Kynurenine Monooxygenase Expression and Activity in Human Astrocytomas

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor. The enzyme indoleamine-2,3-dioxygenase (IDO), which participates in the rate-limiting step of tryptophan catabolism through the kynurenine pathway (KP), is associated with poor prognosis in patients with GBM. The metabolites produced after tryptophan oxidation have immunomodulatory properties that can support the immunosuppressor environment. In this study, mRNA expression, protein expression, and activity of the enzyme kynurenine monooxygenase (KMO) were analyzed in GBM cell lines (A172, LN-18, U87, U373) and patient-derived astrocytoma samples. KMO mRNA expression was assessed by real-time RT-qPCR, KMO protein expression was evaluated by flow cytometry and immunofluorescence, and KMO activity was determined by quantifying 3-hydroxykynurenine by HPLC. Heterogenous patterns of both KMO expression and activity were observed among the GBM cell lines, with the A172 cell line showing the highest KMO expression and activity. Higher KMO mRNA expression was observed in glioma samples than in patients diagnosed with only a neurological disease; high KMO mRNA expression was also observed when using samples from patients with GBM in the TCGA program. The KMO protein expression was localized in GFAP⁺ cells in tumor tissue. These results suggest that KMO is a relevant target to be explored in glioma since it might play a role in supporting tumor metabolism and immune suppression.

Assessment of the safety, pharmacokinetics and pharmacodynamics of GSK3335065, an inhibitor of kynurenine monooxygenase, in a randomised placebo-controlled first-in-human study in healthy volunteers

GSK3335065 is an inhibitor of kynurenine monooxygenase (KMO) being developed for the treatment of acute pancreatitis. Healthy male volunteers were administered ascending doses of GSK3335065 or matched placebo as a single intravenous bolus injection to assess safety, tolerability, pharmacokinetics and pharmacodynamics. GSK3335065 displayed an apparent volume of distribution between 20.6 L and 44.6 L, a clearance between 0.462 L/h and 0.805 L/hr and a terminal half-life between 31.3 and 34.5 hr. In the single subject who received 1.3 mg GSK3335065, changes in tryptophan pathway metabolites were observed consistent with the changes seen in preclinical species suggesting that KMO enzyme activity was partially inhibited. However, a broad complex ventricular tachycardia was observed in this subject, which was judged to be a Serious Adverse Event (SAE) and resulted in early termination of the study. While development of GSK3335065 was subsequently discontinued, significant confounding factors hinder a clear interpretation that the tachycardia was directly related to administration of the compound.

Pharmacophore-Based Virtual Screening of Novel Competitive Inhibitors of the Neurodegenerative Disease Target Kynurenine-3-Monooxygenase

The pathogenesis of several neurodegenerative diseases such as Alzheimer’s or Huntington’s disease has been associated with metabolic dysfunctions caused by imbalances in the brain and cerebral spinal fluid levels of neuroactive metabolites. Kynurenine monooxygenase (KMO) is considered an ideal therapeutic target for the regulation of neuroactive tryptophan metabolites. Despite significant efforts, the known KMO inhibitors lack blood–brain barrier (BBB) permeability and upon the mimicking of the substrate binding mode, are subject to produce reactive oxygen species as a side reaction. The computational drug design is further complicated by the absence of complete crystal structure information for human KMO (hKMO). In the current work, we performed virtual screening of readily available compounds using several protein–ligand complex pharmacophores. Each of the pharmacophores accounts for one of three distinct reported KMO protein-inhibitor binding conformations. As a result, six novel KMO inhibitors were discovered based on an in vitro fluorescence assay. Compounds VS1 and VS6 were predicted to be BBB permeable and avoid the hydrogen peroxide production dilemma, making them valuable, novel hit compounds for further drug property optimization and advancement in the drug design pipeline.

Modulation of Enzyme Activity in the Kynurenine Pathway by Kynurenine Monooxygenase Inhibition

The kynurenine pathway is the major route for tryptophan metabolism in mammals. Several of the metabolites in the kynurenine pathway, however, are potentially toxic, particularly 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and quinolinic acid. Quinolinic acid (QUIN) is an excitotoxic agonist at the NMDA receptor, and has been shown to be elevated in neurodegenerative diseases such as Alzheimer's Disease and Huntington's Disease. Thus, inhibitors of enzymes in the kynurenine pathway may be valuable to treat these diseases. Kynurenine monooxygenase (KMO) is the ideal target for an inhibitor, since inhibition of it would be expected to decrease the toxic metabolites and increase kynurenic acid (KynA), which is neuroprotective. The first generation of KMO inhibitors was based on structural analogs of the substrate, L-kynurenine. These compounds showed reduction of QUIN and increased KynA in vivo in rats. After the determination of the x-ray crystal structure of yeast KMO, inhibitor design has been facilitated. Benzisoxazoles with sub-nM binding to KMO have been developed recently. Some KMO ligands promote the reaction of NADPH with O₂ without hydroxylation, resulting in uncoupled formation of H₂O₂. This potentially toxic side reaction should be avoided in the design of drugs targeting the kynurenine pathway for treatment of neurodegenerative disorders.

Targets for future clinical trials in Huntington's disease: what's in the pipeline?

The known genetic cause of Huntington's disease (HD) has fueled considerable progress in understanding its pathobiology and the development of therapeutic approaches aimed at correcting specific changes linked to the causative mutation. Among the most promising is reducing expression of mutant huntingtin protein (mHTT) with RNA interference or antisense oligonucleotides; human trials are now being planned. Zinc-finger transcriptional repression is another innovative method to reduce mHTT expression. Modulation of mHTT phosphorylation, chaperone upregulation, and autophagy enhancement represent attempts to alter cellular homeostasis to favor removal of mHTT. Inhibition of histone deacetylases (HDACs) remains of interest; recent work affirms HDAC4 as a target but questions the assumed centrality of its catalytic activity in HD. Phosphodiesterase inhibition, aimed at restoring synaptic function, has progressed rapidly to human trials. Deranged cellular signaling provides several tractable targets, but specificity and complexity are challenges. Restoring neurotrophic support in HD remains a key potential therapeutic approach. with several approaches being pursued, including brain-derived neurotrophic factor (BDNF) mimesis through tyrosine receptor kinase B (TrkB) agonism and monoclonal antibodies. An increasing understanding of the role of glial cells in HD has led to several new therapeutic avenues, including kynurenine monooxygenase inhibition, immunomodulation by laquinimod, CB2 agonism, and others. The complex metabolic derangements in HD remain under study, but no clear therapeutic strategy has yet emerged. We conclude that many exciting therapeutics are progressing through the development pipeline, and combining a better understanding of HD biology in human patients, with concerted medicinal chemistry efforts, will be crucial for bringing about an era of effective therapies.