Project Summary Noise-induced hearing loss (NIHL) is the most common sensory disability in humans in industrial countries, affecting people of any age. The total economic cost of hearing loss, including NIHL, is more than $750 billion annually worldwide. Currently, there is no approved medicines available for the prevention or treatment of hearing loss. Several mechanisms – including cochlear inflammation, oxidative stress, Ca2+ overloading, and apoptosis – are important in the pathogenesis of NIHL. Accordingly, most of preclinical or investigational drugs are antioxidants or ion channel blockers. However, these drugs have shown either uncertain results or intolerable side effects. Thus, treatment following NIHL offers a major biomedical challenge. The general aim of this proposed research, in collaboration with Dr. Wu (University of Texas medical branch) and Dr. Serban (University of Montana), is to identify a novel intervention that can promote the survival of cochlear hair cells and ribbon synapses when administrated soon after acoustic trauma. All-trans retinoic acid (atRA) is an important endogenous signaling molecule. In addition to maintaining cochlear homeostasis, RA and synthetic retinoids greatly improves auditory function following acoustic injury, supporting our hypothesis that increasing atRA in the inner ear will improve recovery in NIHL victims. Problematically, atRA alone has an unfavorable pharmacokinetic profile and induces its own clearance, which results in decreased efficacy during long-term treatment. RA clearance is predominantly mediated by cytochrome P450 family 26 enzymes (CYP26). We hypothesize that selective inhibition of CYP26 in the inner ear will reduce atRA enzymatic clearance to drive up available atRA concentrations to promote the survival of cochlear hair cells and ribbon synapses after noise-induced trauma, providing a therapeutic advantage for NIHL victim recovery. Our approach has led to the identification of a novel selective, nanomolar CYP26 inhibitors DX308 with demonstrated efficacy at reducing noise-induced auditory brain stem response threshold shift after systemic treatment. This novel class of compounds is specific for CYP26A1/B1 isoforms without the off-target P450 inhibition associated with previously described azole-based CYP26 inhibitors. The objective of this proposal is in Aim1 is to determine the minimum oral effective dose of DX308 improving cochlear morphological and functional outcomes after noise exposure. In Aim 2, we propose to determine cellular permeability of DX308 across the Tympanic Membrane (TM) and Round Window Membrane (RWM) for potential topical administration. In Aim 3, we will assess the embryo-fetal development toxicity of the active oral dose of DX308. By the end of this project, we will have identified the first-in-class oral CYP26 inhibitor preclinical candidate which exhibits in vivo efficacy at rescuing noise-induced hearing loss, and we will have selected the route of a...