Checkpoint inhibitors have dramatically changed the field of immuno-oncology. Targeting PD-1 in particular hashad a profound effect in a variety of cancers, as a single therapy and in combinations [1]. Currently, four anti-PD-1 antibodies have been approved by the FDA -pembrolizumab, nivolumab, cemiplimab[2], and dostarlimab[3], with several others in development [4]. A model-based analysis comparing the level of target engagement (TE) these antibodies achieve at their efficacious doses can provide insights into their efficacy profiles and guidelines for future anti-PD-1’s in the discovery and development stages.