Understanding resistance to EGFR inhibitors-impact on future treatment strategies

Abstract

EGFR is a tyrosine kinase that participates in the regulation of cellular homeostasis. Following ligand binding, EGFR stimulates downstream cell signaling cascades that influence cell proliferation, apoptosis, migration, survival and complex processes, including angiogenesis and tumorigenesis. EGFR has been strongly implicated in the biology of human epithelial malignancies, with therapeutic applications in cancers of the colon, head and neck, lung, and pancreas. Accordingly, targeting EGFR has been intensely pursued, with the development of a series of promising molecular inhibitors for use in clinical oncology. As is common in cancer therapy, challenges with respect to treatment resistance emerge over time. This situation is certainly true of EGFR inhibitor therapies, where intrinsic and acquired resistance is now well recognized. In this Review, we provide a brief overview regarding the biology of EGFR, preclinical and clinical development of EGFR inhibitors, and molecular mechanisms that underlie the development of treatment resistance. A greater understanding of the mechanisms that lead to EGFR resistance may provide valuable insights to help design new strategies that will enhance the impact of this promising class of inhibitors for the treatment of cancer.

Figure 1

EGFR biology. a | Ligand binding to EGFR causes receptor homodimerization or heterodimerization, which leads to transphosphorylation of the cytoplasmic tail tyrosine residues. Lysine 721 (K721) is the critical site for ATP-binding and kinase activity of EGFR (shown in yellow). Mutation of this amino acid causes the receptor to become inactive., Tyrosine phosphorylation in the C-terminus includes Y974, Y992, Y1045, Y1068, Y1086, Y1148 and Y1173 (shown in orange), or SFKs can phosphorylate Y845 and Y1101 (shown in purple). Reported biological effects of phosphorylation of each tyrosine are noted.– b | EGFR has been consistently detected in the nuclei of cancer cells, primary tumor specimens and highly proliferative tissues.– EGFR binds to STAT3 to increase expression of iNOS, E2F1 to increase expression of B-Myb, and with STAT5 to increase expression of Aurora A. It also increases the expression of cyclin D1. EGFR has kinase-dependent activity within the nucleus of proliferating cells, which includes the phosphorylation of PCNA leading to its stability and enhancing cell proliferation, and translocation and activation of DNA-PK. Abbreviations: AP-2, transcription factor AP-2; B-Myb, Myb-related protein B; CBL, E3 ubiquitin-protein ligase CBL; DNA-PK, DNA-dependent protein kinase catalytic subunit; E2F1, transcription factor E2F1; EGFR, epidermal growth factor receptor; GRB2, growth factor receptor-bound protein 2; iNOS, inducible nitric oxide synthase; MAPK, mitogen-activated protein kinase; P, phosphorylation; PCNA, proliferating cell nuclear antigen; PI3K, phosphatidylinositol 3-kinase; PKC, protein kinase C; PLCγ, 1-phosphatidylinositol-4,5-bisphosphate phosphodiesterase gamma-1; SFK, Src family kinase; SGLT1, sodium/glucose cotransporter 1; SHP1, tyrosine-protein phosphatase non-receptor type 6; SRC, proto-oncogene tyrosine-protein kinase Src; STAT, signal transducer and activator of transcription.

Figure 2

Mechanisms of resistance to EGFR antibodies. a | One mechanism of resistance to cetuximab is overexpression of the EGFR ligand TGFα. b | Overexpression of EGFR has also been implicated in the development of acquired resistance. c | Ubiquitylation is important for mechanisms of escape to cetuximab therapy., d | Modulation of EGFR by SFKs, and increased activity of SFKs in cetuximab-resistant lines have been reported., e | The binding and activation of EGFR or HER2 to HER3 has been reported, which allows prolonged signals to the PI3K/AKT pathway., f | Translocation of EGFR to the nucleus has a role in resistance to cetuximab. g | Increased VEGF production leads to altered angiogenesis and enhanced escape from cetuximab therapy., h | VEGFR1 also contributes to resistance to cetuximab. i–j | Mutations in both PTEN and Ras have been implicated in impaired response to cetuximab therapy. k | Mutations in KRAS keep it in a constant GTP-bound, active state, allowing it to send signals downstream independently from RTK activation. l | EGFRvIII a truncated form of EGFR that is constitutively phosphorylated in a ligand-independent manner., m | MDGI alters trafficking of EGFR, leading to resistance to cetuximab therapy. Abbreviations: B-Myb, Myb-related protein B; CBL, E3 ubiquitin-protein ligase CBL; E2F1, transcription factor E2F1; EGFR, epidermal growth factor receptor; ERK, mitogen-activated protein kinase 3; MDGI, mammary derived growth inhibitor; P, phosphorylation; PCNA, proliferating cell nuclear antigen; PI3K, phosphatidylinositol 3-kinase; PIP3, phosphatidylinositol 3,4,5-trisphosphate PTEN, phosphatase and tensin homolog; RTK, receptor tyrosine kinase; SFK, Src family kinase; STAT3, signal transducer and activator of transcription 3; TGFα, transforming growth factor alpha; Ub, ubiquitylation; VEGF, vascular endothelial growth factor; VEGFR1, vascular endothelial growth factor receptor 1.

Figure 3

Mechanisms of resistance to EGFR TKIs. a | A mutant form of EGFR termed EGFRvIII has an in-frame deletion mutation that produces a truncated 150 kDa protein, which is constitutively phosphorylated in a ligand-independent manner. b | EGFR-dependent tumors that are initially sensitive to EGFR TKIs acquire a mutation at threonine 790. Substitution of this residue in EGFR with a bulky methionine may cause resistance by steric interference with binding of TKIs, including gefitinib and erlotinib.– c | Tumors can become resistant when individual tumor cells undergo an oncogenic shift, which has been noted with several other RTKs, including HGF receptor, AXL and IGF1R. d | In addition to IGF1R as a mechanism of escape, downregulation of the IGF- binding proteins IGFBP3 and IGFBP4, have been implicated in resistance to TKIs. These proteins are crucial for regulating the levels of IGF1R ligands, and loss leads to overactivation of the receptor. e–f | Mutations in both PTEN and Ras have been implicated in impaired response to TKI therapy., g | Cells that developed acquired resistance to gefitinib in vivo were shown to have increased VEGF production leading to altered angiogenesis and enhanced escape from cetuximab therapy. h | VEGFR1 has also been implicated in the contribution to resistance to EGFR TKIs. Abbreviations: AXL, tyrosine-protein kinase receptor UFO (AXL oncogene); EGFR, epidermal growth factor receptor; HGF, hepatocyte growth factor; IGF1R, insulin-like growth factor 1 receptor; IGFBP, insulin-like growth factor-binding protein; kDa, kilodalton; PTEN, phosphatase and tensin homolog; RTK, receptor tyrosine kinase; TKI, tyrosine kinase inhibitor, VEGF, vascular endothelial growth factor; VEGFR1, vascular endothelial growth factor receptor 1.