Resistance mechanisms include new, second-site mutations within EGFR (e.g., T790M and C797S), upregulation of MET kinase, upregulation of insulin growth factor receptor (IGFR), HER2 amplification, increased expression of AXL, BIM modulation, NF-κB activation, histologic switch to small-cell cancer, epithelial-to-mesenchymal transition, PDL1 expression with subsequent immune tolerance, and release of cytokines such as TGFβ and IL6.
At present, secondary resistance mechanisms associated with progression are better known: clonal selection of EGFR resistance mutation (T790M mutation in exon 20), amplification of transmembrane receptors for other growth factors (c-met, HER family, IGF1R, or AXL), downstream molecular alterations in EGFR signaling pathway (PI3K or PTEN), and epithelial-mesenchymal transition or transdifferentiation to small-cell cancer.