Exhibit
10.56  CONFIDENTIAL TREATMENT  FIFTH AMENDMENT TO THE COMMERCIAL SERVICES AGREEMENT  This Fifth Amendment to the Commercial Services
Agreement (this Fifth Amendment) is made this 16th day of October, 2009 (the Amendment Effective Date), by and between
ASTRAZENECA PHARMACEUTICALS LP, a Delaware limited partnership (AstraZeneca), and CUBIST PHARMACEUTICALS, INC., a Delaware
corporation (Cubist).  WHEREAS, AstraZeneca and Cubist previously entered
into that certain Commercial Services Agreement between AstraZeneca and Cubist,
dated as of July 1, 2008 (as amended as of February 26, 2009, May 20,
2009, September 30,2009 and October 15,
2009, the Agreement); and  WHEREAS, AstraZeneca and Cubist desire to amend
certain provisions of the Agreement relating to the CSD Activities and to amend
certain exhibits to the Agreement; and  NOW THEREFORE, in consideration of the premises and
the mutual covenants set forth herein and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged,
the parties hereby agree as follows:  1.Requests for Medical
Information.The Agreement is hereby amended so that Section 3.8(a) is
deleted in its entirety and replaced with the following:  Notwithstanding anything in this Agreement or the
Business Policies to the contrary, no Program Employee may respond to or
otherwise communicate with any Target Prescriber or other Third Party with
respect to any Off-Label Information []*.  2.Specific Cubist Business
Policies.Exhibit B-1 to the Agreement is
hereby amended to add the following standard business procedure and coaching
sheet:  []*  3.CSD Activities.Exhibit C
to the Agreement is hereby amended so that the following language is added
after the last sentence of Section 2 ([]*):  []*.  4.Definitions.All terms used, but not defined, in this Fifth Amendment shall have the respective meanings set forth in
the Agreement.  5.Construction.The principles set forth in Section 1.2 of the Agreement shall
apply to this Fifth Amendment.  
 *Confidential Treatment Requested. 
Omitted portions filed with the Securities and Exchange Commission (the
Commission).


6.Effective Date;
Incorporation of Terms; Continuing Effect.This Fifth
Amendment shall be deemed effective for all purposes as of the Amendment
Effective Date.  The amendment set forth
in this Fifth Amendment shall be deemed to be incorporated in, and made a part
of, the Agreement, and the Agreement and this Fifth Amendment shall be read,
taken and construed as one and the same agreement (including with respect to
the provisions set forth in Article XVI (Miscellaneous) of the Agreement
which shall, as applicable, be deemed to apply to this Fifth Amendment
(including with respect to the governing law)). 
Except as otherwise expressly amended by this Fifth Amendment, the
Agreement shall remain in full force and effect in accordance with its terms
and conditions.  7.Counterparts.This Fifth Amendment may be executed simultaneously in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.  [The remainder of this page has been
intentionally left blank.]  
 *Confidential Treatment Requested. 
Omitted portions filed with the Commission.  2


IN WITNESS WHEREOF, the parties, intending to be
bound, have caused this Fifth Amendment to be executed on their behalf by their
duly authorized agent as of the day and year first above written.  ASTRAZENECA PHARMACEUTICALS LP
  
  CUBIST PHARMACEUTICALS, INC.By:
  /s/
  Marion E. McCourt
  
  By:
  /s/ Michael BonneyName:
  Marion
  E. McCourt
  
  Name:
  Michael BonneyTitle:
  Vice
  President, Cornerstone
  
  Title:
  President and CEO  
 *Confidential Treatment Requested. 
Omitted portions filed with the Commission.  3