Human plasma concentrations of five cytochrome P450 probes extrapolated from pharmacokinetics in dogs and minipigs using physiologically based pharmacokinetic modeling

Single-cell metabolic profiling reveals subgroups of primary human hepatocytes with heterogeneous responses to drug challenge

BACKGROUND

Xenobiotics are primarily metabolized by hepatocytes in the liver, and primary human hepatocytes are the gold standard model for the assessment of drug efficacy, safety, and toxicity in the early phases of drug development. Recent advances in single-cell genomics demonstrate liver zonation and ploidy as main drivers of cellular heterogeneity. However, little is known about the impact of hepatocyte specialization on liver function upon metabolic challenge, including hepatic metabolism, detoxification, and protein synthesis.

RESULTS

Here, we investigate the metabolic capacity of individual human hepatocytes in vitro. We assess how chronic accumulation of lipids enhances cellular heterogeneity and impairs the metabolisms of drugs. Using a phenotyping five-probe cocktail, we identify four functional subgroups of hepatocytes responding differently to drug challenge and fatty acid accumulation. These four subgroups display differential gene expression profiles upon cocktail treatment and xenobiotic metabolism-related specialization. Notably, intracellular fat accumulation leads to increased transcriptional variability and diminishes the drug-related metabolic capacity of hepatocytes.

CONCLUSIONS

Our results demonstrate that, upon a metabolic challenge such as exposure to drugs or intracellular fat accumulation, hepatocyte subgroups display different and heterogeneous transcriptional responses.

Liver microsomal cytochrome P450 3A-dependent drug oxidation activities in individual dogs

Drug oxidations are mediated mainly by cytochromes P450 (P450s or CYPs). CYP3As are an important P450 subfamily and include liver-specific CYP3A12 and intestine-specific CYP3A98 in dogs. Individual differences in drug oxidation activities were investigated, including correlations with immunoreactive CYP3A protein intensities and CYP3A mRNA expression levels in liver microsomes.Pooled and individual dog liver microsomes showed activities toward nifedipine, midazolam, alprazolam, and estradiol, but the levels of catalytic activities varied approximately twofold among the individual dogs. One dog harbored a CYP1A2 variant causing protein deletion, but showed higher activities than the other dogs toward nifedipine oxidation, midazolam 1'-hydroxylation, alprazolam 4-hydroxylation, estradiol 16α-hydroxylation activities, and caffeine C₈-hydroxylation; the latter is used as a reference reaction for CYP1A.In individual dog liver microsomes, the intensities of the immunochemical bands with anti-human CYP3A4 and anti-rat CYP3A2 antibodies along with CYP3A12 and CYP3A26 mRNA expression levels showed good correlations (p < 0.05) with nifedipine oxidation, midazolam 1'- and 4-hydroxylation, alprazolam 1'- and 4-hydroxylation, and estradiol 16α-hydroxylation activities.These results suggest that the oxidation activities of dog liver microsomes toward nifedipine and other typical CYP3A-catalyzed drugs exhibit approximately twofold individual differences and were predominantly mediated by liver-specific CYP3A12 in the dogs.

Pharmacokinetics of primary metabolites 5-hydroxythalidomide and 5'-hydroxythalidomide formed after oral administration of thalidomide in the rabbit, a thalidomide-sensitive species

The teratogenicity of the chemotherapeutic drug thalidomide is species-specific and affects humans, non-human primates, and rabbits. The primary oxidation of thalidomide in previously investigated rodents predominantly resulted in the formation of deactivated 5'-hydroxythalidomide. In the current study, similar in vivo biotransformations to 5-hydroxythalidomide and 5'-hydroxythalidomide were confirmed by the analysis of blood plasma from male rabbits, a thalidomide-sensitive species, after oral administration of thalidomide (2.0 mg/kg). Similar levels of thalidomide in seminal plasma and in blood plasma were detected using liquid chromatography-tandem mass spectrometry at 4 hr and 7 hr after oral doses in male rabbits. Seminal plasma concentrations of 5-hydroxythalidomide and 5'-hydroxythalidomide were also seen in male rabbits in a roughly similar time-dependent manner to those in the blood plasma after oral doses of thalidomide (2.0 mg/kg). Furthermore, the values generated by a simplified physiologically based pharmacokinetic rabbit model were in agreement with the measured in vivo blood plasma data under metabolic ratios of 0.01 for the hepatic intrinsic clearance of thalidomide to both unconjugated 5-hydroxythalidomide and 5'-hydroxythalidomide. These results suggest that metabolic activation of thalidomide may be dependent on rabbit liver enzymes just it was for cytochrome P450 enzymes in humanized-liver mice; in contrast, rodent livers predominantly mediate biotransformation of thalidomide to 5'-hydroxythalidomide. A developmental toxicity test system with experimental animals that involves intravaginal exposures to the chemotherapeutic drug thalidomide via semen should be considered in the future.

Comparison of Canine and Human Physiological Factors: Understanding Interspecies Differences that Impact Drug Pharmacokinetics

This review is a summary of factors affecting the drug pharmacokinetics (PK) of dogs versus humans. Identifying these interspecies differences can facilitate canine-human PK extrapolations while providing mechanistic insights into species-specific drug in vivo behavior. Such a cross-cutting perspective can be particularly useful when developing therapeutics targeting diseases shared between the two species such as cancer, diabetes, cognitive dysfunction, and inflammatory bowel disease. Furthermore, recognizing these differences also supports a reverse PK extrapolations from humans to dogs. To appreciate the canine-human differences that can affect drug absorption, distribution, metabolism, and elimination, this review provides a comparison of the physiology, drug transporter/enzyme location, abundance, activity, and specificity between dogs and humans. Supplemental material provides an in-depth discussion of certain topics, offering additional critical points to consider. Based upon an assessment of available state-of-the-art information, data gaps were identified. The hope is that this manuscript will encourage the research needed to support an understanding of similarities and differences in human versus canine drug PK.

The Neonatal and Juvenile Pig in Pediatric Drug Discovery and Development

Pharmacotherapy in pediatric patients is challenging in view of the maturation of organ systems and processes that affect pharmacokinetics and pharmacodynamics. Especially for the youngest age groups and for pediatric-only indications, neonatal and juvenile animal models can be useful to assess drug safety and to better understand the mechanisms of diseases or conditions. In this respect, the use of neonatal and juvenile pigs in the field of pediatric drug discovery and development is promising, although still limited at this point. This review summarizes the comparative postnatal development of pigs and humans and discusses the advantages of the juvenile pig in view of developmental pharmacology, pediatric diseases, drug discovery and drug safety testing. Furthermore, limitations and unexplored aspects of this large animal model are covered. At this point in time, the potential of the neonatal and juvenile pig as nonclinical safety models for pediatric drug development is underexplored.

Plasma and hepatic concentrations of acetaminophen and its primary conjugates after oral administrations determined in experimental animals and humans and extrapolated by pharmacokinetic modeling

Plasma concentrations of acetaminophen, its glucuronide and sulfate conjugates, and cysteinyl acetaminophen were experimentally determined after oral administrations of 10 mg/kg in humanised-liver mice, control mice, rats, common marmosets, cynomolgus monkeys, and minipigs; the results were compared with reported human pharmacokinetic data. Among the animals tested, only rats predominantly converted acetaminophen to sulfate conjugates, rather than glucuronide conjugates. In contrast, the values of area under the plasma concentration curves of acetaminophen, its glucuronide and sulfate conjugates, and cysteinyl acetaminophen after oral administration of acetaminophen in marmosets and minipigs were consistent with those reported in humans under the present conditions. Physiologically based pharmacokinetic (PBPK) models (consisting of the gut, liver, and central compartments) for acetaminophen and its primary metabolite could reproduce and estimate, respectively, the plasma and hepatic concentrations of acetaminophen in experimental animals and humans after single virtual oral doses. The values of area under the curves of hepatic concentrations of acetaminophen estimated using PBPK models were correlated with the measured levels of cysteinyl acetaminophen (a deactivated metabolite) in plasma fractions in these species. Consequently, using simple PBPK models and plasma data to predict hepatic chemical concentrations after oral doses could be helpful as an indicator of in vivo possible hepatotoxicity of chemicals such as acetaminophen.

Porcine Prediction of Pharmacokinetic Parameters in People: A Pig in a Poke?

The minipig has become an animal of considerable interest in preclinical drug development. It has been used in toxicology research and in examining/establishing regulatory guidelines as a nonrodent animal model. We have reviewed some basic issues that one would want to consider in the development and testing of any animal model for humans. The pig is a reasonable alternative to the dog, but there are some clear limitations and unexplained disparities in the literature, which require further study; primary among these is the need for standardization in choice of breed and sex and routine protocols. The minipig offers numerous advantages over other established animal models, and it has similarities to the human with regard to anatomy, physiology, and biochemistry. The gastrointestinal tract is structurally and functionally similar to humans. This appears to be true for enzymes and transporters in the gut as well, but more study is needed. One major concern is assessment of oral drug absorption, especially with regard to potential food effects due to gastric emptying differences, yet this does not appear to be a consistent observation. Hepatic metabolism seems to reflect enzymatic patterns in humans, with some differences. Kidney function seems similar to humans but requires further study. We have analyzed literature data that suggest the pig would offer a reasonable model for human oral bioavailability and for allometric predictions of clearance. The minipig appears to be the model for dermal absorption in humans, and we discuss this in terms of literature data and our own in-house experience.

Effects of aging and rifampicin pretreatment on the pharmacokinetics of human cytochrome P450 probes caffeine, warfarin, omeprazole, metoprolol and midazolam in common marmosets genotyped for cytochrome P450 2C19

1. The pharmacokinetics were investigated for human cytochrome P450 probes after single intravenous and oral administrations of 0.20 and 1.0 mg/kg, respectively, of caffeine, warfarin, omeprazole, metoprolol and midazolam to aged (10-14 years old, n = 4) or rifampicin-treated/young (3 years old, n = 3) male common marmosets all genotyped as heterozygous for a cytochrome P450 2C19 variant. 2. Slopes of the plasma concentration-time curves after intravenous administration of warfarin and midazolam were slightly, but significantly (two-way analysis of variance), decreased in aged marmosets compared with young marmosets. The mean hepatic clearances determined by in silico fitting for individual pharmacokinetic models of warfarin and midazolam in the aged group were, respectively, 23% and 56% smaller than those for the young group. 3. Significantly enhanced plasma clearances of caffeine, warfarin, omeprazole and midazolam were evident in young marmosets pretreated with rifampicin (25 mg/kg daily for 4 days). Two- to three-fold increases in hepatic intrinsic clearance values were observed in the individual pharmacokinetic models. 4. The in vivo dispositions of multiple simultaneously administered drugs in old, young and P450-enzyme-induced marmosets were elucidated. The results suggest that common marmosets could be experimental models for aged, induced or polymorphic P450 enzymes in P450-dependent drug metabolism studies.

Metabolic profiles of pomalidomide in human plasma simulated with pharmacokinetic data in control and humanized-liver mice

1. Pomalidomide has been shown to be potentially teratogenic in thalidomide-sensitive animal species such as rabbits. Screening for thalidomide analogs devoid of teratogenicity/toxicity - attributable to metabolites formed by cytochrome P450 enzymes - but having immunomodulatory properties is a strategic pathway towards development of new anticancer drugs. 2. In this study, plasma concentrations of pomalidomide, its primary 5-hydroxylated metabolite, and its glucuronide conjugate(s) were investigated in control and humanized-liver mice. Following oral administration of pomalidomide (100 mg/kg), plasma concentrations of 7-hydroxypomalidomide and 5-hydroxypomalidomide glucuronide were slightly higher in humanized-liver mice than in control mice. 3. Simulations of human plasma concentrations of pomalidomide were achieved with simplified physiologically-based pharmacokinetic models in both groups of mice in accordance with reported pomalidomide concentrations after low dose administration in humans. 4. The results indicate that pharmacokinetic profiles of pomalidomide were roughly similar between control mice and humanized-liver mice and that control and humanized-liver mice mediated pomalidomide 5-hydroxylation in vivo. Introducing one aromatic amino group into thalidomide resulted in less species differences in in vivo pharmacokinetics in control and humanized-liver mice.

Simulation of human plasma concentration-time profiles of the partial glucokinase activator PF-04937319 and its disproportionate N-demethylated metabolite using humanized chimeric mice and semi-physiological pharmacokinetic modeling

1. The partial glucokinase activator N,N-dimethyl-5-((2-methyl-6-((5-methylpyrazin-2-yl)carbamoyl)benzofuran-4-yl)oxy)pyrimidine-2-carboxamide (PF-04937319) is biotransformed in humans to N-methyl-5-((2-methyl-6-((5-methylpyrazin-2-yl)carbamoyl)benzofuran-4-yl)oxy)pyrimidine-2-carboxamide (M1), accounting for ∼65% of total exposure at steady state. 2. As the disproportionately abundant nature of M1 could not be reliably predicted from in vitro metabolism studies, we evaluated a chimeric mouse model with humanized liver on TK-NOG background for its ability to retrospectively predict human disposition of PF-04937319. Since livers of chimeric mice were enlarged by hyperplasia and contained remnant mouse hepatocytes, hepatic intrinsic clearances normalized for liver weight, metabolite formation and liver to plasma concentration ratios were plotted against the replacement index by human hepatocytes and extrapolated to those in the virtual chimeric mouse with 100% humanized liver. 3. Semi-physiological pharmacokinetic analyses using the above parameters revealed that simulated concentration curves of PF-04937319 and M1 were approximately superimposed with the observed clinical data in humans. 4. Finally, qualitative profiling of circulating metabolites in humanized chimeric mice dosed with PF-04937319 or M1 also revealed the presence of a carbinolamide metabolite, identified in the clinical study as a human-specific metabolite. The case study demonstrates that humanized chimeric mice may be potentially useful in preclinical discovery towards studying disproportionate or human-specific metabolism of drug candidates.

Combining Chimeric Mice with Humanized Liver, Mass Spectrometry, and Physiologically-Based Pharmacokinetic Modeling in Toxicology

Species differences exist in terms of drug oxidation activities, which are mediated mainly by cytochrome P450 (P450) enzymes. To overcome the problem of species extrapolation, transchromosomic mice containing a human P450 3A cluster or chimeric mice transplanted with human hepatocytes have been introduced into the human toxicology research area. In this review, drug metabolism and disposition mediated by humanized livers in chimeric mice are summarized in terms of biliary/urinary excretions of phthalate and bisphenol A and plasma clearances of the human cocktail probe drugs caffeine, warfarin, omeprazole, metoprolol, and midazolam. Simulation of human plasma concentrations of the teratogen thalidomide and its human metabolites is possible with a simplified physiologically based pharmacokinetic model based on data obtained in chimeric mice, in accordance with reported clinical thalidomide concentrations. In addition, in vivo nonspecific hepatic protein binding parameters of metabolically activated ¹⁴C-drug candidate and hepatotoxic medicines in humanized liver mice can be analyzed by accelerator mass spectrometry and are useful for predictions in humans.