To evaluate the combination effects of anti-onychomycosis drugs, the minimum inhibitory concentrations of topical (efinaconazole, luliconazole, and tavaborole) and oral (itraconazole and terbinafine) drugs for Trichophyton rubrum and Trichophyton interdigitale (8 each, with a total of 16 strains) were determined using the microdilution checkerboard technique based on the Clinical and Laboratory Standard Institute guidelines. No antagonism was observed between the topical and oral drugs against all the tested strains. Efinaconazole with terbinafine exerted a synergistic effect on 43.8% of the strains tested (7/16 strains) and efinaconazole with itraconazole on 12.5% (2/16 strains). Conversely, luliconazole showed no synergistic effect with terbinafine but was synergistically effective with itraconazole against 31.3% of the strains (5/16 strains). Tavaborole showed no synergistic effect with terbinafine and was synergistically effective with itraconazole against 18.8% of the strains (3/16 strains). The results suggest that a combination of topical and oral drugs could be a potential clinical option for onychomycosis treatment, and overall, the efinaconazole and oral drug combination would be the most advantageous among the tested combinations.

To be effective against onychomycosis, topically applied drugs have to reach the infection site at an effective concentration to exert antifungal activity against the parasitic form of dermatophytes. We established a novel in vitro method for predicting drug efficacy at the infection site and verified the method by comparing the efficacy of two azole class topical anti-onychomycosis drugs. To predict drug efficacy in the nail plate, a human nail permeability test was conducted and the activities of the free-drugs in the upper, middle, and lowest layers of the nail plate were determined by measuring the growth inhibitory zone. Efinaconazole permeated the nail more efficiently than luliconazole, and the amount of efinaconazole in the middle and lowest layers was higher compared with that of luliconazole. Efinaconazole demonstrated antifungal activities at the concentrations in all of the nail layers, whereas luliconazole was only active at the concentrations in the upper and middle layers. The results could be explained by differences in their affinity for keratin and nail permeability. The established method enables the evaluation of nail permeability and anti-arthrospore activity of free-drugs in the nail plate to predict drug efficacy. This method will be useful for new topical drug development.