Microemulsion-Based Gel of Terbinafine Hydrochloride for Enhanced Topical Delivery

Abstract

Terbinafine hydrochloride (Terbinafine HCl) was formulated as a microemulsion-based gel to improve its suitability for topical administration. Preformulation studies included meltingpoint determination, UV-visible analysis, solubility screening, and FTIR compatibility assessment. Pseudo-ternary phase diagrams of isopropyl myristate (IPM), Tween 80/PEG 400 (Smix), and water were constructed to identify the microemulsion region. A Tween 80:PEG 400 ratio of 2:1 produced the largest clear and isotropic region and was selected for formulation. Six Terbinafine HCl-loaded microemulsions (F1-F6) were prepared by spontaneous emulsification. F4 was selected as the optimized microemulsion because it remained clear and stable during heating-cooling and freeze-thaw testing, showed transmittance above 98%, and had a drug content of 99.1 ± 1.2%. F4 was incorporated at 80% w/w into Carbopol 934P gels containing 0.5%, 1.0%, or 1.5% polymer (TG1-TG3). TG2, containing 1.0% Carbopol 934P, showed the most favorable balance of pH, viscosity, spreadability, extrudability, drug content, and controlled in vitro release and was selected as the optimized gel. Three-month storage under long-term and accelerated conditions produced only minor changes in the measured physicochemical and release parameters. The study supports the feasibility of a Terbinafine HCl microemulsion-based gel as a stable topical formulation platform, while ex vivo skin permeation and in vivo/clinical efficacy remain to be established.