Impact of Anti-HER2 Therapies on Overall Survival in Patients with HER2-Positive Metastatic Breast Cancer: Focusing on Intracranial Efficacy of Emerging Treatments

Therapies targeting human epidermal growth factor receptor 2 (HER2) have substantially improved overall survival in patients with HER2-positive metastatic breast cancer. Approximately 31% of these patients develop brain metastases, representing a significant therapeutic challenge. This review classifies anti-HER2 therapies into three categories: monoclonal antibodies (MABs), antibody-drug conjugates (ADCs), and tyrosine kinase inhibitors (TKIs). The mechanisms of action and clinical impacts of these agents are examined, with particular attention to intracranial efficacy. The introduction of trastuzumab increased overall survival (OS) from 20.3 to 25.1 months compared to chemotherapy alone. The addition of pertuzumab further extended survival to 57.1 months, as demonstrated in the CLEOPATRA trial. Among ADCs, T-DM1 improved OS to 29.9 months versus 25.9 months in the EMILIA trial, while T-DXd extended OS to 52.6 months in DESTINY-Breast03. T-DXd also demonstrated notable intracranial activity, achieving a 64.9% objective response rate in patients with active brain metastases. In the HER2CLIMB trial, tucatinib reduced intracranial progression by 68% and improved OS (24.7 vs. 19.2 months) in patients with active brain metastases. Recent advances have increased median OS from approximately 20 months prior to trastuzumab to over 50 months with current therapies. Future research should focus on optimizing treatment sequencing, refining biomarker-driven approaches, and developing targeted strategies for brain metastases to further improve long-term survival outcomes.