The Eye as a Drug Target
Ocular anatomy and drug penetration, the routes of administration, eyedrop pharmacology, and the systemic effects of ocular drugs.
The Eye as a Drug Target: Barriers and Routes
The eye is a strange place to deliver a drug. It is tiny, it is sealed off from the bloodstream by barriers as tight as the ones guarding the brain, and it is divided into front and back compartments that almost nothing crosses freely. A drop that floods the surface of the eye may reach the iris beautifully and never touch the retina an inch behind it. Almost every puzzle in ocular pharmacology — why anti-VEGF is injected through the wall of the eye, why glaucoma is treated with drops but macular degeneration is not, why an oral antibiotic barely dents an infection inside the globe — comes back to one question: can the molecule get to the compartment where the disease lives?
Eyedrop Pharmacology: Why Most of the Drop Is Wasted
The eyedrop looks like the simplest drug in medicine — squeeze the bottle, one drop, done. It is in fact one of the least efficient delivery routes in the whole pharmacopoeia. The eye is built to keep things out: the tear film it lands in holds barely a fifth of the drop, blinking and tear drainage flush the rest away within minutes, and only a small percentage ever crosses the cornea to reach its target. Almost everything about how ophthalmic drugs are formulated and prescribed is an attempt to cheat those losses. And the drug that does drain away doesn't vanish — it runs down into the nose and is absorbed straight into the bloodstream, which is how a drop meant for the eye can slow the whole heart.
Two-Way Traffic: When Eye Drops Go Systemic and Systemic Drugs Hit the Eye
It is tempting to think of the eye as a sealed compartment — that a drop you place on the surface stays in the eye, and a tablet you swallow never reaches it. Both assumptions are wrong, and each has killed or blinded patients. A drop of timolol can slow an asthmatic's heart and close their airways. A tablet taken for years for lupus can quietly destroy the retina. The eye and the body are in constant two-way traffic, and a clinician who forgets it will miss the diagnosis in both directions. This chapter is the map; the deep dives into each toxicity live in the Ocular Drug Toxicity subtopic.
