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/cm²) 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/cm²) as compared with Transcutol^® and Maisine^® (~110–150 μg/cm²). 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.

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Materials

MS was purchased from Jiangsu Puyuan Chemical Co., Ltd., Jiangsu, China. Internal standard of MS (lot number: R107Q0) was purchased from US Pharmacopeia, North Bethesda, MD, USA. Nikasol TS-620 (metha-acrylic alkyl ester copolymer aqueous emulsion) (named as “Nikasol” from here onwards) was purchased from Nippon Carbide Industries, Tokyo, Japan. Durabond PC 1879A (aqueous acrylic polymer dispersion) (named as “Durabond” from here onwards) was purchased from H.B. Fuller, Saint Paul, MN, USA. Polyvinyl pyrrolidone (PVP) K-90 (Kovidone^TM) (USP grade, Molecular weight: 360,000) was purchased from Boai NKY Pharmaceuticals Ltd., Tianjin, China. Carboxymethyl cellulose (CMC) 4000 PA 07 (food grade) was purchased from Dow Chemical Company, Lansing, MI, USA. Anhydrous tartaric acid was purchased from Changmao Biochemical Engineering Co Ltd., Jiangsu, China.

D-sorbitol solution (70%) and PG (USP grade) were purchased from Biofresh Green Sdn Bhd, Prai, Malaysia. Tween 80 was purchased from Guangdong Huana Chemistry Co. Ltd., Guangdong, China. Methanol (High-Performance Liquid Chromatography, HPLC grade) was purchased from Thermo Fisher Scientific, Boston, MA, USA. TRC, MAI, LA and PLU were generous gifts by Gattefossé, Saint-Priest, France. IPM (USP grade) was purchased from KLK Oleo, Selangor, Malaysia. Trifluoroacetic acid for synthesis (≥99% purity) was purchased from Merck KGaA, Darmstadt, Germany.

Phosphate buffered saline (PBS) tablets (Dulbecco A; pH 7.3 ± 0.2 at 25 °C) was purchased from Oxoid Limited, Basingstoke, England. High vacuum grease was obtained from Dow Corning, Lansing, MI, USA. Trypsin (extracted from bovine pancreas, BAEE > unit/mg) was purchased from Macklin, Shanghai, China. Sodium bromide (purity > 98%) was purchased from R&M Chemicals, Essex, UK. The backing liner of patches (made of 100% polyethylene) was purchased from Wujiang City Lt Textile Co., Ltd., Jiangsu, China. The release liner of patches (polyethylene plastic) was purchased from Pikulthong Plastic Co., Ltd., Nakornpathom, Thailand. Aluminium foil bags were purchased from Imperial Packaging And Plastics Sdn Bhd, Selangor, Malaysia. Nylon membranes (pore size: 0.45 µm) were purchased from Gelman Sciences, Lansing, MI, USA.

Source: Yeoh, S.C.; Loh, P.L.; Murugaiyah, V.; Goh, C.F. Development and Characterisation of a Topical Methyl Salicylate Patch: Effect of Solvents on Adhesion and Skin Permeation. Pharmaceutics 2022, 14, 2491. https://doi.org/10.3390/pharmaceutics14112491

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