SYNTHETIC BIOLOGICS, INC. CONTROLLED EQUITY OFFERINGSM AmendmentNo. 1 to SALES AGREEMENT

SYNTHETIC BIOLOGICS, INC.

CONTROLLED EQUITY OFFERING^SM

Amendment No. 1 to
SALES AGREEMENT

December 11, 2013

Cantor Fitzgerald & Co.

499 Park Avenue

New York, NY 10022

Ladies and Gentlemen:

Reference is made to the Sales Agreement, dated July 3, 2013 (the “Sales Agreement”), between Cantor Fitzgerald & Co. (“CF&Co”) and Synthetic Biologics, Inc., a Nevada corporation (the “Company”), pursuant to which the Company agreed to sell through CF&Co, as sales agent, up to $15,000,000 of shares of common stock, par value $0.001 per share, of the Company. All capitalized terms used in this Amendment No. 1 to Sales Agreement between CF&Co and the Company (this “Amendment”) and not otherwise defined herein shall have the respective meanings assigned to such terms in the Sales Agreement. CF&Co and the Company agree as follows:

“The Company agrees that, from time to time during the term of this Agreement, on the terms and subject to the conditions set forth herein, it may issue and sell through the Agent, shares of common stock (the “Placement Shares”) of the Company, par value $0.001 per share (the “Common Stock”); provided, however, that in no event shall the Company issue or sell through the Agent such number or dollar amount of Placement Shares that would (a) exceed the number or dollar amount of shares of Common Stock registered on the effective Registration Statement (defined below) pursuant to which the offering is being made, (b) exceed the number of authorized but unissued and unreserved shares of Common Stock, (c) exceed the number or dollar amount of shares of Common Stock permitted to be sold under Form S-3 (including Instruction I.B.6. thereof) or (d) exceed the number or dollar amount of shares of Common Stock for which the Company has filed a Prospectus Supplement (defined below) (the lesser of (a), (b), (c) and (d), the “Maximum Amount”).”

B.          Prospectus Supplement. If required by securities laws, the Company shall file a 424(b) Prospectus Supplement reflecting this Amendment prior to the issuance of any Placement Notice pursuant to the Sales Agreement. The Company and CF&Co agree that the Prospectus Supplement filed to reflect this Amendment, if necessary, will be limited by the dollar amount of shares then available for offer and sale under the then-current Registration Statement and the provisions of Form S-3 (including Instruction I.B.6. thereof), and this Amendment shall not be deemed to be an offer for any sales of shares of Common Stock unless such shares are set forth in a Placement Notice issued by the Company setting forth the number or dollar amount of shares to be sold thereunder and any other parameters in accordance with which the Company desires the Placement Shares to be sold.

C.           No Other Amendments. Except as set forth in Part A above, all the terms and provisions of the Sales Agreement shall continue in full force and effect.

D.           Counterparts. This Amendment may be executed in two or more counterparts, each of which shall be deemed an original, but all of which together shall constitute one and the same instrument. Delivery of an executed Amendment by one party to the other may be made by facsimile or email transmission.

E.            Governing Law. This Amendment shall be governed by, and construed in accordance with, the internal laws of the State of New York without regard to the principles of conflicts of laws.

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If the foregoing correctly sets forth the understanding between the Company and CF&Co, please so indicate in the space provided below for that purpose, whereupon this Amendment No. 1 to Sales Agreement shall constitute a binding agreement between the Company and CF&Co.

ACCEPTED as of the date first-above written:

CANTOR FITZGERALD & CO.