Moser, Justin

Abstract Background: Leukocyte immunoglobulin-like receptor B2 (LILRB2) suppresses immune cell activation, particularly within myeloid cells in the tumor microenvironment (TME), while the PD-1/PD-L1 axis serves as a key immune checkpoint that inhibits T-cell function. SPX-303 is a novel bispecific antibody designed to simultaneously block LILRB2 and PD-L1, thereby revitalizing antitumor immune responses. Here, we present initial findings from the ongoing first-in-human Phase 1 trial of SPX-303 in patients with advanced solid tumors, including safety, pharmacokinetics (PK), and receptor occupancy (RO). Methods: This is an open-label, multicenter Phase 1 study for patients with advanced solid malignancies with 3+3 dose-escalation design, followed by an expansion cohort at the recommended Phase 2 dose (RP2D). SPX-303 is administered intravenously every three weeks and safety is assessed through CTCAE v5.0 criteria. Primary Objectives of this study are to determine the safety, tolerability, and RP2D of SPX-303. Secondary endpoints include antitumor activity as well as characterization of the PK and PD of SPX-303. This study is currently open and enrolling in escalation cohorts. Citation Format: Justin Moser, Hao Xie, Yujie Zhao, Yousef Zakharia, Dongdong Li, Brandon William, Dinh-Duc Nguyen, Jack Hovey, Tiffany Li, Robert Cai, Jichun Ma, Jingdong Qin, Martin Siekierzycki, Anthony Haight, Hua Jin, Weiwei Mao, Kevin Bin Li, Gui-Dong Zhu. A first-in-human phase 1 study on the safety, PK, receptor occupancy, and pharmacodynamic markers of SPX-303, a first-in-class LILRB2×PD-L1 bispecific antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr CT116.