Effect of mdr1a P-Glycoprotein Gene Disruption, Gender, and Substrate Concentration on Brain Uptake of Selected Compounds

Purpose. This study assessed the influence of mdr1aP-glycoprotein (P-gp) gene disruption, gender and concentration on initial brain uptake clearance (Clup) of morphine, quinidine and verapamil. Methods. Clupof radiolabeled substrates was determined in P-gp-competent and deficient [mdr1a(−/−)] mice by in situbrain perfusion. Brain:plasma distribution of substrates after i.v. administration was determined in both strains. Results. Genetic disruption of mdr1aP-gp resulted in 1.3-, 6.6- and 14-fold increases in Clupfor morphine, verapamil and quinidine, respectively. With the exception of small differences for verapamil, gender did not affect Clup. Saturable transport of verapamil and quinidine was observed only in P-gp-competent mice, with apparent IC50values for efflux of 8.6 ± 2.3 μM and 36 ± 2 μM, respectively. VerapamilClupwas ∼50% higher in mdr1a(+/−) vs.mdr1a(+/+) mice; no such difference was observed for quinidine. In P-gp-competent mice, uptake of verapamil and quinidine was unaffected by organic vehicles. Plasma decreased VER Clupto a greater extent in the presence of P-gp. The influence of P-gp in situwas lower than, but correlated with, the effect in vivo. Conclusions. P-gp decreases Clupof morphine, verapamil and quinidine in situwith little or no influence of gender, but this effect cannot fully account for the effects of P-gp in vivo. P-gp is the only saturable transport mechanism for verapamil and quinidine at the murine blood-brain barrier. The influence of protein binding on Clupmay be enhanced by P-gp-mediated efflux.