Combination Chemotherapy in Treating Patients With Colorectal Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed adenocarcinoma of the colon or rectum * Metastatic, locally advanced, or recurrent disease * Patients must have at least 2 generations (parents and grandparents) who belong to one of the following racial groups: * Asian or Pacific Islander (e.g., China, Japan, Korea, the Philippine Islands, or Samoa) * Black (originating from the black racial groups of Africa) * Hispanic (originating from Mexico, Puerto Rico, Cuba, Central or South America, or other Spanish culture) * White (originating from the peoples of Europe, North Africa, or the Middle East) * No patients with parents or grandparents of mixed race or race other than that of the patient PATIENT CHARACTERISTICS: Age: * Not specified Performance status: * CTC 0-2 Life expectancy: * Not specified Hematopoietic: * Granulocyte count at least 1,500/mm3 * Platelet count at least 100,000/mm3 Hepatic: * Bilirubin no greater than 1.5 mg/dL * AST no greater than 3 times upper limit of normal Renal: * Creatinine no greater than 1.5 mg/dL Other: * Not pregnant or nursing * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * Prior adjuvant fluorouracil allowed if relapse occurred at least 6 months after completion of fluorouracil * At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas or mitomycin) * No prior irinotecan * No other concurrent chemotherapy Endocrine therapy: * No concurrent hormones except steroids for adrenal failure, hormones for nondisease related conditions (e.g., insulin for diabetes), or intermittent dexamethasone as an antiemetic * No concurrent prednisone Radiotherapy: * At least 4 weeks since prior radiotherapy (except to bone or soft tissue involving less than 25% of bone marrow) * No concurrent radiotherapy Surgery: * At least 4 weeks since prior major surgery Other: * No concurrent phenobarbital, valproate, or cyclosporine * None of the following concurrently during first course of therapy: * Macrolide antibiotics (e.g., azithromycin, erythromycin, clarithromycin, troleandomycin, dapsone) * Azole antibiotics (e.g., fuconazole, miconazole, itraconazole, ketoconazole) * Triazobenzodiazepines (e.g., alprazolam, midazolam, triazolam) * Antidepressants (e.g., fluoxetine, setraline hydrochloride, fluoxamine, nefazodone hydrochloride) * Quinolone antimicrobials (e.g., ciprofloxacin, ofloxacin) * Imidazole antibiotics (e.g., clotrimazole) * Anti-ulcer medications (e.g., omeprazole, lansoprazole) * Ethinyl estradiol * Diltiazem * Cimetidine hydrochloride * Cisapride * Terfenadine * Rifampin * Glucocorticoids * Antiepileptics * Grapefruit juice