Neratinib +/- Fulvestrant in Metastatic HER2 Non-amplified But HER2 Mutant Breast Cancer

Overexpression of HER2 due to gene amplification is an established therapeutic target in breast cancer for which multiple HER2 targeted drugs are now available. However, the majority of breast cancers are without HER2 overexpression/non-amplified and not currently eligible to receive HER2 targeted drugs. Advances in tumor genome sequencing technology led to the identification of recurrent HER2 mutations (HER2mut) in approximately 2% of HER2 non-amplified primary breast cancers, and 3-5% of metastatic tumors. Importantly, tumor cells harboring HER2mut are sensitive to the anti-tumor effects of HER2-targeted agents in preclinical models, especially neratinib, a potent irreversible pan-HER inhibitor. However, neratinib monotherapy has demonstrated only modest single agent activity in HER2mut,, non-amplified metastatic breast cancer (MBC). Based on the hypothesis that the combination of neratinib and fulvestrant will be more effective than neratinib alone in ER+/HER2mut, non-amplified MBC the investigators conducted a single arm phase II study of neratinib plus fulvestrant with 2 cohorts, fulvestrant (FUL)-treated and FUL-naïve, for patients with ER+/HER2mut, non-amplified MBC to assess the anti-tumor effects of this combination. An exploratory ER-negative (ER-) HER2mut cohort was also included for the efficacy of neratinib monotherapy.