Hydration and N-acetyl-l-cysteine alter the microstructure of human nail and bovine hoof: Implications for drug delivery

Safety assessment for nail cosmetics: Framework for the estimation of systemic exposure through the nail plate

All cosmetics products, including nail care products, must be evaluated for their safety. The assessment of systemic exposure is a key component of the safety assessment. However, data on the exposure, especially via ungual route (nail plate) are limited. Based on the physicochemical properties of human nails and permeability data of topical onychomycosis drugs, the nail plate is considered a good barrier to chemicals. We examine factors impacting penetration of nail care ingredients through the nail plate, including properties of the nails of the ingredients and formulations. The molecular weight, vapor pressure, logP, water solubility, and keratin binding, as well as formulations properties e.g., polymerization of acrylate monomers are considered important factors affecting penetration. To estimate systemic exposure of nail care ingredients through the nail plate, a standardized framework is applied that quantifies the impacts of these properties on penetration with an adjustment factor for each of these influencing properties. All the adjustment factors are then consolidated to derive an integrated adjustment factor which can be used for calculation of the systemic exposure dose for the ingredient. Several case studies are presented to reflect how this framework can be used in the exposure assessment for nail cosmetic products.

Poly(pseudo)rotaxanes formed by mixed micelles and α-cyclodextrin enhance terbinafine nail permeation to deeper layers

This work aimed to develop water-based formulations for onychomycosis topical treatment using micelles of small pegylated surfactants associated with α-cyclodextrin (αCD) to deliver terbinafine to the nail. Kolliphor® RH40 (RH40) and Gelucire® 48/16 (GEL) single and mixed micelles (RH40:GEL 1:1) were prepared. αCD was added to the surfactants dispersions to form poly(pseudo)rotaxanes (PPR). Formulations were characterized in terms of drug solubilization (3 to 34-fold increase), particle size (9–11 nm) and Z-potential (+0.3 − +1.96 mV), blood compatibility (non-hemolytic), rheological behavior (solid-like viscoelastic properties after 5–10% αCD addition), drug release and interaction with the nail plate. GEL micelles and surfactant-10% αCD PPRs notably hydrated the nail plate. The high viscosity of PPR led to a slower drug release, except for RH40:GEL +10% αCD that surprisingly released terbinafine faster. The RH40:GEL +10% αCD formulation delivered twice more amount of terbinafine to deeper regions of nail plate compared to other formulations. The results evidenced the potential of PPR formed by small pegylated surfactants as a water-based formulation for nail drug delivery.

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RH40, GEL and their mixed micelles increased 3-fold and 34-fold TB solubility in water and citrate buffer, respectively.

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Addition of αCD (5–10%) led to PPR formation and viscoelastic supramolecular gels without decrease in TB solubilization.

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PPR formulations with 10% αCD enhanced nail plate hydration, increasing its porosity.

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Compared to mixed micelles, PRP showed slower release rate but promoted TB accumulation in deeper regions of hooves.

Comparative Ungual Drug Uptake Studies: Equine Hoof Membrane vs. Human Nail Plate

Human nail diseases, mostly caused by fungal infections, are common and difficult to treat. The development and testing of new drugs and drug delivery systems for the treatment of nail diseases is often limited by the lack of human nail material for permeation studies. Animal material is frequently used, but there are only few comparative data on the human nail plate, and there is neither a standardized test design nor a nail bed analogue to study drug uptake into the nail. In this study, a new permeation device was developed for permeation studies, and the permeation behavior of three model substances on the human nail plate and a model membrane from the horse hoof was investigated. A linear correlation was found between drug uptake by the human nail plate and the uptake by the equine hoof. The developed and established permeation device is suitable for investigations of ungual drug transport and enables the use of different membrane diameters and the use of a gel-based nail bed analog. The hydrogel-based acceptor medium used ensures adequate stabilization and hydration of the nail membrane.

Efficacy of Bovine Nail Membranes as In Vitro Model for Onychomycosis Infected by Trichophyton Species

Onychomycosis is a fungal infection caused by different etiologic agents, including dermatophytes that specifically colonize keratin-rich substrates. The aim of this work was to investigate mechanical modifications of bovine membranes (used as an in vitro nail model) placed in contact with Trichophyton species. Trichophyton strains were isolated from toenails specimens. The procedure was set up by spreading T. rubrum,T. interdigitale, and T. mentagrophytes strains on Petri dishes with minimal and rich media; after that, bovine membranes were placed in the center. After 27 days, T. interdigitale and T. mentagrophytes significantly reduced the thickness of the colonized membranes, whereas two T. rubrum strains showed the highest degradation limited to the small colonized area. These results were confirmed by SEM images of the colonization profile on membranes. Mechanical analyses performed on membranes were used as an innovative method to evaluate the thickness and structural integrity of membranes variation following fungal colonization. In conclusion, mechanical analyses of substrate may be used as a procedure for the development of a new onychomycosis diagnosis test in order to develop personalized and strain-specific treatment.

Improving Transungual Permeation Study Design by Increased Bovine Hoof Membrane Thickness and Subsequent Infection

Ungual formulations are regularly tested using human nails or animal surrogates in Franz diffusion cell experiments. Membranes sometimes less than 100 µm thick are used, disregarding the higher physiological thickness of human nails and possible fungal infection. In this study, bovine hoof membranes, healthy or infected with Trichophyton rubrum, underwent different imaging techniques highlighting that continuous pores traversed the entire membrane and infection resulted in fungal growth, both superficial, as well as in the membrane’s matrix. These membrane characteristics resulted in substantial differences in the permeation of the antifungal model substance bifonazole, depending on the dosage forms. Increasing the thickness of healthy membranes from 100 µm to 400 µm disproportionally reduced the permeated amount of bifonazole from the liquid and semisolid forms and allowed for a more pronounced assessment of the effects by excipients, such as urea as the permeation enhancer. Similarly, an infection of 400-µm membranes drastically increased the permeated amount. Therefore, the thickness and infection statuses of the membranes in the permeation experiments were essential for a differential readout, and standardized formulation-dependent experimental setups would be highly beneficial.

Enhanced nail delivery of voriconazole-loaded nanomicelles by thioglycolic acid pretreatment: A study of protein dynamics and disulfide bond rupture

This work aimed to select an effective penetration enhancer (PE) for nail pretreatment, develop voriconazole (VOR)-loaded nanomicelles, and evaluate their ability to deliver VOR to the nail. A complete analysis of nail protein dynamics, bond rupture, and microstructure was performed. Alternative methods as electron paramagnetic resonance (EPR) and the Ellman's reagent (DTNB) assay were also evaluated. Nanomicelles were produced and characterized. The PE hydrated the hooves, following the order: urea ≈ cysteine ≈ glycolic acid < thioglycolic acid (TGA) < NaOH. SEM images and methylene blue assay showed enlarged pores and roughness of porcine hooves after incubation with NaOH and TGA. EPR was demonstrated to be the most sensitive technique. DTNB assay quantified higher thiol groups for samples treated with TGA (p < 0.05). A stratigraphic analysis with Raman spectroscopy demonstrated that hooves treated with TGA presented a higher SH/SS ratio at the edges, affecting protein secondary structure. In vitro permeation studies demonstrated significant VOR permeation (29.44 ± 6.13 µg/cm²), 10-fold higher than previous studies with lipid nanoparticles. After TGA pretreatment, VOR permeation was further enhanced (3-fold). TGA pretreatment followed by VOR-loaded nanomicelles demonstrates a promising approach for onychomycosis treatment. The novel methods for protein analysis were straightforward and helpful.

Effect of Penetration Enhancers on Toenail Delivery of Efinaconazole from Hydroalcoholic Preparations

The incorporation of permeation enhancers in topical preparations has been recognized as a simple and valuable approach to improve the penetration of antifungal agents into toenails. In this study, to improve the toenail delivery of efinaconazole (EFN), a triazole derivative for onychomycosis treatment, topical solutions containing different penetration enhancers were designed, and the permeation profiles were evaluated using bovine hoof models. In an in vitro permeation study in a Franz diffusion cell, hydroalcoholic solutions (HSs) containing lipophilic enhancers, particularly prepared with propylene glycol dicaprylocaprate (Labrafac PG), had 41% higher penetration than the HS base. Moreover, the combination of hydroxypropyl-β-cyclodextrin with Labrafac PG further facilitated the penetration of EFN across the hoof membrane. In addition, this novel topical solution prepared with both lipophilic and hydrophilic enhancers was physicochemically stable, with no drug degradation under ambient conditions (25 °C, for 10 months). Therefore, this HS system can be a promising tool for enhancing the toenail permeability and therapeutic efficacy of EFN.

A novel technique to evaluate nail softening effects of different urea formulations

Urea has been incorporated into several topical ungual formulations to hydrate and soften the nail plate. In this study, we employed various characterization techniques (visual observation, scanning electron microscopy, measurement of thickness, transonychial water loss, nail electrical resistance, and mechanical study) to investigate the effect of urea concentration on the hydration of bovine hoof membranes - an in vitro model of infected human nails. We obtained inconsistent results in the thickness, transonychial water loss, nail electrical resistance, and scanning electron microscopy studies. In the mechanical study using a modified Texture Analyzer method, we reported an inverse and linear correlation between urea concentrations in the formulations and the force required to puncture the treated membrane (R² = 0.9582, n ≥ 8). As the urea concentration decreased from 4x to 2x, 1x, and 0x % w/w, the puncture force increased significantly from 0.47 ± 0.07 to 0.77 ± 0.07, 0.91 ± 0.09, and 1.33 ± 0.26 N, respectively (p < 0.05). Thus, urea provided a positive softening effect on the membranes and the puncture force could indicate the urea level in topical formulations. In this study, we provided a novel, efficient, and reliable tool to evaluate the hydration level and physical properties of bovine hoof membranes.

Iontophoresis to Overcome the Challenge of Nail Permeation: Considerations and Optimizations for Successful Ungual Drug Delivery

Iontophoresis is a widely used drug delivery technique that has been used clinically to improve permeation through the skin for drugs and other actives in topical formulations. It is however not commonly used for the treatment of nail diseases despite its potential to improve transungual nail delivery. Instead, treatments for nail diseases are limited to relatively ineffective topical passive permeation techniques, which often result in relapses of nail diseases due to the thickness and hardness of the nail barrier resulting in lower permeation of the actives. Oral systemic antifungal agents that are also used are often associated with various undesirable side effects resulting in low patient compliance. This review article discusses what is currently known about the field of transungual iontophoresis, providing evidence of its efficacy and practicality in delivering drug to the entire surface of the nail for extended treatment periods. It also includes relevant details about the nail structure, the mechanisms of iontophoresis, and the associated in vitro and in vivo studies which have been used to investigate the optimal characteristics for a transungual iontophoretic drug delivery system. Iontophoresis is undoubtedly a promising option to treat nail diseases, and the use of this technique for clinical use will likely improve patient outcomes.Graphical abstract.

Green fabrication of seedbed-like Flammulina velutipes polysaccharides-derived scaffolds accelerating full-thickness skin wound healing accompanied by hair follicle regeneration

A novel seedbed-like scaffold was firstly fabricated by the "frozen sectioning" processing method using Flammulina velutipes as a raw material. The Flammulina velutipes polysaccharides scaffold is composed of a natural structure imitating the "ground" (connected and aligned hollow tubes with porous walls). Meanwhile, its biologically active components include polysaccharides and proteins, mimicking the "plant nutrition" in the seedbed. To further optimize the ground and nutrition components, Flammulina velutipes polysaccharides-derived scaffolds (FPDSs) were fabricated via the treatment of original Flammulina velutipes polysaccharides scaffold (labeled FPS) by NaOH, cysteine (labeled as FPS/NaOH, FPS/Cys, respectively). FPDSs were characterized by SEM, FTIR, XRD, water absorption and retention, and mechanical evaluations. From the results, FPS/NaOH and FPS/Cys lost the characteristic big tubes of original strips and had higher water absorption capacities comparing to FPS. Simultaneously, FPS/NaOH had better ductility, FPS/Cys had showed increased stiffness. Biological activities of FPDSs were tested against different types of bacteria exhibiting excellent anti-bacterial activity, and FPS/NaOH and FPS/Cys had dramatically higher anti-bacterial activity than FPS. The cytocompatibility of FPDSs was evaluated utilizing mouse fibroblast cell line (L929), and all FPDSs showed good cytocompatibility. The FPDSs were further applied to a rat full-thickness skin wound model, and they all exhibited obviously accelerated re-epithelialization, among which FPS/NaOH showed the greatest efficiency. FPS/NaOH could shorten the wound-healing process as evidenced by dynamic alterations of the expression levels of specific stagewise markers in the healing areas. Similarly, FPS/NaOH can efficiently induce hair follicle regeneration in the healing skin tissues. In summary, FPDSs exhibit potential functions as seedbeds to promote the regeneration of the "seed" including hair follicles and injured skin, opening a new avenue for wound healing.

Treatment and management strategies of onychomycosis

Onychomycosis is one of the most prevalent and severe nail fungal infections, which is affecting a wide population across the globe. It leads to variations like nail thickening, disintegration and hardening. Oral and topical drug delivery systems are the most desirable in treating onychomycosis, but the efficacy of the results is low, resulting in a relapse rate of 25-30%. Due to systemic toxicity and various other disadvantages associated with oral therapy like gastrointestinal, hepatotoxicity, topical therapy is commonly used. Topical therapy improves patient compliance and reduces the cost of treatment. However, due to poor penetration of topical therapy across the nail plate, research is focused on different chemical, mechanical and physical methods to improve drug delivery. Penetration enhancers like Thioglycolic acid, Hydroxypropyl-β-cyclodextrin (HP-β-CD), Sodium lauryl sulfate (SLS), carbocysteine, N-acetylcysteine etc. have shown results enhancing the drug penetration across the nail plate. Results with physical techniques such as iontophoresis, laser and Photodynamic therapy are quite promising, but the long-term suitability of these devices is in need to be determined. In this article, a brief analysis of the treatment procedures, factors affecting drug permeation across nail plate, chemical, mechanical and physical devices used to increase the drug delivery through nails for the onychomycosis management has been achieved.

Calcium and Silicon Delivery to Artificial and Human Nails from Nail Polish Formulations

A deteriorating nail standard is a growing problem as the global prevalence of diabetes is increasing. Systemic treatment with mineral supplements may not be recommended, mainly due to the high doses required to deliver optimal therapeutic concentrations. In this work, we evaluate nail polish formulations for the local delivery of strengthening elements to the nail plate. Specifically, we assess calcium and silicon release from nail polish base coat formulations containing three different concentrations of White Portland Cement to water, as well as to artificial and human nails. The delivery of calcium and silicon to the dorsal nail plate was determined by inductively coupled plasma optical emission spectrometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. To the best of our knowledge, this is the first study showing that such dual elemental delivery to human nails can be achieved from nail polish formulations. Hence, this work may form the basis for new inventions where therapeutic functionalities can be integrated with the mechanical and cosmetic properties of a base coat nail polish. Future permeability studies are required to verify long-term effects on the nail standard, induced by the formulations under study.

Optimization of Drug Permeation from 8% Ciclopirox Cyclodextrin/Poloxamer-Soluble Polypseudorotaxane-Based Nail Lacquers

Cyclodextrin/poloxamer-soluble polypseudorotaxane-based nail lacquers have demonstrated significant capacity for promoting the permeation of drugs into the nail plate. Furthermore, previous studies have shown that the use of hydroalcoholic blends as vehicles promotes drug permeation. The work described herein studies the effect of the type of alcohol used in the lacquer preparation, and the composition of the vehicle is optimized to obtain soluble doses of 8% and to promote the diffusion of ciclopirox base and olamine across the nail. Permeation studies on different types of alcohols show that optimum results are achieved with short-chain alcohols, and that results become less satisfactory the higher the number of alcohol carbons. In addition, solubility and penetration studies on the bovine hoof have enabled the composition of the lacquer to be optimized for both forms of ciclopirox. The results suggest that optimized lacquers have better ciclopirox diffusion and penetration properties than the commercial reference lacquer. Lastly, in vivo studies in which optimized ciclopirox olamine lacquer was applied for 45 days to the nails of healthy volunteers showed that it caused no negative effects or changes to the nail surface. These results demonstrate the significant potential of cyclodextrin/poloxamer-soluble polypseudorotaxane-based nail lacquers for the ungual administration of drugs.

Preparation and in vivo evaluation of a highly skin- and nail-permeable efinaconazole topical formulation for enhanced treatment of onychomycosis

Onychomycosis is a progressive fungal infection of the nails that involves the deeper nail layer and nail bed. It is important to maintain sufficient drug concentration in the diseased tissues after topical application. In this study, a stable topical delivery system for efinaconazole (EFN) was designed to enhance absorption potential through the skin and nail plate by incorporating ethanol, diethylene glycol monoethyl ether (Transcutol P) and isopropyl myristate, and cyclomethicone into the topical solution as a delivery vehicle, permeation enhancers, and a wetting agent, respectively. In addition, the stability of EFN in the formulation was significantly improved by adding butylated hydroxytoluene, diethylenetriamine pentaacetic acid, and citric acid as an antioxidant, chelating agent, and pH-adjusting agent, respectively, without discoloration. The optimum EFN formulation (EFN-K) showed 1.46-fold greater human skin permeation than that of the reference control (commercial 10% EFN topical solution). Furthermore, after a 24-hour incubation, the amount of infiltrated EFN from EFN-K in the human nail plate was 4.11-fold greater than that of the reference control, resulting in an 89.7% increase in nail flux at 7 days after treatment. EFN-K significantly accelerated structural recovery of the keratin layer in a Trichophyton mentagrophytes-infected guinea pig onychomycosis model, decreasing the mean viable fungal cell count by 54.3% compared to the vehicle-treated group after once-daily treatment for 4 weeks. Thus, the accelerated skin and nail penetration effect of EFN-K is expected to achieve good patient compliance, and improve the complete cure rate of onychomycosis.

Ciclopirox and Efinaconazole Transungual Permeation, Antifungal Activity, and Proficiency To Induce Resistance in Trichophyton rubrum

Onychomycosis is a nail fungal infection, mostly caused by dermatophytes. The treatment efficacy is impaired by difficulties of reaching effective drug levels at the site of infection; frequent relapses occur after cessation of antifungal therapy. The aim of the study was to compare two commercial products containing ciclopirox or efinaconazole for antimycotic activity and antifungal drug resistance. A study of permeation and penetration through bovine hoof membranes, as a nail model, was performed to evaluate the antimycotic activity of permeates against clinical isolates of selected fungi, and the frequency of spontaneous in vitro Trichophyton rubrum-resistant strains was assessed by broth microdilution assays. The results suggest that ciclopirox creates a depot in the nail, leading to a gradual release of the drug over time with action on both the nail plate and bed. Conversely, efinaconazole, mildly interacting with nail keratin, mainly exerts its antifungal activity in the nail bed. However, in the case of T. rubrum, the antifungal activities of the drugs in the nail plate seem comparable. Finally, efinaconazole showed a potential for induction of resistance in T. rubrum, which may limit its efficacy over time. Ciclopirox did not show any potential to induce resistance in T. rubrum and appears endowed with a more complete activity than efinaconazole in the management of onychomycosis as the nail keratin is a substrate for the growth of fungal cells, and the availability of drug in large concentration just in the nail bed may not be sufficient to guarantee the complete eradication of pathogens.

Physicochemical investigations of native nails and synthetic models for a better understanding of surface adhesion of nail lacquers

The human nail, like any biological material, is not readily available in large amounts and shows some variability from one individual to another. Replacing it by synthetic models is of great interest to perform reproducible and reliable tests in order to assess drug diffusion or nail lacquer adhesion for example. Keratin films, produced at the lab scale from natural hair, and the commercially available Vitro-nail® sheets have been proposed as models of human nails. In this study, we have investigated in detail these two materials. Surface aspect, composition, surface energy and water permeation were determined by SEM-EDS, ATR-FTIR, XPS, DVS and tensiometry and were compared to those of nails clippings. The development of a probe tack test using a rotational rheometer allowed us to measure the adhesion of three different nail lacquers on each substrate and the results were correlated with the surface state. It is shown that except roughness, keratin films exhibit similar composition, water sorption and surface energy as human nails. Vitro-nail® presents a more hydrophilic and permeable behavior than natural nail due to probable higher proportions of amide functions and absence of disulfide bridges. With the aim to improve nail lacquer residence, the importance of adsorption, electrostatic and mechanical adhesions as well as water sorption behavior is highlighted and allowed to show the importance of roughness, a low surface energy, a moderate hydrophobicity and an ability to form hydrogen and electrostatic bonds in order to optimize adhesion.

Potential of Chemical and Physical Enhancers for Transungual Delivery of Amorolfine Hydrochloride

Topical monotherapy of nail infection is limited by poor drug permeability into the human nail plate. Numerous substances and methods are applied to improve the antifungal agent delivery across the nail plate. This work aimed to evaluate the effect of chemical and physical enhancers on the accumulation and permeation of amorolfine hydrochloride through human nail clippings. Polymeric nail lacquers with Eudragit E100 were developed as a potentially suitable delivery system for amorolfine hydrochloride. Incorporating thioglycolic acid and urea into formulations provided increased accumulation of antifungal agent in nail layers of up to 100% and 57%, respectively. Structural changes of nail barrier, induced by fractional CO₂ laser, were visualized by microscopy. The permeation of amorolfine hydrochloride through the nail increased twofold when thioglycolic acid-containing formulation was applied and the nail was pretreated with a fractional CO₂ laser. The results suggest that this novel combination of enhancers has the potential to be an effective option for topical drug delivery through the nail, and increased the efficacy of treatment.

Terbinafine hydrochloride nail lacquer for the management of onychomycosis: formulation, characterization and in vitro evaluation

AIM

The present investigation's intention was to develop an optimized nail lacquer (NL) for the management of onychomycosis.

MATERIALS & METHODS

The NL was optimized statistically adopting 3² full factorial design having different polymer ratios and solvent ratios. The formulations were assessed for drug permeation drying time and peak adhesive strength of the film. Characterization was done using techniques including attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), x-ray diffraction (XRD), etc.

RESULTS & CONCLUSION

The formulation that had 1:1 polymer ratio and 80:20 solvent ratio was chosen as the optimized formulation. In vitro permeation studies showed better penetration (∼3.25-fold) as well as retention (∼11-fold) of the optimized NL formulation in the animal hoof as compared with the commercial formulation. The findings of in vitro and ex vivo studies elucidated the potential of the optimized formulation. [Formula: see text].

Effect of Penetration Enhancers on Drug Nail Permeability from Cyclodextrin/Poloxamer-Soluble Polypseudorotaxane-Based Nail Lacquers

Nail delivery has interest for local treatment of nail diseases. Nevertheless, the low permeability of drugs in the nail plaque precludes the efficacy of local treatments. The use of penetration enhancers can increase drug permeability and improve the efficacy of the treatment of nail pathologies. In this work, different chemical substances have been evaluated as potential penetration enhancers. With this aim, the effect of different substances such as sodium lauryl sulfate (SLS), polyethylene glycol 300 (PEG 300), carbocysteine, N-acetylcysteine, lactic acid, potassium phosphate, Labrasol® and Labrafil® in the microstructure, nail surface and drug permeability has been evaluated. The models obtained by mercury intrusion porosimetry and PoreXpert™ software show a more porous structure in nails treated with different enhancers. Permeation studies with bovine hooves and nails revealed that all the hydroalcoholic lacquers developed, and particularly those prepared with SLS, provide better nail penetration of the drugs ciclopirox olamine and clobetasol propionate. Results have shown that the increase of the drug penetration in the nail is caused by the formation of a porous random microstructure and by the decrease of the contact angle between lacquers and the surface or the nail plaque. The presence of SLS produces an improvement in the spreading of the solution on the nail surface and promotes the penetration of the solution into the nail pores. The hydroalcoholic lacquer, elaborated with cyclodextrin/poloxamer soluble polypseudorotaxane and sodium lauryl sulfate as an enhancer, allowed the rate of diffusion and penetration of the active ingredient within the nail to be significantly higher than obtained with the reference lacquers when using either ciclopirox olamine or clobetasol propionate as the active ingredient.

Polyurethanes as New Excipients in Nail Therapeutics

Onychomycosis affects about 15% of the population. This disease causes physical and psychosocial discomfort to infected patients. Topical treatment (creams, solutions, gels, colloidal carriers, and nail lacquers) is usually the most commonly required due to the high toxicity of oral drugs. Currently, the most common topical formulations (creams and lotions) present a low drug delivery to the nail infection. Nail lacquers appear to increase drug delivery and simultaneously improve the effectiveness of treatment with increased patient compliance. These formulations leave a polymer film on the nail plate after solvent evaporation. The duration of the film residence in the nail constitutes an important property of nail lacquer formulation. In this study, a polyurethane polymer was used to delivery antifungals drugs, such as terbinafine hydrochloride (TH) and ciclopirox olamine (CPX) and the influence of its concentration on the properties of nail lacquer formulations was assessed. The nail lacquer containing the lowest polymer concentration (10%) was the most effective regarding the in vitro release, permeation, and antifungal activity. It has also been demonstrated that the application of PU-based nail lacquer improves the nail plate, making it smooth and uniform and reduces the porosity contributing to the greater effectiveness of these vehicles. To conclude, the use of polyurethane in nail formulations is promising for nail therapeutics.

Effect on Nail Structure and Transungual Permeability of the Ethanol and Poloxamer Ratio from Cyclodextrin-Soluble Polypseudorotaxanes Based Nail Lacquer

Aqueous-based nail lacquers have shown potential in promoting the diffusion of drugs into the nail. In our laboratory, we have recently developed a transungual delivery system based on an aqueous dispersion of cyclodextrin-poloxamer soluble polypseudorotaxanes, supramolecular host−guest assemblies that improves the drug permeation into the nail. However, the high-water content and the rheological and adhesive properties of this lacquer negatively affect properties that play a fundamental role in the patients’ acceptance such as stickiness, nail film formation or drying rate, properties. In this work, we have optimized the composition of these lacquers to improve these properties whilst maintaining good drug permeation profiles. Incorporating ethanol into the vehicle and reducing the proportion of Poloxamer 407 (PL), provided a good strategy. The use of hydro-ethanolic mixtures (>50% ethanol) and the reduction of the poloxamer concentration significantly improved the lacquer drying speed by reducing the stickiness and promoting film formation on the nail surface. Additionally, in a surprising way, the use of hydro-ethanolic vehicles further enhanced the permeation of ciclopirox olamine and clobetasol propionate, used for the treatment of onychomycosis and nail psoriasis respectively, into the nail and hooves.

Chemical and physical strategies in onychomycosis topical treatment: A review

Onychomycosis is a fungal infection of the fingernails or toenails caused by dermatophytes, nondermatophytes, moulds, and yeasts. This condition affects around 10-30% people worldwide, negatively influencing patients' quality of life, with severe outcomes in some cases. Since the nail unit acts as a barrier to exogenous substances, its physiological features hampers drug penetration, turning the onychomycosis treatment a challenge. Currently, there are several oral and topical therapies available; nevertheless, cure rates are still low and relapse rates achieves 10-53%. Also, serious side effects may be developed due to long-term treatment. In light of these facts, researchers have focused on improving topical treatments, either by modifying the vehicle or by using some physical technique to improve drug delivery trough the nail plate, hence increasing therapy effectiveness. Therefore, the aim of this paper is to explain these novel alternative approaches. First, the challenges for drug ungual penetration are presented. Then, the chemical and physical strategies developed for overcoming the barriers for drug penetration are discussed. We hope that the information gathered may be useful for the development of safer and more effective treatments for onychomycosis.

Microstructural alterations in the onychomycotic and psoriatic nail: Relevance in drug delivery

Despite the important nail alterations caused by onychomycosis and psoriasis few studies have characterized the microstructure of the diseased nail plate and the diffusion and penetration of drugs through this altered structure. This work aimed to characterize the microstructure of the healthy, onychomycotic and psoriatic human nail using Raman spectroscopy, scanning electron microscopy, optical microscope profilometry and mercury intrusion porosimetry followed by analysis of the structure with PoreCor® software. The results showed that onychomycotic nails have higher porosity and lower amounts of disulphide bonds compared to healthy nails. This suggests that the presence and action of fungi on the nail plate makes this structure more permeable to water and drugs. Psoriatic nails had increased porosity compared to healthy nails but lower than fungal infected specimens. In vitro permeation studies showed that diseased nails were more permeable to ciclopirox (onychomycosis) and clobetasol (psoriasis) although drug permeation was highly variable and likely to be influenced by the degree of alteration of the nail structure. On the whole, this work provides new and valuable information about the microstructure and porosity of diseased nails and a plausible explanation of the increased drug permeability observed in this work and elsewhere.

Effect of ablative laser on in vitro transungual delivery

Topical therapy of nail psoriasis using methotrexate has not been realized due to the high molecular weight and low permeability of the compound. In this study, we used a 2940nm fractional ablative laser to disrupt the nail barrier to enhance the in vitro transungual delivery of methotrexate. Bovine hoof membrane-an in vitro model of the human nail-was treated by the laser at different energy levels and pore densities. A successful microporation was characterized by mechanical properties, scanning electron microscopy, Fourier transform infrared spectrophotometer, dye binding, histology, pore uniformity, confocal laser microscopy, nail integrity measurement, and permeation studies. No significant difference in the pore dimension was found in different treatment groups (p>0.05). Increases in pore depth corresponded with increases in the laser energy. Laser ablation was found to affect the mechanical properties of the hoof membrane. In in vitro permeation studies, laser ablation resulted in a significant increase in the drug cumulative delivery, flux, and permeability coefficient as compared to the untreated group (n=3, p<0.05). A change in the laser energy and pore density was found to alter the drug permeability. Thus, transungual methotrexate delivery was enhanced by the fractional laser ablation.

Voriconazole-loaded nanostructured lipid carriers (NLC) for drug delivery in deeper regions of the nail plate

Voriconazole-loaded nanostructured lipid carriers (VOR-NLC) were developed and drug penetration evaluated in porcine hooves in vitro. Synergistic effect of urea (Ur), selected among other known chemical enhancers according to hoof hydration potential, was also evaluated. VOR-NLC presented a high encapsulation efficiency (74.52±2.13%), approximate mean diameter of 230nm and were positively charged (+27.32±2.74mV). Stability studies indicated they were stable under refrigeration (4±2°C) for up to 150days. SEM images revealed hooves treated with VOR-NLC and VOR-NLC-Ur suffered a disturbance on the surface depicting high roughness and porosity. Permeation data showed a substantial VOR amount retained in superficial hooves sections independent of the formulation used (2.42±0.26; 2.52±0.36 and 2.41±0.60μg/cm² for unloaded VOR, VOR-NLC and VOR-NLC-Ur, respectively, p>0.05). Still, successive extractions, revealed the amount of VOR retained in deeper regions was significantly higher when VOR-NLC or VOR-NLC-Ur was used (0.17±0.04, 0.47±0.14 and 0.36±0.07μg/cm² for unloaded VOR, VOR-NLC and VOR-NLC-Ur, respectively, p<0.05). Such results indicate NLC are promising formulations for the management of onychomycosis. Further studies in diseased nail plates are necessary.

Transungual Gel of Terbinafine Hydrochloride for the Management of Onychomycosis: Formulation, Optimization, and Evaluation

The present study was aimed to optimize, develop, and evaluate microemulsion and microemulsion-based gel as a vehicle for transungual drug delivery of terbinafine hydrochloride for the treatment of onychomycosis. D-optimal mixture experimental design was adopted to optimize the composition of microemulsion having amount of oil (X ₁), Smix (mixture of surfactant and cosurfactant; X ₂), and water (X ₃) as the independent variables. The formulations were assessed for permeation (micrograms per square centimeter per hour; Y ₁), particle size (nanometer; Y ₂), and solubility of the drug in the formulation (milligrams per milliliter; Y ₃). The microemulsion containing 3.05% oil, 24.98% Smix, and 71.96% water was selected as the optimized formulation. The microemulsion-based gel showed better penetration (∼5 folds) as well as more retention (∼9 fold) in the animal hoof as compared to the commercial cream. The techniques used to screen penetration enhancers (hydration enhancement factor, ATR-FTIR, SEM, and DSC) revealed the synergistic effect of combination of urea and n-acetyl cysteine in disruption of the structure of hoof and hence, leading to enhanced penetration of drug.

Transungual permeation: current insights

Nail disorders are beyond cosmetic concern; besides discomfort in the performance of daily chores, they disturb patients psychologically and affect their quality of life. Fungal nail infection (onychomycosis) is the most prevalent nail-related disorder affecting a major population worldwide. Overcoming the impenetrable nail barrier is the toughest challenge for the development of efficacious topical ungual formulation. Sophisticated techniques such as iontophoresis and photodynamic therapy have been proven to improve transungual permeation. This article provides an updated and concise discussion regarding the conventional approach and upcoming novel approaches focused to alter the nail barrier. A comprehensive description regarding preformulation screening techniques for the identification of potential ungual enhancers is also described in this review while highlighting the current pitfalls for the development of ungual delivery.

Molecular diffusion in the human nail measured by stimulated Raman scattering microscopy

The effective treatment of diseases of the nail remains an important unmet medical need, primarily because of poor drug delivery. To address this challenge, the diffusion, in real time, of topically applied chemicals into the human nail has been visualized and characterized using stimulated Raman scattering (SRS) microscopy. Deuterated water (D2O), propylene glycol (PG-d8), and dimethyl sulphoxide (DMSO-d6) were separately applied to the dorsal surface of human nail samples. SRS microscopy was used to image D2O, PG-d8/DMSO-d6, and the nail through the O-D, -CD2, and -CH2 bond stretching Raman signals, respectively. Signal intensities obtained were measured as functions of time and of depth into the nail. It was observed that the diffusion of D2O was more than an order of magnitude faster than that of PG-d8 and DMSO-d6. Normalization of the Raman signals, to correct in part for scattering and absorption, permitted semiquantitative analysis of the permeation profiles and strongly suggested that solvent diffusion diverged from classical behavior and that derived diffusivities may be concentration dependent. It appeared that the uptake of solvent progressively undermined the integrity of the nail. This previously unreported application of SRS has permitted, therefore, direct visualization and semiquantitation of solvent penetration into the human nail. The kinetics of uptake of the three chemicals studied demonstrated that each altered its own diffusion in the nail in an apparently concentration-dependent fashion. The scale of the unexpected behavior observed may prove beneficial in the design and optimization of drug formulations to treat recalcitrant nail disease.

Progressive development in experimental models of transungual drug delivery of anti-fungal agents

Pre-clinical development comprises of different procedures that relate drug discovery in the laboratory for commencement of human clinical trials. Pre-clinical studies can be designed to recognize a lead candidate from a list to develop the procedure for scale-up, to choose the unsurpassed formulation, to determine the frequency, and duration of exposure; and eventually make the foundation of the anticipated clinical trial design. The foremost aim in the pharmaceutical research and industry is the claim of drug product quality throughout a drug's life cycle. The particulars of the pre-clinical development process for different candidates may vary; however, all have some common features. Typically in vitro, in vivo or ex vivo studies are elements of pre-clinical studies. Human pharmacokinetic in vivo studies are often supposed to serve as the 'gold standard' to assess product performance. On the other hand, when this general assumption is revisited, it appears that in vitro studies are occasionally better than in vivo studies in assessing dosage forms. The present review is compendious of different such models or approaches that can be used for designing and evaluation of formulations for nail delivery with special reference to anti-fungal agents.

Biofunctionalized ceramic with self-assembled networks of nanochannels

Nature designs circulatory systems with hierarchically organized networks of gradually tapered channels ranging from micrometer to nanometer in diameter. In most hard tissues in biological systems, fluid, gases, nutrients and wastes are constantly exchanged through such networks. Here, we developed a biologically inspired, hierarchically organized structure in ceramic to achieve effective permeation with minimum void region, using fabrication methods that create a long-range, highly interconnected nanochannel system in a ceramic biomaterial. This design of a synthetic model-material was implemented through a novel pressurized sintering process formulated to induce a gradual tapering in channel diameter based on pressure-dependent polymer agglomeration. The resulting system allows long-range, efficient transport of fluid and nutrients into sites and interfaces that conventional fluid conduction cannot reach without external force. We demonstrate the ability of mammalian bone-forming cells placed at the distal transport termination of the nanochannel system to proliferate in a manner dependent solely upon the supply of media by the self-powering nanochannels. This approach mimics the significant contribution that nanochannel transport plays in maintaining living hard tissues by providing nutrient supply that facilitates cell growth and differentiation, and thereby makes the ceramic composite "alive".

Development of topical therapeutics for management of onychomycosis and other nail disorders: a pharmaceutical perspective

The human nail plate is a formidable barrier to drug permeation. Development of therapeutics for management of nail diseases thus remains a challenge. This article reviews the current knowledge and recent advances in the field of transungual drug delivery and provides guidance on development of topical/ungual therapeutics for management of nail diseases, with special emphasis on management of onychomycosis, the most common nail disease. Selection of drug candidates, drug delivery approaches, and evaluation of formulations are among the topics discussed. A comprehensive mathematical description for transungual permeation is also introduced.

Matrix based system of isotretinoin as nail lacquer to enhance transungal delivery across human nail plate

The project was aimed at development of isotretinoin nail lacquer and assessment of its penetration efficiency across human nail plate. Preliminary studies (hydration enhancement factor and SEM) aided the selection of thioglycolic acid as permeation and eugenol was selected as local anesthetic in the formulation. The nail lacquer was optimized by 3(2) factorial design and a total of nine formulations were prepared and screened. In vitro adhesion and ex vivo permeation (cumulative drug permeation per unit area (CDP/A) = 6.61 ± 0.57 mg/cm(2)) across bovine hoof guided the selection of F3 as optimized formulation that was improvised. Viscosity adjustments to improve handling characteristics were affected by incorporation of ethyl cellulose (6%; F3M1) that scaled the viscosity to 312.681 cp and insignificantly (p > 0.05) affected CDP/A (6.32 ± 0.45 mg/cm(2)). In comparison to marketed preparation (Retino-A cream) F3M1 afforded two fold increase in CDP/A. The permeation characteristics were defined by Higuchi model (r(2) = 0.964) and flux value of 176 μg/cm(2)/h. Confocal laser scanning microscopy, after 72 h of nail lacquer application, revealed extensive distribution of the fluorescent tracer across the human nail plate in comparison to control that was confined to the top layer. Conclusively, an efficacious and stable nail lacquer of isotretinoin was developed for potential clinical topical use to target the drug to nail bed in treatment of nail psoriasis.

Transungual delivery: deliberations and creeds

Although considered as trifling illness, nail diseases have a reasonably high occurrence and a noteworthy impact on the patients' quality of life. Furthermore, there is a need to improve the topical treatment for nail diseases to avoid drug interactions and to reduce side effects associated with oral therapy. Topical drug delivery to the nails has established amplified consideration lately. Strategies (such as chemical enhancers, formulation strategies, physical and mechanical methods) are being investigated in order to improve drug permeability across the nail plate. The rationale of this review is to present contemporary information on the structure of human nail along with its comparison with animal hooves. Precincts of nail permeability have been briefly discussed with respect to factors like permeant's molecular size, hydrophilicity, charge and the nature of the vehicle. These factors affect drug uptake and permeation through the nail. Formulations like nail lacquers which mimic cosmetic varnish and colloidal carriers along with nail substitutes that can be utilized for transungual delivery have also been discussed.

Insights into drug delivery across the nail plate barrier

Topical therapy is at the forefront in treating nail ailments (especially onychomycosis and nail psoriasis) due to its local effects, which circumvents systemic adverse events, improves patient compliance and reduces treatment cost. However, the success of topical therapy has been hindered due to poor penetration of topical therapeutics across densely keratinized nail plate barrier. For effective topical therapy across nail plate, ungual drug permeation must be enhanced. Present review is designed to provide an insight into prime aspects of transungual drug delivery viz. nail structure and physiology, various onychopathies, techniques of nail permeation enhancement and in vitro models for trans-nail drug permeation studies. Updated list of drug molecules studied across the nail plate and key commercial products have been furnished with sufficient depth. Patents pertinent to, and current clinical status of transungual drug delivery have also been comprehensively reviewed. This is the first systematic critique encompassing the detailed aspects of transungual drug delivery. In our opinion, transungual drug delivery is a promising avenue for researchers to develop novel formulations, augmenting pharmaceutical industries to commercialize the products for nail disorders.

Influence of penetration enhancers and molecular weight in antifungals permeation through bovine hoof membranes and prediction of efficacy in human nails

This work aimed to evaluate the effect of different substances on the permeation of geraniol through bovine hoof membranes. Different penetration enhancers were able to increase the permeability up to 25 times compared to control. It was demonstrated that acetilcysteine in association with ascorbic acid increased the permeation, even in acid formulations. In addition, some antifungal drugs were incorporated into a gel formulation of HPMC containing acetylcysteine 5% and ascorbic acid 0.2% and then the permeation coefficient through bovine hoof membranes was evaluated. The relationship between permeability and molecular weight was established for fluconazole, miconazole, terbinafine, butenafine, geraniol and nerol. Geraniol and nerol, the antifungals with lower molecular weight, had the better permeability results. Permeability coefficients for nail plates were estimated and geraniol demonstrated similar or even better efficacy index values against T. rubrum, T. menthagrophytes and M. canis compared with terbinafine and miconazole.

The human nail--barrier characterisation and permeation enhancement

The human nail remains one of the most challenging membranes for formulation scientists to target and for clinicians to heal. Its formidable barrier properties are the primary reason that oral therapy remains the primary approach to manage ungual infections. This article considers the major structural properties underlying the excellent barrier function of the nail, with particular emphasis on the role of biophysical methods in advancing our knowledge of this appendage. Formulations currently available for management of ungual disease are discussed and their therapeutic efficacy is assessed. Finally, experimental strategies to enhance ungual permeation are reviewed and prospects for future developments in the field are considered.