Exhibit 10.47  CONFIDENTIAL TREATMENT  FIRST AMENDMENT TO THE COMMERCIAL SERVICES AGREEMENT  This First Amendment to the Commercial Services
Agreement (this First Amendment) is made this 26th day of February, 2009, 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 (the Agreement);  WHEREAS, AstraZeneca and Cubist desire to adjust the
Annual Baseline Sales for 2008, pursuant to Section 6.1(b) of the
Agreement, to account for the occurrence of a Significant Market Event in 2008,
namely, []* as contemplated by Section 1.1(rrrrr)(viii) (the []*Significant Market Event), and to set
forth the Annual Amount Due for 2008 based on such adjusted Annual Baseline
Sales; and  WHEREAS, AstraZeneca and Cubist desire to change the
definition of Critical Care Prescriber.  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.Annual Baselines Sales
for 2008.Exhibit A to the Agreement is hereby
amended so that Annual Baseline Sales for Calendar Year 2008 equal []*, and Quarterly
Baseline Sales for each Calendar Quarter of Calendar Year 2008 equal []*. 
Section 6.1(a) of the Agreement is also hereby amended to
delete the following language from the first sentence: (which shall be
prorated to reflect the portion of Calendar Year 2008 commencing on the
Baseline Commencement Date through December 31, 2008).  The parties hereby acknowledge and agree that
such adjustment to the Annual Baseline Sales and Quarterly Baseline Sales for
Calendar Year 2008 is sufficient to take account of the entire impact that the
[]* Significant Market Event is reasonably likely to have on []* for Calendar
Year 2008.  2.Annual Amount Due for
2008.The Agreement
is hereby amended so that the report due by AstraZeneca to Cubist pursuant to Section 3.11(b)(iv) for
Calendar Year 2008 shall be in the form attached hereto as Attachment A(rather than the form set
forth in Exhibit Q to the Agreement). 
Further, Attachment A hereto is hereby deemed to constitute such report
and satisfy AstraZenecas reporting obligation under Section 3.11(b)(iv) with
respect to Calendar Year 2008.  The
Annual Amount Due for Calendar  
 *Confidential Treatment Requested. 
Omitted portions filed with the Securities and Exchange Commission (the Commission).


Year
2008 shall be as calculated in Attachment A hereto and shall not be modified or
adjusted pursuant to Section 6.2(d) of the Agreement or otherwise for
any reason.  3.Critical Care Prescriber. 
The Agreement is hereby amended by removing the prior definition of
Critical Care Prescriber in Section 1.1(gg) and replacing it with the
following:  Critical Care
Prescriber means a Person (including a non-prescriber) who is an
intensivist, pulmonologist, critical care physician, or other healthcare
practitioner who influences prescribing behavior (e.g. a nurse, pharmacist or
physician assistant) who practices, is employed by or otherwise provides
services in the critical care department of a Critical Care Target Account.  4.Definitions.All terms used, but not defined, in this First 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 First Amendment.  6.Effective Date;
Incorporation of Terms; Continuing Effect.This First Amendment shall be deemed effective for
all purposes as of the Effective Date of the Agreement, except that Section 3
of this Amendment shall be deemed effective as of January 1, 2009.  All of the amendments set forth in this First
Amendment shall be deemed to be incorporated in, and made a part of, the
Agreement, and the Agreement, as amended by this First 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
Commercial Services Agreement which shall, as applicable, be deemed to apply to
this First Amendment (including with respect to the governing law with respect
to this First Amendment)).  Except as
otherwise expressly amended by this Amendment, the Agreement shall remain in
full force and effect in accordance with its terms and conditions.  7.Counterparts.This First 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 First 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/ Rich Fante
  
  By:
  /s/ Michael W. BonneyName:
  Rich Fante
  
  Name:
  Michael W. BonneyTitle:
  President
  
  Title:
  President and CEO  
 *Confidential Treatment Requested. 
Omitted portions filed with the Commission.  3


Attachment A  Annual Amount Due for 2008  []*
  
  $
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  *  
 *Confidential Treatment Requested. 
Omitted portions filed with the Commission.  A-1


Calculation of []*  []*
  
  $
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  $
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  []
  *  
 *Confidential Treatment Requested. 
Omitted portions filed with the Commission.  A-2