Many candidate compounds to protect inner ear sensory cells from ototoxicity have been and are currently being tested. The recent work by Chowdhury, et al., is among the first to describe how a candidate otoprotective compound was rationally optimized to reach investigational new drug (IND) status by the U.S. Food and Drug Administration (FDA), thereby providing a direction for others to follow (J Med Chem. 2018 Jan 11;61(1):84). The senior authors previously described a (then) novel screening strategy using zebrafish lateral line neuromast hair cells to identify potential ototoxic compounds and genetic modifiers of ototoxicity (J Assoc Res Otolaryngol. 2008 Jun; 9(2):178; PLoS Genet. 2008 Feb 29;4(2):e1000020). The beauty of their unbiased approach resided in the ability to rapidly screen a large number of single-variable conditions for surviving larval neuromast hair cells that had taken up a fluorescent marker. Thus, the dose-response curve for hair cell survival (the HC50) in numerous compounds could be determined within days, rather than over many months, with a relatively low degree of variability. These studies also identified two candidates that not only protected neuromast hair cells but also mammalian vestibular hair cells in vitro from aminoglycoside-induced cytotoxicity. One compound, PROTO1 (Fig. 1a), the basis of the work of Chowdhury and colleagues, was selected for optimization (J Med Chem. 2018).
The recent work by Chowdhury, et al., is among the first to describe how a candidate otoprotective compound was rationally optimized to reach investigational new drug status.
TESTING AND REFINING
Although PROTO1 provided partial protection of vestibular (extra-striolar) hair cells in vitro against aminoglycoside-induced hair cell death, the authors noted that the drug had several liabilities in vivo, including short half-life and in adequate solubility. Notably, and in common with many other FDA-approved drugs, PROTO1 also inhibited a vital voltage-dependent potassium channel (Kv11.1) that can lead to cardiac arrhythmia (QT interval prolongation) and sudden death. Nonetheless, PROTO1 was uncharacterized with regard to its mechanism of action of binding target. PROTO1 is a urea-thiopene carboxamide, and analogous compounds include HIV inhibitors and modulators of casein kinase, calcineurin, and p53—a tumor suppressor encoded by TP53, the most widely published gene thus far. The authors were able to secure funding to conduct a systemic structure-activity relationship SAR to identify associated analogs that were more efficacious without the liabilities identified in PROTO1.
Using dose-response curves for hair cell survival, the authors identified the analog dose that ensured 50 percent survival of neuromast hair cells (the HC50), where the lower the dose, the more efficacious the drug analog is in protecting hair cells. By systematically varying the side-chains of the aryl group (Fig. 1b), the authors identified that a chlorine side-chain was optimal for neuromast hair cell survival during neomycin exposure. Systematic variation of substituents bound to the terminal nitrogen of the urea group (Fig. 1c) revealed that the aryl group was required for cytoprotection. Loss of the carboxamide (Fig. 1d) and thiopene (Fig. 1e) groups also led to loss of the cytoprotection of hair cells. This left the tetrahydropyridine group as the richest area for exploration of efficacy. From over 50 variants, substitution of the tetrahydropyridine with tropane derivatives led to the identification of compound 90, also known as ORC-13661 and displayed as submicromolar HC50, (0.12 µM), which was considered crucial for efficacy following the systemic delivery and transport across the blood-labyrinth barrier. Maximum (98%) protection of neuromast hair cells was found to be 2.8 micromoles.
OPTIMIZING PROTECTIVE POTENCY
ORC-13661 underwent standard pharmacological analysis for absorption, distribution, metabolism, and excretion kinetics that revealed favorable toxicity profiles. There was also good aqueous solubility that led to strong bioavailability after oral delivery (>50%), with a plasma protein binding of less than 90 percent in humans, which is ideal.
Given these characteristics, ORC-13661 was then tested following oral delivery in a rat model of ototoxicity with 12 daily doses of amikaicin. Threshold shifts at all frequencies were substantially lower than rats treated with amikaicin alone, although protection was not complete. Nonetheless, systematic variation in dosing protocols (e.g., increased dosing, twice-daily dosing) could improve the efficacy of otoprotection afforded by this compound. These data formed the basis of a patent awarded to the University of Washington in 2016. Analogs derived from this process are exclusively licensed to Oricula (www.oricularx.com ), a start-up company founded in 2013 by the principal authors of this study. In 2018, ORC-13661 was granted IND status by the FDA to move forward with early human trials for safety, tolerability, and pharmacokinetics.
The old adage is that if a drug is safe and achieves its intended effect, knowledge of the mechanism of action is not required. Of course, understanding the mechanisms will only strengthen the utility of the compound, and enable researchers to assess whether other compounds can achieve the same or better efficacy via related mechanisms. By charting this path, the authors have also shown others that rational development of otoprotective compounds to prevent ototoxicity works as proof of principle. Studies are now needed to ensure that this candidate is effective for infections typically treated with aminoglycosides, as such infections induce a systemic inflammatory response that potentiates ototoxicity. This is analogous to the search for candidate otoprotectants against noise-induced hearing loss that are tested for efficacy in subjects during and after noise exposure. Equally importantly, the path taken by this group has increased the exposure of regulatory authorities to the importance of developing effective pharmaceutical interventions for hearing loss from ototoxins, noise trauma, and other adventitious or congenital etiologies, thus the significant socioeconomic benefits of preserving auditory and vestibular functions.
Journal Club Highlight
Phenotypic Optimization of Urea-Thiophene Carboxamides To Yield Potent, Well Tolerated, and Orally Active Protective Agents Against Aminoglycoside-Induced Hearing Loss.
By Chowdhury S, Owens KN, Herr RJ, Jiang Q, Chen X, Johnson G, Groppi VE, Raible DW, Rubel EW, Simon JA.
J Med Chem. 2018 Jan 11;61(1):84-97.
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