Evaluation of BPA-Bearing TAT-Lipopeptide-Modified Liposomes for Boron Neutron Capture Therapy from Cell-Associated Boron Delivery to In Vivo Tolerability

Background/Objectives: Boron neutron capture therapy requires the efficient delivery of sufficient amounts of 10B to tumor cells. In this study, we evaluated a liposomal boron-delivery system based on the B5-TAT-lipopeptide, which contains five boronophenylalanine (BPA) residues within its peptide portion and is anchored to the liposomal membrane through a hydrophobic lipid moiety. Methods: B5-TAT-lipopeptide 5% liposomes with a particle size of approximately 100 nm were prepared, and boron delivery to CT26 murine colon carcinoma cells was evaluated. Results: At an exposure concentration of 25 ppm boron, B5-TAT-lipopeptide 5% liposomes achieved a cell-associated boron level of approximately 4.5 µg B per 1 × 106 cells. Separately, sodium mercaptoundecahydro-closo-dodecaborate (BSH)-encapsulated B5-TAT-lipopeptide 5% liposomes achieved approximately 11 µg B per 1 × 106 cells when evaluated at an exposure concentration of 50 ppm. Following thermal neutron irradiation, B5-TAT-lipopeptide 5% liposome-treated cells showed marked loss of clonogenic survival, with surviving fractions of approximately 0.12 and 0.01 after 15 and 45 min of irradiation, respectively. In the preliminary in vivo tolerability study, rapid intravenous administration of B5-TAT-lipopeptide 5% liposomes without encapsulated BSH at 10 mg B/kg resulted in the death of all treated mice within 5–10 min, whereas no mortality occurred following the administration of phosphate-buffered saline (PBS), free BSH, or BSH-encapsulated PEGylated liposomes. Conclusions: These findings demonstrate substantial cell-associated boron delivery and neutron-induced clonogenic cell killing by B5-TAT-lipopeptide-modified liposomes while revealing a critical limitation in acute in vivo tolerability under the examined administration conditions.