PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior topoisomerase 1 inhibitor (topotecan, irinotecan, 9- aminocamptothecin) No more that 1 prior chemotherapy regimen At least 6 weeks since nitrosourea and recovered At least 3 weeks since any other chemotherapy and recovered Endocrine therapy: Not specified Radiotherapy: At least 3 months since completion of most recent course of radiotherapy and recovered Surgery: Not specified Other: No concurrent investigational agents At least 14 days since prior valproic acid No concurrent valproic acid