Targeted Therapies in the Treatment of Colorectal Cancers

Background:

In solid organ malignancies, no tumor type has seen a greater impact from the development of novel targeted therapies in 2004 than metastatic colorectal cancer.

Methods:

We review the current progress to date with the use of monoclonal antibodies in colorectal cancer and look at newer therapies under investigation.

Results:

Two monoclonal antibodies received Food and Drug Administration approval in early 2004, both for the indication of advanced, metastatic colorectal cancer. A large, randomized, placebo-controlled study demonstrated that the addition of a monoclonal antibody to vascular endothelial growth factor, bevacizumab, led to a statistically significant improvement in overall survival, with tolerable additional toxicity. Chimeric monoclonal antibody therapy directed at the epidermal growth factor receptor was associated with radiographic responses in a significant minority of patients with irinotecan-refractory colon cancer in a randomized phase II study of patients with irinotecan-refractory disease.

Conclusions:

These dramatic successes have led to further clinical studies of targeted therapy in colorectal cancer, making it one of the most promising areas of cancer research.

The role of VEGF and EGFR inhibition: implications for combining anti-VEGF and anti-EGFR agents

Multiple cellular pathways influence the growth and metastatic potential of tumors. This creates heterogeneity, redundancy, and the potential for tumors to bypass signaling pathway blockade, resulting in primary or acquired resistance. Combining therapies that inhibit different signaling pathways has the potential to be more effective than inhibition of a single pathway and to overcome tumor resistance. Vascular endothelial growth factor (VEGF) and epidermal growth factor receptor (EGFR) inhibitors have become key therapies in several tumor types. Close relationships between these factors exist: VEGF signaling is up-regulated by EGFR expression and, conversely, VEGF up-regulation independent of EGFR signaling seems to contribute to resistance to EGFR inhibition. Therefore, inhibition of both pathways could improve antitumor efficacy and overcome resistance to EGFR inhibition. Preclinical studies have shown that VEGF and EGFR inhibitors can have additive effects and that combined inhibition is effective in EGFR inhibitor-resistant cell lines. Clinical trials have also produced promising data: combining the anti-VEGF monoclonal antibody bevacizumab with the anti-EGFR antibody cetuximab or the EGFR tyrosine kinase inhibitor erlotinib increases benefit compared with either of these anti-EGFR agents alone or combined with chemotherapy. The potential of this novel approach to anticancer therapy will be elucidated by large, ongoing clinical trials.

Challenges for patient selection with VEGF inhibitors

As targeted therapies for cancer become increasingly integrated into standard practice, appropriate selection of the patients most likely to benefit from these therapies is now receiving critical scrutiny. Early experience with therapies directed at targets that are definitively overactive (e.g. the bcr-abl tyrosine kinase targeted by imatinib) or over-expressed [e.g. the human epidermal growth factor receptor 2 (HER2) targeted by trastuzumab] has generated the perception that pre-treatment target assessment is a pre-requisite for therapy with all targeted agents. However, emerging evidence suggests that this is not presently feasible for anti-angiogenic agents. Despite considerable evidence for the association of intratumoral and/or plasma vascular endothelial growth factor (VEGF) levels with tumor progression and/or poor prognosis, pre-treatment VEGF levels do not appear to be predictive of response to anti-angiogenic therapy. This may possibly be due to the complexity of the angiogenic pathways and the limitations associated with current methods of VEGF detection and quantification; e.g. low assay sensitivity and lack of standardized methods could prevent detection of very small increases in VEGF, which may be clinically important in patients with tumors that are highly dependent on this growth factor. In addition to a general lack of agreement as to the relative clinical relevance of circulating versus tumor VEGF levels, the absence of a 'gold standard' VEGF detection assay and the lack of a predefined, clinically relevant cut-off pose a significant hindrance to the clinical utility of VEGF measurements for therapy selection. Given the fundamental importance of angiogenesis for tumor growth and progression, and the key role of VEGF in these processes, presently it seems appropriate to view anti-VEGF agents such as bevacizumab (Avastin) as having potential utility, independently of pre-treatment screening. Further research is needed to define the relationship between potential surrogate markers of VEGF pathway activity and clinical outcomes.

Systemic Therapy for Metastatic Colorectal Cancer: Current Options, Current Evidence

Combination chemotherapy regimens including irinotecan and oxaliplatin markedly improve response rate and prolong median survival over fluorouracil with leucovorin (FU/LV), and have supplanted FU/LV as the standard systemic approach for metastatic colorectal cancer. The recent availability of five active chemotherapeutic agents has doubled the median overall survival for metastatic colorectal cancer from 10 to 20 months, and though the optimal strategy for incorporation of all drugs is still unclear, current data support the use of chemotherapy doublets in first-line rather than sequential single-agent therapy. Multidrug regimens increase both response rate and the proportion of patients able to undergo potentially curative resection. In addition, as many as 20% to 30% of patients never receive second-line chemotherapy. When used as single agents, bolus and infusional FU/LV and capecitabine are similarly effective but have differing toxicity. Chemotherapy combinations that incorporate infusion of FU are less toxic and more effective than those using bolus FU dosing. Capecitabine is under study as an alternative dosing method for use in combination regimens; however, the optimal dose has not been defined and final safety and efficacy outcomes are being addressed in ongoing phase II and III investigations. Three combinations have shown excellent first-line efficacy in phase III trials—IFL with bevacizumab, FOLFOX, and FOLFIRI—but neither of these combinations is clearly superior. Sound clinical judgment must continue to guide treatment decisions while we await data regarding the optimal combination and sequence of fluorouracil, irinotecan, oxaliplatin, bevacizumab, and cetuximab.

Phase I Study of the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of PTK787/ZK 222584 Administered Twice Daily in Patients With Advanced Cancer

Purpose

PTK787/ZK 222584 (PTK/ZK) is an oral angiogenesis inhibitor targeting all known vascular endothelial growth factor (VEGF) receptor tyrosine kinases, including VEGFR-1/Flt-1, VEGFR-2/KDR, VEGFR-3/Flt-4, the platelet-derived growth factor receptor tyrosine kinase, and the c-kit protein tyrosine kinase. In this phase I dose-escalating study, PTK/ZK was administered bid to exploit the theoretical advantage of maintaining constant drug levels above a threshold known from preclinical data to interfere with VEGF receptor signaling.

Patients and Methods

Forty-three patients with advanced cancers received single-agent PTK/ZK at doses of 150 to 1,000 mg orally bid. Assessments for safety and pharmacokinetics were performed. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was used as a pharmacodynamic marker of response.

Results

At 1,000 mg bid, the dose-limiting toxicity of reversible grade 3 lightheadedness was observed. Dose-related grade 3 fatigue and vomiting were observed but these were not dose-limiting. Pharmacokinetic data confirmed that PTK/ZK exposure increased with increasing dose up to 500 mg bid and appeared to plateau at higher doses. A greater than 40% reduction in the DCE-MRI bidirectional transfer constant (Ki) at day 2 predicted for nonprogression of disease.

Conclusion

The maximum-tolerated oral dose of PTK/ZK is 750 mg orally bid. DCE-MRI and pharmacokinetic data indicate that PTK/ZK ≥ 1,000 mg total daily dose is the biologically active dose.

Combination of antiangiogenic therapy with other anticancer therapies: results, challenges, and open questions

Angiogenesis is necessary for tumor growth. Drug discovery efforts have identified several potential therapeutic targets on endothelial cells and selective inhibitors capable of slowing tumor growth or producing tumor regression by blocking angiogenesis in in vivo tumor models. Certain antiangiogenic therapeutics increase the activity of cytotoxic anticancer treatments in preclinical models. More than 75 antiangiogenic compounds have entered clinical trials. Most of the early clinical testing was conducted in patients with advanced disease resistant to standard therapies. Several phase III trials have been undertaken to compare the efficacy of standard chemotherapy versus the same in combination with an experimental angiogenesis inhibitor. Preliminary results of the clinical studies suggest that single-agent antiangiogenic therapy is poorly active in advanced tumors. Although some of the results of combination trials are controversial, recent positive outcomes with an antivascular endothelial growth factor antibody combined with chemotherapy as front-line therapy of metastatic colorectal cancer have renewed enthusiasm for this therapeutic strategy. This article presents an overview of experimental and clinical studies of combined therapy with antiangiogenic agents and highlights the challenges related to the appropriate strategies for selection of the patients, study design, and choice of proper end points for preclinical and clinical studies using these agents.