DISEASE CHARACTERISTICS:

• Histologically proven advanced or disseminated colorectal cancer

  • Progressive disease on fluorouracil based chemotherapy OR
  • Recurrence of disease within 12 months of adjuvant therapy with fluorouracil
• No known CNS metastases or carcinomatous meningitis

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Performance status:

• SWOG 0-2

Life expectancy:

• At least 12 weeks

Hematopoietic:

• Granulocyte count greater than 1500/mm^3
• Platelet count greater than 100,000/mm^3
• Hemoglobin at least 9.0 g/dL

Hepatic:

• Bilirubin no greater than 2 times upper limit of normal (ULN)
• SGOT no greater than 3 times ULN (no greater than 5 times ULN if liver involved)

Renal:

• Creatinine no greater than 2.0 mg/dL
• Calcium no greater than 12.0 mg/dL

Cardiovascular:

• No myocardial infarction within past 6 months
• No congestive heart failure requiring therapy

Neurologic:

• No severe psychiatric disorders
• No history of seizures

Other:

• No active or uncontrolled infection
• HIV negative
• No prior malignancy within past 5 years except adequately treated basal cell or squamous cell skin cancer or carcinoma in situ of the cervix
• No uncontrolled diabetes mellitus (random blood sugar 200 mg/dL or greater)
• No other severe concurrent disease
• Not pregnant or nursing
• Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• See Disease Characteristics
• Prior oxaliplatin allowed
• No prior irinotecan or topotecan

Endocrine therapy:

• Not specified

Radiotherapy:

• Not specified

Surgery:

• Not specified

Other:

• No concurrent phenytoin, phenobarbital, or other antiepileptic prophylaxis