Belotecan and Cisplatin in Treating Patients With Previously Untreated, Extensive-Stage Small Cell Lung Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed small cell lung cancer meeting the following criteria: * Extensive-stage disease * Previously untreated disease * At least one measurable disease * No brain or leptomeningeal metastasis PATIENT CHARACTERISTICS: * ECOG performance status 0-2 * Life expectancy ≥ 12 weeks * ANC ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 * Hemoglobin ≥ 9.0 g/dL * Total bilirubin ≤ 1.5 mg/dL * ALT and AST ≤ 2.0 times upper normal limit (ULN) (≤ 5.0 times ULN in the presence of liver metastasis) * Alkaline phosphatase ≤ 2.0 times ULN * Creatinine ≤ 1.5 mg/dL OR creatinine clearance ≥ 60 mL/min * No active infection requiring intravenous antibiotics * No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or in situ cancer * No other severe acute or chronic medical condition or laboratory abnormality that may increase the risk associated with study participation or investigational product administration, interfere with the interpretation of study results, or make the patient ineligible for study entry, in the judgment of the investigator PRIOR CONCURRENT THERAPY: * No prior chemotherapy or radiotherapy * No other concurrent chemotherapy, radiotherapy, or immunotherapy