Agatolimod and Trastuzumab in Treating Patients With Locally Advanced or Metastatic Breast Cancer

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