1. 1
ARE P2X RECEPTORS SUITABLE DRUG TARGETS FOR THE TREATMENT OF CHRONIC
INFLAMMATORY AND NEUROPATHIC PAIN?
Duuamene Nyimanu
MSc Molecular Medicine student, University of East Anglia, Norwich NR4 7TJ
ABSTRACT
Several years ago, studies demonstrated that extracellular ATP is important in
pain signalling both at the periphery and in the CNS. This triggered significant
advances in this area resulting in the discovery of the cell-surface receptor, P2X
receptors, as ATP-binding receptors. It was also found that ATP binding to these
receptors results in their activation and signalling in different pain states,
especially chronic (inflammatory and neuropathic) pain. Inflammatory pain is
elicited following inflammatory responses to peripheral nerve injury or an
unspecific immune response, which alters nerve function. Generally, this type of
pain could respond to treatment but neuropathic pain, which develops following
nerve damage resulting in hypersensitivity in the absence of overt stimulus, is
usually refractory to treatment. Several studies demonstrated that ATP-
dependent activation of P2X receptors, particularly P2X3, P2X2/3, P2X4 and
P2X7 receptors, are required for the development of chronic inflammatory and
neuropathic pain, and that blocking these receptors with antagonists or
antisense oligonucleotide silencing or knockout of these receptors in mice
results in significant reduction in hypersensitivity to pain, suggesting that these
receptors could be a potential drug target in managing inflammatory and
neuropathic pain. This review describes the latest evidences for the role of P2X
receptors in chronic inflammatory and neuropathic pain, thereby establishing
why they would be a suitable drug target for pain management and conclude
with a review of different drug-like molecules that have been tested in preclinical
and clinical trial studies for the treatment of these pain states.
Introduction
Pain is an unpleasant sensory and emotional experience associated with actual
or potential tissue damage. It minimises contact with the injurious stimuli
thereby promoting a protective response which includes reflex withdrawal and a
complex behavioural strategy to avoid further pain [1]. Pain is transmitted via
the somatosensory system, a part of the nervous system, which has evolved to
integrate sensory inputs from the body including touch, heat and pain
sensations. These sensory inputs are conducted by the primary afferent neurons
on the dorsal side of the spinal cord, dorsal root ganglion (DRG) neurons from
the peripheral sites (e.g. skin) to the dorsal horn of the spinal cord, from where
they are transmitted to the brain for perception [2], [3]. Studies suggest that ATP
released by activated microglia in sensory neurons promote nociceptor
signalling and produces fast excitatory potentials in the dorsal root ganglion

2. 2
(DRG) neurons [4], [5]. Consequently, Bleehen & Keele (1977), demonstrated
that ATP induced pain when applied to a blister base in human skin. This report
instigated enormous interest in the molecular mechanism by which ATP causes
pain resulting in the discovery of cell-surface receptors, thereby facilitating the
detection of extracellular ATP and other nucleotides on sensory neurons [7]. For
instance, it was observed that ATP or its analogues in primary afferent neurons
produce electrophysiological and biological responses through ligand-gated ion-
channel receptors, called P2X receptors (P2XRs), and G protein-coupled
receptors, called P2Y receptors (P2YRs) [8]–[10]. However, pharmacological
inhibition or suppression of the expression of P2XRs or P2YRs on sensory
neurons or spinal cord had little effect on acute pain evoked by heat or
mechanical pressure in normal animals but inflammatory pain was attenuated
[11], [12], suggesting that the actions of ATP and its receptors may be more
prominent in chronic pain especially inflammatory and neuropathic pain, than in
normal conditions.
Inflammatory pain develops from inflammatory responses to trauma in the
peripheral tissues and may have physiological importance in that it could assist
wound repair since contact with the damaged area is minimised [13] but it could
also result from non-specific immune response which alters nerve function [3].
Also, inflammatory pain may go away after damage is repaired and can generally
be managed by treatment with analgesics [13]. However, neuropathic pain
usually develops following nerve damage, which may be caused by surgery,
cancer, bone compression, diabetes or infection, but does not resolve even when
the damage has been healed [1]. It usually presents as hypersensitivity in the
absence of overt stimulus or can be evoked as in the case of allodynia (pain
resulting from innocuous stimulus) and hyperalgesia (exaggerated pain in
response to noxious stimulus), and is often refractory to treatments including
morphines [1], [14]. Evidences suggest that the damage results in the activation
of microglia cells in the spinal cord leading to cell hypertrophy, proliferation and
altered gene expression [7], [15]. Also, in response to environmental factors, glia
cells evoke various cellular responses including production and release of
various cytokines and neurotrophic factors causing neuroanatomical and
neurochemical transformation in the CNS that results in the hyperexcitability of
dorsal horn neurons [2], [13], [16]. Furthermore, several studies suggest that
P2X3, P2X4 and P2X7 receptors are important in the pathophysiology of chronic
inflammatory and neuropathic pain [15], [17]–[19]. For instance, it had been
reported that P2X3R knockout resulted in enhanced thermal hyperalgesia in
chronic inflammation [11]. Additionally, it has also been shown that stimulation
of P2X4R results in the release of brain-derived neurotrophic factor (BDNF) and
a shift in the neuronal anion gradient in underlying neuropathic pain [20].

3. 3
This review will attempt to answer the question ‘are P2X receptors suitable drug
targets for the treatment of chronic inflammatory and neuropathic pain?’ by
providing evidences for the role of P2X2/3R and P2X3R, P2X4R, and P2X7R in
chronic inflammatory and neuropathic pain. But before this, it will provide some
information about the different P2X receptor subtypes and their signalling
mechanism. The review will then discuss the success and failure of experimental
antagonists for these receptors and conclude with the future perspective on P2X
receptor targeted therapies.
The P2X Receptor subtypes and Signalling
The P2X family of receptors comprises seven subtypes of ATP-gated receptors,
P2X1-7. They were initially designated P2X by Burnstock in 1985 based on their
agonist and antagonist selectivity in different tissues [21]. This was because ATP
analogs such as α,β-methylene-ATP selectively activated P2X receptors while
adenosine 5’-diphosphate with β-sulfur was more selective for the P2Y receptors
[22]. It then became clear that P2X receptors, was activated selectively by ATP,
much less activated by ADP, and insensitive to AMP or adenosine or other
purines and pyrimidines. Additionally, this family of receptors have about 40-
50% amino acid sequence identity and each subunit has two transmembrane
domains (TM1 and TM2), which are separated by a large extracellular cysteine-
rich domain with intracellular N-terminus and C-terminus of considerably
variable length [18], [22]. Also, the channel can form multimers of several
subunits but the most characterised following heterologous expression are
homomeric P2X1, P2X2, P2X3, P2X4, P2X6 and P2X7 channels, and heteromeric
P2X2/3, and P2X1/5. They are abundantly expressed in neurons, glia, epithelia,
endothelial, bone, muscle and hematopoietic tissues and they are involved in
several physiological processes including cell proliferation, differentiation,
motility and death in development, wound healing, restenosis and epithelial cell
turnover aside pain [23], [24].
Furthermore, P2X receptors mediate ATP signalling mainly through three
mechanisms; by forming a ligand-gated Ca2+-permeable cationic channels,
inducing the formation of a large pore, and forming signalling complexes with
interacting proteins and membrane lipids [24]. For instance, as a ligand-gated
Ca2+-permeable cationic channel, ATP-mediated activation of P2XRs has been
found to induce more Ca2+ influx than glutamate ion channel and nicotic
acetycholine ion channels while as large pore-forming channel, some members
of P2X receptor family have been shown to induce the membrane permeability
or pore-formation upon prolonged stimulation and these phenomena has been
observed in P2X2, P2X4, P2X7, P2X2/3, and P2X2/5 [25]; and finally as signalling
complexes, it has been shown that P2X receptors can associate structurally and
functionally with other proteins and lipids to form ATP signalling complexes, an

4. 4
example of which is calmodulin interaction with P2X7 receptor via a calmodulin-
binding motif to form a signalling complex necessary for Ca2+-dependent
enhancement of receptor activity and membrane blebbing [24], [26].
P2X3 And P2X2/3 Receptors In The Pathogenesis Of Inflammatory And
Neuropathic Pain
The P2X3 receptor was the first member of the P2X receptor family to be cloned
and shown to be localised mainly on small nociceptive sensory neurons in the
dorsal root ganglia (DRG) [8]. It was first associated with pain through the
unifying hypothesis for the initiation of pain [10], which stated that high levels of
ATP released from tumour cells during abrasive activity reaches P2X3 receptors
on nociceptive sensory neurons in the DRG [27]. Other studies later used
immunohistochemical approach to show that P2X3 receptors are expressed on
isolectin B4 (IB4) binding subpopulations of small nociceptive neurons and that
it co-localises with the P2X2 receptors on large-diameter neurons in the DRG,
forming a heteromeric P2X2/3 receptor [3]. The binding of ATP to the receptor,
depolarises the DRG by eliciting fast-inactivating currents mediated by the
homomeric P2X3 receptors while the heteromeric P2X2/3 receptors were found
to mediate slow-desensitising currents [28]. It was also found in DRG neurons
isolated from rats with peripheral inflammation induced by complete Freund’s
adjuvant (CFA), that ATP application results in the induction of both fast- and
slow-inactivating currents in control and inflamed neurons, suggesting that the
activation of this receptors in sensory neurons facilitates the transmission of
nociceptive signals from periphery to the spinal cord [28]. The loss of IB4-
binding neurons expressing P2X3 receptors resulted in decreased sensitivity to
noxious stimuli suggesting a critical role for these receptors in acute pain [27].
However, P2X3 and P2X2/3 receptors has now been shown to play a pivotal role
in the signalling pathways involved in chronic inflammatory and neuropathic
pain [15], [29].
Several studies have reported high levels P2X3R-mediated nocifensive behaviour
in rat and human models of inflammatory pain [6], [30]. The stimulation of
P2X3R with ATP or its analogue (α,β-methylene-ATP) in an in-vitro-skin-nerve
model resulted in the excitation of C-mechanoheat polymodal nociceptors, which
was enhanced in the carrageenan-inflamed skin [31], suggesting that not only
are the levels of ATP in inflamed tissues elevated but P2X3 receptors on the
peripheral nerve endings in inflamed tissues could modulate pain transmission.
Also, P2X2 knockout and P2X2/3 knockout mice studies revealed that the double
knockout mice had significant reduction in formalin-induced inflammatory pain,
inability to code the intensity of non-noxious 'warming' stimuli, inability to
rapidly desensitise ATP-induced currents in response to ATP application as well

5. 5
as decreased nociceptive behaviour compared to wildtype [11], [32]. Similarly,
other studies showed that P2X3 antisense oligonucleotides prevented
hyperalgesia in CFA model of chronic inflammatory pain and spinal nerve
ligation model of neuropathic pain, which were correlated with decreased P2X3
expression in the DRG [15], [33]. This suggests that P2X3R and P2X2/3R are
important receptors in nociceptive pain and that therapeutically targeting them
with a selective antagonist could modulate pain state.
Furthermore, increasing evidence suggest that persistent inflammation by CFA is
accompanied by upregulation of both P2X2 and P2X3 receptors in sensory
neurons. It was observed that ATP stimulation of these receptors in inflamed
DRG neurons resulted in elevated expression of P2X2 and P2X3 receptors
resulting in the development of large depolarisation above the threshold for
action potentials compared to control as well as receptor-induced increased
response in DRG neurons observed in vitro and at the peripheral terminals in
vivo [28]. Also, in another study it was shown that intraperitoneal injection of
streptozotocin, a potent P2X3 agonist, in a diabetic neuropathic pain model
results in increased membrane expression of P2X3 receptor and large
enhancement of mechanical allodynia, which was significantly attenuated
following peripheral administration of P2X3 receptor antagonist, pyridoxal-
phosphate-6-azophenyl-2’,4’-disulfonate (PPADs) and TNP-ATP [34]. Similarly,
using highly selective P2X3 and P2X2/3 receptor antagonist A-317491, Jarvis et
al. (2002) showed that intraplantar and intrathecal injection of A-317491 into
rats resulted in antinociceptive effects in CFA-induced chronic hyperalgesia and
nerve injury-induced hyperalgesia. Hence, this collectively demonstrates the
critical role of P2X3 receptors in chronic inflammatory and neuropathic pain and
that relief from these forms of pain could be achieved by pharmacologically
blocking P2X3 or P2X2/3 expression and/or activation.
However, the cellular mechanism by which P2X3R expression and function are
upregulated in sensory neurons is not fully known although it is thought that this
could be mediated by interaction between P2X3 and P2X2/3 receptors, and
inflammatory mediators. This is because various inflammatory mediators such
as substance P, neurokinin B, prostaglandin E2, protons and bradykinin strongly
enhance P2X-mediated responses [7]. It has also been reported that P2X3
receptor activation in peripheral nerve endings of inflamed tissues results in the
activation of ERK in the DRG neurons in rat models of inflammation but not in
normal rats and that administration of PPADs and TNP-ATP, significantly
decreased the mechanical stimulation-evoked activation of ERK in CFA-inflamed
rats but not in normal rats [35]. Moreover, the upregulation of P2X3 and P2X2/3
in pain states have also been associated with growth factors. For instance, it has
been shown that glial cell line-derived neurotrophic factor (GDNF) and nerve
growth factor (NGF) treatment DRG neurons increases the expression of P2X3

6. 6
receptors, with evidence of NGF-mediated de no P2X3 expression in cells that
does not normally express the receptor, suggesting a mechanism of NGF-
mediated hypersensitivity that may contribute to chronic inflammatory pain
[36].
P2X4 Receptors In The Pathogenesis Of Inflammatory And Neuropathic
Pain
The first clue to identifying the role of P2X4 receptors in the spinal cord in
neuropathic pain came from pharmacological investigation of pain behaviour
after nerve injury using the antagonists TNP-ATP and PPADS [37]. They reported
that marked tactile allodynia developed following nerve injury which was
reversed by acutely administering TNP-ATP intrathecally but unaffected by
administering PPADS, suggesting that the tactile allodynia depends on P2X4
receptors in the spinal cord. Also, immunohistochemical analysis showed that
many small cells, identified as microglia, in the dorsal horn of the nerve-injured
side were positive for P2X4 receptor protein, and showed high levels of OX-42
labelling and morphological hypertrophy characteristic of activated microglia.
Additionally, P2X4 receptor antisense oligodeoxynucleotides (ASO) reduced the
up-regulation of P2X4 receptor protein, thereby preventing the development of
nerve-induced tractile allodynia in mice [37]. Moreover, other early studies in a
rat model of neuropathic pain induced by spinal nerve ligation (SNL) reported an
upregulated expression of P2X4 receptor in activated spinal microglia that
mediate tactile allodynia but not in neurons [38]. They observed that P2X4KO
mice were insensitive to SNL-induced neuropathic pain experienced by wild-
type littermates. This collectively suggests that activation of microglia P2X4
receptor is necessary for pain hypersensitivity following nerve injury.
Consequently, efforts to determine how peripheral injury increases the
overexpression of P2X4 receptor in microglia suggest that fibronectin may be
involved. It was observed that microglia cultured on fibronectin-coated dishes
showed a marked increase in P2X4 receptor expression at both mRNA and
protein level while intrathetical delivery of ATP-stimulated microglia to a rat
lumbar spinal cord, showed that microglia treated with fibronectin more
effectively induced allodynia than control microglia [39]. Similarly, it was
observed in a dorsal horn model of neuropathic pain that the level of fibronectin
protein was elevated greatly after nerve injury as P2X4 protein level increased
and pharmacological inhibition of the fibronectin receptor resulted in attenuated
nerve injury-induced P2X4 receptor upregulation and pain hypersensitivity [40].
Additionally, it was shown in Lyn tyrosine kinase knockout mice studies, that
fibronectin could not induce the upregulation of P2X4 receptor in microglia cells
and neuropathic pain in Lyn-deficient mice, suggesting that this kinase may be
important in the molecular mechanism mediating the upregulation of P2X4
receptors in microglia [41].

7. 7
Furthermore, Coull et al. (2005) showed using spinal cord slices from rats that
had displayed pain hypersensitivity following intrathetical administration of
P2X4R-stimulated microglia, that ATP-stimulated microglia positively shifted
the anion reversal potential (Eanion) in lamina I neurons and rendered GABA-
receptor- and glycine-receptor-mediated effects depolarising rather than
hyperpolarising these neurons (fig. 1). Previously, it has been shown in a
peripheral nerve injury model of neuropathic pain that this shift in
transmembrane anion gradient which changes inhibitory currents to excitatory
following nerve injury, was due to trans-synaptic reduction in the expression of
the potassium-chloride exporter KCC2 [42]. Moreover, TNP-ATP which can
reverse nerve-injury induced allodynia, acutely reverses the depolarising Eanion
in the lamina I neurons after peripheral injury [37]. Therefore, the stimulation of
P2X4 receptor on spinal microglia causes neuropathic pain through increased
intracellular chloride (Cl-) in the spinal lamina I neurons (fig.1).
Fig. 1. Illustration of the mechanism by which P2X4R could modulate neuropathic
pain [3]. Damaged sensory neurons release ATP, which binds to P2X4 receptor
resulting in the release of Ca2+ and activation of p38 MAPK, which induces the

8. 8
release of brain derived neurotrophic (BDNF). The BDNF acts on its receptor Trk
and the inhibitory interneurons to release GABA. Also, action potentials from the
primary afferent terminal induce the release of glutamate and consequent opening
of AMPA and NMDA receptors. This collectively results in the depolarisation and
hyperexcitability of the dorsal horn neurons leading to neuropathic pain.
Furthermore, Coull et al. (2005) also observed using brain derived neurotrophic
factor (BDNF) administered intrathecally to normal rats that BDNF induced
tactile allodynia and depolarising shift in Eanion in lamina I neurons by peripheral
nerve injury comparable to those produced by ATP-stimulated microglia.
Moreover, the interruption of signalling between BDNF and its receptor TrkB,
either by pharmacological inhibition or by BDNF-sequestering fusion protein
(TrkB-Fc) prevented tactile allodynia caused by peripheral injury or by
intrathecal administration of P2X4-stimulated microglia [7], [20]. Also, it was
observed that the application of ATP to microglia induced the release of BDNF
but this was abrogated by TNP-ATP, suggesting that P2X4R-stimulated microglia
release BDNF as a signalling factor leading to the collapse of the transmembrane
anion gradient and subsequent neuronal hyperexcitability observed in
neuropathic pain [3]. Additionally, in a study involving P2X4 receptor knockout
mice, primary cultures of dorsal horn microglia showed a reduction in BDNF
staining after ATP stimulation in wild-type cultures, while in cultures from the
P2X4R-mutant mice, application of ATP failed to induce any change [38].
Similarly, other studies involving ATP-stimulation of P2X4 receptors resulted in
SNARE-mediated synthesis and release of BDNF that was dependent on the Ca2+
influx through P2X4 receptors and subsequent p38-MAPK activation (fig. 1) [1],
[43]. Also, GABA receptor-mediated depolarisation could produce an excitation
through voltage sensitive Ca2+ channels and NMDA receptors [3], thereby
suggesting that p38-MAPK as well as GABA and NMDA receptors are important
in the molecular processes involved in P2X4R-mediated development of
neuropathic pain.
Finally, several evidences suggest that P2X4 receptors are important in chronic
inflammatory pain development. For instance, P2X4R knockout mice studies
involving the injection of inflammatory stimuli such as formalin, carrageenan,
and CFA showed the complete loss of tactile allodynia in P2X4R-deficient mice
compared to control [44], [45]. Also, it was observed that P2X4R deficiency
attenuates inflammatory stimuli-induced production of prostaglandin E2 (PGE2),
which usually induces pain hypersensitivity by sensitising and overexciting the
nociceptive neurons, from macrophages. Additionally, the injection of naïve
animals with ATP-primed microglia or macrophages has been shown to induce
neuropathic and chronic inflammatory pain respectively [45], suggesting that
P2X4 receptors mediate the cellular and molecular mechanisms involved in

9. 9
eliciting chronic neuropathic and inflammatory pain, and that selectively
targeting P2X4 receptors could be a strategy for treatment of chronic pain.
P2X7 Receptors In The Pathogenesis Of Inflammatory And Neuropathic
Pain
P2X7 receptors are usually considered the most unusual among the P2X receptor
superfamily in terms of their molecular and functional characteristics due to the
presence of additional 200amino acids in their C-terminal, and the fact that aside
requiring high ATP concentration for activation, prolonged agonist exposure
results in the formation of a larger pore in the membrane [3], [16]. However,
they share a common transmembrane domain with other P2X receptors. They
are predominantly expressed on immune cells including lymphocytes and
peripheral macrophages and have also been described on microglia and
astrocytes. Like other P2XRs, ATP binding activates the receptor resulting in the
opening of the receptor pore for permeation of Ca2+, Na+ and K+, which causes
changes in the intracellular concentration of potassium and consequent release
and activation of interleukin-1β (IL-1β), a potent proinflammatory cytokine (fig.
2) [46]. IL-1β induces a cytokine network resulting in the production of
superoxide products, nitric oxide synthase (iNOS), cyclo-oxygenase and tumour
necrosis factor (TNF)-α, all of which have important roles in the generation and
maintenance of pain [17]. Thus, many studies have been performed to determine
its role in chronic inflammatory and neuropathic pain.

10. 10
Fig. 2 The mechanism of action of P2X7 receptor in IL-1β-mediated inflammation
[47]. ATP binding to the P2X7 receptor activates it resulting in the opening of its
non-selective ion pore and permeation of Ca2+ and K+. The consequent change in
membrane potential and intracellular Ca2+ and K+ results in the assembly of the
inflammasome and conversion of pro-caspase-1 to active caspase-1. Pro-caspase-1
converts the inactive IL-1β into its active form in the lysosome, before it is secreted
out of the cell. It is also believed that lipopolysaccharide (LPS) acts on Toll receptor
4 to activate NFκB and nuclear transcription of IL-1β, later translated into the
inactive form that is secreted into the cytoplasm.
P2X7 receptor knockout mice studies have facilitated the investigation of the
role of this receptor in chronic pain. For instance, using P2X7R-/- mice models of
chronic inflammatory pain (intraplantar Freund’s complete adjuvant) and
neuropathic pain (partial ligation of the sciatic nerve), it was shown that pain
hypersensitivity to both mechanical and thermal stimuli was completely lost in
receptor-deficient mice, while normal nociceptive processing was preserved
[17]. They also reported that the receptor is upregulated in human dorsal root
ganglia and injured nerve obtained from neuropathic pain patients, suggesting
that P2X7 receptor plays an important role in the development of chronic
inflammatory and neuropathic pain via regulation of IL-1β (fig. 2). In another
study investigating the response of blood-derived leukocytes from wildtype and
P2X7R-/- mice to ATP, it was shown that P2X7R-deficiency results in loss of ATP-

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dependent leukocyte functions including IL-1β production [48]. They also
showed in a monoclonal antibody-induced arthritis model, that P2X7R-/- was
associated with significantly attenuated arthritis compared to the severe
arthritic phenotype observed in wildtype mice, suggesting that ATP-dependent
activation of P2X7 receptor was important in chronic inflammatory pain.
Furthermore, the pore-forming property of P2X7R has been associated with
development of chronic neuropathic pain. For instance, in a genome-wide
linkage study, it was shown that mice expressing P2X7 receptors deficient of
inducing pore formation were less sensitive to nerve injury-induced neuropathic
pain than mice expressing the P2X7 receptors that can induce pore formation
[49]. The study also found that within two cohorts of patients; one with pain
after mastectomy and another cohort suffering from osteoarthritis, individuals
expressing P2X7 receptor deficient of pore formation reported lower amount of
pain than those expressing the P2X7 receptor with pore-forming ability. Also, the
administration of a peptide which blocks pore formation but not channel activity
has been shown to selectively reduce nerved injury and inflammatory allodynia
in wildtype mice but not in P2X7R-deficient mice [16]. This suggests that the
pore-forming ability rather than the small ion channel opening alone is key to
the function of P2X7R in chronic inflammatory and neuropathic pain sensitivity
and that selectively targeting pore formation could be a strategy for treatment of
chronic pain.
Additionally, aside its involvement in P2X4R-mediated allodynia, p38 MAPK has
also been shown to mediate P2X7R-induced production of IL-1β, cathespsin S
and TNF-α, which functions in the maintenance of mechanical hypersensitivity in
the spinal cord. Recent studies suggest that the phosphorylation of p38 MAPK via
P2X7 receptor induce hyperalgesia in an orofacial pain model following chronic
constriction injury (CCI) of the infraorbital nerve (CCI-ION) mediated by TNF-α
release from microglia [50]. They also observed that treatment of rats with P2X7
receptor agonist, 3′-O-(4-benzoylbenzoyl) adenosine 5′-triphosphate (BzATP),
induced tactile allodynia through up-regulation of soluble TNF-α and p38 MAPK
in the trigeminal sensory nuclear complex (TNC) which was inhibited by
SB203580 (a phosphorylated p38 MAPK inhibitor) and Etanercept (a TNF-α
inhibitor). This suggests that the activation of p38 MAPK could be a possible
convergence point in the P2X4 and P2X7 receptor signalling pathways during
neuropathic pain. Indeed, although not in pain models, evidence of a structural
and functional interaction between the two receptors had been described [51],
but it is currently unclear whether the heteromeric interaction of these receptors
is critical in chronic inflammatory or neuropathic pain. Nevertheless, the
expression of these receptors on various cell types involved in pain transmission,
suggests a promising target for pharmacological intervention. An example of this
was by Dell’Antonio et al. (2002) who demonstrated in a paw pressure

12. 12
experiment that an irreversible inhibitor of P2X7 receptor, oxidised ATP, had an
anti-hyperalgesic effect on CFA-induced mechanical hyperalgesia.
In summary, this review have thus far discussed the present evidences for the
important role played by P2X3, P2X2/3, P2X4 and P2X7 receptors in mediating
ATP signalling involved in the pathogenesis of chronic inflammatory and
neuropathic pain. This therefore, suggests that these receptors are a promising
target for pain therapies.
P2X Receptors as Therapeutic Targets in Chronic Inflammatory and
Neuropathic Pain
Several attempts have been made to develop molecules that can specifically
target ATP-mediated signalling through different members of P2X receptor
family involved in pain sensitivity. Initial attempts resulted in the identification
of Suramin, a large polysulfonated molecule, that is active at multiple P2 receptor
subtypes and its derivatives such as NF023, which was reported to be ~10-
20fold more selective for P2X receptors; NF279 and NF449 which are potent
P2X1 receptor antagonists [53]. Later, PPADS, a potent coenzyme antagonist
against human P2X1, P2X7 and P2Y1 were discovered along with its derivatives
but the non-selective interaction of these compounds limited their progress as
potential therapeutic agents. Other compounds developed as potential
antagonists include oxidised ATP, Brilliant Blue G, KN-62, NF770 and NF778 but
these compounds where unsuccessful due to the wide diversity of recognition
sites and actions for which ATP is a crucial ligand resulting in their non-selective
interaction with the receptors [21]. Other antagonists developed to target each
of these receptors are described below.
P2X3 and P2X2/3 Receptors
Nucleotide derivatives were developed to modulate the activity of these
receptors. Thus, TNP-ATP, a non-selective but highly potent antagonist of P2X1
and P2X3 receptors was developed and shown to block the pronociceptive
effects of P2X receptor agonists. However, the ability of TNP-ATP to enter
preclinical pain studies for management of P2X3-mediated pain was limited by
its poor metabolic stability in the plasma [21]. A-317491 is another compound
which showed high capability to competitively block homomeric P2X3 and
heteromeric P2X3 receptors when administered in CFA-induced inflammatory
hyperalgesia although it had limited CNS penetration following systemic
administration thereby requiring higher doses or intrathecal administration to
effectively attenuate tactile allodynia following peripheral injury [12], [54].
Other potent P2X2/3 and P2X3 receptor antagonists have been identified
including RO-4, reported to be capable of crossing the blood-brain barrier and
attenuate nerve injury-induced pain models, and has significantly high oral

13. 13
bioavailability and low plasma blood binding as well as good CNS prenetration;
MK-3901, reported to attenuate both neuropathic and chronic inflammatory
pain in experimental models [55]; AZ-2, reported to effectively reverse CFA-
induced mechanical allodynia following systemic and intraplantar dosing but
ineffective at intrathecal dosing; RO85, reported to have high Multi-parameter
optimization (MPO) score and oral bioavailability; and finally AF-219 reported to
have high antagonists potency and selectivity for P2X3 and P2X2/3 receptors,
with moderate protein binding and high oral bioavailability [53]. However,
majority of these drug-like P2X receptor antagonists were unsuccessful in
preclinical studies due to unsatisfactory pharmacological profiles while AF-219
entered advanced clinical trials for treatment of osteoarthritic knee pain and
bladder pain [24].
P2X4 Receptors
The discovery of potent antagonist for P2X4 receptors is still in its infancy
although the crystal structure of the protein has been solved. However, TNP-ATP
has been used as putative antagonists to block P2X4 receptor activation. Also,
Brilliant Blue G is also believed to have antagonistic effects on the receptor
activity as well as the serotonin reuptake inhibitor, paroxetine, a clinically used
antidepressant [53], [56]. Interestingly, N-(benzyloxycarbonyl)phenoxazine was
recently discovered as a potent and selective P2X4 receptor antagonist that
could have potential therapeutic benefit [57].
P2X7 Receptors
The increased characterisation of the role of P2X7 receptor in different
inflammatory diseases not limited to chronic pain, resulted in different
companies initiating a search for selective receptor antagonists (table 1). This
search resulted in the identification of AACBA1, which has been shown to
attenuate collagen-induced arthritis following prophylactic dosing in rats and,
CE-224,535 and AZD9056 which failed phase IIa and phase IIb clinical trial for
the treatment of rheumatoid arthritic pain respectively for lack of efficacy
compared to control although they had acceptable tolerability and safety profile
[58], [59]. Also, systemic screening resulted in the discovery of other selective
antagonists including AZ-11645373, a highly potent P2X7 receptor antagonists
which has been shown to effectively inhibit ATP- and Bz-ATP-elicited currents as
well as A-438079 and A-740003, which has been shown in inflammatory pain
models to reduce thermal hyperalgesia with some success [60], [61]. Similar
effect was also observed on experimental models of neuropathic pain, an effect
that is mediated partly by at least spinal and/or supraspinal sites of action [62].
1 N-(adamantan-1-ylmethyl)-5-[(3R-amino-pyrrolidin-1-yl)methyl]-2-chloro-benzamide, a
hydrochloride salt.

14. 14
Table 1. Recently tested compounds with potential antagonistic activity on P2X7
Receptor [63]
Compound Study Trial
Phase
Completed Observed
Outcome
Side effects
A-438079 Neuropathic
and
inflammatory
pain
Pre-
clinical
Yes Inhibited
mechanical
allodynia and
effective in
the formalin
pain model
Not evaluated.
A-740003 Inflammatory
and
neuropathic
pain
Pre-
clinical
Yes Analgesic
effect in
inflammatory
and
neuropathic
rat models
Not evaluated
A-804598 Neuropathic
and
inflammatory
pain
Pre-
clinical
Yes Analgesic
effect in
inflammatory
and
neuropathic
rat models
Not evaluated
A847227 Inflammatory
and
neuropathic
pain
Pre-
clinical
Yes Antiallodynic
effect and
decreased IL-
1β released in
mouse model
Not evaluated
AZ10606120 Ligand
interaction
and binding
to P2X7R
Pre-
clinical
Yes Had allosteric
effect of
receptor
activity as it
binds non-
ATP binding
site
Not evaluated
AZD9056 Rheumatoid
Arthritis
IIb Yes No significant
efficacy
Gastrointestinal
(vomiting,
nausea and
diarhoea
GSK314118A
Inflammatory
pain
Pre-
clinical
Yes
Analgesic
effect in rat
CFA model of
inflammatory
hyperalgesia
Not related
Natural products as novel source of analgesics for targeting ATP-mediated
P2X receptors
Attempts to overcome the problem of non-selectivity of P2X receptor antagonists
resulted in the search for clues from natural products. Natural products have
been shown to be potentially good source of new specific molecules for the

15. 15
treatment of different pain syndromes. Thus, several natural products have been
developed which has antagonistic properties on P2X receptors in chronic
neuropathic and inflammatory pain, including Emolin, Amentoflavone,
Ligunstrazine, puerarin and purotoxin-1 [64]. For instance, it was shown in rats
that following formalin-induced pain, the herbal product used in Chinese
medicine called, Ligunstrazine (tetramethypyrazine) derived from Ligusticum
wallichii, antagonise P2X3 receptor resulting in the inhibition of membrane
depolarisation induced in DRGs neurons [65]. The same group confirmed this in
another study, where they also found that Ligunstrazine inhibited P2X3 receptor
resulting in reduced ionic currents induced by ATP in the DRG neurons but it
was non-selective since it also induces PKC activation. Similarly, in a neuropathic
pain model it was also found that the Ligunstrazine inhibited the activation of
P2X3 receptors on the primary afferent neurons.
Furthermore, other natural products including puerarin were discovered and
showed to inhibit burn-associated hyperalgesia by preventing the upregulation
of P2X3 receptor expression in the DRG neurons, with similar analgesic effect on
neuropathic pain via the same mechanism [64]. Additionally, Emodin, an
anthraquinone obtained from rhubarb extract did not only show analgesic
activity on neuropathic pain through the inhibition of P2X3 receptors in the
primary sensory neurons, but also had antagonistic activity on the P2X7
receptors [64]. Also, a peptide isolated from the venom of the Asian spider
Geolyscosa species, Purotoxin, has been shown to be a potent and selective
antagonist of P2X3 receptors, as it selectively blocks the P2X3 receptor ion
current in rat neurons [64]. Thus, these evidences collectively suggest that
natural products could offer more potent and selective compounds for
therapeutic targeting of P2X receptors.
Conclusion
Pain is the unpleasant sensory and emotional experience associated with the
actual or potential tissue damage. It can simply be classified into acute and
chronic pain where acute pain describes the actual pain sensation experienced in
response to injury or tissue damage. Chronic pain, which can further be
subdivided in inflammatory and neuropathic pain, occurs due to neurochemical
and phenotypic sensitisation of the peripheral central sensory nerves
characterised by increased sensitivity to painful stimuli (hyperalgesia) and the
perception of pain in response to normally innocuous stimuli (allodynia). Pain
sensitisation usually occurs in response to tissue damage or inflammation and is
mediated by several pronociceptive neurotransmitters and neurotrophic factors
such as ATP or BDNF. Extracellular ATP release has now been shown to elicit and
maintain sensations following inflammation and nerve injury by activating

16. 16
homomeric and heteromeric P2X receptors notably P2X2/3, P2X3, P2X4 and
P2X7 receptors on the peripheral nerves and glial cells (astrocytes and
microglia) in the spinal cord.
Furthermore, several evidences suggest the invaluable role P2X2/3, P2X3, P2X4
and P2X7 receptors in the molecular processes involved in pain sensation, while
inhibition of receptor activity using ASO, gene knockout and chemical
compounds collectively showed that the absence of P2X receptors reverts the
ATP-induced nociceptive pain. Thus, suggesting that inhibition of these receptors
could be a potential therapeutic strategy for development of future analgesics.
Massive studies in this area have resulted in the discovery of P2XR-targetted
therapeutics or molecules. These P2XR antagonists include MK3901, AZ-2,
A740003, AZD9056, AZ10606120, A847227, etc which have been shown to
effectively block receptor activity although majority of them failed clinical trials.
This is mostly due to the widespread expression of all the different receptor
subtypes, which renders the discrimination between beneficial and side effects
extremely complicated. For instance, studies showed that double-knockout
P2X2/P2X3 receptors in the gustatory nerves of mice eliminates taste responses,
although the nerve was responsive to touch, temperature and menthol [66], [67]
while P2X4R-/- in mice had also been shown to cause increased blood pressure
and decreased excretion of nitric oxide products in their urine compared to
wildtype [68]. Similarly, the lack of good subtype-selective agents, and when
selective ligands are available, the lack of an acceptable route of administration
to humans are potential challenges to the development of novel P2XR targets.
Other challenges include the restrictive tissue distribution of some of these
receptors; the ability of channels to form heterotrimers, the ability of the
extracellular domain to undergo substantial conformational rearrangement on
channel opening as well as limited information on the role of P2XRs in
physiological and pathological processes, although this roles are now beginning
to be identified through knockout mice studies [21]. However, the good news is
that crystal structures of some of these receptors are now available in the closed
and ATP-bound (open) states. Hence, in the nearest future we will begin to fully
understand the mechanism of action of these receptors and how to
pharmacologically inhibit them. Further work could thus, explore the potential of
natural products as possible source of future P2XR-targeted therapeutics as well
as identifying possible molecules that could potently inhibit P2X4 receptor.
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