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Scientific Opinion on Evaluation of the Toxicological Relevance of Pesticide Metabolites for Dietary Risk Assessment
The European Food Safety Authority (EFSA) asked the Panel on Plant Protection Products and their Residues (PPR) to develop an opinion on approaches to evaluate the toxicological relevance of metabolites and degradates of pesticide active substances in dietary risk assessment. This opinion identifies the threshold of toxicological concern (TTC) concept as an appropriate screening tool. The TTC values for genotoxic and toxic compounds were found to be sufficiently conservative for chronic exposure, as a result of a validation study with a group of pesticides belonging to different chemical classes. Three critical steps were identified in the application of a TTC scheme: 1) the estimate of the level of the metabolite, 2) the evaluation of genotoxicity alerts and 3) the detection of neurotoxic metabolites. Tentative TTC values for acute exposure were established by the PPR Panel by analysis of the lowest 5th percentiles of No Observed Adverse Effect Levels (NOAELs) used to establish the Acute Reference Doses (ARfD) for the EFSA pesticide data set. Assessment schemes for chronic and acute dietary risk assessment of pesticide metabolites, using the TTC approach and combined (Q)SAR and read across, are proposed. The opinion also proposes how the risk assessment of pesticide metabolites that are stereoisomers should be addressed due to isomer ratio changes reflected in the composition of metabolites. The approach is ready for use, but it is anticipated that on many occasions the outcome of the assessment scheme will be that further testing is needed to reach a firm conclusion on the toxicological relevance of the metabolite. However, the benefit of applying the approach is that it will allow prioritisation of metabolites for subsequent testing.
EFSA will develop a Guidance Document based on the results in this opinion.
© European Food Safety Authority, 2012
The use of pesticides in agriculture may lead to a large number of metabolites being present at low levels in food and feed. Progress in analytical methods and their increasing sensitivity results in the detection of a growing number of metabolites in low amounts. The residue definition for dietary risk assessment should include the active substance and all metabolites of toxicological relevance. A comprehensive toxicological dossier is developed for parent compounds, prior to approval of substances for use within EU (Regulation EC (No) 1107/2009), while often only limited information about the toxicological properties of metabolites is available. In light of these considerations, EFSA asked the PPR Panel to develop an opinion on approaches to evaluate the toxicological relevance of metabolites and degradates of pesticide active substances in dietary risk assessment. The need to minimise the use of laboratory animals where possible was highlighted. The Panel was also asked to consider whether the approaches and methodologies developed for pesticide metabolites are applicable to isomer ratio changes of active substances existing as isomer mixtures or which are used as individual isomers.
A key issue is whether a metabolite would have been tested with the parent compound in laboratory species, due to its formation in vivo. For those metabolites not so tested, because they are unique to plants or livestock, an alternative approach is necessary. The PPR Panel considered relevant publications in the scientific literature and current applications of non-testing approaches in various regulatory contexts. On this basis four projects were outsourced to evaluate the potential impact of metabolic processes on the toxicity of pesticide metabolites and to explore the reliability of the available computational tools. The PPR Panel has developed a strategy to estimate the dietary exposure to pesticide metabolites. Several exposure scenarios were considered covering various possibilities of metabolite ratio extrapolation and the extent of uses. Case studies illustrate the methods that have been used. It is noted that the choice of the scenario has a considerable impact on the estimate of the metabolite level to be used.
The Panels conclusions on these approaches are as follows:
- The potential impact of structural metabolic changes to parent compounds on the toxicological properties of derived metabolites was analysed for the most relevant chemical classes of active substances listed in Annex 1 of Directive 91/414/EEC. Despite the high level of uncertainty due to the heterogeneity of ADME studies and inadequacy of toxicological data on metabolites, the metabolic pathways are in most cases specific for each chemical group and toxification/detoxification potential cannot be reliably attributed to specific metabolic steps.
- The TTC concept is the most appropriate tool for evaluating the toxicological relevance of pesticide metabolites. The existing TTC values for genotoxic and toxic compounds were found to be sufficiently conservative for chronic exposure by a validation study with groups of pesticides belonging to different chemical classes. These values, based on the assumption of continuous exposure during lifetime, are overly conservative for short term exposure duration. Tentative TTC values for acute exposure were established by the analysis of the lowest 5th percentiles of NOAELs used to establish ARfDs for the EFSA pesticide data set. Three critical steps were identified in the application of the TTC scheme in risk assessment of pesticide metabolites: 1) the estimate of the level of the metabolite, 2) the evaluation of genotoxicity alerts and the 3) detection of neurotoxic metabolites arising from a parent compound with a structural alert not covered by the scheme.
- The evaluation of genotoxicity alerts was addressed in an outsourced project involving the application of several (Q)SAR models using the largest dataset available of active ingredients and metabolites. The results showed individual models to have low sensitivities in identifying genotoxic pesticides, while the same tools applied in combination appeared good identifiers of classified mutagens. The low sensitivity was mainly attributed to the heterogeneity of the underlying pesticide database. The PPR Panel concluded that the performance of these applied tools is not satisfactory and cannot support, at the present time, the application of solely (Q)SAR approaches to predict the potential genotoxicity of unknown pesticide metabolites.
- The applicability of (Q)SAR tools, grouping and read-across approaches in the evaluation of developmental and neurotoxic effects of pesticide metabolites was addressed by another outsourced project. The predictivity for neurotoxicity of the (Q)SAR models, tested alone or in combination, is currently inadequate to be applied for pesticide metabolites. (Q)SAR tools alone appeared insufficiently reliable to predict developmental effects, due to their low sensitivity and specificity, but a stepwise approach involving (Q)SAR analysis and read-across, resulted in an improvement in the identification of potential developmental toxicants.
- The results of the (Q)SAR projects allowed the PPR Panel to propose the application of computational methods, involving the separate or sequential use of (Q)SAR and read-across in the prediction of genotoxicity and developmental toxicity, to complement the TTC approach in the assessment scheme for pesticide metabolite exposure.
- Estimates of exposure to pesticide metabolites by the Panel are based mainly on residue metabolism studies. These data have also been adapted using a metabolite to parent ratio applied to the available residue end-points from the supervised trials data to give different estimates of exposure for both chronic and acute exposure. The key issue affecting the results is the potential for extrapolating data, encompassing metabolism groupings and the extent of uses. The approaches tested allowed the Panel to propose a dietary exposure tree for pesticide metabolites. However different methodological approaches produce different outcomes and risk managers would need to advise on the level of protection that is desired.
- The scientific principles that underpin pesticide metabolite exposure calculations (above) are also directly relevant to the derivation of conversion factors which are established during the regulatory evaluation of parent compounds in the framework of Regulation (EC) No 1107/2009 when the residue definitions for monitoring and dietary risk assessment differ. The PPR Panel recognises that currently, there is no unambiguous approach to deriving conversion factors and recommends the developing further guidance in this area.
- Chronic and acute assessment schemes are proposed for the risk assessment of pesticide metabolites considering different strategies for mammalian (rodent or laboratory test species) and plant or livestock specific metabolites. A chronic exposure estimate is necessary in all cases, while an acute exposure assessment is needed only when an Acute Reference Dose (ARfD) has been allocated for the parent compound or structural alerts for acute neurotoxicity and developmental toxicity are detected.
- The chronic assessment scheme involves the comparison of chronic exposure with the corresponding threshold values given in the decision tree. Computational tools involving the combination of (Q)SAR and read-across are proposed in the evaluation of an alert for genotoxicity. If the exposure estimate exceeds the identified TTC values, different approaches are proposed for mammalian rodent and plant or livestock metabolites. A weight of evidence approach is recommended to determine if the toxicological profile of rodent metabolites is covered by the data on parent compound. Plant or livestock specific metabolites need to be assessed using an appropriate testing strategy.
- An acute exposure assessment scheme was developed by the PPR Panel. Ad hoc acute TTC values of 0.3 µg/kg bw/d for substances with a neurotoxicity alert and 5 µg/kg bw/d for substances allocated in Cramer class II and III were derived. A combination of (Q)SAR and read-across approaches is proposed for the prediction of developmental toxicity.
- Where exposure to a metabolite exceeds the respective TTC value, acute and chronic toxicity testing strategies were proposed by the PPR Panel, considering the need to derive health based limits for human exposure.
- The opinion also proposes how the risk assessment of pesticide metabolites that are stereoisomers should be addressed due to isomer ratio changes reflected in the composition of metabolites. The PPR Panel does not propose that the TTC scheme is used for individual stereoiosomers, although the TTC scheme has utility as a screening assessment of the isomer mixture for a metabolite. Further development of (Q)SAR tools would be beneficial, both to predict genotoxicity and to address stereochemistry aspects. Furthermore, metabolism guidelines should require compositional information on stereochemistry to consider the full impact on the dietary risk assessment.
The approaches described in this opinion are ready for use, but it is anticipated that on many occasions the outcome of the assessment scheme will be that further testing is needed to reach a firm conclusion on the toxicological relevance of the pesticide metabolite. However, the benefit of applying the approaches is that it will allow prioritisation of pesticide metabolites for subsequent testing. These approaches should not be used as an alternative to conventional risk assessment for pesticide active substances (parent compounds) themselves occurring as residues in food. They should be assessed prior to authorisation on the basis of dossiers including toxicological tests (Regulation (EC) No 1107/2009).
It is noted that EFSA will develop a Guidance Document based on the results in this opinion.
Pesticide metabolites, dietary exposure, endocrine disruptor, Quantitative Structure Activity Relationship, (Q)SAR, read-across, stereoisomer , Threshold of Toxicological Concern (TTC)