DISEASE CHARACTERISTICS: Histologically or cytologically confirmed limited or extensive stage small cell lung cancer (SCLC) Disease recurring at least 90 days following completion of first line chemotherapy Partial or complete response to first line therapy Must have at least one bidimensionally measurable non CNS lesion May be within a prior radiation port if at least 6 weeks since prior radiotherapy and progressing Brain and/or leptomeningeal metastases allowed if asymptomatic and not requiring corticosteroids

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: At least 2 months Hematopoietic: WBC at least 3,500/mm3 Neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hemoglobin at least 9.0 g/dL (after transfusion, if needed) Hepatic: Bilirubin no greater than 2.0 mg/dL SGOT and SGPT no greater than 2 times upper limit of normal (ULN) (no greater than 5 times ULN if liver metastases present) Alkaline phosphatase no greater than 2 times ULN (no greater than 5 times ULN if liver metastases present) Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance at least 60 mL/min Other: No active uncontrolled infection No other malignancies within the past 5 years except curatively treated basal or squamous cell skin cancer, carcinoma in situ of the cervix, or stage I low grade prostate cancer No other severe medical conditions that would preclude study or cause exposure to extreme risk or decreased life expectancy No uncontrolled emesis No active peptic ulcer, diabetes mellitus, chronic gastritis, significant ascites, or other gastrointestinal (GI) conditions (e.g., removal of a portion of the stomach or recent GI obstruction) that would alter absorption or GI motility No history of allergic reactions to compounds chemically related to topotecan Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception for 3 months prior to, during, and at least 4 weeks after the study

PRIOR CONCURRENT THERAPY: Biologic therapy: At least 3 months since prior immunotherapy No concurrent immunotherapy for SCLC Chemotherapy: See Disease Characteristics No prior topotecan Only one prior chemotherapy regimen allowed No other concurrent chemotherapy for SCLC Endocrine therapy: See Disease Characteristics Radiotherapy: See Disease Characteristics At least 24 hours since prior radiotherapy No concurrent radiotherapy for SCLC Surgery: At least 4 weeks since prior surgery Other: At least 30 days or five half lives since other prior investigational drugs No prior drugs (e.g., cisapride) that would alter absorption or GI motility No other concurrent investigational therapy for SCLC