Computer modelling and in vitro tests in the safety evaluation of chemicals-Strategic applications

The cytochromes P-450 detoxicate most chemicals, but activate carcinogens and other toxic chemicals by oxygenating them to reactive intermediates. A new strategy based on the P-450s has resulted in the development of a computer program (COMPACT) and of a programme of enzyme induction studies, as short-term tests to predict the potential toxicity/carcinogenicity of chemicals. Most carcinogens are metabolically activated by P450 I (P-448); P450 IIE and P450 IV also mediate chemical toxicity by producing oxygen radicals. P450 IIB mostly results in detoxication, but where chemicals are oxygenated with difficulty (e.g. phenobarbitone), the cytochrome is induced and futile cycling and oxygen radical formation may result. Oxygen radicals are highly toxic, mutagenic and carcinogenic and mediate much chemical toxicity in small rodents. As P450 genes of man differ from those of rodents, the latter have limited value as surrogates for man in chemical safety evaluation; many short-term tests, such as the Ames test, although valuable in detecting genotoxic chemicals, are similarly limited by their rat liver microsomal activation (S-9) systems. COMPACT is able to overcome both of these limitations, requires no laboratory animals or tissues, and can be conducted from a knowledge of the structure of the chemical, even prior to its synthesis. As the rodent tumorigenicity assay and the Ames test are currently regarded as the standard procedures for safety evaluation, COMPACT was validated against these in a study of 100 miscellaneous chemicals and showed excellent correlations. A quantitative structure-activity relationship (QSAR) for a series of 14 methyl benzanthracenes shows good correlation of mutagenicity with the energy of the lowest empty molecular orbital [E(LEMO)] values of the chemicals. A strategy for COMPACT and P-450 induction studies in chemical safety evaluation is presented.