PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy No concurrent sargramostim (GM-CSF) Chemotherapy: See Disease Characteristics At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas, mitomycin, or suramin) No prior irinotecan, topotecan, or other topotecan 1 inhibitors No other concurrent chemotherapy Endocrine therapy: Stable or decreasing dosage of corticosteroids within 72 hours of study entry (phase II only) No other concurrent immunosuppressive agents No concurrent hormonal therapy Radiotherapy: At least 4 weeks since prior radiotherapy Patients with prior interstitial brachytherapy or stereotactic radiosurgery must have confirmation of progressive disease No concurrent radiotherapy Surgery: At least 3 weeks since prior resection Other: Acute toxic effects (excluding neurotoxicity or alopecia) of any prior therapy must be resolved No concurrent valproic acid as a single agent Concurrent enzyme-inducing antiepileptic drugs (EIAED) with or without steroids are allowed No concurrent investigational drugs