Folate receptor-mediated liposomal delivery of a lipophilic boron agent to tumor cells in vitro for neutron capture therapy

Purpose. This study was aimed at the in vitro evaluations of folate receptor (FR)-targeted liposomes as carriers for a lipophilic boron agent, K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)], in FR-overexpressing tumor cells for neutron capture therapy. Methods. Large unilamellar vesicles (similar to200 nm in diameter) were prepared with the composition of egg PC/chol/K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)] (2:2:1, mol/mol), with an additional 0.5 mol % of folate-PEG-DSPE or PEG-DSPE added for the FR-targeted or non-targeted liposomal formulations, respectively. Results. Boron-containing, FR-targeted liposomes readily bound to KB cells, an FR-overexpressing cell line, and were internalized via FR-mediated endocytosis. The boron uptake in cells treated with these liposomes was approximately 10 times greater compared with those treated with control liposomes. In contrast, FR-targeted and nontargeted liposomes showed no difference in boron delivery efficiency in F98 cells, which do not express the FR. The subcellular distribution of the boron compound in KB cells treated with the FR-targeted liposomes was investigated by cellular fractionation experiments, which showed that most of the boron compound was found in either the cytosol/endosomal or cell membrane fractions, indicating efficient internalization of the liposomal boron. Conclusion. FR-targeted liposomes incorporating the lipophilic boron agent, K[nido-7-CH(3)(CH(2))(15)-7,8-C(2)B(9)H(11)], into its bilayer were capable of specific receptor binding and receptor-mediated endocytosis in cultured KB cells. Such liposomes warrant further investigations for use in neutron capture therapy.