In vitro Susceptibility Testing of Malassezia furfur against Rilopirox

The in vitro antifungal activity of the new hydroxypyridone antimycotic rilopirox has been evaluated against 29 separate clinical isolates of Mαlαsseziα (M.) furfur obtained from patients with pityriasis versicolor, seborrhoeic dermatitis or dandruff. Minimum inhibitory concentrations (MICs) of rilopirox were measured by the agar dilution technique and, in comparison, by a recently described microdilution method with colorimetric detection of the MIC end points. Rilopirox was found to be able to inhibit growth of all clinical yeast isolates. For the investigated M. furfur strains MIC values from 12.5 to 50 µg ml-1 with a median of 25 µg ml-1 were determined by the agar dilution method. Using the microdilution technique, MIC values between 16 and 128 µg ml-1 (median 32 µg ml-1) were found for the M. furfur isolates. It has to be taken into account that with a 0.3% solution concentrations of 300,000 µg ml-1 are applied to the skin. Furthermore, due to its extreme low penetration rilopirox is long-term available in the skin in inhibiting concentrations. In comparison with rilopirox, the in vitro susceptibility of M. furfur against the systemically applicable triazole antimycotic itraconazole and clotrimazole, an established topical antifungal, was tested. As expected, low MIC values for these azoles were found by the agar dilution method. The median of the MIC of M. furfur was 0.1 µg ml-1 for itraconazole, and 6.25 µg ml-1 for clotrimazole. The inhibitory effectivity of rilopirox against clinical isolates of M. furfur seems to justify its therapeutic evaluation in clinical trials. This new antifungal may be a useful alternative not only in pityriasis versicolor but also in seborrhoeic dermatitis due to the growth inhibition of M. furfur.