Stability determination of an alternative approach to use of liquid drug substance as oil phase in microemulsion formulations: methyl salicylate

Abstract

The objective of this study was to investigate the stability of microemulsions using methyl salicylate as oil phase in the presence and the absence of hydrophobic additives and/or drug after six-month storage at ambient temperature. The investigated additives were menthol, isopropyl palmitate and isopropyl myristate. Indomethacin was as a model drug. The surfactant system of Tween 20-isopropyl alcohol (1:1) was kept constant at 50% while the dispersed phase was at 15% in which weight ratios of methyl salicylate to additive were varied. Formulations were prepared and then characterized. All samples were transparent homogenous liquids with nano-sized droplets and the O/W characteristic. After storage, all samples visually remained unchanged. The analytical results showed methyl salicylate and indomethacin were stable in the prepared microemulsions. The optimized oil phase comprised menthol as additive at a 3:1 weight ratio of methyl salicylate to menthol in the presence and the absence of indomethacin.