EXHIBIT 10.33
                                                                   -------------

                    CONFIDENTIAL TREATMENT HAS BEEN REQUESTED
                     WITH REGARD TO PORTIONS OF THIS EXHIBIT

                   AMENDMENT TO CLINICAL DEVELOPMENT AGREEMENT

      This Amendment is entered into this 27th day of October, 1999 by and
between Vion Pharmaceuticals, Inc., Four Science Park, New Haven, Connecticut
("Vion"), Covance Clinical Research Unit Ltd., Hyde Street, Leeds, United
Kingdom ("Covance"), and Covance Inc., 210 Carnegie Center, Princeton, New
Jersey ("Covance"), and amends the Clinical Development Agreement between Vion
and Covance dated June 6, 1997 ("Original Agreement").

      WHEREAS, the parties to the Original Agreement desire to effect an
amicable and efficient transfer of Covance's responsibilities (except as
provided hereinafter in Section 2(b)) and terminate the Original Agreement.

      NOW THEREFORE, in consideration of the premises and the undertakings of
the parties hereinafter set forth, the parties agree as follows:

1.    DEFINITIONS

      a.     "Affiliate" means an individual, trust, business trust, joint
             venture, partnership, corporation, limited liability company,
             association or any other entity which owns, is owned by or is under
             common ownership with a party. For the purposes of this definition,
             the term "owns" (including, with correlative meanings, the terms
             "owned by" and "under common ownership with") as used with respect
             to any party, shall mean the possession (directly or indirectly) of
             more than fifty percent (50%) of the outstanding voting securities
             of a corporation or comparable equity interest in any other type of
             entity.

      b.     "FDA" means the United States Food and Drug Administration (or its
             substantial equivalent in any foreign country).

      c.     "IND" means Investigational New Drug Application No. 52,804 filed
             for the Drug with the FDA or its substantial equivalent in any
             foreign country.

      d.     All capitalized terms used herein and not otherwise defined, shall
             have the meaning ascribed thereto under the Original Agreement.

      e.     "BIPI" means Boehringer Ingelheim Pharmaceuticals, Inc.

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2.    OBLIGATIONS OF COVANCE

      a.     Covance shall use its best efforts to cooperate with Vion and BIPI
             to transition its responsibilities under the Original Agreement
             according to the Transition Plan, which is attached as Appendix 1
             to this Amendment (and which may be amended from time to time
             during the transition period). For the purposes of this amendment,
             "best efforts" shall be those efforts Covance would apply to a
             similar project for its most important clients in good standing.
             BIPI and Vion agree that they shall accept such responsibilities in
             accordance with the Transition Plan no later than November 30,
             1999.

      b.     Covance shall continue to perform IVRS patient randomization and
             assignment and clinical trial supply management according to the
             methodology and fee structure as under the Original Agreement and
             in accordance with the Roles and Responsibilities for Protocol No.
             PORF-96-001 dated January 15, 1999 and approved January 18, 1999.
             The fee to Covance for IVRS patient randomization and assignment
             and clinical trial supply management shall be []. The parties agree
             that fees described in this Section 2(b) apply to the first []
             patients randomized and assigned. Covance agrees to perform further
             IVRS randomization and assignment and clinical trial supply
             management at BIPI's request on a PRO RATA cost basis.

      c.     Covance will continue to be responsible for site management of each
             individual research site in accordance with the methodology
             described in the Original Agreement and Transition Plan until BIPI
             (or a BIPI Affiliate) is prepared to take over responsibility for
             such tasks but in no event later than November 30, 1999. It is
             understood and agreed that during the transition herein described,
             these tasks may be performed jointly by Covance and BIPI (or a BIPI
             Affiliate). All agreements with Investigators shall be assigned by
             Covance and accepted by BIPI (or a BIPI Affiliate) no later than
             such date pursuant to a written acknowledgment of such assignment
             and acceptance from each individual research site at which point
             Covance's responsibility for such tasks shall cease. After BIPI (or
             a BIPI Affiliate) has executed a contract with an individual
             research site, Covance shall continue to be responsible for
             reporting all (including Pharm-Olam generated) expeditable and/or
             severe adverse events to the FDA (or other relevant agencies) until
             the date upon which BIPI provides written notice to Covance that
             acceptable data bases have been established at BIPI and all
             relevant BIPI Affiliates, but in no event shall Covance be so
             responsible beyond November 30, 1999.

      d.     BIPI and Covance shall enter a contract for performance of the
             activities described in Section 2(b).

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3.    OBLIGATIONS OF VION

      a.     For all activities performed by Covance and not yet paid by Vion
             through September 27, 1999, Vion shall pay Covance a total of [].
             This payment shall be in full settlement of all amounts owed to
             Covance by Vion for Covance activities under the Original Agreement
             through September 27, 1999 and shall be paid to Covance no later
             than November 30, 1999.

      b.     Vion shall reimburse Covance for pass-through Investigator fees
             that have been incurred by Covance through September 27, 1999 but
             not yet reimbursed by Vion. The amount of the pass-through
             Investigator fees incurred through September 27, 1999 is
             approximately []. For all other outstanding pass-through fees
             incurred by Covance through September 27, 1999, Vion shall
             reimburse Covance a total of []. The sums described herein shall be
             paid no later than November 30, 1999.

      c.     For all fees and pass-through costs other than Investigator fees
             after September 27, 1999, including the transition activities
             described in Section 2(a) above and the Transition Plan, Vion shall
             pay Covance a fee of [], such fee payable within fifteen (15) days
             of the date upon which BIPI provides written notice to Covance that
             acceptable data bases have been established at BIPI and all
             relevant BIPI Affiliates, but in no event later than December 15,
             1999. Vion shall continue to reimburse Covance for pass-through
             Investigator fees incurred by Covance within thirty (30) days of
             invoice.

4.    ORIGINAL AGREEMENT

      a.     Upon the date of formal transfer of responsibility for all INDs
             from Vion to BIPI as defined in Section 5 below, the Original
             Agreement will terminate.

      b.     The confidentiality obligations contained in Section 5, the
             indemnification provisions of Sections 2(l) and 3(h) of the
             Original Agreement shall survive termination of the Original
             Agreement. In addition, the parties agree that the obligation to
             comply with all applicable regulations shall also survive
             termination of the Original Agreement.

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5.    RELEASE FROM LIABILITY

      a.     Upon full review of all the data and information relating to
             Covance's work under the Original Agreement and formal transfer of
             responsibility for all INDs from Vion to BIPI, Vion and Covance
             shall execute a mutual release, from further responsibility or
             liability under the Original Agreement (including a waiver of any
             early termination penalties described in Section 8 of the Original
             Agreement). Such release shall not release Vion or Covance from the
             claims of third parties pursuant to Sections 2(l) and 3(h) of the
             Original Agreement. For the purposes of this Amendment, "formal
             transfer of responsibility" is defined, on a country-by-country
             basis, as notification from BIPI (or a BIPI Affiliate) to the FDA
             of the transfer of sponsor responsibility for the IND from Vion to
             BIPI (or a BIPI Affiliate). The review described herein shall be
             completed by BIPI and Vion no later than March 31, 2000. In the
             event of any complaints with respect to the data and information so
             reviewed, BIPI and Vion shall provide Covance with prompt written
             notice of the same and Covance shall be given an opportunity, if
             practicable, to cure any identified deficiency. In the event of a
             dispute among the parties with respect to this review, the parties
             agree to submit the dispute to non-binding mediation in the event
             that executive discussion cannot resolve the issue to everyone's
             satisfaction.





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


VION PHARMACEUTICALS, INC.                   COVANCE CLINICAL
                                             RESEARCH UNIT LTD.

By:   /s/ Thomas Mizelle                     By:   /s/ Charles C. Harwood, Jr.
      ---------------------------                  ---------------------------

Its:  VP Operations                          Its:  Director
      ---------------------------                  ---------------------------

Date: October 27, 1999                       Date: October 28, 1999
      ---------------------------                  ---------------------------


COVANCE INC.


By:   /s/ Charles C. Harwood, Jr.
      ---------------------------

Its:  Senior Vice President and Chief Financial Officer
      -------------------------------------------------

Date: October 28, 1999
      ---------------------------



The undersigned hereby agrees to be bound by the applicable provisions of
Sections 2(a), (c), (d) and 5 hereof.


BOEHRINGER INGELHEIM
PHARMACEUTICALS, INC.


By:   /s/ M. Haehl
      ---------------------------

Its:  Sr. VP Medical/DRA
      ---------------------------

Date: October 27, 1999
      ---------------------------



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