DISEASE CHARACTERISTICS:

• Pathologically confirmed non-small cell lung cancer

  • Stage IIIB (with pleural effusion) or IV disease
• Progressive disease after ≥ 12 weeks of treatment with single-agent erlotinib hydrochloride during which time the patient experienced a clinical benefit as assessed by the treating physician and by radiography (e.g., ≥ 1 CT scan demonstrating stable disease or response after ≥ 4 weeks of treatment with erlotinib hydrochloride monotherapy)
• At least 1 measurable lesion

• Baseline diagnostic tumor specimen available for correlative studies

  • Any diagnostic material is acceptable (e.g., paraffin block, cell block, fine-needle aspirate, etc.)
• No third space fluid that cannot be controlled by drainage

• No active CNS metastases as indicated by clinical symptoms, cerebral edema, or progressive growth

  • Patients with a clinical history of CNS metastases or cord compression are eligible provided they have been definitively treated and are clinically stable for ≥ 4 weeks before the first dose of study treatment if treated with prior whole brain radiotherapy (2 weeks if treated with prior gamma knife therapy)

PATIENT CHARACTERISTICS:

• ECOG performance status 0-2
• Life expectancy ≥ 12 weeks
• ANC ≥ 1,500/mm³
• Platelet count ≥ 100,000/mm³
• Hemoglobin ≥ 8.0 g/dL
• Serum creatinine ≤ 1.5 times upper limit of normal (ULN) OR creatinine clearance ≥ 45 mL/min
• Total bilirubin ≤ 1.5 times ULN
• AST and ALT ≤ 2.5 times ULN
• Willing and able to comply with scheduled visits, treatment plans, laboratory tests, and other study procedures
• Able to take folic acid, vitamin B12, and dexamethasone as per study guidelines
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception
• No other severe acute or chronic medical or psychiatric condition or laboratory abnormality that may increase the risk associated with study participation or study drug administration or that may interfere with the interpretation of study results and, in the judgement of the investigator, would make the patient inappropriate for study entry

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics
• At least 2 weeks since prior erlotinib hydrochloride
• At least 3 weeks since prior major surgery, chemotherapy, radiotherapy, investigational agents, or other cancer therapy
• No prior pemetrexed disodium
• No prior EGFR therapy except erlotinib hydrochloride

• No more than 1 prior cytotoxic chemotherapy regimen for relapsed or metastatic disease

  • Erlotinib hydrochloride is not considered a cytotoxic chemotherapy regimen
• No NSAIDs or salicylates for 2 days before (5 days for long-acting NSAIDs [e.g., naproxen, piroxicam, diflunisal, or nabumetone]), during, and for 2 days after administration of pemetrexed disodium