Combined Actions and Interactions of Chemicals in Mixtures

5 Experimental studies using
simple, well-defined mixtures
Prepared by John Chr. Larsen
5.1 Introduction
In recognising the difficulties in the risk assessment of chemical mixtures the group
around Victor Feron at the TNO Nutrition and Food Research Institute in Zeist,
The Netherlands, initiated a research programme to obtain some basic information
on the toxicological interactions between toxicologically well-characterised
chemicals in well-defined mixtures. The objective was to establish knowledge
about some general principles for the interaction of chemicals in mixtures that
would be useful in the risk assessment of complex mixtures. The group used
relatively simple mixtures (not more than 10 different compounds), which were
tested in short-term repeated-dose toxicity studies in order to examine the concepts
of simple similar action or simple dissimilar action and its implication for the risk
assessment of chemical mixtures.
5.2 Chemicals with different target organs and/or different modes of action
Two four-week studies of the toxicity (clinical chemistry, haematology,
biochemistry, and pathology) in rats were performed on combinations of
compounds with different target organs and/or different modes of actions. The
study designs are given in Tables 5.2.1 and 5.2.2. NOAELs and LOAELs
("minimum-observed-adverse-effect level"), expressed in mg per kg of bodyweight
per day, had been previously established for each single compound in the same
laboratory using the same strain of rats and comparable experimental conditions. In
the first study (Jonker et al. 1990, Feron et al. 1995a) the test compounds were
arbitrarily chosen. Groups of 10 four-week old male and female rats were
administered diets containing stannous chloride, sodium metabisulphite,
metaldehyde, loperamide, Mirex, lysinoalanine, and di-n-octyltin dichloride and
drinking water containing potassium nitrate at levels that for each compound
corresponded to one tenth the NOAEL, one third the NOAEL, the NOAEL, or the
LOAEL (Table 5.2.1). Proper control groups were included.
In the groups receiving one tenth or one third of the NOAEL no treatment related
adverse effects were found. At the NOAEL level slightly decreased haemoglobin
concentration and slightly increased kidney weight in male rats were the only
treatment related adverse effects recorded.
As was expected from the studies of the toxicity of the individual compounds, a
wide range of adverse effects was seen at the LOAEL. The effects included growth
retardation (stannous chloride, loperamide, Mirex), reduced food and water intake
(stannous chloride, metaldehyde, loperamide, Mirex), changes in haematological
(stannous chloride, Mirex) and biochemical parameters (Mirex, di-n-octyltin
dichloride), increased relative testes and thyroid weights (Mirex), increased liver
weights (metaldehyde, Mirex), swollen and vacuolated hepatocytes (stannous
chloride, Mirex), hyperplasia and hyperkeratosis of the forestomach (sodium
metabisulphite), and reduced weight and lymphoid depletion of the thymus (di-n-
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