Single-Dose Toxicity Study of Self-Assembling A6K/Sodium Borocaptate (BSH) Peptide Nanotubes as a New Boron Delivery Agent for Boron Neutron Capture Therapy (BNCT) in Mice

Background: Boron neutron capture therapy (BNCT) is a type of targeted radiotherapy with considerable therapeutic potential that may be combined with immune checkpoint inhibitors (ICIs) to enhance systemic antitumor immunity. Its efficacy relies on the efficient and safe delivery of boron to cancer cells. This study evaluated the acute toxicity of a self-assembling peptide-based boron carrier composed of A6K and sodium borocaptate (BSH) at a 1:10 molar ratio (A6K/BSH boron drug), which had previously shown excellent tumor-selective accumulation and prolonged intracellular retention. Methods: A single-dose intraperitoneal toxicity study was performed in 6-week-old BALB/c mice (n = 6 per group; 3 males and 3 females). Animals received BSH-equivalent doses of 0, 30, 100, 300, or 600 mg/kg and were observed for 14 days. Mortality, clinical signs, body weight, gross necropsy findings, and histopathological characteristics of major organs were then assessed. Results: No mortality or treatment-related clinical signs were observed. Body weight changes were comparable between the control and treated groups. Gross necropsy revealed no treatment-related abnormalities. Histopathology showed mild hepatocellular hypertrophy and granular degeneration without dose dependency. No other organ toxicities or sex-related differences were detected. Conclusions: A single intraperitoneal administration of the A6K/BSH boron drug up to 600 mg/kg (BSH-equivalent) produced no evident acute systemic toxicity, suggesting that the approximate lethal dose exceeds 600 mg/kg for both sexes. This initial safety assessment supports the further development of the A6K/BSH boron drug as a boron delivery agent for BNCT. Further studies are needed to confirm its safety under clinically relevant conditions.