Targeting C797S mutations and beyond in non-small cell lung cancer-a mini-review.

Non-small cell lung cancer (NSCLC) represents over 80% of lung cancer cases and has a high mortality worldwide, however, targeting common epidermal growth-factor receptor (EGFR) alterations (i.e., del19, L858R) has provided a paradigm shift in the treatment of NSCLC. Uncommon EGFR mutations, however, show variable efficacy to EGFR-targeted drugs depending on the molecular alterations within exons 18-21 which underlying biological mechanism are far from being clear. The substitution mutations of G719X in exon 18, L861Q in exon 21, S768I in exon 20, and exon 20 insertions are the most frequent mutations among the uncommon mutations. The development of fourth-generation EGFR-tyrosine kinase inhibitor (TKIs) has gained increased interest as these drugs are able to inhibit resistance mutations (e.g., C797S) often detected in NSCLC patients' resistance to third-generation EGFR TKIs. BDTX-1535 is an orally bioavailable, brain-penetrating, mutation-selective, irreversible EGFR inhibitor with significant antitumour activity in NSCLCs and glioblastomas (phase I/II trials ongoing). It is a fourth-generation EGFR inhibitor that was found to overcome resistance to osimertinib in preclinical models and has shown promising activity in NSCLC patients harbouring C797S mutations. In experimental models BDTX-1535 was found to inhibit all common EGFR mutations and more than 50 of uncommon mutations including T790M, C797S, L718X, E709X, S784F, V834L and A289V, however, exon 20 insertions are inhibited to a much lesser extent. In addition, mutations in the extracellular domain of the EGF receptor (e.g., EGFRvII, III, IV) can be blocked as well. It should be noted that in up to 50% of all NSCLC patients who progress following osimertinib or other EGFR TKI therapy no underlying resistance mechanism can be identified suggesting that non-mutational signal transduction pathways may also be operative, and intratumoural heterogeneity has been found to be a major contributor to resistance and it consists of three main mechanisms: (I) drug-tolerant persister (DTP) cells, (II) chromosomal instability, and (III) extrachromosomal extracellular DNA (ecDNA) (seen in over 50% of NSCLCs) suggesting that novel EGFR TKIs will include many challenges in sufficiently targeting on-target resistance mechanisms. The development of novel drugs that can overcome TKI resistance in NSCLC patients harbouring the C797S mutation and beyond is, therefore, eagerly warranted.