Use of in vitro performance models in the assessment of drug delivery across the human nail for nail disorders

INTRODUCTION: Onychomycosis is the most prevalent nail condition, affecting between 5% and 20% of the population in western countries. Oral therapies are not the treatment of choice due to poor patient compliance, high relapse rates, severe side effects, and contraindication. However, the difficultly in curing the condition using topical delivery is partly due to both the nail structure presenting a complex barrier and the lack of understanding of how best to achieve delivery of drug to the nail bed. AREAS COVERED: Several novel in vitro models incorporating human nails discussed in this article have been developed in recent years that allow for the study of the mechanisms of ungual drug absorption and formulation efficacy to be assessed. EXPERT OPINION: The TurChub zone of inhibition model is a high-throughput performance screen for prototype formulations during development, while the robustness of the ChubTur nail permeation, TurSh nail penetration, and RoMar antifungal efficacy models ensure these can be validated for in vitro bioequivalence testing of generic products to reference listed drugs. With clinical trials being a costly and high-risk approach to generic approval, the described novel in vitro screening and performance testing techniques offer a de-risked and less-expensive route to market.