A plea to bridge the gap between antifungals and the management of onychomycosis

Dermatophyte Resistance to Antifungal Drugs: Mechanisms and Prospectus

Dermatophytes comprise pathogenic fungi that have a high affinity for the keratinized structures present in nails, skin, and hair, causing superficial infections known as dermatophytosis. A reasonable number of antifungal drugs currently exist on the pharmaceutical market to control mycoses; however, their cellular targets are restricted, and fungi may exhibit tolerance or resistance to these agents. For example, the stress caused by antifungal and cytotoxic drugs in sub-inhibitory concentrations promotes compensatory stress responses, with the over-expression of genes involved in cellular detoxification, drug efflux, and signaling pathways being among the various mechanisms that may contribute to drug tolerance. In addition, the ATP-binding cassette transporters in dermatophytes that are responsible for cellular efflux can act synergistically, allowing one to compensate for the absence of the other, revealing the complexity of drug tolerance phenomena. Moreover, mutations in genes coding for target enzymes could lead to substitutions in amino acids involved in the binding of antifungal agents, hindering their performance and leading to treatment failure. The relevance of each one of these mechanisms of resistance to fungal survival is hard to define, mainly because they can act simultaneously in the cell. However, an understanding of the molecular mechanisms involved in the resistance/tolerance processes, the identification of new antifungal targets, as well as the prospective of new antifungal compounds among natural or synthetic products, are expected to bring advances and new insights that facilitate the improvement or development of novel strategies for antifungal therapy.

Potential of Ravuconazole and its Prodrugs as the New OralTherapeutics for Onychomycosis

Onychomycosis is a fungal infection of the nail apparatus caused by dermatophytes, Candida and non-dermatophytic molds. It is highly prevalent in the general population worldwide and also responsible for significant morbidity and complications and does not usually cure itself. Thus, the condition needs to be treated in view of physical and psychological problems produced. Currently, oral medications using terbinafine are the most effective therapy, but it has relatively limited therapeutic success, particularly for long-term management. Such existing oral therapies are associated with high recurrence rates and treatment failure, as well as with potential adverse events and drug-drug interactions. In the light of these issues, development of more efficacious and safer alternatives for the treatment of onychomycosis is warranted.Ravuconazole and its prodrugs are promising new drug candidates for oral therapy of onychomycosis, among which a water-soluble prodrug, mono-lysine phosphoester derivative (E1224 or BFE1224) is in the most advanced stage of clinical development; a Phase II dose-finding study has been successfully completed and Phase III comparative studies are in progress in Japan.This review aims to summarize our current status of knowledge and information on ravuconazole and its prodrugs, particularly BFE1224, as the potential oral treatment option for onychomycosis. It also summarize the clinical features of onychomycosis with particular stress on its etiology, epidemiology, and current therapeutic options and their limitations. Given its clinical usefulness, BFE1224 may become a valuable addition to the current armamentarium for the treatment of onychomycosis.

Improving Cure Rates in Onychomycosis

Background:

Onychomycosis is a persistent fungal nail infection that is notoriously hard to treat. Approximately 20% to 25% of patients with onychomycosis do not respond to treatment, and 10% to 53% of patients relapse. As such, successful treatment is imperative for long-term disease management.

Objective:

To identify ways to improve cure rates for onychomycosis.

Method:

The literature on onychomycosis treatment and recurrence was reviewed to summarize treatment approaches and suggest strategies to increase cure rates.

Results and Conclusion:

To improve treatment success in onychomycosis, we suggest the following measures be followed: (1) onychomycosis must be correctly diagnosed, (2) the treatment regimen should be tailored to the individual patient, (3) the efficacy of antifungals must be maximized, and (4) recurrence must be prevented.

Carum copticum and Thymus vulgaris oils inhibit virulence in Trichophyton rubrum and Aspergillus spp

Emergence of drug-resistant strains has demanded for alternative means of combating fungal infections. Oils of Carum copticum and Thymus vulgaris have long been used in ethnomedicine for ailments of various fungal infections. Since their activity has not been reported in particular against drug-resistant fungi, this study was aimed to evaluate the effects of oils of C. copticum and T. vulgaris on the growth and virulence of drug-resistant strains of Aspergillus spp. and Trichophyton rubrum. The gas chromatography-mass spectrometry analysis revealed thymol constituting 44.71% and 22.82% of T. vulgaris and C. copticum, respectively. Inhibition of mycelial growth by essential oils was recorded in the order of thymol > T. vulgaris > C. copticum against the tested strains. RBC lysis assay showed no tested oils to be toxic even up to concentration two folds higher than their respective MFCs. Thymol exhibited highest synergy in combination with fluconazole against Aspergillus fumigatus MTCC2550 (FICI value 0.187) and T. rubrum IOA9 (0.156) as determined by checkerboard method. Thymol and T. vulgaris essential oil were equally effective against both the macro and arthroconidia growth (MIC 72 μg/mL). A > 80% reduction in elastase activity was recorded for A. fumigatus MTCC2550 by C. copticum, T. vulgaris oils and thymol. The effectiveness of these oils against arthroconidia and synergistic interaction of thymol and T. vulgaris with fluconazole can be exploited to potentiate the antifungal effects of fluconazole against drug-resistant strains of T. rubrum and Aspergillus spp.

Recurrences of Dermatophyte Toenail Onychomycosis during Long-Term Follow-up after Successful Treatments with Mono- and Combined Therapy of Terbinafine and Itraconazole

Background:

The influence of dosing regimens on the recurrence rates of onychomycosis has not been investigated.

Objective:

To compare recurrence rates for toenail dermatophyte onychomycosis between four dosing regimens.

Methods:

A prospective, investigator-blinded, long-term follow-up (1.25 to 7 years postenrolment) study of the following regimens was undertaken with or without booster therapy at week 36: overlapping continuous itraconazole and terbinafine (COMBO), continuous terbinafine (CTERB), intermittent terbinafine (TOT), and pulsed itraconazole (III).

Results:

One hundred six mycologically and 43 completely cured participants at week 48 were included. Recurrence rates (RR) for mycologically and completely cured participants were respectively termed mycologic recurrence (MRR) and complete cure recurrence (CRR) rates. No statistically significant difference was detected between the four regimens for the two rates. However, lower MRRs were obtained for CTERB (32%) and TOT (36%) compared to III (59%) and COMBO (57%). When participants who received booster therapy were excluded from the analysis, the MRR was lower for CTERB (21%) compared to TOT (39%).

Conclusions:

Itraconazole therapy was associated with higher RRs than terbinafine therapy. Combined therapy did not reduce the RRs compared to monotherapies. A difference might exist between continuous and intermittent antifungal regimens, but additional randomized clinical trials are needed for confirmation.

Antifungal resistance mechanisms in dermatophytes

Although fungi do not cause outbreaks or pandemics, the incidence of severe systemic fungal infections has increased significantly, mainly because of the explosive growth in the number of patients with compromised immune system. Thus, drug resistance in pathogenic fungi, including dermatophytes, is gaining importance. The molecular aspects involved in the resistance of dermatophytes to marketed antifungals and other cytotoxic drugs, such as modifications of target enzymes, over-expression of genes encoding ATP-binding cassette (ABC) transporters and stress-response-related proteins are reviewed. Emphasis is placed on the mechanisms used by dermatophytes to overcome the inhibitory action of terbinafine and survival in the host environment. The relevance of identifying new molecular targets, of expanding the understanding about the molecular mechanisms of resistance and of using this information to design new drugs or to modify those that have become ineffective is also discussed.

Emerging therapeutic agents for onychomycosis

Onychomycosis is a frequent disorder that represents the most prevalent fungal infection, particularly among older individuals. Diverse fungi of the dermatophyte, non-dermatophyte mold and yeast families have been reported to be responsible for onychomycosis. The output from the pharmaceutical industry of new antifungals to treat onychomycosis has been limited over the last decade. Present treatment options include both oral and topical drugs, with oral therapies giving better outcomes. However, neither of these treatment options provides high cure rates that are durable. At present, azoles and allylamines are keeping the pivotal roles. New derivatives with a favorable risk-benefit ratio and new formulations of older azoles seem to be promising. Thus, ongoing drug development activities have focused on novel delivery technologies to facilitate incorporation of existing antifungal drugs inside the nail plate and the discovery of new active antifungals.

Effective treatment for dermatophytoses of the foot: effect on restoration of depressed cell-mediated immunity

Superficial infections caused by dermatophyte fungi are highly prevalent throughout the world. Modern antimycotic agents like the azole itraconazole or the synthetic allylamine terbinafine greatly improved treatment outcomes in comparison with former therapeutic options with griseofulvin or older azole preparations like ketoconazole or fluconazole. In randomized trials involving patients with dermotophytoses, a great effectiveness has been shown especially for terbinafine. Oral terbinafine in general is well tolerated, has a low potential for drug interactions and, therefore, may be the most often used therapeutic agent for dermatophyte onychomycosis. However, there is a group of patients suffering from chronic dermatophytoses or early reinfections after antifungal therapy. For these patients, a depression of the delayed-type hypersensitivity reactivity was postulated. Just recently, effective antimycotic treatment, in particular with terbinafine, was shown to enhance and restore cell-mediated immunity, which potentially improves the therapeutic outcome even for this group of patients.

Spores, sporodochia and fomites in onychomycosis

Onychomycosis is a frequent infection. Contagion rarely depends on direct contamination, but rather on environmental propagule dispersion including the intervention of fomites. The potential release of fungal cells in the environment from the affected nails has not been thoroughly studied. Observations made by histomycology suggest that arthroconidia and chlamydoconidia issued from invading fungi can be involved. In addition, the sporodochium, which is an exophytic fungal ball, may be attached underneath the distal free edge of some onychomycoses. It might represent an important source of fungal propagules in patients with poor nail trimming.

Spotlight on nail histomycology

The misdiagnosis of onychomycosis delays the proper treatment of the nail condition. Nail histomycology refers to the microscopic examination of histologic sections prepared from nail clippings for the purpose of detecting the presence of fungi. This laboratory method is the most sensitive procedure for the diagnosis of onychomycosis. It allows for the visualization of the precise location of the invasive fungus inside the nail apparatus. The method also distinguishes various shapes of the fungal cells, in particular filamentous fungi, yeasts, conidia, and sporodochia. The correct diagnosis provided by histomycology combined with culture not only allows for more efficient expenditure of funds and services but also is important for the timely alleviation of the disorder.

Onychomycosis in older patients

Onychomycosis represents the most frequent nail disorder. Its prevalence is influenced by gender, age and several comorbid diseases. Accurate diagnosis relies on laboratory examination, particularly nail histomycology. This method refers to the microscopic examination of histological sections prepared from nail clippings for the purpose of detecting the presence of invading fungi. The pathogens may belong to one or a combination of the groups of dermatophytes, yeasts or nondermatophyte molds. The choice of the diagnostic laboratory method influences the accuracy of the information. Histomycology is recognized to be the most sensitive and specific method. The prevalence of onychomycosis in the elderly population varies according to gender and the nature of the fungal pathogen. Epidemiological figures differ according to the nature of the reference population. In particular, it is of the utmost importance to establish the ratio between the raw numbers of onychomycoses and the total number of individuals in the same age group living in the same geographical area.

Pharmacotherapy of onychomycosis

Fungal infections of the nails are frequent in some segments of the population. Dermatophytes, yeasts and moulds are potential pathogens. A series of antifungal treatments are available to the clinician, differing by both their mechanistic nature and mode of administration. The pharmacodynamic and pharmacokinetic properties of each antifungal agent are distinct. This review focuses on the characteristics of amorolfine, bifonazole, ciclopirox, fluconazole, griseofulvin, itraconazole, ketoconazole, ravuconazole, R126638 and terbinafine. Single drug treatments and combined therapies are presented. None of the current drug regimens have demonstrated reliable efficacy against all cases of onychomycosis. Treatment failures, relapses and reinfections remain stubborn problems in the management of onychomycosis.

New insights into the effect of amorolfine nail lacquer

Despite improvements in antifungal strategies, the outcome of treating onychomycoses often remains uncertain. Several factors account for treatment failure, of which the pharmacokinetics and pharmacodynamics of the antifungal are of importance. The taxonomic nature and ungual location of the fungus cannot be neglected, besides the type of nail and its growth rate. In addition, the biological cycle of the fungus and the metabolic activity of the pathogen likely play a marked influence in drug response. The presence of natural antimicrobial peptides in the nail is also probably a key feature controlling the cure rates. There are many outstanding publications that cover the full spectrum of the field. The purpose of this review is to put in perspective some facets of activity of the topical treatment using amorolfine nail laquer. The antifungal activity of the drug is likely less pronounced in onychomycosis than that expected from conventional in vitro studies. However, the nail laquer formulation should reduce the propensity to form antifungal-resistant spores and limit the risk of reinfection.

Antifungal drug response in anin vitromodel of dermatophyte nail infection

Despite terbinafine being fungicidal against Trichophyton rubrum in standard NCCLS assays and rapidly accumulating in nails in vivo, onychomycosis patients require prolonged terbinafine treatment to be cured. To investigate this, we developed a more clinically relevant onychomycosis in vitro test model. Human nail powder inoculated with T. rubrum and incubated in liquid RPMI 1640 salt medium, which did not support growth alone, developed extensive and invasive mycelial growth. Antifungal drugs were added at different concentrations and cultures incubated for 1 to 4 weeks. Fungal survival was determined by spreading cultures on PDA plates without drug and measuring CFU after 1 to 4 weeks incubation. Drug activity was expressed as the nail minimum fungicidal concentration (Nail-MFC) required for 99.9% elimination of viable fungus. Terbinafine Nail-MFC was 4 microg/ml after 1 week exposure, decreasing to 1 microg/ml after 4 weeks exposure, much higher than MFCs < or = 0.03 microg/ml determined in standard NCCLS MIC assays. In contrast, other clinically used drugs were unable to kill T. rubrum after 4 weeks incubation in this model. Invasive mycelial growth on nail appears to protect T. rubrum from the cidal action of systemic drugs, thus providing a rationale for the long treatment periods in onychomycosis.

Terbinafine: a review of its use in onychomycosis in adults

Terbinafine, an orally and topically active antimycotic agent, inhibits the biosynthesis of the principal sterol in fungi, ergosterol, at the level of squalene epoxidase. Squalene epoxidase inhibition results in ergosterol-depleted fungal cell membranes (fungistatic effect) and the toxic accumulation of intracellular squalene (fungicidal effect). Terbinafine has demonstrated excellent fungicidal activity against the dermatophytes and variable activity against yeasts and non-dermatophyte molds in vitro. Following oral administration, terbinafine is rapidly absorbed and widely distributed to body tissues including the poorly perfused nail matrix. Nail terbinafine concentrations are detected within 1 week after starting therapy and persist for at least 30 weeks after the completion of treatment. Randomized, double-blind trials showed oral terbinafine 250 mg/day for 12 or 16 weeks was more efficacious than itraconazole, fluconazole and griseofulvin in dermatophyte onychomycosis of the toenails. In particular, at 72 weeks' follow-up, the multicenter, multinational, L.I.ON. (Lamisil vs Itraconazole in ONychomycosis) study found that mycologic cure rates (76 vs 38% of patients after 12 weeks' treatment; 81 vs 49% of recipients after 16 weeks' therapy) and complete cure rates were approximately twice as high after terbinafine treatment than after itraconazole (3 or 4 cycles of 400 mg/day for 1 week repeated every 4 weeks) in patients with toenail mycosis. Furthermore, the L.I.ON. Icelandic Extension study demonstrated that terbinafine was more clinically effective than intermittent itraconazole to a statistically significant extent at 5-year follow-up. Terbinafine produced a superior complete cure rate (35 vs 14%), mycologic cure rate (46 vs 13%) and clinical cure rate (42 vs 18%) to that of itraconazole. The mycologic and clinical relapse rates were 23% and 21% in the terbinafine group, respectively, compared with 53% and 48% in the itraconazole group. In comparative clinical trials, oral terbinafine had a better tolerability profile than griseofulvin and a comparable profile to that of itraconazole or fluconazole. Post marketing surveillance confirmed terbinafine's good tolerability profile. Adverse events were experienced by 10.5% of terbinafine recipients, with gastrointestinal complaints being the most common. Unlike the azoles, terbinafine has a low potential for drug-drug interactions. Most pharmacoeconomic evaluations have shown that the greater clinical effectiveness of oral terbinafine in dermatophyte onychomycosis translates into a cost-effectiveness ratio superior to that of itraconazole, fluconazole and griseofulvin.

CONCLUSION

Oral terbinafine has demonstrated greater effectiveness than itraconazole, fluconazole and griseofulvin in randomized trials involving patients with onychomycosis caused by dermatophytes. The drug is generally well tolerated and has a low potential for drug interactions. Therefore, terbinafine is the treatment of choice for dermatophyte onychomycosis.