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)