Cyclooxygenase 2 inhibitors: drug discovery and development
Encyclopedia
Cyclooxygenase
s are enzymes that take part in a complex biosynthetic cascade that results in the conversion of polyunsaturated fatty acids to prostaglandins and thromboxane
(s).
Their main role is to catalyze the transformation of arachidonic acid
into the intermediate prostaglandin H2
, which is the procursor of a variety of prostanoids with diverse and potent biological actions.
Cyclooxygenases have two main isoforms that are called COX-1 and COX-2 (as well as a COX-3
). COX-1 is responsible for the synthesis of prostaglandin and thromboxane in many types of cells, including the gastro-intestinal tract and blood platelets. COX-2 plays a major role in prostaglandin biosynthesis in inflammatory cells and in the central nervous system. Prostaglandin
synthesis in these sites is a key factor in the development of inflammation and hyperalgesia
.
COX-2 inhibitors have analgesic and anti-inflammatory activity by blocking the transformation of arachidonic acid into prostaglandin H2 selectively.
|-
|
|-
|DuP-697
|}
The impetus for development of selective COX-2 inhibitors was the adverse gastrointestinal side-effects of NSAIDs. Soon after the discovery of the mechanism of action of NSAIDs, strong indications emerged for alternative forms of COX, but little supporting evidence was found. COX enzyme proved to be difficult to purify and was not sequenced until 1988. But in 1991 the COX-2 enzyme was cloned and its existence, therefore, confirmed. Before the confirmed existence of COX-2, the Dupont company had developed a compound, DuP-697, that was potent in many anti-inflammatory assays but did not have the ulcerogenic effects of NSAIDs. Once the COX-2 enzyme was identified Dup-697 became the building-block for synthesis of COX-2 inhibitors. Celecoxib and rofecoxib, the first COX-2 inhibitors to reach market, were based on DuP-697. It took less than eight years to develop and market the first COX-2 inhibitor, with Celebrex (celecoxib
) launched in December 1998 and Vioxx (rofecoxib
) launched in May 1999.
Building on those results, scientists started focusing on selective COX-2 inhibitors
. Enormous effort was spent on the development of NSAIDs between the 1960s and 1980 so there were numerous pharmacophores to test when COX-2 was discovered. Early efforts focused on modification on two lead compounds, DuP-697 and NS-398
. These compounds differ greatly from NSAIDs that are arylalkonic acid analogs. Encouraged by the “concept testing
” experiments with selective inhibitors, and armed with several solid leads and clear idea of the nature of the binding site
, development of this field was rapid. In vitro recombinant enzyme assays provided powerful means for assessing COX selectivity and potency
and led to the discovery and clinical development of the first rationally designed COX-2 selective inhibitor, celecoxib. Efforts have been made to convert NSAIDs into selective COX-2 inhibitors such as indometacin
by lengthening of the alkylcarboxylic acid side-chain, but none have been marketed.
moiety. Structure activity relationship (SAR) studies for diaryl heterocyclic compounds have indicated that a cis-stilbene moiety and changes in the para-position of one of the aryl rings play an important role in COX-2 selectivity. Celecoxib and parecoxib have a sulfonamide substituent (SO2NH2) in para-position on one of the aryl rings while etoricoxib and rofecoxib have a methylsulfone (SO2CH3). The oxidation state on the sulfur is important for selectivity; sulfones and sulfonamides are selective for COX-2 but sulfoxides and sulfides are not. The ring system that is fused in this stilbene system has been extensively manipulated to include every imaginable heterocyclic and carbocyclic skeleton of varying ring sizes. It is known that a SO2NHCOCH3 moiety as in parecoxib, which is a prodrug for valdecoxib, is 105 – 106 more reactive acetylating agent of enzyme serine hydroxyl groups than simple amides. Due to the fact that varying kinetic mechanisms affect potency for COX-1 versus COX-2, relying Potency and selectivity in human whole blood is used by many groups and has been accepted as a standard assessment of COX-2 potency and selectivity.
|-
|
|-
|COX-2 receptor site and its amino acid profile along with celecoxib in the binding site
|}
One of the keys to developing COX-2 selective drugs is the larger active site
of COX-2, which makes it possible to make molecules too large to fit into the COX-1 active site but still able to fit the COX-2. The larger active site of COX-2 is partly due to a polar hydrophilic side-pocket that forms because of substitution of Ile
523, His
513, and Ile434 in COX-1 by Val
523, Arg
513, and Val434 in COX-2. Val523 is less bulky than Ile523, which increases the volume of the active site. Substitution of Ile434 for Val434 allows the side-chain of Phe
518 to move back and make some extra space. This side-pocket allows for interactions with Arg513, which is a replacement for His513 of COX-1. Arg513 is thought to be a key residue for diaryl heterocycle inhibitors such as the coxibs. The side-chain of Leu
384, at the top of the receptor channel, is oriented into the active site of COX-1, but, in COX-2, it is oriented away from the active site and makes more space in the apex of the binding site.
The bulky sulfonamide
group in COX-2 inhibitors such as celecoxib and rofecoxib
prevent the molecule from entering the COX-1 channel.
For optimal activity and selectivity of the coxibs, a 4-methylsulfonylphenyl attached to an unsaturated (usually) five-membered ring with a vicinal lipophilic group is required (rofecoxib). The SO2CH3 can be replaced by SO2NH2, wherein the lipophilic pocket is occupied by an optionally substituted phenyl ring or a bulky alkoxy substituent (celecoxib). Within the hydrophilic side-pocket of COX-2, the oxygen of the sulfonamide (or sulfone
) group interacts with Hist90, Arg513, and Gln
192 and forms hydrogen bonds. The substituted phenyl group
at the top of the channel interacts with the side-chains of amino acid residues through hydrophobic and electrostatic interactions. Tyr
385 makes for some sterical restrictions of this side of the binding site so a small substituent of the phenyl group makes for better binding. Degrees of freedom are also important for the binding. The central ring of the coxibs decides the orientation of the aromatic rings and, therefore, the binding to COX enzyme even though it often has no electrostatic interactions with any of the amino acid residues. The high lipophilicity of the active site does require low polarity
of the central scaffold of the coxibs.
(or sulfone
) that fits into the side-pocket of COX-2. This has been studied using SC-58125 (an analogue of celecoxib) and mutated COX-2, wherein the valine 523 residue was replaced by isoleucine 523. The irreversible inhibition did not happen, but reversible inhibition was noticed. A model has been made to explain this three-step mechanism behind the inhibitory effects of selective COX-2 inhibitors. The first step accounts for the contact of the inhibitor with the gate of the hydrophobic channel (called the lobby region). The second step could account for the movement of the inhibitor from the lobby region to the active site of the COX enzyme. The last step probably represents repositioning of the inhibitor at the active site, which leads to strong interactions of the phenylsulfonamide or phenylsulfone group of the inhibitor and the amino acids of the side pocket. It is directly inhibition to postaglanding
effect, and all reduce prostaglandin formation in inflamed joints. All are well absorbed, but peak concentration may differ between the coxibs. The coxibs are highly protein-bound, and the published estimate of half-lives is variable between the coxibs.
was the first specific inhibitor of COX-2 approved to treat patients with rheumatism
and osteoarthritis
. A study showed that the absorption rate, when given orally, is moderate, and peak plasma concentration occurs after about 2–4 hours. However, the extent of absorption
is not well known. Celecoxib has the affinity
to bind extensively to plasma proteins, especially to plasma albumin
. It has an apparent volume of distribution (VD) of 455 +/- 166 L in humans and the area under the plasma concentration-time curve (AUC
) increases proportionally to increased oral doses, between 100 and 800 mg. Celecoxib is metabolized primarily by CYP2C9 isoenzyme to carboxylic acid and also by non-CYP-dependent glucuronidation to glucuronide
metabolites. The metabolites are excreted in urine and feces, with a small proportion of unchanged drug (2%) in the urine. Its elimination half-life is about 11 hours (6–12 hours) in healthy individuals, but racial differences in drug disposition and pharmacokinetic changes in the elderly have been reported. Patients with chronic renal insufficiency appear to have 43% lower plasma concentration compared to healthy individuals, with a 47% increase in apparent clearance, and it can be expected that patients with mild to moderate hepatic impairment have increased steady-state AUC.
sodium is a water-soluble inactive ester amide prodrug
of valdecoxib
, a novel second-generation COX-2-specific inhibitor and the first such agent to be developed for injectable use. It is rapidly converted by hepatic enzymatic hydrolysis
to the active form valdecoxib. The compound then undergoes another conversion, which involves both cytochrome P450-mediated pathway (CYP2C9, CYP3A4) and non-cytochrome P450-mediated pathway, to hydroxylated metabolite and glucuronide metabolite. The hydroxylated metabolite, that also has weak COX-2-specific inhibitory properties, is then further metabolized by non-cytochrome P450 pathway to a glucuronide metabolite. These metabolites are excreted in the urine.
After intra-muscular administration of Parecoxib sodium peak plasma concentration is reached within 15 minutes. The plasma concentration decreases rapidly after administration because of a rather short serum half-life, which is about 15–52 minutes. This can be explained by the rapid formation of Valdecoxib. In contrast to the rapid clearance of Parecoxib, plasma concentration of Valdecoxib declines slowly because of a longer half-life. On the other hand, when Valdecoxib is taken orally it is absorbed rapidly (1–2 hours), but presence of food can delay peak serum concentration. It then undergoes the same metabolism that is described above. It is extensively protein-bound (98%), and the plasma half-life is about 7–8 hours. Note that the half-life can be significantly prolonged in the elderly or those with hepatic impairment, and can lead to drug accumulation.
The hydroxyl metabolite reaches its highest mean plasma concentration within 3 to 4 hours from administration, but it is considerably lower than of Valdecoxib or about 1/10 of the plasma levels of Valdecoxib.
, that is used for patients with chronic arthropathies and musculoskeletal and dental pain, is absorbed moderately when given orally. A study on its pharmacokinetics showed that the plasma peak concentration of etoricoxib occurs after approximately 1 hour. It has shown to be extensively bound to plasma albumin (about 90%), and has an apparent volume of distribution (VD) of 120 L in humans. The area under the plasma concentration-time curve (AUC) increases in proportion to increased dosage (5–120 mg). The elimination half-life is about 20 hours in healthy individuals, and such long half-life enables the choice to have once-daily dosage. Etoricoxib, like the other coxibs, is excreted in urine and feces and also metabolized in likewise manner. CYP3A4
is mostly responsible for biotransformation of etoricoxib to carboxylic acid metabolite, but a non CYP450 metabolism pathway to glucuronide metabolite is also at hand. A very small portion of etoricoxib (<1%) is eliminated unchanged in the urine. Patients with chronic renal insufficiency do not appear to have different plasma concentration curve (AUC) compared to healthy individuals. It has though been reported that patients with moderate hepatic impairment have increased plasma concentration curve (AUC) by approximately 40%. It has been stated that further study is necessary to describe precisely the relevance of pharmacokinetic properties in terms of the clinical benefits and risks of etoricoxib compared to other clinical options.
is unique amongst the coxibs in being a weak acid. It was developed for the treatment of osteoarthritis, rheumatoid arthritis and acute pain. The acidic nature of lumiracoxib allows it to penetrate well into areas of inflammation. It has shown to be rapidly and well absorbed, with peak plasma concentration occurring in about 1–3 hours. A study showed that when a subject was given 400 mg dose, the amount of unchanged drug in the plasma 2.5 hours postdose suggest a modest first pass effect
. The terminal half-life in plasma ranged from 5.4 to 8.6 hours (mean =6.5 hours). The half-life in synovial fluid is considerably longer than in plasma, and the concentration in synovial fluid
24 hours after administration would be expected to result in a substantial COX-2 inhibition. This fact can explain why some users may suffice with once-daily dosage despite a short plasma half-life. The major plasma metabolites are 5-carboxy, 4’-hydroxy, and 4’-hydroxy-5-carboxy derivatives. Lumiracoxib is extensively metabolized before it is excreted, and the excretion routes are in the urine or feces. Peak plasma concentrations exceed those necessary to maximally inhibit COX-2, and that is consistent with a longer pharmacodynamic half-life. In vitro lumiracoxib has demonstrated a greater COX-2 selectivity than any of the other coxibs.
was the second selective COX-2 inhibitor to be marketed, and the first one to be taken off the market. When the pharmacokinetics were studied in healthy human subjects, the peak concentration was achieved in 9 hours with effective half-life of approximately 17 hours . A secondary peak has been observed, which might suggest that the absorption of rofecoxib varies with intestinal motility, hence leading to high variability in time until peak concentration is met. Seventy-one and a half percent of the dose was recovered in urine (less than 1% unmetabolised) and 14,2% was recovered in feces (approximately 1,8% in the bile). Among the metabolites were rofecoxib-3’,4’-dihydrodiol, 4’-hydroxyrofecoxib-O-β-D-glucuronide, 5-hydroxyrofecoxib-O-β-D-glucuronide, 5-hydroxyrofecoxib, rofecoxib-erythro-3,4-dihydrohydroxy acid, rofecoxib-threo-3,4-dihydrohydroxy acid, cis-3,4-dihydrorofecoxib and trans-3,4-dihydrorofecoxib.
. After a short time, it became evident that there was a fivefold higher risk of myocardial infarction
in the rofecoxib group compared to the group that received naproxen. The authors suggested that the difference was due to the cardioprotective effects of naproxen. The APPROVe (Adenomatous Poly Prevention on Vioxx) study was a multicentre, randomized, placebo-controlled, double blind trial aimed to assess the effect of three-year treatment with rofecoxib on recurrence of neoplastic polyps in individuals with a history of colorectal adenomas. In 2000 and 2001, 2587 patients with a history of colorectal adenomas were recruited and followed. The trial was stopped early (2 months before expected completion) on recommendations of its data safety and monitoring board because of concerns about cardiovascular toxicity. When looking at the results of the study, it showed a statistically significant increase in cardiovascular risk when taking rofecoxib compared to placebo beginning after 18 months of treatment. Then on the 30th of September Merck gave out a news release announcing their voluntary worldwide withdrawal of Vioxx.
Some studies of other coxibs have also shown increase in the risk of cardiovascular events, while others have not. For instance, the Adenoma Prevention with Celecoxib study (APC) showed a dose-related increase in risk of cardiovascular death, myocardial infarction, stroke, or heart failure when taking celecoxib compared to placebo; and the Successive Celecoxib Efficacy and Safety Study I (SUCCESS-I) showed increased risk of myocardial infarction when taking 100 mg twice a day of celecoxib compared to diclofenac and naproxen; but taking 200 mg twice a day had lower incidence of myocardial infarction compared to diclofenac and naproxen. Nussmeier et al. (2005) showed in a study increase in incidence of cardiovascular events when taking parecoxib and valdecoxib (compared to placebo) after coronary artery bypass surgery.
Another possible explanation was studied by Li H. et al. (2008). They showed that in spontaneously hypertensive rat
s (SHR) non-selective NSAIDs and the coxibs produce oxidative stress
, indicated by enhanced vascular superoxide
(O2-) content and elevated peroxide
in plasma, which is in tune with enhanced expression of NADPH oxidase, which was noticed with use of diclofenac and naproxen and, to a lesser degree, rofecoxib and celecoxib. Nitrite
in plasma was also decreased suggesting a diminished synthesis of vascular nitric oxide
(NO). This decrease in NO synthesis did not result from decreased expression of endothelial nitric oxide synthase (eNOS
) because expression of eNOS mRNA was not reduced, and even upregulated for some products. The decrease in NO synthesis could, rather, be explained by loss of eNOS function. For eNOS to be normally functional, it needs to form a dimer
and to have its cofactor BH4, which is one of the most potent naturally occurring reducing agents. BH4 is sensitive to oxidation by peroxynitrite
(ONOO-), which is produced when NO reacts with O2-, so it has been hypothesized that depletion of BH4 can occur with excessive oxidative stress (that can be caused be NSAIDs) and, hence, be the cause of eNOS dysfunction. This disfunction, which is referred to as eNOS uncoupling, causes the production of O2- by eNOS, thereby leading to more oxidative stress produced by eNOS. In a study, both the selective COX-2 inhibitors and the non-selective NSAIDs produced oxidative stress, with greater effects seen with non-selective NSAIDs use. This could fit with the hypothesis concerning the prostacyclin/thromboxane imbalance. That is, although the non-selective NSAIDs produce more oxidative stress, they prevent platelet aggregation, whereas the COX-2 inhibitors reduce prostacyclin
production, and, hence, the cardiovascular risk for the non-selective NSAIDs is not higher than for the coxibs.
Among other hypotheses are increased blood pressure, decreased production of epi-lipoxins (which have anti-inflammatory effects), and inhibition of vascular remodeling when using selective COX-2 inhibitors.
Cyclooxygenase
Cyclooxygenase is an enzyme that is responsible for formation of important biological mediators called prostanoids, including prostaglandins, prostacyclin and thromboxane. Pharmacological inhibition of COX can provide relief from the symptoms of inflammation and pain...
s are enzymes that take part in a complex biosynthetic cascade that results in the conversion of polyunsaturated fatty acids to prostaglandins and thromboxane
Thromboxane
Thromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring....
(s).
Their main role is to catalyze the transformation of arachidonic acid
Arachidonic acid
Arachidonic acid is a polyunsaturated omega-6 fatty acid 20:4.It is the counterpart to the saturated arachidic acid found in peanut oil, Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6).It is the counterpart to the saturated arachidic acid found in peanut oil,...
into the intermediate prostaglandin H2
Prostaglandin H2
Prostaglandin H2 is a type of Prostaglandin which is derived from arachidonic acid and is a precursor for many other biologically significant molecules.It is acted upon by:* prostacyclin synthase to create prostacyclin...
, which is the procursor of a variety of prostanoids with diverse and potent biological actions.
Cyclooxygenases have two main isoforms that are called COX-1 and COX-2 (as well as a COX-3
COX-3
COX-3 is an enzyme that is encoded by the PTGS1 gene, but is not functional in humans. COX-3 is the third and most recently discovered cyclooxygenase isozyme, the others being COX-1 and COX-2...
). COX-1 is responsible for the synthesis of prostaglandin and thromboxane in many types of cells, including the gastro-intestinal tract and blood platelets. COX-2 plays a major role in prostaglandin biosynthesis in inflammatory cells and in the central nervous system. Prostaglandin
Prostaglandin
A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....
synthesis in these sites is a key factor in the development of inflammation and hyperalgesia
Hyperalgesia
Hyperalgesia is an increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.-Types:...
.
COX-2 inhibitors have analgesic and anti-inflammatory activity by blocking the transformation of arachidonic acid into prostaglandin H2 selectively.
The rise for development of selective COX-2 inhibitors
{| class="wikitable", align="right"|-
|
|DuP-697
|}
The impetus for development of selective COX-2 inhibitors was the adverse gastrointestinal side-effects of NSAIDs. Soon after the discovery of the mechanism of action of NSAIDs, strong indications emerged for alternative forms of COX, but little supporting evidence was found. COX enzyme proved to be difficult to purify and was not sequenced until 1988. But in 1991 the COX-2 enzyme was cloned and its existence, therefore, confirmed. Before the confirmed existence of COX-2, the Dupont company had developed a compound, DuP-697, that was potent in many anti-inflammatory assays but did not have the ulcerogenic effects of NSAIDs. Once the COX-2 enzyme was identified Dup-697 became the building-block for synthesis of COX-2 inhibitors. Celecoxib and rofecoxib, the first COX-2 inhibitors to reach market, were based on DuP-697. It took less than eight years to develop and market the first COX-2 inhibitor, with Celebrex (celecoxib
Celecoxib
Celecoxib INN is a sulfa non-steroidal anti-inflammatory drug and selective COX-2 inhibitor used in the treatment of osteoarthritis, rheumatoid arthritis, acute pain, painful menstruation and menstrual symptoms, and to reduce numbers of colon and rectum polyps in patients with familial...
) launched in December 1998 and Vioxx (rofecoxib
Rofecoxib
Rofecoxib is a nonsteroidal anti-inflammatory drug that has now been withdrawn over safety concerns. It was marketed by Merck & Co. to treat osteoarthritis, acute pain conditions, and dysmenorrhoea...
) launched in May 1999.
Development of COX-2 inhibitors
Early studies showed that, when inflammation is induced, the affected organ unexpectedly develops an enormous capacity to generate prostaglandins. It was demonstrated that the increase is due to de novo synthesis of fresh enzyme. In 1991, during the investigation of the expression of early-response genes in fibroblasts transformed with Rous sarcoma virus, a novel mRNA transcript that was similar, but not identical, to the seminal COX enzyme was identified. It was suggested that an isoenzyme of COX had been discovered. Another group discovered a novel cDNA species encoding a protein with similar structure to COX-1 while studying phorbol-ester-induced genes in Swiss 3T3 cells. The same laboratory showed that this gene truly expressed a novel COX enzyme. The two enzymes were renamed COX-1, referring to the original enzyme and COX-2.Building on those results, scientists started focusing on selective COX-2 inhibitors
Enzyme inhibitor
An enzyme inhibitor is a molecule that binds to enzymes and decreases their activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides...
. Enormous effort was spent on the development of NSAIDs between the 1960s and 1980 so there were numerous pharmacophores to test when COX-2 was discovered. Early efforts focused on modification on two lead compounds, DuP-697 and NS-398
NS-398
NS-398 is a COX-2 inhibitor used in the study of the function of cyclooxygenases....
. These compounds differ greatly from NSAIDs that are arylalkonic acid analogs. Encouraged by the “concept testing
Concept testing
Concept testing is the process of using quantitative methods and qualitative methods to evaluate consumer response to a product idea prior to the introduction of a product to the market. It can also be used to generate communication designed to alter consumer attitudes toward existing products...
” experiments with selective inhibitors, and armed with several solid leads and clear idea of the nature of the 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...
, development of this field was rapid. In vitro recombinant enzyme assays provided powerful means for assessing COX selectivity and potency
Potency
Potency may refer to:* Potency , a measure of the activity of a drug in a biological system* Virility* Potency is a measure of the differentiation potential of stem cells...
and led to the discovery and clinical development of the first rationally designed COX-2 selective inhibitor, celecoxib. Efforts have been made to convert NSAIDs into selective COX-2 inhibitors such as indometacin
Indometacin
Indometacin or indomethacin is a non-steroidal anti-inflammatory drug commonly used to reduce fever, pain, stiffness, and swelling. It works by inhibiting the production of prostaglandins, molecules known to cause these symptoms...
by lengthening of the alkylcarboxylic acid side-chain, but none have been marketed.
Structure Activity Relationship (SAR)
DuP-697 was a building-block for synthesis of COX-2 inhibitors and served as the basic chemical model for the coxibs that are the only selective COX-2 inhibitors on the market today. DuP-697 is a diaryl heterocycle with cis-stilbeneStilbene
-Stilbene, is a diarylethene, i.e., a hydrocarbon consisting of a trans ethene double bond substituted with a phenyl group on both carbon atoms of the double bond. The name stilbene is derived from the Greek word stilbos, which means shining....
moiety. Structure activity relationship (SAR) studies for diaryl heterocyclic compounds have indicated that a cis-stilbene moiety and changes in the para-position of one of the aryl rings play an important role in COX-2 selectivity. Celecoxib and parecoxib have a sulfonamide substituent (SO2NH2) in para-position on one of the aryl rings while etoricoxib and rofecoxib have a methylsulfone (SO2CH3). The oxidation state on the sulfur is important for selectivity; sulfones and sulfonamides are selective for COX-2 but sulfoxides and sulfides are not. The ring system that is fused in this stilbene system has been extensively manipulated to include every imaginable heterocyclic and carbocyclic skeleton of varying ring sizes. It is known that a SO2NHCOCH3 moiety as in parecoxib, which is a prodrug for valdecoxib, is 105 – 106 more reactive acetylating agent of enzyme serine hydroxyl groups than simple amides. Due to the fact that varying kinetic mechanisms affect potency for COX-1 versus COX-2, relying Potency and selectivity in human whole blood is used by many groups and has been accepted as a standard assessment of COX-2 potency and selectivity.
The relationship between amino acid profile of COX-2 enzyme and inhibition mechanism
{| class="wikitable", align="right"|-
|
|COX-2 receptor site and its amino acid profile along with celecoxib in the binding site
|}
One of the keys to developing COX-2 selective drugs is the larger active site
Active site
In biology the active site is part of an enzyme where substrates bind and undergo a chemical reaction. The majority of enzymes are proteins but RNA enzymes called ribozymes also exist. The active site of an enzyme is usually found in a cleft or pocket that is lined by amino acid residues that...
of COX-2, which makes it possible to make molecules too large to fit into the COX-1 active site but still able to fit the COX-2. The larger active site of COX-2 is partly due to a polar hydrophilic side-pocket that forms because of substitution of Ile
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....
523, His
Histidine
Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
513, and Ile434 in COX-1 by Val
Valine
Valine is an α-amino acid with the chemical formula HO2CCHCH2. L-Valine is one of 20 proteinogenic amino acids. Its codons are GUU, GUC, GUA, and GUG. This essential amino acid is classified as nonpolar...
523, Arg
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
513, and Val434 in COX-2. Val523 is less bulky than Ile523, which increases the volume of the active site. Substitution of Ile434 for Val434 allows the side-chain of Phe
Phenylalanine
Phenylalanine is an α-amino acid with the formula C6H5CH2CHCOOH. This essential amino acid is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. L-Phenylalanine is an electrically neutral amino acid, one of the twenty common amino acids used to biochemically form...
518 to move back and make some extra space. This side-pocket allows for interactions with Arg513, which is a replacement for His513 of COX-1. Arg513 is thought to be a key residue for diaryl heterocycle inhibitors such as the coxibs. The side-chain of Leu
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
384, at the top of the receptor channel, is oriented into the active site of COX-1, but, in COX-2, it is oriented away from the active site and makes more space in the apex of the binding site.
The bulky sulfonamide
Sulfonamide (chemistry)
In chemistry, the sulfonamide functional group is -S2-NH2, a sulfonyl group connected to an amine group.A sulfonamide is a compound that contains this group. The general formula is RSO2NH2, where R is some organic group. For example, "methanesulfonamide" is CH3SO2NH2...
group in COX-2 inhibitors such as celecoxib and rofecoxib
Rofecoxib
Rofecoxib is a nonsteroidal anti-inflammatory drug that has now been withdrawn over safety concerns. It was marketed by Merck & Co. to treat osteoarthritis, acute pain conditions, and dysmenorrhoea...
prevent the molecule from entering the COX-1 channel.
For optimal activity and selectivity of the coxibs, a 4-methylsulfonylphenyl attached to an unsaturated (usually) five-membered ring with a vicinal lipophilic group is required (rofecoxib). The SO2CH3 can be replaced by SO2NH2, wherein the lipophilic pocket is occupied by an optionally substituted phenyl ring or a bulky alkoxy substituent (celecoxib). Within the hydrophilic side-pocket of COX-2, the oxygen of the sulfonamide (or sulfone
Sulfone
A sulfone is a chemical compound containing a sulfonyl functional group attached to two carbon atoms. The central hexavalent sulfur atom is double bonded to each of two oxygen atoms and has a single bond to each of two carbon atoms, usually in two separate hydrocarbon substituents.-IUPAC name and...
) group interacts with Hist90, Arg513, and Gln
Glutamine
Glutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...
192 and forms hydrogen bonds. The substituted phenyl group
Phenyl group
In organic chemistry, the phenyl group or phenyl ring is a cyclic group of atoms with the formula C6H5. Phenyl groups are closely related to benzene. Phenyl groups have six carbon atoms bonded together in a hexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the...
at the top of the channel interacts with the side-chains of amino acid residues through hydrophobic and electrostatic interactions. Tyr
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
385 makes for some sterical restrictions of this side of the binding site so a small substituent of the phenyl group makes for better binding. Degrees of freedom are also important for the binding. The central ring of the coxibs decides the orientation of the aromatic rings and, therefore, the binding to COX enzyme even though it often has no electrostatic interactions with any of the amino acid residues. The high lipophilicity of the active site does require low polarity
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
of the central scaffold of the coxibs.
Mechanism of binding
Studies on the binding mechanism of selective COX-2 inhibitors show that they have two reversible steps with both COX-1 and COX-2, but the selectivity for COX-2 is due to another step that is slow and irreversible and is seen only in the inhibition of COX-2, not COX-1. The irreversible step has been attributed to the presence of the sulfonamideSulfonamide (chemistry)
In chemistry, the sulfonamide functional group is -S2-NH2, a sulfonyl group connected to an amine group.A sulfonamide is a compound that contains this group. The general formula is RSO2NH2, where R is some organic group. For example, "methanesulfonamide" is CH3SO2NH2...
(or sulfone
Sulfone
A sulfone is a chemical compound containing a sulfonyl functional group attached to two carbon atoms. The central hexavalent sulfur atom is double bonded to each of two oxygen atoms and has a single bond to each of two carbon atoms, usually in two separate hydrocarbon substituents.-IUPAC name and...
) that fits into the side-pocket of COX-2. This has been studied using SC-58125 (an analogue of celecoxib) and mutated COX-2, wherein the valine 523 residue was replaced by isoleucine 523. The irreversible inhibition did not happen, but reversible inhibition was noticed. A model has been made to explain this three-step mechanism behind the inhibitory effects of selective COX-2 inhibitors. The first step accounts for the contact of the inhibitor with the gate of the hydrophobic channel (called the lobby region). The second step could account for the movement of the inhibitor from the lobby region to the active site of the COX enzyme. The last step probably represents repositioning of the inhibitor at the active site, which leads to strong interactions of the phenylsulfonamide or phenylsulfone group of the inhibitor and the amino acids of the side pocket. It is directly inhibition to postaglanding
Pharmacokinetics of coxibs
The coxibs are widely distributed throughout the body. All of the coxibs achieve sufficient brain concentrations to have a central analgesicAnalgesic
An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....
effect, and all reduce prostaglandin formation in inflamed joints. All are well absorbed, but peak concentration may differ between the coxibs. The coxibs are highly protein-bound, and the published estimate of half-lives is variable between the coxibs.
Celecoxib
CelecoxibCelecoxib
Celecoxib INN is a sulfa non-steroidal anti-inflammatory drug and selective COX-2 inhibitor used in the treatment of osteoarthritis, rheumatoid arthritis, acute pain, painful menstruation and menstrual symptoms, and to reduce numbers of colon and rectum polyps in patients with familial...
was the first specific inhibitor of COX-2 approved to treat patients with rheumatism
Rheumatism
Rheumatism or rheumatic disorder is a non-specific term for medical problems affecting the joints and connective tissue. The study of, and therapeutic interventions in, such disorders is called rheumatology.-Terminology:...
and osteoarthritis
Osteoarthritis
Osteoarthritis also known as degenerative arthritis or degenerative joint disease, is a group of mechanical abnormalities involving degradation of joints, including articular cartilage and subchondral bone. Symptoms may include joint pain, tenderness, stiffness, locking, and sometimes an effusion...
. A study showed that the absorption rate, when given orally, is moderate, and peak plasma concentration occurs after about 2–4 hours. However, the extent of absorption
Absorption (Pharmacokinetics)
In pharmacology , absorption is the movement of a drug into the bloodstream.Absorption involves several phases...
is not well known. Celecoxib has the affinity
Chemical affinity
In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds...
to bind extensively to plasma proteins, especially to plasma albumin
Albumin
Albumin refers generally to any protein that is water soluble, which is moderately soluble in concentrated salt solutions, and experiences heat denaturation. They are commonly found in blood plasma, and are unique to other blood proteins in that they are not glycosylated...
. It has an apparent volume of distribution (VD) of 455 +/- 166 L in humans and the area under the plasma concentration-time curve (AUC
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
) increases proportionally to increased oral doses, between 100 and 800 mg. Celecoxib is metabolized primarily by CYP2C9 isoenzyme to carboxylic acid and also by non-CYP-dependent glucuronidation to glucuronide
Glucuronide
A glucuronide, also known as glucuronoside, is any substance produced by linking glucuronic acid to another substance via a glycosidic bond...
metabolites. The metabolites are excreted in urine and feces, with a small proportion of unchanged drug (2%) in the urine. Its elimination half-life is about 11 hours (6–12 hours) in healthy individuals, but racial differences in drug disposition and pharmacokinetic changes in the elderly have been reported. Patients with chronic renal insufficiency appear to have 43% lower plasma concentration compared to healthy individuals, with a 47% increase in apparent clearance, and it can be expected that patients with mild to moderate hepatic impairment have increased steady-state AUC.
| Celecoxib | ||
|---|---|---|
| Peak [drug] | 2–4 hours |  |
| Protein binding | 97% | |
| Metabolites | Carboxylic acid and glucuronide conjugates | |
| Half-life [t1/2] | 6–12 hours | |
Parecoxib and valdecoxib
ParecoxibParecoxib
Parecoxib is a water soluble and injectable prodrug of valdecoxib. It is marketed as Dynastat in the European Union. Parecoxib is a COX2 selective inhibitor in the same category as celecoxib and rofecoxib . As it is injectable, it can be used perioperatively when patients are unable to take oral...
sodium is a water-soluble inactive ester amide prodrug
Prodrug
A prodrug is a pharmacological substance administered in an inactive form. Once administered, the prodrug is metabolised in vivo into an active metabolite, a process termed bioactivation. The rationale behind the use of a prodrug is generally for absorption, distribution, metabolism, and...
of valdecoxib
Valdecoxib
Valdecoxib is a non-steroidal anti-inflammatory drug used in the treatment of osteoarthritis, rheumatoid arthritis, and painful menstruation and menstrual symptoms. It is a cyclooxygenase-2 selective inhibitor....
, a novel second-generation COX-2-specific inhibitor and the first such agent to be developed for injectable use. It is rapidly converted by hepatic enzymatic hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
to the active form valdecoxib. The compound then undergoes another conversion, which involves both cytochrome P450-mediated pathway (CYP2C9, CYP3A4) and non-cytochrome P450-mediated pathway, to hydroxylated metabolite and glucuronide metabolite. The hydroxylated metabolite, that also has weak COX-2-specific inhibitory properties, is then further metabolized by non-cytochrome P450 pathway to a glucuronide metabolite. These metabolites are excreted in the urine.
After intra-muscular administration of Parecoxib sodium peak plasma concentration is reached within 15 minutes. The plasma concentration decreases rapidly after administration because of a rather short serum half-life, which is about 15–52 minutes. This can be explained by the rapid formation of Valdecoxib. In contrast to the rapid clearance of Parecoxib, plasma concentration of Valdecoxib declines slowly because of a longer half-life. On the other hand, when Valdecoxib is taken orally it is absorbed rapidly (1–2 hours), but presence of food can delay peak serum concentration. It then undergoes the same metabolism that is described above. It is extensively protein-bound (98%), and the plasma half-life is about 7–8 hours. Note that the half-life can be significantly prolonged in the elderly or those with hepatic impairment, and can lead to drug accumulation.
The hydroxyl metabolite reaches its highest mean plasma concentration within 3 to 4 hours from administration, but it is considerably lower than of Valdecoxib or about 1/10 of the plasma levels of Valdecoxib.
| Parecoxib | ||
|---|---|---|
| Peak [drug] | Within 15 minutes |  |
| Protein binding | N/A | |
| Metabolites | Valdecoxib, after hepatic enzymatic hydrolysis | |
| Half-life [t1/2] | 15–52 minutes | |
| Valdecoxib | ||
|---|---|---|
| Peak [drug] | 2–4 hours, delayed by food |  |
| Protein binding | 98% | |
| Metabolites | Hydroxyl derivatives and glucuronide metabolite | |
| Half-life [t1/2] | 7–8 hours | |
Etoricoxib
EtoricoxibEtoricoxib
Etoricoxib is a COX-2 selective inhibitor from Merck & Co...
, that is used for patients with chronic arthropathies and musculoskeletal and dental pain, is absorbed moderately when given orally. A study on its pharmacokinetics showed that the plasma peak concentration of etoricoxib occurs after approximately 1 hour. It has shown to be extensively bound to plasma albumin (about 90%), and has an apparent volume of distribution (VD) of 120 L in humans. The area under the plasma concentration-time curve (AUC) increases in proportion to increased dosage (5–120 mg). The elimination half-life is about 20 hours in healthy individuals, and such long half-life enables the choice to have once-daily dosage. Etoricoxib, like the other coxibs, is excreted in urine and feces and also metabolized in likewise manner. CYP3A4
CYP3A4
Cytochrome P450 3A4 , a member of the cytochrome P450 mixed-function oxidase system, is one of the most important enzymes involved in the metabolism of xenobiotics in the body. CYP3A4 is involved in the oxidation of the largest range of substrates of all the CYPs. As a result, CYP3A4 is present in...
is mostly responsible for biotransformation of etoricoxib to carboxylic acid metabolite, but a non CYP450 metabolism pathway to glucuronide metabolite is also at hand. A very small portion of etoricoxib (<1%) is eliminated unchanged in the urine. Patients with chronic renal insufficiency do not appear to have different plasma concentration curve (AUC) compared to healthy individuals. It has though been reported that patients with moderate hepatic impairment have increased plasma concentration curve (AUC) by approximately 40%. It has been stated that further study is necessary to describe precisely the relevance of pharmacokinetic properties in terms of the clinical benefits and risks of etoricoxib compared to other clinical options.
| Etoricoxib | ||
|---|---|---|
| Peak [drug] | 1 hour |  |
| Protein binding | 90% | |
| Metabolites | Carboxylic acid metabolite and glucuronide metabolite | |
| Half-life [t1/2] | 20 hours | |
Lumiracoxib
LumiracoxibLumiracoxib
Lumiracoxib is a COX-2 selective inhibitor non-steroidal anti-inflammatory drug, manufactured by Novartis and still sold in few countries, including Mexico, Ecuador and the Dominican Republic, under the trade name Prexige .Lumiracoxib has several distinctive features...
is unique amongst the coxibs in being a weak acid. It was developed for the treatment of osteoarthritis, rheumatoid arthritis and acute pain. The acidic nature of lumiracoxib allows it to penetrate well into areas of inflammation. It has shown to be rapidly and well absorbed, with peak plasma concentration occurring in about 1–3 hours. A study showed that when a subject was given 400 mg dose, the amount of unchanged drug in the plasma 2.5 hours postdose suggest a modest first pass effect
First pass effect
The first-pass effect is a phenomenon of drug metabolism whereby the concentration of a drug is greatly reduced before it reaches the systemic circulation. It is the fraction of lost drug during the process of absorption which is generally related to the liver and gut wall...
. The terminal half-life in plasma ranged from 5.4 to 8.6 hours (mean =6.5 hours). The half-life in synovial fluid is considerably longer than in plasma, and the concentration in synovial fluid
Synovial fluid
Synovial fluid is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency , the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.-Overview:The inner membrane of synovial joints...
24 hours after administration would be expected to result in a substantial COX-2 inhibition. This fact can explain why some users may suffice with once-daily dosage despite a short plasma half-life. The major plasma metabolites are 5-carboxy, 4’-hydroxy, and 4’-hydroxy-5-carboxy derivatives. Lumiracoxib is extensively metabolized before it is excreted, and the excretion routes are in the urine or feces. Peak plasma concentrations exceed those necessary to maximally inhibit COX-2, and that is consistent with a longer pharmacodynamic half-life. In vitro lumiracoxib has demonstrated a greater COX-2 selectivity than any of the other coxibs.
| Lumiracoxib | ||
|---|---|---|
| Peak [drug] | 1–3 hours |  |
| Protein binding | 90% | |
| Metabolites | 5-carboxy, 4’-hydroxy, and 4’-hydroxy-5-carboxy derivatives | |
| Half-life [t1/2] | 6,5 hours | |
Rofecoxib
RofecoxibRofecoxib
Rofecoxib is a nonsteroidal anti-inflammatory drug that has now been withdrawn over safety concerns. It was marketed by Merck & Co. to treat osteoarthritis, acute pain conditions, and dysmenorrhoea...
was the second selective COX-2 inhibitor to be marketed, and the first one to be taken off the market. When the pharmacokinetics were studied in healthy human subjects, the peak concentration was achieved in 9 hours with effective half-life of approximately 17 hours . A secondary peak has been observed, which might suggest that the absorption of rofecoxib varies with intestinal motility, hence leading to high variability in time until peak concentration is met. Seventy-one and a half percent of the dose was recovered in urine (less than 1% unmetabolised) and 14,2% was recovered in feces (approximately 1,8% in the bile). Among the metabolites were rofecoxib-3’,4’-dihydrodiol, 4’-hydroxyrofecoxib-O-β-D-glucuronide, 5-hydroxyrofecoxib-O-β-D-glucuronide, 5-hydroxyrofecoxib, rofecoxib-erythro-3,4-dihydrohydroxy acid, rofecoxib-threo-3,4-dihydrohydroxy acid, cis-3,4-dihydrorofecoxib and trans-3,4-dihydrorofecoxib.
| Rofecoxib | ||
|---|---|---|
| Peak [drug] | 9 hours |  |
| Protein binding | N/A | |
| Metabolites | Major: rofecoxib-threo-3,4-dihydrohydroxy acid and rofecoxib-erythro-3,4-dihydrohydroxy acid | |
| Half-life [t1/2] | 17 hours | |
Cardiovascular events associated with selective COX-2 inhibitors
Even before the first selective COX-2 inhibitor was marketed, specialists began to suspect that there might be a cardiovascular risk associated with this class of medicines. In the VIGOR study (Vioxx Gastrointestinal Outcomes Research), rofecoxib (Vioxx) was compared to naproxenNaproxen
Naproxen sodium is a nonsteroidal anti-inflammatory drug commonly used for the reduction of pain, fever, inflammation and stiffness caused by conditions such as:...
. After a short time, it became evident that there was a fivefold higher risk of myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
in the rofecoxib group compared to the group that received naproxen. The authors suggested that the difference was due to the cardioprotective effects of naproxen. The APPROVe (Adenomatous Poly Prevention on Vioxx) study was a multicentre, randomized, placebo-controlled, double blind trial aimed to assess the effect of three-year treatment with rofecoxib on recurrence of neoplastic polyps in individuals with a history of colorectal adenomas. In 2000 and 2001, 2587 patients with a history of colorectal adenomas were recruited and followed. The trial was stopped early (2 months before expected completion) on recommendations of its data safety and monitoring board because of concerns about cardiovascular toxicity. When looking at the results of the study, it showed a statistically significant increase in cardiovascular risk when taking rofecoxib compared to placebo beginning after 18 months of treatment. Then on the 30th of September Merck gave out a news release announcing their voluntary worldwide withdrawal of Vioxx.
Some studies of other coxibs have also shown increase in the risk of cardiovascular events, while others have not. For instance, the Adenoma Prevention with Celecoxib study (APC) showed a dose-related increase in risk of cardiovascular death, myocardial infarction, stroke, or heart failure when taking celecoxib compared to placebo; and the Successive Celecoxib Efficacy and Safety Study I (SUCCESS-I) showed increased risk of myocardial infarction when taking 100 mg twice a day of celecoxib compared to diclofenac and naproxen; but taking 200 mg twice a day had lower incidence of myocardial infarction compared to diclofenac and naproxen. Nussmeier et al. (2005) showed in a study increase in incidence of cardiovascular events when taking parecoxib and valdecoxib (compared to placebo) after coronary artery bypass surgery.
Possible mechanisms
It has been proposed that COX-2 selectivity could cause imbalance of prostaglandins in the vasculature. If this were the explanation for the increased cardiovascular risk then low-dose aspirin should negate this effect, which was not the case in the APPROVe trial. Also, the non-selective COX inhibitors, have also shown increase in cardiovascular events.Another possible explanation was studied by Li H. et al. (2008). They showed that in spontaneously hypertensive rat
Spontaneously hypertensive rat
Spontaneously hypertensive rat is an animal model of essential hypertension, used to study cardiovascular disease. It is the most studied model of hypertension measured as number of publications...
s (SHR) non-selective NSAIDs and the coxibs produce oxidative stress
Oxidative stress
Oxidative stress represents an imbalance between the production and manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage...
, indicated by enhanced vascular superoxide
Superoxide
A superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
(O2-) content and elevated peroxide
Peroxide
A peroxide is a compound containing an oxygen–oxygen single bond or the peroxide anion .The O−O group is called the peroxide group or peroxo group. In contrast to oxide ions, the oxygen atoms in the peroxide ion have an oxidation state of −1.The simplest stable peroxide is hydrogen peroxide...
in plasma, which is in tune with enhanced expression of NADPH oxidase, which was noticed with use of diclofenac and naproxen and, to a lesser degree, rofecoxib and celecoxib. Nitrite
Nitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
in plasma was also decreased suggesting a diminished synthesis of vascular nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
(NO). This decrease in NO synthesis did not result from decreased expression of endothelial nitric oxide synthase (eNOS
Nitric oxide synthase
Nitric oxide synthases are a family of enzymes that catalyze the production of nitric oxide from L-arginine. NO is an important cellular signaling molecule, having a vital role in many biological processes...
) because expression of eNOS mRNA was not reduced, and even upregulated for some products. The decrease in NO synthesis could, rather, be explained by loss of eNOS function. For eNOS to be normally functional, it needs to form a dimer
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
and to have its cofactor BH4, which is one of the most potent naturally occurring reducing agents. BH4 is sensitive to oxidation by peroxynitrite
Peroxynitrite
Peroxynitrite is the anion with the formula ONOO−. It is an unstable structural isomer of nitrate, NO3−, which has the same formula but a different structure. Although peroxynitrous acid is highly reactive, its conjugate base peroxynitrite is stable in basic solution...
(ONOO-), which is produced when NO reacts with O2-, so it has been hypothesized that depletion of BH4 can occur with excessive oxidative stress (that can be caused be NSAIDs) and, hence, be the cause of eNOS dysfunction. This disfunction, which is referred to as eNOS uncoupling, causes the production of O2- by eNOS, thereby leading to more oxidative stress produced by eNOS. In a study, both the selective COX-2 inhibitors and the non-selective NSAIDs produced oxidative stress, with greater effects seen with non-selective NSAIDs use. This could fit with the hypothesis concerning the prostacyclin/thromboxane imbalance. That is, although the non-selective NSAIDs produce more oxidative stress, they prevent platelet aggregation, whereas the COX-2 inhibitors reduce prostacyclin
Prostacyclin
Prostacyclin is a member of the family of lipid molecules known as eicosanoids.As a drug, it is also known as "epoprostenol". The terms are sometimes used interchangeably.-History:...
production, and, hence, the cardiovascular risk for the non-selective NSAIDs is not higher than for the coxibs.
Among other hypotheses are increased blood pressure, decreased production of epi-lipoxins (which have anti-inflammatory effects), and inhibition of vascular remodeling when using selective COX-2 inhibitors.