Safety and pharmacokinetics of efinaconazole 10% solution in healthy volunteers and patients with severe onychomycosis.

Objectives: To characterize the systemic exposure and pharmacokinetics of efinaconazole 10% solution and assess the potential for drug-drug interaction (DDI) in human volunteers and onychomycosis patients following topical administration.

Methods: Two single-center, open-label studies in healthy volunteers and severe onychomycosis patients. Efinaconazole 10% solution was applied topically to all 10 toenails (0.42 mL total daily dose volume); administered as single and then 7 daily doses to 10 healthy volunteers, and once daily for 28 days to 19 severe onychomycosis patients. Plasma concentrations of efinaconazole and its major metabolite H3 were determined by LC-MS-MS at multiple timepoints. Safety evaluations were carried out throughout both studies.

Results: The mean peak plasma concentrations (Cmax) of efinaconazole and H3 were 0.54 and 1.63 ng/mL, respectively, in healthy volunteers; and 0.67 and 2.36 ng/mL, respectively, in patients. Both parent drug and metabolite accumulated following repeat dosing, and reached steady state in plasma by 14 days. Efinaconazole was well tolerated in both studies; no drug-related adverse events were reported.

Conclusions: Efinaconazole 10% solution resulted in very low systemic exposures to efinaconazole and H3 when applied topically at maximum use conditions to healthy volunteer and onychomycosis patients' toenails. Efinaconazole is a CYP inhibitor like other azole antifungals, and its lowest ki is 91 ng/mL for CYP2C9, a >130-fold higher concentration than the mean steady state Cmax observed in patients. The Cmax/ki ratio was 0.007, well below the threshold for clinical DDI evaluation as recommended in regulatory guidances, thereby suggesting efinaconazole 10% solution has remote potential for drug-drug interactions.