Onychomycosis is a fungal infection of the nail plate – the most visible part of the nail apparatus (see sidebar). This infection is usually caused by species of Epidermophyton, Microsporum and Trichophyton resulting in nails that are opaque, white, thickened, friable and brittle.
Onychomycosis is common, affecting 14% of the human population. Aging increases the incidence signifi cantly, with the rate estimated to be 48% in persons 70 years of age. Discoloration, thickening, hardening or crumbling of the nail may result in pain while wearing shoes.
To cure the infection, the patient is obliged to take an oral systemic medication for months. The currently approved oral medication has such side effects that the cure may be worse than the disease. Oral therapy has the inherent disadvantages of systemic adverse effects and drug interactions, while topical therapy is limited by the low permeability of the nail plates. Traditionally, the topical treatment of onychomycosis has been less than desirable due to the infection’s deep-seated nature and to the ineffective penetration of the deep nail plate by topically applied drugs.
How can topical drugs be delivered into the nail? And, perhaps as importantly, how can one assess the drug content in the human nail to validate nail drug delivery?
The challenge was to develop a system to assay drug content within the inner nail bed where the infection resides. We developed a micrometer-controlled drilling instrument that removes and collects from the inner nail bed a powder sample from which by mass balance recovery we can determine the amount of penetrated radiolabeled drug. With such an assay procedure the effectiveness of topical nail drug delivery can be assessed.