Metabasis Therapeutics (MBRX)

dated as November 2, 2006

This COMMON STOCK PURCHASE AGREEMENT (the “Agreement”) is entered into as of the 1st day of November, 2006 by and between Kingsbridge Capital Limited, an entity organized and existing under the laws of the British Virgin Islands, whose registered address is Palm Grove House, 2nd Floor, Road Town, Tortola, British Virgin Islands (the “Investor”) and Metabasis Therapeutics, Inc., a corporation organized and existing under the laws of the State of Delaware (the “Company”).

WHEREAS, the parties desire that, upon the terms and subject to the conditions and limitations set forth herein, the Company may issue and sell to the Investor, from time to time as provided herein, and the Investor shall purchase from the Company, up to $50 million worth of shares of Common Stock (as defined below); and

WHEREAS, such investments will be made in reliance upon the provisions of Section 4(2) (“Section 4(2)”) and Regulation D (“Regulation D”) of the United States Securities Act of 1933, as amended and the rules and regulations promulgated thereunder (the “Securities Act”), and/or upon such other exemption from the registration requirements of the Securities Act as may be available with respect to any or all of the investments in Common Stock to be made hereunder; and

WHEREAS, the parties hereto are concurrently entering into a Registration Rights Agreement in the form of Exhibit A hereto (the “Registration Rights Agreement”) pursuant to which the Company shall register the Common Stock issued and sold to the Investor under this Agreement and under the Warrant (as defined below), upon the terms and subject to the conditions and limitations set forth therein; and

WHEREAS, in consideration for the Investor’s execution and delivery of, and its performance of its obligations under, this Agreement, the Company is concurrently issuing to the Investor a warrant in the form of Exhibit B hereto (the “Warrant”) pursuant to which the Investor may purchase from the Company up to 260,000 shares of Common Stock, upon the terms and subject to the conditions and limitations set forth therein;

NOW, THEREFORE, the parties hereto agree as follows: