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                                                                   EXHIBIT 10.56

                       LICENSE AND DISTRIBUTION AGREEMENT

This Agreement is between Sumitomo Pharmaceuticals Co., Ltd. ("Sumitomo") and
NeXstar Pharmaceuticals, Inc. ("NeXstar") in connection with registration, sale
and marketing of NeXstar's product AmBisome in Japan. The term "AmBisome"
shall mean, for the purpose of this Agreement, a liposomal antifungal agent
having amphotericin B as an effective substance and other compositions as
defined in Exhibit A attached to this Agreement. NeXstar has developed and is
now selling AmBisome elsewhere in the world. NeXstar has generated, and will
continue to generate, data (the "Product Data") which in its present form is
sufficient to have obtained product registrations for AmBisome from medicines
control agencies in 23 countries. AmBisome is protected in Japan by Patent No.
1890276 and Patent Application No. 284828/88 (Patent Rights), and the trademark
AmBisome is protected in Japan by registered No. 2430891 (Trademark), all in
the name of NeXstar Pharmaceuticals, Inc.  NeXstar and Sumitomo desire to have
the AmBisome product, as it is registered in various countries elsewhere in the
world, developed, registered and marketed in the Japanese market, and therefore
agree as follows. 

1.      LICENSED RIGHTS. NeXstar grants to Sumitomo an exclusive license with a
        right to sublicense under the Patent Rights and Trademark in Japan for 
        the sale and use of AmBisome in Japan for the term of this Agreement. 
        Before Sumitomo exercises its right to sublicense AmBisome to another 
        party, Sumitomo obtains NeXstar's prior approval, such approval not 
        being unreasonably withheld.

2.      SUPPLY OF PRODUCT. All AmBisome to be transferred to Sumitomo under the
        terms of this Agreement (Product) shall be manufactured by NeXstar and
        supplied by NeXstar in nude vials at NeXstar's price of vial in U.S.
        dollars, C.I.F. Osaka.
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        2-1.    [Confidential Material omitted and filed separately with the
                Securities and Exchange Commission pursuant to Section 24b-2 of 
                the Securities Exchange Act of 1934.]

        2-2.    All the product will be packaged and labeled by Sumitomo in
                accordance with all regulatory requirements for use and/or sale
                of AmBisome in Japan.

        2-3.    Order and Forecast

                At the beginning of April and October in every year, Sumitomo
                shall provide to NeXstar a forecast of the number of vials of
                AmBisome required for its sales in the next six (6) months
                starting in April and October respectively. Sumitomo shall place
                firm orders based upon such forecast two (2) months before
                deliveries.

        2-4.    Specification

                All the Product shall meet the specifications set forth in
                Exhibit B attached to this Agreement ("Specification"). The
                analytical test method of Product (Analytical Test Method) for
                the Specification shall be specified in the Exhibit C attached
                to this Agreement. If such Product is found not to meet the
                Specification by Sumitomo using Analytical Test Method and by
                NeXstar with a confirmatory test using the same method, NeXstar
                shall supply Sumitomo with new AmBisome for the replacement at
                NeXstar's cost. In case the test results are different between 
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                Sumitomo and NeXstar, both parties shall discuss the cause of
                the difference and decide how to handle the matter. In case
                any revision or supplement to the Specification and/or
                Analytical Test Method, the parties shall discuss and agree on
                the revision of Exhibit B and C. In case an unexpected incident,
                whatsoever it is, is caused by anything not being prescribed by
                Specification, both parties shall meet and discuss how to deal
                with the situation. NeXstar shall not change or modify the
                manufacturing process of AmBisome without prior written consent
                of Sumitomo.

        2-5.    Sumitomo shall make payment of the Product in U.S. dollars
                within thirty (30) days after the confirmation that Product
                meets the Specification by Sumitomo.

        2-6.    Sumitomo shall have the right to inspect NeXstar's plant to
                check GMP conformance.

3.      INFORMATION AND CLINICAL TRIALS. Sumitomo, with the Product Data and
        cooperation from NeXstar, will seek the registration of AmBisome as a
        licensed pharmaceutical with the Japanese Ministry of Health and Welfare
        (the "Registration") at its own cost. Sumitomo will conduct any
        additional preclinical studies and clinical trials required for the
        Registration. NeXstar will supply up to 63,000 vials of Product for such
        preclinical and/or clinical studies to Sumitomo free of charge. Should
        Sumitomo require more than 63,000 vials for such preclinical and/or
        clinical testing, such further Product will be supplied at a cost to
        Sumitomo of twenty-five dollars ($25.00) per vial. The Product Data to
        be supplied to Sumitomo will be used solely for the purpose of the
        Registration.

        3-1.    Information Exchange

                Until the launch of AmBisome in Japan, both parties will meet at
                least annually to discuss and to exchange information on the
                AmBisome. During the development period Sumitomo shall send to
                NeXstar progress reports semiannually. Sumitomo shall provide
                NeXstar with copies of all data, analyses and reports of
                additional
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                studies conducted by Sumitomo solely for NeXstar's use in
                seeking additional or expanded registrations of AmBisome in the
                territories outside Japan.

        3-2.    Sumitomo shall be responsible for decisions concerning the
                design and execution of the application for the Registration,
                and the conduct of additional preclinical and clinical studies
                required for the Registration. NeXstar shall collaborate with
                Sumitomo for such decision making.

        3-3.    Manufacturing Information

                NeXstar shall provide Sumitomo with sufficient manufacturing
                information on AmBisome for the Registration in Japan. Sumitomo
                agrees to maintain manufacturing information in strict
                confidence and not to disclose the same to any other party 
                without NeXstar's prior written consent and not to use such 
                information for any other purpose than Registration.
        
        3-4.    Adverse Event Report

                Each party shall provide to the other adverse event reports at
                the same time they are provided to the regulatory authorities as
                required by law.

4.      [Confidential Material omitted and filed separately with the 
        Securities and Exchange Commission pursuant to Section 24b-2 of the 
        Securities Exchange Act of 1934.]

5.      ROYALTIES.

        5-1.    [Confidential Material omitted and filed separately with the 
                Securities and Exchange Commission pursuant to Section 24b-2 of 
                the Securities Exchange Act of 1934.]

              
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        5-2.    When Patent Rights no longer exists, the royalty will be
                reduced to two (2) percent of Net Sales.  In the event that a 
                true generic form of AmBisome were to be marketed in Japan the
                royalty would drop to 1%.

        5-3.    Net Sales

                Net Sales shall mean the total of the invoiced sales price of
                AmBisome to wholesalers recorded as revenue by Sumitomo, less

                (1)     actual credited allowances to, and

                (2)     chargebacks from the account of such unaffiliated 
                        customers for damaged or out-dated and returned 
                        product, to the extent such amounts are included in the
                        invoiced sales price,

                (3)     rebates and

                (4)     discount.

        5-4.    Withholding tax.

                All payments to be made under this Agreement shall where
                required by the applicable laws of Japan be made after deduction
                of withholding and other taxes.

6.      MARKETING.  Sumitomo shall distribute the AmBisome only under the
        AmBisome Trademark in Japan, and shall not gain any vested trademark
        rights in the Trademark thereby.  NeXstar shall be responsible for
        keeping the Trademark effective at NeXstar's expense in Japan.  By the
        end of March and September in every year, Sumitomo shall submit to
        NeXstar a marketing plan, which shall explain Sumitomo's proposed
        marketing and distribution efforts for AmBisome for the next 12 months.
        This marketing plan will be formulated with the advice of NeXstar and,
        where possible, will


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        make use of promotional materials developed by NeXstar for use in its
        current markets.  Sumitomo shall market AmBisome in accordance with the
        marketing plan.

7.      PAYMENTS.  Payment of royalties by Sumitomo to NeXstar from sales of 
        AmBisome shall be made within two (2) weeks after reports in section 8
        are sent to NeXstar.  Payments by Sumitomo to NeXstar shall be made in
        U.S. dollars. Payments shall be calculated for each calendar quarter at
        the exchange rate (T.T.S.) on the last business day of such calendar
        quarter published in the United States Edition of the Wall Street
        Journal under the Foreign Exchange listings.  Sumitomo agrees at most
        once in each calendar year, if necessary, to permit an independent
        certified public accounting firm representing NeXstar and which is
        reasonably acceptable to Sumitomo, upon NeXstar's written request, to
        have access during normal business hours to such of the records of
        Sumitomo as may be reasonably necessary to verify the accuracy of the
        determination of royalties for any year ending not more than thirty-six
        (36) months prior to the date of such request at NeXstar's cost. Such
        firm shall report to NeXstar only the information necessary to verify
        the accuracy of royalty payment.

8.      REPORTS.  Sumitomo shall furnish NeXstar with quarterly written reports
        showing

        (a)     the gross sales of all AmBisome sold by Sumitomo and the
                calculation of Net Sales from such gross sales.

        (b)     the royalties payable to NeXstar in U.S. dollars, if any, which
                have accrued based upon Net Sales of AmBisome.

        (c)     withholding taxes, if any, required by law to be deducted in
                respect of such sales.

        (d)     the exchange rates used in determining the amount of U.S.
                dollars.  Reports will be sent to NeXstar within one (1) month 
                after the end of each quarter.
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        All records which Sumitomo must maintain under this Agreement shall be
        full, true and accurate and shall contain all information and data which
        may be necessary to ascertain and verify the royalties payable
        hereunder. Such records shall be maintained for a period of seven (7)
        years from the date of each payment.

9.      CONFIDENTIALITY. Neither Sumitomo nor NeXstar will make any public
        statement or other disclosure of information concerning any aspect of
        this Agreement without the approval of the other party hereto, except
        that each party may make such disclosures as may be required by
        applicable law, including U.S. or Japanese securities laws. For the term
        of this Agreement and five (5) years after the expiration or termination
        of this Agreement, each Party shall hold all financial and technical
        data, know-how or other information received from the other Party in
        connection with the development, or sale of the AmBisome in confidence
        and shall not use such information, except for;

        (1)     information that must be disclosed to government agencies for
                the purpose of registering AmBisome

        (2)     information that is disclosed pursuant to any order or
                requirement of a court, administrative agency, or other
                governmental body

        (3)     information that is or becomes part of the public domain
                through no fault of the receiving party

        (4)     information that is previously known to the receiving party
                at the disclosure as shown by written records

        (5)     information which becomes known to the receiving party from
                another source without breach of secrecy obligation to the
                disclosing party, or

        (6)     information that was or is independently developed by the
                receiving party.



   
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10.     PATENT PROSECUTION AND MAINTENANCE. NeXstar shall take best efforts to
        continue patent prosecution and maintain patent protection as regards
        Patent Rights in Japan at NeXstar's cost.

11.     INFRINGEMENT OF THE PATENT RIGHTS AND TRADEMARK. NeXstar shall have the
        right to initiate an infringement or other appropriate suit or
        proceeding against a third party that has infringed any of the Patent
        Rights and Trademark. NeXstar shall have the sole right to elect counsel
        and, except as provided in the following sentence, shall pay all
        expenses of the suit or proceeding including without limitations
        attorney's fees and court costs. Sumitomo has the right to participate
        and, wherever necessary in order for NeXstar to adequately litigate such
        suit or proceeding, Sumitomo shall offer reasonable assistance to
        NeXstar in connection with any such proceeding at no charge to NeXstar
        except for reimbursement of reasonable out-of-pocket expenses incurred 
        in rendering such assistance. Any damages, royalties, settlement fees or
        other consideration received in any such suit or other proceeding shall
        be allocated first to cover the out-of-pocket expenses, including
        reasonable attorney fees, incurred by either party in connection with
        any such suit or other proceeding, with the remainder to NeXstar.

        If NeXstar decides not to assume responsibility for the conduct such
        suit or other proceeding, Sumitomo shall have the right, but not
        obligation, to conduct it for the infringement claim. In such event
        NeXstar shall pay all expenses of the suit or proceeding including
        without limitations attorney's fees and court costs, and the indemnity
        from the third party shall belong to Sumitomo.     

12.     CLAIMED INFRINGEMENT BY OTHERS. NeXstar shall indemnify, defend and hold
        Sumitomo harmless from any claim by any third party that the sale, the
        offer to sell, or use of AmBisome for the approved indication(s) in
        Japan infringes any patent held or licensed by
<PAGE>   9
        such third party. NeXstar shall bear any and all expenses, costs of
        defense (including without limitation attorney's fees, witness fees,
        damages, judgments, fines, amount paid in settlement or royalties paid 
        to any third party) and any amounts Sumitomo becomes legally obligated
        to pay because of any claimed infringement or claims, provided that
        Sumitomo promptly notifies NeXstar such claimed infringement and agrees
        to cooperate with NeXstar in the defense thereof. This indemnity shall
        survive the termination of this Agreement.

13.     DUE DILIGENCE. Sumitomo shall devote the same degree of attention,
        resources and diligence to its respective obligations in this Agreement
        and its efforts to develop, manufacture, market and sell AmBisome in
        Japan as it devotes to other products of its own development.

14.     TERM AND TERMINATION.

        14-1.   This Agreement shall become effective from the date of the last
                signature set forth herein below and shall continue till the
                later of the expiration date of the last of the patents in the
                Patent Rights or a period of ten (10) years after Sumitomo's
                launch of the Product in the Territory. After such expiration
                date, this Agreement will be automatically extended for every
                succeeding 1 year unless Sumitomo notifies NeXstar of its desire
                to terminate this Agreement at least six (6) months prior to the
                expiry.

        14-2.   Each party may terminate this Agreement without payment of any
                compensation by giving a written notice of termination

                (i)     if the other party files a petition in bankruptcy or a
                        petition to take advantage of any insolvency or
                        reorganization act or, if the other party be adjudicated
                        a bankruptcy, or a court of complete jurisdiction shall
                        enter an order of decree
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                        appointing a receiver of property of the party, and such
                        adjudication, order or decree shall not be vacated or
                        set aside or stayed within thirty (30) days from its
                        entry.

                (ii)    if the other party has failed to perform a material
                        provision of this Agreement and has not corrected such
                        failure within sixty (60) days after receipt of notice
                        describing such failure.

        14-3.   Sumitomo may terminate this Agreement at any time upon six (6)
                months' prior notice to NeXstar and payment of all amounts due
                to NeXstar in such six (6) month period.

        14-4.   In the event that this Agreement is terminated by Sumitomo,
                except for the termination by the reason of NeXstar's failure
                (which shall include the termination by Sumitomo pursuant to
                14-2), Sumitomo will assign and/or transfer any IND, NDA and/or
                Registration for AmBisome to NeXstar of NeXstar's designee. In
                the event of termination under paragraph 14-3, Sumitomo agrees
                to effect the assignment and/or transfer prior to the
                termination date.

15.     ASSIGNMENT. This Agreement shall be binding upon and inure to the
        benefit of the parties hereto and their permitted successors and
        assigns; provided, however, that neither party shall assign any of its
        rights and obligations hereunder except (i) as incident to the merger,
        consolidations, reorganization, or acquisition of stock or assets
        affecting substantially all of the assets or actual voting control of
        the assigning party, or (ii) to an Affiliate; provided, however, that in
        no event shall either party's obligations under the Agreement be
        assigned to an Affiliate without prior written consent of the other
        party. Affiliate shall mean a company of which at least fifty percent
        (50%) of the voting stock is owned or controlled by the party concerned.
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16.     GOVERNING LAW AND ARBITRATION. This Agreement shall be governed by the
        laws of Germany. Any and all disputes, controversies or differences
        which may arise between the parties hereto in relation to the
        interpretation or administration of this Agreement shall finally be
        settled by arbitration in accordance with the rules of the International
        Chamber of Commerce, by which each party hereto is bound. The
        arbitration shall be held in Tokyo, Japan, if demanded by NeXstar, in
        Denver, U.S.A., if demanded by Sumitomo.

17.     NO WARRANTY. NeXstar shall not be liable to Sumitomo, Sumitomo's
        Affiliates, successors or any third party with respect to any claim
        arising from the use of AmBisome, or any claim for loss of profits, loss
        or interruption of business, or for indirect, special or consequential
        damages of any kind, except stipulated otherwise in this Agreement and
        except any claim directly resulted from the willful misconduct or
        failure in manufacturing AmBisome by NeXstar.

18.     FORCE MAJEURE. Neither party shall be responsible for failure or delay
        in the performance of any of its obligations hereunder due to Force
        Majeure. Force Majeure shall mean any circumstance which, due to an
        event or a legal position beyond the parties' reasonable control,
        renders impossible the fulfillment of any of the parties' obligations
        hereunder, such as, but not limited to, acts of God, acts, regulations
        and laws of any government, wars, civil commotions, destructions of
        production facilities or materials by fires, earthquakes or storms,
        labour disturbances, shortages of public utilities, common carriers or
        raw materials, or any other causes of similar effects. The obligations
        hereunder which are rendered impossible by any case of Force Majeure
        shall not be discharged, but only be suspended and the party so affected
        shall continue to perform its obligations as soon as such case of Force
        Majeure is removed or alleviated.
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19.     NOTICES. All notices, reports, requests or demands required or permitted
        under this Agreement shall be forwarded, charges prepaid, by registered
        air mail or by air courier or may be sent by facsimile, with confirmed
        transmission, properly addressed to the respective Parties as follows:

If to NeXstar:

          NeXstar Pharmaceuticals, Inc.                  
          2860 Wilderness Place                          
          Boulder, Colorado 80301                        
          U.S.A.                                         
          (fax) 1-303-444-0672                           
          Attention: Michael T. Burke                    
                     Vice President, Business Development

          With a copy to: Adam Cochran                      
                          Vice President and General Counsel
                          Same address as set forth above   

If to Sumitomo:

          Sumitomo Pharmaceuticals Co. Ltd.                      
          2-8, Doshomachi 2-chome Chuo-ku                        
          Osaka 541, Japan                                       
          (fax) 81-6-233-2399                                    
          Attention: Mr. Shigeru Sakai                           
                     Manager, R & D Coordination Department      
          With a copy to: Mr. Yoshinori Takashima                
                          Manager, Corporate Planning Department 
                          Same address as set forth above        

20.     INDEPENDENT CONTRACTORS. No agency, partnership or joint venture is
        hereby established. Each Party shall act hereunder as an independent
        contractor. Neither Party shall enter into, or incur, or hold itself out
        to third parties as having authority to enter into or incur on behalf of
        the other Party any contractual obligations, expenses or liabilities
        whatsoever.
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Sumitomo Pharmaceuticals Co., Ltd.              NeXstar Pharmaceuticals Inc.  
                                                                              
By: /s/ MASAYASU TAKEUCHI                       By: /s/ PATRICK J. MAHAFFY    
   ---------------------------                     ---------------------------
   Masayasu Takeuchi                                                          
                                                                              
Title: President                                Title: President and Chief    
      ------------------------                        ------------------------
                                                           Executive Officer  
                                                      ------------------------
                                                                              
Date: September 26, 1996                        Date: September 20, 1996      
     -------------------------                        ------------------------

                              
                              
                              
                              
                              
                                            
                              
                              
                              
                              
                              
                              


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                                   EXHIBIT A

                          AMBISOME PRODUCT DESCRIPTION

AmBisome for injection is a sterile lyophilized product for intravenous
infusion. Each vial contains 50 mg of amphotericin B, U.S.P., Ph. Eur.
encapsulated in liposomes consisting of approximately 213 mg hydrogenated soy
phosphatidylcholine, 52 mg cholesterol, U.S.N.F., 84 mg
distearoylphosphatidylglycerol, 0.64 mg alpha tocopherol, U.S.P., Ph. Eur.
together with 900 mg sucrose, U.S.P., Ph. Eur. and 27 mg disodium succinate
hexahydrate as buffer. AmBisome is presented in sterile 50 cc and 30 cc, type
I, USP, EP glass vials. The closure consists of an elastomer stopper and a
crimped aluminum sealing ring fitted with removable plastic cap.



                                EXHIBITS B AND C

[Confidential Material omitted and filed separately with the Securities and 
Exchange Commission pursuant to Section 24b-2 of the Securities Exchange Act 
of 1934.]