Boron neutron capture therapy demonstrated in mice bearing EMT6 tumors following selective delivery of boron by rationally designed liposomes

The application of boron neutron capture therapy (BNCT) following liposomal delivery of a ¹⁰B-enriched polyhedral borane and a carborane against mouse mammary adenocarcinoma solid tumors was investigated. Unilamellar liposomes with a mean diameter of 134 nm or less, composed of an equimolar mixture of cholesterol and 1,2-distearoyl-sn-glycero-3-phosphocholine and incorporating Na₃[1-(2′-B₁₀H₉)-2-NH ₃B₁₀H₈] in the aqueous interior and K[nido-7-CH₃(CH₂)₁₅-7,8-C₂B ₉H₁₁] in the bilayer, were injected into the tail veins of female BALB/c mice bearing right flank EMT6 tumors. Biodistribution studies indicated that two identical injections given 24 h apart resulted in tumor boron levels exceeding 67 μg/g tumorat54h-withtumor/bloodboron ratios beinggreat-est at 96 h (5.68:1; 43 μg boron/g tumor)-following the initial injection. For BNCTexperiments,tumor-bearing micewereirradiated 54 h after the initial injection for 30 min with thermal neutrons, resulting in a total fluence of 1.6 × 10¹² neutrons per cm² (±7%). Significant suppression of tumor growth was observed in mice given BNCT vs. control mice (only 424% increase in tumor volume at 14 d post irradiation vs. 1551% in untreated controls). In a separate experiment in which mice were given a second injection/ irradiation treatment 7 d after the first, the tumor growth was vastly diminished (186% tumor volume increase at 14 d). A similar response was obtained for mice irradiated for 60 min (169% increase at 14 d), suggesting that neutron fluence was the limiting factor controlling BNCT efficacy in this study.