DISEASE CHARACTERISTICS: Histologically confirmed advanced solid tumors for which no standard therapy exists or that is persistent or recurrent following prior therapy Measurable or evaluable disease No hematologic malignancies (e.g., leukemia or lymphoma) No known brain or leptomeningeal disease (unless lesions have been irradiated, are currently untreated with corticosteroids, and have no clinical symptoms)

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: SWOG 0-2 Karnofsky 70-100% Life expectancy: At least 12 weeks Hematopoietic: Absolute neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hemoglobin greater than 9 g/dL Hepatic: Bilirubin less than 1.5 times upper limit of normal (ULN) AST and ALT less than 3 times ULN Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance at least 60 mL/min Calcium no greater than 12 mg/dL Cardiovascular: No myocardial infarction within 6 months No congestive heart failure requiring therapy No unstable angina Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception No active infection No other concurrent severe disease No psychiatric disorders No history of seizure disorders No uncontrolled diabetes (blood sugar no greater than 200 mg/dL)

PRIOR CONCURRENT THERAPY: Biologic therapy: At least 4 weeks since prior biologic therapy and recovered Chemotherapy: At least 4 weeks since prior chemotherapy and recovered No prior mitomycin, irinotecan, or nitrosoureas No more than 6 courses of chemotherapy containing an alkylating agent No more than 4 courses of carboplatin Endocrine therapy: At least 4 weeks since prior endocrine therapy and recovered Radiotherapy: At least 4 weeks since prior radiotherapy and recovered No prior radiotherapy to more than 20% of bone marrow No prior whole pelvis radiotherapy Surgery: Not specified Other: No concurrent phenytoin, phenobarbital, valproic acid, or other antiepileptic prophylaxis No concurrent warfarin