Off-target reactivity in clinical monoclonal antibodies

Biologics, including monoclonal antibodies (mAbs), are widely used therapeutics due to their high specificity, yet off-target interactions remain an underappreciated risk for safety and efficacy. To systematically assess antibody specificity, we applied rapid extracellular antigen profiling (REAP) to evaluate 174 FDA-approved and clinical-stage antibodies against 6,172 human extracellular proteins. We found a substantial burden of off-target reactivity, with 28% of antibodies exhibiting at least one off-target hit. Structural and biophysical analyses revealed that off-target binding arises from antibody intrinsic properties and epitope mimicry, either within related protein families or across structurally unrelated proteins. We further identified new off-target interactions of tanezumab and engineered its variable domains to eliminate off-target binding while preserving target affinity and developability. These findings highlight the prevalence of specific off-target reactivity in therapeutic antibodies and underscore the importance of evaluating specificity early in biologic drug development.