SECOND AMENDMENT TO LEASE

EXHIBIT 10.25

SECOND AMENDMENT TO LEASE

        THIS SECOND AMENDMENT TO LEASE (this "Amendment") is made as of the 18th day of November, 2005 (the "Second Amendment Date"), by and between THE REALTY ASSOCIATES FUND VI, L.P., a Delaware limited partnership ("Landlord") and CUBIST PHARMACEUTICALS, INC., a Delaware corporation ("Tenant").

RECITALS:

        WHEREAS, by a lease (the "Original Lease") dated as of January, 2004, California State Teachers' Retirement System ("Calsters") leased to Tenant approximately 15,475 rentable square feet of space, consisting of a portion of the second and fourth floors in the building known as 45-55 Hayden Avenue, Lexington, Massachusetts (the "Building"); and

        WHEREAS, Landlord has succeeded to the interests of Calsters as landlord under the Lease; and

        WHEREAS, the Original Lease has been amended by a First Amendment to Lease between Landlord and Tenant, dated as of September 29, 2005 (the "First Amendment;" the Original Lease, as so amended, being referred to as the "Lease"), pursuant to which the size of the premises demised under the Original Lease was increased to 46,928 rentable square feet (the "Existing Premises"); and

        WHEREAS, Landlord and Tenant now desire to further amend the Lease to, among other things, temporarily expand the size of the Existing Premises by adding thereto approximately 6,150 rentable square feet of space on the second floor of the Building (the "Comet Space"), and to temporarily adjust the rent and certain provisions, and to provide an option for the Tenant to lease the Comet Space for a longer term, all on the terms and conditions set forth below.

        NOW, THEREFORE, in consideration of the mutual covenants herein contained, and for other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, and intending to be legally bound hereby, Landlord and Tenant hereby agree as follows:

• 1.

  Defined Terms.     All of the foregoing recitals are true and correct. Unless otherwise defined herein, all capitalized terms used in this Amendment shall have the meanings ascribed to them in the Lease, the Lease shall be amended to incorporate any additional definitions provided for in this Amendment, and all references in the Lease to the "Lease" or "this Lease" or "herein" or "hereunder" or similar terms or to any section thereof shall mean the Lease, or such section thereof, as amended by this Amendment.
  
  

  2.

  Additional Terms and Definitions.     (a) From and after the Effective Date (as defined below) and continuing through 11:59 p.m. on May 31, 2006 (the "Comet Expansion Period"), the

• • following terms set forth in "Article 1 Reference Data" of the Lease are hereby amended to have the following meanings:

• 3.

  Effective Date; Delivery and Condition .    (a) Tenant may commence occupancy of the Comet Space on the Effective Date. For the purposes of this Amendment, the "Effective Date" shall be the later to occur of (i) December 1, 2005 and (ii) the date on which the existing tenant vacates the Comet Space and Landlord delivers possession thereof to Tenant.
  
  

  (b)

  Tenant acknowledges that, except as explicitly provided in the Lease, it is leasing the Comet Space during the Comet Expansion Period (as the same may be extended as provided below) in its current AS IS condition, without any representation or warranty whatsoever on the part of Landlord. Tenant currently occupies the Existing Premises and is fully familiar with their condition and that of the common areas of the Building, and Tenant acknowledges that, to the best of Tenant's knowledge (upon reasonable investigation and inquiry), the Existing Premises and the Comet Space are in good condition and suitable for Tenant's uses. Without limiting the foregoing, Tenant agrees that Landlord has no obligation to perform any work in or to either the Existing Premises or the Comet Space to prepare the same for Tenant's continued use and occupancy.
  
  

  (c)

  During the Comet Expansion Period (as such period may be extended hereunder), Tenant shall have, as appurtenant to the Comet Space, the right to use the furniture, furnishings, fixtures, equipment and tenant improvements located in the Comet Space as of the date hereof and belonging to Landlord, including without limitation (i) all telephone and data communications wiring, switches and cabling, and (ii) all furniture listed on Schedule 1 attached hereto and made a part hereof (together, the "Equipment"). Tenant shall keep the Equipment in good and workable condition, reasonable wear and tear excepted, but shall have no obligation to make repairs, perform maintenance thereon or replace the same, other than reasonable and de minimus repairs and maintenance, as determined as necessary by Tenant. Except as provided below, the Equipment shall be covered under Landlord's property insurance, and any insurance proceeds on account of the Equipment shall be payable to Landlord. At the expiration or earlier termination of the Comet Expansion Period, the Equipment shall be surrendered to Landlord in its existing

2

• • • condition, reasonable wear and tear excepted; provided, that, if Tenant extends the lease of the Comet Space pursuant to Section 6 below, all right, title and interest in the Equipment shall be transferred to Tenant, and Landlord shall promptly execute all documentation necessary to effect such transfer. From and after such transfer, the Equipment shall be covered under Tenant's property insurance, and any insurance proceeds on account of the Equipment shall be payable to Tenant.

  4.

  Right of First Offer.     From and after the Second Amendment Date, Exhibit G-1 (Right of First Offer) to the Lease is hereby amended by deleting the schedule of Rights of Existing Building Tenants in its entirety and replacing the same with Schedule 2 attached hereto.
  
  

  5.

  Parking.     During the Comet Expansion Period (as the same may be extended), Section 10.19 of the Lease shall be amended by, in the first sentence thereto:
  
  

  (a)

  Replacing "one hundred and forty-one (141)" with "one hundred fifty-nine (159);" and
  
  

  (b)

  Replacing "46,928" with "53,078."
  
  

  6.

  Extension Option.     Provided that, at the time of such exercise and also at May 31, 2006, (i) the Lease is still in full force and effect, and (ii) there exists no Event of Default, Tenant shall have the right to maintain the Comet Space beyond the Comet Expansion Period as a part of the Premises and the Rentable Floor Area of the Premises for the t