(D)-PPA 1 is a programmed death protein 1/ programmed death-ligand 1 (PD-1/PD-L1) interaction inhibitor. (D)-PPA 1 binds to PD-L1 with a Kd of 0.51 µM and can inhibit interaction at a concentration of 1 mg/mL in flow cytometry experiments. (D)-PPA 1 is a