Design, synthesis and activity evaluation of tetrahydroisoquinoline-based programmed cell death ligand 1 inhibitors

Small molecule blockade of the programmed death receptor 1 (PD-1)/programmed cell death ligand 1 (PD-L1) pathway represents a promising approach for tumor immunotherapy. Based on the previously developed 3D-quantitative structure-activity relationship pharmacophore model for PD-L1 inhibitors, virtual screening followed by homogeneous time-resolved fluorescence (HTRF) activity testing identified the compound {1-[(biphenyl-4-yl)methyl]-7-methoxy-1,2,3,4-tetrahydroisoquinolin-6-yl}acetic acid methyl ester (Compound Y6) as a hit. Analysis of computational docking results of compound Y6 with PD-L1 suggested targeting the methyl acetate substituent at position 6 for optimization. Guided by this, 20 new derivatives were designed and synthesized. The synthesized compounds were subjected to HTRF test and 6 compounds with significant protein blocking effects were screened for subsequent surface plasmon resonance (SPR) test. Subsequent SPR analysis confirmed strong binding of these 6 compounds to hPD-L1 protein with K _D values ranging from 0.24 to 21.31 μM. Several derivatives displayed improved or comparable PD-L1 binding affinity relative to the lead compound Y6 (K _D = 11.3 μM). A co-incubation system (PD-1⁺ Jurkat T/PD-L1⁺ HepG2) was established to evaluate functional immune restoration. This evaluation revealed that compound Y7f effectively promoted HepG2 cells death by restoring T cell immune function. MD simulations identify (1R,16R)-Y7f as the most potent PD-L1 dimerization inducer within the compound Y7f stereoisomer series. The results indicated that the biphenyl-tetrahydroisoquinoline scaffold is a promising structural framework for developing novel PD-1/PD-L1 inhibitors and deserves further investigation.