Increasing resistance in Trichophyton interdigitale to terbinafine
November 20 2018
Researchers in Delhi and Bhopal have found multiple strains of Trichophyton interdigitale causing tinea corporis or tinea cruris that are terbinafine resistant. Ananta Khurana and colleagues analysed 64 strains from patients and 39 (61%) had elevated minimum inhibitory concentrations to terbinafine (MIC >1mg/L). Common substitutions in the target gene squalene epoxidase (ErgA) were F397L and F393F, which usually was associated with MICs of >32 mg/L, with others at 4 and 8 mg/L. These mutations have been described previously, along with others. All isolates were resistant to fluconazole in vitro and almost all susceptible to itraconazole.
The particular interest of this paper is the relationship of MIC with clinical outcome, obtained in 30 patients. For infections caused by isolates with an MIC of <0.5 mg/L, the response rate to terbinafine 250mg daily was 5 of 6 (83%), whereas it was 67% in those with elevated MICs, some of whom received 250mg twice daily for 3 weeks. Itraconazole
Many centres in India are reporting terbinafine failures in dermatophytosis. Notionally attributed to over the counter medication, incomplete course and combined steroid, antifungal and antibacterial creams with incomplete coverage and low exposures. Itraconazole may be preferable, if terbinafine is ineffective.
Read the full paper Khurana et. al. (2018): Correlation of in-vitro susceptibility based on MICs and SQLE mutations with clinical response to terbinafine in patients with tinea corporis/cruris. Antimicrob Agents Chemother. 2018 Oct 1.