Formulation of Topical Drug Delivery Systems Containing a Fixed-Dose Isoniazid–Rifampicin Combination Using the Self-Emulsification Mechanism ^†

Background: Tuberculosis remains a significant global health issue, and the rise of drug-resistant strains is becoming increasingly concerning. Currently, treatment options are limited to systemic regimens; however, developing topical drug delivery systems could offer advantages for treating cutaneous tuberculosis (CTB) when applied directly to the lesions. We developed topical emulsions using the self-emulsification mechanism that combine fixed doses of isoniazid (INH) and rifampicin (RIF) using a quality-by-design approach. Methods: Preformulation studies pertaining to drug solubility in various solvents, the construction of pseudoternary diagrams to identify self-emulsification regions for each tested excipient combination, and the preparation of checkpoint formulations were conducted and visually examined. Formulations displaying no physical instabilities were subsequently exposed to characterization experiments, including droplet size determination, zeta potential, size distribution, viscosity, pH, self-emulsification, cloud point, robustness to dilution, and thermodynamic stability assessment. Three selected formulations were consequently subjected to membrane release experiments, followed by skin diffusion studies, and INH and RIF stability in these emulsions was determined, because these drugs have a known interaction. Conclusions: While incorporating essential oils in a topical formulation improved RIF solubility, it also resulted in several instabilities. RIF exhibited greater susceptibility to degradation under higher temperatures and lower pH conditions. However, drug release from all formulations tested was confirmed. Notably, olive oil microemulsions demonstrated the most favorable characteristics for dermal drug delivery; nonetheless, drug diffusion into and through the skin (which was not desired) could not be quantified. Despite these challenges, the findings indicate that topical drug delivery systems using the self-emulsification process can facilitate the direct treatment of CTB.