Investigation of pre-existing reactivity to biotherapeutics can uncover potential immunogenic epitopes and predict immunogenicity risk.

Despite recent advances in the development of tools to predict immunogenicity risk of biotherapeutic molecules, the ability of a protein to elicit the formation of anti-drug antibodies (ADA) remains one of the most common causes for termination of clinical development programs. In this study, we use ADA assays to detect and measure pre-existing reactivity or the ability of a molecule to produce an ADA-like response in serum from treatment-naïve, healthy donors. We report herein that the magnitude of pre-existing reactivity evaluated pre-clinically and expressed as the 90th percentile of Tier 2 inhibition correlates with the subsequent rate of ADA emergence in the clinic. Furthermore, a multi-domain biotherapeutic (IgG-scFv bispecific antibody) showed the highest pre-existing reactivity and incidence of treatment-emergent ADA (TE-ADA) (57% and 93%, respectively). Using the components of the multidomain molecule in the Tier 2 step of the ADA assay, we were able to identify the scFv as the target of the serum pre-existing reactivity. Most importantly, the domain specificity of pre-existing ADA was the same as that of the TE-ADA from patients treated with the molecule. Based on these data, we propose the evaluation of the magnitude and of the domain specificity of pre-existing reactivity as a powerful tool to understand the immunogenic potential of novel biotherapeutics.