Theriva Biologics, Inc.

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APPENDIX A

AMENDMENT NO. 3

TO THE THERIVA BIOLOGICS, INC. 2020 STOCK INCENTIVE PLAN

Dated: August 29, 2025

WHEREAS, the Board of Directors (the “Board”) of Synthetic 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) by an additional two million (2,000,000) shares of common stock to four million five hundred thousand (4,500,000) 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. 3 to the Plan and has recommended that the stockholders of the Company approve this Amendment No. 3;

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:

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 by an additional two million (2,000,000) shares of common stock to four million five hundred thousand (4,500,000) shares of common stock, so that Section 4(a) reads in its entirety as follows:

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(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 four million five hundred thousand (4,500,000) shares (the “Initial Limit”), all of which may, but need not, be issued in respect of Incentive Stock Options.

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.

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[Signature page follows.]

IN WITNESS WHEREOF, the undersigned has executed this Amendment No. 3 as evidence of its adoption by the Board on the date set forth above.

THERIVA BIOLOGICS, INC.

By:

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Name: Steven A. Shallcross

Title:

Chief Executive Officer, Chief Financial Officer and Director

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Dated:

Signature [PLEASE SIGN WITHIN BOX] Date Signature (Joint Owners) DateTO VOTE, MARK BLOCKS BELOW IN BLUE OR BLACK INK AS FOLLOWS:KEEP THIS PORTION FOR YOUR RECORDSTHIS PROXY CARD IS VALID ONLY WHEN SIGNED AND DATED. DETACH AND RETURN THIS PORTION ONLYV76958-P36287! ! !ForAllWithholdAllFor AllExceptFor Against AbstainFor Against Abstain! ! !! ! !! ! !To withhold authority to vote for any individualnominee(s), mark "For All Except" and write thenumber(s) of the nominee(s) on the line below.THERIVA BIOLOGICS, INCThe Board of Directors recommends you vote FORthe election of each of the following:The Board of Directors recommends youvote 3 YEARS on the following proposal:The Board of Directors recommends you vote FORproposals 2-4:01) Jeffrey J. Kraws02) Steven A. Shallcross03) John Monahan04) Jeffrey Wolf, J.D.Nominees:1. Election of Directors2. To ratify the appointment of BDO USA, P.C. as ourindependent registered public accounting firm forour fiscal year ending on December 31, 2025.3. To approve an amendment to the Company’s2020 Stock Incentive Plan to increase the numberof shares of common stock that we will haveauthority to grant under the plan from 2,500,000to 4,500,000 shares of common stock.4. To approve, on an advisory basis, the compensationof the Company’s named executive officers, asdisclosed in the accompanying proxy statement.5. To recommend, on an advisory basis, thefrequency for holding future advisory voteson executive compensation.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 ownersshould each sign personally. All holders must sign. If a corporation or partnership,please sign in full corporate or partnership name by authorized officer.Note: To transact such other business as may properlycome before the meeting or any adjournments orpostponements of the meeting.! ! ! !1 Year 2 Years 3 Years AbstainSCAN TOVIEW MATERIALS & VOTE wTHERIVA BIOLOGICS, INC9605 MEDICAL CENTER DRIVE, SUITE 270ROCKVILLE, MARYLAND 20850VOTE BY INTERNET - www.proxyvote.com or scan the QR Barcode aboveUse the Internet to transmit your voting instructions and for electronic delivery ofinformation up until 11:59 p.m. Eastern Time the day before the meeting date. Haveyour proxy card in hand when you access the web site and follow the instructions toobtain your records and to create an electronic voting instruction form.ELECTRONIC DELIVERY OF FUTURE PROXY MATERIALSIf you would like to reduce the costs incurred by our company in mailing proxy materials,you can consent to receiving all future proxy statements, proxy cards and annual reportselectronically via e-mail or the Internet. To sign up for electronic delivery, please followthe instructions above to vote using the Internet and, when prompted, indicate thatyou agree to receive or access proxy materials electronically in future years.VOTE BY PHONE -1-800-690-6903 Use any touch-tone telephone to transmit your voting instructions up until 11:59 p.m.Eastern Time the day before the meeting date. Have your proxy card in hand whenyou call and then follow the instructions.VOTE BY MAILMark, sign and date your proxy card and return it in the postage-paid envelope wehave provided or return it to Vote Processing, c/o Broadridge, 51 Mercedes Way,Edgewood, NY 11717.

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting:The Notice and Proxy Statement and our Annual Report on Form 10-K for the year ended December 31, 2024are available at www.proxyvote.com.THERIVA BIOLOGICS, INCANNUAL MEETING OF STOCKHOLDERSAUGUST 29, 2025 AT 3:30 P.M. (LOCAL TIME)THIS PROXY IS SOLICITED ON BEHALF OF THE BOARD OF DIRECTORSThe undersigned stockholder hereby appoints Steven A. Shallcross as proxy, with the power to appoint his substitute, and hereby authorizes him to represent and to vote, as designated on the reverse side of this ballot, all of the shares ofcommon stock of THERIVA BIOLOGICS, INC. that the undersigned is entitled to vote at the Annual Meeting of Stockholders to be held at3:30 p.m. (local time), on August 29, 2025 at the Company’s principal place of business for research and clinical development, located at Carrer Torrent de Can Ninou, naus 5-6, 08150 - Parets del Vallès (Barcelona) Spain, or any adjournment or postponement 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 in accordance with the Board of Directors’ recommendations.Continued and to be signed on reverse side