NOTE

Exhibit 10.91

THIS NOTE HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”). NO SALE, PLEDGE OR DISPOSITION MAY BE EFFECTED EXCEPT IN COMPLIANCE WITH RULE 144 UNDER SAID ACT OR AN EFFECTIVE REGISTRATION STATEMENT RELATED THERETO OR AN OPINION OF COUNSEL FOR THE HOLDER, REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATION IS NOT REQUIRED UNDER THE ACT.

THIS NOTE IS SUBJECT TO THE TERMS AND CONDITIONS OF THE PURCHASE AGREEMENT (AS DEFINED BELOW).

NOTE

For value received, Anesiva, Inc., a Delaware corporation (the “ Company ”), promises to pay to the order of Arcion Therapeutics, Inc. (together with its successors and assigns, the “ Holder ”), the principal sum of Two Million Dollars ($2,000,000), together with interest accrued but unpaid thereon, upon the terms of this Note (the “ Note ”).

Interest shall accrue from the date hereof until maturity at a continuously compounding rate equal to ten percent (10%) per annum payable in cash; provided , however , that, during the occurrence and continuance of an Event of Default (as defined in that certain Secured Note Purchase Agreement, dated as of May 18, 2009 (as amended, restated, supplemented or otherwise modified from time to time, the “ Purchase Agreement ”), among the Company and the Holder), interest shall accrue at a continuously compounding rate equal to fourteen percent (14%) per annum. All computations of interest shall be made on the basis of a year of 365 or 366 days, as the case may be, for the actual number of days (including the first day but excluding the last) occurring in the period for which such interest is payable.

Unless earlier paid pursuant to the terms hereof or the Purchase Agreement or accelerated in connection with an Event of Default, subject to the terms of the Purchase Agreement, the outstanding principal and accrued but unpaid interest shall be immediately due and payable on October 20, 2009 (the “ Maturity Date ”). Company may prepay this Note at any time without penalty or premium.

1. This Note is issued pursuant to the terms of the Purchase Agreement. The Holder is entitled to the benefit of, and is subject to certain restrictions contained in, the Purchase Agreement and the other Related Documents. Capitalized terms not otherwise defined herein shall have the meanings ascribed to them in the Purchase Agreement. The indebtedness evidenced by this Note is secured by certain collateral, as more particularly described in that certain Pledge, Security and Collateral Agent Agreement, dated as of May 18, 2009 (as may be further amended, restated, supplemented or modified from time to time, the “ Security Agreement ”), among the Company, AlgoRx Pharmaceuticals, Inc. and the Collateral Agent and that certain Guaranty, dated as of January 20, 2009 (as may be further amended or modified from time to time, the “ Guaranty ”) by and between AlgoRx Pharmaceuticals, Inc. and the Holder.

Each holder of this Note will be deemed, by its acceptance hereof, to have agreed to the provisions and to have made the representations and warranties set forth in Section 4 the Purchase Agreement. Subject to the terms of the Purchase Agreement, this Note is transferable by surrender hereof at the principal office of the Company, duly endorsed or accompanied by a written instrument of transfer duly executed by the registered holder of this Note or by any other method permitted by the Purchase Agreement.

2. All payments hereunder shall be applied in the order provided for in the Purchase Agreement. Whenever any payment hereunder shall be stated to be due, or whenever any return payment date or any other date specified hereunder would otherwise occur, on a day other than a Business Day, then such payment shall be made, and such return payment date or other date shall occur, on the next succeeding Business Day, and such extension of time shall in such case be included in the computation of payment of interest hereunder.

3. All payments in respect of this Note shall be in immediately available lawful money of the United States of America and shall be sent so as to be received no later than 2 p.m. (Pacific time) on the date of payment, at the address specified in the Purchase Agreement, or at such other address as may be specified from time to time by such Holder in a written notice delivered to the Company. All payments in respect of this Note shall be made unconditionally in full without any deduction, set off, counterclaim or other defense. If any scheduled payment date is not a Business Day, such payment shall be made on the next succeeding Business Day.

4. The Company hereby waives demand, notice, presentment, protest and notice of dishonor.

5. (a) The terms of this Note shall be construed in accordance with the laws of the State of California, as applied to contracts entered into by California residents within the State of California, which contracts are to be performed entirely within the State of California. The Company hereby (i) submits to the exclusive jurisdiction of the courts of the County of San Francisco, State of California and the Federal courts of th