PESTICIDE RESIDUES IN FOOD - 1997
Sponsored jointly by FAO and WHO
with the support of the International Programme
on Chemical Safety (IPCS)
TOXICOLOGICAL AND ENVIRONMENTAL
EVALUATIONS 1994
Joint meeting of the
FAO Panel of Experts on Pesticide Residues
in Food and the Environment
and the
WHO Core Assessment Group
Lyon 22 September - 1 October 1997
The summaries and evaluations contained in this book are, in most
cases, based on unpublished proprietary data submitted for the purpose
of the JMPR assessment. A registration authority should not grant a
registration on the basis of an evaluation unless it has first
received authorization for such use from the owner who submitted the
data for JMPR review or has received the data on which the summaries
are based, either from the owner of the data or from a second party
that has obtained permission from the owner of the data for this
purpose.
METHIDATHION (addendum)
First draft prepared by
A. Moretto
Istituto di medicine del Lavoro Universita degli Studi di Padova
Padua, Italy
Explanation
Evaluation for acute reference dose
Acute toxicity
Observations in humans
Comments
Toxicological evaluation relevant for establishing an acute
reference dose
References
Explanation
Methidathion was last evaluated toxicologically by the 1992 JMPR
(Annex 1, reference 65)when an ADI of 0-0.001 mg/kg bw was established
on the basis of an NOAEL of 0.14 mg/kg bw per day in three-month,
one-year, and two-year studies in dogs. The ADI was based on studies
in which effects on the liver were observed. The CCPR requested the
JMPR to establish an acute reference dose for methidathion in view of
its high acute toxicity. New studies submitted for review were
evaluated by the present Meeting.
Evaluation for acute reference dose
1. Acute toxicity
Rats
Groups of three to six male and female Crl:CD(SD)BR rats were
given technical-grade methidathion (purity, 93.2%) as single doses of
0, 4, 8, 16, 20, 25, or 30 mg/kg bw by gavage in corn oil. The acute
oral LD50 was about 25 mg/kg bw. An abbreviated functional
observational battery of tests, including observations of mortality,
signs in the home cage, during handling, and in the open field, and
righting reflexes in air, were conducted. Signs consistent with
cholinergic poisoning (e.g. lachrymation, salivation, diarrhoea,
tremors, ataxia, and muscle fasciculations) were observed in males at
doses > 8 mg/kg bw and in females at doses > 16 mg/kg bw. Two of
three females at 30, one of five males and three of six females at 25,
and one of six males at 20 mg/kg bw died. The NOAEL for signs of
cholinergic overstimulation was 4 mg/kg bw, and the highest non-lethal
dose was 20 mg/kg bw, when the estimated time to peak effect for toxic
signs was 2 h. Males recovered faster than females, and many of the
signs had abated 4-6 h after treatment (Leahy, 1993).
Groups of five male and five female Crl:CD BR VAF/Plus rats were
given technical-grade methidathion (purity, 93.2%) as single doses of
0 or 0.5-35 mg/kg bw in corn oil by gavage. Body weights, clinical
signs, and liver and brain weights were recorded. Serum, erythrocyte,
and brain chlinesterase activities were measured 4 h after treatment.
All males at 35 mg/kg were dead by day 3, and four females at 20 mg/kg
died within one day. The signs of cholinergic overstimulation included
decreased activity, tremors, absence of pain reflex, excessive
salivation, dyspnoea, lachrymation, red-stained face, discoloured
urogenital area, soft stools, few faeces, meiosis, and fasciculations
and were observed in animals treated with doses > 5 mg/kg bw. The
peak effect was seen 2 h after treatment. There was no meaningful
effect on body-weight gain or on liver or brain weights. The NOAEL for
cholinergic signs was 3 mg/kg bw.
In a subsequent phase, the doses administered were 0, 0.5, 1,
2.5, 5, or 10 mg/kg bw, and cholinesterase activity was measured in
plasma, erythrocytes, and brain about 4 h after treatment. The results
are shown in Table 1. Thus, single oral doses of methidathion caused
acute signs and cholinesterase inhibition. The NOAEL for
cholinesterase inhibition was 1 mg/kg bw (Glaza, 1994).
Crl:CD-BR VAF/Plus rats were given single doses of
technical-grade methidathion (purity, 93.2%) at 0, 1, 4, 8, or 16
mg/kg bw in corn oil by gavage. As a positive control, carbaryl was
administered at 30 mg/kg intraperitoneally. Body weights, food
consumption, and clinical signs were recorded. A complete functional
observational battery of tests and figure-eight maze activity counts
were performed before treatment, at the time of peak effect (1.5 h
after treatment with methidathion and 0.5 h after carbaryl), and one
and two weeks after treatment. Serum and erythrocyte cholinesterase
activities were measured before treatment, at the time of peak effect,
and 14 days after treatment. Acetylcholinesterase activity in regions
of the brain (cerebellum, striatum, and cerebral cortex or
hippocampus) were measured at the time of peak effect and one and two
weeks after treatment. Necropsies were performed, and selected nervous
tissues examined histologically.
All animals survived. In comparison with controls, the
body-weight gain of males at 16 mg/kg bw was decreased. Food
consumption was decreased in animals of each sex at this dose during
the first week. Changes in clinical signs and in the results of
functional observational tests and maze activity were seen only at the
time of peak effect at doses > 8 mg/kg bw in males and > 4 mg/kg
bw in females. The changes consisted of typical cholinergic signs and
decreased body temperature. Similar changes were seen in
carbaryl-treated animals. The cholinesterase activities at the time of
peak effect and two weeks after treatment are reported in Table 2.
Acetylcholinesterase activity in the cerebral cortex or hippocampus
was inhibited in males at 1 mg/kg bw; however, this effect was not
considered to be of toxicological significance because no inhibition
was seen in acetylcholinesterase activity in erythrocytes or in other
regions of the brain at this dose, the activity in controls was
Table 1. Mean cholinesterase activity (mU/ml, standard deviation in parentheses) in rats 4 h after treatment
with methidathion
Dose Males Females
(mg/kg bw)
Plasma Erythrocytes Brain Plasma Erythrocytes Brain
0 316 (53) 1411 (145) 1593 (260) 821 (128) 944 (126) 1659 (63)
0.5 332 (139) 1329 (268) 1715 (152) 857 (181) 1107 (170) 1549 (56)
1.0 300 (41) 1282 (386) 1687 (178) 800 (186) 979 (126) 1626 457)
2.5 282 (26) 1177 (328) 1676 (423) 857 (206) 769 (48) 1016 (102)*
5.0 186 (19)* 1119 (238) 1138 (249)* 668 (109) 559 (97)* 605 (98)*
10.0 169 (26) 1037 (190) 744 (91)* 477 (77)* 618 (t3)* 477 (135)*
* Statistically significant in comparison with the control group, Dunnett's t test, p < 0.05
Table 2. Mean cholinesterase activity (µg/ml) after administration of methidathion to rats, at the time of peak
effect (1.5 h after treatment) and at week 2
Dose Serum Erythrocytes Cerebellum Cortex Striatum
(mg/kg bw)
1.5 h 2 weeks 1.5 h 2 weeks 1.5 h 2 weeks 1.5 h 2 weeks 1.5 h 2 weeks
Males
0 390 359 1530 1488 3.13 3.31 8.22 6.24 25.32 27.95
1 392 415 1482 1516 2.74 3.20 4.81** 8.07 27.00 30.80
4 316 398 646** 968* 1.48** 3.26 2.60** 8.09 7.08** 37.60
8 272** 462 394** 1224 0.99** 3.09 1.01** 6.50 4.12** 19.12
16 231** 394 248** 1084 0.69** 2.79 0.51** 5.94 2.32** 17.88
Females
0 1128 1225 1852 1548 3.10 3.10 7.78 7.26 28.96 15.36
1 874 1128 1690 1456 2.69 2.95 6.74 7.11 26.52 35.84
4 858 1237 610** 1440 1.22** 2.95 2.26** 7.41 7.76** 17.92
8 863 1370 336** 1208 0.77** 2.84 0.82** 6.63 2.32** 28.68
16 800 1320 260** 1252 0.53** 2.81 0.50** 6.16 1.52** 21.52
10 animals at 1.5 h; 5 animals at 2 weeks
Statistically significant in comparison with the control group, Dunnett's t test: * p < 0.05, ** p < 0.01
relatively high; the measurements were variable, and there were no
detectable clinical signs of cholinergic dysfunction. There were no
treatment-related findings at necropsy or histopathologically in the
central or peripheral nervous system. It is concluded that single oral
doses of methidathion caused acute neurobehavioural effects and
cholinesterase inhibition but no histopathological lesions. The
overall NOAEL was 1 mg/kg bw (Chang & Richter, 1994).
2. Observations in humans
No inhibition of plasma or erythrocyte cholinesterase activity or
any other effect was observed in groups of eight men who took daily
oral doses of 0.04 or 0.11 mg/kg bw methidathion in capsules for six
weeks. No low-observed-adverse-effect level (LOAEL) was determined
because cholinesterase was not inhibited (Coulston, 1970; reviewed in
Annex 1, reference 67).
In addition to two cases of massive poisoning (Teitelmann et al.,
1975; Zoppellari et al., 1990) reviewed by the 1992 JMPR (Annex 1,
reference 67), a third clinical case of a massive overdose was
described in the literature. A 52-year-old man ingested an estimated
31 g of a commercial product containing methidathion. He was found
comatose and was treated with gastric lavage, pralidoxime, atropine,
and prolonged mechanical ventilation. Six hours after ingestion, his
plasma cholinesterase activity was 545 IU/ml (normal range, 5500-13
500). The signs had disappeared by day 8, and treatment was
discontinued; however, the patient showed relapse of cholinergic
symptoms, and the treatment was resumed. The patient recovered after
22 days. An electrophysiological study performed on day 15 showed some
evidence of axonal polyneuropathy (no details reported). On day 30,
the plasma cholinesterase activity was still low, at 3390 IU/ml. Three
months after the incident, slight indication of peripheral neuropathy
persisted, as evidenced by hyporeflexia and distal hypoaesthesia. The
plasma cholinesterase activity had returned to 6150 IU/ml by day 90
(Cantais et al., 1993).
In the case reported by Zoppellari et al. (1990, reviewed in
Annex 1, reference 67), there was slight liver involvement, with
transient jaundice (4.3 mg% of total bilirubinas compared with normal
values up to 1.2 mg%) but no other biochemical indication of liver
damage.
Comments
The NOAEL for behavioural changes in rats after a single oral
dose was 3 mg/kg bw. The NOAEL for the inhibition of brain
acetylcholinesterase activity measured 4 h after treatment was 1 mg/kg
bw in males and 2.5 mg/kg bw in females.
In another study of acute neurotoxicity in rats, changes in
clinical signs, the results of a battery of functional observational
tests, and maze activity were observed at the time of peak effect
(about 2 h after treatment) at doses of 8 mg/kg bw and above in males
and 4 mg/kg bw and above in females. Inhibition of
acetylcholinesterase activity in various regions of the brain was
found at doses of 4 mg/kg bw and above. Reduced acetylcholinesterase
activity in the cortex and hyppocampus of a male treated with 1 mg/kg
bw was not considered to be relevant. The overall NOAEL in this study
was 1 mg/kg bw.
In a human case of methidathion poisoning,, the estimated dose
was more than 10 times the LD50 in rats. The patient recovered from
the cholinergic toxicity and developed mild neuropathy; however, no
details were given.
The hepatic changes observed in short-term studies in dogs were
not considered relevant for establishing an acute reference dose for
humans, as no such changes were observed in the volunteer studies
after repeated dosing for up to six weeks. Furthermore, no indication
of hepatic toxicity (except slight, transient jaundice in one case)
was seen in three cases of massive oral overdose which required
atropine and oxime administration and assisted ventilation.
The Meeting established an acute reference dose of 0.01 mg/kg bw,
on the basis of an NOAEL in humans for inhibition of erythrocyte
acetylcholinesterase activity of 0.11 mg/kg bw (highest dose tested)
in a study reviewed by the 1992 JMPR (Annex I, reference 65), and a
safety factor of 10. This value is supported by the NOAEL of 1 mg/kg
bw in rats for the inhibition of brain acetylcholinesterase activity.
Toxicological evaluation relevant for establishing an acute reference
dose
Levels that cause no toxic effect
Rat: 1 mg/kg bw (single oral administration, inhibition of
brain acetylcholinesterase activity)
Human: 0.11 mg /kg bw per day (six-week study in volunteers,
highest dose tested)
Estimate of acute reference dose for humans
0.01 mg/kg bw
References
Cantais, I., Escarment, J., Ledantec, P., Deloffre, I., Stephannazi,
J. & Quinot, J.-F. (1993) Poisoning by an organophosphate:
Methidathion. JEUR, 6, 100-102 (in French).
Chang, J.C.F. & Richter, A.G. (1994) Acute neurotoxicity study with
methidathion technical in rats. Unpublished report No. F-00178, dated
15 February 1994, amended 22 February 1994, from Ciba-Geigy Corp.,
Farmington, Connecticut, USA. Submitted to WHO by Novartis Crop
Protection AG, Basel, Switzerland.
Glaza, S.M. (1994) Acute oral toxicity study of methidathion technical
in rats. Unpublished report No. HWI 6117-235, dated 1 April 1994,
amended 4 May 1994, from Hazleton Wisconsin, Madison, Wisconsin, USA.
Submitted to WHO by Novartis Crop Protection AG, Basel, Switzerland.
Leahy, C.L. (1993) Acute range finding neurotoxicity study with
methidathion technical in rats. Unpublished report No. F-00177, dated
7 December 1993, amended 22 February 1994, from Ciba-Geigy Corp.,
Farmington, Connecticut, USA. Submitted to WHO by Novartis Crop
Protection AG, Basel, Switzerland.
Teitelman, U., Adler, M., Levy, I. & Dikstein, S. (1975) Treatment of
massive poisoning by the organophosphate methidathion. Clin.
Toxicol., 8, 277-282.
Zoppellari, R., Targa, L., Tonini, P. & Zatelli, R. (1990) Acute
poisoning with methidathion: A case. Hum. Exp. Toxicol., 9, 415-419.