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.
EGFR-mutant clones depopulated AZD9291-resistant tumors to below 1% (baseline, 14%-36%) in three patients with progression: one with the loss of EGFR-double mutant clones and two accompanied by transformation to small-cell carcinoma and focal fibroblast growth factor receptor 1 (FGFR1) amplification, respectively.
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.
Immunohistochemical analysis of 140 cases revealed overexpression of EGFR in 37.9% and phosphorylation in 37.1%, but much less in small cell carcinoma.
There were EGFR mutations in 9 (4.5%) of 201 squamous carcinomas, in 1 (2%) of 50 large cell carcinomas, and in 1 (2.3%) of 44 small cell carcinomas that were investigated.