Development of Novel Small Antitumor Compounds Inhibiting PD-1/PD-L1 Binding

Background/aim: Anti-programmed death-1 (PD-1)/PD-ligand 1 (PD-L1) antibody is a successful treatment for patients with solid cancers; however, there are several disadvantages that need to be resolved. Oral small molecule anti-PD-1/PD-L1 inhibitors have been developed and have good bioavailability.

Materials and methods: Potent anti-PD-1/PD-L1 inhibitor candidates from the Shizuoka small compound library were screened and investigated for their antitumor activities in vitro and in vivo using a humanized mouse model. A search for small compounds that inhibit PD-1/PD-L1 binding among 67,395 compounds through three rounds of screening procedures identified six compounds.

Results: The two compounds (SCL-1 and SCL-2), which have as a key chemical structure of triazolopyridazin backbone with a piperazine residue on the aromatic ring and 1,3-diphenyl pyrazoline with hydrazinylphthalazine were selected based on in vitro assays and absorption, distribution, metabolism, and excretion (ADME) scoring and subjected to in vivo experiments using a humanized NOG mouse model. SCL-1 and SCL-2 exhibited moderate inhibitory activities against PD-1/PD-L1 binding compared to an anti-PD-1 antibody, with SCL-1 exerting markedly weaker cytotoxic effects on target cells than the other compounds. In in vivo experiments, SCL-1 exerted significant antitumor effects on PD-L1⁺ SCC-3 tumors, which were dependent on CD8⁺ T cell infiltration and PD-L1 expression in tumors. A pharmacokinetic study revealed that it has good bioavailability and distribution as an oral reagent.

Conclusion: SCL-1 is a novel small compound that inhibits PD-1/PD-L1 binding and exerts potent antitumor effects. Thus, it has potential as an oral reagent for cancer immunotherapy.