The goal of this work was to investigate the use of MDCK (Madin-Darby canine kidney) cells as a possible tool for assessing the membrane permeability properties of early drug discovery compounds. Apparent permeability (Papp) values of 55 compounds with known human absorption values were determined using MDCK cell monolayers. For comparison, Papp values of the same compounds were also determined using Caco-2 cells, a well-characterized in vitro model of intestinal drug absorption. Monolayers were grown on 0. 4-microm Transwell-COL membrane culture inserts. MDCK cells were seeded at high density and cultured for 3 days, and Caco-2 cells were cultured under standard conditions for 21 to 25 days. Compounds were tested using 100 microM donor solutions in transport medium (pH 7.4) containing 1% DMSO. The Papp values in MDCK cells correlated well with those in Caco-2 cells (r2 = 0.79). Spearman's rank correlation coefficient for MDCK Papp and human absorption was 0.58 compared with 0.54 for Caco-2 Papp and human absorption. These results indicate that MDCK cells may be a useful tool for rapid membrane permeability screening.

A method for monitoring formation of latex particle pairs by chemilumnescence is described. Molecular oxygen is excited by a photosensitizer and an antenna dye that are dissolved in one ofthe particles. 1A02dlffse to the second particle and initiates a hi quantum yield chemilu ent reaction of an olefin that is dissolved in it. An alternative homogeneous method utilizes immunochemical aggregation of ligand-or receptor-labeled particles (latex agglutination) (3). Detection of agglutination by conventional light scattering requires formation of large aggregates and has limited sensitivity. Low-angle light scattering measurements provide higher sensitivity but require rigorously exclusion of adventitious particles (4,5).In the present study, nonenzymatic channeling of 1.02 is used to provide exquisitely sensitive real-time monitoring of particle-particle interactions. The prepared by adding concentrated ammonia to 500-kDadextran (Pharmacia) that had been activated by reaction with epichlorohydrin and Zn(BF4)2. The product was purified by precipitation with methanol. Fluorescein-biotin (Fl-biotin) was prepared by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC) coupling of Fl-5'-COOH to 1,12-diamino-4,9-dioxadodecane followed by reaction with biotinyl-6-aminohexanoic acid N-hydroxysuccinimide ester (biotin-LC-NHS, Pierce). Bis-(6-hydroxyhexyl) disulfide and 6-amino-

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