Effect of oil structure on cyclodextrin-based Pickering emulsions for bupivacaine topical application

Cyclodextrins (CDs) coupled with oils forms an insoluble inclusion complex that is able to adsorb to the interface between oils and aqueous phases; it thereby stabilizes Pickering emulsions. Three types of oils (triglyceride, linear chain oil, and ring-structured oil) were chosen to work with CDs to prepare bupivacaine (BPC)-encapsulated Pickering emulsions. We also investigated the relationship between oils and CDs; as well as their influences on stability, drug-releasing capability and skin permeability. Particle sizes and microstructures were determined by dynamic light scattering and scanning electron microscopy, respectively. In vitro drug release studies and in vitro skin permeation studies were evaluated by using Franz diffusion model. Particle sizes of all Pickering emulsions were larger than 1 μm, and the morphology was spherical and covered with rough surfaces. BPC was released over an extended period, and the releasing ratios from Pickering emulsions were only 12.2%–23.1% after 48 h. In skin permeation studies, compared with other formulations, a formula involved with ring-structured oil allowed the highest permeation amount through skin. However, after 24 h of exposure, formulation operated with linear chain oil showed the highest skin-retaining amount. These results suggest that Pickering emulsions could regulate the target site of skin depending on various types of oil used.