Commentary (Patnaik/Rowinsky)-Epidermal Growth Factor Receptor Inhibitors for the Treatment of Colorectal Cancer: A Promise Fulfilled?

The past decade has given rise to an explosion of rationally designed, molecularly targeted therapeutic agents. The epidermal growth factor receptor (EGFR) has served as the principal platform for the development of such novel targeted therapies, resulting in a paradigm shift in the treatment of a vast array of solid malignancies. Damjanov and Meropol have provided a comprehensive and insightful overview of the role of EGFR-directed therapeutics in colorectal cancer. They have chosen to focus their discussion on the compounds that are furthest along in clinical development and, hence, have reviewed the monoclonal antibodies cetuximab (Erbitux), ABX-EGF, and EMD 72000, as well as the small-molecule tyrosine kinase inhibitors gefitinib (Iressa) and erlotinib (Tarceva).

The EGFR Family

The EGFR is a prototypic example of successful translation of science from "the bench to the bedside," capitalizing on a detailed knowledge of the structure and function of its different domains. The EGFR, a type I receptor tyrosine kinase, was the first to be identified of a family of four receptors known as ErbB receptors.[1] A myriad of preclinical studies over the past 2 decades provided proof of principle that this transmembrane protein was implicated in oncogenic transformation to a malignant phenotype, as exemplified by characteristics such as increased cellular growth, invasion, angiogenesis and the development of metastases.

Unlike its close family member ErbB2 (HER2/neu), where gene amplification plays a major role in the dysregulation of the protein, increased signaling via EGFR may be achieved through several means. Overexpression of EGFR has been demonstrated in many human malignancies, including gliomas and cancers of the head and neck, colon, pancreas, breast, ovary, bladder, kidney, and upper aerodigestive tract, without gene amplification. Instead, EGFR overexpression is associated with overexpression of the ligands transforming growth factor-alpha or amphiregulin, resulting in activation of autocrine growth factor loops.[2-5] Other mechanisms of EGFR dysregulation include activating mutations (EGFRvIII), heterologous ligand-dependent mechanisms, and ligand-independent mechanisms via the urokinase plasminogen receptor.[6,7]

The ability to demonstrate overexpression of EGFR by immunohistochemical methods in up to 77% of colorectal cancers and the association of increased expression with a more aggressive phenotype and ultimately a worse prognosis gave way to the consideration of EGFR as a rational therapeutic target in colorectal cancer. There remain a number of challenges in the clinical development of EGFR inhibitors in colorectal cancer despite their impressive preliminary antitumor activity. The successes that have been observed with this strategy have been tempered by a number of mitigating factors, including the lack of therapeutic efficacy of the smallmolecule tyrosine kinase inhibitors, and the inability thus far to prospectively identify patients who will benefit from such therapy.

Monoclonal Antibodies

To date, the most successful anti- EGFR-directed approach has been demonstrated with the use of monoclonal antibodies, which bind to the extracellular domain of EGFR, producing competitive inhibition of ligand-binding, with resultant abrogation of receptor dimerization and subsequent downregulation.[8] The structure of some monoclonal antibodies may allow for the possibility of recruitment of Fc-receptor-expressing effector cells, leading to antibodydependent cellular cytotoxicity.[9-11] While this is a theoretical advantage of the immunoglobulin (Ig)G1 backbone (possessed by cetuximab and EMD 72000), the IgG2 structure of ABX-EGF is completely devoid of effector functions, such as complement- dependent cytotoxicity and antibody-dependent cellular cytotoxicity. The disparate effector functions of monoclonal antibodies thus make it unlikely that their activity as a class can be explained by immunomodulatory effects.

Preclinically, cetuximab-a chimeric (human:murine) monoclonal antibody with a high affinity for EGFR (Kd = 2.0 * 10^-10 M)-demonstrates antitumor activity in colorectal xenograft models and complete regressions if administered with irinotecan (Camptosar). Although it is being evaluated in several malignancies, including cancer of the head and neck, ovary, and pancreas, its development is furthest along in colorectal cancer. A randomized phase II study conducted by Cunningham et al in advanced refractory colorectal cancer was the basis for submission of data to US and European regulatory agencies for accelerated approval. In this study, 329 patients with EGFR-positive, irinotecan-refractory metastatic colorectal cancer were randomized in a 2:1 schema to receive either the combination of cetuximab and irinotecan (218 patients) or cetuximab alone (111 patients). Response rates of 22.9% (95% confidence interval [CI] = 17.5%-29.1%) and 10.8% (95% CI = 5.7%-18.1%) were seen for the combination regimen and single agent, respectively. These results were in fact very similar to those observed in two earlier phase II studies conducted by Saltz et al in advanced colorectal cancer.[12,13] The only predictor of response and survival in these trials has been the development of skin rash, which is now the intense focus of further pharmacodynamic studies. Acneiform skin rash and anaphylactoid or anaphylactic reactions occurred in 2% of patients, while nonneutralizing human antichimeric antibodies were detected in 4% of patients and were not related to allergic ractions. The optimal biologic dose is 200 to 400 mg/m²/wk, based on the saturation of antibody clearance and the demonstration of EGFR and downstream signaling inhibition in biopsy specimens from patients.[14,15]

Humanized Antibodies

Whereas chimeric antibodies contain 34% mouse protein, humanized antibodies such as EMD 72000 contain approximately 5% to 10% mouse protein sequences and are constructed by "implanting" the complementarity- determining regions of a mouse antibody into a human IgG framework. In a phase I study of EMD 72000, partial responses have been observed in 3 of 19 patients with colorectal cancer, with comcomitant demonstration of decreases in phosphorylated EGFR and downstream signaling elements in skin and tumor tissues. Unlike other EGFR-directed therapies, dose-limiting fever and headache have been observed in two phase I studies of EMD 72000, where EGFR-positive patients have been enrolled. Skin rash, the most commonly observed adverse effect, was not dose-limiting.[16,17] Phase II studies have been planned in colorectal cancer and other tumor types.

ABX-EGF is a fully human monoclonal antibody, developed using XenoMouse technology, in which the genes responsible for the production of murine antibody have been deleted and those responsible for human antibody production inserted.[18] Unlike cetuximab, which requires the coadministration of irinotecan for complete tumor growth inhibition to occur preclinically, ABX-EGF (Kd = 5 * 10^-11 M) as a single agent leads to complete prevention of A431 tumor formation and eradication of established tumors. In human tumor xenograft models, ABX-EGF led to significant growth inhibition of tumors expressing 17,000 or more EGFRs per cell and demonstrated no antitumor activity in tumors expressing 11,000 or fewer EGFRs per cell.

The initial phase I studies of ABXEGF were performed with a weekly schedule of administration, demonstrating non-dose-limiting skin rash as the predominant toxicity accompanied by trough concentrations of the agent exceeding those required for EGFR saturation preclinically. A weekly dose of 2.5 mg/kg has been utilized in a phase II study of patients with advanced colorectal cancer who failed previous fluoropyrimide-, irinotecan-, and/or oxaliplatin (Eloxatin)- based therapy. Patients treated in the initial cohort were required to have EGFR expression (2+ or 3+) in more than 10% of tumor cells. A second cohort has also been enrolled, with either 1+ EGFR expression in at least 10% of cells or the sum of 1+, 2+, and 3+ in at least 10% of tumor cells (with the sum of 2+ and 3+ present in less than 10% of tumor cells).[19] Response rates of 10% in a preliminary analysis of 40 patients, with disease stabilization in an additional 55%, make this compound quite attractive from the standpoint of future monotherapy and combination studies. Pharmacokinetic modeling studies have shown that dosing of ABX-EGF every 2 weeks provides preclinical trough concentrations at or above the EGFR sink saturation level (IC90, or the inhibitory concentration at which 90% of EGFR expression is blocked). These data provide the rationale for use of this schedule in future evaluations of ABX-EGF.[20]

Other Anti-EGFR Therapies

Phase II single-agent studies of the small-molecule tyrosine kinase inhibitors in colorectal cancer have been disappointing thus far. Investigators have evaluated both gefitinib and erlotinib in patients with advanced refractory colorectal cancer without demonstrating meaningful antitumor activity. Our own experience with gefitinib in a phase II study of patients with fluoropyrimidine/irinotecanrefractory metastatic colorectal cancer parallels that reported by Damjanov and Meropol with the tyrosine kinase inhibitors (C. Takimoto, personal communication, 2004). Despite negative results observed in single-agent studies of tyrosine kinase inhibitors in colorectal cancer, there is interest in pursuing combination studies with cytotoxic therapy. Data reported by Cho et al suggest intriguing activity of gefitinib combined with the FOLFOX-4 regimen (infusional fluorouracil, leucovorin, oxaliplatin)-a combination known as IFOX-with 73% of patients who had received no prior therapy for metastatic disease achieving a partial response.[21]

Other anti-EGFR-directed therapies currently in phase I/II clinical trials include bispecific monoclonal antibodies. These antibodies possess two differing antigen-binding arms and thus have dual specificity. One arm binds the extracellular domain of EGFR, whereas the other arm binds to an immunologic effector cell, resulting in augmentation of the cellular immune response. Results of studies with bispecific antibodies are eagerly awaited, to determine if they will have utility in the treatment of colorectal cancer.

Remaining Challenges

A number of challenges clearly remain in the development of EGFR inhibitors for colorectal cancer. Although EGFR has been validated as a target for cancer therapy, it is not clear which subsets of patients with colorectal cancer will benefit from EGFR-directed therapeutics. The data from the cetuximab studies show that response rates cannot be predicted from the level of EGFR expression as gauged by immunohistochemistry. Although the lack of a relationship may be due to the loose methodology (ie, quantification of receptors using immunohistochemistry), it is also plausible that EGFR content does not correlate with the functional status of the receptor. The majority of studies have required the presence of EGFR in tumor tissues but have not examined the activation status of the receptor. Therefore, the development of a validated assay that quantifies the activation (phosphorylation) status of EGFR, appears to be a fundamental requirement for the a priori determination of response. Other factors that may predict for response and should be examined include levels of ligand expression and expression levels/activation status of other members of the EGFR family, as well as downstream signaling proteins. Until such assays are available, it behooves us to design studies that do not exclude patients who may potentially benefit from therapy.

Given that skin rash appears to be a pharmacodynamic parameter of response, it may be possible to optimize therapy by approaches such as "doseto- rash" strategies in patients receiving EGFR-directed therapeutics. EGFR-directed studies in colorectal cancer have also shown us that differential responses may be generated in spite of therapies that seemingly target the same receptor. The observation of differing activity profiles for monoclonal antibodies and tyrosine kinase inhibitors suggest that these agents have nonoverlapping mechanisms of action and may provide the basis for studying different classes of agents in combination.[1] Preclinical studies have shown synergy between monoclonal antibodies and tyrosine kinase inhibitors, thus providing the rationale for subsequent clinical evaluations.[22,23]

Conclusions

Colorectal cancer has served as a "fertile ground" for novel therapeutics research and the incorporation of molecularly targeted approaches. Therapeutic advances in the treatment of metastatic colorectal cancer over the past decade have resulted in an incremental survival benefit for patients. In addition to agents such as irinotecan and oxaliplatin, we can now consider a new class of molecularly targeted compounds that are fundamentally different from cytotoxic therapy mechanistically, giving rise to nonoverlapping activity and toxicity profiles. Clinical trials of these drugs are under way, combining targeted therapies with cytotoxic agents in both the metastatic and adjuvant settings for patients with colorectal cancer. These novel agents, which target EGFR or angiogenesis, are providing us with a new platform from which to make future advances. It is not unrealistic to project a median survival of 3 years and beyond for patients with metastatic colorectal cancer. The attenuated toxicity profiles of these new agents are also allowing patients to benefit from improvements in quality of life. Although the monoclonal antibodies have shown more activity than the tyrosine kinase inhibitors in advanced colorectal cancer, it is possible that we have not been able to select the population who may benefit most from the latter agents.

Preclinical data suggest that EGFR tyrosine kinase inhibitors are more effective in preventing mammary hyperplasias than in inhibiting established carcinomas. They may also be superior in reducing the number of colonic polyps but not their size. Such data may imply that these agents could have greater utility in treating preneoplastic lesions or perhaps microscopic metastatic disease, as in the case of adjuvant therapy.[24-26] It is also possible that there may be synergistic interactions between tyrosine kinase inhibitors and chemotherapy that will portend better results with combination therapy than with monotherapy.

The pessimism that existed in the outlook for patients with advanced colorectal cancer has given way to great excitement and anticipation as we await the results of ongoing and future planned studies involving EGFR-directed therapies and other classes of molecularly targeted agents.

Financial Disclosure:The authors have no significant financial interest or other relationship with the manufacturers of any products or providers of any service mentioned in this article.

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