Pemetrexed Disodium Plus Gemcitabine in Treating Patients With Locally Advanced or Metastatic Non-Small Cell Lung Cancer

DISEASE CHARACTERISTICS: Histologically or cytologically confirmed locally advanced or metastatic non-small cell lung cancer Stage IIIB disease that is ineligible for combined modality therapy OR Stage IV disease Measurable disease No clinically detectable (by physical exam) third-space fluid collection (e.g., ascites or pleural effusions) that cannot be controlled by drainage or other procedures No brain metastases (even if treated) PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-1 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 times ULN (5 times ULN if liver metastases present) Alkaline phosphatase no greater than 3 times ULN (5 times ULN if liver metastases present) Renal: Creatinine clearance at least 45 mL/min Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception Able to take folic acid or cyanocobalamin (vitamin B12) supplementation No uncontrolled infection No concurrent chronic debilitating disease No weight loss of 10% or more within the past 6 weeks No other prior malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or noninvasive carcinoma PRIOR CONCURRENT THERAPY: Biologic therapy: No prior biologic or genetic therapy for lung cancer No concurrent immunomodulating agents Chemotherapy: No prior chemotherapy for lung cancer Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy to a different site and recovered No prior radiotherapy to 25% or more of bone marrow No prior radiotherapy to whole pelvis No prior radiotherapy for primary disease No concurrent radiotherapy Surgery: More than 4 weeks since prior major surgery Other: No aspirin or nonsteroidal anti-inflammatory agents 2 days before, during, and for 2 days after pemetrexed disodium administration (5 days for long-acting agents such as piroxicam, naproxen, diflunisal, or nabumetone) No other concurrent cytostatic or cytotoxic therapy