Abstract 2936: Preclinical characterization of XB371, a novel anti-tissue factor antibody-drug conjugate

Abstract Background: Tissue factor (TF) is a transmembrane glycoprotein that plays an important role in the extrinsic coagulation cascade. TF is aberrantly expressed in various cancers, and its expression is generally associated with poor disease outcomes. XB371 is an anti-TF antibody-drug conjugate, developed using the SMARTag® platform, and is designed to deliver a cytotoxic payload to TF-expressing tumors while minimizing adverse events related to the disruption of TF function, notably bleeding. XB371 is composed of a tandem-cleavage topoisomerase inhibitor-based linker-payload conjugated to a monoclonal antibody that binds to TF with high affinity and does not interfere with the clotting cascade.1 Here, we describe the preclinical characterization of XB371, including its in vitro cytotoxicity, bystander, and immunogenic cell death (ICD) activity, and in vivo efficacy in cell line-derived xenograft (CDX) and patient-derived xenograft (PDX) models. Methods: In vitro cytotoxicity of XB371 was evaluated in HPAF-II, BxPC3, and A431 cells using a luminescence assay. To evaluate bystander effect, the potency and maximal effect of XB371 on the viability of non-TF-expressing Jurkat cells were compared for a mono-culture of Jurkat cells and co-cultures with TF-expressing MDA-MB-231 cells. ICD activity was assessed by measuring immunostimulatory molecule release (HMGB1) in A431 cells, and monocyte activation and cytokine release in tumor cell/PBMC co-cultures following XB371 treatment. Tumor suppression was evaluated in HPAF-II, BxPC3, and A431 CDX models using a single dose level or a range of doses of XB371. Tumor growth inhibition was assessed in colorectal cancer (CRC) and non-small cell lung cancer (NSCLC) PDX models using a single dose of XB371 (10 mg/kg intravenously). Results: XB371 displayed potent in vitro cytotoxic activity at subnanomolar concentrations. XB371 demonstrated in vitro bystander cell-killing effect and induced ICD in all assays evaluated. In vivo anti-tumor activity was observed following a single intravenous dose in the HPAF-II, BxPC3, and A431 xenograft models. In the HPAF-II CDX model, XB371 demonstrated dose-dependent anti-tumor activity, with significant increases in median survival and tumor volume quadrupling time at higher doses. A single dose of XB371 inhibited tumor growth in both CRC and NSCLC PDX models. Tumor regression and complete responses were observed in the NSCLC PDX model. Conclusions: XB371 demonstrated in vitro cytotoxicity, bystander, and ICD activity, and in vivo efficacy across multiple xenograft cell lines and PDX models. Taken together, these preclinical results support further development of XB371; IND-enabling studies are ongoing. Reference: 1. Theunissen JW, et al, Mol Cancer Ther. 2018;17:2412-2426. Citation Format: Kathleen Gogas, Brian A. Mendelsohn, Marlene Hennessy, Hui Zhao, Jeffrey Higaki, Maxine Bauzon, Fangjiu Zhang, Stepan Chuprakov, Robyn Barfield,2 Penelope Drake, Seema Kantak. Preclinical characterization of XB371, a novel anti-tissue factor antibody-drug conjugate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2936.