1M Pharm-Pharmacy Practice, College of Pharmaceutical Sciences, Government Medical College, Kozhikode
2Assistant Professor of Pharmacy, College of Pharmaceutical Sciences, Government Medical College, Kozhikode
Dermatophytes, the fungi responsible for skin infections, have been increasingly exhibiting resistance to antifungal treatments, posing significant challenges to clinical management. This review explores the emerging issue of antifungal resistance in dermatophytosis, examining the mechanisms through which dermatophytes develop resistance and how these mechanisms impact treatment efficacy. The article delves into the various antifungal agents used in dermatophyte infections, their resistance profiles, and the implications for patient outcomes. Additionally, the review highlights recent advancements in diagnostic approaches and potential strategies to combat resistance. By synthesizing current research and clinical observations, this review aims to provide a comprehensive overview of antifungal resistance in dermatophytes and offer insights into future directions for improving treatment protocols.
Dermatophytes are fungi responsible for a range of skin, hair, and nail infections collectively known as dermatophytosis. This group encompasses three genera namely Epidermophyton, Microsporum, and Trichophyton. Topical antifungals are still the most frequently suggested treatment for various superficial dermatophytoses. For individuals with widespread skin involvement or those who do not respond to topical treatments, oral medications are considered as an alternative. Oral terbinafine and itraconazole are common treatments. Traditionally, such infections are treated with antifungal medications including azoles, allylamines, and echinocandins. However, recent evidence highlights a concerning increase in antifungal resistance among dermatophytes, which poses challenges for effective treatment and patient management. [1,2] Clinical antifungal resistance is defined as the failure to eradicate a fungal infection despite treatment with an antifungal drug that has in vitro activity against the organism. Resistance in dermatophytes often involves mechanisms such as mutations in drug target genes, enhanced drug efflux, and enzymatic inactivation of antifungals. These mechanisms lead to diminished effectiveness of standard treatments and contribute to prolonged infections and higher transmission rates. Factors such as inappropriate antifungal use, inadequate treatment adherence, and gaps in infection control practices exacerbate the problem. [3] This review aims to provide an up-to-date synthesis of antifungal resistance in dermatophytes, focusing on recent research into resistance mechanisms, treatment impacts, and strategies to address this issue. By reviewing the latest findings, this article seeks to offer insights for improving dermatophytosis management and combating the challenges posed by resistant strains.
RESISTANCE TO VARIOUS ANTIFUNGAL DRUGS
A) AZOLE RESISTANCE
Azole antifungal agents such as itraconazole and fluconazole have been commonly used to treat superficial fungal infections caused by dermatophytes. Although clinical trials have demonstrated fluconazole's effectiveness in treating dermatophytosis, recent in vitro studies, particularly those from India, reveal high minimum inhibitory concentrations. These findings raise concerns about its suitability as the first-choice treatment and suggest that further clinical trials are needed to validate its effectiveness. Resistance to azoles may be associated with the overexpression of genes that code for ABC transporters (e.g., MDR1), leading to increased expulsion of multiple drugs from the cell resulting in reduced drug accumulation and efficacy. A study noted that exposure of T. rubrum to ketoconazole, fluconazole, and itraconazole resulted in an elevated expression level of TruMDR1. This observation suggested that TruMDR1 plays a role in the drug efflux of azoles in this dermatophyte. Changes in the azole target enzyme, lanosterol demethylase, which is involved in ergosterol synthesis, can also contribute to resistance. Specific mutations in genes such as ERG11 (encoding lanosterol demethylase) and other related pathways can lead to altered drug binding and decreased susceptibility. It is important to highlight that ERG11 is the ERG gene most notably overexpressed among fluconazole-resistant T. verrucosum isolates. Researches indicates that ERG3 and ERG6 appear to play a lesser role compared to ERG11 in the development of resistance to azole antifungals in zoophilic dermatophytes. [4,5,6,7,8,13]
B) ALLYLAMINE RESISTANCE
Allylamines, including terbinafine, are key antifungal agents used to treat dermatophytosis, a common superficial fungal infection of the skin, hair, and nails. Resistance to allylamines, although less common than resistance to azoles, is an emerging concern in dermatophyte infections. Terbinafine acts by inhibiting squalene epoxidase, an enzyme critical in the ergosterol biosynthesis pathway. Mutations in the ERG1 gene, which encodes squalene epoxidase, can reduce the drug's binding affinity, leading to resistance. Overexpression of efflux pumps can also contribute to resistance by expelling terbinafine from the fungal cells before it can exert its effect. [8] In 2003, it was first reported by Mukherjee et al. that primary resistance to terbinafine in Trichophyton rubrum led to treatment failure. The isolate was found to have a mutation in the squalene epoxidase gene (L393F). Subsequently, the same research group identified an additional mutation in the squalene epoxidase gene that caused a change in the amino acid sequence F397L. [9] In 2017, Yamada et al. performed antifungal susceptibility tests on 2,056 clinical isolates of Trichophyton rubrum and Trichophyton interdigitale, finding that only 1% of these isolates were resistant to terbinafine. This suggests that terbinafine resistance is relatively uncommon. [10] In 2018, it was reported that 15 out of 87 (17%) Trichophyton interdigitale isolates (which are now classified as Trichophyton mentagrophytes VIII) and five out of 35 (14.3%) Trichophyton rubrum isolates exhibited high resistance to terbinafine. Terbinafine-resistant T. rubrum and T. mentagrophytes are increasingly reported worldwide. However, the prevalence of terbinafine resistance in a large and unbiased dataset of strains (which includes all cases, not just the suspected resistant ones) has only been reported from several countries, including India, Switzerland, Greece, France and USA. [8,11]
Study conducted by Kolarczyková D et al., observed that no resistance was detected in T. rubrum. However, 2.5% of T. mentagrophytes strains exhibited resistance, which was linked to the F397L mutation in the SQLE gene. Resistance levels increased from 1.2% in 2019 to 9.3% in 2020, then dropped back to 0% in 2021. All strains that showed resistance were identified as T. mentagrophytes var. indotineae. These resistant strains had high minimum inhibitory concentrations (MICs) for terbinafine (?4 mg/L), but exhibited low MICs to the other seven antifungal agents tested, with the exception of fluconazole. [12]
MECHANISMS OF ANTIFUNGAL RESISTANCE
Antifungal resistance in dermatophytes involves a variety of mechanisms that can significantly reduce the efficacy of standard treatments. Key resistance mechanisms include:
Genetic Mutations:
Mutations in genes encoding critical fungal enzymes can impair the action of antifungal agents. For instance, changes in the ERG11 gene, which codes for lanosterol demethylase, may result in decreased binding of azoles, a class of antifungals that target this enzyme. These mutations interfere with the production of ergosterol, a critical component of the fungal cell membrane. [13,14,15,16]
Efflux Pumps:
A common resistance strategy involves the activation of membrane-bound efflux pumps, which can identify and expel a wide range of chemicals, leading to multidrug resistance (MDR). Dermatophytes can develop or upregulate efflux pumps, such as those belonging to the ATP-binding cassette (ABC) and major facilitator superfamily (MFS). These pumps actively transport antifungal drugs out of the cell, lowering intracellular drug concentrations and thereby reducing their effectiveness. Increased expression of these pumps can lead to multidrug resistance. [13,15,16]
Enzymatic Degradation:
Some dermatophytes produce enzymes that can chemically modify or degrade antifungal drugs, preventing them from reaching their target. For example, certain fungi can secrete esterase or oxidases that inactivate antifungals before they can disrupt fungal cell function. [16,17]
Fig 1: The primary resistance mechanisms seen in dermatophytes are outlined as follows: In (A), a simplified diagram of ergosterol biosynthesis is shown. Mutations in the squalene epoxidase gene prevent terbinafine from inhibiting the enzyme, which means ergosterol production continues unabated and cell death does not occur, leading to resistance. In (B), the role of ABC transporters in the efflux mechanism is highlighted, particularly in relation to azole resistance in dermatophytes [8]
CLINICAL IMPACT OF ANTIFUNGAL RESISTANCE
The clinical impact of antifungal resistance in dermatophytes is profound and multifaceted:
Prolonged Infections:
Infections caused by resistant strains often require longer durations of treatment due to decreased drug efficacy. This can lead to extended periods of illness, increased discomfort for patients, and additional healthcare costs. Prolonged therapy may also contribute to further resistance development and a greater risk of developing secondary infections or complications. [17,18,19]
Increased Transmission:
Resistant dermatophytes can spread more easily in both community and healthcare settings, as infections that are difficult to treat may not be adequately controlled. This can lead to outbreaks and increased incidence of dermatophytosis, particularly in environments where personal hygiene and infection control measures are insufficient. [18,19,20]
Limited Treatment Options:
The emergence of resistance reduces the effectiveness of standard antifungal therapies, necessitating the use of alternative or more aggressive treatments. These alternatives may not always be available, effective, or well-tolerated by patients, complicating the management of dermatophyte infections. A key factor contributing to the high costs of fungal infections is the limited availability of effective treatments. [18] Addressing these clinical challenges requires a comprehensive approach that includes novel treatment options and effective infection control strategies.
DIAGNOSTIC APPROACHES
Accurate diagnosis of antifungal resistance is essential for effective management of dermatophyte infections. Clinical and Laboratory Standards Institute (CLSI) provides standardized protocols and guidelines for antifungal susceptibility testing, ensuring consistency and reliability in results. Adhering to CLSI standards is essential for accurate interpretation and comparison of phenotypic data. European Committee on Antimicrobial Susceptibility Testing (EUCAST) also offers guidelines for antifungal susceptibility testing, focusing on European standards and practices. [13]
Molecular Methods:
Molecular diagnostic platforms are well-suited for the rapid identification of fungal pathogens and offer a chance to simultaneously create molecular tests that can assess resistance to antifungal drugs. Advances in molecular diagnostics, such as Polymerase Chain Reaction (PCR) and sequencing technologies, enable the identification of specific genetic mutations associated with resistance. These methods offer high sensitivity and specificity for detecting resistance markers and can guide appropriate treatment decisions. [21]
Phenotypic Testing:
Traditional phenotypic methods, such as disk diffusion and microdilution assays, assess the susceptibility of dermatophyte isolates to antifungal agents. Despite being more labor-intensive, these methods remain valuable for evaluating the effectiveness of current treatments and determining the appropriate therapeutic approach. Disk diffusion method involves applying antifungal-impregnated disks to an agar plate inoculated with the fungal strain. The zone of inhibition around each disk indicates the efficacy of the antifungal agent. The size of the inhibition zone helps classify the isolate as susceptible, intermediate, or resistant. In Broth microdilution method, a range of antifungal concentrations is prepared in a liquid growth medium. Fungal isolates are added to each concentration, and growth is assessed after incubation. The minimum inhibitory concentration (MIC) is determined as the lowest concentration at which no visible growth occurs, indicating susceptibility. [21,22]
Whole Genome Sequencing:
Whole genome sequencing provides comprehensive data on the genetic makeup of dermatophytes, allowing for the identification of resistance mechanisms at a detailed level. This approach can reveal new resistance genes and pathways, facilitating the development of targeted therapies and improved diagnostic tools. To elucidate the genetic relationships among T indotineae isolates, comprehensive genomic sequencing was performed. The sequencing data for these isolates have been made publicly available in the Sequence Read Archive under accession numbers SRR27198731 to SRR27198741. Bioinformatic analyses were conducted using the CLC Genomics Workbench with the CLC Microbial Genomics Module software. Single nucleotide variations (SNVs) were identified from the genomic data and used to construct a phylogenetic tree, providing insights into the evolutionary relationships among the isolates. [14,23]
STRATEGIES TO COMBAT ANTIFUNGAL RESISTANCE
Effective strategies to combat antifungal resistance include:
Antifungal Stewardship:
Implementing antifungal stewardship (AFS) programs that promote the rational use of antifungal medications is crucial. These programs aim to optimize treatment regimens, minimize the risk of resistance development, and ensure appropriate use of available therapies. Education for healthcare providers and patients on the importance of adherence to prescribed treatments is also essential. A key aspect of diagnosing fungal infections is verifying suspected cases. Since dermatophytoses often have clinical mimickers, laboratory confirmation is essential for accurate diagnosis. Potassium hydroxide preparations or histopathological techniques using periodic acid-Schiff can confirm the presence of fungal hyphae. To determine the specific fungal species, techniques like fungal cultures or polymerase chain reaction (PCR) are employed. Some PCR assays can detect mutations in the squalene epoxidase gene associated with reduced susceptibility to terbinafine. For identifying the novel and often antimicrobial-resistant organism T indotineae, more advanced molecular techniques are necessary. AFS programs can also prioritize enhancing laboratory capabilities and offer ongoing education that covers bedside diagnostics and advanced treatment strategies. In addition to assessing clinical and mycologic cure, incorporating patient-reported outcome measures could be beneficial within these programs. [24,25]
Infection Control Practices:
Rigorous infection control measures, including proper hygiene and disinfection protocols, can help prevent the spread of resistant dermatophytes. Effective skin hygiene practices involve washing hands regularly, trimming nails, taking frequent baths and thoroughly drying the skin, and wearing non-occlusive shoes along with absorbent socks and powder. It's also important to keep from sharing personal items such as combs, towels, brushes, bedding, and hats, and to refrain from walking barefoot in public restrooms. In healthcare settings, this involves maintaining clean environments and adhering to protocols for managing infected patients. [26,27]
Research and Development:
Investing in research to discover new antifungal agents and treatment modalities is vital. This includes exploring novel compounds, combination therapies, and alternative approaches to overcome existing resistance. Collaborative efforts between researchers, clinicians, and pharmaceutical companies can drive innovation in this field. [28]
By implementing these strategies, healthcare systems can better manage dermatophyte infections and address the growing issue of antifungal resistance.
CONCLUSION:
Antifungal resistance in dermatophytosis is increasingly problematic, making it harder to treat fungal infections. Rising resistance to key medications like azoles and allylamines highlights the need for improved monitoring and updated treatment approaches. The mechanisms underlying this resistance, including target enzyme mutations and efflux pump overexpression, complicate the effectiveness of current therapies and demand a deeper understanding to develop more targeted interventions. Resistance patterns differ by region, so tailored treatment guidelines are crucial. To effectively tackle this issue, ongoing research into new antifungal options and better diagnostic methods is essential. Staying informed and adapting strategies will help manage and reduce the impact of antifungal resistance.
REFERENCES
Fida P. , Sreejith K. , Thabshira M. , P. P. Shahda , Simakh A. P. , Antifungal Resistance In Dermatophytosis Treatment, Int. J. of Pharm. Sci., 2024, Vol 2, Issue 8, 3797-3805. https://doi.org/10.5281/zenodo.13371778
10.5281/zenodo.13371778
