A Physiologically Based in Silico Model for trans-2-Hexenal Detoxification and DNA Adduct Formation in Rat

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Defining in vivo dose-response curves for kidney DNA adduct formation of aristolochic acid I in rat, mouse and human by an in vitro and physiologically based kinetic modeling approach

Aristolochic acid I (AAI) is a well-known genotoxic kidney carcinogen. Metabolic conversion of AAI into the DNA-reactive aristolactam-nitrenium ion is involved in the mode of action of tumor formation. This study aims to predict in vivo AAI-DNA adduct formation in the kidney of rat, mouse and human by translating the in vitro concentration-response curves for AAI-DNA adduct formation to the in vivo situation using physiologically based kinetic (PBK) modeling-based reverse dosimetry. DNA adduct formation in kidney proximal tubular LLC-PK1 cells exposed to AAI was quantified by liquid chromatography-electrospray ionization-tandem mass spectrometry. Subsequently, the in vitro concentration-response curves were converted to predicted in vivo dose-response curves in rat, mouse and human kidney using PBK models. Results obtained revealed a dose-dependent increase in AAI-DNA adduct formation in the rat, mouse and human kidney and the predicted DNA adduct levels were generally within an order of magnitude compared with values reported in the literature. It is concluded that the combined in vitro PBK modeling approach provides a novel way to define in vivo dose-response curves for kidney DNA adduct formation in rat, mouse and human and contributes to the reduction, refinement and replacement of animal testing.

The safety evaluation of food flavoring substances: the role of genotoxicity studies

The Flavor and Extract Manufacturers Association (FEMA) Expert Panel relies on the weight of evidence from all available data in the safety evaluation of flavoring substances. This process includes data from genotoxicity studies designed to assess the potential of a chemical agent to react with DNA or otherwise cause changes to DNA, either in vitro or in vivo. The Panel has reviewed a large number of in vitro and in vivo genotoxicity studies during the course of its ongoing safety evaluations of flavorings. The adherence of genotoxicity studies to standardized protocols and guidelines, the biological relevance of the results from those studies, and the human relevance of these studies are all important considerations in assessing whether the results raise specific concerns for genotoxic potential. The Panel evaluates genotoxicity studies not only for evidence of genotoxicity hazard, but also for the probability of risk to the consumer in the context of exposure from their use as flavoring substances. The majority of flavoring substances have given no indication of genotoxic potential in studies evaluated by the FEMA Expert Panel. Examples illustrating the assessment of genotoxicity data for flavoring substances and the consideration of the factors noted above are provided. The weight of evidence approach adopted by the FEMA Expert Panel leads to a rational assessment of risk associated with consumer intake of flavoring substances under the conditions of use.

Scientific Opinion on Flavouring Group Evaluation 71 Revision 1 (FGE.71Rev1): consideration of aliphatic, linear, α,β-unsaturated alcohols, aldehydes, carboxylic acids, and related esters evaluated by JECFA (63rd and 69th meeting) structurally related to flavouring substances evaluated in FGE.05Rev3

The EFSA Panel on Food Additives and Flavourings was requested to evaluate 39 flavouring substances assigned to the Flavouring Group Evaluation 71 (FGE.71), using the Procedure in Commission Regulation (EC) No 1565/2000. Nine substances have already been considered in FGE.71 [FL-no: 08.054, 08.073, 08.123, 09.037, 09.156, 09.157, 05.158, 09.235, 09.239]. The remaining 30 substances [FL-no: 02.020, 02.050, 02.090, 02.112, 02.137, 02.156, 02.210, 05.037, 05.060, 05.070, 05.073, 05.076, 05.078, 05.102, 05.109, 05.150, 05.171, 05.179, 09.276, 09.277, 09.303, 09.385, 09.394, 09.395, 09.396, 09.397, 09.398, 09.399, 09.678 and 09.841] have been cleared with respect to genotoxicity in FGE.200Rev1 and they are considered in this revision. The substances were evaluated through a stepwise approach that integrates information on the structure-activity relationships, intake from current uses, toxicological threshold of concern (TTC), and available data on metabolism and toxicity. The Panel concluded that none of the 39 substances gives rise to safety concerns at their levels of dietary intake, estimated on the basis of the 'Maximised Survey-derived Daily Intake' (MSDI) approach. Besides the safety assessment of the flavouring substances, the specifications for the materials of commerce have also been considered and found adequate, except for [FL-no: 08.073 and 09.235]. For these two substances, data on the composition of the stereoisomeric mixture should be requested. Normal and maximum use levels should be provided for nine flavouring substances [FL-no: 08.054, 08.073, 08.123, 09.037, 09.156, 09.157, 05.158, 09.235, 09.239]. For two flavouring substances [FL-no: 02.020 and 05.076], the 'modified Theoretical Added Maximum Daily Intake' (mTAMDI) estimates are above the TTC for their structural class I. Therefore, additional information on uses and use levels should be provided for these eleven substances in order to finalise their evaluation.

( E)-2-Hexenal, as a Potential Natural Antifungal Compound, Inhibits Aspergillus flavus Spore Germination by Disrupting Mitochondrial Energy Metabolism

Fungal contamination imposes threats to agriculture and food production and human health. A method to safely and effectively restrict fungal contamination is still needed. Here, we report the effect and mode of action of ( E)-2-hexenal, one of the green leaf volatiles (GLVs), on the spore germination of Aspergillus flavus, which can contaminate a variety of crops. The EC₅₀ value, minimum inhibitory concentration (MIC), and minimum fungicidal concentration (MFC) of ( E)-2-hexenal were 0.26, 1.0, and 4.0 μL/mL, respectively. As observed by scanning electron microscopy (SEM), the surface morphology of A. flavus spores did not change after treatment with the MIC of ( E)-2-hexenal, but the spores were shrunken and depressed upon treatment with the MFC of ( E)-2-hexenal. The MIC and MFC of ( E)-2-hexenal induced evident phosphatidylserine (PS) externalization of A. flavus spores as detected by double staining with Annexin V-FITC and propidium iodide, indicating that early apoptosis was potentially induced. Furthermore, sublethal doses of ( E)-2-hexenal disturbed pyruvate metabolism and reduced the intracellular soluble protein content of A. flavus spores during the early stage of germination, and MIC treatment decreased acetyl-CoA and ATP contents by 65.7 ± 3.7% and 53.9 ± 4.0% ( P < 0.05), respectively. Additionally, the activity of mitochondrial dehydrogenases was dramatically inhibited by 23.8 ± 2.2% ( P < 0.05) at the MIC of ( E)-2-hexenal. Therefore, the disruption of mitochondrial energy metabolism and the induction of early apoptosis are involved in the mechanism of action of ( E)-2-hexenal against A. flavus spore germination.

Scientific Opinion on Flavouring Group Evaluation 200, Revision 1 (FGE.200 Rev.1): 74 α,β-unsaturated aliphatic aldehydes and precursors from chemical subgroup 1.1.1 of FGE.19

The Panel on Food Additives and Flavourings of the European Food Safety Authority was requested to evaluate the genotoxic potential of 74 flavouring substances from subgroup 1.1.1 of FGE.19 in the Flavouring Group Evaluation 200 Revision 1 (FGE.200 Rev1). In FGE.200, genotoxicity studies were provided for one representative substance, namely hex-2(trans)-enal [FL-no: 05.073], and for other two substances in the same subgroup, namely 2-dodecenal [FL-no: 05.037] and 2-nonenal [FL-no: 05.171]. The Panel concluded that the concern still remains with respect to genotoxicity for the substances of this subgroup and requested an in vivo Comet assay performed in duodenum and liver for hex-2(trans)-enal [FL-no: 05.073]. For the two other representative substances of subgroup 1.1.1 (nona-2(trans),6(cis)-dienal [FL-no: 05.058] and oct-2-enal [FL-no: 05.060]), the Panel requested a combined in vivo Comet assay and micronucleus assay. These data have been provided and are evaluated in the present opinion FGE.200 Rev1. Industry submitted genotoxicity studies on trans-2-octenal [FL-no: 05.190], instead of oct-2-enal [FL-no: 05.060]. Based on the available data, the Panel concluded that the concern for genotoxicity can be ruled out for hex-2(trans)-enal [FL-no: 05.073], trans-2-octenal [FL-no: 05.190] and nona-2(trans),6(cis)-dienal [FL-no: 05.058], therefore all the 74 substances [FL-no: 02.020, 02.049, 02.050, 02.090, 02.112, 02.137, 02.156, 02.192, 02.210, 02.231, 05.037, 05.058, 05.060, 05.070, 05.072, 05.073, 05.076, 05.078, 05.102, 05.109, 05.111, 05.114, 05.120, 05.144, 05.150, 05.171, 05.172, 05.179, 05.184, 05.189, 05.190, 05.191, 05.195, 06.025, 06.031, 06.072, 09.054, 09.097, 09.109, 09.119, 09.146, 09.233, 09.244, 09.247, 09.276, 09.277, 09.303, 09.312, 09.385, 09.394, 09.395, 09.396, 09.397, 09.398, 09.399, 09.400, 09.410, 09.411, 09.469, 09.482, 09.489, 09.492, 09.493, 09.498, 09.678, 09.701, 09.719, 09.741, 09.790, 09.841, 09.866, 09.947, 09.948, 13.004] can be evaluated through the Procedure for flavouring substances.

The safety evaluation of food flavouring substances: the role of metabolic studies

The safety assessment of a flavour substance examines several factors, including metabolic and physiological disposition data. The present article provides an overview of the metabolism and disposition of flavour substances by identifying general applicable principles of metabolism to illustrate how information on metabolic fate is taken into account in their safety evaluation. The metabolism of the majority of flavour substances involves a series both of enzymatic and non-enzymatic biotransformation that often results in products that are more hydrophilic and more readily excretable than their precursors. Flavours can undergo metabolic reactions, such as oxidation, reduction, or hydrolysis that alter a functional group relative to the parent compound. The altered functional group may serve as a reaction site for a subsequent metabolic transformation. Metabolic intermediates undergo conjugation with an endogenous agent such as glucuronic acid, sulphate, glutathione, amino acids, or acetate. Such conjugates are typically readily excreted through the kidneys and liver. This paper summarizes the types of metabolic reactions that have been documented for flavour substances that are added to the human food chain, the methodologies available for metabolic studies, and the factors that affect the metabolic fate of a flavour substance.

Dose-dependent DNA adduct formation by cinnamaldehyde and other food-borne α,β-unsaturated aldehydes predicted by physiologically based in silico modelling

Genotoxicity of α,β-unsaturated aldehydes shown in vitro raises a concern for the use of the aldehydes as food flavourings, while at low dose exposures the formation of DNA adducts may be prevented by detoxification. Unlike many α,β-unsaturated aldehydes for which in vivo data are absent, cinnamaldehyde was shown to be not genotoxic or carcinogenic in vivo. The present study aimed at comparing dose-dependent DNA adduct formation by cinnamaldehyde and 18 acyclic food-borne α,β-unsaturated aldehydes using physiologically based kinetic/dynamic (PBK/D) modelling. In rats, cinnamaldehyde was predicted to induce higher DNA adducts levels than 6 out of the 18 α,β-unsaturated aldehydes, indicating that these 6 aldehydes may also test negative in vivo. At the highest cinnamaldehyde dose that tested negative in vivo, cinnamaldehyde was predicted to form at least three orders of magnitude higher levels of DNA adducts than the 18 aldehydes at their respective estimated daily intake. These results suggest that for all the 18 α,β-unsaturated aldehydes DNA adduct formation at doses relevant for human dietary exposure may not raise a concern. The present study illustrates a possible use of physiologically based in silico modelling to facilitate a science-based comparison and read-across on the possible risks posed by DNA reactive agents.

An integrated QSAR-PBK/D modelling approach for predicting detoxification and DNA adduct formation of 18 acyclic food-borne α,β-unsaturated aldehydes

Acyclic α,β-unsaturated aldehydes present in food raise a concern because the α,β-unsaturated aldehyde moiety is considered a structural alert for genotoxicity. However, controversy remains on whether in vivo at realistic dietary exposure DNA adduct formation is significant. The aim of the present study was to develop physiologically based kinetic/dynamic (PBK/D) models to examine dose-dependent detoxification and DNA adduct formation of a group of 18 food-borne acyclic α,β-unsaturated aldehydes without 2- or 3-alkylation, and with no more than one conjugated double bond. Parameters for the PBK/D models were obtained using quantitative structure-activity relationships (QSARs) defined with a training set of six selected aldehydes. Using the QSARs, PBK/D models for the other 12 aldehydes were defined. Results revealed that DNA adduct formation in the liver increases with decreasing bulkiness of the molecule especially due to less efficient detoxification. 2-Propenal (acrolein) was identified to induce the highest DNA adduct levels. At realistic dietary intake, the predicted DNA adduct levels for all aldehydes were two orders of magnitude lower than endogenous background levels observed in disease free human liver, suggesting that for all 18 aldehydes DNA adduct formation is negligible at the relevant levels of dietary intake. The present study provides a proof of principle for the use of QSAR-based PBK/D modelling to facilitate group evaluations and read-across in risk assessment.

Conversion of major soy isoflavone glucosides and aglycones in in vitro intestinal models

SCOPE

This study compares conversion of three major soy isoflavone glucosides and their aglycones in a series of in vitro intestinal models.

METHODS AND RESULTS

In an in vitro human digestion model isoflavone glucosides were not deconjugated, whereas studies in a Caco-2 transwell model confirmed that deconjugation is essential to facilitate transport across the intestinal barrier. Deconjugation was shown upon incubation of the isoflavone glucosides with rat as well as human intestinal S9. In incubations with rat intestinal S9 lactase phlorizin hydrolase, glucocerebrosidase, and cytosolic broad-specific β-glucosidase all contribute significantly to deconjugation, whereas in incubations with human intestinal S9 deconjugation appeared to occur mainly through the activity of broad-specific β-glucosidase. Species differences in glucuronidation and sulfation were limited and generally within an order of magnitude with 7-O-glucuronides being the major metabolites for all three isoflavone aglycones and the glucuronidation during first pass metabolism being more efficient in rats than in humans. Comparison of the catalytic efficiencies reveals that deconjugation is less efficient than conjugation confirming that aglycones are unlikely to enter the systemic circulation.

CONCLUSION

Altogether, the data point at possible differences in the characteristics for intestinal conversion of the major soy isoflavones between rat and human, especially with respect to their deconjugation.

A physiologically based in silico model for trans-2-hexenal detoxification and DNA adduct formation in human including interindividual variation indicates efficient detoxification and a negligible genotoxicity risk

A number of α,β-unsaturated aldehydes are present in food both as natural constituents and as flavouring agents. Their reaction with DNA due to their electrophilic α,β-unsaturated aldehyde moiety may result in genotoxicity as observed in some in vitro models, thereby raising a safety concern. A question that remains is whether in vivo detoxification would be efficient enough to prevent DNA adduct formation and genotoxicity. In this study, a human physiologically based kinetic/dynamic (PBK/D) model of trans-2-hexenal (2-hexenal), a selected model α,β-unsaturated aldehyde, was developed to examine dose-dependent detoxification and DNA adduct formation in humans upon dietary exposure. The kinetic model parameters for detoxification were quantified using relevant pooled human tissue fractions as well as tissue fractions from 11 different individual subjects. In addition, a Monte Carlo simulation was performed so that the impact of interindividual variation in 2-hexenal detoxification on the DNA adduct formation in the population as a whole could be examined. The PBK/D model revealed that DNA adduct formation due to 2-hexenal exposure was 0.039 adducts/10⁸ nucleotides (nt) at the estimated average 2-hexenal dietary intake (0.04 mg 2-hexenal/kg bw) and 0.18 adducts/10⁸ nt at the 95th percentile of the dietary intake (0.178 mg 2-hexenal/kg bw) in the most sensitive people. These levels are three orders of magnitude lower than natural background DNA adduct levels that have been reported in disease-free humans (6.8-110 adducts/10⁸ nt), suggesting that the genotoxicity risk for the human population at realistic dietary daily intakes of 2-hexenal may be negligible.