Document:

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                                                                   Exhibit 10.43

                      FIRST AMENDMENT TO LICENSE AGREEMENT

     This First Amendment is made and entered into as of the thirteenth day of
December, 1999, by and between HESKA CORPORATION, a Delaware corporation having
its principal place of business at 1613 Prospect Parkway, Fort Collins, Colorado
80525 (Facsimile: 970-484-9505) ("HESKA") and IMMULOGIC PHARMACEUTICAL
CORPORATION, a Delaware corporation having its principal place of business at
610 Lincoln Street, Waltham, Massachusetts 02154 (Facsimile: 781-466-6050)
("IMMULOGIC").

     WHEREAS, HESKA and IMMULOGIC are parties to that certain License Agreement
dated as of June 16, 1998 (the "LICENCE AGREEMENT"); and

     WHEREAS, the Licensed Technology described in EXHIBIT A attached hereto is
owned by IMMULOGIC (the "OWNED TECHNOLOGY") and consists of (i) the patent and
patent applications assigned solely to IMMULOGIC and listed on Exhibit A-1 and
(ii) IMMULOGIC's ownership rights in the patents and patent applications
assigned to IMMULOGIC and another party and listed on Exhibit A-2; and

     WHEREAS, IMMULOGIC owns the ALLERVAX(TM) trademark and all registrations
and goodwill associated therewith (also referred to herein as part of the "Owned
Technology"); and

     WHEREAS, IMMULOGIC and Sankyo Co. Ltd. Are parties to that certain Patent
License Agreement dated as of May 22, 1998, a copy of which is attached as
Exhibit E hereto (the "Sankyo License"); and

     WHEREAS, the Licensed Technology described in EXHIBIT B attached hereto is
licensed by IMMULOGIC from third parties and sublicensed to HESKA pursuant to
the License Agreement ("SUBLICENSED TECHNOLOGY") and consists of (i)) the patent
and patent applications assigned to another party and licensed exclusively to
IMMULOGIC and listed on Exhibit B-1 and (ii) IMMULOGIC's exclusive license
rights in the patents and patent applications assigned to IMMULOGIC and another
party and listed on Exhibit B-2; and

     WHEREAS, HESKA and IMMULOGIC desire to amend the License Agreement as set
forth in this First Amendment;

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto agree as
follows:

     1.   DEFINITIONS. Capitalized terms used in this First Amendment (and the
Recitals above) shall have the meanings ascribed to them in the License
Agreement, unless otherwise expressly defined herein.

     2.   AMENDMENTS TO LICENSE AGREEMENT. Upon execution and delivery of this
First Amendment, HESKA shall pay to IMMULOGIC Three Hundred Thousand Dollars
($300,000), in immediately available funds (the "BUYOUT PAYMENT"). In
consideration of the Buyout Payment, the License Agreement is hereby amended as
follows:

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          (a)  CONVEYANCE OF OWNED TECHN0LOGY TO HESKA.

               (i)  IMMULOGIC hereby conveys, transfers and sets over to HESKA
all right title and interest in and to the Owned Technology. IMMULOGIC hereby
agrees, at its own expense, to do all such things and take all such actions, and
to make, execute and deliver all such other documents and instruments, including
but not limited to patent assignments, trademark assignments, copyright
assignments, bills of sale, and other conveyance documents, as shall be
reasonably requested by HESKA to implement the foregoing conveyance of the Owned
Technology and otherwise carry out the provisions, intent and purpose of this
First Amendment.

               (ii) IMMULOGIC hereby represents and warrants to HESKA that
(A) IMMULOGIC has, and conveys to HESKA hereunder, good and marketable title to
the Owned Technology, free and clear of all liens, encumbrances and restrictions
of any kind whatsoever (other than as expressly set forth in EXHIBIT A) and (B)
the representations and warranties of IMMULOGIC set forth in Sections 7.1(b),
(e), (f) and (g) of the License Agreement are true and correct with respect to
the Owned Technology and this First Amendment as if made on the date of this
First Amendment.

          (b)  ASSIGNMENT AND ASSUMPTION OF SUBLICENSED TECHNOLOGY.

               (i)  Promptly after the execution and delivery of this First
Amendment, but in no event after June 30, 2000, IMMULOGIC shall cause each
licensor of the Sublicensed Technology to execute and deliver to HESKA an
Assignment and Assumption Agreement in the form of EXHIBIT C attached hereto,
which shall incorporate the terms and conditions set forth in EXHIBIT D attached
hereto relating to each respective licensor (each, an "Assistant Agreement").

               (ii) If IMMULOGIC shall fail to deliver any Assignment Agreement
when due, HESKA may exercise any and all of its rights and remedies for breach
of the License Agreement, and without limiting the generality of the foregoing,
if IMMULOGIC shall fail to deliver all Assignment Agreements on or before the
first date that a milestone payment is otherwise due and payable under Section
3.1(b)(ii) or Section 3.1(b)(iii) of the License Agreement, HESKA may, by
written notice to IMMULOGIC, terminate its obligations under such Sections
3.1(b)(ii) and 3.1(b)(iii).

               (iii) If IMMULOGIC in good faith seeks to obtain an Assignment
Agreement from any licensor and such licensor refuses in writing to enter into
such Assignment Agreement due to any provisions of the applicable Exhibit D,
IMMULOGIC shall be released from its obligation to deliver the Assignment
Agreement unless HESKA agrees to waive such provision (but not any other
provision) of Exhibit D to which the licensor will not agree within 10 days of
written notice from IMMULOGIC. In the event IMMULOGIC is released from its
obligation to deliver a particular Assignment Agreement under this subparagraph,
the provisions of section 2(b)(ii) shall not apply with respect to that
Assignment Agreement.

          (c)  ELIMINATION OF ROYALTIES AND CERTAIN MILESTONE PAYMENTS. The
following sections of the License Agreement are hereby deleted in their entirety
and shall have no further force and effect: Section 3.1(b)(i), Section 3.2(a),
Section 3.2(b), Section 3.2(c), Section 3.3, Section 3.4(a) and Section 3.4(c).
Without limiting the generality of the foregoing, no further

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consideration shall be payable by HESKA to IMMULOGIC under the License Agreement
except for the amounts payable, if any, pursuant to the terms and conditions of
Section 3.1(b)(ii) and Section 3.1(b)(iii). Article 4 of the License Agreement
shall apply only with respect to payments, if any, due under Section 3.1(b)(ii)
and Section 3.1(b)(iii).

          (d)  COVENANT NOT TO COMPETE. Article 5 of the License Agreement shall
continue to apply to the Owned Technology and Sublicensed Technology, for a
period of five (5) years after the date of this First Amendment. The last
sentence of Article 5 of the License Agreement is hereby deleted in its
entirety.

          (e)  PATENT PROSECUTION: INFRINGEMENT ACTIONS. IMMULOGIC's rights and
obligations under Article 6 of the License Agreement with respect to the Owned
Technology shall be of no further force and effect after the date of this First
Amendment.

          (f)  ASSUMPTION OF SANKYO LICENSE. IMMULOGIC hereby conveys and
assigns to HESKA, as of the date hereof, all of IMMULOGIC's right, title and
interest in and to the Sankyo License, and HESKA hereby assumes the Sankyo
License as licensor thereunder and agree to perform all obligations to be
performed thereunder on and after such date (but not other obligations).
IMMULOGIC represents and warrants that IMMULOGIC has performed all of its
obligations under the Sankyo License through the date hereof.

     3.   EFFECT OF THIS FIRST AMENDMENT. In the event of any conflict between
the terms and conditions of this First Amendment and the License Agreement, this
First Amendment shall govern and control. Upon execution and delivery of this
First Amendment, the License Agreement shall continue in full force and effect,
as amended by the express terms of this First Amendment.

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     IN WITNESS WHEREOF, the parties have executed this First Amendment by their
duly authorized representatives, effective as of the date on which this First
Amendment has been duly signed by both parties, which date will be inserted at
the top of this Agreement.

                                    IMMULOGIC PHARMACEUTICAL
                                    CORPORATION

                                    By /s/ Richard Crowley
                                       ------------------------------
                                       Richard Crowley
                                       President

                                    HESKA CORPORATION

                                    By /s/ Paul Hudnut
                                       ------------------------------
                                       Paul Hudnut
                                       Executive Vice President

EXHIBITS:
--------

A        Owned Technology
B        Sublicensed Technology
C        Form of Assignment and Assumption Agreement
D        Individual Terms and Conditions of Each Assignment Agreement
E        Sankyo License

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                                    EXHIBIT A

                    TO FIRST AMENDMENT TO LICENSE AGREEMENT

                                Owned Technology

PATENT AND PATENT ASSIGNMENTS

The patents and patent applications listed on Exhibit A-1 and IMMULOGIC's
ownership rights in the patents and patent applications assigned to IMMULOGIC
and another party and listed on Exhibit A-2.

TRADEMARKS

ALLERVAX

LIENS AND ENCUMBRANCES

Patent License Agreement, dated May 22, 1998 between ImmuLogic Pharmaceutical
Corporation and Sankyo Co. Ltd., a correct and complete copy of which is
attached to this First Amendment to License Agreement as Exhibit E.

KNOW-HOW AND BIOLOGICAL MATERIALS

As stated in the License Agreement and its exhibits.

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                                                                   Exhibit 10.44

                           SECOND AMENDMENT TO LEASE

     This Second Amendment to Lease ("Amendment") dated as of July 30, 1999 and
effective as of August 1, 1999 and is made by and among Lincoln Street Trust, as
Landlord ("Landlord"), Immulogic Pharmaceutical Corporation ("Immulogic") and
Scriptgen Pharmaceuticals, Inc. ("Scriptgen").

                                    RECITAL

     WHEREAS, Immulogic entered into that certain Lease dated November 29, 1991,
and an Amendment thereto dated May 8, 1992 (as amended, the "Lease") for the
entire building located at 610 Lincoln Street, Waltham, Massachusetts (the
"Premises"):

     WHEREAS, subsequently Immulogic sublet the Premises pursuant to a Sublease
dated January 22, 1998 by and between Immulogic and Scriptgen whereby Scriptgen
subleased substantially all of the Premises (the "Sublease");

     WHEREAS, Immulogic, Scriptgen and Landlord entered into that certain letter
agreement dated February 16, 1998 whereby Scriptgen agreed to certain covenants
in connection with the Sublease (the "Letter Agreement");

     WHEREAS, Immulogic desires to be released of its obligations under the
Lease;

     WHEREAS, Scriptgen desires to become the Tenant under the Lease; and

     WHEREAS, Landlord has agreed to relieve Immulogic of its obligations under
the Lease and recognize Scriptgen as Tenant by amending the Lease and
substituting Scriptgen as the tenant thereunder subject to the following terms
and conditions contained herein.

     NOW, THEREFORE FOR GOOD AND VALUABLE CONSIDERATION, the receipt and
sufficiency of which is hereby acknowledged, the parties agree as follows:

     1.   Immulogic's interest in the Lease is hereby assigned to Scriptgen.
          Scriptgen hereby assumes all liability and obligations of Immulogic as
          Tenant under the Lease on and after the date of this Amendment, and
          Immulogic is hereby released from all liability and obligations under
          the Lease which shall accrue after the date of this Amendment.
          Notwithstanding the foregoing, Scriptgen shall be liable as if
          Scriptgen was the original tenant named in the Lease for any and all
          claims relating to hazardous substances as defined in Section 5.2 of
          the Lease caused by Scriptgen, its agents, employees, invitees,
          licensees and contractors and for any acts or omissions of Scriptgen,
          its agents, employees, invitees, licensees and contractors during
          Scriptgen's occupancy of the Premises under the Sublease. The Sublease
          is hereby terminated. However, the covenants set forth in the Letter

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          Agreement shall survive termination of the Sublease and are hereby
          incorporated in the Lease and shall be binding on Landlord and
          Scriptgen.

     2.   The following shall be deleted following the term "TENANT" in Section
          1.1, "Immulogic Pharmaceutical Corporation" and "SCRIPTGEN
          Pharmaceuticals, Inc." shall be substituted in lieu thereof.

     3.   The following shall be deleted following the term "TENANT'S ORIGINAL
          ADDRESS" in Section 1.1 "One Kendall Square, Building 600, Cambridge,
          MA 02130" and "610 Lincoln Street, Waltham MA 02254" shall be
          substituted in lieu thereof.

     4.   The following shall be deleted after the term "ANNUAL FIXED RENT" in
          Section 1.1:

                  Lease Years 1 and 2:       $1,055,060.50
                  Lease Year 3:              $1,097,777.50
                  Lease Year 4:              $1,183,205.50
                  Lease Year 5:              $1,225,920.50
                  Lease Year 6 through 8:    $1,268,635.50
                  Lease Years 9 and 10:      $1,311,350.60

          and "Annual Fixed Rent through August 31, 2002 is $2,392,040.04
          annually" shall be inserted in lieu thereof. The Annual Fixed Rent
          includes all tenant improvements, fixtures and other property owned by
          Landlord (or transferred to Landlord hereunder) currently used by
          Scriptgen at the Premises.

     5.   The following shall be deleted in Section 1.1 of the Lease: "Security
          Deposit: $750,000," "Broker: R.M. Bradley & Co., Inc. and Lynch,
          Murphy, Walsh and Partners," and "Tenant's Authorized Representative:
          Janet C. Bush."

     6.   The following shall be deleted from Section 1.2 of the Lease: "Exhibit
          B. Construction Costs Disbursement Schedule."

     7.   Article III of the Lease, entitled "Construction of Premises," shall
          be deleted in its entirety.

     8.   The second sentence in Section 5.2 shall be deleted in its entirety
          and replaced with the following: "Tenant shall obtain and maintain all
          permits necessary for all operations conducted in the Premises, and
          shall comply with the terms thereof."

     9.   Section 8.6 of the Lease shall be deleted in its entirety and the
          following shall be substituted in lieu thereof:

          "Each party warrants to the other that it has had no dealings with any
          broker or agent in connection with this Lease. Each party covenants to
          pay, hold harmless

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          and indemnify the other from and against any and all cost, expense or
          liability for any compensation, commissions and charges claimed by any
          broker or agent with respect to this Lease or the negotiation thereof,
          or arising from a breach of the foregoing warranty."

     10.  The following is hereby added as a new section, 8.18:

          8.18 CONSTRUCTION OF ADDITION

               Landlord shall have the right to construct an addition to the
          Premises (the "Addition") and, in connection with the development of
          the Addition, Landlord shall have the right to make alterations to the
          exterior of the Premises and building systems located therein; to
          utilize the Premises for access and egress to and from the Addition or
          utilities serving the Addition; to alter or expand the loading
          facilities at the Premises and to utilize the loading facilities for
          the Addition in common with Premises; to shore up the foundations
          and/or walls of the Premises; to erect scaffolding and protective
          barricades around, within or adjacent to the Premises; and to do any
          other act necessary for the safety of the Premises or the development
          of the Addition. Landlord shall not be liable to Tenant for any
          compensation or reduction of rent by reason of inconvenience or
          annoyance or for loss of business resulting from any act by Landlord
          pursuant to this Section 8.18, provided that Landlord shall not
          materially interfere with Tenant's use of the Premises and shall use
          reasonable efforts to minimize the extent and duration of any
          inconvenience, annoyance or loss of business to Tenant resulting from
          any work pursuant to this Section in or about the Premises, consistent
          with accepted construction practice.

     11.  The Lease is hereby amended by adding a new Article X thereto, to read
          in its entirety as follows, and simultaneously with the execution of
          this Amendment Immulogic shall pay over to Landlord the Security
          Deposit currently held by Immulogic under the Sublease, which shall
          become the Security Deposit under the Lease:

          The Security Deposit will be held by Landlord in an interest-bearing
          account for and during the Term, which deposit shall be returned to
          Tenant within thirty (30) days after the expiration or termination of
          the Lease, provided there exists no breach of any undertaking of
          Tenant. Provided Tenant is not in default of this Lease, all interest
          on the Security Deposit shall be paid to Tenant on an annual basis on
          each anniversary of the Commencement Date. If all or any part of the
          Security Deposit is applied to an obligation of Tenant hereunder,
          Tenant shall immediately, upon request by Landlord, restore the
          Security Deposit to its original amount. Tenant shall not have the
          right to call upon Landlord to apply all or any part of the Security
          Deposit to cure any default or fulfill any obligation of Tenant, by
          such use shall solely be in the discretion of the Landlord. Provided
          Tenant is

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          not in default of any of its obligations hereunder, the Security
          Deposit shall be reduced to $250,000.00 and $250,000.00 returned to
          the Tenant within ten (10) days after Tenant provides evidence
          reasonably satisfactory to Landlord that Tenant has successfully
          completed a public offering of Tenant's common stock, which raised not
          less than $30,000,000.00.

     12.  Exhibit B to the Lease shall be deleted in its entirety.

     13.  Each of Landlord and Immulogic covenants, warrants and represents
          that: (i) the copy of the Lease and the Letter Agreement attached
          hereto as EXHIBIT A is a true and correct copy of the Lease and the
          Letter Agreement and there are no amendments or modifications thereto
          except as included in said EXHIBIT A; (ii) the Lease and the Letter
          Agreement are a complete statement of the agreement of the parties
          thereto with respect to the use and occupancy of the Premises; (iii)
          the Lease and the Letter Agreement are in full force and effect; (iv)
          it has not given or received any notice of default or notice of
          termination of the Lease or the Letter Agreement, and is not in
          default of any monetary obligations or, to the best of its knowledge,
          in the performance of or compliance with any material provisions of
          the Lease and the Letter Agreement, and to the best of its knowledge
          no facts or circumstances exist which, with the giving of notice or
          the passage of time or both, would constitute such a default
          thereunder (Landlord waives its rights in connection with any
          nonmonetary defaults under the Lease of which it is not now aware
          except for defaults arising from the acts or omissions of Scriptgen,
          its agents, employees, invitees, licensees and contractors during
          Scriptgen's occupancy of the Premises under the Sublease), and (v) to
          the best of its knowledge, each other party to the Lease and the
          Letter Agreement is not in default in the performance of or compliance
          with any material provisions of the Lease and the Letter Agreement,
          and to the best of its knowledge no facts or circumstances exist
          which, with the given of notice or the passage of time or would
          constitute such a default thereunder.

     14.  Scriptgen covenants, warrants and represents that (i) it has not given
          or received any notice of default or notice of termination of the
          Sublease or the Letter Agreement and it is not in material default in
          the performance or compliance of any of the material provisions of the
          Sublease or the Letter Agreement and no facts or circumstances exist
          which would, with the giving of notice or the passage of time or both
          would constitute a default thereunder; and (ii) to the best of its
          knowledge, Landlord is not in default in the performance of or
          compliance with any material provisions of the Lease or the Letter
          Agreement, and no facts or circumstances exist which, with the given
          of notice or the passage of time or both, would constitute such a
          default thereunder. Scriptgen hereby agrees that (i) the copy of the
          Lease and the Letter Agreement attached hereto as Exhibit A is a true
          and correct copy of the Lease and the Letter Agreement and there are
          no amendments or modifications thereto except as included in said
          Exhibit A; (ii) the Lease and the Letter Agreement are a complete
          statement of the agreement of the

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          parties thereto with respect to the use and occupancy of the Premises;
          (iii) to the best of Scriptgen's knowledge, the Lease and the Letter
          Agreement are in full force and effect.

     15.  Immulogic hereby transfers, assigns and conveys to Landlord all of its
          right, title and interest in all tenant improvements, fixtures or any
          other personal property of any kind on, under, over or about the
          Premises and hereby warrants and represents that all such property is
          owned by Immulogic free and clear of any liens, security interests, or
          adverse claims (other than rights of Scriptgen under the Sublease).

     16.  All other terms and provisions of the Lease are hereby ratified and
          confirmed.

     17.  Notwithstanding anything contained in this Amendment to the contrary,
          this Amendment shall not be effective and bind the parties until the
          Landlord's lender has consented to this Amendment, it being understood
          that the consent of Landlord's lender is a condition to this
          Amendment.

     18.  This Amendment may be signed in counterparts, all of which taken
          together shall constitute one agreement, provided that it shall not be
          binding upon any party hereto unless and until at least one
          counterpart shall have been executed and delivered by each party
          hereto.

                            SIGNATURE PAGE TO FOLLOW

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     IN WITNESS WHEREOF, the parties have executed this First Amendment to Lease
as of the date first written above as a sealed instrument.

LINCOLN STREET TRUST                         SCRIPTGEN PHARMACEUTICALS, INC.
Landlord

By: /s/ Michael D. Bank                      By: /s/ John Barberich
   ------------------------                     ------------------------
   Name: Michael D. Bank                        Name: John Barberich
   Title: Managing Director                     Title: Vice President & CFO

                                             IMMULOGIC PHARMACEUTICAL
                                             CORPORATION

                                             By: /s/ J. Richard Crowley
                                                ------------------------
                                                Name: J. Richard Crowley
                                                Title: President

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