Advances and limitations of drug delivery systems formulated as eye drops

Abstract Topical instillation of eye drops remains the most common and for most the easiest route of ocular drug administration, representing the treatment of choice for many ocular diseases. Nevertheless, low ocular bioavailability of topically applied drug molecules can considerably limit their efficacy. Over the last several decades, numerous drug delivery systems (DDS) have been developed in order to improve drug bioavailability on the ocular surface. This review systematically covers the most recent advances of DDS applicable by topical instillation, that have shown better performance on in vivo models compared to standard eye drop formulations. These delivery systems are based on in situ forming gels, nanoparticles and combinations of both. Most of the DDS have been developed using natural or synthetic polymers. Polymers offer many advantageous properties for designing advanced DDS including biocompatibility, gelation properties and/or mucoadhesiveness. However, despite the high number of studies published over the last decade, there are several limitations for clinical translation of DDS. The potential challenges for commercialization of new DDS are also presented in this review.