Molecular follow-up of first-line treatment by osimertinib in lung cancer: Importance of using appropriate tools for detecting EGFR resistance mutation C797S.

The C797S mutation encoded by EGFR exon 20 is classically observed as a tertiary event in EGFR-mutant non-small-cell lung carcinoma (NSCLC) primarily treated by first generation tyrosine kinase inhibitors (TKI) and secondarily treated by third-generation TKI, such as osimertinib, if the EGFR-T790M resistance mutation is detected. Recently, significant prolonged progression free survival has been observed following first-line osimertinib, in EGFR-mutant NSLC. While mechanisms of molecular resistance to first-generation TKI have been well studied, little is known about resistance induced by primary third-generation TKI treatments. We report the case of a 65 year-old female treated by first-line osimertinib for a multimetastatic exon 19-EGFR-mutant NSCLC. EGFR-C797S resistance mutation and PIK3CA mutation were detected together with the remaining EGFR-exon 19 deletion. This observation provides insights of acquired resistance to first line-osimertinib. It also highlights the importance of making molecular platforms which perform routine EGFR testing in lung cancer aware of the kind of therapeutic protocols given to the patient. Indeed, for rapid results or low-costs procedures, some targeted methods specifically targeting T790M may be used at relapse and may overlook alterations such as C797S or PIK3CA mutations. Targeted next generation sequencing is therefore a recommended option.