Abstract 1760: Preclinical characterization of XB404, a masked anti-ROR1/2 antibody-drug conjugate

Abstract Background: Receptor tyrosine kinase-like orphan receptors (ROR) 1 and 2 are single-pass transmembrane proteins that are part of the ROR family that mediates Wnt signaling. ROR1 and ROR2 are aberrantly expressed in various cancers, including lung, breast, ovarian, and endometrial cancers, and their expression can be associated with poor disease outcomes. XB404, a masked anti-ROR1/2 antibody-drug conjugate (ADC) developed using the Adagene masking platform and the SMARTag® ADC platform, is designed to deliver a cytotoxic payload to ROR1/2-expressing tumors while minimizing on-target, off-tumor side effects. XB404 is composed of a tandem-cleavage topoisomerase 1 inhibitor-based linker-payload conjugated to a masked monoclonal antibody that binds to both ROR1 and ROR2 with high affinity. Here, we describe the preclinical characterization of XB404, including its in vitro cytotoxicity and internalization and in vivo efficacy in cell line-derived xenograft (CDX) and patient-derived xenograft (PDX) models. Methods: In vitro cytotoxicity of XB404 was evaluated in ROR1- and ROR2-overexpressing cell lines using the CellTiter-Glo® luminescent cell viability assay. In vitro internalization for the unmasked XB404 antibody was assessed in ROR antigen-expressing MDA-MB-231 tumor cells using flow cytometry. A rat pharmacokinetics (PK) study at 5 mg/kg was used to assess and compare PK properties for a non-masked ROR1/2 ADC and XB404. In vivo tumor growth inhibition was evaluated in Jeko-1 and MDA-MB-231 CDX models using XB404 at 7.5 mg/kg in a QWx2 dose schedule. Tumor growth inhibition was assessed in non-small cell lung cancer (NSCLC) and triple-negative breast cancer (TNBC) PDX models using 1, 3, 6, and 10 mg/kg dose levels. Results: Unmasked XB404 displayed potent in vitro cytotoxic activity, and unmasked XB404 antibody demonstrated internalization properties. XB404 demonstrated improved clearance and half-life compared with the non-masked ROR1/2 ADC in rats. In addition, overlapping total antibody and total ADC curves for XB404 indicated good in vivo stability. In vivo antitumor activity was observed in the Jeko-1 and MDA-MB-231 xenograft models. XB404 demonstrated dose-related tumor growth inhibition and improved survival in both NSCLC and TNBC PDX models. Tumor regression was observed in both PDX models and complete responses were observed in the TNBC PDX model. Conclusions: XB404 demonstrated in vitro cytotoxicity and internalization, in vivo stability, and in vivo efficacy across multiple CDX and PDX models. Taken together, these preclinical results support further development of XB404. Investigational New Drug-enabling studies are ongoing. Citation Format: Kathleen R. Gogas, Christine M. Janson, Hui Zhao, Fang Wang, Bee-Cheng Sim, Brian A. Mendelsohn, Penelope M. Drake, Robyn M. Barfield, Thomas Linz, Maxine Bauzon, Dharmaraj Samuel, Minjong Park, Inna Vainshtein, Jackie Cheng, Seema Kantak. Preclinical characterization of XB404, a masked anti-ROR1/2 antibody-drug conjugate abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1760.