ABSTRACT:Thirty-two structurally diverse drugs used for the treatment of various conditions of the central nervous system (CNS), along with two active metabolites, and eight non-CNS drugs were measured in brain, plasma, and cerebrospinal fluid in the P-glycoprotein (P-gp) knockout mouse model after subcutaneous administration, and the data were compared with corresponding data obtained in wild-type mice. Total brain-to-plasma (B/P) ratios for the CNS agents ranged from 0.060 to 24. Of the 34 CNS-active agents, only 7 demonstrated B/P area under the plasma concentration curve ratios between P-gp knockout and wild-type mice that did not differ significantly from unity. Most of the remaining drugs demonstrated 1.1-to 2.6-fold greater B/P ratios in P-gp knockout mice versus wild-type mice. Three, risperidone, its active metabolite 9-hydroxyrisperidone, and metoclopramide, showed marked differences in B/P ratios between knockout and wild-type mice (6.6-to 17-fold). Differences in B/P ratios and cerebrospinal fluid/ plasma ratios between wild-type and knockout animals were correlated. Through the use of this model, it appears that most CNSactive agents demonstrate at least some P-gp-mediated transport that can affect brain concentrations. However, the impact for the majority of agents is probably minor. The example of risperidone illustrates that even good P-gp substrates can still be clinically useful CNS-active agents. However, for such agents, unbound plasma concentrations may need to be greater than values projected using receptor affinity data to achieve adequate receptor occupancy for effect.Active transport mechanisms as determinants of drug absorption, distribution, and clearance have been the focus of considerable research effort over the past decade. Of the numerous transporter proteins recently investigated, the one for which the greatest amount of knowledge exists is P-glycoprotein (MDR1). Originally described as a transporter involved in imparting drug resistance to tumor cells, P-glycoprotein has been demonstrated to be important in reducing absorption of drugs from the intestinal lumen, in active secretion of drugs into urine and bile, and in extrusion of drugs from vital organs such as the brain and reproductive tissues (Troutman et al., 2002). As such, P-glycoprotein-mediated transport has become an important issue in the discovery and development of new drugs. For example, new compounds that are promising with regard to target receptor/ enzyme activity can be severely hampered in their ability to elicit pharmacological effects in vivo should they be good substrates for P-glycoprotein, especially if the route of administration is intended to be oral or the target tissues is one rich in P-glycoprotein activity. Furthermore, the potential for drug-drug interactions arises in the event that the P-glycoprotein substrate is coadministered with another agent that can inhibit P-glycoprotein.Several models have been developed to assess drugs as P-glycoprotein substrates. In vitro models have included the Caco...

ABSTRACT:The role of breast cancer resistance protein (Bcrp) and the combined activities of Bcrp and P-glycoprotein (P-gp, Mdr1a/1b) in limiting the brain penetration of drugs at the blood-brain barrier (BBB) were investigated using wild-type FVB, Mdr1a/1b(؊/؊), (؊/؊), Bcrp(؊/؊), and Mdr1a/1b(؊/؊), (؊/؊)Bcrp(؊/؊) mice. Four drugs, flavopiridol, imatinib mesylate (Gleevec), PF-407288, and prazosin, with different transport specificity for BCRP/Bcrp and MDR1/ Mdr1a were selected, and the drug levels in plasma, cerebrospinal fluid, and brain of mice were determined. Flavopiridol and prazosin were identified as substrates for both mouse Bcrp and Mdr1a with greater transport associated with Bcrp. The brain/plasma (B/P) ratios at 0.5 and 2 h in Mdr1a/1b(؊/؊), (؊/؊) and Bcrp(؊/؊) mice were 1-to 2-fold for both compounds, whereas the ratios in Mdr1a/1b(؊/؊), (؊/؊)Bcrp(؊/؊) mice were more than 5-fold of those observed in FVB mice. For imatinib, a better substrate of P-gp than Bcrp, the B/P ratios in Bcrp(؊/؊) were comparable to those in FVB mice, whereas the B/P ratios in Mdr1a/1b(؊/؊), (؊/؊) and Mdr1a/1b(؊/؊), (؊/؊)Bcrp(؊/؊) mice were more than 4-and 28-fold of those in FVB mice at both time points, respectively. Finally, the Bcrp-specific substrate PF-407288 exhibited comparable B/P ratios in Mdr1a/1b(؊/؊), (؊/؊) and Bcrp(؊/؊) mice and slightly but significantly increased B/P ratios in Mdr1a/ 1b(؊/؊), (؊/؊)Bcrp(؊/؊) mice compared with those in FVB mice. The B/P ratios of compounds in Mdr1a/1b(؊/؊), (؊/؊)Bcrp(؊/؊) mice compared with those in Mdr1a/1b(؊/؊), (؊/؊) mice clearly demonstrate that Bcrp impairs the brain penetration of its substrates. Moreover, P-gp and Bcrp at BBB function synergistically to limit the brain penetration of shared substrates.It is widely recognized that the tight junctions between adjacent brain endothelial cells forming the blood-brain barrier (BBB) restrict the entry of compounds by paracellular diffusion from the blood to the brain. Moreover, the transcellular diffusion of compounds through the brain endothelial cells can also be impeded by transmembrane efflux transporters, such as P-glycoprotein (P-gp, MDR1, ABCB1) and breast cancer resistance protein (BCRP, ABCG2). These efflux transporters can eliminate xenobiotics from the brain against a concentration gradient, thereby limiting central nervous system (CNS) exposure to these compounds. Indeed, the prominent effect of P-gp at the BBB is well established, and P-gp is functionally important in limiting the brain penetration of its substrates (Schinkel et al., 1994;Chen et al., 2003;Scherrmann, 2005). Like P-gp, BCRP is another major member of the ATP-binding cassette family of drug transporters and is highly expressed in the BBB as well. BCRP has been found at the luminal side of human brain capillary endothelial cells (Cooray et al., 2002), and the mRNA level of mouse analog Bcrp was ϳ700 times greater in brain microvessels than in the cortex of the mice (Cisternino et al., 2004), implying that mouse Bcrp may play a key role at the BBB. B...

ABSTRACT:P-glycoprotein is considered to be a major factor impeding effective drug therapy for many diseases of the central nervous system (CNS). Thus, efforts are being made to gain a better understanding of P-glycoprotein's role in drug distribution to brain parenchyma and cerebrospinal fluid (CSF). The goal of this study was to validate and introduce a novel P-glycoprotein-deficient (ABCB1-1⌬) canine model for studying P-glycoprotein-mediated effects of drug distribution to brain tissue and CSF. CSF concentrations of drug are often used to correlate efficacy of CNS drug therapy as a surrogate for determining drug concentration in brain tissue. A secondary goal of this study was to investigate the validity of using CSF concentrations of P-glycoprotein substrates to predict brain tissue concentrations. Loperamide, an opioid that is excluded from the brain by P-glycoprotein, was used to confirm a P-glycoprotein-null phenotype in the dog model. ABCB1-1⌬ dogs experienced CNS depression following loperamide administration, whereas ABCB1 wild-type dogs experienced no CNS depression. In summary, we have validated a novel P-glycoprotein-deficient canine model and have used the model to investigate transport of the P-glycoprotein substrate 99m Tc-sestamibi at the blood-brain barrier and blood-CSF barrier.

Previous studies have indicated that intestinal P-glycoprotein (P-gp) limits the oral bioavailability of substrate drugs and alters systemic pharmacokinetics. In this study, dogs lacking functional P-gp were used to determine the contribution of P-gp to the oral bioavailability and systemic pharmacokinetics of several P-gp substrate drugs. The P-gp substrates quinidine, loperamide, nelfinavir, cyclosporin and the control (non P-gp substrate) drug diazepam were individually administered intravenously and per os to ABCB1-1Δ dogs, which have a P-gp null phenotype and ABCB1 wildtype dogs. ABCB1-1Δ dogs have been shown to have greater brain penetration of P-gp substrates, but limited information is available regarding oral bioavailability of P-gp substrate drugs in this animal model. Plasma drug concentration vs. time curves were generated and pharmacokinetic parameters were calculated for each drug. There were no differences in oral bioavailability between ABCB1-1Δ dogs and ABCB1 wildtype dogs for any of the drugs studied, suggesting that intestinal P-gp does not significantly affect intestinal absorption of these particular substrate drugs in ABCB1-1Δ dogs. However, small sample sizes and individual variability in CYP enzyme activity may have affected the power of the study to detect the impact of P-gp on oral bioavailability.

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