Development and optimization of a methimazole microemulsion for topical application: Formulation characteristics and transdermal permeation

Background Methimazole, an oral antithyroid drug, has recently gained attention for its skin‐brightening effects when applied topically to treat melasma. This study aims to develop, optimize, and characterize a methimazole microemulsion as a novel, safe approach for local melasma treatment. Materials and Methods We prepared microemulsion formulations containing 3% methimazole by combining appropriate amounts of surfactants (Tween 80 and Span 20), propylene glycol cosurfactant, and an oil phase (oleic acid‐transcutol p at a 1:10 ratio). We then assessed droplet size, stability, viscosity, and skin permeation using rat skin models. Results The microemulsions' droplet sizes ranged from 7.06 to 28.13 nm, with viscosities between 120 and 254 centipoises. Our analysis identified droplet size, viscosity, and membrane release as significant independent variables. We determined the permeability parameters of the optimal formulation through rat skin, including steady‐state permeability rate (J ss ), permeability coefficient (p), lag time (T lag ), and apparent diffusion coefficient (D app ). Conclusion We found that the microemulsions' characteristics, physicochemical properties, and in vitro release depended on the surfactant‐to‐cosurfactant ratio, water content, and oil content. We developed an optimal formulation with a high surfactant‐to‐cosurfactant ratio and low water and oil percentages. This formulation shows potential for commercialization and manufacturing of final products.