WARRANT TO PURCHASE COMMON STOCK

Exhibit 10.51

NEITHER THIS WARRANT NOR THE SECURITIES ISSUABLE UPON EXERCISE HEREOF HAVE BEEN THE SUBJECT OF REGISTRATION UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER THE SECURITIES LAWS OF ANY STATE, AND THE SAME HAVE BEEN (OR WILL BE, WITH RESPECT TO THE SECURITIES ISSUABLE UPON EXERCISE HEREOF) ISSUED IN RELIANCE ON EXEMPTIONS FROM THE REGISTRATION REQUIREMENTS OF SAID ACT AND SUCH LAWS. NEITHER THIS WARRANT NOR THE SECURITIES ISSUABLE UPON EXERCISE HEREOF MAY BE SOLD, TRANSFERRED, PLEDGED, HYPOTHECATED OR OTHERWISE DISPOSED OF EXCEPT AS PERMITTED UNDER SUCH SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM.

THE WARRANT EVIDENCED BY THIS CERTIFICATE IS SUBJECT TO CERTAIN RESTRICTIONS ON TRANSFER AS SET FORTH IN THE WARRANT PURCHASE AGREEMENT, DATED AS OF JUNE 15, 2009, COPIES OF WHICH ARE ON FILE AT THE PRINCIPAL EXECUTIVE OFFICES OF THE ISSUER. NO REGISTRATION OF TRANSFER OF THIS WARRANT WILL BE MADE ON THE BOOKS OF THE ISSUER UNLESS AND UNTIL SUCH RESTRICTIONS SHALL HAVE BEEN COMPLIED WITH.

ALEXZA PHARMACEUTICALS, INC.

WARRANT TO PURCHASE COMMON STOCK

Void After August 26, 2014

      THIS CERTIFIES THAT , for value received,            , with its principal office at                      , or its assigns (the “ Holder ”), is entitled to subscribe for and purchase at the Exercise Price (defined below) from Alexza Pharmaceuticals, Inc., a Delaware corporation, with its principal office at 2091 Stierlin Court, Mountain View, CA 94043 (the “ Company ”) up to            (       ) shares of the Common Stock of the Company (the “ Common Stock ”), subject to adjustment as provided herein. This Warrant is being issued pursuant to the terms of the Warrant Purchase Agreement, dated June 15, 2009, by and among the Company and the Holder (the “ Warrant Purchase Agreement ”). Capitalized terms not otherwise defined herein shall have the respective meanings ascribed to such terms in the Warrant Purchase Agreement.

     1.  DEFINITIONS . As used herein, the following terms shall have the following respective meanings:

               (a) “ Exercise Period ” shall mean the period commencing on August 26, 2009 and ending on August 26, 2014, except as otherwise provided below.

               (b) “ Exercise Price ” shall mean $2.26 per share, subject to adjustment pursuant to Section 6 below.

               (c) “ Exercise Shares ” shall mean the shares of the Company’s Common Stock issuable upon exercise of this Warrant, subject to adjustment pursuant to the terms herein, including but not limited to adjustment pursuant to Section 6 below.

     2.  EXERCISE OF WARRANT .

               2.1 Method of Exercise . The rights represented by this Warrant may be exercised in whole or in part at any time during the Exercise Period by delivery of the following to the Company at its address set forth above (or at such other address as it may designate by notice in writing to the Holder):

                    (a) An executed Notice of Exercise in the form attached hereto;

                    (b) Payment of the Exercise Price of the Exercise Shares purchased thereby (i) in cash or by check or wire transfer of immediately available funds, (ii) pursuant to a Cashless Exercise, as described below, or (iii) by a combination of (i) and (ii); and

                    (c) Upon the exercise of the rights represented by this Warrant, shares of Common Stock shall be issued for the Exercise Shares so purchased, and shall be registered in the name of the Holder or persons affiliated with the Holder, if the Holder so designates, within a reasonable amount of time following receipt by the Company of all of the items designated in clauses (a) , (b) and (c) above, but in no event later than thirty (30) days after the date of exercise pursuant to this Section 2.1 . The Company shall (i) upon request of the Holder, if available and if allowed under applicable securities laws, use commercially reasonable efforts to deliver Exercise Shares electronically through the Depository Trust Corporation or another established clearing corporation performing similar functions, or (ii) if requested by the Holder, deliver to the Holder certificates evidencing the Exercise Shares. The person in whose name any Exercise Shares are to be issued upon exercise of this Warrant shall be deemed to have become the holder of record of such shares on the date on which delivery of the Notice of Exercise, delivery of this Warrant and payment of the Exercise Price were made, irrespective of the date of issuance of the shares of Common Stock, except that, if the date of such delivery and payment is a date when the stock transfer books of the Company are closed, such person shall be deemed to have become the holder of such shares at the close of business on the next succeeding date on which the stock transfer books are open.

               2.2 Cashless Exercise . Notwithstanding any provisions herein to the contrary, if, at any time during the Exercise Period, the Current Market Price (as defined below) of one share of Common Stock is greater than the Exercise Price (at the date of calculation as set forth below), in lieu of exercising this Warrant by payment of cash, the Holder may exercise this Warrant in whole or part by a cashless exercise by surrender of this Warrant at the principal office of the Company together with the properly endorsed Notice of Exercise and the Company shall issue to the Holder a number of shares of Common Stock computed using the following formula:

     A =   the Exercise Price.

     B =   the Current Market Price of one share of Common Stock.

     “ Current Market Price ” means on any particular date:

                    (a) if the Common Stock is traded on the Nasdaq SmallCap Market or the Nasdaq Global Market, the average of the closing prices of the Common Stock of the Company on such market over the five (5) trading days ending immediately prior to the applicable date of valuation (in the case of a cashless exercise, the date of valuation will be the exercise date);

                    (b) if the Common Stock is traded on any registered national stock exchange but is not traded on the Nasdaq SmallCap Market or the Nasdaq Global Market, the average of the closing prices of the Common Stock of the Company on such exchange over the five (5) trading days ending immediately prior to the applicable date of valuation (in the case of a cashless exercise, the date of valuation will be the exercise date).

                    (c) if the Common Stock is traded over-the-counter, but not on the Nasdaq SmallCap Market, the Nasdaq Global Market or a registered national stock exchange, the average of the closing bid prices over the five (5) trading day period ending immediately prior to the applicable date of valuation (in the case of a cashless exercise, the date of valuation will be the exercise date); and

                    (d) if there is no active public market for the Common Stock, the value thereof, as determined in good faith by the Board of Directors of the Company upon due consideration of the proposed determination thereof by the Holder.

               2.3 Partial Exercise . If this Warrant is exercised in part only, the Company shall, upon surrender of this Warrant, execute and deliver, within ten (10) days of the date of exercise, a new Warrant evidencing the rights of the Holder, or such other person as shall be designated in the Notice of Exercise, to purchase the balance of the Exercise Shares purchasable hereunder. In no event shall this Warrant be exercised for a fractional Exercise Share, and the Company shall not distribute a Warrant exercisable for a fractional Exercise Share. Fractional Exercise Shares shall be treated as provided in Section 5 hereof.

               2.4 Legend .

                    (a) All certificates evidencing the shares to be issued to the Holder may bear the following legend (provided that no such legend shall be borne by Exercise Shares issued following the valid disposition of such shares pursuant to a registration statement which is effective under the Securities Act):

     “THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR UNDER THE SECURITIES LAWS OF ANY STATE, AND THE SAME HAVE BEEN ISSUED IN RELIANCE ON EXEMPTIONS FROM THE REGISTRATION REQUIREMENTS OF SAID ACT AND SUCH LAWS. SUCH SHARES MAY NOT BE SOLD, TRANSFERRED, PLEDGED, HYPOTHECATED OR OTHERWISE DISPOSED OF EXCEPT AS PERMITTED UNDER SUCH SECURITIES LAWS, PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM.”

                    (b) If the certificates representing shares include the legend set forth in Section 2.4(a) hereof, the Company shall, upon a request from a Holder, or subsequent transferee of a Holder, as soon as practicable but in no event more than thirty (30) days after receiving such request, remove or cause to be removed (i) if the shares cease to be restricted securities, the securities law portion of the legend and/or (ii) in the event of a sale of the shares subject to issuance following the transfer of the shares in compliance with the transfer restrictions, the transfer restriction portion of the legend, from certificates representing the shares delivered by a Holder (or a subsequent transferee).

               2.5 Charges, Taxes and Expenses . Issuance of the Exercise Shares shall be made without charge to the Holder for any issue or transfer tax or other incidental expense in respect of the issuance of any electronic or paper certificate, all of which taxes and expenses shall be paid by the Company, and such certificates shall be issued in the name of the Holder or in such name or names as may be directed by the Holder; provided , however , that in the event Exercise Shares are to be issued in a name other than the name of the Holder, this Warrant when surrendered for exercise shall be accompanied by the Assignment Form attached hereto duly executed by the Holder; and the Company may require, as a condition thereto, the payment of a sum sufficient to reimburse it for any transfer tax incidental thereto. Notwithstanding anything to the contrary in this Section 2.5, all issue or transfer tax or other incidental expenses imposed by a Governmental Authority outside the United States shall be 100% borne by the Holder.

     3.  COVENANTS OF THE COMPANY .

               3.1 Covenants as to Exercise Shares . The Company covenants and agrees that all Exercise Shares that may be issued upon the exercise of the rights represented by this Warrant will, upon issuance, be duly authorized and validly issued and outstanding, fully paid and nonassessable, and free from all taxes, liens and charges with respect to the issuance thereof. The Company further covenants and agrees that the Company will at all times during the Exercise Period, have authorized and reserved, free from preemptive rights, a sufficient number of shares of its Common Stock to provide for the exercise of the rights represented by

this Warrant. If at any time during the Exercise Period the number of authorized but unissued shares of Common Stock shall not be sufficient to permit exercise of this Warrant, the Company will take such corporate action as may, in the opinion of counsel, be necessary to increase its authorized but unissued shares of Common Stock (or other securities as provided herein) to such number of shares as shall be sufficient for such purposes.

               3.2 No Impairment . Except and to the extent as waived or consented to by the Holder in accordance with Section 11 hereof, the Company will not, by amendment of its Certificate of Incorporation (as such may be amended from time to time), or through any means, avoid or seek to avoid the observance or performance of any of the terms to be observed or performed hereunder by the Company, but will at all times in good faith carry out of all the provisions of this Warrant and take all such action as may be necessary or appropriate in order to protect the exercise rights of the Holder against such impairment.

               3.3 Notices of Record Date . If at any time:

                    (a) the Company shall take a record of the holders of Common Stock for the purpose of entitling them to receive a dividend or other distribution, or any right to subscribe for or purchase any evidences of its indebtedness, any shares of stock of any class or any other securities or property, or to receive any other right (other than with respect to any equity or equity equivalent security issued pursuant to a rights plan adopted by the Company’s Board of Directors);

                    (b) there shall be any capital reorganization of the Company, any reclassification or recapitalization of the capital stock of the Company or any consolidation or merger of the Company, or any sale, transfer or other disposition of all or substantially all the property, assets or business of the Company; or

                    (c) there shall be a voluntary or involuntary dissolution, liquidation or winding up of the Company;

then, in any one or more of such cases, the Company shall use commercially reasonable efforts to give to the Holder, provided that such action is available and permitted under the applicable securities laws, at least ten (10) days’ prior written notice of the record date for such dividend, distribution or right or for determining rights to vote in respect of any such reorganization, reclassification, recapitalization, consolidation, merger, sale, transfer, disposition, dissolution, liquidation or winding up of the Company. Any notice provided hereunder shall specify the date on which the holders of Common Stock shall be entitled to any such dividend, distribution or right, and the amount and character thereof, and the then current estimated date for the closing of the transaction contemplated by any proposed reorganization, reclassification, recapitalization, consolidation, merger, sale, transfer, disposition, dissolution, liquidation or winding up of the Company.

     4.  REPRESENTATIONS OF HOLDER .

               4.1 Acquisition of Warrant for Personal Account . The Holder represents and warrants that it is acquiring the Warrant and the Exercise Shares solely for its account for investment and not with a present view toward the public sale or public distribution

of said Warrant or Exercise Shares or any part thereof and has no intention of selling or distributing said Warrant or Exercise Shares or any arrangement or understanding with any other persons regarding the sale or distribution of said Warrant or the Exercise Shares, except as would not result in a violation of the Securities Act. The Holder will not, directly or indirectly, offer, sell, pledge, transfer or otherwise dispose of (or solicit any offers to buy, purchase or otherwise acquire or take a pledge of) the Warrant except in accordance with the provisions of Article VI of the Warrant Purchase Agreement and will not, directly or indirectly, offer, sell, pledge, transfer or otherwise dispose of (or solicit any offers to buy, purchase or otherwise acquire or take a pledge of) the Exercise Shares except in accordance with the provisions of Article VI of the Warrant Purchase Agreement or pursuant to and in accordance with the Securities Act.

               4.2 Securities Are Not Registered .

                    (a) The Holder understands that the offer and sale of neither the Warrant nor the Exercise Shares has been registered under the Securities Act.

                    (b) The Holder recognizes that the Warrant and the Exercise Shares must be held indefinitely unless they are subsequently registered under the Securities Act or an exemption from such registration is available. The Holder recognizes that the Company has no obligation to register the Warrant or, except as provided in the Warrant Purchase Agreement and the Registration Rights Agreement, the Exercise Shares, or to comply with any exemption from such registration.

                    (c) The Holder is aware that neither the Warrant nor the Exercise Shares may be sold pursuant to Rule 144 adopted under the Securities Act unless certain conditions are met, including, among other things, the availability of certain current public information about the Company and the expiration of the required holding period under Rule 144.

               4.3 Disposition of Warrant and Exercise Shares .

                    (a) The Holder further agrees not to make any disposition of all or any part of the Warrant or Exercise Shares in any event unless and until one of the following occurs:

                         (i) The Company shall have received a letter secured by the Holder from the SEC stating that no action will be recommended to the Commission with respect to the proposed disposition;

                         (ii) There is then in effect a registration statement under the Securities Act covering the Exercise Shares and such disposition is made in accordance with said registration statement; or

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