Vol. 288, Issue 3, 1317-1326, March 1999

Increased Lipophilicity and Subsequent Cell Partitioning Decrease Passive Transcellular Diffusion of Novel, Highly Lipophilic Antioxidants

Geri A. Sawada, Craig L. Barsuhn, Barry S. Lutzke, Michael E. Houghton¹ , Guy E. Padbury, Norman F. H. Ho² and Thomas J. Raub

Drug Absorption and Transport (G.A.S., C.L.B., N.F.H., T.J.R.), Discovery Technologies (B.S.L.), Pharmaceutics (M.E.H.), and Drug Metabolism Research (G.E.P.), Pharmacia & Upjohn, Inc., Kalamazoo, Michigan

Oxidative stress is considered a cause or propagator of acute and chronic disorders of the central nervous system. Novel 2,4-diamino-pyrrolo[2,3-d]pyrimidines are potent inhibitors of iron-dependent lipid peroxidation, are cytoprotective in cell culture models of oxidative injury, and are neuroprotective in brain injury and ischemia models. The selection of lead candidates from this series required that they reach target cells deep within brain tissue in efficacious amounts after oral dosing. A homologous series of 26 highly lipophilic pyrrolopyrimidines was examined using cultured cell monolayers to understand the structure-permeability relationship and to use this information to predict brain penetration and residence time. Pyrrolopyrimidines were shown to be a more permeable structural class of membrane-interactive antioxidants where transepithelial permeability was inversely related to lipophilicity or to cell partitioning. Pyrrole substitutions influence cell partitioning where bulky hydrophobic groups increased partitioning and decreased permeability and smaller hydrophobic groups and more hydrophilic groups, especially those capable of weak hydrogen bonding, decreased partitioning, and increased permeability. Transmonolayer diffusion for these membrane-interactive antioxidants was limited mostly by desorption from the receiver-side membrane into the buffer. Thus, in this case, these in vitro cell monolayer models do not adequately mimic the in vivo situation by underestimating in vivo bioavailability of highly lipophilic compounds unless acceptors, such as serum proteins, are added to the receiving buffer.