Series B Preferred Stock Purchase Warrant

Exhibit 10.8

THIS WARRANT AND THE UNDERLYING SHARES OF SERIES B PREFERRED STOCK ISSUABLE UPON ITS EXERCISE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"), OR ANY STATE SECURITIES LAWS, AND NO SALE OR TRANSFER THEREOF MAY BE EFFECTED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT OR AN OPINION OF COUNSEL FOR THE HOLDER, SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATION IS NOT REQUIRED UNDER THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS.

No. W-[    ]

SIRTRIS PHARMACEUTICALS, INC.

Series B Preferred Stock Purchase
Warrant August 1, 2005

        For value received, Sirtris Pharmaceuticals, Inc., a Delaware corporation (the "Company"), hereby grants ARE-770/784/790 Memorial Drive, LLC, a Delaware limited liability company (the "Investor"), subject to the terms set forth below, the right to purchase from the Company at any time or from time to time before the earlier of 5:00 P.M., Boston time, on August 1, 2015, and the termination of the Agreement (as defined below) due to a breach by the Investor, sixty-two thousand five hundred (62,500) fully paid and nonassessable shares of Series B Preferred Stock, $0.001 par value per share, of the Company (the "Series B Preferred Stock"), at a purchase price per share of $0.80 (such purchase price per share as adjusted from time to time as herein provided is referred to herein as the "Purchase Price"). The number and character of such shares of Series B Preferred Stock and the Purchase Price are subject to adjustment as provided herein.

        This Warrant evidences the right to purchase shares of Series B Preferred Stock of the Company, issued pursuant to a certain Lease Agreement (the "Agreement"), dated as of August 1, 2005, between the Company and the Investor, a copy of which is on file at the principal office of the Company.

        As used herein the following terms, unless the context otherwise requires, have the following respective meanings:

•         (a)   The term "Common Stock" includes the Company's Common Stock, $0.001 par value per share.

          (b)   The term "Company" shall includes Sirtris Pharmaceuticals, Inc. and any corporation which shall succeed or assume the obligations of the Company hereunder.

        1.     Exercise of Warrant.     

•         1.1.     Full Cash Exercise.     This Warrant may be exercised in full by the holder hereof by surrender of this Warrant, with the form of subscription at the end hereof duly executed by such holder, to the Company at its principal office, accompanied by payment, in cash or by certified or official bank check payable to the order of the Company, in the amount obtained by multiplying the number of shares of Series B Preferred Stock for which this Warrant is then exercisable by the Purchase Price then in effect.

          1.2.     Partial Exercise.     This Warrant may be exercised in part by surrender of this Warrant in the manner and at the place provided in Section 1.1 except that the amount payable by the holder on such partial cash exercise shall be the amount obtained by multiplying (a) the number of shares of Series B Preferred Stock designated by the holder in the subscription at the end hereof by (b) the Purchase Price then in effect. On any such partial exercise, the Company at its expense will forthwith issue and deliver to the holder hereof a new Warrant or Warrants of like tenor in the name of the holder hereof, calling in the aggregate on the face or faces thereof for the number of shares of Series B Preferred Stock for which such Warrant or Warrants may still be exercised.

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          1.3.     Cashless Exercise.     In addition to and without limiting the rights of the Holder hereof under the terms hereof, at the Holder's option this Warrant may be exercised in whole or in part at any time or from time to time prior to its expiration for a number of shares of Series B Preferred Stock having an aggregate fair market value on the date of such exercise equal to the difference between (a) the fair market value of the number of shares of Common Stock into which such shares of Series B Preferred Stock are convertible on the date of exercise and (b) the aggregate Warrant Price for such shares in effect at such time.

        The "fair market value" of shares of Common Stock shall be calculated on the basis of (a) if the Common Stock is then traded on a securities exchange, the average of the closing prices of the Common Stock on such exchange over the 20 trading day period ending three (3) trading days prior to the date of exercise, (b) if the Common Stock is then regularly traded over-the-counter, the average of the sale prices or, if sale prices for the Common Stock are not regularly and publicly reported, then the closing bid of the Common Stock over the 20 trading day period ending three (3) trading days prior to the date of exercise, or (c) if there is no active public market for the Common Stock, the fair market value thereof shall be determined in good faith by the Company's Board of Directors. If the Holder of this Warrant exercises this Warrant contingent upon the closing of a public offering, the "fair market value" of a share of Common Stock on the date of exercise shall be equal to the initial price to the public specified in the final prospectus with respect to such public offering. The following formula illustrates how many shares would be issued upon exercise of this Warrant pursuant to the "cashless exercise" provisions of this Section 1.5:

No payment of any cash or other consideration to the Company shall be required from the holder of this Warrant in connection with any exercise of this Warrant pursuant to this Section 1.5. Such exercise shall be effective upon the date of receipt by the Company of this Warrant surrendered for cancellation and a written request from the holder hereof that the exercise pursuant to this section be made, or at such later date as may be specified in such request.

•         1.4.     Securities Laws.     The Investor acknowledges and agrees that this warrant may only be exercised by Investor if the Company is satisfied, based on among other things representations by the Investor, that such exercise will not result in any violation by the Company of applicable securities laws.

        2.     Delivery of Stock Certificates, etc. on Exercise.     As soon as practicable after each exercise of this Warrant in full or in part, the Company at its expense will cause to be issued in the name of and delivered to the holder hereof, or as such holder (upon payment by such holder of any applicable transfer taxes and, if requested by the Company, demonstration by such holder of compliance with applicable securities laws) may direct, a certificate or certificates for the number of fully paid and nonassessable shares of Series B Preferred Stock to which such holder shall be entitled on such exercise or such other property or securities to which Investor may be entitled to receive upon exercise.

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        3.     Adjustment for Reorganization, Consolidation, Merger, Recapitalization, etc.     

•         3.1.     Reorganization.     In case at any time or from time to time, the Company shall (a) effect a reorganization, (b) consolidate with or merge into any other person, or (c) effect a recapitalization, then, in each such case, the holder of this Warrant, on the exercise hereof as provided in Section 1 at any time after the consummation of such reorganization, consolidation, merger or recapitalization as the case may be, shall receive, in lieu of the Series B Preferred Stock issuable on such exercise prior to such consummation or such effective date, the stock and other securities and property (including cash) to which such holder would have been entitled upon such consummation, if such holder had so exercised this Warrant immediatel