LY231514 Plus Gemcitabine in Treating Women With Metastatic Breast Cancer

DISEASE CHARACTERISTICS: * Histologically or cytologically confirmed breast cancer with clinical evidence of metastatic disease * Bidimensionally measurable disease * If bisphosphonates used, must have measurable disease site other than bone * No bone only disease * Must have received a prior anthracycline and taxane in the adjuvant and/or metastatic setting * No clinically significant pericardial effusions, pleural effusions, or ascites unless they can be drained * No active CNS metastases * Treated CNS metastasis that has ben stable for at least 8 weeks allowed * Hormone receptor status: * Not specified PATIENT CHARACTERISTICS: Age: * 18 and over Sex: * Female Menopausal status: * Not specified Performance status: * ECOG 0-2 Life expectancy: * At least 3 months Hematopoietic: * Absolute neutrophil count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 Hepatic: * Bilirubin no greater than 2 times upper limit of normal (ULN) * AST no greater than 3 times ULN (5 times ULN if liver metastases) * Albumin at least 3.0 g/dL Renal: * Creatinine clearance at least 45 mL/min Cardiovascular: * No New York Heart Association class III or IV heart disease Other: * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * Able to take folic acid and cyanocobalamin (vitamin B12) supplements * Body surface area less than 3 m\^2 * No uncontrolled infection * No chronic debilitating disease * No other prior malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or adequately treated noninvasive carcinomas PRIOR CONCURRENT THERAPY: Biologic therapy: * At least 4 weeks since prior immunotherapy * At least 4 weeks since prior genetic therapy * No concurrent immunomodulating agents Chemotherapy: * See Disease Characteristics * No more than 3 prior chemotherapy regimens including adjuvant therapy * No more than 1 prior chemotherapy regimen for metastatic disease unless these were a taxane and anthracycline * At least 4 weeks since prior chemotherapy * No prior gemcitabine and/or pemetrexed disodium * No other concurrent cytostatic or cytotoxic chemotherapy Endocrine therapy: * Not specified Radiotherapy: * At least 4 weeks since prior radiotherapy * No prior radiotherapy to greater than 25% of bone marrow * No prior strontium chloride Sr 89 * No concurrent radiotherapy Surgery: * At least 4 weeks since prior major surgery Other: * No aspirin or nonsteroidal antiinflammatory agents 2 days before, the day of, and for 2 days after pemetrexed disodium administration (5 days before for long acting agents such as naproxen, piroxicam, diflunisal, or nabumetone)