ABSTRACT:Glucuronidation is a listed clearance mechanism for 1 in 10 of the top 200 prescribed drugs. The objective of this article is to encourage those studying ligand interactions with UDP-glucuronosyltransferases (UGTs) to adequately consider the potential consequences of in vitro UGT inhibition in humans. Spurred on by interest in developing potent and selective inhibitors for improved confidence around UGT reaction phenotyping, and the increased availability of recombinant forms of human UGTs, several recent studies have reported in vitro inhibition of UGT enzymes. In some cases, the observed potency of UGT inhibitors in vitro has been interpreted as having potential relevance in humans via pharmacokinetic drug-drug interactions. Although there are reported examples of clinically relevant drug-drug interactions for UGT substrates, exposure increases of the aglycone are rarely greater than 100% in the presence of an inhibitor relative to its absence (i.e., AUC i /AUC <2). This small magnitude in change is in contrast to drugs primarily cleared by cytochrome P450 enzymes, where exposures have been reported to increase as much as 35-fold on coadministration with an inhibitor (e.g., ketoconazole inhibition of CYP3A4-catalyzed terfenadine metabolism). In this article the evidence for purported clinical relevance of potent in vitro inhibition of UGT enzymes will be assessed, taking the following into account: in vitro data on the enzymology of glucuronide formation from aglycone, pharmacokinetic principles based on empirical data for inhibition of metabolism, and clinical data on the pharmacokinetic drug-drug interactions of drugs primarily cleared by glucuronidation.
Nonclinical rodent and nonrodent toxicity models used to support clinical trials of candidate drugs may produce discordant results or fail to predict complications in humans, contributing to drug failures in the clinic. Here, we applied microengineered Organs-on-Chips technology to design a rat, dog, and human Liver-Chip containing species-specific primary hepatocytes interfaced with liver sinusoidal endothelial cells, with or without Kupffer cells and hepatic stellate cells, cultured under physiological fluid flow. The Liver-Chip detected diverse phenotypes of liver toxicity, including hepatocellular injury, steatosis, cholestasis, and fibrosis, and species-specific toxicities when treated with tool compounds. A multispecies Liver-Chip may provide a useful platform for prediction of liver toxicity and inform human relevance of liver toxicities detected in animal studies to better determine safety and human risk.
A combined protein and pharmacophore model for cytochrome P450 2D6 (CYP2D6) has been derived using various computational chemistry techniques. A combination of pharmacophore modeling (using 40 substrates), protein modeling, and molecular orbital calculations was necessary to derive a model which incorporated steric, electronic, and chemical stability properties. The initial pharmacophore and protein models used to construct the combined model were derived independently and showed a high level of complementarity. The combined model is in agreement with experimental results concerning the substrates used to derive the model, with site-directed mutagenesis data available for the CYP2D6 protein, and takes into account the site-directed mutagenesis results for a variety of other 2-family P450s.
Aims To characterize the cytochrome P450 (CYP) enzymes responsible for the N-demethylation of sildena®l to its main metabolite, UK-103 320, to investigate the potential inhibitory effects of sildena®l on CYP enzymes and to evaluate the potential of selected drugs to affect sildena®l metabolism. Methods The metabolic pathways of sildena®l N-demethylation were studied using human liver microsomes, as well as microsomes expressing individual human CYP enzymes. Further studies to identify the individual enzymes were performed at 2.5 and 250 mM sildena®l, and employed a combination of chemical inhibition, correlation analysis, and metabolism by expressed recombinant CYP enzymes. In addition, the effect of sildena®l on the activity of the six major drug metabolizing enzymes was investigated.Results Sildena®l conversion was found to be mediated by at least two CYP enzymes, for which the mean kinetic parameters were K m1 =6(t3 mM), K m2 =81(t45 mM),. At 250 mM sildena®l, N-demethylation was primarily mediated through the low-af®nity, high-K m enzyme (approximately 83%), whilst at 2.5 mM there was a greater role for the high-af®nity, low-K m enzyme (approximately 61%). Ketoconazole strongly inhibited metabolism at both sildena®l concentrations and was the only signi®cant inhibitor at 250 mM sildena®l. At the lower sildena®l concentration, sulphaphenazole and quinidine also inhibited formation of UK-103 320. Overall, 75% or more of the N-demethylation of sildena®l at any concentration is probably attributable to CYP3A4. These results were supported by experiments using expressed human CYP enzymes, in which only CYP3A4 and CYP2C9 exhibited substantial sildena®l N-demethylase activity (respective K m values of 221 mM and 27 mM).Sildena®l metabolism was inhibited by potent CYP3A4 inhibitors which are used clinically, but was found to be only a weak inhibitor of drug metabolizing enzymes itself, the strongest inhibition occurring against CYP2C9 (K i =80 mM).Conclusions Evidence is provided for CYP3A4 and to a lesser extent CYP2C9-mediated metabolism of sildena®l. There is the possibility that elevated plasma concentrations of sildena®l could occur with coadministration of known inhibitors of CYP2C9 or CYP3A4. Since peak plasma concentrations of clinical doses of sildena®l are only 200 ng ml x1 (y0.4 mM) it is very unlikely that sildena®l will signi®cantly alter the plasma concentration of other compounds metabolized by cytochrome P450 enzymes.
The results from this evaluation demonstrate the utility of PBPK methodology for the prediction of human pharmacokinetics. This methodology can be applied at different stages to enhance the understanding of the compounds in a particular chemical series, guide experiments, aid candidate selection and inform clinical trial design.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.