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Hearing loss is a side effect of treatment with a subset of antibiotics called aminoglycoside antibiotics (AGAs). Neomycin and kanamycin are examples of AGAs with broad clinical activity against Gram-negative bacteria, but these antibiotics and others in this class are not commonly prescribed in North America and Western Europe because hearing loss and kidney damage can occur. While kidney toxicity is often reversible and can be managed, hearing loss is irreversible and there are currently no FDA approved agents to protect from any type of hearing loss. In this paper, Dr. Julian Simon (Human Biology Division) and collaborators at the University of Washington evaluated and optimized candidates for an orally-active compound to protect against AGA-induced hearing loss.
Little is understood about the mechanism through which AGAs cause hair cell death, and therefore an unbiased screening approach was necessary to find protective candidates. In a previous publication, the authors performed a screen of larval zebrafish to identify compounds that would protect against AGA induced hearing loss. Zebrafish have hair cells in sensory organs on the outside of their bodies called neuromasts. These hair cells function mechanically like mammalian hair cells in the inner ear, and, like their mammalian counterparts, are very sensitive to AGA exposure. Consequently, zebrafish are a reliable screening tool for identifying agents to prevent AGA toxicity.
From this screen, they identified the ORC-001 compound as an agent that conferred dose-dependent protection against hearing loss over a range of AGA concentrations. However, ORC-001 also exhibited some limitations. These included suboptimal solubility, moderate oral availability, and short in vivo half-life. The ORC-001 compound also partially inhibited hERG (human ether-à-go-go-related gene that codes for a potassium ion channel). Inhibition of hERG is associated with cardiac arrhythmias and is a liability in drug development. The aim of the new study was to improve upon this compound by optimizing hearing loss protection and pharmacokinetic properties while minimizing off-target activity that could lead to side effects.
The authors generated a series of derivative compounds based on ORC-001 to explore how structural changes to the molecule affected its activity. Over 400 of these analog compounds were screened using the same larval zebrafish assay used to identify ORC-001 previously. Ten fish per well were treated with the test compound at serial dilutions, followed by treatment with 200uM neomycin. Fish were then stained, anesthetized, and mounted on a microscope slide. Visual analysis of ten neuromasts identified how much hair cell loss had occurred.
After thorough analysis of all the compounds with varying structural groups and their corresponding protective activity, three compounds in addition to ORC-001 were selected to move forward for preclinical evaluation. All four of the compounds showed little toxicity in rats at 50 mg/kg. The three new derivative compounds were far less potent hERG inhibitors than ORC-001. The two compounds with the highest oral bioavailability, ORC-001 and ORC-13661, were moved forward to test for interference with the antibiotic properties of AGAs. ORC-13661 exhibited no interference with the minimal dose of several AGAs against various types of bacteria.
In vivo preclinical testing in rats demonstrated that compound ORC-13661 co-administered orally at 5mg/kg/day with an AGA protected the rats from hearing loss. The protective effect was highly significant at all frequencies tested. Compound ORC-001 was also protective at most frequencies, but the results were less dramatic than for ORC-13661. The authors confirmed by immunohistochemistry that ORC-13661 was preventing AGA-induced hair cell death in the inner ear of the rats.
The results of this study provided the support for an Investigational New Drug (IND) application for ORC-13661. Dr. Julian Simon says, “The results published in our paper are the culmination of a nearly decade-long collaboration with Ed Rubel’s and Dave Raible’s laboratories at the University of Washington. It took an interdisciplinary effort using expertise in auditory neuroscience, zebrafish genetics and medicinal chemistry to turn a screening hit into a viable lead compound for clinical use. The lead compound ORC-13661 will be advanced to FDA IND-filing and clinical trials in the next year by Oricula Therapeutics.” If clinical trials are successful, this drug could make it possible for the AGA class of antibiotics to be prescribed for safe widespread use – a very important prospect considering the rising resistance to currently prescribed antibiotics.
Also contributing to this project from the Fred Hutch was Dr. Sarwat Chowdhury.
Chowdhury S, Owens KN, Herr RJ, Jiang Q, Chen X, Johnson G, Groppi VE, Raible DW, Rubel EW, Simon JA. 2017. Phenotypic Optimization of Urea-Thiophene Carboxamides to Yield Potent, Well Tolerated, and Orally Active Protective Agents against Aminoglycoside-Induced Hearing Loss. J Med Chem. Articles ASAP.
Funding for this study was provided by the National Institute on Deafness and Other Communication Disorders and Life Sciences Discovery Fund.