In vitro and ex vivo corneal penetration and absorption models.

Topical drug administration is the preferred route of drug delivery to the eye despite the poor bioavailability. To develop more efficient drug carriers, reliable in vitro or ex vivo models are required in the early stages of formulation development, with such methods being faster, cheaper, and more ethical alternatives to in vivo studies. In vitro cell culture models are increasingly being used for transcorneal penetration studies, with primary cell cultures and immortalized cell lines now giving way to the development of organotypic corneal constructs for studying ocular drug bioavailability. Artificially cultured human corneal equivalents are still in the early stages of development, but present tremendous potential for corneal penetration studies. Ex vivo models using excised animal tissue are also being used to study corneal penetration with promising results, although significant inter-species variations need to be considered. This review discusses the in vitro and ex vivo models currently being used to study corneal penetration and evaluates their advantages and limitations with a focus on diffusion cell assemblies. In addition to the tissue used, the diffusion cell set-up can significantly influence the penetration profile and should be cautiously adjusted to simulate clinical conditions.