Combined Actions and Interactions of Chemicals in Mixtures

studies using leukaemic cells have shown that 5-FU toxicity could be ameliorated
by nucleoside (thymidine or deoxycytidine) supplementation (Elstein et al. 1997).
Other reports, too, have indicated a protective role of deoxyribonucleotides against
this class of teratogens. However, an in vivo developmental study showed that
supplementation of deoxycytidine and deoxyguonosine had no protective effect on
5-FU induced toxicity. On the contrary, the adverse effect of 5-FU was found to be
potentiated (Lau et al. 1997).
The developmental effect of combinations of methanol and formic acid was studied
in 9-day-old rat embryos (Whole Embryo Culture, WBC). Methanol has shown
teratogenicity in rodents, and in vitro studies have demonstrated that embryonic
development was adversely affected under conditions where methanol was not
metabolised to formate to any appreciable extent. After the embryotoxicity of the
individual substances had been established in previous studies, concentrations of
methanol and formate were chosen which would produce similar effects using the
parameter “Developmental Score” (DEVSC), and isoboles were plotted joining the
equivalently toxic doses. (Andrews et al. 1998). The results showed that low
concentrations of formate together with various concentrations of methanol did not
significantly change the DEVSC from that expected from methanol alone, while
high concentrations of formate resulted in significant reductions of embryonic
DEVSC. The authors suggest that the result was indicative of an antagonistic
interaction. They cite a publication in which in vivo studies indicate that formate
enters the metabolic pathway shared by it and 2-methoxyethanol (ME) or a
metabolite of ME thereby rendering protection of the embryo as a result of a
competitive inhibition. This could indicate that the interaction shown in the present
in vitro study was a result of a complex dissimilar action.
7.4.3 Examples of interaction of reproductive toxicants in vivo
The selection of chemical mixtures as they appear in the environment or in foods
can be used as one approach to test chemical combinations in animal experiments.
An alternative approach is to select chemical mixtures containing agents with equal
target organ, similar modes of actions or similar chemical structure. In addition,
therapeutic use of antidotes as protectors of known toxicants are well described.
The first approach can be illustrated by a study on exposure of rodents to chemical
mixtures representative of groundwater contamination (Heindel et al. 1994). The
potential reproductive and developmental consequences of exposure to pesticides
and fertilisers were evaluated during a continuous breeding study in CD-1 mice,
and during a developmental study in pregnant Sprague-Dawley rats. The animals
were exposed to two pesticide mixtures representing the groundwater
contamination in California and Iowa at dose levels of 1, 10, and 100 times the
concentrations of the pesticides in the water. No detectable reproductive, general or
developmental toxicity of the pesticides mixtures was observed at any of the dose
levels. The authors specified that the purpose of the study was to evaluate the
health effects of realistic concentration of human exposure. Thus, the
concentrations tested were probably considerably lower than the maximally
tolerated doses. Consequently, the low but realistic doses used in this study did not
induce any interaction or combined effect.
In a 5x5x5 full-factorial design, three compounds, commonly found at hazardous
waste sites, were combined using 5 dosages of each agent. Trichloroethylene
(TCE), di-(2-ethylhexyl)phthalate (DEHP) and heptachlor (HEPT) were
administered to rats by oral gavage from days 6-15 during gestation. The selection
of dose levels was based on data from single-agent studies, and linear regression
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