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/ Mark N. Lampert
                                            ----------------------
                                            Mark N. Lampert
                                            President


                  BVF PARTNERS L.P.

                  By:      BVF Inc., its general partner


                           By:      /s/ Mark N. Lampert
                                    ---------------------
                                    Mark N. Lampert
                                    President


                  BVF INC.


                  By:      /s/ Mark N. Lampert
                           ---------------------
                           Mark N. Lampert
                           President