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Acrylamide carcinogenicity and DNA adducts—adduct structure and basepair mismatch
S108
Abstracts / Toxicology Letters 172S (2007) S1–S240
human intake, it is important to take into account species differences but also non-linear effects in toxicokinetics when extrapolating cancer risk from high dose laboratory animal experiments to low dose human exposure. To get more insight in dose dependent bioactivation and detoxification of estragole, a physiologically based biokinetic (PBBK) model was built, providing possibilities to model metabolism of estragole in rats at different oral doses. The kinetics of the phase I and phase II reactions of estragole used in this model were characterized in vitro using tissue fractions of organs that were found to be capable of metabolizing estragole. It is demonstrated that O-demethylation of estragole, leading to detoxification, is a major metabolic route at low doses of estragole. This reaction mainly occurs in the lung and kidney of the rat. Formation of the procarcinogenic metabolite 1 -hydroxyestragole, and the proximate carcinogenic metabolite 1 -sulfooxyestragole, mainly occurs in the liver and their formation reaches saturation at relatively higher doses. This PBBK model provides an instrument to estimate the levels of formation of 1 -hydroxyestragole and 1 -sulfooxyestragole in the liver of rats at estragole dose levels relevant for dietary human intake. doi:10.1016/j.toxlet.2007.05.289 Q20 Acrylamide carcinogenicity and DNA adducts—adduct structure and basepair mismatch Johannes Schulze 1 , Stefan Weiler 2 1 Johann
Wolfgang Goethe University Frankfurt/Main, Frankfurt/Main, Germany; 2 University Luebeck, Luebeck, Germany For acrylamide animal experiments suggest a carcinogenic potential which would result in a lifetime tumor incidence of 1%; for some locations this should be detectable especially in highly exposed cohorts like acrylamide workers. Epidemiological studies do no support this increase, the results in animal experiments have been attributed to cytotoxicity rather than mutagenicity. Correlating mutagenesis data with special reference to adduct structure and organ distribution suggest that neither DNA damage nor specific adduct formation correlate with the observed tumor formation in rodents. Tumor data from animal dosing, epidemiological evidence and DNA adduct pattern may be explained if cytotoxicity is assumed as the leading cellular damage with DNA adducts accumulating as a result of impaired DNA repair in rapidly proliferating cells. Cell proliferation in itself
is carcinogenic as can be seen in scar tumors. However, current risk estimates for carcinogenic compounds still rests on the “one hit” hypothesis. We will suggest criteria for compounds which may likely be carcinogenic by a nonmutagenic mode of action. Risk assessment for these compounds consequently should not be derived from cumulative lifetime exposure. For these compounds it should be expected that a continuous low dose exposure is not carcinogenic as suggested for acrylamide by epidemiological evidence, but may be carcinogenic at high concentrations for a limited time as in many carcinogenesis experiments in laboratory rodents. This difference should translate into exposure limits differing by orders in magnitude comparable to the EPA threshold (1 ng/kg/day) and the acrylamide uptake by foodstuff. doi:10.1016/j.toxlet.2007.05.290 Q21 Human Cytochrome P450 2E1 metabolizes acrylamide to glycidamide Eva Settels, Ulrike Bernauer, Richard Palavinskas, Horst Stefan Klaffke, Ursula Gundert-Remy, Klaus Erich Appel Federal Institute for Risk Assessment, Berlin, Germany The discovery of acrylamide (AA) in heated food has raised concern about possible health risks. In experimental studies AA is genotoxic and carcinogenic. It is also neurotoxic in animals and humans. Investigations with Cytochrome P450 (CYP) 2E1-null mice show that AA is metabolized to glycidamide (GA) primarily by CYP2E1. GA is suggested to be much more reactive than AA with DNA. However, in a study with primary rat hepatocytes, no DNA damage was induced by AA despite substantial expression of CYP2E1. With regard to species extrapolation and risk assessment the relevance of CYP2E1 for the extent of GA generation in various mammalian species, especially in humans, is of paramount importance. Therefore, we studied the extent of GA formation from AA using monkey and human liver microsomes as well as genetically engineered V79 cells stably expressing human CYP2E1 (V79h2E1 cells). AA and formed GA were extracted and refined from microsomal or V79h2E1 suspensions by using organic solvents and by elution through activated carbon–aluminium oxidecolumns. Quantification of GA and AA was performed by gas chromatography/mass spectrometry. AA is metabolized to GA in monkey and human liver microsomes as well as in V79h2E1 cells. MAB-2E1 antibody (monoclonal, raised in mouse, human CYP2E1
Abstracts / Toxicology Letters 172S (2007) S1–S240
human intake, it is important to take into account species differences but also non-linear effects in toxicokinetics when extrapolating cancer risk from high dose laboratory animal experiments to low dose human exposure. To get more insight in dose dependent bioactivation and detoxification of estragole, a physiologically based biokinetic (PBBK) model was built, providing possibilities to model metabolism of estragole in rats at different oral doses. The kinetics of the phase I and phase II reactions of estragole used in this model were characterized in vitro using tissue fractions of organs that were found to be capable of metabolizing estragole. It is demonstrated that O-demethylation of estragole, leading to detoxification, is a major metabolic route at low doses of estragole. This reaction mainly occurs in the lung and kidney of the rat. Formation of the procarcinogenic metabolite 1 -hydroxyestragole, and the proximate carcinogenic metabolite 1 -sulfooxyestragole, mainly occurs in the liver and their formation reaches saturation at relatively higher doses. This PBBK model provides an instrument to estimate the levels of formation of 1 -hydroxyestragole and 1 -sulfooxyestragole in the liver of rats at estragole dose levels relevant for dietary human intake. doi:10.1016/j.toxlet.2007.05.289 Q20 Acrylamide carcinogenicity and DNA adducts—adduct structure and basepair mismatch Johannes Schulze 1 , Stefan Weiler 2 1 Johann
Wolfgang Goethe University Frankfurt/Main, Frankfurt/Main, Germany; 2 University Luebeck, Luebeck, Germany For acrylamide animal experiments suggest a carcinogenic potential which would result in a lifetime tumor incidence of 1%; for some locations this should be detectable especially in highly exposed cohorts like acrylamide workers. Epidemiological studies do no support this increase, the results in animal experiments have been attributed to cytotoxicity rather than mutagenicity. Correlating mutagenesis data with special reference to adduct structure and organ distribution suggest that neither DNA damage nor specific adduct formation correlate with the observed tumor formation in rodents. Tumor data from animal dosing, epidemiological evidence and DNA adduct pattern may be explained if cytotoxicity is assumed as the leading cellular damage with DNA adducts accumulating as a result of impaired DNA repair in rapidly proliferating cells. Cell proliferation in itself
is carcinogenic as can be seen in scar tumors. However, current risk estimates for carcinogenic compounds still rests on the “one hit” hypothesis. We will suggest criteria for compounds which may likely be carcinogenic by a nonmutagenic mode of action. Risk assessment for these compounds consequently should not be derived from cumulative lifetime exposure. For these compounds it should be expected that a continuous low dose exposure is not carcinogenic as suggested for acrylamide by epidemiological evidence, but may be carcinogenic at high concentrations for a limited time as in many carcinogenesis experiments in laboratory rodents. This difference should translate into exposure limits differing by orders in magnitude comparable to the EPA threshold (1 ng/kg/day) and the acrylamide uptake by foodstuff. doi:10.1016/j.toxlet.2007.05.290 Q21 Human Cytochrome P450 2E1 metabolizes acrylamide to glycidamide Eva Settels, Ulrike Bernauer, Richard Palavinskas, Horst Stefan Klaffke, Ursula Gundert-Remy, Klaus Erich Appel Federal Institute for Risk Assessment, Berlin, Germany The discovery of acrylamide (AA) in heated food has raised concern about possible health risks. In experimental studies AA is genotoxic and carcinogenic. It is also neurotoxic in animals and humans. Investigations with Cytochrome P450 (CYP) 2E1-null mice show that AA is metabolized to glycidamide (GA) primarily by CYP2E1. GA is suggested to be much more reactive than AA with DNA. However, in a study with primary rat hepatocytes, no DNA damage was induced by AA despite substantial expression of CYP2E1. With regard to species extrapolation and risk assessment the relevance of CYP2E1 for the extent of GA generation in various mammalian species, especially in humans, is of paramount importance. Therefore, we studied the extent of GA formation from AA using monkey and human liver microsomes as well as genetically engineered V79 cells stably expressing human CYP2E1 (V79h2E1 cells). AA and formed GA were extracted and refined from microsomal or V79h2E1 suspensions by using organic solvents and by elution through activated carbon–aluminium oxidecolumns. Quantification of GA and AA was performed by gas chromatography/mass spectrometry. AA is metabolized to GA in monkey and human liver microsomes as well as in V79h2E1 cells. MAB-2E1 antibody (monoclonal, raised in mouse, human CYP2E1