MEIC evaluation of acute systemic toxicity - Part VII. Prediction of humantoxicity by results from testing of the first 30 reference chemicals with 27 further in vitro assays

The Multicenter Evaluation of In Vitro Cytotoxicity (MEIC) programme was se t up to evaluate the relevance for human acute toxicity of in vitro cytotox icity tests. By the end of the programme in 1996, 39 laboratories had teste d the first 30 reference chemicals in 82 in. vitro toxicity assays, and the last 20 chemicals had been tested in 67 assays. All in vitro results and t he human database have been presented in five previous papers (Parts I-V). Part VI evaluated the in vitro results from the 61 assays used to test all 50 chemicals by comparisons of in vitro IC50 values (concentrations causing 50% inhibition) with LC50 values (blood concentration causing approximatel y 50% lethality) from human single-dose acute poisonings by the chemicals. These comparisons demonstrated a good prediction of human peak LC50 values (peaks from the LC50 curves over time) by most of the 61 assays, notably th e human cell line assays (R-2 = 0.74). The present paper is supplementary t o Part VI, and presents a similar evaluation of the 27 new assays used to t est the first 30 reference chemicals, as well as a less-detailed analysis o f the six new assays used to test the last 20 chemicals. Comparisons betwee n the IC50 values from the 27 assays used to test the first 30 chemicals an d peak LC50 values demonstrated that human cell line tests gave the best me an results (R-2 = 0.76), followed by human primary culture assays (R-2 = 0. 75), animal cell line assays (R-2 = 0.68), animal primary culture assays (R -2 = 0.65), and bacterial assays (R-2 = 0.43), and confirm the findings of Part VI. Some assays were evaluated separately. Firstly, mean IC50 values f rom three human keratinocyte assays were compared with human peak LC50 valu es, resulting in a very good correlation (R-2 = 0.84). When the IC50 values for 19 chemicals which freely pass the blood-brain barrier were divided by a factor of ten (to compensate for a hypothetical extra sensitivity of the central nervous system to cytotoxicity), the correlation improved (R-2 = 0 .87). However, the pat tern of outlier chemicals was found to be different to that of typical outliers for human cell line assays, which makes it diff icult to use keratinocytes for acute systemic toxicity testing. Secondly, m ean IC50 values from two human hepatocyte assays were compared with human p eak LC50 values, resulting in a good direct correlation (R-2 = 0.78) and an even better "blood-brain barrier-compensated" correlation (R-2 = 0.81). Th e hepatocyte assays also had typical outlier chemicals without relevance fo r acute systemic toxicity. To fmd out whether typical hepatocyte responses could be used to predict lethal liver injury, a differential cytotoxicity s tudy was conducted. The relatively high sensitivity of hepatocytes compared with human cell lines, as judged by the differences between the mean IC50 values for the two groups of assays, was compared with reported lethal live r injuries and the basal cytotoxicities of the human lethal blood concentra tions of the 30 compounds. For three chemicals, i.e. methanol, phenol and a rsenic trioxide, high sensitivity of the hepatocytes was indeed correlated with some liver injury. However, there was a much better correlation betwee n a high basal cytotoxicity of the human blood concentration and liver inju ry. Seven of the eight reported liver injuries (from paracetamol, paraquat, iro n [II] sulphate, 2,4-dichlorophenoxyaeetic acid, arsenic trioxide, copper [ II] sulphate and thallium sulphate) were associated with: a slow decline in the body of a peak blood concentration, equivalent to the 24-hour IC50 for human cell lines, preferential accumulation of the chemical in the liver, and/or in vitro cytotoxicity which was considerably accentuated with time. Thus, almost all of the liver injuries were considered to have been caused by the interplay between toxicokinetics and basal cytotoxicity, irrespectiv e of the mechanism of basal. cytotoxicity.