Acoustically active liposomes for drug encapsulation and ultrasound-triggered release

Acoustically active liposomes (AAL), previously developed as ultrasound contrast agents, contain small amounts of air. These AAL have potential to carry pharmaceutics and their acoustic activity could enable them to respond to ultrasound stimulation by releasing their contents. Since liposomes can entrap many kinds of drugs, if such entrapment did not affect their echogenicity, then the release of contents could potentially be controlled by ultrasound stimulation. The aim of this research was to investigate the capacity of acoustically active liposomes for hydrophilic molecule encapsulation and to determine their sensitivity to ultrasound-triggered release. Liposomes, composed of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, and cholesterol, were made acoustically active by hydrating a lipid film, sonication, freezing in the presence of mannitol, lyophilization, and rehydration. As a test molecule, calcein was added in the hydration step. The procedure for generating acoustically active liposomes was compatible with an encapsulation efficiency of 15% or more. The presence of mannitol during freeze-drying was essential not only for generation of acoustic activity but also for efficient encapsulation. Ultrasound-triggered release was achieved by applying 1 MHz ultrasound at 2 W/cm2 for 10 s. The inclusion of 4% diheptanolyphosphatidylcholine (DHPC) increased the sensitivity of liposomes to ultrasound stimulation and resulted in very efficient stimulated release of contents (1/3 released in 10 s, 2/3 released in six such applications). Release of contents was highly correlated with the loss of air induced either by ultrasound or rapid pressure reduction. These encapsulation and triggered release techniques are highly efficient, and hence may be applicable to drug delivery.