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                                                                EXHIBIT 10.15

                            [SCHERING LETTERHEAD]

NeXstar Pharmaceuticals, Inc.                                        Schering AG
Attn. Mr. Mahaffy
2860 Wilderness Place
Boulder, CO 80301
USA

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<S>         <C>                <C>                                        <C>                                 <C>
Your Ref.   Your letter dated  Our Ref. (please indicate when replying)   Telephone: (030)488-0/468 4380      Date
                               Legal Department                           Telefax: (030) 468 4086              1997-02-12
                               Christiane Seiler/Pa
                               (7B2801-2)
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AMENDMENT TO THE COLLABORATIVE RESEARCH AGREEMENT AND LICENSE AGREEMENT
OF NOVEMBER 1993

Dear Mr. Mahaffy:

We refer to your discussions with Dr. Scholz about an amendment of the above
referenced Agreements with respect to additional research funding. We have
further noted that a few of the terms of the Collaborative Research Agreement
and of the License Agreement should be clarified. We propose to agree to the
provisions set forth below. All capitalized terms in this letter shall, if not
otherwise expressly indicated, have the meaning defined in the Collaborative
Research Agreement.

1.          SCHERING AKTIENGESELLSCHAFT ("SCHERING") and NEXAGEN, INC, a
            predecessor to NEXSTAR PHARMACEUTICALS, INC ("NEXSTAR"), entered
            into a Collaborative Research Agreement and a License Agreement in
            November, 1993. SCHERING and NEXSTAR now wish to enter into this
            letter agreement to amend certain provisions of the Collaborative
            Research Agreement and the License Agreement effective February 1,
            1997.
        
2.          SCHERING and NEXSTAR wish to intensify their research collaboration
            under the Collaborative Research Agreement with respect to certain
            projects for tumor imaging within the Field (the "New Projects").
            The parties acknowledge that the New Projects may or may not be
            successful and may be terminated prematurely.
        
2.1         SCHERING agrees to make additional annual payments of U.S.  
            $1,400,000 (one million and four hundred thousand U.S. dollars) to
            NEXSTAR for each twelve (12) month period beginning on February 1st
            of a calendar year ("Amendment Year") which is intended to
            compensate NEXSTAR for the research performed by seven (7)
            additional FTE's. The first Amendment Year will begin on February
            1, 1997. The seven (7) additional FTE's shall focus on the New
            Projects which New Projects shall be incorporated in the Research
            Program and the Annual Research Plan.
        
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                                                                        SCHERING
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Address                                   Our Ref.      Date           Page
NeXstar Pharmaceuticals, Inc.             RA/Se/Pa      1997-02-12     2
Attn. Mr. Mahaffy

2.2         SCHERING shall make the annual payments of U.S. $1,400,000 (one
            million and four hundred thousand U.S. dollars) per Amendment 
            Year in four (4) equal instalments of U.S. $350,000 (three hundred
            and fifty thousand U.S.dollars) to be made quarterly within five
            (5) days of the beginning of the calendar months February, May,
            August and November of each Amendment Year. However, the first
            instalment shall only be due ten (10) days after receipt of the 
            counter-signed original of this letter by SCHERING.

2.3         SCHERING's obligation to make the payments referenced in Section
            2.1 and 2.2 of this letter shall terminate:

            (a)   completely in the event that the Collaborative Research 
                  Agreement is terminated by either party for whatever reason;
                  or

            (b)   completely or partially if the Joint Management Committee or
                  either SCHERING or NEXSTAR determines and gives the other 
                  party six (6) months notice of such determination in writing
                  that any of the New Projects shall not be further pursued; or

            (c)   at the latest and automatically, without requiring
                  a termination notice, on January 31, 1999 if it has not been
                  terminated previously according to this Section 2.3.

3.          In the event NEXSTAR incurs extraordinary expenses relating to
            oligonucleotide synthesis for any Research Program carried out
            under the Annual Research Plans, SCHERING and NEXSTAR agree that 
            they will discuss and SCHERING will reasonably consider 
            reimbursement to NEXSTAR of some or all of these extraordinary 
            expenses. However, the parties understand that SCHERING shall be
            under no obligation to make such reimbursement to NEXSTAR.

4.          The parties also wish to clarify the following with respect to the
            Collaborative Research Agreement:

            4.1   For reference purposes, the date of the Collaborative 
                  Research Agreement shall be deemed to be November 16, 1993.

            4.2   Effective Date means November 16, 1993.

            4.3   Commitment Year means a twelve month period commencing each 
                  November 16.

            4.4   Notwithstanding the provisions of Section 1.2 of
                  the original form of the Collaborative Research Agreement
                  (providing that after the first three Commitment Years the
                  amount per FTE for each Commitment Year shall increase by the
                  percentage increase in the Consumer Price Index, National,
                  All Products, for the prior Commitment Year), the Annual
                  Commitment shall continue to be U.S. $1,000,000 (one million
                  U.S. dollars) for each subsequent Commitment Year effective
                  November 17, 1996 and is intended to continue to compensate
                  NEXSTAR for the research performed by five (5) FTE's per
                  year. 
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                                                                        SCHERING
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Address                                   Our Ref.       Date            Page 
NeXstar Pharmaceuticals, Inc.             RA/Se/Pa       1997-02-12      3 
Attn. Mr. Mahaffy

5.   All terms of the License Agreement between the parties of November 16, 1993
     shall remain in full force and effect, except that the date of October 1,
     1998 set out in the first line of Section 4.2 (a) of the License Agreement
     shall not be applicable for any result deriving from a New Project. With
     respect to New Projects, the date referred to in the first line of Section
     4.2 (a) of the License Agreement shall be January 31, 2002.
        
6.   All terms and conditions of the Collaborative Research Agreement and the 
     License Agreement which are not expressly changed by this letter shall
     remain in full force and effect and shall govern the provisions set forth
     in this amendment letter. Effective February 1, 1997 the Collaborative
     Research Agreement and the License Agreement shall be amended as set out
     in this amendment letter.
        
If you agree with the provisions set forth above, please countersign the
attached original and return it to us.



Yours sincerely,
SCHERING AKTIENGESELLSCHAFT
                    ppa.



/s/ PROF. STOCK               /s/ DR. SCHOLZ
Prof. Stock                   DR. Scholz



                                          Date:
                                          Accepted by:




                                          /s/ PATRICK J. MAHAFFY
                                          -----------------------------
                                          NEXSTAR PHARMACEUTICALS, INC.