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ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION

Role of Site-Specific Binding to Plasma Albumin in Drug Availability to Brain

Department of Pharmaceutical Sciences, School of Pharmacy, Texas Tech University Health Sciences Center, Amarillo, Texas (H.M., J.M.R.P., Q.R.S.); and Department of Pharmaceutical Sciences, University of Michigan, College of Pharmacy, Ann Arbor, Michigan (R.F.)

Many studies have reported greater drug uptake into brain than that predicted based upon existing models using the free fraction (f_u) of drug in arterial serum. To explain this difference, circulating plasma proteins have been suggested to interact with capillary membrane in vivo to produce a conformational change that favors net drug dissociation and elevation of f_u. Albumin, the principal binding protein in plasma, has two main drug binding sites, Sudlow I and II. We tested this hypothesis using drugs that bind selectively to either site I (warfarin) or site II (ibuprofen), as well as mixed ligands that have affinity for both sites (tolbutamide and valproate). Brain uptake was determined in the presence and absence of albumin using the in situ rat brain perfusion technique. Unidirectional brain uptake transfer constants (K_in) were measured and compared with those predicted using the modified Kety-Crone-Renkin model: K_in = F(1 – e^{–f_u x PS_u/F}), where F is perfusion flow and PS_u is the permeability-surface area product to free drug of brain capillaries. The results demonstrated good agreement between measured and predicted K_in over a 100-fold range in perfusion fluid albumin concentration using albumin from three different species (i.e., human, bovine, and rat), as well as whole-rat serum. K_in decreased in the presence of albumin in direct proportion to perfusion fluid f_u with constant PS_u. The results show that brain uptake of selected Sudlow site I and II ligands matches that predicted by the modified Kety-Crone-Renkin model with no evidence for enhanced dissociation.

Address correspondence to: Dr. Quentin R. Smith, Department of Pharmaceutical Sciences, School of Pharmacy, Texas Tech University Health Sciences Center, 1300 Coulter, Amarillo, TX 79106. E-mail: quentin.smith{at}ttuhsc.edu

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