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