2 ABSTRACTPurpose: Clinical study has previously revealed that plasma concentration of , an active metabolite of irinotecan, was higher in patients with end-stage renal failure than those with normal kidney function although SN-38 is mainly eliminated in the liver. Here, we focused on inhibition by uremic toxins of hepatic SN-38 uptake and down-regulation of uptake transporter(s) by uremic plasma in humans. Methods:We evaluated SN-38 uptake and its inhibition by uremic toxins, 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF), indoxyl sulfate (Indox), hippuric acid (HA) and indole acetate (IA), with cryopreserved human hepatocytes and HEK293 cells stably expressing hepatic uptake transporters, organic anion transporting polypeptides (OATPs). We also collected plasma samples from patients with severe renal failure to examine their effects on mRNA level of OATPs in primary cultured human hepatocytes.Results: SN-38 was taken up by hepatocytes, which showed biphasic saturation patterns. The SN-38 uptake by hepatocytes was significantly inhibited by a uremic toxin mixture including clinically relevant concentrations of CMPF, Indox, HA and IA. 3Kinetic analyses for OATP-mediated transport revealed that the uptake of SN-38 by OATP1B1 was the highest, followed by OATP1B3. Among the uremic toxins, CMPF exhibited most potent inhibition of OATP1B1-mediated SN-38 uptake and directly inhibited the uptake of SN-38 also in hepatocytes. In addition, gene expression of OATP1B1 and OATP1B3 in hepatocytes was significantly down-regulated by the treatment with the uremic plasma.Conclusions: OATP1B1-mediated hepatic uptake of SN-38 was inhibited by uremic toxins, and gene expression of OATP1B1 was down-regulated by uremic plasma.

ABSTRACT:We clarified that major human cytochrome P450 (P450) enzymes were expressed in a chimeric mouse line established recently in Japan, in which the liver could be replaced by more than 80% with human hepatocytes. In this study, we investigated major human phase II enzymes such as UDP-glucuronosyltransferase (UGT), sulfotransferase (SULT), N-acetyltransferase (NAT), and glutathione S-transferase (GST) in the livers of chimeric mice by mRNA, protein, and enzyme activity using reverse transcription-polymerase chain reaction, Western blot analysis, and high-performance liquid chromatography, respectively. Human UGT, SULT, NAT, and GST mRNA were expressed in the liver of the chimeric mice, and UGT2B7, SULT1E1, SULT2A1, and GSTA1 proteins could be detected. The expression of mRNA and protein was correlated with the human albumin (hAlb) concentration in mouse blood, the replacement of which by human hepatocytes could be estimated by the hAlb concentration in the blood of the chimeric mice, because the chimeric mice produce human albumin. The enzyme activities, such as morphine 6-glucuronosyltransferase activity and estrone 3-sulfotransferase activity, activities that are specific to humans but not to mice, were increased in a hAlb concentration-dependent manner. The chimeric mice with humanized liver with nearly 90% replacement by human hepatocytes demonstrated almost the same protein contents of human phase II enzymes and enzyme activities as those of the donor. In conclusion, the chimeric mice exhibited an efficient capacity of drug conjugation similar to that in humans. These chimeric mice expressed human phase II enzymes as well as P450s, suggesting that they could be a useful animal model in drug development.Drug metabolism, drug interactions involving drug-metabolizing enzymes, and genetic polymorphisms of such enzymes have been well studied. To clarify the mechanism of the changed pharmacokinetics in studies of drug interactions, we usually focus on cytochrome P450 (P450) enzymes. However, a major metabolic pathway of a compound with polar functional groups is sometimes a conjugation reaction. The contribution of phase II conjugation to the clearance of a drug was estimated to be approximately 30% or higher (Bjornsson et al., 2003). Major hepatic phase II enzymes in humans are UDP-glucuronosyltransferase (UGT), sulfotransferase (SULT), N-acetyltransferase (NAT), and glutathione S-transferase (GST). There have been few reports on drug interactions that focused on phase II enzymes. Recently, several advances in the understanding of the inhibition and induction of such phase II enzymes have been made, especially concerning UGT. The effects of many drugs on glucuronidation in humans were clarified and listed by Kiang et al. (2005). Moreover, genetic polymorphisms of UGT, NAT, SULT, and GST have been demonstrated and the allelic frequency and effects of such mutations toward enzyme activities have become clearer. The UGT1A1*28 allele showed reduced UGT1A1 enzyme activity and was suggested to be a significant ri...

The liver of a chimeric urokinase-type plasminogen activator (uPA)(+/+)/severe combined immunodeficient (SCID) mouse line recently established in Japan could be replaced by more than 80% with human hepatocytes. We previously reported that the chimeric mice with humanized liver could be useful as a human model in studies on drug metabolism and pharmacokinetics. In the present study, the humanization of an excretory pathway was investigated in the chimeric mice. Cefmetazole (CMZ) was used as a probe drug. The CMZ excretions in urine and feces were 81.0 and 5.9% of the dose, respectively, in chimeric mice and were 23.7 and 59.4% of the dose, respectively, in control uPA(-/-)/SCID mice. Because CMZ is mainly excreted in urine in humans, the excretory profile of chimeric mice was demonstrated to be similar to that of humans. In the chimeric mice, the hepatic mRNA expression of human drug transporters could be quantified. On the other hand, the hepatic mRNA expression of mouse drug transporters in the chimeric mice was significantly lower than in the control uPA(-/-)/SCID mice. In conclusion, chimeric mice exhibited a humanized profile of drug excretion, suggesting that this chimeric mouse line would be a useful animal model in excretory studies.

The AUC(u) of SN-38 in SRF cancer patients is much greater than that of normal kidney patients primarily because of the reduced hepatic uptake of SN-38.

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