A Study to Characterize Colon Pathology in Patients With HER2 Amplified Breast Cancer Treated With Neratinib

This is an open-label, phase 2 study that will investigate colon pathology in patients with HER2-positive breast cancer treated with neratinib as monotherapy. All patients will receive neratinib for the first 28 days as a single daily dose of 240 mg. Colonoscopy will be performed after eligibility has been confirmed, but prior to administration of the first dose of neratinib and at Day 30 (± 3 days) the conclusion of Cycle 1 (28 days). Following the second study colonoscopy procedure: * For patients being treated for stage 1 to 3c breast cancer in the extended adjuvant setting, neratinib will continue to be administered at a single daily dose of 240 mg until completion of one year of therapy from start of treatment, or until disease recurrence (as determined by the Investigator), death, unacceptable toxicity, or other specified withdrawal criterion. * For patients being treated for metastatic breast cancer (mBC), capecitabine will be introduced after the second study colonoscopy procedure at a dose of 750mg/m2 twice daily for 14 days of each 21 day treatment cycle, with neratinib administered continuously throughout at 240 mg daily, until disease progression, death, unacceptable toxicity, or other specified withdrawal criterion. All patients will receive loperamide diarrhea prophylaxis daily for one 28-day cycle and then as needed.