An Acquired <i>HER2</i>  T798I Gatekeeper Mutation Induces Resistance to Neratinib in a Patient with HER2 Mutant–Driven Breast Cancer

We report a <i>HER2</i>^T798I gatekeeper mutation in a patient with <i>HER2</i>^L869R-mutant breast cancer with acquired resistance to neratinib. Laboratory studies suggested that <i>HER2</i>^L869R is a neratinib-sensitive, gain-of-function mutation that upon dimerization with mutant HER3^E928G, also present in the breast cancer, amplifies HER2 signaling. The patient was treated with neratinib and exhibited a sustained partial response. Upon clinical progression, <i>HER2</i>^T798I was detected in plasma tumor cell-free DNA. Structural modeling of this acquired mutation suggested that the increased bulk of isoleucine in HER2^T798I reduces neratinib binding. Neratinib blocked HER2-mediated signaling and growth in cells expressing HER2^L869R but not HER2^L869R/T798I In contrast, afatinib and the osimertinib metabolite AZ5104 strongly suppressed HER2^L869R/T798I-induced signaling and cell growth. Acquisition of HER2^T798I upon development of resistance to neratinib in a breast cancer with an initial activating <i>HER2</i> mutation suggests <i>HER2</i>^L869R is a driver mutation. HER2^T798I-mediated neratinib resistance may be overcome by other irreversible HER2 inhibitors like afatinib.<b>Significance:</b> We found an acquired <i>HER2</i> gatekeeper mutation in a patient with <i>HER2</i>-mutant breast cancer upon clinical progression on neratinib. We speculate that <i>HER2</i>^T798I may arise as a secondary mutation following response to effective HER2 tyrosine kinase inhibitors (TKI) in other cancers with <i>HER2</i>-activating mutations. This resistance may be overcome by other irreversible HER2 TKIs, such as afatinib. <i>Cancer Discov; 7(6); 575-85. ©2017 AACR.</i><i>This article is highlighted in the In This Issue feature, p. 539</i>.