Onychomycosis (OM; fungal nail infection) is a common and contagious fungal infection of the nail plate or nail bed, leading to the gradual destruction of the nail plate. OM is considered the most prevalent of the nail ailments, accounting for about 50% of all diseased nails and about 30% of cutaneous fungal infections. The prevalence of OM is reported to be 23% across Europe, 13.8% in North America and approximately 10% in Japan, with the prevalence increasing in Western countries, presumably due to lifestyle changes and the ageing of the population.
Distal subungual onychomycosis (DSO) is the most common form of OM, characterised by invasion of the nail bed and underside of the nail plate beginning at the hyponychium. This infection is visualised as nails with normal surface texture and thickness but variable "bays" of white nail that extend from the distal nail tip proximally into the area of the nail bed. The vast majority of cases of OM are caused by dermatophyte fungi. In 80% to 98% of affected individuals, Trichophyton rubrum with additional infections caused by Trichophyton mentagrophytes var interdigitale (commonly referred to as Trichophyton mentagrophytes) or Epidermophyton floccosum. The dermatophyte Trichophyton rubrum is the major cause of tinea pedis (TP; athlete's foot) and OM. OM is a progressive disease; if left untreated, OM can lead to permanent nail damage and associated discomfort.
Onychomycosis is currently treated with surgery, medical devices (eg laser therapy), oral or topical anti-fungal agents or a combination of both. Oral terbinafine, when used for a minimum of 3 months, can achieve an efficacy of 38% or higher if treatment is extended but has a significant drug-interaction and side effect profile, including liver failure, and periodic monitoring (after 4-6 weeks of treatment) of liver function test is recommended. Topical agents are used for up to 18 months but have significantly lower efficacy rates of less than 20%.
Terbinafine is a well-established synthetic allylamine anti-fungal developed by Sandoz (now Novartis) and commercially available worldwide for more than 25 years in different oral and topical formulations as a dermal cream (1%), emulsion gel (1%), solution/spray (1%), as well as oral tablets (250 mg and 125 mg). It is highly hydrophobic in nature and tends to accumulate in skin, nails, and fatty tissues. Terbinafine's anti-fungal mechanism of action is by inhibition of squalene epoxidase in the fungal cell membrane. This leads to a deficiency in ergosterol and an intracellular accumulation of squalene, resulting in fungal cell rupture/lysis (fungicidal activity).
Dermatophytoses of nails, in contrast to those at other body sites, are particularly difficult to eradicate with drug treatment. This is the consequence of factors intrinsic to the nail (the hard, protective nail plate, sequestration of pathogens between the nail bed and plate, and slow growth of the nail). The unique barrier properties of the nail plate, which hampers the passage of anti-fungal drugs in a concentration required to eradicate the deeply-seated causative fungi in the nail bed, is a specific challenge.
As such, there is a recognized need for a simple, effective and curative topical treatment for OM. Furthermore, topical treatment may result in minimal adverse systemic events and possibly improved adherence.
