The kinetics and toxicity of leupeptin in the inner ear of the chinchilla

Abstract Problem: One strategy to prevent injury to the inner ear after loud noise exposure or other toxic insults is to arrest the neurochemical pathway leading to inner hair cell apoptosis. Leupeptin is a novel antagonist of calpain, a described component of the damage pathway. We desired to evaluate the safety and kinetics of leupeptin in an animal model. Methods: Using a novel surgical approach, we placed middle ear microwicks in 23 chinchillas. The wicks were saturated with a standard amount and concentration of leupeptin and were left in place for 1 of 4 time periods (4 hours, 24 hours, 72 hours, and 7 days.) Additionally, 2 control animals underwent placement of saline-soaked microwicks for 7 days. All animals underwent either air- or bone-conducted preoperative auditory brainstem response (ABR) testing. At the end of the appropriate time period, the contralateral inner ear was surgically ablated and the animals underwent ABR of the test ear. Bilateral perilymph and peripheral blood samples were collected from all animals for evaluation of leupeptin concentration by mass spectrometry. Results: Two animals died during the perioperative period, and 1 animal was excluded because of technical errors. The remaining 20 test animals were distributed among the time points as follows: 4 hr × 4; 24 hr × 5; 72 hr × 5; 7 day × 6. One animal demonstrated hearing loss at 24 hours, 1 at 72 hours, and 1 at 7 days; 2 additional animals at the 7-day time point were deaf on post-exposure testing. Neither of the 7-day saline-control animals demonstrated significant hearing loss. The kinetics of leupeptin will be reviewed. Conclusion: Leupeptin may cause hearing loss in chinchillas. Significance: Inhibitors of damage pathways have great clinical promise in treating acute toxic injuries to the inner ear, but Leupeptin warrants further research in animal models to establish its safety. Support: None reported.