Structural characteristics of macrocyclic peptide as a potent modulator of PD‐1/PD‐L1 immune checkpoint axis

Blocking therapy targeted towards the immune checkpoint PD‐1/PD‐L1 pathway has proven to be a very effective approach for treatment of a number of tumor types and has become an important tool in immunotherapy harnessing therapeutic antibodies. Although successful, these applications face several constraints such as potential immunogenicity, poor solid tumor penetration or monoclonal antibody‐related toxicity for example immune‐related adverse events. Potential therapies involving several groups of small‐molecule inhibitors targeting immune checkpoints could completely avoid the aforementioned problems. Small‐molecules such as macrocyclic peptides act as PD‐1 agonists and not only exhibit high affinity and specificity of binding but also and even more importantly lack immunogenicity. Here, we report characterization of 3 rd class of macrocyclic peptide inhibitors capable of binding the distal extracellular domain of PD‐L1. Based on the crystal structure we determined the details of the macrocyclic peptide ligand‐PD‐L1 interactions revealing their hydrophobic and electrostatic nature. We also report sulfur binding mode between PD‐L1 and the ligand involving interaction between sulfur atom of methionine from PD‐L1 and phenyl aromatic ring of phenylalanine from the ligand which to date has been only scarcely described in the case of one of the macrocycles binding modes. Our results clearly indicate wide diversity among 3 rd class of macrocycle peptides.