Mitoxantrone and Prednisone With or Without Leflunomide in Treating Patients With Stage IV Prostate Cancer

RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Combining more than one drug may kill more tumor cells. It is not yet known if mitoxantrone and prednisone are more effective with or without leflunomide for treating prostate cancer. PURPOSE: Randomized phase II/III trial to compare the effectiveness of mitoxantrone and prednisone with or without leflunomide in treating patients who have stage IV prostate cancer that has not responded to hormone therapy.