Kinetics and disposition in toxicology. Example: carcinogenic risk estimate for ethylene

Review and priority setting for substances that are listed without a specific migration limit in Table 1 of Annex 1 of Regulation 10/2011 on plastic materials and articles intended to come into contact with food

The EFSA Panel on Food Contact Materials, Enzymes and Processing Aids (CEP) was requested by the European Commission to review the substances for which a Specific Migration Limit (SML) is not assigned in Regulation (EU) No 10/2011. These substances had been covered by the Generic SML of 60 mg/kg food, but with Regulation (EU) 2016/1416 it was removed, necessitating their re-examination. EFSA was requested to identify those substances requiring an SML to ensure the authorisation is sufficiently protective to health, grouping them in high, medium and low priority to serve as the basis for future re-evaluations of individual substances. The CEP Panel established a stepwise procedure. This took into account existing hazard assessments for each substance on carcinogenicity/mutagenicity/reprotoxicity (CMR), bioaccumulation and endocrine disruptor (ED) properties along with the use of in silico generated predictions on genotoxicity. Molecular weights and boiling points were considered with regard to their effect on potential consumer exposure. This prioritisation procedure was applied to a total of 451 substances, from which 78 substances were eliminated at the outset, as they had previously been evaluated by EFSA as food contact substances. For 89 substances, the Panel concluded that a migration limit should not be needed. These are in the lists 0 and 1 of the Scientific Committee for Food (SCF), defined as substances for which an Acceptable Daily Intake (ADI) does not need to be established, along with substances that are controlled by existing restrictions and/or generic limits. Of the remaining 284 substances, 179 were placed into the low priority group, 102 were placed into the medium priority group and 3 were placed into the high priority group, i.e. salicylic acid (FCM No 121), styrene (FCM No 193) and lauric acid, vinyl ester (FCM No 436).

Understanding the importance of low-molecular weight (ethylene oxide- and propylene oxide-induced) DNA adducts and mutations in risk assessment: Insights from 15 years of research and collaborative discussions

The interpretation and significance of DNA adduct data, their causal relationship to mutations, and their role in risk assessment have been debated for many years. An extended effort to identify key questions and collect relevant data to address them was focused on the ubiquitous low MW N7-alkyl/hydroxyalkylguanine adducts. Several academic, governmental, and industrial laboratories collaborated to gather new data aimed at better understanding the role and potential impact of these adducts in quantifiable genotoxic events (gene mutations/micronucleus). This review summarizes and evaluates the status of dose-response data for DNA adducts and mutations from recent experimental work with standard mutagenic agents and ethylene oxide and propylene oxide, and the importance for risk assessment. This body of evidence demonstrates that small N7-alkyl/hydroxyalkylguanine adducts are not pro-mutagenic and, therefore, adduct formation alone is not adequate evidence to support a mutagenic mode of action. Quantitative methods for dose-response analysis and derivation of thresholds, benchmark dose (BMD), or other points-of-departure (POD) for genotoxic events are now available. Integration of such analyses of genetox data is necessary to properly assess any role for DNA adducts in risk assessment. Regulatory acceptance and application of these insights remain key challenges that only the regulatory community can address by applying the many learnings from recent research. The necessary tools, such as BMDs and PODs, and the example datasets, are now available and sufficiently mature for use by the regulatory community. Environ. Mol. Mutagen. 60: 100-121, 2019. © 2018 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Dephenylation of the rubber chemical N-phenyl-2-naphthylamine to carcinogenic 2-naphthylamine: a classical problem revisited

N-phenyl-2-naphthylamine (PBNA) represents an example of a suspected carcinogen that is found negative in mutagenicity and clastogenicity testing as well as in long-term animal carcinogenicity bioassays in several species, but for which a carcinogenic risk cannot be excluded because of its metabolic conversion to the known human carcinogen 2-naphthylamine. Also, epidemiologic studies failed to indicate an elevated bladder cancer risk in humans occupationally exposed to PBNA. The amounts of 2-naphthylamine found in the urine of different species including humans after exposure to PBNA indicate unequivocally that PBNA is dephenylated to some extent. These are not explained by the 2-naphthylamine impurities in technical-grade PBNA. To explain the metabolic dephenylation process, it has been suggested that PBNA is metabolized by cytochrome P-450 (CYP) enzymes to the phenolic derivative 4'-hydroxy-N-phenyl-2-naphthylamine, followed by its further oxidation to the quinone imine, which subsequently hydrolyses to form the dephenylation product 2-naphthylamine. Phenolic metabolites from the initial CYP-mediated activation step are rapidly conjugated. Quantitatively, dephenylation of PBNA to 2-naphthylamine is a minor pathway. The dog represents an animal model that appears to approximate the human metabolism and biological activation of PBNA. Based on published data, a worst-case scenario indicates that about 1% of total PBNA taken up is transferred into 2-naphthylamine. However, in vitro as well as in vivo findings with PBNA may point to a significantly smaller conversion rate, as metabolites anticipated from the metabolism of 2-naphthylamine were not detected so far. The assumption, which may well be an overestimation, is compatible with findings in animal experiments, and explains the lack of direct evidence of carcinogenicity of PBNA in both experimental and epidemiological studies.

Carcinogenicity and genotoxicity of ethylene oxide: new aspects and recent advances

Long-term inhalation studies in rodents have presented unequivocal evidence of experimental carcinogenicity of ethylene oxide, based on the formation of malignant tumors at multiple sites. However, despite a considerable body of epidemiological data only limited evidence has been obtained of its carcinogenicity in humans. Ethylene oxide is not only an important exogenous toxicant, but it is also formed from ethylene as a biological precursor. Ethylene is a normal body constituent; its endogenous formation is evidenced by exhalation in rats and in humans. Consequently, ethylene oxide must also be regarded as a physiological compound. The most abundant DNA adduct of ethylene oxide is 7-(2-hydroxyethyl)guanine (HOEtG). Open questions are the nature and role of tissue-specific factors in ethylene oxide carcinogenesis and the physiological and quantitative role of DNA repair mechanisms. The detection of remarkable individual differences in the susceptibility of humans has promoted research into genetic factors that influence the metabolism of ethylene oxide. With this background it appears that current PBPK models for trans-species extrapolation of ethylene oxide toxicity need to be refined further. For a cancer risk assessment at low levels of DNA damage, exposure-related adducts must be discussed in relation to background DNA damage as well as to inter- and intraindividual variability. In rats, subacute ethylene oxide exposures on the order of 1 ppm (1.83 mg/m3) cause DNA adduct levels (HOEtG) of the same magnitude as produced by endogenous ethylene oxide. Based on very recent studies the endogenous background levels of HOEtG in DNA of humans are comparable to those that are produced in rodents by repetitive exogenous ethylene oxide exposures of about 10 ppm (18.3 mg/m3). Experimentally, ethylene oxide has revealed only weak mutagenic effects in vivo, which are confined to higher doses. It has been concluded that long-term human occupational exposure to low airborne concentrations to ethylene oxide, at or below current occupational exposure limits of 1 ppm (1.83 mg/m3), would not produce unacceptable increased genotoxic risks. However, critical questions remain that need further discussions relating to the coherence of animal and human data of experimental data in vitro vs. in vivo and to species-specific dynamics of DNA lesions.

Biomarkers of exposure and effect as indicators of potential carcinogenic risk arising from in vivo metabolism of ethylene to ethylene oxide

The purposes of the present study were: (i) to investigate the potential use of several biomarkers as quantitative indicators of the in vivo conversion of ethylene (ET) to ethylene oxide (EO); (ii) to produce molecular dosimetry data that might improve assessment of human risk from exogenous ET exposures. Groups (n = 7/group) of male F344 rats and B6C3F1 mice were exposed by inhalation to 0 and 3000 p. p.m. ET for 1, 2 or 4 weeks (6 h/day, 5 days/week) or to 0, 40, 1000 and 3000 p.p.m. ET for 4 weeks. N:-(2-hydroxyethyl)valine (HEV), N:7-(2-hydroxyethyl) guanine (N7-HEG) and HPRT: mutant frequencies were assessed as potential biomarkers for determining the molecular dose of EO resulting from exogenous ET exposures of rats and mice, compared with background biomarker values. N7-HEG was quantified by gas chromatography coupled with high resolution mass spectrometry (GC-HRMS), HEV was determined by Edman degradation and GC-HRMS and HPRT: mutant frequencies were measured by the T cell cloning assay. N7-HEG accumulated in DNA with repeated exposure of rodents to 3000 p.p.m. ET, reaching steady-state concentrations around 1 week of exposure in most tissues evaluated (brain, liver, lung and spleen). The dose-response curves for N7-HEG and HEV were supralinear in exposed rats and mice, indicating that metabolic activation of ET was saturated at exposures >/=1000 p.p.m. ET. Exposures of mice and rats to 200 p.p.m. EO for 4 weeks (as positive treatment controls) led to significant increases in HPRT: mutant frequencies over background in splenic T cells from exposed rats and mice, however, no significant mutagenic response was observed in the HPRT: gene of ET-exposed animals. Comparisons between the biomarker data for both unexposed and ET-exposed animals, the dose-response curves for the same biomarkers in EO-exposed rats and mice and the results of the rodent carcinogenicity studies of ET and EO suggest that too little EO arises from exogenous ET exposure to produce a significant mutagenic response or a carcinogenic response under standard bioassay conditions.

A physiological toxicokinetic model for exogenous and endogenous ethylene and ethylene oxide in rat, mouse, and human: formation of 2-hydroxyethyl adducts with hemoglobin and DNA

Ethylene (ET) is a gaseous olefin of considerable industrial importance. It is also ubiquitous in the environment and is produced in plants, mammals, and humans. Uptake of exogenous ET occurs via inhalation. ET is biotransformed to ethylene oxide (EO), which is also an important volatile industrial chemical. This epoxide forms hydroxyethyl adducts with macromolecules such as hemoglobin and DNA and is mutagenic in vivo and in vitro and carcinogenic in experimental animals. It is metabolically eliminated by epoxide hydrolase and glutathione S-transferase and a small fraction is exhaled unchanged. To estimate the body burden of EO in rodents and human resulting from exposures to EO and ET, we developed a physiological toxicokinetic model. It describes uptake of ET and EO following inhalation and intraperitoneal administration, endogenous production of ET, enzyme-mediated oxidation of ET to EO, bioavailability of EO, EO metabolism, and formation of 2-hydroxyethyl adducts of hemoglobin and DNA. The model includes compartments representing arterial, venous, and pulmonary blood, liver, muscle, fat, and richly perfused tissues. Partition coefficients and metabolic parameters were derived from experimental data or published values. Model simulations were compared with a series of data collected in rodents or humans. The model describes well the uptake, elimination, and endogenous production of ET in all three species. Simulations of EO concentrations in blood and exhaled air of rodents and humans exposed to EO or ET were in good agreement with measured data. Using published rate constants for the formation of 2-hydroxyethyl adducts with hemoglobin and DNA, adduct levels were predicted and compared with values reported. In humans, predicted hemoglobin adducts resulting from exposure to EO or ET are in agreement with measured values. In rodents, simulated and measured DNA adduct levels agreed generally well, but hemoglobin adducts were underpredicted by a factor of 2 to 3. Obviously, there are inconsistencies between measured DNA and hemoglobin adduct levels.

The carcinogenic risk of ethene (ethylene)

On occasion of the 25th year of publication of Toxicologic Pathology, the Editor has asked for a report about recent progress in the area addressed by an article entitled “Olefinic Hydrocarbons: A First Risk Estimate,” one of the top 10 most frequently cited papers of the journal (3). Because general issues of risk assessment have very recently been addressed in this journal (6), I will focus on new specific aspects of ethene carcinogenicity.

Toxicokinetic models for volatile industrial chemicals and reactive metabolites

Two approaches of compartmental toxicokinetic modeling of gaseous compounds are presented which are suitable for kinetic analysis of concentration-time data measured in the air of closed exposure systems. The first approach is based on a two-compartment model with physiological gas uptake, the second on a physiologically-based toxicokinetic model. Both models can be used for the description of inhalation, accumulation, exhalation and metabolism of gaseous compounds together with the toxicokinetics of metabolites. Interspecies extrapolation is based on physicochemical, physiological and biochemical parameters. The advantage of the two-compartment model is its limited number of variables and its experimentally easy applicability. Its disadvantage is the impossibility to predict tissue specific concentrations. The advantage of the physiologically-based model is its usability for predictions and for the description of tissue specific concentrations. However, it entails great effort, since a series of parameters has to be determined before meaningful model calculations can be carried out.

Metabolic transformation of halogenated and other alkenes--a theoretical approach. Estimation of metabolic reactivities for in vivo conditions

Olefinic hydrocarbons are metabolized in vivo by cytochrome P450-dependent monooxygenases to the corresponding epoxides. The maximum in vivo metabolic rate, which is an important toxicokinetic parameter, has been used to define the apparent rate constant (kapp) describing in vivo metabolic reactivity of alkenes. To derive kapp, the metabolic rate normalized per body weight was divided by the corresponding average alkene concentration in the body at saturation conditions of 90%. Toxicokinetic data obtained in rats for 13 compounds (ethene, 1-fluoroethene, 1,1-difluoroethene, 1-chloroethene, 1,1-dichloroethene, cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1,2-trichloroethene, perchloroethene, propene, isoprene, 1,3-butadiene and styrene) have been used to calculate kapp values. A theoretical model, based on the assumption that in vivo epoxidation can be described as a cytochrome P450-mediated electrophilic reaction, has been developed. Using the olefinic hydrocarbons as an example it has been shown that kapp can be explained solely by the following molecular parameters: ionization potential, dipole moment and pi-electron density. These molecular parameters were calculated by a quantum chemical method or were taken from the literature. Furthermore, the model was tested also by predicting kapp for isobutene, an alkene which was not used for the model development. The predicted value of kapp agrees with the one derived experimentally, demonstrating that molecular parameters of halogenated and other alkenes can be used to predict in vivo metabolic reactivity. The model presented here is a first contribution to the ultimate goal to predict toxicokinetic parameters for in vivo conditions based on physicochemical parameters of enzymes and compounds exclusively.

Uptake and metabolism of ethene studied in a smoke-stop experiment

Knowledge of the relationships between exposure levels and levels of hemoglobin adducts are essential when the latter are to be used for exposure monitoring or risk estimation, the hygienic control being based on measurements of exposure. These ratios are mostly very uncertain, mainly due to difficulties of determining the time-weighted average exposure concentration. A solution to this problem has been suggested involving adduct measurement before and after two consecutive periods of about 1 week, the first with absence from exposure, the second with careful measurement of exposure. This model was tested in two smokers who abstained from smoking for one week. Analysis of inhaled ethene and of adducts from ethylene oxide (EO) to N-terminal valine of hemoglobin are compatible with metabolism of 2% of inhaled ethene to EO and a detoxification rate of 1 h-1 of EO.

Effects of ethylene on micronucleus formation in the bone marrow of rats and mice following four weeks of inhalation exposure

Male Fischer 344 rats and male B6C3F1 mice (10/species/group) were exposed to ethylene 6 h/day, 5 days/week, for 4 weeks. The ethylene target concentrations were 0, 40, 1000, and 3000 ppm. An ethylene oxide (EO) control group for each species was exposed under the same conditions at a target concentration of 200 ppm. Bone marrow was collected approximately 24 h after the final exposure. Polychromatic erythrocyte (PCE) to normochromatic erythrocyte (NCE) ratios were determined and 2000 PCE/animal were scored for the presence of micronuclei. Ethylene did not produce statistically significant, exposure-related increases in the frequency of micronucleated PCE (MNPCE) in the bone marrow of either rats or mice when compared to air-exposed control animals. As expected, EO exposure resulted in significant increases in the frequencies of MNPCE in both species.

Pharmacokinetics of ethylene in man; body burden with ethylene oxide and hydroxyethylation of hemoglobin due to endogenous and environmental ethylene

The inhalation pharmacokinetics and the endogenous production of ethylene has been determined in healthy volunteers with respect to the formation of the carcinogen ethylene oxide. Ethylene showed a low degree of accumulation in the body determined in six subjects, the thermodynamic partition coefficient "body/air" being 0.53 +/- 0.23 (mean +/- SD) and the accumulation factor "body/air" at steady-state being 0.33 +/- 0.13 (mean +/- SD). The rate of metabolism was directly proportional to the exposure concentration. Only 2% of ethylene inhaled was metabolized to ethylene oxide, whereas 98% of ethylene was exhaled unchanged. The rate of the endogenous production of ethylene was 32 +/- 12 nmol/h (mean +/- SD), as calculated from exhalation data from 14 subjects. The resulting body burden was 0.44 +/- 0.19 nmol/kg (mean +/- SD). By analyzing published data on ethylene oxide in man its half-life was estimated to be 42 min. Using the pharmacokinetic parameters of ethylene and ethylene oxide, the body burden of ethylene oxide due to the sum of the exposure to environmental ethylene of about 15 ppb and to endogenous ethylene exposure of 0.44 nmol/kg was predicted to be 0.25 nmol/kg. In the blood of five non-smokers and one smoker the hemoglobin adduct resulting from the reaction of ethylene oxide with the N-terminal valine, N-(2-hydroxyethyl)valine, was quantified by gas chromatography/mass spectrometry. The value of 20 +/- 5 pmol/g Hb (mean +/- SD) found in the non-smokers corroborated the steady-state level of 18 +/- 3 pmol/g Hb (mean +/- SD) calculated from the pharmacokinetic approach.

The closed chamber technique--uptake, endogenous production, excretion, steady-state kinetics and rates of metabolism of gases and vapors

The "closed chamber technique" (CCT) is presented. It allows investigation of pharmacokinetics of volatile substances in vivo in animals and in man and in vitro using tissue fractions. During the exposure period only the atmospheric concentrations of the substance are measured. The concentration-time data obtained are pharmacokinetically analyzed by a two compartment model describing uptake, endogenous production and excretion of the unchanged substance and its metabolic elimination. Using this model, pharmacokinetics of ethylene have been determined in rats and man. For both species, the results compared well with an estimation based on an allometric species scaling. Furthermore, the applicability of CCT is demonstrated in vivo on several other gases and vapors of solvents, e.g. trichloroethylene and 1,1,1-trichloroethane, and in vitro on 1,2-epoxybutene-3.

Investigation of species differences in isobutene (2-methylpropene) metabolism between mice and rats

Metabolism of isobutene (2-methylpropene) in rats (Sprague Dawley) and mice (B6C3F1) follows kinetics according to Michaelis-Menten. The maximal metabolic elimination rates are 340 mumol/kg/h for rats and 560 mumol/kg/h for mice. The atmospheric concentration at which Vmax/2 is reached is 1200 ppm for rats and 1800 ppm for mice. At steady state, below atmospheric concentrations of about 500 ppm the rate of metabolism of isobutene is direct proportional to its concentration. 1,1-Dimethyloxirane is formed as a primary reactive intermediate during metabolism of isobutene in rats and can be detected in the exhaled air of the animals. Under conditions of saturation of isobutene metabolism the concentration of 1,1-dimethyloxirane in the atmosphere of a closed exposure system is only about 1/15 of that observed for ethene oxide and about 1/100 of that observed for 1,2-epoxy-3-butene as intermediates in the metabolism of ethene or 1,3-butadiene.

Hemoglobin adducts in animals exposed to gasoline and diesel exhausts. 1. Alkenes

Blood samples from rats and hamsters exposed to automotive engine exhausts in the Committee of Common Market Automobile Constructors long-term inhalation study at Battelle-Geneva were analysed for the levels of 2-hydroxyethylvaline (HOEtVal) and 2-hydroxypropylvaline (HOPrVal) in hemoglobin (Hb). These adducts to the N-terminus of the Hb chains were determined by gas chromatography-mass spectrometry of derivatives obtained by a modified Edman degradation that specifically cleaves off alkylated N-terminal amino acids (valine in Hb). The adduct levels found correspond to the metabolic conversion of about 5-10% of inhaled ethene and propene to ethylene oxide and propylene oxide, respectively, in agreement with results from earlier studies on mice inhaling radio-labelled alkenes. It is concluded that the alkenes, via epoxides, are the main sources of the observed HOEtVal and HOPrVal. From calculated doses and estimates of genotoxic potency the contribution from ethene in urban air to human cancer risk is discussed.

Inhaled ethylene oxide induces preneoplastic foci in rat liver

The metabolite of E, EO, has been shown to be an extrahepatic carcinogen in rats in long-term studies. By means of a rat liver foci bioassay with 3 to 4 days old Sprague-Dawley rats, EO showed an initiating capacity in the livers of female, but not of male rats, measured as incidence of foci deficient in ATPase. After inhalation of 55 and 100 ppm EO, 8 h daily, 5 days weekly, and over 3 weeks, 1 week of pause, and another 8 weeks of promotion with polychlorinated biphenyls, foci incidence was generally low. But it was concentration dependently higher than in controls 12 weeks after starting the experiment. A linear concentration-effect relationship existed with a correlation coefficient of r = 0.991. With 33 ppm EO the number of foci was not enhanced significantly. The administration of 10,000 ppm E did not result in an enhanced foci incidence. In general the carcinogenic potential of EO, which has not been shown so far to cause hepatic tumors in rats, could be demonstrated in rat liver using a sensitive rat liver foci bioassay.

Pharmacokinetics of isoprene in mice and rats

Pharmacokinetic analysis of isoprene inhaled by male Wistar rats and male B6C3F1 mice showed saturation kinetics in both species. Below atmospheric concentrations of 300 ppm in rats and in mice the rate of metabolism is directly proportional to the concentration. The low accumulation of isoprene in the body at low atmospheric concentrations suggests transport limitation of the metabolism. Only small amounts of isoprene taken up are exhaled as unchanged substance (15% in rats and 25% in mice). Its half life in rats is 6.8 min and in mice 4.4 min. At concentrations above 300 ppm the rate of metabolism does not increase further in proportion to the atmospheric concentration. It finally approaches maximal values of 130 mumol/(h X kg) body weight at atmospheric concentrations above 1500 ppm in rats, and 400 mumol/(h X kg) body weight at concentrations above 2000 ppm in mice. This indicates limited production of the two possible mono-epoxides of isoprene at high concentrations. Isoprene is endogenously produced and is systemically available. Its production rate is 1.9 mumol/(h X kg) in rats, and 0.4 mumol/(h X kg) in mice, respectively. Part of the endogenous isoprene is exhaled by the animals but it is metabolized to a greater extent: the rate of metabolism of endogenously produced and systemically available isoprene is 1.6 mumol/(h X kg) (rats) and 0.3 mumol/(h X kg) (mice).