Identifying the therapeutic potential of niclosamide in overcoming IFN-gamma dependent cancer immune evasion in the tumor microenvironment.

Introduction: Tumor cells frequently develop immune resistance through interferon-γ (IFN-γ)-induced PD-L1 expression, acquisition of cancer stem cell (CSC)-like features, and adaptation to hypoxia within the tumor microenvironment (TME). Although IFN-γ activates both STAT1 and STAT3, how these pathways interact to regulate immune evasion under hypoxia remains unclear. Methods: Using the MC38 murine colorectal cancer model and T cell-tumor spheroid co-culture assays, we examined how IFN-γ signaling through STAT1 and STAT3 regulates PD-L1 expression, CSC plasticity, and cytotoxic T cell function under normoxic and hypoxic conditions. Pharmacologic inhibitors and siRNA-mediated knockdown were used to dissect pathway function. Niclosamide, an FDA-approved anthelmintic, was evaluated as a dual STAT1/STAT3 inhibitor. Results: IFN-γ primarily induced PD-L1 expression through STAT1 activation, whereas CSC plasticity was associated with STAT3 signaling. STAT1 and STAT3 displayed reciprocal regulation, whereby inhibition of one enhanced activation of the other. Niclosamide effectively inhibited phosphorylation of both STAT1 and STAT3, resulting in suppressed PD-L1 upregulation, reduced CSC enrichment, and partial inhibition of hypoxia-induced HIF-1α expression. In co-culture assays, Niclosamide enhanced T cell infiltration, reduced exhaustion under hypoxic conditions, and improved T cell-mediated tumor killing. Discussion: These findings identify Niclosamide as a potent dual STAT1/STAT3 inhibitor capable of reversing IFN-γ- and hypoxia-driven immune evasion. Repurposing Niclosamide may represent a promising strategy to enhance the efficacy of immune checkpoint blockade in solid tumors.