From anti-drug antibody incidence to clinical relevance: interpreting highly sensitive immunogenicity assays in biosimilars.

BACKGROUND: Highly sensitive anti-drug antibody (ADA) assays in biosimilar comparative trials can generate high apparent positivity clustered near decision thresholds without corresponding clinical impact. Using approved biosimilars of ustekinumab and denosumab, this work retrospectively examines alternate analytical strategies to improve the biological interpretability of ADA findings. METHODS: Clinical ADA datasets were re-analyzed using a variability-anchored minimum screening cut point (mSCP), magnitude-aware log S/N versus percent inhibition contour visualization, and subject-level classification into preexisting, treatment-boosted, and treatment-emergent categories. A subset of the data was also subjected to longitudinal change from baseline evaluation. RESULTS: Conventional tiered analysis produced overall screening sample positivity of 48-82% across all studies and subject-level ADA incidence >80% in 10 of the 11 arms (range, 64-100%), without any corresponding impact on pharmacokinetics, safety, or efficacy. The mSCP-based approach reduced cut point-adjacent noise-consistent signals, enabling subject-level classification that is clinically more relevant. Longitudinal analysis further separated persistent from transient responses. CONCLUSION: Biosimilar immunogenicity testing is a confirmatory exercise against an already-approved reference, not an investigative characterization. As regulatory frameworks move toward reduced reliance on comparative Phase 3 studies, identifying persistent ADA responses at Phase 1 becomes increasingly important; a fit-for-purpose, comparability-anchored analytical strategy warrants collective regulatory rethinking.Clinical trial registration: www.isrctn.com; identifier: ISRCTN1142400EudraCT Number: 2021-006668-25;ClinicalTrials.gov: NCT05335356 ClinicalTrials.gov: NCT05323708EudraCT number: 2021-006545-36, CT.gov number: NCT05345691.