Chapter 12 Ametryn in Rats, Lactating Goats, and Laying Hens Metabolic Fate 1
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Jinn Wu , DaveD.W.Liu , Robert A. Robinson , William Itterly , and Thomas M. Capps 2
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XenoBiotic Laboratories, Inc., P.O. Box 3205, Princeton, NJ 08543-3205 Agricultural Division, Ciba-Geigy Corporation, P.O. Box 18300, Greensboro,NC27419 2
C]-Ametryn was administered to rats, lactating goats, and laying [hens. Excreta from all three species, daily milk from the goats, and 14
egg samples from the hens were collected. In addition, selected tissues were taken following animal sacrifice. All samples were analyzed to determine the total radioactive residues, and metabolites were identified and quantified in excreta and selected tissues. The results showed that, in all three species, the majority of the administered dose was eliminated in excreta. Residues were observed in milk, eggs, tissues, and organs; liver and kidney showed the highest total radioactive residue levels. Metabolites in the rat were isolated, purified, and identified by thin-layer chromatography, high performance liquid chromatography, and various mass spectroscopic techniques. Ametryn appears to undergo extensive metabolic transformation by N-dealkylation, oxidation/hydroxylation, and conjugation (with sulfate, glutathione derivatives, and glucuronic acid). The results obtained from the rat were compared with those obtained from the ruminants and poultry. Animal metabolism provides both a scientific and a regulatory contribution to the data base needed to register or re-register an agrochemical. Absorption, distribution, metabolism, and excretion (ADME) studies in the rat are required in the overall toxicological assessment of a chemical's safety. Metabolism studies in livestock are needed when pesticide use, either by direct treatment of animal feed items or by premises application, can result in residues in milk, meat, or eggs. The primary focus of a rat metabolism study is the identification of metabolites in excretory products (urine and/or feces); and in a ruminant (goat or cow) or laying hen study, the primary focus is the qualitative nature of the residues (NOR) in milk, meat, and tissues. Analysis of excreta from livestock, though not critical from a regulatory perspective, can often aid in tissue identification by contributing an alternate source of an important metabolite in greater abundance than can be obtained from the tissue itself. Isolation and identification of a "marker" metabolite from goat, hen, or, in many instances, rat excreta, followed by a thorough chromatographic comparison with the tissue metabolite, can be handled in a more timely manner with less analytical frustration. 0097-6156/92/0503-0168S06.50/0 © 1992 American Chemical Society Hutson et al.; Xenobiotics and Food-Producing Animals ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
12. WU ET AL.
Ametryn in Rats, Lactating Goats, and Laying Hens
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[ C]-Ametryn [2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-5-triazine], uniformly labeled on the triazine ring, is a selective herbicide registered for control of broad-leaved and grassy weeds. {See chemical structure below.) To fulfill the re-registration requirement of the Pesticide Assessment Guidelines, the metabolic fate of the titled compound in three different animal species, namely, rats, lactating goats, and laying hens, was investigated. SCH,
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Downloaded by FUDAN UNIV on January 14, 2017 | http://pubs.acs.org Publication Date: August 24, 1992 | doi: 10.1021/bk-1992-0503.ch012
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P C]-Ametryn Study Design Rat metabolism work was initiated first on the premise that one or more metabolites might be present in excreta, which might be useful in subsequent goat and hen studies. A preliminary rat metabolism study was conducted (1) to generate excreta samples in order to isolate and identify unknown metabolites and (2) to assess the possibility of carbon dioxide elimination. A more detailed and definitive study was subsequently initiated according to U.S. Environmental Protection Agency (EPA) guidelines. Detailed information on the dosing regimens for rats, goats, and hens is summarized in Table I. Table I. Ametryn Dosing Summary Animal Rat
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Single Oral Low Dose Multiple Oral Low Dose Single Intravenous Dose Single Oral High Dose
0.5 200
Goat Hen a b c
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Type
Dosing Level (mglkg)
1.5 (50 ppm) 3 (50 ppm)
Multiple Oral Dose c
Multiple Oral Dose c
Dosing followed by 7-day depuration. Animal sacrificed within 24 hours after the final dosing. Estimated dietary exposure.
Hutson et al.; Xenobiotics and Food-Producing Animals ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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XENOBIOTICS
AND FOOD-PRODUCING
ANIMALS
A total of eight rats in two different dosing groups (two males and two females in each group), i.e., single oral low dose (0.5 mg/kg of rat body weight) and single oral high dose (200 mg/kg), were used in the preliminary study. In the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) rat study, a total of 44 rats were utilized. Ten rats (five per sex) were used in each of the four treated groups; i.e., single oral low dose (0.5 mg/kg), multiple oral low dose (0.5 mg/kg), single oral high dose (200 mg/kg), and intravenous dose groups (0.5 mg/kg). In addition, four rats (two per sex) were used as controls. Sample collection periods were 4 days for the preliminary study and 7 days for the definitive study. For the goat metabolism study, three lactating goats (one control and two treated) were dosed for a period of 3 days. The goats were orally administered approximately 75 mg/day (-50 ppm dietary intake) of [ C]-ametryn for 3 consecutive days. For the hen metabolism study, 15 laying hens (10 treated and 5 controls) were orally administered approximately 6 mg/day (-50 ppm dietary intake) of [ C]-ametryn for 3 consecutive days. Excreta from all three species, as well as daily milk and egg samples, were collected. Also, selected tissues were taken following animal sacrifice.
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Total Material Balance and Total Radioactive Residues Table II summarizes the total recovery of the administered dose, which is expressed as a percentage of the total administered dose, in all three species of test animals. Rats excreted nearly comparable levels in the urine and feces, with about 2% of the dose remaining in the carcass and tissues within 7 days of the radiochemical dose. A summary of the total recovery of the administered dose in goats and hens is also given in Table II. The C recovered in goat urine, feces, daily milk, and tissues was approximately 61%, 8%, 1%, and 6%; the radiocarbon recovery profile in hen excreta, egg, and organs/tissues was similar, namely, 73%,
