Combined Actions and Interactions of Chemicals in Mixtures

Finally, Gray has very recently published an abstract saying that the combined
effect of vinclozolin and procymidone in an in vivo Hershberger test was found to
be additive.
Furthermore, Safe (1998) has suggested using the TEF approach for estimating
total intakes of potential dietary and environmental estrogens
7.5.3 Comments on studies dealing with interactions of EDCs
In exploring the combined effect of estrogenic compounds many authors (Soto et
al., 1994; Sumpter and Jobling, 1995; Arnold et al., 1997; Soto et al., 1997) have
adopted an experimental design where the single agents as well as the mixture were
tested at only one dose level. This approach is intuitively appealing but is regarded
as inappropiate in most cases because it only allows assessments of combination
effects, which are based on expectations of effect summation. However, the
application of the method of effect summation is limited to combinations of agents,
which produce linear dose-response curves that pass through the origin of the doseeffect
plot. Linear dose-response curves are rarely seen in toxicology (Kortenkamp
and Altenburger, 1998). They may occur in the low-dose range of otherwise nonlinear
curves such as for carcinogens. In most of the above-mentioned studies
linear dose-response relationships have not been demonstrated.
Kortenkamp and Altenburger (1999) have recently reevaluated studies of
combination effects within the field of EDCs. They concluded that the debate in the
estrogen field has taken no account of the concepts for assessing combination
effects that have evolved during the last 100 years in pharmacology and
toxicology. Furthermore, they have demonstrated that some studies, which
purportedly showed absence of synergy, have in fact overlooked synergistic
effects. There were also unidentified antagonisms, but the overwhelming majority
of studies were inconclusive. These problems were traced to an undue focus on
measuring effects of mixtures at only one dose level and to a general unawareness
of the necessity to use criteria that define clearly what constitutes synergism,
additivity or antagonism.
The emphasis on synergisms has diverted attention away from the possible
implications of seemingly less spectacular additive combination effects. Silva et al.
(2002) have provided support for an additive effect of estrogenic compounds in
vitro. They tested mixtures of eight estrogenic compounds, including hydroxylated
PCBs, benzophenones, parabenes, bisphenol A, and genistein, in a recombinant
yeast estrogen screen (YES). To ensure that no chemical contributed
disproportionately to the overall combination effect, a mixture was prepared at a
mixture ratio proportional to the potency of each individual component. The
performance of four approaches for the calculation of additive combination effects
(concentration addition, toxicity equivalency factors, effect summation and
independent action) was compared. The authors concluded that concentration
addition and its application, the toxicity equivalency factor approach, were valid
methods for the calculation of additive mixture effects. There were excellent
agreement between prediction and observation. Use of the concepts of independent
action and effect summation led to clear underestimations of the experimentally
observed responses. Substantial mixture effects were reported even though each
chemical was present at levels well below its no-observed-effect-concentration
(NOEC) or EC01. The authors concluded that estrogenic agents are able to act
together to produce significant effect when combined at concentrations below their
NOECs.
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