Susceptibility testing of amorolfine, bifonazole and ciclopiroxolamine against Trichophyton rubrum in an in vitro model of dermatophyte nail infection

Antimycotic nail lacquers are effective and safe for the treatment of onychomycosis. To assess the effi cacy of three topical agents we studied the minimum inhibitory and fungicidal concentration of amorolfi ne, bifonazole and ciclopiroxolamine. Amorolfi ne showed the most effective fungistatic and fungicidal activity in vitro against seven clinical Trichophyton rubrum nail isolates, followed in descending order by ciclopiroxolamine and bifonazole. To mimic a nail infection more appropriately, the nail minimum fungicidal concentration (Nail-MFC) was determined in an onychomycosis model. Amorolfine and ciclopiroxolamine had Nail-MFCs ranging from 2–32 μg/ml and 16–32 μg/ml, respectively. In contrast, bifonazole was unable to kill T. rubrum in this model. Statistical analyses of the results show a signifi cant difference between the two treatments with amorolfi ne and ciclopiroxolamine ( P � 0.001). For amorolfi ne a mean concentration of 12.28 μg/ml (95%-CI � [8.66, 17.41]) was suffi cient to kill all strains, while for ciclopiroxolamine about twice that concentration was needed, i.e., 24.13 μg/ml (95%-CI = [17.06, 34.13]). The individual sensitivity of six of the seven T. rubrum strains was higher for amorolfi ne. These data demonstrate that both amorolfi ne and ciclopiroxolamine effectively kill T. rubrum growing on nail powder and suggest a better cidal action for amorolfi ne. Further investigation would be required to determine if these in vitro data can partially explain the clinical observation of signifi cantly higher cure rates in onychomycosis following a therapy with an amorolfi ne-containing nail lacquer formulation.