Ultrasound, microbubbles and the blood–brain barrier

The blood–brain barrier (BBB) is a specialized system of capillary endothelial cells that protects the brain from harmful substances in the blood stream, while supplying the brain with the required nutrients for proper function. The BBB controls transport through both tight junctions and metabolic barriers and is often a rate-limiting factor in determining permeation of therapeutic drugs into the brain. It is a significant obstacle for delivery of both small molecules and macromolecular agents. Although many drugs could be potentially used to treat brain disease, there has been no method that allows non-invasive-targeted delivery through the BBB. Recently, promising studies indicate that ultrasound can be used to locally deliver a drug or gene to a specific region of interest in the brain. If microbubbles are combined with ultrasound exposure, the effects of ultrasound can be focused upon the vasculature to reduce the acoustic intensity needed to produce BBB opening. Several avenues of transcapillary passage after ultrasound sonication have been identified including transcytosis, passage through endothelial cell cytoplasmic openings, opening of tight junctions and free passage through injured endothelium. This article reviews the topic of transient disruption of the BBB with ultrasound and microbubbles and addresses related safety issues. It also discusses possible roles of the BBB in brain disease and potential interactions with ultrasound and microbubbles in such disease states.