Abstract Blockade of the protein–protein interaction between the transmembrane protein programmed cell death protein 1 (PD‐1) and its ligand PD‐L1 has emerged as a promising immunotherapy for treating cancers. Using the technology of mirror‐image phage display, we developed the first hydrolysis‐resistant D ‐peptide antagonists to target the PD‐1/PD‐L1 pathway. The optimized compound D PPA‐1 could bind PD‐L1 at an affinity of 0.51 μ M in vitro. A blockade assay at the cellular level and tumor‐bearing mice experiments indicated that D PPA‐1 could also effectively disrupt the PD‐1/PD‐L1 interaction in vivo. Thus D ‐peptide antagonists may provide novel low‐molecular‐weight drug candidates for cancer immunotherapy.

Blocking of the PD‐1/PD‐L1 Interaction by a D‐Peptide Antagonist for Cancer Immunotherapy