A Fly-over the Current Drugs Used in Ophthalmology

Although the external eye is readily accessible for drug administration, efficient drug delivery to this organ is difficult because it is a unique organ, in that it has several func- tional and structural protective mechanisms, such as blinking, permanent lacrimation and lacrimal drainage. the globe itself is protected physically by tough layers of sclera and cornea and the internal structures of the eye are isolated anatomically from the sys- temic circulation by blood-ocular barriers. the cornea will become more permeable when ulcerated and the uvea may increase the leakage of drugs when inflamed. topical drug use is not benign and should always be limited to the lowest effective concentra- tion, frequency and duration. the preservatives in ophthalmic drugs (such as Benzalko- nium chloride) are toxic to mammalian cells and will interfere with isolation and growth of microbes for diagnostic purposes. Care must be taken in the choice of drugs, so that the chosen drug will pass the cornea, conjunctiva and sclera or blood-ocular barrier and that the action of the drug will be limited to the ocular tissue. Drugs required in high concentration in the cornea, conjunctiva or uvea will be administered more frequently or by sub-conjunctival injection. Systemic administration can also be chosen for the uvea. if high concentrations are required within orbital tissues, systemic administration will be used.

ophthalmic solutions and suspensions (“drops”)
Drops are quickly diluted and eliminated from the eye by tears, so they should be used more often than ointment, certainly when increased lacrimation is present. the drugs will be absorbed by the conjunctiva and can be detected in the blood. ophthalmic solu- tions must fall within a pH between 4,5 to 9 to be comfortable and prevent increased tearing and blinking. the drop volume delivered by dropper bottles is in average about 40μL and the palpebral fissure is capable of holding only 25-30μL. Delivering more than one drop will only result in increased systemic absorbtion through the lacrimal duct and conjunctiva, waste of drug and irritation. the applied fluid is estimated to be completely washed out from the ocular surface within 10 minutes of application. the rate of wash out will be increased by larger volumes of drops. for the same reason, applying a drop of a second drug directly after the first drug, will reduce the availability of the first drug. i would recommend an interval of time between instillation of 2 drugs of at least 10 minutes. [...]