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. Copper ionophores combined with anti–PD-1 therapy were evaluated in colorectal cancer models exhibiting distinct copper handling states. Model-specific differences in copper accumulation and isotopic composition were associated with distinct immune and tumor growth responses over time, interpreted within each tumor–host system. • Anti-PD-1 enhancement by copper depended on the chemical identity of ionophores. • Copper delivery in models with different copper profiles raised or reduced immunity. • Plasma and tumor copper after treatment were associated with model specific response. • Isotopic copper ratio was identified as a potential biomarker for anti-PD-1 response.