ophthalmic delivery

Ophthalmic delivery is the art of getting medicine onto or into the eye to treat eye conditions — infections, inflammation, dry eye, glaucoma. Because the eye is delicate and exquisitely sensitive, everything placed in it must be sterile and gentle, and the eye fights back against retention almost as soon as you instill a drop.

The central challenge is that the eye is built to flush things out. The tear film and blinking, plus rapid drainage through the tear ducts into the nose, clear most of an eye drop within a minute or two, so typically only a few percent of the dose actually penetrates the cornea to reach the target tissues. The cornea itself is a layered barrier — a lipid-rich epithelium sandwiching a watery stroma — that favours drugs able to be both somewhat fat- and water-soluble. Formulations must also match the eye's tonicity and a comfortable pH to avoid stinging and reflex tearing that wash the drug away even faster.

Dosage forms range from simple sterile eye drops and ointments to gels, suspensions, in-situ gelling solutions that thicken on contact, and inserts or implants that sit in the eye and release drug for weeks. Preservatives keep multi-dose bottles sterile but can irritate, which is why single-use, preservative-free units exist. Delivery to the back of the eye (retina) is especially hard and often still requires injection into the eye.