Exploiting the Innate Plasticity of the Programmed Cell Death‐1 (PD1) Receptor to Design Pembrolizumab H3 Loop Mimics**

Abstract Checkpoint blockade of the immunoreceptor programmed cell death‐1 (PD1) with its ligand‐1 (PDL1) by monoclonal antibodies such as pembrolizumab provided compelling clinical results in various cancer types, yet the molecular mechanism by which this drug blocks the PD1/PDL1 interface remains unclear. To address this question, we examined the conformational motion of PD1 associated with the binding of pembrolizumab. Our results revealed that the innate plasticity of both C'D and FG loops is crucial to form a deep binding groove (371 Å 3 ) across several distant epitopes of PD1. This analysis ultimately provided a rational‐design to create pembrolizumab H3 loop mimics [RDYRFDMGFD] into β ‐hairpin scaffolds. As a result, a 20‐residue long β ‐hairpin peptide 1 e was identified as a first‐in‐class potent PD1‐inhibitor (EC 50 of 0.29 μM; K i of 41 nM).