Understanding drug distribution and release in ophthalmic emulsions through quantitative evaluation of formulation-associated variables

Establishing bioequivalence (BE) of ophthalmic emulsions in the absence of in vivo data is challenging. In these emulsions, drug release is a complex process due to drug distribution among various phases which are difficult to characterize. The objective of this study is to investigate the process of drug distribution and mechanism of drug release in the context of formulation-associated variables. A previously reported kinetic method for determining drug partitioning was used to quantitatively evaluate the drug distribution within a simplified biphasic (emulsion) system employing cyclosporine and difluprednate as model drugs. The impacts of formulation variables, such as the amount of polysorbate 80, glycerin, and carbomer copolymer as well as the area of oil-water interface were investigated. Polysorbate 80 was found to have the greatest influence on the drug distribution. It enhanced both the rate and extent of the drug distribution from oil to aqueous phase. Glycerin was found to slightly reduce the rate and extent of drug distribution of cyclosporine into the aqueous phase, probably by suppressing the solubilization capability of the micelles. Carbomer slowed down the diffusion of drug into the oil phase and shifted the equilibrium drug distribution towards the aqueous phase. Furthermore, increase in the interfacial area significantly increased the rate of drug diffusion across the oil-aqueous interface but had negligible effect on the extent of drug distribution. It is noteworthy that the experimental setup utilized a planar interface rather than an interface with curvature, which may have slightly underestimated the influence of globule size on equilibrium drug distribution. The findings of this study give insight into the drug distribution and diffusion in complex ophthalmic emulsions and assist with formulation design as well as development of in vitro methods to support BE assessment of ophthalmic emulsions.