Photodynamic therapy: A treatment option for terbinafine resistant Trichophyton species

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.

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.

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).

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.