Inclusion Criteria:

- Histologically or cytologically confirmed metastatic breast cancer

- Documented expression of CD117 (c-kit) or platelet-derived growth factor receptor

- Adequate tumor tissue from either the primary tumor and/or metastatic disease
available for evaluation

- Must have received prior chemotherapy with an anthracycline (doxorubicin or
epirubicin) and/or taxane (paclitaxel or docetaxel) as adjuvant or for advanced
disease

- At least 1 unidimensionally measurable lesion

- At least 20 mm by conventional techniques OR at least 10 mm by spiral CT scan

- Bone disease may not be only source of measurable disease

- Pleural or peritoneal ascites are not considered measurable disease

- No known brain metastases

- Hormone receptor status:

- Not specified

- Female or male

- Not specified

- Performance status - ECOG 0-2

- Performance status - Karnofsky 60-100%

- More than 12 weeks

- Absolute neutrophil count at least 1,500/mm^3

- WBC at least 3,000/mm^3

- Platelet count at least 100,000/mm^3

- Bilirubin normal

- AST or ALT no greater than 2.5 times upper limit of normal

- Creatinine normal

- Creatinine clearance at least 60 mL/min

- No symptomatic congestive heart failure

- No unstable angina pectoris

- No cardiac arrhythmia

- No other uncontrolled concurrent illness

- No ongoing or active infection

- No prior allergic reaction attributed to compounds of similar chemical or biologic
composition to imatinib mesylate

- No other malignancy within the past 5 years except basal cell skin cancer or
carcinoma in situ of the cervix

- No psychiatric illness or social situation that would preclude study compliance

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective barrier contraception during and for 1 week
after study

- No concurrent biologic agents

- No more than 2 prior chemotherapy regimens for metastatic disease

- Therapy with high-dose regimens or bone marrow transplantation considered 1
regimen

- At least 4 weeks since prior chemotherapy (6 weeks for carmustine or mitomycin) and
recovered

- No concurrent chemotherapy

- Prior hormonal therapy for stage IV disease and/or as adjuvant therapy allowed

- At least 4 weeks since prior radiotherapy and recovered

- Prior localized radiotherapy that does not influence the signal of the evaluable
lesion is allowed

- At least 2 weeks since prior minor surgery

- At least 4 weeks since prior major surgery

- Recovered from prior surgery

- Low-molecular weight heparin or heparin allowed for anticoagulation

- No concurrent warfarin

- No concurrent combination antiretroviral therapy for HIV-positive patients

- No concurrent investigational therapies or agents

- No other concurrent anticancer therapy

- No concurrent intake of cola, orange juice, grapefruit, or orange or grapefruit
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