Ocular Drug Delivery Systems: Barriers, Nanocarrier-Based Approaches, In Situ Gelling Formulations, and Recent Clinical Advances

Abstract

Background: Ocular drug delivery remains one of the most challenging problems in pharmaceutical science, constrained by a unique set of anatomical and physiological barriers that limit conventional eye drop bioavailability to less than 5% of the instilled dose. The eye possesses a dual compartment anatomy — anterior and posterior segments — separated by anatomical barriers including the lens, ciliary body, and vitreous humor, each imposing distinct pharmacokinetic challenges for topical, intravitreal, periocular, and systemic drug administration routes. The rising global burden of posterior segment diseases including age-related macular degeneration, diabetic retinopathy, glaucoma, and uveitis, combined with the inadequacy of monthly or bimonthly intravitreal injection regimens for maintaining long-term drug levels in the vitreous, has created an urgent need for advanced drug delivery systems that extend dosing intervals, improve patient compliance, and reduce the risks associated with repeated intraocular injections.

Objective: This review critically examines the anatomical and physiological barriers to ocular drug delivery, the classification and formulation design of ocular drug delivery systems including topical formulations, nanoparticles, liposomes, in situ gelling systems, ocular implants, and contact lens-based delivery, mechanisms of ocular drug absorption, therapeutic applications across anterior and posterior segment diseases, and recent clinical advances through 2023.

Results and Discussion: Nanocarrier-based ophthalmic formulations including polymeric nanoparticles, solid lipid nanoparticles, liposomes, cubosomes, and dendrimers achieve significantly greater corneal drug penetration and precorneal residence time compared to conventional eye drops, with in vivo bioavailability improvements of two- to tenfold documented in animal models. In situ gelling systems — responsive to pH, temperature, or ion concentration — extend the precorneal residence time from minutes to hours by forming a viscous gel depot upon contact with the ocular surface, improving bioavailability without the patient compliance limitations of viscous eye drops. For posterior segment delivery, biodegradable implants and drug-eluting devices have demonstrated sustained intravitreal drug levels for three to thirty-six months, dramatically reducing injection frequency compared to monthly intravitreal injections.

Conclusion: Advanced ocular drug delivery systems represent a scientifically mature field with multiple clinically approved products demonstrating meaningful clinical benefits, including substantial reductions in intravitreal injection frequency and sustained visual acuity improvements. The principal unmet needs are in truly non-invasive posterior segment drug delivery and in universal patient acceptance of contact lens-based and sustained-release topical delivery systems.