Incidence and Severity of Diarrhea in Patients With HER2 Positive Breast Cancer Treated With Trastuzumab and Neratinib

PRIMARY OBJECTIVE: I. To characterize the incidence and severity of diarrhea in patients with early stage breast cancer receiving adjuvant neratinib with or without trastuzumab in the setting of anti-diarrheal strategies. SECONDARY OBJECTIVES: I. To evaluate the incidence of grade 3 or higher diarrhea using the dose-escalation strategy and anti-diarrhea medications as needed (prn) in patients who received prior trastuzumab emtansine (T-DM1). II. To assess neratinib adherence, holds, delays, and early discontinuation throughout the course of study therapy which includes patients receiving neratinib for \> 1 year. III. To assess overall toxicity including constipation and cardiac toxicity with concomitant neratinib and trastuzumab. OUTLINE: This is a dose-escalation study of neratinib. Patients receive one of the following treatment regimen: NERATINIB MONOTHERAPY: Patients receive neratinib orally (PO) once daily (QD) on days 1-21. Cycles repeats every 21 days for up to 55 weeks in the absence of disease progression or unacceptable toxicity. Patients may receive loperamide and/or diphenoxylate hydrochloride/atropine sulfate as needed per physician discretion for symptom management. NERATINIB AND TRASTUZUMAB: Patients receive neratinib PO QD on days 1-21. Patients also receive trastuzumab maintenance therapy as determined by treating physician. Treatment repeats every 21 days for up to 55 weeks in the absence of disease progression or unacceptable toxicity. After completion of trastuzumab treatment, patients may continue on neratinib monotherapy for the remainder of the 55 weeks. Patients may receive loperamide and/or diphenoxylate hydrochloride/atropine sulfate as needed per physician discretion for symptom management. After completion of studies treatment, patients are followed up for 28 days.