Quantitative Determination of Endoplasmic Reticulum Viscosity during Immunogenic Cell Death by a Theranostic Rhenium Complex

The endoplasmic reticulum (ER) is an important targeting organelle for metal-based immunogenic cell death (ICD) inducers. Metal complexes can induce ER stress by causing protein misfolding, which can be reflected by alternations in microenvironmental parameters, including viscosity. We present here a theranostic Re(I) complex (Re1) that shows viscosity-dependent emission intensity and lifetime. Re1 can trigger immunogenic cell death (ICD) in MDA-MB-231 cells by localizing in the ER and causing ER stress. We demonstrate that Re1 can simultaneously induce and monitor the gradual increase in the ER viscosity quantitatively.