Irinotecan and Gemcitabine in Treating Patients With Recurrent or Progressive Stage III or Stage IV Non-Small Cell Lung Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed non-small cell lung cancer (NSCLC), meeting 1 of the following stage criteria: * Stage IIIA or IIIB disease * Progressive disease * Stage IV disease * Failed 1 prior platinum-based chemotherapy regimen, including adjuvant therapy * Measurable or evaluable disease, as defined by 1 of the following criteria: * At least 1 unidimensionally measurable lesion ≥ 20 mm by conventional techniques OR \> 10 mm by spiral CT scan * Lesions apparent on CT scan that do not meet the criterion for measurability * Brain metastases allowed provided patient has received definitive therapy for metastases, is asymptomatic, and has extra-CNS metastases PATIENT CHARACTERISTICS: Age * 18 and over Performance status * ECOG 0-1 Life expectancy * More than 12 weeks Hematopoietic * WBC ≥ 3,000/mm\^3 * Absolute neutrophil count ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 Hepatic * AST and ALT ≤ 1.5 times normal * Alkaline phosphatase \< 1.5 times normal * Bilirubin ≤ 1.3 mg/dL Renal * Creatinine ≤ 1.6 mg/dL OR * Creatinine clearance ≥ 50 mL/min Cardiovascular * No unstable angina * No congestive heart failure * No myocardial infarction within the past 3 months * No life-threatening ventricular arrhythmia requiring maintenance therapy Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective barrier contraception during and for 3 months after completion of study treatment * No uncontrolled seizure disorder * No uncontrolled diabetes mellitus * No active infection requiring systemic therapy * No other malignancy within the past 5 years except nonmelanoma skin cancer or carcinoma in situ of the cervix * No other unstable or serious condition PRIOR CONCURRENT THERAPY: Biologic therapy * Not specified Chemotherapy * See Disease Characteristics * At least 3 weeks since prior chemotherapy * Prior irinotecan allowed * No prior gemcitabine * No more than 1 prior chemotherapy regimen for NSCLC, excluding gefitinib * No other concurrent chemotherapy Endocrine therapy * Not specified Radiotherapy * At least 4 weeks since prior radiotherapy * No concurrent radiotherapy Surgery * Not specified Other * More than 1 month since prior participation in another clinical trial using an investigational agent * No other concurrent investigational agents