MEIC Evaluation of Acute Systemic Toxicity

Abstract

The Multicenter Evaluation of In Vitro Cytotoxicity (MEIC) programme was set up to evaluate the relevance for human acute toxicity of in vitro cytotoxicity tests. At the end of the project in 1996, 29 laboratories had tested all 50 reference chemicals in 61 cytotoxicity assays. Five previous articles have presented the in vitro data and the human database to be used in the evaluation. This article presents three important 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 display of the correlations between IC50 (concentration causing 50% inhibition) values from all 61 assays and three independent sets of human acute lethal blood concentrations, i.e. clinical lethal concentrations, forensic lethal concentrations, and peak concentrations; and c) a series of comparisons between average IC50 values from ten human cell line 24-hour assays and human lethal blood concentrations. In the latter comparisons, results from correlations were linked with known human toxicity data for the chemicals, to provide an understanding of correlative results. This correlative/mechanistic approach had the double purpose of assessing the relevance of the in vitro cytotoxicities, and of testing a series of hypotheses connected with the basal cytotoxicity concept. The results of the studies were as follows. Rat LD50 predictions of human lethal dosage were only relatively good (R2 = 0.61), while mouse LD50s gave a somewhat better prediction (R2 = 0.65). Comparisons performed between IC50 values from the 61 assays and the human lethal peak concentrations demonstrated that human ceil line tests gave the best average results (R2 = 0.64), while mammalian and fish cell tests correlated less well (R2 = 0.52–0.58), followed by non-fish ecotoxicological tests (R2 = 0.36). Most of the 61 assays underpredicted human toxicity for digoxin, malathion, carbon tetrachloride and atropine sulphate. In the correlative/mechanistic study, the 50 chemicals were first separated into three groups: A = fast-acting chemicals with a restricted passage across the blood–brain barrier; B = slow-acting chemicals with a restricted passage across the blood–brain barrier; and C = chemicals which cross the blood–brain barrier freely, while inducing a non-specific excitation/depression of the central nervous system (CNS). The IC50 values for chemicals in group C were divided by a factor 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 concentrations for groups A and C, and 48-hour concentrations for group B). As a result, in vitro toxicity and in vivo toxicity correlated very well for all groups (R2 = 0.98, 0.82 and 0.85, respectively). No clear overprediction of human toxicity was made by the human cell tests. The human cell line tests underpredicted human toxicity for only four of the 50 chemicals. These outlier chemicals were digoxin, malathion, nicotine and atropine sulphate, all of which have a lethal action in man through interaction with specific target sites not usually found in cell lines. Potassium cyanide has a cellular human lethal action which cannot be measured by standard anaerobic cell lines. The good prediction of the human lethal whole-blood concentration of this chemical was not 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 cell line cytotoxicities are relevant for the human acute lethal action for 43 of the 50 chemicals. The results strongly support the basal cytotoxicity concept, and further point to the non-specific CNS depression being the obligatory reaction of humans to cytotoxic concentrations of chemicals, provided that the chemicals are able to pass the blood–brain barrier.

The Prediction of Human Acute Systemic Toxicity by the EDIT/MEIC In Vitro Test Battery: The Importance of Protein Binding and of Partitioning into Lipids

“…All the relevant case reports on human poisoning available in the literature were consulted, in order to enable the construction of approximate 50% lethal blood concentration curves (LC50 curves) over time for all the MEIC reference chemicals (21). The peak concentration of these LC50 curves, expressed as logM, appears to be the best measure of human acute toxicity for use in comparisons with in vitro results (6). For that reason, the peak concentrations of the LC50 curves were used for the comparison made in the two studies.…”

“…The results from the MEIC study showed that the test battery of three human cell line tests (the original MEIC test battery) predicted acute human systemic toxicity better (R 2 = 0.74) than the prediction of human lethal doses provided by LD50 values for rats and mice (R 2 = 0.60-0.66). The results from the study also showed that specific mechanisms exist, which could only be measured by using organotypic in vitro toxicity tests (6). Furthermore, the results indicated that the inclusion of in vitro methods for various biokinetic events, such as passage across the blood-brain bar-rier (BBB), protein binding, biotransformation, and absorption in the gut, could improve the predictability of the test battery (6,8).…”

“…The results from the study also showed that specific mechanisms exist, which could only be measured by using organotypic in vitro toxicity tests (6). Furthermore, the results indicated that the inclusion of in vitro methods for various biokinetic events, such as passage across the blood-brain bar-rier (BBB), protein binding, biotransformation, and absorption in the gut, could improve the predictability of the test battery (6,8). Thus, the results implied the need to further optimise the test battery by including these types of tests.…”

“…All the IC50 values (concentrations resulting in a 50% inhibition) for the 32 chemicals that cross the BBB freely were divided by a factor of ten, in order to compensate for the hypothetical extra vulnerability of the central nervous system (CNS) to cytotoxicity. However, the algorithm was shown to be less than optimal (6). Furthermore, whether or not most compounds pass the BBB is not known.…”

“…In the Multicentre Evaluation of In Vitro Cytotoxicity (MEIC) programme, the relevance of using in vitro toxicity assays for the prediction of human acute systemic toxicity was evaluated (1)(2)(3)(4)(5)(6)(7)(8)(9). As a practical outcome of the programme (8), a battery of three 24-hour human cell line assays was selected (cell/endpoint [method no.…”