PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: More than 12 weeks Hematopoietic: Absolute neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hemoglobin at least 9 g/dL Hepatic: Bilirubin no greater than 2 times upper limit of normal (ULN) SGOT and SGPT no greater than 2 times ULN (5 times ULN if documented liver metastases) Alkaline phosphatase no greater than 2 times ULN (5 times ULN if documented liver metastases) Renal: Blood urea nitrogen no greater than 1.5 times ULN Creatinine no greater than 1.5 times ULN Other: No active nonmalignant systemic disease that would increase risk No frequent vomiting or medical condition that would interfere with oral medication intake (e.g., partial bowel obstruction) No other malignancy within the past 5 years except surgically cured carcinoma in situ of the cervix or basal cell or squamous cell skin cancer HIV negative No AIDS-related illness Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception