In vitro Transscleral Permeability of Radiolabeled Compounds Correlated With Scleral Thickness in Human, Cynomologus Monkey, Dog, and Rabbit Sclera

Abstract

Purpose: : To determine the in vitro scleral permeability of several compounds with varied molecular weights in 4 different species (human, monkey, dog, rabbit) and correlate the permeability to scleral thickness.

Methods: : Scleral sections excised from moist chamber stored human poles or obtained from euthanized NZW rabbit, beagle dog, or cynomolgus monkey eyes were mounted in a two compartment perfusion chamber. An adjacent piece of sclera was excised and placed in fixative for H&E histology. Before excising the sclera (in the animal whole globes), scleral thickness measurements were taken using an Accupath 5 ultrasonic pachymeter. A small depot (150 ul) of either ³H-water, ³H-dexamethasone (392 MW), or ¹⁴C-dextran (70 kD) was added to the episcleral surface while perfusing BSS slowly across the choroidal side. The perfusate was collected every 30 min. for 5 hrs, the samples were counted on a liquid scintillation counter and scleral permeability (ktrans) was calculated. H&E slides, from the adjacent scleral piece, were also used to determine scleral thickness using digital images analyzed with Image J.

Results: : Cynomolgus monkey sclera measured the thinnest, consistently by ultrasound and H&E measurements, followed in order by dog, rabbit, and human. This thickness data inversely correlated with the largest MW compound (Dextran 70 kD) ktrans data, where Ktrans values were greatest in monkey, followed by dog, rabbit, and human. There was also an inverse correlation in the dexamethasone (392 MW) data with the largest ktrans values in monkey, followed by dog, human, and rabbit. For tritiated water, all of the ktrans values were similar, regardless of the species.

Conclusions: : Scleral permeability ktrans measurements can be correlated with scleral thickness data and indicate that the rabbit and dog may be better transscleral animal models to compare with human than cynomolgus monkeys. This model can be used to screen ophthalmic drugs intended to reach the back of the eye.

Keywords: sclera • retina • drug toxicity/drug effects