Monitoring Liposome Payload Release by Photoacoustic Spectroscopy of Indocyanine Green

Remote monitoring of liposome payload release is of relevance to drug delivery and molecular imaging. However, conventional methods of monitoring release (e.g. fluorescence dequenching, luciferase) are difficult to apply at depths> 5 mm in vivo. In this study, we evaluated the photoacoustic (PA) imaging response of a novel formulation of indocyanine-green (ICG) loaded liposomes. Specifically, we prepared two liposomes formulations of different mean diameter (180 nm vs 900 nm) and characterized the PA response before and after liposome destruction to determine if dye release resulted in a change in the PA response of the dye. Results indicated that dye loading efficiency was 90% in the 180 nm liposome formulation and 25% in the 900 nm formulation. When intact, both liposomes exhibited a narrow optical absorption peak centered near 780 nm that broadened and shifted to 750 nm following liposome rupture. This liposome preparation may be used as blood pool contrast agents, molecular imaging agents, or markers of drug delivery processes in PA or optical imaging applications.