PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 3 weeks since prior chemotherapy (6 weeks for nitrosoureas or mitomycin) and recovered No persistent chronic toxicity from prior chemotherapy greater than grade 1 Endocrine therapy: Not specified Radiotherapy: At least 3 weeks since prior radiotherapy Concurrent radiotherapy to single site of progressive disease allowed during first course of study treatment Surgery: Not specified Other: At least 3 weeks since any prior treatment for malignancy and recovered No other concurrent investigational drug