PD-L1 targeted antibody-polymer-Epirubicin conjugate prolongs survival in a preclinical murine model of advanced ovarian cancer

Following successful design of polymer enhanced rituximab-epirubicin (EPI) conjugates targeted to non-Hodgkin lymphoma (Zhang et al. 2017), we developed U6244-051 that consists of anti-PD-L1 antibody (αPD-L1) and semitelechelic N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-epirubicin (EPI) conjugates (ST-P-EPI); the latter is attached to αPD-L1 via Cu-free azide/alkyne cycloaddition. This new polymer-enhanced antibody-drug conjugate (pADC) not only exhibits a high drug-to-antibody ratio (DAR ∼ 30-40) but also integrates immune checkpoint blockade with long-lasting immunogenic anticancer chemotherapy, providing an innovative chemo-immuno combination modality. The biological properties of U6244-051 were evaluated using ID8-Luc murine ovarian cancer cells in vitro and in vivo. In vitro, U6244-051 treatment induced immunomodulatory changes, including upregulation of calreticulin, PD-L1, and MHC I, suggesting enhanced tumor cell visibility to the immune system. In vivo efficacy was assessed in a syngeneic murine model (C57BL/6J mice inoculated with 5 × 10⁶ ID8-Luc cells/mouse). U6244-051 treatment resulted in 100 % survival at day 100, despite initiation at an advanced disease stage. Treatment modulated the tumor immune microenvironment by reducing immunosuppressive populations (TAMs and MDSCs) and enhancing T cell recruitment and activation. A decrease in PD-L1 expression and upregulation of MHC I correlated with enhanced immune-mediated tumor clearance. Additionally, reduced Treg levels and increased CD8⁺ T cell activation contributed to a more effective antitumor response. Repeated dosing amplified immunomodulatory effects, leading to durable immunity. These results highlight U6244-051 as a next-generation pADC with high translational potential, offering enhanced efficacy and reduced on-target, off-tumor toxicity.