ERBB2 and brain neoplasm: However, with improved control of systemic disease and prolonged survival, the incidence of brain metastases is increasing in these patients.2, 3 Currently, as many as half of patients with HER2‐positive, metastatic breast cancer develop brain metastases over time.4 Treatment of these brain tumors is a growing clinical challenge, in large part due to the poor penetration of HER2‐targeted agents through the blood–brain barrier (BBB).4, 5