Synthesis of oligo-closo-dodecaborates by Hüisgen click reaction as encapsulated agents for the preparation of high-boron-content liposomes for neutron capture therapy
High-boron-content compounds, 8, 10, and 13, were designed and synthesized as boron agents encapsulated in liposomes by the Hüisgen click cycloaddition of closo-dodecaborate-containing azides and alkynes. These compounds have relatively low cytotoxicities and their GI50 values for B16, CT26, and HeLa cells are higher than 0.14 mM, indicating toxicities similar to that of BSH. High-boron-content liposomes were prepared using 13, which has the largest number of boron atoms in a molecule among the synthesized compounds. The final boron concentration in 13-encapsulating liposomes reached 11.0 × 103 ppm with a B/P ratio of 5.1. A significantly high tumor boron accumulation (72.2 ppm) was observed in tumor-bearing mice 36 h after the injection of 13-encapsulating liposomes at a dose of 15 mgB kg−1 body weight with the tumor/blood (T/B) ratio of approximately 14.