Circulating and imaging markers for angiogenesis.

Abundant preclinical and indirect clinical data have for several decades convincingly supported the notion that anti-angiogenesis is an effective strategy for the inhibition of tumor growth. The recent success achieved in patients with metastatic colon carcinoma using a neutralizing antibody directed against vascular endothelial growth factor (VEGF) has translated preclinical optimism into a clinical reality.With this transformation in the field of angiogenesis has come a need for reliable surrogate markers. A surrogate marker by definition serves as a substitute for the underlying process in question, and in the case of angiogenesis, microvessel density (usually in so-called "hot-spots") has until now been the most widely used parameter. However, this parameter is more akin to a static "snap-shot" and does not lend itself either to the dynamic in situ assessment of the status of the tumor microvasculature or to the molecular factors that regulate its growth and involution. This has led to an acute need for developing circulating and imaging markers of angiogenesis that can be monitored in vivo at repeated intervals in large number of patients with a variety of tumors in a non-invasive manner. Such markers of angiogenesis are the subject of this review.