PRIOR CONCURRENT THERAPY: Biologic therapy: At least 4 weeks since prior immunotherapy and recovered from toxic effects No concurrent immunotherapy Chemotherapy: At least 4 weeks since prior chemotherapy (at least 6 weeks since prior mitomycin or nitrosoureas) and recovered No other concurrent chemotherapy Endocrine therapy: Concurrent hormonal therapy allowed if patient has progressive disease while receiving hormonal therapy for at least three months Radiotherapy: At least 4 weeks since prior radiotherapy and recovered No concurrent radiation therapy Surgery: At least 21 days since prior major surgery Other: Any drugs that affect hepatic or renal function must be given as a stable dose and should not be initiated after patient enters study