Department of Medical Oncology, Flinders Medical Centre, Bedford Park, South Australia.
Our understanding of the molecular pathways that mediate cancer cell proliferation has increased significantly and with this comes the rapid development of molecular targeted therapies. The epidermal growth factor receptor and the vascular endothelial growth factor are two such targets that have proven to be important in the treatment of advanced colorectal cancer. Successful inhibition of these targets, utilising monoclonal antibodies bevacizumab, cetuximab and panitumumab, has led to improved patient outcomes. Prolongation of patient survival and improvement in quality of life has been associated with the use of these antibodies. Such therapies are now becoming part of standard management of advanced colorectal malignancy. Predictive biomarkers that allow for a more rational and effective utilisation of these new molecular targeted therapies are being discovered. The number of potential molecular targets seems infinite as new drugs are rapidly processed through new accelerated clinical trial designs and drug development programs. Many challenges remain in the successful development of molecular targeted therapies, including overcoming mechanisms of resistance, optimal drug delivery, the issues of the financial cost of these new drugs and equitable access to the new therapies.
Colorectal carcinoma is the third most common malignancy of both sexes in developed countries.1 The spread of colorectal cancer to distant sites (Duke’s stage D) represents essentially incurable disease, except for selected cases where complete surgical resection can be applied. Chemotherapy for advanced colorectal cancer can prolong survival and provide symptomatic benefit and quality of life improvement.2-8 Over the last decade new cytotoxics, including irinotecan and oxaliplatin, have produced further survival benefit.9-13 Therapeutic options for patients with metastatic colorectal cancer who have failed these treatments are limited.
The epidermal growth factor receptor (EGFR) and the vascular endothelial growth factor (VEGF) represent the two molecular structures and associated pathways that have proven to be successful targets in the development of new drugs. Other targets are being evaluated in ongoing clinical trials. We will outline the results obtained in trials that have evaluated molecular targeted strategies and briefly outline the challenges of successful implementation of such treatment, in the context of advanced colorectal cancer.
Cetuximab is a chimeric immunoglobulin G1 monoclonal antibody that binds to the EGFR with high affinity.14 Panitumumab is a humanised monoclonal antibody that has the same effect on EGFR.15 This association of antibody and receptor competitively inhibits ligand binding and leads to inhibition of phosphorylation and subsequent activation of downstream signalling pathways. These antibodies also stimulate EGFR internalisation, effectively removing the receptor from the cell surface.14 Blocking EGFR can lead to cell cycle arrest in the G1 phase,16 and cell death via apoptosis.17
Single-agent therapy with cetuximab or panitumumab has demonstrated activity in patients with refractory, metastatic EGFR-positive colorectal carcinoma. The objective response rates observed in these studies vary between 8 to 12% for single-agent EGFR directed therapy.18-21 The response rate appears similar (10%) when cetuximab is used as a single agent as first line therapy for previously untreated patients.22 Two Phase III randomised control trials have confirmed a benefit of EGFR directed therapy when compared to best supportive care. Cetuximab demonstrated an overall survival advantage, and a quality of life benefit.20 Panitumumab was associated with a progression free survival benefit, but not an overall survival advantage although the trial design allowed for cross-over from best supportive care to panitumumab on disease progression.19 An acneiform skin rash is the principal side-effect of such a treatment approach. An increased severity of skin rash has been associated with a greater response rate.20,23 The problem with using rash as predictive markers is that the drug must first be administered to observe the rash. It is therefore not a predictive factor that can be used prior to therapy, and cannot be used to select patients for initiation of therapy.
Efficacy has also been demonstrated with the combination of either panitumumab or cetuximab and irinotecan in patients with irinotecan-refractory EGFR-positive metastatic colorectal cancer.23,24 Response rates of 19 to 23% with median duration of response of four to six months have been observed, but the median survival of approximately six to eight months remains similar to that observed in the single agent EGFR antibodies.20,23,25 The combination of FOLFIRI and cetuximab was associated with a modest prolongation of progression-free survival when used as a first line treatment for advanced colorectal cancer. A trial comparing irinotecan plus cetuximab versus irinotecan alone as second line treatment, showed no overall survival difference between the two arms.26 The combination of oxaliplatin, fluoropyrimidines and EGFR directed antibody has also been evaluated, with evidence of higher response rates with the antibody-chemotherapy combination approach.
Targeting the EGFR pathway via the associated tyrosine kinase has been tried, with initial studies suggesting possible efficacy. Single agent tyrosine kinase inhibitor (TKI) therapy was associated with stable disease in almost 40% of patients, but no objective responses.27 Phase I and II trials combining TKI with chemotherapy initially suggested safety and possible efficacy, but subsequent studies have been disappointing.28,29 The addition of gefitinib does not overcome fluoropyrimidine resistance.30 In a randomised Phase II trial the addition of gefitinib to FOLFIRI did not provide benefit,31 and in another Phase II study the combination of irinotecan and gefitinib was associated with increased toxicity.32
Angiogenesis plays an important role in the growth and progression of cancers like colorectal cancers. VEGF pathways play a key role in this process. The two major players are VEGF receptors (VEGFR) and their ligands, VEGF glycoproteins. There are five major ligands, VEGF- A through E, while the receptors include VEGFR-1, 2 and 3. The binding of VEGF ligands to the receptors triggers a series of events involving endothelial cell proliferation, migration and survival, in addition to altering vascular permeability, thereby controlling the physiological and tumour angiogenesis.
Angiogenesis plays an important role in cancer survival and progression.33 There are two major anti-angiogenic approaches – monoclonal antibodies or small molecules directed against the VEGF pathways. Other approaches like antisense oligonucleotides and aptamers are still in early research phase. Worldwide, several such drugs like bevacizumb, sunitinib and sorafenib, are approved for routine clinical use to treat patients with colorectal cancer, renal cancers, lung and breast cancers, based on efficacy data from well conducted Phase III trials. In this review, we present the evidence to show how targeting angiogenesis has changed the way we manage colorectal cancer.
Bevacizumab, a recombinant humanised monoclonal antibody against VEGF-A ligand, was the first anti-angiogenic drug to show impressive survival benefit in clinical trials. Most studies indicate that bevacizumab in combination with chemotherapy is better than chemotherapy alone in terms of survival for metastatic colorectal cancer. Bevacizumab acts as a chemo-sensitiser by reducing new blood vessel formation and inducing apoptosis in addition to normalising the tumour vasculature, improving delivery of chemotherapy.34
Hurwitz et al compared the benefit of adding bevacizumab to irinotecan and 5-fluorouracil in patients with previously untreated metastatic colorectal cancer.35 There was a significant difference in response rates, overall survival and progression free survival in favour of the bevacizumab arm.
The consistent survival benefit of adding bevacizumab to other chemotherapy regimens like bolus 5-FU/LV,36 FOLFOX37 and capecitabine/oxaliplatin38 confirmed that the approach of combining anti-angiogenic drugs and chemotherapy is beneficial. The survival advantage ranged from 1.4 to 4.7 months. Interestingly, the NO16966 study showed a progression-free survival benefit, but no significant differences in overall survival and response rates.37 Early cessation of bevacizumab in this trial has been postulated as a reason for the smaller observed benefit, and has led to recommendations that bevacizumab should be continued beyond the completion of first-line chemotherapy.39 Results from trials in which bevacizumab has been added to chemotherapy in the first line and second line setting are summarised in tables 1 and 2.
Chen et al studied the efficacy of bevacizumab and 5-FU (bolus or infusion) in heavily pre-treated patients who were refractory to irinotecan and oxaliplatin in a single arm Phase II trial.41 The progression-free survival was 3.5 months and response rate was just 1%. The Eastern Co-operative Oncology Group trial (E3200) was a Phase III trial comparing FOLFOX with or without bevacizumab in irinotecan refractory patients. A survival benefit was demonstrated in the bevacizumab treated patients.40
There are several ongoing studies which evaluate the role of bevacizumab to the standard adjuvant chemotherapy with FOLFOX for Duke’s C or high risk Duke’s B colorectal cancer. Since bevacizumab is a potent radio-sensitiser through its normalisation of tumour vasculature, it is being evaluated in combination with radiotherapy for rectal cancers. The results of these trials are eagerly awaited before bevacizumab can be recommended in this setting.
Targeting angiogenesis with small molecule TKIs has not met with such success as the antibody-based approach. Vatalinib is one such TKI of multiple targets including VEGFR, platelet derived growth factor and C-KIT. The advantage of this drug is oral administration in addition to multi-targeting activity. Vatalinib has been studied extensively in very large Phase III trials (Colorectal Oral Novel Therapy for the Inhibition of Angiogenesis and Retarding of Metastases – CONFIRM 1 and 2) in combination with chemotherapy in the first line and second line setting.42,43 There was no benefit in survival or response rates. A subsequent meta-analysis showed that the addition of vatalinib improved the progression-free survival in patients with elevated lactate dehydrogenase.44 The valuable lessons learnt from these studies indicate the need for better patient selection using validated predictive factors.
There are several ongoing studies with other anti-angiogenic drugs including sunitinib, sorafenib and cediranib in combination with chemotherapy drugs. The results of these trials are still awaited.
There is a well recognised toxicity profile related to anti-angiogenic drugs, including hypertension, proteinuria and bleeding. The trials of bevacizumab have identified proteinuria (28%), hypertension (25%), haemorrhage (2 to 9%), arterial thromboembolism (0 to 3.8%), wound healing complications (2%) and gastrointestinal perforation (1.5%).45 Several Phase IV studies of bevacizumab in combination with various chemotherapy agents in community practice have highlighted similar incidence of adverse events.46-48 Early identification and prompt therapy of these complications cannot be overemphasised.
Preliminary results from Phase II and III trials have failed to demonstrate a benefit in combining bevacizumab with EGFR-directed monoclonal antibodies.49-51 These studies raise concerns about increased toxicity and reduced treatment efficacy when combining these molecular targeted therapies. We have much to learn about the interaction of these drugs and our understanding of ‘multi-targeting’ remains rudimentary.
Colorectal cancer is a multi-step process characterised by a sequence of genetic alterations in cell growth regulator genes, such as K-RAS, p53 and DCC genes.52 EGFR is a logical potential biomarker, but as measured by immunohistochemistry, is not a useful predictive factor. There is no significant relationship between EGFR expression as determined by immunohistochemistry and the likelihood of response to cetuximab.23,25,53,54 K-RAS gene mutations occur early in the stages of carcinogenesis, as the colorectal adenoma progresses to develop into a carcinoma. The RAS/RAF/MAP kinase and the PTEN/PI3K/AKT signalling pathways are activated by ligand binding and activation of EGFR, and these pathways form a network that plays a central role in cancer progression and survival.55 Mutations in K-RAS can lead to constitutive activation of the pathway, and this may render inhibitors of components of the cascade upstream of EGFR ineffective. K-RAS mutations are found in 30-50% of colorectal cancers with most mutations found in exon 2 of the K-RAS gene.56,57-64
Previous studies have compared the efficacy of EGFR-directed monoclonal antibodies across wild-type K-RAS and mutant K-RAS tumours. Median survival was longer and responses were seen almost exclusively in the wild-type K-RAS subsets.58-60,65 In a randomised control trial, panitumumab benefit when compared to best supportive care was confined to patients with wild-type K-RAS tumours.56 For patients with K-RAS mutant tumours, there was no difference in progression-free survival between the panitumumab and best supportive care groups, but median progression-free survival for patients with wild-type K-RAS tumours was 12.3 weeks with panitumumab and 7.3 weeks with best supportive care. All of the objective responses occurred in patients with panitumumab treated wild-type K-RAS tumours.56
In the first line setting, K-RAS mutation status has a similar predictive significance. Results of the CRYSTAL study demonstrated that benefit through the addition of cetuximab to FOLFIRI chemotherapy is restricted to patients with wild-type K-RAS tumours. Patients with colorectal tumours that contain K-RAS mutations did not obtain a benefit with cetuximab.66 Similar results were observed in patients receiving the FOLFOX chemotherapy combination as first line therapy, and the progression free survival was trending lower in those patients with K-RAS mutations when treatment included cetuximab versus chemotherapy alone.
Other gene mutations, such as mutations involving the PTEN, BRAF or PI3KCA genes can also lead to unrestricted cancer cell growth and may also be useful predictive biomarkers. Loss of PTEN activity, for example, has been associated with lack of efficacy as measured by radiological response, with none of 11 patients responding to cetuximab in a recently reported series.60 Moving upstream in the signalling pathway, high EGFR ligand expression, particularly amphiregulin and epiregulin, has been observed in tumours responding to cetuximab and these ligands represent attractive targets for future biomarker research.67 So far, there are no reliable biomarkers to predict benefit from bevacizumab.
Over the last decade there has been a paradigm shift in the way we manage patients with advanced colorectal cancer. Multi-agent chemotherapy and multiple lines of therapy are now part of optimal treatment strategies. Anti-angiogenic therapy has contributed to improving outcomes, particularly prolonging disease control with an associated prolongation of overall survival. The benefit has been observed when bevacizumab is used as part of either first or second line therapy. EGFR directed therapy, particularly utilising cetuximab or panitumumab, also prolongs survival, but this appears to be restricted to patients with tumours that are have wild-type K-RAS. These new targeted therapies are relatively expensive. In the UK National Health Service review, bevacizumab was not considered to be cost effective in combination with chemotherapy,68 and in 2008 bevacizumab and the EGFR-directed antibodies are not funded on the Pharmaceutical Benefits Scheme in Australia. The cost effectiveness improves when these treatments can be delivered to patients with a higher chance of benefiting. Avoiding therapy in patients that have little chance of responding can help to eliminate toxicity of ineffective therapy and allow other treatment approaches to be pursued. Accurate and reliable biomarkers that allow selection of patients with advanced colorectal cancer, who will benefit from new therapies, would represent a significant advance in the clinical management of this disease. The K-RAS correlative analyses have identified a biomarker that can effectively exclude a significant proportion of patients, 40% with tumours that have K-RAS mutations, from EGFR monoclonal antibody therapy. Other prognostic and predictive variables, preferably ones that are reliably and easily measured, need to be identified.
Conflict of interest statement: Christos Karapetis has served on advisory boards for Merck Serono, Astra Zeneca, AMGEN and Roche.
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