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that only stimulates endothelial function. At present, delivery strategies that can accomplish prolonged and targeted exposure include either sustained-release delivery of growth factor proteins or systemic administration of organor tissue-specific agents.
5.3. Monitoring of neovascularization
Monitoring the effect of angiogenic therapy has been a long-standing challenge. In principle, this can be accomplished by either directly monitoring blood vessel growth or evaluating the functional effects of the therapy. In any case, it needs to be sensitive enough to detect small changes of vascularization that can account for the symptomatic improvement of the patient. The development of monitoring modality capable of non-invasively assessing neovascularization is essential for the success of therapeutic angiogenesis.
Recently molecular imaging of angiogenesis has received considerable attention, with a number of reports demonstrating the feasibility of detecting blood vessel growth in tissues by targeting “angiogenic” endothelial cell-specific antigens.13 Among them, αvβ3 integrin, a member of the family of adhesion molecules, has been studied as a marker of tumor angiogenesis. This integrin is known to bind to matrix proteins with an exposed RGD (arginine-glycine-aspartate) sequence. Recently, a clinical study has shown that Galacto-RGD with positronemission tomography (PET) enables non-invasive quantitative assessment of the αvβ3 expression pattern on tumor and endothelial cells in cancer patients.14
Direct visualization of new vasculature is a convincing way to detect collateral vessel growth, although it does not necessarily address the functional consequences of the new vessel growth. Large collaterals (>130 µm diameter) can be observed and perhaps quantified with standard angiographic techniques; however, a number of artifacts influence the angiographic appearance of vessels, including vascular tone, amount of the injected contrast, force of injection, and medication. A particularly interesting technology is the 3D reconstruction of microCT angiographic images, which enables depiction of patterns of arterial growth and quantification of blood vessel density and volume.15 However, micro-CT is unsuitable for repeated non-invasive measurements of vessels, as it requires long scan periods and high X-ray doses. Volumetric
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computed tomography (VCT) which combines advantages of microCT and clinical CT scanners is tested in experimental and preclinical applications.16 Finally, magnetic resonance angiography can also provide visualization of collaterals.17 Although relatively effective in the limb, the sensitivity of MRI coronary reconstruction is not yet sufficient.
The alternative to direct visualization of the vasculature is the assessment of the physiological consequences of vessel growth, such as improvement in tissue perfusion, oxygenation, or function. In coronary artery disease trials, nuclear perfusion scan imaging has been used as a surrogate end-point. Remarkably little effect was observed with this imaging modality even when clinical symptoms of patients appeared to be improved. This raised questions about the spatial resolution and sensitivity of single-photon emission computed tomography (SPECT) imaging. Positron-emission tomography (PET) and MRI are the main alternatives to SPECT imaging. PET boasts somewhat higher spatial resolution, elimination of attenuation, and quantitative assessment of perfusion, providing comprehensive insights into vascular development. A variety of PET imaging probes for imaging neovascular development have been synthesized, including perfusion tracers such as 15O-labeled water, monoclonal anti-VEGF antibody, and ligands for αvβ3 integrin.13 MRI can determine perfusion and blood volume changes in response to treatment; however, no agreement has been reached with regard to the methodology of the measurement.18 One approach relies on assessing relative differences in perfusion between normal and ischemic zones, thereby providing an assessment of the ischemic zone size. Alternative approaches attempt to measure perfusion by the first-pass technique. The advantages of MRI are its high spatial resolution and sensitivity to small changes in flow. Similar to PET, however, experience with the use of MRI perfusion in large clinical trials is limited.
5.4. Study design
Appropriate study design is essential for translating findings of basic research into clinical setting. An important part of therapeutic angiogenesis trials is the selection of an appropriate patient population.