Pharmacology Ch 42 740-764
Pharmacology of Eicosanoids
-Autocoids – substances that are synthesized rapidly in response to specific stimuli and act in
immediate environment before degradation shortly after
-Eicosanoids – autocoids derived from arachidonic acid metabolism, and they are involved in
inflammatory, neoplastic, and cardiovascular physiology and pathology
Generation of Arachidonic Acid and Omega-3 Fatty Acid
-Arachidonic acid is synthesized from linoleic acid, which can only be obtained through diet
-Omega-3 fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are
precursors to resolvins, protectins, and maresins
-can be obtained from diet or biotransformed from a-linoleic acid
-omega-3 fatty acids have double bond between 3rd and 4th carbons from the terminus
-phospholipase A2 releases arachidonic acid from cellular phospholipids, and is the RATELIMITING step in generation of eicosanoids
-various isoforms of PLA2 exist and are stimulated by cytokines and growth factors.
-glucocorticoids inhibit PLA2 through lipocortins such as annexin 1, which mediates
anti-inflammatory actions of glucocorticoids
Cyclooxygenase Pathway – generates prostaglandins, prostacyclin, and thromboxanes
-(lipoxygenase pathway forms leukotrienes and lipoxins, and epoxygenase pathway leads to
epoxyeicosatetraenoic acids)
-Cyclooxygenases are membrane bound heme enzymes that come in two isoforms: COX-1 and
COX-2, which are similar but play various roles in two sequential reactions
1. cycolooxygenase step converting AA  prostaglandin G2 (PGG2)
2. peroxidase step converts PGG2 to PGH2
-COX1 and COX2 produce different eicosanoid products
-COX1 is constitutively expressed and is used in physiologic housekeeping: vascular homeostasis,
renal and GI blood flow, renal function, intestinal mucosal proliferation, platelet function, and
antithrombogenesis
-COX2 is only induced for specialized functions such as inflammation, pain, fever, mitogenesis,
renal adaptation to stress, trabecular bone, ovulation, placentation
-COX2 constitutive in some parts of brain like hippocampus, hypothalamus, amygdala
Prostaglandins – come from a 20 carbon structure called a prostanoid and are divided into PG1,
PG2, and PG3, with the number indicating how many double bonds in the molecule
-PG2 most prevalent because they are direct derivatives of arachidonic acid and derive from
eicosatetraenoic acid
-PG1 derives from arachidonic acid precursor DHGLA (eicosatrienoic acid), while PG3 derives
from eicosapentaenoic acid
-PGH2 is the critical juncture of cyclooxygenase pathway, giving rise to PGD2, PGE2, PGF2a,
thromboxane A2 (TxA2), and prostacyclin (PGI2)
-PGD2 – secreted by mast cells/neurons, function as bronchoconstrictors, sleep
-PGE2 – pain, vasodilation, bronchoconstriction, cytoprotective (shielded from
ischemia), fever, mucus, inflammation (PGE2 by COX2 in brain induces fever)
-PGF2a – vascular tone, reproductive, bronchoconstriction
Thromboxane and Prostacyclin – platelets express thromboxane synthase but not prostacyclin
synthase
-TxA2 is chief eicosanoid of platelets, and only lives 10-20 seconds, acts as a vasoconstrictor and
promoter of platelet adhesion and aggregation
-PGI2 is expressed on vascular endothelium, which does not express thromboxane synthase, and
functions to vasodilate, venodilate, and inhibit platelet aggregation
-PGI2 is the antagonist of TxA2
-balance between TxA2 and PGI2 regulates blood pressure and thrombogenesis
-imbalances lead to hypertension, ischemia, thrombosis, coagulopathy, MI, stroke
-EPA is converted to TxA3 and PGI3, but TxA3 is a weak vasoconstrictor
Lipoxygenase Pathway – second fate of arachidonic acid which leads to leukotriene and lipoxin
-5-lipoxygenase, 12-lipoxygenase, and 15-lipoxygenase (5-LOX) are most important
-immediate products of lipoxygenases are HPETEs, which are reduced to HETES by glutathione
peroxidase
-5-LOX  5-HPETE  leukotrine A4 (LTA4)  all other leukotrienes
-15-LOX  15-HETE  Lipoxins
-5-LOX requires translocation to nuclear membrane for activity, helped by FLAP protein
Leukotrienes – 5-LOX converts AA  5-HPETE and also 5-HPETE  LTA4
-LTA4 is converted to either LTB4 or LTC4
-LTA4 (LTA4 hydrolase)  LTB4 in neutrophils/RBC, and LTC4 in other leukocytes
-LTC, D, E, and F4 are cysteinyl leukotrienes
1. 5-LOX – produces 5-HPETE/5-HETE, LTA4, and epoxytetraene (neutrophils, macrophages, etc)
2. 12-LOX – produces 12-HPETE/12-HETE, epoxytetraene (platelets, tumors, skin, macrophages)
3. 15-LOX – 15-HPETE/15-HETE, lipoxins
-LTB4 acts via receptors BLT1 (host defense and inflammation, leading to proinflammatory
chemotaxis, aggregation), regulates neutrophil lysosome function and creates ROS, enhances
cytokine production
-BLT2 binds COX product 12-HHT and evokes chemotaxis of leukocytes
-LTC4 and LTD4 bind CysLT1 receptors and cause vasoconstriction, bronchospasm, and vascular
permeability
Lipoxins – derivatives of AA containing 4 double bonds and 3 –OH groups
-LXA4 and LXB4 are the main lipoxins and modulate leukotrienes and cytokines and are
important in resolution of inflammation
-at sites of inflammation, inverse amounts of lipoxin and leukotriene present, meaning lipoxin
are negative regulators of leukotrienes
-stop neutrophil chemotaxis, adhesion, and diapedesis by decreasing P-selectin, limit eosinophil
recruitment, stimulates vasodilation (through PGI2 and PGE2), inhibit LTC4 and LTD4
vasoconstriction, inhibit LTB4 inflammatory effects
-stimulate uptake and clearance of apoptotic neutrophils by macrophages
-imbalance of lipoxin-leukotriene balance can lead to inflammatory disease
-Local mediators such as resolvins, protectins, and maresins generally limit neutrophil
recruitment
-Resolvins – regulate neutrophil infiltration, dendritic cell function and IL-12 production,
promotes resolution, reduces colitis, protects from osteoclast-mediated bone destruction
-Protectin – regulates neutrophil and T cell infiltration, regulates TNF and IFN production,
promotes resolution, reduces peritonitis and airway inflammation, protects brain from
ischemia/reperfusion injury, mitigates kidney ischemia injury
-Maresin – regulates neutrophil infiltration and promotes resolution
Epoxygenase Pathway – cytochrome P450 epoxygenases oxygenate AA to form EET and
hydroxyacid derivatives
-important in tissues that do not express COX or LOX like cells in kidneys
-regulate vascular tone by inhibiting Na+/K+ ATPase in vascular smooth muscle, and may affect
renal absorption
Isoprostanes – AA is susceptible to free radical peroxidation and release of modified lipids by
PLA2 gives rise to isoprostanes
-found in blood during oxidative stress, can be vasoconstrictors and activate NF-kB,
phospholipase C, protein kinase C, and Ca flux
Metabolic Inactivation of Local Eicosanoids – eicosanoids are turned off by hydroxylation, Boxidation, and omega-oxidation that make them more hydrophilic and urine-excretable
Integrated Inflammation Schema – leukotrienes, lipoxins, thromboxanes, prostaglandins,
prostacyclins are all critical in mediating inflammation
-damagecytokine cascadeCOX2 levels increaseincrease levels of eicosanoids
-local PGE2, LTB4, and cysteinyl leukotrienes promote accumulation and infiltration of
inflammatory cells by increasing blood flow and vascular permeability
-LTB4 and 5-HETE are important in attracting neutrophils
-Transcellular biosynthetic routes show eicosanoid intermediates donated from one cell type to
another to generate larger diversity of mediators
Asthma – Chronic inflammation, hyper-reactivity, constriction, obstruction
-antigens in lungs stimulate generation of prostaglandin PGD2 and leukotrienes
-LTB4 attracts inflammatory cells and promotes aggregation particularly in B cells
-LTB4 also promotes expression of IgE receptors on mast cells and basophils
-LTC4 and LTD4 bronchoconstric the lungs and cause airway to secrete mucus
Inflammatory Bowel Disease – Crohn’s and ulcerative colitis are idiopathic, but show elevated
LTB4 production in mucosa, resulting in abnormal leukocyte infiltration into parenchyma
Rheumatoid Arthritis – autoimmune inflammatory disease that affects the joints, skin,
cardiovascular system, lungs, and muscles
-local increase in cytokines, TNF, growth factors, interleukins, all of which induce COX-2
-Levels of COX-2 and PGE2 are elevated in synovial fluid of affected joints
-PGE2 stimulates pain pathways and other COX2 derived eicosanoids and 5-LOX leukotrienes
activate surrounding endothelium to recruit inflammatory cells
-Macrophages release collagenase and protease and lymphocyte activity leads to immune
complex formation
Glomerulonephritis – inflammatory renal condition where local complement activation
promotes neutrophil and macrophage infiltration
-abnormal levels of LTB4, and LTA4 promotes LTC4 and LTD4.
-LTC4 and LTD4 decrease renal blood flow and glomerular filtration rate by vasoconstriction
Cancer – Chronic NSAID therapy decreases risk for colorectal cancer
-colorectal carcinomas express excess COX-2 believed to generate PGE2 and other eicosanoids
to promote tumor growth
-some eicosanoids can bind retinoic acid receptor RXR which is involved in regulating cell growth
and differentiation
-overexpressing COX-2 could generate eicosanoids that can flood XR signaling to provide
excessive growth stimuli
Cardiovascular Disease – TxA2 mediates thrombosis in acute coronary syndromes and other
cardiovascular disease
-aspirin is a COX inhibitor that is an antiplatelet agent in prophylaxis and treatment of these
diseases
Phospholipase inhibitors – inhibition of phospholipase A2 prevents release of AA from cell
phospholipids, the rate limiting step in eicosanoid synthesis, limiting inflammation
-glucocorticoids such as prednisone, prednisolone, and dexamethasone are therapy in
autoimmune and inflammatory diseases
-function by inducing lipocortins which interfere with action of PLA2 to inhibit AA
availability
-annexins induced by glucocorticoids and block inflammatory responses and enhance
endogenous anti-inflammatory mechanisms (such as lipoxin A4)
-glucocorticoids inhibit COX-2 gene expression, cytokine release, and limiting pool of
COX-2 substrate
Cyclooxygenase Inhibitors – most common are NSAIDs and acetaminophen
NSAIDs – have combined anti-inflammatory, antipyretic, and analgesic properties.
-goal is inhibit COX-mediated generation of proinflammatory eicosanoids limiting inflammation
-antipyretic activity due to decreasing levels of PGE2 in brain around hypothalamus
-NSAIDs alleviate the signs of inflammatory response but do not resolve the response
-All NSAIDs are reversible COX inhibitors except for aspirin, preventing conversion of AA to PGG2
-chronic NSAID use can lead to NSAID-induced gastropathy, including dyspepsia, gastrotoxicity,
subepithelial damage and hemorrhage, gastric mucosal erosion, ulceration, and necrosis
-NSAIDs are organic acids, and are completely absorbed by the gut, binding to plasma albumin,
accumulation at sites of inflammation, and renal excretion
-NSAIDs are divided into short (<6 hours) and long (>10 hours) half-life classes (naproxen,
salicylate, piroxicam, and phenylbutazone are LONG half-lives)
Salicylates – include ASPIRIN and its derivatives, used to treat pain, headache, myalgia, and
arthralgia
-aspirin is irreversible by acetylating active site serine on both COX1 and 2
-inhibition of the COX1 prevents formation of prostaglandins, thromboxanes and prostacyclin
-Salicylates may also inhibit neutrophil oxidative burst by reducing NADPH activity
-low-dose aspirin daily is used as antithrombotic as well as prophylaxis of acute coronary
syndromes and ischemic stroke
-antithrombotic because it prevents COX from synthesizing TxA2 for platelets,
irreversibly inhibited for 10 days
-endothelial cells can still synthesize PGI2 and a single dose of aspirin decreases amount of
thromboxane that can be generated, shifting vascular TxA2-PGI2 balance toward PGI2 mediated
vasodilation, platelet inhibition, and antithrombogenesis
-aspirin inhibits COX-2 generation of prostaglandins, but COX-2 can still form a stereoisomer of
15-HETE called 15-(R)-HETE from AA
-5-LOX convertes 15-(R)-HETE to 15-epi-lipoxin (stable isomer of lipoxin) called aspirintriggered lipoxins (ATLs), which mimic lipoxin function as anti-inflammatory agents
-long term aspirin therapy leads to GI ulceration, hemorrhage, nephrotoxicity, hepatic injury
-Aspirin-induced airway hyperreactivity and Reye’s Syndrome are unique toxicities of aspirin
-Reye’s Syndrome – hepatic encephalopathy and liver steatosis in young children, arises from
aspirin use during a viral infection
Propionic Acid Derivatives – ibuprofen (potent analgesic in rheumatoid arthritis, osteoarthritis,
akylosing spondylitis, gout, primary dysmenorrhea), naproxen (long plasma half life, directly
inhibits leukocytes, less severe GI effects), ketoprofen, flurbiprofen
Acetic Acid Derivatives – indomethacin, sulindac, etodolac, diclofenac, ketorolac
-besides inhibiting COX, many of these NSAIDs promote incorporating AA into triglyceride to
reduce availability of substrate for COX and LOX
-Indomethacin is a direct inhibitor of neutrophil motility, not tolerated as well as
ibuprofen
-Diclofenac reduces intracellular arachidonic acid by altering fatty acid transport and is
used in treatment of pain associated with renal stones
-Ketorolac is employed for strong analgesic properties in postsurgical patients (no more
than 3-5 days allowed use)
-Acetic acid derivatives used for treatment of arthritis and other musculoskeletal disorders
-can cause GI ulceration, hepatitis, and jaundice
Oxicam Derivatives – Piroxicam treats rheumatoid arthritis and osteoarthritis, but may be
better tolerated than aspirin, naproxen, and ibuprofen
-additional effect of in modulation of neutrophil function by inhibiting collagenase,
proteoglycanase, and the oxidative burst
-piroxicam displays GI ulceration, prolongs bleeding time
Fenamate Derivatives – Mefenamate and Meclofenamate, both inhibit COX but also antagonize
prostanoid receptors and have less anti-inflammatory activity/more toxic, little point of use
Ketone NSAIDs – Nabumetone ketone prodrug that is oxidized to active form in-vivo and
prefers inhibiting COX-2
Acetaminophen – sometimes classified as an NSAID, but it is not – it has analgesic and
antipyretic effects like aspirin, but has insignificant anti-inflammatory effect because of weak
COX inhibition – given to children, but can cause hepatotoxicity
COX-2 Inhibitors – hypothesized that selective COX2 inhibition could eliminate chemical
mediators of inflammation while maintaining cytoprotective products of COX1
COX2 Selective Inhibitors – Celecoxib, rofecoxib, valdecoxib, and meloxicam
-all are sulfonic acid derivatives that are selective for COX2, and have anti-inflammatory,
antipyretic, and analgesic properties without antiplatelet actions of COX1 inhibitors
-inhibition of COX2 may generate problems in wound healing, angiogenesis, and resolution of
inflammation
-Celecoxib is the only FDA approved COX2 sleective inhibitor for osteoarthritis, rheumatoid
arthritis, ankylosing spondylitis, acute pain, and primary dysmenorrhea
-also approved to reduce adenomatous colorectal polyps in people with familial
adenomatous polyposis, hopefully preventing colon cancer
-celecoxib decreases activity of peroxisome proliferator activated receptor PPAR, a
transcription factor that heterodimerizes with RXR factors for growth regulation
-risk of hypertension, edema, heart failure, stroke, MI, contraindicated for bypass
Cytokine Inhibitors – TNF and IL-1 enhance prostaglandin production and upregulate COX2, so
inhibiting these cytokines may inhibit initiation of COX2 mediated inflammatory response
-Etanercept, infliximab, adalimumab, golimumab, and certolizumab are antibody based TNF-a
antagonists
-first approved for treatment of rheumatoid arthritis, these drugs halt joint destruction and
bone erosion, decrease pain, calm swollen and tender joints, and limit disease progression
-increased risk of serious infections including extrapulmonary tuberculosis, fungal infections,
hep B reactivation; also risk of lymphoma, demyelinating disease, heart failure
-Lipoxins, ATLs block TNF-a to provide new approach: Anakinra – is a recombinant form of IL-1
receptorapproved for rheumatoid arthritis who have failed other agents and antagonizes TNF
Thromboxane Antagonists – TxA2 antagonists and thromboxane synthase inhibitors could be
powerful antiplatelet agents protecting against thrombosis and vascular disease
-Dazoxibem and primagrel inhibit thromboxane synthase, and ridogrel is a TxA2
receptor antagonist
Leukotriene Inhibition
1. Lipoxygenase Inhibition – potential to treat diseases involving leukotrienes, such as asthma,
IBD, and rheumatoid arthritis
-drugs that impair lipoxygenase from using nonheme iron would inhibit enzyme, such as
zileuton,a benzothiophene derivative of N-hydroxyurea that inhibits 5-LOX by chelating
its nonheme iron
-in asthma, Zileuton induces bronchodilation, improves symptoms and
generates lifelong improvement
2. 5-LOX Activating Protein (FLAP) Inhibition – interfering with role of FLAP could inhibit 5-LOX
activity and leukotriene function
-5-LOX translocates to nuclear membrane and docks with FLAP which binds to AA
released by PLA2 and shuttles 5-LOX to active site
-No FLAP inhibitors are available
3. Leukotriene Synthesis Inhibitors – no inhibitors of enzyme involved in leukotriene synthesis
are available
-adenosine, acting on neutrophils, inhibits LTB4 synthesis by regulating AA release, and
could be used to control inflammation
4. Leukotriene Receptor Antagonists – inhibit leukotriene-mediated bronchoconstriction, and
cysteinyl leukotriene receptor (CysLT1) antagonists are effective against asthma induced by
antigen, exercise, cold, or aspirin
-Montelukase and Zafirlukast are the only available cysteinyl leukotriene receptor
antagonists to treat asthma