Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically or cytologically confirmed breast cancer

- Locally advanced or metastatic disease

- HER2-overexpressing tumor, defined as 3+ overexpression by IHC and/or HER2 amplified
by FISH

- Non-measurable disease allowed

- Achieved partial response, complete response, or stable disease (i.e., no disease
progression for ≥ 12 weeks) while on trastuzumab (Herceptin®) and chemotherapy,
hormonal therapy alone, or trastuzumab alone

- Last dose of trastuzumab must have been administered within the past 16 weeks

- No unstable brain metastases

- Patients with brain metastases are eligible provided they have been stable for ≥
1 month after surgery or radiotherapy/radiosurgery AND off corticosteroids and
anticonvulsants for ≥ 4 weeks

- Hormone receptor status unspecified

PATIENT CHARACTERISTICS:

- ECOG(Eastern Cooperative Oncology Group)performance status (PS) 0-2 (Karnofsky PS
70-100%)

- Absolute neutrophil count ≥ 1,500/mm³

- Hemoglobin > 8 g/dL (transfusion/epoetin alfa allowed)

- Platelet count ≥ 100,000/mm³

- Total bilirubin < 1.5 times upper limit of normal (ULN)

- AST and ALT ≤ 2.5 times ULN (≤ 5.0 times ULN if known liver metastases)

- Creatinine < 2 mg/mL

- Ejection fraction ≥ 50% by echocardiogram or MUGA

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception before, during, and for ≥ 3 months
after completion of study treatment

- No ongoing or active infection requiring oral or IV antibiotics

- No known autoimmune disorders or antibody-mediated disorders

- No known HIV positivity

- No known history of hepatitis B or C (active and/or previously treated)

- No other malignancies within the past 5 years except nonmelanoma skin cancer or
cervical cancer in situ

- No history of allergic reactions attributed to compounds of similar chemical or
biologic composition to study drugs

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- More than 12 weeks since prior chloroquine

- More than 4 weeks since prior growth factors

- More than 4 weeks since prior systemic corticosteroids

- More than 4 weeks since prior chemotherapy, radiotherapy, or monoclonal antibody
therapy (except trastuzumab)

- No prior agatolimod sodium

- No prior allogeneic stem cell transplantation

- No prior continuous treatment with single-agent trastuzumab for > 6 months

- No more than 3 prior chemotherapy regimens for metastatic breast cancer

- Any number of prior hormonal therapies allowed

- No other concurrent investigational agents or monoclonal antibodies

- No other concurrent anticancer agents or therapies

- Concurrent bisphosphonates for skeletal metastasis allowed