Terbinafine is considered the gold standard in the treatment of dermatophytosis, but pathogenic dermatophytes that are resistant to terbinafine pose a new threat. A Swiss research team involving the Department of Dermatology at Lausanne University Hospital (CHUV) conducted a study to determine the proportion of resistant skin fungi, analyze the molecular mechanisms of terbinafine resistance and validate a method for reliable and rapid identification.
Fungal skin infections are among the most common dermatoses [1]. Dermatophytoses associated with Trichophyton rubrum (T. rubrum) and/or Trichophyton interdigitale (T. interdigitale) are the most common reason for seeking dermatological treatment [2]. Terbinafine is the drug of choice for dermatophyte infections [3]. Documented terbinafine resistance in T. rubrum was first reported in 2003 [4]. Resistance in a T. rubrum strain was found to be caused by a missense single point mutation in the squalene epoxidase (SQLE) gene leading to an amino acid substitution, L393F, [5,6]. Since then, cases of terbinafine-resistant Trichophyton strains have become more frequent and there have been several reports, including from Switzerland and Germany [1].
Methodology
From 2013 to 2021, samples were collected from patients with suspected dermatophyte infections and analyzed mycologically [1]. For this study, all isolated strains of Trichophyton rubrum and the complex T. mentagrophytes/T. interdigital considered. Dermatophytes were identified as pathogenic fungi in 15.6% of the 41,513 dermatological samples. The researchers examined a total of 5634 isolated Trichophyton strains (4229 T. rubrum, 1405 T. mentagrophytes/T. interdigital) for their antifungal resistance. The latter was determined by hyphal growth on Sabouraud-Dextros agar medium with 0.2 μg/ml terbinafine. All trichophyte isolates whose growth ability was maintained in the presence of terbinafine were subjected to SQLE sequencing. The minimum inhibitory concentrations (MIC) were determined by microdilution.
Results of the analysis at a glance
Over a period of 8 years, the proportion of fungal skin infections that were resistant to terbinafine increased from 0.63% in 2013 to 1.3% in 2021 [1]. Routine phenotypic in vitro screening analysis identified 0.83% (n=47/5634) of trichophyte strains with in vitro terbinafine resistance. Molecular screening revealed a mutation in the SQLE in all cases. The mutations L393F, L393S, F397L, F397I, F397V, Q408K, F415I, F415S, F415V, H440Y or the deletion A398A399G400 were detected in T. rubrum . The mutations L393F and F397L were the most frequent.
In contrast, in the strains of the T. mentagrophytes/ T. interdigitale complex examined, all mutations corresponded to F397L, except for one strain with L393S. All 47 strains had significantly higher minimum inhibitory concentrations (MIC) than the terbinafine-sensitive controls. The mutation-related MIC range was between 0.004 and 16.0 μg/ml, whereby an MIC of only 0.015 μg/ml already resulted in clinical resistance to the standard terbinafine dosage.
Conclusion
Based on the available data, the authors suggest an MIC of 0.015 μg/ml as the minimum limit for predicting clinically relevant failure of terbinafine treatment after standard oral dosing in dermatophyte infection [1]. Furthermore, they recommend growth on Sabouraud dextrose agar medium with 0.2 μg/ml terbinafine and SQLE sequencing as fungal spore-independent methods [1].
Literature:
- Blanchard G, et al: Reliable and rapid identification of terbinafine resistance in dermatophytic nail and skin infections. J Eur Acad Dermatol Venereol 2023; 37(10): 2080-2089.
- Hay RJ, et al: The global burden of skin disease in 2010: an analysis of the prevalence and impact of skin conditions. J Invest Dermatol 2014; 134(6): 1527-1534.
- Gupta AK, et al: Terbinafine in the treatment of dermatophyte toenail onychomycosis: a meta-analysis of efficacy for continuous and intermittent regimens. J Eur Acad Dermatol Venereol 2013; 27(3): 267-272.
- Mukherjee PK, et al: Clinical Trichophyton rubrum strain exhibiting primary resistance to terbinafine. Antimicrob Agents Chemother 2003; 47(1): 82-86.
- Osborne CS, et al: Amino acid substitution in Trichophyton rubrum squalene epoxidase associated with resistance to terbinafine. Antimicrob Agents Chemother 2005; 49(7): 2840-2844.
- Osborne CS, et al: Biological, biochemical, and molecular characterization of a new clinical Trichophyton rubrum isolate resistant to terbinafine. Antimicrob Agents Chemother 2006; 50(6): 2234-2236.
- Castellanos J, et al: Unusual Inflammatory Tinea Infections: Majocchi’s Granuloma and Deep/Systemic Dermatophytosis. J Fungi 2021, 7, 929.
www.mdpi.com/2309-608X/7/11/929, (last accessed 12/04/2023).
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