PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: See Disease Characteristics Greater than 4 weeks since prior chemotherapy No prior irinotecan or topotecan Endocrine therapy: Not specified Radiotherapy: Greater than 4 weeks since prior radiotherapy No radiotherapy to greater than 30% of bone marrow Surgery: Not specified Other: No concurrent phenytoin, phenobarbitol, or other antiepileptic prophylaxis