PRIOR CONCURRENT THERAPY: Biologic therapy: Prior bone marrow transplantation (BMT) or peripheral blood stem cell transplantation (PBSCT) allowed and recovered At least 2 weeks since prior cytokine therapy and recovered No concurrent immune modulator therapy No concurrent cytokines including interleukin-11, interleukin-2, and epoetin alfa Chemotherapy: At least 2 weeks since prior chemotherapy (4 weeks for nitrosoureas) and recovered No more than 3 prior chemotherapy regimens No other concurrent chemotherapy Endocrine therapy: No concurrent steroids Radiotherapy: No prior craniospinal radiotherapy Prior total body irradiation allowed as part of BMT or PBSCT and recovered Concurrent radiotherapy for localized painful lesions allowed Surgery: Not specified Other: No concurrent metoclopramide or cisapride to maintain motility or gastric emptying No concurrent H2 antagonists No concurrent proton pump inhibitors No concurrent antacids for gastritis, gastroesophageal reflux, or ulcers (gastric or duodenal) No antacid therapy for 6 hours before and for 90 minutes after topotecan administration