<Page>


EXHIBIT 10.23

                       Confidential treatment requested.

                  Confidential portions indicated by "[*****]"
                     have been omitted and filed separately
                        with the Securities and Exchange
                                   Commission.

                                AMENDMENT NO. 5
                                       TO
                                LICENSE AGREEMENT

AMENDMENT NO. 5 TO LICENSE AGREEMENT (this "AMENDMENT NO. 5") is made this 3rd
day of March, 2003, by and between YALE UNIVERSITY, a corporation organized and
existing under and by virtue of a charter granted by the general assembly of the
Colony and State of Connecticut and located in New Haven, Connecticut ("YALE"),
and VION PHARMACEUTICALS, INC. (f/k/a OncoRx Inc.), a corporation organized and
existing under the laws of the State of Delaware and with principal offices
located in New Haven, Connecticut ("LICENSEE").

                              W I T N E S S E T H:

WHEREAS, YALE and LICENSEE are parties to a license agreement dated August 31,
1994 (the "ORIGINAL LICENSE AGREEMENT"), under which YALE exclusively licensed
to LICENSEE a series of YALE owned Inventions relating to potential anti-tumor
and antiviral compounds;

WHEREAS, YALE and LICENSEE amended the Original License Agreement pursuant to an
Agreement dated November 15, 1995 to make an additional patent and patent
application subject to the Original License Agreement and provide the terms of
compensation to YALE therefor;

WHEREAS, YALE and LICENSEE again amended the ORIGINAL LICENSE AGREEMENT, as
amended, pursuant to Amendment No. 1 to License Agreement, dated as of June 12,
1997, to reduce the earned royalties payable on SUBLICENSE INCOME and Amendment
No. 2 to License Agreement, dated as of June 12, 1997, to reduce the earned
royalties payable on SUBLICENSE INCOME, and to make certain other changes to the
Original License Agreement;

WHEREAS, YALE and LICENSEE again amended the ORIGINAL LICENSE AGREEMENT, as
amended, pursuant to Amendment No. 3 to License Agreement, dated as of September
25, 1998 ("AMENDMENT NO. 3"), to modify and clarify the patents and technology
covered by the ORIGINAL LICENSE AGREEMENT and to add certain additional terms
with respect to the antiviral compound [B]-L-FD4C;

WHEREAS, YALE and LICENSEE again amended the ORIGINAL LICENSE AGREEMENT, as
amended, pursuant to Amendment No. 4 to License Agreement, dated as of January
31, 2000 ("AMENDMENT NO. 4"), to provide for an exclusive sublicense to
Achillion Pharmaceuticals, Inc. of LICENSEE's rights in Inventions relating to
(beta)-L-FD4C; and

WHEREAS, YALE and LICENSEE desire to further amend the ORIGINAL LICENSE
AGREEMENT, as amended.

NOW, THEREFORE, in consideration of the premises and of the mutual covenants
herein contained and for other good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, YALE and LICENSEE hereby agree as
follows:


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1. Exhibit B of the ORIGINAL LICENSE AGREEMENT, as subsequently amended (as so
amended, the "LICENSE AGREEMENT"), shall be deleted and replaced in its entirety
with the Attachment I hereto.

2. Exhibit C of the LICENSE AGREEMENT shall be deleted and replaced in its
entirety with the Attachment II hereto.

3. Exhibit D of the LICENSE AGREEMENT shall be deleted.

4. Exhibit E of the LICENSE AGREEMENT (see AMENDMENT NO. 3) shall be deleted and
replaced in its entirety with the Attachment III hereto.

5. The defined term "RESEARCH" used in the LICENSE AGREEMENT shall be replaced
in all places it appears by the undefined term "research".

6. LICENSEE shall have a worldwide non-exclusive license to the INVENTIONS set
forth in Exhibit E.

7. The definition of "INVENTIONS" in Section 1.1 of the LICENSE AGREEMENT shall
be deleted and replaced with the following new Section 1.1:

      1.1 "INVENTIONS" shall mean each of the patent and patent applications
described in Exhibit B, Exhibit C and Exhibit E attached hereto.

8. The definition of "LICENSED PRODUCTS" in Section 1.3 of the LICENSE AGREEMENT
shall be deleted and replaced with the following new Section 1.3:

      1.3 "LICENSED PRODUCTS" shall mean all products for research, therapeutic
or diagnostic uses that may derive from or which result from the manufacture and
production or use of a claim of the LICENSED PATENTS, wherever used or sold.

9. A new Section 2.1.1 shall be added to the LICENSE AGREEMENT and it shall read
as follows:

      2.1.1 YALE hereby grants to LICENSEE, subject to all of the terms and
conditions of this Agreement, a non-transferable, non-exclusive worldwide
license for the term defined by ARTICLE III to make, have made, use, sell and
practice the INVENTIONS as described in Exhibit E.

10. A new Section 4.5 shall be added to the LICENSE AGREEMENT and it shall read
as follows:

      4.5 LICENSEE shall pay to YALE (A) a one time payment of [*****] upon
[*****] or [*****] and (B) a one time payment of [*****] upon [*****] or [*****]
for its first product related to any of the following U.S. PATENTS: [*****], as
well as any continuations, divisionals, continuations-in-part, reissue,
reexaminations or extensions thereof or substitutes therefor of any of these
U.S. PATENTS and any international equivalents of the foregoing. For purposes of
clarity, the aggregate amount of potential payment by LICENSEE shall be [*****]
under this provision. This payment is apart from any other consideration which
may be due under this agreement.

11. Section 7.1 of the LICENSE AGREEMENT shall be deleted in its entirety and
shall be replaced with the following:

      7.1 Each of Exhibits B, C and E describes patent protection in the United
States and, if applicable, any foreign country for INVENTIONS listed thereon.

12. Exhibit 1 of the LICENSE AGREEMENT (see AMENDMENT NO. 4) shall be deleted
and replace in its entirety with the Attachment IV hereto.

13. LICENSEE agrees [*****] from all of its licensees, sublicensees and
transferees [*****], based on U.S. Patent Nos. [*****] (and any continuations,
divisionals, continuations-in-part,


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reissues, reexaminations or extensions thereof or substitutes therefor, and
the internationalequivalents of the foregoing) (the "[*****]" and "[*****]"
patents, respectively) [*****], to [*****] with respect to patents or other
technology licensed from YALE, provided that [*****].

14. Where an individual licensed product or its use is covered by more than one
patent or patent application as set forth in the accompanying exhibits, LICENSEE
shall be responsible for paying royalties under one patent or patent application
only. In the event that there are different royalty rates for the applicable
patents, LICENSEE shall pay the higher rate.

15. Section (11.1(iv)) of the LICENSE AGREEMENT shall be amended by replacing
ninety (90) days with [*****] days in line 3.

16. Except as expressly amended hereby, the LICENSE AGREEMENT remains unchanged
and in full force and effect.

17. LICENSEE hereby agrees to transfer all Promycin assets listed on Exhibit F
to YALE.

18. Any reference to the LICENSE AGREEMENT contained in any notice, request,
certificate, or other document executed concurrently with or after the execution
and delivery of this AMENDMENT NO. 5 shall be deemed to include this AMENDMENT
NO. 5 unless the context shall otherwise require.

                                      * * *

IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 5 to be
executed and delivered in duplicate originals by their duly authorized
representatives.

YALE UNIVERSITY            VION PHARMACEUTICALS, INC.


By: /s/ Jon Soderstrom              By: /s/ Howard B. Johnson
    ----------------------              ---------------------
Name: Jon Soderstrom                Name: Howard B. Johnson
      ---------------------               -----------------
Title: Managing Director,           Title: Chief Financial Officer
       --------------------                -----------------------
       Office Cooperative Research
       ---------------------------

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                                  ATTACHMENT I
                                       TO
                                 AMENDMENT NO. 5
                                       TO
                                LICENSE AGREEMENT

                                    EXHIBIT B
                                 PATENT LISTING

U.S. Patent No. [*****], Issued [*****]
[*****]

All international patents and patent applications, if any, related to [*****]

U.S. Patent No. [*****], Issued [*****]
[*****]

All international patents and patent applications , if any, related to [*****]

U.S. Patent No. [*****], Issued [*****]
[*****]

All international patents and patent applications , if any, related to [*****]

U.S. Patent No. [*****], Issued [*****]
[*****]

All international patents and patent applications , if any, related to [*****]

U.S. Patent No. [*****], Issued [*****]
[*****]

All international patents and patent applications, if any, related to [*****]

U.S. Patent No. [*****], Issued [*****]
[*****]

International Publication Number [*****]
[*****]

All other international patents and patent applications, if any, related to
[*****]

Yale's interest in the following Patents and patent applications:

U.S. Patent No. [*****], Issued [*****]
[*****]

All international patents and patent applications, if any, related to [*****]

U.S. Patent Application Serial No. [*****]


                                        4



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[*****]

All international patents and patent applications, if any, related to [*****]


                                       5



<Page>

                                  ATTACHMENT II
                                       TO
                                 AMENDMENT NO. 5
                                       TO
                                LICENSE AGREEMENT


                                    EXHIBIT C
                                 PATENT LISTING

U.S. Patent Application Serial Number [*****]
[*****]

U.S. Patent Application Serial Number [*****]
[*****]

U.S. Patent Application Serial Number [*****]
[*****]

U.S. Patent Application Serial Number [*****]
[*****]

All international patents and patent applications, if any, related to [*****]


                                       6



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                                 ATTACHMENT III
                                       TO
                                 AMENDMENT NO. 5
                                       TO
                                LICENSE AGREEMENT

                                    EXHIBIT E
                                 PATENT LISTING

U.S. Patent Number [*****], Issued [*****]
[*****]

All international patents and patent applications , if any, related to [*****]

U.S. Patent Number [*****], Issued [*****]
[*****]

All international patents and patent applications , if any, related to [*****]


                                        7



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                                 ATTACHMENT III
                                       TO
                                 AMENDMENT NO. 5
                                       TO
                                LICENSE AGREEMENT


                                    EXHIBIT F
                                 PROMYCIN ASSETS

Subject to Vion's agreements with Boehringer Ingelheim International, the
following Promycin assets are available for transfer:

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]

[*****]


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[*****]

[*****]

[*****]

[*****]

[*****]


                                       9



<Page>

                                  ATTACHMENT IV
                                       TO
                                 AMENDMENT NO. 5
                                       TO
                                LICENSE AGREEMENT


                                    EXHIBIT 1
                               SUBLICENSED PATENTS

1. U.S. Patent Application Serial Number [*****] entitled [*****]

2. U.S. Patent Application Serial Number [*****] entitled [*****]


                                       10