Validation of bovine hoof slices as a model for infected human toenails: in vitro ciclopirox transungual permeation

Background Topical therapy has recently been proposed for treating onychomycosis and other nail disturbances. However, the clinical outcome may be limited by the difficulty of active ingredients effectively penetrating the nail plate. Bovine hoof membranes have been widely used to predict in vitro efficacy of drug products in nail diseases. Many studies have compared bovine hooves with human healthy nails, considering the difference between healthy and unhealthy nails to be negligible. Objectives To validate bovine hoof slices as a model for human unhealthy nails by investigating the transungual permeation/retention of ciclopirox (CPX) through bovine hoof slices and excised infected human toenails after application of a new film-forming formulation (P-3051). To investigate the ability of CPX to achieve fungicidal concentrations in and through infected toenails. Methods A new experimental technique based on a permeation unit allowed analysis by high-performance liquid chromatography of the amounts of CPX permeating through and retained in the membranes. The efficacy index was evaluated as follows: amount of permeated CPX/Trichophyton rubrum minimum inhibiting concentration. Results Extrapolated CPX flux through bovine hoof slices was about 14-fold higher than through infected human toenails, the difference being mainly due to the fourfold higher thickness of the toenails. In toenails, the CPX efficacy index for T. rubrum was positive (> 1.0) soon after P-3051 application. Conclusions This study confirms the validity of bovine hoof slices as a model for infected human nails, and suggests a substantial equivalence between the two models. Following P-3051 application, CPX reaches fungicidal concentrations in and through human infected toenails.