PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent biologic therapy Chemotherapy: See Disease Characteristics At least 4 weeks since prior chemotherapy and recovered No prior camptothecin analogues No other concurrent chemotherapy Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy and recovered No concurrent radiotherapy Surgery: At least 4 weeks since prior major surgery and recovered No concurrent surgery Other: No other concurrent anticancer therapy At least 4 weeks since prior investigational drugs (including analgesics or antiemetics) No other investigational drugs during or within 28 days after final dose of study drug