DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed advanced or metastatic colorectal cancer

  • Unresectable disease

• At least 1 bidimensionally measurable lesion

  • At least 2 cm in perpendicular diameters
• No evidence of CNS metastases

PATIENT CHARACTERISTICS:

Age:

• 18 to 75

Performance status:

• 0-1

Life expectancy:

• At least 3 months

Hematopoietic:

• WBC at least 3,500/mm^3
• Platelet count at least 100,000/mm^3

Hepatic:

• Bilirubin normal
• SGOT/SGPT no greater than 2.5 times upper limit of normal (ULN)
• Alkaline phosphatase no greater than 2.5 times ULN

Renal:

• Creatinine no greater than 1.25 times ULN

Cardiovascular:

• No clinically significant cardiac disease
• No congestive heart failure
• No New York Heart Association class III or IV heart disease
• No symptomatic coronary artery disease
• No uncontrolled cardiac arrhythmia
• No myocardial infarction within the past year

Gastrointestinal:

• No evidence of dysphagia
• No malabsorption or intestinal obstruction that would affect absorption or excretion of study drugs
• No chronic diarrhea

Other:

• No other malignancy within the past 5 years except adequately treated carcinoma in situ of the cervix or localized nonmelanoma skin cancer
• No psychiatric disability that would preclude study compliance
• No other significant medical condition
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• No prior chemotherapy for advanced or metastatic disease

Endocrine therapy:

• Not specified

Radiotherapy:

• Not specified

Surgery:

• At least 4 weeks since prior major surgery to the gastrointestinal tract

Other:

• No concurrent therapy for history of seizures or CNS disorder