Drug distribution in the vitreous humor of the human eye: the effects of intravitreal injection position and volume.

PURPOSE: The purpose of this study was to determine how the position and volume of an intravitreal injection affect the distribution and elimination of drug from the vitreous humor. METHODS: A mathematical model that had been developed and used previously to study drug distribution in the vitreous humor of the rabbit eye was modified to match the physiology of the human eye. Fluorescein and fluorescein glucuronide were used as the model compounds for these studies. Four extreme injection locations were considered: a central injection, an injection displaced towards the retina, an injection displaced towards the lens, and an injection displaced toward the hyaloid membrane. Injections containing an equal mass of drug dissolved in volumes of either 15 microL or 100 microL were compared. RESULTS: The location of an intravitreal injection was found to have a substantial effect on elimination and distribution in the vitreous. Peak concentrations at different vitreous locations varied by over three orders of magnitude, depending on the injection location. The mean concentration of drug remaining in the vitreous 24 hours after the intravitreal injection varied by up to a factor of 3.8, depending on the injection location. Changing the volume of the injection from 15 microL to 100 microL dampened the effects of the initial injection location; however, meant concentrations at 24 hours still varied by up to a factor of 2.5. CONCLUSIONS: Careful control of the conditions of an intravitreal injection could reduce treatment variability, improve bioavailability, and reduce the possibility of retinal toxicity.