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Results from tests on the Multicentre Evaluation of In Vitro Cytotoxicity (MEIC) reference chemicals 31–50 in 67 different in vitro toxicity assays are presented in this paper as a prerequisite to in vitro/in vivo comparisons for all MEIC in vitro toxicity data in forthcoming papers, i.e. the final MEIC evaluation of the relevance of the tests. With the aim of increasing knowledge about the relative significance of some in vitro methodological factors, the strategies and methods of the preceding parts in the MEIC series (Parts II and III) were again employed to enable comparative cytotoxicity analysis of the new in vitro results presented in this paper. A principal components analysis (PCA) of the results from tests of the 20 chemicals in 67 assays demonstrated a dominating first component describing as much as 74% of the variance in the toxicity data, indicating a similar ranking of the cytotoxicities of the chemicals in most of the tests. The influence on the general variability of the results of a few, key methodological factors was also evaluated by using linear regression comparisons of the results of all pairs of methods available in the study, i.e. methods which were similar in all respects except for the factor being analysed. Results from this “random probe” analysis were: a) the cytotoxicities of 11 of the 20 chemicals increased considerably with exposure time (> 10 times over 4–168 hours); b) in general, human cell line toxicity was well predicted by cytotoxicity in animal cells; c) prediction of human cell line toxicity by most ecotoxicological tests was only fairly good; d) 14 comparisons of similar assays with different cell lines showed similar toxicities (mean R2 = 0.83); e) nine comparisons of similar assays employing different primary cultures and cell lines shared similar toxicities (mean R2 = 0.71); and f) 16 comparisons of similar assays with different growth/viability endpoints showed similar toxicities (mean R2 = 0.71). Results b, d, e and f must contribute to the PCA-documented high general similarity of the in vitro toxicity data. Results a and c, together with factors which were not analysed, such as different protocols and inter-laboratory variability of tests, could explain the 26% dissimilarity. To provide background information to the planned final MEIC evaluation of the relevance of the 61 methods in which all 50 chemicals have been tested, an additional PCA was made of the 50 chemical-61 assay in vitro database (from Parts II and III and the present paper). This supplementary PCA demonstrated an 80% similarity of results. Compared with the previous analysis of the tests of the first 30 MEIC reference chemicals (MEIC Part III), the present analysis of the tests of the last 20 MEIC chemicals indicates a somewhat higher variation in the results. Correspondingly, some deviating endpoint measurements and cell line responses were demonstrated by the pairwise comparisons in the present study. As a result, the analysis revealed a high correlation (R2 = 0.73) between the average human cell line toxicity and the results from a new protein denaturation test. These preliminary results suggest that intracellular protein denaturation may be a frequently occurring mechanism in basal cytotoxicity.

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This study was designed to compare the cytotoxic concentrations of chemicals, determined with three independent in vitro cytotoxicity testing protocols, with each other and with established animal LD50 values, and against human toxic concentrations for the same chemicals. Ultimately, these comparisons allow us to evaluate the potential of in vitro cell culture methods for the ability to screen a variety of chemicals for prediction of human toxicity. Each laboratory independently tested 50 chemicals with known human lethal plasma concentrations and LD50 values. Two of the methods used monolayer cell cultures to measure the incorporation of radiolabeled amino acids into newly synthesized proteins and cellular protein content, while the third technique used the pollen tube growth test. The latter is based on the photometric quantification of pollen tube mass production in suspension culture. Experiments were performed in the absence or presence of increasing doses of the test chemical, during an 18- to 24-h incubation. Inhibitory concentrations were extrapolated from concentration-effect curves after linear regression analysis. Comparison of the cytotoxic concentrations confirms previous independent findings that the experimental IC50 values are more accurate predictors of human toxicity than equivalent toxic blood concentrations (HETC values) derived from rodent LD50s. In addition, there were no conclusive statistical differences among the methods. It is anticipated that, together, these procedures can be used as a battery of tests to supplement or replace currently used animal protocols for human risk assessment.