Background: Epidermal growth factor receptor (EGFR) T790M mutation is well known as primary and acquired resistance mechanisms to the 1st and 2nd generation EGFR tyrosine kinase inhibitors (TKIs). However, a de novo and clonal EGFR T790M mutation is rarely observed in 0.8-2% of patients with EGFR-mutant NSCLC. The 3rd generation EGFR TKIs including osimertinib are expected to be effective against de novo, clonal EGFRT790M-positive NSCLC. However, the efficacy and resistance mechanisms of osimertinib have been limited in patients with de novo and clonal EGFRT790M-positive NSCLC.

Methods: The seven (1%) of 690 patients with EGFR-mutant NSCLC were identified as having de novo and clonal EGFRT790M mutations between December 2002 and July 2014 in Seoul National University Hospital. Three with de novo EGFRL858R/T790M mutation received osimertinib at a dose of 80mg or 160mg once daily (NCT01802632) and two showed acquired resistance to osimertinib. Fresh tumor samples were obtained before and after treatment with osimertinib and analyzed by whole-exome and -transcriptome sequencings as well as droplet digital polymerase chain reaction (ddPCR). MET amplification was determined by fluorescence in situhybridization (FISH) in formalin-fixed paraffin-embedded tumor tissues.

Results: Three patients with de novo EGFRL858R/T790M mutation received osimertinib and two (LC1 and LC2) showed acquired resistance to osimertinib at 16 and 30 months, respectively (Table 1). As of November 2018, the LC3 patient received osimertinib without disease progression. Novel MTOR L1433S mutation within FAT domain that was not seen at initial tumors (ploidy 2.3) was clonally acquired at osimertinib-resistant tumors (ploidy 2.4) in the LC1 patient. In addition, a novel and clonal MGA V124D mutation within T-box was found at osimertinib-resistant tumors (ploidy 5.9) of the LC2 patient. MET copy number was 14 with high fpkm level (721.9) at osimertinib-resistant tumors in the LC2 patient. Average MET gene copies per nucleus by FISH were 2.74 at initial tumors and 5.25 at osimertinib-resistant tumors in LC2 patient, resulting in MET amplification as a resistance mechanism. Subclonal MGA V124D mutation was found at initial tumors of the LC2 patient using ddPCR. The Ba/F3 systems confirmed that novel mutants were oncogenic.

Conclusions: Novel MTOR and MGA mutations were functional and clonally acquired as resistance mechanisms to osimertinib in patients with de novo EGFRL858R/T790M-mutant NSCLC. In addition, MET amplification contributed to acquired resistance to osimertinib. Taken together, acquired resistance mechanisms of osimertinib might be mainly mediated by alternative pathway activation in de novo EGFRT790M-positive NSCLC.

Citation Format: Ha-Ram Park, Yusoo Lee, Tae Min Kim, Soyeon Kim, Chan-Young Ock, Miso Kim, Bhumsuk Keam, Yoon Kyung Jeon, Dong-Wan Kim, Dae Seog Heo. Acquired resistance to osimertinib in patients with de novo EGFRT790M-positive non-small cell lung cancer (NSCLC) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3032.