PRIOR CONCURRENT THERAPY: Biologic therapy: At least 4 weeks since prior biologic therapy and recovered No concurrent immunotherapy Chemotherapy: No more than 3 prior chemotherapy regimens for metastatic disease At least 4 weeks since prior chemotherapy (6 weeks for mitomycin and nitrosoureas) and recovered No other concurrent chemotherapy Endocrine therapy: At least 4 weeks since prior anticancer hormonal therapy and recovered No concurrent anticancer hormonal therapy Radiotherapy: At least 4 weeks since prior radiotherapy and recovered No concurrent radiotherapy Surgery: Not specified Other: At least 4 weeks since prior investigational therapy No other concurrent experimental medications