• © 2004 by American Society of Clinical Oncology

Targeting the Epidermal Growth Factor Receptor: An Important Incremental Step in the Battle Against Colorectal Cancer

Although colorectal cancer continues to be a major cause of cancer death worldwide, considerable improvement has been achieved in the last decade with the advent of newer chemotherapy agents such as irinotecan and oxaliplatin. This advance is most obvious in the treatment of patients with metastatic disease, in whom the median survival now exceeds 20 months.¹ Nevertheless, we may be approaching the limit on the benefits achievable with conventional cytotoxic agents, and the last few years have witnessed growing interest and significant advances in the use of targeted therapy for colorectal cancer.^2,3 One such targeted therapeutic agent is cetuximab, a chimeric monoclonal antibody directed against the epidermal growth factor receptor (EGFR).⁴

In this issue of the Journal of Clinical Oncology, Saltz et al⁵ report on the results of a phase II trial in which patients with EGFR-positive colorectal cancer, refractory to irinotecan, received cetuximab alone. In a recent study presented at the 2003 Annual Meeting of the American Society of Clinical Oncology, Dr Cunningham, the lead investigator on the BOND trial, also reported on the activity of cetuximab as a single agent.⁶ The evaluation of cetuximab as monotherapy is important, since initial clinical studies with cetuximab were done in combination with irinotecan, making it difficult to know the independent contribution of the antibody to the activity of the regimen. The combination of irinotecan and cetuximab in irinotecan-refractory patients was quite effective, with an overall response rate of approximately 23% in two separate studies.^6,7 The ability of this antibody to reverse resistance to irinotecan can partially be explained by biologic principles. Although EGFR is commonly thought to mediate proliferation signals in tumor cells, it also regulates many other processes. Activation of the EGFR leads to downstream signaling that activates the mitogenic and survival pathways, such as mitogen-activated protein (MAP) kinases and phosphatidyl-inositol-3 kinase (P-I-3K)/AKT pathways.⁸ Inhibition of these pathways by EGFR antagonists, such as cetuximab, can lead to induction of BAX, activation of caspase-8, and downregulation of BCL-2 and NFκB.^9,10 The above alterations may render cells more sensitive to apoptotic stimuli, such as chemotherapy. Tumor cells that may have been activated by EGFR ligands, such as epidermal growth factor and transforming growth factor-alpha, may then be rendered chemosensitive by EGFR inhibition and activation of these proapoptotic pathways.

These biologic principles can explain why cetuximab may be able to sensitize tumor cells to chemotherapy to which they had become resistant. However, one should question how a so-called cytostatic therapy, given as a single agent, can result in tumor regression. In colon carcinoma, an autocrine loop exists whereby ligands for the EGFR will activate the receptor. It is also known that the tumor microenvironment is a relatively caustic environment, with low pH and pO₂ tension,¹¹ and survival signals are necessary for tumor cells to live in this adverse environment. By inhibiting the autocrine or paracrine activation of the EGFR, tumor cells that may typically survive in this caustic environment may undergo spontaneous apoptosis. Thus, the term “cytostatic” therapy may underestimate the therapeutic potential of targeted therapy. This does not imply that anti-EGFR therapy cannot lead to tumor stabilization, but inhibition of critical signaling pathways can also lead to tumor cell apoptosis and regression

The results of this single-agent study by Saltz et al, with a response rate of 9% and a median survival of 6.4 months, are remarkably similar to the results reported in the single-agent cetuximab arm in the more recent Bond trial.¹² This consistency of results confirms the importance of exploiting the cell surface EGFR as a therapeutic target in colorectal cancer, and potentially provides oncologists with another tool in combating this once dismal disease. Although the questions of activity of cetuximab as a single agent in patients who had tumor progression on prior therapy seem to be adequately settled, the findings from these trials lead to many difficult biologic and clinical questions that will only be answered by close collaboration between clinical investigators and basic science researchers.

The most obvious of these questions ask why the EGFR expression level does not seem to be important in determining which tumors will respond, and why only a minority of all the EGFR-positive tumors responded to the therapy. Unfortunately, we do not know the biologic significance of the wide spectrum of expression of EGFR levels on colorectal cancer cells. Currently, the US Food and Drug Administration is evaluating an EGFR immunohistochemical kit that may be used in conjunction with EGFR-directed therapy. The standardization of EGFR positivity is an important first step in providing oncologists and pathologists with a common reference by which patients can be properly selected or “phenotyped” for clinical trials and therapeutic interventions.

In most anti-EGFR clinical trials, any EGFR-positive staining cells, no matter how weak or how few, renders a tumor EGFR “positive.” Although one would assume that the more EGFR, the better the response to anti-EGFR therapy (as was observed in preclinical studies), detailed analyses of the Saltz and BOND trials do not demonstrate that the level of EGFR expression has any impact on response rates. It is intriguing that a significant number of EGFR-positive tumors are resistant to cetuximab. Although activation of the EGFR may be an important signal for tumor cell growth and survival, it must be recognized that other growth factor receptors such as cMET (hepatocyte growth factor receptor), the platelet-derived growth factor receptor, and the insulin-like growth factor receptor-1, among others, are present and important for the growth and dissemination of tumor cells. Activation of these receptors may activate alternative or even redundant downstream signaling pathways. In addition, constitutive activation of signaling pathways such as Ras and Src can initiate downstream signaling that leads to cell proliferation and inhibition of apoptosis. Therefore, if a tumor cell possesses one or more of these receptors or activated signaling intermediates, targeting a single cell surface protein tyrosine kinase such as EGFR may not be enough to inhibit growth or survival signals.

Another considerable challenge is the lack of a predictive marker that would allow us to select patients who are most likely to benefit from cetuximab therapy. EGFR would be the obvious choice, but it does not seem to be particularly helpful. In fact, since all epithelial cells express the EGFR, it is unknown whether those tumors that are now considered EGFR-negative may respond to cetuximab, especially if the tumor microenvironment is rich in the EGFR ligands transforming growth factor-alpha and epidermal growth factor. In addition, some EGFR activity may be mediated through heterodimerization with Her-2-neu and cell-surface receptor cross talk.

Many investigators have hypothesized that one could predict response to anti-EGFR therapy by the presence of unique adverse effects that demonstrate target modification. For example, patients who develop a rash during therapy are more likely to respond than patients who do not. This suggests that skin rash could be used as a “poor man's test” to optimize anti-EGFR therapy, with dose escalation planned until the desired effect is achieved—in this case a follicular rash. Although demonstration of inhibition of activation of the EGFR in normal tissues, such as skin, may not accurately represent the dynamics in the tumor, if one does not observe activity in the skin, it is less likely that the EGFR will be inhibited in the tumor. The above hypothesis is being tested in clinical trials. More sophisticated molecular markers, such as phosphorylation of downstream signaling molecules, are also being investigated as predictive markers for therapy. However, since the tumor microenvironment often expresses a wide range of growth stimulatory signals, it is unknown whether the signaling pathways analyzed in the primary tumor are representative of the signaling pathways in metastases, the common target of systemic therapy.

Other unanswered questions include the role of cetuximab in combination with chemotherapy in first-line treatment for colorectal cancer and the choice of the most appropriate regimen for an individual patient. These questions are made more difficult by the availability of newer cytotoxic drugs and the likely addition of several biologic agents, including cetuximab, to our armamentarium for the treatment of colorectal cancer. Currently, there are no reliable molecular or clinical markers of efficacy for anti–growth factor receptor or antiangiogenic therapies in patients with colorectal cancer. The initial use of cetuximab in combination with chemotherapy will likely be similar to the paradigm established for cytotoxic drugs, in which combinations of agents leads to incremental improvement in efficacy. Ironically, the development of targeted therapy for colorectal cancer is surprisingly similar to the development of cytotoxic therapies—we have been unable to identify markers of sensitivity, and yet, fortunately, we continue to make small incremental advances.