• ≥ 18 years old at the time of informed consent

• Ability to provide written informed consent and HIPAA authorization CTO-IUSCC-0864

• Histologically confirmed HER2 positive breast cancer

∙ Any Estrogen Receptor/Progesterone Receptor status is allowed.

‣ HER2 positive is defined as HER2 3+ by immunohistochemistry (IHC) or 2+ by IHC associated with a fluorescence in situ hybridization (FISH) ratio of \> 2.0 or \> 6 total HER2 gene copies per cell.

• High-risk disease defined as one of the following:

∙ Any residual invasive carcinoma in the breast or axillary nodes in the final pathology from resected tumor following neoadjuvant taxane and trastuzumab-based chemotherapy

‣ Inflammatory phenotype at the time of diagnosis per the treating physician

‣ Clinical stage III disease at the time of diagnosis per the treating physician and/or clinical imaging

‣ Locally recurrent disease and have undergone definitive local therapy

• Received at least six months of HER2 targeted therapy with trastuzmab +/- pertuzumab TDM-1, or others in the neoadjuvant or adjuvant setting

• a. Any combination of HER2 targeted therapy in the curative setting is allowed, including neratinib or others on a clinical trial

• Completed last dose of HER2 targeted therapy no more than 6 months prior to registration

• Completed last dose of cytotoxic chemotherapy or radiation at least 30 days prior to registration with resolution of any prior toxicity to ≤ 2 with the exception of alopecia

• ECOG performance status of 0 to 2

• Adequate organ function as indicated by:

∙ Total bilirubin \< 1.5 mg/dL (except in patients with documented Gilbert's disease, who must have a total bilirubin \< 3.0 mg/dL)

‣ Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) \< 2.0 x ULN

‣ Calculated creatinine clearance of \> 60 mL/min using the Cockcroft-Gault formula

‣ Absolute neutrophil count (ANC) \> 1.0 K/mm3

‣ Platelets \> 100 K/ mm3

⁃ Adequate cardiac function defined as left ventricular ejection fraction (LVEF) above the institutional lower limit of normal by echocardiogram or MUGA obtained within 90 days of registration

⁃ Women of childbearing potential must have a negative serum pregnancy test within 14 days of protocol registration. Women are considered to have childbearing potential (regardless of sexual orientation, having undergone a tubal ligation, or remaining celibate by choice) unless they meet one of the following criteria:

• Has undergone a hysterectomy or bilateral oophorectomy; or

∙ Has been naturally amenorrheic for at least 12 consecutive months.

⁃ Women of childbearing potential and men must agree to use one effective contraception throughout the study and for 6 months after the last study treatment.

⁃ Note: Acceptable methods of birth control include abstinence, partner with previous vasectomy, placement of an intrauterine device (IUD), condom with spermicidal foam/gel/film/cream/suppository, diaphragm or cervical vault cap, or hormonal birth control (pills or injections).