Theriva Biologics, Inc.

Appendix A

**AMENDMENT NO. 4

TO THE THERIVA BIOLOGICS, INC. 2020 STOCK INCENTIVE PLAN

Dated: August 3, 2026

WHEREAS, the Board of Directors (the “Board”) of Theriva Biologics, Inc. (the “Company”) heretofore established the Synthetic Biologics, Inc. 2020 Stock Incentive Plan (the “Plan”); and

WHEREAS, the Board desires to amend the Plan to increase the maximum number of shares of common stock of the Company available for grants of Awards thereunder (as of the date of this amendment, previously adopted the Plan) to Six Million Five Hundred Thousand (6,500,000) shares of common stock; and

WHEREAS, pursuant to Section 17.2 of the Plan, the Board has the right to amend the Plan with respect to certain matters; and

WHEREAS, the Board has approved and authorized this Amendment No. 4 to the Plan and has recommended that the stockholders of the Company approve this Amendment No. 4;

NOW, THEREFORE, BE IT RESOLVED, that the Plan is hereby amended, subject to and effective as of the date of stockholder approval hereof, in the following particulars:

1. Subject to approval of the Company’s stockholders, Section 4(a) of the Plan is hereby amended by increasing the share references in such section to Six Million Five Hundred Thousand (6,500,000) shares of common stock, so that Section 4(a) reads in its entirety as follows:

“(a) Subject to adjustment pursuant to Section 4.3 hereof, the maximum aggregate number of shares of Common Stock which may be issued under all Awards granted to Participants under the Plan shall be Six Million Five Hundred Thousand (6,500,000) shares (the “Initial Limit”), all of which may, but need not, be issued in respect of Incentive Stock Options.”

2. Subject to approval of the Company’s stockholders, Section 4.2 of the Plan is hereby amended by increasing the share references in such section from Two Hundred Fifty Thousand (250,000) shares of common stock to Seven Hundred Fifty Thousand (750,000) shares of common stock, so that Section 4.2 reads in its entirety as follows:

“(a) Subject to adjustment pursuant to Section 4.3 hereof, the number of shares of Common Stock with respect to which Awards may be granted during any calendar year to any one Eligible Person who is a non-employee director of the Board shall not exceed Seven Hundred Fifty Thousand (750,000).”

3. Except as specifically set forth herein, the terms of the Plan shall be and remain unchanged, and the Plan as amended shall remain in full force and effect.

[Signature page follows.]

IN WITNESS WHEREOF, the undersigned has executed this Amendment No. 4 as evidence of its adoption by the Board on the date set forth above.

THERIVA BIOLOGICS, INC.

By:

Name: Steven A. Shallcross

Title: Chief Executive Officer, Chief Financial Officer and Director

Dated: