Purpose: The purpose of this study was to evaluate partitioning into and transport across posterior segment tissues (sclera, retinal pigment epithelium (RPE)-choroid) of AL-4940, the active metabolite of angiostatic cortisene anecortave acetate (AL-3789).
Methods: Transport of [(14)C]-AL-4940 was measured through RPE-choroid-sclera (RCS) and sclera, excised from Dutch Belted pigmented rabbits' eyes, in the directions of scleral to vitreal (S-->V) and vitreal to scleral (V-->S) for 3 h at 37 degrees C using Ussing chambers. Tissue integrity was monitored by transepithelial electrical resistance (TEER), potential difference (PD), and biochemical assay (LDH). Partitioning in RPE-choroid and sclera was determined separately for both [(14)C]-AL-4940 and [(14)C]-AL-3789. Mathematical analysis for bilaminate membranes used partitioning and transport data to derive diffusion coefficients for 2 tissue layers sclera and RPE-choroid.
Results: Partitioning of drug in tissue was comparable for both [(14)C]-AL-4940 and [(14)C]-AL-3789. Partition coefficients of drug in tissue were 2.2 for sclera and about 4 for RPE-choroid. Permeability through sclera alone was about 3 x 10(-5) cm/s and about 1 x 10(-5) cm/s through the RCS tissue, irrespective of the direction of transport (S-->V) or (V-->S). Results from bioelectrical and biochemical evaluation of tissue with modified LDH assay provided evidence that the RCS tissue preparation remained viable during the period of transport study.
Conclusions: The thin RPE-choroid layer contributes significantly to resistance to drug transport, and diffusivity in this layer is 10 times less than in sclera. This experimental scheme is proposed as an important component for the development of a general ocular physiologically based pharmacokinetic model.