The presence of the T790M mutation during first or second-generation EGFR-TKI treatments is observed in 50-60% of patients. This mutation hinders the drug’s binding to the mutant EGFR protein. Osimertinib, however, can covalently bind to the T790M and cysteine-797 (C797) residue at the protein’s ATP binding site, overcoming resistance mechanisms.
Analysis of circulating tumor (ct) DNA at the time of treatment discontinuation or progression showed that 49% of patients in the AURA3 trial lacked the T790M mutation. Often, the loss of T790M is associated with the development of alternative competitive resistance mechanisms. In patients with T790M mutant NSCLC receiving osimertinib, 63% experienced loss of T790M at progression, commonly linked to histological transdifferentiation, KRAS mutations, or gene fusions. In the same study, those with T790M loss had a shorter treatment duration (6.1 months vs. 15.2 months, p = 0.01). T790M loss has been linked to earlier resistance and poorer survival in other cohorts. The presence of T790M loss and plasma EGFR-activating mutations is associated with the shortest PFS (average 2.6 months, 95% CI 1.3, not reached) in genomic analysis of post-progression samples from patients receiving second-line osimertinib. However, when osimertinib is used first-line, the T790M mutation is usually absent before or after progression, reducing its significance with increased frontline use.
Another genetic change at the C797 residue in the ATP binding region of EGFR’s exon 20 has emerged as a key mechanism of resistance to osimertinib. This often involves a cysteine-to-serine (C797S) substitution, and rarely, a cysteine-to-glycine (C797G) change. Mechanistically, osimertinib overcomes T790M resistance by forming a bond with the C797 residue in the ATP pocket; thus, a mutation at C797 hinders its covalent bond with mutant EGFR.
Specifically, C797S is the most common mechanism of EGFR-dependent resistance to osimertinib across treatment lines, often emerging in the second line. In the AURA3 trial, 15% of patients had C797 mutations at disease progression, while in the FLAURA trial, 7% of patients progressing on first-line osimertinib had these mutations. Real-world cohorts report a prevalence of C797 mutations between 11% and 29%.
Importantly, a triple mutant scenario with EGFR L858R/T790M/C797S or EGFRexon19del/T790M/C797S genotype may occur in patients with T790-positive NSCLC receiving osimertinib in second or later lines. Interestingly, the allelic context of acquired C797S is significant and may predict response to alternative treatments. If C797S and T790M mutations are on different alleles (in trans), tumor cells may be resistant to osimertinib but sensitive to a combination of first and third-generation TKIs. If mutations are on the same allele (in cis), current EGFR-TKIs, alone or in combination, do not show antitumor activity. Preclinical studies have shown that cells with EGFR C797S in trans with T790M are sensitive to a combination of first and third-generation EGFR TKIs
. Clinical and radiographic responses have been associated with cases of patients having EGFR C797S in trans with T790M, treated with a combination of a first-generation TKI and osimertinib. Conversely, the emergence of C797S in cis with T790M negates sensitivity to first and second-generation TKIs. Preclinical data suggest that cancer cells retain sensitivity to early-generation inhibitors upon the emergence of C797S during initial osimertinib use, but this may be temporary, and additional resistance mechanisms can develop alongside C797S.
Mutations at the L718 residue in exon 18’s P-loop, which corresponds to the region of the protein forming a hydrophobic sandwich with osimertinib’s phenyl aromatic ring, have been shown to confer drug resistance. Reported changes include L718Q and L718V. NSCLC patients with EGFR L858R/T790M/L718 mutations are resistant to first and second-generation inhibitors, but tumors with L858R/L718 may be sensitive to afatinib when osimertinib is used as the first line.
Moreover, the G724S mutation in exon 18 has been identified as a resistance mechanism in cases treated with second-line osimertinib. Structural analysis and computational modeling indicate that the EGFR G724S mutation alters the protein’s glycine-rich loop, disrupting covalent bond formation with osimertinib. Studies have shown that afatinib reduces tumor growth in cells driven by G724S in the context of concurrent T790M loss. The effectiveness of afatinib in cases of T790M loss and emerging G724S mutation has also been reported in a case study.
Interestingly, another study, using simulations, in vitro experiments, and patient genomic profiling, proposed that the G724S mutation selectively confers resistance, especially in the Ex19Del context, but L858R/G724S cancer cells remain sensitive to osimertinib.
G796R and L792 are rare mutations reported in NSCLC treated with osimertinib and may interfere with the drug’s binding, thereby reducing its efficacy. Finally, T790M/M766Q double mutant cells have been shown to be resistant to osimertinib but sensitive to neratinib and poziotinib (dual inhibitors of human epidermal growth factor receptor 2 (HER2) and EGFR kinase).
Reference:
Gomatou, G., Syrigos, N., & Kotteas, E. (2023). Osimertinib Resistance: Molecular Mechanisms and Emerging Treatment Options. Cancers, 15(3), 841.
