RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Combining more than one drug may kill more tumor cells.

PURPOSE: Phase II trial to study the effectiveness of combination chemotherapy in treating patients who have metastatic colorectal cancer.

OBJECTIVES:

• Determine the response rate of patients with metastatic colorectal carcinoma treated with O6-benzylguanine and carmustine.
• Evaluate tumor tissue biochemical and immunohistochemical predictors of response in patients treated with this regimen.
• Correlate carcinoembryonic antigen levels in the peripheral blood with response in patients treated with this regimen.

OUTLINE: This is a multicenter study.

Patients receive O6-benzylguanine (BG) IV over 1 hour and carmustine IV over 1 hour (beginning 1 hour after completion of BG infusion) on day 1. Treatment continues every 6 weeks in the absence of disease progression or unacceptable toxicity.

Patients are followed every 3 months.

PROJECTED ACCRUAL: A total of 19-36 patients will be accrued for this study within 1.7-3 years.

DISEASE CHARACTERISTICS:

• Diagnosis of metastatic colorectal carcinoma
• Bidimensionally measurable disease

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Performance status:

• ECOG 0-2

Life expectancy:

• At least 12 weeks

Hematopoietic:

• WBC at least 4,000/mm^3
• Granulocyte count at least 1,500/mm^3
• Absolute neutrophil count at least 1,500/mm^3
• Platelet count at least 100,000/mm^3
• Hemoglobin greater than 8 g/dL

Hepatic:

• Bilirubin no greater than 1.2 mg/dL
• AST and ALT less than 2.5 times upper limit of normal (ULN)
• PT no greater than ULN (not on anticoagulation therapy)

Renal:

• Creatinine no greater than 1.5 mg/dL
• Creatine clearance greater than 60 mL/min

Pulmonary:

• DLCO at least 60%

Other:

• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception during and for 2 months after study
• No other concurrent active malignancies
• Prior malignancies presumed to be cured allowed
• No other concurrent uncontrolled severe medical problem that would preclude study participation

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• At least 6 months since prior adjuvant chemotherapy without disease recurrence
• No more than 1 prior chemotherapy regimen for metastatic disease

Endocrine therapy:

• Not specified

Radiotherapy:

• Prior adjuvant radiotherapy allowed
• No prior radiotherapy to more than 25% of total bone marrow

Surgery:

• Not specified

Other:

• No other prior therapy for advanced disease