Eye drops, ointments, intravitreal eye injections and sustained release implants are all common methods of delivery for medication to the eye. They differ in the route in which they are administered.
Systemic routes of treatment, in the form of pills (oral route) and intravenous injections, are rarely used because most of the medication is blocked by the blood-brain barrier and does not get inside the eye. In addition, high doses of systemic medications increase the chances of systemic complications.
The objective for all methods of delivery is to maximize the amount of medication that reaches the site or target tissue and minimize side effects in the rest of the body. Some delivery methods that work for one eye problem will not work for others.
Eye drops are commonly used to treat eye infections, allergies, glaucoma, inflammation, and to provide lubrication for dry eyes. Eye drops are great for anything affecting the outside of the eye and have some difficulty penetrating the eye.
The conjunctiva, cornea, and sclera are most effectively treated with solution or suspension eye drops. The difference between the two is particle sizes. A solution has extremely small molecules which makes it clear. A suspension has larger particles that makes it look cloudy.
Application tip: Each eye drop has a volume of about 32 microliters. ( A microliter is one millionth of a liter). The surface of the eye will accommodate only about 28 microliters. That means that one eye drop is more than the amount needed to cover the surface of your eye. So, if you need to instill two drops in your eye, wait about 5 minutes for the first drop to be absorbed before applying the second; otherwise, that second drop will just run down your cheek and be wasted.
Contact lenses and collagen shields soaked in antibiotics are also delivery methods for treating corneal infections. They act as a reservoir.
Ophthalmic ointments simply last longer than drops. Ointments are in a semisolid form that melts when applied. Your body heat makes the ointment melt and cover your eye.
Eye ointments are the preferred delivery method for treating eye lid problems and for nighttime applications of medicine that should have longer contact with the eye surface. Ointments are greasy and difficult to apply but are preferred for infants and very young children because they can be instilled more reliably and remain in the eye for longer.
These injections are used to treat problems with the retina. Eye drops, ointments, IV solutions, and oral medications will not penetrate the blood brain barrier and reach the retina. The only way to get medication to the retina is to inject it into the vitreous humor and allow the medications to diffuse to the retina.
Intravitreal injections are used to deliver anti-VEGF medication for the treatment of wet macular degeneration.
Durysta is now injected to treat glaucoma.
Sustained Release Drug Delivery Systems
This is a subtype of intravitreal injection that lasts long-term and reaches the retina. A small biodegradable device is injected into the eye where it releases medication over many months and then biodegrades. Currently these delivery systems are available for intraocular pressure-reducing medication for glaucoma (Durysta) and steroid treatment for retinal vascular occlusions (Ozurdex, Iluvien).