Lometrexol Plus Folic Acid in Treating Patients With Stage IIIB or Stage IV Non-Small Cell Lung Cancer

DISEASE CHARACTERISTICS: Histologically confirmed non-small cell lung cancer (NSCLC) Stage IIIB or IV Squamous cell carcinoma Adenocarcinoma Large cell carcinoma Adenosquamous carcinoma Progressive disease after receiving 1 cisplatin- or carboplatin-containing regimen for stage IIIB or IV disease Measurable disease At least 1 lesion at least 1 cm x 1 cm by CT scan either not in a previously irradiated field or showing clear evidence of disease progression after radiation No symptomatic or rapidly increasing, moderate or large amounts of pleural effusion or ascites No prior or concurrent CNS metastases (brain or meningeal) PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 70-100% Life expectancy: At least 3 months Hematopoietic: Absolute neutrophil count at least 1,500/mm^3* Platelet count at least 100,000/mm^3* Hemoglobin at least 9.0 g/dL* *Without transfusions or growth factors in the previous 7 days Hepatic: Bilirubin no greater than 1.5 times upper limit of normal (ULN) (regardless of liver involvement secondary to tumor) AST and ALT no greater than 2.5 times ULN (5 times ULN if liver has tumor involvement) Renal: Calculated creatinine clearance at least 60 mL/min using Cockroft and Gault formula Gastrointestinal: No inflammatory bowel disease No radiation enteritis No malabsorption syndrome No inability to absorb folic acid Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception during study No known untreated vitamin B12 deficiency HIV negative No drug abusers No prior malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or carcinoma in situ of the cervix No severe concurrent disease or major comorbidity that would preclude study participation PRIOR CONCURRENT THERAPY: Biologic therapy: At least 3 weeks since prior biologic therapy for NSCLC and recovered from acute side effects Prior treatment with an experimental vaccine allowed No concurrent routine or prophylactic filgrastim (G-CSF), sargramostim (GM- CSF), or epoetin alfa Chemotherapy: See Disease Characteristics At least 3 weeks since prior chemotherapy for NSCLC (6 weeks for mitomycin or nitrosourea) and recovered from acute side effects Prior adjuvant or neoadjuvant chemotherapy allowed Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics Recovered from acute side effects of prior radiotherapy No prior radiotherapy to 25% or more of bone marrow No prior whole pelvic irradiation Surgery: At least 3 weeks since prior major surgery and recovered Other: At least 3 weeks since prior investigational agent No concurrent proguanil, trimethoprim, co-trimoxazole, or pyrimethamine