Ocular drug delivery systems or dosage forms range from the most common eye drops and other conventional formulations that are dosed daily to more complex implant systems that can be dosed once every few years. Conventional dosage forms like solutions, suspensions, emulsions, and ointments are only able to treat a limited number of ocular diseases. Each ocular tissue layer might act like a barrier based on drug physicochemical properties, drug carrier properties, and clearance mechanisms of a given route of administration. Thus, a delivery system or approach should be optimized for a given target tissue. For drug delivery purposes, the eye can be divided into two major segments, the anterior segment from the front of the eye to the lens and the posterior segment including eye tissues beyond the lens.
Ocular barriers are generally specific for application route. The main administration routes for ocular drug delivery include topical, periocular, intraocular, and systemic.
Topical administration is the most common route for treating diseases of the anterior segment of the eye, due to ease of application, drug localization and adequate efficacy, and low cost. However, only about 30-50 pl of ophthalmic solution is delivered using a dropper, due to limited holding capacity of the precorneal area. Oral and parenteral applications are the most common methods for systemic delivery, with the oral route being more convenient. Even though systemic admin-istration might be useful in treating posterior segment eye diseases, high doses and frequent dosing may be required since there are various limitations including extensive drug dilution in the blood, low cardiac output to the eye, and blood- ocular barriers that restrict drug permeability.
Several transporters including influx and efflux transporters are present in the cornea, conjunctiva, retina and blood-ocular barriers, which may influence drug bioavailability [1, 2]. Modifications targeting these transporters might be an alternative approach to improve ocular bioavailability of drugs. Increasing the retention time in the precorneal area is one of the main approaches to enhance ocular bioavailability. Mucoadhesion, which refers to attachment to mucus either by hydrogen bonding or electrostatic binding with mucin layer, may influ-ence drug absorption . Hydrogels have a variety of applications in ophthalmology including in situ gelling formulations, soft contact lenses, foldable intraocular lenses, and ocular adhesives for wound repair. High water content of hydrogels may be advantageous in preserving peptide/protein stability. Chemically cross-linked temperature-sensi-tive hydrogels that have high water content and retain transparency have been used as in situ forming gels .
The enzymes in the ocular tissues play an important role in the conversion of prodrug to drug. Esterases and amidases are the most common enzymes in ocular tissues with high enzyme activity detected in the iris-ciliary body, cornea, and aqueous humor [5, 6].
Contact lenses provide an alternative approach for sustained drug delivery on the ocular surface and beyond. Polymethyl methacrylate was the first widely used polymer for the production of contact lenses, which were not able to allow adequate oxygen permeation for the cornea and had to be removed at night. This was a limitation for the use of contact lenses as a long-term drug delivery system. Highly oxygen-permeable silicone hydrogel contact lenses have overcome this issue, and contact lenses are now more promising as drug delivery systems [7, 8, 9].
Conclusions. Effective drug delivery for the treatment of ocular diseases has always been a challenge especially for the posterior segment, due to the anatomy of the eye, the ocular barriers, and the physiological changes caused by the nature of the diseases. Scientists continue to work on new drug delivery systems to enhance target access, extent of delivery, and duration of drug exposure in order to improve drug efficacy while reducing side effects, in the hope to ultimately improve patient benefit and convenience.
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