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EC number: 202-443-7
CAS number: 95-71-6
The range of NOAEL for congeners of Methylhydroquinone in rodents (rats,
mice) after oral administration is in the range of 20 – 402 mg/kg/d
repeated oral doses considering the duration of experiments of 28d up to
The targeted toxicological endpoints are primarily decreased body weight
and liver toxicity. Regarding the effects on the thyroid gland function,
it appears that the OH-groups in para-position, like in
methylhydroquinone, are inactive. Mild, transient tremors and reduced
home-cage activity were also considered.
Since the smallest experimental NOAEL obtained in animals are 20 mg/kg
bw./ day, the experts set the NOAEL for the targeted substance
Methylhydroquinone on 20 mg/kg bw./day. Nevertheless, it appears that a
value of 50 mg/kg/day is also acceptable since the tremors obtained in
the study of Topping, D.C. et al. 2007, are reversible and disappear
during long term use.
Repetitive doses of HQ resulted in transient tremors and reduced motor
activity levels following daily treatment and during the functional
observational battery examinations performed shortly after
administration of P64 mg/kg HQ, but not 20 mg/kg. The threshold for CNS
stimulation appears to be very close to 64 mg/kg because the effects
were just barely detectible in this study and were not detectible in
F-344 rats given 100 mg/kg for 13 weeks or 50 mg/kg for 2 years. In this
study, repetitive acute CNS stimulation did not result in evidence of
subchronic neurotoxicity as assessed by FOB examinations, quantitative
grip strength measurement, brain weight, or neuropathology examinations.
In addition, no nephropathy was evident in Sprague-Dawley rats given 200
mg/kg HQ; a dose level that is nephrotoxic to F-344 rats.
Discussion in the study report pertaining to the NOAEL:
At the remaining dose level of 125 and 45 mg/kg/day, treatment-related
changes were confined to males and involved an increased severity of
extramedullary erythropoiesis in the spleen. There was no difference in
the amount of haemosidin pigment present in the spleen at these dose
compared with that found in the spleen of control maschon in der
Tabellele rats but the increased severity of extramedullary
erythropoiesis could indicate that an underlying anaemia was present in
these animals, or at least had developed at some time during the dosing
period. The absence of any haematological evidence of an anaemia
indicates however that the normally expected physiological response of
increased erythrogenesis was able to fully compensate for any increase
in the rate of erythrocyte destruction at these two dose levels. The
increased erythrogenesis in these animals was not accompanied by a
measurable increase in the number of polychromatic erythrocytes.
Females dosed at 125 or 45 mg/kg/ day were unaffected by repeated
administration of No. 240 Trimethylhydrochinon.
The reference substance trimethylhydoquinone has the same functional
groups as the target substane 2 -methylhydroquinone and is therefore a
structural analogue of the targeted substance methylhydroquinone
Both substances are analogues since they have similar structural and
functinal properties and similar toxicological preperties are expected.
The subchronic (sub-acute) 28-day oral totxicity study in the rat may
therefore serve as reference substance for forcast of the toxicological
properties of the targeted 2 -methylhydroquinone after oral use, i.e.the
NOAEL value in particular.
The substance is not classified for specific target organ toxicity (RE)
since no data from a 90 day repeated dose study are available.
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