Encapsulated ultrasound microbubbles: Therapeutic application in drug/gene delivery

Encapsulated gas microbubbles are well known as ultrasound contrast agents for medical ultrasound imaging. Nonetheless, not only do these microbubbles help to image, but they can also be used as drug/gene carriers. The microbubbles as drug/gene carriers have an average size less than that of red blood cells, i.e. they are capable of penetrating even into the small blood capillaries and releasing drug and genes under the action of ultrasound field. The application of ultrasound and microbubbles to targeted drug and gene delivery has been the subject of intense experimental research. Under exposure of sufficiently high-amplitude ultrasound, these targeted microbubbles would rupture, spewing drugs or genes, which are contained in its encapsulating layer, to targeted cells or tissues. Recently, targeting ligands are attached to the surface of the microbubbles (i.e. targeted-microbubbles), which have been widely used in cardiovascular system and tumor diagnosis and therapy. In this paper, the characterization of novel targeted ultrasonic contrast agents or microbubbles and their potential applications in drug delivery or gene therapy are reviewed.