CHLORPYRIFOS-METHYL
First draft prepared by Dr. S. Caroldi,
University of Padua,
Padua, Italy
EXPLANATION
Chlorpyrifos-methyl was previously evaluated by the Joint Meeting
in 1975 and an ADI of 0.01 mg/kg was allocated (Annex I, 24). The
Meeting stated that further desirable work included appropriate
mutagenicity study and neurotoxicity study with histological
examination of nervous tissue. Six acute toxicity studies,
short-term studies in mice, rats and dogs, a carcinogenicity study and
a long-term study in mice, two special studies on delayed
neurotoxicity, and five teratogenicity and mutagenicity studies were
submitted for evaluation.
EVALUATION FOR ACCEPTABLE INTAKE
Toxicological studies
Acute toxicity studies
Eye irritation in the rabbit
Chlorpyrifos-methyl (100 mg ground into fine powder) was placed
into the left eye of three female New Zealand White rabbits (Ranch
Rabbits, Crawley 11-20 weeks old). The right eye served as a control.
Both eyes were examined for signs of irritation 1, 24, 48 and 72 hours
after instillation. Slight transient conjunctivitis was observed 1
hour after treatment but this effect disappeared within 24 hours. The
test article was non-irritant for the eyes (Jones 1985c).
Dermal irritation in the rabbit
Chlorpyrifos-methyl (500 mg moistened with distilled water) was
applied under occlusive dressing topically to the skin (approximately
6 cm2) of three New Zealand White rabbits (Ranch Rabbits, Crawley,
at least 12 weeks old) for a contact period of 4 hours. Skin was
evaluated at 1, 24, 48 and 72 hours after removal of patches. Very
slight redness was observed 1 hour after removal of patches in two
animals but skin was normal from 24 hours. Chlorpyrifos-methyl was
not irritating for the skin (Jones, 1984).
Dermal sensitization in the guinea pig
Twenty-eight young albino Dunkin-Hartley guinea pigs (14 males,
14 females) were dosed with technical chlorpyrifos-methyl. Induction:
a lint pad (2.5 cm x 2.5 cm) wetted with test article (0.3 ml) at a
concentration of 100% w/v in polyethylene glycol was applied to the
left flank of each guinea pig for a contact period of 6 hours on days
1, 7, 14. Challenge: On day 29 chlorpyrifos-methyl (100% w/v) was
applied on the right flank of animals induced as described above and
of controls. No adverse skin reactions were noted after exposure to
chlorpyrifos-methyl either test or control animals.
Chlorpyrifos-methyl does not cause delayed contact hypersensitivity in
the guinea pig using the Buehler Test (Jones, 1985d).
Table 1. Acute toxicity of Chlorpyrifos-methyl*
Species Sex Route LD50 LC50 Reference
(mg/kg bw) (mg/m3)
Rats M oral 2680 (1) Lackenby (1985)
F 3069 (1)
M&F oral > 5000 (2) Jones (1985a)
M&F dermal > 2000 (2) Jones (1985b)
(24 h exp.)
M&F inhalation > 670 (3) Hardy & Jackson
(4 h exp.) (1984)
* Analysis of formulations not reported
1) Signs of toxicity were detected from 2000 mg/kg bw which disappeared in survivors
within 4 days. Pathology of animals that died before scheduled sacrifice showed
abnormalities of the gastrointestinal tract and of the liver.
2) No spontaneous deaths, minor toxicity signs disappeared within 24 hr of intoxication.
3) No spontaneous deaths. Signs of respiratory tract and eye irritation during exposure.
Short-term studies
Mice
Twelve male and 12 female ICR mice (4 weeks old at the beginning
of the study) were orally dosed with chlorpyrifos-methyl (91.8%
purity) at concentrations of 0, 1, 5, 10, 1000 or 10 000 ppm, equal to
0, 0.125, 0.651, 1.3, 122, 523 mg/kg bw/day for males and 0, 0.14,
0.75, 1.45, 139, 318 mg/kg bw/day for females (calculated as average
daily intake throughout the duration of the study) for 28 days.
Chlorpyrifos-methyl was incorporated into the basal diet (fresh
preparation performed weekly) up to nominal concentrations. The
actual content of the test substance in the formulations was 89% of
nominal (mean of 2 determinations at each dose level). Clinical
signs, mortality, body weight, food consumption, and water consumption
in each group were recorded during treatment. Gross pathology was
performed on all animals; full histopathology on all mice in the 0,
1000, and 10 000 ppm groups and histopathology of adrenal glands in
the 1, 5 and 10 ppm groups.
All animals of the high dose group died within 9 days of
treatment. Marked reduction of body weight and of food consumption
were observed. At 1000 ppm, mortality rate was not different from
that of controls and clinical signs of toxicity were not observed. A
slight decrease of body weight was observed in both sexes with respect
to controls (less than 10% in both sexes at the end of the study)
which corresponded to lower food intake (less than 10%) and lower
water intake (7% and 18% in males and females, respectively). The
results of urinalysis, haematological and biochemical tests were not
different from controls apart from a slight increase of alanine
aminotransferase and total cholesterol observed in both sexes
(statistically significant in females only). Cholinesterase activity
was reduced in plasma, erythrocytes and brain in both sexes. Organ
weight data showed significant increases of absolute and relative
liver weights in males and relative liver weight in females, though no
gross nor microscopical abnormalities were detected. Significant
increases in absolute and relative adrenal weights which corresponded
to swelling of cortical cells on microscopic examination were observed
in males. At 10 ppm, significant reductions of both plasma (53%) and
erythrocyte (33%) cholinesterase activities were observed in male rats
and of plasma (23%) cholinesterase activity only in females. Brain
cholinesterase activities were within normal values in both sexes. No
other parameters were different from control. At 5 ppm, trivial
reduction of plasma cholinesterase activities was observed in both
sexes. No other parameters were different from controls. At 1 ppm,
no parameters were different from controls. The NOAEL in this study
was 10 ppm for both sexes corresponding to 1.3 and 1.5 mg/kg bw/day
for males and females, respectively (Yoshida et al., 1985).
Rats
Ten male and 10 female Fischer-344 rats (4 weeks old at the
beginning of the study) were treated with chlorpyrifos-methyl
(technical grade 94.2% purity) at concentrations of 0, 0.1, 1, 10, 250
mg/kg bw/day for 13 weeks (primary groups). Ten additional rats/sex/
fed with 0 or 250 mg/kg bw/day after the 13 weeks of treatment were
allowed to recover for 4 weeks (recovery groups). The concentrations
of the test substance in the diets were adjusted weekly. Homogeneity,
stability and correspondence to targeted concentrations of the diets
were checked and found acceptable.
No animal died during the study. At 250 mg/kg bw/day, body
weight and body weight gain were significantly reduced throughout the
duration of the study and this effect was still detectable during the
4 week recovery period. The mean body weight gain over the 13 week
period was decreased 16% in males and 23% in females. Food
consumption was not reduced. At the end of the 13-week test period,
packed cell volume, red blood cells and haemoglobin values were
decreased and the platelet count was increased in both sexes. Alanine
amino-transferase, alkaline phosphatase and aspartate aminotransferase
were decreased, cholesterol values were increased and triglycerides
were decreased in both sexes. Plasma, erythrocyte and brain
cholinesterase activities were significantly reduced at the 6th and
13th week of the study. At the 17th week plasma cholinesterase
activity almost recovered to control values but erythrocyte and brain
cholinesterase activities were still reduced. Urine staining, worse
condition of the hair coat and increased lacrimation were observed.
Adrenals and liver weights were increased at the end of the study.
Histology showed alterations in adrenal glands and liver. In adrenals
changes consisted of varying degrees of hypertrophy and vacuolation of
the cells of the zona fasciculata and scattered necrotic foci. The
incidence of these lesions was reduced after stopping treatment.
Slight hypertrophy of centrilobular hepatocytes was observed in males
only which disappeared after recovery period.
At 10 mg/kg bw/day, no differences of body weight or body weight
gain were detected. Haematological and biochemical parameters were
not different from controls. Plasma cholinesterases were reduced at
6 and 13 weeks of the study while erythrocyte cholinesterase was
reduced at week 6 but had recovered at the end of the study. Brain
cholinesterases were reduced at the end of the study by 17% and 15% in
males and females, respectively. Histology showed alterations in the
adrenals similar to those found in animals at the higher dose level.
At 1 mg/kg bw/day, only plasma cholinesterase activity was slightly
reduced in both sexes while erythrocyte and brain cholinesterase
activities were not different from controls. Other parameters and
results of pathology were not different from controls. At 0.1 mg/kg
bw/day, all parameters were unaffected. On the basis of slight
inhibition of brain cholinesterase and histological alterations
detected in adrenals at 10 mg/kg bw/day, the NOAEL in this study was
1 mg/kg bw/day for both sexes (Barna-Lloyd et al., 1990).
Dogs
Four male and 4 female beagle dogs (16 weeks old) were treated
with chlorpyrifos-methyl (purity 94.2%) at concentrations of 0, 0.1,
10 or 50 mg/kg bw/day for 13 weeks. Homogeneity, stability and
correspondence of the diets to the target concentrations of the test
substance were checked throughout the study and found acceptable. At
50 mg/kg bw/day, marked reduction of food consumption, body weight and
body weight gain were observed in both sexes. Female dogs showed
weakness and generalized muscle wasting at the end of the study.
Significant differences of some haematological and biochemical
parameters were detected at the end of the study. Plasma and
erythrocyte cholinesterase activities were reduced during and at the
end of the study. Brain cholinesterase activity at the end of the
study was 34% of the control value. Increased absolute and relative
liver weights were observed in both sexes. Histology showed slight
hypertrophy of centrilobular hepatocytes. At 10 mg/kg bw/day,
reductions of plasma and, to a lesser extent, erythrocyte
cholinesterase activities, but not of brain cholinesterase activity,
were measured. At 0.1 mg/kg bw/day, slight reduction of plasma
cholinesterase was found. Based on brain cholinesterase inhibition
and on clinical effects at 50 mg/kg bw/day, the NOAEL level in this
study was 10 mg/kg bw/day (Szabo & Davis 1990).
Long-term carcinogenicity studies
Mice
Fifty-two male and 52 female ICR mice (Charles River Japan, Inc.)
were treated with chlorpyrifos-methyl (97.4% pure) at concentrations
of 0, 1, 5, 50, or 500 ppm, equal to 0, 0.08, 0.42, 4.4 or 44.0 mg/kg
bw/day for males and 0, 0.08, 0.4, 3.9 or 41.5 mg/kg bw/day for
females (calculated as average daily intake throughout the duration of
the study) for 78 weeks. Forty-four additional mice/sex/dose level
were treated for 26 and 52 weeks, then sacrificed for intermediate
investigations. Mice were 5 weeks old at the beginning of the study.
Standard diet and tap water were available ad libitum. The test
compound was incorporated into basal diet (diet freshly prepared 2-3
times a week) up to the nominal concentrations. The actual content of
chlorpyrifos-methyl in the formulations was checked monthly and
results showed 94%, 91%, 95%, and 94% of nominal (mean of the 19
determinations) for 1, 5, 50, and 500 ppm, respectively. Urinalysis,
haematological examinations, biochemical tests in blood,
cholinesterase activity determinations in plasma, erythrocyte and
brain were performed in 10 mice/sex/group at 26, 52, and 78 weeks.
Pathology was performed on all animals, organ weights were recorded at
26, 52, and 78 weeks in 10 mice/sex/group. After 52 weeks full
histopathology was performed only on mice in the 0 and 500 ppm groups
(on lung, liver, kidneys, adrenals, and gross lesions in mice of the
other groups). Histopathology was also performed on all survivors at
the end of treatment and on mice found dead during the treatment.
Mortality rate at the end of treatment was 38%, 33%, 33%, 38% and 27%
in males and 29%, 37%, 27%, 29% and 25% in females at 0, 1, 5, 50, and
500 ppm, respectively. Small reductions of body weight (less than
10%) were detectable at the highest dose level in both sexes (in
females during the first year of treatment only) which corresponded to
a small statistically significant reduction in food and water intake.
No other treatment-related signs of toxicity were detectable in either
sex at any dose level. At 26, 52 and 78 weeks total cholesterol was
increased in females in the group at the highest dose but not in
males; no biologically significant alterations of other parameters
were observed for either sex at any dose level. Plasma and erythrocyte
cholinesterases were significantly reduced in both sexes at 50 and 500
ppm throughout the duration of the study. At 500 ppm cholinesterase
activity in brain was consistently reduced throughout the study (the
average percent of control activity in 3 determinations performed
after 26, 52 and 78 weeks was 43 and 52 in males and females,
respectively). Cholinesterase activity in brain was not affected at
lower dose levels. The incidence of gross lesions and neoplastic
histological lesions was not different among groups. Some
non-neoplastic histological alterations as centrilobular
hepatocellular fatty change and cortical cell swelling in the adrenals
were increased at 500 ppm in both sexes. The incidence of renal
tubular atrophy and cortical cyst(s) was increased in male only. The
NOAEL level in this study is 50 ppm equal to 4.4 mg/kg bw/day for
males and 3.94 mg/kg bw/day for females (Yoshida et al., 1988).
Fifty-six male and 56 female CD-1 mice were treated with
chlorpyrifos-methyl (96% purity) at concentration of 0, 15, 30, 60 ppm
for two years. Dietary concentrations of the test substances were
prepared at approximately 3 to 4 week intervals. The actual content
of chlorpyrifos methyl in the formulations was checked 15 times
throughout the duration of the study and showed 196% (70-580), 128%
(80-420), 138% (56-579) of nominal (mean and range of 15
determinations) for 15, 30, and 60 ppm, respectively. Food
consumption and body weight were not influenced by administration of
the test substance up to the highest dose. No relevant clinical
observations were recorded. Mortality rate at the end of the study
was 70%, 71%, 61% and 61% in male and 55%, 66%, 48% and 59% in females
at 0, 15, 30, 60 ppm, respectively. There were no statistically
significant treatment-related differences in organ weights. A variety
of non-neoplastic and neoplastic lesions was observed in treated and
control groups of both sexes. The lesions were of types commonly found
in mice and the incidence of neoplasms in treated mice was not
different from that in controls. Under the conditions of this study,
chlorpyrifos-methyl was not carcinogenic to CD-1 mice. The
determination of a NOAEL is hampered in this study by poor consistency
of the concentrations of the test substance in the diet (Molello
et al., 1980).
Rats
Sixty male and 60 female Fischer 344 rats/dose level were
administered chlorpyrifos-methyl (94-95% pure) at dietary
concentrations of 0, 1, 2, 20, and 1000 ppm for two years. Ten
additional rats/sex/dose level were treated for 12 months and then
sacrificed for intermediate investigations. Rats were 5-6 weeks old
at the beginning of the study. Rats were housed one/cage and feed and
tap water were available ad libitum. The actual content of
chlorpyrifos-methyl in the diet was checked on days 1, 97, 181, 272,
363, 454, 545, 636, and 727, and concentrations were found to be
within 2% of nominal. Animals were observed daily and weighed on a
weekly basis for the first 13 weeks and then every 4 weeks.
Urinalysis, haematological examinations, clinical chemistry,
cholinesterase activity determinations in plasma, erythrocyte and
brain were run at 26, 52, 78 and 104 weeks. Gross pathology was
performed on all animals, and organ weights were recorded at terminal
sacrifice. Full histopathology was performed only on rats in the
control and high-dose groups and those animals sacrificed in moribund
condition or found dead. Sections of adrenal glands, brain,
epididymides, kidneys, liver, lungs, pituitary, spinal cord, sciatic
nerve, testes, tibial nerve and gross lesions were histologically
examined in animals from other dose levels at terminal sacrifice.
Small but statistically significant reductions of body weight
(less than 10%) were detectable at the highest dose level in both
sexes after the first year on test. There was no compound-related
effect on mortality in either sex. Plasma cholinesterase was
significantly reduced in both sexes at 20 and 1000 ppm throughout the
duration of the study. At 1000 ppm acetylcholinesterase activity in
erythrocyte and brain was consistently reduced throughout the study.
Slightly lower erythrocyte cholinesterase activity was observed at the
6 month evaluation but was not considered to be biologically
significant. Cholinesterase activity in brain was not affected at
lower dose levels (< 1000 ppm). The incidences of gross lesions and
neoplasms were similar among groups. Biologically significant changes
in organ weights were not found with the exception of the adrenal,
which was increased (absolute and relative) in both high-dose males
and females after one year and at final sacrifice. Urinalysis and
clinical chemistry measurements were unremarkable. Although RBC, Hb
and PCV were decreased and platelets increased at the high-dose level
at intervals prior to final sacrifice in both males and females, no
change was found at the final measurement. An increased incidence of
diffuse, moderate vacuolation consistent with lipid accumulation was
found in the adrenal glands of both males and females at the high-dose
level at the interim sacrifice and in the 20 ppm groups at the final
sacrifice.
Based on the increased incidence of adrenal, vacuolation observed
at the 20 and 1000 ppm dietary concentrations, the NOAEL was 2 ppm,
equivalent to 0.1 mg/kg bw/day (Barna-Lloyd et al., 1991).
Special studies on delayed neurotoxicity
Four groups of 10 adult White Leghorn hens were treated orally
with 30 mg/kg of atropine sulfate. One group received no additional
treatment (negative controls), the second group received 250 mg/kg of
tri-o-tolyl phosphate orally (positive controls). The remaining
groups received either 2500 or 5000 mg/kg of chlorpyrifos-methyl
orally (95.9% purity). In a previous study 6 hens had survived a
single oral dose of chlorpyrifos-methyl of 6400 mg/kg. Hens were
observed daily for abnormal gait, they were weighed weekly and killed
21 days after dosing and histology performed on sciatic nerves and
cervical, thoracic and lumbar sections of the spinal cord.
Cholinesterase activity was not measured but 4 out of 10 hens
developed acute symptoms (likely cholinergic) during the first 5 days
after receiving 5000 mg/kg chlorpyrifos-methyl. None of the negative
controls and all of the positive controls showed clinical (starting
day 11-15 after dosing) and histological evidence (in spinal cord and
sciatic nerves) of delayed neurotoxicity. None of the
chlorpyrifos-methyl dosed birds showed clinical evidence of delayed
neurotoxicity. At 2500 mg/kg one hen showed histological alterations
in sciatic nerve while spinal cord was unaffected. Six out of 10
birds receiving 5000 mg/kg of chlorpyrifos-methyl showed positive
and/or equivocal histological evidence of neurotoxicity of the spinal
cord and/or sciatic nerve(s) (Clark et al., 1979).
Groups of 10 adult White Leghorn hens were orally dosed with
chlorpyrifos-methyl (97% purity) at concentrations of 0, 5, 50 or 500
mg/kg in corn oil for 5 days a week for 13 weeks. Two additional
groups received tri-ortho-cresyl phosphate (TOCP) p.o. at
concentration of 10 or 30 mg/kg/day for 5 days a week. Dosing was
discontinued after 7 weeks of treatment at 30 mg/kg TOCP when 6 hens
were killed because of pronounced signs of neurotoxicity. Actual
concentrations of the test substance in the diets, homogeneity and
stability were acceptable. Hens were observed daily and body weight
recorded weekly. Histopathology of brain, of spinal cord and of
sciatic nerve was performed on all hens at the end of the study.
Apart from a consistent reduction of mean body weight in hens dosed
with chlorpyrifos-methyl at the highest dose level, no other signs of
toxicity were observed. The body weight reduction was apparent from
day 3 of the study up to termination and early onset was considered
indicative of acute toxicity at this dose level. Cholinesterase
activity was not measured. Clinical signs of delayed neuropathy
developed only in hens dosed with TOCP at the highest dose level
starting in the 4th week of treatment. Histological signs of delayed
neuropathy were observed in spinal cord and sciatic nerve of each
animal dosed with TOCP. Hens dosed with chlorpyrifos-methyl did not
show clinical nor histological signs of delayed neuropathy
(Barna-Lloyd et al., 1984).
Special study on embryotoxicity and teratogencity
Ten female rabbits (Japan white, six to seven months old) were
dosed with chlorpyrifos-methyl orally by stomach catheter at
concentrations of 0, 4, 8, 16 mg/kg bw/day on days 6-18 of gestation.
Selection of dose levels was based on approximately 20% inhibition of
plasma cholinesterase observed in a preliminary study at 16 mg/kg
bw/day of chlorpyrifos-methyl for 2 weeks. All rabbits were checked
daily for toxicity. Maternal body weights, food and water consumption
were recorded daily. All of the pregnant rabbits were sacrificed on
day 29 of gestation. Body weight was not affected by
chlorpyrifos-methyl treatment even though a slight reduction of food
intake was observed in rabbits at the 8 and 16 mg/kg/day dose levels
from 7 to 20 days of gestation. Number of corpora lutea and
implantations was not different between dosed rabbits and controls.
No significant differences were detected in the number of dead embryos
and fetuses pre- and post-implantation, number of live fetuses, nor
individual average pup weights. No abnormalities related to treatment
were found in external, visceral or skeletal examinations.
Chlorpyrifos-methyl produced no effects on teratology, mortality or
weight loss. However, a higher dose level could have been utilized
based on the lack of maternal toxicity at any tested dose level (Asai
et al., 1976).
Table 2. Results of genotoxicity assays on chlorpyrifos-methyl
Test system Test object Concentration of test Purity Results Reference
substance
Ames test (both S. typhimurium 1, 10, 100, 500, 1000, 99.9% Negative (1) De Graff (1983)
with and without TA98, TA100, TA1535, 2500, 5000, 10 000 痢/plate
activation) TA1537, TA1538 dissolved in DMSO
CHO/HGPRT assay Chinese hamster ovary 5, 20, 40, 60, 80, 100 95.2% Negative (2) Mendrala (1985)
(both with and cells (CHO-K1-BH4) 然/plate dissolved in DMSO
without metabolic
activation)
Chromosome aberration Chinese hamster ovary Nonactivated: 95.2% Negative Gollapudi (1985)
assay (both with and cells (CHO-K1-CCL61) 4, 12, 40 痢/ml
without metabolic Activated: Positive (3)
activation) 5, 15, 50 痢/ml
dissolved in DMSO
Unscheduled DNA Male CDF Fischer rat 1 x 10-4 M 95.2% Negative (4) Mendrala & Dryzga (1985b)
synthesis liver primary cell 3.16 x 10-5 M
cultures 1 x 10-5 M
3.16 x 10-6 M
1 x 10-6 M
Table 2 (contd).
Test system Test object Concentration of test Purity Results Reference
substance
Mouse micronucleus Male/female CD-1 146 mg/kg bw 95.2% Negative (5) Bruce et al. (1985)
test BR mice 460 mg/kg bw
1460 mg/kg bw
(1) Positive control (without activation: TA-100/TA-1535,sodium azide at 10 痢/plate; TA-98/TA-1538,2-nitrofluorene at 10 痢/plate;
TA-1537, 9-aminoacridine at 50 痢/plate. With activation: 2-anthramine at 2.5 痢/plate for all strains) gave expected positive
response.
(2) Positive control (3 mM EMS; 100 然 3-MC) yielded expected positive response. The non-activation assay gave positive response
at 80 然 concentration but not at 100 然 concentration. A second run with addition of 30 然 concentration gave negative response
at all concentration levels.
(3) Positive control (1242 痢/ml EMS; 21 痢/ml CP) yielded expected positive response. A dose-related increase of the frequency of
aberrations was observed only in the assay with activation.
(4) Positive control (2-AAF 1x10-5,1x10-6,1x10-7 M) yielded expected positive response.
(5) Positive control (120 mg/kg bw CP) yielded expected positive response.
COMMENTS
Chlorpyrifos-methyl was evaluated by the 1975 JMPR which
allocated an ADI of 0-0.01 mg/kg bw/day. Chlorpyrifos-methyl is
moderately acutely toxic by the oral route. No significant
differences between sexes were observed.
In a 28-day study in mice at dietary concentrations of 0, 1, 5,
10, 1000 or 10 000 ppm, the NOAEL was 10 ppm, equal to 1.4 and 1.5
mg/kg bw/day for males and females respectively based on brain
cholinesterase inhibition and alterations in the adrenal glands at
1000 ppm.
In a 13-week study in rats at dietary concentrations yielding
doses of 0, 0.1, 1, 10, or 250 mg/kg bw/day, the NOAEL was 1 mg/kg
bw/day based on histological alterations in the adrenals at 10 mg/kg
bw/day.
In a 13-week study in dogs at dietary concentrations yielding
doses of 0, 0.1, 10, or 50 mg/kg bw/day, the NOAEL was 10 mg/kg bw/day
based on brain acetyl-cholinesterase inhibition, increased liver
weight and reduction of body weight gain at 50 mg/kg bw/day.
In a 78-week study in mice at dietary concentrations of 0, 1, 5,
50 or 500 ppm the NOAEL was 50 ppm, equal to 4.4 and 3.9 mg/kg bw/day
in males and females, respectively. At 500 ppm, 50% inhibition of
brain acetylcholinesterase occurred; other effects noted at this
dietary concentration were centrilobular hepatocellular fatty change
and cortical cellular swelling of the adrenals. The incidence of
neoplastic lesions was similar in all groups.
In a 2-year dietary study in rats the NOAEL was 0.1 mg/kg bw/day
based on dose-related alterations in adrenal glands detected at 1 and
50 mg/kg bw/day. There was no evidence of carcinogenicity in rodents.
Chlorpyrifos-methyl did not cause delayed neurotoxicity in hens.
A teratology study in rabbits was negative at all doses tested.
The NOAEL was 16 mg/kg bw/day, the highest dose tested.
After consideration of all available in vitro and in vivo
genotoxicity data, the Meeting concluded that chlorpyrifos-methyl was
not genotoxic, despite a significant clastogenic response in an
in vitro study.
The ADI was based on the results of the 2-year study in rats
using a 100-fold safety factor. The Meeting was not able to use the
human acetylcholinesterase inhibition data reviewed in 1975 as the
basis for the ADI because adverse effects on adrenals were observed in
rats in the absence of cholinesterase inhibition.
TOXICOLOGICAL EVALUATION
Level causing no toxicological effect
Mouse: 50 ppm, equal to 3.9 mg/kg bw/day
Rat: 0.1 mg/kg bw/day
Human: 0.1 mg/kg bw/day
Estimate of acceptable daily intake for humans
0-0.001 mg/kg bw
Studies which will provide information valuable in the
continued evaluation of the compound
Further observations in humans
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