Selpercatinib combination with the mitochondria-targeted antioxidant MitoQ effectively suppresses RET –mutant thyroid cancer

Genetic alternation of REarranged during Transfection ( RET ) that leads to constitutive RET activation is a crucial etiological factor for thyroid cancer. RET is known to regulate mitochondrial processes, although the underlying molecular mechanisms remain unclear. We previously showed that the multi-kinase inhibitors vandetanib and cabozantinib increase the mitochondrial membrane potential (Δψ m ) in RET -mutated thyroid tumor cells and that this effect can be exploited to increase mitochondrial enrichment of Δψ m -sensitive agents in the tumor cells. In this study, we hypothesized that the RET-selective inhibitor, selpercatinib, can increase Δψ m and, subsequently, tumor cell uptake of the mitochondria-targeted ubiquinone (MitoQ) to the level to break the mitochondrial homeostasis and induce lethal responses in RET -mutated thyroid tumor cells. We show that selpercatinib significantly increased Δψ m , and its combination with MitoQ synergistically suppressed RET -mutated human thyroid tumor cells, which we validated using RET-targeted genetic approaches. Selpercatinib and MitoQ, in combination, also suppressed CCDC6-RET fusion cell line xenografts in mice and prolonged animal survival more effectively than single treatments of each agent. Moreover, we treated two patients with CCDC6-RET or RET M918T thyroid cancer, who could not take selpercatinib at regular doses due to adverse effects, with a dose-reduced selpercatinib and MitoQ combination. In response to this combination therapy, both patients showed tumor reduction. The quality of life of one patient significantly improved over a year until the tumor relapsed. This combination of selpercatinib with MitoQ may have therapeutic potential for patients with RET -mutated tumors and intolerant to regular selpercatinib doses.