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Royalty Stream Purchase Agreement - Pharmaceutical Partners LLC, Bioventure Investments Kft, Pharmaceutical Royalties LLC, Seragen Inc. and Ligand Pharmaceuticals Inc.

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                        ROYALTY STREAM PURCHASE AGREEMENT

                                      AMONG

                         PHARMACEUTICAL PARTNERS, L.L.C.

                           BIOVENTURE INVESTMENTS, Kft

                        PHARMACEUTICAL ROYALTIES, L.L.C.

                                       and

                                  SERAGEN, INC.

                       LIGAND PHARMACEUTICALS INCORPORATED

                          Dated as of December 31, 1999





*** Certain confidential portions of this Exhibit were omitted by means of
blackout of the text (the "Mark"). This Exhibit has been filed separately with
the Secretary of the Commission without the Mark pursuant to the Company's
Application Requesting Confidential Treatment under Rule 24b-2 under the 1934
Act.



<PAGE>




                                TABLE OF CONTENTS
                                                                            
ARTICLE I - DEFINITIONS..........................................................1
         1.01     Definitions....................................................1

ARTICLE II - PURCHASE AND SALE...................................................2
         2.01     Purchase and Sale..............................................2
         2.02     No Assumed Obligations.........................................3
         2.03     Excluded Assets................................................3
         2.04     Initial Purchase Price.........................................3
         2.05     Contingent Purchase Price......................................3

ARTICLE III - REPRESENTATIONS AND WARRANTIES OF SELLER AND STOCKHOLDERS..........4
         3.01     Corporate Existence and Power..................................4
         3.02     Corporate Authorization........................................4
         3.03     Corporate Authorization Governmental Authorization.............4
         3.04     Corporate Authorization Non-Contravention......................4
         3.05     No Undisclosed Material Liabilities............................5
         3.06     Litigation.....................................................5
         3.07     Compliance with Laws...........................................5
         3.08     Intellectual Property..........................................5
         3.09     Finders' Fees..................................................6
         3.10     Other Information..............................................6

ARTICLE IV - REPRESENTATIONS AND WARRANTIES OF BUYER.............................6
         4.01     Organization and Existence.....................................6
         4.02     Corporate Authorization........................................6
         4.03     Governmental Authorization.....................................6
         4.04     Non-Contravention..............................................6
         4.05     Finders' Fees..................................................6
         4.06     Financing......................................................7
         4.07     Litigation.....................................................7

ARTICLE V - COVENANTS............................................................7
         5.01     Maintenance of Enabling Agreements.............................7
         5.02     Confidentiality................................................7
         5.03     Public Announcement............................................8
         5.04     Payment of Seragen Royalty.....................................8
         5.05     Roche and Novartis Royalty Reports.............................8
         5.06     Roche and Novartis Audits......................................8
         5.07     Breach of Roche or Novartis Licenses...........................8

ARTICLE VI - REPRESENTATIONS AND WARRANTIES OF BUYER.............................9
         6.01     Indemnification................................................9
         6.02     Procedures; No Waiver; Exclusivity.............................9



<PAGE>


                                                                            
ARTICLE VII - TERM...............................................................10
         7.01     Term...........................................................10
         7.02     Termination by Seller..........................................10

ARTICLE VIII - MISCELLANEOUS.....................................................10
         8.01     Notices........................................................10
         8.02     Amendments; No Waivers.........................................11
         8.03     Expenses.......................................................11
         8.04     Successors and Assigns.........................................11
         8.05     Governing Law; Jurisdiction....................................11
         8.06     Counterparts; Effectiveness....................................12
         8.07     Entire Agreement...............................................12
         8.08     Captions.......................................................12

ARTICLE IX        ...............................................................12
         9.1      The Guaranty...................................................12
         9.2      Guaranty Unconditional.........................................12
         9.3      Discharge Only Upon Payment in Full; Reinstatement in Certain
                  Circumstances..................................................13
         9.4      Waiver by Ligand...............................................13

Schedule
--------

Schedule A        Patent Matters



<PAGE>


                        ROYALTY STREAM PURCHASE AGREEMENT


     AGREEMENT dated as of December 31, 1999 among Seragen, Inc., a Delaware
corporation ("Seller"), with respect to Article IX, Ligand Pharmaceuticals
Incorporated, a Delaware corporation ("Ligand"), as guarantor of Seller's
obligations, Pharmaceutical Partners, L.L.C. , a Delaware limited liability
company ("Pharma Partners"), Bioventure Investments, Kft, an affiliate of Pharma
Partners ("Bioventure"), and Pharmaceutical Royalties, LLC, an affiliate of
Pharma Partners ("Pharma Royalties"), as assignees. Bioventure and Pharma
Royalties are hereinafter collectively referred to as the "Pharma Affiliates".
Pharma Partners and the Pharma Affiliates are collectively referred to
hereinafter as "Buyer".

                              W I T N E S S E T H:

     WHEREAS, Buyer desires to purchase certain assets of Seller from Seller,
and Seller desires to sell, assign and transfer such assets to Buyer, upon the
terms and subject to the conditions hereinafter set forth; and

     WHEREAS, Buyer desires that Ligand unconditionally guaranty the obligations
of Seller set forth in this Agreement, and Ligand, in order to induce Buyer to
enter into this Agreement, agrees to such guaranty, upon the terms and subject
to the conditions hereinafter set forth.

     NOW, THEREFORE, in consideration of the foregoing and the representations,
warranties, covenants and agreements herein contained, the parties hereto agree
as follows:

                                    ARTICLE I

                                   DEFINITIONS

     1.01 Definitions. The following terms, as used herein, have the following
meanings:

     "Affiliate" means with respect to any Person, any Person directly or
indirectly controlling, controlled by or under common control with such other
Person.

     "Closing" has the meaning set forth in Section 2.01.

     "Beth Israel Agreement" means the License and Royalty Agreement between
Beth Israel Hospital Association ("Beth Israel") and Seller dated as of June 1,
1990.

     "Beth Israel Royalty" means ***% of the Gross Royalty less deductions
therefrom permitted under the Beth Israel Agreement.

     "Enabling Agreements" means the Beth Israel Agreement, the Novartis License
and the Roche License.


   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.


<PAGE>

     "Excluded Liabilities and Obligations" has the meaning set forth in Section
2.02.

     "Governmental Authority" means any government, court, regulatory or
administrative agency or commission, or other governmental authority, agency or
instrumentality, whether federal, state or local (domestic or foreign),
including, without limitation, the U.S. Patent and Trademark Office (the "PTO")
and the U.S. National Institutes of Health.

     "Gross Royalty" means all royalty payments made to Seller or any other
Person under the Novartis License and the Roche License after January 1, 2001;
provided that, for purposes of this Agreement, Gross Royalty shall not include
any milestone payment made to Seller pursuant to Section 3.2 of the Roche
License.

     "Lien" means, with respect to any agreement or other asset, any mortgage,
lien, pledge, charge, security interest or encumbrance of any kind in respect of
such asset.

     "Novartis License" means that License Agreement entered into between Seller
and Sandoz Pharma, Ltd. executed by Seller on September 4, 1996 and by Sandoz on
August 27, 1996.

     "Patents" means the patents and applications in Schedule A hereto of which
Seller is the exclusive licensee under the Beth Israel Agreement and which are
subject to the Novartis License and the Roche License.

     "Person" means an individual, corporation, partnership, association, trust
or other entity or organization, but not including a government or political
subdivision or any agency or instrumentality of such government or political
subdivision.

     "Purchase Price" has the meaning set forth in Section 2.04.

     "Roche License" means that Nonexclusive Sublicense Agreement of September
8, 1999 by and between Seller on the one hand and Hoffmann-La Roche Inc. and F.
Hoffmann-La Roche Ltd. on the other.

     "Seragen Royalty" means ***% of the Gross Royalty.

                                   ARTICLE II

                      PURCHASE AND SALE OF SERAGEN ROYALTY

     2.01 Purchase and Sale. Upon the terms and subject to the conditions of
this Agreement: (a) Pharma Partners agrees to cause the Pharma Affiliates to
purchase from Seller, and Seller agrees to sell and transfer to the Pharma
Affiliates, upon execution of this Agreement, free and clear of all Liens, the
Seragen Royalty. For purposes of this Agreement, Bioventure shall purchase ***%
and Pharma Royalties shall purchase ***%, respectively, of the Seragen


   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.

                                       2

<PAGE>

Royalty. The payment of the Purchase Price by the Pharma Affiliates to Seller
is sometimes hereinafter referred to as the "Closing".

     (b) At the Closing, Seller shall cause to be delivered to Pharma Partners
and the Pharma Affiliates:

               (i)......a certified copy of the Board of Directors of Seller
          authorizing the Agreement and the transactions contemplated thereby;
          and

               (ii) a receipt for the Purchase Price.

At and after the Closing, if requested by Pharma Partners, Seller will execute
and deliver to Pharma Partners or the Pharma Affiliates such instruments and
documents as may be reasonably requested by Pharma Partners in order to evidence
the Pharma Affiliates' ownership of the Seragen Royalty, including without
limitation such UCC registration forms as Pharma Partners may request.

     2.02 No Assumed Obligations. Notwithstanding any provision in this
Agreement or any other writing to the contrary, the Pharma Affiliates are
acquiring only the Seragen Royalty and are not assuming any liability or
obligation of Seller of whatever nature, whether presently in existence or
arising or asserted hereafter, whether under any of the Enabling Agreements or
otherwise. All such liabilities and obligations shall be retained by and remain
obligations and liabilities of Seller (the "Excluded Liabilities and
Obligations").

     2.03 Excluded Assets. Buyer does not, by purchase of the Seragen Royalty,
acquire any assets or contract rights of Seller under the Enabling Agreements
except all rights, title and interest to the Seragen Royalty. Buyer acknowledges
that milestone payments made to Seller pursuant to Section 3.2 of the Roche
License are not included in the Seragen Royalty. Notwithstanding the foregoing
in this Section 2.03, after the Closing, at the request of Pharma Partners,
Seller and Pharma Partners agree to develop and make jointly in good faith an
approach to the licensees under the Novartis License and the Roche License in
order to facilitate the direct payment by such licensees of the Seragen Royalty
to the Pharma Affiliates.

     2.04 Initial Purchase Price. Upon execution and delivery of this Agreement,
the Pharma Affiliates shall pay to Seller $3,250,000.00 (the "Purchase Price").
Bioventure shall make ***% of such payment and Pharma Royalties shall make ***%
of such payment. The payment will be made by federal funds wire transfer at the
Closing pursuant to wiring instructions received from Seller.

     2.05 Contingent Purchase Price. In the circumstance where net sales (as
defined in the Roche License) reported by Roche to Seller under the Roche
License for any of calendar years 2001, 2002, 2003 or 2004 exceed *** , the
Pharma Affiliates will make a one-time payment to Seller equal to $3,250,000.00
within thirty (30) days of receipt from Seller of an invoice therefor; provided
that (a) Seller shall include with its invoice a copy (certified as true and
correct by an executive officer of Seller) of all applicable documentation
provided by


   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.

                                       3

<PAGE>

Roche to Seller evidencing the amount of such net sales, (b) the Roche
License remains in full force and effect and there shall have been no breach or
default by Seller thereunder and Buyer shall have received a certificate signed
by an executive officer of Seller to such effect and (c) the payment obligation
of the Pharma Affiliates under this Section 2.05 shall not be triggered until 30
days after the date on which the Pharma Affiliates shall have received an amount
equal to the Seragen Royalty for such *** net sales for the applicable calendar
year. Bioventure shall make ***% of any such payment and Pharma Royalties shall
make ***% of any such payment.

                                  ARTICLE III

                    REPRESENTATIONS AND WARRANTIES OF SELLER

     Seller hereby represents and warrants to Buyer that:

     3.01 Corporate Existence and Power. Seller is a corporation duly
incorporated, validly existing and in good standing under the laws of the State
of Delaware, and has all corporate powers and all licenses, authorizations,
consents and approvals required to carry on its business as now conducted.

     3.02 Corporate Authorization. The execution, delivery and performance by
Seller of this Agreement, and the consummation by Seller of the transactions
contemplated hereby are within Seller's corporate powers and have been duly
authorized by all necessary corporate action on the part of Seller. This
Agreement has been duly executed and delivered and constitutes a valid and
binding agreement of Seller.

     3.03 Corporate Authorization Governmental Authorization. The execution,
delivery and performance by Seller of this Agreement does not require any notice
to, action or consent by or in respect of, or filing with, any Governmental
Authority.

     3.04 Corporate Authorization Non-Contravention. (a) The execution, delivery
and performance by Seller of this Agreement does not and will not (i) contravene
or conflict with the corporate charter or bylaws of Seller, (ii) contravene or
conflict with or constitute a violation of any provision of any law or
regulation binding upon or applicable to Seller or the Seragen Royalty which
contravention, conflict or violation could reasonably be expected to have a
material adverse effect on the Seragen Royalty; (iii) contravene or conflict
with or constitute a violation of any judgment, injunction, order or decree
binding upon or applicable to either of Seller or the Seragen Royalty which
contravention, conflict or violation could reasonably be expected to have a
material adverse effect on the Seragen Royalty; (iv) constitute a default under
or give rise to any right of termination, cancellation or acceleration of any
right or obligation of Seller or to a loss of any benefit relating to the
Seragen Royalty, or (v) result in the creation or imposition of any Lien on the
Seragen Royalty (except for any Lien in favor of the Buyer).

   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.

                                       4

<PAGE>

     (b) Seller has not granted, and there does not currently exist, any Lien on
the Seragen Royalty, on any of the Enabling Agreements or on any amounts payable
to Seller under the Novartis License or the Roche License.

     3.05 No Undisclosed Material Liabilities. There are no material liabilities
related to Seragen Royalty of any kind whatsoever, whether accrued, contingent,
absolute, determined, determinable or otherwise, and there is no existing
condition, situation or set of circumstances which could reasonably be expected
to result in such a liability.

     3.06 Litigation. There is no action, suit, investigation or proceeding (or
any basis therefor), of which Seller has received notice, pending or, to the
knowledge of Seller, threatened, before any Governmental Authority or arbitrator
that has or could materially affect the Seragen Royalty. There have been no
claims made by any Person with respect to, and no actions, suits or other
proceedings relating to the Seragen Royalty which could reasonably be expected
to have a material adverse effect thereon.

     3.07 Compliance with Laws. Seller is not in violation of, has not violated,
and to the knowledge of Seller, is not under investigation with respect to and
has not been threatened to be charged with or given notice of any violation of,
any law, rule, ordinance or regulation, or judgment, order or decree entered by
any Governmental Authority applicable to the Seragen Royalty which could
reasonably be expected to have a material adverse effect thereon.

     3.08 Intellectual Property. (a) Schedule A sets forth a true and complete
list of the Patents specifying as to each, as applicable (i) the owner of such
Patent; and (ii) the jurisdictions by or in which each Patent has issued or an
application for patent has been filed, including the respective patent or
application numbers.

     (b) Seller has the right under the Beth Israel Agreement to procure and
maintain the Patents and has taken all material measures required to protect the
value of the Patents.

     (c) Seller has an exclusive license to the Patents under the Beth Israel
Agreement. To its knowledge, Seller, Beth Israel, and each inventor of the
Patents has complied with the PTO duty of candor and good faith in dealing with
the PTO, including the duty to disclose to the PTO all information known to be
material to the patentability of each claim of the U.S. Patents. All assignments
from each inventor to, as the case may be, the owner thereof or to a predecessor
in interest to the owner thereof, have been executed and recorded with the PTO
for each of the U.S. Patents.

     (d) The copies of the Enabling Agreements as provided by Seller to Buyer
are true and correct copies. There have been no amendments or modifications to
any of the Enabling Agreements. The Gross Royalty is not subject to any claim of
off-set for any other liability or obligation of Seller. Seller is in material
compliance with the Beth Israel Agreement and is not in breach of its
obligations with respect thereto which breach could reasonably be expected to
have a material adverse effect on its rights thereunder. Roche and Novartis are,
to the knowledge of Seller, in compliance with, respectively, the Roche License
and the Novartis License and Seller has no reason to believe that either Roche
or Novartis does not intend to comply with its obligations pursuant to the Roche
License and the Novartis License, respectively, including their

                                       5

<PAGE>

respective obligations to pay royalties on products covered thereby. Except
for the Roche License and the Novartis License, Seller has not granted any
licenses or other rights and has no obligations to grant licenses or other
rights with respect to the Patents, and, except for the Enabling Agreements,
there are no other contracts, arrangements, or understandings relating to the
Seragen Royalty.

     (e) Seller has taken all reasonable actions under all applicable foreign
jurisdictions to protect its license interests in the Patents in each such
jurisdiction where such Patents are filed.

     3.09 Finders' Fees. There is no investment banker, broker, finder or other
intermediary which has been retained by or is authorized to act on behalf of
Seller who might be entitled to any fee or commission from Buyer or any of its
Affiliates upon consummation of the transactions contemplated by this Agreement.

     3.10 Other Information. Neither this Agreement nor any of the exhibits and
schedules appended hereto contains any untrue statement of a material fact or
omits to state a material fact necessary in order to make the statements
contained therein not misleading.


                                   ARTICLE IV

                     REPRESENTATIONS AND WARRANTIES OF BUYER

     Buyer hereby represents and warrants to Seller that:

     4.01 Organization and Existence. Pharma Partners and each of the Pharma
Affiliates is duly organized, validly existing and in good standing under the
laws of its jurisdiction of organization and has all applicable powers and all
material governmental licenses, authorizations, consents and approvals required
to carry on its business as now conducted.

     4.02 Corporate Authorization. The execution, delivery and performance by
Buyer of this Agreement and the consummation by Buyer of the transactions
contemplated hereby are within the powers of Buyer and have been duly authorized
by all necessary action on the part of Buyer. This Agreement constitutes a valid
and binding agreement of Buyer.

     4.03 Governmental Authorization. The execution, delivery and performance by
Buyer of this Agreement does not require any action by or in respect of, or
filing with, any Governmental Authority.

     4.04 Non-Contravention. The execution, delivery and performance by Buyer of
this Agreement does not and will not (i) contravene or conflict with the
organizational documents of Pharma Partners or either of the Pharma Affiliates,
(ii) contravene or conflict with or constitute a violation of any provision of
any law or regulation binding upon or applicable to Buyer; or (iii) contravene
or conflict with or constitute a violation of any judgment, injunction, order or
decree binding upon or applicable to Buyer.

     4.05 Finders' Fees. There is no investment banker, broker, finder or other
intermediary that has been retained by or is authorized to act on behalf of
Buyer who might be

                                       6

<PAGE>

entitled to any fee or commission from Seller upon consummation of the
transactions contemplated by this Agreement.

     4.06 Financing. At the Closing, Bioventure will have sufficient funds
available to pay ***% of the Purchase Price and Pharma Royalties will have
sufficient funds available to pay ***% of the Purchase Price.

     4.07 Litigation. There is no action, suit, investigation or proceeding
pending against, or to the knowledge of Buyer threatened against or affecting,
Buyer before any court or arbitrator or any governmental body, agency or
official which in any matter challenges or seeks to prevent, enjoin, alter or
materially delay the transactions contemplated hereby.

                                   ARTICLE V

                                   COVENANTS

     Buyer and Seller agree that:

     5.01 Maintenance of Enabling Agreements. (a) Seller shall exercise fully
all of its rights, and comply fully with all of its obligations, under the
Enabling Agreements and shall not, without Pharma Partners' prior written
approval (not to be unreasonably withheld), permit any amendment or take any
other action (or omit to take any action) with respect thereto which could
reasonably be expected to impair the Seragen Royalty. For purposes of this
Section 5.01, it shall be reasonable for Pharma Partners to withhold its
approval with respect to any amendment, action or omission if, in the reasonable
opinion of Pharma Partners, such amendment, action or omission could have the
effect of reducing the Seragen Royalty.

     (b) Without Pharma Partners' prior written approval, Seller shall not sell,
transfer, assign or otherwise dispose of, or grant any Lien on, the Novartis
License or the Roche License. Without Pharma Partners' prior written approval,
Seller shall not sell, transfer, assign or otherwise dispose of, or grant any
Lien on, the Beth Israel Agreement if such sale, transfer, assignment or
disposal could have the effect of reducing the Gross Royalty payable to Seragen.

     (c) Seller shall pay all maintenance or annuity fees necessary to maintain
each issued patent included in the Patents in force for the full term of each
such patent. Seller shall in good faith exercise reasonable judgment in the
continued prosecution of each patent application included in the Patents, and of
any continuation or divisional patent application thereof. If Seller elects to
abandon any patent application included within the Patents, or of any
continuation or divisional patent application thereof, Seller shall notify Buyer
not less than ninety (90) days prior to such action.

     5.02 Confidentiality. After the Closing, Buyer and Seller will hold, and
will use reasonable commercial efforts to cause their officers, directors,
employees, accountants, counsel, consultants, advisors and agents to hold, in
confidence, unless compelled to disclose by judicial or administrative process
or unless required by law or the rules and regulations of the Securities


   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.

                                       7

<PAGE>

and Exchange Commission or any securities exchange or trading system, all
confidential documents and information concerning Buyer, Seller and the Seragen
Royalty.

     5.03 Public Announcement. Except as required by law or the rules and
regulations of the Securities and Exchange Commission or any securities exchange
or trading system, the parties agree to consult with each other before issuing
any press release or making any public statement with respect to Buyer's
acquisition of the Seragen Royalty. Such press release or public statement
shall, to the extent possible, be a joint release.

     5.04 Payment of Seragen Royalty. Within five (5) business days of a Payment
Event (as defined below) Seller will remit by federal funds wire transfer (a)
***% of the Seragen Royalty to Bioventure and (b) ***% of the Seragen Royalty to
Pharma Royalties. The amount of the Seragen Royalty payment shall be determined
based upon the amount of the Gross Royalty. Each remittance shall be made by
wire transfer pursuant to instructions received from Pharma Partners. Any
payment of the Seragen Royalty which is not paid when due shall bear interest at
the prime interest rate as announced by Citibank, N.A. plus ***%. By notice to
Seller, Pharma Partners may change the instructions or the amounts payable
hereunder to each of the Pharma Affiliates. As used herein, "Payment Event"
means the receipt by Seller or any of its Affiliates of a Gross Royalty payment
or, in the absence of such receipt, the making by Novartis or Roche of a Gross
Royalty payment to any other Person.

     5.05 Roche and Novartis Royalty Reports. Seller has the right to receive
reports concerning royalties payable to Seller under the Roche and Novartis
Licenses. Seller shall provide Buyer with a copy (certified by an executive
officer of Seller) of each such report upon making the Seragen Royalty payment
applicable thereto which reports will be subject to the provisions of Section
5.02 and applicable confidentiality provisions of the Novartis and Roche
Licenses.

     5.06 Roche and Novartis Audits. Seller has the right under the Roche and
Novartis Licenses to perform audits relative to assuring the accuracy of reports
related to royalty payments made thereunder. At Buyer's request and at Buyer's
expense, Seller shall cause such audits to be conducted on the terms provided in
the Roche and Novartis Licenses.

     5.07 Breach of Roche or Novartis Licenses. Upon any occurrence of a breach
by Roche or Novartis under the Roche and Novartis Licenses, respectively, which
is not cured as provided in the applicable agreement and which affects the
Seragen Royalty, at Buyer's request and Buyer's expense, using counsel selected
by Buyer, Seller shall seek to enforce the applicable agreement with respect to
the breach thereof. Buyer shall be entitled to control such litigation,
including any counterclaim alleging invalidity of the Patents or otherwise
alleging that the Novartis License or Roche License is invalid or unenforceable.
Seller shall cooperate with Buyer and Buyer's counsel in such litigation
including, without limitation, (a) if requested by Buyer or its counsel, Seller
shall make available to Buyer and its counsel at Seller's offices all of
Seller's books and records reasonably related to such litigation, including
copies thereof and (b) if requested by Buyer or its counsel, Seller shall cause
its officers, directors, employees and agents (i) to execute and deliver all
true and correct affidavits and other documents as may be


   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.

                                       8

<PAGE>

requested by Buyer or its counsel and (ii) to appear and testify at any
proceedings relating to such litigation, including any depositions or interim
appearances. All recoveries obtained by such enforcement shall be for the
benefit of Buyer and, if received by Seller, shall be immediately remitted,
without off-set or deduction, to the Pharma Affiliates. Notwithstanding the
foregoing in this Section 5.07, if Roche or Novartis makes any claim against
Seller or Ligand (whether as a counterclaim or otherwise) for damages which
could result in Seller or Ligand suffering an out-of-pocket loss in the form of
a damages award against Seller or Ligand in favor of Roche or Novartis, then
Seller or Ligand shall be entitled to control the defense of such claim, using
counsel selected by Ligand and at Seller's or Ligand's expense. In no event
shall Pharma Partners or any of the Pharma Affiliates be liable for any judgment
or damages awarded against Seller or Ligand in such circumstances.

                                   ARTICLE VI

                           SURVIVAL; INDEMNIFICATION

     6.01 Indemnification. Seller hereby indemnifies Buyer and its Affiliates
against and agrees to hold each of them harmless from any and all damage, loss,
liability and expense (including, without limitation, reasonable expenses of
investigation and reasonable attorneys' fees and expenses in connection with any
action, suit or proceeding) (collectively, "Loss") incurred or suffered by Buyer
or any of its Affiliates arising out of any misrepresentation or breach of
warranty, covenant or agreement made or to be performed by Seller pursuant to
this Agreement, including any failure by Seller to satisfy any of the Excluded
Liabilities and Obligations.

     6.02 Procedures; No Waiver; Exclusivity. (a) The party seeking
indemnification under Section 6.01 (the "Indemnified Party") agrees to give
prompt notice to the party against whom indemnity is sought (the "Indemnifying
Party") of the assertion of any claim, or the commencement of any suit, action
or proceeding in respect of which indemnity may be sought under such Section;
provided that the failure to give such notice shall not affect the Indemnified
Party's rights hereunder except to the extent the Indemnifying Party is
materially prejudiced by such failure. The Indemnifying Party may, and at the
request of the Indemnified Party shall, participate in and control the defense
of any such third party suit, action or proceeding at its own expense. The
Indemnifying Party shall not be liable under Section 6.01 for any settlement
effected without its prior consent of any claim, litigation or proceeding in
respect of which indemnity may be sought hereunder; provided that such consent
may not be unreasonably withheld.

     (b) No investigation by Buyer of the Enabling Agreements or otherwise shall
limit Buyer's rights to indemnification hereunder.

     (c) After the Closing, Section 6.01 will provide the exclusive remedy for
any misrepresentation, breach of warranty, covenant or other agreement or other
claim arising out of this Agreement or the transactions contemplated hereby.

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     (d) The representations, warranties, covenants and agreements contained
herein shall survive the Closing. The expiration of any term of this Agreement
shall not excuse any party hereto from its liability in respect of any breach
hereof prior to such expiration.

                                  ARTICLE VII

                                      TERM

     7.01 Term. This Agreement will expire 90 days after the termination or
expiration of the Roche License and the Novartis License; provided that the
Pharma Affiliates shall have received all applicable Seragen Royalty payments.

     7.02 Termination by Seller. This Agreement may be terminated by Seller
solely in the event that Buyer fails to make timely payment of the contingent
purchase price pursuant to Section 2.05 hereof, which failure is not cured
within sixty (60) days of written notice given by Seller to Buyer.

                                  ARTICLE VIII

                                 MISCELLANEOUS

     8.01 Notices. All notices, requests and other communications to either
party hereunder shall be in writing (including telex, telecopy, or similar
writing) and shall be given,

         (a)      if to Pharma Partners or either of the Pharma Affiliates, to:


                  c/o Pharmaceutical Partners, L.L.C.
                  675 Third Avenue
                  Suite 3000
                  New York, NY 10017
                  Attention:        Pablo Legorreta
                                    David Madden
                  Telecopy:         (917) 368-0021

                  with a copy to:

                  Testa, Hurwitz & Thibeault, LLP
                  125 High Street
                  Boston, MA  02110
                  Attention:        F. George Davitt
                  Telecopy:         (617) 248-7100

         (b)      if to Seller, to:

                  Seragen, Inc.
                  10275 Science Center Drive
                  San Diego, CA  92121

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                  Attention: General Counsel
                  Facsimile:        (858) 550-1825

         (c)      if to Ligand, to:

                  Ligand Pharmaceuticals Incorporated
                  10275 Science Center Drive
                  San Diego, CA  92121
                  Attention: General Counsel
                  Facsimile:        (858) 550-1825

or to such other address as any party may have furnished to the others in
writing in accordance herewith, except that notices of change of address shall
only be effective upon receipt. All notices and other communications given to
any party hereto in accordance with the provisions of this Agreement shall be
deemed to have been given on the date of receipt if delivered by hand or
overnight courier service or sent by fax prior to 4:00 p.m. (New York time) or
on the date five business days after dispatch by certified or registered mail if
mailed, in each case delivered, sent or mailed (properly addressed) to such
party as provided in this Section 10.01.

     8.02 Amendments; No Waivers. (a) Any provisions of this Agreement may be
amended or waived if, and only if, such amendment or waiver is in writing and
signed, in the case of an amendment, by Pharma Partners and each of the Pharma
Affiliates, Seller and, with respect to Article IX hereof, Ligand, or in the
case of a waiver, by the party against whom the waiver is to be effective.

     (b) No failure or delay by either party in exercising any right, power or
privilege hereunder shall operate as a waiver thereof nor shall any single or
partial exercise thereof preclude any other or further exercise thereof or the
exercise of any other right, power or privilege. The rights and remedies herein
provided shall be cumulative and not exclusive of any rights or remedies
provided by law.

     8.03 Expenses. Except as otherwise provided herein, all costs and expenses
incurred in connection with this Agreement shall be paid by the party incurring
such cost or expense.

     8.04 Successors and Assigns. The provisions of this Agreement shall be
binding upon and inure to the benefit of the parties hereto and their respective
successors and assigns. After the Closing, without limiting the generality of
the foregoing, nothing herein shall prohibit or restrict Pharma Partners or any
of the Pharma Affiliates from assigning any of its rights and obligations
hereunder to any Affiliate of Pharma Partners or any other Person; provided
that, without the consent of Seller and Ligand, no such assignment to a Person
who is not an Affiliate of Pharma Partners or the Pharma Affiliates shall
relieve Pharma Partners or the Pharma Affiliates from their obligations
hereunder.

     8.05 Governing Law; Jurisdiction. This Agreement shall be construed in
accordance with and governed by the law of the State of New York. Process in any
such suit, action or proceeding may be served on any party anywhere in the
world, whether within or without the jurisdiction of any such court.

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<PAGE>

     8.06 Counterparts; Effectiveness. This Agreement may be signed in any
number of counterparts, each of which shall be an original, with the same effect
as if the signatures thereto and hereto were upon the same instrument. This
Agreement shall become effective when each party hereto shall have received a
counterpart hereof signed by the other party hereto.

     8.07 Entire Agreement. This Agreement and the Exhibits and Schedules hereto
constitute the entire agreement between the parties with respect to the subject
matter hereof and supersede all prior agreements, understandings and
negotiations, both written and oral, between the parties with respect to the
subject matter of this Agreement. No representation, inducement, promise,
understanding, condition or warranty not set forth herein has been made or
relied upon by either party hereto. None of this Agreement, nor any provision
hereof, is intended to confer upon any Person other than the parties hereto any
rights or remedies hereunder.

     8.08 Captions. The captions herein are included for convenience of
reference only and shall be ignored in the construction or interpretation
hereof.


                                   ARTICLE IX

                                    GUARANTY

     9.1 The Guaranty. Ligand hereby unconditionally guarantees the full and
punctual performance of the obligations of Seller under this Agreement. Upon
failure by the Seller to pay punctually any such amount, Ligand shall forthwith
on demand pay the amount not so paid at the place and in the manner specified in
this Agreement.

     9.2 Guaranty Unconditional. The obligations of Ligand hereunder shall be
unconditional and absolute and, without limiting the generality of the
foregoing, shall not be released, discharged or otherwise affected by:

     (a) any extension, renewal, settlement, compromise, waiver or release in
respect of any obligation of Seller under this Agreement, by operation of law or
otherwise;

     (b) any modification or amendment of or supplement to this Agreement or the
Enabling Agreements;

     (c) any change in the corporate existence, structure or ownership of
Seller, or any insolvency, bankruptcy, reorganization or other similar
proceeding affecting the Seller or its assets or any resulting release or
discharge of any obligation of Seller contained in this Agreement;

     (d) the existence of any claim, set-off or other rights which Ligand may
have at any time against Seller;

     (e) any invalidity or unenforceability relating to or against Seller for
any reason of this Agreement, or any provision of applicable law or regulation
purporting to prohibit the payment by Seller of the Seragen Royalty; or

                                       12

<PAGE>

     (f) any other act or omission to act or delay of any kind by Seller or any
other corporation or Person or any other circumstance whatsoever which might,
but for the provisions of this paragraph, constitute a legal or equitable
discharge of Ligand's obligations hereunder.

     9.3 Discharge Only Upon Payment in Full; Reinstatement in Certain
Circumstances. Ligand's obligations hereunder shall remain in full force and
effect until this Agreement shall have terminated and all amounts payable by
Seller under this Agreement shall have been paid in full. If any time any
payment of the Seragen Royalty or any other amount payable by the Seller under
this Agreement is rescinded or must be otherwise restored or returned upon the
insolvency, bankruptcy or reorganization of Seller or otherwise, Ligand's
obligations hereunder with respect to such payment shall be reinstated as though
such payment had been due but not made at such time.

     9.4 Waiver by Ligand. Ligand irrevocably waives acceptance hereof,
presentment, demand, protest and any notice not provided for herein, as well as
any requirement that at any time any action be taken by any corporation or
Person against Seller or any other corporation or Person.




                                       13

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed by their respective authorized officers as of the day and year
first above written.

                     SERAGEN, INC.


                     By:      /s/ William L. Respess
                              Name:     William L. Respess
                              Title:   Vice President, General Counsel


                     LIGAND PHARMACEUTICALS INCORPORATED


                     By:      /s/ William L. Respess
                              Name:     William L. Respess
                              Title:   Senior Vice President, General
                                       Counsel, Government Relations


                     PHARMACEUTICAL PARTNERS, L.L.C.


                     By:      /s/ David Madden
                              David Madden
                              Managing Member


                     BIOVENTURE INVESTMENTS, Kft


                     By:      /s/ illegible
                              Name:  illegible
                              Title:  Managing Director


                     PHARMACEUTICAL ROYALTIES, LLC

                     By:  PHARMACEUTICAL ROYALTIES, LLC
                           Managing Member

                           By:  /s/ David Madden
                                 David Madden
                                 Managing Member

<PAGE>
                                                                      SCHEDULE A

                                 PATENT MATTERS


U.S. Pat. No. 5,011,684
U.S. Pat. No. 5,336,489
U.S. Pat. No. 5,510,105
U.S. Pat. No. 5,587,162
U.S. Pat. No. 5,607,675
U.S. Pat. No. 5,674,494
U.S. Pat. No. 5,916,559
         ***

Australian Pat. No. 575,210
Canadian Pat. No. 1,275,951
New Zealand Pat. No. 213,983

All patents and patent applications are owned by Beth Israel Hospital
Association (now known as Beth Israel Deaconess Medical Center).


   *** Portions of this page have been omitted pursuant to a request for
Confidential Treatment and filed separately with the Commission.

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