Copper ionophores drive divergent responses to immune checkpoint inhibition across colorectal tumor models.

Copper-binding agents can trigger immunogenic cell death (ICD) and modulate responses to immune checkpoint inhibitors (ICIs), but how tumor copper biology, agent chemistry, and tumor microenvironment shape these effects is unclear. We evaluated diethyldithiocarbamate [Cu(DDC)2] and clioquinol [Cu(CQ)2] in two syngeneic colorectal cancer models with distinct copper handling. In vitro transcriptomic and cytokine profiling showed compound-specific activation of cuproptosis and immune pathways. In vivo, copper accumulation and isotopic fractionation correlated with immune features of the tumor microenvironment, including immune cell infiltration and calreticulin exposure. In CT26 tumors, Cu(CQ)2 transiently enhanced programmed cell death protein 1 (PD-1) blockade, then promoted immune suppression with prolonged dosing. In MC38 tumors, combination therapy showed sustained antagonism or no added benefit. These data indicate that tumor-intrinsic copper handling and ionophore identity jointly determine immune dynamics and therapeutic outcome, and they suggest copper isotopic metrics as candidate biomarkers warranting further evaluation in the context of copper-based strategies with ICIs.