METHACRIFOS
EXPLANATION
Methacrifos was evaluated for acceptable daily intake by the
Joint Meeting in 1980, and reviewed in 1982 (Annex 1, FAO/WHO, 1981a
and 1983a). A toxicological monograph was published after the 1980
Meeting (Annex 1, FAO/WHO, 1981b) and a monograph addendum was
published after the 1982 Meeting (Annex 1, FAO/WHO, 1983b). A
temporary ADI was allocated in 1980, which was extended in 1982,
pending the submission of a teratogenicity study in rats and an
appropriate reproduction study. Information on the sensitivity of
chinchilla rabbits to known teratogens and further observations on man
were considered desirable. The results of these studies have been
submitted and are summarized in this monograph addendum.
EVALUATION FOR ACCEPTABLE INTAKE
BIOLOGICAL DATA
Toxicological studies
Special study on reproduction
In a 2-litter, 2-generation reproduction study, groups of 28
(F0) or 24 (F1a) Sprague-Dawley derived rats received methacrifos
(purity 98%) admixed in the diet at 0, 75, 225, or 450 ppm. Dietary
analyses generally indicated that exposure levels were less than
nominal values as follows: 75 ppm (60 - 90%), 225 ppm (71 - 90%), and
450 ppm (70 - 88%). However, analyses of the diet consumed during
mating, gestation, and part of the weaning period of the F2a litters
showed that exposure levels were significantly reduced from nominal
values (75 ppm (< 8%), 225 ppm (24%), 450 ppm (< 36%)).
Significant body-weight depression occurred in the 225 and
450 ppm dams during lactation in the first generation, but only at the
high dose in the second generation. Depressed body-weight gain was
associated with decreased food consumption. The fertility indices in
both matings were comparable. There was a statistically-insignificant
decrease in the implantation rate of the F0 parents (combined data
for both litters). Litter sizes of F1a litters were significantly
reduced, but not those of F1b litters. Sex ratios were normal in
both litters. Pup weights were reduced at 14 days post-partum in all
litters and continued to be depressed in F1a and F1b pups at
450 ppm. At 225 ppm, pup weights were reduced at 28 and 35 days
post-partum in F1a offspring only. Pup mortality was increased at
225 and 450 ppm in F1a and F1b litters. There was a dose-related
increase in pup mortality in all generations.
In behavioural tests, negative geotaxis was observed in F1a and
F1b offspring, with a positive dose-response relationship at 225 and
450 ppm; however, this attained statistical significance only in F1a
pups at 450 ppm. In the F1a parents, female body weights were
reduced during lactation at 225 and 450 ppm in both matings. A very
slight body-weight decrease also occurred at 75 ppm. The body-weight
gain of the pups was reduced at 225 and 450 ppm. There were no
consistent effects on gross or histopathological findings, on absolute
or relative organ weights, or on body weights of either adults or pups
at necropsy. Although the above-mentioned dietary analysis confounds
interpretation of the results of this study, a no-observed-effect
level could not be established because of the observed increase in pup
mortality in all generations (Fritz et al., 1985).
Special studies on teratogenicity
Rats
The teratogenic potential of methacrifos (98% purity in 0.5%
carboxymethylcellulose aqueous solution) was investigated in Tif:
RAI(SPF) rats by daily oral intubation from days 6 to 15 of gestation
inclusively. As a pilot study had indicated marked maternal toxicity
at 60 mg/kg b.w./day, daily doses of 0, 7.5, 22.5, or 45 mg/kg b.w.
were given to groups of 24 pregnant females. The rats were sacrificed
after 21 days and necropsied. Maternal toxicity (reduced food intake
and body-weight gain) occurred at the high dose and to a lesser extent
in the mid-dose group. Some high-dose dams adopted a prone position
for up to 2 days during treatment, but no abortions occurred. The
following parameters were not influenced by treatment in each group:
implantation rates, embryonal or fetal deaths, the number of live
fetuses, or sex ratios. At necropsy, 1 fetus from the control group
exhibited partial pulmonary aplasia, but no visceral abnormalities
were found in the fetuses of treatment groups. However, retarded
skeletal ossification, particularly of hind-limb phalangeal nuclei and
the fifth sternebrae, occurred in high-dose fetuses; mid-dose fetuses
also exhibited delayed ossification of the fifth sternebrae.
Accordingly, the results of this study indicate a no-observed-effect
level of 7.5 mg/kg b.w./day (Giese et al., 1985).
Rabbits
To ensure the susceptibility of chinchilla rabbits to a known
teratogen, thalidomide was suspended in 2% aqueous carboxymethyl
cellulose solution and was administered by intubation to groups of
12 - 14 acclimatized 4- to 5-month old pregnant female rabbits at
dose levels of 100 or 250 mg/kg b.w./day. The day of mating was
deemed to be day 0 of gestation. Dosing was performed on days 6 -
10 inclusive, and the dams were sacrificed on day 28. Terminal
maternal body weights (i.e., terminal weight minus gravid uterine
weight) were reduced in both test groups. Total abortions occurred in
1 rabbit at 100 mg/kg b.w./day and in 2 rabbits at 250 mg/kg b.w./day.
There were dose-related increases in embryonal and fetal resorptions
and in terata with arthryogryposes, agenesis of the kidney and ureter,
hydrocephaly, and microphthalmia. Several anomalies (cleft palate,
micrognathia, cranioschesis, and renal hypoplasia) were observed at
the top dose level only. The study indicates that the chinchilla
rabbit shows embryo- and fetotoxicity as well as increased incidences
of several malformations following oral exposure to thalidomide
(Fritz, 1982).
Observations in humans
Nine male volunteers, aged 24 - 45 years (mean 39.9 years), took
encapsulated methacrifos (5 mg; purity unspecified) daily with
breakfast for 28 consecutive days. Blood was collected on 3 occasions
in the week preceding exposure so that baseline cholinesterase
activity could be determined. Haematological and biochemical
parameters were monitored at 3-day intervals during the study.
Urinalysis was conducted daily and urine microscopy was performed
twice weekly during exposure and 4, 8, and 15 days after cessation of
exposure. Electrocardiograms were conducted twice weekly for 5 weeks
after initial exposure. Blood was taken 6 - 7 hours after breakfast
for the determination of plasma and erythrocyte cholinesterase
activities daily for the first week, twice weekly during the rest of
the exposure period, and 3 days after cessation of exposure. The mean
daily exposure was 0.063 mg/kg b.w. (range 0.054 - 0.075 mg/kg b.w.).
Three of 9 subjects experienced mild indigestion, lasting up to
2 hours, after exposure; some developed anorexia. There were no
adverse clinical findings at 10 regular physical examinations
conducted during and 2 weeks after the study. Plasma and erythrocyte
cholinesterase activities were unaffected, as were standard
haematological and biochemical parameters. Urinalysis microscopy and
electrocardiography revealed no abnormal findings (Shephard &
Kyffin, 1984).
COMMENTS
As previously found, methacrifos was not teratogenic in either
chinchilla rabbits or Sprague-Dawley rats. The induction of several
malformations by thalidomide in chinchilla rabbits indicated the
validity of that animal model for teratogenicity testing. The
consistent deaths of rat pups observed in 2-generation reproduction
studies warrant clarification, particularly in view of the
embryotoxicity in rats noted by the 1980 Joint Meeting.
A further study with human volunteers, each receiving 5 mg/day,
indicated that neither plasma nor erythrocyte cholinesterase
activities were affected. Although 3 of the 9 volunteers experienced
mild indigestion, sometimes with mild anorexia, there were no
significant findings on clinical examination.
The Meeting noted that previous Joint Meetings had accepted the
validity of chronic toxicity and carcinogenicity studies originally
conducted by Industrial Bio-test Laboratories (IBT). In view of the
difficulties experienced in achieving nominal dietary concentrations
of methacrifos in the 2-generation reproduction study, the Meeting
concluded that the full details of these studies, including the
details of diet preparation and dietary analyses, should be
re-evaluated. Accordingly, the Meeting agreed to extend the temprorary
ADI.
TOXICOLOGICAL EVALUATION
LEVEL CAUSING NO TOXICOLOGICAL EFFECT
Rat: 1 ppm in the diet, equivalent to 0.05 mg/kg b.w./day.
Dog: 1 ppm in the diet, equivalent to 0.025 mg/kg b.w./day.
ESTIMATE OF TEMPORARY ACCEPTABLE DAILY INTAKE FOR MAN
0 - 0.0003 mg/kg b.w.
STUDIES WITHOUT WHICH THE DETERMINATION OF A FULL ADI IS
IMPRACTICABLE, TO BE SUBMITTED TO WHO BY 1987
1. Submission of all dietary preparation and analytical data for
the chronic oral toxicity study in rats and the carcinogenicity study
in mice conducted by IBT.
TO BE SUBMITTED TO WHO BY 1989:
2. An adequate multigeneration study in rats.
STUDIES WHICH WILL PROVIDE INFORMATION VALUABLE FOR THE CONTINUED
EVALUATION OF THE COMPOUND
Further observations in man.
REFERENCES
Fritz, H. Report on thalidomide teratology study - nonstandard in
1982 rabbits (chinchilla-type). Unpublished report No. K-S.80
from Ciba-Geigy Ltd., Basle, Switzerland. Submitted to WHO
by Ciba-Geigy Ltd., Basle, Switzerland.
Fritz, H., Suter, H.P., Zak, F., Krinke, G., & Hess, R. Two
1985 generation, two litter reproduction toxicity study in rats
with CGA 20,168. Unpublished report No. 83.0814 from
Ciba-Geigy Ltd., Basle, Switzerland. Submitted to WHO by
Ciba-Geigy Ltd., Basle, Switzerland.
Giese, K., Suter, H.P., & Gfeller, W. Report on CGA 20,168 tech:
1985 teratology study in rats. Unpublished report No. 83.0813
from Ciba-Geigy Ltd., Basle, Switzerland. Submitted to WHO
by Ciba-Geigy Ltd., Basle, Switzerland.
Shephard, N.W. & Kyffin, P. A study of the tolerance to methacrifos
1984 administered orally in a dose of between 0.06 mg/kg/day and
0.1 mg/kg/day for a period of 28 days to normal volunteers.
Unpublished report No. CG/METHAC/83/1 from Medical Science
Research, Beaconsfield, Bucks., UK. Submitted to WHO by
Ciba-Geigy Ltd., Basle, Switzerland.