Elesclomol rescues mitochondrial copper deficiency in disease models without triggering cuproptosis

Copper (Cu) is an essential cofactor for metalloenzymes such as cytochrome c oxidase (CcO), the terminal enzyme of the mitochondrial electron transport chain. Mutations that directly or indirectly prevent Cu transport to mitochondria result in lethal pediatric diseases, such as Menkes disease. There is no clinically approved treatment for Menkes disease. We recently discovered that an investigational chemotherapy drug, elesclomol (ES), when complexed with Cu (ES-Cu), rescues mitochondrial Cu deficiency, activates CcO, and prevents perinatal lethality in a mouse model of Menkes disease. However, ES-Cu also has the potential to trigger cuproptosis, a type of Cu-dependent cell death. Therefore, to develop ES-Cu as a therapeutic agent for Menkes disease, it is critical to determine the therapeutic index of ES-Cu in Cu-deficient models. To this end, we used a Cu-deficient rat cardiomyocyte cell line and a mottled-brindled mouse model of severe Menkes disease to determine the toxicity and efficacy of ES-Cu. Our cell culture studies demonstrated that the EC₅₀ of ES-Cu is ∼50-fold lower than IC₅₀. Moreover, the biomarkers of Cu toxicity, including lipoylated proteins and a subset of iron-sulfur cluster-containing proteins of mitochondria, are activated only when ES-Cu is used at ∼10-fold to 25-fold higher than its EC₅₀. Importantly, none of these biomarkers are activated in mottled-brindled mice treated with therapeutic doses of ES-Cu. Our study shows that ES-Cu can deliver Cu to CcO both in vitro and in vivo without triggering cuproptosis, a finding that could facilitate its use in Cu deficiency disorders, such as Menkes disease. SIGNIFICANCE STATEMENT: Genetic copper (Cu) deficiency causes lethal pediatric diseases, such as Menkes disease, which lacks approved treatment. Recently, the therapeutic potential of elesclomol (ES), a Cu-transporting chemotherapeutic drug, in a mouse model of Menkes disease has been reported. Because of the potential risk of Cu-induced toxicity from ES-Cu, it is crucial to determine its therapeutic index. Here, the biomarkers of ES-Cu efficacy and toxicity in Cu-deficient disease models were measured to demonstrate that ES-Cu can restore cuproenzymes without triggering toxicity biomarkers.