RocR1 and RocR2 showed KRAS and EGFR gene amplifications, respectively, with loss of exon 19 deletion and T790M in EGFR following acquisition of rociletinib resistance.
ARTICLE ABSTRACTThe critical T790M mutation in EGFR, which mediates resistance to first- and second-generation EGFR tyrosine kinase inhibitors (TKI; gefitinib, erlotinib, and afatinib), has facilitated the development of third-generation mutation-selective EGFR TKIs (rociletinib and osimertinib). We previously reported heterogeneous afatinib-resistant mechanisms, including emergence of T790M-EGFR, and responses to third-generation EGFR TKIs. Here, we used afatinib-resistant lung adenocarcinoma cells [AfaR (formerly AFR3) cells], carrying exon 19 deletion/T790M in EGFR. To identify the novel resistance mechanisms in post-afatinib treatment, RocR1/RocR2 and OsiR1/OsiR2 cells were established using increasing concentrations of rociletinib and osimertinib, respectively. Attenuation of exon 19 deletion and T790M was confirmed in both rociletinib-resistant cells; in addition, EGFR and KRAS amplification was observed in RocR1 and RocR2, respectively. Significant KRAS amplification was observed in the osimertinib-resistant cell lines, indicating a linear and reversible increase with increased osimertinib concentrations in OsiR1 and OsiR2 cells. OsiR1 cells maintained osimertinib resistance with KRAS amplification after osimertinib withdrawal for 2 months. OsiR2 cells exhibited KRAS attenuation, and osimertinib sensitivity was entirely recovered. Phospho-EGFR (Y1068) and growth factor receptor–bound protein 2 (GRB2)/son of sevenless homolog 1 (SOS1) complex was found to mediate osimertinib resistance in OsiR1 cells with sustained KRAS activation. After 2 months of osimertinib withdrawal, this complex was dissociated, and the EGFR signal, but not the GRB2/SOS1 signal, was activated. Concomitant inhibition of MAPK kinase and EGFR could overcome osimertinib resistance. Thus, we identified a heterogeneous acquired resistance mechanism for third-generation EGFR TKIs, providing insights into the development of novel treatment strategies.