Development and Characterisation of a Topical Methyl Salicylate Patch: Effect of Solvents on Adhesion and Skin Permeation

The advent of skin patch formulation design and technology has enabled the commercialisation of methyl salicylate (MS) as a topical patch. However, the most fundamental aspect of skin permeation is unknown at present. The study aims to investigate the effect of solvent choice on the skin permeation of MS in a neat solvent system and patch formulation with an emphasis on patch adhesion. MS in six selected solvents (propylene glycol (PG), Transcutol(®), isopropyl myristate, Labrasol(®), Plurol(®) oleique CC 497 and Maisine(®) CC) was characterised and in vitro permeation studies were also performed. An ATR-FTIR analysis on solvent-treated skin was conudcted. Patch formulation was prepared and characterised for adhesion, in vitro drug release and skin permeation studies. The highest MS permeation was found in neat PG over 24 h (~90 μg/cm(2)) due to its strong skin protein conformation effect. Transcutol(®) and isopropyl myristate showed better skin deposition and formulation retention, respectively. Nevertheless, PG enhanced the patch adhesion despite having a lower cumulative amount of MS permeated (~80 μg/cm(2)) as compared with Transcutol(®) and Maisine(®) (~110–150 μg/cm(2)). These two solvents, however, demonstrated better skin deposition and formulation retention but a lower patch adhesion. The unpredictable influence of the solvent on patch adhesion highlights the importance of the trade-off between patch adhesion and skin permeation during formulation design.