Polymer-based carriers for ophthalmic drug delivery

Despite the wide range of diseases affecting the eye, ocular bioavailability remains a challenge in ophthalmic drug delivery. Nowadays an extensive variety of polymers are being explored to develop colloidal drug carriers which show better performance than the more popular drug solutions. For instance, regardless of the type of polymer used, these systems prolong the residence time of the drug in the absorption site with respect to conventional aqueous eye drops which are rapidly cleared from eye surface. Furthermore, colloidal drug carriers can be internalized by cells. In addition, positively charged particles penetrate the cornea more effectively than neutral or negatively charged ones. These phenomena lead to higher ocular bioavailability. This review overviews the different polymers available to produce drug-loaded gels, microparticles and nanoparticles, highlighting the advantageous features and biocompatibility of each polymer and the major achievements in the field of ocular delivery. In addition, the design of more complex delivery systems that combine several delivery platforms is presented. Finally, regulatory aspects relevant to the clinical translation of advanced ophthalmic drug delivery systems are also discussed. All together, this manuscript is aimed at guiding pharmaceutical research and development towards the rationale polymer selection to produce drug delivery systems that improve the performance of drugs for the therapy of ophthalmic diseases.