Design, Synthesis, Evaluation, and SAR of 5-Phenylisoindoline Derivatives, a Potent Class of Small-Molecule Inhibitors Targeting the Programmed Cell Death-1/Programmed Cell Death-Ligand 1 (PD-1/PD-L1) Interaction

A novel series of 5-phenylisoindoline derivatives were designed, synthesized, and evaluated for their activity to inhibit the interaction of PD-1/PD-L1 through the homogeneous time-resolved fluorescence assay. Meanwhile, structure-activity relationships were discussed according to both experiments and calculations. Several compounds exhibited potent activity with an IC₅₀ value less than 10 nM, especially <b>D6</b> (4.8 nM). <b>D6</b> could promote IFN-γ secretion and reduce the proportion of PD-L1 late apoptosis at 100 nM in the coculture model of peripheral blood mononuclear cells and hPD-L1-FC. Beyond this, the in vitro model showed <b>D6</b> could lead to the weakening of migration caused by the PD-1/PD-L1 axis. Furthermore, <b>D6</b> also displayed dose-dependent and low-toxic efficacy in the MC38 mouse tumor model with the tumor growth inhibition of 52.8% (20 mg/kg, ip) and 64.4% (160 mg/kg, i.g.). Mechanistic investigations suggested that <b>D6</b> could activate the immune microenvironment in the tumor. Thus, <b>D6</b> is a promising small molecule lead for blocking PD-1/PD-L1.