A synergistic effect of P-glycoprotein (P-gp)/Abcb1a and breast cancer resistance protein (Bcrp)/Abcg2 was reported to limit the brain penetration of their common substrates. This study investigated this based on pharmacokinetics using Mdr1a/1b(Ϫ/Ϫ), Bcrp(Ϫ/Ϫ), and Mdr1a/1b(Ϫ/Ϫ)/ Bcrp(Ϫ/Ϫ) mice. Comparison of the brain-and testis-toplasma ratios (C brain /C plasma and C testis /C plasma , respectively) of the reference compounds quinidine and dantrolene for P-gp and Bcrp, respectively, indicates that impairment of either P-gp and Bcrp did not cause any change in the efflux activities of Bcrp or P-gp, respectively, at both the bloodbrain barrier (BBB) and blood-testis barrier (BTB). The C brain / C plasma and C testis /C plasma of the common substrates erlotinib, flavopiridol, and mitoxantrone were markedly increased in Mdr1a/1b(Ϫ/Ϫ)/Bcrp(Ϫ/Ϫ) mice even compared with Mdr1a/1b(Ϫ/Ϫ) and Bcrp(Ϫ/Ϫ) mice. Efflux activities by P-gp and Bcrp relative to passive diffusion at the BBB and BTB were separately evaluated based on the C brain /C plasma and C testis /C plasma in the knockout strains to the wild-type strain. P-gp made a larger contribution than Bcrp to the net efflux of the common substrates, but Bcrp activities were also significantly larger than passive diffusion. These parameters could reasonably account for the marked increase in C brain /C plasma and C testis /C plasma in the Mdr1a/1b(Ϫ/Ϫ)/ Bcrp(Ϫ/Ϫ) mice. In conclusion, the synergistic effect of P-gp and Bcrp on C brain /C plasma and C testis /C plasma can be explained by their contribution to the net efflux at the BBB and BTB without any interaction between P-gp and Bcrp.It is well accepted that the penetration of xenobiotic compounds into the brain and testis is restricted by the blood-brain barrier (BBB) and blood-testis barrier (BTB), respectively. The BBB is formed by brain capillary endothelial cells, whereas, in addition to endothelial cells, myoid and Sertoli cells form the BTB (Bart et al., 2002;Kusuhara and Sugiyama, 2005). Tight junctions between adjacent cells in the BBB and BTB are highly developed and limit the penetration of substances via the paracellular route. Moreover, drug transporters act as active barriers to limit the tissue penetration of substrates from the blood by extruding them back into the blood in the BBB and BTB and, thereby, modulating pharmacological or adverse reactions. It has been shown that ATP binding cassette (ABC) transporters, which are known to mediate resistance to anticancer drugs and antiviral drugs, are expressed in the BBB and BTB. These include P-glycoprotein (P-gp/MDR1/ABCB1), breast cancer resistance protein (BCRP/ABCG2), multidrug resistance-associated protein (MRP)-1/ABCC1, MRP2/ABCC2, MRP4/ABCC4, and MRP5/ABCC5 (Leggas et al., 2004;Zhang et al., 2004;Lee et al., 2005). In particular, P-gp is a well known transporter that plays a pivotal role in barrier function, and disruption of the Mdr1a gene, a predominant isoform expressed in the barriers, causes accumulation of a number of its substrates (S...

UCN-01 can be administered safely as an initial 72-hour CIV with subsequent monthly doses administered as 36-hour infusions.

This study investigated the impact of the active efflux mediated by P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) at the blood-brain barrier (BBB) on the predictability of the unbound brain concentration (C u,brain ) by the concentration in the cerebrospinal fluid (CSF) (C u,CSF ) in rats. C u,brain is obtained as the product of the total brain concentration and unbound fraction in the brain (f u,brain ) determined in vitro in brain slices. Twenty-five compounds, including P-gp and/or Bcrp substrates, were given a constant intravenous infusion, and their plasma, brain, and CSF concentrations were determined. P-gp and/or Bcrp substrates, such as verapamil, loperamide, flavopiridol, genistein, quinidine, dantrolene, daidzein, cimetidine, and pefloxacin, showed a higher CSF-to-brain unbound concentration ratio (K p,uu,CSF/brain ) compared with non-P-gp and non-Bcrp substrates. K p,uu,CSF/brain values of P-gp-specific (quinidine and verapamil) and Bcrp-specific (daidzein and genistein) substrates were significantly decreased in Mdr1a/ 1b(Ϫ/Ϫ) and Bcrp(Ϫ/Ϫ) mice, respectively. Furthermore, consistent with the contribution of P-gp and Bcrp to the net efflux at the BBB, K p,uu,CSF/brain values of the common substrates (flavopiridol and erlotinib) were markedly decreased in Mdr1a/ 1b(Ϫ/Ϫ)/Bcrp(Ϫ/Ϫ) mice, but only moderately or weakly in Mdr1a/1b(Ϫ/Ϫ) mice and negligibly in Bcrp(Ϫ/Ϫ) mice. In conclusion, predictability of C u,brain by C u,CSF decreases along with the net transport activities by P-gp and Bcrp at the BBB. C u,CSF of non-P-gp and non-Bcrp substrates can be a reliable surrogate of C u,brain for lipophilic compounds.

A clinical drug-drug interaction between famotidine (a H 2 receptor antagonist) and probenecid has not been reproduced in rats. The present study hypothesized that the species-dependent probenecid sensitivity is due to a species difference in the contribution of renal organic anion and cation transporters. The transport activities of the H 2 receptor antagonists (cimetidine, famotidine, and ranitidine) by rat and human basolateral organic anion and cation transporters [human organic anion transporter (hOAT) 1, hOAT2, r/hOAT3, rat organic cation transporter (rOct) 1, and r/hOCT2] were compared using their cDNA transfectants. The transport activities (V max /K m ) of famotidine (K m , 345 M) by rOat3 were 8-and 15-fold lower than those of cimetidine (K m , 91 M) and ranitidine (K m , 155 M), respectively, whereas the activity by hOAT3 (K m , 124 M) was 3-fold lower than that of cimetidine (K m , 149 M) but similar to that of ranitidine (K m , 234 M). Comparison of the relative transport activity with regard to that of cimetidine suggests that famotidine was more efficiently transported by hOAT3 than rOat3, and vice versa, for ranitidine. Only ranitidine was efficiently transported by hOAT2 (K m , 396 M). rOct1 accepts all of the H 2 receptor antagonists with a similar activity, whereas the transport activities of ranitidine and famotidine (K m , 61/56 M) by r/hOCT2 were markedly lower than that of cimetidine (K m , 69/73 M). Probenecid was a potent inhibitor of r/OAT3 (K i , 2.6 -5.8 M), whereas it did not interact with OCTs. These results suggest that, in addition to the absence of OCT1 in human kidney, a species difference in the transport activity by hOAT3 and rOat3 accounts, at least in part, for the species difference in the drug-drug interaction between famotidine and probenecid.The kidney plays important roles in the detoxification of xenobiotics and endogenous wastes as well as maintaining the correct balance of ions and nutrients in the body. Urinary excretion is the major detoxification mechanism in the kidney, and this is governed by glomerular filtration, tubular secretion across the proximal tubules, and reabsorption. The renal uptake of organic anions and cations on the basolateral membrane of the proximal tubules has been characterized by multispecific organic anion and cation transporters (OAT/ SLC22 and OCT/SLC22), respectively (Lee and Kim, 2004;Wright and Dantzler, 2004;Shitara et al., 2005).Molecular cloning of basolateral transporters from different species allows examination of differences in their substrate specificities and transport activities, leading to a better understanding of the molecular mechanisms of species differences in drug disposition. For OCTs, the isoform expressed in the kidney differs between rodents and humans. Both Oct1 (Slc22a1) and Oct2 (Slc22a2) are involved in the renal uptake of organic cations on the basolateral membrane of the proximal tubules in rodents, whereas OCT2 is the predominant isoform in the human kidney (Koepsell, 2004;Lee and Kim, 2004;Wright a...

ABSTRACT:Fexofenadine is a selective, nonsedating H 1 -receptor antagonist approved for symptoms of allergic conditions, which is mainly excreted into feces via biliary excretion. The purpose of this study is to investigate its pharmacokinetics in mice and rats to determine the role of P-glycoprotein (P-gp) in its biliary excretion. In mice, biliary excretion clearance (17 ml/min/kg) accounted for almost 60% of the total body clearance (30 ml/min/kg). Comparing the pharmacokinetics after intravenous and oral administration indicated that the bioavailability of fexofenadine was at most 2% in mice. Knockout of Mdr1a/1b P-gp did not affect the biliary excretion clearance with regard to both plasma and liver concentrations, whereas the absence of P-gp caused a 6-fold increase in the plasma concentration after oral administration. In addition, the steady-state brain-to-plasma concentration ratio of fexofenadine was approximately 3-fold higher in Mdr1a/1b P-gp knockout mice than in wild-type mice. Together, these results show that P-glycoprotein plays an important role in efflux transport in the brain and small intestine but only a limited role in biliary excretion in mice. In addition, there was no difference in the biliary excretion between normal and hereditarily multidrug resistance-associated protein 2 (Mrp2)-deficient mutant rats (Eisai hyperbilirubinemic rats) and between wild-type and breast cancer resistance protein (Bcrp) knockout mice. These results suggest that the biliary excretion of fexofenadine is mediated by unknown transporters distinct from P-gp, Mrp2, and Bcrp.Fexofenadine is an orally active nonsedating histamine H1-receptor antagonist that is prescribed for oral treatment of allergic rhinitis and chronic idiopathic urticaria. After oral administration of [ 14 C]fexofenadine, the radioactivity was recovered in the urine (2.5%), bile (28%), and feces (69%) of rats whose bile duct had been cannulated, and thus, its oral absorption is at most 30%, and biliary excretion likely plays a major role in its elimination (Common Technical Document for the Registration of Pharmaceuticals for Human Use).It has been suggested that transporters play important roles in the disposition of fexofenadine. Fexofenadine has been shown to be a substrate of P-glycoprotein (P-gp) (Cvetkovic et al., 1999;Perloff et al., 2002). Vectorial transport of fexofenadine in the basal-to-apical direction was observed in Caco-2 cells, and this was inhibited by inhibitors of P-gp, such as ritonavir and verapamil (Perloff et al., 2002). Transfection of MDR1 P-gp cDNA into LLC-PK1 cells increased the vectorial transport of fexofenadine in the basal-to-apical direction (Cvetkovic et al., 1999). Furthermore, knockout of Mdr1a P-gp caused a significant increase in the plasma concentration of fexofenadine at 4 h after intravenous and oral administration. In addition, among the tissues examined, the brain-to-plasma concentration ratio of [ 14 C]fexofenadine compared with that in wild-type mice was also increased (Cvetkovic et al., 1999). The...

UCN-01 is a protein kinase inhibitor under development as a novel anticancer drug. The initial pharmacologic features in patients were not predicted from preclinical experiments. The distribution volume and the systemic clearance were much lower than those in experimental animals (mice, rats, and dogs), and the elimination half-life was unusually long (>200 hours). The unbound fraction in human plasma was also much smaller than that in dogs, rats and mice, as was the binding of UCN-01 to human alpha-1 acid glycoprotein much stronger than that to human serum albumin or human gamma-globulin. The association constants for alpha-1 acid glycoprotein and human plasma were approximately 8 x 10(8)(mol/L)(-1), indicating extremely high affinity. In this review article, the authors discuss the pharmacologic features of UCN-01 across species and provide a perspective on how this information could be applied prospectively to the future development of this agent.

These results suggest that monkey is a good predictor of the renal uptake of organic anions in the human.

UCN-01 (7-hydroxystaurosporine; NSC 638850) is a protein kinase antagonist selected for clinical trial based in part on evidence of efficacy in a preclinical renal carcinoma xenograft model. Schedule studies and in vitro studies suggested that a 72-h continuous infusion would be appropriate. In rats and dogs, maximum tolerated doses produced peak plasma concentrations of approximately 0.2-0.3 microM. However, concentrations 10-fold greater are well tolerated in humans, and the compound has a markedly prolonged T1/2. Specific binding to human alpha1-acidic glycoprotein has been demonstrated. These findings reinforce the need to consider actual clinical pharmacology data in "real time" with phase I studies.