TOXICOLOGY
Fenamiphos was first evaluated for toxicological effects by the JMPR in 1974, at which time an ADI of 0-0.0006 mg/kg bw was established. In 1985, following a direct request for re-evaluation by a Member State, additional data were reviewed and the ADI was reduced to 0-0.0003 mg/kg bw and made temporary because of concern about fetotoxicity seen in a study in rabbits. The 1985 JMPR requested submission of the results of an ongoing study of oncogenicity in rats, a full, legible report and raw data from the study of developmental toxicity in rats, and a new study of developmental toxicity in rabbits to clarify the observation of fetotoxicity at low dietary levels. The results of these studies were considered by the 1987 JMPR, which established an ADI of 0-0.0005 mg/kg bw. Fenamiphos was evaluated at the present Meeting within the CCPR periodic review programme.
In rats, fenamiphos was rapidly excreted, with over 96% of the administered dose eliminated within 48 h. Excretion was primarily in the urine, with less than 4% of the dose eliminated in the faeces. At 48 h the levels of tissue residues were below the limit of quantification except after a high dose (3 mg/kg bw), when the maximal tissue levels observed were 3.5-8.4 m g/kg in the liver, 1.6-2.1 m g/kg in the kidney, and 1.6-3.5 m g/kg in the skin. Fenamiphos was completely metabolized in rats. Metabolites retaining anticholinesterase activity, such as fenamiphos sulfoxide and desisopropyl-fenamiphos sulfoxide, were seen in variable but generally low proportions (rarely greater than 3%). Most of the products were dephosphorylated phenol, phenol sulfoxide, or phenol sulfone metabolites and their corresponding sulfates.
Fenamiphos is extremely hazardous after single oral doses to rats, mice, rabbits, cats, dogs, and chickens (LC50 values 2.4-23 mg/kg bw) and highly hazardous after dermal administration to rats and rabbits (LD50 values 75-230 mg/kg bw). It is moderately hazardous after inhalation in rats and mice (LC50 values < 100 m g/l, 4 h). WHO has classified fenamiphos as "extremely hazardous".
The sulfoxide, sulfone, and desisopropylated sulfone metabolites of fenamiphos are similarly toxic to rats after oral administration (LD50 values 1.4-4.1 mg/kg bw). The phenol sulfoxide and sulfone metabolites are only slightly toxic to rats after oral administration, with LD50 values ranging from 1200 to 1900 mg/kg bw.
Fenamiphos inhibited plasma cholinesterase more effectively than erythrocyte acetylcholinesterase, both in vitro and in vivo. Fenamiphos sulfoxide, fenamiphos sulfone, desisopropyl fenamiphos, desisopropyl fenamiphos sulfoxide, and desisopropyl fenamiphos sulfone inhibited plasma and erythrocyte cholinesterase in vitro more effectively than fenamiphos itself.
In evaluating the following studies, inhibition of erythrocyte acetylcholinesterase activity was not used as an indicator of adverse effects in the nervous system when information on brain acetylcholinesterase activity was also available. In the absence of this information, NOAELs were determined on the basis of inhibition of erythrocyte acetylcholinesterase (of ³ 20%). Statistical significance was used as a criterion for considering depression of brain acetylcholinesterase activity to be adverse.
In a three-week study in which rats were exposed by inhalation to atmospheres containing 0, 0.03, 0.25, or 3.5 m g fenamiphos/litre for 6 h/d, five days per week, the only finding was inhibition of plasma cholinesterase at the highest dose. Erythrocyte and brain acetylcholinesterase were unaffected. The no-observed-adverse-effect concentration (NOAEC) was 3.5 m g/litre.
In a three-month study in rats, fenamiphos given at dietary concentrations of 0, 0.37, 0.57, or 0.91 ppm inhibited plasma cholinesterase activity only at the highest dose. No treatment-related changes in erythrocyte or brain acetylcholinesterase activity were seen at any dose. In a second study of short-term toxicity, rats were fed diets containing 0, 4, 8, 16, or 32 ppm fenamiphos for three months. Erythrocyte acetylcholinesterase activity was inhibited at doses of 16 ppm (equivalent to 0.8 mg/kg bw per day) and above. This was considered an adverse effect as brain acetylcholinesterase was not measured in this study. The overall NOAEL was 8 ppm, equivalent to 0.4 mg/kg bw per day.
Rabbits received fenamiphos by dermal application at doses of 0, 0.5, 2.5, or 10 mg/kg bw per day for three weeks (6 h/d, five days per week). Body-weight gain was slightly reduced in animals of each sex at 10 mg/kg bw per day. In females, reductions in cholinesterase activity in the brain (by 20%) and plasma were noted at 2.5 mg/kg bw per day and above. Erythrocyte acetylcholinesterase activity, however, was affected only at 10 mg/kg bw per day. In males, the only findings were decreased plasma and erythrocyte cholinesterase activity at 10 mg/kg bw per day. The NOAEL was 0.5 mg/kg bw per day.
In a series of studies, dogs were fed diets containing 0, 0.5, 0.6, 1, 1.7, 2, 3, 5, 6, 10, 12, or 18 ppm fenamiphos for periods ranging from three months to two years. In dogs treated at 18 ppm (equivalent to 0.45 mg/kg bw per day) for three months, muscle tremors were seen. In dogs treated at 12 ppm (equal to 0.31 mg/kg bw per day) for one year, brain acetylcholinesterase activity was inhibited in females (by 17%), and males showed slight anaemia. Erythrocyte acetylcholinesterase activity was inhibited at doses of 3 ppm (equal to 0.083 mg/kg bw per day) and above for one year. Plasma cholinesterase activity was inhibited at doses of 1.7 ppm (equivalent to 0.042 mg/kg bw per day) and above in a three-month study. No other parameters were affected. Since no information on brain acetylcholinesterase activity was available at doses between 3 and 12 ppm, the Meeting considered 3 ppm (equal to 0.083 mg/kg bw per day) to be the overall NOAEL in dogs.
In mice fed diets containing 0, 2, 10, or 50 ppm fenamiphos for 20 months, there were marginal decreases in survival and body-weight gain at 50 ppm (equal to 7.4 mg/kg bw per day). The relative ovarian and spleen weights were reduced at 10 ppm (equal to 1.4 mg/kg bw per day) and above. There were no non-neoplastic changes that could be attributed to treatment, and fenamiphos was not carcinogenic at any dose. Cholinesterase activity was not measured at any dose. The NOAEL was 2 ppm, equal to 0.3 mg/kg bw per day.
In rats fed diets containing 0, 3, 10, or 30 ppm fenamiphos for two years, the only treatment-related effects were inhibition of erythrocyte acetylcholinesterase activity throughout the study and behavioural changes during the first six weeks of the study at 30 ppm. Brain acetylcholinesterase activity was not measured. The NOAEL was 10 ppm, equal to 0.56 mg/kg bw per day.
Rats were fed diets containing 0, 1.7, 7.8, or 37 ppm fenamiphos for two years. At 37 ppm, equal to 2.5 mg/kg bw per day, body-weight gain in both males and females was decreased. Erythrocyte acetylcholinesterase activity was inhibited at 7.8 ppm (equal to 0.46 mg/kg bw per day) and above. Brain acetylcholinesterase activity was inhibited only at 37 ppm; inhibition was 25% in animals of each sex killed after one year, and 14% in males at study termination. Animals of each sex at 37 ppm also had an increased frequency of non-neoplastic inflammatory lesions of the nasal, laryngeal, and lung tissues and increased relative weights of the brain, heart, and lungs. Fenamiphos was not carcinogenic at any dose. The NOAEL was 7.8 ppm, equal to 0.46 mg/kg bw per day.
Fenamiphos was adequately tested in a battery of tests for genotoxicity. It was found to be mildly clastogenic at cytotoxic doses in vitro but not in vivo. It did not cause reverse or forward mutation, unscheduled DNA synthesis, or sister chromatid exchange in vitro. The Meeting concluded that fenamiphos is not genotoxic.
In a two-generation study of reproductive toxicity, rats were treated with 0, 2.5, 10, or 40 ppm fenamiphos. Parental toxicity was characterized by reduced weight gain in F0 and F1 dams at 40 ppm (equal to 2.8 mg/kg bw per day) during lactation, and in F1 males at 10 ppm (equal to 0.64 mg/kg bw per day) and above before mating. Pathological changes in the salivary gland were seen in F0 males and females at 40 ppm. Erythrocyte acetylcholinesterase activity was consistently inhibited at 10 ppm and above in females but only at 40 ppm in males. Brain acetylcholinesterase activity was inhibited at the highest dose in adult F0 and F1 females (by 21-29%) and in F1 males (by 6%) but not in pups of either sex. In pups at 40 ppm, erythrocyte acetylcholinesterase activity was inhibited only on day 21 of lactation. The only reproductive effect was decreased weight gain of F1 and F2 pups at 40 ppm, beginning on day 7 of lactation. The NOAEL for systemic toxicity was 2.5 ppm, equal to 0.17 mg/kg bw per day. The NOAEL for reproductive toxicity was 10 ppm, equal to 0.64 mg/kg bw per day.
In a study of developmental toxicity, mated rats were treated with 0, 0.3, 1, or 3 mg/kg bw per day on days 6-15 of gestation. Maternal toxicity was seen at the highest dose, characterized by mortality, tremors, and reduced weight gain. The fetuses were not affected at any dose. Cholinesterase activity was not measured in this study. The NOAELs were 1 mg/kg bw per day for maternal toxicity and 3 mg/kg bw per day for developmental toxicity.
Fenamiphos was also administered to mated rats at doses of 0, 0.25, 0.85, or 3 mg/kg bw per day on days 6-15 of gestation. The highest dose resulted in maternal deaths, tremors, salivation, lacrimation, urine staining, and hypoactivity. Body-weight gain and food consumption were also significantly reduced. Erythrocyte acetylcholinesterase activity was reduced at this dose, but the changes in brain acetylcholinesterase activity were not statistically significant or dose-related. The fetuses were unaffected at 3 mg/kg bw per day. The NOAELs were 0.85 mg/kg bw per day for maternal toxicity and 3 mg/kg bw per day for developmental toxicity.
In a study of developmental toxicity in rabbits, animals received 0, 0.1, 0.3, or 1 mg/kg bw per day on days 6-18 of gestation. At 0.3 mg/kg bw per day and above, fenamiphos was maternally toxic, resulting in decreased body-weight gain, bloody nasal discharge, and white ocular discharge. Fetotoxicity, characterized by chain fusion of the sternebrae, was seen only at 1 mg/kg bw per day. The NOAEL for maternal toxicity was 0.1 mg/kg bw per day, and that for developmental toxicity was 0.3 mg/kg bw per day. Cholinesterase activity was not measured in this study.
In a second study, mated rabbits were treated with 0, 0.1, 0.5, or 2.5 mg/kg bw per day on days 6-18 of gestation. Clear signs of maternal toxicity seen at the highest dose included mortality, salivation, dyspnoea, ataxia, diarrhoea, and decreased weight gain and food consumption during treatment. Although some questions remain about the doses that were actually administered (because of uncertain homogeneity), no embryotoxic or teratogenic effects were seen at the maternally toxic dose of 2.5 mg/kg bw per day.
A single dose of 25 mg/kg bw fenamiphos had no effect on neuropathy target esterase activity in the brains or spinal cords of hens under atropine protection.
Fenamiphos did not induce delayed neuropathy in three studies in hens, when tested at doses of 0, 2, 5, 16, or 26 mg/kg bw per day for 30 days or when given once at doses of 0 or 25 mg/kg bw.
Fenamiphos was minimally irritating to rabbit skin and moderately irritating to rabbit eyes and was a mild skin sensitizer in guinea-pigs.
In a study of acute neurotoxicity, rats were given single doses of 0, 0.37, 1.5, or 2.3 mg/kg bw fenamiphos by gavage. Erythrocyte acetylcholinesterase activity was inhibited at the lowest dose tested in males only, and in both males and females at higher doses. At 1.5 mg/kg bw, males showed unco-ordinated gait and muscle fasciculation. At the highest dose, decreased motor activity was also seen in males, and clinical signs, decreased grip strength, and deaths were observed in rats of each sex. There was no effect on brain acetylcholinesterase activity at any dose. The NOAEL was 0.37 mg/kg bw per day.
In another study of neurotoxicity rats were fed diets containing 0, 1, 10, or 50 ppm fenamiphos for 13 weeks. At 10 ppm and above, erythrocyte acetylcholinesterase activity was inhibited in animals of each sex. At the highest dose brain acetylcholinesterase activity was inhibited (by 12%) in females only. A battery of functional observational and motor activity tests revealed no treatment-related effects. The NOAEL was 10 ppm, equal to 0.61 mg/kg bw per day.
An ADI of 0-0.0008 mg/kg bw was established on the basis of an overall NOAEL of 0.083 mg/kg bw per day in the dog, using a safety factor of 100.
A toxicological monograph was prepared, summarizing the data received since the previous evaluation and including relevant summaries from the previous monograph and monograph addenda on fenamiphos.
TOXICOLOGICAL EVALUATION
Levels that cause no toxic effect
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Mouse: |
2 ppm in the diet, equal to 0.3 mg/kg bw per day (20-month study of toxicity and carcinogenicity) |
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Rat: |
0.37 mg/kg (single doses, study of neurotoxicity) |
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10 ppm, equal to 0.61 mg/kg bw per day (three-month study of neurotoxicity) |
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2.5 ppm, equal to 0.17 mg/kg bw per day (parental toxicity in a study of reproductive toxicity) |
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10 ppm, equal to 0.64 mg/kg bw per day (study of reproductive toxicity) |
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0.85 mg/kg bw per day (maternal toxicity in a study of developmental toxicity) |
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3 mg/kg bw per day (developmental toxicity in a study of developmental toxicity) |
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7.8 ppm, equal to 0.46 mg/kg bw per day (two-year study of toxicity and carcinogenicity) |
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Rabbit: |
0.1 mg/kg bw per day (maternal toxicity in a study of developmental toxicity) |
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0.3 mg/kg bw per day (fetotoxicity in a study of developmental toxicity) |
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Dog: |
3 ppm in the diet, equal to 0.083 mg/kg bw per day (overall assessment) |
Estimate of acceptable daily intake for humans
0-0.0008 mg/kg bw
Estimate of acute reference dose
The available data did not permit the Meeting to establish an acute reference dose different from the ADI (0-0.0008 mg/kg bw). Although the results of a study of neurotoxicity in rats given single doses were available, the dog was found to be the more sensitive species. Information on acute effects in dogs may allow the establishment of an acute reference dose in the future.
Studies that would provide information useful for the continued evaluation of the compound
1. Effects of single doses in dogs (with appropriate evaluation of functional changes in the cholinergic nervous system, including brain acetylcholinesterase activity).2. Observations in humans.
Toxicological criteria for estimating guidance values for dietary and non-dietary exposure to fenamiphos
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Human exposure |
Relevant route, study type, species |
Results, remarks |
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Short-term |
Oral neurotoxicity, rat |
NOAEL = 0.37 mg/kg bw per day: effects observed during battery of functional observational tests |
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Oral toxicity, rat (fasted) |
LD50 = 2.4-6 mg/kg bw |
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Inhalation toxicity, 4 h, rat |
LC50 = 91-100 fig/I |
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Inhalation toxicity, 5 days, rat |
NOAEL = 4 m g/l |
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Dermal toxicity, rat |
LD50 = 72-92 mg/kg bw |
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Dermal irritation, rabbit |
Minimally irritating |
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Ocular irritation, rabbit |
Moderately irritating |
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Dermal sensitization, guinea-pig |
Mildly sensitizing |
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Medium-term |
Repeated inhalation toxicity, 3 weeks, rat |
NOAEL = 3.5 m g/l (highest dose tested) |
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Repeated dermal toxicity, 3 weeks, rabbit |
NOAEL = 0.5 mg/kg bw per day: inhibition of brain acetylcholinesterase activity |
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Repeated oral, reproductive toxicity, rat |
NOAEL = 0.17 mg/kg bw per day: parental toxicity |
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NOAEL = 0.64 mg/kg bw per day: reproductive toxicity |
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Repeated oral, developmental toxicity, rabbit |
NOAEL = 0.1 mg/kg bw per day: maternal toxicity |
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NOAEL = 0.3 mg/kg bw per day: developmental toxicity |
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Long-term |
Repeated oral, 1-2 years, dog |
NOAEL = 0.083 mg/kg bw per day: inhibition of brain acetylcholinesterase activity, anaemia |
