Targeting dihydroorotate dehydrogenase in acute myeloid leukemia

With their discovery of a promising inhibitor of dihydroorotate dehydrogenase (DHODH), Wu and Wong et al. have made progress toward effective differentiation therapy for acute myeloid leukemia (AML).1 Their paper entitled “Pharmacological inhibition of dihydroorotate dehydrogenase induces apoptosis and differentiation of acute myeloid leukemia cells,” published in this issue of Haematologica, reports that this inhibitor had significant anti-leukemia efficacy in vitro and in vivo.1

DHODH is an enzyme that catalyzes the oxidation of dihydroorotate into orotate in intracellular de novo pyrimidine synthesis. Pyrimidine bases are essential for cellular metabolism and cell growth, and are considered important precursors in nucleotide, glycoprotein, and phospholipid biosynthesis and nucleotide recycling.2 Dysregulation of and functional dependency on pyrimidine biosynthesis have been found in diverse solid tumors.3 As a key enzyme regulating this process, DHODH has been identified as a synthetically lethal target in tumors that carry specific genetic mutations, including PTEN-deficient and triple-negative breast cancer,54 BRAF–mutant melanoma,6 RAS/LKB1 double-mutant lung cancer,7 and KRAS-mutated pancreatic cancer.8

AML is a highly heterogeneous hematologic malignancy, characterized by uncontrolled growth of immature hematopoietic stem/progenitor cells and impaired differentiation.109 The long-term survival rate of AML patients who receive standard therapies remains poor.11 Differentiation therapy was proposed decades ago with the goal of promoting the normal process of hematopoietic maturation from self-renewing progenitors to terminally differentiated effector cells,12 but its success has been limited except in specific genetic subtypes of AML. For example, all-trans retinoic acid induces differentiation and remissions in patients with acute promyelocytic leukemia.13 Inhibitors of isocitrate dehydrogenase (IDH) have been approved more recently for treatment of AML carrying IDH1/2 mutations, and the activity of these inhibitors is associated with clinical and morphological signs of myeloid differentiation.14

Recent pre-clinical findings by Sykes et al. showed that DHODH is a metabolic regulator in the pyrimidine synthesis pathway and a new metabolic target in differentiation therapy for AML.15 Inhibition of DHODH effectively enabled cell differentiation and had anti-leukemia efficacy in vitro and in vivo.15 This discovery opens up a new perspective in differentiation therapy for AML carrying complex and heterogeneous combinations of gene mutations. It leads to pharmacological intervention via development of novel, potent, and optimized DHODH inhibitors for clinical applications and to studies aiming to determine the mechanisms underlying DHODH inhibition–mediated AML differentiation.

In this issue of Haematologica, Wu and Wang et al. confirm the essential role of DHODH in AML survival and differentiation.1 Using CRISPR-Cas9–mediated gene knockout, the authors observed induction of apoptosis and cellular differentiation in DHODH-knockout AML cells. Through analysis of RNA sequencing data for 173 primary AML samples, they found that DHODH and MYC, a key regulator of myeloid cell proliferation and differentiation, are coordinately expressed in AML. Using structure-based virtual screening of 337 natural products, the research team identified isobavachalcone, a chalcone derived from the traditional Chinese medicine Psoralea corylifolia, as a potent and selective inhibitor of DHODH. Isobavachalcone, a competitive inhibitor of CoQ₀ and a noncompetitive inhibitor of the DHODH substrate dihydroorotate, binds the docking site of the “ubiquinone channel” of the DHODH complex, interacting structurally with and stabilizing DHODH. Treatment with isobavachalcone led to dose-dependent induction of apoptosis and myeloid differentiation in the cell lines tested. The on-target effect mediated by isobavachalcone’s inhibition of DHODH is supported by the findings that uridine, a downstream metabolite of DHODH in the pyrimidine synthesis pathway, was depleted in isobavachalcone-treated cells and that growth inhibition was rescued by uridine supplementation.1 Mechanistically, the apoptosis and cell differentiation induced by isobavachalcone’s inhibition of DHODH was found to occur, at least in part, through c-MYC suppression, by directly reducing its gene transcription and degrading c-MYC protein via proteasome dependent degradation and downregulation of the enzyme O-linked N-acetylglucosamine transferase that normally stabilizes MYC via transfer of GlcNAc from uridine diphosphate-GlcNAc to MYC.1716 Notably, O-GlcNAc post-translational modification similarly stabilizes additional proteins such as AKT and the TET family of proteins.18 In a subcutaneous AML cell line xenograft model, isobavachalcone treatment suppressed tumor growth, and this effect was accompanied by inhibition of DHODH activity and induction of apoptosis of tumor cells.1 Oral administration of isobavachalcone was well tolerated in mice. A combination of isobavachalcone and adriamycin (doxorubicin), an agent widely used in AML therapy, synergistically induced cell death and restored sensitivity of doxorubicin-resistant AML cells in vitro; this combination also led to cell differentiation and prolonged mouse survival in vivo in a systemic AML cell line xenograft leukemia model, analogous to the report of the efficacy of doxorubicin combined with the approved DHODH inhibitor leflunomide in triple-negative breast cancer.5 These data support the idea that isobavachalcone, an orally active DHODH inhibitor, might have clinical benefit in AML differentiation therapy.

The work by Wu and Wang et al. advances our knowledge of DHODH regulation in AML and offers a novel inhibitor with promise for differentiation therapy. The detailed mechanisms underlying this activity, such as how the functional interplay between DHODH and c-MYC in AML triggers differentiation and whether this is dependent on cellular context, is worthy of future exploration. Importantly, the capacity of isobavachalcone to facilitate blast differentiation in patient-derived xenografts and AML patients with different cytogenetic abnormalities needs to be addressed. In studies similar to that of Wu and Wong et al., two groups recently reported new findings related to anti-leukemia mechanisms of DHODH inhibition in different types of hematologic malignancies. DHODH blockade by the chiral tetrahydroindazole ([R]-HZ00), a newly developed DHODH inhibitor, increased p53 activation and enhanced the anti-leukemic effect of the MDM2 inhibitor, Nutlin3a, in p53-WT chronic myeloid leukemia.19 PTC299, identified as a VEGFA inhibitor, targeted DHODH, resulting in cell growth inhibition and differentiation in leukemias, including AML, linking DHODH regulation and stress-induced VEGFA and angiogenesis.20 Notably, these studies demonstrated that DHODH inhibition alone is not sufficient to eliminate leukemia, requiring combination with standard chemotherapeutic agents or target-specific inhibitors. Given the genetic complexity of AML, future use of DHODH inhibitors in combination with selected drugs with non-overlapping mechanisms of action may tackle specific aspects of the complex pathogenesis of AML and ultimately improve patients’ outcomes.

Figure 1.Isobavachalcone in the regulation of dihydroorotate dehydrogenase in pyrimidine biosynthesis and myeloid blast differentiation. Left, the pyrimidine biosynthesis pathway and its biological functions. Isobavachalcone (IBC) prevents DHODH from catalyzing dihydroorotate into orotate, blocking pyrimidine biosynthesis, resulting in myeloid lineage differentiation and induction of apoptosis. The role of DHODH and the negative impact of c-MYC on myeloid blast differentiation is indicated by the dotted line/arrow. DHODH: dihydroorotate dehydrogenase; UMP: uridine 5′-monophosphate; UTP: uridine triphosphate; CTP: cytidine triphosphate; dTTP: deoxythymidine triphosphate; UDP-GlcNAc: uridine diphosphate N-acetylglucosamine; CDP-DAG: cytidine diphosphate diacylglycerol; CDP-choline: cytidine diphosphate choline; O-GlcNAc: O-linked N-acetylglucosamine; OGT: O-GlcNAc transferase. CTP to dTTP requires multiple steps.