Hydraulic flow and vascular clearance influences on intravitreal drug delivery.

PURPOSE: A recent paper proposed a model for hydraulic flow inthe eye, claiming this could affect intravitreal drug administration. The impact of flow on various modes of administration was investigated in a physiologically accurate ocular model of the rabbit eye.
METHODS: Hydraulic flow initiated at the hyaloid was simulated in a three-dimensional finite element model including effects of convection and episcleral efflux. The interrelation between hydraulic and vascular clearance was treated using a method in which choroidal clearance is effected by simple boundary conditions, diminishing computing requirements. Drug diffusion coefficient and clearance rates for the choroid and anterior chamber were varied.
RESULTS: Volumes and velocities of fluid flow permeating the vitreous agreed with literature values. Hydraulic flow impacted clearance of compounds not eliminated by the choroid; agreement with experimental data justified assuming perfect aqueous humor mixing. Hypertensive pressure produced up to a maximum 4-fold change in vitreal drug content from an intravitreal device depending upon location, orientation of the releasing surface, but was less important than vascular clearance strength and diffusion coefficient.
CONCLUSIONS: The influence of intraocular pressure ([OP)-induced hydraulic flow is not likely to be of clinical significance for low molecular weight drugs that are efficiently cleared by the choroid.