Abstract Hearing loss caused by noise, aging and chemotherapy affects seven hundred million people worldwide, but there are no FDA-approved drugs to prevent it. This research will test the potential to repurpose a small molecule BRAF inhibitor, dabrafenib (TAFINLAR), an FDA-approved drug for several cancers, for new use in preventing cisplatin-induced hearing loss. Dabrafenib was a top hit in our unbiased high-throughput screens of 4,385 bioactive compounds and 187 specific kinase inhibitors for cisplatin-induced cell-death protection in an inner ear cell line. We found that dabrafenib fully protected the outer hair cells against cisplatin toxicity in mouse cochlear explants with IC50 of 30 nM and an excellent therapeutic index (LD50/IC50) of >2000. Mechanistically, we identified dabrafenib and three additional BRAF inhibitors, two MEK1/2 inhibitors, and an ERK1/2 inhibitor immediately downstream of BRAF in the cellular pathway, supporting the role of BRAF in cisplatin-induced hair cell death. Cisplatin treatment of the inner ear cell line caused upregulation of phospho- BRAF, phospho-MEK1/2 and phospho-ERK1/2 that was inhibited by co-treatment with dabrafenib. Moreover, cisplatin treatment of cochlear explants or noise exposure in vivo caused up-regulation of phospho-ERK1/2 short time after damage in supporting cells (inner phalangeal and Deiters’ cells) that was mitigated by dabrafenib treatment. Furthermore, at 100 nM dabrafenib protected zebrafish lateral line neuromasts from cisplatin-induced death in vivo and, importantly, significant protection was achieved with oral delivery of dabrafenib for three consecutive days in mouse models against cisplatin-induced hearing loss. The daily dose of dabrafenib administered to the mice was in the range approved for long-term human treatment. In this proposal, we will test the protection provided by dabrafenib for cisplatin-induced hearing loss in a multiple low dose cisplatin regimen that mimics closely the cisplatin treatment of cancer patients in the clinic. Functional auditory performance and inner ear morphology will be assessed. Two doses of dabrafenib will be tested to evaluate the therapeutic window of the drug in vivo. We will also determine dabrafenib’s interference with cisplatin tumor killing efficacy in tumor cell lines and mouse tumor models in which cisplatin is the standard treatment, neuroblastoma and lung cancer. We will confirm BRAF kinase is the molecular target of dabrafenib in cisplatin-induced hearing loss by generating a supporting cell specific conditional knockout mouse in which BRAF is specifically deleted in the supporting cells of the inner ear starting at postnatal day 28 and testing its resistance to cisplatin. Supporting cells’ ERK phosphorylation can serve as an in vivo biomarker for cisplatin damage and evaluating treatment with BRAF inhibitors and can be utilized to determine dabrafenib’s PK/PD properties. Our study will reveal a new cellular pathway and molecular target BRAF kinase for otoprotection and will provide the crucial data needed for advancing dabrafenib to clinical trials in humans for prevention of cisplatin-induced hearing loss.

Project Narrative This research will test the potential to repurpose a small molecule BRAF kinase inhibitor, dabrafenib, an FDA- approved drug for several cancers, for new use in preventing cisplatin- induced hearing loss. Significant protection against cisplatin-induced hearing loss has been achieved with oral delivery of dabrafenib in mouse models in doses approved in humans. The successful completion of this study will provide the crucial data needed for rapidly advancing dabrafenib to clinical studies in humans for prevention of hearing loss.