data suggest that STRO-003 has potent antitumor activity and potential for a differentiated safety profile. Preparations are underway for IND-enabling studies for STRO-003, which we expect will be completed in the first quarter of 2024.

Beyond these programs and collaborations, we are developing a broader pipeline of next-generation protein therapeutics using our XpressCF® and XpressCF+® platforms. Our protein engineering and chemistry efforts are focused on maximizing therapeutic indices, and our technology allows us to rapidly test our therapeutic hypothesis in significantly more product candidates than conventional protein synthesis allows in order to identify the best molecule to advance to the clinic. We are also actively pursuing the discovery and development of other novel ADCs, including iADCs, bispecific ADCs, and ADC2s.

Our goal is to use our proprietary XpressCF® platform to create product candidates primarily against clinically validated targets. Key elements of our strategy are to:

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Advance luvelta through clinical development. We began enrolling patients in a luvelta Phase 1 trial focused on advanced ovarian and endometrial cancers in March 2019 and dosed the first patient in the dose expansion portion of the Phase 1 trial in January 2021. We reported updated dose-escalation data in December 2020 and May 2021 and reported dose-expansion data in January 2022 and January 2023. In December 2022, we announced that data was presented resulting from compassionate use of luvelta in pediatric patients with relapsed/refractory CBF/GLIS AML. Given that FolRα is a clinically validated target for ovarian cancer, along with luvelta’s homogeneous design, we believe it has the potential to be a best-in-class FolRα-targeted ADC and provide benefit to a broader patient population, as well as potentially greater activity, stability and safety as compared to other investigational agents in development.

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Maintain worldwide rights or, where appropriate, U.S. rights to our core product candidates to the extent possible and collaborate with partners to develop or co-develop and commercialize or co-commercialize our core product candidates in certain territories. We have assembled a management team with extensive experience in the biopharmaceutical industry, including drug discovery and development through commercialization, and our plan is to independently pursue the development and commercialization of our product candidates. As we continue to advance our products, we may opportunistically pursue additional strategic partnerships that maximize the value of our pipeline, including relationships to potentially co-develop and co-commercialize one or more of our product candidates.

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Develop a diverse pipeline of novel product candidates with optimal therapeutic profiles. We intend to build a broad pipeline of optimally designed, next-generation protein therapeutics, initially for cancer, using our XpressCF® platform. Our cell-free-based protein synthesis system enables the rapid and systematic evaluation of protein structure-activity relationships, which we believe will accelerate the discovery and development of molecules. We aim to take advantage of the most potent modalities,

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Strategically pursue additional collaborations to broaden the reach of our XpressCF® platform. To maximize the value of our XpressCF® platform technology, we have entered into multi-target, product-focused collaborations with leaders in the field of oncology, including an iADC collaboration with Astellas, a cytokine derivatives collaboration with Merck, a BCMA ADC collaboration with BMS and a MUC1-EGFR ADC collaboration with EMD Serono. We intend to selectively enter into additional collaborations with partners who are seeking efficient and effective drug discovery and manufacturing capabilities for the development of novel therapeutics. We intend to retain to the extent possible certain development and commercial rights to maximize the future potential value of product candidates discovered and developed using our XpressCF® platform.

Cancer is the second leading cause of mortality in the United States and is the leading cause of death for those under 65 years of age. The American Cancer Society estimated that there would be greater than 1.9 million new cases of cancer diagnosed and approximately 610,000 people would die of cancer in the United States in 2023.

Cancer treatment has traditionally included chemotherapy, radiation, surgery, or a combination of these approaches. Chemotherapy agents and other small molecule targeted therapies can be effective in certain types of cancer, but they can also cause toxicities that may lead to life-threatening consequences, lower quality of life or early termination of treatment. Furthermore, these agents offer limited efficacy in many types of cancer.

Over the last twenty years, new paradigms of cancer research and treatment have emerged to address the limitations of existing treatments. Some of the most promising new approaches involve biologic therapies, including monoclonal antibodies. Monoclonal antibodies are proteins that bind to antigen targets on tumor cells and inhibit tumor growth, or block processes that provide nourishment for the tumor. As a drug class, monoclonal antibodies have transformed the treatment of oncology and represent some of the top selling therapies on the market, resulting in more than $67 billion in sales in 2021 across all oncology indications.

Despite the success of conventional monoclonal antibodies, they still have limitations. For example, the response seen with monoclonal antibodies can be variable, with some patients responding, while others do not. In addition, the response is often not durable, and many patients relapse or become refractory to treatment. Also, safety and tolerability concerns often limit the use of higher, potentially more efficacious doses. Additional modalities such as ADCs have shown promise over the last decade with fourteen approvals and over a hundred ADC candidates investigated in the clinic. ADCs use the foundation of monoclonal antibodies and small molecule drugs by targeting the delivery of chemotherapeutics to the tumor. They have shown clinical benefit in hematological and solid tumors, and often have a better safety profile than systemically delivered chemotherapeutics. We believe our XpressCF® platform will provide enhanced therapeutic approaches for treating cancer to address these unmet needs and are exploring next generation biologics, including ADCs, iADCs, and ADC 2 s. The expectation is that multiple therapeutic modalities will be used in novel combinations to treat patients and provide the most potent anti-cancer effect.

After more than two decades of industry efforts, several new modalities of highly potent monoclonal antibody-based therapies have emerged, including ADCs. The key components of ADCs include an antibody, a stable linker, and a cytotoxic agent (warhead). The antibody is used to target and deliver cytotoxic agents to tumor cells. ADCs can be mono, bispecific, or multi-specific. The intended result of this powerful and targeted approach is greater tumor cell death and less systemic tolerability issues as compared to traditional chemotherapy. The following diagram shows the component parts of an ADC.

Currently, there are more than 180 ADCs being explored in clinical development. Kadcyla and Adcetris were the first of the new generation of ADCs to be approved for the treatment of specific subsets of breast cancer and lymphoma, respectively. Several more ADCs are currently on the market in the U.S.: Besponsa, Mylotarg, Lumoxiti, Polivy, Zynlonta, and Zevalin were approved for the treatment of specific subsets of leukemia and lymphoma; Padcev was approved for the treatment of bladder and urinary tract cancers; Enhertu and Trodelvy were approved for the treatment of breast cancer; Tivdak was approved for the treatment of cervical cancer; and ELAHERE™ was approved for the treatment of ovarian cancer. All of these approved therapies demonstrate that ADCs have an emerging role in the armamentarium of cancer therapeutics.

Despite the approvals of these ADCs, there have been challenges in achieving the full clinical potential of this modality. We believe these challenges are directly related to the following:

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Heterogeneity as a Result of Imprecise and Variable Conjugation. Many ADCs, both those approved and those in development, use imprecise technologies that opportunistically attach the cytotoxic payload to naturally occurring amino acids within the antibody and result in a heterogeneous mixture. In these mixtures, the number and site location of the linker-warhead can vary significantly from antibody to antibody within the single ADC product. These many different forms in the final product are likely to perform differently, with some forms carrying insufficient cytotoxin to kill the tumor, and some forms carrying too high a load resulting in unintended toxicities. The overall performance of the heterogeneous ADC is therefore the average activity of the different species within the ADC mixture, which may limit both efficacy and tolerability. For these reasons, we believe this current class of ADCs, which are heterogeneous mixtures, are suboptimal for effective cancer treatment. The figure below compares homogeneous and heterogeneous ADCs.

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Suboptimal Linker-Warhead Positioning. Conventional ADC technologies use conjugation chemistry to attach linker-warheads to naturally occurring amino acids within an antibody; therefore, the position is dictated by the pre-existing amino acid sequence. Published research studies have demonstrated that linker-warhead positioning along an antibody can have significant effect on the ability of an ADC to kill tumor cells, with some positions resulting in suboptimal killing. This position effect also contributes to the challenge of a heterogeneous ADC mixture. We believe that superior ADCs can be developed using technologies that allow linker-warhead positioning to be fine-tuned to empirically determined sites for maximal therapeutic benefit.

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Lack of Tumor Specificity Due to Linker Design. One of the major challenges in ADC technology has been to develop linking chemistries that ensure that warheads are only released from the antibody within the tumor microenvironment, and not released within the blood or healthy tissue as the ADC is delivered systemically and travels through the body. We believe that safer ADCs can be developed by utilizing non-natural amino acids that enable state-of-the-art chemistries to ensure that the warhead is not prematurely released. In addition, linker chemistries that rely on proteinases preferentially expressed in the tumor such as cathepsin and B-Glucuronidase, can provide more tumor specific release of the active catabolites and a resulting better safety profile.

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Mechanism of Action of Cytotoxin Payloads. Beyond potent cytotoxic activity of ADC payloads, there are additional attributes that lead to better efficacy and more durable responses. Payloads that induce bystander activity which is dependent on the ADC target engagement, but also kills surrounding cells within the tumor, are thought to result in broader activity. Additionally, some payloads can induce immunogenic cell death pathways. These pathways not only cause potent tumor cell killing but also produce an immunological phenotype in the cancer cells that allow a secondary immune response and killing of the tumor.

XpressCF® enables the incorporation of non-natural amino acids into antibody sequences and results in site specific conjugation of drug payloads. More recently, we have developed technology to enable incorporation of two different non-natural amino acids that allows for the site-specific conjugation of two different payloads, providing the opportunity to combine pharmacology into a single molecule. We believe this is the first use of dual conjugation combining a conventional cytotoxin with an immune stimulatory payload to drive not only direct killing of the tumor cells but an immune response against the tumor. These iADC molecules utilize immune agonists such as TLR 7, TLR 8 and STING to induce activation of innate immune cells within the tumor microenvironment and resulted in more complete responses and protective anti-tumor immune responses in preclinical tumor models. This dual conjugation approach is the basis for our research collaboration with Astellas that is focused on the discovery of iADC molecules for solid tumors. In addition to immune modulators, additional payloads can be incorporated into our dual conjugation approach. These ADC2 payloads are focused on targets that are upregulated tumors that do not respond well to existing therapies. Our goal is to provide more durable responses in hard-to-treat tumors by combining two payloads that may offset resistance mechanisms.

Cytokines are small biologically active proteins that play an essential role in immune cell function. Cytokines are important for cell-to-cell communication and are responsible for controlling immune cell growth and differentiation. Recombinant human cytokines were among the first biotechnology products engineered for therapeutic use and, in the field of oncology, cytokines that stimulate the immune system to attack cancer cells have been viewed as a potential new approach.

Certain cytokines play a central role in T cell function, contributing to the careful balance between helpful and harmful immune responses. These can be powerful activators of the immune system but can also suppress immune responses through certain specialized T cells that have suppressive functions. A previously approved cytokine therapeutic Proleukin® had shown therapeutic benefit in a small number of cancer patients, but its therapeutic use was limited due to toxicity. Scientists at other companies have focused research on finding ways to modify cytokines so as to reduce toxicity while maintaining therapeutic benefit. The observed efficacy of a modified cytokine, in combination with an immune checkpoint inhibitor, indicates the potential of this new approach. In light of these data and our prior research into cytokines, we commenced a cytokine-based research program using our XpressCF® and XpressCF+® platform technologies to engineer cytokines aimed at better exposure and tolerability profiles. Our collaboration with Merck focused on developing cytokine derivatives yielded an IL-2 derivative that entered Phase 1 in 2022. We believe that recent advances in immuno-oncology combined with new protein engineering technologies create opportunities to identify novel cytokine-based therapeutics with superior therapeutic indexes.

While ADCs, iADCs, ADC2s and engineered cytokines hold significant promise, drug developers working with these complex biologics face significant design and development challenges. Optimizing these complex biological structures is a challenging, trial and error process that requires the refinement of several properties in tandem. This iterative process is cumbersome and fraught with significant limitations. As a result, the drug candidate nominated for development is often plagued by inefficient design properties, which then translates to a suboptimal therapeutic index when investigated in the clinic.

Our XpressCF® platform seeks to address these significant shortcomings. We believe our cell-free-based protein synthesis technology allows for efficient and proper design exploration to be conducted prior to nominating a lead drug candidate. In addition, we believe we can optimally design these types of complex biologics in a

manner that is ideal for subsequent production at relevant scale and manufacture. We believe we are the only company with products in clinical development that has the capability to produce cell-free-based protein synthesis at scale. We believe we have a significant advantage over other development approaches in this space.

Our XpressCF® platform is fundamentally different from the conventional cell-based protein synthesis approach in that we separate the production of the cell mass from the production of the protein.

We first generate a cellular mass from our proprietary cell line from which we harvest the inner cellular machinery for making proteins. The cellular mass is generated from our highly engineered variant of Escherichia coli, or E.coli bacteria, and has been optimized to make extract that produces complex mammalian proteins. These cells are grown over the course of several days, harvested, broken apart, clarified, and stored as a cell mass for future production of our protein therapeutics. We refer to this proprietary cell mass as extract, or XtractCF®. The extract includes necessary components for energy production, transcription and translation and can be used to support cell-free protein synthesis. This extract can then be used agnostically to manufacture a wide variety of therapeutic proteins and protein fragments without the need to generate further cell lines.

As a result, protein synthesis then becomes a predictable and reproducible biochemical reaction, independent of the constraints of a cell. A specific DNA sequence is added to the extract, which results in the coding and expression of the desired protein in less than 24 hours. Using this process, we express hundreds or thousands of DNA sequences simultaneously within the same cell-free extract system and therefore can make and purify hundreds or thousands of unique proteins at the same time. This allows us to perform rapid expression, testing and characterization of many variants early in discovery to elucidate structure-activity relationships. Structure-activity relationship refers to how changes to the structure of a protein can lead to improvements in a molecule’s properties, such as binding, internalization, functional activity and stability, which are properties that are key to the therapeutic protein’s efficacy and tolerability in the patient. We are thereby able to optimize many properties with high specificity, including: binding efficiency to each antigen target, spatial orientation, linker design, target killing efficiency, immunological activity, protein expression, and folding efficiency and stability.

We believe the advantages of our cell-free-based protein synthesis technology platform include:

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Ability to Rapidly Produce and Evaluate a Wide Variety of Protein Structures In-house. By decoupling the production of the cell-free extract from the production of the protein, we are able to stockpile large quantities of cell-free extract from which we are able to manufacture a wide variety of proteins without the need to generate individual cell lines, including cytokine-based immuno-oncology therapeutics, ADCs, iADCs and bispecific antibodies. Additionally, our dual conjugation ADC2 technology could enable “mixed payload” ADCs that combine two distinct small molecules with different pharmacologies onto a single antibody.

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Ability to Incorporate Non-Natural Amino Acids. Our technology allows for efficient incorporation of a non-natural amino acid in any location in an antibody or protein with high precision and fidelity, which we believe allows for the design of optimized protein conjugates. Further, our non-natural amino acid conjugation technology permits complete and rapid stable linkage between our linker components and the non-natural amino acid, resulting in a single species without loss of efficiency as the conjugates become increasingly complex.

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Faster Cycle Time. Our ability to produce thousands of protein variants in parallel overnight allows us to rapidly express, test and characterize many variants early in discovery to elucidate structure-activity relationships and identify opportunities for superior therapeutic profiles, as well as new intellectual property. We are therefore able to efficiently optimize many properties with high specificity in parallel.

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Efficient Drug Discovery and Early Pharmacology and Safety Assessment. Our cell-free technology creates the opportunity for accelerated pharmacology and safety assessments during the design and discovery phase of product development. This approach allows us to generate optimized proteins early in our discovery process, which can be transitioned seamlessly to clinical scale production using the same cell-free process.

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Rapid and Predictable Scalability. Our cell-free extract does not need to be modified in any manner as we scale from research to preclinical to clinical to commercial production. This enables us to move more rapidly to the clinic by eliminating master cell banking activities and significantly de-risks scale-up to manufacturing.

As a result, we believe our technology enables new approaches to ADCs, iADCs, bispecific ADCs, and ADC2 drug discovery, development and manufacturing. Key attributes are:

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Homogeneous Design. Our XpressCF+® platform enables precise and specific placement of non-natural amino acids in defined numbers and positions within our engineered proteins. These non-natural amino acids then serve as highly stable attachment sites, also known as conjugation sites, for chemical functional groups. For example, we attach linker-warheads to non-natural amino acids within our antibodies to create single-species, tumor-killing ADCs. Similarly, we can attach polyethylene glycol polymers onto non-natural amino acids within our cytokine-based therapeutics to create single-species immunotherapies designed for extended pharmacokinetics and safety.

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Experimentally Defined Structure-Activity Relationships. Our cell-free technology enables rational design of protein therapeutics through a rapid, reiterative process that experimentally defines structure-activity relationship for cytokine-based therapeutics, ADCs, iADCs, bispecific ADCs and ADC2s. This approach allows us to explore a wide variety of structural features and formats in parallel as we optimize therapeutic candidates. For example, the precise location of chemical conjugation sites directly affects the activity of both ADCs and cytokine-based therapeutics. Our proprietary technology is key to our ability to define the best number and positions of non-natural amino acids for conjugation based on: conjugation efficiency; functional activity/pharmacological properties; and pharmacokinetics and safety. This design flexibility is also an important aspect of our discovery approach to other protein therapeutics. For example, we are able to make and directly compare a variety of pairings and structural formats for our ADC molecules to ensure that we have optimized sites of conjugation, the number of payloads on each antibody (DAR) and linker chemistry. We have examples where changing just one of these parameters can significantly impact the safety, efficacy and stability of the ADC.

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Efficient Transition from Research Scale to Development Scale Protein Production. Protein therapeutics can encounter obstacles, or even fail, during the transition from research cell lines to cGMP cell lines appropriate for clinical development and commercialization. Our XpressCF® platform can rapidly produce different protein types from a single proprietary extract, which can be scaled for discovery, development and ultimately, we believe, commercialization of cytokine-based immuno-oncology therapeutics, ADCs, iADCs, bispecific ADCs and ADC2s.

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Manufacturable Dual Conjugations. Our XpressCF+® platform allows us to manufacture antibodies that contain two different non-natural amino acids that are substrates for mutually orthogonal site-specific conjugation reactions. This advantage permits dual conjugation, resulting in homogenous iADC or ADC2 dual conjugate molecules with two different precisely placed payloads.

Accordingly, we use our XpressCF® platform to discover and develop cancer therapeutics by empirically determining the optimum structure-activity relationships for cytokine-based immuno-oncology therapeutics, ADCs, iADCs, bispecific ADCs and ADC2s and transitioning those products to cGMP compliant manufacturing.

Our XpressCF® platform has garnered the attention of leading pharmaceutical and biopharmaceutical companies and resulted in collaborations to discover and develop novel therapeutics. We have leveraged these strategic partnerships to extend our own capabilities and broaden the scope of our XpressCF® platform. Through December 31, 2022, all of our collaborations have provided us with an aggregate of approximately $621 million in payments, which includes approximately $54 million in investments in our stock. Our collaborations include:

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Vaxcyte Relationship. We have granted Vaxcyte the right to discover and develop vaccines for the prophylaxis and treatment of infectious diseases. Vaxcyte’s most advanced product candidate is VAX-24, a 24-valent pneumococcal conjugate vaccine candidate under investigation for the prevention of invasive pneumococcal disease in adults. In addition, we have granted Vaxcyte an option to obtain development and manufacturing rights for XtractCF® that, when exercised, would grant Vaxcyte the right to make and source our cell-free extract for research, development, and manufacture of vaccines for the prophylaxis and treatment of infectious disease.

Our current product candidates and Discovery and Preclinical stage programs, all based on our proprietary XpressCF® platform, are summarized in the chart below:

Luveltamab tazevibulin (luvelta), an ADC Directed Against the Target Folate Receptor-Alpha (FolRα)

We are developing luveltamab tazevibulin, or luvelta, an optimally designed ADC directed against the cancer target FolRα, initially focused on ovarian and endometrial cancers. Luvelta was designed and optimized for an improved therapeutic index by placing a precise number of linker-warheads at four specific locations within the antibody using our proprietary XpressCF+® platform. Phase 1 trial enrollment, focused on ovarian and endometrial cancers, began in March 2019, with updated dose-escalation data presented in December 2020 and May 2021. We reported initial dose-expansion data in January 2022 and a near-final dataset in January 2023. Based on such reported data, Luvelta exhibited a manageable safety profile and promising preliminary efficacy data. Dose escalation in the Phase 1 trial has been fully enrolled and the dose-expansion portion of the trial is ongoing to assess the efficacy, safety, and tolerability of luvelta at dose levels of 4.3 and 5.2 mg/kg. For the dose-expansion portion of the Phase 1 trial, we dosed the first patient in January 2021 and have enrolled less heavily pre-treated ovarian cancer patients. Additionally, a combination cohort in ovarian cancer, assessing the combination of luvelta with bevacizumab, opened for enrollment in December 2021, and an expansion cohort for FolRa-selected endometrial cancer opened and began enrolling patients in the fourth quarter of 2021. An expansion cohort assessing the effects of administration of prophylactic pegfilgrastim in combination with luvelta opened for enrollment in the second quarter of 2022; interim results from this cohort were also presented in January 2023. In August 2021, we were granted Fast Track designation for luvelta by the FDA for the treatment of patients with platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, who have received one to three prior lines of systemic therapy. In December 2021, we entered into a collaboration and exclusive license agreement with Tasly to develop and commercialize luvelta in Greater China.

Luvelta has been provided through compassionate use to pediatric patients with relapsed/refractory CBF/GLIS AML, which data were presented at ASH 2022. The data showed that luvelta was well tolerated as a monotherapy agent and in combination with standard cancer therapies. In December 2022, luvelta was granted Orphan Drug Designation by the FDA for this pediatric indication.

Ovarian cancer is the most common cause of cancer death from gynecologic tumors in the United States, and the fifth most common cause of cancer death in women. In the United States alone, the American Cancer Society estimates that 19,000 new cases of ovarian cancer would be diagnosed in 2023, and approximately 13,270 women would die of this disease. Given that early stages of the disease cause minimal, nonspecific symptoms or are asymptomatic, approximately 75% of patients with ovarian cancer are diagnosed as stage III and IV, for which the prognosis is poor. Standard pre- or post-operative chemotherapy for ovarian cancer is combination therapy with a platinum compound and a taxane, for example, carboplatin and paclitaxel, with or without bevacizumab which achieves a complete or partial response in between 70% to 80% of patients. Increasingly, PARP inhibitors are being used in the maintenance setting. Patients who are refractory or resistant to platinum-based treatments are then treated with a host of additional palliative chemotherapeutic agents, each showing only marginal benefit with response rates to single agent chemotherapy of 10-12% and progression free survival of 3-4 months. This represents a significant unmet need.

There is also a significant unmet need in the treatment of recurrent or metastatic endometrial cancer. In the United States alone, the American Cancer Society estimated 66,000 new cases of endometrial cancer in 2023, and that approximately 13,000 women would die of this disease. First-line treatment for stage III/IV disease is commonly paclitaxel/carboplatin. Recently, the combination of lenvatinib and pembrolizumab was approved for the treatment of patients with advanced, metastatic endometrial cancer who have disease progression following prior systemic therapy with a platinum doublet. With the lack of available therapies for patients who progress after standard of care therapies, long-term survival prospects are poor and novel treatments offering even a modest improvement in progression-free survival or overall survival may be considered for expedited regulatory approval.

There remains a significant unmet need in the treatment of CBF/GLIS AML in pediatric patients. The CBF/GLIS subtype of AML is a rare, aggressive form of AML that has never been found in adult AML patients, and typically affects pediatric patients with a median age of 1.5 years. The prevalence of CBF/GLIS AML is 1%-3% in childhood AML, and in recent studies the incidence was determined to be 1.3%-1.8% of pediatric AML patients. The prognosis for this disease is grim, with a 5-year overall survival of 15-30%. The first-line treatment for this disease is chemotherapy with a goal of reducing disease burden to the point that the patient can receive a bone marrow transplant. Patients who are refractory to primary chemotherapy or who relapse following bone marrow transplant have no additional treatment options and have very poor treatment outcomes. Given the lack of treatment options for these patients, a novel treatment that offers an opportunity for these patients to become eligible for bone marrow transplantation may be considered for expedited regulatory review.

There have been a number of FolRα-targeted therapies in development, including naked antibodies, small molecule drug conjugates, ADCs, and T cell retargeting molecules. The most advanced clinically active agent targeting FolRα to date has been Immunogen’s ELAHERE™ (mirvetuximab soravtansine IMGN853), an ADC composed of a FolRα-binding antibody linked to the tubulin-disrupting maytansinoid, DM4, via a cleavable linker.

In November 2022, the FDA granted accelerated approval to Immunogen for ELAHERE™ for the treatment of adult patients with FRα-positive, platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer, who have received one to three prior systemic treatment regimens. Patients are selected for therapy based on an FDA-approved test that defines FRα-positivity by 2+ and/or 3+ IHC staining in ≥75% of tumor cells.

BMS and Eisai Co., Ltd., or Eisai, are also co-developing a FolRα-targeted ADC for the treatment of cancers, including ovarian cancers, identified as MORAb-202. MORAb-202 is an ADC made from the anti-FolRα antibody farletuzumab conjugated to an average of four eribulin molecules via a cleavable linker. Public information about MORAb-202 development is limited, but a Phase II efficacy study of MORAb-202 was described as open and enrolling on the clinical trials.gov website in the fourth quarter of 2022. Also, Elucida Oncology is developing a nanoparticle C’Dot drug conjugate targeting FolRα.

Luvelta targets FolRα, a surface protein with limited expression on normal tissue and overexpressed in multiple cancers, including ovarian cancer, which makes FolRα a promising ADC approach.

Luvelta employs a cleavable linker that releases a cytotoxic drug inside tumor cells, while being stable and resistant to cleavage in general circulation. The cytotoxic drug used is our proprietary hemiasterlin moiety. From a safety perspective, we designed luvelta to have what we believe to be the optimal potency-to-safety ratio. We therefore rationally selected a homogenous ADC with an optimized DAR of four.

Based on preclinical findings, we believe our efficient homogeneous design of luvelta could provide anti-tumor activity, stability, and safety with the potential to minimize off-target damage and improve clinical benefit. We believe an improved therapeutic index could differentiate luvelta from conventional technology for the treatment of ovarian cancer and endometrial cancer. To test this, we have created a benchmark FolRα-targeting surrogate molecule based on conventional technology that has a heterogeneous ADC, with a similar DAR utilizing a DM4 linker-warhead. We have tested this benchmark molecule against luvelta in multiple preclinical models. However, additional preclinical and clinical testing will be needed to determine the safety and efficacy of luvelta and to obtain regulatory approval, if ever obtained.

Our first Phase 1 trial for luvelta is an open-label study evaluating luvelta as a monotherapy for patients with ovarian and endometrial cancers. This trial is being conducted in two-parts, dose escalation and dose expansion. We began enrolling ovarian cancer patients in March 2019, with updated data for the dose escalation cohort reported in December 2020 and May 2021. The primary objectives of the clinical trial are to determine the safety and tolerability profile, to define the recommended Phase 2 dose level and interval, and to evaluate preliminary anti-tumor activity. Our secondary objectives are to characterize human pharmacokinetics and additional safety, tolerability, and efficacy measures.

We initially enrolled adult patients with advanced and/or refractory ovarian cancer, for whom no suitable treatment exists. These patients are considered to have incurable disease and need repeated courses of life-prolonging and palliative treatment. The initial Phase 1 trial enrolled ovarian cancer patients regardless of their FolRα expression levels. These ovarian cancer patients were enrolled in a dose escalation cohort, with luvelta administered on day one of a 21-day cycle. Since anti-tumor activity was observed during the fully enrolled dose escalation portion of the Phase 1 trial, we initiated enrollment of patients in the dose expansion portion of this clinical study in January 2021 and are treating less heavily pre-treated ovarian cancer patients. The dose expansion portion of this Phase 1 study of luvelta is fully enrolled and currently ongoing.

In May 2021, we announced updated data from the dose-escalation portion of our ongoing Phase 1 clinical trial of STRO-002 in patients with ovarian cancer. The dose-escalation portion of the trial was fully enrolled with 39 patients in August 2020. Patients were heavily pre-treated and had a median of six prior lines of therapy, including standard of care platinum-based regimens, bevacizumab, PARP inhibitors, and checkpoint inhibitors.

The dose-escalation portion of the Phase 1 trial included 34 patients treated with clinically active dose levels, 2.9 mg/kg or higher, of which 31 patients had post-baseline scans and were evaluable for RECIST response. At the data cutoff of April 23, 2021, results out of 31 evaluable patients included:

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86% of treatment-emergent adverse events, or TEAEs, were Grade 1 or 2. The most common Grade 3 and 4 AEs were neutropenia (64%), arthralgia (13%), fatigue (10%), neuropathy (8%), and abdominal pain (8%), all of which were managed with standard medical treatment, dose reductions, or dose delays.

Based on the results of the dose escalation portion of the Phase 1 trial discussed above, we identified dose levels of 4.3 and 5.2 mg/kg to study in the dose-expansion portion of the Phase 1 trial. For the dose-expansion portion, we dosed the first patient in January 2021 and are treating less heavily pre-treated ovarian cancer patients. We reported initial dose-expansion data in January 2022 and preliminary final data in January 2023. We also initiated an exploratory dose expansion cohort of 15 patients to assess the safety of treatment with luvelta at 5.2 mg/kg in combination with prophylactic pegfilgrastim, and interim results from this cohort were also presented in January 2023.

The dose-expansion cohort for ovarian cancer fully enrolled 44 patients, who had experienced up to three prior lines of therapy and were randomized into dose levels starting at 4.3 mg/kg (N=23) and 5.2 mg/kg (n=21). 81% of the patients were platinum-resistant, and 66% and 82% of the patients had been treated previously with bevacizumab and PARP inhibitors, respectively.

The patients were also assessed for FolRα expression. Higher FolRα expression levels calculated using tumor proportion scores, or TPS, correlated with higher response rates. We have identified TPS as a potentially appropriate scoring algorithm for luvelta with respect to the biomarker enrichment strategy. Of the 44 patients in this cohort, 9 had a TPS score of less than or equal to 25%, while 35 had a TPS score of greater than 25%. Of these 35 patients, as of the data cutoff date of November 8, 2022, 32 had at least one post-baseline scan, and therefore were evaluable for RECIST v1.1 responses.

The results demonstrated that luvelta provided substantial clinical benefit in FolRα-selected patients, defined by TPS of >25%, with a 37.5% overall response rate (ORR), median DOR of 5.5 months, and median PFS of 6.1 months, regardless of starting dose. Results also demonstrated the higher starting dose of 5.2 mg/kg provided greater patient benefit compared to the lower starting dose of 4.3mg/kg. FolRα-selected patients account for approximately 80% of the patient population in advanced ovarian cancer, as represented in the patient stratification in the Phase 1 study.

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Luvelta, when given to FolRα-selected patients at a starting dose of 5.2 mg/kg, provided greater patient benefit than a starting dose of 4.3 mg/kg, with the 5.2 mg/kg dose of luvelta demonstrating an ORR of 43.8% (n=16), a median DOR of 5.4 months (n=7) and a median PFS of 6.6 months (n=16)

Safety signals from the 44 patients at the 5.2 mg/kg and 4.3 mg/kg starting dose levels, were consistent with data from the dose-escalation cohort, including:

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Neutropenia was the leading TEAE that resulted in a treatment delay or a dose reduction. The majority of the cases of neutropenia were generally asymptomatic and resolved with a one-week dose delay or, in other cases, with standard medical treatment, including the use of G-CSF.

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There were limited observed cases of febrile neutropenia, including one Grade 5 event at the 5.2 mg/kg starting dose level and one Grade 3 event at the 4.3 mg/kg starting dose level. The trial protocol was subsequently updated to require dose reduction for Grade 4 neutropenia.

In 2022 we initiated an exploratory cohort, or cohort C, of 15 patients to assess the safety of treatment with luvelta at 5.2 mg/kg in combination with prophylactic pegfilgrastim and presented preliminary data from ten patients from this cohort in January 2023. Early results from these initial 10 patients in cohort C, when compared to patients who were not given prophylactic pegfilgrastim in the dose-expansion cohort at the 5.2 mg/kg dose (n=21) demonstrated substantial reductions in Grade 3+ neutropenia and instances of dose delays. In particular:

Additionally, we opened for enrollment a Phase 1 trial to assess the combination of STRO-002 and bevacizumab for treatment of ovarian cancer in December 2021 and began enrolling patients in an expansion cohort for FolRα-selected endometrial cancer in the fourth quarter of 2021. We expect to present initial results from each of these studies in the second half of 2023. Further, we plan to submit an IND for the treatment of NSCLC with luvelta by the end of 2023 or in early 2024.

We are also seeking to develop luvelta for the treatment of CBF/GLIS AML in pediatric patients. Initial access to luvelta in this indication has been provided through compassionate use. Initial data on the anti-leukemic activity of luvelta in 17 pediatric patients with relapsed/refractory CBF/GLIS AML was presented at ASH 2022.

The ASH 2022 presentation included results from 17 patients treated with luvelta that were relapsed or refractory to standard of care treatments. The median age of the patients treated was two years old and the median number of prior therapies was two. Eight of the patients had previously undergone a stem cell transplant (SCT). Luvelta was well-tolerated as a monotherapy agent and in combination with standard of care therapies. In the 17 patients treated, Best Overall Response (BOR) included eight patients with complete remission (CR), of which seven patients were minimal residual disease (MRD) negative. 47% of the patients achieved complete remission and 53% of the patients achieved partial response or stable disease. Responders were observed in varying contexts, including those with or without prior stem cell transplant and in monotherapy or in combination with cytotoxic therapy.

We met with the FDA to discuss our clinical development plan for luvelta to treat pediatric CFB/GLIS AML in January 2023 and plan to submit an IND for this indication in the first half of 2023.

We are developing STRO-001, an optimally designed ADC directed against the cancer target CD74, for multiple myeloma and NHL. STRO-001 was designed and optimized for maximal therapeutic index by placing linker-warheads at specific locations within the antibody using our proprietary XpressCF+® platform. We have completed enrollment for STRO-001 in a Phase 1 trial for multiple myeloma and NHL for which we reported updated data in December 2020. Based on such reported data, STRO-001 has been generally well-tolerated and no ocular toxicity signals have been observed, with no patients receiving prophylactic corticosteroid eye drops. The maximum tolerated dose of STRO-001 has been identified. We have paused further enrollment of patients in the STRO-001 Phase 1 trial following identification of the maximum tolerated dose. In October 2018, we were granted Orphan Drug Designation by the FDA for STRO-001 for the treatment of multiple myeloma. In October 2021, we entered into the BioNova Option Agreement.

CD74 is a transmembrane glycoprotein, or a protein with an attached sugar that spans the inside and outside of a cell. While normal tissues appear to have minimal CD74 expression levels, CD74 is an important B cell target for multiple myelomas and lymphomas. CD74 is expressed in approximately 90% of B cell cancers, including multiple myeloma and lymphoma.

Currently, there are no approved therapeutics that specifically target CD74 for treatment of B cell malignancies. We believe earlier ADCs being developed against the target CD74 were ineffective either because

they failed to achieve sufficient killing of malignant B cells or they were unable to achieve a sufficient therapeutic benefit before toxicities limited further dose escalations.

B cell malignancy tumor subtypes include multiple myeloma and NHL, which includes mantle cell lymphoma, diffuse large B cell lymphoma, or DLBCL, and follicular lymphoma. In the United States alone in 2019, the prevalence of multiple myeloma and NHL was estimated at more than 920,000 cases, and the American Cancer Society estimated that there would be more than 116,000 new cases of multiple myeloma and NHL in 2023. Although several therapeutics have recently been approved for the treatment of specific B cell malignancies, including immunotherapies, targeted kinase inhibitors, ADC and CAR-T cell therapies, unmet need persists. Many of these therapeutics are typically used in combination with other agents to provide the most potent anti-cancer effect. While these new therapies have demonstrated improvements in survival, the majority of these patients ultimately relapse during treatment and some experience a resistance to therapy.

Our first internally developed product candidate is STRO-001, which we believe has the potential to be a first-in-class and best-in-class ADC directed against the cancer target CD74, an antigen that is highly expressed in many B cell malignancies and is an attractive target for an ADC therapeutic, given its rapid internalization by the cell. STRO-001 is an ADC targeting the CD74 protein antigen that was developed using our proprietary XpressCF® and XpressCF+® platforms. STRO-001 is composed of an antibody stably conjugated to a highly potent cytotoxic drug, a maytansinoid derivative, at two specific sites on the antibody using a non-cleavable linker. STRO-001 degrades inside of tumor cells to release very potent intracellular catabolites whose hydrophilic nature results in poor permeability into surrounding cells. We believe this decreases the potential of off-target effect in normal tissues. From a safety perspective, we designed STRO-001 to have an optimal potency to toxicity ratio. We rationally selected a homogeneous ADC with a drug-antibody ratio, or DAR, of two. Heterogeneous ADCs typically have DARs that range from zero to eight, with lower DARs generally being associated with less potency and higher DARs generally being associated with a negative impact on pharmacokinetics and toxicity. We chose a DAR of two after demonstrating that DARs of four or six did not increase the preclinical efficacy of STRO-001. In October 2018, we were granted orphan drug designation by the FDA, for STRO-001 for the treatment of multiple myeloma.

The Phase 1 trial for STRO-001 is an open-label study that is evaluating STRO-001 as a monotherapy for patients with multiple myeloma and NHL. The trial is being conducted in two parts: dose escalation and dose expansion. The primary objectives of the trial are to determine the safety and tolerability profile of STRO-001, determine the recommended Phase 2 dose and interval and evaluate preliminary anti-tumor activity. The secondary objectives are to characterize the human pharmacokinetics of STRO-001 and additional safety, tolerability and efficacy measures.

Our Phase 1 trial of STRO-001 has been enrolling adult patients with advanced and/or refractory multiple myeloma and NHL (including DLBCL, mantle cell lymphoma and follicular lymphoma) who are refractory to, or intolerant of, all established therapies known to provide clinical benefit for their condition. Multiple myeloma and NHL patients have been enrolled in two separate dose escalation cohorts, starting initially with an accelerated dose titration design. Treatment is currently scheduled on day one of a 21-day cycle.

We submitted our IND for STRO-001 in December 2017 and the first patient was dosed in April 2018. In October 2018, we were granted Orphan Drug Designation by the FDA for STRO-001 for the treatment of multiple myeloma.

In December 2020, we reported data from the NHL cohort from the dose escalation portion of the Phase 1 trial as of October 30, 2020:

We have reached the maximum tolerated dose for STRO-001 and have paused further enrollment of patients in the Phase 1 study. The next phase of development for STRO-001 includes further dose optimization by BioNova in the Greater China territory. BioNova announced in February 2023 the dosing of the first patient in a phase 1 clinical study of STRO-001 for the treatment of advanced NHL.

In October 2021, we entered into the BioNova Option Agreement to confer BioNova the option to obtain exclusive rights to develop and commercialize STRO-001 in Greater China. BioNova intends to pursue the clinical development, regulatory approval, and commercialization of STRO-001 in multiple indications, including NHL, multiple myeloma, and leukemia in the licensed territory. We will retain development and commercial rights of STRO-001 globally outside of Greater China, including the United States.

We have recently progressed the development candidacy of STRO-003, a ROR1 ADC for the treatment of ROR1-expressing solid tumors, including triple negative breast cancer, or TNBC, NSCLC, and ovarian cancer. STRO-003 is an anti-ROR1 human IgG1 antibody conjugated using our XpressCF+® platform technology to a cleavable DBCO-PEGylated β-glucuronide-exatecan linker-payload, at a DAR of approximately 8. Currently, there are no therapeutics approved that specifically target ROR1, although there is one ROR1-targeting ADC, zilovertamab vedotin, or ZV (also known as MK-2140, VLS-101), in Phase 2 testing targeting DLBCL, mantle cell lymphoma, or MCL, NSCLC, and breast cancer. Based on preclinical in vitro and in vivo data, we believe that STRO-003 has the potential for an improved therapeutic index compared to ZV. We believe these features present a unique opportunity for clinical development of STRO-003 to address unmet medical needs in hematological malignancies, ovarian cancer, TNBC and NSCLC.

We believe STRO-003 has been precisely designed and optimized to provide the potential for a best-in-class ADC targeting ROR-1. Our proprietary non-natural amino acid, which provides the substrate for conjugation to our proprietary β-glucuronidase cleavable exatecan linker warhead, have been placed at what we believe are the optimal sites in the amino acid sequence of our high affinity anti-ROR1 antibody, resulting in enhanced performance and stability in preclinical in vitro and in vivo models. These models also suggest that our β-glucuronidase cleavable linkers may provide greater tumor specificity and enhanced tolerability relative to a protease-cleavable linker delivering an exatecan payload. In particular, in a non-human primate safety study, we did not observe neutropenia, ocular toxicity signals or lung toxicity signals even in the highest dose cohort for STRO-003. Finally, our preclinical testing has shown that the exatecan payload delivered by STRO-003 elicits potent tumor cell killing, bystander activity and immunogenic cell death, which we believe may provide meaningful clinical benefit to patients.

We believe ROR1 is a favorable target for an ADC due to its limited normal tissue expression, as well as its prevalence in solid tumors and B cell malignancies, including CLL, DLBCL, MCL, TNBC, NSCLC, and ovarian cancer. Its expression is correlated with poor prognosis in different cancers. Currently, there are no approved therapeutics that specifically target ROR1, but it is a target of increasing interest with several clinical-stage ADCs in development, including ZV (Phase 2), NBE-002 (Phase 1), and CS5001 (Phase 1). We have developed site-specific ADCs targeting ROR1 and have nominated STRO-003 for further development.

Our technology allows us to rapidly incorporate non-natural amino acids in varying numbers and positions, to identify the best cytokine modification for pharmacological activity, pharmacokinetics, and safety. Furthermore, our technology enables rapid preclinical development and transition to cGMP manufacturing, ensuring speed to clinic in a promising field. Our drug discovery teams are exploring novel immuno-oncology therapies, including cytokine-based therapies.

We are also actively researching to identify new ADCs to add to our pipeline. We have multiple ADC discovery programs ongoing using our XpressCF+® platform. Our protein engineering and chemistry efforts are focused on maximizing therapeutic indices, and our technology allows us to rapidly test our therapeutic hypothesis in significantly more product candidates than conventional protein synthesis allows in order to identify the best molecule to advance to the clinic. We have also expanded our ADC technology platform to include iADCs. Our XpressCF+® platform has enabled a groundbreaking technology to engineer homogeneous, dually conjugated immunostimulant and cytotoxic warheads on a single ADC molecule. Our novel iADC design is intended to deliver two different drugs directly to the tumor, and not only kill tumor cells but also locally prime an immune response to the patient’s particular tumor cells. We believe that our iADC approach creates a new therapeutic opportunity by combining the best features of an ADC with the biology of a personalized vaccine.

In addition, development of our XpressCF+® platform to enable homogenous, dually-conjugated iADCs also enables us to discover, develop and manufacture ADC2 molecules. In these ADC2 molecules, two different linker-warheads are precisely conjugated at specific positions to deliver two different small molecule payloads to a single cancer cell. We are actively investigating different combinations of payloads to identify synergistic pairings with differentiated toxicity profiles. We believe such ADC2 molecules have the potential to provide the next generation of highly potent cancer therapeutics with acceptable safety and tolerability.

Our bispecific antibody drug discovery programs are focused on bispecific ADCs. We believe such compounds can provide improved specificity for tumors and could potentially spare healthy tissues expressing one, but not both, of the antigens targeted by the bispecific ADC.

In July 2018, we entered into an Exclusive Patent License and Research Collaboration Agreement (the “2018 Merck Agreement”) with Merck to jointly develop up to three research programs focusing on cytokine derivatives for cancer and autoimmune disorders.

Under the 2018 Merck Agreement, we received from Merck a non-refundable, non-creditable, upfront payment of $60.0 million in August 2018 for access to our technology and the identification and preclinical research and development of two target programs, with an option for Merck to engage us to continue these activities for a third program upon the payment of an additional amount. The option to expand activities to a third program expired in January 2021. In December 2021, Merck did not extend the research term for the second research program of the collaboration and that research program reverted to us. The first program of the collaboration is focused on MK-1484, a distinct cytokine derivative molecule for the treatment of cancer.

In March 2020, Merck exercised its option to extend the research term of the collaboration’s first cytokine-derivative program by one year, which, pursuant to the terms of the 2018 Merck Agreement, triggered a payment

of $5.0 million to us. In the second quarter of 2021, we earned a $15.0 million contingent payment for the initiation by Merck of the first IND-enabling toxicology study under the first cytokine-derivative program in the collaboration. In September 2021, Merck agreed to extend the research term of this program for an additional two years to facilitate completion of preclinical research and development activities for a second candidate molecule in the program, which has a novel design and approach. In connection with the extension, we received an initial $2.5 million payment. Merck decided not to pursue further development of a second molecule under the first cytokine-derivative program of the collaboration and therefore allowed the option to extend the period for nomination of additional clinical candidates to expire in June 2022.

In July 2022, the first patient was dosed with MK-1484 in a Phase 1 study. As a result of this achievement, we received a $10.0 million contingent payment from Merck.

In August 2020, we entered into a Pre-Clinical and Clinical Supply Agreement with Merck, wherein Merck requested us to provide manufacturing services, including clinical product supply, upon completion of the research programs under the 2018 Merck Agreement. The consideration for the services is based on an agreed-upon level of FTE personnel effort and related reimbursement rate in addition to agreed-upon pricing for the clinical product supply.

We are also eligible to receive aggregate contingent payments of up to approximately $0.5 billion for the target program selected by Merck, assuming the development and sale of the related therapeutic candidate and all possible indications identified under the collaboration. If one or more products from the target program is developed for non-oncology or a single indication, we will be eligible for reduced aggregate milestone payments. In addition, we are eligible to receive tiered royalties ranging from mid-single digit to low teen percentages on the worldwide sales of any commercial products that may result from the collaboration.

Merck may terminate the 2018 Merck Agreement at any time with 60 days’ prior written notice. Either we or Merck has the right to terminate the 2018 Merck Agreement based on the other party’s uncured material breach or bankruptcy.

In November 2019, BMS acquired Celgene, and Celgene became a wholly owned subsidiary of BMS. In connection with such acquisition, BMS assumed the rights and obligations of the 2014 Celgene Agreement, 2017 Celgene Agreement and 2018 Celgene Master Services Agreement. Throughout this Annual Report, we refer to Celgene as BMS and our agreements with Celgene as the BMS Agreement and the 2018 BMS Master Services Agreement.

In September 2014, we signed a Collaboration and License Agreement with BMS to discover and develop bispecific antibodies and/or ADCs, focused primarily on the field of immuno-oncology, using our proprietary integrated cell-free protein synthesis platform, XpressCF®. In August 2017, we entered into an amended and restated collaboration and license agreement with BMS to refocus the collaboration on four programs that were advancing through preclinical development, including an ADC program targeting B cell maturation antigen, or the BCMA ADC, CC-99712.

Upon signing the BMS Agreement in 2014, we received an upfront, nonrefundable payment totaling $83.1 million.

In March 2015, we received a $15.0 million contingent payment from BMS that provided BMS a right to access certain of our technology for use in conjunction with certain BMS intellectual property. In June 2016, we received a $25.0 million milestone upon completion of certain preclinical activities. Additionally, in June 2016, we earned a $10.0 million substantive milestone for certain manufacturing accomplishments.

In August 2017, we received an option fee payment of $12.5 million from BMS. In each of October 2017 and December 2018, we received a $10.0 million milestone for certain manufacturing accomplishments.

In 2019, BMS initiated the Phase 1 clinical trial for CC-99712, a BCMA ADC which was discovered and is being manufactured by us. In the second half of 2021, BMS expanded their Phase 1 trial to include combination treatment with a gamma secretase inhibitor. BMS has worldwide development and commercialization rights with respect to CC-99712. We will continue to be responsible for clinical supply manufacturing and certain development services for the BCMA ADC and are eligible to receive from BMS aggregate development and regulatory contingent payments of up to $275.0 million, if approved in multiple indications, and tiered royalties ranging from mid to high single digit percentages on worldwide sales of any resulting commercial products.

In March 2018, we entered into a Master Development and Clinical Manufacturing Services Agreement, or the 2018 BMS Master Services Agreement, with BMS, wherein BMS requested us to provide development, manufacturing and supply chain management services, including clinical product supply. The consideration for the services is based on an agreed-upon level of FTE personnel effort and related reimbursement rate in addition to agreed-upon pricing for clinical product supply for use in Phase 1 clinical trials.

BMS may terminate the BMS Agreement at any time with 120 days’ prior written notice. Either we or BMS has the right to terminate the BMS Agreement based on the other party’s uncured material breach, challenge of the validity and enforceability of intellectual property, or bankruptcy.

We signed a Collaboration Agreement and a License Agreement with EMD Serono in May 2014 and September 2014, respectively, which were entered into in contemplation of each other. The Collaboration Agreement was subsumed into the License Agreement (the “MDA Agreement”), which agreement is to develop ADCs for multiple cancer targets. Our collaboration with EMD Serono has yielded a novel bispecific ADC product candidate targeting EGFR and MUC1, known as M1231, for which an IND submission was filed in the second half of 2020. Recently, EMD Serono decided to close the Phase 1a trial of M1231 in patients with solid tumors and not initiate a previously planned expansion study. EMD Serono stated that the decision was based on strategic portfolio considerations.

Upon signing the Collaboration Agreement, we received an upfront, nonrefundable, non-creditable payment totaling $10.0 million. Upon signing the MDA Agreement, we received an additional upfront, nonrefundable, non-creditable payment totaling $10.0 million and will receive financial support for research and development services to be provided by us, based on an agreed-upon level of FTE personnel effort and related reimbursement rate. Under a supply agreement with EMD Serono, we provide them with product candidate materials for IND-enabling and Phase 1 clinical studies. The consideration for any related services is based on an agreed-upon level of FTE personnel effort and related reimbursement rate in addition to agreed-upon pricing for providing the materials.

We are eligible to receive up to $52.5 million for M1231 under the MDA Agreement, primarily from pre-commercial contingent payments, of which we have earned and received a $1.5 million payment, a $1.0 million payment, and a $2.0 million payment in 2019, 2020 and 2021, respectively. In addition, we are eligible to receive tiered royalties ranging from low-to-mid single digit percentages, along with certain additional one-time royalties, on worldwide sales of any commercial products that may result from the MDA Agreement. The MDA Agreement term expires on a product-by-product and country-by-country basis. Upon expiration, EMD Serono will have a fully paid-up, royalty-free, perpetual, and irrevocable non-exclusive license, with the right to grant sublicenses, under certain of our intellectual property rights. EMD Serono may terminate the MDA Agreement at any time with 90 days’ prior written notice or upon the inability of us to provide EMD Serono access to a specified number of cancer drug targets. Either we or EMD Serono has the right to terminate the MDA Agreement based on the other party’s uncured material breach or bankruptcy.

In June 2022, we entered into a license and collaboration agreement with Astellas, or the Astellas Agreement, for the development of immunostimulatory antibody-drug conjugates for up to three biological targets, to be identified by Astellas. We will conduct research and pre-clinical development of any compound (as designated by Astellas) in each of the three programs in accordance with the terms of a research plan between us and Astellas. Astellas will have an exclusive worldwide license to develop and commercialize any such designated compound, subject to our rights to participate in cost and profit sharing in the United States, as described below.

Pursuant to the Astellas Agreement, we received from Astellas a one-time, nonrefundable, non-creditable, upfront payment of $90.0 million during the year ended December 31, 2022.

We are also eligible to receive up to $422.5 million in development, regulatory and commercial milestones for each product candidate, and tiered royalties ranging from low double-digit to mid-teen percentages on worldwide sales of any commercial products that may result from the collaboration, subject to customary deductions under certain circumstances. We can also elect to convert any product candidate into a cost and profit-sharing arrangement, for the United States only. In the event we make such election, we will share commercialization costs and profits relating to such product candidate equally with Astellas in the United States, and no royalties will be due from Astellas for net sales of such product candidates in the United States.

The Astellas Agreement contains customary provisions for termination, including by Astellas for convenience upon 30 days’ written notice and by either party for cause, including for material breach (subject to cure). We have certain reversion rights as to product candidates in connection with certain termination events.

In October 2007, we entered into an Amended and Restated Exclusive Agreement, or the Stanford License, with the Board of Trustees of the Leland Stanford Junior University (Stanford), that grants us an exclusive license, with the right to sublicense, under the patent rights owned by Stanford covering certain technology rights related to our XpressCF® expression system.

We were required to make milestone payments to Stanford of approximately $930,000 on the accomplishment of certain development and regulatory milestones, which total amount has been paid as of December 31, 2021. No additional milestone payments are due under the Stanford License. Additionally, we owe Stanford annual license maintenance fees of $75,000, which may be creditable against earned royalties in such year and are required to reimburse Stanford for ongoing patent-related costs. We are also required to pay to Stanford low single digit royalties on net sales and to share any sublicensing income received related to the licensed technology. We may terminate the agreement at any time upon 30 days’ written notice.

In 2013, we and Johnson & Johnson Innovation, through the Johnson & Johnson Development Corporation, provided initial co-funding for Vaxcyte, Inc., or Vaxcyte, with which we have a license agreement, a supply agreement and an option agreement related to certain development and manufacturing rights. Under the license agreement, Vaxcyte has the right to use the XpressCF® and XpressCF+®platforms to discover and develop vaccine candidates for the treatment or prophylaxis of infectious diseases. The lead program for Vaxcyte is VAX-24, its 24-valent pneumococcal conjugate vaccine candidate. Vaxcyte is responsible for performing all research and development activities, and we provide technical support and supply XtractCF® and other materials to Vaxcyte. In the first quarter of 2022, Vaxcyte announced initiation of a Phase 1/2 clinical proof-of-concept study of VAX-24, under investigation for the prevention of invasive pneumococcal disease in adults and announced in October 2022 positive topline data from such study in adults aged 18-64.

In May 2018, we entered into a Supply Agreement with Vaxcyte, wherein Vaxcyte engaged us to supply extracts and custom reagents, as requested by Vaxcyte. The pricing is based on an agreed upon cost plus arrangement.

In December 2022, we entered into a letter agreement (the “Vaxcyte Agreement”) with Vaxcyte and granted Vaxcyte an option to obtain development and manufacturing rights for XtractCF® that, when exercised, would grant Vaxcyte the right to make and source our cell-free extract for research, development, and manufacture of vaccines for the prophylaxis and treatment of infectious disease.

Pursuant to the Vaxcyte Agreement, we received a one-time, nonrefundable, non-creditable, upfront payment of $10.0 million in cash, and 167,780 shares of Vaxcyte common stock with a fair value of $7.5 million in December 2022. We will receive an additional nonrefundable, non-creditable payment of $5.0 million after we and Vaxcyte mutually agree in writing upon the Form Definitive Agreement that will become effective upon Vaxcyte’s exercise of the option. In the event that Vaxcyte elects to exercise the option, Vaxcyte will pay us $75.0 million in cash in two installments, and upon the occurrence of certain regulatory milestones, certain additional milestone

payments totaling up to $60.0 million. In the event that Vaxcyte undergoes a change of control, and subsequently exercises the option, a substantial majority of the milestone payments are accelerated.

We hold 0.7 million shares of common stock of Vaxcyte and are eligible for four percent royalties on worldwide net sales of any vaccine candidates for human health use under the license agreement. Also, we retain the right to discover and develop vaccines for the treatment or prophylaxis of any disease that is not caused by an infectious pathogen, including cancer.

Vaxcyte has the right to terminate the Vaxcyte license agreement for convenience upon prior written notice. Either party may terminate for the other party’s material uncured breach under certain circumstances.

In December 2021, we entered into the Tasly License Agreement with Tasly to grant an exclusive license to develop and commercialize STRO-002 in Greater China. Tasly will pursue the clinical development, regulatory approval, and commercialization of STRO-002 in multiple indications, including ovarian cancer, non-small cell lung cancer, triple-negative breast cancer, and other indications in Greater China. We retained development and commercial rights of STRO-002 globally outside of Greater China, including the United States.

Under the Tasly License Agreement, Tasly was obligated to make an initial payment to us of $40.0 million, with additional potential payments totaling up to $345.0 million related to development, regulatory and commercialization contingent payments and milestones. We will provide STRO-002 to Tasly under appropriate clinical and commercial supply service agreements. Upon commercialization, we will receive tiered royalties, ranging from low- to mid-teen percentages based on annual net sales of STRO-002 in Greater China for at least ten years following the first commercial sale of STRO-002 in Greater China. In February 2022, Tasly indicated that it would like to discuss and renegotiate the terms of the Tasly License Agreement.

In April 2022, we entered amendment No. 1, or the Tasly Amendment, to the Tasly License Agreement. Pursuant to the Tasly Amendment, the initial nonrefundable upfront payment due by Tasly was amended to $25.0 million, and a $15.0 million payment will become payable to us upon the achievement of certain regulatory milestones. The Tasly Amendment also added an additional regulatory milestone payment to the Tasly License Agreement, providing additional potential payments totaling up to $350.0 million related to development, regulatory and commercialization milestones, beyond the payments described above, and made certain other ministerial edits.

Tasly has the right to terminate the Tasly License Agreement for convenience or other reasons specified in the Tasly License Agreement, upon prior written notice.

In October 2021, we entered into the BioNova Option Agreement to confer BioNova the right to obtain exclusive rights to develop and commercialize STRO-001 in Greater China and amended the BioNova Option Agreement with BioNova in the first quarter of 2023. BioNova will pursue the clinical development, regulatory approval, and commercialization of STRO-001 in multiple indications, including NHL, multiple myeloma, and leukemia in the licensed territory. We retain development and commercial rights of STRO-001 globally outside of Greater China, including the United States.

Under the BioNova Option Agreement, BioNova paid the Company an initial licensing option payment of $4.0 million, with potential payments totaling up to $199.0 million related to the initial payment, option exercise, development, regulatory, and commercial milestones, including the right to exercise the license option for a payment of $12.0 million. We will provide STRO-001 to BioNova under appropriate clinical and commercial supply service agreements. Upon commercialization, we will receive tiered royalties ranging from low- to mid-teen percentages based on annual net sales of STRO-001 in Greater China for at least ten years following the first commercial sale of STRO-001 in Greater China.

BioNova has the right to terminate the BioNova Option Agreement for convenience or other reasons specified in the BioNova Option Agreement, upon prior written notice.

We have significant expertise in the production of therapeutic biologics. Our proprietary XpressCF® platform is a cell-free protein synthesis technology that enables rapid and systematic process development, streamlined scale-up and GMP manufacturing.

We manufacture our cell-free extract and related reagents in our GMP manufacturing facility in San Carlos, California for our clinical trials and supply commitments. We have identified a contract manufacturing organization, or CMO, to serve as our strategic partner for the production of cell-free extract and have initiated technology transfer to this CMO. Similarly, we have identified a CMO to produce custom reagents used in our cell-free production and have initiated this technology transfer as well. We expect both technology transfers to be substantially complete in 2023.

Our process development and manufacturing strategies are tailored to rapidly advance our product candidates, including the use of a supply chain of established CMOs to ensure successful execution. The production of antibodies will be done by either us or CMOs, depending on our internal cGMP production capacity. We have identified a CMO to produce the antibody component of our products at scale and technology transfer of the manufacturing process is underway. The production of all other necessary elements for the manufacture of our ADC product candidates, and the final manufacture of the ADC drug product, will be handled entirely by CMOs. Our XpressCF+® platform has been successfully used for manufacturing several antibodies containing non-natural amino acids and requires minimal process optimization to support early clinical phase manufacturing. We utilize industry established production steps for the purification of our antibodies. The CMOs we have selected have strong track records in cGMP manufacturing with expertise in clinical or commercial drug manufacturing for cytotoxic agents, large scale manufacture of antibodies, conjugation and fill-finish of therapeutic biologics. All activities from cell-free extract production to formulated drug product are performed to maintain aggressive timelines and minimize delays.

The biotechnology and biopharmaceutical industries, and the immuno-oncology subsector, are characterized by rapid evolution of technologies, fierce competition, and strong defense of intellectual property. Any product candidates that we successfully develop and commercialize will have to compete with existing therapies and new therapies that may become available in the future. While we believe that our proprietary XpressCF® platform and scientific expertise in the field of biologics and immuno-oncology provide us with competitive advantages, a wide variety of institutions, including large biopharmaceutical companies, specialty biotechnology companies, academic research departments and public and private research institutions, are actively developing potentially competitive products and technologies. We face substantial competition from biotechnology and biopharmaceutical companies developing products in immuno-oncology. Our competitors include larger and better funded biopharmaceutical, biotechnological and therapeutics companies, including companies focused on cancer immunotherapies, such as AstraZeneca PLC, BMS, GlaxoSmithKline PLC, Johnson & Johnson, Merck Sharp & Dohme LLC, Novartis AG, Pfizer Inc., or Pfizer, Roche Holding Ltd, Sanofi S.A., and companies focused on ADCs, such as BMS, Pfizer, GlaxoSmithKline PLC, Daiichi Sankyo Company, Limited, Eisai, Co., Ltd., ImmunoGen, Inc., Eli Lilly & Company, Pfizer, Exelixis, Inc., Seagen, Inc., Astellas Pharma Inc., Genentech, Inc., or Genentech, Gilead Sciences Inc., Mersana Therapeutics, Inc., and ADC Therapeutics SA, as well as numerous small companies. Moreover, we also compete with current and future therapeutics developed at universities and other research institutions.

If our most advanced product candidates are approved, they will compete with a range of therapeutic treatments that are either in development or currently marketed. Currently marketed oncology therapeutics include a range of biologic modalities with the top selling products by class spanning tumor targeting monoclonal antibodies, such as Johnson & Johnson’s Darzalex; to ADCs, such as Genentech’s Kadcyla; to immune checkpoint inhibitors, such as Merck’s Keytruda; to T cell-engager immunotherapies, such as Amgen, Inc.’s Blincyto; and to CAR-T cell therapies, such as Gilead’s Yescarta. In addition, numerous compounds are in clinical development for cancer treatment. The clinical development pipeline for cancer includes small molecules, antibodies, vaccines, cell therapies and immunotherapies from a variety of companies and institutions.

We also face substantial competition from biotechnology and biopharmaceutical companies developing products with FolRα-targeted therapies, including naked antibodies, small molecule drug conjugates, ADCs, and T cell retargeting molecules. The most advanced clinically active agent targeting FolRα to date has been Immunogen’s ELAHERE™ (mirvetuximab soravtansine IMGN853), an ADC composed of a FolRα-binding antibody linked to the tubulin-disrupting maytansinoid, DM4, via a cleavable linker. BMS and Eisai, are also co-developing a FolRα-targeted ADC for the treatment of cancers, including ovarian cancers, identified as MORAb-202.

Many of our competitors, either alone or with strategic partners, have substantially greater financial, technical, manufacturing, marketing, sales, supply and human resources or experience than we have. Accordingly, our competitors may be more successful than us in obtaining approval for treatments and achieving widespread market acceptance, rendering our treatments obsolete or non-competitive. Accelerated merger and acquisition activity in the biotechnology and biopharmaceutical industries may result in even more resources being concentrated among a smaller number of our competitors. These companies also compete with us in recruiting and retaining qualified scientific and management personnel, establishing clinical trial sites and patient registration for clinical trials, and acquiring technologies complementary to, or necessary for, our programs. Smaller or early-stage companies may also prove to be significant competitors, particularly through collaborative arrangements with large and established companies. Our commercial opportunity could be substantially limited in the event that our competitors develop and commercialize products that are more effective, safer, less toxic, more convenient or less expensive than our comparable products. In geographies that are critical to our commercial success, competitors may also obtain regulatory approvals before us, resulting in our competitors building a strong market position in advance of the entry of our products. We believe the factors determining the success of our programs will be the efficacy, safety and convenience of our product candidates.

The regulations that govern pricing and reimbursement for new drugs and therapeutic biologics vary widely from country to country. Some countries require approval of the sale price of a drug or therapeutic biologic before it can be marketed. In many countries, the pricing review period begins after marketing approval is granted. In some foreign markets, prescription biopharmaceutical pricing remains subject to continuing governmental control even after initial approval is granted. As a result, a drug company can obtain regulatory approval for a product in a particular country, but then be subject to price regulations that delay commercial launch of that product.

A drug company’s ability to commercialize any products successfully will also depend in part on the extent to which coverage and adequate reimbursement for these products and related treatments will be available from government authorities, private health insurers and other organizations. Even if one or more products are successfully brought to the market, these products may not be considered cost-effective, and the amount reimbursed for such products may be insufficient to allow them to be sold on a competitive basis. Increasingly, third-party payors who reimburse patients or healthcare providers, such as government and private insurance plans, are requiring that drug companies provide them with predetermined discounts from list prices and are seeking to reduce the prices charged or the amounts reimbursed for biopharmaceutical products.

Significant delays can occur in obtaining reimbursement for newly approved drugs or therapeutic biologics, and coverage may be more limited than the purposes for which the drug or therapeutic biologic is approved by the FDA or similar foreign regulatory authorities. Moreover, eligibility for reimbursement does not imply that any drug or therapeutic biologic will be reimbursed in all cases or at a rate that covers a drug company’s costs, including research, development, manufacture, sale and distribution.

Interim reimbursement levels for new drugs or therapeutic biologics, if applicable, may also be insufficient to cover a drug company’s costs and may not be made permanent. Reimbursement rates may be based on payments allowed for lower cost drugs or therapeutic biologics that are already reimbursed, may be incorporated into existing payments for other services and may reflect budgetary constraints or imperfections in Medicare data. Net prices for drugs or therapeutic biologics may be reduced by mandatory discounts or rebates required by government healthcare programs or private payors and by any future relaxation of laws that presently restrict imports of drugs or therapeutic biologics from countries where they may be sold at lower prices than in the United States. Further, no uniform policy for coverage and reimbursement exists in the United States. Third-party payors often rely upon Medicare coverage policy and payment limitations in setting their own reimbursement rates, but also have their own methods and approval processes apart from Medicare determinations. Therefore, coverage and reimbursement can differ significantly from payor to payor.

We strive to protect and enhance the proprietary technology, inventions, and improvements that are commercially important to our business, including seeking, maintaining, and defending patent rights, whether developed internally or licensed from third parties. Our policy is to seek to protect our proprietary position by, among other methods, pursuing and obtaining patent protection in the United States and in jurisdictions outside of the United States related to our proprietary technology, inventions, improvements, platforms, and product candidates that are important to the development and implementation of our business. Our patent portfolio is intended to cover, but is not limited to, our technology platforms, our product candidates, and components thereof, their methods of use and processes for their manufacture, our proprietary reagents and assays, and any other inventions that are commercially important to our business. We also rely on trade secret protection of our confidential information and know-how relating to our proprietary technology, platforms, and product candidates, continuing innovation, and in-licensing opportunities to develop, strengthen, and maintain our proprietary position in our XpressCF® platform and product candidates. We expect to rely on data exclusivity, market exclusivity, patent term adjustment and patent term extensions when available. Our commercial success may depend in part on our ability to obtain and maintain patent and other proprietary protection for our technology, inventions, and improvements; to preserve the confidentiality of our trade secrets; to maintain our licenses to use intellectual property owned or controlled by third parties; to defend and enforce our proprietary rights, including our patents; to defend against and challenge the assertion by third parties of their purported intellectual property rights; and to operate without the unauthorized infringement on the valid and enforceable patents and other proprietary rights of third parties.

We believe that we have a strong global intellectual property position and substantial know-how and trade secrets relating to our XpressCF® platform technology, platform, and product candidates. Our patent portfolio as of December 31, 2022, contained 26 U.S. issued patents and 235 patents issued in ex-U.S. jurisdictions, including Europe, China, Japan, Australia and Singapore, and 42 U.S. pending applications, as well as 97 patent applications pending in ex-U.S. jurisdictions, including Europe, China, Japan, Australia and Singapore owned solely by us. These patents and patent applications include claims relating to:

Our issued patents, and any patents that may issue from our pending patent applications, in our solely owned patent portfolio are expected to expire between January 2030 and November 2043, absent any patent term adjustments or extensions.

In addition, we have exclusively licensed the following patent portfolio from Stanford: 10 U.S. issued patents and 35 patents issued in ex-U.S. jurisdictions, including Europe, China, Canada, India, Australia, South Korea, Eurasia and Singapore. This patent portfolio includes claims relating to methods related to in vitro protein synthesis that we use in our XpressCF® platform when discovering, developing and manufacturing our product candidates.

Remaining patents in our patent portfolio licensed from Stanford are expected to expire between August 2023 and January 2028, absent any patent term adjustments or extensions.

As for the XpressCF® platform, product candidates and processes we develop and commercialize, in the normal course of business, we intend to pursue, where appropriate, patent protection or trade secret protection relating to compositions, methods of manufacture, assay methods, methods of use, treatment of indications, dosing and formulations. We may also pursue patent protection with respect to product development processes and technology.

The following table describes the material patents and patent applications owned or licensed by us.

We continually assess and refine our intellectual property strategy as we develop new platform technologies and product candidates. To that end, we are prepared to file additional patent applications if our intellectual property strategy requires such filings, or where we seek to adapt to competition or seize business opportunities. Further, we are prepared to file patent applications, as we consider appropriate under the circumstances relating to the new technologies that we develop. In addition to filing and prosecuting patent applications in the United States, we often file counterpart patent applications in the European Union and in additional countries where we believe such foreign filing is likely to be beneficial, including but not limited to any or all of Australia, Brazil, Canada, China, Hong Kong, India, Israel, Japan, Mexico, New Zealand, Singapore, South Africa, South Korea, and Taiwan.

The term of individual patents depends upon the laws of the countries in which they are obtained. In most countries in which we file, the patent term is 20 years from the earliest date of filing of a non-provisional patent application. However, the term of United States patents may be extended for delays incurred due to compliance with the FDA requirements or by delays encountered during prosecution that are caused by the United States Patent and Trademark Office, or the USPTO. For example, the Hatch-Waxman Act permits a patent term extension for FDA-approved drugs of up to five years beyond the expiration of the patent. The length of the patent term extension is related to the length of time the drug is under regulatory review. Patent extension cannot extend the remaining term of a patent beyond a total of 14 years from the date of product approval, and only one patent applicable to an approved drug may be extended. Similar provisions are available in Europe and other jurisdictions to extend the term of a patent that covers an approved drug. In the future, if and when our biopharmaceutical product candidates receive FDA approval, we expect to apply for patent term extensions on patents covering those product candidates. We intend to seek patent term extensions to any of our issued patents in any jurisdiction where these are available; however, there is no guarantee that the applicable authorities, including the USPTO and FDA, will agree with our assessment of whether such extensions should be granted, and even if granted, the length of such extensions. Our currently issued patents will likely expire on dates ranging from 2030 to 2039, unless we receive patent term extension or patent term adjustment, or both. If patents are issued on our pending patent applications, the resulting patents are projected to expire on dates ranging from 2033 to 2043, unless we receive patent term extension or patent term adjustment, or both. However, the actual protection afforded by a patent varies on a product-by-product basis, from country-to-country, and depends upon many factors, including the type of patent, the scope of its coverage, the availability of regulatory-related extensions, the availability of legal remedies in a particular country and the validity and enforceability of the patent.

The patent positions of companies like ours are generally uncertain and involve complex legal and factual questions. No consistent policy regarding the scope of claims allowable in patents in the field of immunotherapy has emerged in the United States. The patent situation outside of the United States is even more uncertain. Changes in the patent laws and rules, either by legislation, judicial decisions, or regulatory interpretation in the United States and other countries may diminish our ability to protect our inventions and enforce our intellectual property rights, and more generally could affect the value of our intellectual property. In particular, our ability to stop third parties from making, using, selling, offering to sell, or importing any of our patented inventions, either directly or indirectly, will depend in part on our success in obtaining, defending, and enforcing patent claims that cover our technology, inventions, and improvements. With respect to both licensed and company-owned intellectual property, we cannot be sure that patents will be granted with respect to any of our pending patent applications or with respect to any patent applications filed by us in the future, nor can we be sure that any of our existing patents or any patents that may be granted to us in the future will be commercially useful in protecting our platforms and product candidates and the methods used to manufacture those platforms and product candidates. Moreover, even our issued patents do not guarantee us the right to practice our technology in relation to the commercialization of our platform’s product candidates. However, the area of patent and other intellectual property rights in biotechnology is an evolving one with many risks and uncertainties, and third parties may have blocking patents that could be used to prevent us from commercializing our patented XpressCF® technology, platforms and product candidates and practicing our proprietary technology. Our issued patents and those that may issue in the future may be challenged, invalidated, or circumvented, which could limit our ability to stop competitors from marketing related platforms or product candidates or limit the length of the term of patent protection that we may have for our XpressCF® technology, platforms, and product candidates. In addition, the rights granted under any issued patents may not provide us with protection or competitive advantages against competitors with similar technology. Furthermore, our competitors may independently develop similar technologies. For these reasons, we may have competition for our XpressCF® technology, platforms, and product candidates. Moreover, because of the extensive time required for development, testing and regulatory review of a potential product, it is possible that, before any particular product candidate can be commercialized, any related patent may expire or remain in force for only a short period following commercialization, thereby reducing any advantage of the patent. For this and more comprehensive risks related to our proprietary technology, inventions, improvements, platforms, and product candidates, please see the section entitled “Risk Factors—Risks Related to Intellectual Property.”

We intend to file applications for trademark registrations in connection with our product candidates in various jurisdictions, including the United States. We have filed for trademark protection of the Sutro Biopharma marks, the XpressCF® mark and the XpressCF+® mark with the USPTO. Additionally, we filed for trademark protection of the ProteinSARTM mark, XpressPDF® mark, XpressRNAP® mark, XpressRS® mark, XpresstRNA® mark and

XtractCF® mark with the USPTO. XpressCF® refers to our cell-free protein synthesis technology as a whole, and XpressCF+® refers specifically to cell-free protein synthesis incorporating one or more non-natural amino acids. The Sutro Biopharma marks were registered by the USPTO in 2014 and 2018, the XpressCF® mark was registered by the USPTO in 2017, and XpressCF+® mark was registered by the USPTO in 2017. The XpressRNAP® mark, the XpressRS® mark, and the XpresstRNA® mark were registered in the USPTO in 2021. The XpressPDF® mark and the XtractCF® mark were registered in the USPTO in 2022.

We also rely on trade secret protection for our confidential and proprietary information. Although we take steps to protect our confidential and proprietary information as trade secrets, including through contractual means with our employees and consultants, third parties may independently develop substantially equivalent proprietary information and techniques or otherwise gain access to our trade secrets or disclose our technology. Thus, we may not be able to meaningfully protect our trade secrets. It is our policy to require our employees, consultants, outside scientific collaborators, sponsored researchers and other advisors to execute confidentiality agreements upon the commencement of employment or consulting relationships with us. These agreements provide that all confidential information concerning our business or financial affairs developed or made known to the individual during the course of the individual’s relationship with us is to be kept confidential and not disclosed to third parties except in specific circumstances. In the case of employees, the agreements provide that all inventions conceived by the individual, and which are related to our current or planned business or research and development or made during normal working hours, on our premises or using our equipment or proprietary information, are our exclusive property. In many cases our confidentiality and other agreements with consultants, outside scientific collaborators, sponsored researchers and other advisors require them to assign or grant us licenses to inventions they invent as a result of the work or services they render under such agreements or grant us an option to negotiate a license to use such inventions.

We seek to preserve the integrity and confidentiality of our proprietary technology and processes by maintaining physical security of our premises and physical and electronic security of our information technology systems. Our Infosec Governance Committee, comprising senior executives and facilities and information technology employees, and under the supervision of our Audit Committee of our Board of Directors, is responsible for designing, implementing, monitoring and improving the security of our confidential and/or proprietary information. We conduct regular audits of our information security systems, including our on-site and cloud-based information systems and strive to continuously improve the robustness of our security and information recovery systems in the event of, for example, a cyberattack or natural disaster that compromises our data integrity. In addition, we conduct regular training and testing of our employees to identify, and report cyberattacks, including phishing and other forms of social engineering. We also maintain a limited insurance policy against cyberattacks that may provide a measure of compensation in the event that we are harmed by an information security attack. Although we have confidence in these individuals, organizations, and systems, our security measures have been breached in the past and may again be breached in the future, and we may not have adequate remedies for any breach. To the extent that our employees, contractors, consultants, collaborators, and advisors use intellectual property owned by others in their work for us, disputes may arise as to the rights in related or resulting know-how and inventions.

Government authorities in the United States, at the federal, state and local level, and in other countries and jurisdictions extensively regulate, among other things, the research, development, testing, manufacture, quality control, approval, packaging, storage, recordkeeping, labeling, advertising, promotion, distribution, marketing, post-approval monitoring and reporting, and import and export of pharmaceutical products. The processes for obtaining regulatory approvals in the United States and in foreign countries and jurisdictions, along with subsequent compliance with applicable statutes and regulations and other regulatory authorities, require the expenditure of substantial time and financial resources.

In the United States, pharmaceutical products are subject to extensive regulation by the FDA. The Federal Food, Drug, and Cosmetic Act, or the FDC Act, and other federal and state statutes and regulations, govern, among other things, the research, development, testing, manufacture, storage, recordkeeping, approval, labeling,

promotion and marketing, distribution, post-approval monitoring and reporting, sampling, and import and export of pharmaceutical products. Biological products used for the prevention, treatment, or cure of a disease or condition of a human being are subject to regulation under the FDC Act, except the section of the FDC Act which governs the approval of new drug applications, or NDAs. Biological products are approved for marketing under provisions of the Public Health Service Act, or PHS Act, via a Biologics License Application, or BLA. However, the application process and requirements for approval of BLAs are very similar to those for NDAs, and biologics are associated with similar approval risks and costs as drugs. Failure to comply with applicable U.S. requirements may subject a company to a variety of administrative or judicial sanctions, such as clinical hold, FDA refusal to approve pending BLAs, warning or untitled letters, product recalls, product seizures, total or partial suspension of production or distribution, injunctions, fines, civil penalties, and criminal prosecution.

Biological product development for a new product or certain changes to an approved product in the United States typically involves preclinical laboratory and animal tests, the submission to the FDA of an IND, which must become effective before clinical testing may commence, and adequate and well-controlled clinical trials to establish the safety and effectiveness of the biologic for each indication for which FDA approval is sought. Satisfaction of FDA pre-market approval requirements typically takes many years and the actual time required may vary substantially based upon the type, complexity, and novelty of the product or disease.

Preclinical tests include laboratory evaluation of product chemistry, formulation, and toxicity, as well as animal trials to assess the characteristics and potential safety and efficacy of the product. The conduct of the preclinical tests must comply with federal regulations and requirements, including good laboratory practices. The results of preclinical testing are submitted to the FDA as part of an IND along with other information, including information about product chemistry, manufacturing and controls, and a proposed clinical trial protocol. Long-term preclinical tests, such as animal tests of reproductive toxicity and carcinogenicity, may continue after the IND is submitted. A 30-day waiting period after the submission of each IND is required prior to the commencement of clinical testing in humans. If the FDA has neither commented on nor questioned the IND within this 30-day period, the clinical trial proposed in the IND may begin. Clinical trials involve the administration of the investigational biologic to healthy volunteers or patients under the supervision of a qualified investigator. Clinical trials must be conducted: (i) in compliance with federal regulations; (ii) in compliance with good clinical practice, or GCP, an international standard meant to protect the rights and health of patients and to define the roles of clinical trial sponsors, administrators, and monitors; as well as (iii) under protocols detailing the objectives of the trial, the parameters to be used in monitoring safety, and the effectiveness criteria to be evaluated. Each protocol involving testing on U.S. patients and subsequent protocol amendments must be submitted to the FDA as part of the IND.

The FDA may order the temporary, or permanent, discontinuation of a clinical trial at any time, or impose other sanctions, if it believes that the clinical trial either is not being conducted in accordance with FDA requirements or presents an unacceptable risk to the clinical trial patients. The trial protocol and informed consent information for patients in clinical trials must also be submitted to an institutional review board, or IRB, for approval. An IRB may also require the clinical trial at the site to be halted, either temporarily or permanently, for failure to comply with the IRB’s requirements, or may impose other conditions.

Clinical trials to support BLAs for marketing approval are typically conducted in three sequential phases, but the phases may overlap. In Phase 1, the initial introduction of the biologic into healthy human subjects or patients, the product is tested to assess metabolism, pharmacokinetics, pharmacological actions, side effects associated with increasing doses, and, if possible, early evidence on effectiveness. In oncology clinical trials, efficacy endpoints are also often explored in Phase 1. Phase 2 usually involves trials in a limited patient population to determine the effectiveness of the drug or biologic for a particular indication, dosage tolerance, and optimum dosage, and to identify common adverse effects and safety risks. If a compound demonstrates evidence of effectiveness and an acceptable safety profile in Phase 2 evaluations, Phase 3 trials are undertaken to obtain the additional information about clinical efficacy and safety in a larger number of patients, typically at geographically dispersed clinical trial sites, to permit the FDA to evaluate the overall benefit-risk relationship of the drug or biologic and to provide adequate information for the labeling of the product. In some instances, trial phases may be truncated or combined into one or more combined-phase or adaptive design trials. In most cases, the FDA requires two adequate and well-controlled Phase 3 clinical trials to demonstrate the efficacy of the biologic. A single Phase 3 trial with other confirmatory evidence may be sufficient in certain oncological conditions where the trial is a large multicenter trial demonstrating internal consistency and a statistically very persuasive finding of a clinically meaningful effect on mortality, irreversible morbidity or prevention of a disease with a potentially serious outcome and confirmation of the result in a second trial would be practically or ethically impossible.

The manufacturer of an investigational drug in a Phase 2 or 3 clinical trial for a serious or life-threatening disease is required to make available, such as by posting on its website, its policy on evaluating and responding to requests for expanded access.

After completion of the required clinical testing, a BLA is prepared and submitted to the FDA. FDA approval of the BLA is required before marketing of the product may begin in the United States. The BLA must include the results of all preclinical, clinical, and other testing and a compilation of data relating to the product’s pharmacology, chemistry, manufacture, and controls. The cost of preparing and submitting a BLA is substantial. The submission of most BLAs is additionally subject to a substantial application user fee, currently exceeding $3,242,000 for Fiscal Year 2023. The applicant under an approved BLA is also subject to an annual program fee, currently exceeding $393,000 per prescription drug product for Fiscal Year 2023. These fees are typically increased annually. The FDA has 60 days from its receipt of a BLA to determine whether the application will be filed based on the agency’s threshold determination that it is sufficiently complete to permit substantive review. Once the submission is filed, the FDA begins an in-depth review. The FDA has agreed to certain performance goals in the review of BLAs. Most such applications for standard review biologic products are reviewed within 10 months of the date the FDA files the BLA; most applications for priority review biologics are reviewed within six months of the date the FDA files the BLA. Priority review can be applied to a biologic that the FDA determines has the potential to treat a serious or life-threatening condition and, if approved, would be a significant improvement in safety or effectiveness compared to available therapies. The review process for both standard and priority review may be extended by the FDA for three additional months to consider certain late-submitted information, or information intended to clarify information already provided in the submission.

The FDA may also refer applications for novel biologic products, or biologic products that present difficult questions of safety or efficacy, to an advisory committee—typically a panel that includes clinicians and other experts—for review, evaluation, and a recommendation as to whether the application should be approved. The FDA is not bound by the recommendation of an advisory committee, but it generally follows such recommendations. Before approving a BLA, the FDA will typically inspect one or more clinical sites to assure compliance with GCP. Additionally, the FDA will inspect the facility or the facilities at which the biologic product is manufactured. The FDA will not approve the product unless compliance with current Good Manufacturing Practices, or cGMPs, is satisfactory and the BLA contains data that provide substantial evidence that the biologic is safe, pure, potent and effective in the indication studied.

After the FDA evaluates the BLA and the manufacturing facilities, it issues either an approval letter or a complete response letter. A complete response letter generally outlines the deficiencies in the submission and may require substantial additional testing, or information, in order for the FDA to reconsider the application. If, or when, those deficiencies have been addressed to the FDA’s satisfaction in a resubmission of the BLA, the FDA will issue an approval letter. The FDA has committed to reviewing such resubmissions in two or six months depending on the type of information included. An approval letter authorizes commercial marketing of the biologic with specific prescribing information for specific indications. As a condition of BLA approval, the FDA may require a risk evaluation and mitigation strategy, or REMS, to help ensure that the benefits of the biologic outweigh the potential risks. REMS can include medication guides, communication plans for healthcare professionals, and elements to assure safe use, or ETASU. ETASU can include, but are not limited to, special training or certification for prescribing or dispensing, dispensing only under certain circumstances, special monitoring, and the use of patient registries. The requirement for a REMS can materially affect the potential market and profitability of the product. Moreover, product approval may require substantial post-approval testing and surveillance to monitor the product’s safety or efficacy.

Once granted, product approvals may be withdrawn if compliance with regulatory standards is not maintained, or problems are identified following initial marketing. Changes to some of the conditions established in an approved application, including changes in indications, labeling, or manufacturing processes or facilities, require submission and FDA approval of a new BLA or BLA supplement before the change can be implemented. A BLA supplement for a new indication typically requires clinical data similar to that in the original application, and the FDA uses the same procedures and actions in reviewing BLA supplements as it does in reviewing BLAs.

The FDA is required to facilitate the development, and expedite the review, of biologics that are intended for the treatment of a serious or life-threatening disease or condition for which there is no effective treatment and which demonstrate the potential to address unmet medical needs for the condition. Under the fast track program, the sponsor of a new biologic candidate may request that the FDA designate the candidate for a specific indication as a fast track biologic concurrent with, or after, the submission of the IND for the candidate. The FDA must determine if the biologic candidate qualifies for fast track designation within 60 days of receipt of the sponsor’s request. In addition to other benefits, such as the ability to engage in more frequent interactions with the FDA, the FDA may initiate review of sections of a fast track product’s BLA before the application is complete. This rolling review is available if the applicant provides, and the FDA approves, a schedule for the submission of the remaining information and the applicant pays applicable user fees. However, the FDA’s time period goal for reviewing an application does not begin until the last section of the BLA is submitted. Additionally, the fast track designation may be withdrawn by the FDA if the FDA believes that the designation is no longer supported by data emerging in the clinical trial process.

Under the FDA’s accelerated approval regulations, the FDA may approve a biologic for a serious or life-threatening illness that provides meaningful therapeutic benefit to patients over existing treatments based upon a surrogate endpoint that is reasonably likely to predict clinical benefit, or on a clinical endpoint that can be measured earlier than irreversible morbidity or mortality, that is reasonably likely to predict an effect on irreversible morbidity or mortality or other clinical benefit, taking into account the severity, rarity, or prevalence of the condition and the availability or lack of alternative treatments.

In clinical trials, a surrogate endpoint is a measurement of laboratory or clinical signs of a disease or condition that substitutes for a direct measurement of how a patient feels, functions, or survives. Surrogate endpoints can often be measured more easily or more rapidly than clinical endpoints. A biologic candidate approved on this basis is subject to rigorous post-marketing compliance requirements, including the completion of Phase 4 or post-approval clinical trials to confirm the effect on the clinical endpoint. Failure to conduct required post-approval trials, or confirm a clinical benefit during post-marketing trials, will allow the FDA to withdraw the biologic from the market on an expedited basis. All promotional materials for biologic candidates approved under accelerated regulations are subject to prior review by the FDA. The Food and Drug Omnibus Reform Act, or FDORA, was recently enacted, which included provisions related to the accelerated approval pathway. Pursuant to FDORA, the FDA is authorized to require a post-approval study to be underway prior to approval or within a specified time period following approval. FDORA also requires the FDA to specify conditions of any required post-approval study, which may include milestones such as a target date of study completion and requires sponsors to submit progress reports for required post-approval studies and any conditions required by the FDA not later than 180 days following approval and not less frequently than every 180 days thereafter until completion or termination of the study. FDORA enables the FDA to initiate enforcement action for the failure to conduct with due diligence a required post-approval study, including a failure to meet any required conditions specified by the FDA or to submit timely reports.

Under the Orphan Drug Act, the FDA may grant orphan drug designation to biological products intended to treat a rare disease or condition—generally a disease or condition that affects fewer than 200,000 individuals in the United States, or if it affects more than 200,000 individuals in the United States, there is no reasonable expectation that the cost of developing and making a product available in the United States for such disease or condition will be recovered from sales of the product.

Orphan drug designation must be requested before submitting a BLA. After the FDA grants orphan drug designation, the generic identity of the biological product and its potential orphan use are disclosed publicly by the FDA. Orphan drug designation does not convey any advantage in, or shorten the duration of, the regulatory review and approval process. The first BLA applicant to receive FDA approval for a product with particular principal molecular structural features to treat a particular disease with FDA orphan drug designation is entitled to a seven-year exclusive marketing period in the United States for that product for that indication. During the seven-year exclusivity period, the FDA may not approve any other applications to market the same drug for the same disease, except in limited circumstances, such as a showing of clinical superiority to the product with orphan drug exclusivity. A product is clinically superior if it is safer, more effective or makes a major contribution to patient care. In the case of a biological product, the same drug is a drug that contains the same principal molecular features, Orphan drug exclusivity does not prevent the FDA from approving a different drug or biological product

for the same disease or condition, or the same biological product for a different disease or condition. Among the other benefits of orphan drug designation are tax credits for certain research and a waiver of the BLA user fee.

Sponsors of clinical trials of FDA-regulated products, including biological products, are required to register and disclose certain clinical trial information. Information related to the product, patient population, phase of investigation, trial sites and investigators, and other aspects of the clinical trial are then made public as part of the registration. Sponsors are also obligated to discuss the results of their clinical trials after completion. Disclosure of the results of these trials can be delayed in certain circumstances for up to two years after the date of completion of the trial. Competitors may use this publicly available information to gain knowledge regarding the progress of development programs.

Under the Pediatric Research Equity Act, or PREA, BLAs or supplements to BLAs must contain data to assess the safety and effectiveness of the biological product for the claimed indications in all relevant pediatric subpopulations and to support dosing and administration for each pediatric subpopulation for which the biological product is safe and effective. The FDA may grant full or partial waivers, or deferrals, for submission of data. Unless otherwise required by regulation, PREA does not apply to any biological product for an indication for which orphan designation has been granted, except a product with a new active ingredient that is molecularly targeted cancer product intended for the treatment of an adult cancer and directed at a molecular target determined by FDA to be substantially relevant to the growth or progression of a pediatric cancer.

To help reduce the increased risk of the introduction of adventitious agents, the PHS Act emphasizes the importance of manufacturing controls for products whose attributes cannot be precisely defined. The PHS Act also provides authority to the FDA to immediately suspend licenses in situations where there exists a danger to public health, to prepare or procure products in the event of shortages and critical public health needs, and to authorize the creation and enforcement of regulations to prevent the introduction or spread of communicable diseases in the United States and between states.

After a BLA is approved, the product may also be subject to official lot release as a condition of approval. As part of the manufacturing process, the manufacturer is required to perform certain tests on each lot of the product before it is released for distribution. If the product is subject to official release by the FDA, the manufacturer submits samples of each lot of products to the FDA together with a release protocol showing a summary of the history of manufacture of the lot and the results of all of the manufacturer’s tests performed on the lot. The FDA may also perform certain confirmatory tests on lots of some products, such as viral vaccines, before releasing the lots for distribution by the manufacturer. In addition, the FDA conducts laboratory research related to the regulatory standards on the safety, purity, potency, and effectiveness of biological products. As with drugs, after approval of biologics, manufacturers must address any safety issues that arise, are subject to recalls or a halt in manufacturing, and are subject to periodic inspection after approval.

Once a BLA is approved, a product will be subject to certain post-approval requirements. For instance, the FDA closely regulates the post-approval marketing and promotion of biologics, including standards and regulations for direct-to-consumer advertising, off-label promotion, industry-sponsored scientific and educational activities and promotional activities involving the internet. Biologics may be marketed only for the approved indications and in accordance with the provisions of the approved labeling.

Adverse event reporting and submission of periodic reports is required following FDA approval of a BLA. The FDA also may require post-marketing testing, known as Phase 4 testing, REMS, and surveillance to monitor the effects of an approved product, or the FDA may place conditions on an approval that could restrict the distribution or use of the product. In addition, quality control, biological product manufacture, packaging, and labeling procedures must continue to conform to cGMPs after approval. Biologic manufacturers and certain of their subcontractors are required to register their establishments with the FDA and certain state agencies. Registration with the FDA subjects' entities to periodic unannounced inspections by the FDA, during which the agency inspects manufacturing facilities to assess compliance with cGMPs. Accordingly, manufacturers must continue to expend time, money, and effort in the areas of production and quality-control to maintain compliance with cGMPs. Regulatory authorities may withdraw product approvals or request product recalls if a company fails to comply with regulatory standards, if it encounters problems following initial marketing, or if previously unrecognized problems are subsequently discovered.

A biologic product may rely upon an in vitro companion diagnostic for use in selecting the patients that will respond to a therapy. If an in vitro diagnostic is essential to the safe and effective use of the therapeutic product, then the FDA generally will require approval or clearance of the diagnostic at the same time that FDA approves the therapeutic product.

Pursuing FDA approval of an in vitro companion diagnostic usually would require a pre-market approval, or PMA, for that diagnostic. Based on a final FDA guidance document, and the FDA’s past treatment of companion diagnostics, the FDA will likely require PMA approval of an in vitro companion diagnostics to identify patient populations suitable for a cancer therapy. The review of these in vitro companion diagnostics involves coordination of review by the FDA’s Center for Biologics Evaluation and Research and by the FDA’s Center for Devices and Radiological Health. Approval of a companion diagnostic is generally required at the time of new drug approval.

The PMA process, including the gathering of clinical and nonclinical data and the submission to and review by the FDA, can take several years or longer. The applicant must prepare and provide the FDA with reasonable assurance of the device’s safety and effectiveness, including information about the device and its components regarding, among other things, device design, manufacturing and labeling. PMA applications are subject to an application fee, which exceeds $441,000 for most PMAs for Fiscal Year 2023. In addition, PMAs for devices must generally include the results from extensive preclinical and adequate and well-controlled clinical trials to establish the safety and effectiveness of the device for each indication for which FDA approval is sought. In particular, for a diagnostic, the applicant must demonstrate that the diagnostic produces reproducible results between multiple users at multiple laboratories. As part of the PMA review, the FDA will typically inspect the manufacturer’s facilities for compliance with the Quality System Regulation, or QSR, which imposes elaborate testing, control, documentation and other quality assurance requirements.

PMA approval is not guaranteed, and the FDA may ultimately respond to a PMA submission with a not approvable determination based on deficiencies in the application and require additional clinical trial or other data that may be expensive and time consuming to generate and that can substantially delay or prevent approval. If the FDA concludes that the applicable criteria have been met, the FDA will issue a PMA for the approved indications, which can be more limited than those originally sought by the applicant. The PMA can include post-approval conditions that the FDA believes necessary to ensure the safety and effectiveness of the device, including, among other things, restrictions on labeling, promotion, sale and distribution.

After a device is placed on the market, it remains subject to significant regulatory requirements. Medical devices may be marketed only for the uses and indications for which they are cleared or approved. Device manufacturers must also register with FDA and list their devices. A medical device manufacturer’s manufacturing processes are required to comply with the applicable portions of the QSR, which cover the methods and documentation of the design, testing, production, processes, controls, quality assurance, labeling, packaging and shipping of medical devices. Domestic facility records and manufacturing processes are subject to periodic inspections by the FDA.

Failure to comply with applicable regulatory requirements can result in enforcement action by the FDA, which may include any of the following sanctions: warning or untitled letters, fines, injunctions, civil or criminal penalties, recall or seizure of current or future products, operating restrictions, partial suspension or total shutdown of production, denial of submissions for new products, or withdrawal of PMA approvals.

In addition to FDA restrictions on marketing of pharmaceutical products, several other types of state and federal laws have been applied to restrict certain general business and marketing practices in the pharmaceutical industry in recent years. These laws include anti-kickback statutes, false claims statutes and other healthcare laws and regulations.

The federal Anti-Kickback Statute prohibits, among other things, knowingly and willfully offering, paying, soliciting or receiving remuneration to induce, or in return for, purchasing, leasing, ordering or arranging for the purchase, lease or order of any healthcare item or service reimbursable under Medicare, Medicaid, or other federally financed healthcare programs. The Patient Protection and Affordable Care Act as amended by the Health Care and Education Reconciliation Act, collectively, the ACA, amended the intent element of the federal statute so that a person or entity no longer needs to have actual knowledge of the statute or specific intent to violate it in order to commit a violation. This statute has been interpreted to apply to arrangements between pharmaceutical manufacturers on the one hand and prescribers, purchasers and formulary managers on the other. Although there are a number of statutory exceptions and regulatory safe harbors protecting certain common activities from prosecution or other regulatory sanctions, the exceptions and safe harbors are drawn narrowly, and practices that involve remuneration intended to induce prescribing, purchases or recommendations may be subject to scrutiny if they do not qualify for an exception or safe harbor.

Federal civil and criminal false claims laws, including the federal civil False Claims Act, prohibit any person or entity from knowingly presenting, or causing to be presented, a false claim for payment to the federal government, or knowingly making, or causing to be made, a false statement to have a false claim paid. This includes claims made to programs where the federal government reimburses, such as Medicaid, as well as programs where the federal government is a direct purchaser, such as when it purchases off the Federal Supply Schedule. Recently, several pharmaceutical and other healthcare companies have been prosecuted under these laws for allegedly inflating drug prices they report to pricing services, which in turn were used by the government to set Medicare and Medicaid reimbursement rates, and for allegedly providing free product to customers with the expectation that the customers would bill federal programs for the product. In addition, certain marketing practices, including off-label promotion, may also violate false claims laws. Additionally, the ACA amended the federal Anti-Kickback Statute such that a violation of that statute can serve as a basis for liability under the federal False Claims Act. The majority of states also have statutes or regulations similar to the federal Anti-Kickback Statute and False Claims Act, which apply to items and services reimbursed under Medicaid and other state programs, or, in several states, apply regardless of the payor.

Other federal statutes pertaining to healthcare fraud and abuse include the civil monetary penalties statute, which prohibits, among other things, the offer or payment of remuneration to a Medicaid or Medicare beneficiary that the offeror or payor knows or should know is likely to influence the beneficiary to order a receive a reimbursable item or service from a particular supplier, and the additional federal criminal statutes created by the Health Insurance Portability and Accountability Act of 1996, or HIPAA, which prohibits, among other things, knowingly and willfully executing or attempting to execute a scheme to defraud any healthcare benefit program or obtain by means of false or fraudulent pretenses, representations or promises, any money or property owned by or under the control of any healthcare benefit program in connection with the delivery of or payment for healthcare benefits, items or services.

In addition, HIPAA, as amended by the Health Information Technology for Economic and Clinical Health Act of 2009, or HITECH, imposes obligations on certain healthcare providers, health plans, and healthcare clearinghouses, known as covered entities, as well as their business associates that perform certain services involving the storage, use or disclosure of individually identifiable health information, including mandatory contractual terms, with respect to safeguarding the privacy, security, and transmission of individually identifiable health information, and require notification to affected individuals and regulatory authorities of certain breaches of security of individually identifiable health information.

Further, pursuant to the ACA, the Centers for Medicare & Medicaid Services, or CMS, has issued a final rule that requires manufacturers of prescription drugs to collect and report information on certain payments or transfers of value to physicians, physician assistants, certain types of advanced practice nurses and teaching hospitals, as well as investment interests held by physicians and their immediate family members. The reports must be submitted on an annual basis and the reported data are posted in searchable form on a public website on an annual basis. Failure to submit required information may result in civil monetary penalties.

In addition, several states now require prescription drug companies to report certain expenses relating to the marketing and promotion of drug products and to report gifts and payments to individual healthcare practitioners in these states. Other states prohibit various marketing-related activities, such as the provision of certain kinds of gifts or meals. Still other states require the posting of information relating to clinical studies and their outcomes. Some states require the reporting of certain pricing information, including information pertaining to and justifying price increases. In addition, certain states require pharmaceutical companies to implement compliance programs and/or marketing codes. Additional jurisdictions, such as the City of Chicago and the District of Columbia, require pharmaceutical sales representatives to be licensed and meet continuing education requirements. Certain states and local jurisdictions also require the registration of pharmaceutical sales representatives. Compliance with these laws is difficult and time consuming, and companies that do not comply with these state laws face civil penalties.

Efforts to ensure that business arrangements with third parties comply with applicable healthcare laws and regulations involve substantial costs. If a drug company’s operations are found to be in violation of any such requirements, it may be subject to significant penalties, including civil, criminal and administrative penalties, damages, fines, disgorgement, imprisonment, the curtailment or restructuring of its operations, loss of eligibility to obtain approvals from the FDA, exclusion from participation in government contracting, healthcare reimbursement or other government programs, including Medicare and Medicaid, integrity oversight and reporting obligations and reputational harm. Although effective compliance programs can mitigate the risk of investigation and prosecution for violations of these laws, these risks cannot be entirely eliminated. Any action for an alleged or suspected violation can cause a drug company to incur significant legal expenses and divert management’s attention from the operation of the business, even if such action is successfully defended.

Healthcare reforms that have been adopted, and that may be adopted in the future, could result in further reductions in coverage and levels of reimbursement for pharmaceutical products, increases in rebates payable under U.S. government rebate programs and additional downward pressure on pharmaceutical product prices. On September 9, 2021, the Biden administration published a wide-ranging list of policy proposals, most of which would need to be carried out by Congress, to reduce drug prices and drug payment. Healthcare reform proposals recently culminated in the enactment of Inflation Reduction Act, or IRA, which will, among other things, allow the Department of Health and Human Services, or HHS, to negotiate the selling price of certain drugs and biologics that CMS reimburses under Medicare Part B and Part D (excluding drugs and biologics that are designated and approved for only one rare disease or condition), although only high-expenditure single-source biologics that have been approved for at least 11 years (7 years for drugs) can be selected by CMS for negotiation, with the negotiated price taking effect two years after the selection year. The negotiated prices, which will first become effective in 2026, will be capped at a statutory ceiling price. Beginning in October 2022 for Medicare Part D and January 2023 for Medicare Part B, the IRA will also penalize drug manufacturers that increase prices of Medicare Part D and Part B drugs at a rate greater than the rate of inflation. The IRA will also eliminate, beginning in 2025, the coverage gap under Medicare Part D by significantly lowering the enrollee maximum out-of-pocket cost and requiring manufacturers to subsidize, through a newly established manufacturer discount program, 10% of Part D enrollees’ prescription costs for brand drugs below the out-of-pocket maximum, and 20% once the out-of-pocket maximum has been reached. The IRA permits the Secretary of HHS to implement many of these provisions through guidance, as opposed to regulation, for the initial years. Manufacturers that fail to comply with the IRA may be subject to various penalties, including civil monetary penalties. The IRA also extends enhanced subsidies for individuals purchasing health insurance coverage in ACA marketplaces through plan year 2025. These provisions will take effect progressively starting in 2023, although they may be subject to legal challenges. It is unclear to what extent other statutory, regulatory, and administrative initiatives will be enacted and implemented.

As of December 31, 2022, we had 278 full-time employees and 17 full-time contract employees. Of these employees, 72 have an M.D. or a Ph.D. None of our employees are represented by a labor union or covered by collective bargaining agreements, and we believe our relationship with our employees is good.

We recognize that attracting, motivating, and retaining talent at all levels is vital to continuing our success. We invest in our employees in many ways, including through high-quality benefits and various health and wellness initiatives and offer competitive compensation packages (base salary and incentive plans), ensuring fairness in internal compensation practices. The principal purposes of our incentive plans (bonus and equity) are to provide retention incentives that align with the long-term interests of our stakeholders and stockholders.

To further engage and incentivize our workforce, we also offer a range of opportunities to support professional development and growth. We support ongoing education by providing an appropriate level of reimbursement for courses which are related to an individual’s current or future position, we support our scientific team through encouraging their in-person and/or virtual attendance at conferences and symposia which further their development and we have a robust internal transfer practice to engage our current talent in growth opportunities within and outside of their functional areas. We embarked upon a Company-wide leadership development program which offered the opportunity for every employee to continue to build upon their learning. For our talent pipeline assessment and development, we work closely with individual scientific and business functional leaders to identify our high-performing and high-potential employees, by conducting a company-wide talent assessment and calibration. This assessment is completed annually to ensure we tie together our incentives, development, and recognition to retain and attract the people we need to drive our success.

We provide our team with ongoing resources aimed at both mental and physical health. We work closely with our Employee Assistance Plan which provides important mental health services and resources. We have a health and wellness initiative which encourages healthy behaviors aimed at creating positive life-long habits. We have a culture of collaboration and collaborative principles which we are intentional about fostering. Our initiatives on Diversity, Equity, Inclusion and Belonging aim to learn, listen and act in support of these principles. We are actively involved in our community through, among other things, mentoring underserved communities and supporting the philanthropic interests of our employees and patients.

We also recognize that maintaining continuity of management in the event of the departure of one or more of our senior executives is critical to the continued success of the organization. To this end, we have prepared a formal written succession plan for our senior executives and to provide guidance for the next generation of our leaders to ensure an orderly and smooth transition in the event of an executive departure. While senior management is primarily responsible for developing our succession plan, our Nominating and Corporate Governance Committee of our Board of Directors (with respect to the CEO) and Compensation Committee of our Board of Directors (with respect to other executives) oversee and guide our process and thinking.

We were incorporated under the laws of the State of Delaware in April 2003 under the name Fundamental Applied Biology, Inc. We subsequently changed our name to Sutro Biopharma, Inc. Our principal executive offices are located at 111 Oyster Point Boulevard, South San Francisco, California 94080, and our telephone number is (650) 881- 6500. Our website address is www.sutrobio.com. The information contained on, or that can be accessed through, our website is not part of, and is not incorporated by reference into, this report.

We file annual, quarterly and current reports, proxy statements and other documents with the Securities and Exchange Commission, or SEC, under the Securities Exchange Act of 1934, as amended, or Exchange Act. The SEC maintains an Internet website that contains reports, proxy and information statements, and other information regarding issuers, including us, that file electronically with the SEC. The public can obtain any documents that we file with the SEC at www.sec.gov. Copies of each of our filings with the SEC can also be viewed and downloaded free of charge at our website, ir.sutrobio.com, after the reports and amendments are electronically filed with or furnished to the SEC.

Investing in our common stock involves a high degree of risk. Before making your decision to invest in shares of our common stock, you should carefully consider the risks described below, together with the other information contained in this annual report, including our financial statements and the related notes and “Management’s Discussion and Analysis of Financial Condition and Results of Operations.” The risks and uncertainties described below are not the only ones we face. Additional risks and uncertainties that we are unaware of, or that we currently believe are not material, may also become important factors that affect us. We cannot assure you that any of the events discussed below will not occur. These events could have a material and adverse impact on our business, financial condition, results of operations and prospects. If that were to happen, the trading price of our common stock could decline, and you could lose all or part of your investment.

We are a clinical stage biopharmaceutical company with a limited operating history and no products approved for commercial sale. We have a history of significant losses, expect to continue to incur significant losses for the foreseeable future and may never achieve or maintain profitability, which could result in a decline in the market value of our common stock.

We are a clinical stage biopharmaceutical company with a limited operating history on which to base your investment decision. Biotechnology product development is a highly speculative undertaking and involves a substantial degree of risk.

To date, we have enrolled a limited number of patients in our initial clinical trials, have no products approved for commercial sale, have not generated any revenue from commercial product sales and, as of December 31, 2022, had an accumulated deficit of $452.6 million. For the years ended December 31, 2022, and December 31, 2021, our net loss was $119.2 million and $105.5 million, respectively. Substantially all of our losses have resulted from expenses incurred in connection with our research and development programs and from general and administrative costs associated with our operations. In addition, our expenses could increase beyond expectations if we are required by the FDA, or foreign regulatory agencies, to perform studies or clinical trials in addition to those studies and clinical trials that we currently anticipate conducting for our product candidates, or if there are any delays in our or our partners completing clinical trials or the development of any of our product candidates. Our technologies and product candidates are in early stages of development, and we are subject to the risks of failure inherent in the development of product candidates based on novel technologies. In addition, we have limited experience as a clinical stage company and have not yet demonstrated an ability to successfully overcome many of the risks and uncertainties frequently encountered by companies in new and rapidly evolving fields, particularly in the biotechnology industry. Furthermore, we do not expect to generate any revenue from commercial product sales for the foreseeable future, and we expect to continue to incur significant operating losses for the foreseeable future due to the cost of research and development, preclinical studies and clinical trials and the regulatory approval process for our product candidates. We expect our net losses to increase substantially as we progress further into clinical development of our lead programs and create additional infrastructure to support operations as a public company. However, the amount of our future losses is uncertain. We may never generate revenues from the commercial sale of our or our collaborators’ products. Our ability to achieve profitability, if ever, will depend on, among other things, our, or our existing or future collaborators’, successful development of product candidates, evaluating the related commercial opportunities, obtaining regulatory approvals to market and commercialize product candidates, manufacturing any approved products on commercially reasonable terms, establishing a sales and marketing organization or suitable third-party alternatives for any approved product, and raising sufficient funds to finance business activities. If we, or our existing or future collaborators, are unable to develop our technologies and commercialize one or more of our product candidates or if sales revenue from any product candidate that receives approval is insufficient, we will not achieve profitability, which could have a material and adverse effect on our business, financial condition, results of operations and prospects. Even if we achieve profitability in the future, we may not be able to sustain profitability in subsequent periods.

We will need substantial additional funds to advance development of our product candidates. This additional financing may not be available on acceptable terms, or at all. Failure to obtain this necessary capital when needed may force us to delay, limit or terminate our product development programs, commercialization efforts or other operations. Continued unfavorable conditions in the capital markets may pose difficulties to us in accessing additional capital on reasonable, or even any, terms to continue our product and platform development or other operations. In addition, market volatility, high levels of inflation and interest rate fluctuations may increase our cost of capital or otherwise restrict our access to potential sources of future liquidity.

The development of biopharmaceutical product candidates is capital-intensive. If our product candidates enter and advance through preclinical studies and clinical trials, we will need substantial additional funds to expand our development, regulatory, manufacturing, marketing and sales capabilities. We have used substantial funds to develop our technology and product candidates and will require significant funds to conduct further research and development and preclinical testing and clinical trials of our product candidates, to seek regulatory approvals for our product candidates, to manufacture extract and products, if any, which may be approved for commercial sale, to establish marketing and sales capabilities to commercialize our product candidates, and to provide support to our collaborators in the development of their products. In addition, we expect to continue to incur additional costs associated with operating as a public company.

Since our inception, we have invested a significant portion of our efforts and financial resources in research and development activities for our two proprietary clinical-stage product candidates luvelta and STRO-001, and the development of our technology platform, including our in-house manufacturing capabilities. Clinical trials for our product candidates have required substantial funds to date and will continue to require substantial funds to complete. As of December 31, 2022, we had $302.3 million in cash, cash equivalents and marketable securities. We expect to incur substantial expenditures in the foreseeable future as we seek to advance luvelta, STRO-001 and STRO-003 and any future product candidates through clinical development, manufacturing, the regulatory approval process and, if approved, commercial launch activities, as well as in connection with the continued development of our technology platform and manufacturing capabilities. Based on our current operating plan, we believe that our available cash, cash equivalents and marketable securities will be sufficient to fund our operations through at least the next 12 months. However, our future capital requirements and the period for which we expect our existing resources to support our operations may vary significantly from what we expect, and we may need to seek additional funds sooner than planned. Our monthly spending levels vary based on new and ongoing research and development and other corporate activities. Because the length of time and activities associated with successful research and development of our product candidates is highly uncertain, we are unable to estimate the actual funds we will require for development and any marketing and commercialization activities for approved products. The timing and amount of our operating expenditures will depend largely on:

If we are unable to obtain funding on a timely basis or on acceptable terms, we may have to delay, reduce or terminate our research and development programs and preclinical studies or clinical trials, limit strategic opportunities or undergo reductions in our workforce or other corporate restructuring activities. We also could be required to seek funds through arrangements with collaborators or others that may require us to relinquish rights to some of our technologies or product candidates that we would otherwise pursue on our own. We cannot provide assurance that anticipated collaborator payments will, in fact, be received. We do not expect to realize revenue from sales of commercial products or royalties from licensed products in the foreseeable future, if at all, and, in no event, before our product candidates are clinically tested, approved for commercialization and successfully marketed. To date, we have primarily financed our operations through payments received under our collaboration and other associated agreements, the sale of equity securities and debt financing. We will be required to seek additional funding in the future and currently intend to do so through additional collaborations and/or licensing agreements, public or private equity offerings or debt financings, credit or loan facilities, or a combination of one or more of these funding sources. Our ability to raise additional funds will depend on financial, economic and other factors, many of which are beyond our control, including the factors impacting potential interest rates for any debt financings. Additional funds may not be available to us on acceptable terms or at all.

In addition, current macroeconomic conditions have caused turmoil in the banking sector. For example, on March 10, 2023, SVB one of our banking partners and lenders, was closed by the California Department of Financial Protection and Innovation, which appointed the Federal Deposit Insurance Corporation, or FDIC, as receiver. Under the terms of our Loan and Security Agreement, we were required to keep substantially all of our cash and investments with SVB, the substantial majority of which was held in a custodial account with another institution, for which SVB Asset Management was the advisor. While we were afforded full access to our cash and investments with SVB on March 13, 2023, and have since amended our Loan and Security Agreement to provide us with greater cash management flexibility, we may be impacted by other disruptions to the U.S. banking system, including potential delays in our ability to transfer funds whether held with SVB or otherwise and in the short-term potential delays in making payments to vendors while new banking relationships are established.

Subject to limited exceptions, our Loan and Security Agreement with Oxford and SVB prohibits us from incurring indebtedness without the prior written consent of Oxford and SVB. If we raise additional funds by issuing equity securities, our stockholders will suffer dilution and the terms of any financing may adversely affect the rights of our stockholders. If we raise additional funds through licensing or collaboration arrangements with third parties, we may have to relinquish valuable rights to our product candidates, or grant licenses on terms that are not favorable to us. In addition, as a condition to providing additional funds to us, future investors may demand, and may be granted, rights superior to those of existing stockholders. Our current debt financing involves, and future debt financings, if available, are likely to involve, restrictive covenants limiting our flexibility in conducting future business activities, and, in the event of insolvency, debt holders would be repaid before holders of our equity securities receive any distribution of our corporate assets. Failure to obtain capital when needed on acceptable terms may force us to delay, limit or terminate our product development and commercialization of our current or future product candidates, which could have a material and adverse effect on our business, financial condition, results of operations and prospects.

Our product candidates are in early stages of development and may fail in development or be impacted by competitive products or suffer delays that materially and adversely affect their commercial viability. If we or our collaborators are unable to complete development of or commercialize our product candidates or experience significant delays in doing so, our business will be materially harmed.

We have no products on the market and all of our product candidates for cancer therapy are in early stages of development. In particular, our most advanced product candidate, luvelta, is in the dose expansion phase of its Phase 1 clinical trials. Also, enrollment began in the second half of 2019 for patients in the Phase 1 clinical trial for CC-99712, a BCMA ADC candidate resulting from our BMS collaboration. Merck initiated patient dosing in a Phase 1 clinical trial for MK-1484 in July 2022, a product candidate resulting from our cytokine-derivative collaboration. In the first quarter of 2022, Vaxcyte announced that it had initiated a Phase 1/2 clinical proof-of-concept study of its lead product candidate, VAX-24, its 24-valent pneumococcal conjugate vaccine candidate, under investigation for the prevention of invasive pneumococcal disease in adults, and announced initial data in October 2022. Additionally, we have programs that are in earlier stages of discovery and preclinical development and may never advance to clinical-stage development. Our ability to achieve and sustain profitability depends on obtaining regulatory approvals for and successfully commercializing our product candidates, either alone or with third parties, and we cannot guarantee you that we will ever obtain regulatory approval for any of our product candidates. We have limited experience in conducting and managing the clinical trials necessary to obtain regulatory approvals, including approval by the FDA. Before obtaining regulatory approval for the commercial distribution of our product candidates, we or an existing or future collaborator must conduct extensive preclinical tests and clinical trials to demonstrate the safety and efficacy in humans of our product candidates.

We may not have the financial resources to continue development of, or to modify existing or enter into new collaborations for, a product candidate if we experience any issues that delay or prevent regulatory approval of, or our ability to commercialize, product candidates, including:

•

inability of us or any third-party contract manufacturer to scale up manufacturing of our product candidates and those of our collaborators to supply the needs of clinical trials and commercial sales, and to manufacture such products in conformity with regulatory requirements using our proprietary XpressCF® and XpressCF+® platforms;

We or our collaborators’ inability to complete development of or commercialize our product candidates or significant delays in doing so due to one or more of these factors, could have a material and adverse effect on our business, financial condition, results of operations and prospects.

Our business is dependent on the success of our product candidates based on our proprietary XpressCF® and XpressCF+® platforms and, in particular, our proprietary product candidates, luvelta, STRO-001 and STRO-003. Existing and future preclinical studies and clinical trials of our product candidates may not be successful. If we are unable to commercialize our product candidates or experience significant delays in doing so, our business will be materially harmed.

We have invested a significant portion of our efforts and financial resources in the development of our proprietary XpressCF® and XpressCF+® platforms and our proprietary product candidates, luvelta, STRO-001 and STRO-003. Our ability to generate commercial product revenues, which we do not expect will occur for many years, if ever, will depend heavily on the successful development and eventual commercialization of luvelta, STRO-001 and STRO-003. We have not previously submitted a new drug application, or NDA, or a biologics license application, or BLA, to the FDA, or similar regulatory approval filings to comparable foreign authorities, for any product candidate, and we cannot be certain that our product candidates will be successful in clinical trials or receive regulatory approval. Further, our product candidates may not receive regulatory approval even if they are successful in clinical trials. If we do not receive regulatory approvals for our product candidates, we may not be able to continue our operations. Even if we successfully obtain regulatory approvals to market our product candidates, our revenues will be dependent, in part, upon the size of the markets in the territories for which we gain regulatory approval and have commercial rights. If the markets for patient subsets that we are targeting are not as significant as we estimate, we may not generate significant revenues from sales of such products, if approved.

We plan to seek regulatory approval to commercialize our product candidates both in the United States and in selected foreign countries. While the scope of regulatory approvals generally is similar in other countries, in order to obtain separate regulatory approvals in other countries, we must comply with numerous and varying regulatory requirements of such countries regarding safety and efficacy. Other countries also have their own regulations governing, among other things, clinical trials and commercial sales, as well as pricing and distribution of our product candidates, and we may be required to expend significant resources to obtain regulatory approval and to comply with ongoing regulations in these jurisdictions.

The success of luvelta, STRO-001 and STRO-003 and our other future product candidates will depend on many factors, including the following:

If we do not achieve one or more of these factors in a timely manner or at all, we could experience significant delays or an inability to successfully commercialize our product candidates, which would materially harm our business, financial condition, results of operations and prospects.

Additionally, we have in the past and may in the future create benchmark molecules for comparative purposes. For example, we have created a benchmark FolRα targeting antibody-drug conjugate, or ADC, using conventional technology that results in a heterogeneous ADC mixture. We have compared luvelta to this benchmark molecule in multiple preclinical models. We believe the results of these tests help us understand how the therapeutic index of luvelta compares to competitors’ product candidates. However, we cannot be certain that any benchmark molecule that we create is the same as the molecule we are attempting to recreate, and the results of the tests comparing any such benchmark molecule to any other potential or current product candidate may be different than the actual results of a head-to-head test of any such other potential or current product candidate against a competitor molecule. Additional preclinical and clinical testing will be needed to evaluate the therapeutic index of our potential or current product candidates, and to understand their therapeutic potential relative to other product candidates in development. While we believe our ADCs may be superior to other investigative agents in development, without head-to-head comparative data, we will not be able to make claims of superiority to other products in our promotional materials, if our product candidates are approved.

If we do not achieve our projected development goals in the time frames we anticipate and project, the commercialization of our products may be delayed and our stock price may decline.

From time to time, we estimate the timing of the anticipated accomplishment of various scientific, clinical, regulatory, commercial and other product development goals, which we sometimes refer to as milestones. These milestones may include the commencement or completion of scientific studies and clinical trials and the submission of regulatory filings. From time to time, we may publicly announce the expected timing of some of these milestones. All of these milestones are and will be based on numerous assumptions. The actual timing of these milestones can vary dramatically compared to our estimates, in some cases for reasons beyond our control. If we do not meet these milestones as publicly announced, or at all, the commercialization of our products maybe delayed or never achieved and, as a result, our stock price may decline.

For example, even with the approval of vaccines for COVID-19, the COVID-19 pandemic may further delay enrollment in trials due to prioritization of hospital resources toward the pandemic, restrictions on travel, and some patients may be unwilling to enroll in our trials or be unable to comply with clinical trial protocols if quarantines or


Delaware		47-0926186
(State or other jurisdiction of		(I.R.S. Employer
incorporation or organization)		Identification No.)


111 Oyster Point Blvd.
South San Francisco, California		94080
(Address of principal executive offices)		(Zip Code)


		Page

PART I		6

ITEM 1.	Business	6

ITEM 1A.	Risk Factors	44

ITEM 1B.	Unresolved Staff Comments	102

ITEM 2.	Properties	102

ITEM 3.	Legal Proceedings	102

ITEM 4.	Mine Safety Disclosures	102

PART II		103

ITEM 5.	Market for Registrant’s Common Equity, Related Stockholder Matters, and Issuer Purchases of Equity Securities	103

ITEM 6.	[Reserved]	105

ITEM 7.	Management’s Discussion and Analysis of Financial Condition and Results of Operations	106

ITEM 7A.	Quantitative and Qualitative Disclosures About Market Risk	122

ITEM 8.	Financial Statements and Supplementary Data	123

ITEM 9.	Changes in and Disagreements with Accountants on Accounting and Financial Disclosure	161

ITEM 9A.	Controls and Procedures	161

ITEM 9B.	Other Information	162

ITEM 9C.	Disclosure Regarding Foreign Jurisdictions that Prevent Inspections	162

PART III		163

ITEM 10.	Directors, Executive Officers of the Registrant and Corporate Governance	163

ITEM 11.	Executive Compensation	163

ITEM 12.	Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters	163

ITEM 13.	Certain Relationships and Related Transactions, and Director Independence	163

ITEM 14.	Principal Accounting Fees and Services	163

PART IV		164

ITEM 15.	Exhibits and Financial Statement Schedules	164

ITEM 16.	Form 10-K Summary	168

	Signatures	169


Patent Relevance	Ownership	Type of Patent Protection	Expiration or Anticipated Expiration (Absent patent term extension or adjustment)	Pending Jurisdictions	Issued Jurisdictions
XpressCF® platform	In licensed from Stanford	Utility	2023	None	US, AU, CA, EP, JP
XpressCF® platform	Owned by Sutro	Utility	2033	CA	US, AU, CN, EP, IL, IN, JP, KR, SG
XpressCF® platform	Owned by Sutro	Utility	2034	US, CA, HK, SG	US, AU, CN, EP, IL, IN, JP, KR
XpressCF® platform	Owned by Sutro	Utility	2034	None	US, EP
XpressCF® platform	Owned by Sutro	Utility	2035	EP	US
XpressCF® platform	Owned by Sutro	Utility	2041	US, TW, PCT	None
XpressCF® platform	Owned by Sutro	Provisional	2043	US	None
STRO-001 and STRO-002	Owned by Sutro	Utility	2033	US, BR, CA, EP, IN	US, AU, CN, EP, HK, IL, JP, KR, SG
STRO-001 and STRO-002	Owned by Sutro	Utility	2033	US, BR, CA, EP	US, AU, EP, CN, HK, IL, IN, JP, KR, SG
STRO-001	Owned by Sutro	Utility	2035	US, EP	US, EP
STRO-001	Owned by Sutro	Utility	2037	US, EP, HK	None
STRO-001	Owned by Sutro	Utility	2037	AU, BR, CA, IN, IL, JP, KR, MX, NZ, SG, ZA	CN, MO
STRO-001	Owned by Sutro	Utility	2038	US, EP	None
STRO-002	Owned by Sutro	Utility	2037	US, EP	None
STRO-002	Owned by Sutro	Utility	2038	US, AU, BR, CA, CN, EP, HK, IL, IN, JP, KR, MX, NZ, SG, ZA	US
STRO-002	Owned by Sutro	Utility	2036	US, BR, CA, CN, EP, KR	US, AU, EP, IL, IN, JP, SG
STRO-002	Owned by Sutro	Utility	2039	US, EP, JP	None
STRO-002	Owned by Sutro	Utility	2042	PCT, TW	None
STRO-002	Owned by Sutro	Utility	2042	PCT	None
STRO-002	Co-owned by Sutro	Provisional	2043	US	None
STRO-003	Owned by Sutro	Provisional	2043	US	None
STRO-003	Owned by Sutro	Provisional	2043	US	None


☒	ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934


☐	TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934


Large accelerated filer	☐	Accelerated filer	☐
Non-accelerated filer	☒	Smaller reporting company	☒
		Emerging growth company	☐