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. Dated: July 6, 2005

Dated:  July 6, 2005

                                PERSEUS-XXXXX BIOPHARMACEUTICAL FUND, LP

                                By:     Perseus-Xxxxx Partners, LLC,
                                        General Partner

                                By:     SFM Participation, L.P.,
                                        Managing Member

                                By:     SFM AH LLC
                                        General Partner

                                By:     Xxxxx Fund Management LLC,
                                        Managing Member

                                By:     /s/ Xxx Xxxxxxxxxxxx
                                        ---------------------------------------
                                        Name:  Xxx Xxxxxxxxxxxx
                                        Title: Assistant General Counsel


                                PERSEUS-XXXXX PARTNERS, LLC

                                By:     SFM Participation, L.P.
                                        Managing Member

                                By:     SFM AH LLC
                                        General Partner

                                By:     Xxxxx Fund Management LLC,
                                        Managing Member

                                By:     /s/ Xxx Xxxxxxxxxxxx
                                        ---------------------------------------
                                        Name:  Xxx Xxxxxxxxxxxx
                                        Title: Assistant General Counsel


                                PERSEUS BIOTECH FUND PARTNERS, LLC

                                By:     Perseuspur, L.L.C.
                                        Managing Member

                                By:     /s/ Xxxx Xxxxxxx
                                        ---------------------------------------
                                        Name:  Xxxx Xxxxxxx
                                        Title: Secretary and Treasurer







                                PERSEUSPUR, L.L.C.

                                By:     /s/ Xxxx Xxxxxxx
                                        ---------------------------------------
                                        Name:  Xxxx Xxxxxxx
                                        Title: Secretary and Treasurer






                                XX. XXXXX X. XXXXX

                                By:     /s/ Xxxx Xxxxxxx
                                        ----------------------------------------
                                        Name:  Xxxx Xxxxxxx
                                        Title: Attorney-in-Fact


                                SFM PARTICIPATION, L.P.

                                By:     SFM AH LLC
                                        General Partner

                                By:     Xxxxx Fund Management LLC,
                                        Managing Member

                                By:     /s/ Xxx Xxxxxxxxxxxx
                                        ---------------------------------------
                                        Name:  Xxx Xxxxxxxxxxxx
                                        Title: Assistant General Counsel


                                SFM AH LLC

                                By:     Xxxxx Fund Management LLC,
                                        Managing Member

                                By:     /s/ Xxx Xxxxxxxxxxxx
                                        ---------------------------------------
                                        Name:  Xxx Xxxxxxxxxxxx
                                        Title: Assistant General Counsel


                                MR. XXXXXX XXXXX

                                By:     /s/  Xxx Xxxxxxxxxxxx
                                        ---------------------------------------
                                        Name:  Xxx Xxxxxxxxxxxx
                                        Title: Attorney-in-Fact


                                XXXXX FUND MANAGEMENT LLC

                                By:     /s/ Xxx Xxxxxxxxxxxx
                                        ---------------------------------------
                                        Name:  Xxx Xxxxxxxxxxxx
                                        Title: Assistant General Counsel