Drug penetration to the back of the eye is poor due to the anterior barriers -cornea, aqueous turnover, lens, and blood flow in the anterior uvea. Also, penetration of a systemically circulating drug to the vitreous humour is poor. The blood-vitreous barrier consists of the retinal pigment epithelium and the endothelium of the retinal capillaries (Maurice and Mishima, 1984). Consequently, local intravitreal drug administration has gained increasing interest as a possible method for drug administration of antibiotics in endo-pthalmitis and also in drug treatment of proliferative vitreoretinopathy (Niesman, 1992).
Intravitreally injected drug spreads gradually to the vitreous space. Diffusion in the vitreous is rapid because the collagen concentration in the vitreous gel is low (Maurice and Mishima, 1984). Drugs are eliminated from the vitreous humour via the retina and via the anterior chamber. The rate of drug elimination via the anterior route is determined by molecular weight. The half-life of vitreal elimination for a relatively small molecule such as gentamicin is 1 day, but the half-lives for macromolecules are several days (Maurice and Mishima, 1984). The retinal route contains both passive and active components. Lipid-soluble drugs may be transported across the blood-vitreous barrier and active transport has been described for indomethacin. Both factors may increase the elimination rate so that the half-lives of many antibiotics in the vitreous humour are only a few hours (Maurice and Mishima, 1984).
Sustained release medications have been explored for intravitreal administration because of the rapid drug elimination from the vitreous humour. A second aim is to avoid the retinal drug toxicity that is related to high drug concentrations after intravitreal injection of drug solution.
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