KRAS and EGFR amplifications mediate resistance to rociletinib and osimertinib in acquired afatinib-resistant NSCLC harboring exon 19 deletion/T790M in EGFR

The critical T790M mutation in epidermal growth factor receptor (EGFR), which mediates resistance to first- and second-generation EGFR tyrosine kinase inhibitors (TKIs; gefitinib, erlotinib, and afatinib), has facilitated the development of third-generation mutation-selective EGFR TKIs (rociletinib and osimertinib). We previously reported heterogeneous afatinib-resistance 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 mitogen-activated protein kinase 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.