<PAGE>
 
                                                                 EXHIBIT 10.40.1



                                    May 5, 1997


Robert L. Capizzi, M.D.
100 Turnbridge Circle
Haverford, PA  19041

          Re:  AGREEMENT DATED AS OF MARCH 4, 1996 (THE "AGREEMENT")
               -----------------------------------------------------

Dear Dr Capizzi:

The purpose of this letter is to confirm our arrangements to extend your
consulting relationship with U.S. Bioscience by modifying the Agreement in
certain respects, as follows.  All capitalized terms used herein are used as
defined in the Agreement.

As provided in the Agreement, your Term as an employee and Senior Scientific
Advisor would terminate at the close of business on May 31, 1997.  We are very
pleased that you have agreed to continue to serve as a consultant to U.S.
Bioscience from May 31, 1997 through May 31, 1998 (the "Consulting Term"), which
Consulting Term may be extended by mutual agreement of the parties, under the
following terms and conditions:

     (a)  All of the stock options granted to you by U.S. Bioscience on or
          before February 21, 1995 will continue to vest during the Consulting
          Term, and you will be entitled to exercise (including cashless
          exercise) these options, to the extent vested, during the Consulting
          Term and for a period of three months thereafter.  The options granted
          to you on March 11, 1996 will continue to vest and will remain
          exercisable in accordance with the terms set forth in the Non-
          Statutory Stock Option dated March 11, 1996.

     (b)  At all times during the Consulting Term when you are also a director
          of U.S. Bioscience, you will have the same rights as a director of
          U.S. Bioscience as those afforded, from time to time, to the other
          directors. For purposes of determining your eligibility for annual
          director's fees payable to non-employee directors, or options in lieu
          thereof under the 1996 Non-employee Directors Stock Option Plan, you
          will be considered to be a non-employee director of U.S. Bioscience
          from and after the date of May 31, 1997.

     (c) You will provide at the request of U.S. Bioscience consulting services
         during the Consulting Term. The parties acknowledge that it is
         difficult to predict with
<PAGE>
 
Robert L. Capizzi, M.D.
May 5, 1997
Page 2

         precision the exact number of hours of consulting services that will be
         requested and provided under this Agreement, however, you shall be
         required to provide at least 40 hours of consulting services per year
         hereunder. Such consulting services may include, but shall not be
         limited to: (i) speaking on behalf of U.S. Bioscience or its products;
         (ii) assistance in addressing clinical or preclinical questions; (iii)
         assistance in preparing position papers; (iv) contacting potential
         investigators on behalf of U.S. Bioscience and assisting in study
         design; and (v) assisting U.S. Bioscience to assign, obtain, maintain
         and enforce proprietary rights relating to inventions conceived or
         reduced to practice during your employment with U.S. Bioscience which
         relate to any of its products, services or activities. U.S. Bioscience
         will make reasonable efforts to provide you with advance notice when
         requesting your services and you will make reasonable efforts to adapt
         your schedule to comply with U.S. Bioscience's reasonable requests.
         Certain of the consulting services may be performed by telephone and/or
         by written communication and other such services may require your
         presence, whether at U.S. Bioscience's facilities, potential
         investigators' facilities, meeting with regulatory authorities,
         professional meetings, meetings arranged by commercial partners, or
         otherwise. It is expressly understood and agreed that during the
         Consulting Term you may be employed on a full-time basis. You represent
         to U.S. Bioscience that the terms of this letter do not, and will not,
         conflict with the terms of any of your employment or consulting
         arrangements. Consulting services in excess of 80 hours per year during
         the Consulting Term shall be compensated at a rate to be mutually
         agreed by the parties. U.S. Bioscience shall advance to you or
         reimburse promptly all reasonable costs and expenses related to such
         consulting services, including but not limited to secretarial support
         services, literature searches, slide preparation, conference
         registration, travel and accommodations, and other reasonable expenses
         appropriate to your representation of U.S. Bioscience at conferences
         and professional meetings. It is understood that such expenses shall be
         subject to written approval in advance by U.S. Bioscience.

     (d) In rendering consulting services hereunder, you will make appropriate
         disclosure to third parties (e.g., IRB's, potential investigators,
         research subjects, regulatory authorities, professionals) of all
         financial ties and relationships between you and U.S. Bioscience. You
         also agree to make appropriate disclosure in connection with any of
         your publications which relate to any U.S. Bioscience compound.

     (e) You will comply with federal securities laws and regulations with
         respect to transactions in U.S. Bioscience securities, as well as the
         procedures and practices 
<PAGE>
 
Robert L. Capizzi, M.D.
May 5, 1997
Page 3

         set forth in the U.S. Bioscience Securities Law Guidebook, as amended
         from time to time.

     (f) The parties confirm that at the termination of your Term as an
         employee, May 31, 1997, you shall no longer be eligible to receive the
         benefits set forth in the Agreement under Section 2(a), 2(c) or 2(d).

Except as modified by this letter, the parties agree that the Agreement is
confirmed to be and shall remain in full force and effect.

Please indicate your acceptance of the terms set forth in this letter in the
space provided below and return it to me.  An additional executed copy of this
letter is enclosed for your file.

                         Sincerely yours,

                         U.S. Bioscience, Inc.



                         By:  /s/ Philip S. Schein
                            -----------------------------
                               Philip S. Schein, M.D.
                               Chairman and Chief Executive Officer



Accepted and agreed as of the 8th day of May, 1997:
                              ---                  



/s/ Robert L. Capizzi
----------------------------
Robert L. Capizzi, M.D.