Miniprotein engineering for inhibition of PD-1/PD-L1 interaction.

Miniproteins constitute an excellent basis for the development of structurally demanding functional molecules. The engrailed homeodomain, a three-helix-containing miniprotein, was applied as a scaffold for constructing programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) interaction inhibitors. PD-L1 binders were initially designed using the computer-aided approach and subsequently optimized iteratively. The conformational stability was assessed for each obtained miniprotein using circular dichroism spectroscopy, indicating that numerous mutations could be introduced. The formation of a sizable hydrophobic surface at the inhibitor that fits the molecular target imposed the necessity for the incorporation of additional charged amino acid residues to retain its appropriate solubility. Finally, the miniprotein effectively binding to PD-L1 (K_D = 51.4 nM) that inhibits PD-1/PD-L1 interaction in cell-based studies with EC₅₀ = 3.9 μM, was discovered.