New targeted probes for radioimaging of angiogenesis

Angiogenesis, the formation of new blood vessels, is a multi-step process regulated by pro- and anti-angiogenic factors. In order to grow and metastasize, tumors need a constant supply of oxygen and nutrients. For their growth beyond the size of 1–2 mm tumors are dependent on angiogenesis. Recently, various new anti-cancer agents (e.g. bevacizumab, sorafenib and sunitinib) have become available that specifically inhibit angiogenesis in tumors. To evaluate the effects of these new anti-angiogenic agents it would be of interest to scintigraphically image the process of angiogenesis in tumors. Several markers have been described that are preferentially expressed on newly formed blood vessels in tumors (αvβ3 integrin, vascular endothelial growth factor and its receptor, prostate-specific membrane antigen) and in the extracellular matrix surrounding newly formed blood vessels (extra-domain B of fibronectin, Tenascin-C, matrix metalloproteinases, Robo-4). Several ligands targeting these markers have been tested as a radiotracer for imaging angiogenesis in tumors. The potential of some of these tracers such as radiolabeled cyclic RGD peptides and radiolabeled anti-PSMA antibodies has already been tested cancer patients, while for markers such as Robo-4 the ligand has not yet been identified. Here the preclinical and clinical studies with these new tracers to image angiogenesis in tumors are reviewed.