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The Oncologist, Vol. 10, No. 4, 250-261, April 2005; doi:10.1634/theoncologist.10-4-250
© 2005 AlphaMed Press

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Critical Evaluation of Current Treatments in Metastatic Colorectal Cancer

Alan Venook

Division of Medical Oncology, Clinical Research Office, University of California Cancer Center, San Francisco, California, USA

Correspondence: Alan Venook, M.D., Division of Medical Oncology, Clinical Research Office, University of California Cancer Center, Box 1705, 1600 Divisadero, San Francisco, California 94115-1705, USA. Telephone: 415-353-9888; Fax: 415-353-9959; e-mail: venook{at}cc.ucsf.edu


   LEARNING OBJECTIVES
Top
 Learning Objectives
Abstract
 Introduction
 Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
 Conclusions
 Disclosure of Potential...
 References
 
After completing this course, the reader will be able to:

  1. Describe the evolution of cancer chemotherapy for patients with colorectal cancer.
  2. Discuss the current relevance of VEGF as a therapeutic target in colorectal cancer.
  3. Discuss the current relevance of EGFR as a therapeutic target in colorectal cancer.

Access and take the CME test online and receive 1 hour of AMA PRA category 1 credit at CME.TheOncologist.com


   ABSTRACT
Top
 Learning Objectives
 Abstract
Introduction
 Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
 Conclusions
 Disclosure of Potential...
 References
 
Fluorouracil (FU) has been the mainstay of treatment for metastatic colorectal cancer (mCRC) for many years. However, in recent years, newer chemotherapeutic agents, particularly irinotecan (Campostar®; Pfizer Pharmaceuticals, New York, NY, http://www.pfizer.com) and more recently oxaliplatin (Eloxatin®; Sanofi-Aventis Inc., New York, NY, http://www.sanofi-aventis.com), have been shown to improve survival in combination with FU-based therapies. These agents were therefore incorporated into first- and second-line treatment strategies. The development of targeted agents that are tumor specific with better toxicity profiles than chemotherapeutic agents has widened the spectrum of therapies for this disease. The U.S. Food and Drug Administration (FDA) recently approved two targeted agents for treating mCRC: an antivascular endothelial growth factor monoclonal antibody (mAb), bevacizumab (Avastin®; Genentech, Inc., South San Francisco, CA, http://www.gene.com), in combination with first-line 5-FU-based chemotherapy regimens and the human epidermal growth factor receptor (HER-1/EGFR)-targeted mAb cetuximab (Erbitux®; ImClone Systems, Inc., New York, NY, http://www.imclone.com) as monotherapy or in combination with irinotecan as second-line therapy in refractory cancer. These newer, more effective agents are improving clinical outcome for patients with mCRC. However, as the number of agents has increased, choosing the most effective treatment strategy has become increasingly complex. This review discusses the role of the individual agents in the treatment of mCRC and identifies the most effective regimens.

Key Words. Fluorouracil • Irinotecan • Oxaliplatin • Bevacizumab • Cetuximab • Vascular endothelial growth factor • VEGF


   INTRODUCTION
Top
 Learning Objectives
 Abstract
 Introduction
Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
 Conclusions
 Disclosure of Potential...
 References
 
Colorectal cancer is the third most common cancer in the U.S., with approximately 145,000 new cases expected in 2005 [1]. Estimated 5-year survival rates range from 90% for patients with stage I disease to <10% for patients with metastatic colorectal cancer (mCRC) [1]. For many years, the standard treatment for patients with mCRC was systemic chemotherapy with fluorouracil (FU). In the past 5 years, however, therapies for mCRC have progressed dramatically and shifted from monotherapy to combination therapy and, more recently, to sequential combination therapy. Because these more efficacious treatment regimens allow patients to survive longer and receive more lines of therapy, choosing the best treatment regimen is becoming increasingly complex. This review discusses the role of various agents/regimens in the treatment of mCRC and focuses on identifying the most effective regimens based on the available data.


   CHEMOTHERAPEUTIC AGENTS/REGIMENS FOR MCRC
Top
 Learning Objectives
 Abstract
 Introduction
 Chemotherapeutic Agents/Regimens...
Targeted Agents in Combination...
 Conclusions
 Disclosure of Potential...
 References
 
Fluorouracil/Leucovorin
For more than 40 years, FU was the standard of care for patients with mCRC. Because FU has a schedule-dependent mode of action, various strategies have been explored to maximize its cytotoxic effects, such as changing the dose, schedule, or route of administration. A brief overview of studies follows, comparing the efficacy and toxicity profiles of different modalities.

A pooled analysis, incorporating data from six randomized trials, showed that FU administered as a continuous i.v. infusion led to a significantly higher response rate than bolus i.v. FU (Mayo Clinic protocol) (22% versus 14%, respectively; odds ratio, 0.55; p = .0002), although the median survival times were similar (infusional FU 12.1 months versus bolus FU 11.3 months; hazards ratio, 0.88; p = .04) [2]. FU administered by continuous infusion allows for the delivery of more drug than bolus FU and shifts the dose-limiting toxicity from myelosuppression to hand-foot syndrome, which results in a more favorable toxicity profile [2, 3]. The change in toxicity profile is a result of a higher concentration of FU in the bone marrow following bolus administration versus continuous infusion [4].

The activity of FU also has been enhanced by the addition of the biochemical modulator folinic acid (leucovorin [LV]). In a recent meta-analysis of 3,300 patients randomized in 19 trials, the response rate was twofold greater in patients receiving FU/LV than in those receiving FU alone (21% versus 11%, respectively; p < .0001) [5]. There also was a slightly, but significantly, longer survival time in those receiving FU/LV [5]. Biomodulation of FU with LV also has, however, been associated with higher incidences of grade 3 and 4 diarrhea, stomatitis, and hematologic toxicities than either bolus or infusional FU [6].

Based on better response rates, bolus FU/LV therapy became the standard of care for patients with mCRC in the U.S. and remained as such until 2000. In Europe, oncologists were, however, more likely to use a modulated infusion regimen. The most commonly used approach there is the de Gramont regimen, an every-2-weeks protocol (LVFU2) that combines bolus with infusional FU/LV. De Gramont and colleagues reported that this regimen was more effective and had a better safety profile than the LV/FU bolus regimen [7]. However, U.S. oncologists have been reluctant to use infusional FU because of the inconvenience and higher costs associated with infusion access and pumps. Figure 1Go shows key publications demonstrating the efficacy of bolus versus infusional 5-FU therapy.



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  		Figure 1. FU: the cornerstone of therapy for mCRC [2, 5, 59–67]. Abbreviations: CALGB = Cancer and Leukemia Group B; ECOG = Eastern Cooperative Oncology Group; FU = fluorouracil; LV = leucovorin; mCRC = metastatic colorectal cancer.

 
Irinotecan, Oxaliplatin, and Capecitabine
In the past few years, newer chemotherapeutic agents, such as irinotecan (Camptosar®; Pfizer Pharmaceuticals, New York, NY, http://www.pfizer.com), oxaliplatin (Eloxatin®; Sanofi-Aventis Inc., Paris, France, http://www.sanofi-aventis.com), and capecitabine (Xeloda®; Hoffmann-La Roche Inc., Nutley, NJ, http://www.roche.us), used as monotherapy or in combination with 5-FU-based therapy have had a significant impact on treatment strategies for patients with mCRC.

Irinotecan, a topoisomerase I inhibitor, was initially introduced as monotherapy for patients with mCRC refractory to FU [8, 9]. In two pivotal phase III trials, therapy with single-agent irinotecan resulted in a longer survival time than best supportive care or infusional FU/LV therapy in FU-refractory patients [10, 11] (Table 1Go). Therefore, based on these data, irinotecan was approved for patients with mCRC who had failed treatment with FU/LV (Table 2Go).


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  		Table 1. The efficacy of chemotherapeutic regimens, bevacizumab in combination with IFL, and single-agent cetuximab in first- or second-line mCRC
 

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  		Table 2. FDA-approved regimens for mCRC
 
Researchers then evaluated the addition of irinotecan to FU/LV as first-line treatment for mCRC. Saltz and colleagues showed that combining irinotecan with bolus FU/LV (IFL) resulted in a higher response rate, longer progression-free survival (PFS) time, and longer overall survival time in first-line mCRC treatment compared with bolus FU/LV alone [12] (Table 1Go). As expected, the incidence of grade 3 diarrhea was more common with irinotecan-based therapy than with bolus FU/LV alone. Similarly, in a European trial, Douillard and colleagues also showed that adding irinotecan to weekly or every-2-weeks regimens of FU/LV by infusion (FOLFIRI) improved the response rate and increased the PFS and survival times [13] (Table 1Go). Based on data from these trials, the U.S. Food and Drug Administration (FDA) and European regulatory authorities approved irinotecan in combination with both bolus and infusional FU/LV as first-line therapy for mCRC, replacing FU/LV as the standard of care (Table 2Go). U.S. oncologists were, however, partial to the bolus regimen, and European oncologists continued to use infusional FU/LV. Interestingly, a second European trial also examined irinotecan in combination with the AIO regimen (infusional FU/LV given weekly for 6 weeks) as first-line mCRC treatment [14]. Survival times in that trial were longer than those in the Saltz et al. and Douillard et al. trials (Table 1Go), but this may be explained by the extensive use of second- and third-line therapies, as both oxaliplatin and irinotecan were readily available in Europe at the time.

Oxaliplatin is a cisplatin derivative with a similar mechanism of action to other platinum compounds, although its antitumor profile differs from those of cisplatin (Platinol®; Bristol-Myers Squibb, Princeton, NJ, http://www.bms.com) and carboplatin (Paraplatin®; Bristol-Myers Squibb, http://www.bms.com) [15]. Indeed, experimental studies have shown that oxaliplatin inhibits colorectal cancer tumor cell lines that are resistant to cisplatin and carboplatin [16].

In a phase III trial, oxaliplatin was evaluated in combination with infusional FULV2 (FOLFOX regimen) as first-line therapy for patients with mCRC. The findings showed a higher response rate, longer median PFS time, and longer survival than those observed with FU/LV alone, although the survival difference was not statistically significant (Table 1Go) [17]. The lack of difference in survival benefit between the two groups may be explained by the small sample and crossover treatments that might have obscured any difference in survival [17]. Toxicities of oxaliplatin in combination with FU/LV2 also were mild, although dose-dependent and mostly reversible sensory neuropathy was a cumulative dose-limiting toxicity in the oxaliplatin arm. These data were submitted to the FDA in 2002, but oxaliplatin was denied approval because of the lack of a statistically significant survival benefit. A phase III trial demonstrated the efficacy of oxaliplatin-based therapy as second-line treatment for mCRC [18]. Rothenberg and colleagues showed a higher response rate and longer time to progression (TTP) with oxaliplatin plus infusional plus bolus FU than with oxaliplatin monotherapy and infusional FU/LV alone, although single-agent oxaliplatin and FU/LV had no meaningful activity (Table 1Go). These data led to the approval of oxaliplatin in combination with infusional FU/LV as second-line therapy following irinotecan and FU/LV for mCRC in the U.S. (Table 2Go).

A phase III trial recently suggested that FOLFOX first-line therapy is superior in efficacy and safety to IFL [19]. Goldberg and colleagues showed that FOLFOX produced a higher response rate (45%) and longer median survival time (19.5 months) than IFL (31% and 15 months, respectively) and than irinotecan and oxaliplatin given every 3 weeks (IROX; 35% and 17.4% months, respectively) (Table 1Go). With the exception of peripheral neuropathy, the toxicity profile for FOLFOX also was more favorable than that of either IFL or IROX.

The conclusion that the FOLFOX regimen is more effective than IFL or IROX for first-line mCRC therapy must be tempered by several nuances in the trial design. Notably, that an infusional FU-based combination regimen was compared with a bolus FU-based combination regimen, and therefore the individual contributions of irinotecan and oxaliplatin to an FU-based regimen cannot be determined. Also, crossover to second-line therapy was imbalanced because oxaliplatin was not readily available in the U.S. at the time of the trial, and therefore, most patients who received FOLFOX initially, as first-line therapy, received irinotecan at progression, whereas only a few patients who received first-line IFL received oxaliplatin at progression. The FDA, however, recently approved oxaliplatin as first-line therapy for mCRC (Table 2Go). In Europe, a second trial examined the AIO regimen plus oxaliplatin (FUFOX) versus bolus FU/LV in patients with advanced CRC [20]. It was shown that the oxaliplatin-based regimen significantly improved response rate, but not overall survival, although survival was longer (20.4 months versus 16.1 months) with the FUFOX regimen (Table 1Go). As with other European trials during that period, the long overall survival time may be attributed to the 75% of patients who received oxaliplatin- and/or irinotecan-based salvage therapies [20].

FOLFOX and FOLFIRI appear to be the most effective in terms of efficacy and tolerability. In a recent randomized trial, Colucci and colleagues showed that the two regimens were comparable [21]. Larger randomized studies comparing FOLFOX with FOLFIRI are ongoing and will help us to evaluate which is the more effective regimen.

Tournigand and colleagues also evaluated the FOLFOX and FOLFIRI regimens to find the best sequence for treating patients with mCRC [22]. Those authors showed that a sequence of first-line FOLFOX followed by second-line FOLFIRI resulted in a similar survival time to that produced by the reverse sequence (Table 1Go). However, as at least 30% of patients did not receive second-line therapy, the authors highlighted the importance of choosing the most appropriate first-line therapy. Although both first-line therapies achieved similar response rates (FOLFIRI 56% versus FOLFOX 54%), second-line FOLFIRI achieved a significantly lower response rate than did FOLFOX (4% versus 15%). The toxicity profiles for the two regimens were also different. As expected from previous studies, grade 3/4 mucositis, nausea/vomiting, and grade 2 alopecia were more common with FOLFIRI, whereas grade 3/4 neutropenia and neurosensory toxicity were more common with FOLFOX. The authors noted that future studies should focus on the limitations of the trial, notably that neurotoxicity forces many patients to stop oxaliplatin before tumor resistance develops and that FOLFIRI has a relatively poor efficacy as a second-line therapy.

Importantly, it should be noted that phase III trials evaluating combination therapy have reported median overall survival times in the range of 14.8–21.5 months. As these were large, randomized trials conducted over a relatively short period, it is unlikely that patient selection or other factors outside the treatment strategy were responsible for these differences. It is most likely that second- or third-line therapies had an impact on overall survival. A recent meta-analysis analyzed results from seven phase III trials in mCRC to correlate the percentage of patients receiving: A) second-line therapy, and B) all three agents with the reported median overall survival [23]. The median overall survival was significantly correlated with the percentage of patients receiving all three agents over the disease course, but not with the proportion of patients receiving second-line therapy. To optimize clinical outcome, the authors suggested that it is important to make FU/LV, irinotecan, and oxaliplatin available to all patients with mCRC who are candidates for this type of therapy. However, possible study bias was noted, that is, patients who lived longer had a better chance of receiving all therapies, while patients with poorer performance statuses and shorter life expectancies were probably excluded from second-line therapy. Also, this analysis could not evaluate the best sequence of treatment, although ongoing and future randomized trials comparing different sequences should address these questions. Finally, as sequential therapies cannot be predefined in treatment protocols, overall survival may no longer be regarded as the most sensitive end point for assessing the efficacy of first-line therapy; other factors, such as PFS and TTP, should be considered.

Researchers are investigating alternative approaches to optimizing FU therapy. Capecitabine is an oral fluoropyrimidine carbamate, initially designed to deliver FU predominantly to tumor cells [24, 25]. In two randomized trials, the efficacy and toxicity of capecitabine as first-line treatment in patients with mCRC were evaluated [26, 27] (Table 1Go). Those trials showed that capecitabine had an efficacy that was at least equivalent to that of i.v. FU/LV. Grade 3 hand-foot syndrome was reported more frequently with capecitabine, although the condition was tolerated with a reduced dose. A higher incidence of grade 3/4 hyperbilirubinemia also was reported, but all cases were reversible. Against this background, capecitabine appears to offer equivalent efficacy and a better toxicity profile than the Mayo regimen, with the added convenience of an oral agent. Based on these data, capecitabine was approved in the U.S. as first-line therapy for patients with mCRC for whom combination therapy is not warranted (Table 2Go). Other phase II studies have examined the addition of oxaliplatin or irinotecan to capecitabine [28, 29]. So far, data have been encouraging, with toxicity profiles similar to that of infusional FU, and it appears that oxaliplatin/capecitabine is equivalent to irinotecan/capecitabine therapy. Based on these data, randomized, phase III trials are planned to evaluate oxaliplatin or irinotecan in combination with capecitabine in patients with mCRC.


   TARGETED AGENTS IN COMBINATION WITH CHEMOTHERAPY—MOVING FORWARD
Top
 Learning Objectives
 Abstract
 Introduction
 Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
Conclusions
 Disclosure of Potential...
 References
 
Advances in chemotherapeutic agents have led to better outcomes for patients with mCRC. Chemotherapies, however, are limited by their lack of specificity and are often associated with frequent and potentially severe dose-limiting toxicities. Therefore, there is an urgent need for more effective, better-tolerated treatments that specifically target the processes pivotal to tumorigenesis and metastasis. Further advances in the understanding of molecular biology have led to the development of target-specific agents. The FDA recently approved two targeted agents: an antivascular endothelial growth factor (anti-VEGF) monoclonal antibody (mAb), bevacizumab (Avastin®; Genentech, Inc., South San Francisco, CA, http://www.gene.com), and a human epidermal growth factor receptor (HER-1/EGFR)-targeted mAb, cetuximab (Erbitux®; Imclone Systems, Inc., New York, NY, http://www.imclone.com), as first- and second-line mCRC therapy, respectively. These two agents are already having a significant impact on mCRC treatment strategies.

Bevacizumab

Rationale for Targeting VEGF
Over 20 years ago, researchers showed that tumors cannot grow beyond 1–2 mm without the establishment of a blood supply [30]. The formation of tumor blood vessels, or angiogenesis, not only allows tumors to absorb nutrients and oxygen for their further growth and development but also allows a pathway for migrating tumor cells to access the systemic circulation and establish metastases. The transition, or switch, of a tumor to an angiogenic phenotype is caused by an increase in proangiogenic factors, including VEGF, basic fibroblast growth factor (bFGF), and transforming growth factor beta (TGF-ß), and a decrease in antiangiogenic factors, such as interferon-{alpha} [31].

VEGF is a specific mitogen for the endothelial cell and one of the most potent proangiogenic factors. VEGF acts as a survival factor for endothelial cells through the inhibition of apoptosis and plays an important role in mobilizing endothelial cell precursors to sites of angiogenesis [32, 33]. VEGF is upregulated in most human tumors, including colorectal cancer [33]. This has been correlated with increased tumor invasion, microvessel density, disease recurrence, and a poor prognosis [34, 35]. Based on these observations, and its low levels in healthy tissue, VEGF appears to be a particularly attractive target for anticancer therapy.

There are a variety of strategies that target VEGF, although VEGF blockade with mAbs is the most studied approach. Bevacizumab is an anti-VEGF, humanized mAb that is the most advanced agent of its class in clinical development. Preclinical data show that this agent is active in colorectal cancer and other types of solid tumors and is better tolerated than conventional chemotherapeutic agents [36, 37]. Preclinical studies have also shown that combining anti-VEGF therapy with chemotherapeutic agents results in augmented antitumor activity [38, 39]. The mechanism by which bevacizumab enhances the efficacy of chemotherapy is not well understood, although it has been proposed that, as tumor blood vessels are chaotic, irregular, and leaky, relatively low doses of anti-VEGF therapy "normalize" tumor vasculature, reducing intratumoral pressure and allowing better delivery of therapeutic agents to the tumor, thereby maximizing antitumor activity [40]. Against this background, it was suggested that the most effective use of bevacizumab is in combination with chemotherapy.

Clinical Studies
Several studies have examined bevacizumab in combination with chemotherapy in the first- and second-line settings in patients with mCRC. Phase II and III trials of bevacizumab in combination with FU/LV and IFL are completed or ongoing (Table 1Go and Table 3Go). Studies of bevacizumab in combination with oxaliplatin-based therapies are ongoing.


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  		Table 3. Completed or ongoing trials evaluating bevacizumab in combination with bolus FU/LV therapy in mCRC
 
The phase III trial was the first phase III evaluation of the efficacy and safety of first-line bevacizumab and led to the recent approval of this agent in combination with i.v. FU-based therapy as first-line therapy for patients with mCRC (Table 2Go) [41]. In this trial, Hurwitz and colleagues examined bevacizumab in combination with IFL as first-line therapy for patients with mCRC. Over 900 patients were randomized to one of three treatments: IFL/placebo, IFL/bevacizumab, or FU/LV/bevacizumab. The FU/LV/bevacizumab arm was included because no previous study had examined IFL in combination with bevacizumab, but this arm was stopped when the safety of bevacizumab plus IFL was demonstrated in an interim analysis. Treatment could continue until disease progression, unacceptable toxicity, or 96 weeks. At progression, patients could receive second-line therapy. Oncologists treating patients in a bevacizumab-containing arm could choose to continue with bevacizumab during second-line therapy.

The addition of bevacizumab to IFL resulted in a significantly longer survival time, by almost 5 months (30% increase in survival) (20.3 months versus 15.6 months; p < .001) (Table 1Go and Table 3Go). The addition of bevacizumab to IFL also resulted in a significantly greater overall response rate, duration of response, and PFS time. Survival benefit was observed for all patient subgroups and was independent of second-line therapy. For the subgroup of patients who received second-line therapy with oxaliplatin-containing regimens, overall survival times were 25.1 months and 22.2 months for the IFL/bevacizumab and IFL/placebo arms, respectively [42].

Overall, IFL with bevacizumab was generally well tolerated, with no overlapping toxicities. Although a number of adverse events, including bleeding, thrombosis, proteinuria, and hypertension, were observed in phase I/II trials, only hypertension (easily managed with standard blood pressure medications) had a higher incidence in the IFL/bevacizumab arm. However, six patients (1.5%) receiving IFL/bevacizumab had gastrointestinal events, including bowel perforation. Although this event was uncommon given the size of the trial, the risk of such events may be increased with bevacizumab. Studies are ongoing to understand further bevacizumab-related gastrointestinal perforation.

Phase II and III studies have evaluated the addition of bevacizumab to FU/LV as another standard option for the first-line treatment of mCRC [43–45] (Table 3Go). Indeed, these studies showed that FU/LV/bevacizumab compares favorably with FU/LV. Therefore, bevacizumab should be considered for the subgroup of patients for whom irinotecan- or oxaliplatin-based therapy is not recommended, as this subgroup of patients has few treatment options available.

Clinical trials are in progress or planned to evaluate the addition of bevacizumab to FOLFOX or FOLFIRI. For example, a recently completed phase III trial evaluated the addition of bevacizumab to FOLFOX in the second-line treatment of patients who have failed previous irinotecan plus FU therapy. Analyses of the findings are ongoing, but these data demonstrated that patients receiving bevacizumab plus FOLFOX had a 17% longer survival time than those receiving FOLFOX alone (12.5 months versus 10.7 months) and this regimen also had an acceptable toxicity profile [46]. Therefore, adding bevacizumab to chemotherapy results in a significant survival benefit for patients with untreated or relapsed mCRC. Figure 2Go shows the survival benefit of bevacizumab plus chemotherapy relative to other regimens and best supportive care.



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  		Figure 2. Survival benefit of bevacizumab plus first-line chemotherapy relative to other treatment strategies in patients with mCRC. Abbreviations: BSC = best supportive care; FOLFIRI = fluorouracil/leucovorin plus irinotecan; FOLFOX = every-2-weeks chemotherapy regimen combining bolus with infusional fluorouracil/leucovorin plus oxaliplatin; FU/LV = fluorouracil/leucovorin; IFL = bolus fluorouracil/leucovorin plus irinotecan; mCRC = metastatic colorectal cancer; VEGF = vascular endothelial growth factor; XELOX = capecitabine plus oxaliplatin.

 
Other ongoing trials will provide further information on the efficacy and safety of bevacizumab in different settings. These studies will help us to define the extent to which bevacizumab can be used to treat mCRC. Finally, studies are examining bevacizumab plus FOLFOX in the adjuvant setting. This is a particularly important setting for antiangiogenic therapy, as targeting tumor neovasculature during early disease may contribute to curative treatment.

Cetuximab

Rationale for Targeting HER-1/EGFR
The HER-1/EGFR signaling pathway is thought to play a pivotal role in tumor growth and progression of various cancers, including colorectal cancer [47]. HER-1/EGFR belongs to the HER family of receptors. These receptors are transmembrane glycoproteins, comprising an extracellular ligand-binding domain, an intracellular tyrosine-kinase (TK) domain, and a transmembrane segment [47]. Various ligands can bind to the HER-1/EGFR extracellular domain inducing receptor homo- or heterodimerization with another HER-1/EGFR receptor or other HER family members. This results in activation of the receptor’s TK activity, initiating a downstream signaling cascade that ultimately leads to tumor cell proliferation, migration, adhesion and angiogenesis, and inhibition of apoptosis [48].

Various reports have shown that HER-1/EGFR signaling is dysregulated in colorectal cancer and other tumor types [49]. HER-1/EGFR overexpression has also been correlated with disease progression, poor prognosis, and reduced sensitivity to chemotherapy [50]. Therefore, several strategies have been developed to target HER-1/EGFR, including small-molecule TK inhibitors and mAbs. Cetuximab is the most advanced mAb targeting HER-1/EGFR in clinical development.

Cetuximab exerts its antitumor effects through ligand-independent processes, stimulating receptor internalization [51]. Preclinical and early clinical studies have shown that single-agent cetuximab primarily leads to cytostatic activity, whereas combining cetuximab with chemotherapeutic agents, such as cisplatin, topotecan (Hycamtin®; GlaxoSmithKline, Philadelphia, PA, http://www.gsk.com), and irinotecan leads to synergistic antitumor activity [51–54]. One hypothesis for this synergy is that, for the majority of cell lines, blocking HER-1/EGFR signaling is insufficient for cytotoxicity, but HER-1/EGFR inhibition may leave the cells more vulnerable to chemotherapy. Importantly, these studies consistently suggested that, at least in these models, HER-1/EGFR expression is required for cetuximab activity.

Clinical Data
A phase II study evaluated the activity and safety of cetuximab plus irinotecan in patients with irinotecan-refractory CRC. The response rate was 22.5% in 120 patients who had progressive disease on irinotecan [55]. Cunningham and colleagues also evaluated cetuximab alone and cetuximab plus irinotecan in patients with irinotecan-refractory CRC [56]. The response rates were 10.8% for cetuximab alone and 22.9% for cetuximab plus irinotecan. Finally, a more recent phase II trial also assessed the safety and efficacy of single-agent cetuximab in patients with chemotherapy-refractory mCRC who express HER-1/EGFR [57]. The findings were similar to those of previous trials (Table 1Go). Unfortunately, response did not correlate with the degree of HER-1/EGFR expression as predicted by preclinical studies. This may be a result of the imprecision of quantifying HER-1/EGFR expression or possibly because HER-1/EGFR may not be the most appropriate marker for predicting response to therapy. Techniques such as gene expression profiling may help us to identify predictive markers of response, allowing the selection of patients most likely to respond to therapy.

Although different trials, particularly those with different regimens, should be compared with caution, the response rate from this trial was equivalent to the level of activity observed with second-line FOLFOX (9.9%) [18]. Based on these data, cetuximab was recently approved as second-line therapy for patients with mCRC in the U.S. and Europe (Table 2Go).

The side effects observed in this trial were manageable and similar to those previously reported. The most commonly reported grade 3/4 adverse events were severe hypersensitivity (managed with standard medications) and asthenia, fatigue, malaise, or lethargy. An acne-like rash typically associated with HER-1/EGFR inhibition was also observed in almost every patient, but none of the patients discontinued treatment because of the rash.

Finally, pilot trials have evaluated cetuximab in combination with IFL as first-line therapy for mCRC [58], and randomized phase III trials are examining cetuximab in combination with chemotherapy as first-line and adjuvant therapy. These studies will provide further information on the role of cetuximab in the treatment of CRC.


   CONCLUSIONS
Top
 Learning Objectives
 Abstract
 Introduction
 Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
 Conclusions
Disclosure of Potential...
 References
 
We have discussed some of the results from recently completed trials examining various chemotherapeutic regimens and/or targeted agents as first- and second-line mCRC therapy. We can conclude that the development of newer agents and the shift in treatment from monotherapy to sequential combination chemotherapy has improved clinical outcome for patients with mCRC, but the most appropriate setting for each of these agents/regimens still needs to be determined.

In terms of efficacy and tolerability, FOLFOX, FOLFIRI, and IFL plus bevacizumab are the most effective first-line regimens. However, we still cannot confirm which of these regimens is the most effective for individual patients, although bevacizumab plus FOLFOX or FOLFIRI is likely to have the most clinical benefit (Fig. 2Go). In addition, important issues regarding the most effective sequence for these agents/regimens in the second- and third-line settings have yet to be clarified. For example, if patients progress after IFL, their options for second-line treatment include bevacizumab plus FOLFOX, cetuximab plus irinotecan, and capecitabine plus oxaliplatin. Recently, Grothey and colleagues [20] suggested that sequence is less important if all treatments are made available to all patients with mCRC. Ongoing, well-designed, comparative trials will hopefully provide a better understanding of how each regimen should be used to achieve maximum clinical benefit.

In addition, the optimal chemotherapy combinations, doses, and sequences of administration have yet to be defined. The addition of targeted agents has only added to the complexity. Indeed, it is likely that the optimal dose may vary with tumor location, growth rate, and previous therapy. Therefore, further studies are required to define the optimal dose and regimens for these agents in mCRC.

FU has been the cornerstone of treatment for mCRC for over 40 years. In the past few years, the introduction of more effective chemotherapeutic agents and targeted agents with their promising activities and mild toxicity profiles has pushed the overall median survival time from 12 months to 2 years. However, as discussed, there are still many challenges facing oncologists. Research is ongoing to understand these issues, and significant advances are expected through the implementation of well-designed clinical trials and continued preclinical investigation.


   DISCLOSURE OF POTENTIAL CONFLICTS OF INTEREST
Top
 Learning Objectives
 Abstract
 Introduction
 Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
 Conclusions
 Disclosure of Potential...
References
 
The author indicated no potential conflicts of interest.


   REFERENCES
Top
 Learning Objectives
 Abstract
 Introduction
 Chemotherapeutic Agents/Regimens...
 Targeted Agents in Combination...
 Conclusions
 Disclosure of Potential...
 References
 

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