CONTEXT: Contrast-enhanced ultrasound (CEU) using targeted microbubble agents has already been proved effective for imaging angiogenesis in tumors and the vasculature. Building on this technique, researchers in the cardiovascular division of the University of Virginia in Charlottesville have demonstrated that microbubbles targeted to endothelial integrins can provide early CEU evaluation of ischemia and response to proangiogenic growth factor therapies-before recovery of tissue perfusion.
RESULTS: To create a model that would mimic limb ischemia in humans, scientists induced iliac artery ligation in rats. Half the animals were treated with fibroblast growth factor-2 (FGF-2), promoting the growth of new blood vessels. The researchers also produced targeted microbubbles (MBE) conjugated with the disintegrin echistatin to bind with av integrins that are expressed from the surface of endothelial cells lining these new blood vessels. CEU perfusion ultrasound imaging was performed immediately after ligation and at regular intervals of four, seven, 14, and 28 days. Before each procedure, the rats received MBE and nontargeted control microbubbles (MBc) in random order 20 minutes apart.
IMAGE: Color-coded CEU image of angiogenesis in response to ischemia with and without fibroblast growth factor with av-targeted microbubbles in rat hind limb skeletal muscle. Other images acquired during the study showed that signals from MBE were more intense compared with MBc, and they were strongest days before blood flow recovery. Although blood flow and oxygen tensions in untreated ischemic tissue partially recovered by days 14 to 28, the signal from MBE peaked at days four to seven. Signal intensity from MBE was greatest in FGF-2-treated animals.
IMPLICATIONS: The protocol detects the molecular signal of angiogenesis long before the end result of perfusion changes can be seen, according to lead researcher Dr. Jonathan Lindner. Molecular events are recorded at the level of microcirculation. The early detection of angiogenesis through targeted CEU will improve cardiovascular treatment and help develop guided therapies tailored to an individual's angiogenic response, Lindner said.