Breast Cancer Clinical Trials

• Have pathologically documented BC that:

‣ Is locally advanced, unresectable or metastatic.

⁃ Has a confirmed HER2 status as determined by the local laboratory as standard of care testing prior to study screening (Part 1, Part 2 Cohorts 2 and 4) or the central laboratory (Part 2, Cohorts 1 and 3) from the most recently collected pre-randomization tumor sample.

⁃ Has a documented history of HER2 expression consistent with the subgroup definitions (i.e., HER2-low, HER2-ultralow, HER2-null, HER2-positive, or TNBC) as per current American Society of Clinical Oncology/College of American Pathologists guidelines.

• Have measurable disease defined by RECIST v1.1.

• Has left ventricular ejection fraction ≥55% by either echocardiography or multi-gated acquisition (scanning) within 28 days before randomization/enrollment.