<PAGE>   1
                                                                  EXHIBIT 10.15D

                                 AMENDMENT NO. 1
                                       TO
                              CONSULTING AGREEMENT

              THIS AMENDMENT NO. 1 ("Amendment") is effective as of September 1,
1998 ("Effective Date"), by and between Guilford Pharmaceuticals Inc., a
Delaware corporation (the "Company"), and Solomon H. Snyder, M.D., an individual
residing in the State of Maryland (the "Consultant").

              WHEREAS, the Company and the Consultant are parties to a
Consulting Agreement dated September 1, 1995 ("Consulting Agreement"), pursuant
to which the Consultant has provided consulting services to the Company; and

              WHEREAS, the parties desire to extend such consulting relationship
on the terms and conditions contained herein;

              NOW THEREFORE, in consideration of the foregoing, the mutual
promises of the parties hereunder, and other good and valuable consideration,
the receipt and sufficiency of which are hereby acknowledged, the Consultant and
the Company hereby agree as follows:

       1.     The term of the Consulting Agreement is hereby extended for the
              period commencing September 1, 1998 through December 31, 1998 (the
              "First Extension").

       2.     In consideration of the performance of the Services by the
              Consultant during the First Extension, the Company agrees to pay
              the Consultant at a rate of $14,166.67 per month, payable in
              arrears.

       3.     Except as explicitly set forth in paragraph 2 above and in the
              Consulting Agreement, Consultant shall receive no other
              compensation (whether in cash, stock options or otherwise) for his
              Services during the First Extension.

       4.     The Consultant represents and warrants to the Company that the
              Consultant is, as of the Effective Date, under no contractual or
              other restriction or obligation, including agreements or
              understandings with third parties, which conflicts with the
              Consulting Agreement as amended hereby, the performance of his
              duties under the Consulting Agreement as amended hereby, or the
              other obligations of the Consultant to the Company. Attached
              hereto as Exhibit A is a current list of Consultant's board of
              directors, scientific advisory board and/or other consulting
              arrangements and affiliations.

       5.     The parties acknowledge that they may enter into a further
              amendment of the Consulting Agreement or a new superseding
              consulting agreement in the future, the


<PAGE>   2


              effect of which may be to make retroactive modifications to the
              terms (including those regarding compensation) of the Consulting
              Agreement and/or this Amendment. The foregoing notwithstanding,
              the parties agree that this Amendment and the Consulting Agreement
              shall not be amended or supplemented except by a written document
              executed and delivered by both parties hereto. The failure on the
              part of either party to enforce, or any delay in enforcing, any
              right, power or remedy that such party may have under this
              Amendment or the Consulting Agreement shall not constitute a
              waiver of any such right, power or remedy, or release the other
              party from any obligations under this Amendment or the Consulting
              Agreement, except by a written document signed by the party
              against whom such waiver or release is sought to be enforced.

       6.     Except as specifically set forth above in this Amendment, the
              terms of the Consulting Agreement remain unchanged and in full
              force and effect as set forth therein.

       7.     Capitalized terms used in this Amendment that have not been
              defined herein shall have the meanings ascribed in the Consulting
              Agreement.

       IN WITNESS WHEREOF, the parties hereto have duly executed and delivered
this Amendment No. 1 as of the Effective Date.

GUILFORD PHARMACEUTICALS INC.


By:    /s/ Craig R. Smith, M.D.
   -----------------------------------------
Name:  Craig R. Smith, M.D.
Title: President and Chief Executive Officer


CONSULTANT


     /s/ Solomon H. Snyder, M.D.
--------------------------------------------
          Solomon H. Snyder, M.D.

Address:      3801 Canterbury Road, No. 1001
              Baltimore, Maryland 21218





                                       2
<PAGE>   3


                                 AMENDMENT NO. 2
                                       TO
                              CONSULTING AGREEMENT

              THIS AMENDMENT NO. 2 ("Amendment") is effective as of January 1,
1999 ("Effective Date"), by and between Guilford Pharmaceuticals Inc., a
Delaware corporation (the "Company"), and Solomon H. Snyder, M.D., an individual
residing in the State of Maryland (the "Consultant").

              WHEREAS, the Company and the Consultant are parties to a
Consulting Agreement dated September 1, 1995, as amended June 7, 1996 and
September 1, 1998 ("Consulting Agreement"), pursuant to which the Consultant has
provided consulting services to the Company; and

              WHEREAS, the parties desire to extend such consulting relationship
on the terms and conditions contained herein;

              NOW THEREFORE, in consideration of the foregoing, the mutual
promises of the parties hereunder, and other good and valuable consideration,
the receipt and sufficiency of which are hereby acknowledged, the Consultant and
the Company hereby agree as follows:

       8.     The term of the Consulting Agreement is hereby extended for the
              period commencing January 1, 1999 through March 31, 1999 (the
              "Second Extension").

       9.     In consideration of the performance of the Services by the
              Consultant during the First Extension, the Company agrees to pay
              the Consultant at a rate of $14,166.67 per month, payable in
              arrears.

       10.    Except as explicitly set forth in paragraph 2 above and in the
              Consulting Agreement, Consultant shall receive no other
              compensation (whether in cash, stock options or otherwise) for his
              Services during the First Extension.

       11.    The Consultant represents and warrants to the Company that the
              Consultant is, as of the Effective Date, under no contractual or
              other restriction or obligation, including agreements or
              understandings with third parties, which conflicts with the
              Consulting Agreement as amended hereby, the performance of his
              duties under the Consulting Agreement as amended hereby, or the
              other obligations of the Consultant to the Company.

       12.    The parties acknowledge that they may enter into a further
              amendment of the Consulting Agreement or a new superseding
              consulting agreement in the future, the effect of which may be to
              make retroactive modifications to the terms (including


                                       3
<PAGE>   4


              those regarding compensation) of the Consulting Agreement and/or
              this Amendment. The foregoing notwithstanding, the parties agree
              that this Amendment and the Consulting Agreement shall not be
              amended or supplemented except by a written document executed and
              delivered by both parties hereto. The failure on the part of
              either party to enforce, or any delay in enforcing, any right,
              power or remedy that such party may have under this Amendment or
              the Consulting Agreement shall not constitute a waiver of any such
              right, power or remedy, or release the other party from any
              obligations under this Amendment or the Consulting Agreement,
              except by a written document signed by the party against whom such
              waiver or release is sought to be enforced.

       13.    Except as specifically set forth above in this Amendment, the
              terms of the Consulting Agreement remain unchanged and in full
              force and effect as set forth therein.

       14.    Capitalized terms used in this Amendment that have not been
              defined herein shall have the meanings ascribed in the Consulting
              Agreement.

       IN WITNESS WHEREOF, the parties hereto have duly executed and delivered
this Amendment No. 2 as of the Effective Date.

GUILFORD PHARMACEUTICALS INC.


By:    /s/ Craig R. Smith, M.D.
   -----------------------------------------
Name:  Craig R. Smith, M.D.
Title: President and Chief Executive Officer


CONSULTANT


     /s/ Solomon H. Snyder, M.D.
--------------------------------------------
          Solomon H. Snyder, M.D.

Address:      3801 Canterbury Road, No. 1001
              Baltimore, Maryland 21218





                                       4