Food for Thought ... Integrated Testing Strategies for Safety Assessments

Replacing a test on a living organism with a cellular, chemicoanalytical, or computational approach obviously is reductionistic. Sometimes this might work well, e.g., when an extreme pH is a clear indication of corrosivity. In general, however, it is naïve to expect a single system to substitute for all mechanisms, the entire applicability domain (substance classes), and degrees of severity. Still, toxicology has long neglected this when requesting a one-to-one replacement to substitute for the traditional animal test. We might even extend this to say it is similarly naïve to address an entire human health effect with a single animal experiment using inbred, young rodents... the only way to approximate human relevance is to mimic the complexity and responsiveness of the organ situation and to model the respective kinetics, i.e., the target hoped for from the human-on-a-chip approach (Hartung and Zurlo, 2012). everything else requires making use of several information sources, if not compromising the coverage of the test. Genotoxicity is a nice example, where patches have continuously been added to cover the various mechanisms. However, here the simplest possible strategy, i.e., a battery of tests where every positive result is considered a liability, causes problems. We have seen where the inevitable accumulation of false-positives leads (Kirkland et al., 2005), ultimately undermining the credibility of in vitro approaches. Summary Despite the fact that toxicology uses many stand-alone tests, a systematic combination of several information sources very often is required: Examples include: when not all possible outcomes of interest (e.g., modes of action), classes of test substances (applicability domains), or severity classes of effect are covered in a single test; when the positive test result is rare (low prevalence leading to excessive falsepositive results); when the gold standard test is too costly or uses too many animals, creating a need for prioritization by screening. Similarly, tests are combined when the human predictivity of a single test is not satisfactory or when existing data and evidence from various tests will be integrated. Increasingly, kinetic information also will be integrated to make an in vivo extrapolation from in vitro data. Integrated Testing Strategies (ITS) offer the solution to these problems. ITS have been discussed for more than a decade, and some attempts have been made in test guidance for regulations. Despite their obvious potential for revamping regulatory toxicology, however, we still have little guidance on the composition, validation, and adaptation of ITS for different purposes. Similarly, Weight of Evidence and Evidence-based Toxicology approaches require different pieces of evidence and test data to be weighed and combined. ITS also represent the logical way of combining pathway-based tests, as suggested in Toxicology for the 21st Century. This paper describes the state of the art of ITS and makes suggestions as to the definition, systematic combination, and quality assurance of ITS.