Acquired resistance to third-generation EGFR-TKIs and emerging next-generation EGFR inhibitors

Abstract

The discovery that mutations in the EGFR gene are detected in up to 50% of lung adenocarcinoma patients, along with the development of highly efficacious epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), has revolutionized the treatment of this frequently occurring lung malignancy. Indeed, the clinical success of these TKIs constitutes a critical milestone in targeted cancer therapy. Three generations of EGFR-TKIs are currently approved for the treatment of EGFR mutation-positive non-small cell lung cancer (NSCLC). The first-generation TKIs include erlotinib, gefitinib, lapatinib, and icotinib; the second-generation ErbB family blockers include afatinib, neratinib, and dacomitinib; whereas osimertinib, approved by the FDA on 2015, is a third-generation TKI targeting EGFR harboring specific mutations. Compared with the first- and second-generation TKIs, third-generation EGFR inhibitors display a significant advantage in terms of patient survival. For example, the median overall survival in NSCLC patients receiving osimertinib reached 38.6 months. Unfortunately, however, like other targeted therapies, new EGFR mutations, as well as additional drug-resistance mechanisms emerge rapidly after treatment, posing formidable obstacles to cancer therapeutics aimed at surmounting this chemoresistance. In this review, we summarize the molecular mechanisms underlying resistance to third-generation EGFR inhibitors and the ongoing efforts to address and overcome this chemoresistance. We also discuss the current status of fourth-generation EGFR inhibitors, which are of great value in overcoming resistance to EGFR inhibitors that appear to have greater therapeutic benefits in the clinic.

Keywords: Cancer; EGFR inhibitors; chemoresistance mechanisms; drug resistance; mutations; surmounting chemoresistance; targeted therapy.

Graphical abstract

Figure 1

Conformational movement occurs in human EGFR tyrosine kinase domains between inactive and active states (A and B) Crystal structure of the inactive state (PDB: 1XKK) and active state of the EGFR tyrosine kinase domain (PDB: 2ITX) were visualized by using PyMol 2.4.1. When in the inactive conformation, residues Lys745 and Glu762 are separated, the C-helix is in the out conformation, and the A-loop is in its closed conformation. When active, residues Lys745 and Glu762 are adjacent, the C-helix is in the in conformation, and the A-loop is in its open conformation.

Figure 2

Global launched EGFR inhibitors The launching time and chemical structures of three generations of EGFR-TKIs are displayed. Moreover, the related multi-targeted TKIs are also listed for reference. CN, China; US, United States.

Figure 3

Summary of EGFR mutations associated with resistance to third-generation EGFR-TKIs Shown are the various EGFR mutations that result in resistance to TKIs. The localization of these mutations on the various domains of the EGFR protein and exons are also depicted. L1, L2, ligand-binding domains; CR1, CR2, cysteine-rich domains; TM, transmembrane region.

Figure 4

EGFR-dependent mechanisms of resistance to third-generation EGFR-TKIs (A) Mutations located in the EGFR kinase domain induce resistance to third-generation EGFR inhibitors. (B) Amplification of the EGFR^WT and EGF. (C) Deletion of the T790M mutation. (D) Clathrin-mediated internalization could not induce the degradation of mutant EGFR. Internalized mutant EGFR still harbors the capacity to activate downstream signaling pathways. (E) Heterodimerization of EGFR and HER2 or AXL activates PKCδ via phosphorylation of Tyr1173. Activated PKCδ triggers nuclear translocation of PKCδ and activates AKT and NF-κB signaling.

Figure 5

EGFR-independent mechanisms of resistance to third-generation EGFR-TKIs (A) Activation of downstream EGFR signaling pathways. NRAS mutation, KRAS mutation, BRAF mutation, MEK1 mutation, deletion of NF1, or amplification of the CRKL gene sustains the activation of the MAPK/ERK signaling pathway. Deletion of the PTEN or PIK3CA mutation results in constitutive activation of the PI3K/AKT signaling pathway. (B) Oncogene fusions trigger sustained activation of survival-related signaling pathways. (C–G) Amplification of tyrosine kinase receptors including MET (C), HER2/3 (D), IGF1R (E), AXL (F), and FGFR1 (G). (H) Alteration of cell-cycle-related genes. (I) Aberrant phosphorylation of ACK1 enhances the activation of the AKT signaling pathway, leading to decreased BIM levels. (J) Increased nuclear translocation of YAP upregulates the expression of FOXM1, which promotes the expression of SAC members. (K) Epithelial-mesenchymal transition (EMT). (L) Transformation from NSCLC to SCLC.

Figure 6

Crystal structure determination of the fourth-generation EGFR-TKIs (A) The published chemical structure of the fourth-generation EGFR-TKIs. (B–D) EAI045-EGFR T790M/C797S/V948R (PDB: 5ZWJ) (B), JBJ-04-125-02-EGFR T790M/V948R (PDB: 6DUK) (C), and CH7233163-EGFR L858R/T790M/C797S (PDB: 6LUB) (D) crystal structures are shown as a cartoon diagram by PyMol 2.4.1. Compounds are shown as a color-coded stick model (C, green; O, red; N, blue; S, yellow; F, pale-cyan). Distinct hydrogen bonds are shown as yellow dashed lines.