Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances

Abstracts

JUSTIFICATIVA E OBJETIVOS: Os antiinflamatórios não-esteroidais (AINE) estão entre as drogas mais prescritas e usadas no mundo, incluindo a utilização em Anestesiologia. O propósito desta revisão é discutir alguns aspectos atuais da bioquímica da ciclooxigenase, que vem servindo de base para o desenvolvimento dos novos AINE. CONTEÚDO: Estas drogas exercem suas ações principalmente através da inibição da ciclooxigenase (COX), a enzima chave que catalisa a conversão de ácido araquidônico em prostaglandinas e tromboxanos. Pelo menos duas isoformas da COX já foram identificadas, a COX-1, que é constitutivamente expressa na maioria dos tecidos, e a COX-2, que é uma forma induzível da enzima localizada principalmente nas células e tecidos envolvidos em processos inflamatórios. Com a descoberta da COX-2 e a determinação de sua estrutura, foi possível desenvolver drogas mais seletivas que reduzem a inflamação sem afetar a COX-1, protetora do estômago e rins, dando origem a uma nova geração de compostos antiinflamatórios denominados de inibidores específicos da COX-2. CONCLUSÕES: Embora estes compostos de última geração apresentem menor toxicidade para o trato gastrintestinal, outros efeitos adversos graves têm sido observados, incluindo insuficiência renal e efeitos cardiovasculares, como o infarto agudo do miocárdio e a trombose. A despeito destes efeitos colaterais, estes novos fármacos estão sendo testados em outras condições clínicas, principalmente no tratamento preventivo do câncer e da doença de Alzheimer.

ANTIINFLAMATÓRIOS; DOR; DOR

REVIEW ARTICLE

Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances^*

Analgésicos inhibidores específicos de la ciclooxigenase-2: avanzos terapéuticos

Wilson Andrade Carvalho, M.D.^I; Rosemary Duarte Sales Carvalho, M.D.^II; Fabrício Rios-Santos, M.D.^III

^IProfessor Titular de Farmacologia da UESC e Professor Adjunto da UFBA, Anestesiologista do Hospital São Rafael

^IIProfessora de Farmacologia da Faculdade de Farmácia da UFBA

^IIIProfessor Assistente de Farmacologia do Departamento de Saúde/ FMUESC

^{Correspondence}

SUMMARY

BACKGROUND AND OBJECTIVES: Nonsteroidal Anti-inflammatory Drugs (NSAIDs) are among the most widely prescribed drugs, including for Anesthesiology. This review aimed at discussing some current cycloxygenase biochemical aspects, which have provided the basis for the development of new analgesic and anti-inflammatory drugs.

CONTENTS: These drugs primarily act by inhibiting cycloxygenase (COX), which is the key-enzyme catalyzing the conversion of arachidonic acid into prostaglandins and thromboxane. At least two COX isoforms have already been identified: COX-1, which is constitutively expressed in most tissues, and the inducible enzyme COX-2, which is primarily found in inflammatory cells and tissues. The discovery of COX-2 has enabled the development of more selective drugs to decrease inflammation without affecting COX-1 that protects stomach and kidneys and giving origin to a new generation of anti-inflammatory compounds called specific COX-2 inhibitors.

CONCLUSIONS: Although there is significantly lower gastrointestinal toxicity in patients treated with selective COX-2 inhibitors, other severe adverse effects have been observed, including renal failure and cardiovascular effects, such as myocardial infarction acute and thrombosis. Despite these potential side effects, these new drugs are being tested in different clinical conditions, especially in cancer prevention and Alzheimer's disease.

Key Words: ANTI-INFLAMMATORY: COX-2 inhibitors; PAIN, Chronic: cancer

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: Los antiinflamatorios no-esteroidales (AINE) están entre las drogas más prescritas y usadas en el mundo, incluyendo la utilización en Anestesiología. El propósito de esta revisión es discutir algunos aspectos actuales de la bioquímica de la ciclooxigenasis, que viene sirviendo de base para el desenvolvimiento de los nuevos AINE.

CONTENIDO: Estas drogas ejercen su acción principalmente a través de la inhibición de la ciclooxigenasis (COX), la enzima clave que catalisa la conversión de ácido araquidónico en prostaglandinas y tromboxanos. Por lo menos dos isoformas de la COX ya fueron identificadas, la COX-1, que es constitutivamente expresa en la mayoría de los tejidos, y la COX-2, que es una forma inducible de la enzima localizada principalmente en células y tejidos envueltos en procesos inflamatorios. Con la descubierta de la COX-2 y la determinación de su estructura, fue posible desenvolver drogas más selectivas que reducen la inflamación sin afectar la COX-1, protectora del estomago y riñones, dando origen a una nueva generación de compuestos antiinflamatorios denominados de inhibidores específicos de la COX-2.

CONCLUSIONES: No entanto estos compuestos de última generación presenten menor toxicidad para el trato gastrointestinal, otros efectos adversos graves han sido observados, incluyendo insuficiencia renal y efectos cardiovasculares, como el infarto del miocardio y la trombosis. A despecho de estos efectos colaterales, estos nuevos fármacos están siendo testados en otras condiciones clínicas, principalmente en el tratamiento preventivo del cáncer y de la enfermedad de Alzheimer.

INTRODUCTION

Since 1893, when German chemist Felix Hoffman has motivated Bayer to produce acetylsalicylic acid, patented as Aspirin, non-steroidal anti-inflammatory drugs (NSAIDs) have become the most widely prescribed and used drugs worldwide. It is estimated that in the United States alone, approximately 50 million people spend around 5 to 10 billion dollars/year with those drugs ¹.

However, and despite the wide use of these agents, their mechanism of action has been only explained in 1971 when John Vane ², that has been awarded the Nobel Prize for his discovery, has proposed that aspirin-like anti-inflammatory drugs would suppress inflammatory processes by cycloxygenase (COX) inhibition, thus preventing prostaglandins synthesis.

COX, key-enzyme catalyzing prostaglandins biosynthesis and also known as Prostaglandin Synthetase or Prostaglandin Endoperoxide Synthetase, was isolated in 1976 and cloned in 1988. In 1991 a gene was identified which would code a second enzyme isoform, then called cycloxygenase-2. Today it is known that the two genes express two different isoforms of the enzyme, called: cycloxygenase-1 (COX-1) and cycloxygenase-2 (COX-2). Both isoforms have similar primary protein structure and essentially catalyze the same reaction ^3-8.

With the discovery of COX-2, induced isoform predominantly expressed during inflammatory processes, a new therapeutic perspective came to light for the development of more selective drugs with less adverse effects. These agents' group has originated a new generation of anti-inflammatory drugs (selective COX-2 inhibitors), called Coxibs ^9,10. More recently, new motivations for clinical use and research were found with the description of a third cycloxygenase variant called COX-3 ¹¹.

CYCLOXYGENASE BIOCHEMISTRY AND PROSTAGLANDINS SYSNTHESIS

Prostaglandin and leucotriens compounds are called eicosanoids, for being derived from essential fatty acids of twenty carbons esterified into cell membrane phospholipids. Eicosanoids synthesis may be triggered by different stimulations activating membrane receptors, coupled to a regulatory protein bound to a guanidine nucleotide (G protein), resulting in the activation of phospholipase A₂ or increased intracellular Ca²⁺ concentration. Phospholipase A₂ hydrolyzes membrane phospholipids, especially phosphatidylcoline and phosphatidylethanolamine , releasing arachidonic acid ^10-14.

Released arachidonic acid is the substract for two different enzymatic pathways: cycloxygenases pathway, which triggers prostaglandins and thromboxanes biosynthesis, and lipoxygenases pathway, responsible for leucotriens, lipoxins and other compounds synthesis ^15-18 (Figure 1).

Brazilian Journal of Anesthesiology, 2004; 54: 3: 448 - 464

Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances

Wilson Andrade Carvalho; Rosemary Duarte Sales Carvalho; Fabrício Rios-Santos

The presence of an inducible COX isoform was first described by Needleman et al., who have shown its induction by inflammatory stimulation and cytokines ¹⁹. Due to different expression characteristics, two isoforms were then accepted: COX-1 and COX-2 ^5,19-21. COX-1 was the first to be described and is constitutively expressed, that is, it is present in cells in physiologic conditions, especially blood vessels, platelets, stomach and kidneys. COX-2 may be induced by cytokines (interleukin-1, interleukin-2 and tumor necrosis factor a), phorbol esters, growth factors and endotoxins, being primarily expressed by cells involved in the inflammatory process, such as macrophages, monocytes and synoviocytes. On the other hand, COX-2 expression may be inhibited by glucocorticoids, interkeukin-4, interleukin-14 and interleukin-10, while prostaglandin E₂ (PGE₂) promotes COX-2 expression up-regulation ^4,5,9,22-25. COX-2 may also be post-transcriptionally regulated. Loss of COX-2 post-transcriptional regulation by mutation of proteins specifically interacting with some messenger RNA elements of the enzyme may result in increased COX-2 expression, the mechanism of which has been proposed as a critical factor involved in bowel cancer ²⁶ (Figure 2).

Brazilian Journal of Anesthesiology, 2004; 54: 3: 448 - 464

Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances

Wilson Andrade Carvalho; Rosemary Duarte Sales Carvalho; Fabrício Rios-Santos

COX-1 and COX-2 are integral proteins located within the internal leaflet of the liposoluble biolayer of membrane phospholipids. The number of aminoacids of both isoenzymes is very similar, varying 599 aminoacids for COX-1 and 604 aminoacids for COX-2 ²⁴. Cycloxygenases structure has three different domains: one amino terminal domain similar to epidermal growth factor (EGF), followed by membrane binding domain and terminal carboxylic catalytic domain, which contains active sites of cycloxygenase and peroxidase activity. Structural COX analysis reveals differences in the amino-terminal signaling peptide region and in the terminal carboxylic peptide chain, however of unknown importance. COX-1 has a sequence of 17 aminoacids in its amino-terminal peptide chain, which is not present in COX-2, while COX-2 has an additional insertion of 18 aminoacids in its carboxyl termination. Remaining COX-1 and COX-2 nucleus sequence, however, is approximately 75% identical and all residues identified as essential for catalytic activity are maintained ^4,6,26-28.

Cycloxygenases have two different activities. One endoperoxide reductase activity which oxides and cycles non-esterified precursor fatty acid to form cyclic endoperoxide PGG (prostaglandin G), and a peroxidase activity which converts PGG into prostaglandin H (PGH). Active cycloxygenase site is located in a long hydrophobic canal formed in the core of membrane-associated a-helixes and provides direct access of arachidonic acid to the active site without leaving the membrane. Peroxidase function is located on the other side of the enzyme and is similar for both isoforms. Prostaglandin synthesis, then, starts with cycloxygenase activity catalyzing the addition of molecular oxygen to arachidonic acid to initially form intermediate endoperoxide prostaglandin G₂ (PGG). The same enzyme by its peroxidase activity, catalyzes this prostaglandin reduction to form PGH₂. PGG and PGH have weak activity and serve as the substract to form different active prostaglandins and thromboxanes, including PSD₂, PGE₂, PGF_2a, prostacyclins (PGI₂) and thromboxanes (TX) (Figure 1) ^{3,6,8,24,27,29}.

Although remarkable differences were observed in DNA and RNA of COX genes structure and regulation, protein structure and enzymatic function are very similar. COX-1 and COX-2 have approximately 60% genetic homology in their coding regions, and their genes are located on chromosomes 9 and 1, respectively ^7,8,24.

Prostaglandins formed as from cycloxygenase action bind to prostanoid receptors located in the cell membrane, coupled to G protein. G protein activation results in the stimulation of effector systems responsible for second messengers release in different tissues. These receptors have already been cloned and are organized in five groups according to the PG to which they have more affinity, called DP (PGD₂), FP (PGF₂), IP (PGI₂), TP (PXA₂) and EP (PGE₂). Although most prostaglandin receptors are cell membrane surface receptors and make up the super-family of G protein-coupled receptors, some may be located in the nuclear membrane, the ligants of which act as transcription factors changing genetic cell expression. It has been recently shown that some eicosanoids, including PGI₂, series J prostaglandins, 15 desoxy-D ^12-14 -PGJ₂(d15-PGJ₂) and B₄ leucotrien (LTB₄) are endogenous ligands of a family of nuclear receptors called Peroxisome Proliferator-Activated Receptors (PPARS), which regulate lipid metabolism, cell differentiation and proliferation. There are currently three known PPAR receptor isoforms, called a, d and g ^13,30,31.

At least two effector systems acting through second messengers release are being associated to prostaglandin action. One is adenylate cyclase, the activation of which stimulates cyclic adenosine-3',5'-monophosphate (cyclic AMP). PGI₂, PGE₂ and PGD₂ activate adenylate cyclase increasing cyclic AMP concentration, while TXA₂ decreases such activity. The other effector system is phospholipase C which, when activated by prostaglandins, increases diacylglycerol and 1,4,5- inositol triphosphate resulting in cascade activation of kinase proteins and increased intracellular Ca^{++ 13,14,32,33}.

IS THERE A COX-3?

The expression of a third catalytic COX variant (COX-3) has been shown in in vitro studies with macrophage strains ^11,34,35. The uniqueness of this variant expression is that its origin are not pro-inflammatory prostaglandins but a member of the cyclopentatones family, 15desoxy-D ^12-14 PGJ₂, Peroxisome Proliferator-Activated Receptors (PPARS) with anti-inflammatory activity ^34-40. If the hypothesis of a third cycloxygenase with anti-inflammatory activity is correct, its expression may result in typical inflammation remission periods, as it has been observed with some chronic diseases, such as rheumatoid arthritis.

COX-3, possibly a COX-1 variant (for deriving from the same gene) is primarily distributed in cerebral cortex, spinal cord and heart, being more sensitive to acetaminophen (paracetamol) as compared to COX-1 and COX-2. It has been proposed that COX-3 inhibition could represent the primary central mechanism through which NSAID-type analgesic and antipyretic drugs would decrease pain and fever ¹¹.

CYCLOXYGENASE PHISIOLOGICAL AND PATHOLOGICAL FUNCTIONS

Prostaglandins are involved in different physiological and pathological processes, including vasodilation or vasoconstriction; bronchial or uterine muscles contraction or relaxation; hypotension; ovulation; bone metabolism; renal blood flow increase (resulting in diuresis, natriuresis, kaliuresis and renin secretion stimulation); gastric mucosa protection and local blood flow regulation; gastric acid secretion inhibition; nervous growth and development; immune response; hyperalgesia; cell chemotactic activity regulation; endocrine response; angiogenesis; metastatic progression, among others ^{10,14,17,18,40-46}.

Family E prostaglandins (PGEs) are potent vasodilators for most vascular beds. Vasodilating activity primarily involves arterioles, pre-capillary sphincters and post-capillary venulae. PGD₂, in general, dilates mesenteric, coronary and renal vessels, and constricts pulmonary circulation. PGI₂ is an effective vasodilator and may lead to major arterial hypotension, while TXA₂ is a potent vasoconstrictor. In blood, prostaglandins also modulate platelet function. PGE₁, PGD₂ and PGI₂ are platelet aggregation inhibitors, while thromboxane A₂ is a major aggregation inducer. PGI₂ is synthesized by vascular endothelium, controls cells adhesion to endothelium and platelet aggregation and contributes to intact vascular wall antithrombogenic mechanism ⁴⁴.

PGEs and PGI₂ inhibit gastric acid secretion stimulated by food, histamine or gastrins and decrease pepsin secretion, acidity and content. Prostaglandins are gastric mucosa vasodilators and seem to be involved with local blood flow regulation. Stomach and small bowel mucus secretion is increased by PGEs. These effects help maintaining gastric mucosa integrity, protect epithelial cells and are referred to as cytoprotecting properties of COX-1 synthesized prostaglandins. In fact, adverse gastrointestinal NSAIDs effects are associated to the suppression of COX-1 constitutive expression, resulting in gastric damage, hemorrhage and ulceration ^13,45,47.

Prostaglandins also influence renal blood flow distribution, sodium and water reabsorption and renin release. PGI₁, PGE₂ and PGD₂ determine renin secretion in renal cortex, probably by direct effect on juxtaglomerular cells. COX-2 is found in renal vessels, cortical macula densa, interstitial renal cells, collector duct and the small portion of Henle's loop, with increased expression in certain areas with aging ^7,9,13,14,45.

Prostaglandins and leucotriens, when released, also play a critical role in inflammatory process signs and symptoms genesis ^{10,15,16,17,22}. Ferreira (1979) ⁴² has observed that intradermal prostaglandin induces hyperalgesia. It has been shown that PGE and PGI₂ hypersensitize C fibers polimodal nociceptors to mechanical and chemical stimulations ^17,18,42,43.

It has been shown that prostaglandins are produced in CNS neurons and vessels with major participation in several central functions, including sleep and emergence cycle control, febrile thermogenesis and nociceptive transmission. Prostaglandins and cytokines (interleukine-6) are also implied in the pathophysiology of some degenerative brain diseases, such as multiple sclerosis, AIDS-associated dementia and Alzheimer's disease ^6,8,24,27,48.

It is known that lipopolysaccharides (LPS) and cytokines may induce COX-2 in different brain regions, and that COX-1 is constitutively expressed in several neurons. COX-1 is predominantly distributed in the forebrain where prostaglandins may be involved in integration functions, such as autonomous nervous system and sensorial transmission pathways modulation. Interestingly, COX-2 is constitutively expressed in some brain regions, especially cortex, hippocampus, tonsila-hippocampus complex (important for memory and behavior), hypothalamus and spinal cord. Among neural circuits, fever response regulatory pathway is the best known. Interleukine-1b (IL-1b) release as from pirogenic stimulation promotes central PGE₂ synthesis which, in turn, activates the thermoregulating center located in anterior hypothalamus pre-optic area and triggering fever ^6,49-51.

Since the eye is ontogenetically generated from neuroepithelium and shares many CNS characteristics, it has called the attention of investigators to study COX-2 distribution in this organ. The study of primary open angle glaucoma treatment with prostaglandins has led to the observation that COX-2 expression is totally decreased in the non-pigmented secretory epithelium of the ciliary body of glaucoma patients, remaining unchanged in other parts of the eye ²⁶.

SPECIFIC CYCLOXYGENASE-2 INHIBITORS

Since 1844 when salicylic acid was isolated from gaultheria oil by Cahours, to the introduction of acetylsalicylic acid in 1899 by Dreser, the search for new NSAIDs is in overt development, currently counting on a large number of compounds available in several countries of the world ¹⁴ (Chart I). Although older anti-inflammatory drugs, such as salicylates, are very effective analgesic, antipyretic and anti-inflammatory drugs, their prolonged use is limited in most patients for developing highly uncomfortable gastrointestinal effects, such as dyspepsia, abdominal pain, bleeding, gastric or duodenal ulcer or perforation. Gastrointestinal side effects were attributed to the low COX-2 inhibition specificity of these compounds ^8,9,18,28.

Brazilian Journal of Anesthesiology, 2004; 54: 3: 448 - 464

Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances

Wilson Andrade Carvalho; Rosemary Duarte Sales Carvalho; Fabrício Rios-Santos

COX-2 specificity has been primarily evaluated in in vitro studies for simplicity and speed reasons. Several systems have been tested with this aim, including studies with human recombinant enzymes, cell cultures and whole blood. In whole blood studies, platelet thromboxane synthesis during clot formation is used as COX-1 activity index, while PGE₂ synthesis (primarily by monocytes) in whole blood exposed to lipopolysaccharides is used as COX-2 activity index. Most studies determine 50% inhibitory concentration (IC₅₀) representing drug concentration which, in controlled conditions, inhibits 50% of COX activity. NSAIDs are evaluated by a relative specificity index between COX isoforms, expressed as IC₅₀ for COX-2/COX-1. According to this criteria, the lower the relative coefficient or COX-2/COX-1 ratio, the higher the drug specificity to COX-2. These proportional inhibition indices of both isoforms are used to compare drug potency, and in theory should also be correlated to clinical predisposition to gastrointestinal adverse effects. These in vitro studies, however, have poorly reproducible results, probably reflecting study differences. In addition, IC₅₀ does not necessarily reflect drug-enzyme interaction complexity and real in vivo inhibition and for such should not be used to measure clinical efficacy. Different NSAIDs inhibitory activity is shown in table I ²⁸.

Brazilian Journal of Anesthesiology, 2004; 54: 3: 448 - 464

Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances

Wilson Andrade Carvalho; Rosemary Duarte Sales Carvalho; Fabrício Rios-Santos

Clinical evaluation of these compounds specificity is based on the use of techniques allowing the observation of clinical toxicity, such as endoscopy for gastric ulcers visualization, or observation of therapeutic effects of anti-inflammatory doses of the agent. It is important to consider, however, that several factors determine clinical COX-2 inhibitor response, including among the most relevant, genetic variability of target-protein or metabolizing enzymes, drugs interaction and patient's characteristics, which may influence both efficacy and adverse effects during clinical trials ^8,9,21,28. Aspirin promotes irreversible inhibition of cycloxygenase activity by covalent binding with a serine molecule of the target site. Aspirin acetylates serine in position 530 of COX-1 structure preventing arachidonic acid binding to enzyme active site. In COX-2, aspirin acetylates a serine molecule in position 516. Although not being recommended as first choice for chronic inflammatory processes, such as rheumatism and arthritis due to its potent inhibitory action on COX-1, aspirin is still a major alternative to prevent high thromboembolic risk diseases, such as myocardial infarction, in these cases benefiting from the anti-COX-1 mechanism by preventing platelet aggregation. This irreversible aspirin effect is particularly important in Anesthesiology when patients in chronic use of the drug present higher intra and postoperative bleeding risk. Since platelet cycloxygenase inhibition is present throughout platelet life, corresponding to 8 to 10 days, it is recommended that patients withdraw the drug at least one week before surgery, unless risk-benefit ratio justifies otherwise ^13,14,18.

Others NSAIDs, such as mefenemate, diclofenac and ibuprofen are reversible competitive inhibitors of both isoforms, competing with arachidonic acid binding in COX target-site. A third class of NSAIDs, represented by flurbiprofen and indomethacin, promotes time-dependent slow reversible COX-1 and COX-2 inhibition as result of a saline bridge between drug's carboxylate and arginine in position 120 of target-site, followed by conformation changes ²⁶.

COX-2 specific inhibitors first generation is represented by nimesulide, etodalak and meloxicam. These products selectivity was in fact observed after their commercialization and was primarily result of clinical and experimental observations of low incidence of gastrointestinal side effects which were further proven by in vitro studies. Nimesulide is the aberrant NSAID example, with good in vivo potency in inflammatory models, but with weak in vitro inhibition of COX preparations. Nimesulide, in addition to COX-2 specificity, has additional effects that intensify its anti-inflammatory activity, such as neutrophils activation inhibition and antioxidant properties. Based on in vitro studies it has initially been suggested that meloxicam would selectively inhibit COX-2. However, when tested in vivo, its COX-2 specificity has been just approximately 10 times higher as compared to COX-1, in addition to presenting platelet inhibition ⁵². Molecular changes in these products, especially nimesulide, aiming at increasing COX-2 specificity, have originated structures without carboxylic cluster and with the presence of sulfonamide or sulfone clusters, originating the so-called specificity or specific second generation inhibitors. This group includes celecoxib, rofecoxib, valdecoxib, parecoxib (valdecoxib pro-drug), APHS [o-(acetoxyphenyl)hept-2-ynyl sulfide] and etoricoxib, being the two former already available in the market ⁸ (Chart I, Table I and Figure 3).

Brazilian Journal of Anesthesiology, 2004; 54: 3: 448 - 464

Specific cyclooxygenase-2 inhibitor analgesics: therapeutic advances

Wilson Andrade Carvalho; Rosemary Duarte Sales Carvalho; Fabrício Rios-Santos

The relatively recent X-rays crystallography technique is gradually elucidating the action mechanism through which NSAIDs inhibit cycloxygenase. Picot et al. (1994) ⁵³ using this technique have reported COX-1 tridimensional structure starting a new understanding of COX inhibitors therapeutic actions. The understanding of COX-1 and COX-2 structures and their active sites is the fundamental basis for the development of more specific COX-2 inhibitors and for studies on the structure-activity ratio of such products. During enzyme activity, arachidonic acid is bound to arginine in position 120 and to serine in position 530. The transfer of tyrosine electrons in position 385 to an oxidized heme, which is also bound to the enzyme, starts cycloxygenase reaction. Several studies have tried to explain how and where NSAIDs act in cycloxygenase to block prostaglandins synthesis. Inside COX hydrophobic channel, a difference of aminoacid in positon 523 (isoleukin in COX-1 and valine in COX-2) may be critical for the specificity of several drugs ^19,24,28,29.

Currently, since binding sites for specific COX-2 inhibitors have already been described, and enzyme tridimensional structure is well-established, modern molecular modeling techniques may develop new compounds with high affinity and specificity, but probably without sulfonamide and sulfone clusters of second generation compounds. Compound RS57067000 may be one of the first in this class of really specific COX-2 inhibitors, thus representing the birth of a third generation of specific COX-2 inhibitors ^6,27.

NEW SPECIFIC CYCLOXYGENASE-2 INHIBITORS APPLICATIONS

Human brain cells are able to start and amplify characteristic brain inflammatory response involving cytokine synthesis, acute phase proteins, complement activation, prostaglandins and oxygen radicals release. In Alzheimer's disease, all signs of microglia inflammation and astroglia activation with amyloidal protein deposition are associated to the pathogenesis of the disease. A critical event in this pathology is that b-amyloidal protein is able to activate microglia resulting in increased neuronal COX-2 expression, potentiating amyloidal protein-mediated oxidative stress. Recent information suggests that COX-2-derived prostaglandins increase neurodegenerative inflammatory process, induce pro-inflammatory cytokines synthesis in astroglia cells and potentiate glutamate excitotoxicity accelerating neurodegeneration. Selective COX-2 inhibitors may be a major alternative for preventing central prostaglandin production in those patients ^1,26,54,55.

Several epidemiological human, animal and in vitro studies have observed the involvement of COX-2 in neoplastic processes opening the perspective of using selective COX-2 inhibitors to treat several types of cancer. Epidemiological studies have shown that aspirin is able to decrease the incidence of colon cancer in 40% to 50% ^56-58. Several other NSAIDs, including specific COX-2 inhibitors, have shown excellent results in preventing several types of cancer, including pancreas, liver, esophagus, intestine, stomach, lungs, breast and prostate, among others ^57-64.

Molecular basis for the development of NSAIDs chemoprotective activity in neoplastic processes is primarily related to high COX-2 expression and production by tumor tissue, as it is the case with esophageal carcinomas and colo-rectal neoplasias. During colo-rectal cancer development, a sequence of multiple genetic changes peaks with the transformation of a polyp in cancer. Initial genetic change is a mutation in tumor suppressor gene APC (Adenomatous Polyposis Coli). APC gene codes a protein able to decrease concentration of a second cytoplasm protein called b-catenine, which plays a major role in cell adhesion and development. Mutation substantially increases b-catenine which is displaced toward the nucleus forming a complex with another protein called T-cell Factor 4 (TCF-4). This complex then binds to DNA and induces the expression of genes promoting cell growth and proliferation. The complex also induces the expression of Peroxisome Proliferator-Activated Receptors d (PPARd). After interacting with the ligant, PPARd forms a complex with other nuclear receptor called Retinoid X Receptor (RXR) able to couple to DNA and activate target-genes. PPARd ligants include some eicosanoids originated from the activation of COX-2 and some fatty acids ^64-66.

Molecular genetics tests may be very useful in determining colo-rectal neoplastic mutations and in guiding patients and their relatives ⁶⁷.

A proposed mechanism for NSAIDs antineoplastic activity would be the inhibition of PPARd binding to cell DNA, preventing the activation of genes responsible for cell development, metabolism, growth and differentiation. A different mechanism would be NSAIDs ability to induce cancer cells apoptosis by inhibiting PPARd function. In addition, by inhibiting COX-2, NSAIDs would also be preventing PGE₂ formation in tumor tissue, preventing stimulation of Vascular Endothelial Growth Factor (VEGF), which induces angiogenesis by indirectly stimulating neoplastic cell growth and expansion ^57,64-66. Increased COX-2 expression and negative PPARd regulation with subsequent modulation in PGE₂ and 15desoxyD ^12-14 PGJ₂ concentration may influence breast cancer development and progression to metastasis ⁶⁸. These data indirectly suggest that COX-3 may participate as a source of 15desoxyD^12-14 PGJ₂. This is however mere speculation.

Eibl et al. (2003) ⁶⁹ have shown that the antimitotic effect of nimesulide, selective COX-2 inhibitor, on pancreas cancer cells is indirect, inducing apoptosis regardless of COX-2 expression. According to these findings, rofecoxib, specific COX-2 inhibitor, has resulted in vitro in dose-dependent apoptosis increase and time-dependent tumor cell proliferation inhibition, as reviewed by other authors ⁷⁰.

Several NSAIDs are used in Anesthesiology to treat cancer pain, especially in association with opioids. It is believed that patients in those situations may have additional benefits from the use of selective COX-2 inhibitors to replace traditionally used and low selectivity NSAIDs.

SPECIFIC COX-2 INHIBITORS SIDE EFFECTS

COX-1-derived prostaglandins are considered to offer cytoprotection to gastrointestinal tract. Two major multicentric clinical trials were performed aiming at evaluating coxibes (celecoxib and rofecoxib) clinical efficacy and gastrointestinal complications: the so-called Vioxx Gastrointestinal Outcomes Research (VIGOR) and Celecoxib Long-Term Arthritis Safety Study (CLASS). VIGOR trial has evaluated 8076 rheumatoid arthritis patients treated in average for nine months with daily 50 mg rofecoxib as compared to 500 mg naproxene twice a day. In this group, mean age was 58 years and 80% were females. Approximately 60% had been treated with glucocorticoids for a long period and 8% had history of intestinal perforation, hemorrhage or peptic ulcer symptoms. This study has revealed perforation and gastrointestinal hemorrhage or peptic ulcer symptoms in 4.5 per 100 patients/year in the naproxene group as compared to 2.1 per 100 patients/year in the rofecoxib group, with statistically significant difference of 54% ⁹.

CLASS trial consisted of two separate studies. One has compared celecoxib (400 mg twice a day) and diclofenac (75 mg twice a day). The other has compared celecoxib and ibuprofen (800 mg three times a day). From studied patients, 72% had osteo-arthritis and 68.5% were females. Study has lasted 13 months and aspirin was allowed until 325 mg/day. There were no significant differences between groups in the incidence of gastric ulcer, upper gastrointestinal bleeding or obstruction ⁹.

According to ADRAC (Adverse Drug Reactions Advisory Committee), since the introduction of celecoxib (celebra) in the market in 1999, more than 919 reports on related adverse and side effects have been recorded. As expected, few patients have reported gastrointestinal changes such as nausea, abdominal pain, diarrhea and dyspepsia. However, more prominent adverse effects were observed with celecoxib and other COX-2 specific inhibitors, such as hives, headache, allergy (such as tongue edema and angioedema) and renal failure ^71-79.

VIGOR trial results have shown a higher risk for thrombotic cardiovascular events with rofecoxib, including myocardial infarction, unstable angina, heart clots, sudden death, ischemic attacks and transient ischemic attacks. Other studies have shown similar results between rofecoxib and celecoxib ^71,72. Some studies have shown that COX-2 is essential for starting ductus arteriosus closing during pregnancy, suggesting that maternal use of COX-2 inhibitors very close to labor would increase the incidence of patent ductus arteriosus after delivery ²⁶.

Scheider et al. ⁷⁹ have reported allergic vasculitis with diffuse necrotizing purpura followed by multiple organ failure associated to clinical use of celecoxib.

Although not conclusive, some authors call the attention to the noxious pharmacodynamic interaction of aspirin with reversible COX inhibitors, such as ibuprofen and diclofenac, due to competitive active site inhibition by reversible inhibitors, preventing aspirin to reach its target in position 530 of the enzyme, resulting in antagonism of aspirin cardioprotective effect on patients with established cardiovascular disease ^80,81.

Specific COX-2 inhibitors, similarly to other NSAIDs, may promote renal function changes primarily resulting in peripheral edema, arterial hypertension, renal water and sodium excretion inhibition and hyperkalemia. Hyperkalemia may result from decreased prostaglandin-mediated rennin release, which in turn promotes aldosterone decrease and distal tubule potassium excretion decrease. Celecoxib and rofecoxib promote mild hyperkalemia ²⁶.

Although not knowing the real impact of celecoxib and rofecoxib on lithium plasma concentration, patients under combined treatment should monitor their lithium plasma levels since it is widely known that other non-selective NSAIDs decrease renal lithium excretion ²⁶.

In conclusion, we stress that newer generations of spefic COX-2 inhibitor anti-inflammatory drugs are still of limited success especially due to their side-effects and contraindications. However, thanks to the discovery of different cycloxygenase variants, to the use of molecular modeling to obtain more specif compounds, and to new applications for those agents, including cancer treatment and Alzheimer's disease, the interest on NSAIDs will remain as current as 100 years ago.

REFERENCES

Submitted for publication May 5, 2003

Accepted for publication September 1, 2003

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*

Received from Hospital São Rafael, Faculdade de Farmácia da Universidade Federal da Bahia (UFBA) e no Departamento de Saúde da Universidade Estadual de Santa Cruz (UESC)

Publication Dates

• Publication in this collection
  02 July 2004
• Date of issue
  June 2004

History

• Accepted
  01 Sept 2003
• Received
  05 May 2003