Resistance to EGF-R (erbB-1) and VEGF-R modulating agents

Abstract

In an effort to improve the survival of cancer patients, new therapeutic approaches focusing on the molecular mechanisms that mediate tumour cell growth or survival have gained much attention. In particular, EGF-R and VEGF/VEGF-R have been extensively investigated as targets for anti-neoplastic therapy. Agents that selectively target EGF-R, erbB-2, VEGF-R-2 or VEGF have shown promising activity in clinical trials, and several are now approved for use in selected cancer indications. However, all patients ultimately develop resistance to these drugs. Thus, there is a great need to understand how patients become resistant to effective therapies for these cancers since this approach may lead to improvements in therapies that target EGF-R and VEGF/VEGF-R. Pre-clinical studies have begun to shed light on the mechanisms of resistance to anti-angiogenetic drugs and to date four mechanisms of resistance have been identified (1) upregulation of bFGF, (2) overexpression of MMP-9, (3) increased levels of SDF-1α and (4) HIF-1α-induced recruitment of bone marrow-derived CD45+ myeloid cells. In addition, the molecular mechanisms of resistance to EGF-R modulating agents can be attributed to several general processes: (1) activation of alternative tyrosine kinase inhibitors that bypass the EGF-R pathway (e.g. c-MET and IGF-1R), (2) increased angiogenesis, (3) constitutive activation of downstream mediators (e.g. PTEN and K-ras) and (4) the existence of specific EGF-R mutations. K-ras mutations have been significantly associated with a lack of response to EGF-R tyrosine kinase inhibitors in patients with NSCLC and with a lack of response to cetuximab or to panitumumab in patients with advanced colorectal cancer. The identification of these resistance mechanisms has led to clinical trials using newly designed targeted therapies that can overcome resistance and have shown promise in laboratory studies. Ongoing research efforts will likely continue to identify additional resistance mechanisms, and these findings will hopefully translate into effective therapies for different cancers.

Introduction

Despite advances in chemotherapy, most patients with cancer that has metastasised will succumb to the disease within 2 years of diagnosis. In an effort to improve survival, new therapeutic approaches focusing on the molecular mechanisms that mediate tumour cell growth or survival have gained much attention. In particular, the epidermal growth factor receptor (EGF-R) and the vascular endothelium growth factor receptor (VEGF-R) have been extensively investigated as targets for anti-neoplastic therapy.

Receptor tyrosine kinases (RTKs) such as EGF-R or VEGF-R are transmembrane proteins with an extracellular ligand binding domain and an intracellular tyrosine kinase catalytic domain. On binding to their cognate ligands, most RTKs dimerise and become activated through autophosphorylation of intracellular tyrosine residues. Activation of RTKs results in upregulation of multiple cellular signalling pathways that promote cell growth, survival and angiogenesis or environmental stimuli. Inappropriate activation of RTKs via mutation, overexpression or ectopic ligand production is a frequent feature of human tumour development and progression, and is thought to be a major mechanism by which cancer cells subvert normal growth controls.1, 2, 3 Consequently, in recent years modulation of RTK signal transduction has been an active area in oncology drug discovery. EGF-R (also called erbB1) and other erbB family RTKs (erbB-2/HER-2-neu, erbB-3/HER-3 and erbB-4/HER-4) encoded by the c-erbB proto-oncogenes have been strongly implicated in cancer development and progression as reviewed by 4, 2. Several mechanisms can cause aberrant receptor activation, resulting in tyrosine kinase activity, which is observed in cancer, including receptor overexpression, mutation, ligand-dependent receptor dimerisation and ligand-independent activation. For erbB-2, where a specific ligand has not been identified, activation occurs by homo- or hetero-dimerisation alone, whereas erbB-3 does not have significant kinase activity.4, 5 However, on activation, all 4 receptors are capable of signal transduction, causing activation of the ras/MAP kinase pathway, the PI3K/Akt pathway, src family kinases and STAT proteins. Activation of these pathways promotes cell proliferation, survival and angiogenesis.⁶

VEGF is the prototype of a large family of angiogenic and lymphangiogenic growth factors, which includes 6 structurally homologous, secreted glycoproteins called VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E and placenta growth factor.¹ VEGF-A (commonly referred to as VEGF) was the first such molecule to be identified by the virtue of its ability to induce vascular permeability.⁷ The VEGF ligands trigger biological effects on their interaction with specific cell-surface receptors. The diversity of these receptors also adds to the biological complexity of angiogenesis and lymphangiogenesis. Two receptors were originally identified on vascular endothelial cells: VEGF-R-1 (a 180-kD transmembrane protein, also called Flt-1) and VEGF-R-2 (a 200-kD transmembrane protein, also called KDR). A third structurally related tyrosine kinase receptor is the 180-kD VEGF-R-3 (also called Flt-4), which is expressed broadly on endothelial cells during early embryogenesis.⁸ VEGF-R-2 is expressed in most, if not all, adult vascular endothelial cells as well as on circulating endothelial progenitor cells. Interestingly, both epithelial and mesenchymal tumour cells more typically express VEGF-R-1 than VEGF-R-2⁹; however, in several experimental tumour models tumour cell-specific VEGF-R-2 expression has been shown to be the critical driver in the pathogenesis of tumours.1, 3 VEGF binding induces conformational changes within VEGF-R-2 followed by receptor dimerisation and autophosphorylation of tyrosine residues in the intracellular kinase domain. These tyrosine residues (Tyr⁹⁵¹, Tyr⁹⁹⁶, Tyr¹⁰⁵⁴ and Tyr¹⁰⁵⁹) serve as high-affinity docking sites for a variety of signalling proteins, including phospholipase Cγ, ras-GAP, focal adhesion kinase, src family of tyrosine kinases, PI3K. Akt, PK-C, Raf-1 and MAPs. The interaction of 1 or more of these molecules with VEGF-R-2 may lead to alterations in cell proliferation, migration, differentiation, tube formation, and increase in vascular permeability and vascular integrity.³

Agents that selectively target EGF-R, erbB-2, VEGF-R-2 or VEGF have shown promising activity in clinical trials, and several are now approved for use in selected cancer indications (Table 1, Table 2). All patients, however, ultimately develop resistance to anti-EGF-R-and anti-VEGF(-R)-targeted therapies. Thus, there is a great need to understand how patients become resistant to effective therapies for these cancers.