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EC number: 205-746-2
CAS number: 149-74-6
There are no in vivo data on the
toxicokinetics of dichloro(methyl)(phenyl)silane.
The following summary has therefore been
prepared based on the physicochemical properties of the substance itself
and its hydrolysis productsand using this data in
algorithms that are the basis of many computer-based physiologically
based pharmacokinetic or toxicokinetic (PBTK) prediction models. The
main input variable for the majority of these algorithms is log Kowso
by using this, and other where appropriate, known or predicted
physicochemical properties ofdichloro(methyl)(phenyl)silaneor
its hydrolysis products, reasonable predictions or statements may be
made about their potential absorption, distribution, metabolism and
excretion (ADME) properties.
Dichloro(methyl)(phenyl)silane is a
moisture-sensitive liquid that hydrolyses very rapidly in contact with
water (half-life <1 minute at pH 7), generating hydrochloric
acid and methylphenylsilanediol. Human
exposure can occur via the inhalation or dermal routes. Relevant
inhalation exposure would be to the hydrolysis products (hydrolysis
would occur rapidly when inhaled, even if a mixture of parent and
hydrolysis products were present in air). The substance would also
hydrolyse rapidly in contact with moist skin. The resultinghydrochloric
acidhydrolysis product would be
severely irritating or corrosive.
Potential systemic exposure to hydrochloric acid is not discussed.
Significant oral exposure is not expected for
this corrosive substance.
However, oral exposure to humans via the environment may be
relevant for the hydrolysis product, methylphenylsilanediol.When
oral exposure takes place it is necessary to assume that except for the
most extreme of insoluble substances, that uptake through intestinal
walls into the blood takes place. Uptake from intestines can be assumed
to be possible for all substances that have appreciable solubility in
water or lipid. Other mechanisms by which substances can be absorbed in
the gastrointestinal tract include the passage of small water-soluble
molecules (molecular weight up to around 200) through aqueous pores or
carriage of such molecules across membranes with the bulk passage of
water (Renwick, 1993).
As methylphenylsilanediol is very water
soluble (34000 mg/l)and has a molecular weight of approximately
154 it meets both of these criteria, so should oral exposure occur it is
reasonable to assume systemic exposure will occur also.
The fat solubility and therefore potential dermal penetration of a
substance can be estimated by using the water solubility and log Kowvalues.
Substances with log Kowvalues between 1 and 4 favour dermal
absorption (values between 2 and 3 are optimal) particularly if water
solubility is high. Due to the likely very rapid hydrolysis ofdichloro(methyl)(phenyl)silaneon
contact with skin, systemic exposure via this route is predicted to be
minimal. After or during deposition of a liquid on the skin, evaporation
of the substance and dermal absorption occur simultaneously so the
vapour pressure of a substance is also relevant and becausedichloro(methyl)(phenyl)silaneis
volatile this would further limit the potential for absorption.
Although the water solubility (34000 mg/l) of the hydrolysis
favourable for absorption, the predictedlog Kowof
0.8 is less so although still close to the favourable range. Therefore
it is considered some dermal uptakeof
the hydrolysis product is likely.
Since the other hydrolysis product, hydrochloric
acidis corrosive to the skin, damage
to the skin might increase penetration of either the parent or
hydrolysis product. There are no reliable studies to check for signs of
There is a QSPR to estimate the blood:air partition coefficient
for human subjects as published by Meulenberg and Vijverberg (2000). The
resulting algorithm uses the dimensionless Henry coefficient and the
octanol:air partition coefficient (Koct:air) as independent
variables. Using these values for the hydrolysis product,methylphenylsilanediol,
results in a very high blood:air partition coefficient (approximately
7E+07:1) so once hydrolysis has occurred, as it would be expected to in
the lungs, then significant uptake would be expected into the systemic
circulation. However, the high water solubility ofmethylphenylsilanediolmay
lead to some of it being retained in the mucus of the lungs so once
hydrolysis has occurred, absorption is likely to slow down.
As with dermal exposure, damage to membranes
caused by the corrosive nature of the hydrochloric acid hydrolysis
product might enhance the uptake. There are no reliable studies to check
for signs of inhalation toxicity.
For blood:tissue partitioning a QSPR algorithm has been developed
by DeJongh et al. (1997) in which the distribution of compounds
between blood and human body tissues as a function of water and lipid
content of tissues and the n-octanol:water partition coefficient (Kow)
is described. Using this value for the hydrolysis product, methylphenylsilanediol,
predicts that distribution would be minimal except for some distribution
Table 1: Tissue:blood partition coefficients
Hydrogen and chloride ions will enter the
body’s natural homeostatic processes.
Dichloro(methyl)(phenyl)silane is rapidly
hydrolysed to methylphenylsilanediol and hydrochloric acid in
the presence of moisture. Most if not all of this will have occurred
before absorption into the body. There are no data regarding the
metabolism of methylphenylsilanediol. Genetic toxicity tests in vitro showed
no observable differences in effects with and without metabolic
activation for dichloro(methyl)(phenyl)silane.
A determinant of the extent of urinary excretion is the soluble fraction
in blood. QPSR’s as developed by DeJonghet al. (1997) using
log Kowas an input parameter, calculate the solubility in
blood based on lipid fractions in the blood assuming that human blood
contains 0.7% lipids.
Using this algorithm, the soluble fraction ofmethylphenylsilanediolin
blood is approximately 96% suggesting it is likely to be effectively
eliminated via the kidneys in urine and accumulation is very unlikely.
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