Development of Liposomal Formulations Composed of Thermoresponsive Polymers to Enhance Tumor Accumulation in Combining with Local Tumor Heating

Liposomes, nanosized vesicles consisting of a lipid bilayer membrane, have been widely investigated as versatile drug delivery systems. Clinically, several liposomal formulations have been used as anticancer drug carriers owing to their enhanced permeability and retention (EPR) effects. Nevertheless, the accumulation of conventional liposomes in tumors remains insufficient to achieve optimal therapeutic efficacy. Therefore, the development of innovative liposomes with improved tumor selectivity and delivery efficiency is required. In this study, we designed and evaluated novel liposomes prepared from thermoresponsive polyoxazoline (POZ)-lipid conjugates (DSPE-POZ), cholesterol, and hydrogenated soybean phosphatidylcholine (HSPC). The obtained liposomes had a particle size of 50-60 nm and a slightly negative ζ-potential. POZ has a lower critical solution temperature (LCST), which enables liposomes to alter their physicochemical properties in response to mild hyperthermia. The fixed aqueous layer thickness (FALT) of the DSPE-POZ liposomes dramatically decreased above the LCST, indicating increased hydrophobicity of the liposomal surface. A cellular uptake study using mouse colon cancer cells (Colon26) demonstrated that DSPE-POZ liposomes containing POZ with an LCST of 37-38 °C were significantly highly taken up by tumor cells at 43 °C (above LCST) compared to 37 °C. Furthermore, biodistribution studies using Colon26 tumor-bearing mice revealed that DSPE-POZ liposomes were cleared from the blood and preferentially accumulated in heated tumors, achieving a 7-fold higher uptake than in nonheated tumors. These results indicate that liposomes composed of POZ-lipid conjugates represent a promising drug delivery platform capable of improving tumor accumulation when combined with local tumor heating.