Diarrhea Prevention and Prophylaxis With Crofelemer in HER2 Positive Breast Cancer Patients

Inclusion Criteria: 1. Willing and able to provide written informed consent; 2. Men and women ≥18 years of age; 3. Pathologically confirmed diagnosis of HER2 positive breast cancer of any stage (previous treatment is allowed without limits on lines of prior therapy); 4. Scheduled to receive at least 3 consecutive cycles of THP or TCHP; 5. Performance status of 0-2 according to the ECOG scale; 6. Negative pregnancy test at time of informed consent for women of childbearing potential; 7. Able to read, understand, follow the study procedure and complete crofelemer, rescue medication, and bowel movement diaries; 8. Patients may enroll simultaneously on this study and other studies, including but not limited to NSABP B52; 9. Patients with brain metastases (including concurrent steroid treatment) are allowed on this study. 10. Left Ventricular Ejection Fraction (LVEF) greater or equal to 50% at baseline as determined by either ECHO or MUGA Exclusion Criteria: 1. Pregnant and/or breastfeeding; 2. Ongoing irritable bowel syndrome (IBS) or colitis (including but not limited to ulcerative colitis, Crohn's disease, microscopic colitis, etc.); 3. Use of investigational drugs within 3 weeks of signing consent or foreseen use during the study; 4. Use of chemotherapy, trastuzumab, or pertuzumab within the past 3 weeks; 5. Use of antibiotics within the past 7 days (up to 2 prophylactic doses of antibiotics for procedures including, but not limited to port placement, is permitted); 6. Any type of ostomy; 7. Total colectomy; 8. Fecal incontinence; 9. Ongoing radiation induced diarrhea or constipation or planned radiotherapy to the abdomen or pelvis while on study; 10. Active systemic infection requiring ongoing intervention, including but not limited to oral and intravenous antibiotics, anti-fungals, anti-parasites, anti-virals; 11. Abdominal or pelvic surgery without recovery of bowel function; 12. Inadequate organ function for starting THP or TCHP, which may include the following laboratory results within 28 days prior to signing consent: 1. Total bilirubin \> upper limit of normal (ULN) (unless the patient has documented Gilbert's syndrome) 2. Serum creatinine \> 2.0 mg/dL or 177 μmol/L 3. AST (SGOT) and ALT (SPGT) \> 2.5 ULN.