Strategies for toxicological evaluation of mixtures

Different strategies for the toxicological evaluation of mixtures are presented. The purpose is to determine the effects of each component (variable) in the mixture, and possible interactions between variables. The examples presented have 3–5 predictor variables and 1–3 responses, and are based on statistical experimental design, multivariate data analysis and modelling. The following examples are presented: (1) inhalation experiments with synthetic vapour mixtures of hydrocarbons formulated by means of mixture design at different vapour concentrations (the experimental region covers both partial and complete evaporation of the liquid mixtures); (2) combination of refinery streams for fuel blending by means of mixture design with constraints, followed by engine tests and determination of exhaust particles; (3) fractionation of organic extracts of diesel exhaust particles, and recombination of the extracts by means of mixture design, followed by mutagenicity testing of the recombined extracts in the Ames Salmonella assay; (4) spiking complex mixtures with individual compounds using factorial design, and subsequent mutagenicity testing. The data obtained from these four examples were analysed by means of Projections to Latent Structures (PLS). The effects of each variable and possible interactions, were quantified by means of PLS regression coefficients. Furthermore, the empirical models obtained were evaluated by means of correlation coefficients, cross validation and predictions.