PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Chemotherapy: At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas and mitomycin) and recovered No other concurrent chemotherapy Phase I: At least 4 weeks since prior irinotecan and recovered Phase II: No more than 2 prior fluorouracil-based regimens for metastatic disease No prior irinotecan Endocrine therapy: See Disease Characteristics No concurrent anticancer hormonal therapy Radiotherapy: No concurrent radiotherapy Surgery: See Disease Characteristics Other: No other concurrent experimental medications