Discovery and development of TRPV1 antagonists
Encyclopedia
Chronic pain remains a recognized unmet medical need. Consequently, the search for new analgesic agents is being intensively studied by the pharmaceutical industry. The TRPV1 receptor is an ion channel that has been implicated in mediation of many types of pain and therefore studied most extensively. The first competitive antagonist, capsazepine
, was first described in 1990, since then development of novel TRPV1 antagonists has come a long way. This effort has led to the identification of several TRPV1 antagonists that have entered clinical trials as analgesic agents. Should these new chemical entities relieve symptoms of chronic pain then this class of compounds may offer one of the first novel mechanisms for the treatment of pain, in many years.
Capsaicin
, the active ingredient in chilli pepper was first isolated over a century ago. In 1919 the exact chemical structure of capsaicin
was determined and the complete synthesis of the compound was achieved a decade later. Capsaicin has been used as an analgesic
for decades, but the therapeutic potential of capsaicin was first recognized as early as 1850. The effects of the pungent chemical, capsaicin, is mediated through the ligand gated ion channel TRPV1
. This knowledge set the stage for further research of the function of the TRPV1 receptor
and preclinical studies showed evidence of its importance in numerous human diseases. These are the first agents acting by this mechanism that made their way into clinic for evaluation of their use as possible analgesics and therefore important targets for drug development
. Many discoveries are yet to be made, both in terms of the range of potential therapeutic applications in addition to analgesia for TRPV1 antagonists
and it was only in the last decade where there has been a full understanding of the molecular mechanism. In the years to come it will be clearer if TRPV1 antagonists can fulfill their potential.
. The receptor is made of four identical subunits each with six transmembrane segments, S1-S6, and between fifth and sixth segment is an aqueous pore. This region forms the channel conductive pore and contains the N- and C-termini on the cytosolic side of the cell membrane
.
Capsaicin
and RTX
, elicit burning pain by activating a non-selective cation channel expressed on sensory nerve endings. When capsaicin
was found to have analgesic effects in preclinical studies much emphasis was put into the research of the receptor/channel that capsaicin binds to and activates. Besides being activated by capsaicin, TRPV1 also responds to a wide range of exogenous and endogenous chemical ligands as well as physical stimuli such as heat over 42°C and changes in more diverse activators such as protons (acid, pH<6). TRPV1
is also subject to regulation by changes in membrane potential
and this intrinsic voltage-dependence is thought to underlie the gating
mechanism of this non-selective cation channel which leads to the influx of sodium and calcium ions. Importantly, TRPV1
activity is also subject to regulation by a host of intracellular signaling cascades such as G-protein coupled receptor signaling, that are implicated in the responses to algogenic agents, inflammatory mediators and injury.
is primarily expressed on, small myelinated and unmyelinated medium size, sensory neurons in dorsal root
and trigeminal ganglia, where sensory neurons cluster. TRPV1 receptors are also found in muscles, joints, the urinary bladder and kidneys. The functional activity of TRPV1 has been demonstrated, within the central nervous system, in the spinal cord
and specific sites in the brain including the hypothalamus
, cerebellum
, locus coeruleus, periaqueductal grey and cortex
. Activation of TRPV1 sets off an influx of calcium and sodium ions which in turn initiates a cascade of events that result in membrane depolarization
, neuronal firing and transduction of neural impulses. TRPV1 phosphorylates as a response to several algesic agents, resulting in a lower threshold of channel activation. Some substances such as bradykinin
, nerve growth factor
and protons have been reported to sensitize the TRPV1 receptor. Activation of TRPV1 results in the release of pro-nociceptive peptides, which decreases when treated with TRPV1 antagonists. In general, most channel antagonists bind in the pore region, interacting with residues from all four monomers of the tetrameric channel.
side. This is a unique property of TRPV1, where ligands of other ligand-gated channels bind from the extracellular
space. Capsaicin is highly lipophilic
and can pass the plasma membrane easily. It is generally accepted that capsaicin acts on and binds to the TRPV1 receptor from the intracellular side prior to activation. The critical sites for capsaicin binding are Arg 114 and Glu 761 at the N- and C-termini of the receptor, respectively. Because these two amino acids are charged and located in the cytosolic part of TRPV1 receptor, the two regions are likely to be implicated in hydrophilic interaction of TRPV1 with vanilloids such as capsaicin and RTX
.
In addition to these sites in N- and C-termini of TRPV1, a region in the intracellular linker sited in the transmembrane domain, called ‘the TM3 region’, has been shown to be critical for hydrophobic interaction with vanilloids. The TM3 region is considered to be necessary for binding to vanilloids. It is surrounded by the hydrophobic environment because of its placement in the plasma membrane. Now it is recognized as an important link in hydrophobic interaction with capsaicin. The binding sites Arg 114 and Glu 761 and the TM3 region in TRPV1, together consist of a binding pocket to vanilloids.
(RTX) is another naturally occurring vanilloid that exhibits TRPV1 agonistic activity. It is more potent than capsaicin and is currently in development as a sensory neuron desensitizing agent.
Initially, agonists were the major focus of the TRPV1 ligand development due to the analgesic effect resulting from desensitization
of the receptor. However, because of an initial burning effect of all natural vanilloid receptor agonists, including capsaicin, therapy becomes complicated and perhaps ineffective. Attempts to make synthetic
agonists with good separation between excitatory effects and the analgesic effects have not been successful. To avoid this persisting side effects of TRPV1 agonists, a focused consideration has been given to competitive antagonists as novel analgesic drugs.
. Antagonists that bind to the agonist binding site
, and lock the channel in the closed, nonconductive state are competitive antagonists. In contrast, antagonists that interact with additional binding sites on the receptor structure preventing receptor opening by the agonist or blocking its aqueous pore are non-competitive antagonists. Non-competitive antagonists acting as open channel blockers are therapeutically attractive because of their recognition of over-activated TRPV1 channels, which can reduce the potential of unwanted side effects.
model for TRPV1 antagonists consists of three essential features: a hydrogen-bond acceptor, a hydrogen-bond donor, and a ring feature. In addition, the TRPV1 antagonists have been superimposed
in such a way that they could fit in the volume of the TRPV1 pore. When the homology model is considered, appropriate interaction sites are found in the receptor pore. The hydrogen-bond acceptor on the ligand is proposed to interact with Tyr 667 (helix S6) on the receptor as a hydrogen-bond donor, and the hydrogen-bond donor on the ligand is proposed to interact with Tyr 667 on the opposite monomer
of the tetramer
on the receptor as a hydrogen-bond acceptor. The ring feature of the pharmacophore is proposed to fit in the hydrophobic space formed by the aromatic rings of the four Tyr 667 residues of the four monomers. Consistent with the critical role played by Tyr 667 in the interaction with key elements of the TPRV1 antagonist pharmacophore, site-directed mutagenesis
studies have shown that exchanging this tyrosine for alanine in the rat TRPV1 receptor abolishes functional activity of TRPV1. The lipophilic end in antagonist is varied in character and volume and interacts with the lower end of transmembrane helices S5 and S6. Because the intracellular ends of these helices extend past the membrane, they are likely to be flexible and may be part of the channel opening and closing process. The combined use of a pharmacophore model, assembled from highly optimized TRPV1 antagonists, with a homology model of the protein has enhanced understanding of the observed structure–activity relationships of many series of current TRPV1 antagonists, and should be useful in the discovery of new classes of antagonists.
or amide
linker and the aliphatic C region where a lipophilic octanyl moiety is associated with the highest potency. The homovanillyl motif and amide bond regions contain dipolar groups which are implicated in hydrogen bonding interactions.
Phenolic hydroxide and amide moieties appear to be vital for inducing capsaicin responses. Removal of the phenolic hydroxide or amide bond in capsaicin analogues leads to reduction of potency. The phenolic hydroxide and amide moieties in capsaicin share potential multiple hydrogen bond interactions with the TRPV1 receptor. Capsaicinoids and capsinoids
are characterized by an oxygenated aromatic moiety bound via an amide(capsaicinoids) or ester (capsinoids) linker to a lipophilic acyl group. The vanillyl
and carbonyl
linker contain polar groups capable of forming hydrogen bonds essential for activity, whereas the lipophilic moiety interacts with a corresponding cleft of the vanilloid binding site on TRPV1. Replacement of the medium-sized branched fatty acid
of capsaicin with longer fatty acids is damaging for activity, but the presence of unsaturations restores and potentiates activity e.g. oleoylvanillamine (olvanil)(fig. 3a), is 10-fold more potent than capsaicin in TRPV1 activation assays.
1,3-Di(arylalkyl)thioureas
Capsazepine
(fig. 4a), the first competitive vanilloid antagonist, reported by Novartis
group, was
aimed at assessing the effect of conformational constraint on the lipophilic C-region of capsaicin. In capsazepine the amide bond of the capsaicin is replaced by a thiourea
moiety and a propylidene linker between the aromatic vanillyl 2-carbon A ring and the B-linker amide nitrogen forces the aromatic ring in an orthogonal orientation with respect to the thiourea bond. This constraint has long been considered as the distinctive characteristic of vanilloid antagonism. Capsazepine competes for the capsaicin-binding site on TRPV1 however, due to low metabolic stability and poor pharmacokinetic properties the compound did not reach into clinical development. It was observed later on that this tether was not critical for activity as powerful antagonists free from this structural feature were developed, with 1,3-di(arylalkyl)thioureas emerging as one of the most promising non-vanilloid class of TRPV1 antagonist showing excellent therapeautical potential in pain regulation. Within these compounds, the replacement of the guaiacyl moiety of capsaicinoids with a 3-fluoro-4-sulfonylamido group found critical to revert activity. This, lead to the design of C-region moiety mimicked on RTX, led to compound seen in figure 4b,that showed excellent analgesic activity in mice. An alternative optimization of the lipophilic C region led to JYL1421 (fig. 4c), another promising clinical candidate.
Di(arylalkyl)- and Aryl(arylakyl)ureas
Several capsaicin analogs of the urea type were developed by acylation
of homovanillylamine and related amines with different 4-(α-pyridyl)piperidine-1-acyl chlorides. The presence of a polar amino moiety in the hydrophobic C region of capsacinoids was crucial to couple potency and hydrophilicity, mimicking similar observations that led to the discovery of phenylacetylrinvanil (fig. 3b) from olvanil(fig. 3a). Phenylacetylrinvanil is the most potent capsaicinoid reported to date, ~500-fold more potent than capsaicin. Several other ureas emerged as remarkably active TRPV1 antagonists. Compared with capsazepine, the piperazinyl urea (fig. 5a and 5b) showed a higher selectivity profile against a wide variety of enzymes and channels whereas the related very potent and specific TRPV1 antagonist A-425619 (fig. 5c) could reduce pain associated with inflammation and tissue injury in rats. Further research has led to a variety of small-molecule antagonists of TRPV1, including the ureas SB-705498 (fig. 5d), SB-452533 (fig. 5e)[16,17] and ABT-102(fig. 5f), compounds that have entered clinical trials.
Cinnamides
N-Arylcinnamides have emerged as potent and important class of TRPV1 antagonists, Compound SB-366791, (fig. 6a) shows competitive and specific activity in both human and rat TRPV1 receptors overall profile of receptor selectivity much better than that of capsazepine.
Within this series of compounds, AMG9810 (fig. 6b) exhibited high antagonist potency showing good oral bio-availability in rats and a promising pharmacokinetic profile, boding well for clinical efficacy. Another potent blocker from this group is AMG0347(fig. 6c)that was shown in a postoperative pain trial to be able to decrease capsaicin-induced heat and mechanical hyperalgesia and to block central TRPV1 receptors.
Carboxamides
Several TRPV1 antagonists of the carboxamide
type have been discovered. They are structurally quite heterogenous, as exemplified by comparison of the nicotinamide
derivative SB-782443 (fig. 7a), the thiazolylcarboxamide (fig. 7b), and the tetrahydropyridylcarboxamide (fig. 7c). SB-782443 (fig. 7a) showed excellent potency at human, guinea pig, and rat TRPV1, a favorable in vitro drug metabolism and pharmacokinetics profile, and remarkable in vivo
activity in an inflammatory pain model. Based on their in vitro profile, several compounds of this class qualified for preclinical development.
Other derivatives
Nonclassic antagonists lack the urea, thiourea, or amide groups typical of the classic TRPV1 ligands. Two major structural types of nonclassic antagonists have been discovered. First there are the imidazole
derivatives. Starting from a 4,6-disubstituted benzimidazole lead structure, a series of 4,5-biarylimidazoles capable to block both capsaicin and acid-induced calcium influx in TRPV1-expressing Chinese hamster ovary cells. Imidazole (fig. 8a) was identified as a highly potent and orally bioavailable TRPV1. Another class are the diaryl ethers and amines. Compounds from a quinazoline
series can be considered as conformationally restricted analogs of a biarylamide series. In terms of activity 5-isoquinoline was found the most active among and ranked in the order of 5-isoquinoline > 8-quinoline> 8-quinazoline> 8-isoquinoline≥ cinnoline> phthalazine> quinoxaline> 5-quinoline e.g. AMG517 (fig. 8b),although it lacks any recognizable carbonyl motif it still potently blocks capsaicin, proton, and heat activation of TRPV1 in vitro and shows a good tolerability profile. Also, the clinical candidates from Janssen, Abott and Merck pharmaceuticals (fig. 8c) having a 5-aminoisoquinoline group as a common feature suggesting that there is a key interaction of this group at the receptor site for TRPV1 antagonist activity.
Agonists
NeurogesX has successfully completed three Phase III clinical studies of Qutenza (NGX-4010) that met studies primary endpoints. Qutenza is a synthetic trans-capsaicin and drug delivery is by a rapid-delivery patch application system NeurogesX plans to launch Qutenza in the United States in the first half of November 2010.
Anesiva, another biotechnology company, has completed two Phase III trials of Adlea (ALGRX 4975), an injectable capsaicin. Adlea is promising as a pain reliever and both trials showed that Adlea’s safety profile of adverse events, wound healing, and wound sensory function were similar to placebo over the study duration.
Antagonists
At least seven orally active TRPV1 antagonist substances have progressed into clinical development and several more are in preclinical development. The ligand GRC 6211, by Eli Lilly and Company
-Glenmark
is the most advanced and is currently in phase IIb clinical trials. GlaxoSmithKline
, Merck
-Neurogen, Amgen
and AstraZeneca
are all developing TRPV1 antagonist and all are developing substances that have completed phase I trials successfully.
Capsazepine
Capsazepine is a synthetic analogue of capsaicin.-Pharmacaology:Capsazepine blocks the painful sensation of heat caused by capsaicin which activates the TRPV1 ion channel. It is therefore considered to be a capsaicin antagonist. The TRPV1 channel functions as a pain and temperature sensor in...
, was first described in 1990, since then development of novel TRPV1 antagonists has come a long way. This effort has led to the identification of several TRPV1 antagonists that have entered clinical trials as analgesic agents. Should these new chemical entities relieve symptoms of chronic pain then this class of compounds may offer one of the first novel mechanisms for the treatment of pain, in many years.
History
Capsaicin
Capsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
, the active ingredient in chilli pepper was first isolated over a century ago. In 1919 the exact chemical structure of capsaicin
Capsaicin
Capsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
was determined and the complete synthesis of the compound was achieved a decade later. Capsaicin has been used as an analgesic
Analgesic
An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....
for decades, but the therapeutic potential of capsaicin was first recognized as early as 1850. The effects of the pungent chemical, capsaicin, is mediated through the ligand gated ion channel TRPV1
TRPV1
The transient receptor potential cation channel subfamily V member 1 ', also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene...
. This knowledge set the stage for further research of the function of the TRPV1 receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
and preclinical studies showed evidence of its importance in numerous human diseases. These are the first agents acting by this mechanism that made their way into clinic for evaluation of their use as possible analgesics and therefore important targets for drug development
Drug development
Drug development is a blanket term used to define the process of bringing a new drug to the market once a lead compound has been identified through the process of drug discovery...
. Many discoveries are yet to be made, both in terms of the range of potential therapeutic applications in addition to analgesia for TRPV1 antagonists
Receptor antagonist
A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses...
and it was only in the last decade where there has been a full understanding of the molecular mechanism. In the years to come it will be clearer if TRPV1 antagonists can fulfill their potential.
Vanilloid receptor 1 (VR1/TRPV1 receptor)
The vanilloid receptor (TRPV1) is one of six sub-members that belong to the transient receptor potential channel (TRP) superfamily. TRPV1 was the first mammalian member to be discovered and is a non-selective cation channel permeable for calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
. The receptor is made of four identical subunits each with six transmembrane segments, S1-S6, and between fifth and sixth segment is an aqueous pore. This region forms the channel conductive pore and contains the N- and C-termini on the cytosolic side of the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
.
Capsaicin
Capsaicin
Capsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
and RTX
Resiniferatoxin
Resiniferatoxin is a naturally occurring, ultrapotent capsaicin analog that activates the vanilloid receptor in a subpopulation of primary afferent sensory neurons involved in nociception...
, elicit burning pain by activating a non-selective cation channel expressed on sensory nerve endings. When capsaicin
Capsaicin
Capsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
was found to have analgesic effects in preclinical studies much emphasis was put into the research of the receptor/channel that capsaicin binds to and activates. Besides being activated by capsaicin, TRPV1 also responds to a wide range of exogenous and endogenous chemical ligands as well as physical stimuli such as heat over 42°C and changes in more diverse activators such as protons (acid, pH<6). TRPV1
TRPV1
The transient receptor potential cation channel subfamily V member 1 ', also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene...
is also subject to regulation by changes in membrane potential
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
and this intrinsic voltage-dependence is thought to underlie the gating
Gating
-Neurobiology:*Gating , the opening or closing of ion channels*Sensory gating, an automatic process by which the brain adjusts to stimuli*Synaptic gating, neural circuits suppressing inputs through synapses...
mechanism of this non-selective cation channel which leads to the influx of sodium and calcium ions. Importantly, TRPV1
TRPV1
The transient receptor potential cation channel subfamily V member 1 ', also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene...
activity is also subject to regulation by a host of intracellular signaling cascades such as G-protein coupled receptor signaling, that are implicated in the responses to algogenic agents, inflammatory mediators and injury.
Mechanism of action
TRPV1TRPV1
The transient receptor potential cation channel subfamily V member 1 ', also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene...
is primarily expressed on, small myelinated and unmyelinated medium size, sensory neurons in dorsal root
Dorsal root
In anatomy and neurology, the dorsal root is the afferentsensory root of a spinal nerve.At the distal end of the dorsal root is the dorsal root ganglion, which contains the neuron cell bodies of the nerve fibres conveyed by the root.If the dorsal root of a spinal nerve were severed it would lead...
and trigeminal ganglia, where sensory neurons cluster. TRPV1 receptors are also found in muscles, joints, the urinary bladder and kidneys. The functional activity of TRPV1 has been demonstrated, within the central nervous system, in the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
and specific sites in the brain including the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
, cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, locus coeruleus, periaqueductal grey and cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. Activation of TRPV1 sets off an influx of calcium and sodium ions which in turn initiates a cascade of events that result in membrane depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
, neuronal firing and transduction of neural impulses. TRPV1 phosphorylates as a response to several algesic agents, resulting in a lower threshold of channel activation. Some substances such as bradykinin
Bradykinin
Bradykinin is a peptide that causes blood vessels to dilate , and therefore causes blood pressure to lower. A class of drugs called ACE inhibitors, which are used to lower blood pressure, increase bradykinin further lowering blood pressure...
, nerve growth factor
Nerve growth factor
Nerve growth factor is a small secreted protein that is important for the growth, maintenance, and survival of certain target neurons . It also functions as a signaling molecule. It is perhaps the prototypical growth factor, in that it is one of the first to be described...
and protons have been reported to sensitize the TRPV1 receptor. Activation of TRPV1 results in the release of pro-nociceptive peptides, which decreases when treated with TRPV1 antagonists. In general, most channel antagonists bind in the pore region, interacting with residues from all four monomers of the tetrameric channel.
Binding
Ligands of the TRPV1 receptor seem to act from the intracellularIntracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
side. This is a unique property of TRPV1, where ligands of other ligand-gated channels bind from the extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
space. Capsaicin is highly lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...
and can pass the plasma membrane easily. It is generally accepted that capsaicin acts on and binds to the TRPV1 receptor from the intracellular side prior to activation. The critical sites for capsaicin binding are Arg 114 and Glu 761 at the N- and C-termini of the receptor, respectively. Because these two amino acids are charged and located in the cytosolic part of TRPV1 receptor, the two regions are likely to be implicated in hydrophilic interaction of TRPV1 with vanilloids such as capsaicin and RTX
Resiniferatoxin
Resiniferatoxin is a naturally occurring, ultrapotent capsaicin analog that activates the vanilloid receptor in a subpopulation of primary afferent sensory neurons involved in nociception...
.
In addition to these sites in N- and C-termini of TRPV1, a region in the intracellular linker sited in the transmembrane domain, called ‘the TM3 region’, has been shown to be critical for hydrophobic interaction with vanilloids. The TM3 region is considered to be necessary for binding to vanilloids. It is surrounded by the hydrophobic environment because of its placement in the plasma membrane. Now it is recognized as an important link in hydrophobic interaction with capsaicin. The binding sites Arg 114 and Glu 761 and the TM3 region in TRPV1, together consist of a binding pocket to vanilloids.
Agonists
Capsaicin (fig. 2), a naturally occurring vanilloid, is the best known TRPV1 agonist. ResiniferatoxinResiniferatoxin
Resiniferatoxin is a naturally occurring, ultrapotent capsaicin analog that activates the vanilloid receptor in a subpopulation of primary afferent sensory neurons involved in nociception...
(RTX) is another naturally occurring vanilloid that exhibits TRPV1 agonistic activity. It is more potent than capsaicin and is currently in development as a sensory neuron desensitizing agent.
Initially, agonists were the major focus of the TRPV1 ligand development due to the analgesic effect resulting from desensitization
Desensitization (medicine)
For medical purposes, desensitization is a method to reduce or eliminate an organism's negative reaction to a substance or stimulus.For example, if a person with diabetes mellitus has a bad allergic reaction to taking a full dose of beef insulin, the doctor gives the person a very small amount of...
of the receptor. However, because of an initial burning effect of all natural vanilloid receptor agonists, including capsaicin, therapy becomes complicated and perhaps ineffective. Attempts to make synthetic
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
agonists with good separation between excitatory effects and the analgesic effects have not been successful. To avoid this persisting side effects of TRPV1 agonists, a focused consideration has been given to competitive antagonists as novel analgesic drugs.
Antagonists
Intense efforts have been carried out to design both competitive and non-competitive TRPV1 antagonistsReceptor antagonist
A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses...
. Antagonists that bind to the agonist binding site
Binding site
In biochemistry, a binding site is a region on a protein, DNA, or RNA to which specific other molecules and ions—in this context collectively called ligands—form a chemical bond...
, and lock the channel in the closed, nonconductive state are competitive antagonists. In contrast, antagonists that interact with additional binding sites on the receptor structure preventing receptor opening by the agonist or blocking its aqueous pore are non-competitive antagonists. Non-competitive antagonists acting as open channel blockers are therapeutically attractive because of their recognition of over-activated TRPV1 channels, which can reduce the potential of unwanted side effects.
Pharmacophore
The pharmacophorePharmacophore
thumb|right|300px|An example of a pharmacophore model.A pharmacophore is an abstract description of molecular features which are necessary for molecular recognition of a ligand by a biological macromolecule....
model for TRPV1 antagonists consists of three essential features: a hydrogen-bond acceptor, a hydrogen-bond donor, and a ring feature. In addition, the TRPV1 antagonists have been superimposed
Superimposed
Superimposed is an Indie Metal band based in Manchester, England. The exact membership of the band is subject to speculation, as the number of members appearing at gigs varies, and their identity is heavily masked...
in such a way that they could fit in the volume of the TRPV1 pore. When the homology model is considered, appropriate interaction sites are found in the receptor pore. The hydrogen-bond acceptor on the ligand is proposed to interact with Tyr 667 (helix S6) on the receptor as a hydrogen-bond donor, and the hydrogen-bond donor on the ligand is proposed to interact with Tyr 667 on the opposite monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
of the tetramer
Tetramer
A tetramer is a protein with four subunits . There are homotetramers such as glutathione S-transferase or single-strand binding protein, dimers of hetero-dimers such as hemoglobin , and heterotetramers, where each subunit is different.-Subunit interactions in tetramers:The interactions between...
on the receptor as a hydrogen-bond acceptor. The ring feature of the pharmacophore is proposed to fit in the hydrophobic space formed by the aromatic rings of the four Tyr 667 residues of the four monomers. Consistent with the critical role played by Tyr 667 in the interaction with key elements of the TPRV1 antagonist pharmacophore, site-directed mutagenesis
Site-directed mutagenesis
Site-directed mutagenesis, also called site-specific mutagenesis or oligonucleotide-directed mutagenesis, is a molecular biology technique in which a mutation is created at a defined site in a DNA molecule. In general, this form of mutagenesis requires that the wild type gene sequence be known...
studies have shown that exchanging this tyrosine for alanine in the rat TRPV1 receptor abolishes functional activity of TRPV1. The lipophilic end in antagonist is varied in character and volume and interacts with the lower end of transmembrane helices S5 and S6. Because the intracellular ends of these helices extend past the membrane, they are likely to be flexible and may be part of the channel opening and closing process. The combined use of a pharmacophore model, assembled from highly optimized TRPV1 antagonists, with a homology model of the protein has enhanced understanding of the observed structure–activity relationships of many series of current TRPV1 antagonists, and should be useful in the discovery of new classes of antagonists.
Structure activity relationship
Capsaicin (fig. 2) has three functional regions: an aromatic A region where a parent homovanillyl (3-methoxy 4-hydroxybenzyl) group is optimal, a B region known as the esterEster
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
or amide
Amide
In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
linker and the aliphatic C region where a lipophilic octanyl moiety is associated with the highest potency. The homovanillyl motif and amide bond regions contain dipolar groups which are implicated in hydrogen bonding interactions.
Phenolic hydroxide and amide moieties appear to be vital for inducing capsaicin responses. Removal of the phenolic hydroxide or amide bond in capsaicin analogues leads to reduction of potency. The phenolic hydroxide and amide moieties in capsaicin share potential multiple hydrogen bond interactions with the TRPV1 receptor. Capsaicinoids and capsinoids
Capsinoids
Capsinoids, which include capsiate, dihydrocapsiate, and nordihydrocapsiate, are substances naturally present in chili peppers. Although they are structurally similar to capsaicin, the substance that causes pungency in hot peppers, they largely lack that characteristic. Capsinoids have an...
are characterized by an oxygenated aromatic moiety bound via an amide(capsaicinoids) or ester (capsinoids) linker to a lipophilic acyl group. The vanillyl
Vanillyl
In organic chemistry, vanillyl is a functional group. Compounds containing a vanillyl group are called vanilloids, and include vanillin, vanillic acid, capsaicin and VMA....
and carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
linker contain polar groups capable of forming hydrogen bonds essential for activity, whereas the lipophilic moiety interacts with a corresponding cleft of the vanilloid binding site on TRPV1. Replacement of the medium-sized branched fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
of capsaicin with longer fatty acids is damaging for activity, but the presence of unsaturations restores and potentiates activity e.g. oleoylvanillamine (olvanil)(fig. 3a), is 10-fold more potent than capsaicin in TRPV1 activation assays.
1,3-Di(arylalkyl)thioureas
Capsazepine
Capsazepine
Capsazepine is a synthetic analogue of capsaicin.-Pharmacaology:Capsazepine blocks the painful sensation of heat caused by capsaicin which activates the TRPV1 ion channel. It is therefore considered to be a capsaicin antagonist. The TRPV1 channel functions as a pain and temperature sensor in...
(fig. 4a), the first competitive vanilloid antagonist, reported by Novartis
Novartis
Novartis International AG is a multinational pharmaceutical company based in Basel, Switzerland, ranking number three in sales among the world-wide industry...
group, was
aimed at assessing the effect of conformational constraint on the lipophilic C-region of capsaicin. In capsazepine the amide bond of the capsaicin is replaced by a thiourea
Thiourea
Thiourea is an organosulfur compound of with the formula SC2 . It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. "Thioureas" refers to a broad...
moiety and a propylidene linker between the aromatic vanillyl 2-carbon A ring and the B-linker amide nitrogen forces the aromatic ring in an orthogonal orientation with respect to the thiourea bond. This constraint has long been considered as the distinctive characteristic of vanilloid antagonism. Capsazepine competes for the capsaicin-binding site on TRPV1 however, due to low metabolic stability and poor pharmacokinetic properties the compound did not reach into clinical development. It was observed later on that this tether was not critical for activity as powerful antagonists free from this structural feature were developed, with 1,3-di(arylalkyl)thioureas emerging as one of the most promising non-vanilloid class of TRPV1 antagonist showing excellent therapeautical potential in pain regulation. Within these compounds, the replacement of the guaiacyl moiety of capsaicinoids with a 3-fluoro-4-sulfonylamido group found critical to revert activity. This, lead to the design of C-region moiety mimicked on RTX, led to compound seen in figure 4b,that showed excellent analgesic activity in mice. An alternative optimization of the lipophilic C region led to JYL1421 (fig. 4c), another promising clinical candidate.
Di(arylalkyl)- and Aryl(arylakyl)ureas
Several capsaicin analogs of the urea type were developed by acylation
Acylation
In chemistry, acylation is the process of adding an acyl group to a compound. The compound providing the acyl group is called the acylating agent....
of homovanillylamine and related amines with different 4-(α-pyridyl)piperidine-1-acyl chlorides. The presence of a polar amino moiety in the hydrophobic C region of capsacinoids was crucial to couple potency and hydrophilicity, mimicking similar observations that led to the discovery of phenylacetylrinvanil (fig. 3b) from olvanil(fig. 3a). Phenylacetylrinvanil is the most potent capsaicinoid reported to date, ~500-fold more potent than capsaicin. Several other ureas emerged as remarkably active TRPV1 antagonists. Compared with capsazepine, the piperazinyl urea (fig. 5a and 5b) showed a higher selectivity profile against a wide variety of enzymes and channels whereas the related very potent and specific TRPV1 antagonist A-425619 (fig. 5c) could reduce pain associated with inflammation and tissue injury in rats. Further research has led to a variety of small-molecule antagonists of TRPV1, including the ureas SB-705498 (fig. 5d), SB-452533 (fig. 5e)[16,17] and ABT-102(fig. 5f), compounds that have entered clinical trials.
Cinnamides
N-Arylcinnamides have emerged as potent and important class of TRPV1 antagonists, Compound SB-366791, (fig. 6a) shows competitive and specific activity in both human and rat TRPV1 receptors overall profile of receptor selectivity much better than that of capsazepine.
Within this series of compounds, AMG9810 (fig. 6b) exhibited high antagonist potency showing good oral bio-availability in rats and a promising pharmacokinetic profile, boding well for clinical efficacy. Another potent blocker from this group is AMG0347(fig. 6c)that was shown in a postoperative pain trial to be able to decrease capsaicin-induced heat and mechanical hyperalgesia and to block central TRPV1 receptors.
Carboxamides
Several TRPV1 antagonists of the carboxamide
Carboxamide
In organic chemistry carboxamides are functional groups with the general structure R-CO-NH2 with R as an organic substituent.Two amino acids, asparagine and glutamine, have a carboxamide group in them....
type have been discovered. They are structurally quite heterogenous, as exemplified by comparison of the nicotinamide
Nicotinamide
Nicotinamide, also known as niacinamide and nicotinic acid amide, is the amide of nicotinic acid . Nicotinamide is a water-soluble vitamin and is part of the vitamin B group...
derivative SB-782443 (fig. 7a), the thiazolylcarboxamide (fig. 7b), and the tetrahydropyridylcarboxamide (fig. 7c). SB-782443 (fig. 7a) showed excellent potency at human, guinea pig, and rat TRPV1, a favorable in vitro drug metabolism and pharmacokinetics profile, and remarkable in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
activity in an inflammatory pain model. Based on their in vitro profile, several compounds of this class qualified for preclinical development.
Other derivatives
Nonclassic antagonists lack the urea, thiourea, or amide groups typical of the classic TRPV1 ligands. Two major structural types of nonclassic antagonists have been discovered. First there are the imidazole
Imidazole
Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents...
derivatives. Starting from a 4,6-disubstituted benzimidazole lead structure, a series of 4,5-biarylimidazoles capable to block both capsaicin and acid-induced calcium influx in TRPV1-expressing Chinese hamster ovary cells. Imidazole (fig. 8a) was identified as a highly potent and orally bioavailable TRPV1. Another class are the diaryl ethers and amines. Compounds from a quinazoline
Quinazoline
Quinazoline is a compound made up of two fused six-membered simple aromatic rings, a benzene ring and a pyrimidine ring. Its chemical formula is C8H6N2. Quinazoline is yellow and crystalline...
series can be considered as conformationally restricted analogs of a biarylamide series. In terms of activity 5-isoquinoline was found the most active among and ranked in the order of 5-isoquinoline > 8-quinoline> 8-quinazoline> 8-isoquinoline≥ cinnoline> phthalazine> quinoxaline> 5-quinoline e.g. AMG517 (fig. 8b),although it lacks any recognizable carbonyl motif it still potently blocks capsaicin, proton, and heat activation of TRPV1 in vitro and shows a good tolerability profile. Also, the clinical candidates from Janssen, Abott and Merck pharmaceuticals (fig. 8c) having a 5-aminoisoquinoline group as a common feature suggesting that there is a key interaction of this group at the receptor site for TRPV1 antagonist activity.
Clinical trials
As of late 2009 no vanilloid receptor ligands are on the market but available public information suggests that quite a few are in clinical trials. Several biotechnology and pharmaceutical companies are developing TRPV1 ligands and the emphasis seems to be on both agonists and antagonists. Although the agonists appear to be further along in clinical development.Agonists
NeurogesX has successfully completed three Phase III clinical studies of Qutenza (NGX-4010) that met studies primary endpoints. Qutenza is a synthetic trans-capsaicin and drug delivery is by a rapid-delivery patch application system NeurogesX plans to launch Qutenza in the United States in the first half of November 2010.
Anesiva, another biotechnology company, has completed two Phase III trials of Adlea (ALGRX 4975), an injectable capsaicin. Adlea is promising as a pain reliever and both trials showed that Adlea’s safety profile of adverse events, wound healing, and wound sensory function were similar to placebo over the study duration.
Antagonists
At least seven orally active TRPV1 antagonist substances have progressed into clinical development and several more are in preclinical development. The ligand GRC 6211, by Eli Lilly and Company
Eli Lilly and Company
Eli Lilly and Company is a global pharmaceutical company. Eli Lilly's global headquarters is located in Indianapolis, Indiana, in the United States...
-Glenmark
Glenmark Pharmaceuticals
Glenmark Pharmaceuticals is one of the leading pharmaceutical company headquarteredin Mumbai, India. It manufactures and markets generic formulation products and active pharmaceutical ingredients , both in the domestic and international markets...
is the most advanced and is currently in phase IIb clinical trials. GlaxoSmithKline
GlaxoSmithKline
GlaxoSmithKline plc is a global pharmaceutical, biologics, vaccines and consumer healthcare company headquartered in London, United Kingdom...
, Merck
Merck & Co.
Merck & Co., Inc. , also known as Merck Sharp & Dohme or MSD outside the United States and Canada, is one of the largest pharmaceutical companies in the world. The Merck headquarters is located in Whitehouse Station, New Jersey, an unincorporated area in Readington Township...
-Neurogen, Amgen
Amgen
Amgen Inc. is an international biotechnology company headquartered in Thousand Oaks, California. Located in the Conejo Valley, Amgen is the world's largest independent biotech firm. The company employs approximately 17,000 staff members. Its products include Epogen, Aranesp, Enbrel, Kineret,...
and AstraZeneca
AstraZeneca
AstraZeneca plc is a global pharmaceutical and biologics company headquartered in London, United Kingdom. It is the world's seventh-largest pharmaceutical company measured by revenues and has operations in over 100 countries...
are all developing TRPV1 antagonist and all are developing substances that have completed phase I trials successfully.
See also
- Vanilloid receptor TRPVTRPVTRPV is a family of transient receptor potential ion channels. These channels are selective for calcium and magnesium over sodium ions. Like other members of the TRP superfamily, TRPV channels can be activated through seemingly disparate mechanisms...
- Vanilloid receptor subtype 1 TRPV1TRPV1The transient receptor potential cation channel subfamily V member 1 ', also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene...
- Transient receptor potential channel
- CapsaicinCapsaicinCapsaicin 2CHCH=CH4CONHCH2C6H3-4--3- ) is the active component of chili peppers, which are plants belonging to the genus Capsicum. It is an irritant for mammals, including humans, and produces a sensation of burning in any tissue with which it comes into contact...
- ResiniferatoxinResiniferatoxinResiniferatoxin is a naturally occurring, ultrapotent capsaicin analog that activates the vanilloid receptor in a subpopulation of primary afferent sensory neurons involved in nociception...
- CapsazepineCapsazepineCapsazepine is a synthetic analogue of capsaicin.-Pharmacaology:Capsazepine blocks the painful sensation of heat caused by capsaicin which activates the TRPV1 ion channel. It is therefore considered to be a capsaicin antagonist. The TRPV1 channel functions as a pain and temperature sensor in...