<PAGE>

                                                                   EXHIBIT 10.15

                            THIRD AMENDMENT TO LEASE
                                  PREMIER SOUTH

This THIRD AMENDMENT TO LEASE (this "Third Amendment") is made this 15th day of
April, 2002, by and between TEACHERS INSURANCE & ANNUITY ASSOCIATION OF AMERICA,
INC., a New York corporation ("Landlord") and ICOS CORPORATION, a Delaware
corporation ("Tenant").

                                    RECITALS

      Landlord is the landlord and Tenant is the tenant under that certain Lease
dated December 20, 1996 (the "Initial Lease"), for premises located at 22025
20th Avenue S.E., Bothell, Washington, 98021, Premier South (legally described
on Exhibit A hereto), as modified by First Amendment to Lease dated for
reference purposes August 5, 1997 (the "First Amendment"), and by Second
Amendment to Lease dated for reference purposes October 30, 1998 (the "Second
Amendment"). As used herein the "Amended Lease" shall mean the Initial Lease as
modified by the First Amendment and the Second Amendment, and the "Lease" shall
mean the Amended Lease as further amended by this Third Amendment.

      The parties desire to further amend the Amended Lease as set forth below.

      Landlord and Tenant are simultaneously entering into a lease amendment
(the "Premier North Third Amendment") for premises located at 22025 20th Avenue
S.E., Bothell, Washington, 98021, commonly known as Premier North.

      Except as otherwise specifically defined herein all capitalized terms
shall have the meanings assigned in the Amended Lease.

                                    AGREEMENT

      Now, therefore, for good and valuable consideration, the parties agree as
follows:

1.    Extension. Section 1.05 of the Lease is amended to provide that the
      expiration date of the Lease shall be December 31, 2007.

2.    Rent Adjustments. Section 1.13 (a) of the Lease shall be amended to
      provide the following Base Rent adjustments from and after February 10,
      2004:

<TABLE>
<CAPTION>
                        Effective Date of Rent
                        Increase                        New Base Monthly Rent
<S>                                                     <C>
                        February 10, 2004               $     0.00
                        April  11,  2004                $58,715.00
                        May 1, 2005                     $60,476.00
                        May 1, 2006                     $62,291.00
                        May 1, 2007                     $64,159.00
                        January 1, 2007                 $91,755.00
                        January 1, 2008                 $94,508.00
</TABLE>

3.    Liability Insurance. Section 4.04(a) of the Lease is deleted and replaced
      in its entirety with the following:

      (a)   Liability Insurance. From and after September 15, 2002, Tenant shall
            maintain a policy of comprehensive general liability insurance at
            Tenant's expense, insuring Landlord and its management contractor
            against liability arising out of the ownership, use, occupancy, or
            maintenance of the Property. The initial amount of such insurance
            shall be $1,000,000.00 per occurrence and an excess umbrella


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<PAGE>

            liability of $5,000,000.00. However, the amount of such insurance
            shall not limit Tenant's liability nor relieve Tenant of any
            obligation hereunder. The policy shall contain cross-liability
            endorsements, if applicable, and shall insure Tenant's performance
            of the indemnity provisions of Section 5.04, subsections (a), (b)
            and (e). Tenant shall, at Tenant's expense, maintain such other
            liability insurance as Tenant deems necessary to protect Tenant.
            Tenant's insurance shall be primary and non-contributing, and Tenant
            shall name Landlord and its management contractor as additional
            insureds and provide copies upon renewal, or at least once annually.

4.    Commissions. Sections 1.09 and 14.01 of the Lease are amended to reflect
      that, with respect to this Third Amendment, each party represents that it
      has not had dealings with any real estate broker, finder or other person
      with respect to this Lease in any manner, except for Insignia/Kidder
      Mathews, who represented Tenant and who shall be compensated by Landlord
      pursuant to separate agreement. Each party agrees to indemnify, defend and
      hold the other party harmless from any commissions claimed by any broker
      through the indemnifying party. Landlord and Tenant recognize that it is
      possible that they may hereafter make additional agreements regarding
      further extensions or renewal of this Lease or a new lease or leases for
      all or one or more parts of the Premises or other space in the Project for
      a term or terms commencing after the Commencement Date of this Lease.
      Landlord and Tenant recognize that it is also possible that they may
      hereafter modify this Lease to add additional space or to substitute space
      as part of the Premises. If any such additional agreements, new leases or
      modifications to this Lease are made, Landlord shall not have any
      obligation to pay any compensation to any real estate broker or to any
      other third person engaged by Tenant to render services to Tenant in
      connection with negotiating such matters, regardless of whether under the
      circumstances such person is or is not regarded by the law as an agent of
      Landlord.

5.    Option to Renew. All provisions under the Lease and any exhibits thereto
      relating to renewal of the Lease are hereby replaced by the following:

      Tenant is granted the right to extend the term of this Lease beyond the
      expiration date of the term as set forth in this Third Amendment for one
      period of sixty (60) months (the "Extended Term"). Tenant may not exercise
      its Extension Right if it is then in default beyond any applicable cure
      period or if it has ever been in default beyond any applicable cure period
      more than two (2) times in any twelve (12) month period. Tenant may
      exercise its Extension Right by delivering written notice thereof to
      Landlord not later than twelve (12) months prior to the expiration of the
      term. In the Extended Term, all terms and conditions of this Lease shall
      apply, except (i) there shall be no tenant improvement allowance, no right
      of first opportunity, and no further renewal rights, and (ii) the Base
      Monthly Rent for the first thirty-six (36) months of the Extended Term
      shall be ninety-five percent (95%) of the then prevailing market rate for
      a similar lease and term for similarly situated and improved space in the
      Bothell-Woodinville High-Tech Industrial Market (the unadjusted number
      being referred to herein as the "Fair Market Rent"), provided that in no
      event shall the Base Monthly Rent for the Extended Term be less than the
      Base Monthly Rent for the last month of the initial term, and (iii) at the
      beginning of the thirty-seventh (37th) month of the Extended Term, Base
      Rent for the balance of the Extended Term shall be adjusted based upon the
      increase in the Consumer Price Index, all Urban Consumers, for the
      Seattle/Tacoma SMSA, published by the United States Department of Labor,
      Bureau of Labor Statistics ("Index") which is in effect on the first day
      of the Extended Term ("Beginning Index"). The corresponding index
      ("Extension Index") which is in effect on the first day of the
      thirty-seventh (37th) month of the Extended Term shall be used as a
      comparison in determining the amount of the Base Rent Increase. In
      calculating the CPI increase, the Base Monthly Rent shall be increased to
      equal the product achieved by multiplying the Base Monthly Rent due with
      respect to that first paying month of the Extended Term by a fraction, the
      numerator of which is the Extension Index and the denominator of which is
      the Beginning Index. If the method of computing the Index is changed from
      that in effect when the Beginning Index was


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<PAGE>

      established, then the Beginning Index and all Extension Indexes shall be
      converted in accordance with the conversion factor published by the United
      States Department of Labor, Bureau of Labor Statistics. If during the
      Term, the Index is (a) discontinued or (b) revised without such a
      Conversion Factor being so published, such other government index or
      computation with which it is replaced shall be used in order to obtain
      substantially the same result as would be obtained if the Index had not
      been discontinued or revised.

      Extension Rights shall apply to all of the Premises then under lease to
      Tenant. Tenant's Extension Right is personal and may not be exercised by
      any assignee or sub lessee other than an Affiliate of Tenant or a
      successor by merger or consolidation.

      If Landlord and Tenant are not able to agree on the Fair Market Rent
      component of the Base Rent for the Extended Term within thirty days after
      Tenant's notice of election to renew, then such Fair Market Rent shall be
      determined by arbitration as follows. Landlord and Tenant shall each
      select an appraiser with at least ten years experience in the
      office/high-tech industrial market in the eastside area. If the two
      appraisers are unable to agree within ten days after their selection, they
      shall select a similarly qualified third appraiser (the "Neutral
      Appraiser"). If the three appraisers are unable to agree unanimously on
      Fair Market Rent within fourteen (14) days after appointment of the
      neutral Appraiser, the issue shall be resolved by the three appraisers in
      accordance with the following procedure. The appraiser selected by each of
      the parties shall state in writing his determination of the Fair Market
      Rent supported by the reasons therefore with counterpart copies to each
      party. The appraisers shall arrange for a simultaneous exchange of such
      proposed resolutions. The role of the Neutral Appraiser shall be to select
      which of the two proposed resolutions most closely approximates his
      determination of Fair Market Rent. The Neutral Appraiser shall have no
      right to propose a middle ground or any modification of either of the two
      proposed resolutions. The resolution he chooses as most closely
      approximating his determination shall constitute the decision of the
      appraisers and be final and binding upon the parties. Each party shall
      bear cost of its own appraiser, and the non-prevailing party shall pay the
      costs of the Neutral Appraiser.

6.    Landlord's Third Amendment Work. Landlord hereby agrees to perform, at
      Landlord's expense, the following work:

      (a)   Repaint the exterior of the Building during 2003.

      (b)   Pressure wash all walkways during 2002.

      (c)   Install double check valves as required by Alderwood Water District
            during 2002.

7.    Combined Tenant Improvement Allowance. Within Sixty (60) days after full
      execution of this Third Amendment and the Premier North Third Amendment,
      Landlord shall advance Tenant two hundred fifty thousand dollars
      ($250,000) in total for both buildings (the "Third Amendment Allowance")
      for use in construction of improvements to the Premises and to Premier
      North. All such improvements shall be constructed pursuant to Article Six
      of the Lease. The Third Amendment Allowance may be used only for the
      following costs

      (a)   Payment of the cost of preparing the space plan and the final
            working drawings and specifications, including mechanical,
            electrical, plumbing and structural drawings and of all other
            aspects of the tenant improvement plans.

      (b)   The payment of plan check, permit and license fees relating to
            construction of the Third Amendment Improvements.

      (c)   Construction of the Third Amendment Improvements, including, without
            limitation, the following:


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<PAGE>

            (1)   Installation within the Premises of all partitioning, doors,
                  floor coverings, ceilings, wall coverings and painting,
                  millwork and similar items.

            (2)   All electrical wiring, lighting fixtures, outlets and
                  switches, and other electrical work to be installed within the
                  Premises.

            (3)   The furnishing and installation of all duct work, terminal
                  boxes, diffusers and accessories required for the completion
                  of the heating, ventilation and air conditioning systems
                  within the Premises, I including the cost of meter and key
                  control for after-hour air conditioning.

            (4)   Any additional Tenant requirements including, but not limited
                  to, odor control, special heating, ventilation and air
                  conditioning, noise or vibration control or other special
                  systems.

            (5)   All fire and life safety control systems such as fire walls,
                  sprinklers, halon,fire alarms, including piping, wiring and
                  accessories installed within the Premises.

            (6)   All plumbing, fixtures, pipes and accessories to be installed
                  within the Premises.

            (7)   Testing and inspection costs.

            (8)   Contractor's fees, including but not limited to any fees based
                  on general conditions.

            Upon completion of the improvements, but not later than December 31,
      2003, Tenant shall provide Landlord with an accounting of the uses of the
      Third Amendment Allowance. Any funds not used for permitted purposes as of
      that date shall be refunded to Landlord.

8.    No Other Modification. Except as expressly modified herein, all of the
      terms, covenants and conditions of the Lease shall continue in full force
      and effect and are hereby ratified by the parties.

Landlord:  Teachers Insurance & Annuity Association
           of America, Inc.

         By:   /s/ James P. Garofalo
            ------------------------------------------

         Its:  Assistant Secretary
             -----------------------------------------


Tenant:  Icos Corporation, a Delaware corporation

         By:  /s/ Gary Wilcox
            ------------------------------------------

         Its: Exec. Vice President
             -----------------------------------------


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<PAGE>

                                    EXHIBIT A

LEGAL DESCRIPTION - (Premier North and South)

SECTION 30, TOWNSHIP 27 NORTH, RANGE 5 EAST, W.M., SNOHOMISH COUNTY, WASHINGTON.
RECORDED UNDER AF # 8505105032 BEING A PORTION OF NE1/4 SE 1/4 SECTION 30
TOWNSHIP 27 RANGE 5 EAST AND PORTION NW1/4 SW1/4 SECTION 29 TOWNSHIP 27 RANGE 5
EAST.


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<PAGE>

STATE OF New York   )
                    ) ss.
COUNTY OF New York  )

      I certify that I know or have satisfactory evidence that James P. Garofalo
signed this instrument, on oath stated that (he/she) was authorized to execute
the instrument and acknowledged it in (his/her) capacity as Assistant Secretary
of TEACHERS INSURANCE & ANNUITY ASSOCIATION OF AMERICA, INC. to be the free and
voluntary act of such party for the uses and purposes mentioned in the
instrument.

      GIVEN under my hand and official seal this 17th day of June, 2002.


                                     /s/ Loretta M. Monahan
                                     -------------------------------------------
                                     NOTARY PUBLIC in and for the
                                     State of New York, residing
                                     at Bronx;
                                     My commission expires:  1-12-2006
                                     Loretta M. Monahan
                                     -------------------------------------------
                                     [Type or Print Notary Name]

STATE OF           )
                   ) ss.
COUNTY OF          )

      I certify that I know or have satisfactory evidence that Gary L. Wilcox
signed this instrument, on oath stated that (he/she) was authorized to execute
the instrument and acknowledged it in (his/her) capacity as EVP of Operations
of ICOS CORPORATION to be the free and voluntary act of such party for the uses
and purposes mentioned in the instrument.

      GIVEN under my hand and official seal this 6th day of June, 2002.


                                     /s/ Lovena Laycock
                                     -------------------------------------------
                                     NOTARY PUBLIC in and for the
                                     State of Washington, residing
                                     at Bothell;
                                     My commission expires:  10-9-05
                                     Lovena Laycock
                                     -------------------------------------------
                                     [Type or Print Notary Name]


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