The in vitro activities of luliconazole, amorolfine, ciclopirox, and terbinafine were determined against 320 dermatophyte isolates from large toenails of onychomycosis patients enrolled into an ongoing phase 2b/3 clinical study. The geometric mean MIC for luliconazole was 0.00022 g/ml against all isolates, compared to 0.0194 to 0.3107 g/ml for the three other agents. The in vitro potency of luliconazole was maintained regardless of the dermatophyte species.O nychomycosis (tinea unguium) is a fungal infection of the nail bed and or plate and is associated with significant morbidity (1). Besides causing cosmetic concerns, this mycosis can result in pain or discomfort, acute bacterial cellulitis associated with spread of infection to the skin, and social stigma (2, 3). The prevalence of onychomycosis is estimated to be between 2 and 14%, with a rate of 13.8% reported in North America (1, 4). Risk factors for infection include increasing age, male gender, dystrophic nails, tinea pedis, and poor peripheral circulation (1, 3, 4). These infections are caused primarily by dermatophytes, including Trichophyton rubrum and T. mentagrophytes, and to a lesser extent by other dermatophytes (e.g., Epidermophyton floccosum), Candida species, and nondermatophytic molds (e.g., Acremonium, Fusarium, and Scopulariopsis species) (5, 6). Luliconazole is a novel imidazole currently under development. Pharmacokinetic and safety results from phase 1 studies in patients with onychomycosis have demonstrated high concentrations of luliconazole within the nail plates of the great toe and have shown that this agent is well tolerated when administered as a 10% solution (7). In vitro studies with a limited number of isolates have also reported potent activity of this agent against dermatophytes and other causative agents of dermatophytosis, including onychomycosis (8-10). Results from animal models and small clinical studies have also suggested in vivo efficacy against dermatophytosis (11-13). Our objective was to measure the in vitro activity of luliconazole against dermatophytes isolated at screening from toenails in an ongoing phase 2b/3 study of patients with mild to moderate toenail onychomycosis (www.clinicaltrials.gov, identifier NCT01431820). In addition, the in vitro activities of amorolfine, ciclopirox, and terbinafine, three products approved and most often used for treatment of onychomycosis in the United States and the European Union, were also determined.Three hundred twenty dermatophytes obtained from patients with confirmed toenail onychomycosis (positive KOH and culture and clinical diagnosis) during screening were used in this study. This included 308 Trichophyton rubrum and 10 T. mentagrophytes isolates and one each of Trichophyton tonsurans and Epidermophyton floccosum. All isolates were clinical strains that had been freshly subcultured and not previously frozen. Stock solutions of amorolfine, ciclopirox, terbinafine, and luliconazole were prepared in dimethyl sulfoxide (DMSO) and further diluted in RPMI 1640 buffered to a...