Abstract P3-03-03: An acquired HER2 T798I gatekeeper mutation induces resistance to neratinib in a patient with HER2 mutant-driven breast cancer
Authors: Hanker AB , Red Brewer M , Sheehan JH , Koch JP , Lanman R , Hyman DM , Cutler RE , Lalani AS , Cross D , Lovly CM , Meiler J , Arteaga CL 
Published: March 2017

Abstract
ERBB2, the gene encoding HER2, is mutated in 2-4% of breast cancers. The HER2 irreversible tyrosine kinase inhibitor (TKI) neratinib has shown clinical activity against breast cancer cells harboring HER2 activating mutations. Here, we report for the first time an acquired gatekeeper HER2T798I mutation in a patient with HER2-mutant breast cancer after an initial exceptional response to neratinib.
A patient with ER+/PR+/HER2-negative invasive lobular breast cancer progressing on standard therapy was found to harbor a L869R kinase domain mutation in HER2. HER2L869R is homologous to the known activating mutation EGFRL861R/Q. MCF10A breast epithelial cells expressing HER2L869R displayed enhanced HER2-mediated signaling and were resistant to lapatinib and trastuzumab but sensitive to neratinib. The patient was enrolled in the phase II SUMMIT trial (NCT01953926) and treated with neratinib, achieving a partial response lasting 16 months before developing progression. Next gen sequencing of DNA from both a new skin metastasis and plasma cell-free DNA (cfDNA) identified HER2L869R (8.7% cfDNA), whereas a novel HER2T798I mutation was detected only in plasma at 1.3%. Deep sequencing of pre-therapy tumor tissue and plasma did not detect HER2T798I, suggesting that this mutation arose upon resistance. HER2T798I has not been reported in TCGA, COSMIC, or among plasma samples from 17,345 cancer patients subjected to digital DNA sequencing using the Guardant360 assay.
HER2T798I is homologous to the EGFRT790M, KITT670I and BCR-ABLT315I gatekeeper mutations known to mediate resistance to erlotinib/gefitinib and imatinib. To examine if HER2T798I mediates resistance to neratinib, we employed biochemical and biological assays and molecular modeling of wild-type (WT) HER2 and HER2T798I. Structural modeling showed the increased bulk of the isoleucine at position 798 would result in a steric clash with neratinib, thus reducing drug binding. We stably expressed HER2WT, HER2T798I, HER2L869R and HER2L869R/T798I in MCF10A cells and NR6 mouse fibroblasts. Neratinib (10-100 nM) blocked HER2-mediated signaling in cells expressing HER2WT or HER2L869R but did not in cells expressing HER2T798I. The EGFR irreversible TKI osimertinib (100 nM), which isselective for mutant EGFR (including EGFRT790M) and approved for treatment of NSCLC expressing EGFRT790M, failed to inhibit HER2WT, HER2L869R or HER2T798I. In contrast, either the EGFR/HER2 irreversible TKI afatinib or AZ5104, a metabolite of osimertinib, strongly blocked signaling induced by HER2WT, HER2L869R or HER2T798I. Cells expressing HER2T798M displayed a significantly higher IC50 to neratinib than cells expressing HER2WT, whereas afatinib or AZ5014 were very active against all cells (IC50&lt;10 nM).
Conclusions: The acquisition of a T798I gatekeeper mutation in HER2 upon development of clinical resistance to neratinib in a breast cancer with an initial activating mutation in HER2 strongly suggests that HER2L869R is a driver mutation. We speculate that HER2T798I may arise as a secondary mutation following response to effective HER2 TKIs in other cancers with HER2 activating mutations. Certain irreversible EGFR inhibitors may be effective in patients with HER2-driven breast cancer resistant to neratinib.
Citation Format: Hanker AB, Red Brewer M, Sheehan JH, Koch JP, Lanman R, Hyman DM, Cutler, Jr. RE, Lalani AS, Cross D, Lovly CM, Meiler J, Arteaga CL. An acquired HER2 T798I gatekeeper mutation induces resistance to neratinib in a patient with HER2 mutant-driven breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-03-03.