WARRANT TO PURCHASE STOCK

Exhibit 4.3

THIS WARRANT AND THE SHARES ISSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED OR OTHERWISE TRANSFERRED EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AN EXEMPTION FROM SUCH REGISTRATION, OR IN ACCORDANCE WITH APPLICABLE LAW.

WARRANT TO PURCHASE STOCK

THIS WARRANT CERTIFIES THAT, for good and valuable consideration, the receipt of which is hereby acknowledged, SQUARE 1 BANK or its assignee (“Holder”) is entitled to purchase the number of fully paid and nonassessable shares of the class of securities (the “Shares”) of AKESIS PHARMACEUTICALS, INC. (the “Company”) at the initial exercise price per Share (the “Warrant Price”) all as set forth above and as adjusted pursuant to Article 2 of this warrant (this “Warrant”), subject to the provisions and upon the terms and conditions set forth in this Warrant.

ARTICLE 1

EXERCISE

1.1 Method of Exercise. Holder may exercise this Warrant by delivering this Warrant and a duly executed Notice of Exercise in substantially the form attached as Appendix 1 to the principal office of the Company. Unless Holder is exercising the conversion right set forth in Section 1.2, Holder shall also deliver to the Company a check for the aggregate Warrant Price for the Shares being purchased.

1.2 Conversion Right. In lieu of exercising this Warrant as specified in Section 1.1, Holder may from time to time, by delivering this Warrant and a duly executed Notice of Exercise in substantially the form attached as Appendix 1 to the principal office of the Company, convert this Warrant, in whole or in part, into a number of Shares determined by dividing (a) the aggregate fair market value of the Shares or other securities otherwise issuable upon exercise of this Warrant minus the aggregate Warrant Price of such Shares by (b) the fair market value of one Share. The fair market value of one Share shall be determined pursuant to Section 1.4.

1.3 Intentionally Omitted.

1.4 Fair Market Value. The fair market value of one Share shall be equal to the average of the closing bid and asked prices of one Share quoted in the over-the-counter market in which the Shares are traded or the closing price quoted on any exchange on which the Shares are listed, whichever is applicable, as published in the Western Edition of The Wall Street Journal for the ten (10) trading days prior to the date of determination of fair market value (or such shorter period of time during which such stock was traded over-the-counter or on such exchange). If the Shares are not traded on the over-the-counter market or on an exchange, the fair market value shall be the price per Share that the Company could obtain from a willing buyer for Shares sold by the Company from authorized but unissued Shares, as such prices shall be determined in good faith by the Company’s Board of Directors.

1.5 Delivery of Certificate and New Warrant. Promptly after Holder exercises or converts this Warrant, the Company shall deliver to Holder certificates for the Shares acquired and, if this Warrant has not been fully exercised or converted and has not expired, a new warrant, substantially identical in form and substance to this Warrant, representing the Shares not so acquired.

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1.6 Replacement of Warrants. On receipt of evidence reasonably satisfactory to the Company of the loss, theft, destruction or mutilation of this Warrant and, in the case of loss, theft or destruction, on delivery of an indemnity agreement reasonably satisfactory in form and amount to the Company or, in the case of mutilation, on surrender and cancellation of this Warrant, the Company at its expense shall execute and deliver, in lieu of this Warrant, a new Warrant substantially identical in form and substance to this Warrant.

1.7 Repurchase on Sale, Merger, or Consolidation of the Company.

1.7.1 “Acquisition.” For the purpose of this warrant, “Acquisition” means (a) any sale, license, or other disposition of all or substantially all of the assets (including intellectual property) of the Company, (b) any sale or disposition of all or substantially all of the capital stock of the Company, or (c) any reorganization, consolidation, merger or sale of the voting securities of the Company or any other transaction where the holders of the Company’s securities before the transaction beneficially own less than 50% of the outstanding voting securities of the surviving entity after the transaction.

1.7.2 Assumption of Warrant. If upon the closing of any Acquisition the successor entity assumes the obligations of this warrant, then this warrant shall be exercisable for the same securities, cash, and property as would be payable for the Shares issuable upon exercise of the unexercised portion of this warrant as if such Shares were outstanding on the record date for the Acquisition and subsequent closing. The Warrant Price shall be adjusted accordingly. The Company shall use reasonable efforts to cause the surviving corporation to assume the obligations of this warrant.

1.7.3 Nonassumption. If upon the closing of any Acquisition the successor entity does not assume the obligations of this warrant and Holder has not otherwise exercised this warrant in full, then this warrant shall be deemed to have been automatically converted pursuant to Section 1.2 and thereafter Holder shall participate in the Acquisition on the same terms as other holders of the same class of securities of the Company

ARTICLE 2

ADJUSTMENTS TO THE SHARES

2.1 Stock Dividends, Splits, Etc. If the Company declares or pays a dividend on its common stock payable in common stock, or other securities, or subdivides the outstanding common stock into a greater amount of common stock, then upon exercise of this Warrant, for each Share acquired, Holder shall receive, without cost to Holder, the total number and kind of securities to which Holder would have been entitled had Holder owned the Shares of record as of the date the dividend or subdivision occurred.

2.2 Reclassification, Exchange or Substitution. Upon any reclassification, exchange, substitution, or other event that results in a change of the number and/or class of the securities issuable upon exercise or conversion of this Warrant (other than as a result of a dividend or subdivision provided for in Section 2.1 or a combination provided for in Section 2.3), Holder shall be entitled to receive, upon exercise or conversion of this Warrant, the number and kind of securities and property that Holder would have received for the Shares if this Warrant had been exercised immediately before such reclassification, exchange, substitution, or other event. The Company or its successor shall promptly issue to Holder a new warrant for such new securities or other property. The new warrant shall provide for adjustments which shall be as nearly equivalent as may be practicable to the adjustments provided for in this Article 2 including, without limitation, adjustments to the Warrant Price and to the number of securities or property issuable upon exercise of the new warrant. The provisions of this Section 2.2 shall similarly apply to successive reclassifications, exchanges, substitutions, or other events.

2.3 Adjustments for Combinations, Etc. If the outstanding Shares are combined or consolidated, by reclassification or otherwise, into a lesser number of shares, the Warrant Price shall be proportionately increased.

2.4 Intentionally Omitted.

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2.5 Certificate as to Adjustments. Upon each adjustment of the Warrant Price, the Company at its expense shall promptly compute such adjustment, and furnish Holder with a certificate of its Chief Financial Officer setting forth such adjustment and the facts upon which such adjustment is based. The Company shall, upon written request, furnish Holder a certificate setting forth the Warrant Price in effect upon the date thereof and the series of adjustments leading to such Warrant Price.

2.6 Fractional Shares. No fractional Shares shall be issuable upon exercise or conversion of the Warrant and the Number of Shares to be issued shall be rounded down to the nearest whole Share. If a fractional share interest arises upon any exercise or conversion of the Warrant, the Company shall eliminate such fractional share interest by paying Holder amount computed by multiplying the fractional interest by the fair market value of a full Share.

ARTICLE 3

REPRESENTATIONS AND COVENANTS

3.1 Representations and Warranties of the Company. The Company hereby represents and warrants to the Holder as follows:

(a) The initial Warrant Price referenced on the first page of this Warrant is the per share price paid in Company’s most recent equity financing.

(b) All Shares which may be issued upon the exercise of this Warrant, shall, upon issuance, be duly authorized, validly issued, fully paid and nonassessable, and free of any liens and encumbrances except for restrictions on transfer provided for herein or under applicable federal and state securities laws.

(c) As of the Issue Date, the authorized capital of the Company consists of: (i) 50,000,000 shares of Common Stock, 21,197,552 of which shares are issued and outstanding, and (ii) 10,000,000 shares of Preferred Stock, none of which shares are issued and outstanding.

3.2 Notice of Certain Events. The Company shall provide Holder with not less than 10 days prior written notice, including a description of the material facts surrounding, any of the following events: (a) declaration of any dividend or distribution upon its common stock, whether in cash, property, stock, or other securities and whether or not a regular cash dividend; (b) offering for subscription pro rata to the holders of any class or series of its stock any additional shares of stock of any class or series or other rights; (c) effecting any reclassification or recapitalization of common stock; or (d) an Acquisition.

3.3 Holder Investment Representations. Holder makes the representations to the Company set forth in Exhibit A hereof in connection with the issuance of this Warrant and the Shares (collectively, the “Securities”).

ARTICLE 4

MISCELLANEOUS

4.1 Term: Exercise Upon Expiration. This Warrant is exercisable in whole or in part, at any time and from time to time on or before the Expiration Date set forth above. If this warrant has not been exercised prior to the Expiration Date, and if the fair market value of one Share (as determined in accordance with Section 1.4) is then greater than the then-applicable per share exercise price, then this warrant shall be deemed to have been automatically exercised on the Expiration Date by “cashless” conversion pursuant to Section 1.2.

4.2 Legends. The Shares shall be imprinted with a legend in substantially the following form as well as any additional legends that may be required by any applicable federal or state securities laws:

THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR INVESTMENT AND NOT WITH A VIEW TO, OR IN CONNECTION WITH, THE SALE OR DISTRIBUTION THEREOF, AND HAVE NOT BEEN REGISTERED

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UNDER THE SECURITIES ACT OF 1933, AS AMENDED. SUCH SHARES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN EXEMPTION THEREFROM UNDER THE ACT AND ANY APPLICABLE STATE SECURITIES LAWS.

4.3 Compliance with Securities Laws on Transfer. This Warrant and the Shares issuable upon exercise of this Warrant may not be transferred or assigned in whole or in part without compliance with applicable federal and state securities laws by the transferor and the transferee and unless the transferor shall have: (i) notified the Company of the proposed transfer; (ii) furnished the Company with a detailed statement of the circumstances surrounding the proposed disposition; and (iii) furnished the Company with an opinion of counsel, reasonably satisfactory to the Company, that such transfer will be exempt from registration or qualification under any applicable federal or state securities laws; provided, however, that the Company shall not require Holder to provide such an opinion of counsel if (a) the transfer is to an affiliate of Holder or (b) there is no material question as to the availability of current information as referenced in Rule 144(c), Holder represents that it has complied with Rule 144(d) and Rule 144(e) in reasonable detail, the selling broker represents that it has complied with Rule 144(f), and the Company is provided with a copy of Holder’s notice of proposed sale.

4.4 Transfer Procedure. Subject to the provisions of Section 4.3, Holder may transfer all or part of this Warrant or the Shares issued upon exercise of this Warrant by giving the Company notice of the portion of the Warrant being transferred setting forth the name, address and taxpayer identification number of the transferee and surrendering this Warrant to the Company for reissuance to the transferee(s) (and Holder, if applicable). No surrender or reissuance shall be required if the transfer is to an affiliate of Holder.

4.5 Notices. All notices and other communications from the Company to the Holder, or vice versa, shall be deemed delivered and effective when given personally or mailed by first-class registered or certified mail, postage prepaid, at such address as may have been furnished to the Company or the Holder, as the case may be, in writing by the Company or such Holder from time to time. All notices to the Company or to Holder, as applicable, shall, until further notice delivered in accordance with the foregoing, be addressed as follows:

If to the Company:

Akesis Pharmaceuticals, Inc.

888 Prospect Street, Suite 320

La Jolla, CA 92037

Attn: Chief Financial Officer

With a copy (which shall not constitute notice) to:

Paul Hastings Janofsky & Walker LLP

3579 Valley Centre Drive

San Diego, CA 92130

Attn: Carl Sanchez, Esq.

If to Holder:

Square 1 Bank

Attn: Warrant Administrator

406 Blackwell Street, Suite 240

Crowe Building

Durham, NC 27701

4.6 Amendments. This Warrant and any term hereof may be changed, waived, discharged or terminated only by an instrument in writing signed by the party against which enforcement of such change, waiver, discharge or termination is sought.

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4.7 Attorneys’ Fees. In the event of any dispute between the parties concerning the terms and provisions of this Warrant, the party prevailing in such dispute shall be entitled to collect from the other party all costs incurred in such dispute, including reasonable attorneys’ fees.

4.8 Governing Law. This Warrant shall be governed by and construed in accordance with the laws of the State of California, without giving effect to its principles regarding conflicts of law.

Acknowledged and agreed to by:

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APPENDIX 1

NOTICE OF EXERCISE

1. The undersigned hereby elects to purchase                      shares (the “Shares”) of the Common Stock of AKESIS PHARMACEUTICALS, INC. (the “Company”) pursuant to the terms of the attached Warrant, and tenders herewith payment of the purchase price of such shares in full.

1. The undersigned hereby elects to convert the attached Warrant into shares (the “Shares”) of the Common Stock of AKESIS PHARMACEUTICALS, INC. (the “Company”) in the manner specified in Section 1.2 of the attached Warrant. This conversion is exercised with respect to                      of the aggregate number of shares of Common Stock of the Company covered by the Warrant.

[Strike paragraph that does not apply.]

2. Please issue a certificate or certificates representing said shares in the name of the undersigned or in such other name as is specified below:

Square 1 Bank

Attn: Warrant Administrator

406 Blackwell Street, Suite 240

Crowe Building

Durham, NC 27701

3. The undersigned makes the representations to the Company set forth in Exhibit A hereof in connection with the issuance of the Securities.

SQUARE 1 BANK or Registered Assignee

EXHIBIT A

INVESTMENT REPRESENTATIONS

(a) The undersigned is aware of the Company’s business affairs and financial condition, and has acquired sufficient information about the Company to reach an informed and knowledgeable decision to acquire the Securities. The undersigned is purchasing the Securities for its own account for investment purposes only and not with a view towards, or for resale in connection with, any “distribution” thereof for purposes of the Securities Act of 1933, as amended (the “Securities Act”). The undersigned has such knowledge and experience in financial business matters and the undersigned is capable of evaluating the merits and risks of the purchase of the Securities and of protecting its interests in connection therewith.

(b) The undersigned understands that the Securities have not been registered under the Securities Act in reliance upon a specific exemption therefrom, which exemption depends upon, among other things, the bona fide nature of the undersigned’s investment intent as expressed herein.

(c) The undersigned further understands that the Securities must be held indefinitely, and the undersigned must therefore bear the economic risk therewith, unless the Securities are subsequently registered under the Securities Act or unless an exemption from registration is otherwise available. In addition, the undersigned understands that the certificate evidencing the Securities will be imprinted with a legend which prohibits the transfer of the Securities unless they are registered or such registration is not required.

(d) The undersigned is familiar with the provisions of Rule 144, promulgated pursuant to the Securities Act, which, in substance, permits limited public resale of “restricted securities” acquired, directly or indirectly, from the issuer thereof, in a non-public offering subject to the satisfaction of certain conditions.

(e) The Securities may be resold in certain limited circumstances subject to the provisions of Rule 144, which requires, among other things, the existence of a public market for the Securities, the availability of certain current public information about the Company, the resale occurring not less than one year after a party has purchased and paid for the security to be sold, the sales being effected through a “broker’s transaction” or in transactions directly with a “market maker” and the number of securities being sold during any three-month period not exceeding specified limitations.

(f) The undersigned further understands that in the event that all of the applicable requirements of Rule 144 are not satisfied, registration under the Securities Act, compliance with Regulation A, or some other registration exemption will be required.

(g) The undersigned is an “accredited investor” as defined in Rule 501(a) of Regulation D promulgated under the Securities Act.

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