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P-Glycoprotein Contributes to the Blood-Brain, but Not Blood-Cerebrospinal Fluid, Barrier in a Spontaneous Canine P-Glycoprotein Knockout Model

Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, Washington (K.L.M., S.G., R.B., J.G., R.T., P.G.); and Department of Pharmacokinetics, Dynamics, and Drug Metabolism, Pfizer Global Research and Development, Groton Laboratories, Groton, Connecticut (K.S., F.N.)

P-glycoprotein is considered to be a major factor impeding effective drug therapy for many diseases of the central nervous system (CNS). Thus, efforts are being made to gain a better understanding of P-glycoprotein's role in drug distribution to brain parenchyma and cerebrospinal fluid (CSF). The goal of this study was to validate and introduce a novel P-glycoprotein–deficient (ABCB1-1) canine model for studying P-glycoprotein–mediated effects of drug distribution to brain tissue and CSF. CSF concentrations of drug are often used to correlate efficacy of CNS drug therapy as a surrogate for determining drug concentration in brain tissue. A secondary goal of this study was to investigate the validity of using CSF concentrations of P-glycoprotein substrates to predict brain tissue concentrations. Loperamide, an opioid that is excluded from the brain by P-glycoprotein, was used to confirm a P-glycoprotein–null phenotype in the dog model. ABCB1-1 dogs experienced CNS depression following loperamide administration, whereas ABCB1 wild-type dogs experienced no CNS depression. In summary, we have validated a novel P-glycoprotein–deficient canine model and have used the model to investigate transport of the P-glycoprotein substrate ^99mTc-sestamibi at the blood-brain barrier and blood-CSF barrier.