Fit-for-purpose assay

Some examples of in vitro assays for immunotoxicity assessment follow (Table 13.1), but this list is not intended to be comprehensive. It should be borne in mind that this is a rapidly growing area of research, and many of the assays employed in immunotoxicity or immunosafety evaluation of pharmaceuticals are fit-for-purpose assays based on mechanism or target and are beyond the scope of this chapter. Evaluation of effects on individual cell types and mechanisms (e.g., neutrophil, macrophage, cytotoxic T lymphocyte, and NK function) will not be discussed, as these are rarely employed unless driven by STS findings or intended mechanism of action. 

System suitability Use of appropriate assay controls to determine if a particular run is valid. May also include assay fit-for-purpose evaluations, such as stability indicating methods, etc. 

TABLE 2.1. Recommendations to Engineer Quality into Fit-for-Purpose Bioanalytical Assay Characterization in Drug Discovery... 

A variety of biomarkers have been shown to be valuable individually for one or several toxicant or disease situations. Few of these biomarkers have been systematically evaluated for the plethora of situations that might provoke false positive responses. Acceleration of the current pace of biomarker evaluation and qualification demands (a) the availability of panels of biomarker-assays that can be comparatively evaluated on well-defined common sample sets, (b) fit-for-purpose performance evaluation in controlled animal studies with carefully benchmarked histological endpoints and samples from well-defined focused clinical trial cohorts, and (c) ready availability of banked blood and urine sample archives from clinical trial populations with carefully documented morbidities such as the Framingham Heart Study,45 or the Drug-Induced Liver Injury Network (DILIN) prospective study,46 to name a few. Availability of such panels of validated biomarker assays and well-documented preclinical and clinical samples, as well as increased cooperation between animal model researchers and clinical researchers will enable individual biomarkers to be qualified for sensitivity of specifically defined adverse events, qualified for appropriate specificity using samples of defined benign events, and collected into panels that yield complementary information about the health and safety of animals and patients. 

FIGURE 6.1 Conceptual diagram of fit for purpose biomarkers method validation. The method validation processes include four activity circles prevalidation (preanalytical consid eration and method development), exploratory method validation, in study method validation and advanced method validation. The processes are continuous and iterative, dictated by the purpose of the biomarker application. The solid arrows depict the normal flow of biomarker development (prevalidation), method validation (exploratory or advanced), and application (in study method validation). The process could include moving the chosen biomarkers from exploratory mechanistic pilot studies to advanced validation and confirmatory studies, or from exploratory validation to advanced validation after changes in critical business decision. The broken arrows represent scenarios where validation data do not satisfy study requirements, necessitating assay refinement or modification. 

Careful understauding of the effects of each preanalytical variable on the biomarker data is complex and necessitates a staged approach conceptually similar to the fit-for-purpose analytical validation of a biomarker assay (see Chapter 41). In the early exploratory phase of biomarker investigation, standardization of procedures with a defined protocol for sample collection and handling will permit comparative interpretation and analysis of the data within study and/or between studies. Minimally, variables that should be experimentally evaluated to optimize sample collection for a specific biomarker will include matrix type, preservation... 

BEST PRACTICES IN NOVEL BIOMARKER ASSAY FIT-FOR-PURPOSE TESTING... 

In deciding on uncertainty tolerances the fitness for purpose criterion becomes crucially important. For example, when developing and validating a bioanalytical method, if the achievable accuracy and precision lead the analyst to conclude that the current method is not suitable relative to the original assay requirements, the method in its current form might be discarded altogether, used as the basis for further development of a method that does satisfy the specific criteria, and/or retained as a useful method that can be applied to studies conducted in a less demanding environment. 

Elucidation of how the general principles underlying the concept of validation should be expressed in practice is an evolving process, as exemplified by the ongoing evolution of validation requirements for bioanalytical assays in the pharmaceutical industry (Shah 1992, 2000 FDA 2001 Viswanathan 2007). The complementary principle of fitness for purpose (Section 9.2) applies not only to the assay method but also to the validation process itself. Procedures that are considered to be fit for purpose in validation of an analytical method to be used in drug development, for example, need not necessarily apply to, e.g., methods used to screen pesticide residues in foodstuffs. As noted in Section 9.2, this point of view appears to be consistent with the definition of validation applied to all measurements (ISO 1994) Validation Confirmation by examination and provision of objective evidence that the particular requirements for a specified intended use are fulfilled. Of course, some basic principles are common to all validation schemes. 

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