PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: Prior topotecan, paclitaxel, and carboplatin allowed if progression observed greater than 3 months after last dose No prior mitomycin or nitrosoureas No more than 1 prior chemotherapy regimen At least 4 weeks since chemotherapy and recovered (alopecia allowed) Endocrine therapy: Not specified Radiotherapy: No prior radiotherapy to more than 25% of bone marrow At least 4 weeks since radiotherapy and recovered (alopecia allowed) Surgery: Not specified