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

Inclusion Criteria:

Willing and able to provide written informed consent;
Men and women ≥18 years of age;
Pathologically confirmed diagnosis of HER2 positive breast cancer of any stage (previous treatment is allowed without limits on lines of prior therapy);
Scheduled to receive at least 3 consecutive cycles of THP or TCHP;
Performance status of 0-2 according to the ECOG scale;
Negative pregnancy test at time of informed consent for women of childbearing potential;
Able to read, understand, follow the study procedure and complete crofelemer, rescue medication, and bowel movement diaries;
Patients may enroll simultaneously on this study and other studies, including but not limited to NSABP B52;
Patients with brain metastases (including concurrent steroid treatment) are allowed on this study.
Left Ventricular Ejection Fraction (LVEF) greater or equal to 50% at baseline as determined by either ECHO or MUGA

Exclusion Criteria:

Pregnant and/or breastfeeding;
Ongoing irritable bowel syndrome (IBS) or colitis (including but not limited to ulcerative colitis, Crohn's disease, microscopic colitis, etc.);
Use of investigational drugs within 3 weeks of signing consent or foreseen use during the study;
Use of chemotherapy, trastuzumab, or pertuzumab within the past 3 weeks;
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);
Any type of ostomy;
Total colectomy;
Fecal incontinence;
Ongoing radiation induced diarrhea or constipation or planned radiotherapy to the abdomen or pelvis while on study;
Active systemic infection requiring ongoing intervention, including but not limited to oral and intravenous antibiotics, anti-fungals, anti-parasites, anti-virals;
Abdominal or pelvic surgery without recovery of bowel function;

Inadequate organ function for starting THP or TCHP, which may include the following laboratory results within 28 days prior to signing consent:

Total bilirubin > upper limit of normal (ULN) (unless the patient has documented Gilbert's syndrome)
Serum creatinine > 2.0 mg/dL or 177 μmol/L
AST (SGOT) and ALT (SPGT) > 2.5 ULN.