Vaccine Therapy With or Without Polysaccharide-K in Patients With Stage IV HER2 Positive Breast Cancer Receiving HER2-Targeted Monoclonal Antibody Therapy

PRIMARY OBJECTIVES: I. To evaluate the safety of polysaccharide-K (PSK) when given with HER2-directed immunotherapy. SECONDARY OBJECTIVES: I. To evaluate the effect of PSK on natural killer (NK) cell functional activity when given with HER2-directed immunotherapy. TERTIARY OBJECTIVES: I. To investigate the effect of PSK when given with HER2-directed immunotherapy on: serum levels of pro-inflammatory cytokine and/or chemokines; intermolecular epitope spreading; serum transforming growth factor (TGF)-beta levels; progression free survival (PFS) and overall survival (OS). OUTLINE: Patients are randomized to 1 of 2 treatment arms. ARM I: Patients receive HER2 ICD peptide-based vaccine intradermally (ID) once monthly for 3 months, trastuzumab (or trastuzumab and pertuzumab) per standard of care, and placebo orally (PO) twice daily (BID) for 4 months. ARM II: Patients receive HER2 ICD peptide-based vaccine ID and trastuzumab (or trastuzumab and pertuzumab) as in Arm I and polysaccharide-K PO BID for 4 months. After completion of study treatment, patients are followed up for 9 months and then twice annually for 3 years.