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Validation
Validation forms the interface between the development and optimisation of a test method and its regulatory acceptance and serves the knowledge-based assessment of the suitability of a method for a specific regulatory purpose. Validation is a process used to demonstrate the reproducibility and relevance of a method. Reproducibility means that the method provides comparable results regardless of place, time or experimenter. Relevance describes how well and to what extent the selected test system and method reflects an important biological process described in the (human) organism and can be used as a model for an adverse effect.
In 2005, the OECD published internationally agreed principles and criteria for conducting validation studies in Guidance document (GD) No. 34. Together with the OECD GD No. 1 for the development of test guidelines, the document helps users and regulators to understand the benefits and limitations of a method. In addition, in 2018 the OECD published a guide to “Good in vitro Method Practice (GIVIMP)” in order to optimise the development process and improve the transfer of methods from the academic field to regulation. The EU has also developed eLearning modules in which the contents of the GIVIMP document are explained in more detail to interested scientists.
The validation process consists of 6 to 7 modules.
The first four modules serve to demonstrate reproducibility:
1. Definition of the Method
First, the method is sufficiently characterised and standardised in order to be able to plausibly demonstrate that the methodology is suitable for demonstrating a defined effect. This also includes the development of a “Standard Operating Procedure” (SOP).
2. Reproducibility Within a Laboratory
The SOP shows that the method can be reproduced in the developing laboratory.
3. Transferability
Another laboratory checks whether the SOP allows other laboratories to generate the same results as the developing laboratory.
4. Reproducibility Between Laboratories
In a ring study (2-3 laboratories) it is checked whether laboratories can achieve a comparable reproducibility of the results by testing (blinded) reference substances based on the SOP.
Modules 5 and 6 serve to demonstrate the relevance of the method
5. Predictive Capacity
Using a sufficient number of reference substances, another ring study demonstrates that the method is suitable for detecting potentially harmful effects on health. This includes the definition of sensitivity (reliability of detecting substances that have a defined activity) and specificity (reliability of detecting substances which do not have a defined activity).
6. Applicability Domain
Based on the experience from the characterisation of the test system and the testing of reference substances, limitations of the methodology with regard to physico-chemical properties (e.g. solubility) or substance classes whose effect may not be represented in the model system are defined.
In order to facilitate the validation of comparable, so-called me-too methods and therefore the availability of the methodology worldwide, performance standards are usually subsequently defined.
7. Definition of Performance Standards
A list of reference substances and quality criteria is developed that me-too methods must meet in order to be accepted without having to go through the entire validation process.
Particularly important for a successful validation is, among other things, the selection of suitable reference substances whose (existing or non-existing) effects are known from other alternative methods, historical animal test data or, if available, from humans and whose quality must be ensured. For the implementation of the validation it is also important that the relevant procedures of the participating laboratories correspond to GLP or comparable criteria. In addition, the selection of test substances, data collection and statistical evaluation should be carried out independently of one another. At the end, validation is assessed and confirmed by independent experts. Examples of successful validation studies but also of performance standards can be found on the OECD's “Series on Testing and Assessment” website.
On the OECD level it is currently discussed how the uptake new approach methodologies (NAM) in the tet guideline programme can be facilitated. A first workshop on this topic was held last year, the corresponding information can be assessed here.
At the same time, method developers should also be made aware that their methods can become part of a testing strategy and help to replace animal testing. Sufficient standardization is particularly important here. In various projects, criteria have been developed to determine the readiness of a method for validation or transfer to regulation. In addition, it is also important to promote the validation of suitable methods; a call by the OECD for funding can be found here.
