An in situ mouse brain perfusion model predictive of passive and carrier-mediated transport across the blood-brain barrier (BBB) was developed and applied to mdr1a P-glycoprotein (Pgp)-deficient mice [mdr1a(-/-)]. Cerebral flow was estimated from diazepam uptake. Physical integrity of the BBB was assessed with sucrose/inulin spaces; functional integrity was assessed with glucose uptake, which was saturable with a Km of approximately 17 mmol/L and Vmax of 310 mmol x 100 g(-1) x min(-1). Brain uptake of a Pgp substrate (colchicine) was significantly enhanced (two- to fourfold) in mdr1a(-/-) mice. These data suggest that the model is applicable to elucidating the effects of efflux transporters, including Pgp, on brain uptake.

No abstract

ABSTRACT:The P-glycoprotein (P-gp)-deficient mouse model is used to assess the influence of P-gp-mediated efflux on the central nervous system ( The efflux transporter P-glycoprotein (P-gp) attenuates the central nervous system (CNS) distribution of many drugs, including opioids, triptans, protease inhibitors, and antihistamines. One method used to assess the influence of P-gp on the CNS distribution of compounds is the P-gp-deficient mouse model. The P-gp efflux ratio, calculated from the ratio of brain/plasma partition coefficient (K p,brain ) in P-gpdeficient (mdr1aϪ/Ϫ) mice to K p,brain in P-gp-competent (mdr1aϩ/ϩ) mice, reflects the degree to which P-gp-mediated efflux attenuates CNS distribution. However, when other processes influence CNS distribution, the P-gp efflux ratio may be a poor indicator of the degree to which CNS distribution of a compound is impaired.

The ATP-driven drug export pump, P-glycoprotein, is a primary gatekeeper of the blood-brain barrier and a major impediment to central nervous system (CNS) pharmacotherapy. Reducing P-glycoprotein activity dramatically increases penetration of many therapeutic drugs into the CNS. Previous studies in rat showed that brain capillary P-glycoprotein was transcriptionally up-regulated by the pregnane X receptor (PXR), a xenobioticactivated nuclear receptor. Here we used a transgenic mouse expressing human PXR (hPXR) to determine the consequences of increased blood-brain barrier P-glycoprotein activity. P-glycoprotein expression and transport activity in brain capillaries from transgenic mice was significantly increased when capillaries were exposed to the hPXR ligands, rifampin and hyperforin, in vitro and when the mice were dosed with rifampin in vivo. Plasma rifampin levels in induced mice were comparable with literature values for patients. We also administered methadone, a CNS-acting, P-glycoprotein substrate, to control and rifampin-induced transgenic mice and measured the drug's antinociceptive effect. In rifampin-induced mice, the methadone effect was reduced by approximately 70%, even though plasma methadone levels were similar to those found in transgenic controls not exposed to rifampin. Thus, hPXR activation in vivo increased P-glycoprotein activity and tightened the blood-brain barrier to methadone, reducing the drug's CNS efficacy. This is the first demonstration of the ability of bloodbrain barrier PXR to alter the efficacy of a CNS-acting drug.

Abstract. The opioids are commonly used to treat acute and severe pain. Long-term opioid administration eventually reaches a dose ceiling that is attributable to the rapid onset of analgesic tolerance coupled with the slow development of tolerance to the untoward side effects of respiratory depression, nausea and decreased gastrointestinal motility. The need for effective-long term analgesia remains. In order to develop new therapeutics and novel strategies for use of current analgesics, the processes that mediate tolerance must be understood. This review highlights potential pharmacokinetic (changes in metabolite production, metabolizing enzyme expression, and transporter function) and pharmacodynamic (receptor type, location and functionality; alterations in signaling pathways and crosstolerance) aspects of opioid tolerance development, and presents several pharmacodynamic modeling strategies that have been used to characterize time-dependent attenuation of opioid analgesia.