Polyborane-encapsulated PEGylated Liposomes Prepared Using Post-insertion Technique for Boron Neutron Capture Therapy

PEGylated liposomes are one of the useful boron carriers for boron neutron capture therapy (BNCT). Recently, a method of adding PEG after liposome formation (post-insertion) was reported. In this study, we prepared polyborane-encapsulated PEGylated liposomes for BNCT with half the amount of DSPE-PEG of the conventional method using post-insertion technique (post-PEG liposomes), and their usefulness were evaluated in comparison with conventional PEGylated liposomes (pre-PEG liposomes). From the results of physicochemical property measurements, it was confirmed that particle size distributions, surface charge densities, and fixed aqueous layer thicknesses of these liposomes were equivalent. In vitro cytotoxicity and cell uptake tests were also carried out using B16 melanoma and RAW264.7 cells. Polyborane solution and bare liposomes were used for comparison. From the results of these tests, we confirmed that post-PEG liposomes and pre-PEG liposomes have the same influence of PEGylation. To evaluate biodistribution properties at 24 h post-administration, these liposomes and polyborane solution were injected into the tail veins of tumor-bearing mice. Boron concentration and tumor/blood ratios of PEGylated liposomes were 73.2-77.6 µg/g of tumor tissue and 5.5-5.8, respectively. From these results, it was found that by using post-insertion technique, liposomes for BNCT having same effect as the liposome prepared using the conventional method can be prepared with half amount of DSPE-PEG.