PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Concurrent colony stimulating factors (e.g., filgrastim (G-CSF), sargramostim (GM-CSF)) allowed only during time off treatment during each course Chemotherapy: No prior chemotherapy for metastatic disease At least 6 months since prior adjuvant chemotherapy and recovered Prior adjuvant fluorouracil allowed provided not infusional No prior fluorouracil-uracil with or without leucovorin calcium, capecitabine, S-1, or ethynyl uracil No other concurrent chemotherapy Endocrine therapy: Prior hormonal therapy for metastatic disease or in adjuvant setting allowed Recovered from toxicity No concurrent hormonal anticancer therapy Radiotherapy: Prior radiotherapy for metastatic disease or in adjuvant setting allowed At least 2 weeks since prior radiotherapy and recovered No prior radiotherapy to greater than 30% of bone marrow No concurrent radiotherapy except for palliation of painful bone metastases, pathologic fractures of known lytic disease, or brain lesions that develop Surgery: Not specified Other: No other concurrent investigational therapy No other concurrent anticancer drugs No concurrent halogenated antiviral agents (e.g., lodenosine, fialuridine, L-FMAU, emtricitabine, or sorivudine) No concurrent initiation of bisphosphonate therapy for development of new bone lesions or progression of existing bone lesions