1 Homozygously mdr1a gene disrupted mice (mdr1a(7/7) mice) and wild type mice (mdr1a(+/+) mice) were used to develop a method for P-glycoprotein (P-gp) function imaging non-invasively and to study the eect of a P-gp reversal agent on its function in vivo. ]verapamil accumulation in the brain of mdr1a(+/+) mice was increased by cyclosporin A to levels comparable with those in mdr1a(7/7) mice, indicating that reversal of P-gp mediated eux can be monitored by PET. 5 We conclude that cyclosporin A can fully block the P-gp function in the blood brain barrier and the testes and that PET enables the in vivo measurement of P-gp function and reversal of its function noninvasively.

Background and purpose: Lopinavir is extensively metabolized by cytochrome P450 3A (CYP3A) and is considered to be a substrate for the drug transporters ABCB1 (P-glycoprotein) and ABCC2 (MRP2). Here, we have assessed the individual and combined effects of CYP3A, ABCB1 and ABCC2 on the pharmacokinetics of lopinavir and the relative importance of intestinal and hepatic metabolism. We also evaluated whether ritonavir increases lopinavir oral bioavailability by inhibition of CYP3A, ABCB1 and/or ABCC2. Experimental approach: Lopinavir transport was measured in Madin-Darby canine kidney cells expressing ABCB1 or ABCC2. Oral lopinavir kinetics (+/-ritonavir) was studied in mice with genetic deletions of Cyp3a, Abcb1a/b and/or Abcc2, or in transgenic mice expressing human CYP3A4 exclusively in the liver and/or intestine. Key results: Lopinavir was transported by ABCB1 but not by ABCC2 in vitro. Lopinavir area under the plasma concentration -time curve (AUC)oral was increased in Abcb1a/b -/-mice (approximately ninefold vs. wild-type) but not in Abcc2 -/-mice. Increased lopinavir AUCoral (>2000-fold) was observed in cytochrome P450 3A knockout (Cyp3a -/-and Cyp3a/Abcb1a/b/Abcc2 -/-mice was observed. CYP3A4 activity in intestine or liver, separately, reduced lopinavir AUCoral (>100-fold), compared with Cyp3a -/-mice. Ritonavir markedly increased lopinavir AUCoral in all CYP3A-containing mouse strains. Conclusions and implications: CYP3A was the major determinant of lopinavir pharmacokinetics, far more than Abcb1a/b. Both intestinal and hepatic CYP3A activity contributed importantly to low oral bioavailability of lopinavir. Ritonavir increased lopinavir bioavailability primarily by inhibiting CYP3A. Effects of Abcb1a/b were only detectable in the presence of CYP3A, suggesting saturation of Abcb1a/b in the absence of CYP3A activity.

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The multidrug transporters breast cancer resistance protein (BCRP), multidrug-resistance protein 1 (MDR1), and multidrugresistance-associated protein (MRP) 2 and 3 eliminate toxic compounds from tissues and the body and affect the pharmacokinetics of many drugs and other potentially toxic compounds. The food-derived carcinogen PhIP (2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine) is transported by BCRP, MDR1, and MRP2. To investigate the overlapping functions of Bcrp1, Mdr1a/b, and Mrp2 in vivo, we generated Bcrp1;Mdr1a/b;Mrp2 2/2 mice, which are viable and fertile. These mice, together with Bcrp1;Mrp2;Mrp3 2/2 mice, were used to study the effects of the multidrug transporters on the pharmacokinetics of PhIP and its metabolites. Thirty minutes after oral or intravenous administration of PhIP (1 mg/kg), the PhIP levels in the small intestine were reduced 4-to 6-fold in Bcrp1;Mdr1a/b;Mrp2 2/ 2 and Bcrp1;Mrp2;Mrp3 2/2 mice compared with wild-type mice. Fecal excretion of PhIP was reduced 8-to 20-fold in knockouts. Biliary PhIP excretion was reduced 41-fold in Bcrp1;Mdr1a/b;Mrp2 2/2 mice. Biliary and small intestine levels of PhIP metabolites were reduced in Bcrp1;Mrp2-deficient mice. Furthermore, in both knockout strains, kidney levels and urinary excretion of genotoxic PhIPmetabolites were significantly increased, suggesting that reduced biliary excretion of PhIP and PhIP metabolites leads to increased urinary excretion of these metabolites and increased systemic exposure. Bcrp1 and Mdr1a limited PhIP brain accumulation. In Bcrp1;Mrp2;Mrp3 2/2 , but not Bcrp1; Mdr1a/b;Mrp 2/2 mice, the carcinogenic metabolites N2-OHPhIP (2-hydroxyamino-1-methyl-6-phenylimidazo [4,5-b]pyridine) and PhIP-5-sulfate (a genotoxicity marker) accumulated in liver tissue, indicating that Mrp3 is involved in the sinusoidal secretion of these compounds. We conclude that Bcrp1, Mdr1a/b, Mrp2, and Mrp3 significantly affect tissue disposition and biliary and fecal elimination of PhIP and its carcinogenic metabolites and may affect PhIP-induced carcinogenesis as a result.