* New compoundN-tert-butyl-N'-(4-ethylbenzoyl)-3,5-dimethylbenzohydrazide
Tebufenozide is a fat-soluble insecticide used to control Lepidoptera pests in fruits, vegetables and other crops. It has a novel mode of action in that it mimics the action of the insect moulting hormone, ecdysone. Lepidoptera larvae cease to feed within hours of exposure and then undergo a lethal, unsuccessful moult.
Tebufenozide was evaluated for the first time by the present Meeting.
TOXICOLOGY
Oral administration to rats of single doses of 3 or 250 mg/kg bw of 14C-labelled tebufenozide resulted in rapid absorption and excretion in urine and faeces, only trace amounts of 14C being recovered in expired air. The excretion profiles were similar, regardless of the position of the 14C label, the dose, the sex, or whether the rats had been pretreated with 30 ppm of unlabelled tebufenozide in the diet for two weeks. A mean total of 87-104% of the administered radioactivity was eliminated within 48 h, primarily via the faeces which accounted for 90% of the 14C that was excreted; only minor amounts (1-8%) were excreted in urine and trace amounts (0.1-0.4%) in expired air. In animals at 3 mg/kg bw, absorption accounted for 35-39% of the administered radioactivity; 30-34% was excreted in the bile and about 5% in the urine. At 250 mg/kg bw, only about 4% of the administered dose was absorbed and metabolized. The highest levels of 14C in the blood were measured 0.5-12 h after dosing, and clearance of the radiolabel from the circulation was rapid. Tissue retention of 14C was low, suggesting that there is little or no bioaccumulation of tebufenozide in the body.
Most of the 14C excreted in the faeces was in the form of unabsorbed (parent) tebufenozide, which accounted for about 60 and 90% of administered doses of 3 and 250 mg/kg bw per day respectively; no unchanged tebufenozide was detected in the urine. The absorbed [14C]tebufenozide was extensively metabolized in rats. There were no significant qualitative differences in the metabolic profiles associated with the position of the 14C label, the dose, the sex, or whether rats were pretreated with unlabelled tebufenozide. In general, the 13-15 metabolites identified in the urine, faeces, and bile were identical. The main route of metabolism of tebufenozide appeared to be oxidation of the benzylic carbons (A- or B-ring), resulting in a number of metabolites with various combinations of oxidation state at the three oxidized carbon centres and one metabolite produced by oxidation of the non-benzylic, terminal carbon on the A-ring ethyl group.
Tebufenozide was of low acute toxicity after administration to mice orally or to rats by the oral, dermal or inhalation route. The oral LD50 in mice and rats was >5000 mg/kg bw; the dermal LD50 in rats was >5000 mg/kg bw, and the inhalation LC50 in rats was > 4.3 mg/litre. The metabolites were also of low acute toxicity to mice after oral administration. Tebufenozide was not irritating to the skin and was minimally irritating to the eyes of male rabbits; it was not a skin sensitizer in guinea-pigs. WHO has not classified tebufenozide for acute toxicity.
Repeated short-term oral administration of tebufenozide to mice (2 and 13 weeks), rats (2, 4, and 13 weeks), and dogs (2, 6, 13, and 52 weeks) resulted primarily in haematotoxic effects (regenerative haemolytic anaemia and compensatory responses from the haematopoietic tissues). The NOAEL for these effects was 200 ppm, equal to 35 mg/kg bw per day, in mice in a 13-week study (0, 20, 200, 2000 and 20,000 ppm tested); 200 ppm, equal to 13 mg/kg bw per day, in rats in a 13-week study (0, 20, 200, 2000, and 20,000 ppm tested); 50 ppm, equal to 2.0 mg/kg bw per day, in dogs in a 13-week study (0, 50, 500, and 5000 ppm tested), and 50 ppm, equal to 1.8 mg/kg bw per day, in a one-year study of toxicity in dogs (0, 15, 50, 250, and 1500 ppm tested). Repeated dermal applications of tebufenozide to rats for four weeks caused no systemic toxicity at doses up to 1000 mg/kg bw per day. The dog appeared to be the most sensitive species for both short-term and long-term toxicity.
In an 18-month study of toxicity and carcinogenicity in mice administered tebufenozide in the diet at concentrations of 0, 5, 50, 500, or 1000 ppm, the NOAEL for systemic toxicity was 50 ppm, equal to 7.8 mg/kg bw per day, on the basis of a slightly reduced survival rate and mild regenerative haemolytic anaemia at higher doses. In a two-year study of toxicity and carcinogenicity in rats administered tebufenozide in the diet at 0, 10, 100, 1000, or 2000 ppm, the NOAEL was 100 ppm, equal to 4.8 mg/kg bw per day, on the basis of decreased body weight and food consumption and mild regenerative haemolytic anaemia at higher doses. Tebufenozide was not carcinogenic in mice or rats under the conditions of the studies.
Tebufenozide and its metabolites have been adequately tested for genotoxicity in a range of assays both in vitro and in vivo. The Meeting concluded that neither tebufenozide nor its metabolites were genotoxic.
In two two-generation studies of reproductive toxicity in rats, with one litter per generation, concentrations of 0, 10, 150, or 2000 ppm and 0, 25, 200, or 2000 ppm were administered. The NOAEL for systemic (parental) toxicity was 25 ppm, equal to 1.6 mg/kg bw per day, on the basis of a consistent increase in the incidence of gross and histopathological lesions in the spleens (congestion, pigment, and extramedullary haematopoiesis) of F0 and F1 parental animals at higher doses (200 and 2000 ppm). The NOAEL for reproductive toxicity was 13 mg/kg bw per day on the basis of potential or minor reproductive effects (decreased mean number of implantation sites, prolonged gestation, a slightly greater frequency of total resorptions, and a small increase in the number of dams that died during delivery) at the high dose of 2000 ppm in dams in the first study and in lactating pups (decreased mean weight gain on lactation days 14 and 21) in the second study.
In studies of developmental toxicity in rats and rabbits, doses of 0, 50, 250, or 1000 mg/kg bw per day were administered. There was no evidence of teratogenic potential. The NOAEL for maternal, embryo- and fetotoxicity and teratogenicity was 1000 mg/kg bw per day, the highest dose tested, in both species.
In a study of acute neurotoxicity in rats, no treatment-related effects were seen when single doses of 0, 500, 1000, or 2000 mg/kg bw were administered. The NOAEL for acute neurotoxicity and neuropathological effects was 2000 mg/kg bw, the highest dose tested.
In summary, exposure to tebufenozide by the oral route results primarily in haematotoxicity. The main target of its action is the peripheral haematopoietic system; the pivotal toxicological end-point of concern, which is seen consistently across all species tested, is mild regenerative haemolytic anaemia with compensatory responses from the haematopoietic tissues.
An ADI of 0-0.02 mg/kg bw was established for tebufenozide on the basis of the NOAELs for haematotoxicity of 1.8 mg/kg bw per day in the one-year study in dogs and 1.6 mg/kg bw per day in a two-generation study of reproductive toxicity in rats, using a safety factor of 100.
A toxicological monograph was prepared, summarizing the data that were reviewed at the present Meeting.
TOXICOLOGICAL EVALUATION
Levels that cause no toxic effect
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Mouse: |
200 ppm, equal to 35 mg/kg bw per day (13-week study of toxicity) |
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50 ppm, equal to 7.8 mg/kg bw per day (haematotoxicity in an 18-month study of toxicity and carcinogenicity) |
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Rat: |
200 ppm, equal to 13 mg/kg bw per day (13-week study of toxicity) |
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100 ppm, equal to 4.8 mg/kg bw per day (haematotoxicity in a two-year study of toxicity and carcinogenicity) |
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25 ppm, equal to 1.6 mg/kg bw per day (maternal haematotoxicity in a two-generation study of reproductive toxicity) |
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200 ppm, equal to 13 mg/kg bw per day (reproductive toxicity in a two-generation study) |
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1000 mg/kg bw per day, the highest dose tested (maternal, embryo-, and fetotoxicity and teratogenicity in a study of developmental toxicity) |
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Rabbit: |
1000 mg/kg bw per day, the highest dose tested (maternal, embryo-, and fetotoxicity and teratogenicity in a study of developmental toxicity) |
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Dog: |
50 ppm, equal to 1.8 mg/kg bw per day (haematotoxicity in a one-year study of toxicity) |
Estimate of acceptable daily intake for humans
0-0.02 mg/kg bw
Studies that would provide information useful for the continued evaluation of the compound
1. Observations in humans.
2. Studies on the mechanism of haematotoxicity.
Toxicological criteria for setting guidance values for dietary and non-dietary exposure to tebufenozide
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EXPOSURE |
RELEVANT ROUTE, STUDY TYPE, SPECIES |
RESULT, REMARKS |
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Short-term (1-7 days)
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Oral toxicity, rat |
LD50 >5000 mg/kg bw |
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Dermal toxicity, rat |
LD50 >5000 mg/kg bw |
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Inhalation, 4 h, toxicity, rat |
LC50 >4.3 mg/litre |
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Dermal irritation, rabbit |
Not irritating |
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Ocular irritation, rabbit |
Minimally irritating |
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Dermal sensitization, guinea-pig |
Not sensitizing |
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Medium-term (1-26 weeks)
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Repeated dietary, 90 days, toxicity, dog |
NOAEL = 2.0 mg/kg bw per day, primarily haematotoxicity |
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Repeated dermal, 28 days, toxicity, rat |
NOAEL = 1000 mg/kg bw per day, highest dose tested |
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Repeated dietary, reproductive toxicity, rat |
NOAEL = 13 mg/kg bw per day, minor reproductive effects |
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Repeated gavage, developmental toxicity, rat and rabbit |
NOAEL = 1000 mg/kg bw per day (highest dose tested), maternal, embryo- and fetal toxicity and teratogenicity |
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Long-term (> one year) |
Repeated dietary, one year, toxicity, dog |
NOAEL = 1.8 mg/kg bw per day, primarily haematotoxicity |
RESIDUE AND ANALYTICAL ASPECTS
The Meeting was provided with information on registered uses of tebuconazole on fruits, vegetables and other crops, and received extensive information on metabolism, environmental fate in soil, methods of residue analysis, the stability of residues in stored analytical samples, supervised residue trials, animal transfer studies and the fate of residues during processing. The metabolism studies were on rats, lactating goats, laying hens, fish, apples, grapes, rice and sugar beet. The information on environmental fate included studies of field dissipation and biodegradation in water/sediment systems.
Residues of tebufenozide can be determined by HPLC with UV detection or by GLC with NP detection after methylating the residues. Limits of determination are usually 0.01-0.05 mg/kg in a range of commodities, 0.02 mg/kg in soil and 0.1 mg/l in water.
The Meeting agreed that the residue should be defined as tebufenozide.
The Meeting evaluated residue data from supervised trials and estimated maximum residue levels for apples, grapes, walnuts, rice and pecans.
Information on the fate of tebufenozide during the processing of apples, grapes and tea was provided. In one study the total residue of tebufenozide in apple juice was about 15% of that in the apples. In a number of studies of vinification the mean residue in wine was 36% of that in the grapes. Infusions of tea contained 5-31% of the tebufenozide in the dry tea, with a mean of 17%.
Maximum residue levels estimated by the Meeting which are recommended for establishing MRLs are recorded in Annex I, together with STMR levels.
FURTHER WORK OR INFORMATION
Desirable
1. Information on tebufenozide residues in raisins, raisin culls and rice hulls.
2. Information on residues of tebufenozide in foods in commerce or at consumption.
3. A transfer study on poultry.
4. The results of a cow-feeding study which the Meeting was informed was in progress.
5. Data on residues in paddy rice and on the stability of residues in analytical samples of rice stored for longer periods than the 20-21 days already reported.
6. A detailed report of the completed study of uptake by rotational crops that the Meeting was informed was available.
7. Representative data on the storage stability of residues on leafy vegetables for the full duration of the studies that the Meeting was informed are in progress.
