An NIR‐Fluorophore‐Based Therapeutic Endoplasmic Reticulum Stress Inducer

: The endoplasmic reticulum (ER) stress signaling or unfolded protein response (UPR) is a common feature of many human diseases, including cancer. Excessive activation of ER stress directly induces cell death, holding a new promising strategy for the therapeutic intervention of cancer. Current ER-stress-inducing agents mainly target UPR components or proteasomes, which exert limited treatment efficacy and undesired side effects due to unselective ER stress and poor tumor-specific distribution. In this study, a unique near-infrared (NIR) fluorophore, IR-34, is synthesized and identified to selectively and efficiently trigger tumoricidal ER stress by targeting the mitochondrial protein NDUFS1. IR-34 is demonstrated to specifically accumulate in living cancer cells for tumor NIR imaging and drastically inhibit tumor growth and recurrence without causing apparent toxicity. Thus, this multifunctional NIR fluorophore may represent a novel theranostic agent for tumor imaging-guided treatment and also strengthens the idea that mitochondria could be a useful target for therapeutic ER stress in cancer cells.