Abstract
Mild hyperthermia generated using high intensity focused ultrasound (HIFU) and microbubbles (MBs) can improve tumor drug delivery from non-thermosensitive liposomes (NTSLs) and low temperature sensitive liposomes (LTSLs). However, MB and HIFU are limited by the half-life of the contrast agent and challenges in accurate control of large volume tumor hyperthermia for longer duration (>30min.). The objectives of this study were to: 1) synthesize and characterized long-circulating echogenic nanobubble encapsulated LTSLs (ELTSLs) and NTSLs (ENTSLs), 2) evaluate in vivo drug release following short duration (~20min each) HIFU treatments administered sequentially over an hour in a large volume of mouse xenograft colon tumor, and 3) determine the impact of the HIFU/nanobubble combination on intratumoral drug distribution. LTSLs and NTSLs containing doxorubicin (Dox) were co-loaded with a nanobubble contrast agent (perfluoropentane, PFP) using a one-step sonoporation method to create ELTSLs and ENTSLs, which then were characterized for size, release in a physiological buffer, and ability to encapsulate PFP. For the HIFU group, mild hyperthermia (40-42°C) was completed within 90min after liposome infusion administered sequentially in three regions of the tumor. Fluorescence microscopy and high performance liquid chromatography analysis were performed to determine the spatial distribution and concentration of Dox in the treated regions. PFP encapsulation within ELTSLs and ENTSLs did not impact size or caused premature drug release in physiological buffer. As time progressed, the delivery of Dox decreased in HIFU-treated tumors with ELTSLs, but this phenomenon was absent in the LTSL, NTSL, and ENTSL groups. Most importantly, PFP encapsulation improved Dox penetration in the tumor periphery and core and did not impact the distribution of Dox in non-tumor organs/tissues. Data from this study suggest that short duration and sequential HIFU treatment could have significant benefits and that its action can be potentiated by nanobubble agents to result in improved drug penetration.