Model Studies Directed Toward the Boron Neutron-Capture Therapy of Cancer: Boron Delivery to Murine Tumors with Liposomes

The successful treatment of cancer by boron neutron-capture therapy (BNCT) requires the selective concentration of boron-10 within malignant tumors. The potential of liposomes to deliver boron-rich compounds to tumors has been assessed by the examination of the biodistribution of boron delivered by liposomes in tumor-bearing mice. Small unilamellar vesicles with mean diameters of 70 nm or less, composed of a pure synthetic phospholipid (distearoyl phosphatidylcholine) and cholesterol, have been found to stably encapsulate high concentrations of water-soluble ionic boron compounds. The hydrolytically stable borane anions B10H2- 10, B12H11SH2-, B20H17OH4-, B20H3- 19, and the normal form and photoisomer of B20H2- 18were encapsulated in liposomes as their soluble sodium salts. The tissue concentration of boron in tumor-bearing mice was measured at several time points over 48 h after i.v. injection of emulsions of liposomes containing the borane anions. Although the boron compounds used do not exhibit an affinity for tumors and are normally rapidly cleared from the body, liposomes were observed to selectively deliver the borane anions to tumors. The highest tumor concentrations achieved reached the therapeutic range (>15 μg of boron per g of tumor) while maintaining high tumor-boron/blood-boron ratios (>3). The most favorable results were obtained with the two isomers of B20H2- 18. These boron compounds have the capability to react with intracellular components after they have been deposited within tumor cells by the liposome, thereby preventing the borane ion from being released into blood.