UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): December 8, 2020

SUTRO BIOPHARMA, INC.

(Exact name of registrant as specified in its charter)

310 Utah Avenue, Suite 150,

South San Francisco, California, 94080

(Address of principal executive offices) (Zip Code)

(650) 392-8412

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company ☒

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. ☐

Item 1.01. Entry into a Material Definitive Agreement

On December 8, 2020, Sutro Biopharma, Inc. (“Sutro”) entered into an underwriting agreement (the “Underwriting Agreement”) with Cowen and Company, LLC, Piper Sandler & Co. and Wells Fargo Securities, LLC, as representatives of the several underwriters named therein (the “Underwriters”), pursuant to which Sutro agreed to issue and sell 6,000,000 shares of its common stock (the “Shares”) to the Underwriters (the “Offering”). The Shares will be sold at the public offering price of $21.00 per share. Pursuant to the Underwriting Agreement, Sutro has also granted the Underwriters a 30-day option to purchase up to an additional 900,000 shares of its common stock. The Underwriting Agreement contains customary representations and warranties, conditions to closing, market standoff provisions, termination provisions and indemnification obligations, including for liabilities under the Securities Act of 1933, as amended. The Offering is being made pursuant to the shelf registration statement on Form S-3 (File No. 333-234101) that was filed by Sutro with the Securities and Exchange Commission (“SEC”) on October 4, 2019, and declared effective by the SEC on October 11, 2019, and the registration statement on Form S-3MEF (File No. 333-251206) that was filed by Sutro pursuant to Rule 462(b) with the SEC on December 8, 2020, and a related prospectus supplement.

Sutro estimates that net proceeds from the Offering will be approximately $118.0 million, after deducting underwriting discounts and commissions and estimated Offering expenses, and assuming no exercise of the Underwriters’ option to purchase additional shares. Sutro intends to use the net proceeds from the Offering, together with its existing cash, cash equivalents and marketable securities, to fund the continued clinical development of STRO-001 and STRO-002 and the remainder to fund the further development of its technology platform, including manufacturing, to broaden its pipeline of product candidates and for working capital and general corporate purposes. Sutro expects the Offering to close on December 11, 2020, subject to the satisfaction of customary closing conditions.

The Underwriting Agreement is filed as Exhibit 1.1 to this report and the foregoing description of the terms of the Underwriting Agreement is qualified in its entirety by reference to such exhibit. A copy of the opinion of Fenwick & West LLP, relating to the validity of the Shares in connection with the Offering, is filed with this Current Report on Form 8-K as Exhibit 5.1.

Item 9.01.Financial Statements and Exhibits

(d) Exhibits.

Forward-Looking Statements

This Current Report on Form 8-K contains forward-looking statements within the meaning of the U.S. Private Securities Litigation Reform Act of 1995 and other federal securities laws. Any statements contained herein that do not describe historical facts, including, but not limited to, statements regarding the expected net proceeds of the Offering, the anticipated use of proceeds of the Offering, and the satisfaction of customary closing conditions related to the Offering, are forward-looking statements that involve risks and uncertainties that could cause actual results to differ materially from those discussed in such forward-looking statements. Such risks and uncertainties include, among others, the risks identified in Sutro’s filings with the SEC, including its Quarterly Report on Form 10-Q for the three months ended September 30, 2020, filed with the SEC on November 5, 2020, the prospectus supplement related to the Offering, and subsequent filings with the SEC. Any of these risks and uncertainties could materially and adversely affect Sutro’s results of operations, which would, in turn, have a significant and adverse impact on Sutro’s stock price. Sutro cautions you not to place undue reliance on any forward-looking statements, which speak only as of the date they are made. Sutro undertakes no obligation to update publicly any forward-looking statements to reflect new information, events or circumstances after the date they were made or to reflect the occurrence of unanticipated events.

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.