HER2HEART-US: Prevention of Cardiotoxicity in Breast Cancer Patients Receiving HER2-directed Therapy

Inclusion Criteria: * Histologically or cytologically confirmed HER2+ breast cancer by ASCO/CAP guidelines of any clinical or pathologic stage. * Planning to commence standard of care HER2-directed therapy or started HER2-directed therapy within 6 months prior to randomization * At least 18 years of age. * Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 * For patients newly commencing HER2-directed therapy, left ventricular ejection fraction (LVEF) ≥ 50% up to 30 days prior to enrollment detected by echocardiogram. For patients already receiving HER2-directed therapy, LVEF ≥ 50% after the last cycle of therapy prior to enrollment. (Patient will be enrolled at the time of their next cycle after the echocardiogram.) * Systolic blood pressure ≥ 100 mmHg and resting heart rate ≥ 60 bpm. * eGFR \> 30 mL/min/1.73m\^2. * Women of childbearing potential must agree to use adequate contraception prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. * Patients with an acceptable support system (as determined by the treating medical team). * Ability to understand and willingness to sign an Institutional Review Board (IRB) approved written informed consent document. Legally authorized representatives may sign and give informed consent on behalf of study participants. Exclusion Criteria: * Prior exposure to mantle c…