In vitro activity of R126638 and ketoconazole against Malassezia species.

The in vitro activity of a new triazole R126638 against Malassezia yeasts was compared with that of ketoconazole. With the agar dilution technique, minimal inhibitory concentrations were lower for R126638 compared with ketoconazole against Malassezia globosa, M. obtusa, M. slooffiae, M. restricta and two strains of M. sympodialis. On human stratum corneum in vitro, both R126638 and ketoconazole were very effective in reducing the production of hyphae from 15% to 1% with R126638 and to 2% with ketoconazole. Scanning electron microscopy did not reveal obvious surface differences between untreated cultures and cultures exposed to ketoconazole or R126638 in the concentration range 0.01-1 microg/ml. However, transmission electron microscopy showed partial to complete necrosis of the cytoplasmic organelles of Malassezia yeasts. The combined scanning electron microscopy and transmission electron microscopy findings confirm earlier observations of the "mummifying" effect of azoles against Malassezia spp. In conclusion, R126638 is an interesting new triazole with high activity against the Malassezia yeasts, which are involved in pityriasis versicolor and seborrhoeic dermatitis.