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Drug Metabolism and Disposition Fast Forward
First published on January 19, 2007; DOI: 10.1124/dmd.106.012294

0090-9556/07/3504-660-666$20.00
DMD 35:660-666, 2007

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Use of Plasma and Brain Unbound Fractions to Assess the Extent of Brain Distribution of 34 Drugs: Comparison of Unbound Concentration Ratios to in Vivo P-Glycoprotein Efflux Ratios

J. Cory Kalvass, Tristan S. Maurer, and Gary M. Pollack

School of Pharmacy, University of North Carolina, Chapel Hill, North Carolina (J.C.K., G.M.P.); and Department of Pharmacokinetics, Dynamics, and Metabolism, Pfizer Global Research and Development, Groton, Connecticut (T.S.M.)

The P-glycoprotein (P-gp)-deficient mouse model is used to assess the influence of P-gp-mediated efflux on the central nervous system (CNS) distribution of drugs. The steady-state unbound plasma/unbound brain concentration ratio ([plasma],u/[brain],u) is an alternative method for assessing CNS distribution of drugs independent of the mechanism(s) involved. The objective of this study was to compare the degree of CNS distributional impairment determined from the in vivo P-gp efflux ratio with that determined from the [plasma],u/[brain],u ratio. CNS distribution of 34 drugs, including opioids, triptans, protease inhibitors, antihistamines, and other clinically relevant drugs with either poor CNS distribution or blood-brain barrier efflux, was studied. Plasma and brain unbound fractions were determined by equilibrium dialysis. Kp,brain and the P-gp efflux ratio were obtained from the literature or determined experimentally. The P-gp efflux ratio and the [plasma],u/[brain],u ratio were in concurrence (<3-fold difference) for 21 of the 34 drugs. However, the [plasma],u/[brain],u ratio exceeded the P-gp efflux ratio substantially (>4-fold) for 10 of the 34 drugs, suggesting that other, non-P-gp-mediated mechanism(s) may limit the CNS distribution of these drugs. The P-gp efflux ratio exceeded the [plasma],u/[brain],u ratio by more than 3-fold for three drugs, suggesting the presence of active uptake mechanism(s). These observations indicate that when mechanisms other than P-gp affect CNS distribution (non-P-gp-mediated efflux, poor passive permeability, cerebrospinal fluid bulk flow, metabolism, or active uptake), the P-gp efflux ratio may underestimate or overestimate CNS distributional impairment. The [plasma],u/[brain],u ratio provides a simple mechanism-independent alternative for assessing the CNS distribution of drugs.

Address correspondence to: Gary M. Pollack, C.B. 7360, University of North Carolina, Chapel Hill, NC 27599-7360. E-mail: gary_pollack{at}unc.edu




This article has been cited by other articles:


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 J. Pharmacol. Exp. Ther.Home page
J. C. Kalvass, E. R. Olson, M. P. Cassidy, D. E. Selley, and G. M. Pollack
Pharmacokinetics and Pharmacodynamics of Seven Opioids in P-Glycoprotein-Competent Mice: Assessment of Unbound Brain EC50,u and Correlation of in Vitro, Preclinical, and Clinical Data
J. Pharmacol. Exp. Ther., October 1, 2007; 323(1): 346 - 355.
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