Molecular imaging of tumor angiogenesis.
The emergence of angiogenesis as an important target for cancer therapy has led to increased research aimed at understanding the mechanisms underlying the development, maintenance, and destruction of tumor vasculature. Concurrently, molecular imaging technologies have been developed and are being incorporated as integral components of biomedical research due to their ability to noninvasively monitor in vivo molecular events. With the evaluation of numerous anti-angiogenic agents in clinical trials, the adaptation and validation of molecular imaging modalities for monitoring angiogenesis is actively being pursued. The importance of selecting appropriate molecular targets in the study of angiogenesis has become increasingly complex due to the pleiotropy of vascular phenotypes. Furthermore, due to both the relatively low abundance of endothelial cells in tumor tissue and the inherent difficulties of detecting molecular events, molecular imaging of vasculature necessitates continued improvements in achieving higher sensitivity. While several studies have been published that set the groundwork for imaging angiogenesis, much has yet to be accomplished. Various tumor models and transgenic mice provide an excellent resource for developing molecular imaging technologies for the understanding of angiogenesis. This research may play a particularly crucial role in evaluating mechanism and efficacy during pre-clinical testing of anti-angiogenic drugs. Due to practical limitations, however, the implementation of angiogenesis-directed molecular imaging may not extend beyond highly specialized clinical trials. That is, imaging modalities that evaluate angiogenesis at a functional level may prove more appropriate. Despite future technical challenges, molecular imaging will become an important research and clinical tool in evaluating tumor angiogenesis.