<PAGE>   1
                                                                    EXHIBIT 10.6

               [WYETH LEDERLE VACCINES AND PEDIATRICS LETTERHEAD]

March 30, 1999

Biodelivery Sciences, Inc.
University Heights Science Park
111 Lock Street
Newark, NJ 07103

         Re: AMENDMENT TO LICENSE AGREEMENT

Dear Sirs:

         Reference is made to the License Agreement effective July 1, 1996 by
and between Biodelivery Sciences, Inc. (hereinafter "BDS") and American Cyanamid
Company (hereinafter "WLV"). Pursuant to Section 15, it is proposed to amend the
Agreement as follows:

1.       BDS will provide at least 40 mg of an encochleated mixture of plasmids,
         rIL-12p35 and rIL-12p40 to WLV. BDS will provide 10 mg of the
         encochleated mixture of plasmids rIL-12p35 and rIL-12p40 to WLV by May
         1, 1999 and the remaining encochleated plasmids by June 21, 1999. WLV
         will supply no more than 35 mg of each plasmid to BDS within two (2)
         business days of execution of this Amendment. WLV will supply 40 mg of
         each plasmid within two (2) business days of receipt of at least 10 mg
         of the encochleated mixture of plasmid rIL-12p35 and rIL-12p4O. WLV
         will accept the BDS analytical characterization of the encochleated
         plasmid preparations in order to allow prompt initiation of the
         experiment described in Paragraph 4 below.

2.       BDS will provide at least 20 mg of encochleated plasmid gD2 to WLV by
         June 21, 1999. WLV will supply no more than 75 mg of the plasmid to BDS
         within two (2) business days of WLV's receipt of at least 10 mg of the
         encochleated mixture of plasmid rIL-12p35 and rIL-12p40. WLV will
         accept the BDS analytical characterization of the encochleated plasmid
         preparations in order to allow prompt initiation of the experiment
         described in Paragraph 4 below.

3.       In accordance with Section 4.1.2 of the Agreement, WLV will make the
         1st quarter payment of year 4, $325,000, on July 1, 1999.

4.       WLV will use the encochleated plasmids produced by BDS to evaluate
         their performance in a primate CTL immunogenicity study for eliciting a
         Significant CTL Response in an assay with a background below 30%
         spontaneous lysis and a


<PAGE>   2

BDS License Amendment
March 30, 1999
Page 2 of 3

         positive control 15% above background. Such Significant CTL Response is
         defined as a level of response to encochleated plasmid 15% over
         background for two (2) consecutive samples in 50% of the animals. It is
         expected that this study including establishing cell lines for each
         primate, vaccination, sampling, running assays, and data analysis will
         be completed by December 31, 1999 if such a study is begun by July 1,
         1999. Such study can begin at this time only if the encochleated
         plasmids identified in Paragraph 1 and 2 above are provided by BDS to
         WLV on or before the indicated dates.

5.       If the encochleated plasmids identified in Paragraphs 1 and 2 are not
         received by WLV by the indicated dates, then

         a)       the December 31, 1999 date for WLV to complete the primate
                  immunogenicity study will be extended by the same amount of
                  time as the difference between when the encochleated plasmids
                  are received by WLV and the date(s) indicated in Paragraphs 1
                  and 2; or

         b)       WLV may determine not to complete the study.

6.       If the primate immunogenicity study is completed and shows Significant
         CTL Response

         a)       and WLV chooses to continue the License Agreement by December
                  31, 1999, WLV would owe BDS $325,000 as the payment of the 2nd
                  quarter of year 4, payable to BDS on January 10, 2000, and
                  agrees to pay the 3rd and 4th quarterly payment for year 4,
                  the 3rd quarter of year 4, payable to BDS on January 15, 2000,
                  and the 4th quarter of year 4, payable to BDS on April 1,
                  2000; or

         b)       and WLV terminates the License Agreement by December 31, 1999,
                  WLV would owe BDS a $325,000 termination fee payable to BDS on
                  January 10, 2000.

7.       If the primate immunogenicity study does not show a Significant CTL
         Response

         a)       and WLV continues the License Agreement, WLV will proceed as
                  per 6 a) above; or

         b)       WLV can terminate the Agreement without any further
                  obligations to BDS.

8.       If WLV does not initiate the primate immunogenicity study due to not
         receiving encochleated plasmids by the specified dates in Paragraphs 1
         and 2, then WLV may terminate the License Agreement without any further
         financial obligation except that identified in Paragraph 3 of this
         Amendment.


<PAGE>   3

BDS License Amendment
March 30, 1999
Page 3 of 3

9.       If BDS provides the encochleated plasmids as per Paragraphs 1 and 2
         above, and WLV does not complete the primate immunogenicity study by
         December 31, 1999, then

         a)       WLV may choose to continue the study and extend the Amended
                  Agreement until no later than March 31, 2000 with a payment of
                  $325,000 as of January 10, 2000, and if WLV chooses to
                  continue the License Agreement after such extension, WLV would
                  owe BDS $325,000 as payment of the 2nd quarter of year 4,
                  payable to BDS within ten (10) business days and the 4th
                  quarterly payment on April 1, 2000; or

         b)       If WLV does not complete the study by December 31, 1999 and
                  chooses to terminate, WLV will owe $325,000 to BDS as a
                  termination fee, payable to BDS on January 10, 2000.

10.      All other terms and conditions of the License Agreement remain the
         same.

         If you are in agreement, please sign below to accept the amendment set
forth herein.

                                             Very truly yours,

                                             American Cyanamid Company

                                             By: /s/ George R. Siber
                                                 -------------------------------
                                                 George R. Siber, M.D.
                                                 Vice President and Chief
                                                 Scientific Officer
                                                 Wyeth-Lederle Vaccines

ACKNOWLEDGED, AGREED TO
AND ACCEPTED:

BIODELIVERY SCIENCES, INC.

By: /s/ Raphael J. Mannino
    -----------------------------
Name: Raphael J. Mannino
Title: President & Chief Executive Officer
Date: April 16, 1999