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

DISEASE CHARACTERISTICS: Histologically proven hormone refractory stage IV prostate cancer Hormone refractory disease is defined as: Progressive measurable disease OR Progressive disease by bone scan OR Increase in PSA by 50% over nadir level confirmed twice and measured at least two weeks apart Prior treatment with primary androgen ablative therapy with castrate levels of testosterone Minimum score of 2 on the McGill 6 point pain scale secondary to metastatic bony pain with an analgesic score of at least 4 No CNS metastases PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 70-100% Life expectancy: Greater than 16 weeks Hematopoietic: Absolute neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hemoglobin at least 8.0 g/dL (without blood transfusion(s) within 2 weeks prior to study) Hepatic: Bilirubin less than 1.5 times upper limit of normal (ULN) AST no greater than 2.5 times ULN Renal: Creatinine no greater than 2.0 mg/dL Cardiovascular: No cardiac failure No myocardial infarction within the past 6 months No uncontrolled hypertension LVEF greater than 50% Other: Fertile patients must use effective barrier contraception during and for 3 months after study No known hypersensitivity to polysorbate or polyethylene glycol No other malignancies within past 5 years, except basal cell skin cancer No other acute or chronic medical, psychiatric, or lab abnormality that would prevent compliance No uncontrolled peptic ulcer No active infection No contraindication to mitoxantrone therapy No contraindication to prednisone therapy PRIOR CONCURRENT THERAPY: Biologic therapy: At least 4 weeks since prior biologic response modifiers At least 4 weeks since prior immunotherapy No concurrent immunotherapy Chemotherapy: No prior SU101 or mitoxantrone No prior cytotoxic chemotherapy for prostate cancer No other concurrent chemotherapy Endocrine therapy: See Disease Characteristics At least 4 weeks since prior antiandrogen therapy and recovered Concurrent primary androgen ablation therapy (orchidectomy, luteinizing hormone releasing hormone (LHRH) agonist (if stable dose), estrogen, or cyproterone acetate) allowed No concurrent antiandrogen therapy (except LHRH) No concurrent cholestyramine Radiotherapy: At least 4 weeks since prior radiotherapy (8 weeks since strontium 89 and samarium 153) Prior palliative radiotherapy to metastatic sites allowed No prior radiotherapy to greater than 50% of bone marrow No concurrent radiotherapy except for palliation of bone pain Surgery: At least 2 weeks since prior major surgery No concurrent surgery for prostate cancer Other: At least 4 weeks since prior investigational therapy At least 4 weeks since prior antiangiogenesis therapy At least 6 weeks since prior bicalutamide No other concurrent investigational therapy