The Eicosanoids: Prostaglandins, Thromboxanes, Leukotrienes, & Related Compounds: Introduction

The eicosanoids are oxygenation products of polyunsaturated long-chain fatty acids. They are ubiquitous in the animal kingdom and are also found—together with their precursors—in a variety of plants. They constitute a very large family of compounds that are highly potent and display an extraordinarily wide spectrum of biologic activity. Because of their biologic activity, the eicosanoids, their specific receptor antagonists and enzyme inhibitors, and their plant and fish oil precursors have great therapeutic potential.

Arachidonic Acid & Other Polyunsaturated Precursors

Arachidonic acid (AA) or 5,8,11,14-eicosatetraenoic acid, the most abundant of the eicosanoid precursors, is a 20-carbon (C20) fatty acid containing four double bonds (designated C20:4–6). AA must first be released or mobilized from the sn-2 position of membrane phospholipids by one or more lipases of the phospholipase A₂ (PLA₂) type (Figure 18–1) for eicosanoid synthesis to occur. At least three phospholipases mediate arachidonate release from membrane lipids: cytosolic (c) PLA₂, secretory (s) PLA₂, and calcium-independent (i) PLA₂. cPLA₂ dominates in the acute release of AA while the inducible sPLA₂ contributes under conditions of sustained or intense stimulation of AA production. AA can also be released by a combination of phospholipase C and diglyceride lipase.

Following mobilization, AA is oxygenated by four separate routes: the cyclooxygenase (COX), lipoxygenase, P450 epoxygenase, and isoeicosanoid pathways (Figure 18–1). Several factors determine the type of eicosanoid synthesized: (1) the substrate lipid species, (2) the type of cell, and (3) the cell's particular phenotype. The pattern of eicosanoids synthesized also frequently reflects (4) the manner in which the cell is stimulated. Distinct but related products can be formed from precursors other than AA. For example, homo--linoleic acid (C20:3–6) or eicosapentaenoic acid (C20:5–3, EPA) yield products that differ quantitatively and qualitatively from those derived from AA. This shift in product formation is the basis for using fatty acids obtained from cold-water fish or from plants as nutritional supplements in humans. For example, thromboxane (TXA₂), a powerful vasoconstrictor and platelet agonist, is synthesized from AA via the COX pathway. COX metabolism of EPA yields TXA₃, which is relatively inactive. The hypothesis that dietary eicosapentaenoate substitution for arachidonate could reduce the incidence of cardiovascular events via effects that minimize thrombosis and arrhythmias, and reduce blood pressure, is a focus of current investigation.

Synthesis of Eicosanoids

Products of Prostaglandin Endoperoxide Synthases (Cyclooxygenases)

Two unique COX isozymes convert AA into prostaglandin endoperoxides. Prostaglandin (PG) H synthase-1 (COX-1) is expressed constitutively in most cells. In contrast, PGH synthase-2 (COX-2) is inducible; its expression varies markedly depending on the stimulus. COX-2 is an immediate early-response gene product that is markedly up-regulated by shear stress, growth factors, tumor promoters, and cytokines. COX-1 generates prostanoids for "housekeeping" such as gastric epithelial cytoprotection, whereas COX-2 is the major source of prostanoids in inflammation and cancer. This distinction is overly simplistic, however; there are both physiologic and pathophysiologic processes in which each enzyme is uniquely involved and others in which they function coordinately. For example, endothelial COX-2 is the primary source of vascular prostacyclin (PGI2), whereas renal COX-2-derived prostanoids are important for normal renal development and maintenance of function. An additional COX variant, termed COX-3, has been described in dogs but the relevance of this and other COX-1 splice variants to human biology remains to be determined.

Nonsteroidal anti-inflammatory drugs (NSAIDs; see Chapter 36) exert their therapeutic effects through inhibition of the COXs. Indomethacin and sulindac are slightly selective for COX-1. Meclofenamate and ibuprofen are approximately equipotent on COX-1 and COX-2, whereas celecoxib = diclofenac < rofecoxib = lumiracoxib < etoricoxib in inhibition of COX-2 (listed in order of increasing average selectivity). Aspirin acetylates and inhibits both enzymes covalently. Low doses (< 100 mg/day) inhibit preferentially, but not exclusively, platelet COX-1, whereas higher doses inhibit both systemic COX-1 and COX-2.

Both COX-1 and COX-2 promote the uptake of two molecules of oxygen by cyclization of arachidonic acid to yield a C₉–C₁₁ endoperoxide C₁₅ hydroperoxide (Figure 18–2). This product is PGG₂, which is then rapidly modified by the peroxidase moiety of the COX enzyme to add a 15-hydroxyl group that is essential for biologic activity. This product is PGH₂. Both endoperoxides are highly unstable. Analogous families—PGH₁ and PGH₃ and all their subsequent products—are derived from homo--linolenic acid and eicosapentaenoic acid, respectively.

The prostaglandins, thromboxane, and prostacyclin, collectively termed the prostanoids, are generated from PGH₂ through the action of downstream isomerases and synthases. These terminal enzymes are expressed in a relatively cell-specific fashion, such that most cells make one or two dominant prostanoids. The prostaglandins differ from each other in two ways: (1) in the substituents of the pentane ring (indicated by the last letter, eg, E and F in PGE and PGF) and (2) in the number of double bonds in the side chains (indicated by the subscript, eg, PGE1, PGE2). PGH₂ is metabolized by prostacyclin, thromboxane, and PGF synthases (S) to PGI2, TXA₂, and PGF₂, respectively. Two additional enzymes, 9,11-endoperoxide reductase and 9-ketoreductase, provide for PGF₂ synthesis from PGH₂ and PGE₂, respectively. At least three PGE₂ synthases have been identified: microsomal (m) PGES-1, the more readily inducible mPGES-2, and cytosolic PGES. There are two distinct PGDS isoforms, the lipocalin-type PGDS and the hematopoietic PGDS.

Several products of the arachidonate series are of current clinical importance. Alprostadil (PGE1) may be used for its smooth muscle relaxing effects to maintain the ductus arteriosus patent in some neonates awaiting cardiac surgery and in the treatment of impotence. Misoprostol, a PGE₁ derivative, is a cytoprotective prostaglandin used in preventing peptic ulcer and in combination with mifepristone (RU-486) for terminating early pregnancies. PGE₂ and PGF₂ are used in obstetrics to induce labor. Latanoprost and several similar compounds are topically active PGF₂ derivatives used in ophthalmology to treat open angle glaucoma. Prostacyclin (PGI2, epoprostenol) is synthesized mainly by the vascular endothelium and is a powerful vasodilator and inhibitor of platelet aggregation. It is used clinically to treat pulmonary hypertension and portopulmonary hypertension. In contrast, thromboxane(TXA₂) has undesirable properties (aggregation of platelets, vasoconstriction). Therefore TXA₂-receptor antagonists and synthesis inhibitors have been developed for cardiovascular indications, although these (except for aspirin) have yet to establish a place in clinical usage.

All the naturally occurring COX products undergo rapid metabolism to inactive products either by hydration (for PGI2 and TXA₂) or by oxidation of the key 15-hydroxyl group to the corresponding ketone by prostaglandin 15-OH dehydrogenase. Further metabolism is by ¹³ reduction, -oxidation, and -oxidation. The inactive metabolites can be determined in blood and urine by immunoassay or mass spectrometry as a measure of the in vivo synthesis of their parent compounds.

Products of Lipoxygenase

The metabolism of AA by the 5-, 12-, and 15-lipoxygenases (LOX) results in the production of hydroperoxyeicosatetraenoic acids (HPETEs), which rapidly convert to hydroxy derivatives (HETEs) and leukotrienes (Figure 18–3). The most actively investigated leukotrienes are those produced by the 5-LOX present in leukocytes (neutrophils, basophils, eosinophils, and monocyte-macrophages) and other inflammatory cells such as mast cells and dendritic cells. This pathway is of great interest since it is associated with asthma, anaphylactic shock, and cardiovascular disease. Stimulation of these cells elevates intracellular Ca²⁺ and releases arachidonate; incorporation of molecular oxygen by 5-LOX, in association with 5-LOX-activating protein (FLAP), then yields the unstable epoxide leukotriene A₄ (LTA₄). This intermediate either converts to the dihydroxy leukotriene B₄ (LTB₄) or conjugates with glutathione to yield leukotriene C₄ (LTC₄), which undergoes sequential degradation of the glutathione moiety by peptidases to yield LTD₄ and LTE₄. These three products are called cysteinyl leukotrienes. Although leukotrienes are predominantly generated in leukocytes, nonleukocyte cells (eg, endothelial cells) that express enzymes downstream of 5-LOX/FLAP can take up and convert leukocyte-derived LTA₄ in a process termed transcellular biosynthesis. Transcellular formation of prostaglandins has also been shown; for example, endothelial cells can use platelet PGH₂ to form PGI2.

LTC₄ and LTD₄ are potent bronchoconstrictors and are recognized as the primary components of the slow-reacting substance of anaphylaxis (SRS-A) that is secreted in asthma and anaphylaxis. There are four current approaches to antileukotriene drug development: 5-LOX enzyme inhibitors, leukotriene-receptor antagonists, inhibitors of FLAP, and phospholipase A₂ inhibitors.

LTA₄, the primary product of 5-LOX, can be converted via 12-LOX in platelets to the lipoxins  LXA₄ and LXB₄. These mediators can also arise through 5-LOX metabolism of 15-HETE, the product of 15-LOX-2 metabolism of arachidonic acid. 15-LOX-1 prefers linoleic acid as a substrate forming 15S -hydroxyoctadecadienoic acid. The stereochemical isomer, 15R- HETE, may be derived from the action of aspirin-acetylated COX-2 and further transformed in leukocytes by 5-LOX to 15-epi-LXA₄ or 15-epi-LXB₄, the so-called aspirin-triggered lipoxins. 12-HETE, a product of 12-LOX, can also undergo a catalyzed molecular rearrangement to epoxy-hydroxyeicosatrienoic acids called hepoxilins. Although these compounds can be formed in vitro and when synthesized may have potent biologic effects, the importance of the endogenous compounds in human biology remains ill defined.

The LOXs located in epidermal cells are distinct from "conventional" enzymes—arachidonic acid and linoleic acid are apparently not the natural substrates for epidermal LOX. Epidermal accumulation of 12R-HETE is a feature of psoriasis and ichthyosis and inhibitors of 12R-LOX are under investigation for the treatment of these proliferative skin disorders.

Epoxygenase Products

Specific isozymes of microsomal cytochrome P450 monooxygenases convert AA to hydroxy- or epoxyeicosatrienoic acids (Figures 18–1 and 18–3). The products are 20-HETE, generated by the CYP hydroxylases (CYP3A, 4A, 4F) and the 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs), which arise from the CYP epoxygenase (2J, 2C). Their biosynthesis can be altered by pharmacologic, nutritional, and genetic factors that affect P450 expression. The biologic actions of the EETs are reduced by their conversion to the corresponding, and biologically less active, dihydroxyeicosatrienoic acids (DHETs) through the action of epoxide hydrolases. Unlike the prostaglandins, the EETs and DHETs can be incorporated into phospholipids, which then act as storage sites. Intracellular fatty acid-binding proteins may differentially bind EETs and DHETs, thus modulating their metabolism, activities, and targeting. EETS are synthesized in endothelial cells and cause vasodilation in a number of vascular beds by activating the smooth muscle large conductance Ca²⁺-activated K⁺ channels. This results in smooth muscle cell hyperpolarization and vasodilation, leading to reduced blood pressure. Substantial evidence indicates that EETs may function as endothelium-derived hyperpolarizing factors, particularly in the coronary circulation. Consequently there is interest in inhibitors of soluble epoxide hydrolase as potential antithrombotic and antihypertensive drugs. Anti-inflammatory, antiapoptotic and proangiogenic actions of the EETs have also been reported.

Isoeicosanoids

The isoeicosanoids, a family of eicosanoid isomers, are formed nonenzymatically by direct free radical–based action on AA and related lipid substrates. Isoprostanes are prostaglandin stereoisomers. Because prostaglandins have many asymmetric centers, they have a large number of potential stereoisomers. COX is not needed for the formation of the isoprostanes, and its inhibition with aspirin or other NSAIDs should not affect the isoprostane pathway. The primary epimerization mechanism is peroxidation of arachidonate by free radicals. Peroxidation occurs while arachidonic acid is still esterified to the membrane phospholipids. Thus, unlike prostaglandins, these stereoisomers are "stored" as part of the membrane. They are then cleaved by phospholipases, circulate, and are excreted in urine. Isoprostanes are present in relatively large amounts (tenfold greater in blood and urine than the COX-derived prostaglandins). They have potent vasoconstrictor effects when infused into renal and other vascular beds and may activate prostanoid receptors. They also may modulate other aspects of vascular function, including leukocyte and platelet adhesive interactions and angiogenesis. It has been speculated that they may contribute to the pathophysiology of inflammatory responses in a manner insensitive to COX inhibitors. A particular difficulty in assessing the likely biologic functions of isoprostanes—several of which have been shown to serve as incidental ligands at prostaglandin receptors—is that while high concentrations of individual isoprostanes may be necessary to elicit a response, multiple compounds are formed coincidentally in vivo under conditions of oxidant stress. Analogous leukotriene and EET isomers have been described.

*The authors acknowledge the contributions of the previous authors of this chapter, Drs. Marie L. Foegh and Peter W. Ramwell.

Basic Pharmacology of Eicosanoids

Mechanisms & Effects of Eicosanoids

Receptor Mechanisms

As a result of their short half-lives, the eicosanoids act in both an autocrine and a paracrine fashion, ie, close to the site of their synthesis and not as circulating hormones. These ligands bind to receptors on the cell surface, and pharmacologic specificity is determined by receptor density and type on different cells (Figure 18–4). A single gene product has been identified for the PGI2 (IP), PGF₂ (FP), and TXA₂ (TP) receptors, while four distinct PGE2 receptors (EPs 1–4) and two PGD₂ receptors (DP₁ and DP₂) have been cloned. Additional isoforms of the human TP ( and ), FP (A and B), and EP₃ (I, II, III, IV, V, VI, e, and f)) receptors can arise through differential mRNA splicing. Two receptors exist for both LTB₄ (BLT₁ and BLT₂) and the cysteinyl leukotrienes (cysLT₁ and cysLT₂). The formyl peptide (fMPL)-1 receptor can be activated by lipoxin A4 and consequently has been termed the ALX receptor. All of these receptors are G protein-coupled; properties of the best-studied receptors are listed in Table 18–1.

EP₂, EP₄, IP, and DP₁ receptors activate adenylyl cyclase via G_s. This leads to increased intracellular cAMP levels, which in turn activates specific protein kinases (see Chapter 2). EP₁, FP, and TP activate phosphatidylinositol metabolism, leading to the formation of inositol trisphosphate, with subsequent mobilization of Ca²⁺ stores and an increase of free intracellular Ca²⁺. TP also couples to multiple G proteins, including G_12/13 and G₁₆, to stimulate small G protein signaling pathways, and may activate or inhibit adenylyl cyclase via G_s (TP) or G_i (TP), respectively. EP₃ isoforms can couple to both elevation of intracellular calcium and to increased or decreased cAMP. The DP₂ receptor (also known as the chemoattractant receptor-homologous molecule expressed on TH2 cells, or CRTH2), which is unrelated to the other prostanoid receptors, is a member of the fMLP (formylated MetLeuPhe) receptor superfamily. This receptor couples through a G_i-type G protein and leads to inhibition of cAMP synthesis and increases in intracellular Ca²⁺ in a variety of cell types.

LTB₄ also causes inositol trisphosphate release via the BLT₁ receptor, causing activation, degranulation, and superoxide anion generation in leukocytes. The BLT₂ receptor, a low-affinity receptor for LTB₄, is also bound with reasonable affinity by 12S- and 12R-HETE, although the biologic relevance of this observation is not clear. CysLT₁ and cysLT₂ couple to G_q, leading to increased intracellular Ca²⁺. Studies have also placed G_i downstream of cysLT₂.

Although prostanoids can activate peroxisome proliferator-activated receptors (PPARs) if added in sufficient concentration in vitro, it remains questionable whether these compounds attain concentrations sufficient to function as endogenous nuclear-receptor ligands in vivo.

Effects of Prostaglandins & Thromboxanes

The prostaglandins and thromboxanes have major effects on smooth muscle in the vasculature, airways, and gastrointestinal and reproductive tracts. Contraction of smooth muscle is mediated by the release of calcium, while relaxing effects are mediated by the generation of cAMP. Many of the eicosanoids' contractile effects on smooth muscle can be inhibited by lowering extracellular calcium or by using calcium channel blocking drugs. Other important targets include platelets and monocytes, kidneys, the central nervous system, autonomic presynaptic nerve terminals, sensory nerve endings, endocrine organs, adipose tissue, and the eye (the effects on the eye may involve smooth muscle).

Smooth Muscle

Vascular

TXA₂ is a potent vasoconstrictor. It is also a smooth muscle cell mitogen and is the only eicosanoid that has convincingly been shown to have this effect. The mitogenic effect is potentiated by exposure of smooth muscle cells to testosterone, which up-regulates smooth muscle cell TP expression. PGF₂ is also a vasoconstrictor but is not a smooth muscle mitogen. Another vasoconstrictor is the isoprostane 8-iso-PGF₂, also known as iPF₂III, which may act via the TP receptor.

Vasodilator prostaglandins, especially PGI2 and PGE2, promote vasodilation by increasing cAMP and decreasing smooth muscle intracellular calcium, primarily via the IP and EP₄ receptors. Vascular PGI₂ is synthesized by both smooth muscle and endothelial cells, with the COX-2 isoform in the latter cell type being the major contributor. In the microcirculation, PGE₂ is a vasodilator produced by endothelial cells. PGD₂ may also function as a vasodilator—in particular as a dominant mediator of flushing induced by the lipid-lowering drug niacin—although the role of this prostanoid in the cardiovascular system remains under investigation.

Gastrointestinal Tract

Most of the prostaglandins and thromboxanes activate gastrointestinal smooth muscle. Longitudinal muscle is contracted by PGE2 (via EP₃) and PGF₂ (via FP), whereas circular muscle is contracted strongly by PGF₂ and weakly by PGI2, and is relaxed by PGE₂ (via EP₄). Administration of either PGE₂ or PGF₂ results in colicky cramps (see Clinical Pharmacology of Eicosanoids, below). The leukotrienes also have powerful contractile effects.

Airways

Respiratory smooth muscle is relaxed by PGE2 and PGI2 and contracted by PGD₂, TXA₂, and PGF₂. Studies of DP₁ and DP₂ knockout mice suggest an important role of this prostanoid in asthma, although contradictory findings in DP₂-deficient mice suggest significant complexity in the function of PGD₂ in airway inflammation. The cysteinyl leukotrienes are also bronchoconstrictors. They act principally on smooth muscle in peripheral airways and are a thousand times more potent than histamine, both in vitro and in vivo. They also stimulate bronchial mucus secretion and cause mucosal edema. Bronchospasm occurs in about 10% of people taking NSAIDs, possibly because of a shift in arachidonate metabolism from COX metabolism to leukotriene formation.

Reproductive

The actions of prostaglandins on reproductive smooth muscle are discussed below under section D, Reproductive Organs.

Platelets

Platelet aggregation is markedly affected by eicosanoids. Low concentrations of PGE2 enhance (via EP₃), whereas higher concentrations inhibit (via IP), platelet aggregation. Both PGD₂ and PGI2 inhibit aggregation via, respectively, DP₁- and IP-dependent elevation in cAMP generation. Unlike their human counterparts, mouse platelets do not express DP₁. TXA₂ is the major product of COX-1, the only COX isoform expressed in mature platelets. Itself a platelet aggregator, TXA₂ amplifies the effects of other, more potent, platelet agonists such as thrombin. The TP-G_q signaling pathway elevates intracellular Ca²⁺ and activates protein kinase C, facilitating platelet aggregation and TXA₂ biosynthesis. Activation of G₁₂/G₁₃ induces Rho/Rho-kinase-dependent regulation of myosin light chain phosphorylation leading to platelet shape change. A single point mutation in the human TP results in a mild bleeding disorder. The platelet actions of TXA₂ are restrained in vivo by PGI₂, which inhibits platelet aggregation by all recognized agonists. Platelet COX-1-derived TXA₂ biosynthesis is increased during platelet activation and aggregation and is irreversibly inhibited by chronic administration of aspirin at low doses. Urinary metabolites of TXA₂ increase in clinical syndromes of platelet activation such as myocardial infarction and stroke. Macrophage COX-2 appears to contribute roughly 10% of the increment in TXA₂ biosynthesis observed in smokers, while the rest is derived from platelet COX-1. A variable contribution, presumably from macrophage COX-2, may be insensitive to the effects of low-dose aspirin. Comparative trials of the cardioprotective actions of low- and high-dose aspirin have not been performed. However, indirect comparisons across placebo-controlled trials do not suggest an increasing benefit with dose; in fact, they suggest an inverse dose-response relationship, perhaps reflecting increasing inhibition of PGI₂ synthesis at higher doses of aspirin.

Kidney

Both the medulla and the cortex of the kidney synthesize prostaglandins, the medulla substantially more than the cortex. COX-1 is expressed mainly in cortical and medullary collecting ducts and mesangial cells, arteriolar endothelium, and epithelial cells of Bowman's capsule. COX-2 is restricted to the renal medullary interstitial cells, the macula densa, and the cortical thick ascending limb.

The major renal eicosanoid products are PGE2 and PGI2, followed by PGF₂ and TXA₂. The kidney also synthesizes several hydroxyeicosatetraenoic acids, leukotrienes, cytochrome P450 products, and epoxides. Prostaglandins play important roles in maintaining blood pressure and regulating renal function, particularly in marginally functioning kidneys and volume-contracted states. Under these circumstances, renal cortical COX-2-derived PGE₂ and PGI₂ maintain renal blood flow and glomerular filtration rate through their local vasodilating effects. These prostaglandins also modulate systemic blood pressure through regulation of water and sodium excretion. Expression of medullary COX-2 and mPGES-1 is increased under conditions of high salt intake. COX-2-derived prostanoids increase medullary blood flow and inhibit tubular sodium reabsorption, while COX-1-derived products promote salt excretion in the collecting ducts. Increased water clearance probably results from an attenuation of the action of antidiuretic hormone (ADH) on adenylyl cyclase. Loss of these effects may underlie the systemic or salt-sensitive hypertension often associated with COX inhibition. A common misperception—often articulated in discussion of the cardiovascular toxicity of drugs such as rofecoxib—is that hypertension secondary to NSAID administration is somehow independent of the inhibition of prostaglandins. Loop diuretics, eg, furosemide, produce some of their effect by stimulating COX activity. In the normal kidney, this increases the synthesis of the vasodilator prostaglandins. Therefore, patient response to a loop diuretic is diminished if a COX inhibitor is administered concurrently (see Chapter 15).

There is an additional layer of complexity associated with the effects of renal prostaglandins. In contrast to the medullary enzyme, cortical COX-2 expression is increased by low salt intake, leading to increased renin release. This elevates glomerular filtration rate and contributes to enhanced sodium reabsorption and a rise in blood pressure. PGE2 is thought to stimulate renin release through activation of EP₄ or EP₂. PGI2 can also stimulate renin release and this may be relevant to maintenance of blood pressure in volume-contracted conditions and to the pathogenesis of renovascularhypertension. Inhibition of COX-2 may reduce blood pressure in these settings.

TXA₂ causes intrarenal vasoconstriction (and perhaps an ADH-like effect), resulting in a decline in renal function. The normal kidney synthesizes only small amounts of TXA₂. However, in renal conditions involving inflammatory cell infiltration (such as glomerulonephritis and renal transplant rejection), the inflammatory cells (monocyte-macrophages) release substantial amounts of TXA₂. Theoretically, TXA₂ synthase inhibitors or receptor antagonists should improve renal function in these patients, but no such drug is clinically available. Hypertension is associated with increased TXA₂ and decreased PGE2 and PGI2 synthesis in some animal models, eg, the Goldblatt kidney model. It is not known whether these changes are primary contributing factors or secondary responses. Similarly, increased TXA₂ formation has been reported in cyclosporine-induced nephrotoxicity, but no causal relationship has been established.

Reproductive Organs

Female Reproductive Organs

Animal studies demonstrate a role for PGE2 and PGF₂ in early reproductive processes such as ovulation, luteolysis, and fertilization. Uterine muscle is contracted by PGF₂, TXA₂, and low concentrations of PGE₂; PGI2 and high concentrations of PGE₂ cause relaxation. PGF₂, together with oxytocin, is essential for the onset of parturition. The effects of prostaglandins on uterine function are discussed below (see Clinical Pharmacology of Eicosanoids).

Male Reproductive Organs

Despite the discovery of prostaglandins in seminal fluid, and their uterotropic effects, the role of prostaglandins in semen is still conjectural. The major source of these prostaglandins is the seminal vesicle; the prostate, despite the name "prostaglandin," and the testes synthesize only small amounts. The factors that regulate the concentration of prostaglandins in human seminal plasma are not known in detail, but testosterone does promote prostaglandin production. Thromboxane and leukotrienes have not been found in seminal plasma. Men with a low seminal fluid concentration of prostaglandins are relatively infertile.

Smooth muscle–relaxing prostaglandins such as PGE1 enhance penile erection by relaxing the smooth muscle of the corpora cavernosa (see Clinical Pharmacology of Eicosanoids).

Central and Peripheral Nervous Systems

Fever

PGE2 increases body temperature, predominantly via EP₃, although EP₁ also plays a role, especially when administered directly into the cerebral ventricles. Exogenous PGF₂ and PGI2 induce fever, whereas PGD₂ and TXA₂ do not. Endogenous pyrogens release interleukin-1, which in turn promotes the synthesis and release of PGE₂. This synthesis is blocked by aspirin and other antipyretic compounds.

Sleep

When infused into the cerebral ventricles, PGD₂ induces natural sleep (as determined by electroencephalographic analysis) via activation of DP₁ receptors and secondary release of adenosine. PGE2 infusion into the posterior hypothalamus causes wakefulness.

Neurotransmission

PGE compounds inhibit the release of norepinephrine from postganglionic sympathetic nerve endings. Moreover, NSAIDs increase norepinephrine release in vivo, suggesting that the prostaglandins play a physiologic role in this process. Thus, vasoconstriction observed during treatment with COX inhibitors may be due, in part, to increased release of norepinephrine as well as to inhibition of the endothelial synthesis of the vasodilators PGE2 and PGI2. PGE₂ and PGI₂ sensitize the peripheral nerve endings to painful stimuli by increasing their terminal membrane excitability. Prostaglandins also modulate pain centrally. Both COX-1 and COX-2 are expressed in the spinal cord and release prostaglandins in response to peripheral pain stimuli. PGE₂, and perhaps also PGD₂, PGI₂, and PGF₂, contribute to so-called central sensitization, an increase in excitability of spinal dorsal horn neurons, that augments pain intensity, widens the area of pain perception, and results in pain from innocuous stimuli.

Inflammation and Immunity

PGE2 and PGI2 are the predominant prostanoids associated with inflammation. Both markedly enhance edema formation and leukocyte infiltration by promoting blood flow in the inflamed region. PGE₂ and PGI₂, through activation of EP₂ and IP, respectively, increase vascular permeability and leukocyte infiltration. Through its action as a platelet agonist, TXA₂ can also increase platelet-leukocyte interactions. Although probably not made by lymphocytes, prostaglandins may contribute positively or negatively to lymphocyte function. PGE₂ suppresses the immunologic response by inhibiting differentiation of B lymphocytes into antibody-secreting plasma cells, thus depressing the humoral antibody response. It also inhibits mitogen-stimulated proliferation of T lymphocytes and the release of cytokines by sensitized TH1 lymphocytes. PGE₂ and TXA₂ may also play a role in T-lymphocyte development by regulating apoptosis of immature thymocytes. PGD₂, a major product of mast cells, is a potent chemoattractant for eosinophils in which it also induces degranulation and leukotriene biosynthesis. PGD₂ also induces chemotaxis and migration of TH2 lymphocytes mainly via activation of DP₂ although a role for DP₁ has also been established. It remains unclear how these two receptors coordinate the actions of PGD₂ in inflammation and immunity. A degradation product of PGD₂, 15d-PGJ₂, at concentrations actually formed in vivo, may also activate eosinophils via the DP₂ (CRTH₂) receptor.

Bone Metabolism

Prostaglandins are abundant in skeletal tissue and are produced by osteoblasts and adjacent hematopoietic cells. The major effect of prostaglandins (especially PGE2, acting on EP₄) in vivo is to increase bone turnover, ie, stimulation of bone resorption and formation. EP₄ receptor deletion in mice results in an imbalance between bone resorption and formation, leading to a negative balance of bone mass and density in older animals. Prostaglandins may mediate the effects of mechanical forces on bones and changes in bone during inflammation. EP₄-receptor deletion and inhibition of prostaglandin biosynthesis have both been associated with impaired fracture healing in animal models. COX inhibitors can also slow skeletal muscle healing by interfering with prostaglandin effects on myocyte proliferation, differentiation, and fibrosis in response to injury. Prostaglandins may contribute to the bone loss that occurs at menopause; it has been speculated that NSAIDs may be of therapeutic value in osteoporosis and bone loss prevention in older women. However, controlled evaluation of such therapeutic interventions remains to be carried out. NSAIDs, especially those specific for inhibition of COX-2, delay bone healing in experimental models of fracture.

Eye

PGE and PGF derivatives lower intraocular pressure. The mechanism of this action is unclear but probably involves increased outflow of aqueous humor from the anterior chamber via the uveoscleral pathway (see Clinical Pharmacology of Eicosanoids).

Cancer

There has been significant interest in the role of prostaglandins, and in particular the COX-2 pathway, in the development of malignancies. Pharmacologic inhibition or genetic deletion of COX-2 restrains tumor formation in models of colon, breast, lung, and other cancers. Large human epidemiologic studies have found that the incidental use of NSAIDs is associated with significant reductions in relative risk for developing these and other cancers. In patients with familial polyposis coli, COX inhibitors significantly decrease polyp formation. Polymorphisms in COX-2 have been associated with increased risk of some cancers. Several studies have suggested that COX-2 expression is associated with markers of tumor progression in breast cancer. In mouse mammary tissue, COX-2 is pro-oncogenic whereas NSAID use is associated with a reduced risk of breast cancer in women, especially for hormone receptor-positive tumors. Despite the support for COX-2 as the predominant source of pro-oncogenic prostaglandins, randomized clinical trials have not been performed to determine whether superior anti-oncogenic effects occur with selective inhibition of COX-2, compared with nonselective NSAIDs. Indeed data from animal models and epidemiologic studies in humans are consistent with a role for COX-1 as well as COX-2 in the production of pro-oncogenic prostanoids.

PGE2, which is considered the principal pro-oncogenic prostanoid, facilitates tumor initiation, progression, and metastasis through multiple biologic effects, increasing proliferation and angiogenesis, inhibiting apoptosis, augmenting cellular invasiveness, and modulating immunosuppression. The pro- and anti-oncogenic roles of other prostanoids remain under investigation, with TXA₂ emerging as another likely procarcinogenic mediator, deriving either from macrophage COX-2 or platelet COX-1. Studies in mice lacking EP₁, EP₂, or EP₄ receptors confirm reduced disease in multiple carcinogenesis models. EP₃, in contrast, plays no role or may even play a protective role in some cancers. Transactivation of epidermal growth factor receptor (EGFR) has been linked with the pro-oncogenic activity of PGE₂.

Effects of Lipoxygenase & Cytochrome P450-Derived Metabolites

The actions of lipoxygenases generate compounds that can regulate specific cellular responses important in inflammation and immunity. Cytochrome P450-derived metabolites affect nephron transport functions either directly or via metabolism to active compounds (see below). The biologic functions of the various forms of hydroxy- and hydroperoxyeicosaenoic acids are largely unknown, but their pharmacologic potency is impressive.

Blood Cells and Inflammation

LTB₄, acting at the BLT₁, is a potent chemoattractant for T lymphocytes, eosinophils, monocytes, and possibly mast cells; the cysteinyl leukotrienes are potent chemoattractants for eosinophils and T lymphocytes. Cysteinyl leukotrienes may also generate distinct sets of cytokines through activation of mast cell cysLT₁ and cysLT₂. At higher concentrations, these leukotrienes also promote eosinophil adherence, degranulation, cytokine or chemokine release, and oxygen radical formation. Cysteinyl leukotrienes also contribute to inflammation by increasing endothelial permeability, thus promoting migration of inflammatory cells to the site of inflammation. The leukotrienes have been strongly implicated in the pathogenesis of inflammation, especially in chronic diseases such as asthma and inflammatory bowel disease.

Lipoxins have diverse effects on leukocytes, including activation of monocytes and macrophages and inhibition of neutrophil, eosinophil, and lymphocyte activation. Both lipoxin A and lipoxin B inhibit natural killer cell cytotoxicity.

Heart and Smooth Muscle

Cardiovascular

12(S)-HETE promotes vascular smooth muscle cell proliferation and migration at low concentrations; it may play a role in myointimal proliferation that occurs after vascular injury such as that caused by angioplasty. Its stereoisomer, 12(R)-HETE, is not a chemoattractant, but is a potent inhibitor of the Na⁺,K⁺ ATPase in the cornea. LTC₄ and LTD₄ reduce myocardial contractility and coronary blood flow, leading to cardiac depression. Lipoxin A and lipoxin B exert coronary vasoconstrictor effects in vitro.

Gastrointestinal

Human colonic epithelial cells synthesize LTB₄, a chemoattractant for neutrophils. The colonic mucosa of patients with inflammatory bowel disease contains substantially increased amounts of LTB₄.

Airways

The cysteinyl leukotrienes, particularly LTC₄ and LTD₄, are potent bronchoconstrictors and cause increased microvascular permeability, plasma exudation, and mucus secretion in the airways. Controversies exist over whether the pattern and specificity of the leukotriene receptors differ in animal models and humans. LTC₄-specific receptors have not been found in human lung tissue, whereas both high- and low-affinity LTD₄ receptors are present.

Renal System

There is substantial evidence for a role of the epoxygenase products in regulating renal function although their exact role in the human kidney remains unclear. Both 20-HETE and the EETs are generated in renal tissue. 20-HETE, which potently blocks the smooth muscle cell Ca²⁺-activated K⁺ channel and leads to vasoconstriction of the renal arteries, has been implicated in the pathogenesis of hypertension. In contrast, studies support an antihypertensive effect of the EETs because of their vasodilating and natriuretic actions. Inhibitors of soluble epoxide hydrolase, which prolong the biologic activities of the EETs, have been developed as potential new antihypertensive drugs. In vitro studies, and work in animal models, support targeting soluble epoxide hydrolase for blood pressure control.

Miscellaneous

The effects of these products on the reproductive organs remain to be elucidated. Similarly, actions on the nervous system have been suggested but not confirmed. 12-HETE stimulates the release of aldosterone from the adrenal cortex and mediates a portion of the aldosterone release stimulated by angiotensin II but not that by adrenocorticotropic hormone. Very low concentrations of LTC₄ increase and higher concentrations of arachidonate-derived epoxides augment luteinizing hormone (LH) and LH-releasing hormone release from isolated rat anterior pituitary cells.

Inhibition of Eicosanoid Synthesis

Corticosteroids block all the known pathways of eicosanoid synthesis, perhaps in part by stimulating the synthesis of several inhibitory proteins collectively called annexins or lipocortins. They inhibit phospholipase A₂ activity, probably by interfering with phospholipid binding, thus preventing the release of arachidonic acid.

The NSAIDs (eg, indomethacin, ibuprofen; see Chapter 36) block both prostaglandin and thromboxane formation by reversibly inhibiting COX activity. The traditional NSAIDs are not selective for COX-1 or COX-2. Selective COX-2 inhibitors, which were developed more recently, vary—as do the older drugs—in their degree of selectivity. Indeed, there is considerable variability between (and within) individuals in the selectivity attained by the same dose of the same NSAID. Aspirin is an irreversible COX inhibitor. In platelets, which are anuclear, COX-1 (the only isoform expressed in mature platelets) cannot be restored via protein biosynthesis, resulting in extended inhibition of TXA₂ biosynthesis.

EP-receptor agonists and antagonists are under evaluation in the treatment of bone fracture and osteoporosis, whereas TP-receptor antagonists are being investigated for usefulness in treatment of cardiovascular syndromes. Direct inhibition of PGE2 biosynthesis through selective inhibition of the inducible mPGES-1 isoform is also under examination for potential therapeutic efficacy in pain and inflammation, cardiovascular disease, and chemoprevention of cancer.

Although they remain less effective than inhaled corticosteroids, a 5-LOX inhibitor (zileuton) and selective antagonists of the CysLT1 receptor for leukotrienes (zafirlukast, montelukast, and pranlukast; see Chapter 20) are used clinically in mild to moderate asthma. Growing evidence for a role of the leukotrienes in cardiovascular disease has expanded the potential clinical applications of leukotriene modifiers. Conflicting data have been reported in animal studies depending on the disease model used and the molecular target (5-LOX versus FLAP). Human genetic studies have demonstrated a link between cardiovascular disease and polymorphisms in the leukotriene biosynthetic enzymes, in particular FLAP, in some populations.

NSAIDs usually do not inhibit lipoxygenase activity at concentrations that inhibit COX activity. In fact, by preventing arachidonic acid conversion via the COX pathway, NSAIDs may cause more substrate to be metabolized through the lipoxygenase pathways, leading to an increased formation of the inflammatory leukotrienes. Even among the COX-dependent pathways, inhibiting the synthesis of one derivative may increase the synthesis of an enzymatically related product. Therefore, drugs that inhibit both COX and lipoxygenase are being developed.

Clinical Pharmacology of Eicosanoids

Several approaches have been used in the clinical application of eicosanoids. First, stable oral or parenteral long-acting analogs of the naturally occurring prostaglandins have been developed. Several such compounds have been approved for clinical use overseas and are being introduced in the USA (Figure 18–5). Second, enzyme inhibitors and receptor antagonists have been developed to interfere with the synthesis or effects of the eicosanoids. The discovery of COX-2 as a major source of inflammatory prostanoids led to the development of selective COX-2 inhibitors in an effort to preserve the gastrointestinal and renal functions directed through COX-1, thereby reducing toxicity. However, it is apparent that the marked decrease in biosynthesis of PGI2 that follows COX-2 inhibition occurring without a concurrent inhibition of platelet COX-1-derived TXA₂, removes a protective constraint on endogenous mediators of cardiovascular dysfunction and leads to an increase in cardiovascular events in patients taking selective COX-2 inhibitors. Third, efforts at dietary manipulation—to change the polyunsaturated fatty acid precursors in the cell membrane phospholipids and so change eicosanoid synthesis—is used extensively in over-the-counter products and in diets emphasizing increased consumption of cold-water fish.

Female Reproductive System

Studies with knockout mice have confirmed a role for prostaglandins in reproduction and parturition. COX-1-derived PGF₂ appears important for luteolysis, consistent with delayed parturition in COX-1-deficient mice. A complex interplay between PGF₂ and oxytocin is critical to the onset of labor. EP₂ receptor-deficient mice demonstrate a preimplantation defect, which underlies some of the breeding difficulties seen in COX-2 knockouts.

Abortion

PGE2 and PGF₂ have potent oxytocic actions. The ability of the E and F prostaglandins and their analogs to terminate pregnancy at any stage by promoting uterine contractions has been adapted to common clinical use. Many studies worldwide have established that prostaglandin administration efficiently terminates pregnancy. The drugs are used for first- and second-trimester abortion and for priming or ripening the cervix before abortion. These prostaglandins appear to soften the cervix by increasing proteoglycan content and changing the biophysical properties of collagen.

Dinoprostone, a synthetic preparation of PGE2, is administered vaginally for oxytocic use. In the USA, it is approved for inducing abortion in the second trimester of pregnancy, for missed abortion, for benign hydatidiform mole, and for ripening of the cervix for induction of labor in patients at or near term.

Dinoprostone stimulates the contraction of the uterus throughout pregnancy. As the pregnancy progresses, the uterus increases its contractile response, and the contractile effect of oxytocin is potentiated as well. Dinoprostone also directly affects the collagenase of the cervix, resulting in softening. The vaginal dose enters the maternal circulation, and a small amount is absorbed directly by the uterus via the cervix and the lymphatic system. Dinoprostone is metabolized in local tissues and on the first pass through the lungs (about 95%). The metabolites are mainly excreted in the urine. The plasma half-life is 2.5–5 minutes.

For the induction of labor, dinoprostone is used either as a gel (0.5 mg PGE2) or as a controlled-release formulation (10 mg PGE₂) that releases PGE₂ in vivo at a rate of about 0.3 mg/h over 12 hours. An advantage of the controlled-release formulation is a lower incidence of gastrointestinal side effects (< 1%).

For abortifacient purposes, the recommended dosage is a 20-mg dinoprostone vaginal suppository repeated at 3- to 5-hour intervals depending on the response of the uterus. The mean time to abortion is 17 hours, but in more than 25% of cases the abortion is incomplete and requires additional intervention.

For softening of the cervix at term, the preparations used are either a single vaginal insert containing 10 mg PGE2 or a vaginal gel containing 0.5 mg PGE₂ administered every 6 hours. The softening of the cervix for induction of labor substantially shortens the time to onset of labor and the delivery time.

Antiprogestins (eg, mifepristone) have been combined with an oral oxytocic synthetic analog of PGE1 (misoprostol) to produce early abortion. This regimen is available in the USA and Europe (see Chapter 39). The ease of use and the effectiveness of the combination have aroused considerable opposition in some quarters. The major toxicities are cramping pain and diarrhea. The oral and vaginal routes of administration are equally effective, but the vaginal route has been associated with an increased incidence of sepsis, so the oral route is now recommended.

An analog of PGF₂ is also used in obstetrics. This drug, carboprost tromethamine (15-methyl-PGF₂; the 15-methyl group prolongs the duration of action) is used to induce second-trimester abortions and to control postpartum hemorrhage that is not responding to conventional methods of management. The success rate is approximately 80%. It is administered as a single 250-mcg intramuscular injection, repeated if necessary. Vomiting and diarrhea occur commonly, probably because of gastrointestinal smooth muscle stimulation. In some patients transient bronchoconstriction can occur. Transient elevations in temperature are seen in approximately one eighth of patients.

Facilitation of Labor

Numerous studies have shown that PGE2, PGF₂, and their analogs effectively initiate and stimulate labor, but PGF₂ is one tenth as potent as PGE₂. There appears to be no difference in the efficacy of PGE₂ and PGF₂ when they are administered intravenously; however, the most common usage is local application of PGE₂ analogs (dinoprostone) to promote labor through ripening of the cervix. These agents and oxytocin have similar success rates and comparable induction-to-delivery intervals. The adverse effects of the prostaglandins are moderate, with a slightly higher incidence of nausea, vomiting, and diarrhea than that produced by oxytocin. PGF₂ has more gastrointestinal toxicity than PGE₂. Neither drug has significant maternal cardiovascular toxicity in the recommended doses. In fact, PGE₂ must be infused at a rate about 20 times faster than that used for induction of labor to decrease blood pressure and increase heart rate. PGF₂ is a bronchoconstrictor and should be used with caution in women with asthma; however, neither asthma attacks nor bronchoconstriction have been observed during the induction of labor. Although both PGE₂ and PGF₂ pass the fetoplacental barrier, fetal toxicity is uncommon.

The effects of oral PGE2 administration (0.5–1.5 mg/h) have been compared with those of intravenous oxytocin and oral demoxytocin, an oxytocin derivative, in the induction of labor. Oral PGE₂ is superior to the oral oxytocin derivative and in most studies is as efficient as intravenous oxytocin. Oral PGF₂ causes too much gastrointestinal toxicity to be useful by this route.

Theoretically, PGE2 and PGF₂ should be superior to oxytocin for inducing labor in women with preeclampsia-eclampsia or cardiac and renal diseases because, unlike oxytocin, they have no antidiuretic effect. In addition, PGE₂ has natriuretic effects. However, the clinical benefits of these effects have not been documented. In cases of intrauterine fetal death, the prostaglandins alone or with oxytocin seem to cause delivery effectively.

Dysmenorrhea

Primary dysmenorrhea is attributable to increased endometrial synthesis of PGE2 and PGF₂ during menstruation, with contractions of the uterus that lead to ischemic pain. NSAIDs successfully inhibit the formation of these prostaglandins (see Chapter 36) and so relieve dysmenorrhea in 75–85% of cases. Some of these drugs are available over the counter. Aspirin is also effective in dysmenorrhea, but because it has low potency and is quickly hydrolyzed, large doses and frequent administration are necessary. In addition, the acetylation of platelet COX, causing irreversible inhibition of platelet TXA₂ synthesis, may increase the amount of menstrual bleeding.

Male Reproductive System

Intracavernosal injection or urethral suppository therapy with alprostadil (PGE1) is a second-line treatment for erectile dysfunction. Doses of 2.5–25 mcg are used. Penile pain is a frequent side effect, which may be related to the algesic effects of PGE derivatives; however, only a few patients discontinue the use because of pain. Prolonged erection and priapism are side effects that occur in less than 4% of patients and are minimized by careful titration to the minimal effective dose. When given by injection, alprostadil may be used as monotherapy or in combination with either papaverine or phentolamine.

Renal System

Increased biosynthesis of prostaglandins has been associated with one form of Bartter's syndrome. This is a rare disease characterized by low-to-normal blood pressure, decreased sensitivity to angiotensin, hyperreninemia, hyperaldosteronism, and excessive loss of K⁺. There also is an increased excretion of prostaglandins, especially PGE metabolites, in the urine. After long-term administration of COX inhibitors, sensitivity to angiotensin, plasma renin values, and the concentration of aldosterone in plasma return to normal. Although plasma K⁺ rises, it remains low, and urinary wasting of K⁺ persists. Whether an increase in prostaglandin biosynthesis is the cause of Bartter's syndrome or a reflection of a more basic physiologic defect is not yet known.

Cardiovascular System

The vasodilator effects of PGE compounds have been studied extensively in hypertensive patients. These compounds also promote sodium diuresis. Practical application will require derivatives with oral activity, longer half-lives, and fewer adverse effects.

Pulmonary Hypertension

PGI2 lowers peripheral, pulmonary, and coronary resistance. It has been used to treat both primary pulmonary hypertension and secondary pulmonary hypertension, which sometimes occurs after mitral valve surgery. In addition, prostacyclin has been used successfully to treat portopulmonary hypertension, which arises secondary to liver disease. The first commercial preparation of PGI₂ (epoprostenol) approved for treatment of primary pulmonary hypertension improves symptoms, prolongs survival, and delays or prevents the need for lung or lung-heart transplantation. Side effects include flushing, headache, hypotension, nausea, and diarrhea. The extremely short plasma half-life (3–5 minutes) of epoprostenol necessitates continuous intravenous infusion through a central line for long-term treatment, which is its greatest limitation. Several prostacyclin analogs with longer half-lives have been developed and used clinically. Iloprost (half-life about 30 minutes), is usually inhaled six to nine times per day although it has been delivered by intravenous administration outside the USA. Treprostinil (half-life about 4 hours) may be delivered by subcutaneous or intravenous infusion.

Peripheral Vascular Disease

A number of studies have investigated the use of PGE1 and PGI2 compounds in Raynaud's phenomenon and peripheral arterial disease. However, these studies are mostly small and uncontrolled, and these therapies do not have an established place in the treatment of this disease.

Patent Ductus Arteriosus

Patency of the fetal ductus arteriosus depends on COX-2–derived PGE2 acting on the EP₄ receptor. At birth, reduced PGE₂ levels, a consequence of increased PGE₂ metabolism, allow ductus arteriosus closure. In certain types of congenital heart disease (eg, transposition of the great arteries, pulmonary atresia, pulmonary artery stenosis), it is important to maintain the patency of the neonate's ductus arteriosus before corrective surgery. This can be achieved with alprostadil (PGE1). Like PGE₂, PGE₁ is a vasodilator and an inhibitor of platelet aggregation, and it contracts uterine and intestinal smooth muscle. Adverse effects include apnea, bradycardia, hypotension, and hyperpyrexia. Because of rapid pulmonary clearance (the half-life is about 5–10 minutes in healthy adults and neonates), the drug must be continuously infused at an initial rate of 0.05–0.1 mcg/kg/min, which may be increased to 0.4 mcg/kg/min. Prolonged treatment has been associated with ductal fragility and rupture.

In delayed closure of the ductus arteriosus, COX inhibitors are often used to inhibit synthesis of PGE2 and so close the ductus. Premature infants in whom respiratory distress develops due to failure of ductus closure can be treated with a high degree of success with indomethacin. This treatment often precludes the need for surgical closure of the ductus.

Blood

As noted above, eicosanoids are involved in thrombosis because TXA₂ promotes platelet aggregation while PGI2, and perhaps also PGE2 and PGD₂, are platelet antagonists. Chronic administration of low-dose aspirin (81 mg/d) selectively and irreversibly inhibits platelet COX-1 without modifying the activity of systemic COX-1 or COX-2 (see Chapter 34). Because their effects are reversible within the typical dosing interval, nonselective NSAIDs (eg, ibuprofen) do not reproduce this effect. TXA₂, in addition to activating platelets, amplifies the response to other platelet agonists; hence inhibition of its synthesis inhibits secondary aggregation of platelets induced by ADP, by low concentrations of thrombin and collagen and by epinephrine. Not surprisingly, selective COX-2 inhibitors do not alter platelet TXA₂ biosynthesis and are not platelet inhibitors. However, COX-2-derived PGI₂ generation is substantially suppressed during selective COX-2 inhibition removing a restraint on the cardiovascular action of TXA₂, and other platelet agonists. It is highly likely that selective depression of PGI₂ generation contributes to the increased thrombotic events in humans treated with selective COX-2 inhibitors.

Overview analyses have shown that low-dose aspirin reduces the secondary incidence of heart attack and stroke by about 25%. However, it elevates the low risk of serious gastrointestinal bleeding about twofold over placebo. Low-dose aspirin also reduces the incidence of first myocardial infarction. However, in this case, the benefit versus risk of gastrointestinal bleeding is less clear. The effects of aspirin on platelet function are discussed in greater detail in Chapter 34.

Respiratory System

PGE2 is a powerful bronchodilator when given in aerosol form. Unfortunately, it also promotes coughing, and an analog that possesses only the bronchodilator properties has been difficult to obtain.

PGF₂ and TXA₂ are both strong bronchoconstrictors and were once thought to be primary mediators in asthma. Polymorphisms in the genes for PGD₂ synthase and the TP have been linked with asthma in humans, and deletion of DP₁ sharply reduces allergen-induced infiltration of lymphocytes and eosinophils and airway hyperreactivity. However, the cysteinyl leukotrienes—LTC₄, LTD₄, and LTE₄—probably dominate during asthmatic constriction of the airway. As described in Chapter 20, leukotriene-receptor inhibitors (eg, zafirlukast, montelukast) are effective in asthma. A lipoxygenase inhibitor (zileuton) has also been used in asthma but is not as popular as the receptor inhibitors. It remains unclear whether leukotrienes are partially responsible for acute respiratory distress syndrome.

Corticosteroids and cromolyn are also useful in asthma. Corticosteroids inhibit eicosanoid synthesis and thus limit the amounts of eicosanoid mediator available for release. Cromolyn appears to inhibit the release of eicosanoids and other mediators such as histamine and platelet-activating factor from mast cells.

Gastrointestinal System

The word "cytoprotection" was coined to signify the remarkable protective effect of the E prostaglandins against peptic ulcers in animals at doses that do not reduce acid secretion. Since then, numerous experimental and clinical investigations have shown that the PGE compounds and their analogs protect against peptic ulcers produced by either steroids or NSAIDs. Misoprostol is an orally active synthetic analog of PGE1. The FDA-approved indication is for prevention of NSAID-induced peptic ulcers. The drug is administered at a dosage of 200 mcg four times daily with food. This and other PGE analogs (eg, enprostil) are cytoprotective at low doses and inhibit gastric acid secretion at higher doses. Misoprostol use is low, probably because of its adverse effects including abdominal discomfort and occasional diarrhea. Dose-dependent bone pain and hyperostosis have been described in patients with liver disease who were given long-term PGE treatment.

Selective COX-2 inhibitors were developed in an effort to spare gastric COX-1 so that the natural cytoprotection by locally synthesized PGE2 and PGI2 is undisturbed (see Chapter 36). However, this benefit is seen only with highly selective inhibitors and may be offset by increased cardiovascular toxicity.

Immune System

Cells of the immune system contribute substantially to eicosanoid biosynthesis during an immune reaction. T and B lymphocytes are not primary synthetic sources; however, they may supply arachidonic acid to monocyte-macrophages for eicosanoid synthesis. In addition, there is evidence for eicosanoid-mediated cell-cell interaction by platelets, erythrocytes, leukocytes, and endothelial cells.

The eicosanoids modulate the effects of the immune system. PGE2 and PGI2 limit T-lymphocyte proliferation in vitro, as do corticosteroids. PGE₂ also inhibits B-lymphocyte differentiation, suppressing the immune response. T-cell clonal expansion is attenuated through inhibition of interleukin-1 and interleukin-2 and class II antigen expression by macrophages or other antigen-presenting cells. The leukotrienes, TXA₂, and platelet-activating factor stimulate T-cell clonal expansion. These compounds stimulate the formation of interleukin-1 and interleukin-2 as well as the expression of interleukin-2 receptors. The leukotrienes also promote interferon- release and can replace interleukin-2 as a stimulator of interferon-. PGD₂ induces chemotaxis and migration of TH2 lymphocytes. These in vitro effects of the eicosanoids agree with in vivo findings in animals with acute organ transplant rejection, as described below.

Cell-Mediated Organ Transplant Rejection

Acute organ transplant rejection is a cell-mediated immune response (see Chapter 56). Administration of PGI2 to renal transplant patients has reversed the rejection process in some cases. Experimental in vitro and in vivo data show that PGE2 and PGI₂ can attenuate T-cell proliferation and rejection, which can also be seen with drugs that inhibit TXA₂ and leukotriene formation. In organ transplant patients, urinary excretion of metabolites of TXA₂ increases during acute rejection. Corticosteroids, the first-line drugs used for treatment of acute rejection because of their lymphotoxic effects, inhibit both phospholipase and COX-2 activity.

Inflammation

Aspirin has been used to treat arthritis for approximately 100 years, but its mechanism of action—inhibition of COX activity—was not discovered until 1971. COX-2 appears to be the form of the enzyme most associated with cells involved in the inflammatory process although, as outlined above, COX-1 also contributes significantly to prostaglandin biosynthesis during inflammation. Aspirin and other anti-inflammatory agents that inhibit COX are discussed in Chapter 36.

Rheumatoid Arthritis

In rheumatoid arthritis, immune complexes are deposited in the affected joints, causing an inflammatory response that is amplified by eicosanoids. Lymphocytes and macrophages accumulate in the synovium, whereas leukocytes localize mainly in the synovial fluid. The major eicosanoids produced by leukocytes are leukotrienes, which facilitate T-cell proliferation and act as chemoattractants. Human macrophages synthesize the COX products PGE2 and TXA₂ and large amounts of leukotrienes.

Infection

The relationship of eicosanoids to infection is not well defined. The association between the use of the anti-inflammatory steroids and increased risk of infection is well established. However, NSAIDs do not seem to alter patient responses to infection.

Glaucoma

Latanoprost, a stable long-acting PGF₂ derivative, was the first prostanoid used for glaucoma. The success of latanoprost has stimulated development of similar prostanoids with ocular hypotensive effects, and bimatoprost, travoprost, and unoprostone are now available. These drugs act at the FP receptor and are administered as drops into the conjunctival sac once or twice daily. Adverse effects include irreversible brown pigmentation of the iris and eyelashes, drying of the eyes, and conjunctivitis.

Dietary Manipulation of Arachidonic Acid Metabolism

Because arachidonic acid is derived from dietary linoleic and -linolenic acids, which are essential fatty acids, the effects of dietary manipulation on arachidonic acid metabolism have been extensively studied. Two approaches have been used. The first adds corn, safflower, and sunflower oils, which contain linoleic acid (C18:2), to the diet. The second approach adds oils containing eicosapentaenoic (C20:5) and docosahexaenoic acids (C22:6), so-called omega-3 fatty acids, from cold-water fish. Both types of diet change the phospholipid composition of cell membranes by replacing arachidonic acid with the dietary fatty acids. Diets high in fish oils have been shown to impact ex vivo indices of platelet and leukocyte function, blood pressure, and triglycerides with different dose-response relationships. There is an abundance of epidemiologic data relating diets high in fatty fish to a reduction in the incidence of myocardial infarction and sudden cardiac death although there is more ambiguity about stroke. Of course, epidemiologic data may confound such diets with a reduction in saturated fats and other elements of a "healthy" lifestyle. In addition, some data from prospective randomized trials suggest that such dietary interventions may reduce the incidence of sudden death. Experiments in vitro suggest that fish oils protect against experimentally induced arrhythmogenesis, aggregation, vasomotor spasm, and cholesterol metabolism.

Preparations Available

Nonsteroidal anti-inflammatory drugs are listed in Chapter 36.

References