Sutro Biopharma Inc — Current Report (Form 8-K)

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 1, 2025

SUTRO BIOPHARMA, INC.

(Exact name of registrant as specified in its charter)

111 Oyster Point Blvd,

South San Francisco, California, 94080

(Address of principal executive offices) (Zip Code)

(650) 881-6500

(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:

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

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 3.03 Material Modification to the Rights of Security Holders

The information set forth in Item 5.03 of this Current Report on Form 8-K is incorporated by reference herein.

Item 5.03 Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year

On December 2, 2025, Sutro Biopharma, Inc. (the “Company”) filed a Certificate of Amendment (the “Amendment”) to its Restated Certificate of Incorporation (the “Certificate of Incorporation”) with the Secretary of State of the State of Delaware to implement a 1-for-10 reverse stock split (the “Reverse Stock Split”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”) to be effective as of 12:01 am Eastern time on December 3, 2025 (the “Effective Time”). As previously disclosed on the Company’s Current Report on Form 8-K filed with the Securities and Exchange Commission on June 10, 2025, the Company’s stockholders approved a proposal to authorize the Company’s Board of Directors (the “Board”) to implement, at the Board's discretion, a reverse stock split at a ratio not less than 1-for-5 and not greater than 1-for-25, with the exact ratio to be determined within that range at the discretion of the Board.

The foregoing description of the Amendment does not purport to be complete and is qualified in its entirety by the full text of the Amendment, a copy of which is filed as Exhibit 3.1 to this Current Report on Form 8-K and is incorporated herein by reference.

Item 8.01 Other Events

On November 20, 2025, the Board approved the Reverse Stock Split at a ratio of 1-for-10. On December 3, the Common Stock will begin trading on a split-adjusted basis under a new CUSIP number, 869367201.

Upon the Effective Time, every ten shares of the Company’s issued and outstanding Common Stock will be automatically combined and reclassified into one issued and outstanding share of Common Stock.

No fractional shares will be issued in connection with the Reverse Stock Split. Stockholders who would otherwise be entitled to receive fractional shares will receive the number of shares of Common Stock as rounded up to the nearest whole share. The Reverse Stock Split will not affect any stockholder’s ownership percentage of Common Stock (except to the extent that the Reverse Stock Split results in such stockholder owning a fractional share which will be rounded up to the nearest full share), alter the par value of the Common Stock, change the number of authorized shares, or modify any voting rights or other terms of the Common Stock. In addition, a proportionate adjustment will be made to the per share exercise price and the number of shares issuable upon the exercise of all outstanding stock options to purchase shares of Common Stock and the number of shares reserved for issuance pursuant to the Company’s equity incentive and inducement plans, and employee stock purchase plan. Immediately prior to the Reverse Stock Split, there were approximately 85,193,912 shares of Common Stock outstanding. After the Reverse Stock Split, there will be approximately 8,519,392 shares outstanding.

On December 1, 2025, the Company issued a press release announcing that the Board approved the Reverse Stock Split. A copy of the press release is filed herewith as Exhibit 99.1 and is incorporated by reference into this Item 8.01.

Item 9.01. Financial Statements and Exhibits

(d) Exhibits.

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.