| The factors can be divided into drug factors, the natural barriers
of the eyes, the health status of the eyes and mode of drug administration..
The drug factors
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size of molecule: smaller ones tend to penetrate better than big one
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protein binding: highly protein bound drugs does not penetrate well
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lipid solubility: highly lipid soluble drugs penetrate better than water
soluble one
Ocular barriers influencing drug penetration
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Epithelial barrier: the corneal epithelium restricts the entry of
water-soluble drugs into the cornea and aqueous humour. It is breached
by an epithelial defect or if the epithelium is intact, is by-passed by
subconjunctival injection
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Aqueous-vitreous barrier: bulk flow of aqueous humour from the eye
and the presence of intact lens and zonule retard the diffusion of drugs
from the anterior chamber into the vitreous humour
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Blood-aqueous barrier: this limits entry into the aqueous from the
blood. The epithelium of the iris and ciliary body pump anionic drugs from
the aqueous into the blood stream
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Blood-retinal barrier: limits the entry of drugs into the eye from
the systemic circulation: external , the pigment epithelial barrier, internal,
the retinal capillary endothelial barrier. There is an outward pumping
anions across the retina by the RPE and the endothelial cells of the retinal
vessels.
Health status of the eye
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Inflammation or infection can lead to break down of the ocular barriers
leading to increased drug penetration
Mode of delivery
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Drug penetration of the eye can be increased by directly delivering the
drugs into the compartment desired. For example intravitreous antiobiotics
in endophthalmitis
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