Mode of action based risk assessment of the botanical food-borne alkenylbenzene apiol from parsley using physiologically based kinetic (PBK) modelling and read-across from safrole.

The present study developed physiologically-based kinetic (PBK) models for the alkenylbenzene apiol in order to facilitate risk assessment based on read-across from the related alkenylbenzene safrole. Model predictions indicate that in rat liver the formation of the 1'-sulfoxy metabolite is about 3 times lower for apiol than for safrole. These data support that the lower confidence limit of the benchmark dose resulting in a 10% extra cancer incidence (BMDL10) that would be obtained in a rodent carcinogenicity study with apiol may be 3-fold higher for apiol than for safrole. These results enable a preliminary risk assessment for apiol, for which tumor data are not available, using a BMDL10 value of 3 times the BMDL10 for safrole. Based on an estimated BMDL10 for apiol of 5.7-15.3 mg/kg body wt per day and an estimated daily intake of 4 × 10(-5) mg/kg body wt per day, the margin of exposure (MOE) would amount to 140,000-385,000. This indicates a low priority for risk management. The present study shows how PBK modelling can contribute to the development of alternatives for animal testing, facilitating read-across from compounds for which in vivo toxicity studies on tumor formation are available to compounds for which these data are unavailable.

Evaluation of the interindividual human variation in bioactivation of methyleugenol using physiologically based kinetic modeling and Monte Carlo simulations.

The present study aims at predicting the level of formation of the ultimate carcinogenic metabolite of methyleugenol, 1'-sulfooxymethyleugenol, in the human population by taking variability in key bioactivation and detoxification reactions into account using Monte Carlo simulations. Depending on the metabolic route, variation was simulated based on kinetic constants obtained from incubations with a range of individual human liver fractions or by combining kinetic constants obtained for specific isoenzymes with literature reported human variation in the activity of these enzymes. The results of the study indicate that formation of 1'-sulfooxymethyleugenol is predominantly affected by variation in i) P450 1A2-catalyzed bioactivation of methyleugenol to 1'-hydroxymethyleugenol, ii) P450 2B6-catalyzed epoxidation of methyleugenol, iii) the apparent kinetic constants for oxidation of 1'-hydroxymethyleugenol, and iv) the apparent kinetic constants for sulfation of 1'-hydroxymethyleugenol. Based on the Monte Carlo simulations a so-called chemical-specific adjustment factor (CSAF) for intraspecies variation could be derived by dividing different percentiles by the 50th percentile of the predicted population distribution for 1'-sulfooxymethyleugenol formation. The obtained CSAF value at the 90th percentile was 3.2, indicating that the default uncertainty factor of 3.16 for human variability in kinetics may adequately cover the variation within 90% of the population. Covering 99% of the population requires a larger uncertainty factor of 6.4. In conclusion, the results showed that adequate predictions on interindividual human variation can be made with Monte Carlo-based PBK modeling. For methyleugenol this variation was observed to be in line with the default variation generally assumed in risk assessment.

Inhibition of methyleugenol bioactivation by the herb-based constituent nevadensin and prediction of possible in vivo consequences using physiologically based kinetic modeling.

Methyleugenol (ME) occurs naturally in a variety of spices, herbs, including basil, and their essential oils. ME induces hepatomas in rodent bioassays following its conversion to a DNA reactive metabolite. In the present study, the basil constituent nevadensin was shown to be able to inhibit SULT-mediated DNA adduct formation in HepG2 cells exposed to the proximate carcinogen 1'-hydroxymethyleugenol in the presence of nevadensin. To investigate possible in vivo implications of SULT inhibition by nevadensin on ME bioactivation, the rat physiologically based kinetic (PBK) model developed in our previous work to describe the dose-dependent bioactivation and detoxification of ME in male rat was combined with the recently developed PBK model describing the dose-dependent kinetics of nevadensin in male rat. The resulting binary ME-nevadensin PBK model was used to predict the possible nevadensin mediated reduction in ME DNA adduct formation and resulting carcinogenicity at the doses of ME used by the NTP carcinogenicity study. Using these data an updated risk assessment using the Margin of Exposure (MOE) approach was performed. The results obtained point at a potential reduction of the cancer risk when rodents are orally exposed to ME within a relevant food matrix containing SULT inhibitors compared to exposure to pure ME.

Physiologically based kinetic modeling of bioactivation and detoxification of the alkenylbenzene methyleugenol in human as compared with rat.

This study defines a physiologically based kinetic (PBK) model for methyleugenol (ME) in human based on in vitro and in silico derived parameters. With the model obtained, bioactivation and detoxification of methyleugenol (ME) at different doses levels could be investigated. The outcomes of the current model were compared with those of a previously developed PBK model for methyleugenol (ME) in male rat. The results obtained reveal that formation of 1'-hydroxymethyleugenol glucuronide (1'HMEG), a major metabolic pathway in male rat liver, appears to represent a minor metabolic pathway in human liver whereas in human liver a significantly higher formation of 1'-oxomethyleugenol (1'OME) compared with male rat liver is observed. Furthermore, formation of 1'-sulfooxymethyleugenol (1'HMES), which readily undergoes desulfonation to a reactive carbonium ion (CA) that can form DNA or protein adducts (DA), is predicted to be the same in the liver of both human and male rat at oral doses of 0.0034 and 300 mg/kg bw. Altogether despite a significant difference in especially the metabolic pathways of the proximate carcinogenic metabolite 1'-hydroxymethyleugenol (1'HME) between human and male rat, the influence of species differences on the ultimate overall bioactivation of methyleugenol (ME) to 1'-sulfooxymethyleugenol (1'HMES) appears to be negligible. Moreover, the PBK model predicted the formation of 1'-sulfooxymethyleugenol (1'HMES) in the liver of human and rat to be linear from doses as high as the benchmark dose (BMD10) down to as low as the virtual safe dose (VSD). This study shows that kinetic data do not provide a reason to argue against linear extrapolation from the rat tumor data to the human situation.

Physiologically based biokinetic model of bioactivation and detoxification of the alkenylbenzene methyleugenol in rat.

The present study defines a physiologically based biokinetic (PBBK) model for the alkenylbenzene methyleugenol in rat based on in vitro metabolic parameters determined using relevant tissue fractions, in silico derived partition coefficients, and physiological parameters derived from the literature. The model was based on the model previously developed for the related alkenylbenzene estragole and consists of eight compartments including liver, lung, and kidney as metabolizing compartments, and separate compartments for fat, arterial blood, venous blood, richly perfused and slowly perfused tissues. Evaluation of the model was performed by comparing the PBBK predicted concentration of methyleugenol in the venous compartment to methyleugenol plasma levels reported in the literature, by comparing the PBBK predicted dose-dependent percentage of formation of 2-hydroxy-4,5-dimethoxyallylbenzene, 3-hydroxy-4-methoxyallylbenzene, and 1'-hydroxymethyleugenol glucuronide to the corresponding percentage of metabolites excreted in urine reported in the literature, which were demonstrated to be in the same order of magnitude. With the model obtained the relative extent of bioactivation and detoxification of methyleugenol at different oral doses was examined. At low doses, formation of 3-(3,4-dimethoxyphenyl)-2-propen-1-ol and methyleugenol-2',3'-oxide leading to detoxification appear to be the major metabolic pathways, occurring in the liver. At high doses, the model reveals a relative increase in the formation of the proximate carcinogenic metabolite 1'-hydroxymethyleugenol, occurring in the liver. This relative increase in formation of 1'-hydroxymethyleugenol leads to a relative increase in formation of 1'-hydroxymethyleugenol glucuronide, 1'-oxomethyleugenol, and 1'-sulfooxymethyleugenol the latter being the ultimate carcinogenic metabolite of methyleugenol. These results indicate that the relative importance of different metabolic pathways of methyleugenol may vary in a dose-dependent way, leading to a relative increase in bioactiviation of methyleugenol at higher doses.