The epidermal growth factor receptor (EGFR) is the prototypical receptor tyrosine kinase (RTK) and one of the most comprehensively studied molecular targets in clinical oncology.
EGF denotes epidermal growth factor; HB, heparin-binding; NRG, neuregulin; bFGF, basic fibroblast growth factor; P, phosphate; TGF, transforming growth factor.
Adapted from Ciardiello F, Tortora G. EGFR antagonists in cancer treatment. N Engl J Med. 2008;358:1160-1174.
The epidermal growth factor receptor (EGFR) is the prototypical receptor tyrosine kinase (RTK) and one of the most comprehensively studied molecular targets in clinical oncology. Therapeutic agents specifically targeting EGFR have shown highly promising activity in the clinic.
However, resistance to these agents and the question of which patients are most likely to derive therapeutic benefit from them have posed significant challenges to their optimal use. A greater understanding of the molecular mechanisms underlying EGFR signaling is now beginning to stimulate the development of new diagnostic and therapeutic strategies to enhance the impact of EGFR-targeted therapies.
EGFR was the first of the RTKs to be identified, following the Nobel Prize-winning discovery of its principal ligand EGF in the early 1960s. It is a member of the ERBB family of receptors and can be activated by numerous other ligands besides EGF, including transforming growth factor (TGF) α.
As with other members of the ERBB family, ligand binding drives the formation of receptor pairs, either between 2 EGFR molecules or between EGFR and another member of the family (homo-/heterodimerization), which stimulates the intrinsic tyrosine kinase activity of the receptor. Among the key intracellular signal transduction cascades subsequently activated by the EGFR are the MAPK cascade and the phosphatidylinositol 3-kinase/ protein kinase B (PI3K/Akt) cascade.
Under normal conditions, EGFR activation is tightly regulated by the availability of the ligand. Dysregulation of the EGFR pathway by overexpression or constitutive activation can promote 5 of the 6 hallmarks of cancer. Defects in the EGFR signaling pathway have now been implicated in a diverse range of cancers, including colorectal, head and neck, lung, breast, and pancreatic cancers.
Epidermal growth factor receptor (EGFR) tyrosine kinase domain with 4-anilinoquinazoline inhibitor erlotinib (Tarceva). Surface (gray) of EGFR and kinase; Tarceva in ball-and-stick (carbon-green, oxygen-red, nitrogen-blue).
EGFR-specific treatments that have been developed thus far fall into 2 distinct categories: (1) small-molecule tyrosine kinase inhibitors (TKIs) that block the adenosine triphosphate (ATP)-binding site and prevent tyrosine kinase activity; and (2) monoclonal antibodies that block extracellular ligand binding. Four of these agents are currently approved by the FDA.
Gefitinib (Iressa, AstraZeneca) was the first EGFR-specific TKI to be introduced and was originally fast-tracked for FDA approval in 2003 for the treatment of patients with non-small cell lung cancer (NSCLC). Following its subsequent poor performance in the INTACT 1 and 2 phase III trials, which showed no improvement in overall survival (OS) or time to progression (TTP) in the first-line setting, the FDA limited the indication for gefitinib to patients who are currently or have previously benefited from gefitinib.
A second TKI, erlotinib (Tarceva, Genentech), was approved in 2004 as a monotherapy for patients with NSCLC, and subsequently in combination with gemcitabine for advanced pancreatic cancer in 2005.
Two monoclonal antibodies, cetuximab (Erbitux, Bristol-Myers Squibb), a chimeric mouse/human antibody, and panitumumab (Vectibix, Amgen), a fully humanized antibody, are currently approved for the treatment of metastatic colorectal cancer. Cetuximab is also approved for the treatment of recurrent head and neck cancers.
Lapatinib (Tykerb, GlaxoSmithKline) and vandetanib (Caprelsa, AstraZeneca) are FDAapproved, multitargeted TKIs that include EGFR among their targets.
Other agents under development include the TKI afatinib (Boehringer Ingelheim) and the monoclonal antibody nimotuzumab (YM BioSciences Inc). Phase I trials of a recombinant antibody, Sym004 (Symphogen), a 1:1 mixture of 2 chimeric anti-EGFR antibodies, were also reported at the 2011 American Society of Clinical Oncology (ASCO) meeting.
A common problem with targeted agents is the development of resistance, both intrinsic and acquired, and EGFR-targeted therapies are no exception. Much research has focused on understanding the molecular mechanisms underlying the development of resistance. A surprising finding was that while some mutations in the EGFR gene are activating and drive tumor formation, others could actually be governing intrinsic resistance against EGFR inhibitors. Intrinsic resistance is also reported to be conferred by the presence of mutations in the KRAS gene.
In patients with NSCLC, acquired resistance to TKIs occurs after a median of 10 to 14 months of treatment. Several mechanisms of acquired resistance have been reported.
The most common was identified in a landmark paper describing a gatekeeper mutation, T790M, which restores ATP binding and permits tyrosine kinase activity even in the presence of inhibitors. T790M is responsible for TKI resistance in more than 50% of all cases. A number of novel agents are being developed in response to this mutation, including irreversible inhibitors and EGFR mutation-specific antibodies.
Another major mechanism of acquired resistance is compensatory amplification of the MET gene; one study suggested a prevalence of more than 20%. MET is able to activate an alternate member of the ERBB family, ERBB3, and sustain activation of the PI3K/Akt cascade.
Furthermore, other theories have been posited as explanations for acquired resistance to EGFR inhibitors. These concepts include ubiquitination, which targets the EGFR for destruction in the cell; the epithelial-to-mesenchymal transition, in recognition of the fact that mesenchymal cells are more resistant; oncogenic shift, involving increased expression of alternative ERBB receptors; and activation of alternative signaling pathways, including the Akt/mTOR cascade.
As a result, EGFR inhibitors are also being tested in combination with other agents, including vascular endothelial growth factor receptor and MET TKIs.
In addition to the high incidence of resistance to EGFR inhibitors, there is an urgent need for improved methods with which to select specific patients who will best respond to these agents.
There has been substantial variability in phase III trials of EGFR inhibitors, with some studies demonstrating no improvement in OS or TTP. Crucially, according to the Iressa Pan-Asia Study (IPASS), gefitinib has only about a 1% response rate in patients with NSCLC who do not have activating mutations of EGFR, compared with an almost 25% response rate for chemotherapy, demonstrating that they gained no benefit from gefitinib therapy.
Protein overexpression has proved to be a useful biomarker of response to targeted therapies for other oncogenic pathways. The conundrum facing EGFR researchers is that most studies have failed to show any significant relationship between EGFR expression levels and response rates.
Recent research has suggested that one reason for this may be the identification of multiple isoforms of the EGFR, adding a new and unappreciated level of complexity to EGFR signal transduction. These isoforms may be an unexpected target of EGFR-directed antibodies in humans and may subsequently interfere with accurate pharmacodynamics and pharmacokinetic measurements, moderate therapeutic efficacy, and contribute to the disparity between observed EGFR expression levels and responsiveness to EGFR-targeted therapies.
Nita Maihle, PhD, professor of Obstetrics, Gynecology and Reproductive Sciences at Yale University, New Haven, Connecticut, has been studying EGFR isoforms as biomarkers in ovarian cancer. In an interview with OncLive, she said, “There are three known isoforms so far, in addition to the full-length receptor, p60 soluble (s)EGFR, p70 sEGFR, and p90-110 sEGFR, which are ubiquitously expressed in human tissues and blood.”
The tide of clinical research is now turning toward the identification of better biomarkers of response. Among promising recent advances discussed at this year’s ASCO meeting are EGFR mutation-specific antibodies and the phosphorylation of EGFR tyrosine residue 1068.
Jane de Lartigue, PhD, is a freelance medical writer and editor based in the United Kingdom.