EXHIBIT 10.43  SEVENTH AMENDMENT TO LEASE  THIS SEVENTH AMENDMENT TO
LEASE (this Seventh Amendment) is made as of the 18th day of November, 2008 (the Seventh
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 (Suite 2201-55) and fourth (Suite 4201-55)
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), by a Second Amendment to Lease between Landlord
and Tenant dated as of November 18, 2005 (the Second Amendment), by a
Third Amendment to Lease between Landlord and Tenant dated as of June 20,
2007 (the Third Amendment), by a Fourth Amendment to Lease dated as of October 25,
2007, by a Fifth Amendment to Lease (the Fifth Amendment) dated as of December 18,
2007, and by a Sixth Amendment to Lease (the Sixth Amendment) dated as of July 31,
2008 (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 131,143 rentable square feet (the Existing Premises). The
Existing Premises consist of Suites 2201-55, 2200-55, 3000-55, 3200-55,
3201-55, 4201-55, 4200-55, 4600-45, and 1000-55; and  WHEREAS, by letter dated as of April 27, 2006,
and pursuant to the Second Amendment, Tenant elected to include Suite 2200-55
(also known as the Comet Space) in the Existing Premises for the remainder of
the Lease Term; and  WHEREAS, Landlord and Tenant now desire to further
amend the Lease to, among other things, grant to Tenant the exclusive right to
use approximately 1,754 square feet of space on the ground floor of the 55
Hayden Avenue portion of the Building (the Lobby Space), and to adjust the
rent and certain provisions, 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


1.             Defined Terms.
All of the foregoing recitals are true and correct. Unless otherwise defined
herein, all capitalized terms used in this Seventh 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 Seventh 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 Seventh Amendment.  2.             Lobby Space. Effective
from and after the Seventh Amendment Date, and for so long as Tenant continues
to occupy all of the Premises in the 55 Hayden Avenue portion of the Building,
Landlord hereby grants to Tenant the exclusive right to use, as appurtenant to
the Premises, the Lobby Space, shown on Exhibit A-7, attached
hereto and incorporated herein by reference. The Lobby Space shall be used by
Tenant as a reception area and lobby for Tenants employees and invitees to
Tenants offices in the Building. From and after the Seventh Amendment Date,
the Lobby Space shall be treated as if it were a part of the Premises for all
purposes of the Lease, including without limitation Article 6 thereof.  3.             Annual
Rent for Lobby Space. From after the Seventh Amendment Date, and in
addition to Annual Rent and additional rent payable with respect to the
Existing Premises, the Tenant shall pay Annual Rent for the Lobby Space as
provided below:  RENTALPERIOD
  
  TOTALANNUALRENT
  
  MONTHLYPAYMENT
  
  RENTALRATE/SFThrough 4/30/09
  
  $
  56,566.50
  
  $
  4,713.88
  
  $
  32.255/1/09  4/30/10
  
  $
  58,320.50
  
  $
  4,860.04
  
  $
  33.255/1/10  4/30/11
  
  $
  60,074.50
  
  $
  5,006.21
  
  $
  34.255/1/11 4/30/12
  
  $
  61,828.50
  
  $
  5,152.38
  
  $
  35.255/1/12  4/30/13
  
  $
  63,582.50
  
  $
  5,298.54
  
  $
  36.255/1/13  4/30/14
  
  $
  65,336.50
  
  $
  5,444.71
  
  $
  37.255/1/144/30/15
  
  $
  67,090.50
  
  $
  5,590.88
  
  $
  38.255/1/15  4/30/16
  
  $
  68,844.50
  
  $
  5,737.04
  
  $
  39.25  4.             Delivery and
Condition of Lobby Space. (a) Tenant acknowledges that Tenant will
accept possession of the Lobby Space on the Seventh Amendment Date 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 the condition of the Lobby Space, and Tenant acknowledges
that the Lobby Space is in good condition and suitable for Tenants intended
uses.  (b) Without limiting any other provision of the
Lease, all plans and specifications for any alterations or improvements that
Tenant desires to make in the Lobby Space (together, the Lobby Improvements),
will be prepared by Tenant and submitted to Landlord for its review and
approval. All Lobby Improvements shall be  2


undertaken by Tenant at Tenants sole cost and
expense, in a good and workmanlike manner and in strict accordance with the
applicable requirements of the Lease (including without limitation Sections 3.3
and 3.4). Tenant shall coordinate the timing of any Lobby Improvements with
Landlord, and shall perform the same in such a manner as to minimize any
disturbance to, or interference with the business operations of, other tenants
or occupants of the Building.  (c)             At such time as Tenant shall cease
to occupy all of the Premises in the 55 Hayden Avenue portion of the Building,
Tenant shall at Landlords election, remove any Lobby Improvements made by
Tenant and restore such other of the Lobby Improvements as Landlord may deem
reasonably necessary to return the Lobby Space to use as a part of the Building
common area.  (d)             Without in any way limiting Tenants
repair and maintenance obligations under the Lease with respect to the Premises
or the Lobby Space (as if it were a part of the Premises), Tenant shall at all
times that Tenant has the right to use the Lobby Space, repair, maintain and
(as necessary) replace: (i) the exterior entry system leading to the Lobby
Space (including without limitation the door(s), exterior and interior glass,
closers, locks and lighting); (ii) carpeting and floor tiles in the Lobby
Space; (iii) heating, ventilation and air conditioning equipment in the
Lobby Space (as well as any supplemental systems serving only the Lobby Space);
and (iv) life safety alarms, systems and equipment located in the Lobby
Space.  (e) Promptly upon the
Seventh Amendment Date, Landlord shall (i) revise the existing signage on
the monument sign to reflect that Tenants Premise are in Building 55 and that
other tenants in the Building are in Building 45, (ii) install necessary
exterior directional signage indicating the proper entrances to Building 45 and
to Building 55, and (iii) to the extent not already done, make necessary
changes to the existing elevator card readers to as to prevent tenants in other
portions of the Building from using their cards to access the elevators in Building
55, and to satisfy any applicable life safety or other codes and legal
requirements. Tenant acknowledges that the mail and other delivery service drop
boxes for the Building are currently located outside of the Building, adjacent
to the Lobby Space entrance. Landlord will investigate whether it will be
preferable for the Building as a whole to relocate the existing drop boxes to a
more central location, or to leave the existing boxes and install additional
drop boxes in a new location. Tenant shall reimburse Landlord within 30 days
after receipt of invoices for the costs and expenses reasonably incurred in
connection with all of the foregoing actions described in this paragraph (e),
up to an aggregate maximum reimbursement amount of Ten Thousand Dollars
($10,000).  5. Brokers. Tenant covenants, represents and
warrants to Landlord that Tenant has had no dealings or communications with any
broker or agent (other than Grubb & Ellis Company and Richards Barry
Joyce & Partners) in connection with this Seventh Amendment, and
Tenant covenants and agrees to pay, hold harmless and indemnify the Landlord
from and against any and all cost, expense (including reasonable attorneys  3


fees) or liability for any compensation, commission
or charges to any broker or agent (other than the foregoing named brokers)
claiming through the Tenant with respect hereto.  6.               Successors.
This Seventh Amendment shall be binding upon and inure to the benefit of the
parties hereto and their respective successors and assigns, subject to the
provisions of the Lease regarding assignment or other transfers of each partys
rights under the Lease.  7.               Authority.
Each party represents and warrants to the other that each person executing this
Seventh Amendment on behalf of such party has the authority to do so and that
such execution has fully obligated and bound such party to all terms and
provisions of this Seventh Amendment.  8.               No
Further Amendment. It is understood and agreed that all other conditions
and terms contained in the Lease not herein specifically amended shall remain
unmodified and in full force and effect, and the Lease, as modified by this
Seventh Amendment, is hereby ratified and confirmed.  9.               Tenant
Representations. As a material inducement to Landlord entering into this
Seventh Amendment, Tenant represents and certifies to Landlord that as of the
date hereof: (i) the Lease is in full force and effect, (ii) to the
best of Tenants knowledge, Landlord is not in default (continuing beyond the
expiration of any applicable notice or grace periods) in any respect in any of
the terms, covenants and conditions of the Lease; and (iii) Tenant has no
existing setoffs, counterclaims or defenses against Landlord under the Lease.  10.             Landlord
Representations. As a material inducement to Tenant entering into this
Seventh Amendment, Landlord represents and certifies to Tenant that as of the
date hereof: (i) the Lease is in full force and effect, an d(ii) to
the best of Landlords knowledge, there exists no Event of Default on the part
of Tenant in any respect in any of the terms, covenants and conditions of the
Lease.  11.             Governing
Law. The Lease, this Seventh Amendment and the rights and obligations of
both parties thereunder and hereunder shall be governed by the laws of The
Commonwealth of Massachusetts.  12.             Counterparts. This Seventh Amendment may be
executed in counterparts, each of which shall be an original and all of which
counterparts taken together shall constitute one and the same instrument.  [Text Ends Here]  4


IN WITNESS WHEREOF, the
undersigned have hereunto se their hands and seals as of the date first above
written.  LANDLORD:The
  Realty Associates Fund VI, L.P., a Delaware limited partnershipBy:
  Realty Associates Fund VI LLC, a Massachusetts limited liability company,
  general partnerBy:      Realty
  Associates Advisors LLC, a Delaware limited liability company, ManagerBy:
  Realty Associates Advisors Trust, a Massachusetts business trust, ManagerBy:
  /s/ Heather L. HohenthalOfficer:
  Regional DirectorBy:
  Realty Associates Fund VI Texas Corporation, a Texas corporation, general
  partnerBy:
  /s/ Heather L. HohenthalOfficer:
  Regional DirectorTENANT:CUBIST
  PHARMACEUTICALS, INC.By:
  /s/ David W.J. McGirrName:
  David W.J. McGirrTitle:
  Senior Vice President and Chief Financial Officer  5


Exhibit A-7  Plans
of Lobby Space    6