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EC number: 258-904-8
CAS number: 53988-10-6
In a subacute oral toxicity study with MB2
(2-mercaptomethylbenzimidazole; also named MMBI; 1:1 mixture of 4-ethyl
and 5-methyl isomers) male and female rats were treated by gavage at
doses of 0 (corn oil), 4, 20 and 100 mg/kg bw/day for 28 consecutive
days followed by a 2-week recovery period for the control and highest
dose groups. Body weight and food consumption, clinical signs, organ
weights, clinical biochemistry and haematological parameters including
clotting times, and micronuclei induction in bone marrow erytropoetic
cells, and histopathology were examined (Saitoh et al., 1999).
“Relative organ weights of lung, liver and kidney, and serum cholesterol
and phospholipid significantly increased in male rats treated with MMBIs
at doses of 20 and 100 mg/kg. Males rats administered 100 mg/kg MMBIs
exhibited a 1.8 fold increase in thyroid weight associated with
histopathological changes but not altered serum thyroid hormone levels.
Female rats administered 100 mg MMBIs/kg exhibited significant increases
of liver and kidney weights, and serum cholesterol level.
No-observed-effect levels for male and female rats were found to be 4
and 20 mg/kg, respectively, in this subacute oral toxicity study”.
Relative lung, liver and kidney weights were significantly increased in
male rats treated with 20 mg/kg bw/day. However, absolute lung, liver
and kidney weights in male rats were not increased significantly in male
rats treated with 20 mg/kg bw/day. The increase in relative organ
weights is slight and not evident in the recovery groups at 100 mg/kg
bw/day. There is no correlate with histopathology finding in lung, liver
and kidneys up to the highest dose of 100 mg/kg bw/day. The effects seen
in relative organ weights at 20 mg/kg bw/day are regarded as secondary
effects due to the slightly (2.4%) decreased body weight.
Taking the above information into account, the relative organ weight
effects at 20 mg/kg bw/day are not considered adverse and the NOAEL for
male rats is concluded to be 20 mg/kg bw/day.
In female rats relative and absolute organ weights of lung, liver and
kidney were not increased at a dose of 20 mg/kg bw/day.
Therefore, the NOAEL for males and females is 20 mg/kg bw/day.
The PL (phospholipid) level was statistically increased in male rats
only at 100 mg/kg bw/day and no effect was observed in female rats at
A slight but significant higher T-CHO (total cholesterol) level was
observed in male rats at doses of 20 mg/kg bw/day and 100 mg/kg bw/day.
The increase was approx. 25% and 50 %, respectively. In female rats the
T-CHO level was increased at 100 mg/kg bw/day (approx. 25%) only andno
significant increase in T-CHO level was observed in female rats at 20
mg/kg bw/day and below.Increased cholesterol levels in this study are
accompanied by an increase of liver weights in male and female rats. The
increase of cholesterol is interconnected with an increase of the liver
weights. The increase of the liver weights is dose dependent and a
result of the exposure of the rats to the test substance. Liver weight
changes in repeated dose studies in mammalians are often caused by liver
enzyme induction. An important function of the liver is to produce and
clear cholesterol in the body. If the liver is not working properly, it
can cause cholesterol to build up in the body.
In the subacute study by Saitoh et al. (1999), the T-CHO values were
only slightly increased in male rats at 20 mg/kg bw/day. The effect was
reversible in the recovery group at 100 mg/kg bw/day; the T-CHO values
in the control and the 100 mg/kg bw/day groups were similar and there
were no significant differences in liver or thyroid weights in the
control and the 100 mg/kg bw/day groups. These data indicate, that the
elevated cholesterol levels at 20 mg/kg bw/day are adaptive and should
be considered non-adverse.
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