PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: See Disease Characteristics No more than one prior chemotherapy regimen At least 4 weeks since prior chemotherapy Prior nitrosourea required At least 6 weeks since nitrosourea Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics More than 3 months since radiotherapy Surgery: Not specified Other: No concurrent treatment with other investigational agents or other antitumor agents