<PAGE>   1
                                                                    EXHIBIT 99.6


                       FORMATION AND PERFORMANCE AGREEMENT


         THIS FORMATION AND PERFORMANCE AGREEMENT (this "Formation Agreement")
is made and entered into effective as of June 6, 2000 by and between ICOS
CORPORATION, a Delaware corporation ("ICOS"), and TEXAS BIOTECHNOLOGY
CORPORATION, a Delaware corporation ("TBC") (separately, a "Party," and
collectively, the "Parties").

                                    RECITALS

         A. ICOS-ET-LP LLC, a Washington limited liability company ("ICOS-LP"),
and TBC, both as limited partners, and ICOS-ET-GP LLC, a Washington limited
liability company ("ICOS-GP"), and TBC-ET, Inc., a Delaware corporation
("TBC-GP"), both as general partners, will form ICOS-Texas Biotechnology L.P., a
Delaware limited partnership (the "Partnership"), and will execute the Agreement
of Limited Partnership of the Partnership, to be dated effective as of the date
hereof (the "Partnership Agreement"). Capitalized terms used but not otherwise
defined in this Formation Agreement shall have the meanings set forth in the
Partnership Agreement.

         B. ICOS-LP and ICOS-GP are wholly owned by ICOS. TBC-GP is wholly owned
by TBC.

         C. ICOS-LP, ICOS-GP, TBC-GP and any Affiliate of a Party who acquires a
Partnership Interest are hereafter referred to collectively as the
"Subsidiaries" and individually as a "Subsidiary." ICOS and TBC are hereafter
referred to collectively as the "Parent Companies" and individually as a "Parent
Company."

         D. TBC intends to transfer to the Partnership the Endothelin License
Agreement, pursuant to which the Partnership (as licensee) will receive a
license to the Background Technology, in return for a forty-nine and nine-tenths
percent (49.9%) Partnership Interest.

         E. TBC further intends to contribute certain assets to TBC-GP and then
cause TBC-GP to contribute all such assets to the Partnership within [ * ] of
the effective date of its formation.

         F. ICOS intends to contribute certain assets to ICOS-LP and ICOS-GP and
then cause each of them to contribute all such assets to the Partnership within
[ * ] of the effective date of its formation.

         G. Each of TBC and ICOS intends to provide assurances to the other that
it will cause its respective Subsidiaries to perform all obligations and duties
as a Partner of the


[*]  DESIGNATES PORTIONS OF THIS DOCUMENT THAT HAVE BEEN OMITTED PURSUANT TO A
     REQUEST FOR CONFIDENTIAL TREATMENT FILED SEPARATELY WITH THE COMMISSION.


                                      -1-
<PAGE>   2
Partnership as provided in the Partnership Agreement and that it will cause its
respective Subsidiaries or Affiliates who are providing services to the
Partnership to perform all of their obligations and duties under any contracts
with the Partnership, upon the terms set forth herein.

         NOW, THEREFORE, in consideration of the mutual covenants of the
Parties, each to the other, and of good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, the Parties agree as follows:

                                    AGREEMENT

1.       PERFORMANCE BY THE SUBSIDIARIES

         For so long as a Parent Company's Subsidiary owns a Partnership
Interest in the Partnership, and thereafter for so long as a Subsidiary has an
accrued but unperformed obligation to the Partnership pursuant to the
Partnership Agreement, such Parent Company [ * ], to cause the Subsidiary to
perform all of the Subsidiary's agreements, covenants and obligations under the
Partnership Agreement, subject to the terms and conditions thereof. Each Parent
Company [ * ] agreements with the Partnership to which the Subsidiaries or
Affiliates are parties or become parties, regardless of when such agreements are
created. Without limiting the generality of the foregoing, each Parent Company
[ * ]. The Parties acknowledge and agree that each Parent Company [ * ] pursuant
to the Partnership Agreement, and such other agreements to which the Subsidiary,
or any of its Affiliates, and the Partnership are parties, and not with respect
to the performance of the Subsidiary or Affiliate of any obligations or duties
owed to any other party, whether person or entity.

2.       FURTHER ASSURANCES

         In addition to executing this Formation Agreement, causing its
respective Subsidiaries to execute the Partnership Agreement [ * ] Subsidiaries
thereunder, each Parent Company agrees to [ * ].

3.       REPRESENTATIONS AND WARRANTIES

         Each Parent Company represents and warrants to the other Parent Company
as follows:

         (a) The Parent Company shall provide its Subsidiaries with full power
and authority to execute the Partnership Agreement and to perform all functions
as a Partner of the Partnership contemplated therein, and take all reasonable
additional steps as shall be necessary to cause each Subsidiary to perform all
obligations, duties and transactions as provided in the Partnership Agreement.

         (b) The Parent Company shall maintain each of its Subsidiaries as a
duly organized and validly existing Entity under the laws of the jurisdiction in
which it was organized.


[*]  DESIGNATES PORTIONS OF THIS DOCUMENT THAT HAVE BEEN OMITTED PURSUANT TO A
     REQUEST FOR CONFIDENTIAL TREATMENT FILED SEPARATELY WITH THE COMMISSION.


                                       -2-
<PAGE>   3
4.       [INTENTIONALLY LEFT BLANK.]

5.       BREACH

         If any Party breaches or fails to perform any obligation required to be
performed by it pursuant to this Formation Agreement and does not cure such
breach or failure within thirty (30) days after it has received written notice
thereof from the other Party or the Management Committee (the "Defaulting
Party"), the non-Defaulting Party shall have all remedies available to it at law
or in equity against the Defaulting Party as a result of its breach or failure
to perform its obligations under this Formation Agreement.

6.       DISPUTE RESOLUTION

         In the event a dispute arises between the Parties regarding the
application or interpretation of any provision of this Formation Agreement (a
"Dispute"), then upon the written request of any Party that includes a summary
of the Dispute, [ * ].

         Any Dispute that the Parties are unable to resolve [ * ]. The Parties
will mutually determine the mediator from a list of mediators obtained from the
American Arbitration Association office located in the city in which the
proceeding will take place. If the Parties are unable to agree on the mediator,
the mediator will be selected by the American Arbitration Association with a
preference for selecting a retired federal judge or state supreme court judge as
the mediator.

         If any Dispute goes unresolved following the foregoing procedures,
[ * ].

7.       NOTICES

         Any notice, request, instruction or other document required or
permitted under this Formation Agreement shall be given either in writing,
delivered in person, or mailed by certified or registered mail, return receipt
requested, or transmitted by facsimile transmission with electronic confirmation
of receipt to the Parties' last known addresses. Notices shall be deemed to have
been given (a) on the date of service, if served personally on the Party to whom
notice is to be given, (b) on the first day after transmission by facsimile
transmission, if transmitted as set forth above, or (c) on the third day after
mailing, if mailed as set forth above.

8.       DISCLAIMER OF AGENCY

         This Formation Agreement does not create any entity or relationship
beyond the scope set forth herein and, except as otherwise expressly provided
herein, this Formation Agreement shall not constitute any Party the legal
representative or agent of the other, nor shall any Party have the right or
authority to assume, create or incur any liability or obligation, express or
implied, against, in the name of or on behalf of any other Party, another
Party's Subsidiary, the Partnership or its Affiliates.


[*]  DESIGNATES PORTIONS OF THIS DOCUMENT THAT HAVE BEEN OMITTED PURSUANT TO A
     REQUEST FOR CONFIDENTIAL TREATMENT FILED SEPARATELY WITH THE COMMISSION.


                                      -3-
<PAGE>   4
9.       WAIVERS

         No waiver of any breach of the terms of this Formation Agreement shall
be effective unless such waiver is in writing and signed by the Party against
whom such waiver is claimed. No waiver of any breach shall be deemed to be a
waiver of any other or subsequent breach.

10.      AMENDMENTS

         This Agreement may be amended only in a writing signed by both Parties.

11.      SEVERABILITY

         If any provision of this Formation Agreement shall be held to be
invalid, illegal or unenforceable, the validity, legality and enforceability of
the remaining provisions shall not in any way be affected or impaired thereby.

12.      SUCCESSORS AND ASSIGNS

         This Agreement shall be binding on and inure to the benefit of the
Parties and their respective successors and permitted assigns.

13.      THIRD PARTIES

         This Agreement is entered into between the Parties for the exclusive
benefit of themselves and their successors and permitted assigns. This Agreement
is not intended for the benefit of, and shall not be enforceable by, any
creditor of a Party, Subsidiary or the Partnership or any other third party,
whether person or entity. Except to the extent provided by applicable statute,
and then only to that extent, no third party shall have any rights under this
Formation Agreement or under any other agreement between the Parties,
Subsidiaries and/or the Partnership.

14.      COUNTERPARTS

         This Formation Agreement may be executed in any number of counterparts,
each of which shall be deemed an original and all of which shall constitute one
and the same instrument.

15.      ATTORNEYS' FEES

         In the event of a dispute between the Parties regarding the enforcement
or interpretation of this Formation Agreement that is arbitrated or litigated,
the nonprevailing Party or Parties shall pay the reasonable costs and attorneys'
fees of the prevailing Party or Parties, including the reasonable costs and
attorneys' fees incurred in the appeal of any final or interlocutory judgment.


[*]  DESIGNATES PORTIONS OF THIS DOCUMENT THAT HAVE BEEN OMITTED PURSUANT TO A
     REQUEST FOR CONFIDENTIAL TREATMENT FILED SEPARATELY WITH THE COMMISSION.


                                      -4-
<PAGE>   5
16.      RIGHTS AND REMEDIES

         The rights and remedies provided by this Formation Agreement are
cumulative, and the use of any one right or remedy shall not preclude or waive
the right to use any or all other remedies. These rights and remedies are given
in addition to any other rights the Parties may have by law, statute, ordinance
or otherwise.

17.      GOVERNING LAW; JURISDICTION

         This Formation Agreement shall be governed by and construed in
accordance with the laws of the State of Delaware, without regard to the choice
of law provisions of the State of Delaware or any other jurisdiction.


                                           ICOS CORPORATION

                                           By: /s/ Paul Clark
                                              ----------------------------------
                                           Name: Paul Clark
                                                --------------------------------
                                           Title: Chairman & CEO
                                                 -------------------------------



                                           TEXAS BIOTECHNOLOGY CORPORATION

                                           By: /s/ David B. McWilliams
                                              ----------------------------------
                                           Name: David B. McWilliams
                                                --------------------------------
                                           Title: President/CEO
                                                 -------------------------------


[*]  DESIGNATES PORTIONS OF THIS DOCUMENT THAT HAVE BEEN OMITTED PURSUANT TO A
     REQUEST FOR CONFIDENTIAL TREATMENT FILED SEPARATELY WITH THE COMMISSION.


                                      -5-