Drug handling in the eye Flashcards
What is the major route of absorption in the eye?
Cornea
What are the methods of drug administration in the eye?
Corneal Non corneal (episclera, conjunctival Direct intra-ocular administration
What factors affect intra-ocular penetration of drugs?
- permeability of cornea to drug
- lacrimation
- tear drainage
- composition of tear film
What is the most common route of drug application in the eye?
Inferior fornix of the conjunctiva
What is the capacity of the conjunctival sac?
15-30 uL
What volume does the tear film occupy in the conjunctival sac?
7-8uL
What is the normal blink rate?
15-20 blinks per minute
What volume do solution applications normally apply?
50-100uL per drop
Hence, a lot of the drug is lost to overspill
Where can drugs drain into from the inferior fornix?
They drain into the nasolacrimal duct and into the nose, at which point they can be absorbed by the nasal and nasopharyngeal mucosa; hence the drug can enter the systemic system. This can be avoided by changing the composition of the drug from a liquid to an ointment or by occluding the drainage into the naso-lacrimal drug for 5 minutes
What is the pH of normal tears?
6.5 to 7.6
What are the layers of the tear film?
Outer lipid layer
Middle aqueous layer
Inner mucin layer: mucin layer contributes to the stability of the tear film
Is the corneal epithelium lipophilic or phobic?
lipophilic
What is the most important barrier to intraocular penetration of drug?
corneal epithelium: desmosomes between the cells and hydrophobic cell membrane
Is transport across the endothelium of cornea free or restricted?
Free, gap junction between cells, so substance can easily move into the aqueous
How can a drug be altered to make it more soluble in the cornea?
Increase the pH: so it is non-ionised
Lipid soluble
