Abstract 3137: The combination of anti-PD1 and a mouse analog of PYX-201, an antibody-drug conjugate targeting the extra-domain B splice variant of fibronectin (EDB+FN), shows greater anti-tumor efficacy than either treatment alone

Abstract Antibody-drug conjugates (ADC) are transforming cancer therapy, with some showing potential for enhanced efficacy with immunotherapy (Hoimes et al., 2023; Powles et al., 2024). PYX-201 is a first-in-concept, non-cellular ADC targeting EDB+FN, an extracellular matrix protein that is highly expressed in the stroma of various solid tumors but minimally present in normal adult tissues, making it an ideal drug delivery target. Currently, PYX-201 is being evaluated as a monotherapy in a phase 1 clinical trial for various solid tumors (NCT05720117). Preclinical studies have shown that PYX-201 exhibits broad anti-tumor activity across an array of patient-derived xenograft mouse models (Severe et al., 2024). PYX-201 operates through multiple mechanisms, including modulating the tumor immune compartment and increasing sensitivity to immunotherapy (Hooper et al., 2022). Combining PYX-201 with immunotherapy could be a potent strategy for tackling difficult-to-treat cancers. This study aims to demonstrate that combining a mouse analog of PYX-201 with anti-PD1 is more effective than either treatment alone in a syngeneic mouse breast tumor model. The mouse analog of PYX-201 was generated using the variable regions of the heavy and light chains of an anti-human EDB+FN monoclonal antibody (L19), fused to mouse IgG2a constant regions, and conjugated to an optimized Auristatin0101 payload via an mcValCitPABC linker, with an average drug-to-antibody ratio of 4 (Hooper et al., 2022). For efficacy studies, immunocompetent mice bearing subcutaneous EMT6 breast tumors were treated with the mouse analog of PYX-201 (3 mg/kg) and anti-PD1 (10 mg/kg), either alone or in combination. Tumor volume and body weight were measured bi-weekly, with tumors collected for pharmacodynamic analysis and immune profiling by flow cytometry and imaging. Treatment with either anti-PD1 or the mouse analog of PYX-201 resulted in significant tumor growth inhibition (TGI) and some complete responders (CR) compared to the vehicle control group (anti-PD1: 53.76% TGI, 3/15 CR; mouse analog of PYX-201: 65.87% TGI, 3/15 CR). Notably, the combination of the mouse analog of PYX-201 and anti-PD1 resulted in significantly greater tumor regression and clearance than either treatment alone (93.21% TGI, 8/15 CR). Flow cytometry and image analysis revealed an increased number of infiltrating lymphocytes post-treatment with the mouse analog of PYX-201. Further studies are underway to characterize changes in the activation status and tumor localization of these lymphocytes after treatment. Overall, these results provide compelling evidence that combining PYX-201 with anti-PD1 is a highly effective and promising strategy for improving outcomes in patients with difficult-to-treat cancers. Citation Format: Anthony Burt Rodriguez, Amanda Facklam, Matthew Iovino, Chuan Shen, Justin Trickett, Sara Lewandowski, Frank Wang, Marsha Crochiere, Nicolas Severe, Jan Pinkas. The combination of anti-PD1 and a mouse analog of PYX-201, an antibody-drug conjugate targeting the extra-domain B splice variant of fibronectin (EDB+FN), shows greater anti-tumor efficacy than either treatment alone [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 3137.