The kynurenine 3-hydroxylase inhibitor Ro 61-8048 improves dystonia in a genetic model of paroxysmal dyskinesia

The effects of the novel kynurenine 3-hydroxylase inhibitor 3,4-dimethoxy-N-[4-(3-nitrophenyl)thiazol-2-yl]benzenesulfonamide (Ro 61-8048) on severity of dystonia were examined in dt(sz) mutant hamsters, an animal model of paroxysmal dystonia, in which stress precipitates dystonic episodes. Ro 61-8048 (50, 100 and 150 mg/kg i.p.) significantly reduced the severity of dystonia in dt(sz) hamsters without leading to marked central side effects. Determinations of kynurenic acid concentrations in brain homogenates demonstrated that Ro 61-8048 (100 mg/kg i.p.) provoked a two- to threefold increase of the endogeneous broad spectrum glutamate receptor antagonist kynurenic acid in the striatum, cerebellum and brainstem of mutant hamsters. The antidystonic efficacy of Ro 61-8048 at well-tolerated doses suggests that kynurenine 3-hydroxylase inhibitors should be considered as new therapeutic candidates for the treatment of dyskinesias.     

The NMDA receptor NR2B subtype selective antagonist Ro 25-6981 aggravates paroxysmal dyskinesia in the dt(sz) mutant

Previously, enhanced levels of spermine which stimulates N-methyl-D-aspartate (NMDA) receptors, particularly those containing the NR2B subunit, were found in brains of dt(sz) mutant hamsters, a model of paroxysmal dyskinesia in which dystonic episodes occur in response to stress. Therefore, the effects of the NR2B selective NMDA receptor antagonist Ro 25-6981 ([R-(R,S)]-alpha-(4-hydroxyphenyl)-beta-methyl-4-phenyl-methyl)-1-piperidine-propanol] on severity of dystonia were investigated in the dt(sz) hamster. Ro 25-6981 failed to exert antidystonic effects, but even caused a moderate aggravation at higher doses (10.0, 12.5 mg/kg). This result indicates that overstimulation of receptors that include the NR2B subunit by polyamines is not involved in the dystonic syndrome. NR2B-selective NMDA receptor antagonists seem not to provide a novel approach in the treatment of hereditary paroxysmal dyskinesias.     

Paradoxical aggravation of paroxysmal dystonia during chronic treatment with phenobarbital in a genetic rodent model

Recent studies in mutant hamsters (dt(sz)), an animal model of primary paroxysmal dystonia, indicated that altered function of the gamma-aminobutyric acid (GABA)ergic system plays a critical role in the pathogenesis of dystonia. In the present study, dt(sz) hamsters were chronically treated with phenobarbital, which has been found to exert antidystonic effects in mutant hamsters after acute administration. In untreated dt(sz) hamsters, the severity of dystonia follows an age-dependent time course with a maximum between the 30th and 40th day of life, followed by a continuous decline of severity until complete remission occurs at the age of about 70 days. In contrast to acute effects, chronic treatment with phenobarbital via drinking water starting at an age of 21 days (i.e., after weaning) worsened dystonia and retarded the spontaneous remission. The unexpected prodystonic effect was more marked after administration of higher doses and when chronic treatment with phenobarbital started at an age of 1 day (neonatal administration via breast milk). After withdrawal of phenobarbital at the age of 70 days, the severity rapidly declined in all treated groups. When phenobarbital was readministered 1 week later, the hamsters again exhibited severe dystonia. The mechanism of these unexpected findings is unknown. Tentatively, activity-dependent GABA-mediated excitation caused by chronic treatment with phenobarbital may be important for the prodystonic effects under pathological conditions in dt(sz) hamsters.     

Quinolinic acid formation in immune-activated mice: studies with (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(-3-nitrophenyl)thiazol-2yl]-benzenesul fonamide (Ro 61-8048), two potent and selective inhibitors of kynurenine hydroxylase.

The role of kynurenine hydroxylase activity in the neo-formation of the excitotoxin quinolinic acid (QUIN) has been studied in mice by using (m-nitrobenzoyl)-alanine (mNBA) and 3,4-dimethoxy-[-N-4-(-3-nitrophenyl)thiazol-2yl]-benzenesulf onamide (Ro 61-8048), two potent and selective inhibitors of this enzyme. Immune-stimulation with pokeweed mitogen (PWM, 200 microg i.v., 12 h) induced a robust increase in kynurenine (KYN) and its metabolites kynurenic acid (KYNA) and QUIN in blood and brain. When incubated in a medium containing KYN but not tryptophan, spleen, lung and liver (but not brain) slices accumulated a measurable amount of QUIN in the supernatant. Slices obtained from PWM treated animals had a ten-fold increase in QUIN accumulation in spleen, no changes in lung and a 40% decrease in liver, suggesting that the spleen contributes to the increased QUIN levels found in the blood and brain of immune-stimulated mice. Large doses of kynurenine hydroxylase inhibitors increased KYN and KYNA, but unexpectedly did not decrease QUIN content in control blood and brain. When tested in organ slices obtained from either controls or immune-stimulated animals, mNBA (1-1000 microM) and Ro 61-8048 (0.1-100 microM) strongly reduced QUIN neo-formation, suggesting that, in vitro, kynurenine hydroxylase activity is required for QUIN neosynthesis. Indeed, after repeated doses of mNBA or Ro 61-8048, QUIN content in blood and brain of immune-stimulated animals significantly decreased. Our results suggest that, under basal conditions, sufficient QUIN synthesis may occur through kynurenine hydroxylase-independent pathways. In immune-stimulated animals, however, kynurenine hydroxylase inhibitors significantly reduce blood and brain accumulation of QUIN.     

The carbonic anhydrase inhibitor acetazolamide exerts antidystonic effects in the dt(sz) mutant hamster

Previous studies suggested an involvement of gamma-aminobutyric acid (GABA)-mediated excitation by an enhanced efflux of bicarbonate ions in addition to retarded development of GABAergic inhibition in the syndrome of dt(sz) mutant hamsters, a model of paroxysmal dyskinesia in which dystonic episodes occur in response to stress. Acetazolamide blocks bicarbonate regeneration in neurons and can thereby reduce GABA-mediating excitation without affecting GABA-mediated inhibition. In the present study, the effects of acetazolamide (15-60 mg/kg, i.p.) on severity of dystonia were therefore examined in dt(sz) hamsters. Acetazolamide significantly reduced the severity of dystonia at a dose of 60 mg/kg. These data are in line with several case reports from patients with paroxysmal dystonia, suggesting that acetazolamide can be useful in the treatment of this movement disorder. The mechanism of the antidystonic efficacy of acetazolamide has to be examined by further studies.     

Effects of adenosine receptor agonists and antagonists in a genetic animal model of primary paroxysmal dystonia

1. Recent studies have shown beneficial effects of an adenosine A(2A) receptor agonist in dt(sz) mutant hamsters, an animal model of paroxysmal dystonia, in which stress and consumption of coffee can precipitate dystonic attacks. This prompted us to examine the effects of adenosine receptor agonists and antagonists on severity of dystonia in dt(sz) hamsters in more detail. 2. The non-selective adenosine A(1)/A(2A) receptor antagonists, caffeine (10 - 20 mg kg(-1) i.p.) and theophylline (10 - 30 mg kg(-1) s.c.), worsened the dystonia in dt(sz) hamsters. 3. Aggravation of dystonia was also caused by the selective adenosine A(1)/A(2A) antagonist CGS 15943 (9-chloro2-2-furyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine) at a dose of 30 mg kg(-1) i.p. and by the adenosine A(1) antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine; 20 - 30 mg kg(-1) i.p.), while the A(2) antagonist DMPX (3,7-dimethyl-1-propargylxanthine; 2 - 4 mg kg(-1) i.p.) and the highly selective A(2A) antagonist ZM 241385 (4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol; 2 - 5 mg kg(-1) i.p.) failed to exert any effects on dystonia. 4. In contrast to the antagonists, both the adenosine A(1) receptor agonist CPA (N(6)-cyclopentyladenosine; 0.1 - 1.0 mg kg(-1) i.p.) and the A(2A) agonist CGS 21680 (2p-(2carboxyethylphen-ethylamino-5'-N-ethylcarboxamindoadenosine; 0.1 - 2.0 mg kg(-1) i.p.) exerted a striking improvement of dystonia. 5. These data suggest that the precipitating effects of methylxanthines are, at least in part, related to their adenosine receptor antagonistic action. 6. Although adenosine receptor agonists can be regarded as interesting candidates for the therapy of paroxysmal dystonia, adverse effects may limit the therapeutic potential of adenosine A(1) agonists, while beneficial effects of the adenosine A(2A) agonist CGS 21680 were already found at well tolerated doses.     

Effects of pharmacological manipulations of cannabinoid receptors on severity of dystonia in a genetic model of paroxysmal dyskinesia

Previous studies have shown beneficial effects of the cannabinoid CB(1)/CB(2) receptor agonist (R)-4,5-dihydro-2-methyl-4-(4-morpholinylmethyl)-1-(1-naphthalenylcarbonyl)-6H-pyrrolo [3,2,1-ij]quinolin-6-one mesylate (WIN 55,212-2) in dt(sz) mutant hamsters, a model of idiopathic paroxysmal dystonia (dyskinesia). To examine the pathophysiological significance of the cannabinergic system in the dystonic syndrome, the effect of the cannabinoid CB(1) receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR 141716A) on severity of dystonia was investigated in dt(sz) mutants which exhibit episodes of dystonic and choreoathetotic disturbances in response to mild stress. SR 141716A (5 and 10 mg/kg i.p.) failed to exert any effects on the severity of dystonia. While the antidystonic efficacy of WIN 55,212-2 (5 mg/kg i.p.) was confirmed, cannabidiol (which has low affinity to cannabinoid receptors) tended to delay the progression of dystonia only at a high dose (150 mg/kg i.p.). The antidystonic and cataleptic effects of WIN 55,212-2 (5 mg/kg i.p.) were completely antagonized by pretreatment with SR 141716A at doses of 2.5 mg/kg (catalepsy) and 10 mg/kg (antidystonic efficacy). These data indicate that the antidystonic efficacy of WIN 55,212-2 is selectively mediated via CB(1) receptors. The lack of prodystonic effects of SR 141716A together with only moderate antidystonic effects of WIN 55,212-2 suggests that reduced activation of cannabinoid CB(1) receptors by endocannabinoids is not critically involved in the dystonic syndrome. In view of previous pathophysiological findings in mutant hamsters, the antidystonic efficacy of WIN 55,212-2 can be explained by modulation of different neurotransmitter systems within the basal ganglia.     

Kynurenine 3-mono-oxygenase inhibitors reduce glutamate concentration in the extracellular spaces of the basal ganglia but not in those of the cortex or hippocampus

Kynurenine 3-mono-oxygenase (KMO, kynurenine hydroxylase) inhibitors increase brain kynurenic acid (KYNA) synthesis and cause pharmacological actions possibly mediated by a reduced activity of excitatory synapses. We used in vivo microdialysis and passive avoidance to study the effects of local KYNA or systemic KMO inhibitor administration on glutamate (GLU) neurotransmission. Local application of KYNA (30-100 nM) through reverse microdialysis reduced GLU content in caudate and cortical dialysates by 75 and 55%, respectively. No changes were found in the hippocampus. Systemic administration of Ro 61-8048 (4-40 mg/kg) increased KYNA levels in dialysates obtained from the cortex (from 10.3 +/- 1.9 to 45.5 +/- 15 nM), caudate (from 2.4 +/- 0.8 to 9.5 +/- 0.9 nM) and hippocampus (from 7.7 +/- 1.7 to 19.2 +/- 3.5 nM). It also caused a parallel robust decrease in GLU levels in the dialysates collected from the caudate (from 2.2 +/- 0.5 to 0.63 +/- 0.05 microM) but not in those collected from the parietal cortex or the hippocampus. In a passive avoidance paradigm, the administration of the NMDA receptor antagonist MK-801 (0.1 mg/kg) reduced, while Ro 61-8048 (4-80 mg/kg) did not change the latency time of entering into the dark compartment on the recall trial. Our data show that KMO inhibitors increase brain KYNA synthesis and selectively reduce GLU extracellular concentration in the basal ganglia.     

Gabapentin decreases the severity of dystonia at low doses in a genetic animal model of paroxysmal dystonic choreoathetosis

The effects of the gamma-aminobutyric acid (GABA)-potentiating drug gabapentin (1-(aminomethyl) cyclohexaneacetic acid) on severity of dystonia were examined in a hamster model of idiopathic paroxysmal dystonic choreoathetosis. In the genetically dystonic hamster (dt(sz)) recent pharmacological and neurochemical studies suggested that disturbed GABAergic inhibition is involved in the pathogenesis. In line with a case report of beneficial effects in human paroxysmal dystonic choreoathetosis, gabapentin reduced the severity of dystonia in mutant hamsters at doses of 5 and 10 mg kg(-1) i.p. At higher doses (20 and 100 mg kg(-1)), gabapentin, however, failed to exert antidystonic effects. The GABApotentiating activity of gabapentin could explain the antidystonic effects of low doses, while the loss of efficacy at higher doses may be due to other mechanisms of gabapentin.     

Effects of striatal injections of GABA(A) receptor agonists and antagonists in a genetic animal model of paroxysmal dystonia

The underlying mechanisms of idiopathic dystonias are poorly understood. The dystonic phenotype in the dt(sz) mutant hamster, a model of paroxysmal dystonia, has been suggested to be based on a deficit of gamma-aminobutyric acid (GABA)ergic interneurons and changes of the GABA(A)-benzodiazepine receptor complex in the striatum. In order to confirm and extend previous observations, the effects of compounds which bind to different sites of the GABA(A) receptor on the severity of dystonia were determined after striatal microinjections in comparison to systemic treatments in dt(sz) mutants. The GABA(A) receptor agonist (muscimol) and the benzodiazepine (flurazepam) reduced the severity of dystonia after striatal and systemic injections. The antidystonic effects of the barbiturate phenobarbital were less marked both after striatal and intraperitoneal administration of drugs. Intrastriatal injections of GABA delayed the onset of dystonic attacks. Striatal and systemic treatments with the GABA(A) receptor antagonist, bicuculline, and with pentylenetetrazole, which reduces GABAergic function, accelerated the onset of dystonia at subconvulsant doses. The benzodiazepine receptor antagonists flumazenil aggravated dystonia after systemic and intrastriatal injections. In all, the present data substantiate the relevance of striatal GABAergic disinhibition in the pathogenesis of paroxysmal dystonia in dt(sz) mutants.     

Effect of non-dopaminergic drug treatment on Levodopa induced dyskinesias in MPTP monkeys: Common implication of striatal neuropeptides

Dopamine denervation in Parkinson's disease and repeated Levodopa (L-DOPA) administration that induces dyskinesias are associated with an enhancement of basal ganglia neuropeptide transmission. Various adjunct non-dopaminergic treatments to Levodopa were shown to reduce and/or prevent dyskinesias. The aim of this study was to seek if non-dopaminergic drug treatments to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioned monkeys combined with L-DOPA to prevent dyskinesia were associated with changes of striatal neuropeptides. Chronic treatment with Ro 61-8048 a kynurenine hydroxylase inhibitor, docosahexaenoic acid (DHA) a polyunsaturated fatty acid (omega-3), naltrexone an opioidergic antagonist and CI-1041 an N-methyl-d-aspartate (NMDA) glutamate receptor antagonist with L-DOPA prevented dyskinesias to various extents except naltrexone whereas all MPTP monkeys treated with L-DOPA alone developed dyskinesias. Striatal preproenkephalin (PPE), preprodynorphin (PPD) and preprotachykinin A (PPT-A) mRNA levels were measured by in situ hybridization. An increase of PPE and PPD mRNA levels was observed in anterior caudate nucleus of L-DOPA treated MPTP monkeys compared to controls and to Saline-treated MPTP monkeys whereas PPT-A mRNA levels were unchanged. Striatal PPE and PPD mRNA levels remained elevated in L-DOPA plus naltrexone-treated MPTP monkeys, while co-treatment with DHA, CI-1041 or Ro 61-8048 prevented their increase to various extents. Maximal dyskinesias scores of MPTP monkeys correlated significantly with striatal PPE and PPD mRNA levels but not with PPT-A mRNA levels. These results show that drugs displaying a wide range of pharmacological activities can modulate L-DOPA induced dyskinesias and this activity is correlated with striatal PPD and PPE mRNA levels suggesting a convergent mechanism.     

Antidystonic effects of Kv7 (KCNQ) channel openers in the dt sz mutant, an animal model of primary paroxysmal dystonia

BACKGROUND AND PURPOSE: Mutations in neuronal Kv7 (KCNQ) potassium channels can cause episodic neurological disorders. Paroxysmal dyskinesias with dystonia are a group of movement disorders which are regarded as ion channelopathies, but the role of Kv7 channels in the pathogenesis and as targets for the treatment have so far not been examined. EXPERIMENTAL APPROACH: In the present study, we therefore examined the effects of the activators of neuronal Kv7.2/7.3 channels retigabine (5, 7.5, 10 mg kg(-1) i.p. and 10, 20 mg kg(-1) p.o.) and flupirtine (10, 20 mg kg(-1) i.p.) and of the channel blocker 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991, 3 and 6 mg kg(-1) i.p.) in the dt sz mutant hamster, a model of paroxysmal dyskinesia in which dystonic episodes occur in response to stress. KEY RESULTS: Retigabine (10 mg kg(-1) i.p., 20 mg kg(-1) p.o.) and flupirtine (20 mg kg(-1) i.p.) significantly improved dystonia, while XE-991 caused a significant aggravation in the dt sz mutant. The antidystonic effect of retigabine (10 mg kg(-1) i.p.) was counteracted by XE-991 (3 mg kg(-1) i.p.). CONCLUSIONS AND IMPLICATIONS: These data indicate that dysfunctions of neuronal Kv7 channels deserve attention in dyskinesias. Since retigabine and flupirtine are well tolerated in humans, the present finding of pronounced antidystonic efficacy in the dt sz mutant suggests that neuronal Kv7 channel activators are interesting candidates for the treatment of dystonia-associated dyskinesias and probably of other types of dystonias. The established analgesic effects of Kv7 channel openers might contribute to improvement of these disorders which are often accompanied by painful muscle spasms.     

Early effects of 2-acetylaminofluorene and N-hydroxy-2-acetylaminofluorene on mitochondrial outer membrane enzyme activities of rat liver

During the early stages of the carcinogen 2-acetylaminofluorene (2-AAF) feeding, there were marked decreases of hepatic mitochondrial type B monoamine oxidase (MAO) and kynurenine 3-hydroxylase activities in male rats, but not in female rats. The administration of N-hydroxy-2-AAF (N-OH-2-AAF), a proximate carcinogenic metabolite of 2-AAF, to male rats also resulted in the decrease of both enzyme activities. The findings indicate that type B MAO and kynurenine 3-hydroxylase which reside in the outer mitochondrial membranes are susceptible to 2-AAF during the early stages of its hepatocarcinogenesis.     

Some characteristics of hamster liver and lung microsomal aryl hydrocarbon (biphenyl and benzo(a)pyrene) hydroxylation reactions

Aryl hydrocarbon hydroxylation (AHH) reactions were compared using liver and lung microsomes of corn oil- and 3-methylcholanthrene (3-MC)-treated hamsters, employing benzo(a)pyrene (BAP) and biphenyl as substrates. The predominant AHH activity of liver and lung microsomes from corn oil- or 3-MC-treated hamsters was biphenyl 4-hydroxylase. Biphenyl 2-hydroxylase and BAP-hydroxylase activities were approximately 50 per cent as active as biphenyl 4-hydroxylase in liver and approximately 1–3 per cent as active as biphenyl 4-hydroxylase in lung microsomes. Biphenyl 4-hydroxylase activity was 70–80 per cent as active in lung as in liver microsomes. Treatment with 3-MC in vivo induced the biphenyl 4-hydroxylation reaction in liver but not in lung microsomes, the biphenyl 2-hydroxylation reaction both in lung and liver microsomes, and the BAP hydroxylation reaction in lung but not in liver microsomes. Biphenyl 2- and 4-hydroxylase activities of liver microsomes displayed similar sensitivities to inhibition by a number of chemical inhibitors in vitro. Inhibition of biphenyl hydroxylation reactions by metyrapone or carbon monoxide did not distinguish between lung or liver microsomal mono-oxygenases of corn oil- or 3-MC-treated hamsters. While small differences were expressed by inhibition with ethylmorphine, large differences became apparent through inhibition studies with BAP or α-naphthoflavone. It is concluded that the major aromatic hydroxylase activity of lung microsomes from corn oil- or 3-MC-treated hamsters resembles the constitutive (uninduced) AHH of the liver microsomes and that the minor aromatic hydroxylase activity of lung microsomes from corn oil- or 3-MC-treated hamsters resembles the induced AHH of the liver microsomes.     

Antidystonic efficacy of gamma-aminobutyric acid uptake inhibitors in the dtsz mutant

In the dtsz mutant hamster, a model of paroxysmal dyskinesia in which dystonic episodes occur in response to stress, previous studies suggested that retarded development of gamma-aminobutyric acid (GABA)ergic inhibition plays a critical role in the pathogenesis. In the present study, we therefore examined the effects of selective GABA uptake inhibitors on severity of dystonia in dtsz hamsters. R(-)N-(4,4-di(3-methylthien-2-yl)-but-3-enyl) nipecotic acid hydrochloride (tiagabine, 5-20 mg/kg i.p.) and 1-[2-[[(diphenylmethylene) imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NNC-711, 1-10 mg/kg i.p.) significantly reduced the severity of dystonia. These data suggest that GABA uptake inhibitors may provide novel therapeutic approaches for paroxysmal dyskinesias.     

Enzymatic studies on tryptophan metabolism disorder in rats chronically exposed to carbon disulfide

Eight-week-old female Wistar rats were exposed to carbon disulfide for 6 hr a day, 5 days a week, for 12 weeks in inhalation chambers. Then the activities of the main enzymes of tryptophan metabolism (i.e., L-tryptophan 2,3-dioxygenase, indoleamine 2,3-dioxygenase, kynurenine 3-hydroxylase, kynureninase, and kynurenine aminotransferases) in their tissues were determined. The results showed that exposure to carbon disulfide caused a significant increase in the activities of kynureninase and kynurenine-2-oxoglutarate aminotransferase in the kidneys, but only a slight increase in their activities in the liver. The activities of L-tryptophan 2,3-dioxygenase and kynurenine 3-hydroxylase also tended to increase, but the increases were not statistically significant. These results suggests that the kynurenine pathway of tryptophan metabolism in the kidneys of rats exposed to carbon disulfide is activated and that the increased activities of kynurenine-2-oxoglutarate aminotransferase in the kidneys may cause the increased excretion of tryptophan metabolites after tryptophan loading as shown in an earlier study (A. Okayama, L. Fun, A. Yamatodani, Y. Ogawa, H. Wada, and S. Goto, 1987, Arch. Toxicol. 60, 460-463.     

Benzodiazepine Ro 5-4864 increases coronary flow

Ro 5-4864 (chlorodiazepam) increased coronary flow in isolated retrograde perfused Langendorff rat heart preparations without affecting heart rate and left ventricular contractility (dP/dt). On the other hand Ro 5-4023 (clonazepam) produced very little effect. PK 11195 which has been shown to inhibit the binding of Ro 5-4864 to cardiac muscle did not antagonize this vasodilatory effect of Ro 5-4864 but increased coronary flow by itself. The data indicate a specific vasodilatory effect of certain benzodiazepines. The mechanism of action remains unknown.     

Pyrrolo[3,2-c]quinoline derivatives: a new class of kynurenine-3-hydroxylase inhibitors.

A series of pyrrolo[3,2-c]quinoline derivatives were synthesised and evaluated as inhibitors of selected enzymes of the kynurenine pathway. 7-Chloro-3-methyl-1H-pyrrolo[3,2-c]quinoline-4-carboxylic acid (7a) was found to be a relatively potent and selective inhibitor of kynurenine-3-hydroxylase (KYN-3-OHase). A molecular modelling study showed a good superimposition of 7a with PNU-156561 and kynurenine the natural substrate of KYN-3-OHase.