PRIOR CONCURRENT THERAPY: Biologic therapy: At least 6 months since prior adjuvant immunotherapy for colorectal cancer No concurrent immunotherapy Chemotherapy: At least 6 months since prior adjuvant chemotherapy for colorectal cancer No other concurrent chemotherapy Endocrine therapy: No concurrent hormonal therapy Radiotherapy: See Disease Characteristics At least 6 months since prior adjuvant radiotherapy for colorectal cancer Prior radiotherapy for locally symptomatic disease allowed Surgery: Prior surgery allowed Other: No prior therapy for metastatic disease No other concurrent investigational drugs No concurrent halogenated antiviral agents (e.g., lodenosine, fialuridine, FMAU, emtricitabine, and sorivudine)