kynurenine pathway inhibition
TRANSCRIPT
This is an Accepted Article that has been peer-reviewed and approved for publication in the FEBS Journal, but has yet to undergo copy-editing and proof correction. Please cite this article as an “Accepted Article”; doi: 10.1111/j.1742-4658.2012.08487.x
Kynurenine pathway inhibition as a therapeutic strategy for neuroprotection
Trevor W Stone1, Caroline M Forrest1, L Gail Darlington2
1Institute for Neuroscience and Psychology,
College of Medical, Veterinary & Life Sciences,
West Medical Building,
University of Glasgow,
Glasgow G12 8QQ, U.K.
and
2Epsom General Hospital
Epsom
Surrey KT18 7EG
Article type : Minireview [email protected]
Running title:-
Kynurenines and neuronal viability
Correspondence:-
Prof. T W Stone, West Medical Building, University of Glasgow, Glasgow G12 8QQ,
UK
Key-words:-
Tryptophan; kynurenine; quinolinic acid; kynurenic acid; neurodegeneration;
neuroprotection;
Abstract
The oxidative pathway for the metabolism of tryptophan along the kynurenine
pathway generates quinolinic acid, an agonist at N-methyl-D-aspartate (NMDA)
receptors, as well as kynurenic acid which is an antagonist at glutamate and nicotinic
receptors. The pathway has become recognised as a key player in the mechanisms
of neuronal damage and neurodegenerative disorders. As a result, manipulation of
the pathway, so that the balance between the levels of components of the pathway
can be modified, has become an attractive target for the development of
pharmacological agents with the potential to treat those disorders. This review
summarises some of the relevant background information on the pathway itself
before identifying some of the chemical strategies for its modification, with examples
of their successful application in animal models of infection, stroke, traumatic brain
damage, cerebral malaria and cerebral trypanosomiasis.
Key-words:-
Tryptophan; kynurenine; quinolinic acid; kynurenic acid; neurodegeneration;
neuroprotection;
Abbreviations
AA: anthranilic acid
AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid.
CNS : central nervous system
CSF: cerebrospinal fluid
FCE28833A: 3,4-dichlorobenzoylalanine
3HAA: 3-hydroxyanthranilic acid
IDO: indoleamine-2,3-dioxygenase
IL-1β : interleukin-1β
KAT: kynurenine aminotransferase
KMO: kynurenine-3-monoxygenase
L689,560: 2-carboxy-5,7-dichloro-4-[[(N-phenylamino)-carbonyl]amino]-1,2,3,4-
tetrahydroquinoline
L701,252: 4-hydroxy-3-(cyclopropylcarbonyl)-7-chloroquinoline-2(1H)-one
L701,324: 4-hydroxy-7-chloro-3-(3-phenyloxy)phenyl-quinoline-2(1H)-one
MDL 100,748: 4-[(carboxymethyl)amino]-5,7-dichloroquinoline-2-carboxylic acid
MDL29,951: 3-(4,6-dichloro-2-carboxyindole-3-yl)propionic acid
MPP+ : 1-methyl-phenylpyridinium.
NAD: nicotinamide adenine dinucleotide
NMDA: N-methyl-D-aspartate
Ro61-8048: 3,4-dimethoxy-N-[4-(3-nitrophenyl)-thiazol-2-yl]-benzenesulfonamide
TDO: tryptophan-2,3-dioxygenase
Th cells: T helper cells
ZD9379: 7-chloro-4-hydroxy-2-(4-methoxy-2-methylphenyl)-1,2,5,10-
tetrahydropyridazino-[4,5b]quinoline-1,10-dione sodium
Introduction
Although quinolinic acid (2,3-pyridine-dicarboxylic acid) was recognised as a
metabolite of tryptophan for many years, it was thought to be merely an inactive
precursor in the synthesis of nicotinic acid and, thus, nicotinamide and the ubiquitous
enzyme co-factor nicotinamide adenine dinucleotide (NAD). It was known that direct
administration of high concentrations into the brain could induce convulsions [1],
although the mechanism was unknown. This situation changed when quinolinic acid
was shown to activate selectively the population of glutamate receptors that are also
sensitive to N-methyl-D-aspartate (NMDA) [2], receptors which were soon thereafter
to be implicated in synaptic transmission and neuronal plasticity phenomena such as
long-term potentiation and long-term depression. This discovery led to the direct
demonstration that over-activation of NMDA receptors by quinolinic acid could
produce neuronal degeneration in the brain [3], raising the possibility of a role in
several forms of accidental or disease-related brain injury [4-6].
After exploring the potential neuro-activity of other tryptophan metabolites along the
same pathway (kynurenines) it was later found that kynurenic acid was an antagonist
at NMDA receptors, with a lesser ability to block other glutamate receptors
responding to kainate and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
(AMPA) [7]. Several families of glutamate receptor blockers for potential therapeutic
use were designed based on the structure of kynurenic acid [8-10], as will be
discussed below.
Excitotoxicity
Although quinolinic acid can produce damage by its activation of NMDA receptors
[3,11] it can also induce, or facilitate the production of, reactive oxygen species such
as hydrogen peroxide, superoxide and hydroxyl radicals [12,13]. Indeed these
molecular species may contribute to, and potentiate, the neurotoxic effects of
quinolinic acid in vivo [14]. The kynurenine pathway and quinolinic acid generation
are activated by pro-inflammatory factors, and this interaction may be relevant to the
notion that inflammation triggered within the central nervous system (CNS) may
contribute to the development of neurodegenerative disorders such as Alzheimer's
disease and Huntington's disease. This possibility is strengthened by data showing a
potentiation of quinolinic acid neurotoxicity by the pro-inflammatory cytokine
interleukin-1β (IL-1β) [15], which has itself been implicated in the damaging effects of
stroke. The cytokine may produce neurodegeneration itself, and increase cell death
produced by quinolinic acid or other NMDA receptor agonists, at least partly by
potentiating amino acid-induced calcium influx [16]. This is, however, a controversial
area, since there is some evidence that IL-1β can inhibit excitotoxic neuronal damage
[17] and a deficiency may increase neuronal injury [18]. Some of these differences
may be attributable to the use of different experimental models and cytokine
concentrations. Interestingly, the toxicity of this combination can be prevented by
antagonists at the A2A receptors for adenosine, compounds which have been shown
to be neuroprotective against injurious stimuli and chemical insults in a variety of
conditions [19]. This might therefore be consistent with a role for quinolinic acid in
neurotoxicity induced by inflammatory stimuli or a range of alternative triggers such
as the dopaminergic toxin 1-methyl-phenylpyridinium (MPP+) and the glutamate
receptor agonist kainic acid.
A commonly voiced concern about the possible role of quinolinic acid in neuronal
damage is that its concentrations in the blood and cerebrospinal fluid (CSF), which
are usually less than 50nM, are much lower than the millimolar levels often required
to induce neuronal damage. However, the levels of compounds in body fluids
represent substantial dilutions from their cellular sites of origin and release, and the
local concentrations of quinolinic acid may be many times greater in the extracellular
space around those activated glial cells and macrophages which produce it. Indeed,
since quinolinic acid is generated in the same types of immune-competent cells that
can generate IL-1β [20,21], their combined presence at higher levels could further
enhance local toxicity. These possibilities are entirely consistent with the increasing
evidence that microglial activation makes a substantial contribution to various form of
brain damage.
In addition, even after its dilution from its sites of generation, quinolinic acid can
attain potentially toxic levels in the blood or cerebrospinal fluid (CSF) following
cerebral insults such as trauma, ischaemia [11, 22, 23] or microbial infections [24].
Finally, there may be subsets of neurons which are particularly sensitive to quinolinic
acid toxicity. Some cells can be killed by maintained exposure to concentrations of
around 100nM quinolinic acid, levels only slightly higher than the resting levels of
approximately 10-50nM.
The relationship between quinolinic acid and neurodegenerative disorders has
attracted a great deal of interest. Special interest has developed into a possible role
in Huntington's disease in the light of molecular studies in yeast models [25] and a
strong association between activity along the kynurenine pathway and the length of
the CAG triplet nucleotide repeat sequence in patients at different stages of the
disorder [26]. This link between neurotoxicity and neurodegeneration is addressed in
other chapters in this issue.
Other kynurenines.
Although the emphasis to date in the understanding of neurodegeneration has
been focussed on quinolinic acid, this compound represents only one component of
the kynurenine pathway of tryptophan oxidation (Figure 1)[10, 11, 27]. The pathway
also generates a glutamate antagonist, kynurenic acid [10, 28] and highly redox-
active compounds such as 3-hydroxykynurenine and 3-hydroxyanthranilic acid.
Kynurenic acid is recognised largely for its ability to block glutamate receptors,
especially the NMDAR at which it blocks the actions of the co-agonist glycine. In
studies of cultured neurons as well as brain slices, kynurenic acid blocked the effects
of exogenously applied acetylcholine or α7-nicotinic receptor-selective agonists [29].
Subsequent work revealed that kynurenic acid could block that component of
excitatory post-synaptic potentials (epsps) mediated by the activation of cholinergic,
nicotinic α7 receptors in hippocampal interneurons [30] - epsps also blocked by
methyl-lycaconitine, an established antagonist at the α7 receptor. Dihydro-β-
erythroidine, which blocks α4β2 receptors, was ineffective, confirming the importance
of the α7 receptors. Kynurenic acid reduced the amplitude of these epsps with an
EC50 of 136 μM and was more potent in blocking the nicotinic epsps than the full,
glutamate-mediated epsps. The ability of kynurenic acid to block nicotinic synaptic
transmission may be important to kynurenic acid pharmacology in the hippocampus,
although it is less potent than when used to block exogenously applied
cholinomimetics in culture. The difference in potency could also be a result of the
different state of differentiation of cells in culture, the relative absence of glial cells or
the different relationships between synaptic terminals and glia with their complement
of enzymes and transporters.
Several clinical studies have examined the levels of kynurenic acid in patient
groups. The levels in blood increased significantly in a sub-population of patients who
died within 21 days of a stroke compared with patients who survived for a longer
period [31]. A possible relationship between the levels of kynurenic acid and patient
mortality has been noted by several groups [32, 33], possibly analogous to reports
that indoleamine-2,3-dioxygenase (IDO; Fig. 1) activity increases with age. One
reason for such a relationship may lie in the fact that, in addition to its formation by
kynurenine aminotransferase (KAT), kynurenic acid can be formed by the non-
enzymic oxidation of kynurenine and tryptophan via indole-3-pyruvic acid [34], a
reaction which is increased by oxidative stress. Increased levels of nitric oxide have
been noted after brain injury, and this can inhibit superoxide dismutase. The resulting
increase in superoxide anions could oxidise indolepyruvate to kynurenic acid,
consistent with reports that nitric oxide donors increase kynurenic acid production
[35].
In fact, KAT exists in two major forms, KAT I being primarily cytosolic in location,
and KAT II being primarily mitochondrial. The latter is identical to α-aminoadipate
aminotransferase. A third form of KAT has been identified, although much less is
known about its selectivity and activity. There is some influence of diet and nutritional
status on this and other kynurenine pathway enzymes, since KAT, kynureninase and
kynurenine-3-monoxygenase are at least partly dependent on the availability of
vitamin B6 (pyridoxine; PLP) for their activity.
3-hydroxyanthranilic acid can generate reactive oxygen species such as hydrogen
peroxide and superoxide when in the presence of transition metal ions but it is also a
highly efficient scavenger of free radicals [36]. Under physiological conditions, 3-
hydroxyanthranilic acid can auto-oxidise to quinoneimines, but the reaction products
can then oxidise other molecules [37, 38]. The balance of this redox cycling
behaviour will depend on local concentrations of iron and copper, the levels and
activities of other free radical generators and anti-oxidants, and pH, which influences
both redox activity and metal ion availability [39].
Some of the kynurenine compounds are of increasing interest in disorders of brain
function. The experimental evidence for a role of kynurenines in stroke injuries is
firmly based on the demonstration that inhibition of the pathway reduces the neuronal
damage which follows cerebral vessel occlusion in rodents [40]. In a recent study on
humans, blood samples were taken from patients as soon as possible after entering
hospital in the immediate aftermath of a stroke and for up to 14 days thereafter [31].
As expected from earlier work by others, the existence of brain damage was
indicated by an early increase in the levels of protein S100B, although this persisted
for several days, perhaps reflecting the progressive development of delayed neuronal
damage. Neopterin levels, a well-established marker of inflammation [41] were also
substantially elevated, together with activation of the kynurenine pathway, since
blood levels of kynurenine and tryptophan revealed an increased kynurenine:
tryptophan ratio consistent with activation of the initial enzymes of the pathway –
[IDO] and tryptophan-2,3-dioxygenase [TDO]. There was also a highly significant
decrease in the ratio of 3-hydroxyanthranilic acid: anthranilic acid which was strongly
correlated with infarct volume indicated in computed tomography brain scans [31].
The reason for the changed ratio is not clear, though the reciprocal changes
suggests a biochemical connection such as the conversion of anthranilic acid (AA)
into 3-hydroxyanthranilic acid (3HAA) [42]. The changed ratio, observed now in
several disorders including osteoporosis, chronic brain injury, Huntington’s disease,
coronary heart disease, thoracic disease, stroke and depression [43] could then
indicate that inflammation generates a decrease in that conversion.
The loss of 3HAA may have important consequences for the immune system. 3-
hydroxyanthranilic acid inhibits the proliferation of CD8+ T cells [44]. It can also
suppress the responses of T cells to allogeneic stimuli [45], acting primarily on Th1
rather than Th2 cells [46], and decreases the ability of dendritic cells to activate T
cells [47]. The overall result of the changed 3HAA:AA ratio, therefore, would seem to
be protective, limiting the inflammatory response, including the activation of microglia
which are thought to contribute to brain damage following stroke.
Anthranilic acid interacts with copper to form an anti-inflammatory complex able to
remove highly injurious reactive oxygen species [48,49]. Several anthranilic acid
derivatives have similar, marked anti-inflammatory activity [50] and it is an intriguing
possibility that the high levels of anthranilic acid in some disorders such as strokes
might be converted to anti-inflammatory compounds as a mechanism to reduce
tissue damage.
The kynurenine pathway as a pharmacological target
Interest in the kynurenine pathway as a potential site of drug action has centred
around the possibility of modifying the balance between the endogenous
concentrations of quinolinic acid and it’s antagonist, kynurenic acid. This interest is
expanding in view of the wide range of clinical disorders in which abnormalities in the
pathway have been proposed, including AIDS-related dementia [51, 52] and stroke
[31], but with probably the greatest interest in the neurodegenerative disorders. One
of the popular hypotheses for the aetiology of disorders such as Alzheimer's disease,
Parkinson's disease and Huntington's disease is that there is an ongoing over-
activation of glutamate receptors, especially NMDA receptors. This hypothesis has
received much support in principle from the discovery that the spinal degenerative
disorder amyotrophic lateral sclerosis, or motoneurone disease, is the result of a
defective glutamate transporter. The resulting accumulation of extracellular glutamate
generates oxidative stress that ultimately leads to the demise of the motoneurones.
By analogy, a loss of transporters or increased presence of receptors for glutamate in
localised regions of the brain such as the nucleus basalis, neostriatum or substantia
nigra could cause or contribute to the neuronal death in Alzheimer's disease,
Huntington's disease and Parkinson's disease respectively. It is probable also that
much of the chronic brain damage occurring after stroke injury is the result of
‘delayed neurodegeneration’ which is caused, not by the initial insult itself, but by
secondary processes entrained by that insult.
These secondary processes may be classified generally as inflammatory, since
there is growing evidence that they are mediated by cytokines and chemokines
produced by glial cells that are immunologically activated by products of the initial
insult. These compounds play a key role in attracting and modulating the activity of
peripheral monocytes and macrophages that invade the CNS and participate in the
removal of damaged tissue and the control of potential infections. One consequence
of this central inflammatory response, however, seems to be the enhancement of
neuronal damage to some extent (as noted above). The activation of immune-
competent cells – peripheral macrophages or central microglia – also includes
induction of the kynurenine pathway, so that quinolinic acid will also be generated
and could contribute to the ‘inflammatory’ response and the later phases of damage
(known as ‘delayed degeneration’). This view is supported by the demonstration that
the excitotoxic response to kainic acid – a response generally attributed almost
exclusively to the direct activation of kainate receptors and the consequent calcium
influx – can be reduced significantly by the co-administration of m-nitrobenzoyl-
alanine, an inhibitor of kynurenine-3-monoxygenase (KMO; Fig. 1) which reduces the
production of quinolinic acid [53]. The implication is that the secondary activation of
microglia includes the generation of increased levels of quinolinic acid or other
kynurenines (see below) that exacerbate neuronal damage.
Infections of the CNS.
By activating IDO, viral components or bacterial lipopolysaccharides increase the
production of several kynurenines. There are substantially elevated levels of
quinolinic acid in the brains of children with bacterial infections of the CNS, changes
which correlate well with markers of immune activation such as neopterin [52].
Septicaemia is similarly associated with increased serum and CSF quinolinic acid
(10-fold in serum and 30-fold in CSF) and kynurenine. Infection of mice by Herpes
simplex virus type 1 raised the levels of quinolinic acid in mice, in parallel with
paralysis.
More recently, attention has focussed on parasitic infections, with reports that the
KMO inhibitor 3,4-dimethoxy-N-[4-(3-nitrophenyl)-thiazol-2-yl]-benzenesulfonamide
(Ro61-8048) described by Roever et al. [54] could prevent death and ataxia in mice
infected with the malaria parasite Plasmodium [55]. This protection was associated
with the predictably raised levels of kynurenic acid, and also of anthranilic acid and
the chemotactic monocyte chemoattractant protein-1. A similar protection has now
been demonstrated in trypanosomiasis (sleeping sickness), with a significant
reduction by Ro61-8048 of the later stages of brain pathology occurring in mice
infected with Trypanosoma brucei parasites [56].
Ischaemic damage.
In addition to the work on stroke introduced above, a delayed increase of quinolinic
acid was noted in gerbils subjected to a period of cerebral ischaemia, with quinolinic
acid levels rising to 50-fold their basal value after 7 days [57]. There was an
accompanying increase in the activity of several of the kynurenine pathway enzymes
in those brain regions experiencing an interrupted blood supply. Intracisternally
applied tryptophan was converted to quinolinic acid in damaged but not normal areas
of brain, consistent with a local production at the sites of injury. There was no change
of kynurenine aminotransferase activity and, as a result, there was an increased ratio
of quinolinic acid: kynurenic acid which would tend to exacerbate the degree of
neuronal injury. Central microglia and macrophages may contribute to delayed
neuronal death after cerebral ischaemia [58] since they possess and secrete
quinolinic acid as noted above and as reflected in the existence of quinolinic acid-
positive microglia in the brain following transient global ischaemia [59].
Huntington's disease
The injection or infusion of quinolinic acid into the rodent striatum has become a
widely accepted paradigm for generating electrophysiological, neuropathological and
behavioural changes closely resembling those seen in patients with Huntington's
disease [60], particularly symptoms which develop in the early stages of the disorder
[61]. Even in non-human primates quinolinic acid is able to induce motor disabilities
very similar to the involuntary movements of Huntington’s disease [62, 63].
When considered together with the clinical evidence for parallel changes in the
CAG triplet repeat sequence, symptoms, and kynurenine metabolism [26], there is a
real possibility that interference with the pathway could slow or prevent the
development of symptoms or the progression of the disorder. Inhibition of KMO, with
the resulting increase of kynurenic acid levels and possible lowering of quinolinic acid
production, represents a promising avenue for this therapeutic approach.
A therapeutic strategy
Kynurenic acid analogues
A major effort to develop antagonists acting directly at the glutamate binding sites
resulted in a large number of compounds with therapeutic promise, although some
exhibited neurotoxic and psychological side effects, including neuronal vacuolisation
or disturbing psychotomimetic effects. These problems contributed to a shift in
emphasis towards the glycine-B co-agonist site on the NMDA receptor, with the
beneficial corollary that kynurenic acid analogues generally cross the blood–brain
barrier more easily than many of the glutamate site quinoxaline ligands.
The most obvious way to base a therapeutic strategy for neuroprotection on the
kynurenine pathway is to mimic the glutamate blocking activity of kynurenic acid,
since over-activation of the various glutamate receptors may be a key characteristic
of brain damage in stroke or neurodegeneration. To this end, several approaches
have been reported in which the kynurenic acid molecule itself is modified by the
addition of halogen atoms. This can generate potent antagonistic analogues such as
5,7-dichlorokynurenic acid with an IC50 of only 80 nM as an antagonist at the
glycine-B site on the NMDA receptor. The potency of these compounds is increased
further if the 4-hydroxy group of kynurenic acid is substituted by acetic acid or similar
moieties. Such additions have led to amido- and thio-substituted compounds such as
MDL 100,748 (4-[(carboxymethyl)amino]-5,7-dichloroquinoline-2-carboxylic acid) [64,
65] (Fig. 2). Some of these compounds have shown clear therapeutic potential as
neuroprotectants or anticonvulsants, while L689,560 (2-carboxy-5,7-dichloro-4-[[(N-
phenylamino)-carbonyl]amino]-1,2,3,4-tetrahydroquinoline) (Fig. 2) has been used
extensively to displace compounds at the strychnine-resistant (NMDA-linked) glycine
binding site. A valuable discovery was made when it was found that 3-phenyl
substituents retained potent activity at the NMDA/glycine site but they were also
more lipophilic than earlier compounds and, presumably partly as a result of this
property, were bioavailable after oral administration [66]. The presence of a 3-keto
grouping was retained in quinones such as L701,252 (4-hydroxy-3-
(cyclopropylcarbonyl)-7-chloroquinoline-2(1H)-one) (Fig. 2) which showed nanomolar
potency at displacing L689,560 binding, and was an effective anticonvulsant in mice.
Kynurenic acid has also been converted into 2-quinolone sulphonamide analogues
with good antagonistic potency.
A breakthrough in kynurenic acid pharmacology arrived with the demonstration that
the quinoline nucleus could be replaced by the indole nucleus with a retention of
glutamate antagonism. Of many subsequent analogues of this structure MDL29,951
(3-(4,6-dichloro-2-carboxyindole-3-yl)propionic acid) [67, 68] (Fig. 2)proved to be
highly effective, although oral bioavailability was reduced. Lipid solubility and blood–
brain barrier penetration are increased if the 3-position of the kynurenate nucleus is
occupied by highly lipophilic substituents, as in L701,324 (4-hydroxy-7-chloro-3-(3-
phenyloxy)phenyl-quinoline-2(1H)-one (Fig. 2) [69]. Both this compound and a
sulphur-containing analogue have good antagonistic activity at the glycine-B co-
agonist site and acceptable systemic and oral bioavailability. The B ring of the
kynurenate nucleus is also amenable to modification by a range of substituents.
Neuronal damage produced by focal cerebral ischaemia can be reduced by a
number of compounds in rats, even when administered several hours after the insult.
A long half-life, leading to greater persistence of compounds in the brain may
account for the efficacy of the Zeneca compound ZD9379 (7-chloro-4-hydroxy-2-(4-
methoxy-2-methylphenyl)-1,2,5,10-tetrahydropyridazino-[4,5b]quinoline-1,10-dione
sodium) (Fig. 2) which, with a half-life of 34 hours in rats, was able to protect the
brain up to at least 24 hours after middle cerebral artery occlusion [70].
Replacement of the nitrogenous ring of kynurenate by a seven-membered ring to
yield benzazepinedione compounds allows retention of antagonism at NMDA
receptors, protection against cerebral ischaemia and the ability to displace
strychnine-resistant glycine binding in vitro or ex vivo [71].
Pro-drugs
The use of pro-drugs to deliver kynurenic acid or its analogues directly into the
brain provides an alternative approach to overcoming the limitations of the blood-
brain barrier. Esterified analogues of kynurenic acid penetrate into the CNS
significantly more rapidly than kynurenic acid itself. Once within the brain
parenchyma, such compounds can be converted to kynurenic acid itself. Similarly,
esterified 4-amino analogues are converted into kynurenic acid in the brain, with
retention or increase in their functional activity, while precursor analogues such as L-
4-chloro-kynurenine and 4,6-dichlorokynurenine are metabolised to the highly potent
kynurenic acid derivatives 7-chlorokynurenic acid and 5,7-dichlorokynurenic acid [72,
73].
Enzyme inhibitors
A different approach is to interfere with the enzymes of the kynurenine pathway so
as to modify the ratio between quinolinic acid and kynurenic acid levels, or to alter
the relative concentrations of other components of the pathway. The ratio of
quinolinic acid: kynurenic acid is largely determined by KMO (Fig. 1) and a number of
investigators have attempted to define the molecular features of molecules required
to achieve inhibition of this enzyme without having non-specific effects on related
flavine mono-oxygenases. A similar shift of balance should be attainable by inhibiting
kynureninase.
The feasibility of this approach was first demonstrated by the development of
nicotinylalanine as an inhibitor of kynureninase and KMO [74-76] (Fig. 1). The
Inhibition of KMO reduced the levels of endogenous quinolinic acid but increased the
conversion of kynurenine to kynurenic acid. These neurochemical changes were
substantially greater when nicotinylalanine was administered together with
kynurenine and the acidic transport inhibitor probenecid, which limits the efflux of
kynurenic acid formed within the brain. The change in the ratio of quinolinic acid:
kynurenic acid was assumed to underlie the anticonvulsant and neuroprotective
properties of nicotinylalanine.
Related compounds developed since this initial proof of concept work generated
meta-nitrobenzoylalanine which preferentially inhibits KMO, while ortho-
methoxybenzoylalanine preferentially inhibits kynureninase [ 77, 78]. The former
produces higher levels of kynurenine and kynurenic acid in the brain and peripheral
tissues, but both have been shown to increase the amount of kynurenic acid in the
hippocampus in vivo. This probably accounts for the decrease of locomotion and
suppression of seizures in sensitive strains of mice [79]. The inhibition of KMO
produced the expected fall in 3-hydroxykynurenine levels together with the increase
of kynurenic acid.
FCE28833A (3,4-dichlorobenzoylalanine) (Fig. 3) is active after systemic
administration and inhibits KMO more effectively than meta-nitrobenzoylalanine
leading to an increase of kynurenine and kynurenic acid in the brain. Concentrations
of both metabolites were increased substantially in the rat hippocampus after a single
systemic injection and, interestingly from a therapeutic viewpoint, the levels of
kynurenic acid remained high for 24 h after the injection [80].
The compound used almost exclusively at the present time to inhibit KMO is one of
a series of N-(4-phenylthiazol-2-yl) benzenesulphonamides. This compound, Ro61-
8048 (Fig. 3) inhibits KMO with an IC50 of only 37 nM. It also increased the levels of
kynurenic acid in the extracellular fluid of gerbil brain after oral administration [54].
A second approach to preventing the synthesis of quinolinic acid is to inhibit 3-
hydroxyanthranilic acid 3,4-dioxygenase. Good inhibition is produced by a series of
4-halo-3-hydroxyanthranilic acids which produce a corresponding reduction in the
formation of quinolinic acid [81, 82].
Conclusion
In summary, the discovery that tryptophan metabolites have direct agonist or
antagonist activity at glutamate and nicotinic receptors in the CNS has resulted in an
enormous amount of work to generate more effective or more brain-penetrant
analogues of those metabolites, or to develop approaches which modify their
endogenous concentrations. The proof of principle that interference with the
kynurenine pathway is indeed neuroprotective is now well established in models for
conditions as different as epileptic seizures, cerebral malaria, stroke and
trypanosomiasis. The over-riding advantage of persevering to manipulate the
kynurenine pathway as a key site of action for new therapeutic agents lies in the fact
that the pathway is activated primarily, if not exclusively, in those areas which have
been subjected to damage or are undergoing degeneration. In this important respect,
modulators of the kynurenine pathway will have inestimable advantages over
compounds which block non-specifically all glutamate or nicotinic receptors
throughout the brain and, indeed, throughout peripheral tissues where they are also
physiologically important.
References
[1] Lapin, I.P. (1989) Behavioural and convulsant effects of kynurenines. In Quinolinic
acid and the Kynurenines ed. Stone, T.W., pp 193-211 CRC Press, Boca Raton,
Florida, 1989.
[2] Stone, T.W. and Perkins, M.N. (1981) Quinolinic acid: a potent endogenous
excitant at amino acid receptors in CNS. Eur. J. Pharmacol. 72, 411-412.
[3] Schwarcz, R., Whetsell, W.O. & Mangano, R.M. (1983) Quinolinic acid - an
endogenous metabolite that produces axon-sparing lesions in rat-brain. Science 219,
316-318.
[4] Guillemin, G.J. (2011) Quinolinic acid, the inescapable neurotoxin. FEBS J. (this
issue)
[5] Kandanearatchi, A. and Brew, B. J. (2011) The kynurenine pathway and quinolinic
acid: pivotal roles in HIV and associated neurocognitive disorders. FEBS J. (this
issue).
[6] Perez-de-la-Cruz, V. and Santamaria, A. (2011) Quinolinic acid, an endogenous
molecule combining excitotoxicity, oxidative stress and other toxic mechanisms.
FEBS J. (this issue)
[7] Perkins, M.N. and Stone, T.W. (1983) Quinolinic acid: regional variations in
neuronal sensitivity. Brain Res. 259, 172-176.
[8] Stone, T.W. (2000) The development and therapeutic potential of kynurenic acid
and kynurenine derivatives for CNS neuroprotection. Trends. Pharmacol. Sci. 21,
149-154
[9] Stone, T.W. (2000) Inhibitors of the kynurenine pathway. Eur. J. Med. Chem. 35,
179-186
[10] Stone, T.W. and Darlington, L.G. (2002) Endogenous kynurenines as targets for
drug discovery and development. Nature Reviews Drug Discovery 1, 609-620.
[11] Stone, T.W. (2001) Kynurenines in the CNS - from obscurity to therapeutic
importance. Progr. in Neurobiol. 64, 185-218.
[12] Rios C. and Santamaria, A. (1991) Quinolinic acid is a potent lipid peroxidant in
rat brain homogenates. Neurochem. Res. 16, 1139-1143.
[13] Santamaria, A., Jimenez-Capdeville, M.E., Camacho, A., Rodriguez-Martinez,
E., Flores, A., Galvan-Arzate, S. (1991) In vivo hydroxyl radical formation after
quinolinic acid infusion into rat corpus striatum. Neuroreport 12, 2693-2696.
[14] Behan, W.M.H., McDonald, M., Darlington, L.G. and Stone, T.W. (1999)
Oxidative stress as a mechanism for quinolinic acid-induced hippocampal damage:
protection by melatonin and deprenyl. Brit. J. Pharmacol. 128, 1754-1760.
[15] Stone, T.W. and Behan, W.M.B. (2007) Interleukin-1β but not TNF-α potentiates
neuronal damage by quinolinic acid: protection by an adenosine A2A receptor
antagonist. J. Neurosci. Res. 85, 1077-1085.
[16] Viviani, B., Bartesaghi, S., Gardoni, F., Vezzani, A., Behrens, M.M., Bartfai, T.,
Binaglia, M., Corsini, E., Di Luca, M., Galli, C.L. and Marinovich M. (2003)
Interleukin-1β enhances NMDA receptor-mediated intracellular calcium increase
through activation of the Src family of kinases. J. Neurosci. 34, 8692-8700.
[17] Strijbos, P.J.L.M. and Rothwell, N.J. (1995) Interleukin-1β attenuates excitatory
amino acid-induced neurodegneration in vitro – involvement of nerve growth factor. J.
Neurosci. 15, 3468-3474.
[18] Pelidou, S.H., Schultzberg, M. and Iverfeldt, K. (2002) Increased sensitivity to
NMDA receptor-induced excitotoxicity in cerebellar granule cells from interleukin-1
receptor type 1-deficient mice. J. Neuroimmunol. 133, 108-115.
[19] Stone, T.W., Ceruti, S. and Abbracchio, M.P. (2009) Adenosine Receptors and
Neurological Disease: Neuroprotection and Neurodegeneration. In Handbook of
Experimental Pharmacology, Vol., 193: Adenosine Receptors in Health and Disease,
eds. CN Wilson & SJ Mustafa. Chapter 17, pp. 535 - 589. Springer.
[20] Moffett, J.R., Espey, M.G. and Namboodiri, M.A.A. (1994) Antibodies to
quinolinic acid and the determination of its cellular distribution within the rat immune
system. Cell and Tissue Res. 278, 461-469.
[21] Heyes, M.P., Achim, C.L., Wiley, C.A., Major, E.O., Saito, K., and Markey, S.P.
(1996) Human microglia convert L-tryptophan into the neurotoxin quinolinic acid.
Biochem. J. 320, 595-597.
[22] Heyes, M.P. and Nowak, T.S. Jnr. Delayed increases in regional brain quinolinic
acid follow transient ischemia in the gerbil. J. Cereb. Blood Flow Metab. 10: 660-667,
1990.
[23] Blight, A.R., Leroy, E.C. Jnr. and Heyes, M.P. (1997) Quinolinic acid
accumulation in injured spinal cord: time course, distribution, and species differences
between rat and guinea pig. J. Neurotrauma 14, 89-98.
[24] Heyes, M.R., Brew, B.J., Martin, A., Price, R.W., Salazar, A.M., Sidtis, J.J., Yergey,
J.A., Mourdian, M.M., Sadler, A.E., Keilp, J., Rubinow, D. and Markey, S.P. (1991)
Quinolinic acid in cerebrospinal fluid and serum in HIV-1 infection: relationship to clinical
and neurologic status. Ann. Neurol. 29: 202-209.
[25] Giorgini, F., Guidetti, P., Nguyen, Q.V., Bennett, S.C., Muchowski, P.J. (2005) A
genomic screen in yeast implicates kynurenine 3-monooxygenase as a therapeutic
target for Huntington disease. Nat. Genet. 37, 526-531.
[26] Forrest, C.M., Mackay, G.M., Stoy, N., Spiden, S.L., Taylor, R., Stone, T.W. and
Darlington, L.G. (2010) Blood levels of kynurenines, interleukin IL-23 and sHLA-G at
different stages of Huntington’s disease. J. Neurochem. 112, 112-122.
[27] Stone, T.W. (1993) The neuropharmacology of quinolinic and kynurenic acids.
Pharmacol. Revs. 45, 309-379.
[28] Perkins, M.N. and Stone, T.W. (1982) An iontophoretic investigation of the action
of convulsant kynurenines and their interaction with the endogenous excitant
quinolinic acid. Brain Res. 247, 184-187.
[29] Hilmas, C., Pereira, E.F.R., Alkondon, M., Rassoulpour, A., Schwarcz, R. and
Albuquerque, E.X. (2001) The brain metabolite kynurenic acid inhibits α7 nicotinic
receptor activity and increases non-α7 nicotinic receptor expression: physiopathological
implications. J. Neurosci. 21, 7463-7473.
[30] Stone, T.W. (2007) Kynurenic acid blocks nicotinic synaptic transmission to
hippocampal interneurons in young rats. Eur. J. Neurosci. 25, 2656-2665.
[31] Darlington, L.G., Mackay, G.M., Forrest, C.M., Stoy, N., George, C. and Stone,
T.W. (2007) Altered kynurenine metabolism correlates with infarct volume in stroke.
Eur. J. Neurosci. 26, 2211-2221.
[32] Pertovaara, M., Raitala, A., Lehtimaki, T., Karhunen, P.J., Oja, S.S., Jylha, M.,
Hervonen, A. and Hurme, M. (2006) Indoleamine 2,3-dioxygenase activity in
nonagenarians is markedly increased and predicts mortality. Mech. Ageing and
Develop. 127, 497-499.
[33] Baran, H., Kepplinger, B., Herrera-Marschitz, M., Stolze, K., Lubec, G. and Nohl,
H. (2001) Increased kynurenic acid in the brain after neonatal asphyxia. Life Sci. 69,
1249-1256.
[34] Politi, V., Deluca, G., Gallai, V., Puca, O., Comin, M. (1999) Clinical experiences
with the use of indole-3-pyruvic acid. Adv. Exp. Med. Biol. 467, 227-232.
[35] Luchowski, P. and Urbanska, E.M. (2007). SNAP and SIN-1 increase brain
production of kynurenic acid. Eur. J. Pharmacol. 563, 130-133.
[36] Weiss, G., Diez-Ruiz, A., Murr, C., Theurl, I. and Fuchs, D. (2002) Tryptophan
metabolites as scavengers of reactive oxygen and chlorine species. Pteridines 13,
140-144
[37] Giles, G.I., Collins, C.A., Stone, T.W. and Jacob, C. (2003) Electrochemical and
in vitro evaluation of the redox properties of kynurenine species. Biochem. Biophys.
Res. Comm. 300, 719-724
[38] Leipnitz, G., Schumacher, C., Dalcin, K.B., Scussiato, K., Solano, A., Funchal,
C., Dutra-Filho, C.S., Wyse, A.T.S., Wannmacher, C.M.D., Latini, A. and Wajner, M.
(2006). In vitro evidence for an antioxidant role of 3-hydroxykynurenine and 3-
hydroxyanthranilic acid in the brain. Neurochem. Internat. 50, 83-94.
[39] Goldstein, L.E., Leopold, M.C., Huang, X.D., Atwood, C.S., Saunders, A.J.,
Hartshorn, M., Lim, J.T., Faget, K.Y., Muffat, J.A., Scarpa, R.C., Chylack, L.T.,
Bowden, E.F., Tanzi, R.E. and Bush, A.I. (2000) 3-Hydroxykynurenine and 3-
hydroxyanthranilic acid generate hydrogen peroxide and promote alpha-crystallin
cross-linking by metal ion reduction. Biochemistry 39, 7266-7275.
[40] Cozzi, A., Carpenedo, R. and Moroni, F. (1999) Kynurenine hydroxylase
inhibitors reduce ischemic brain damage: Studies with (m-nitrobenzoyl)-alanine
(mNBA) and 3,4-dimethoxy-[-N-4-(nitrophenyl)thiazol-2YL]-benzenesulfonamide (Ro
61-8048) in models of focal or global brain ischemia. J. Cereb. Blood Flow Metab. 19
, 771-777.
[41] Murr, C., Widner, B., Wirleitner, B. and Fuchs, D. (2002) Neopterin as a marker
for immune system activation. Curr. Drug Metab. 3, 175-187.
[42] Baran, H. and Schwarcz, R. (1990) Presence of 3-hydroxyanthranilic acid in rat-
tissues and evidence for its production from anthranilic acid in the brain. J.
Neurochem. 55, 738-744
[43] Darlington, L.G., Forrest, C.M., Mackay, G.M., Stoy, N., Smith, R.A., Smith, A.J.
and Stone, T.W. (2010) On the biological significance of the 3-hydroxyanthranilic
acid:anthranilic acid ratio. Internat. J. Tryptophan Res. 3, 51-59.
[44] Weber, W.P., Feder-Mengus, C., Chiarugi, A., Rosenthal, R., Reschner, A.,
Schumacher, R., Zajaz, P., Misteli, H., Frey, D.M., Oertli, D., Heberer, M. and
Spagnoli, G.C. (2006) Differential effects of the tryptophan metabolite 3-
hydroxyanthranilic acid on the proliferation of human CD8(+) T cells induced by TCR
triggering or homeostatic cytokies. Eur. J. Immunol. 36, 296-304
[45] Terness, P., Bauer, T.M., Rose, L., Dufter, C., Watzlik, A., Simon, H. and Opelz,
G. (2002) Inhibition of allogeneic T cell proliferation by indoleamine 2,3-dioxygenase-
expressing dendritic cells: Mediation of suppression by tryptophan metabolites. J.
Exp. Med. 196, 447-457.
[46] Fallarino, F., Grohmann, U., Vacca, C., Bianchi, R., Orabona, C., Spreca, A.,
Fioretti, M.C. and Puccetti, P. (2002) T cell apoptosis by tryptophan catabolism. Cell
Death Diff. 9, 1069-1077.
[47] Lopez, A,S., Alegre, E., LeMaoult, J., Carosella, E. and Gonzalez, A. (2006)
Regulatory role of tryptophan degradation pathway in HLA-G expression by human
monocyte-derived dendritic cells. Molec. Immunol. 43, 2151-2160
[48] Miche, H., Brumas, V. and Berthon, G. (1997) Copper(II) interactions with
nonsteroidal antiinflammatory agents. 2. Anthranilic acid as a potential (OH)-O-center
dot-inactivating ligand. J. Inorg. Biochem. 68, 27-38.
[49] Halova-Lajoie, B., Brumas, W., Fiallo, M.M.L. and Berthon, G. (2006) Copper(II)
interactions with non-steroidal anti-inflammatory agents. III - 3-Methoxyanthranilic
acid as a potential (OH)-O-center dot-inactivating ligand: A quantitative investigation
of its copper handling role in vivo. J. Inorg. Biochem. 100, 362-373.
[50] Platten, M., Ho, P.P., Youssef, S., Fontoura, P., Garren, H., Hur, E.M., Gupta,
R., Lee, L.Y., Kidd, B.A., Robinson, W.H., Sobel, R.A., Selley, M.L. and Steinman, L.
(2005) Treatment of autoimmune neuroinflammation with a synthetic tryptophan
metabolite. Science 310, 850-855..
[51] Heyes, M.P., Mefford, I.N., Quearry, B.J., Dedhia, M., and Lackner, A. (1990)
Increased ratio of quinolinic acid to kynurenic acid in cerebrospinal fluid of D retrovirus-
infected Rhesus Macaques: relationship to clinical and viral status. Ann. Neurol. 27,
666-675.
[52] Heyes, M.P., Brew, B.J., Saito, K., Quearry, B.J., Price, R.W., Lee, K., Bhalla,
R.B., Der, M. and Markey, S.P. (1992) Inter-relationships between neuroactive
kynurenines, neopterin and β2-microglobulin in CSF and serum of HIV-1 infected
patients. J. Neuroimmunol. 40, 71-80
[53] Behan, W.M.H. and Stone, T.W. (2000) Role of kynurenines in the neurotoxic
actions of kainic acid. Brit. J. Pharmacol. 129, 1764-1770.
[54] Rover, S., Cesura, A.M., Hugenin, P., Kettler, R. and Szente, A. (1997)
Synthesis and biochemical evaluation of N-(4-phenylthiazol-2-
yl)benzenesulfonamides as high-affinity inhibitors of kynurenine 3-hydroxylase. J.
Med. Chem. 40, 4378-4385.
[55] Clark, C.J., Mackay, G.M.., Smythe, G.A., Bustamante, S., Stone, T.W. and
Phillips, R.S. (2005) Prolonged survival of a murine model of cerebral malaria by
kynurenine pathway inhibition. Infection and Immunity, 73, 5249-5251.
[56] Rodgers, J., Stone, T. W., Barratt, M.P., Bradley, B., Kennedy, P.G. (2009)
Kynurenine pathway inhibition reduces central nervous system inflammation in a
model of human African trypanosomiasis. Brain 132, 1259-1267.
[57] Heyes, M.P. and Nowak, T.S. Jnr. (1990) Delayed increases in regional brain
quinolinic acid follow transient ischemia in the gerbil. J. Cereb. Blood Flow Metab. 10,
660-667.
[58] Lees, G.J. (1993) The possible contribution of microglia and macrophages to
delayed neuronal death after ischemia. J. Neurol. Sci. 114, 119-122.
[59] Baratte, S., Molinari, A., Veneroni, O., Speciale, C., Benatti, L. and Salvati, P.
(1998) Temporal and spatial changes of quinolinic acid immunoreactivity in the gerbil
hippocampus following transient cerebral ischemia. Mol. Brain Res. 59, 50-57.
[60] Shear, D.A., Dong, J., Haik-Oreguer, K.L., Bazzett, T.J., Albin, R.L. and Dunbar,
G.L. (1998) Chronic administration of quinolinic acid in the rat striatum causes
spatial learning deficits in a radial arm water maze task. Exp. Neurol. 150, 305-311.
[61] Shear, D.A., Dong, J., Gundy, C.D., Haik-Creguer, K.L. and Dunbar, G.L. (1998)
Comparison of intrastriatal injections of quinolinic acid and 3-nitropropionic acid for
use in animal models of Huntington's disease. Prog. Neuropsychopharmacol. Biol.
Psychiat. 22, 1217-1240.
[62] Storey, E., Cipolloni, P.B., Ferrante, R.J., Kowall, N.W. and Beal, M.F. (1994)
Movement disorder following excitotoxin lesions in primates. Neuroreport 5, 1259-
1261.
[63] Burns, L.H., Pakzaban, P., Deacon, T.W., Brownell, A.L., Tatter, S.B., Jenkins,
B.G. and Isacson, O. (1995) Selective putaminal excitotoxic lesions in non-human
primates model the movement disorder of Huntington disease. Neurosci. 64, 1007-
1017.
[64] Harrison, B.L., Baron, B.M., Cousino, D.M., McDonald, I.A. (1990) 4-
[(carboxymethyl)oxy]-5,7-dichloroquinoline-2-carboxylic and 4-
[(carboxymethyl)amino]-5,7-dichloroquinoline-2-carboxylic acid - new antagonists of
the strychnine-insensitive glycine binding-site on the N-methyl-D-aspartate receptor
complex. J. Med. Chem. 33, 3130-3132.
[65] Baron, B. M., Harrison, B.L., McDonald, I.A., Meldrum, B.S., Palfreyman, M.G.,
Salituro, F.G., Siegel, B.W., Slone, A.L., Turner, J.P. and White, H.S. (1992) Potent
indoleline and quinoline-containing N-methyl-D-aspartate antagonists acting at the
strychnine-insensitive glycine binding-site. J. Pharmacol. Exp. Ther. 262, 947-956.
[66] Carling, R.W., Leeson, P.D., Moore, K.W., Moyes, C.R., Duncton, M., Hudson,
M.L., Baker, R., Foster, A.C., Grimwood, S., Kemp, J.A., Marshall, G.R.,
Tricklebank, M.D. and Saywell, K.L.(1997) 4-substituted-3-phenylquinolin-2(1H)-
ones: acidic and nonacidic glycine site N-methyl-D-aspartate antagonists with in vivo
activity. J. Med. Chem. 40 (1997) 754-765.
[67] Salituro, F.G., Harrison, B.L., Baron, B.B., Nyce, P.L., Stewart, K.T. and
McDonald, I.A. (1990) 3-(2-carboxyindol-3-yl)propionic acid-derivatives - antagonists
of the strychnine-insensitive glycine receptor associated with the N-methyl-D-
aspartate receptor complex. J. Med. Chem. 33, 2944-2946.
[68] Cai, S.X., Zhou, Z.L., Huang, J.C., Whittemore, E.R., Egbuwoku, Z.O.,
Hawkinson, J.E., Woodward, R.M., Weber, E. and Keana, J.F.W. (1996) Structure -
activity relationships of 4-hydroxy-3-nitroquinolin-2(1H)-ones as novel antagonists at
the glycine site of N-methyl-D-aspartate receptors. J. Med. Chem. 39, 4682-4686.
[69] Bristow, L.J., Flatman, K.L., Hutson, P.H., Kulagowski, J.J., Leeson, P.D.,
Young, L. and Tricklebank, M.D. (1996) The atypical neuroleptic profile of the
glycine/N-methyl-D-aspartate receptor antagonist, L-701,324, in rodents. J.
Pharmacol. Exp. Therap. 277, 578-585.
[70] Takano, K., Tatlisumak, T., Formato, J.E., Carano, R.A.D., Bergmann, A.G. ,
Pullan, L.M., Bare, T.M., Sotak, C.H. and Fisher, M. (1997) Glycine site antagonist
attenuates infarct size in experimental focal ischemia - postmortem and diffusion
mapping studies. Stroke 28, 1255-1262.
[71] Jackson, P.F., Davenport, T.W., Garcia, L., McKinney, J.A., Melville, M.G.,
Harris, G.G., Chapdelaine, M.J., Damewood, J.R., Pullan, L.M. and Goldstein, J.M.
(1995) Synthesis and biological activity of a series of 4-aryl substituted
benz[b]azepines: Antagonists at the strychnine-insensitive glycine site. Bioorg. Med.
Chem. Lett. 5, 3097-3100.
[72] Hokari, M., Wu, H.-Q., Schwarcz, R., Smith, Q.R. (1996) Facilitated brain uptake
of 4-chlorokynurenine and conversion to 7-chlorokynurenic acid. Neuroreport 8, 15-
18.
[73] Moore, L. W., Leeson, P.D., Carling, R.W., Tricklebank, M.D., Singh, L. (1993)
Anticonvulsant activity of glycine-site NMDA antagonists. 1: 2-carboxyl prodrugs of
5,7-dichlorokynurenic acid. Bioorg. Med. Chem. Lett. 3, 61-64.
[74] Connick, J.H., Heywood, G.C., Sills, G.J., Thompson, G.G., Brodie, M.J. and
Stone, T.W. (1992) Nicotinylalanine increases cerebral kynurenic acid content and
has anticonvulsant activity. Gen. Pharmacol. 23, 235-239.
[75] Russi, P., Alesiani, M., Lombardi, G., Davolio, P., Pellicciari, R. and Moroni, F.
(1992) Nicotinylalanine increases the formation of kynurenic acid in the brain and
antagonizes convulsions. J. Neurochem. 59, 2076-2080.
[76] Moroni, F., Russi, P., Gallo-Mezo, M.A., Moneti, G. and Pellicciari, R J. (1991)
Modulation of quinolinic and kynurenic acid content in the rat-brain - effects of
endotoxins and nicotinylalanine. J. Neurochem. 57, 1630-1635.
[77] Pellicciari, R., Natalini, B., Costantino, G., Mahmoud, M.R., Mattoli, L. and
Sadeghpour, B.M., Moroni, F., Chiarugi, A. and Carpenedo, R. (1994) Modulation of
the kynurenine pathway in search for new neuroprotective agents - synthesis and
preliminary evaluation of (m-nitrobenzoyl)alanine, a potent inhibitor of kynurenine-3-
hydroxylase. J. Med. Chem. 37, 647-655.
[78] Natalini, B., Mattoli, L., Pellicciari, R., Carpenedo, R., Chiarugi, A. and Moroni, F.
(1995) Synthesis and activity of enantiopure (s)(m-nitrobenzoyl) alanine, a potent
kynurenine-3-hydroxylase inhibitor. Bioorg. Med. Chem Lett. 5, 1451-1454.
[79] Chiarugi, A., Carpenedo, R., Molina, M.T., Mattoli, L., Pellicciari, R. and Moroni,
F. (1995) Comparison of the neurochemical and behavioral-effects resulting from the
inhibition of kynurenine hydroxylase and/or kynureninase. J. Neurochem. 65, 1176-
1183.
[80] Speciale, C., Wu, H.Q., Cini, M., Marconi, M., Varasi, M. and Schwarcz, R.
(1996) (R,S)-3,4-dichlorobenzoylalanine (FCE 28833A) causes a large and persistent
increase in brain kynurenic acid levels in rats. Eur. J. Pharmacol. 315, 263-267.
[81] Walsh, J.L., Todd, W.P., Carpenter, B.K. and Schwarcz, R. (1991) 4-halo-3-
hydroxyanthranilic acids - potent competitive inhibitors of 3-hydroxy-anthranilic acid
oxygenase invitro. Biochem. Pharmacol. 42, 985-990.
[82] Naritsin, D.B., Saito, K., Markey, S.P. and Heyes, M.P. (1995) Metabolism of L-
tryptophan to kynurenate and quinolinate in the central-nervous-system - effects of 6-
chlorotryptophan and 4-chloro-3-hydroxyanthranilate. J. Neurochem. 65, 2217-2226.
Figure 1
Diagrammatic summary of the major component compounds and enzymes of the
kynurenine pathway for the oxidation of tryptophan.
Sequence of steps in the pathway from tryptophan to nicotinic acid and NAD,
including the compounds quinolinic acid and kynurenic acid which are of primary
relevance to the text.
Figure 2
Chemical structures of compounds discussed
The structures are shown of several of the glutamate receptor blocking compounds
based on the structure of kynurenic acid. Most act at the glycine-B receptor site on
the NMDA receptor, the preferred site of action of kynurenic acid.
Figure 3
Chemical structures of compounds discussed.
The structures are shown of the two main inhibitors of the kynurenine pathway that
are neuroprotective and prevent excitoxicity by blocking kynureninase or kynurenine-
3-monoxygenase (KMO).