Background: CBT-501 (Genolimzumab, GB-226) is a novel humanized mAb (IgG4k) against human Programmed Death-1 (PD-1). CBT-501 demonstrated highly specific binding to PD-1 of human (K_D= 505 pM) and cynomolgus (K_D= 7.2 nM). It did not completely block the binding of nivolumab or pembrolizumab, suggesting a novel epitope. CBT-501 efficiently inhibited the binding of PD-L1/L2 to PD-1 through a competitive action for both human and monkey. It enhanced human T-cell activation in the Mix Lymphocyte Reaction (MLR), as shown by increased release of IL-2 and INF-γ. The Fc-fragment bound to FcRn dose-dependently without any effector function. The PK/PD studies in cynomolgus showed a linear dose exposure, with a T_1/2of 115-142 h for single and repeat dosing. The PD-1 receptor occupancy after a single dose was dose-dependent reaching 100% at peak, maintained after multiple dosing and continued for 6 wk after last dose. Single and repeat dose toxicology studies in cynomolgus showed weight gain of the thyroid and mild expansion of the thyroid follicles at the highest dose of 100 mg/kg which were reversible. No abnormal drug-related toxicity was found. In rhesus vaccinated with adenovirus-mediated Simian Immunodeficiency Virus, biweekly IV CBT-501 promoted the antigen-specific T-cell response, as shown by ELISPOT and intracellular staining of IL-2, INF-γ and TNF-α in PBMC and T-cell subpopulation. Due to lack of immunoselectivity to mouse PD-1, investigation of the in vivo anti-tumor activity in rodent model is challenging. Methods: Crown Bioscience has developed humanized genetically engineered mouse model expressing human PD-1 extracellular domain with a fully functional murine immune system. Subcutaneous colon adenocarcinoma MC38 syngeneic tumor draft was established in mice. Results: CBT-501 injected IP q2w×3wk demonstrated significant anti-tumor activity. The tumor growth reduction was dose dependent with comparable or improved activity over nivolumab, and with the highest inhibition rate at 83%. Conclusions: Based on the strong nonclinical data and differentiation, and an acceptable safety profile, a first in human Phase 1 trial is planned in the Asia-Pacific region.