Among the cohort of the FLAURA trial, the most common resistance marker found in the samples of 91 patients which underwent molecular analysis when post-primary osimertinib progression occurred, was MET amplification (14/91; 15%) while EGFR-C797S mutation was detected in 6 patients (7%) [[14]Ramalingam SS Cheng Y Zhou C et al.Mechanisms of acquired resistance to first-line osimertinib: Preliminary data from the phase III FLAURA study.]. Here, we report the first detailed molecular analysis of a progressive multimetastatic Del19 NSCLC following first-line osimertinib treatment. We observed EGFR-C797S and PIK3CA-E545K mutations without EGFR-T790M. Amplification of MET or any other amplification was not detected. This observation strengthens the importance of using molecular methods able to detect C797S mutation in such a context. Indeed, because of rapid and low-cost results, methods such as pyrosequencing or real-time PCR only focused on investigating EFGR exon 19 and 21 main mutations or T790M are sometimes preferentially used instead of NGS at relapse. However, such methods may overlook emerging mutations related to first-line osimertinib, such as EGFR-C797S or L718V/Q. Although there is currently no treatment specifically targeting EGFR-C797S resistance mutation, this information is important both for advances in research of novel drugs active in resistant tumors and for care optimization of each patient. In preclinical models, EGFR-C797S mutant NSCL showed response to first-generation TKI [[15]Rangachari D To C Shpilsky JE VanderLaan PA Kobayashi SS Mushajiang M et al.EGFR Mutated Lung Cancers Resistant to Osimertinib through EGFR C797S Respond to First-Generation Reversible EGFR Inhibitors but Eventually Acquire EGFR T790M/C797S in Preclinical Models and Clinical Samples.]. However this response was transitory and T790M eventually occurred. Interestingly, Starrett et al [[13]Starrett JH Guernet AA Cuomo ME et al.Drug Sensitivity and Allele Specificity of First-Line Osimertinib Resistance EGFR Mutations.] showed that in preclinical models, erlotinib was effective in cells with C797S but not with 718L/D mutations and that combination of first-line erlotinib and osimertinib prevents secondary mutations. Moreover, these authors, in a retrospective study of circulating free DNA of EGFR-mutant patient's blood samples showed that C797S mutation was more often associated to Del19 whereas L718V/Q mutations were correlated to L858R. Unfortunately, in this study data on treatment given to the patients were not available. Nevertheless, if confirmed, such data might be useful for orientating the search of resistance mutations. The PIK3CA mutation that was associated with C797S in the present case has also been described in a few cases following second-line osimertinib treatment [[4]Rosell R Carcereny E Gervais R et al.Erlotinib versus standard chemotherapy as first-line treatment for European patients with advanced EGFR mutation-positive non-small-cell lung cancer (EURTAC): a multicentre, open-label, randomised phase 3 trial.]. This mutation might also be considered for alternative therapeutic options though PIK3CA inhibitors demonstrated only a modest clinical efficacy as single-agent early clinical studies [[16]Rodon J Dienstmann R Serra V et al.Development of PI3K inhibitors: lessons learned from early clinical trials.]. The role of other somatic alterations observed in the present case is still unclear. CTNNB1 mutations, such as the one present in our case in pre- and post osimertinib samples has been described to limit EGFR-inhibitor response in EGFR-mutant lung cancers [[17]Blakely CM Watkins TBK Wu W Gini B Chabon JJ McCoach CE et al.Evolution and clinical impact of co-occurring genetic alterations in advanced-stage EGFR-mutant lung cancers.]. Targeting the Wnt-catenin pathway might therefore be used for counteracting TKI-resistance. In addition, we observed RB1 and CDKN2A homozygous deletions. Both alterations may have played a role in the tumor progression. RB1 deletion has been described to be a marker of transformation from NSCLC into small cell lung carcinoma [[18]Taniguchi H Sen T Rudin CM. Targeted Therapies and Biomarkers in Small Cell Lung Cancer.] while association of CDKN2A deletion with an activating EGFR mutation has been correlated to a poor response to TKI [[19]Jiang J Gu Y Liu J Wu R Fu L Zhao J et al.Coexistence of p16/CDKN2A homozygous deletions and activating EGFR mutations in lung adenocarcinoma patients signifies a poor response to EGFR-TKIs.].

To the best of our knowledge, this is the first detailed case report with genomic data which shows EGFR-C797S mutation without EGFR T790M, after first-line osimertinib in a metastatic Del19 NSCLC. Our report underlines the importance for molecular platforms to be fully informed on therapeutic protocols prescribed to NSCLC patients, in order to make them able to use the optimal methods of molecular detection when they start the molecular screening of a tumor sample. Targeted NGS appears as a recommended option since it detects main resistance EGFR mutations and is also able to identify MET amplification. This is crucial both for precision medicine and for understanding resistance mechanisms to novel TKI protocols, such as first-line osimertinib.