P450 2E1 expression in liver, kidney, and lung of rats treated with single or combined inducers

Risk characterization of N-nitrosodimethylamine in pharmaceuticals

Since 2018, N-nitrosodimethylamine (NDMA) has been a reported contaminant in numerous pharmaceutical products. To guide the pharmaceutical industry, FDA identified an acceptable intake (AI) of 96 ng/day NDMA. The approach assumed a linear extrapolation from the Carcinogenic Potency Database (CPDB) harmonic-mean TD50 identified in chronic studies in rats. Although NDMA has been thought to act as a mutagenic carcinogen in experimental animals, it has not been classified as a known human carcinogen by any regulatory agency. Humans are exposed to high daily exogenous and endogenous doses of NDMA. Due to the likelihood of a threshold dose for NDMA-related tumors in animals, we believe that there is ample scientific basis to utilize the threshold-based benchmark dose or point-of-departure (POD) approach when estimating a Permissible Daily Exposure limit (PDE) for NDMA. We estimated that 29,000 ng/kg/day was an appropriate POD for calculating a PDE. Assuming an average bodyweight of 50 kg, we expect that human exposures to NDMA at doses below 5800 ng/day in pharmaceuticals would not result in an increased risk of liver cancer, and that there is little, if any, risk for any other type of cancer, when accounting for the mode-of-action in humans.

The impact of CYP2E1 genetic variability on risk assessment of VOC mixtures

Humans are simultaneously exposed to multiple chemicals in the environment. Many of the chemicals use the same enzymes in their metabolic pathways. Competitive inhibition may occur as one of the possible interactions between the xenobiotics in human body. For example, many volatile organic compounds (VOCs) are metabolized using P450 enzymes, specifically CYP2E1. Inheritable gene alterations may result in changes of function of the enzymes in different human subpopulations. Variations in quantity and/or quality of particular isoenzymes may cause differences in the metabolism of VOCs. These variations may cause higher sensitivity in certain populations. Using examples of three different mixtures, this review paper outlines the variances in CYP2E1 isoenzymes, effect of exposure to such mixtures on sensitive populations, and approaches to mixtures risk assessment.

The effect of ethanol and nitric oxide on the N-nitrosodimethylamine formation in HepG2 cells overexpressing CYP2E1

The influence of lipopolysaccharide (LPS) and the nitric oxide synthase (iNOS) inhibitor--N-nitro-L-arginine methyl ester (L-NAME)--on the formation of N-nitrosodimethylamine (NDMA) by HepG2 cells, engineered to overexpress CYP2E1, was assessed and compared with data from empty vector-transfected cells. HepG2 cells produced significant amounts of NDMA but its levels in the culture media of cells overexpressing CYP2E1 was significantly lower than in empty-vector transfected cells. LPS increased the formation of NDMA, the expression of the iNOS and the production of the nitric oxide (NO). On the other hand, L-NAME significantly decreased NDMA levels. The results above indicate that the synthesis of NDMA by HepG2 cells depends on NO production. Furthermore, ethanol did not affect iNOS expression but decreased NDMA levels in CYP2E1-transfected cells below the detection limit. It is probably caused by the increased N-nitrosodimethylamine metabolism. In conclusion, HepG2 cells' ability to synthesize NO with simultaneous CYP2E1 activation may lead to an increase of carcinogenic products of the NDMA metabolism.

The use of in vitro metabolic parameters and physiologically based pharmacokinetic (PBPK) modeling to explore the risk assessment of trichloroethylene

A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,1,2-trichloroethene, TRI) for rat and humans, based on in vitro metabolic parameters. These were obtained using individual cytochrome P450 and glutathione S-transferase enzymes. The main enzymes involved both for rats and humans are CYP2E1 and the μ- and π-class glutathione S-transferases. Validation experiments were performed in order to test the predictive value of the enzyme kinetic parameters to describe 'whole-body' disposition. Male Wistar rats were dosed orally or intravenously with different doses of trichloroethylene. Obtained exhaled radioactivity, excreted radioactivity in urine, and obtained blood concentration-time curves of trichloroethylene for all dosing groups were compared to predictions from the PBPK model. Subsequently, using the scaling factor derived from the rat experiments predictions were made for the extreme cases to be expected in humans, based on interindividual variations of the key enzymes involved. On comparing these predictions with literature data a very close match was found. This illustrates the potential application of in vitro metabolic parameters in risk assessment, through the use of PBPK modeling as a tool to understand and predict in vivo data. From a hypothetical 8 h exposure scenario to 35 ppm trichloroethylene in rats and humans, and assuming that the glutathione S-transferase pathway is responsible for the toxicity of trichloroethylene, it was concluded that humans are less sensitive for trichloroethylene toxicity than rats.

A physiologically-based pharmacokinetic(PB-PK) model for ethylene dibromide: relevance of extrahepatic metabolism

A physiologically-based pharmacokinetic (PB-PK) model was developed for ethylene dibromide (1,2-dibromoethane, EDB) for rats and humans, partly based on previously published in vitro data (Ploemen et al., 1997). In the present study, this PB-PK model has been validated for the rat. In addition, new data were used for the human class ThetaGST T1-1. Validation experiments are described in order to test the predictive value of kinetics to describe "whole-body" metabolism. For the validation experiments, groups of cannulated rats were dosed orally or intravenously with different doses of EDB. Obtained blood concentration-time curves of EDB for all dosing groups were compared to model predictions. It appeared that metabolism, which previously was assumed to be restricted to the liver, was underestimated. Therefore, we extended the PB-PK model to include all the extrahepatic organs, in which the enzymes involved in EDB metabolism have been detected and quantified. With this extended model, the blood concentrations were much more accurately described compared to the predictions of the "liver-model". Therefore, extrahepatic metabolism was also included in the human model. The present study illustrates the potential application of in vitro metabolic parameters in risk assessment, as well as the use of PB-PK modelling as a tool to understand and predict in vivo data.

Formation of 20-hydroxyeicosatetraenoic acid, a vasoactive and natriuretic eicosanoid, in human kidney. Role of Cyp4F2 and Cyp4A11

20-hydroxyeicosatetraenoic acid (20-HETE), an omega-hydroxylated arachidonic acid (AA) metabolite, elicits specific effects on kidney vascular and tubular function that, in turn, influence blood pressure control. The human kidney's capacity to convert AA to 20-HETE is unclear, however, as is the underlying P450 catalyst. Microsomes from human kidney cortex were found to convert AA to a single major product, namely 20-HETE, but failed to catalyze AA epoxygenation and midchain hydroxylation. Despite the monophasic nature of renal AA omega-hydroxylation kinetics, immunochemical studies revealed participation of two P450s, CYP4F2 and CYP4A11, since antibodies to these enzymes inhibited 20-HETE formation by 65. 9 +/- 17 and 32.5 +/- 14%, respectively. Western blotting confirmed abundant expression of these CYP4 proteins in human kidney and revealed that other AA-oxidizing P450s, including CYP2C8, CYP2C9, and CYP2E1, were not expressed. Immunocytochemistry showed CYP4F2 and CYP4A11 expression in only the S2 and S3 segments of proximal tubules in cortex and outer medulla. Our results demonstrate that CYP4F2 and CYP4A11 underlie conversion of AA to 20-HETE, a natriuretic and vasoactive eicosanoid, in human kidney. Considering their proximal tubular localization, these P450 enzymes may partake in pivotal renal functions, including the regulation of salt and water balance, and arterial blood pressure itself.

Evaluation of rat hepatic 2E1 activity in function of age, sex and inducers: choice of an experimental model capable of testing the hepatotoxicity of low molecular weight compounds

The aim of this work on rat hepatic P450 2E1 activity was to seek the most suitable experimental model to study the role of cytochrome P450 2E1 in the metabolism of industrial chemicals. Two sets of experiments were devoted to selecting the age and sex of animals and to estimating the response of male and female rats to different inducers. In the first set, the effect of three inducers (fasting; ethanol; acetone) was studied in male rats aged 5, 7 and 9 weeks. In the second set, the effect of different inducers, namely beta-naphthoflavone (BNF), phenobarbital (PB), ethanol, acetone and pyridine, on PNP and chlorzoxazone (CLZO) hydroxylase activities was studied in 7 week old male and female rats. The results demonstrate firstly that microsomal p-nitrophenol (PNP) hydroxylase activity significantly decreases in control male rats in inverse function of age, and secondly that induction by ethanol decreases with age. The PNP hydroxylase activity level of controls and the significant increases in PNP hydroxylase activity observed in 7 week old male rats show that this is the most suitable age for the second set of experiments. In this second set, it was shown that P450 1A (induced by BNF) is involved in CLZO hydroxylase activity only. PB increased the hydroxylase activities in male and female rats by about 1.5 and 1.7 times those of the controls, respectively. The effects of P450 2E1 inducers in function of sex show that male rats exhibited more significant increases in PNP and CLZO hydroxylase activities than female. The specificity of these two substrates is discussed. Neither of these two reactions was specifically catalysed by P450 2E1, but PNP may be considered as the most specific and the least sensitive substrate. In addition, the linear relationship observed between the two substrates (PNP and CLZO) showed a good correlation between their activities (r = 0.90, P < 0.001). In conclusion, these results suggest the use of the 7 week old male rat as the experimental model to study the role of cytochrome P450 2E1 in the hepatotoxicity of low molecular weight industrial chemicals.

Effect of pyrazole and dexamethasone administration on cytochrome P450 2E1 and 3A isoforms in rat liver and kidney: lack of specificity of p-nitrophenol as a substrate of P450 2E1

The induction effects of pyrazole and dexamethasone (known to be specific to P450 2E1 and 3A enzymes, respectively), given alone or simultaneously, were studied in rat liver and kidney microsomes. Pyrazole treatment induced the catalytic activity and the amount of P450 2E1 enzyme in both organs. Immunoreactive P450 2E1 and 4-nitrophenol 2-hydroxylation increased 8- and 13-fold, respectively (versus control), in the kidney, but only 2.4- and 2.7-fold (versus control) in the liver after pyrazole treatment. As assessed by nifedipine oxidation activity, dexamethasone treatment increased the P450 3A catalytic activity approximately 4-fold (versus control) in the liver, but not in the kidney, suggesting that P450 3A was not inducible in the kidney. Pyrazole decreased P450 3A activity in the liver but did not modify it in the kidney. A combination of both chemicals induced both enzymes, but to a lesser extent than treatment with each single chemical compound. Furthermore, the 2-hydroxylation of p-nitrophenol, considered one of the most specific substrates for monitoring the level of P450 2E1, was mediated also by P450 3A, at least in dexamethasone-treated rats. Finally, this experimental work demonstrated that P450 3A induction is organ-specific, and it also demonstrated the lack of specificity of p-nitrophenol as a P450 2E1 substrate.

Noninvolvement of CYP2E1 in the (omega-1)-hydroxylation of fatty acids in rat kidney microsomes

Pyrazole, acetone, and ethanol are known to induce cytochrome P450 2E1 (CYP2E1) and fatty acid (omega-1)-hydroxylation in rat liver microsomes. However, the nature of the P450 enzyme involved in this (omega-1)-hydroxylation has not been clearly established in extrahepatic tissues such as kidney. Four enzymatic activities (hydroxylations of chlorzoxazone, 4-nitrophenol, and two fatty acids) were assayed in kidney microsomal preparations of rats treated with CYP2E1 inducers. Per os treatment resulted in large increases (threefold to fivefold) in the chlorzoxazone and 4-nitrophenol hydroxylations, and up to a ninefold increase when ethanol was administered by inhalation. However, neither the omega-hydroxylation nor the (omega-1)-hydroxylation of fatty acids was modified. Immunoinhibition specific to CYP2E1 did not significantly decrease the omega and (omega-1)-lauric acid hydroxylations, while the polyclonal anti-CYP4A1 antibody inhibited in part both the omega- and (omega-1)-hydroxylations. Chemical inhibitions using either CYP2E1 competitive inhibitors (such as chlorzoxazone, DMSO, and ethanol) or P450 mechanism-based inhibitors (such as diethyldithiocarbamate and 17-octadecynoic acid) led to a partial inhibition of the hydroxylations. All these results suggest that fatty acid (omega-1)-hydroxylation, a highly specific probe for CYP2E1 in rat and human liver microsomes, is not mediated by CYP2E1 in rat kidney microsomes. In contrast to liver, where two different P450 enzymes are involved in fatty acid omega- and (omega-1)-hydroxylations, the same P450 enzyme, mainly a member of the CYP4A family, was involved in both hydroxylations in rat renal microsomes.