On January 23, 2019, FDA issued final guidance on immunogenicity testing of therapeutic protein products. The guidance contains FDA’s nonbinding recommendations for developing and validating assays for anti-drug antibody (ADA) detection, “a key aspect of therapeutic protein product development.”
FDA advises sponsors to use a multi-tiered approach to detect ADAs. This begins with a sensitive screening assay to discover ADAs that bind to the product. A false-positive rate of approximately 5% is desirable for the screening assay to maximize detection of true positives. Next, a confirmatory assay eliminates false-positives identified in the screening assay by using a higher specificity and at least as good selectivity. If a therapeutic protein product has multiple functional domains, FDA recommends that sponsors perform screening and confirmatory tests against the whole product. ADAs identified from the screening and confirmatory assays can be further characterized by, for example, titration assays to quantify the ADA response or neutralization assays to assess the ADAs’ potential for inhibiting the product’s therapeutic activity.
For each assay, sponsors should consider the applicability of the following exemplary assay design elements:
- Assay Cut-Point: the level of response that is considered positive or negative.
- Sensitivity: the lowest concentration that produces a positive result or a result equal to the cut-point.
- Specificity: the ability for the assay to uniquely detect the target ADA.
- Selectivity: the assay’s ability to recognize target ADAs in a mixture of different components.
- Precision: the variability between measurements of the same material.
- Reproducibility: the ability for a separate laboratory to reach a comparable result.
- Robustness and Sample Stability: the effect of small variations in method and instrument performance on the assay.
- Selection of Format: weighing advantages and disadvantages to select an appropriate format.
- Selection of Reagents: understanding which standard components and which unique components to use for each assay.
- Reporting Results for Qualitative and Quasi-Quantitative Assays: selecting an appropriate approach for reporting positive antibody responses.
Once the characteristics of an ADA assay are selected, they should be validated through specific laboratory investigations. FDA recommends that sponsors submitting BLAs provide data demonstrating that the assays are fully validated. The guidance also provides recommendations related to implementing ADA assays and documenting results.