DISEASE CHARACTERISTICS: Histologically confirmed platinum-refractory small cell lung cancer No response or progression during or within 6 months of completing platinum based therapy Measurable disease No symptomatic brain or leptomeningeal metastases

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: SWOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Granulocyte count at least 1,500/mm3 Platelet count at least 100,000/mm3 No hemolytic anemia Hepatic: Bilirubin normal AST no greater than 2.5 times upper limit of normal Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance at least 60 mL/min Cardiovascular: No history of cardiac arrhythmias Other: No other active malignancy requiring concurrent treatment No allergy to sulfonamides Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: See Disease Characteristics No more than 2 prior chemotherapy regimens At least 3 weeks since prior chemotherapy and recovered Endocrine therapy: No concurrent corticosteroids to control symptoms of brain and/or leptomeningeal metastases Radiotherapy: At least 4 weeks since prior radiotherapy No concurrent radiotherapy to measurable lesions Surgery: Not specified Other: No concurrent hypoglycemic agents (including insulin)