Efficient Endolymphatic Sac-Directed Gene Delivery Using AAV8BP2 and Posterior Semicircular Canal Injection

Mutations in SLC26A4, which encodes the anion transporter pendrin, represent one of the most common genetic causes of hereditary hearing loss, including Pendred syndrome and DFNB4. Pendrin plays a critical role in maintaining ion homeostasis within the inner ear, particularly in the endolymphatic sac (ES), making it an important therapeutic target for gene replacement strategies. However, efficient delivery of therapeutic genes to relevant inner ear structures remains a major challenge for clinical translation. In this study, we evaluated the inner ear transduction profile of AAV8BP2, an engineered AAV8-derived capsid, in comparison with AAV2.7m8 and AAV8 in mice. Neonatal mice received posterior semicircular canal (PSCC) injections at postnatal day 0 (P0), and viral transduction in the cochlea and ES was assessed at P7. To examine age-dependent differences in viral transduction, additional experiments were performed in adult mice injected at P21 and analyzed at P28. We also compared three surgical delivery routes for inner ear gene transfer: PSCC injection, round window membrane (RWM) injection, and RWM injection combined with PSCC fenestration. AAV8BP2 effectively transduced the ES in both neonatal and adult mice and showed greater GFP expression in the spiral prominence than AAV8 following neonatal administration. Among the three delivery routes evaluated in adult mice, PSCC injection achieved the highest ES transduction while maintaining cochlear hair cell transduction comparable to that achieved with RWM-based approaches. Together, these findings define the relative transduction profiles of the tested AAV capsids and delivery routes and provide a basis for selecting vector-delivery route combinations for SLC26A4-targeted inner ear gene therapy.