Ocular Pharmacology Flashcards
why are eyelids amenable to systemic meds?
what about topical?
- extensive vasculature
- epithelium of eyelids act as barriers to absorption of topical medications
systemic medications and the cornea - do they mix? which?
- cornea is an avascular structure, systemic meds have little effect
- The only systemic antibiotic known to concentrate in the tear film is tetracycline
what general type of antibiotics do we need for corneal ulcers and why?
- corneal ulceration requires topical bactericidal
antibiotics to limit destruction of tissue - stroma has no defense aside from epithelium, tear film, and blinking
- topical meds for cornea - issue with quantity
- The tear film dilutes ophthalmic drops because the conjunctival fornix can only
accommodate an additional 10-25 µl - maximum volume the conjunctival fornix is 30 ul after drop instillation, the dosing of one drop (50µl) far exceeds what amount that can be contained and therefore loss occurs through drainage
- The excess fluid volume is shunted to the nasolacrimal system
- what properties must a topical drug have to penetrate the conrea in the anterior chamber
- corneal epithelium and endothelium are lipophilic and hydrophobic
- corneal stroma is hydrophilic and lipophobic
- medications with both these
properties will better penetrate the cornea into the anterior chamber (in other words, the drug
must be lipid and water-soluble to fully penetrate the cornea into the anterior chamber)
topicals - drug ionization and molecular size, affect on penetration
- when might penetration be better?
- Unionized drugs readily penetrate intact epithelium
- water-soluble drugs will not penetrate
epithelium
<><> - tight junction of the epithelium and endothelium prevent entrance to large
molecules - epithelial abrasions or ulceration will enhance topical drug entrance
- Corneal neovascularization will also open a passageway to systemically administered medications
what drugs can be delivered to the anterior chamber?
- when is it easiest?
- low molecular weight nonprotein-bound
lipophilic drug molecules can penetrate the blood-aqueous barrier of the anterior uvea
<><> - In cases of uveitis, where uveal blood
vessels become permeable, systemically administered medications can gain entrance into the anterior chamber and attain therapeutic levels
how to deliver drugs to the posterior segment?
- what makes this easier?
- The blood retinal barrier will limit passage of systemic drug molecules into the posterior segment tissues and vitreous
<><> - Posterior uveitis (choroidal and
retinal inflammation) can allow drug molecules to enter posterior segment tissues as well as
the vitreal chamber
With topical ophthalmic medications, independently of formulation, drug penetration is directly proportional to:
- drug concentration
- Frequency of application will also increase drug penetration
topical solutions
- how much is carried away when applied?
- is there systemic absorption?
- a portion is absorbed by the conjunctiva and cornea
- the majority (80%) is carried away into the nasolacrimal system
<><><><> - Systemic absorption of topical solutions can be clinically significant
topical solutions in cases of excessive lacrimation
- tear film dilutes ophthalmic drops prior to absorption by the surrounding tissues
<><> - drug concentration can be increased (2% versus 1%)
- frequency of administration can be
increased (every 2-4 hours) - passage through epithelial cells can be facilitated with the addition of benzakolium chloride
how fast are topical drugs absorbed?
do therapeutic levels reach all parts of the eye?
ophthalmic drops are absorbed within 5-10 minutes of administration
<><>
- Therapeutic levels of ophthalmic drops can be detected in the anterior chamber and as far posterior as the ciliary body; however, adequate drug concentrations may not reach the posterior segment tissues.
what to do before applying aqueous suspensions?
aqueous suspensions require repeated shaking of the bottle (40 times) for maximal deliverable effects.
advantages of topical oitments
- longer contact time of drug to surface tissues (therefore potentially fewer treatments per day)
- lack of dilution by tears
- smaller amount of drug entering the nasolacrimal system (therefore potentially less systemic effects)
- lubrication and protection of the cornea due to its petroleum vehicle
blinking effect on topical oitments
Blinking boosts the release
of drug from ointments
