Effect of Ion Pairing with Alkylamines on the In-vitro Dermal Penetration and Local Tissue Disposition of Salicylates

Abstract

Abstract Hydrophilic ionic drugs can be rendered lipophilic by ion-pair formation with hydrophobic counter-ions. This study examines the value of forming ion pairs between anionic salicylate and a series of amines as model cationic counter-ions to facilitate topical delivery and skin penetration. The in-vitro translocation of salicylate ions from a non-aqueous vehicle through human epidermis was estimated in the presence or absence of amines. The distribution into, and accumulation of the salicylate ion in various tissues following topical application to anaesthetised rats were also investigated. Although the epidermal permeation constants of the salicylate-amine ion pairs were lower than that of salicylate itself (enhancement ratios: 0–74–0.87), salicylate retention and localisation in the underlying rat tissues increased in the presence of some of the counter-ions studied. Salicylate concentrations (μg (g tissue)−1) in the dermis were 877.2 ± 78.6 for salicylate alone and 1098 ± 121.9–2586 ± 332.5 for salicylate-amine ion pairs. The levels of salicylate in tissues up to the top muscle layer were 1.2–3.7-fold higher in the presence of the counter-ions. It is concluded that, although amine counter-ions have the ability to influence the penetration of salicylate, in-vitro permeability studies do not reflect the in-vivo increases in tissue concentrations resulting from probable changes in systemic clearance.