A multimodal approach to assess a liposomal carrier's biodistribution, stability, and clearance.

Liposomal carriers, used for site-specific drug delivery, are being investigated for diagnostic approaches by replacing the therapeutic with an imaging contrast agent, exploring potential for selective treatment planning. There remains a critical need to improve assessment of biodistribution, stability, and clearance kinetics of liposomal carriers. This pilot study presents a multimodal approach in which liposome-encapsulated J-aggregated indocyanine green (ICG) dye (Lipo-JICG) is imaged with high spatial resolution using both photoacoustic (PA) imaging, to assess the absorbance characteristics of JICG and monomeric ICG, and cryofluorescence tomography (CFT), to measure ICG fluorescence. An assay comparing the relationship between absorbance and fluorescence of Lipo-JICG and ICG demonstrated that the absorbance peak shifted from 780 to 895 nm as the Lipo-JICG:ICG ratio increased; meanwhile, the fluorescence decreased drastically as the Lipo-JICG:ICG ratio increased, demonstrating that J-aggregation quenches fluorescence. Twelve mice were then PA imaged pre-injection, then up to 6 days after Lipo-JICG injection. Unmixed Lipo-JICG signal peaked at 30 min post-injection in both liver and spleen; unmixed ICG signal peaked post-injection, decreasing over time in both organs and increasing at 6 days in the spleen. With CFT, ICG fluorescence followed a similar trend, with a maximum at 30 min in liver and at 6 days in spleen, implying that Lipo-JICG continued to break down and excrete through the hepatic system over 6 days post-injection. Future studies will continue to develop this methodology to assess biodistribution, stability, and clearance of liposomal carriers in tumor-bearing murine models.