MEIC EVALUATION OF ACUTE SYSTEMIC TOXICITY - PART-6 - THE PREDICTION OF HUMAN TOXICITY BY RODENT LD50 VALUES AND RESULTS FROM 61 IN-VITRO METHODS

The Multicenter Evaluation of In Vitro Cytotoxicity (MEIC) programme w as set up to evaluate the relevance for human acute toxicity of in vit ro cytotoxicity tests. At the end of the project in 1996, 29 laborator ies had tested all 50 reference chemicals in 61 cytotoxicity assays. F ive previous articles have presented the in vitro data and the human d atabase to be used in the evaluation. This article presents three impo rtant parts of the final evaluation: a) a comparison of rat and mouse oral LD50 with human acute lethal doses for all 50 chemicals; b) a dis play of the correlations between IC50 (concentration causing 50% inhib ition) values from ail 61 assays and three independent sets of human a cute lethal blood concentrations, i.e. clinical lethal concentrations, forensic lethal concentrations, and peak concentrations; and c) a ser ies of comparisons between average IC50 values from ten human cell lin e 24-hour assays and human lethal blood concentrations. In the latter comparisons, results from correlations were linked with known human to xicity data for the chemicals, to provide an understanding of correlat ive results. This correlative/mechanistic approach had the double purp ose of assessing the relevance of the in vitro cytotoxicities, and of testing a series of hypotheses connected with the basal cytotoxicity c oncept. The results of the studies were as follows. Rat LD50 predictio ns of human lethal dosage were only relatively good (R-2 = 0.61), whil e mouse LD50s gave a somewhat better prediction (R-2 = 0.65). Comparis ons performed between IC50 values from the 61 assays and the human let hal peak concentrations demonstrated that human cell line tests gave t he best average results (R-2 = 0.64), while mammalian and fish cell te sts correlated less well (R->2 = 0.52-0.58), followed by non-fish ecot oxicological tests (R-2 = 0.36). Most of the 61 assays underpredicted human toxicity for digoxin, malathion, carbon tetrachloride and atropi ne sulphate. In the correlative/mechanistic study, the 50 chemicals we re first separated into three groups: A = fast-acting chemicals with a restricted passage across the blood-brain barrier; B = slow-acting ch emicals with a restricted passage across the blood-brain barrier; and C = chemicals which cross the blood-brain barrier freely, while induci ng a non-specific excitation/depression of the central nervous system (CNS). The IC50 values for chemicals in group C were divided by a fact or of ten to compensate for a hypothetical extra vulnerability of the CNS to cytotoxicity. Finally, the average human cell line IC50 values (24-hour IC50 for groups A and C, and after 48-hour for group B) were compared with relevant human lethal blood concentrations (peak concent rations for groups A and C, and 48-hour concentrations for group B). A s a result, in vitro toxicity and in vivo toxicity correlated very wel l for all groups (R-2 = 0.98, 0.82 and 0.85, respectively). No clear o verprediction of human toxicity was made by the human cell tests. The human cell line tests underpredicted human toxicity for only four of t he 50 chemicals. These outlier chemicals were digoxin, malathion, nico tine and atropine sulphate, all of which have a lethal action in man t hrough interaction with specific target sites not usually found in cel l lines. Potassium cyanide has a cellular human lethal action which ca nnot be measured by standard anaerobic cell lines. The good prediction of the human lethal whole-blood concentration of this chemical was no t conclusive, i.e. was probably a ''false good correlation''. Another two chemicals in group C resulted in ''false good correlations'', i.e. paracetamol and paraquat. The comparisons thus indicated that human c ell line cytotoxicities are relevant for the human acute lethal action for 43 of the 50 chemicals. The results strongly support the basal cy totoxicity concept, and further point to the non-specific CNS depressi on being the obligatory reaction of humans to cytotoxic concentrations of chemicals, provided that the chemicals are able to pass the blood- brain barrier.