Acceptance of in vitro testing by regulatory authorities

National regulatory authorities have responsibility for taking decisions that possibly affect the health of whole populations and it is therefore to be expected that they will be reluctant to substitute alternative in vitro toxicity test methods for conventional animal studies unless the new procedures have been demonstrated to be reliable and have been adequately validated. Validation of in vitro methods presents particularly difficult problems because whilst they tend to produce consistent and objective results, the test systems used are incapable of mirroring the complexity of the biochemical processes seen in animals. As a result, a single animal study would need a large battery of in vitro studies to replace it to cover the various endpoints that are involved in the in vivo study; each of these in vitro tests would need to be validated with regard to the specific endpoint that it is investigating. Although great advances have been made in recent years in the development of alternative methods, few have been validated to an extent that makes them acceptable to regulatory authorities as replacements for in vivo studies. Rather, they are largely seen and used as screening techniques whereby decisions can be taken on the value of further development of newly discovered compounds, and as aids in the interpretation of animal studies and in their extrapolation to man, that is, they are of value in ‘mechanism of action’ studies. Nevertheless, certain in vitro procedures are already accepted by regulatory authorities and their use, for example, in ‘screening out’ compounds that have severe irritant properties, and in identifying compounds with potential mutagenic and carcinogenic activity, has had a profound effect on both the number of animal studies carried out and on the welfare of those animals still used.