Effect of mdr1a P-glycoprotein gene disruption, gender, and substrate concentration on brain uptake of selected compounds

Purpose. This study assessed the influence of mdr1a P-glycoprotein (P-gp) gene disruption, gender and concentration on initial brain uptake clearance (Cl_up) of morphine, quinidine and verapamil.Methods. Cl_up of radiolabeled substrates was determined in P-gp-competent and deficient [mdr1a(–/–)] mice by in situ brain perfusion. Brain:plasma distribution of substrates after i.v. administration was determined in both strains.Results. Genetic disruption of mdr1a P-gp resulted in 1.3-, 6.6- and 14-fold increases in Cl_up for morphine, verapamil and quinidine, respectively. With the exception of small differences for verapamil, gender did not affect Cl_up. Saturable transport of verapamil and quinidine was observed only in P-gp-competent mice, with apparent IC₅₀ values for efflux of 8.6 ± 2.3 M and 36 ± 2 M, respectively. VerapamilCl_up was 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 Cl_up to a greater extent in the presence of P-gp. The influence of P-gp in situ was lower than, but correlated with, the effect in vivo.Conclusions. P-gp decreases Cl_up of morphine, verapamil and quinidine in situ with 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 Cl_up may be enhanced by P-gp-mediated efflux.