Combined Actions and Interactions of Chemicals in Mixtures

glutathione, which may be reduced by compounds that compete for the glutathione-
S-transferases and/or glutathione (Feron et al. 1995c).
Another important possibility for interactions is induction or inhibition of the drug
metabolising enzymes. Inducers or inhibitors of the microsomal cytochrome P450
oxidative systems may either potentiate (via increased production of active
metabolites) or reduce (via increased detoxification) the toxicity of other
chemicals. Thus, ketones like acetone and methyl n-butyl ketone and methyl
isobutyl ketone can potentiate the hepatotoxicity of carbon tetrachloride and 1,2dichlorobenzene
by induction of cytochrome P450. On the other hand, inhibition of
cytochrome P450 by disulfiram strongly enhances the carcinogenicity of ethylene
dichloride and dibromide by forcing their biotransformation through the
glutathione pathway, leading to enhanced formation of the ultimate carcinogenic
glutathione conjugate. The principle of enhancing the toxicity of some pesticides
by adding an inhibitor of cytochrome P450 (e.g. piperonyl butoxide) in the
formulation is well known (Feron et al. 1995c).
A review of the literature (Krishnan & Brodeur 1991) demonstrated that the
majority of toxicokinetic interaction results from metabolic induction or inhibition
caused by some components of the mixture. These interactions may alter tissue
dosimetry and thereby the toxicity of components in the mixture. The tissue doses
of chemicals in mixture can be predicted with physiologically based toxicokinetic
(PBTK) models when the binary interactions between all of the components in the
mixture are known (Haddad et al. 1999a, 1999b, 2000a, 2000b). However, the
quantitative characteristics of each of these binary interactions have to be
determined by experimentation. Given the complexity of the mixtures, to which
humans are exposed, this would obviously require an unrealistic large number of
experiments in order to characterise the qualitative and quantitative nature of the
possible interactions.
Haddad et al. (2000b) addressed this problem by using the theoretical limits of the
PBTK modulation of tissue dose that would arise from hypothetical metabolic
interactions between 10 volatile organic compounds (VOCs) in the male rat. The
VOCs used were dichloromethane, benzene, trichloroethylene, toluene,
tetrachloroethylene, ethylbenzene, styrene, and para-, ortho-, and meta-xylene. All
rat physiological parameters and physico-chemical (partition coefficient) and
biochemical (metabolic constants) parameters used in the PBTK models were taken
from the vast literature on these compounds. All model equations, except those
describing metabolism, were taken from Ramsey and Andersen (1984). PBTK
models predicting the blood concentrations of each mixture component were
simulated using either the description of saturable metabolism presented in chapter
2.4.2 (equation 1) or the description using the hepatic extraction ratio (equation 3).
In the latter case the numerical value of E was set to either 1 (maximal enzyme
induction) or 0 (maximal enzyme inhibition). Data on blood concentration kinetics
following exposure to binary, quaternary, quinternary, octernary and decernary
mixtures of the VOCs were obtained in rats exposed for fours hours by inhalation
(50 – 100 ppm each). For all chemicals the simulation lines obtained using E = 1
and E = 0 formed the boundary lines, whereas the one obtained using Vmax and Km
values was in between. The kinetic data from mixture exposures were within the
simulated boundaries of blood concentrations. However, with increasing
complexity of the mixtures the impact on the blood kinetics of the single
components became progressively more important, i.e. blood concentrations of
unchanged parent chemicals increased with mixture complexity. This is consistent
with the occurrence of metabolic inhibition among the chemicals in the mixture. In
a second experiment rats were pre-exposed to the mixture of all ten chemical (50
ppm each) four hours a day for three consecutive days. On day four the rats were
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