HER2 Activating Mutations in Estrogen Receptor Positive Breast Cancer

Purpose of Review: HER2 activating mutations are a new, druggable mutation identified by next-generation DNA sequencing (NGS) of breast cancer. Here, we review the recent data on the diagnosis and treatment of HER2 mutated, metastatic breast cancer. Recent Findings: Pre-clinical studies have shown that HER2 activating mutations accelerate tumor growth and can be inhibited by HER2 targeted drugs, including trastuzumab and the second-generation, pan-HER tyrosine kinase inhibitor, neratinib. HER2 mutations can be diagnosed by NGS testing on either a tumor biopsy or circulating tumor DNA obtained from peripheral blood. Case reports provided initial evidence that HER2 targeted therapies can effectively treat patients with HER2 mutated, metastatic breast cancer. Two phase II clinical trials, MutHER and SUMMIT, both demonstrate that neratinib monotherapy has clinical efficacy for these patients, with clinical benefit rate of 31–40%. Summary: HER2 targeted therapies are effective for HER2 mutated breast cancer but emergence of drug resistance remains a problem. Clinical trials are now testing neratinib-containing drug combination regimens for HER2 mutated, metastatic breast cancer patients.