AS PER AMENDMENT 1/22/97: The study has been extended for approximately 16 additional weeks beyond the current 6-month extension. Subjects will be unblinded to their assigned regimen beginning 2/21/97 and will continue therapy for up to 16 weeks in open-label fashion. AS PER AMENDMENT 5/9/97: The study has been extended for an additional 8 weeks; study drug will not be provided after 9/15/97.