DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed relapsed or progressive small cell lung cancer

  • Classical or intermediate variant OR
  • Relapsed or progressive extrapulmonary small cell carcinoma of unknown origin

• Bidimensionally measurable disease

  • At least 1 cm by 1 cm by physical exam or radiologic exam
  • Outside prior radiation port unless clinical evidence of disease progression
• Previously radiated brain metastases allowed provided stable or improved

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Performance status:

• ECOG 0-2

Life expectancy:

• More than 12 weeks

Hematopoietic:

• Absolute neutrophil count at least 1,500/mm3
• Platelet count at least 100,000/mm3

Hepatic:

• Bilirubin no greater than 2 times upper limit of normal (ULN)
• SGOT/SGPT no greater than 3 times ULN (5 times ULN if liver metastases present)
• Alkaline phosphatase no greater than 5 times ULN

Renal:

• Creatinine no greater than 2 mg/dL

Other:

• HIV negative
• No AIDS-related illness
• No frequent vomiting or medical condition that would interfere with oral medication administration (e.g., partial bowel obstruction)
• No active nonmalignant systemic disease that would preclude study
• No other active invasive malignancy within the past year or concurrently requiring ongoing treatment
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception for at least 1 month before, during, and for at least 3 months after study

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• At least 4 weeks since prior biologic therapy
• No concurrent immunotherapy
• No concurrent biologic therapy
• Concurrent epoetin alfa allowed

Chemotherapy:

• At least 4 weeks since prior chemotherapy
• No more than 1 prior continuous or discontinuous chemotherapy regimen for metastatic disease
• No other concurrent chemotherapy

Endocrine therapy:

• Concurrent hormonal therapy to boost appetite allowed (e.g., corticosteroids or medroxyprogesterone)

Radiotherapy:

• See Disease Characteristics
• At least 4 weeks since prior radiotherapy to 15% or more of bone marrow
• At least 1 week since prior radiotherapy to less than 15% of bone marrow
• No prior radiotherapy to 50% or more of bone marrow
• No concurrent radiotherapy

Surgery:

• Not specified

Other:

• Recovered from prior therapy
• No other concurrent investigational drugs
• Concurrent pamidronate allowed