Drug delivery to the nail: therapeutic options and challenges for onychomycosis.

Onychomycosis is one of the most common nail disorders. It affects 10-30% of the world population and is caused by dermatophytes, non-dermatophytes, molds, and yeasts. Present treatment methods of onychomycosis include oral therapy, topical therapy, and a combination of both; they have mild-to-moderate efficacy, with a relapse and reinfection rate of 20-25%. For oral therapy, newer antifungal compounds (azole class and allylamine class) are being investigated to increase efficacy and minimize side effects. Oral therapy with antifungal agents have severe side effects, with lesser bioavailability and longer duration of treatment. By contrast, topical therapy of onychomycosis is associated with greater patient compliance and fewer systemic side effects and drug interactions. Current topical treatment options of onychomycosis are nail lacquers, ointments, lotions, solutions, and gels, but these formulations have been unsuccessful due to poor penetration and distribution of drugs at the infected site. Therefore, novel therapeutic options are constantly being researched to improve the efficacy of onychomycosis treatment by enhancing the permeation of the drug across the nail to reach the infected site. Various physical and chemical enhancement methods have been studied to increase the permeation of drugs across the nail plate to the nail bed. Device-based therapeutic options have also been investigated to increase the antifungal drug concentration and its effects in the onychomycotic nail. Randomized clinical trials of these novel therapies have demonstrated better efficacy. The present review discusses the anatomy of the human nail, onychomycosis and its types, onycholysis, and conventional and novel therapies. We also review patents granted as well as design challenges facing optimal drug formulation for onychomycosis treatment.