JOINT FILING AGREEMENT

                                    EXHIBIT 6

                             JOINT FILING AGREEMENT


         Each of the undersigned hereby acknowledges and agrees, in compliance
with the provisions of Rule 13d-1(k)(1) promulgated under the Securities
Exchange Act of 1934, as amended, that the Schedule 13D to which this Agreement
is attached as an Exhibit (the "Schedule 13D"), and any amendments thereto, will
be filed with the Securities and Exchange Commission jointly on behalf of the
undersigned.

         This Agreement may be executed in one or more counterparts.

                                    PERSEUS-XXXXX BIOPHARMACEUTICAL FUND,
                                    LP

                                    By:     Perseus-Xxxxx Partners, LLC,
                                            General Partner

                                    By:     SFM Participation, LP,
                                            Member

                                    By:     SFM AH, Inc.,
                                            General Partner


                                    By:   /S/ XXXXXXX X. XXXX
                                        -------------------------------
                                        Name:  Xxxxxxx X. Xxxx
                                        Title: Vice President

                                    PERSEUS-XXXXX PARTNERS, LLC

                                    By:     SFM Participation, L.P.,
                                            Member

                                    By:     SFM AH, Inc.,
                                            General Partner


                                    By:   /S/ XXXXXXX X. XXXX
                                        --------------------------------
                                        Name:  Xxxxxxx X. Xxxx
                                        Title: Vice President




CUSIP No. 000000000                                                Page 2 of 3




                                    PERSEUS BIOTECH FUND PARTNERS, LLC


                                    By:   /S/ XXXXXXX X. XXXXX
                                       ---------------------------------
                                         Name: Xxxxxxx X. Xxxxx
                                         Title: Member

                                    XX. XXXXX X. XXXXX


                                    By:   /S/ XXXXX X. XXXXX
                                         -------------------------------
                                         Name:  Xxxxx X. Xxxxx

                                    XX. XXXXXXX X. XXXXX


                                    By:   /S/ XXXXXXX X. XXXXX
                                         --------------------------------
                                         Name: Xxxxxxx X. Xxxxx

                                    SFM PARTICIPATION, L.P.

                                    By:     SFM AH, Inc.,
                                            General Partner


                                    By:   /S/ XXXXXXX X. XXXX
                                         --------------------------------
                                         Name:  Xxxxxxx X. Xxxx
                                         Title: Vice President

                                    SFM AH, INC.


                                    By:   /S/ XXXXXXX X. XXXX
                                         ---------------------------------
                                         Name:  Xxxxxxx X. Xxxx
                                         Title:  Vice President

                                    MR. XXXXXX XXXXX


                                    By:   /S/ XXXXXXX X. XXXX
                                         ----------------------------------
                                         Name:  Xxxxxxx X. Xxxx
                                         Title:  Attorney-in-fact



CUSIP No.  000000000                                               Page 3 of 3


                                    XXXXX FUND MANAGEMENT LLC


                                    By:   /S/ XXXXXXX X. XXXX
                                       ------------------------------------
                                         Name:  Xxxxxxx X. Xxxx
                                         Title: Assistant General Counsel

                                    XX. XXXXXXX X. XXXXXXXXXXXXX


                                    By:   /S/ XXXXXXX X. XXXX
                                        -----------------------------------
                                         Name:  Xxxxxxx X. Xxxx
                                         Title:  Attorney-in-fact