Monitoring epidermal growth factor receptor C797S mutation in Japanese non–small cell lung cancer patients with serial cell‐free DNA evaluation using digital droplet PCR

Osimertinib is a third‐generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR‐TKI) that is effective in treating both naïve and T790M‐mutated EGFR‐TKI‐resistant non–small cell lung cancer patients. The EGFR C797S mutation is the major osimertinib resistance mechanism. The present study monitored the EGFR C797S mutation during osimertinib treatment in Japanese patients using droplet digital PCR (ddPCR). In our first cohort, C797S detection was validated with tumor specimens and/or plasma samples from 26 patients using ddPCR with custom‐designed probes detecting and discriminating T790M and C797S in cis and trans positions. In our second cohort, 18 patients with EGFR‐T790M who were going to start osimertinib were analyzed using ddPCR by collecting the plasma samples every month from the beginning of the course of osimertinib. In the first cohort, C797S was detected in 15.4% of patients. C797S and T790M in cis and trans positions were distinguished using ddPCR. In the second cohort, serial cfDNA evaluation revealed that the rate of EGFR mutation changes with disease state. Increases of EGFR mutation were detected, including C797S several months before the diagnosis of disease progression. As with the first cohort, C797S and T790M in cis and trans position were distinguished by ddPCR at disease progression. Coincidentally, in the first cohort, next generation sequencing detected NRAS Q61K mutation and the resistance with NRAS Q61K mutation was overcome by trametinib. In the second cohort, serial cfDNA analysis was useful for evaluating bone oligo‐progression and local radiation therapy.