The pharmacokinetics, biotransformation and hepatic transporter effects of troglitazone were investigated following daily oral dosing, at 300 and 600 mg/kg, for 7 days to control (SCID) and chimeric (PXB) mice with humanized livers. Clinical chemistry revealed no consistent pattern of changes associated with troglitazone treatment in the PXB mouse. Human MRP2 but not mouse mrp2 was down-regulated following troglitazone treatment. Pharmacokinetic analysis revealed similar T(max) values for troglitazone in both mouse groups, a mono- and bi-phasic elimination phase in PXB and SCID mice, respectively, but a 3- to 5- and 2- to 5-fold higher C(max) and AUC, respectively, in PXB mice. Oxidative and conjugative metabolic pathways were identified, with the sulfate being the predominant metabolite in PXB compared to SCID mice (4- to 13-fold increase in liver and blood, respectively). The glucuronide conjugate was predominant in SCID mice. There was no evidence of glutathione conjugation. The primary oxidative pathways were mono- and di-oxidations which may also be attributed to quinone or hydroquinone derivatives. Several metabolites were observed in PXB mice only. As the troglitazone metabolic profiles in the PXB mouse were similar to reported human data the PXB mouse model can provide a useful first insight into circulating human metabolites of xenobiotics metabolized in the liver.

We investigated effects of human (h) GH on the proliferation of h-hepatocytes that had been engrafted in the liver of albumin enhancer/promoter driven-urokinase plasminogen activator transgenic/severe combined immunodeficiency disease (uPA/SCID) mice (chimeric mice). The h-hepatocytes therein were considered to be deficient in GH, because hGH receptor (hGHR) is unresponsive to mouse GH. Actually, hIGF-1 was undetectable in chimeric mouse sera. The uPA/SCID mice were transplanted with h-hepatocytes from a 6-year (6Y)-old donor, and were injected with recombinant hGH (rhGH). rhGH stimulated the repopulation speed of h-hepatocytes; and up-regulated hIGF-1, human signal transducers and activators of transcription (hSTAT) 3, and cell cycle regulatory genes such as human forkhead box M1, human cell division cycle 25A, and human cyclin D1. To confirm the reproducibility of these effects of rhGH, similar experiments were run using h-hepatocytes from a 46-year (46Y)-old donor. rhGH similarly enhanced their repopulation speed and up-regulated the expression of the abovetested genes, especially hIGF-1 and hSTAT1. The extent of the enhancement by rhGH was much less than that in 6Y-hepatocyte-chimeric mice most probably due to the difference in GHR expression levels between the two donors. In conclusion, this study clearly demonstrated that rhGH stimulates the proliferation of h-hepatocytes in vivo.

Human-specific or disproportionately abundant human metabolites of drug candidates that are not adequately formed and qualified in preclinical safety assessment species pose an important drug development challenge. Furthermore, the overall metabolic profile of drug candidates in humans is an important determinant of their drug-drug interaction susceptibility. These risks can be effectively assessed and/or mitigated if human metabolic profile of the drug candidate could reliably be determined in early development. However, currently available in vitro human models (e.g., liver microsomes, hepatocytes) are often inadequate in this regard. Furthermore, the conduct of definitive radiolabeled human ADME studies is an expensive and time-consuming endeavor that is more suited for later in development when the risk of failure has been reduced. We evaluated a recently developed chimeric mouse model with humanized liver on uPA/SCID background for its ability to predict human disposition of four model drugs (lamotrigine, diclofenac, MRK-A, and propafenone) that are known to exhibit human-specific metabolism. The results from these studies demonstrate that chimeric mice were able to reproduce the human-specific metabolite profile for lamotrigine, diclofenac, and MRK-A. In the case of propafenone, however, the human-specific metabolism was not detected as a predominant pathway, and the metabolite profiles in native and humanized mice were similar; this was attributed to the presence of residual highly active propafenone-metabolizing mouse enzymes in chimeric mice. Overall, the data indicate that the chimeric mice with humanized liver have the potential to be a useful tool for the prediction of human-specific metabolism of xenobiotics and warrant further investigation.

The aims of this study were to assess the utility of the PXB mouse model of a chimeric human/mouse liver in studying human-specific effects of an important human hepatotoxic drug, the PPARg agonist, troglitazone. When given orally by gavage for 7 days, at dose levels of 300 and 600 ppm, troglitazone induced specific changes in the human hepatocytes of the chimeric liver without an effect on the murine hepatic portions. The human hepatocytes, in the vehicle-treated PXB mouse, showed an accumulation of electron-dense lipid droplets that appeared as clear vacuoles under the light microscope in H&E-stained sections. Following dosing with troglitazone, there was a loss of the large lipid droplets in the human hepatocytes, a decrease in the amount of lipid as observed in frozen sections of liver stained by Oil-red-O, and a decrease in the expression of two bile acid transporters, BSEP and MRP2. None of these changes were observed in the murine remnants of the chimeric liver. No changes were observed in the expression of three CYPs, CYP 3A2, CYP 1A1, and CYP 2B1, in either the human or murine hepatocytes, even though the baseline expression of the enzymes differed significantly between the two hepatocyte species with the mouse hepatocytes consistently showing increased expression of the protein of all three enzymes. This study has shown that the human hepatocytes, in the PXB chimeric mouse liver, retain an essentially normal phenotype in the mouse liver and, the albeit limited CYP enzymes studied show a more human, rather than a murine, expression pattern. In line with this conclusion, the study has shown a differential response of the human versus the mouse hepatocytes, and the effects observed are highly suggestive of a differential handling of the compound by the two hepatocyte species although the exact reasons are not as yet clear. The PXB chimeric mouse system therefore holds the clear potential to explore human hepatic-specific features, such as metabolism, prior to dosing human subjects, and as such should have considerable utility in drug discovery and development.

ABSTRACT:The expression of drug transporters and metabolizing enzymes is a primary determinant of drug disposition. Chimeric mice with humanized liver, including PXB mice, are an available model that is permissive to the in vivo infection of hepatitis C virus (HCV), thus being a promising tool for investigational studies in development of new antiviral molecules. To investigate the potential of HCV infection to alter the pharmacokinetics of small molecule antiviral therapeutic agents in PXB mice, we have comprehensively determined the mRNA expression profiles of human ATP-binding cassette (ABC) transporters, solute carrier (SLC) transporters, and cytochrome P450 (P450) enzymes in the livers of these mice under noninfected and HCV-infected conditions. Infection of PXB mice with HCV resulted in an increase in the mRNA expression levels of a series of interferon-stimulated genes in the liver. For the majority of genes involved in drug disposition, minor differences in the mRNA expression of ABC and SLC transporters as well as P450s between the noninfected and HCV-infected groups were observed. The exceptions were statistically significantly higher expression of multidrug resistance-associated protein 4 and organic anion-transporting polypeptide 2B1 and lower expression of organic cation transporter 1 and CYP2D6 in HCV-infected mice. Furthermore, the enzymatic activities of the major human P450s were, in general, comparable in the two experimental groups. These data suggest that the pharmacokinetic properties of small molecule antiviral therapies in HCV-infected PXB mice are likely to be similar to those in noninfected PXB mice. However, caution is needed in the translation of this relationship to HCV-infected patients as the PXB mouse model does not accurately reflect the pathology of patients with chronic HCV infection.