The programmed cell death protein 1(PD-1)is an immune-checkpoint that negatively regulates the immune system and a key mechanism that tumors utilize to escape from immune surveillance.PD-1 antibodies can block the interaction of PD-1 with its ligands(PD-L1 and PD-L2),restore T cells activation,and elicit antitumor activity.In this paper,we reported a novel PD-1 monoclonal antibody(mAb)CS1003,which is a humanized IgG4 PD-1 mAb generated by conventional hybridoma technology,and currently being developed in multiple clinical trials as monotherapy or in combination with other anticancer agents.We showed that CS1003 bound to recombinant human,cynomolgus monkey,and mouse PD-1 with EC50 values of 0.1757,0.2459,and 0.3664 nM,respectively.CS1003 blocked PD-1 interaction with its ligands,dose-dependently enhanced T cell proliferation and secretion of cytokines(IL-2 and IFN-γ)to the levels comparable to the reference antibody pembrolizumab.Intraperitoneal administration of CS1003(0.1,0.5,2.5 mg/kg,once every 3 days)dose-dependently suppressed the growth of MC38-hPD-L1 colon cancer in hPD-1 knock-in mice.Pharmacokinetics(PK)study revealed a linear PK profile within the dose range of 2-18 mg/kg following single intravenous administration in cynomolgus monkey.These data provide a comprehensive preclinical characterization of CS1003 that supports its clinical development for cancer immunotherapy.