c Current therapies used to treat dermatophytoses such as onychomycosis are effective but display room for improvement in efficacy, safety, and convenience of dosing. We report here that the investigational agent VT-1161 displays potent in vitro antifungal activity against dermatophytes, with MIC values in the range of <0.016 to 0.5 g/ml. In pharmacokinetic studies supporting testing in a guinea pig model of dermatophytosis, VT-1161 plasma concentrations following single oral doses were dose proportional and persisted at or above the MIC values for at least 48 h, indicating potential in vivo efficacy with once-daily and possibly once-weekly dosing. Subsequently, in a guinea pig dermatophytosis model utilizing Trichophyton mentagrophytes and at oral doses of 5, 10, or 25 mg/kg of body weight once daily or 70 mg/kg once weekly, VT-1161 was statistically superior to untreated controls in fungal burden reduction (P < 0.001) and improvement in clinical scores (P < 0.001). The efficacy profile of VT-1161 was equivalent to those for doses and regimens of itraconazole and terbinafine except that VT-1161 was superior to itraconazole when each drug was dosed once weekly (P < 0.05). VT-1161 was distributed into skin and hair, with plasma and tissue concentrations in all treatment and regimen groups ranging from 0.8 to 40 g/ml (or g/g), at or above the MIC against the isolate used in the model (0.5 g/ml). These data strongly support the clinical development of VT-1161 for the oral treatment of onychomycosis using either once-daily or once-weekly dosing regimens. F ungal infections of the skin, nails, and hair by dermatophytes, yeasts, and nondermatophyte molds are a common occurrence (1-3), with an estimated worldwide prevalence of 20% to 25% (1). In a survey of ambulatory records in the United States, these infections necessitated an average of 4 million physician visits per year from 1995 to 2004 (4), and the majority of these infections were caused by dermatophytes. The worldwide distribution of causative fungi and types of infection are dependent on geography, environment, and cultural factors (1, 2). Consistent with most of Europe and the United States (1, 2), a recent retrospective study found that onychomycosis (also referred to as tinea unguium) was the most common dermatophyte infection and that the absolute number of onychomycosis cases increased 10-fold over a 40-year period (5). This increase can be partly explained by the increases of two at-risk populations, the elderly (6) and diabetics (7). The most common species causing onychomycosis in Europe and the United States are Trichophyton rubrum, Trichophyton mentagrophytes, and Epidermophyton floccosum (8).Current oral treatment options for superficial fungal infections include allylamines, such as terbinafine, and azole-based fungal sterol 14␣-demethylase (CYP51) inhibitors (8, 9). The azoles fall into two classes defined by the moiety that binds the heme iron within the active site of CYP51, the imidazoles (e.g., ketoconazole), which are rarely used orally due...