We report a HER2T798I gatekeeper mutation in a patient with HER2L869R-mutant breast cancer with acquired resistance to neratinib. Laboratory studies suggested that HER2L869R is a neratinib-sensitive, gain-of-function mutation that upon dimerization with mutant HER3E928G, also present in the breast cancer, amplifies HER2 signaling. The patient was treated with neratinib and exhibited a sustained partial response. Upon clinical progression, HER2T798I was detected in plasma tumor cell-free DNA. Structural modeling of this acquired mutation suggested that the increased bulk of isoleucine in HER2T798I reduces neratinib binding. Neratinib blocked HER2-mediated signaling and growth in cells expressing HER2L869R but not HER2L869R/T798I. In contrast, afatinib and the osimertinib metabolite AZ5104 strongly suppressed HER2L869R/T798I-induced signaling and cell growth. Acquisition of HER2T798I upon development of resistance to neratinib in a breast cancer with an initial activating HER2 mutation suggests HER2L869R is a driver mutation. HER2T798I-mediated neratinib resistance may be overcome by other irreversible HER2 inhibitors like afatinib.
- Received December 20, 2016.
- Revision received February 26, 2017.
- Accepted March 1, 2017.
- Copyright ©2017, American Association for Cancer Research.