Quantitative Evaluation of the Impact of Active Efflux by P-Glycoprotein and Breast Cancer Resistance Protein at the Blood-Brain Barrier on the Predictability of the Unbound Concentrations of Drugs in the Brain Using Cerebrospinal Fluid Concentration as a Surrogate

Abstract

This study investigated the impact of the active efflux mediated by P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) at the blood-brain barrier (BBB) on the predictability of the unbound brain concentration (C u,brain ) by the concentration in the cerebrospinal fluid (CSF) (C u,CSF ) in rats. C u,brain is obtained as the product of the total brain concentration and unbound fraction in the brain (f u,brain ) determined in vitro in brain slices. Twenty-five compounds, including P-gp and/or Bcrp substrates, were given a constant intravenous infusion, and their plasma, brain, and CSF concentrations were determined. P-gp and/or Bcrp substrates, such as verapamil, loperamide, flavopiridol, genistein, quinidine, dantrolene, daidzein, cimetidine, and pefloxacin, showed a higher CSF-to-brain unbound concentration ratio (K p,uu,CSF/brain ) compared with non-P-gp and non-Bcrp substrates. K p,uu,CSF/brain values of P-gp-specific (quinidine and verapamil) and Bcrp-specific (daidzein and genistein) substrates were significantly decreased in Mdr1a/ 1b(Ϫ/Ϫ) and Bcrp(Ϫ/Ϫ) mice, respectively. Furthermore, consistent with the contribution of P-gp and Bcrp to the net efflux at the BBB, K p,uu,CSF/brain values of the common substrates (flavopiridol and erlotinib) were markedly decreased in Mdr1a/ 1b(Ϫ/Ϫ)/Bcrp(Ϫ/Ϫ) mice, but only moderately or weakly in Mdr1a/1b(Ϫ/Ϫ) mice and negligibly in Bcrp(Ϫ/Ϫ) mice. In conclusion, predictability of C u,brain by C u,CSF decreases along with the net transport activities by P-gp and Bcrp at the BBB. C u,CSF of non-P-gp and non-Bcrp substrates can be a reliable surrogate of C u,brain for lipophilic compounds.

An Evaluation of Using Rat-Derived Single-Dose Neuropharmacokinetic Parameters to Project Accurately Large Animal Unbound Brain Drug Concentrations

“…3; Table 3). For thiopental, the one outlier compound, our C b,u /C p,u (0.86) is equivalent to the steady-state value (0.91) shown by Kodaira et al (2011). This fact, and the comparison of all thiopental interneurocom- (Table 6) by the rat acute dose-derived ratio (Table 2).…”

“…3). It is interesting to note that disclosed (Kodaira et al, 2011) steady-state-derived Sprague-Dawley rat C b /C p (0.66) and C b,u / C p,u (0.91) for thiopental are within 1.5-and 1.1-fold, respectively, of our acute dose-derived ratios, but 2.5-and 4.0-fold higher than those ratios reported by Liu et al (2009a) (Table 2). These data, combined with equivalent C CSF /C p,u [0.60 (Liu et al, 2009a) dose-and steady-state-derived interneurocompartmental parameters are indeed equal in rats.…”

Drug Transporters on Arachnoid Barrier Cells Contribute to the Blood–Cerebrospinal Fluid Barrier

“…For P-gp substrates, conventional knowledge posits that CSF drug concentration is only influenced by BBB and CP P-gp (Kodaira et al, 2011). Our studies put forth a new paradigm: CSF drug concentration is strongly influenced by AB P-gp and BCRP.…”

“…It would be expected that drug transporters expressed in AB cells would influence the concentration of intrathecally administered drugs that remain in the CSF, their rate of egress from the CSF, and hence, the concentration available to the CNS. Second, the concentration of drugs in the CSF is often used as a surrogate for that of unbound drug in the interstitial fluid in the brain and, hence, available to drug targets (Kodaira et al, 2011). Thus, identification of transporters localized in the AB cells adjacent to the CSF space is necessary to correctly interpret exposure-response relationships for CNS-acting drugs.…”

In Vitro P-glycoprotein Efflux Ratio Can Predict the In Vivo Brain Penetration Regardless of Biopharmaceutics Drug Disposition Classification System Class

“…In the BBB classification by Broccatelli et al, human unbound CSF/plasma ratio data trumped brain penetration data from other species when the unbound brain/plasma ratio in humans was unavailable. This could potentially lead to a misclassification in the BBB assignment because unbound CSF/plasma ratio is recognized as a poor surrogate for brain penetration for compounds that are actively extruded from the brain by efflux transporters (Kodaira et al, 2011); i.e., a drug concentration in the CSF may severely overestimate the unbound concentration in the brain for P-gp substrates because P-gp is not expressed on the plasma membrane of choroid plexus epithelial cells facing the blood, unlike the BBB.…”