DISEASE CHARACTERISTICS: Histologically proven advanced solid tumor for which no curative therapy exists No leukemias or lymphomas Brain or leptomeningeal disease allowed only if prior irradiation of these lesions, no concurrent corticosteroids, and no clinical symptoms

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Absolute neutrophil count at least 2,000/mm3 Platelet count at least 150,000/mm3 Hemoglobin at least 9.0 g/dL Hepatic: Bilirubin no greater than 1.5 mg/dL regardless of liver tumor presence SGOT no greater than 3 times upper limit of normal (ULN) (5 times ULN if due to liver tumor) Renal: Creatinine no greater than 2.0 mg/dL Cardiovascular: No uncontrolled high blood pressure No unstable angina No symptomatic congestive heart failure No myocardial infarction within past 6 months No serious uncontrolled cardiac arrhythmia Gastrointestinal: No active inflammatory bowel disease No significant bowel obstruction No chronic malabsorption Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception No active infection HIV negative No AIDS related illness No significant alcoholism or substance abuse No severe medical or psychiatric illness that would preclude participation in study

PRIOR CONCURRENT THERAPY: At least 4 weeks since prior therapy for cancer and recovered Biologic therapy: No concurrent immunotherapy No concurrent filgrastim (G-CSF), sargramostim (GM-CSF), or epoetin alfa Chemotherapy: No prior nitrosoureas, mitomycin, or irinotecan No prior high dose chemotherapy in support of peripheral blood stem cell transplantation No other concurrent chemotherapy Endocrine therapy: No concurrent hormonal therapy Radiotherapy: No prior irradiation of pelvis No concurrent radiotherapy Surgery: No prior gastrectomy or total colectomy Other: At least 1 week since prior anticonvulsants No other concurrent investigational drugs No combination anticholinergic medications containing barbiturates (e.g., Donnatol)