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,...

 
                                   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:  April 24, 1998

          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