Combined Activity of the Redox-Modulating Compound Setanaxib (GKT137831) with Cytotoxic Agents in the Killing of Acute Myeloid Leukemia Cells  
pathologies, including _Diabetes mellitus_, neurodegenerative diseases, ischemia-reperfusion injury, e.g., related to ischemic stroke and chronic inflammatory conditions such as chronic obstructive pulmonary disease (COPD) [1; 2; 3]. High levels of reactive oxygen species (ROS) have also been observed in cancer cells of tumors of different origin and have, in several cases, been causally linked with malignancy. For example, ROS-associated damage to mitochondrial DNA has been linked to breast cancer progression [4]. To enable cell survival in the presence of high ROS levels, elevated activity of ROS detoxifying mechanisms is likewise common in cancer cells. Two different pharmacological strategies have been attempted to exploit these features for a selective cancer therapy: Inhibition of ROS formation, using antioxidants or--more specifically--inhibitors of ROS-generating enzymes, or promotion of ROS production to cytotoxic levels. Until now, none of these approaches has advanced to clinical use. However, cytotoxic activity of several established cancer drugs has been partially linked to the promotion of excessive ROS formation [5; 6; 7]. Acute myeloid leukemia (AML) is a heterogenous cancer entity based on rather different genetic traits [8; 9]. It has often unfavorable prognosis, related to its occurrence at higher age, therapy-limiting comorbidities and adverse cytogenetics. The commonly used 'induction chemotherapy' involves treatment with anthracyclines (daunorubicin, idarubicin or mitoxantrone) and the antimetabolite cytarabine, followed by either consolidation chemotherapy or, if indicated, allogeneic stem-cell transplantation [10; 11]. A subset of 25-30% of patients are diagnosed with a mutation in the gene encoding Fms-like tyrosine kinase 3 (FLT3), rendering FLT3 constitutively active. The predominant mutation-type is described as 'internal tandem duplications (ITD)' of amino acid sequence in a region of the kinase which, in the physiological setting, regulates activity negatively. Presence of the ITD mutation causes strong FLT3 activation and mislocalization into intracellular compartments such as the ER. FLT3-ITD mutations are associated with an aggressive disease and particularly poor prognosis [12; 13; 14; 15; 16]. This prompted the development of FLT3 tyrosine kinase inhibitors, of which several have been recently approved for clinical treatment of AML with FLT3 mutations [17; 18]. The first approved inhibitor midostaurin represented one of the first targeted therapies for AML [19]. However, as 5-year overall survival rates for AML continue to be below 40%, especially in the elderly, there is a clear need for new therapeutic options. Over the last few years, several groups including our own, have demonstrated elevated ROS levels in AML cells, leading to the speculation that altered ROS metabolism may permit novel therapeutic approaches [20; 21; 22; 23; 24]. In FLT3-ITD-positive cells, part of the ROS elevation is driven by FLT3 activity, involving a heightened level and activity of the ROS-producing enzyme NADPH-oxidase 4 (NOX4) [25]. We therefore considered the possibility that inhibition of NOX4 activity may be a strategy of targeting FLT3-ITD-positive AML cells. Setanaxib (GKT137831) has initially been described as a dual NOX1/4 inhibitor with potency in the submicromolar range [26]. Because NOX4 has been established as a mediator of different chronic inflammatory and fibrotic pathologies, the orally available compound has been assessed in vivo for the treatment of related diseases including fibrotic kidney disease, idiopathic pulmonary fibrosis, and primary biliary cholangitis [27]. Even rather high doses of Setanaxib are well tolerated in animals and humans, and clinical trials with Setanaxib for treatment of primary biliary cholangitis have advanced to stage II/III (ClinicalTrials.gov identifier: NCT05014672).