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Imaging techniques targeting tumor angiogenesis have been investigated for past decade. Of these, the radiolabeled Arg-Gly-Asp (RGD) peptide have been focused because it has high affinity and selectivity for integrin alpha(v)beta3. Integrin alpha(v)beta3 is expressed on the plasma membrane in an active status in which it binds its ligands and transduce signals when exposed activating external stimuli of tumor angiogenesis such as vascular endothelial growth factor (VEGF). Many linear or cyclic RGD peptides were developed for positron emission tomography (PET). In this review, we focused on currently developed RGD peptides as PET probes for non-invasive detection of integrin alpha(v)beta3 expression. Because antiangiogenic agents may stop tumor progression rather than cause tumor shrinkage, reliable biomarker to detect early response to antiangiogenic therapy should be developed to use in the clinical trials. Antiangiogenic therapies are designed to normalize abnormal blood vessels found in tumors and hemodynamic parameters (e.g. blood flow and vascular permeability). Imaging techniques are needed to define therapeutic effect of antiangiogenic agents and monitor response to antiangiogenic agents for patient selection. In the past years, most major imaging techniques have been used for this purpose with dynamic contrast enhanced magnetic resonance imaging (DCE-MRI), dynamic contrast-enhanced computed tomography (CT), ultrasound, and [15O]-water positron emission tomography (PET). Since integrin alpha(v)beta3 overexpression is important factor determining invasiveness and metastatic potential of tumor, practical application of patient selection for use of integrin alpha(v)beta3 antagonist becomes the focus of public attention. If we can observe significant uptake of alpha(v)beta3 ligand before antagonist therapy, we are able to determine appropriate selection of patients and optimal dose of antagonists.

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