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