FENAMIPHOS
EXPLANATION
Fenamiphos was examined for toxicological evaluation by the JMPR
in 1974 and 1985 (Annex 1, FAO/WHO, 1975a and 1986a). The 1974 Meeting
estimated an ADI of 0.0006 mg/kg bw. In 1985, following a direct
request for reevaluation received from a Member State and a review of
additional data, the JMPR converted and reduced the ADI to a temporary
ADI of 0.0003 mg/kg bw because of concern for foetotoxicity
demonstrated in a rabbit teratology study. The 1985 JMPR required
submission of the results of an on-going rat oncogenicity study, of a
full legible report and raw data for the rat teratology study and a
new rabbit teratology study to clarify the foetotoxicity observed at
low dietary levels. These studies have been submitted and are reviewed
in this monograph addendum.
EVALUATION FOR ACCEPTABLE INTAKE
BIOLOGICAL DATA
Toxicological studies
Special Studies on Carcinogenicity
Rats
Groups of 50 male and 50 female Fischer 344 rats were fed diets
containing mean effective concentrations of 0, 1.7, 7.8 or 37 ppm
(equal to approximately 0, 0.1, 0.50.6 and 2.5-3.4 mg/kg bw/day)
technical fenamiphos (89.3% pure) for two years. Clinical observation,
mortality, food and test article consumption, hematology, blood
chemistry, urinalysis and body weight were monitored throughout the
study. Additional groups of 10 animals/sex/group were given 0 or
37 ppm fenamiphos for one year. At this time they were terminated and
brain cholinesterase activity was determined. All rats, including
those found dead or sacrificed in extremis during the study and
those terminated at 12 or 24 months, underwent gross macroscopic
observation, organ weight measurement and microscopic evaluation of
more than forty tissues. Ophthalmological examination was done on 10
rats/sex/group prior to initiation and termination of the study.
Percent of survival at termination was not treatment-related and
ranged from 58-88% in males and 62-84% in females. Clinical
observation showed a possibly treatment-related higher incidence of
rough coat and alopecia in females of the high dose group when
compared to controls. A statistically significant decrease of body
weight was noted throughout the study for both male and female rats of
the high dose group. Although no treatment-related effect on feed
consumption was noted, there was a dose-related statistically
significant decrease of plasma cholinesterase activity in rats of both
sexes in all dose groups throughout the study. Plasma cholinesterase
inhibition in male rats was 7 to 38% at 1.7 ppm, 24 to 69% at 2.8 ppm
and 56 to 88% at 37 ppm. In females plasma cholinesterase inhibition
ranged from 22 to 96% throughout the study. Erythrocyte cholinesterase
activity (EChE) was also statistically significantly inhibited in both
sexes at all dose levels when compared with controls. EChE inhibition
was 5 to 7% at 1.7 ppm, 12 to 25% at 7.8 ppm and 56 to 81% at 37 ppm
in males and 3 to 11% at 1.7 ppm, 21 to 43% at 7.8 ppm and 62 to 81%
at 37 ppm in females. A statistically significant decrease of brain
cholinesterase activity was observed in male rats of the high dose
level at termination (-14%) and in both sexes (-25% in males and -24%
in females) at the interim sacrifices after 1 year of treatment.
There was a statistically significant increase in relative brain,
heart and lung to body weight for both males and females of the high
dose group at termination. In high dose females, the relative kidney
to body weight was also significantly increased when compared to
controls. At interim sacrifices females also showed a statistically
significant increase of relative organ to body weights for adrenals,
brain, heart, kidneys and ovaries. These changes were considered to be
related to the decrease in body weight observed in rats of both sexes
at the 37 ppm dose level. The only statistically significant change in
absolute organ weight at termination was a decrease in liver weight
and an increase in lung weight in both sexes at 37 ppm. Hematological
examination showed several statistically significant differences in
treated animals when compared to controls in both sexes at 3, 6 and
12 month examinations. These differences were randomly distributed and
were not dose-related and therefore they were not considered to be
treatment-related. No treatment-related changes were observed during
ophthalmological examination or in food consumption, clinical
chemistry, urinalysis, gross pathology and tissue masses in
fenamiphos-treated rats when compared to controls.
No treatment-related neoplastic lesions were observed during
histopathological examination. No increase in the incidence of any
tumor, total tumors, or total number of animals with tumors was
observed at any dose level in either sex in fenamiphos-treated rats at
termination or interim sacrifice when compared to controls. A
statistically significant higher incidence of non-neoplastic
inflammatory lesions were observed in the nasal,, laryngeal and lung
tissue of rats receiving 37 ppm fenamiphos in the diet when compared
to controls. These changes were attributed by the author of the study
to the marked cholinesterase inhibition present in these animals. No
treatment-related non-neoplastic changes were noted at 1.7 and
7.8 ppm.
The authors of the study concluded that under the conditions of
the study fenamiphos was not oncogenic to male or female Fischer 344
rats. Based on the serum and erythrocyte cholinesterase activity
inhibition present at all dose levels tested a NOAEL was not found in
this study (Hayes, 1986a).
Special Studies on Embryotoxicity and Teratogenicity
Rats
Four groups of 25 female FB30 rats mated overnight with untreated
males (in the ratio of 1 male to 2 females) were given by gavage a
daily dose of 0, 0.3, 1 and 3 mg/kg bw/day fenamiphos (92.5% pure) in
a aqueous Cremophor emulsion from day 6 to day 15 of gestation
(presence of sperm in the smear obtained in the morning after mating).
Control females received the same volumes (10 ml/kg bw) of the aqueous
emulsion. On day 20 of gestation the dams were narcotized with CO2
and the fetuses were removed by Caesarian section and examined for
litter size, average fetus weight per litter, sex, external and
visceral abnormalities and skeletal malformations and development.
Eighteen out of 25 dams receiving 3 mg/kg bw/day fenamiphos
showed signs of toxicity (trembling and laying on one side) and 2
died. Time of death was not given and cause of death could not be
established. No treatment-related signs of toxicity or deaths were
observed in rats receiving 0, 0.3 or 1 mg/kg bw/day fenamiphos. A 15%
reduction of average weight gain during the treatment period was found
in dams receiving 3 mg/kg bw/day fenamiphos when compared to controls.
The difference was reported to be not statistically significant by the
author of the study. A total of 6 females were not fertilized in this
study (1, 2, 2 and 1 in the 0, 0.3, 1 and 3 mg/kg bw/day dose group,
respectively). The average placenta weight of the high dose group was
statistically significantly (p < 0.05) lower than that of the control
group.
There were no treatment-related effects on the number of
fertilized or pregnant females, litter size, number of resorptions,
number of studied fetuses, average fetus weight, sex ratio, incidence
of alterations of development, type or number of malformations. The
most frequent manifestations observed were nodulations on ribs which
were present in 2 fetuses from one litter of the 0.3 mg/kg bw/day
group and in 4 fetuses from 3 litters of the 1 mg/kg bw/day dose
group. The other malformations observed were general edema, abdominal
fissure and anophthalmia in one fetus of the 0.3 mg/kg bw/day dose
group. No malformation was observed in fetuses of the high dose group.
The lower placenta weight observed in the high dose group was not
considered to be toxicologically significant because it was considered
to be still within the normal range and because no effects on
embryonic and fetal development were seen in the study. It was
concluded that fenamiphos at doses up to and including 3 mg/kg bw/day
was not embryotoxic or teratogenic to FB30 rats. It was, however,
toxic to mothers at the dose level of 3 mg/kg bw/day (Schulter, 1981).
Rabbits
In a dose finding embryotoxicity (including teratogenicity)
study, groups of 3 mated (1:1) female Chinchilla rabbits were given
single daily doses of 0, 0.1, 0.8, or 3 mg/kg bw/day fenamiphos (91%
pure) in distilled water with 0.5% Cremophor EL by gavage from day 6
through day 18 post coitum. Cholinesterase activity was measured in
plasma and erythrocytes before the first dose and just after the last
dose administration. On day 28 post coitum all females were terminated
and the fetuses removed by Caesarian section, sexed, weighed and
examined for gross external and internal abnormalities.
One female of the high dose group had body weight loss from day
10 and died on day 12 post coitum. A mean body weight loss of 3.7% and
a reduction of food consumption (-33.1%) were noted during the
treatment period in high dose animals when compared to controls. A
statistically significant inhibition of both plasma and erythrocyte
cholinesterase activity of 89 and 90%, respectively, was noted in
rabbits of the high dose group on day 18 post coitum. Five
pre-implantation losses and 1 fetal resorption were observed in the
high dose group but not in the 2 lower dose groups or in controls. No
treatment-related differences were noted in number of corpora lutea,
implantations, live and dead fetuses or fetal weight in treated groups
when compared to controls.
It was concluded by the authors of the study that fenamiphos
orally administered to mated female Chinchilla rabbits was toxic to
the mothers at dose levels of 0.8 and 3 mg/kg bw/day but not at the
dose level of 0.1 mg/kg bw/day. Based on these results, dose levels of
0.1, 0.5 and 2.5 mg/kg bw/day were proposed for the main study
(Becker et al., 1986).
In the main study 4 groups of 16 single-mated female Chinchilla
rabbits were administered by gavage single daily doses of 0, 0.1, 0.5
and 2.5 mg/kg bw/day fenamiphos (91% pure) in distilled water from day
6 through day 18 post coitum. On day 28 post coitum the dams were
killed and fetuses were removed by Caesarian section. Only dams with
at least one living fetus were used for calculation of body weight
gain, food consumption and reproduction data. Dams were examined for
position of fetuses in the uterus, number of corpora lutea,
implantations, resorptions, live and dead fetuses. Fetuses were
weighed and examined for sex, external and internal malformations,
skeletal abnormalities and development.
Three females (one in each of the 0, 0.1 and 0.5 mg/kg bw/day
dose group) were not pregnant. Four females of the 2.5 mg/kg bw/day
dose group died as a result of fenamiphos treatment, after 3, 5 and
10 days of treatment and on day 21 post coitum. Treatment-related
signs of toxicity (salivation and dyspnoea) were observed in these
4 females and in 5 other females of the high dose group from the
second till the last day of treatment (day 7 and 18 post coitum,
respectively). In 2 females which died, ataxia was noted and in
another, diarrhea was observed. No signs of toxicity were noted in any
female of the 0, 0.1 and 0.5 mg/kg bw/day dose group. A
treatment-related statistically significant decrease of mean food
consumption (29.4%) and a treatment-related reduction of body weight
gain (56%) was observed during the treatment period in dams of the
high dose group when compared to controls. Food consumption was
significantly increased in this group from day 24 to day 28.
No treatment-related or statistically significant differences
were observed in the mean number of implantations, corpora lutea, live
or dead fetuses and resorptions. In fetuses no effect which could be
attributed to the administration of fenamiphos was seen in sex ratio,
body weight, external or internal malformations, skeletal
abnormalities or development. In one fetus of the high dose group,
encephalocele with reduced size of the brain was noted. This finding
was not considered to be treatment-related. A number of
treatment-unrelated skeletal changes was noted in fetuses of all
groups.
Based on the maternal toxicity observed at the high dose level
the NOAEL for the females under the conditions of this study was
0.5 mg/kg bw/day. Under the conditions of the study fenamiphos was not
considered to be embryotoxic or teratogenic when administered to mated
female Chinchilla rabbits (Becker, 1986).
Short term studies
Rats
Groups of 209 male and female Fischer 344 rats were given diets
containing 0, 0.37, 0.57, or 0.91 ppm fenamiphos (89% pure) for
14 weeks. Rats were monitored for clinical signs of toxicity,
mortality, abnormality, masses, food consumption and body weight
throughout the study. Cholinesterase activity was determined in
10 rats/sex/group in plasma and erythrocytes at 5, 9, 12 and 14 weeks
and in brain at termination of the study.
No effects attributable to the administration of fenamiphos were
noted during clinical observation nor in food consumption and body
weight measurements. Statistically but not toxicologically significant
decreases of both plasma and erythrocyte cholinesterase activity were
observed in fenamiphos-treated rats of both sexes when compared to
controls. Maximum plasma cholinesterase inhibition as compared to
controls was 17% at week 14 in males of the 0.37 ppm dose group and
24% at week 9 in females of the high dose group. Erythrocyte
cholinesterase inhibition was in all cases < 10%. No toxicologically
significant treatment-related change was noted for brain
cholinesterase activity. It was concluded that no biologically
significant cholinesterase activity inhibition was found in this
study. The NOAEL for fenamiphos in this study was 0.9 ppm equal to
0.07 and 0.08 mg/kg bw/day for male and female rats, respectively
(Hayes, 1986b).
COMMENTS
The oncogenicity study in rats, the full report of the teratology
study in rats and the new teratology study in rabbits required by the
1985 JMPR have been submitted and evaluated.
Fenamiphos was not oncogenic to Fischer 344 rats in a combined
chronic toxicity/oncogenicity study. Erythrocyte cholinesterase
activity was inhibited in both sexes at all dose levels tested. No
biologically significant cholinesterase inhibition was seen, however,
in a 90-day rat study at doses up to and including 0.9 ppm, equal to
0.07-0.08 mg/kg bw/day.
Despite some inconsistences noted in the full report of the rat
embryotoxicity/teratogenicity study required by the 1985 JMPR, this
study was considered to be adequate. The compound was not teratogenic
to Long Evans FB 30 rats, but it was toxic to the dams at the highest
dose of 3 mg/kg bw/day.
In the required new teratology study (JMPR, 1985), which was
performed in Chinchilla rabbits, fenamiphos was toxic to mothers but
it was not embryotoxic or teratogenic. This was in contrast to the
results of a previous teratology study in New Zealand rabbits, where
fenamiphos administered at similar or lower doses was found to be both
fetotoxic and teratogenic.
A re-evaluation and comparison of data from all these studies
indicate that fenamiphos is toxic to female rats and rabbits. However,
the concern of the 1985 Meeting about the fetotoxicity and
teratogenicity observed in New Zealand rabbits (Mackenzie, 1982) was
partially alleviated by the results of two new studies. A 3-generation
reproduction study previously evaluated by the JMPR was negative.
On the basis of the results of the oncogenicity study in rats and
on the species- and strain-dependent results of the fetotoxicity/
teratogenicity studies, an ADI was estimated.
TOXICOLOGICAL EVALUATION
LEVEL CAUSING NO TOXICOLOGICAL EFFECT
Rat: 0.9 ppm in the diet, equal to 0.07 mg/kg bw/day in males
and 0.08 mg/kg bw/day in females
Dog: 2 ppm in the diet, equivalent to 0.05 mg/kg bw/day
ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN
0-0.0005 mg/kg bw.
STUDIES WHICH WiLL PROVIDE INFORMATION VALUABLE IN THE CONTINUED
EVALUATION OF THE COMPOUND
Observations in man.
REFERENCES
Becker, H., Mueller, E., Luetkemeir, H. & Sachsse, K., 1986.
Dose-finding embryotoxicity (including teratogenicity) study with SRA
3886 in the rabbit. Unpublished report No. 065250 from Research &
Consulting Co. AG, Itingen, Switzerland. Submitted to WHO by Bayer AG.
Leverkusen, FRG.
Becker, H., 1986. Embryotoxicity (including teratogenicity) study with
SRA 3886 in the rabbit. Unpublished report No. R 3917 from Research &
Consulting Co. AG, Itingen, Switzerland. Submitted to WHO by Bayer AG.
Leverkusen, FRG.
Hayes, R.H., 1986a. Combined chronic toxicity/oncogenicity study of
technical fenamiphos (Nemacur) with rats. Unpublished report No. 721
from Corporate Toxicology Dept., Mobay Chemical Co., Stilwell, KA,
USA. Submitted to WHO by Bayer AG.
Hayes, R.H., 1986b. Ninety-day cholinesterase study on rats with
technical fenamiphos (Nemacur) in diet. Unpublished report No. 717
from Corporate Toxicology Dept., Mobay Chemical Co., Stilwell, KA,
USA. Submitted to WHO by Bayer AG. Leverkusen, FRG.
Mac Kenzie, K.M., 1982. Teratology study with Nemacur in rabbits.
Unpublished report No. 308 from Hazelton Raltech, Inc. Submitted to
WHO by Bayer AG. Leverkusen, FRG.
Schulter, G., 1981. SRA 3886 (Nemacur): Evaluation for embryotoxicity
and teratogenic effects in orally dosed rats. Unpublished report
No. 9785 from Institute for Toxicology, Bayer AG. Submitted to WHO by
Bayer AG. Leverkusen, FRG.