AMENDMENT TO AGREEMENT AND PLAN OF MERGER

EXHIBIT 2.2

AMENDMENT TO AGREEMENT AND PLAN OF MERGER

THIS AMENDMENT TO AGREEMENT AND PLAN OF MERGER (this “Amendment”), is dated as of the 19th day of December, 2002, by and among INCYTE GENOMICS, INC., a Delaware corporation (“Parent”), MONACO ACQUISITION CORPORATION, a Delaware corporation and a wholly owned subsidiary of Parent (“Merger Subsidiary”), MAXIA PHARMACEUTICALS, INC., a Delaware corporation (the “Company”), and MAXIA PHARMACEUTICALS, LLC., a Delaware limited liability company (the “LLC”).

WITNESSETH:

WHEREAS, Parent, Merger Subsidiary, the Company and the LLC entered into an Agreement and Plan of Merger dated as of the 11th day of November, 2002 (the “Merger Agreement”); and

WHEREAS, the parties hereto desire to amend the Merger Agreement as set forth below.

NOW, THEREFORE, in consideration of the premises and of the mutual agreements, provisions and covenants herein contained, the parties hereby agree as follows:

The authorized capital stock of the Company consists of 20,000,000 shares of common stock, $0.0001 par value (“Company Common”), 2,455,491 shares of

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Series A Preferred Stock, $0.0001 par value (“Company Series A Preferred”), 5,900,000 shares of Series B Preferred Stock, $0.0001 par value (“Company Series B Preferred”), and 1,326,260 shares of Series C Preferred Stock, $0.0001 par value (“Company Series C Preferred”, and together with the Company Series A Preferred, Company Series B Preferred and Company Series C Preferred, the “Company Preferred”). As of the date of this Agreement, there were issued and outstanding 7,518,583 shares of Company Common, 2,057,612 shares of Company Series A Preferred, 2,569,627 shares of Company Series B Preferred and 1,326,260 shares of Company Series C Preferred. Each share of Company Preferred is convertible into one share of Company Common subject to certain adjustments specified in the Company’s Restated Certificate of Incorporation as in effect on the date of this Agreement. As of the date of this Agreement, there were an aggregate of 8,481,751 shares of Company Common reserved for issuance upon conversion of Company Preferred. The Company Common, Company Preferred and Additional Company Stock (as defined in Section 4.2(b)) are referred to herein collectively as the “Company Stock.” The rights, preferences and privileges of Company Common and Company Preferred are as set forth in the Company’s Fifth Amended and Restated Certificate of Incorporation as in effect on the date of this Agreement and will be amended as set forth in the Amended Company Certificate. The rights, preferences and privileges of Additional Company Stock are set forth in the Amended Company Certificate.

As of immediately before the Effective Time, the Company shall have caused the Amended Company Certificate to be filed and the authorized capital stock of the Company, upon its effectiveness, shall consist of (i) three series of common stock divided as follows: 10,935,677 shares of Company Common, of which 7,518,583 shares will be issued and outstanding, 2,733,920 shares of common stock—Additional Payments, $0.0001 par value (“Company Common Additional Payments”), of which 1,879,646 shares will be issued and outstanding, and 109,357 shares of common stock—Earn-Out, $0.0001 par value (“Company Common—Earn Out”), of which 75,186 shares will be issued and outstanding, (ii) three series of Series A Preferred divided as follows: 3,257,612 shares of Company Series A Preferred, of which 2,057,612 shares will be issued and outstanding, 814,403 shares of Series A Preferred Stock-Additional Payments $0.0001 par value (“Company Series A—Additional Payments”), of which 514,403 shares will be issued and outstanding, 32,577 shares of Series A Preferred Stock-Earn-Out, $0.0001 par value (“Company Series A—Earn Out”), of which 20,577 shares will be issued and outstanding, (iii) three series of Series B Preferred divided as follows: 5,900,000 shares of Company Series B Preferred, of which 2,569,627 shares will be issued and outstanding, 1,475,000 shares of Series B Preferred Stock-Additional Payments, $0.0001 par value (“Company Series B—Additional Payments”), of which 642,407 shares will be issued and outstanding, 59,000 shares of Series B Preferred Stock-Earn-Out, $0.0001 par value (“Company Series B—Earn Out”), of which 25,697 shares will be issued and outstanding, and (iv) three series of Series C Preferred divided as follows: 1,326,260 shares of Company Series C Preferred, all of which will be issued

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and outstanding, 331,565 shares of Series C Preferred Stock—Additional Payments, $0.0001 par value (“Company Series C Preferred—Additional Payments”), all of which will be issued and outstanding, and 13,263 shares of Series C Preferred Stock—Earn-Out, $0.0001 par value (“Company Series C Preferred—Earn Out”), all of which will be issued and outstanding. The Company Common Additional Payments, Company Common—Earn Out, Company Series A—Additional Payments, Company Series A—Earn Out, Company Series B—Additional Payments, Company Series B—Earn Out, Company Series C Preferred—Additional Payments and Company Series C Preferred—Earn Out described in this subsection are referred to herein as “Additional Company Stock”).

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IN WITNESS WHEREOF, the parties have caused this Amendment to be executed, all as of the date first above written.

[Signature Page to Amendment to Agreement and Plan of Merger]