Acquired Resistance ofEGFR-Mutant Lung Cancer to a T790M-Specific EGFR Inhibitor

EGFR-mutant lung cancers represent a paradigm for the use of tyrosine kinase inhibitors (TKIs) to treat molecular subsets of cancer, with randomized trials demonstrating the efficacy of first-generation, reversible epidermal growth factor receptor (EGFR) TKIs. However, acquired resistance invariably develops, and rebiopsy of patients with clinical progression has elucidated EGFR T790M mutation as the major resistance mechanism.1 To address this clinical challenge, third-generation pyrimidine-based EGFR TKIs have been designed to have selectivity for EGFR T790M over wild-type EGFR.2 They form a covalent bond with cysteine 797 in the adenosine triphosphate–binding cleft. Early-phase clinical trials have demonstrated their efficacy in patients with double-mutant tumors (EGFR L858R/T790M and ex19del/T790M) and acquired resistance to first-generation EGFR inhibitors.3 Specificity for EGFR T790M may be the result of hydrophobic interactions between the bulky methionine moiety of EGFR T790M and these pyrimidine-based drugs. Mechanisms of resistance to these third-generation EGFR TKIs have been identified in preclinical in vitro models, but none have so far been identified in patients. Mutations at the EGFR C797 codon, located within the kinase-binding site, are a predicted resistance mechanism to irreversible EGFR inhibitors also confirmed in vitro.4 Loss of the potential for covalent bond formation at position 797 by the missense mutation C797S results in a markedly reduced cellular potency of this class of TKIs.5 Interestingly, an analogous cysteine-to-serine mutation is a mechanism of resistance to the irreversible Bruton tyrosine kinase inhibitor ibrutinib in patients with chronic lymphocytic leukemia.6