PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Chemotherapy: See Disease Characteristics Prior adjuvant fluorouracil (5-FU) allowed No more than 2 prior 5-FU regimens (no more than 1 for recurrent or metastatic disease) No other concurrent chemotherapy Endocrine therapy: No concurrent hormonal therapy (except contraceptives or replacement steroids) No concurrent IV steroids Radiotherapy: Prior radiotherapy allowed if measurable disease outside radiation port At least 4 weeks since prior radiotherapy and recovered No concurrent radiotherapy Surgery: Not specified Other: No concurrent anticonvulsant therapy No other concurrent experimental medications