DISEASE CHARACTERISTICS: Histologically proven metastatic, advanced, or locally recurrent fluorouracil (5-FU) refractory adenocarcinoma of the colon or rectum Progression of disease within 6 months of receiving adjuvant 5-FU based chemotherapy OR Progression of disease during or following completion of 5-FU based chemotherapy for metastatic disease No more than 1 failure with a 5-FU regimen for recurrent, advanced, or metastatic disease Failure on 5-FU adjuvant therapy must accompany failure on 5-FU therapy for metastatic disease Bidimensionally measurable disease CNS metastases allowed only with measurable disease in other sites Prior treatment for CNS metastases required

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: WBC at least 3500/mm3 Absolute neutrophil count at least 1500/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.5 mg/dL AST and ALT no greater than 3 times upper limit of normal Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance at least 60 mL/min Other: No other significant, uncontrolled underlying medical or psychiatric condition No serious active infection Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception Neurologically stable No other prior malignancy within past 5 years except curatively treated nonmelanomatous skin cancer

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