Cysteine-reactive covalent chloro-N-acetamide ligands induce ferroptosis mediated cell death

Covalent inhibitors are an attractive targeting strategy that has expanded the development of degraders to target poorly druggable proteins including the E3 ligase RNF4. We show that RNF4 is a potential vulnerability of AML. High RNF4 expression levels correlate with poor patient survival and depletion of RNF4 results in increased sensitivity of AML cells to antileukemic drugs. Therefore, we aimed to develop chemical degraders (PROTACs) of RNF4 using a known covalent RNF4 ligand (CCW16), containing a chloro-N-acetamide group, as well as established E3 ligands targeting CRBN or VHL. However, while CCW16 and CCW16-derived PROTACs react potently with cysteines in recombinant RNF4, in cells, CCW16 forms covalent bonds with a large number of proteins, including peroxiredoxins. Consequently, CCW16 based PROTACs do not trigger degradation of RNF4, but induce the ferroptosis marker heme oxygenase-1 and impair cell viability in a distinct, RNF4-independent, ferroptotic cell death pathway. We hypothesize that other chloro-N-acetamide-containing E3 ligase ligands would also induce ferroptosis. Indeed, the RNF114 ligand EN219 also strongly induces ferroptosis, suggesting that ligands harboring this electrophile induce undesired off-target toxicity.