Eicosanoids

These are 20-carbon compounds that include: prostaglandins, thromboxanes, leukotrienes, & related compounds. They are generated from phospholipid precursors. They function primarily as hormone-like chemical signals between cells. They are implicated in the control of many physiological processes, and are among the most important mediators and modulators of the inflammatory reaction. They are a very significant target for drug action.

Structure and biosynthesis In mammals, the main eicosanoid precursor is arachidonic acid (5,8,11,14-eicosatetraenoic acid), a 20-carbon unsaturated fatty acid containing four double bonds. The initial and rate-limiting step in eicosanoid synthesis is the liberation of arachidonate from cell membrane phospholipids by the enzyme phospholipase A2 (PLA2). Several species exist, but the most important is probably the highly regulated cytosolic PLA2. Cytosolic PLA2 is activated (and hence arachidonic acid liberated) by phosphorylation. This occurs in response to signal transduction events triggered by many stimuli, such as thrombin action on platelets, bradykinin action on fibroblasts, antigen-antibody reactions on mast cells. General cell damage also triggers the activation process.

The free arachidonic acid is metabolized separately (or sometimes jointly)by several pathways, including the following: Fatty acid cyclo-oxygenase (COX). Two main isoform forms, COX-1 and COX-2.These are highly homologous enzymes but are regulated in different and tissue-specific ways. They enzymatically combine arachidonic substrates with molecular oxygen to form unstable intermediates, which can subsequently be transformed by other enzymes to different prostanoids (prostaglandins and thromboxanes). The COX enzymes are blocked by nonsteroidal anti-inflammatory drugs (NSAIDs) as aspirin Lipoxygenases. Several subtypes synthesize leukotrienes, lipoxins or other compounds. Among factors determining the type of eicosanoid synthesized are (1) the substrate lipid species, (2) the type of cell, and (3) the manner in which the cell is stimulated.

COX-1 COX-2 Tissue expression Constitutive enzyme expressed in most tissues (housekeeping function). Inducible in many tissues by many stimuli including growth factors (TNF-α) and cytokines (IL-1). Constitutive in brain, kidney and vessels. Function Regulates normal cellular processes , such as platelet aggregation, GIT cytoprotection, renal blood flow auto-regulation, and initiation of labor. Inflammation, fever, pain, renal function, production of vascular PGI2 Inhibitors Most of classical NSAIDs (diclofenac, ibuprofen, indomethacin). Many NSAIDs drugs and also selective COX-2 inhibitors like celecoxib

Inhibition of synthesis Synthesis of COX products can be inhibited by nonsteroidal antiinflammatory drugs (NSAIDs). Aspirin acetylates COX at a serine residue and causes irreversible inhibition while other NSAIDs are competitive and reversible inhibitors. Most NSAIDs are nonselective COX-1 and COX-2 inhibitors, but some later ones like celecoxib, etoricoxib are selective for COX-2. The sensitivity of COX in different tissues to inhibition by these drugs varies; selective inhibition of formation of certain products may be possible at lower doses. NSAIDs do not inhibit the production of LTs: this may even be increased since all the arachidonic acid becomes available to the LOX pathway. Glucocorticosteroids inhibit the release of arachidonic acid from membrane lipids (by stimulating production of proteins called annexins which inhibit phospholipase A2). So they indirectly reduce production of all eicosanoids—PGs, TXs and LTs. Moreover, they inhibit the induction of COX-2 by cytokines at the site of inflammation.

Prostanoids The term prostanoids encompasses the prostaglandins and the thromboxanes. Cyclo-oxygenases (COXs) oxidise arachidonate, producing the unstable intermediates prostaglandin (PG) G2 and PGH2. These are rapidly transformed by isomerase or synthase enzymes to PGE2, PGI2, PGD2, PGF2α and TXA2. There are two main COX isoforms: COX-1, a constitutive enzyme which produces prostanoids that act as homeostatic regulators (e.g. modulating vascular responses), and COX-2, which is often induced by inflammatory stimuli. PGI2 (prostacyclin), acts on IP receptors, predominantly from vascular endothelium producing vasodilatation &inhibition of platelet aggregation . Thromboxane (TX) A2, predominantly from platelets causing platelet aggregation and vasoconstriction.

PGE2 is prominent in inflammatory responses and is a mediator of fever PGE2 is prominent in inflammatory responses and is a mediator of fever. Main effects are: EP1 receptors: contraction of bronchial and gastrointestinal tract (GIT) smooth muscle EP2 receptors: relaxation of bronchial, vascular and GIT smooth muscle EP3 receptors: inhibition of gastric acid secretion, increased gastric mucus secretion, contraction of pregnant uterus and of GIT smooth muscle, inhibition of lipolysis and of autonomic neurotransmitter release. EP4 receptors: vasodilation, smooth muscle relaxation. PGE1 is cytoprotective , vasodilator and an inhibitor of platelet aggregation, and it contracts uterine and intestinal smooth muscle. PGF2α acts on FP receptors, found in uterine (and other) smooth muscle, and corpus luteum, producing contraction of the uterus . PGD2 ,acts on DP receptors, is derived particularly from mast cells causing vasodilatation and inhibition of platelet aggregation and contraction of respiratory smooth muscle

The role of prostanoids in inflammation: The inflammatory response is inevitably accompanied by the release of prostanoids. PGE2 predominates, although PGI2 is also important. In areas of acute inflammation, PGE2 and PGI2 are generated by the local tissues and blood vessels, while mast cells release mainly PGD2. In chronic inflammation, cells of the monocyte/macrophage series also release PGE2 and TXA2. PGE2, PGI2 and PGD2 are powerful vasodilators. Prostanoids do not directly increase the permeability of the postcapillary venules, but potentiate this effect of histamine and bradykinin. Similarly, they do not themselves produce pain, but potentiate the effect of bradykinin by sensitising afferent C fibres. The anti-inflammatory effects of the NSAIDs stem largely from their ability to block these actions of the prostaglandins.

Prostaglandins of the E series are also pyrogenic Prostaglandins of the E series are also pyrogenic. NSAIDs exert antipyretic actions by inhibiting PGE2 synthesis in the hypothalamus. Catabolism of prostanoids : is by a multistep process. After carrier-mediated uptake, most prostaglandins are rapidly inactivated by 'prostaglandin-specific' dehydrogenase and reductase enzymes and the inactive products are further degraded by general fatty acid-oxidising enzymes and excreted in the urine The prostaglandin-specific enzymes are present in high concentration in the lung.

Clinical uses of prostanoids: 1. Gynecological ,obstetric : Termination of pregnancy: dinoprostone which is a synthetic preparation of PGE2 that is approved for inducing abortion in the second trimester of pregnancy. It is administered vaginally as a suppository. S/Es: nausea, vomiting, and diarrhea -Antiprogestins (eg, mifepristone) have been combined with an oral prostaglandin (e.g., misoprostol,PGE1 analogue) to produce early abortions. Induction of labour: dinoprostone (PGE2) or misoprostol are used either as a gel or as a controlled-release formulation. Postpartum hemorrhage: carboprost (PGF2α) It is administered as a single intramuscular injection. S/Es: Vomiting , diarrhea and transient bronchoconstriction. 2-Male Reproductive System: Tretment of impotence: alprostadil (PGE1) (by Intracavernosal injection or urethral suppository therapy)enhance penile erection by relaxing the smooth muscle of the corpora cavernosa.

3. Gastrointestinal: 4. Cardiovascular: To prevent ulcer associated with NSAIDs use: misoprostol (PGE1):Ii is orally active to decrease acid secretion and increase mucus secretion. S/Es: abdominal discomfort and occasional diarrhea. Treatment of chronic idiopathic constipation and irritable bowel syndrome with constipation: Lubiprostone is a PGE1 derivative that stimulates chloride channels so increases intestinal fluid secretion ,softens the stool and increases intestinal motility. S/E: nausea, headache and abdominal pain. 4. Cardiovascular: To maintain the patency of the ductus arteriosus: until surgical correction of the defect in babies with certain congenital heart malformations: Patency of the fetal ductus arteriosus depends on COX-2–derived PGE. Alprostadil (PGE1)given through continuous intravenous infusion(because of its short half-life).S/Es: apnea, bradycardia, hypotension, and hyperpyrexia. To inhibit platelet aggregation (during hemodialysis): epoprostenol (PGI2).

Primary pulmonary hypertension: Epoprostenol, the pharmaceutical form of naturally occurring prostacyclin, and the synthetic analogs of prostacyclin (iloprost and treprostinil) are potent pulmonary vasodilators and inhibitors of proliferation of smooth muscle cells that are used for the treatment of pulmonary arterial hypertension. These drugs mimic the effects of prostacyclin in endothelial cells, producing a significant reduction in pulmonary arterial resistance with a subsequent increase in cardiac index and oxygen delivery. These agents all have a short half-life. Epoprostenol and treprostinil are administered as a continuous intravenous infusion, and treprostinil may also be administered orally or via inhalation or subcutaneous infusion. Inhaled iloprost requires frequent dosing due to the short half-life A/Es: Dizziness, headache, flushing, and fainting are the most common adverse effects . Bronchospasm and cough can also occur after inhalation of iloprost.

5. Ophthalmic: Open‐angle glaucoma: latanoprost (PGF2α analogue) eye drops acting on FP receptors to increase aqueous humor outflow via the uveoscleral pathway. Other synthetic FP receptor agonists for open-angle glaucoma or ocular hypertension are bimatoprost, tafluprost and travoprost. Bimatoprost increases eyelash prominence, length, and darkness and is approved for the treatment of eye- lash hypotrichosis. S/Es: blurred vision, iris color change (increased brown pigmentation), increased number and pigment of eyelashes, ocular irritation, and foreign body sensation.

Leukotrienes Leukotrienes are synthesised from arachidonic acid by lipoxygenase- catalysed pathways. 5-Lipoxygenase oxidises arachidonate to give 5- hydroperoxyeicosatetraenoic acid (5-HPETE), which is converted to leukotriene (LT) A4. This, in turn, can be converted to either LTB4 or to a series of glutathione adducts, the cysteinyl-leukotrienes (also referred to as the sulfidopeptide leukotrienes):LTC4, LTD4 and LTE4. These cysteinyl leukotrienes are produced mainly by eosinophils, mast cells, basophils and macrophages. LTB4 is produced mainly by neutrophils, acting on specific receptors, causes adherence, chemotaxis and activation of polymorphs and monocytes, and stimulates proliferation and cytokine production from macrophages and lymphocytes. It is found in inflammatory exudates and tissues in many inflammatory conditions, including rheumatoid arthritis, psoriasis and ulcerative colitis.

The cysteinyl-leukotrienes cause: contraction of bronchial muscle ,mucus secretion & mucosal edema. vasodilatation in most vessels & may mediate the cardiovascular changes of acute anaphylaxis, so are called: the slow-reacting substances of anaphylaxis (SRS-A). LTB4 is an important mediator in all types of inflammation; The cysteinyl-leukotrienes are of particular importance in asthma: Cysteinyl-leukotrienes ‐receptor antagonists (zafirlukast and montelukast) are now in use in the treatment of asthma. Zileutin, a 5‐lipoxygenase enzyme inhibitor, is developed as antiasthmatic agent. Leukotriene B4 is metabolised by a unique membrane-bound P450 enzyme in neutrophils, and then further oxidised to 20-carboxy-LTB4. LTC4 and LTD4 are metabolised to LTE4, which is excreted in the urine.

Lipoxins They are metabolites of trihydroxy arachidonate the products of the lipoxygenase enzymes. They act on specific receptors on polymorphonuclear leucocytes to oppose the action of proinflammatory stimuli, supplying what might be called 'stop signals' to inflammation. Aspirin stimulates the synthesis of these substances, perhaps contributing to its other anti-inflammatory effects. Resolvins, as the name implies, are a series of compounds that fulfil a similar function, but unlike lipoxins, their precursor fatty acid is eicosapentaenoic acid. Fish oils are rich in this fatty acid and it is likely that at least some of their anti- inflammatory benefit is produced through conversion to these highly active species.

Endothelin-1 antagonists Endothelin-1 (ET-1) is a peptide autacoid produced by vascular endothelial cells. It activates ETA and ETB receptors in vascular smooth muscle and other tissues. The results of ETA receptor activation are vasoconstriction and cell proliferation while ETB receptors mediate vasodilation, antiproliferation and increased ET-1 clearance. ET-1 may serve physiologically to counteract the vasodilation produced by the endothelin-relaxing factor (nitric oxide). Levels of ET-1 peptide are increased 10-fold in pulmonary arteries of patients with pulmonary arterial hypertension. Bosentan is an endothelin-1 receptor antagonist that is approved for treating pulmonary arterial hypertension. Clinical trials have shown that bosentan significantly decreases pulmonary vascular resistance and dyspnea.

Bosentan is administered orally and is generally well tolerated but 11% of patients have experienced elevated serum aminotransferase levels. For this reason, liver function tests should be monitored at baseline and then monthly in persons taking bosentan. Based on animal studies, bosentan is very likely to cause major birth defects if used by pregnant women, and it is contraindicated in pregnancy. A second ET receptor antagonist, ambrisentan, was recently approved for the treatment of pulmonary arterial hypertension. As with bosentan, warnings are made regarding hepatic function and enzyme level monitoring.