Part 4 – Metabolism and Kinetics
1. INTRODUCTION
Metabolism studies are used to assess the nature and disposition of chemical residues in or on food or feed commodities. The composition of the residues (parent compound and/or metabolites) and where they occur in a crop or farm animal must be known so that supervised residue trials, processing trials and analytical methods deal with the relevant residues.
There should be a comparison of the metabolites which are formed in experimental animals, and target animals or crops. Similarities and differences in the degradation of the chemical should be discussed in the light of the residues which may be present in food commodities for human consumption in order to determine appropriate residue definitions. These studies are also used to facilitate the development of analytical methods for determining residue levels. Metabolism studies should also provide information on residues that have the potential to pass into a processed commodity, e.g. a water-soluble conjugate has a higher potential to contaminate sugar or wine.
If a metabolite appears in plants and is not present in animals, additional toxicological data may be required on it. Animal metabolites are normally considered as being covered by the toxicological studies on the parent compound because the metabolites will be formed during the feeding studies.
1.1. Reference materials
The details of documents referred to in these instructions (including codes and standards) are given in the References section. Bibliographic details and, where appropriate, ISBN and purchase information are given. Applicants should be aware that many of these documents are updated regularly (hence dates are not supplied in the text). It is therefore important to ensure that the latest editions of reference materials are obtained.
2. GENERAL INSTRUCTIONS
Plant metabolism studies are required when use of the active ingredient on edible (human and animal) crops is proposed. Animal metabolism and kinetic studies are required for any proposed use. Data are required for both laboratory animals and livestock (target animals).
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Metabolism studies in plants show the chemical transformations and distributions that take place after exposure to an agricultural chemical.
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Metabolism studies in animals are used to assess the fate of the administered chemical, including the chemical identification and location of the residue.
- Toxicokinetic or pharmacokinetic studies allow the quantitation and determination of the time-course of absorption, distribution, biotransformation and excretion of xenobiotic chemical agents in animals.
These studies are often conducted using radiolabeled substances and should be conducted according to recognised test protocols, including those of the OECD, the European Union, the Food and Agriculture Organization or the US Environment Protection Agency. If studies are conducted to non-standard testing protocols, advice should be sought from the APVMA as to the suitability of such studies for registration purposes.
Metabolism studies should be derived from animal species/crops representative of those on which the product is to be used; or on species likely to come in contact with the material, e.g. animals grazing and/or fed treated crops.
If original detailed studies are performed using laboratory animals, it will be necessary to show that similar metabolic pathways are followed in animals exposed to the chemical or its residues, and whose meat, milk and eggs are destined for human consumption.There should be a comparison of the metabolites that are formed in experimental animals, and target animals or crops. Similarities and differences in the degradation of the chemical should be discussed in the light of the residues that may be present in food commodities for human consumption.
These instructions describe the aims and scope of the studies required but do not give details of individual study protocols. Kinetics data that are obtained during toxicity studies should be submitted in Part 3 (Toxicology) of the application.
3. OUTLINE OF DATA REQUIREMENTS AND APPLICATION LAYOUT
A check list of data requirements for Part 4 (Metabolism and Kinetics ) of an application for agricultural chemical products, and the way in which they should be set out, are described below. Note the format used supersedes that described in the Appendix E of the Ag Manual: Requirements Code for Agricultural Chemicals.
Table 1: Data requirements for Part 4 (Metabolism and Kinetics)
| METABOLISM AND KINETICS Table of contents Metabolism studies in target plants Metabolism and toxicokinetic studies in laboratory animals Metabolism and pharmacokinetic studies in target animals Metabolism database |
4. METABOLISM STUDIES IN TARGET PLANTS
The amount of active substance used in plant metabolism and compound distribution trials should be chosen according to the proposed practical application rates and methods (e.g. foliar application, seed or soil treatment), or, if necessary to obtain sufficient quantifiable residues, at exaggerated rates.
Plant metabolism studies are usually carried out on crops typical of those to which the compound will be applied. If the metabolism is the same or similar in crops from three crop groups, no further studies are required. The exception is that use on sugar cane will require a sugar cane or sugar beet study. Crops can be considered under the following groups:
- grasses: e.g. cereals, sugar cane, pastures;
- leafy crops: e.g. vegetables, lucerne, clovers;
- pulses and oilseeds;
- root vegetables; and
- fruits.
The APVMA should be consulted on the applicability of the studies for the proposed registration if the studies have not been conducted on crops similar to the major target crops. The aims of plant metabolism studies are:
- to provide an estimate of the total terminal residues in the edible parts (food and feedstuff) of treated crops according the proposed use pattern;
- to identify the major components of the total terminal residue (e.g. parent and metabolites);
- to indicate the distribution of residues within the various crop fractions including whether the compound is absorbed through the roots and/or foliage, is entirely surface deposited and whether the residues are translocated;
- to show efficiency of extraction procedures for various components of the residue;
- to provide guidance on the possible fate of residues during subsequent food processing, e.g. if the residue is entirely a surface residue it may be substantially removed by washing or peeling of fruit; and
- to assist in determining residue definitions for enforcement and risk assessment.
5. METABOLISM AND TOXICOKINETIC STUDIES IN LABORATORY ANIMALS
Studies are required on the fate of the substance in experimental animals. The following information would normally be expected:
The degree and rate of absorption after oral administration in at least one mammalian species. An investigation of the extent of absorption after dermal application is desirable. The vehicle chosen for the dermal study should resemble closely that proposed for the product.
- The extent and rate of distribution and storage in the tissue of animals, or any bioaccumulation that may occur.
- Biotransformation in animals, including the rate and degree of such biotransformation, together with a description of any metabolites produced.
- The mode and extent of excretion or elimination of the parent compound, and/or its degradation products, in animals including the rate at which such excretion occurs.
6. METABOLISM AND PHARMACOKINETIC STUDIES IN TARGET ANIMALS
Metabolism studies using a radiolabelled active substance with, optionally, a pharmacokinetic component, should be conducted on animal species representative of those likely to come in contact with the material, e.g. animals directly treated or grazing and/or fed treated crops or crop commodities. Studies are usually conducted using goats or cows (to represent the ruminant) and chickens. A monogastric animal (e.g. pig) study is not normally required as data are available from studies with rats. However, if metabolism in the rat is different from that in the ruminant and chicken, a pig study will be required.
The aims of livestock animal metabolism studies are:
- to provide an estimate of the total terminal residues in the edible commodity of animals;
- to identify the major components of the total terminal residues;
- to indicate the distribution and nature of residues in muscle, fat, milk, eggs, liver and kidney to identify target tissues and to determine if the residues are fat-soluble;
- to show efficiency of extraction procedures for various components of the residues;
- to identify bound residues;.
- to assist in determining residue definitions for enforcement and risk assessment.
Measuring the mode and extent of excretion or elimination of the parent compound, and/or its degradation products, in livestock to identify any potential for bioaccumulation is optional.
If this aspect is included, and the study is designed to show residue levels have reached a plateau, and those in the tissues are less than 0.1 mg/kg, animal feeding studies may not be required. (If contemplating this approach the APVMA should be consulted before electing not to conduct animal feeding studies.) Also, the applicant is reminded that a validated non-radiolabel method of residue determination is still required for enforcement purposes.
Further advice can be obtained from the Food and Agriculture Organization publication, Guidelines on Producing Pesticide Residues Data from Supervised Trials.
7. METABOLISM DATABASE
A metabolism database should also be included, similar in format to the toxicological database described in Part 3 (Toxicology), Section 3-8.
GLOSSARY
Agricultural chemical product
A substance or mixture of substances that fits the legal definition in the Agricultural and Veterinary Chemicals Code Act 1994 (see Ag Manual, Module A).
Bioaccumulation
The process by which the amount of a substance in a living organism (or its parts) increases with time.
Biotransformation
The process by which xenobiotics are converted into metabolites by an animal or plant, including the enzymatic conversion of chemicals into compounds that can be more easily excreted. This term is often used synonymously with the term 'metabolism'.
Laboratory/experimental animals
Animals (usually rats, mice, rabbits, dogs) bred specifically for testing of xenobiotic agents.
Metabolism
In plant studies, the term 'metabolism' is used in a wider context than in animals studies, to include the formation of all products (degradates) of the pesticide in or on the plant, regardless of whether they result from internal plant metabolic processes, from chemical reactions (hydrolysis and photolysis) or biological processes outside the plant (eg microbial degradation in the soil).
Pharmacokinetics
The study of the movement of drugs within the body (i.e. the absorption, distribution via the blood, metabolism and excretion). The term is usually applied to medicines or therapeutic agents tested at doses related to their therapeutic action but can be applied to any xenobiotic chemicals. See also Toxicokinetics.
Product
A formulation containing one or more active constituent(s), and possibly non-active constituent(s), which is intended for application and administration, with or without dilution before use, and which is labelled with directions for use.
Registration
The process whereby the APVMA approves the sale and use of a formulated product, after the evaluation and assessment of appropriate scientific data demonstrating that the product is effective and not unduly hazardous to human health, the environment, or target plants and animals, and that it will not adversely affect trade.
Residue
The remains of the active constituent(s) in a chemical product persisting in or on food, agricultural commodities or plants, soil, water or other environmental components, together with all derivatives, metabolites, and degradation products of the active constituent(s) arising from their use.
Target animals
Animals that will be exposed to an agricultural or veterinary product, either by direct administration (as for veterinary products) or in contaminated feedstuff.
Target plant
Crops that will be exposed to an agricultural product.
Target organ
The organ(s) or tissues, in an animal/plant in which, owing to the chemical and kinetic properties of a xenobiotic agent, the pharmacological or toxic effects are seen.
Toxicokinetics
The study of the movement of xenobiotic chemical agents within the body (i.e. the absorption, distribution via the blood, metabolism and excretion), to provide key information for the assessment of possible toxicity at elevated doses. The term is sometimes used interchangeably with 'pharmacokinetics' (see above) but the latter term applies more specifically to medicines or therapeutic agents.
Xenobiotic chemicals
Compounds 'foreign' to an organism (e.g. environmental contaminants, agricultural and veterinary chemicals, drugs).
REVISION HISTORY
| Revision Date | Description of Revision |
|---|---|
| 1 July 2005 | First edition
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| 1 October 2005 | Second edition
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| 1 April 2006 | Third edition
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| 1 July 2007 | Fourth edition
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