Boron-containing folate receptor-targeted liposomes as potential delivery agents for neutron capture therapy

Boron neutron capture therapy (BNCT) depends on the selective delivery of a sufficient number of B-10 atoms to tumor cells to sustain a lethal B-10(n,alpha)Li-7 reaction. Expression of FR frequently is amplified among human tumors. The goal of the present study was to investigate folate receptor (FR)-targeted liposomes as potential carriers for a series of boron-containing agents. Two highly ionized boron compounds, Na-2[B12H11SH] and Na-3 (B20H17NH3), were incorporated into liposomes by passive loading with encapsulation efficiencies of 6% and 15%, respectively. In addition, five weakly basic boronated polyamines were investigated. Two were the spermidine derivatives: N-5-(4-carboranylbutyl)spermidine-3HCl (SPD-5), N-5-[4-(2-aminoethyl-o-carboranyl)butyl]spermidine.4HCl (ASPD-5). Three were the spermine derivatives: N-5-(4-carboranylbutyl)spermine.4HCl (SPM-5), N-5-[4-(2-aminoethyl-o-carboranyl)butyl]spermine.5HCl (ASPM-5), and (NN10)-N-5-bis(4-carboranylbutyl)spermine.4 HCl (SPM-5,10). These were incorporated into liposomes by a pH-gradient-driven remote-loading method with varying loading efficiencies, which were influenced by the specific trapping agent and the structure of the boron compound. Greater loading efficiencies were obtained with lower molecular weight boron derivatives, using ammonium sulfate as the trapping agent, compared to those obtained with sodium citrate. The in vitro uptake of folate-derivatized, boronated liposomes was investigated using human KB squamous epithelial cancer cells, which have amplified FR expression. Higher cellular boron uptake (up to 1584 mug per 10(9) cells) was observed with FR-targeted liposomes than with nontargeted control liposomes (up to 154 mug per 10(9) cells), irrespective of the chemical form of the boron and the method used for liposomal preparation. KB cell binding of the FR-targeted liposomes was saturable and could be blocked by 1 mM free folic acid. Our findings suggest that further evaluation of FR-targeted liposomes is warranted to assess their potential as boron carriers for neutron capture therapy.