Exploration of alternative methods for toxicity assessment of pesticide metabolites

Transcription

1 Exploration of alternative methods for toxicity assessment of pesticide metabolites Alternative in vitro methods to characterize the role of endocrine active substances (EAS) in hormone-targeted tissues, Istituto Superiore di Sanita`, Rome, 17 th December 2012

2 Background Regulation (EC) No 1107/2009 concerning the placing of PPPs on the market requires: identification of metabolites determination of their toxicological relevance for in/exclusion in the residue definition

3 Background OECD Guidance Document on the Definition of Residue Proposes residue definition for risk assessment, reflecting the actual toxicological burden consisting of active substance and relevant metabolites Requires quantitative and qualitative assessment of metabolites

4 Pesticide metabolites as residues In contrast to active substances for which a full toxicological dossier needs to be provided usually very limited or no toxicological data are available although their toxic potency and toxicological profile might differ substantially from the parent compound Reference values of parent compounds are applied for metabolites present in food commodities

5 Assessment tools Conventional testing of metabolites? Number of metabolites Difficulties in synthesising sufficient amounts Costs Research capacities Animal welfare concerns

6 Assessment tools Preparatory Work for the Toxicology Part of the Opinion Project: Impact of Metabolism on Toxicity (AGES, Austria) Project on Threshold of Toxicological Concern (TTC) Concept (CRD, UK) 2 projects on (Quantitative) Structure-Activity Relationships (Q)SARs (JRC, EC)

7 Assessment tools Impact of Metabolism Impact of metabolic and degradation processes on the toxicological properties of residues of pesticides in food commodities (published 6 May 2010)

8 Assessment tools Impact of Metabolism Review of transformation pathways (11 pesticide classes) and comparison with toxicity data (DARs, public literature) Based on lack of other information analyses largely based on acute tox studies In total more than 140 chemical changes identified and analysed Conclusions: Toxification/detoxification cannot be attributed to certain metabolic steps (Conjugates not necessary less toxic due to cleavage) Metabolite > 10% to be considered as contributing significantly to toxicity Improvement of ADME studies (more accuracy, more information) necessary

9 Assessment tools - TTC Applicability of thresholds of toxicological concern in the dietary risk assessment of metabolites, degradation and reaction products of pesticides (published 24 March 2010)

10 Assessment tools TTC Threshold of Toxicological Concern (TTC) Concept Level of exposure below which no appreciable risk is expected Based on comparison of chemical structures and chronic data (CPDB data base - Gold et al. 1984, 1989) So far only applicable for chronic exposures Already in use for assessment of food flavourings

11 Assessment tools - TTC Threshold of Toxicological Concern (TTC) Concept Classification TTC threshold (µg/person/d) TTC threshold (µg/kg bw/d) Structural features Cramer class I Simple structure + Metabolism Cramer class II Less innocuous than class I Cramer Class III Suggestive of significant toxicity Functional groups Cramer class III + Neurotoxicity Parent compound neurotoxic OP + Carbamate structure Cramer class III + Genotoxicity Parent compound genotoxic Identification with QSAR

12 Assessment tools - TTC Review on the TTC concept and current use Validation of TTC for pesticides: Comparison of 100 pesticide ADIs with TTC classification Case study with 79 metabolites of 15 selected pesticides Conclusions: TTC appropriate for assessment of metabolites Exposure to metabolites considered to be covered by TTC thresholds allocated critical is the genotoxicity threshold QSARs predictions for genotoxicity considered reliable

13 Assessment tools QSAR I Applicability of QSAR analysis to the evaluation of the toxicological relevance of metabolites and degradates of pesticide active substances for dietary risk assessment (published 7 May 2010)

14 Assessment tools QSAR I Review on the use of QSAR for regulatory purposes Extensive evaluation of potentially useful QSARs (e.g. Toxtree, Lazar, Derek, Hazard Expert, Caesar, Topkat) using different data sets (Pesticide metabolites, EU classified substances, CPDB) Case studies focused on genotoxicity based on the outcome of the TTC project Conclusions: Currently limited use of QSARs for regulatory purposes Recommendations for the application of different models covering all relevant toxicological endpoints - i.e. combination of models recommended to optimise sensitivity and specificity in regard to genotoxicity

15 Assessment tools QSAR II Applicability of QSAR analysis in the evaluation of developmental and neurotoxicity effects for the assessment of the toxicological relevance of metabolites and degradates of pesticide active substances for dietary risk assessment (published 16 June 2011)

16 Assessment tools QSAR II Evaluation of QSAR models (Derek, Caesar, Topkat, Leadscope, Hazard Expert, PASS, ADME predictor, Accord) and read across (OECD toolbox) for assessment of developmental effects and neurotoxicity Conclusions: No appropriate QSAR models for neurotoxicity identified Stepwise approach for developmental effects combining read across and QSAR might be promising

17 Extension of the mandate including assessment of isomers (September 2011) In order to: investigate if the approaches proposed for assessment of metabolites are applicable for isomers identify specific issues, suitable methodologies, research needs for isomers

18 Scientific Opinion on Evaluation of the Toxicological Relevance of Pesticide Metabolites for Dietary Risk Assessment (published 26 July 2012) Available from: ub/2799.htm

19 Scientific Opinion Toxicology Part Chapters: Presentation and evaluation of projects on Impact of Metabolism, TTC and QSARs Development of a single TTC value for acute exposures (based on short term NOAELs) Proposal for toxicity assessment and for acute and chronic exposures (decision trees) Evaluation of the assessment tools for isomer assessment Critical Issues/Uncertainties

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21 Is an ARfD assigned to parent substance NO YES Has the parent compound a neurotoxic mechanism of action NO QSAR analysis for developmental toxicity See chapter 7.2 YES No further testing or ** The metabol ite is not relevant No Acute exposure estimate of metabolite See chapter 8 Exposure > 0.3 µg/kg bw/d See chapter OP,carbamate toxicophore See chapter 5 No YES YES Compounds predicted positive Compounds predicted positive /inconclusive Compounds predicted negative/inconclusive Read-across See chapter 6.3 No Is the metabolite in Cramer Class II or III? YES No Non TTC approach see chronic Compounds predicted negative Exposure > 5 µg/kg bw/d (modified acute value) See chapter No Is the metabolite in Cramer Class I? YES Exposure > 30 µg/kg bw/d (modified acute value) See chapter YES No acute RA necessary

22 Conclusions/Recommendations Toxification/detoxification not attributable to specific metabolic steps Use of PBPK models recommended for assessment of ADME TTC concept is an appropriate and sufficiently conservative evaluation tool for metabolites, but should not be used for parent compounds For acute exposures an acute TTC threshold of 5 µg/kg bw/d can be applied Where exposure exceeds TTC values, targeted testing can be applied TTC can in principle used for evaluation of endocrine active substances (except steroids) but should preferably be based on testing if data already indicate that a compound has endocrine mediated adverse effects

23 Conclusions/Recommendations Once an EU agreed approach for defining and assessing of endocrine active substances is available, the use of the TTC approach for EAS needs to be revisited In principle the TTC concept and QSARs could be used for assessment of stereoisomers present in mixtures, however, in practice information on stereochemistry is not provided in the data sets underlying these tools QSARs should not be used as a stand alone tool for assessments but can together with the application of read-across complement the TTC approach The usability of QSARs for the assessment of endocrine active active substance has not been investigated based on lack of a definition and appropriate data bases A guidance should be developed based on the scientific opinion

24 Thank you!