Exhibit 10.55  CONFIDENTIAL TREATMENT  FOURTH AMENDMENT TO THE COMMERCIAL SERVICES AGREEMENT  This Fourth Amendment to the Commercial Services
Agreement (this Fourth Amendment) is made this 15th 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 and September 30,2009,
the Agreement); and  WHEREAS, AstraZeneca and Cubist desire to amend
certain provisions of the Agreement relating to the Detailing of the Product to
Critical Care Target Accounts, as set forth herein.  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.Sales Force Obligations.The Agreement is hereby amended so that the third
sentence of Section 3.1(a) is hereby deleted and replaced with the
following:  Cubist shall
ensure that (A) each Detail by a Representative to []* is a Primary
Detail, and (B) each Detail by a Representative to []* is a Primary Detail
or a Secondary Detail.  2.Critical Care Target
Accounts.The Agreement is hereby amended so thatSection 1
of the Promotion Plan (Critical Care Target Accounts) set forth in Exhibit M
to the Agreement is hereby deleted in its entirety and replaced with the
following:  1.Critical Care Target
Accounts  []*.  Notwithstanding anything contained in the
Agreement, for purposes of a Detail to []*, Detail
shall mean a Representative of the Sales Force meeting with []* or providing an
in-service for []* at which []* is in attendance, such that the relevant
characteristics of the Product are described by the Representative consistent
with the requirements of this Agreement and Applicable Law and in a manner that
is customary in the industry for the purpose of promoting a prescription
pharmaceutical product.  3.Exhibit J (Form of
Detail Report).The Agreement is hereby amended so that Exhibit J
to theAgreement is hereby deleted and replaced with the Exhibit J
attached hereto as Attachment I.  
 *Confidential Treatment Requested.  Omitted portions filed with the Securities
and Exchange Commission (the Commission).  1


4.Definitions.All terms used, but not defined, in this Fourth 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 Fourth Amendment.  6.Effective Date;
Incorporation of Terms; Continuing Effect.This Fourth
Amendment shall be deemed effective for all purposes as of the Amendment
Effective Date.  The amendments set forth
in this Fourth Amendment shall be deemed to be incorporated in, and made a part
of, the Agreement, and the Agreement and this Fourth 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 Fourth Amendment
(including with respect to the governing law)). 
Except as otherwise expressly amended by this Fourth Amendment, the
Agreement shall remain in full force and effect in accordance with its terms
and conditions.  7.Counterparts.This Fourth 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 Fourth Amendment to be executed on their behalf by
their duly authorized agent as of the Amendment Effective Date.  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


ATTACHMENT I  EXHIBIT J FORM OF DETAIL
REPORT  Monthly/Quarterly
Report Reporting Period:                              to  Prescriber
Level Reporting  []*
  
  []*
  
  []*
  
  []*
  
  []*[]*
  
  
  
  
  
  
  
  
  
  []*
  
  []*[]*[]*[]*[]*  []*  
 *Confidential Treatment Requested. 
Omitted portions filed with the Commission.  J-1


Monthly/Quarterly
Report Reporting Period:                              to  Account
Level Reporting  []*
  
  []*
  
  []*
  
  []*
  
  []*[]*
  
  
  
  
  
  
  
  
  
  []*
  
  []*[]*
  
  
  
  
  
  
  
  
  
  []*
  
  []*  []*  Report Created:  
 *Confidential Treatment Requested.  Omitted portions filed with the Commission.  J-2