Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically or cytologically confirmed small cell lung cancer (SCLC)

- Extensive stage, including malignant pleural effusion

- Measurable disease

- At least 1 unidimensionally measurable lesion at least 20 mm by conventional
techniques or 10 mm by spiral CT scan*

- Chest x-rays are acceptable if the lesions are clearly defined and
surrounded by aerated lung

- Clinically detected lesions must be superficial (e.g., skin nodules or
palpable lymph nodes) to be considered measurable

- Skin lesions are to be documented by color photography, including a ruler
to estimate the size of the lesion NOTE: *A solitary measurable lesion must
be histologically or cytologically confirmed

- No symptomatic brain metastases

- No clinically relevant third-space fluid collections (e.g., ascites or pleural
effusions) by physical examination that cannot be controlled by drainage or other
procedures

PATIENT CHARACTERISTICS:

Age

- 18 and over

Performance status

- ECOG 0-2

Life expectancy

- At least 12 weeks

Hematopoietic

- Absolute neutrophil count at least 1,500/mm^3

- Platelet count at least 100,000/mm^3

- Hemoglobin at least 9 g/dL

Hepatic

- Bilirubin no greater than 1.5 times upper limit of normal (ULN)

- AST and ALT no greater than 3.0 times ULN*

- Alkaline phosphatase no greater than 3.0 times ULN* NOTE: *5 times ULN if liver has
tumor involvement

Renal

- Creatinine clearance at least 45 mL/min

Other

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception during and for 3 months after study
treatment

- Able and willing to take folic acid, cobalamin (vitamin B_12), or dexamethasone

- Able to tolerate interruption of aspirin or other nonsteroidal anti-inflammatory
agents for a 5-day period (8-day period for long-acting agents such as piroxicam)

- No second primary malignancy except the following:

- Carcinoma in situ of the cervix

- Adequately treated nonmelanoma skin cancer

- Prior low-grade (Gleason score no greater than 6) localized prostate cancer

- Other previously treated malignancy with no evidence of recurrence within the
past 5 years

- No active infection

- No other serious concurrent systemic disorder that would preclude patient safety or
study completion

PRIOR CONCURRENT THERAPY:

Biologic therapy

- No prior systemic biologic therapy for SCLC

- No prior systemic immunotherapy for SCLC

- No concurrent prophylactic filgrastim (G-CSF)

- No concurrent thrombopoiesis stimulators

- No concurrent immunotherapy

Chemotherapy

- No prior systemic chemotherapy for small cell lung cancer

- No prior pemetrexed disodium

- No other concurrent chemotherapy

Endocrine therapy

- No concurrent hormonal cancer therapy

Radiotherapy

- No prior radiotherapy

- No concurrent radiotherapy

Surgery

- No concurrent anticancer surgery

Other

- More than 30 days since prior investigational drugs

- No prior enrollment on this study

- No other concurrent experimental medications (except thymidine)

- No other concurrent anticancer therapy