Organ interactions resulting from drug, metabolite or xenobiotic transport between organs are key components of human metabolism that impact therapeutic action and toxic side effects. Preclinical animal testing often fails to predict adverse outcomes arising from sequential, multi-organ metabolism of drugs and xenobiotics. Human microphysiological systems (MPS) can model these interactions and are predicted to dramatically improve the efficiency of the drug development process. In this study, five human MPS models were evaluated for functional coupling, defined as the determination of organ interactions via an in vivo-like sequential, organ-to-organ transfer of media. MPS models representing the major absorption, metabolism and clearance organs (the jejunum, liver and kidney) were evaluated, along with skeletal muscle and neurovascular models. Three compounds were evaluated for organ-specific processing: terfenadine for pharmacokinetics (PK) and toxicity; trimethylamine (TMA) as a potentially toxic microbiome metabolite; and vitamin D3. We show that the organ-specific processing of these compounds was consistent with clinical data, and discovered that trimethylamine-N-oxide (TMAO) crosses the blood-brain barrier. These studies demonstrate the potential of human MPS for multi-organ toxicity and absorption, distribution, metabolism and excretion (ADME), provide guidance for physically coupling MPS, and offer an approach to coupling MPS with distinct media and perfusion requirements.

The kidney proximal tubule is the primary site in the nephron for excretion of waste products through a combination of active uptake and secretory processes, and is also a primary target of drug-induced nephrotoxicity. Here, we describe the development and functional characterization of a 3-dimensional flow-directed human kidney proximal tubule microphysiological system. The system replicates the polarity of the proximal tubule, expresses appropriate marker proteins, exhibits biochemical and synthetic activities, as well as secretory and reabsorptive processes associated with proximal tubule function in vivo. This microphysiological system can serve as an ideal platform for ex vivo modeling of renal drug clearance and drug-induced nephrotoxicity. Additionally, this novel system can be used for preclinical screening of new chemical compounds prior to initiating human clinical trials.

The flavin-containing monooxygenases (FMOs) are important for the metabolism of numerous therapeutics and toxicants. Six mammalian FMO genes (FMO1-6) have been identified, each exhibiting developmental and tissue-and species-specific expression patterns. Previous studies demonstrated that human hepatic FMO1 is restricted to the fetus whereas FMO3 is the major adult isoform. These studies failed to describe temporal expression patterns, the precise timing of the FMO1/FMO3 switch, or potential control mechanisms. To address these questions, FMO1 and FMO3 were quantified in microsomal fractions from 240 human liver samples representing ages from 8 wk gestation to 18 y using Western blotting. FMO1 expression was highest in the embryo (8 -15 wk gestation; 7.8 Ϯ 5.3 pmol/mg protein). Low levels of FMO3 expression also were detectable in the embryo, but not in the fetus. FMO1 suppression occurred within 3 d postpartum in a process tightly coupled to birth, but not gestational age. The onset of FMO3 expression was highly variable, with most individuals failing to express this isoform during the neonatal period. FMO3 was detectable in most individuals by 1-2 y of age and was expressed at intermediate levels until 11 y (12.7 Ϯ 8.0 pmol/mg protein). These data suggest that birth is necessary, but not sufficient for the onset of FMO3 expression. A gender-independent increase in FMO3 expression was observed from 11 to 18 y of age (26.9 Ϯ 8.6 pmol/mg protein). Finally, 2-to 20-fold interindividual variation in FMO1 and FMO3 protein levels were observed, depending on the age bracket. The FMOs (EC 1.14.13.8) are important for the NADPHdependent oxidative metabolism of a wide variety of compounds containing nucleophilic nitrogen-, sulfur-, selenium-, and phosphorous-heteroatoms (1-4). Examples of known substrates of relevance to pediatric therapeutics include the antipsychotic chlorpromazine (5), the antihistaminics promethazine (6) and brompheniramine (7), the H 2 -receptor antagonists cimetidine (8) and ranitidine (9), and the gastroprokinetic agent itopride (10). However, given the prevalence of nitrogen-and sulfur-heteroatoms in medicinals, this short list is likely a gross underestimate of FMO's contribution to pediatric drug disposition. Environmental agents of particular concern include several thioether-containing organophosphorous pesticides (11), the carcinogen 2-aminofluorene (12), and the neurotoxicants nicotine (13) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (14). Finally, a few dietary and/or endogenous FMO substrates have been identified, including trimethylamine, a break-down product of dietary choline (15), cysteamine (16), methionine, and several cysteine-Sconjugates (3).Unlike the numerous cytochrome P450-dependent monooxygenases, there are only six mammalian FMO enzymes, each encoded by a distinct gene located on the long arm of human chromosome 1 (17, 18). As such, the FMO are considered more versatile with regard to substrate specificity than the cytochrome P450-dependent monooxygenases, a feat...

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The pharmacokinetics of non-renally cleared drugs in patients with chronic kidney disease is often unpredictable. Some of this variability may be due to alterations in the expression and activity of extra-renal drug metabolizing enzymes and transporters, primarily localized in the liver and intestine. Studies conducted in rodent models of renal failure have shown decreased mRNA and protein expression of many members of the cytochrome P450 enzyme (CYP) gene family and the ATP-Binding Cassette (ABC) and Solute Carrier (SLC) gene families of drug transporters. Uremic toxins interfere with transcriptional activation, cause down-regulation of gene expression mediated by proinflammatory cytokines, and directly inhibit the activity of the cytochrome P450s and drug transporters. While much has been learned about the effects of kidney disease on non-renal drug disposition, important questions remain regarding the mechanisms of these effects, as well as the interplay between drug metabolizing enzymes and drug transporters in the uremic milieu. In this review, we have highlighted the existing gaps in our knowledge and understanding of the impact of chronic kidney disease on non-renal drug clearance, and identified areas of opportunity for future research.

Retinoic acid (RA) is a critical signaling molecule that performs multiple functions required to maintain cellular viability. It is also used in the treatment of some cancers. Enzymes in the CYP26 family are thought to be responsible for the elimination of RA, and CYP26A1 appears to serve the most critical functions in this family. In spite of its importance, CYP26A1 has neither been heterologously expressed nor been characterized kinetically. We expressed the rCYP26A1 in baculovirus infected insect cells and purified the hexahistidine tagged protein to homogeneity. Heme incorporation was determined by carbon monoxide difference spectrum and a type 1 spectrum was observed with RA binding to CYP26A1. We found that RA is a tight binding ligand of CYP26A1 with low nM binding affinity. CYP26A1 oxidized RA efficiently (depletion K m 9.4 ± 3.3 nM and V max 11.3 ± 4.3 pmoles/min/pmole P450) when supplemented with P450 oxidoreductase and NADPH but was independent of cytochrome b5. 4-Hydroxy-RA (4-OH-RA) was the major metabolite produced by rCYP26A1 but two other primary products were also formed. 4-OH-RA was further metabolized by CYP26A1 to more polar metabolites and this sequential metabolism of RA occurred in part without 4-OH-RA leaving the active site of CYP26A1. The high efficiency of CYP26A1 in eliminating both RA and its potentially active metabolites supports the major role of this enzyme in regulating RA clearance in vivo. These results provide a biochemical framework for CYP26A1 function and offer insight into the role of CYP26A1 as a drug target as well as in fetal development and cell cycle regulation.

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The potential of metabolites to contribute to drug-drug interactions is not well defined. The aim of this study was to determine the quantitative role of circulating metabolites in inhibitory drug-drug interactions in vivo. The AUC data for at least one circulating metabolite was available for 71% of the 102 inhibitors identified. 78% of the 80 metabolites characterized at steady state had AUCs greater than 10% of the parent drug. A comparison of the metabolite’s and parent’s [I]/Ki showed that 17 of the 21 (80%) reversible inhibitors studied had metabolites that are likely to contribute to in vivo drug-drug interactions, with some metabolites predicted to be the major inhibitors. The in vivo drug interaction risk of amiodarone, bupropion and sertraline could only be identified from in vitro data when metabolites were included into predictions. In conclusion, circulating metabolites are common with CYP inhibitors and do contribute to clinically observed CYP inhibition.