Abstract 1330: Osimertinib, an irreversible next generation EGFR tyrosine kinase inhibitor, exerts anti-tumor activity in various preclinical NSCLC models harboring G719X mutant-EGFR
Authors: Floch Nicolas , Bickerton Sue , Martin Matthew J. , Cross Darren A. , Smith Paul D. 
Published: August 2019

Abstract
Two first-generation (erlotinib & gefitinib), two second generation (afatinib & dacomitinib) and a third generation (osimertinib) EGFR-TKIs are currently available for the management of EGFR mutation-positive NSCLC. All these TKIs are effective in patients with NSCLC whose tumors harbor the in-frame deletions in exon 19 and the L858R point mutation in exon 21. These two mutations represent 90% of all EGFR mutations. Osimertinib, when used in the front-line setting, has shown more favorable treatment outcomes than first-generation EGFR-TKIs. In approximately 50% of patients, resistance to first and second generation TKI is mediated by the acquisition of the ‘gatekeeper’ mutation T790M. Currently, osimertinib is the only registered EGFR TKI that is active against exon 19 deletions and L858R mutation, regardless of the presence of T790M mutation.
While the efficacy of EGFR TKIs for the common EGFR mutations is well established, much less is known about rare EGFR mutations such as exon 20 insertions, G719X, L861Q, S768I, as most of the data consist of single case reports or small case series. This work describes the therapeutic potential of osimertinib in tumors harboring the G719X mutation alone or in combination with L861Q and S768I.
Using available patient-derived xenografts (PDX) and cell lines derived from two of these PDX which habor the G719X mutation, we have evaluated in vitro and in vivo the pre-clinical activity of osimertinib.
We show that osimertinib inhibits signalling pathways and cellular growth of G719X cell lines in vitro. This translates into sustained tumor growth inhibition in vivo in 3 out of 4 PDX explored (87%, p&lt;0.001, 158% p&lt;0.001 & 181%, p&lt;0.001 respectively at day 14) when compared to the control group. Importantly, in these in vivo models, osimertinib achieved exposure consistent with those observed in patients treated with an 80 mg clinical dose. Moreover, osimertinib demonstrates superior activity to afatinib (145%, p&lt;0.001, & 58% p&lt;0.01 respectively at day 14) in a model harboring the G719A. Out of the 4 PDX models explored in vivo, one model appeared to be inherently resistant to osimertinib. Further genomic characterisation revealed that the model displayed amplification of c-MET, a well know mechanism of resistance to EGFR-TKI. Subsequent in vivo pre-clinical study shows that the model is sensitive to c-MET inhibition. In addition, we performed pharmacodynamic studies to explore the relationship between efficacy and target/pathway modulation. These studies establish a clear relationship between depth and duration of inhibition of the phopshorylation of EGFR and anti-tumor efficacy.
The work presented herein demonstrates that osimertinib has the potential to improve upon the current treatment options for NSCLC patients whose tumors harbor a G719X mutation, and warrants further clinical investigation.
Citation Format: Nicolas Floch, Sue Bickerton, Matthew J. Martin, Darren A. Cross, Paul D. Smith. Osimertinib, an irreversible next generation EGFR tyrosine kinase inhibitor, exerts anti-tumor activity in various preclinical NSCLC models harboring G719X mutant-EGFR [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 1330.