EXHIBIT 4.2

Execution Version

FIRST AMENDMENT TO THE

PURCHASE AGREEMENT

July 20, 2011

This First Amendment to the Purchase Agreement (the “First Amendment”) is dated as of the date hereof, between Icagen, Inc., a Delaware corporation with offices at 4222 Emperor Boulevard, Suite 350, Durham, North Carolina 27703 (“Icagen”), and Pfizer Inc., a Delaware corporation with offices at 235 East 42nd Street, New York, New York 10017 (“Pfizer”).

Capitalized terms used but not otherwise defined herein shall have the same meaning ascribed to such terms in the Merger Agreement (as defined below).

WHEREAS, Pfizer and Icagen entered into a Purchase Agreement (the “Purchase Agreement”) as of August 13, 2007, whereby Pfizer agreed to purchase from Icagen, and Icagen agreed to issue and sell to Pfizer, upon the terms and conditions stated in the Purchase Agreement, up to a maximum of $15,000,000 of Icagen’s common stock, par value $0.001 per share (the “Common Stock”);

WHEREAS, on August 20, 2007, in a first closing under the Purchase Agreement, Icagen sold shares of Common Stock with an aggregate purchase price of approximately $15,000,000 to Pfizer and in a subsequent closing under the Purchase Agreement on February 13, 2008, Icagen sold shares of Common Stock to Pfizer with an aggregate purchase price of approximately $10,000,000;

WHEREAS, under Section 3.6 of the Purchase Agreement, Pfizer agreed to appoint, subject to certain exceptions, each of the President and the Treasurer of Icagen, with full power of substitution, as the proxies of Pfizer with respect to certain matters on which Pfizer is entitled to vote as a holder of Common Stock;

WHEREAS, concurrently with the execution and delivery of this First Amendment, Icagen, Pfizer and Eclipse Acquisition Corp., a Delaware corporation and a direct or indirect wholly-owned subsidiary of Pfizer, are entering into an Agreement and Plan of Merger, dated as of the date hereof (the “Merger Agreement”); and

WHEREAS, the parties hereto desire to amend the Purchase Agreement as set forth herein.

NOW, THEREFORE, in consideration of the mutual covenants contained herein, and for other good and valuable consideration, the parties hereto, intending to be legally bound, hereby agree as follows:

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[Signature page follows]

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

FIRST AMENDMENT TO PURCHASE AGREEMENT