Effect of Edge Activator Combinations in Transethosomal Formulations for Skin Delivery of Thymoquinone via Langmuir Technique

Thymoquinone (TQ), a bioactive compound found in Nigella sativa seeds, possesses diverse therapeutic properties for skin conditions. However, formulating TQ presents challenges due to its hydrophobic nature and chemical instability, which hinder its skin penetration. Transethosomes, as a formulation, offer an environment conducive to enhancing TQ’s solubility, stability, and skin permeation. To optimize TQ transethosomal formulations, we introduced a combination of ionic and nonionic surfactants, namely Tween 20 and sodium lauryl sulfate (SLS) or sodium lauroyl glutamate (SLG). Surfactants play a crucial role in stabilizing the formulation, reducing aggregation, improving biocompatibility, and minimizing potential toxicity. We fine-tuned the formulation composition and gained insights into its interfacial behavior using the Langmuir monolayer technique. This method elucidated the interfacial properties and behavior of phospholipids in ethosome and transethosome formulations. Our findings suggest that monolayer studies can serve as the initial step in selecting surfactants for nanocarrier formulations based on their interfacial dilational rheology studies. It was found that the addition of surfactant to the formulation increased the elasticity considering the capability of transethosomes to significantly decrease their radius when permeating the skin barrier. The results of the dilational rheology experiments were most relevant to drug permeation through the skin for the largest amplitude of deformation. The combination of Tween 20 and SLS efficiently modified the rheological behavior of lipids, increasing their elasticity. This conclusion was supported by in vitro studies, where formulation F2 composed of Tween 20 and SLS demonstrated the highest permeation after 24 h (300.23 µg/cm2). Furthermore, the F2 formulation showed the highest encapsulation efficiency (EE) of 94%, surpassing those of the control and ethosomal formulations. Additionally, this transethosomal formulation exhibited antimicrobial activity against S. aureus, with a zone of inhibition of 26.4 ± 0.3 mm. Importantly, we assessed the cytotoxicity of both ethosomes and transethosomes at concentrations ranging from 3.5 µM to 50 µM on HaCaT cell lines and found no cytotoxic effects compared to TQ hydroethanolic solution. These results suggest the potential safety and efficacy of TQ transethosomal formulations.