Drug Delivery To The Eye Flashcards
Protection mechanisms used in the eye
To protect cornea:
- eye lids
- reflex blinking
- tear film
- 3 layers
- upper = lipidic (prevents evaporation),
- intermediate = aqueous
- lower = mucus (prevents cornea from drying)
- drained into nasal cavity through nasolacrimal duct
How do we deliver drug to eye?
via Opthalmic delivery
Describe opthalmic delivery to anterior + posterior segment of the eye
Anterior segment
- accessible through cornea
- 90% of the dose is WASTED
- only 1 - 5% absorbed across cornea into anterior chamber
Posterior segment
- not accessible by topical delivery
- systemic delivery of very high doses = side effects
- intraocular injections = highly invasive
- polymeric implants for sustained release over months = surgery needed
Why is there low absorption into anterior segment of the eye?
Tear drainage
Absorption into systemic circulation
Protein binding
Corneal barrier
Formulations for drug delivery to the eye
eye drops
eye ointments
eye lotions
Describe the tear drainage mechanism
eye drops induce blinking
lost dose on the cheeks
excess drained into nose
contact time = 1 - 2 min
drug diluted by tear film = low concentration in contact with cornea
Why does Protein binding affect absorption into anterior segment?
It’s in tear film
Also in cornea + in aqueous binding humor
How can drug be absorbed into systemic circulation from the eye?
From the nasal mucosa
From GI tract after damage to the nose
From conjunctiva
How does Corneal barrier affect absorption into anterior segment?
Small surface area
Not permeable (multi-layered epithelium with tight junctions)
Drugs must cross cell layers (lipophilic) + stroma (hydrophilic) = correct hydrolipophilicity
How are Eye drops formulated?
Aqueous solutions/ suspensions
- solution must be free of particles
- prolonged effect w/ suspensions but risk of irritancy
Formulation is critical as cornea is sensitive
- pH close to 7.4
- Isotonic
Must be sterile
- preservatives required in mulidose containers (benzalkonium chloride)
- single dose containers oreferred
- max volume in each container = 10mL
- Max use = 1 month (7 days in hospital)
Drawback of eye drops
Very short residence time
Viscosity enhancers increase retention time
How to prolong drug retention using eye drops
Correct application
- drop inside lower lid, eye closed/nasolacrimal duct occluded by finger
Decrease drop volume
Inserts
- slow release of the drug
- not popular
Formulation as ointments or gels
- viscous formulations are retained in the eye corner
- blinking spreads them over cornea
- blur vision
Discuss eye ointments
Drug particles (max. 50 micromoles) dispersed in a semi-solid greasy base
Packed sterile in plastic/aluminium tubes fitted w/sterile canula
Preservatives not required as bacterial growth is unlikely (no water in med)
Use for 1 month
Discuss Eye lotions
Sterile + isotonic aqueous solutions used to wash eyes (to remove particles + irritants)
Packed in large container + supplied w/ plastic eye bath
Why isn’t preservatives required in eye ointments
Bacterial growth is unlikely due to no water in medicine
