METHACRIFOS
EXPLANATION
First draft prepared by Dr S. Caroldi
University of Padua, Padua, Italy
Methacrifos toxicity has been reviewed four times by the JMPR
between 1980 and 1988 (Annex 1, FAO/WHO 1981ab, 1983ab, 1986d, 1987a,
1988c, and 1989a). Data include pharmacokinetic studies in rats;
in vitro and in vivo genotoxicity studies; acute studies in mice,
rats, rabbits, dogs, and hens; short-term toxicity studies in rats,
dogs, pigs, and quail; carcinogenicity studies in mice and rats,
teratogenicity studies in rats and rabbits; delayed neurotoxicity
tests in hens; and a four-week oral study in humans.
A temporary ADI was allocated in 1980 which was extended in 1982
upon receipt of some of the required data. In 1986, further data were
received including the study in humans. The temporary ADI was again
extended, pending receipt of a multigeneration reproduction study. In
addition, further data pertaining to the long-term rodent studies were
required. The data on the long-term studies were reviewed in 1988,
and the temporary ADI was again extended. The use of the data from
the study in humans permitted the temporary ADI to be increased.
EVALUATION FOR ACCEPTABLE INTAKE
BIOLOGICAL DATA
Toxicological studies
Reproduction study
In a 2-litter, 2-generation reproduction study, groups of 32
(F0) male and female rats (Charles River strain) approximately 6
weeks old received methacrifos (purity 95.1%) a mixed in the diet at
0, 10, 100, and 200 then 400 ppm. Depending on body weight
variations, the calculated daily intake of methacrifos varied between
0.5-1.5 mg/kg bw at 10 ppm, 4.1-15.5 mg/kg bw at 100 ppm, 8.4-34.1
mg/kg bw at 200 ppm and 18.7-49.2 mg/kg bw at 400 ppm. The highest
dose corresponds to a daily intake lower than 1/10 of oral LD50 in
rats (678 mg/kg). Due to the instability of methacrifos in dietary
preparations, test diets were prepared biweekly. Dietary analysis
performed throughout the whole test showed variations of methacrifos
concentration within 15% of nominal values. Rats were maintained on
their respective diets for at least 70 days prior to mating. The
highest concentration of methacrifos was maintained for the F0
generation up to weaning of F1a litters. Then it was reduced to 200
ppm two weeks before remating of F0 generation because selection of
a F1 generation was not possible. The F1b generations were
maintained on their respective diets for 84 days prior to mating.
Histopathology of the reproductive tract was performed on F0 and F1b
control adults, on animals dosed with the highest methacrifos dose and
in any apparently infertile rats. Occasional body tremors were
present at different times in 3 out of 32 females of the 400 ppm
group. Cholinesterase activities were not measured. In rats
maintained on the 400 ppm methacrifos diet, body weight gain during
lactation was lower than in controls (this corresponded to lower food
consumption). Lower body weight gain was also recorded at the
beginning of exposure and during the mating period in this group.
Mean body weight was not different between groups except for males and
females of F1b generation at the 4th week of age at the highest dose
(lower than in controls). Mating performance and the duration of
gestation were normal. Fertility index was slightly reduced at
400/200 ppm for F0 generation at both matings.
Litter parameters were severely affected at 400 ppm. There were
6 total litter losses post-birth with reduction of total and live
litter weight at birth. The viability index and lactation index were
reduced. Development during the pre-weaning stage was reduced;
reduced stomach contents and reduced adipose tissue at sacrifice of
weanlings indicated a nutritional deficit. Selection of a subsequent
generation at 400 ppm was not possible and the dose was reduced to 200
ppm. At 200 ppm total litter losses over the three matings were 3, 1,
and 3 respectively. Litter size, viability index and lactation index
(only F1b generation) were reduced. Mean pup weight was lower during
lactation, the difference becoming more pronounced by weaning. At 100
ppm total litter losses over the four matings were 2, 0, 3, and 2,
respectively. A decreased fertility index was observed in the F1b
parental females. At 100 ppm a difference in litter size was
occasionally observed (F1b generation only). Other litter parameters
were not different from controls. At 10 ppm no litter losses occurred
and all litter parameters were unaffected. Two litter losses were
recorded in controls, one for each mating of the F0 generation.
Sporadic significant delay to achieve surface, startle and air
righting reflexes at 100 ppm and 200 ppm were considered as secondary
effects related to lower pup weights. Organ weight analysis showed
lower heart, spleen and ovary weights for females at 400/200 ppm.
Microscopic examinations of reproductive tracts of controls and of F0
(400/200 ppm) and F1b (200 ppm) generations did not reveal treatment
related changes.
Toxic effects were observed in adults and resulting offspring at
dose levels in the diet of 100 ppm and above and thus 10 ppm (equal
to 0.5 mg/kg bw/day) represented a NOAEL in terms of both adults and
offspring (Penny et al., 1989).
COMMENTS
The present Meeting reviewed a reproduction study in rats. The
NOAEL was 10 ppm. At higher levels, an increased incidence of total
litter losses occurred, and in the F1b generation, reduced fertility
and litter size were observed at the next dose level.
The evaluation of the reproduction study alleviated previous
concerns regarding reproductive toxicity. The Meeting therefore
allocated an ADI, based on the study in humans using a safety factor
of 10.
TOXICOLOGICAL EVALUATION
Level causing no toxicological effect
Mouse: 100 ppm in the diet, equivalent to 15 mg/kg bw/day
Rat: 10 ppm in the diet, equivalent to 0.5 mg/kg bw/day
Dog: 100 ppm in the diet, equivalent to 2.5 mg/kg bw/day
Man: 0.06 mg/kg bw/day
Estimate of acceptable daily intake for humans
0-0.006 mg/kg bw
Studies which will provide information valuable in
the continued evaluation of the compound
Further observations in humans
REFERENCE
Penny, J., Keith, J., Parker, C.A., Offer, J.M., Anderson, A., Dawe,
I.S., (1989). The effect of technical CGA 20168 on reproductive
function of two generations in the rat. Performed by Huntingdon
Research Centre Ltd. CBG 449/89533. Submitted to WHO by Ciba-Geigy
Corp.