LAUSR

The first (1G) and second (2G) generations of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) show differential inhibitory capacities towards EGFR T790M-mutated non-small-cell lung cancer (NSCLC) cells. To assess the ratio of the allele fractions of T790M (AFT790M) to EGFR-activating mutations (AFmEGFR) in patients treated with 1G and 2G EGFR TKIs who acquired T790M-mediated resistance and to determine the relationship between AF and the later efficacy of osimertinib. The efficacy of osimertinib was reviewed for 54 T790M-positive EGFR-mutated NSCLC patients grouped by the generation of prior EGFR TKI use (1G vs. 2G). AFmEGFR and AFT790M were determined by QuantStudio digital PCR using tissues obtained upon acquired resistance. The progression-free survival (PFS; 20.3 vs. 11.6 months, p = 0.031) and the 1-year PFS rate (63.2 vs. 37.5%, p = 0.029) for osimertinib were significantly better for group 1G compared to group 2G. The ratio of AFT790M to AFmEGFR in group 1G was significantly higher than in group 2G (46.16 ± 5.40% vs. 25.86 ± 4.25%, p = 0.009). An unbiased analysis revealed three AF-associated clusters (ARCs) suggesting the ratio of AFT790M to AFmEGFR correlates with the efficacy of osimertinib. We found all patients in ARC2 having the highest ratio of AFT790M to AFmEGFR to have previously been treated with a 1G EGFR TKI and to show the longest osimertinib PFS compared to ARC3 (NR vs. 11.9 months, p = 0.060) and ARC1 (NR vs. 12.4 month, p = 0.045). Acquired T790M fraction of EGFR-mutated NSCLC is linked to different generations of prior EGFR TKI use and the later efficacy of osimertinib.