EGFR-mutated lung cancer is a significant subgroup of non–small cell lung cancer. To inhibit EGFR-mediated signals, multiple EGFR tyrosine kinase inhibitors (EGFR-TKI) have been developed; however, approximately one third of patients with EGFR-mutated lung cancer do not respond to EGFR-TKIs. More effective inhibition of EGFR-mediated signals is therefore necessary. For cancers expressing mutated EGFR, including EGFR T790M, which confers resistance to first- (gefitinib and erlotinib) and second- (afatinib) generation EGFR-TKIs, the synergistic efficacy of afatinib and cetuximab combination therapy has been reported in preclinical and clinical studies; however, the mechanisms underlying this effect remain elusive. In this study, we evaluated the effects of multiple EGFR-TKIs on the EGFR monomer–dimer equilibrium by inducing dimerization-impairing mutations in cells expressing EGFR. Interestingly, we found that afatinib and dacomitinib exhibit a monomer preference: cells expressing dimerization-impaired EGFR mutants exhibited increased sensitivity to afatinib and dacomitinib relative to those with dimerization-competent EGFR mutants. Although EGFR-TKIs themselves induce dimerization of EGFR, the inhibition of dimerization by cetuximab overcame EGFR-TKI–induced dimerization. By shifting the monomer–dimer equilibrium toward monomer dominance using cetuximab, the effectiveness of afatinib and dacomitinib improved significantly. We report a novel and clinically relevant phenomenon, the monomer preference of EGFR-TKIs, which can explain the mechanism underlying the synergism observed in afatinib and cetuximab combination therapy. In addition, we propose the novel concept that monomer–dimer equilibrium is an important factor in determining EGFR-TKI efficacy. These findings provide novel insights into treatment strategies for EGFR-TKI–refractory non–small cell lung cancer.
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