Abstract
Currently used methodology for determining unbound drug exposure in brain combines measurement of the total drug concentration in the whole brain in vivo with estimation of brain tissue binding from one of two available in vitro methods: equilibrium dialysis of brain homogenate and the brain slice uptake method. This study of 56 compounds compares the fraction of unbound drug in brain (fu,brain), determined using the brain homogenate method, with the unbound volume of distribution in brain (Vu,brain), determined using the brain slice method. Discrepancies were frequent and were primarily related to drug pH partitioning, attributable to the preservation of cellular structures in the slice that are absent in the homogenate. A mathematical model for pH partitioning into acidic intracellular compartments was derived to predict the slice Vu,brain from measurements of fu,brain and drug pKa. This model allowed prediction of Vu,brain from fu,brain within a 2.2-fold error range for 95% of the drugs compared with a 4.5-fold error range using the brain homogenate fu,brain method alone. The greatest discrepancies between the methods occurred with compounds that are actively transported into brain cells, including gabapentin, metformin, and prototypic organic cation transporter substrates. It was concluded that intrabrain drug distribution is governed by several diverse mechanisms in addition to nonspecific binding and that the slice method is therefore more reliable than the homogenate method. As an alternative, predictions of Vu,brain can be made from homogenate fu,brain using the pH partition model presented, although this model does not take into consideration possible active brain cell uptake.
Footnotes
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.110.035998.
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The online version of this article (available at http://dmd.aspetjournals.org) contains supplemental material.
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ABBREVIATIONS:
- BBB
- blood-brain barrier
- ISF
- interstitial fluid
- MPP
- 1-methyl-4-phenylpyridinium
- P-gp
- P-glycoprotein
- BCRP
- breast cancer resistance-associated protein
- MRP
- multidrug resistance protein
- TEA
- tetraethylammonium
- D22
- decynium-22
- OCT
- organic cation transporter
- MK-571
- 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid
- MS
- mass spectrometry
- CIR
- confidence interval ratio.
- Received August 20, 2010.
- Accepted December 13, 2010.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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