3. What are the factors that govern the penetration of drugs into the eye? 
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

  • size of molecule: smaller ones tend to penetrate better than big one
  • protein binding: highly protein bound drugs does not penetrate well
  • lipid solubility: highly lipid soluble drugs penetrate better than water soluble one
Ocular barriers influencing drug penetration
  • 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
  • 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
  • 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
  • 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

  • Inflammation or infection can lead to break down of the ocular barriers leading to increased drug penetration

Mode of delivery

  • 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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