EXHIBIT A AGREEMENT REGARDING JOINT FILING The undersigned, Biotechnology Value Fund, L.P., a Delaware limited partnership, and BVF Partners L.P., a Delaware limited partnership, and BVF Inc., a Delaware corporation, hereby agree and acknowledge that...

                                    EXHIBIT A


                        AGREEMENT REGARDING JOINT FILING



         The undersigned, Biotechnology Value Fund, L.P., a Delaware limited
partnership, and BVF Partners L.P., a Delaware limited partnership, and BVF
Inc., a Delaware corporation, hereby agree and acknowledge that the information
required by Schedule 13G, to which this Agreement is attached as an exhibit, is
filed on behalf of each of them. The undersigned further agree that any
amendments or supplements thereto shall also be filed on behalf of each of them.


Dated:  November 8, 1999


                  BIOTECHNOLOGY VALUE FUND, L.P.

                  By:      BVF Partners L.P., its general partner

                           By:      BVF Inc., its general partner


                                    By:     /s/ Xxxx X. Xxxxxxx
                                            ----------------------
                                            Xxxx X. Xxxxxxx
                                            President


                  BVF PARTNERS L.P.

                  By:      BVF Inc., its general partner


                           By:      /s/ Xxxx X. Xxxxxxx
                                    ---------------------
                                    Xxxx X. Xxxxxxx
                                    President


                  BVF INC.


                  By:      /s/ Xxxx X. Xxxxxxx
                           ---------------------
                           Xxxx X. Xxxxxxx
                           President