The Emerging Terbinafine-Resistant Trichophyton Epidemic: What Is the Role of Antifungal Susceptibility Testing?

BACKGROUND: Dermatophytosis is commonly encountered in the dermatological clinics. The main aetiological agents in dermatophytosis of skin and nails in humans are Trichophyton (T.) rubrum, T. mentagrophytes and T. interdigitale (former T. mentagrophytes-complex). Terbinafine therapy is usually effective in eradicating infections due to these species by inhibiting their squalene epoxidase (SQLE) enzyme, but increasing numbers of clinically resistant cases and mutations in the SQLE gene have been documented recently. Resistance to antimycotics is phenotypically determined by antifungal susceptibility testing (AFST). However, AFST is not routinely performed for dermatophytes and no breakpoints classifying isolates as susceptible or resistant are available, making it difficult to interpret the clinical impact of a minimal inhibitory concentration (MIC). SUMMARY: PubMed was systematically searched for terbinafine susceptibility testing of dermatophytes on October 20, 2020, by two individual researchers. The inclusion criteria were in vitro terbinafine susceptibility testing of Trichophyton (T.) rubrum, T. mentagrophytes and T. interdigitale with the broth microdilution technique. The exclusion criteria were non-English written papers. Outcomes were reported as MIC range, geometric mean, modal MIC and MIC50 and MIC90 in which 50 or 90% of isolates were inhibited, respectively. The reported MICs ranged from <0.001 to >64 mg/L. The huge variation in MIC is partly explained by the heterogeneity of the Trichophyton isolates, where some originated from routine specimens (wild types) whereas others came from non-responding patients with a known SQLE gene mutation. Another reason for the great variation in MIC is the use of different AFST methods where MIC values are not directly comparable. High MICs were reported particularly in isolates with SQLE gene mutation. The following SQLE alterations were reported: F397L, L393F, L393S, H440Y, F393I, F393V, F415I, F415S, F415V, S443P, A448T, L335F/A448T, S395P/A448T, L393S/A448T, Q408L/A448T, F397L/A448T, I121M/V237I and H440Y/F484Y in terbinafine-resistant isolates.

Photodynamic therapy: A treatment option for terbinafine resistant Trichophyton species.

BACKGROUND: Terbinafine is a first-line agent against Trichophyton-infections. However, treatment failure and resistance due to squalene epoxidase (SQLE) alterations are increasingly being reported. Photodynamic therapy (PDT) is based on combining a photosensitizer, light and oxygen to create photo-activated reactive oxygen species. It has demonstrated in vitro and in vivo activity against various microorganisms including dermatophytes. We investigated if PDT is equally effective against terbinafine resistant and susceptible strains. METHODS: Minimum inhibitory concentrations (MIC) of methylene blue (MB)-PDT against wildtype and resistant Trichophyton rubrum and Trichophyton interdigitale were determined in duplicate in microtitre plates following EUCAST E.Def 11.0 reference methodology. Included mutants harboured F397L, L393F, L393S, F415S or F397I SQLE-alterations. Illumination with red diode light was performed after <3 min, 30 min and 3 h of incubation, respectively, and plates were cultured at 25 °C for 5 days. Geometric mean MICs and MIC ranges were calculated for each isolate. RESULTS: MB-PDT led to complete inhibition of all isolates at geometric mean concentrations of 1-16 mg/L. Efficacy was independent of incubation time prior to illumination, terbinafine susceptibility (MICs ≤0.004-4 mg/L) and presence of SQLE mutations. However, the MB-PDT MIC was slightly elevated (MB: 2-8 mg/L and 8-16 mg/L) in isolates from two pigmented cultures of Trichophyton interdigitale (one wildtype and one harbouring L393F) with a darker color when compared to unpigmented cultures (MB: 0.5-4 mg/L). CONCLUSION: Terbinafine resistant and susceptible strains are equally susceptible to MB-PDT. Lower efficacy was observed against dark coloured isolates which we speculate may be due to melanisation interfering with photo-activation due to preferential light absorption.

Photodynamic therapy treatment of superficial fungal infections: A systematic review.

BACKGROUND: Fungal infections in skin, hair and nails affect up to 25 % of the global population. Conventional antifungal treatment is effective but due to resistance, treatment failure, drug interactions, and treatment related toxicity, there is a need for alternative treatments. Photodynamic therapy (PDT) has shown antimicrobial properties and is used increasingly for fungal infections. This review investigates the reported efficacy and side effects of PDT of superficial mycoses. METHODS: Pubmed and Embase were searched 26-01-2020 for "superficial fungal infections" and "photodynamic therapy" in "Human subjects" using a predefined search string. Criteria for inclusion were: clinical trials and cases involving PDT-treated patients with primary fungal infections in skin, hair and nails. Criteria for exclusion were: languages other than English, animal models, in vitro trials, secondary fungal infections, reviews and guidelines. RESULTS: 541 records were identified and 34 papers fulfilled the criteria. PDT of onychomycosis (n = 380 patients) found treatment with methylene blue (MB) photosensitizer (PS) more efficacious with complete cure rates of 70 %-80 % than 5-aminolevulinic acid (ALA)-PDT (mycological cure rates of 17 %-57 %) and methyl aminolevulinate (MAL)-PDT (mycological cure rate of 32 %). Other PDT-treated fungal diseases included (n = 55): foot infections (n = 19), tinea cruris (n = 10), scalp infections (n = 2), Malassezia infections (n = 9) and subcutaneous fungal infections (n = 15) achieved promising effect. CONCLUSION: PDT-treatment of superficial mycoses can be efficacious as salvage therapy. In the light of increasing resistance and few licensed treatment alternatives, larger randomized controlled trials investigations and optimization of the PDT-treatment protocol are warranted to evaluate PDT's potential as a future antifungal treatment.