Abstract 3431: Dose range finding study in non-human primates confirms the unique mechanism of action of CTX-8371, a novel bispecific antibody blocking PD-1 and PD-L1 (2022)

Abstract Multiple antibodies blocking PD-1 or PD-L1 have been approved for clinical use. However, not all patients successfully respond to monotherapy and the need for combinations has become more apparent. CTX-8371 combines PD-1/PD-L1 targeting in one bispecific, tetravalent molecule. CTX-8371 potently blocks PD-1/PD-L1 in vitro and in vivo. This robust activity may be explained by the unique mechanism of action (MOA) of CTX-8371 which involves cell surface PD-1 cleavage. Here we present further in vivo evidence of PD-1 cleavage by CTX-8371 detected during a dose range finding (DRF) study in cynomolgus macaques. Results of this study as well as in vitro receptor occupancy (RO) and the PK/PD profile of CTX-8371 in tumor-bearing mice are described. CTX-8371 was administered to female cynomolgus monkeys (n=3/group) in a single intravenous infusion at 2 mg/kg or weekly infusions at 10 or 50 mg/kg, for a total of 2 doses. PBMCs were isolated before, 24 hours after the first dose, or 24 hours after the second dose (for 10 and 50 mg/kg groups). Immunophenotyping, including PD-1 and PD-L1, was performed on PBMCs by flow cytometry. Mean serum concentration-time data was analyzed to derive PK parameters using noncompartmental and two-compartment models. PK was also determined in tumor-bearing mice using an MSD ELISA-based assay and non-compartmental analysis. In monkeys, CTX-8371 administration resulted in a marked decrease of PD-1+CD4+ and PD-1+CD8+ T cells in PBMCs on days 2 and 9 at all dose levels, indicating PD-1 cleavage from the cell surface. These results confirm in vitro, and in vivo MOA data obtained from mouse studies. Clearance and half-life of CTX-8371 were calculated to be ~6.87 mL/day/kg and 5.45 days, respectively, which fall within the expected ranges for a human IgG1 antibody in non-human primates (NHP) with linear PK. CTX-8371 infusion was well tolerated in cynomolgus monkeys. In tumor-bearing mice, the dose which results in 50% of maximum tumor growth inhibition was found to be 1-2 mg/kg. The minimum effective dose, at which at least 50% of animals exhibit greater than 50% tumor growth inhibition over two consecutive days, was 1mg/kg. It was observed that 11% (1/9), 78% (7/9) and 100% (9/9) of mice in 0.1 mg/kg, 1 mg/kg and 10 mg/kg dose groups, respectively, meet the above criteria. CTX-8371 also showed dose-dependent in vitro RO on T cells from peripheral blood of human, cynomolgus monkey, and transgenic hPD-1hPD-L1 mice. Data presented here confirm the unique MOA of CTX-8371, cleavage of PD-1 from the cell surface. The dual blockade of PD-1 and PD-L1, combined with PD-1 cleavage, may offer superior benefit compared to current checkpoint inhibitor therapies. RO data as well as murine and NHP PK/PD analyses will guide a projected human dose for future development. Citation Format: Diana I. Albu, Xianzhe Wang, Yan Qin, Vivian Li, Purushothama Nanjappa, Amy Daniel Ulumben, Ruturaj Jadhav, Jason Kong, Austin Ablicki, Neal Schilling, Thomas Schuetz, Susan Kalled, Bing Gong, Nelly Kuklin. Dose range finding study in non-human primates confirms the unique mechanism of action of CTX-8371, a novel bispecific antibody blocking PD-1 and PD-L1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3431.