In Vitro Human Onychopharmacokinetic and Pharmacodynamic Analyses of ME1111, a New Topical Agent for Onychomycosis

Structure-activity relationship studies of ME1111, a novel antifungal agent for topical treatment of onychomycosis

Onychomycosis is a prevalent disease in many areas of the world, affecting approximately 5.5% of the global population. Among several subtypes of onychomycosis, distal-lateral-subungual onychomycosis is the most common, and topical onychomycosis agents effective against this pathogenesis require properties such as high nail penetration and low affinity for keratin, the main component of the nail. To develop novel and highly effective antifungal agents with such properties, we first established an efficient ex vivo evaluation method using bovine hoof slices and human nails, and then used this method to screen an in-house compound library. Using this strategy, we identified 1, a structure with a phenyl-pyrazole skeleton. In subsequent analyses, we investigated the structure-activity relationship of 1, permitting the identification of 28 (Development Code ME1111).

Transungual Penetration and Antifungal Activity of Prescription and Over-the-Counter Topical Antifungals: Ex Vivo Comparison

INTRODUCTION

Topical antifungals for toenail onychomycosis must penetrate the nail to deliver an inhibitory concentration of free drug to the site of infection. In two ex vivo experiments, we tested the ability of topical antifungals to inhibit growth of Trichophyton rubrum and Trichophyton mentagrophytes, the most common causative fungi in toenail onychomycosis.

METHODS

Seven topical antifungals were tested: three U.S. Food and Drug Administration-approved products indicated for onychomycosis (ciclopirox 8% lacquer; efinaconazole 10% solution; tavaborole 5% solution) and four over-the-counter (OTC) products for fungal infections (tolnaftate 1% and/or undecylenic acid 25% solutions). The ability to inhibit fungal growth was tested in the presence and absence of keratin. Products were applied either to human cadaverous nails or keratin-free cellulose disks prior to placement on an agar plate (radius: 85 mm) seeded with a clinical isolate of T. rubrum or T. mentagrophytes. After incubation, the zone of inhibition (ZI), defined as the radius of the area of no fungal growth, was recorded.

RESULTS

In the nail penetration assay, average ZIs for efinaconazole (T. rubrum: 82.1 mm; T. mentagrophytes: 63.8 mm) were significantly greater than those for tavaborole (63.5 mm; 39.1 mm), ciclopirox (7.4 mm; 3.6 mm) and all OTC products (range: 10.5-34.2 mm against both species; all P < 0.001). In the cellulose disk diffusion assay, efinaconazole and tavaborole demonstrated maximal antifungal activity against both species (ZIs = 85 mm); average ZIs against T. rubrum and T. mentagrophytes were smaller for ciclopirox (59.0 and 55.7 mm, respectively) and OTC products (range: 31.2-57.8 mm and 25.7-47.7 mm, respectively).

CONCLUSIONS

Among all antifungals tested, the ability to penetrate human toenails to inhibit growth of both T. rubrum and T. mentagrophytes was greatest for efinaconazole, followed by tavaborole. These results indicate superior transungual penetration of efinaconazole compared to the other antifungals, suggesting lower keratin binding in the nail.

Antidermatophyte activity and PK/PD of ME1111 in a guinea pig model of tinea corporis

Onychomycosis, a superficial fungal infection of the nails, is prevalent in many areas of the world. Topical agents for onychomycosis need to reach the subungual layer and nail bed to exert antifungal activity in the presence of keratin, the major component of the nail. It is difficult to evaluate the efficacy and pharmacodynamics of topical agents for onychomycosis in a non-clinical evaluation system. No consistent animal model has yet been established to predict the efficacy of topical agents for onychomycosis. In this study, we evaluated the pharmacokinetics and pharmacodynamics of ME1111 in a guinea pig model of tinea corporis designed to predict the efficacy of topical medication for onychomycosis in the vicinity of the nail bed. Trichophyton mentagrophytes TIMM1189 was infected on the back skin of guinea pigs, and ME1111 solution (5%, 10%, or 15%) was administered topically, once daily for 14 consecutive days. Following the completion of dosing, segments of skin from the site of infection were excised and cultured. The concentration of ME1111 in the back skin of guinea pigs increased with formulation concentration and correlated with mycological efficacy. We revealed the concentration required for ME1111 to be effective at the site of infection. Further analysis is needed to predict the efficacy of topical agents for onychomycosis by analyzing the relationship between PK/PD around the nail bed and factors such as subungual penetration and permeability.

Novel and Investigational Treatments for Onychomycosis

Onychomycosis is a common nail disease caused by fungi. The primary pathogens are dermatophytes; however, yeasts, non-dermatophyte moulds, and mixed fungal populations may also contribute to the development of a recalcitrant condition, usually accompanied by difficulties in everyday life and severe emotional stress. Treatment failure and relapse of the infection are the most frequent problems, though new issues have become the new challenges in the therapeutic approach to onychomycosis. Resistance to antifungals, an increasing number of comorbidities, and polydrug use among the ageing population are imperatives that impose a shift to safer drugs. Topical antifungals are considered less toxic and minimally interact with other drugs. The development of new topical drugs for onychomycosis is driven by the unmet need for effective agents with prolonged post-treatment disease-free time and a lack of systemic impact on the patients’ health. Efinaconazole, Tavaborole, and Luliconazole have been added to physicians’ weaponry during the last decade, though launched on the market of a limited number of countries. The pipeline is either developing new products (e.g., ME-1111 and NP213) with an appealing combination of pharmacokinetic, efficacy, and safety properties or reformulating old, well-known drugs (Terbinafine and Amphotericin B) by using new excipients as penetration enhancers.

Vibrational Properties of Benzoxaboroles and Their Interactions with Candida albicans’ LeuRS

Benzoxaboroles have emerged over the past decade mainly due to their growing medicinal importance. Regarding the wide application of IR spectroscopy in the pharmaceutical industry, the vibrational properties of over a dozen of benzoxaboroles were described, based on results of DFT calculations as well as IR and Raman spectra measurements. Investigated series of compounds included the currently available antifungal drug (Tavaborole, AN2690) as well as its derivatives. An intense and well-isolated band corresponding to the B-OH group stretching vibrations was present in all experimental IR spectra in the range of 1446–1414 cm−1 and can be considered as characteristic for benzoxaboroles. The vibrational properties of benzoxaboroles are shown to be affected by the formation of intramolecular as well as intermolecular hydrogen bonds, which should also influence the interactions of benzoxaboroles with biomolecules and impact on their biological functions. Docking studies of the benzoxaboroles’ adenosine monophosphate (AMP) spiroboronates into the Candida albicans leucyl-RS synthetase binding pocket showed that the introduction of an amine substituent has a strong influence on their binding. The determined values of inhibition constants manifest high potential of some of the investigated molecules as possible inhibitors of that enzyme.

Synthesis and Influence of 3-Amino Benzoxaboroles Structure on Their Activity against Candida albicans

Benzoxaboroles emerged recently as molecules of high medicinal potential with Kerydin® (Tavaborole) and Eucrisa® (Crisaborole) currently in clinical practice as antifungal and anti-inflammatory drugs, respectively. Over a dozen of 3-amino benzoxaboroles, including Tavaborole's derivatives, have been synthetized and characterized in terms of their activity against Candida albicans as a model pathogenic fungus. The studied compounds broaden considerably the structural diversity of reported benzoxaboroles, enabling determination of the influence of the introduction of a heterocyclic amine, a fluorine substituent as well as the formyl group on antifungal activity of those compounds. The determined zones of the growth inhibition of examined microorganism indicate high diffusion of majority of the studied compounds within the applied media as well as their reasonable activity. The Minimum Inhibitory Concentration (MIC) values show that the introduction of an amine substituent in position "3" of the benzoxaborole heterocyclic ring results in a considerable drop in activity in comparison with Tavaborole (AN2690) as well as unsubstituted benzoxaborole (AN2679). In all studied cases the presence of a fluorine substituent at position para to the boron atom results in lower MIC values (higher activity). Interestingly, introduction of a fluorine substituent in the more distant piperazine phenyl ring does not influence MIC values. As determined by X-ray studies, introduction of a formyl group in proximity of the boron atom results in a considerable change of the boronic group geometry. The presence of a formyl group next to the benzoxaborole unit is also detrimental for activity against Candida albicans.

Assessment of the nail penetration of antifungal agents, with different physico-chemical properties

Onychomycosis, or fungal nail infection, is a common fungal infection largely caused by dermatophyte fungi, such as Trichophyton rubrum or Trichophyton mentagrophytes, which affects a significant number of people. Treatment is either through oral antifungal medicines, which are efficacious but have significant safety concerns, or with topical antifungal treatments that require long treatment regimens and have only limited efficacy. Thus, an efficacious topical therapy remains an unmet medical need. Among the barriers to topical delivery through the nail are the physico-chemical properties of the antifungal drugs. Here, we explore the ability of a range of antifungal compounds with different hydrophilicities to penetrate the nail. Human nail discs were clamped within static diffusion (Franz) cells and dosed with equimolar concentrations of antifungal drugs. Using LC-MS/MS we quantified the amount of drug that passed through the nail disc and that which remained associated with the nail. Our data identified increased drug flux through the nail for the more hydrophilic compounds (caffeine as a hydrophilic control and fluconazole, with LogP -0.07 and 0.5, respectively), while less hydrophilic efinaconazole, amorolfine and terbinafine (LogP 2.7, 5.6 and 5.9 respectively) had much lower flux through the nail. On the other hand, hydrophilicity alone did not account for the amount of drug associated with/bound to the nail itself. While there are other factors that are likely to combine to dictate nail penetration, this work supports earlier studies that implicate compound hydrophilicity as a critical factor for nail penetration.

Onychomycosis: Treatment and prevention of recurrence

Onychomycosis is a fungal nail infection caused by dermatophytes, nondermatophytes, and yeast, and is the most common nail disorder seen in clinical practice. It is an important problem because it may cause local pain, paresthesias, difficulties performing activities of daily living, and impair social interactions. The epidemiology, risk factors, and clinical presentation and diagnosis of onychomycosis were discussed in the first article in this continuing medical education series. In this article, we review the prognosis and response to onychomycosis treatment, medications for onychomycosis that have been approved by the US Food and Drug Administration, and off-label therapies and devices. Methods to prevent onychomycosis recurrences and emerging therapies are also described.