false 0001382101 DEFA14A 0001382101 2024-01-01 2024-12-31

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(AMENDMENT NO. )

Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐ Preliminary Proxy Statement

☐ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material Pursuant to §240.14a-12

SUTRO BIOPHARMA, INC.

(Name of Registrant as Specified In Its Charter)

N/A

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

☒ No fee required.

☐ Fee paid previously with preliminary materials.

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11.

SUTRO BIOPHARMA Your Vote Counts! SUTRO BIOPHARMA, INC. 2025 Annual Meeting Vote by June 05, 2025 11:59 PM ET SUTRO BIOPHARMA, INC. 111 OYSTER POINT BOULEVARD SOUTH SAN FRANCISCO, CALIFORNIA 94080 Ricky Campana P.O. Box 123456 Suite 500 51 Mercedes Way Edgewood, NY 11717 QAARV1-P12345 148,294 322,224 1 OF 30# 4 You invested in SUTRO BIOPHARMA, INC. and it's time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy material for the shareholder meeting to be held on June 06, 2025. Get informed before you vote View the Notice & Proxy Statement, Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to May 23, 2025. If you would like to request a copy of the material(s) for this and/or future shareholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. PV For complete information and to vote, visit www.ProxyVote.com Control # XXXX XXXX XXXX XXXX Smartphone users = Point your camera here and vote without entering a control number Virtually at: *Please check the meeting materials for any special requirements for meeting attendance. Vote Virtually at the Meeting* June 06, 2025 3:00 PM PDT www.virtualshareholdermeeting.com/STRO2025 V2.1

Vote at www.ProxyVote.com THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming shareholder meeting. Please follow the instructions on the reverse side to vote these important matters. Voting Items 1. 2. Election of Class | Directors Nominees: 01) Heidi Hunter Control # XXXX XXXX XXXX XXXX 02) Jon Wigginton, M.D. SHARE CLASSES REPRESENTED FOR VOTING THE COMPANY NAME INC. - COMMON ASDFGHJKL 123456789.1234 THE COMPANY NAME INC. - CLASS A THE COMPANY NAME INC. - CLASS B 123456789.1234 123456789.1234 THE COMPANY NAME INC. - CLASS C THE COMPANY NAME INC. - CLASS D THE COMPANY NAME INC. - CLASS E THE COMPANY NAME INC. - CLASS F THE COMPANY NAME INC. - 401 K 123456789.1234 123456789.1234 123456789.1234 123456789.1234 123456789.1234 03) Michael Dybbs, Ph.D. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2025. Board Recommends For For For 3. To approve, on a non-binding, advisory basis, the compensation of our named executive officers. 4. To approve an amendment to our Restated Certificate of Incorporation to effect a reverse stock split of our outstanding shares of common stock at a ratio of any whole number between 1-for-5 and 1-for-25, with the exact ratio determined by the board of directors in its discretion, subject to the board of directors' authority to abandon such amendment. NOTE: Such other business as may properly come before the meeting or any adjournment thereof. For Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click "Delivery Settings". QAARV2-P12345 1.00000 322,224 148,294 CLT ID

BIOPHARMA SUTRO BIOPHARMA, INC. 111 OYSTER POINT BOULEVARD SOUTH SAN FRANCISCO, CALIFORNIA 94080 Investor Address Line 1 Investor Address Line 2 Investor Address Line 3 Investor Address Line 4 Investor Address Line 5 John Sample 1234 ANYWHERE STREET ANY CITY, ON A1A 1A1 1 1 1 OF 2 SCAN TO VIEW MATERIALS & VOTE VOTE BY INTERNET - www.proxyvote.com or scan the QR Barcode above Use the Internet to transmit your voting instructions and for electronic delivery of information. Vote by 11:59 P.M. ET on 06/05/2025. Have your proxy card in hand when you access the web site and follow the instructions to obtain your records and to create an electronic voting instruction form. During The Meeting - Go to www.virtualshareholdermeeting.com/STRO2025 You may attend the meeting via the Internet and vote during the meeting. Have the information that is printed in the box marked by the arrow available and follow the instructions. VOTE BY PHONE 1-800-690-6903 Use any touch-tone telephone to transmit your voting instructions. Vote by 11:59 P.M. ET on 06/05/2025. Have your proxy card in hand when you call and then follow the instructions. VOTE BY MAIL Mark, sign and date your proxy card and return it in the postage-paid envelope we have provided or return it to Vote Processing, c/o Broadridge, 51 Mercedes Way, Edgewood, NY 11717. CONTROL # 0000000000000000 NAME THE COMPANY NAME INC. - COMMON THE COMPANY NAME INC. - CLASS A THE COMPANY NAME INC. - CLASS B THE COMPANY NAME INC. - CLASS C THE COMPANY NAME INC. - CLASS D THE COMPANY NAME INC. - CLASS E THE COMPANY NAME INC. - CLASS F THE COMPANY NAME INC. - 401 K SHARES 123,456,789,012.12345 123,456,789,012.12345 123,456,789,012.12345 123,456,789,012.12345 123,456,789,012.12345 123,456,789,012.12345 123,456,789,012.12345 123,456,789,012.12345 PAGE 1 OF 2 TO VOTE, MARK BLOCKS BELOW IN BLUE OR BLACK INK AS FOLLOWS: THIS PROXY CARD IS VALID ONLY WHEN SIGNED AND DATED. The Board of Directors recommends you vote FOR the following: 1. Election of Class I Directors All For Withhold For All All Except 0 0 0 To withhold authority to vote for any individual nominee(s), mark "For Alí Except" and write the number (s) of the nominee(s) on the line below. KEEP THIS PORTION FOR YOUR RECORDS DETACH AND RETURN THIS PORTION ONLY 0000675103_1 R1.0.0.2 Nominees 01) Heidi Hunter 02) Jon Wigginton, M.D. 03) Michael Dybbs, Ph.D. The Board of Directors recommends you vote FOR proposals 2, 3 and 4. 2. 3. 4. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2025. To approve, on a non-binding, advisory basis, the compensation of our named executive officers. To approve an amendment to our Restated Certificate of Incorporation to effect a reverse stock split of our outstanding shares of common stock at a ratio of any whole number between 1-for-5 and 1-for-25, with the exact ratio determined by the board of directors in its discretion, subject to the board of directors' authority to abandon such amendment. NOTE: Such other business as may properly come before the meeting or any adjournment thereof. For Against Abstain O O O 0 O 0 0 0 Investor Address Line 1 Investor Address Line 2 Investor Address Line 3 Investor Address Line 4 Investor Address Line 5 John Sample Please sign exactly as your name(s) appear (s) hereon. When signing as attorney, executor, administrator, or other fiduciary, please give full title as such. Joint owners should each sign personally. All holders must sign. If a corporation or partnership, please sign in full corporate or partnership name by authorized officer. 1234 ANYWHERE STREET ANY CITY, ON A1A 1A1 02 0000000000 SHARES CUSIP # SEQUENCE # JOB # Signature [PLEASE SIGN WITHIN BOX] Date Signature (Joint Owners) Date

0000675103_2 R1.0.0.2 Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting: The Notice and Proxy Statement and Form 10-K are available at www.proxyvote.com SUTRO BIOPHARMA, INC. Annual Meeting of Shareholders June 6, 2025 3:00 PM PDT This proxy is solicited by the Board of Directors The undersigned hereby appoints Jane Chung, R.Ph and David Pauling, and each of them, with full power of substitution and power to act alone, as proxies to vote all the shares of Common Stock which the undersigned would be entitled to vote if personally present and acting at the Annual Meeting of Shareholders of SUTRO BIOPHARMA, INC., to be held on June 6, 2025, via a live webcast at www.virtualshareholdermeeting.com/STRO2025, and at any adjournments or postponements thereof. This proxy, when properly executed, will be voted in the manner directed herein. If no such direction is made, this proxy will be voted FOR ALL NOMINEES in Proposal 1, FOR Proposal 2, FOR Proposal 3 and FOR Proposal 4. Continued and to be signed on reverse side