UVB Radiation and Selected Tryptophan-Derived AhR Ligands-Potential Biological Interactions in Melanoma Cells.

Excessive UV exposure is considered the major environmental factor in melanoma progression. Human skin is constantly exposed to selected tryptophan-derived aryl hydrocarbon receptor (AhR) ligands, including kynurenine (KYN) and kynurenic acid (KYNA), as they are endogenously produced and present in various tissues and body fluids. Importantly, recent studies confirmed the biological activity of KYN and KYNA toward melanoma cells in vitro. Thus, in this study, the potential biological interactions between UVB and tryptophan metabolites KYN and KYNA were studied in melanoma A375, SK-MEL-3, and RPMI-7951 cells. It was shown that UVB enhanced the antiproliferative activity of KYN and KYNA in melanoma cells. Importantly, selected tryptophan-derived AhR ligands did not affect the invasiveness of A375 and RPMI-7951 cells; however, the stimulatory effect was observed in SK-MEL-3 cells exposed to UVB. Thus, the effect of tryptophan metabolites on metabolic activity, cell cycle regulation, and cell death in SK-MEL-3 cells exposed to UVB was assessed. In conclusion, taking into account that both UVB radiation and tryptophan-derived AhR ligands may have a crucial effect on skin cancer formation and progression, these results may have a significant impact, revealing the potential biological interactions in melanoma cells in vitro.

Phase 1 study to access safety, tolerability, pharmacokinetics, and pharmacodynamics of kynurenine in healthy volunteers.

The kynurenine pathway (KP) is the main path for tryptophan metabolism, and it represents a multitude of potential sites for drug discovery in neuroscience, including pain, stroke, and epilepsy. L-kynurenine (LKYN), the first active metabolite in the pathway, emerges to be a prodrug targeting glutamate receptors. The safety, tolerability, pharmacokinetics, and pharmacodynamics of LKYN in humans have not been previously investigated. In an open-label, single ascending dose study, six participants received an intravenous infusion of 50, 100, and 150 µg/kg LKYN and new six participants received an intravenous infusion of 0.3, 0.5, 1, and 5 mg/kg LKYN. To compare the pharmacological effects between species, we investigated in vivo the vascular effects of LKYN in rats. In humans, LKYN was safe and well-tolerated at all dose levels examined. After infusion, LKYN plasma concentration increased significantly over time 3.23 ± 1.12 µg/mL (after 50 µg/kg), 4.04 ± 1.1 µg/mL (after 100 µg/kg), and 5.25 ± 1.01 µg/mL (after 150 µg/kg) (p ≤ 0.001). We observed no vascular changes after infusion compared with baseline. In rats, LKYN had no effect on HR and MAP and caused no dilation of dural and pial arteries. This first-in-human study of LKYN showed that LKYN was safe and well-tolerated after intravenous infusion up to 5 mg/kg over 20 minutes. The lack of change in LKYN metabolites in plasma suggests a relatively slow metabolism of LKYN and no or little feed-back effect of LKYN on its synthesis. The therapeutic potential of LKYN in stroke and epilepsy should be explored in future studies in humans.

Progress in the development of kynurenine and quinoline-3-carboxamide-derived drugs.

INTRODUCTION: The diverse neuro- and immunomodulatory effects of kynurenine pathway (KP) enzymes and metabolites exert offer possibilities for intervention in diseases such as autoimmunity, neurodegeneration, and neoplastic processes. AREAS COVERED: This review focuses on data obtained from the preclinical and clinical use of a KP metabolite analog and structurally related compounds. 4-Cl-KYN has completed clinical trials in depression without success. However, the good safety data give hope for further trials in suicide prevention, neuropathic pain, and dyskinesia. Quinoline-3-carboxamide derivatives laquinimod, paquinimod, and tasquinimod show structural similarities to kynurenines. Laquinimod and paquinimod show promising results in the treatment of autoimmune diseases, tasquinimod is considered primarily as an anti-cancer drug. Data available until 31 May 2020 at Clinicaltrials.gov and PubMed have been reviewed. EXPERT OPINION: The failure of 4-Cl-KYN for use as an anti-depressant may be related to inadequate concentration, or that the ketamine-like rapid anti-depressant effect is not produced via NMDAR modulation. Further clarification may emerge from studies involving higher drug concentration, and/or from identification of ketamine targets. Clinical application trials in very diverse indications of structurally related quinoline-3-carboxamides and the wide range of their mode of action warrant further studies permitting direct comparison of effects and better target identification.

Kynurenines in the Pathogenesis of Multiple Sclerosis: Therapeutic Perspectives.

Over the past years, an increasing amount of evidence has emerged in support of the kynurenine pathway's (KP) pivotal role in the pathogenesis of several neurodegenerative, psychiatric, vascular and autoimmune diseases. Different neuroactive metabolites of the KP are known to exert opposite effects on neurons, some being neuroprotective (e.g., picolinic acid, kynurenic acid, and the cofactor nicotinamide adenine dinucleotide), while others are toxic to neurons (e.g., 3-hydroxykynurenine, quinolinic acid). Not only the alterations in the levels of the metabolites but also disturbances in their ratio (quinolinic acid/kynurenic acid) have been reported in several diseases. In addition to the metabolites, the enzymes participating in the KP have been unearthed to be involved in modulation of the immune system, the energetic upkeep of neurons and have been shown to influence redox processes and inflammatory cascades, revealing a sophisticated, intertwined system. This review considers various methods through which enzymes and metabolites of the kynurenine pathway influence the immune system, the roles they play in the pathogenesis of neuroinflammatory diseases based on current evidence with a focus on their involvement in multiple sclerosis, as well as therapeutic approaches.

A Randomized Trial of the N-Methyl-d-Aspartate Receptor Glycine Site Antagonist Prodrug 4-Chlorokynurenine in Treatment-Resistant Depression.

BACKGROUND: Ketamine has rapid-acting antidepressant effects but is associated with psychotomimetic and other adverse effects. A 7-chlorokynurenic acid is a potent and specific glycine site N-methyl-d-aspartate receptor antagonist but crosses the blood-brain barrier inefficiently. Its prodrug, L-4-chlorokynurenine (4-Cl-KYN), exerts acute and sustained antidepressant-like effects in rodents and has no reported psychotomimetic effects in either rodents or healthy volunteers. This study examined whether 4-Cl-KYN has rapid antidepressant effects in individuals with treatment-resistant depression. METHODS: After a 2-week drug-free period, 19 participants with treatment-resistant depression were randomized to receive daily oral doses of 4-Cl-KYN monotherapy (1080 mg/d for 7 days, then 1440 mg/d for 7 days) or placebo for 14 days in a randomized, placebo-controlled, double-blind, crossover manner. The primary outcome measure was the Hamilton Depression Rating Scale score, assessed at several time points over a 2-week period; secondary outcome measures included additional rating scale scores. Pharmacokinetic measures of 7-chlorokynurenic acid and 4-Cl-KYN and pharmacodynamic assessments were obtained longitudinally and included 1H-magnetic resonance spectroscopy brain glutamate levels, resting-state functional magnetic resonance imaging, and plasma and cerebrospinal fluid measures of kynurenine metabolites and neurotrophic factors. RESULTS: Linear mixed models detected no treatment effects, as assessed by primary and secondary outcome measures. No difference was observed for any of the peripheral or central biological indices or for adverse effects at any time between groups. A 4-Cl-KYN was safe and well-tolerated, with generally minimal associated adverse events. CONCLUSIONS: In this small crossover trial, 4-Cl-KYN monotherapy exerted no antidepressant effects at the doses and treatment duration studied.ClinicalTrials.gov identifier: NCT02484456.

Metabolomic association between venous thromboembolism in critically ill trauma patients and kynurenine pathway of tryptophan metabolism.

OBJECTIVE: Incidence of venous thromboembolism (VTE) in critically ill patients remains unacceptably high despite widespread use of thromboprophylaxis. A systems biology approach may be useful in understanding disease pathology and predicting response to treatment. Metabolite profile under specific environmental conditions provides the closest link to phenotype, but the relationship between metabolomics and risk of VTE in critically ill patients is unknown. In this study, metabolomics signatures are compared in patients with and without VTE. DESIGN: Multicenter case-control study using prospectively collected data from the Inflammation and Host Response to Injury program, with pathway and in silico gene expression analyses. SETTING: Eight level 1 US trauma centers. PATIENTS: Critically ill adults with blunt trauma who developed VTE within the first 28 days of hospitalization compared to patients without VTE (N-VTE). INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Patients included in the study (n = 20 VTE, n = 20 N-VTE) were mean age of 34 years, injury severity score of 35, and VTE diagnosed a median of 10.5 days after admission. Global metabolomics revealed two kynurenine metabolites, N-formylkynurenine (AUC = 0.77; 95% CI: 0.59-0.89) and 5-hydroxy-N-formylkynurenine (AUC = 0.80; 95% CI:0.63-0.90) significantly discriminated VTE and N-VTE; ratio between N-formylkynurenine/5-hydroxy-N-formylkynurenine improved predictive power (AUC = 0.87; 95% CI: 0.74-0.95). In the pathway analysis, tryptophan was the only significant metabolic pathway including N-formylkynurenine and 5-hydroxy-N-formylkynurenine (p < 0.001), and 8 proteins directly or indirectly interacted with these metabolites in the interaction network analysis. Of the 8 genes tested in the in silico gene expression analyses, KYNU (p < 0.001), CCBL1 (p < 0.001), and CCBL2 (p = 0.001) were significantly different between VTE and N-VTE, controlling for age and sex. CONCLUSIONS: Two novel kynurenine metabolites in the tryptophan pathway associated with hospital-acquired VTE, and 3 candidate genes were identified via pathway and interaction network analyses. Future studies are warranted to validate these findings in diverse populations using a multi-omics approach.

Sleep disturbance and kynurenine metabolism in depression.

OBJECTIVE: Although the interrelationships between sleep disturbance, inflammation, and depression have been found, molecular mechanisms that link these conditions are largely unknown. Kynurenine metabolism is hypothesized to be a key mechanism that links inflammation and depression. Inflammation activates the kynurenine pathway, leading to increases in 3-hydroxykynurenine (3HK) and quinolinic acid (QA), potentially neurotoxic metabolites, and decreases in kynurenic acid (KynA), a potentially neuroprotective compound. This relative neurotoxic shift in the balance of kynurenine metabolites has been associated with depression, but never been examined regarding sleep disturbance. We tested the association between sleep disturbance and this relative neurotoxic shift in 68 currently depressed, 26 previously depressed, and 66 never depressed subjects. METHODS: Sleep disturbance was assessed using the Pittsburgh Sleep Quality Index. Serum concentrations of kynurenine metabolites were measured using high performance liquid chromatography. Putative neuroprotective indices reflecting the relative activity of neuroprotective and neurotoxic kynurenine metabolites were calculated as KynA/QA and KynA/3HK (primary outcomes). RESULTS: Sleep disturbance was associated with reduced KynA/QA in the currently depressed group only (unadjusted beta -0.43, p<0.001). This association remained significant even after controlling for age, sex, analysis batch, body-mass index, and depressive symptoms in currently depressed subjects (adjusted beta -0.30, p=0.02). There was no significant association between sleep disturbance and KynA/3HK in any of the groups. Sleep disturbance was associated with increased C-reactive protein in currently depressed subjects only (unadjusted beta 0.38, p=0.007; adjusted beta 0.33, p=0.02). CONCLUSION: These data support the hypothesis that altered kynurenine metabolism may molecularly link sleep disturbance and depression.

The Prodrug 4-Chlorokynurenine Causes Ketamine-Like Antidepressant Effects, but Not Side Effects, by NMDA/GlycineB-Site Inhibition.

Currently approved antidepressant drug treatment typically takes several weeks to be effective. The noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist ketamine has shown efficacy as a rapid-acting treatment of depression, but its use is associated with significant side effects. We assessed effects following blockade of the glycineB co-agonist site of the NMDA receptor, located on the GluN1 subunit, by the selective full antagonist 7-chloro-kynurenic acid (7-Cl-KYNA), delivered by systemic administration of its brain-penetrant prodrug 4-chlorokynurenine (4-Cl-KYN) in mice. Following administration of 4-Cl-KYN, 7-Cl-KYNA was promptly recovered extracellularly in hippocampal microdialysate of freely moving animals. The behavioral responses of the animals were assessed using measures of ketamine-sensitive antidepressant efficacy (including the 24-hour forced swim test, learned helplessness test, and novelty-suppressed feeding test). In these tests, distinct from fluoxetine, and similar to ketamine, 4-Cl-KYN administration resulted in rapid, dose-dependent and persistent antidepressant-like effects following a single treatment. The antidepressant effects of 4-Cl-KYN were prevented by pretreatment with glycine or the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX). 4-Cl-KYN administration was not associated with the rewarding and psychotomimetic effects of ketamine, and did not induce locomotor sensitization or stereotypic behaviors. Our results provide further support for antagonism of the glycineB site for the rapid treatment of treatment-resistant depression without the negative side effects seen with ketamine or other channel-blocking NMDA receptor antagonists.

Kynurenine causes vasodilation and hypotension induced by activation of KCNQ-encoded voltage-dependent K(+) channels.

Kynurenine is a potential contributor to hypotension in animal and human sepsis. The present study was designed to examine whether the voltage-dependent K(+) channels encoded by the KCNQ gene family (Kv7 channels) mediate vasodilator effects of kynurenine and whether modulation of these channels ameliorates hypotension caused by this compound. Rat aortas and mesenteric arteries or human omental arteries without endothelium were used. Some rings were incubated with the selective Kv7 channel inhibitor linopirdine (10 µM). l-Kynurenine (10 µM-1 mM) induced concentration-dependent relaxation in rat aortas and mesenteric arteries as well as human omental arteries, whereas linopirdine abolished the relaxation. l-Kynurenine (1 mM) produced hyperpolarization of vascular smooth muscle, which was reversed by linopirdine (10 µM). Wistar rats received l-kynurenine (1 mM) iv and subsequent linopirdine (10 µM) iv under 3% sevoflurane inhalation. l-Kynurenine iv caused hypotension, whereas linopirdine iv partially reversed it. In conclusion, kynurenine dilates arteries from rats as well as humans via Kv7 channels in the vascular smooth muscle. In rats, this tryptophan metabolite causes hypotension, which is partly counteracted by Kv7 channel inhibition. These results suggest that modulation of Kv7 channels may be a novel strategy to treat hypotension induced by the kynurenine.

Exposure to kynurenic acid during adolescence produces memory deficits in adulthood.

The glia-derived molecule kynurenic acid (KYNA) is an antagonist of α7 nicotinic acetylcholine receptors and the glycine(B) binding site on n-methyl-d-aspartateglutamate receptors, both of which have critical roles in neural plasticity as well as learning and memory. KYNA levels are increased in the brains and cerebral spinal fluid of persons with schizophrenia, leading to the notion that changes in KYNA concentration might contribute to cognitive dysfunction associated with this disorder. Indeed, recent studies indicate that increasing endogenous KYNA concentration by administering l-kynurenine (L-KYN, the precursor of KYNA) impairs spatial as well as contextual learning and memory in adult rats. In the present study, rats were treated with L-KYN (100 mg/kg) throughout adolescence to increase endogenous KYNA concentration during this critical time in brain development. Rats were then tested drug-free as adults to test the hypothesis that exposure to elevated levels of KYNA during development may contribute to cognitive dysfunction later in life. Consistent with prior studies in which adult rats were treated acutely with L-KYN, juvenile rats exposed to increased KYNA concentration during adolescence exhibited deficits in contextual fear memory, but cue-specific fear memory was not impaired. In addition, rats treated with L-KYN as adolescents were impaired on a novel object recognition memory task when tested as adults. The memory deficits could not be explained by drug-induced changes in locomotor activity or shock sensitivity. Together, these findings add to the growing literature supporting the notion that exposure to increased concentration of KYNA may contribute to cognitive deficits typically observed in schizophrenia.

Lipopolysaccharide-induced depressive-like behavior is mediated by indoleamine 2,3-dioxygenase activation in mice.

Although elevated activity of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) has been proposed to mediate comorbid depression in inflammatory disorders, its causative role has never been tested. We report that peripheral administration of lipopolysaccharide (LPS) activates IDO and culminates in a distinct depressive-like behavioral syndrome, measured by increased duration of immobility in both the forced-swim and tail suspension tests. Blockade of IDO activation either indirectly with the anti-inflammatory tetracycline derivative minocycline, that attenuates LPS-induced expression of proinflammatory cytokines, or directly with the IDO antagonist 1-methyltryptophan (1-MT), prevents development of depressive-like behavior. Both minocycline and 1-MT normalize the kynurenine/tryptophan ratio in the plasma and brain of LPS-treated mice without changing the LPS-induced increase in turnover of brain serotonin. Administration of L-kynurenine, a metabolite of tryptophan that is generated by IDO, to naive mice dose dependently induces depressive-like behavior. These results implicate IDO as a critical molecular mediator of inflammation-induced depressive-like behavior, probably through the catabolism of tryptophan along the kynurenine pathway.

The kynurenine metabolic pathway in the eye: studies on 3-hydroxykynurenine, a putative cataractogenic compound.

The rabbit lens has an elevated content of 3-hydroxykynurenine (30HKYN) in spite of a very low activity of the enzymes leading to its synthesis. The iris/ciliary body, on the contrary, has very high activity of 30HKYN synthesizing enzymes but a content of 30HKYN lower than that of the lens. These observations suggest that 30HKYN is formed in the iris/ ciliary body, released into the aqueous humor and then taken up into the lens where it may be used for the synthesis of UV filtering products. An excessive accumulation of 30HKYN in the lens has been associated with cataract formation. We found that available selective inhibitors of kynurenine hydroxylase reduced 30HKYN synthesis in both the lens and the iris/ciliary body.

Kynurenines and seizures.

Recent data and concepts concerning the convulsant effects of kynurenines, neuroactive metabolites of tryptophan, in mice, rats, and frogs are reviewed. Myoclonic seizures of the hindlegs are induced in mice by l- and d,l-kynurenine. Both l- and d,l-kynurenine exhibit a selective synergism with strychnine. The convulsant effect of l-kynurenine is selectively antagonized by taurine, less selectively by l-glycine, and not at all by gamma-aminobutyric acid (GABA) or the GABA agonist muscimol. Derivatives of GABA and some standard anticonvulsant drugs alter seizures induced by l-kynurenine and quinolinic acid in different ways. The involvement of brian kynurenines in the genesis of epileptic seizures is suggested.