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 cytotoxicies 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 compounds synthesized. The final boron concentration in 13-encapsulating liposomes reached 11.0 x 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 body weight with the tumor/blood (T/B) ratio of approximately 14.