Usefulness of liposome-targeting transferrin as a boron delivery agent in boron neutron capture therapy for C6 glioma

Boron neutron capture therapy (BNCT) is a tumor-targeting particle radiation therapy. Successful BNCT requires both a sufficient amount of 10B atoms in tumor tissue and their selective accumulation. To achieve this, we demonstrate the usefulness of a new drug-delivery system composed of sodium mercaptoundecahydrododecaborate (BSH)-encapsulated transferrin (TF) that targets polyethylene glycol (PEG)-coated liposomes (TF-PEG-liposome (BSH)). TF-PEG-liposome (BSH) delivered sufficient amounts of 10B atoms in vitro to C6 rodent glioma cells in comparison with bare BSH or transferrin-non-decorated PEG-liposome (BSH). In addition, TF-PEG-liposome (BSH) showed a marked retention of 10B in tumor cells in vitro. The cell-selective accumulation of 10B was inhibited by the addition of free TF. This result demonstrates that the cell-selective accumulation of 10B by this drug delivery system can be ascribed to receptor-mediated endocytosis. In conclusion, the results of the present study suggest that TF-PEG-liposome (BSH) can achieve a good contrast of 10B in tumor tissue in comparison with that in blood and normal tissue in vivo. TF-PEG-liposome (BSH) is expected to be useful as an intracellular targeting carrier in clinical BNCT for malignant gliomas. (author)