PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 80-100% Life expectancy: Not specified Hematopoietic: WBC at least 3000/mm3 Neutrophil count at least 1500/mm3 Platelet count at least 100,000/mm3 Hemoglobin at least 10 g/dL (red cell transfusions allowed) Hepatic: Bilirubin no greater than 1.5 times the upper limit of normal (ULN) (except for patients with Gilbert's syndrome) ALT and/or AST no greater than 2.5 times ULN (no greater than 4 times ULN with liver metastases) Alkaline phosphatase no greater than 2.5 times ULN (no greater than 4 times ULN with liver metastases) Renal: Creatinine no greater than 1.5 times ULN Cardiovascular: No history of severe cardiac disease: New York Heart Association class II (if not due to cancer), III, or IV Myocardial infarction within the past 6 months Ventricular tachyarrhythmias requiring ongoing treatment Unstable angina Other: Not pregnant or nursing Fertile patients must use effective contraception No other prior or concurrent malignancy except basal cell carcinoma of the skin, carcinoma in situ of the cervix, or any other malignancy from which the patient has been disease free for at least 2 years No history of medically significant psychiatric disease, especially depression No history or evidence of retinopathy No seizure disorders