DISEASE CHARACTERISTICS:

• Histologically or cytologically determined metastatic colorectal cancer*

  • Primary lesion confirmed endoscopically, surgically, or radiologically NOTE: * Patients with a history of colorectal cancer treated by surgical resection who develop radiological or clinical evidence of metastatic cancer do not require separate histological or cytological confirmation of metastatic disease, unless more than 5 years between primary surgery and development of metastatic disease OR primary cancer was stage I

• Measurable disease, defined as ≥ 1 unidimensionally measurable lesion ≥ 20 mm by conventional techniques OR ≥ 10 mm by spiral CT scan

  • Nonmeasurable lesions include the following:

    • Bone lesions
    • Leptomeningeal disease
    • Ascites
    • Pleural/pericardial effusion
    • Lymphangitis cutis/pulmonis
    • Abdominal masses not confirmed and followed by imaging techniques
    • Cystic lesions

• UGT1A1*1 homozygous or UGT1A1*28 heterozygous genotype status

  • Patients with homozygous UGT1A1*28 genotype not eligible
• Received at least 1 prior regimen with oxaliplatin for metastatic disease
• Recurrent disease following prior adjuvant therapy allowed

PATIENT CHARACTERISTICS:

• ECOG performance status 0-1
• Granulocyte count ≥ 1,500/mm^3
• Platelet count ≥ 100,000/mm^3
• Creatinine normal
• Bilirubin normal
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception duration and for 3 months after completion of study treatment
• No known Gilbert's disease or other chronic liver disease
• No colonic or small bowel disorders (e.g., inflammatory bowel disease, Crohn's disease, or ulcerative colitis) that predispose the patients to uncontrolled diarrhea (i.e., > 3 watery or soft stools daily at baseline in patients without a colostomy or ileostomy)

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics
• Recovered from prior therapy
• No prior irinotecan
• Prior pelvic radiotherapy allowed as long as measurable lesion is outside irradiated field
• No concurrent palliative radiotherapy
• No other concurrent chemotherapy
• No concurrent steroids except those given for adrenal failure, hormones for non-disease-related conditions (e.g., insulin for diabetes), or intermittent dexamethasone as an antiemetic or for prevention of infusion reaction