PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent prophylactic growth factors Chemotherapy: One prior adjuvant or neoadjuvant 5-FU-based chemotherapy regimen allowed At least 4 weeks since prior adjuvant or neoadjuvant chemotherapy (6 weeks if nitrosoureas or mitomycin) and recovered No prior therapy with a topoisomerase I inhibitor No other concurrent chemotherapy Endocrine therapy: Not specified Radiotherapy: One prior radiotherapy regimen allowed No prior radiation therapy to only site of measurable disease At least 4 weeks since prior radiation therapy and recovered No concurrent palliative radiation therapy Surgery: Not specified Other: See Disease Characteristics No other concurrent investigational antineoplastic therapy