Combined Actions and Interactions of Chemicals in Mixtures

Since no compounds are known to cause mutations in vivo without being initiating
carcinogens, the OECD-guideline tests for these effects (see section on
mutagenesis) could be used to identify some initiating carcinogens.
More recently, the newborn mouse and rat assays have become popular. The
animals are treated with the test compound at the age of one day and one or two
weeks intraperitoneally, and tumours are scored at about six to eight weeks later.
Since these assays use the natural growth of the animals to enhance the effects of
the test compounds they may be presumed to be extremely sensitive to tumour
initiators. However they are not operationally defined as test systems to identify
initiating compounds.
Several initiators may work in concert to elicit a response, which is stronger or
weaker than that of the expected (additive) effect. A stronger response has been
observed with mixtures of heterocyclic aromatic amines (HAAs) from fried meat in
rat liver tumourigenesis (Hasegawa et al., 1996a; Hasegawa et al., 1996b), whereas
both stronger (synergistic) and weaker (antagonistic) responses have been
observed with PAH mixtures (Iversen, 1994; Warshawsky et al., 1993; Horton and
Christian, 1974). Such modulated responses can be caused by several mechanisms.
The most straightforward is interaction at the level of uptake, where compounds
may compete during absorption. A similar mechanism is competition at the level of
enzymatic metabolism and activation. Competition for activating enzymes may
direct a larger proportion of the dose to detoxication pathways and vice versa.
Moreover, there may be threshold doses for enzyme induction leading to increased
activation or deactivation. Such mechanisms can also lead to non-linearity of the
dose-response curves for individual compounds.
Initiators acting by different genetic mechanisms may also act synergistically. This
is often used for chemotherapy, where genotoxic compounds leading to mutation
act in synergy with spindle poisons and topoisomerase inhibitors (Fukuda et al.,
1996; Raymond et al., 1996).
Formal guideline tests for initiation do not exist, but a range of models has been
used in the scientific literature and could be used to inspire the setup of formal tests
for initiation and co-initiation. Genotoxic compounds (see chapter on mutagenesis)
are potential tumour initiators, and when such compounds are found also to be
carcinogenic they are often described as genotoxic carcinogens.
7.3.2.2 Co-carcinogenesis (co-initiation)
Co-carcinogenesis is defined as a mechanism by which a non-carcinogen increases
the action of a carcinogen, most often an initiating carcinogen. No formal test
systems exist for this effect. Co-carcinogenesis can be affected by at least three
distinct mechanisms. One is by compounds, which increase the penetration of
carcinogenes through epithelial barriers. Examples include organic solvents, which
help PAHs through the skin barrier, and fatty diets, which increase the uptake of
unpolar substances through the gut wall. Dimethylsulphoxide (DMSO) is well
known as a solvent which destroys the skin barrier and increases the penetration of
toxic compounds, and it has also been shown to increase the tumourigenic response
to benzo[a]pyrene, but the solvent has several effects in skin, and DMSO can
therefore be both increase and decrease carcinogenesis (Jacoby and Weiss, 1986).
The DMSO-induced decrease seems to be mainly related to the promotion phase of
carcinogenesis, however.
Co-carcinogenesis can also be elicited by compounds, which increase the
activation or the impact of genetic damage of the initiator. Benzo[e]pyrene is a
well-known example, acing as a co-carcinogen with benzo[a]pyrene by increasing
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