Validation—Lessons learned from practical experience

Dynamic Qualities of Validation and the Evolution of New In Vitro Toxicological Tests

This review summarises some aspects of the dynamics of the evolution of toxicological test methods based on cell biology. Within the multicentre evaluation of in vitro cytotoxicity (MEIC) programme, some general principles for validation have been proposed: a) a human database should be used as the source of reference data in the validation of new methods aimed at predicting human toxicity; b) the relevance of a test should be determined before its reliability is assessed; c) a parallel validation of methods is preferable to a serial validation; and d) toxicokinetic data should be included in the validation process to improve the predictivity of cytotoxicity test results. These toxicokinetic data can be used to extrapolate the cytotoxic in vitro concentrations to provide human toxic exposure levels. As part of test development, the cytotoxic concentration can also be compared directly with the critical toxic human blood or tissue concentration. This approach is being explored in the ERGATT/CFN integrated toxicity testing scheme (ECITTS) prevalidation project. The critical toxic concentrations are determined by using a set of neurospecific cellular tests, chosen and combined on the basis of knowledge of common neurotoxicity mechanisms. Another approach to selecting tests for prevalidation is through the development of tests that are found to be necessary and complementary to existing tests. Such a programme has been initiated on the basis of the results of the MEIC study. The progress made so far in this “missing tests programme” is presented in this paper.

Validation in Support of Internationally Harmonised OECD Test Guidelines for Assessing the Safety of Chemicals

Ten years elapsed since the OECD published the Guidance document on the validation and international regulatory acceptance of test methods for hazard assessment. Much experience has been gained since then in validation centres, in countries and at the OECD on a variety of test methods that were subjected to validation studies. This chapter reviews validation principles and highlights common features that appear to be important for further regulatory acceptance across studies. Existing OECD-agreed validation principles will most likely generally remain relevant and applicable to address challenges associated with the validation of future test methods. Some adaptations may be needed to take into account the level of technique introduced in test systems, but demonstration of relevance and reliability will continue to play a central role as pre-requisite for the regulatory acceptance. Demonstration of relevance will become more challenging for test methods that form part of a set of predictive tools and methods, and that do not stand alone. OECD is keen on ensuring that while these concepts evolve, countries can continue to rely on valid methods and harmonised approaches for an efficient testing and assessment of chemicals.

Validation of Alternative In Vitro Methods to Animal Testing: Concepts, Challenges, Processes and Tools

This chapter explores the concepts, processes, tools and challenges relating to the validation of alternative methods for toxicity and safety testing. In general terms, validation is the process of assessing the appropriateness and usefulness of a tool for its intended purpose. Validation is routinely used in various contexts in science, technology, the manufacturing and services sectors. It serves to assess the fitness-for-purpose of devices, systems, software up to entire methodologies. In the area of toxicity testing, validation plays an indispensable role: "alternative approaches" are increasingly replacing animal models as predictive tools and it needs to be demonstrated that these novel methods are fit for purpose. Alternative approaches include in vitro test methods, non-testing approaches such as predictive computer models up to entire testing and assessment strategies composed of method suites, data sources and decision-aiding tools. Data generated with alternative approaches are ultimately used for decision-making on public health and the protection of the environment. It is therefore essential that the underlying methods and methodologies are thoroughly characterised, assessed and transparently documented through validation studies involving impartial actors. Importantly, validation serves as a filter to ensure that only test methods able to produce data that help to address legislative requirements (e.g. EU's REACH legislation) are accepted as official testing tools and, owing to the globalisation of markets, recognised on international level (e.g. through inclusion in OECD test guidelines). Since validation creates a credible and transparent evidence base on test methods, it provides a quality stamp, supporting companies developing and marketing alternative methods and creating considerable business opportunities. Validation of alternative methods is conducted through scientific studies assessing two key hypotheses, reliability and relevance of the test method for a given purpose. Relevance encapsulates the scientific basis of the test method, its capacity to predict adverse effects in the "target system" (i.e. human health or the environment) as well as its applicability for the intended purpose. In this chapter we focus on the validation of non-animal in vitro alternative testing methods and review the concepts, challenges, processes and tools fundamental to the validation of in vitro methods intended for hazard testing of chemicals. We explore major challenges and peculiarities of validation in this area. Based on the notion that validation per se is a scientific endeavour that needs to adhere to key scientific principles, namely objectivity and appropriate choice of methodology, we examine basic aspects of study design and management, and provide illustrations of statistical approaches to describe predictive performance of validated test methods as well as their reliability.

Applicability of rat precision-cut lung slices in evaluating nanomaterial cytotoxicity, apoptosis, oxidative stress, and inflammation

The applicability of rat precision-cut lung slices (PCLuS) in detecting nanomaterial (NM) toxicity to the respiratory tract was investigated evaluating sixteen OECD reference NMs (TiO₂, ZnO, CeO₂, SiO₂, Ag, multi-walled carbon nanotubes (MWCNTs)). Upon 24-hour test substance exposure, the PCLuS system was able to detect early events of NM toxicity: total protein, reduction in mitochondrial activity, caspase-3/-7 activation, glutathione depletion/increase, cytokine induction, and histopathological evaluation. Ion shedding NMS (ZnO and Ag) induced severe tissue destruction detected by the loss of total protein. Two anatase TiO₂ NMs, CeO₂ NMs, and two MWCNT caused significant (determined by trend analysis) cytotoxicity in the WST-1 assay. At non-cytotoxic concentrations, different TiO₂ NMs and one MWCNT increased GSH levels, presumably a defense response to reactive oxygen species, and these substances further induced a variety of cytokines. One of the SiO₂ NMs increased caspase-3/-7 activities at non-cytotoxic levels, and one rutile TiO₂ only induced cytokines. Investigating these effects is, however, not sufficient to predict apical effects found in vivo. Reproducibility of test substance measurements was not fully satisfactory, especially in the GSH and cytokine assays. Effects were frequently observed in negative controls pointing to tissue slice vulnerability even though prepared and handled with utmost care. Comparisons of the effects observed in the PCLuS to in vivo effects reveal some concordances for the metal oxide NMs, but less so for the MWCNT. The highest effective dosages, however, exceeded those reported for rat short-term inhalation studies. To become applicable for NM testing, the PCLuS system requires test protocol optimization.

Defining the role of ECVAM in the development, validation and acceptance of alternative tests and testing strategies

The background to the establishment of the European Centre for the Validation of Alternative Methods (ECVAM) is reviewed, and the main events at the opening of the Centre and an ECVAM symposium on practical aspects of validation are summarized. Finally, recommendations made to ECVAM for consideration in developing the Centre's strategy are listed.

Precision determination for the dynamic respirometric index (DRI) method used for biological stability evaluation on municipal solid waste and derived products

Dynamic respiration index (DRI) is an effective respirometric method to measure the biological stability of municipal solid waste (MSW). It allows testing MSW biological stability under standardized conditions and is now used as a routine analytical method. However, the method needs to be studied for precision parameters to ensure the quality of results generated. This work reports on a DRI validation study, detecting repeatability (r) and reproducibility limits (R). To perform the study, 4-6 Italian laboratories took part in an interlaboratory test for the validation of the DRI method on four different municipal solid wastes from different mechanical-biological treatment full-scale plants. Precision values (r and R) of DRI, expressed as relative standard deviation, were in the range of 3.6% and 15.5%, and were acceptable when compared with previous data obtained in another respirometric test. On the other hand, no regressions were found between r and R, and DRI, and as a consequence prediction of precision values was not possible a priori for different DRI levels, unless the same typology of waste was considered.

Precision determination for the specific oxygen uptake rate (SOUR) method used for biological stability evaluation of compost and biostabilized products

This work represents the first attempt to evaluate the precision of the specific oxygen uptake rate method expressed in terms of repeatability (r) and reproducibility limits (R). Three laboratories were involved in an inter-laboratory test for the validation of respiration analyses on six biomass samples (three composts and three biostabilized products) having different degrees of biological stability. Both the maximum specific oxygen uptake rate peak (SOUR) and the cumulative oxygen demand after 12 h (OD(12)) and 20 h (OD(20)) of respiration test were investigated. Precisions expressed as the relative standard deviation were in the range of 9-41%. Linear regressions found for r and R, versus OD(12) and OD(20), enabled derivation of precision values (r and R) for all respirometric levels within the operating range. The OD(12) and OD(20) indices were found to be more adequate to indicate biological stability since they were less influenced by random errors than the SOUR index.