Ultrasound-Responsive Liposomes

Ultrasound-responsive liposomes are drug-loaded liposomes that contain a small amount of gas (often air). Co-encapsulation of a pharmaceutic along with this gas renders the liposomes acoustically active, allowing for ultrasound imaging as well as controlled release of the contents through ultrasound stimulation. Methods for the facile production of gas-containing liposomes with simultaneous drug encapsulation are available. Conventional procedures are used to prepare liposomes composed of phospholipid and cholesterol, namely, hydration of the lipid film followed by sonication. After sonication, the gas is introduced by one of two methods. The first method involves freezing and lyophilizing the sonicated liposomes in the presence of mannitol, the relevant property of which appears to be that it accentuates freezing damage to the lipid membranes. The other technique employs freezing of liposomes under elevated pressure of the desired gas. The concept of ultrasound-mediated drug delivery has many potential applications to specific clinical conditions such as cancer, thrombus, arterial restenosis, myocardial infarction, and angiogenesis because of its ability to localize the delivery of therapeutic agents that would cause side effects if given in large amounts systemically.