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.