Topical Administration: Ointments
Another strategy for increasing the contact time of ocular medications is through the use of ointments. Commercial oil-based ointments usually consist of petrolatum and mineral oil. The mineral oil allows the ointment to melt at body temperature. Both ingredients are also effective lipid solvents. However, most water-soluble medications are insoluble in the ointment and are present as microcrystals. Only those microcrystals on the surface of the ointment dissolve in the tears; the rest are trapped until the ointment melts. Such protracted, slow release may prevent the drug from reaching a therapeutic level in the tears. Only when the drug has high lipid solubility (which allows it to diffuse through the ointment) and some water solubility can it escape from the ointment into both the corneal epithelium and the tears. Fluorometholone, chloramphenicol, and tetracycline are examples of drugs that achieve higher aqueous levels when administered as ointment than as drops.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.