Cell-Permeable Morpholino Inhibits Programmed Death-Ligand 1 at mRNA Level and Potentiates Antitumor Immunity in Breast Cancer

Abstract Morpholino-based ASOs with nuclease-resistant backbone and high sequence specificity could modulate the PD-L1/PD-1 axis to overcome T regulatory cell mediated inhibition but poor cell-permeability limits their clinical applications. Here, we develop a guanidinium-modified cell-permeable morpholino specifically targeting PD-L1 in breast cancer. It suppresses PD-L1 expression through effectively mitigating cytokine induced PD-L1 upregulation and enhancing autophagic flux. Notably, the ASO synergistically with durvalumab (anti-PD-L1 antibody) disrupt, hyperglycosylated PD-L1 by mRNA-level interference and also reduced PD-1⁺ T-cell populations. Importantly, systemic and intra-tumoral administration of ASO demonstrated no abnormalities in cardiac tissues which show no off-target toxicity but it reprogrammed the tumor microenvironment by upregulating inducible nitric-oxide synthase and suppressed regulatory cells, collectively enhancing antitumor immune responses. Together, these findings establish morpholino ASO as a promising targeting therapeutic strategy to overcome tumor-induced immunosuppression, opening new avenues for precision treatment in breast-cancer.