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 Amendment
containing the information required by Schedule 13D, to which this Agreement is
attached as an exhibit, is filed on behalf of each of them.  The undersigned
further agree that any further 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