The preformulation and formulation development for the transungual delivery of the antifungal drug econazole nitrate

Abstract

Onychomycosis is fungal infection of toe nails or fingernails caused by a fungal microbe that invades the nail bed. It is the most common disease of the nails and constitutes about a half of all nail abnormalities and may affect toenails or fingernails, but toenail infections are particularly common. It occurs in about 10 percent of the adult population. The most common symptoms of fungal nail infection are thickening and discoloration of the nail. Treatment of onychomycosis is challenging because the infection is embedded in the nail making it difficult for the drug to diffuse to the site of infection. Onychomycosis is an opportunistic infection in people with compromised immune function and in those with diabetes, psoriasis, HIV/AIDS etc. Onychomycosis affects patients’ physical and psychological health and has a negative impact on overall quality of life. Oral administration of antifungal agents has been the mainstay in treatment of onychomycosis such as griseofulvin, terbinafine and itraconazole, but has limitations of systemic adverse events and drug interactions, whereas several drugs have been approved for topical administration but their efficacy is limited by the low permeability of the nail plate. This study evaluated the preformulation transungual permeability of econazole nitrate and formulation development of a transungual topical patch utilizing penetration enhancers in combination with econazole nitrate to optimize the delivery and penetration through the nail. The objectives of this project were to: 1) determine the critical factors for the in vitro transungual delivery of econazole nitrate, 2) design and develop a transungual formulation containing econazole nitrate and selection of the penetration enhancers, and 3) characterize the physical characteristics and functional properties of a novel transungual formulation. There were ten penetration enhancers being screened in this project according to the enhancement for saturation solubility, in vitro nail penetration and in vitro skin permeation and penetration of the antifungal drug econazole nitrate. Unlike transdermal drug delivery, the selection requirements for skin penetration enhancer were to increase drug accumulation in the epidermis and decrease the amount in the dermis to avoid unnecessary systermic absorption (Palliyil, et al. 2013). Thiourea (TU) improved the solubility and nail penetration of econazole nitrate. It also produced enhancement in the transungual diffusion of the drug. It was selected as the nail permeation enhancer and skin penetration enhancer for econazole nitrate. In the pH study, pH 5 ammonium phosphate buffer was the most effective pH for both enhancing the amount of drug in the nail and decreasing keratin binding. This resulted in increased accumulated of free drug in the target nail. In the formulation screening study, pressure sensitive adhesives (PSA), polyisobutylene, polysiloxane and polyacrylate classes of adhesives, were screened to develop a monolithic drug-in-adhesive type nail patch. Increasing the concentration of TU from 1% to 2.5% resulted in drug crystallization in the dry patch, therefore the concentration of 1% (w/w) TU was selected for all further screening. The concentration of econazole nitrate, propylene glycol and triethyl citrate were screened at 2.5%, 10% and 10% accordingly to ensure high drug release rate with no drug crystallization. The in vitro drug release rate of EN from the patch was improved with propylene glycol and hydrophobic plasticizer triethyl citrate. Polyvinylpyrrolidone was added to the patch formulation to lower the pH of the patch. This resulted in a greater concentration of ionized EN. The nail permeation and penetration of EN were studied in vitro using human cadaver toenails mounted in Franz diffusion cells. Thiourea, when formulated in the novel nail patch, was shown to deliver higher amount of EN into target tissues with a shorter permeation lag time compared to formulations which did not utilize thiourea.