Case report: primary resistance to osimertinib in erlotinib-pretreated lung adenocarcinoma with EGFR T790 M mutation

BACKGROUND: Among non-small cell lung cancer (NSCLC) patients with acquired T790 M mutation resistance to first-generation epidermal growth factor receptor-tyrosine kinase inhibitor (EGFR-TKI), 71% are likely to benefit from osimertinib. There have been several reports about the secondary resistance to osimertinib treatment in T790 M-positive patients, while primary resistance to osimertinib has been rarely reported. CASE PRESENTATION: A 62-year-old Asian male never smoker who presented with stage IV EGFR L858R-positive adenocarcinoma developed EGFR T790 M mutation after 14 months of treatment with erlotinib combined with thoracic radiotherapy as first-line therapy. The patient was initiated on osimertinib treatment with T790 M mutation detected (14.4%), but disease progressed 2 months later. CONCLUSION: The mechanism of primary resistance to osimertinib remains unclear. There may be an association between T790 M mutation disappearance, TP53 mutation and radiotherapy, but further researches are needed to confirm this.