1. Autacoids and related drugs Course: Pharmacology I Course Code: PHR 213 Course Instructor: Md. Samiul Alam Rajib Senior Lecturer Department of Pharmacy BRAC University

2. Autacoids Autacoids are chemical agents/ substances which are produced in our body and show their important physiological functions in other organs or tissues in our body and thus helps to protect our body in some adverse situation. The functions of autacoids are: Microcirculation Drug reaction Allergic reaction Inflammation Anaphylaxis-severe allergic reaction The autacoids are histamine, serotonins, bradykinins, prostaglandins, Substance- P, SRS-A, SRS-C etc. These are some autacoids present in our body.

3. Histamine Role of Histamine in inflammation or allergic reaction: Histamine causes vasodilation and enhances capillary permeability causing loss of plasma to the extracellular fluid resulting in edema formation. Histamine causes migration of WBC and macrophages to the inflammed area, stimulate phagocytosis process and thus increases production of inflammatory mediator substances. Histamine also increases migration of inflammatory cells to other organs or tissue in our body. Histamine potentiates the pain producing capacity of Bradykinin Histamine helps in the release of proteolytic and hydrolytic enzymes from lysosome.

4. Location, synthesis and release of Histamine Location:  All tissues, especially lungs, tissue, blood vessels (smooth muscle) and GI tracts.  High concentration at Mast cells and Basophils  Functions as a neurotransmitter in the brain.  Component of venom and insect stings. Synthesis:

5. Location, synthesis and release of Histamine (Cont.) Release:  Released in response to stimuli (Allergens and another chemical stimuli)  Due to destruction of cells because of cold, toxins from organisms, venoms of insects, trauma  Can be leased in large amounts due to allergens and anaphylaxis.

6. Pharmacological properties of histamine Effects on Cardiovascular system: Histamine decreases cardiac function by stimulating vagal and thus resulted effects are: Decrease autorythmicity Decrease conductivity Decrease excitability Decrease force of contraction of cardiac muscle Decrease heart rate Decrease cardiac output Decrease blood pressure Histamine causes vasodilation. So total peripheral resistance is decreased resulting in fall of BP. BP=Total peripheral resistance (TPR)*Cardiac output (CO) Effects on Glands: Histamine stimulates secretion from gastric gland, pancreatic gland, intestinal gland, sweat gland and other exocrine glands including bile secretion and bronchial secretion.

7. Pharmacological properties of histamine (cont.) Effects on smooth muscle: Histamine causes contraction of GI smooth muscle, Bronchial smooth muscle of urinary bladder & others except vascular smooth muscle. Effect on skin: The effects of histamine on skin is called triple response which is characterized by: Local red spot due to capillary dilatation and increased local blood flow. Surrounding flash and flare: it occurs due to dilatation of arteriole (blood vessels) locally (due to nerve reflex activity) resulting in red spot with irregular shape surrounding the red spot. Itching and pain occurs due to allergic reaction of histamin and stimulation of cutanous nerve endings. Edema formation: occurs due to loss of plasma to the extracellular fluids.

8. Therapeutic indication of histamine Diagnosis of pernicious anaemia Diagnosis of acid secreting capacity of gastric gland Diagnosis/ test of hyperactivity of bronchial in asthma patients Diagnosis of phaechromo cytoma Contraindication of histamine Asthma patients Peptic ulcer disease patients Patients with GI bleeding Hypotensive patients Ischaemic heart disease or Angina Pectoris patients

9. Histamine receptors: There are 3 types of histamine receptors present in our body. They are- H1 receptors H2 receptors H3 receptors H4 receptors Distribution of receptors: H1 receptors are present in GIT, bronchial smooth muscle, uterine smooth muscle, urinary bladder, blood vessels and adrenal medulla. H2 receptors are present in cardiac muscle, gastric gland, some blood vessels. H3 receptors are present in brain, spinal cord, liver, spleen, GIT, lungs and some blood vessels. H 4 is highly expressed in bone marrow and white blood cells and regulates neutrophil release from bone marrow. It is also expressed in the colon, liver, lung, small intestine, spleen, testes, thymus, tonsils, and trachea.

10. Antihistaminic drugs Antihistaminic drugs are the agents which can interfere with the pharmacological properties of histamine. Antihistaminic drugs can competitively bind to the histaminic receptors and thus can produce their antihistaminic pharmacological properties.

11. Mechanism of action of H1 receptor blockers Displaces histamine from the H1 receptor, which is a G-protein coupled receptor Histamine leads to formation of IP3 and a release of stored Ca++, followed by a cascade of other events. (IP3/DAG pathway) H1 receptor blockade prevents this activity and leads to a decrease in Ca++ inside of the cell

12. Classification of antihistaminic drugs Broadly antihistaminic drugs can be classified into 3 types: 1. H1 blocker 2. H2 blocker 3. H3 blocker H1 blockers can be subclassified into 2 groups: 1st generation/ sedative groups: e.g. Promethazine, Diphenhydramine, Chloropheniramine 2nd generation/ non-sedative group: e.g. Cetirijine, Loratadine, Doxepine, Desloratadine H2 blockers: e.g. Cimetidine, Ranitidine, Famotidine, Nizatidine, Roxatidine, Loxatidine H3 blockers: e.g. Thioperamide Lunder clinical trial Betahistine is a selective H3 agonist used in Menicres disease but contraindicated in ulcer asthma

13. First Generation Antihistamines Small, lipophilic molecules that could cross the BBB Not specific to the H1 receptor Groups: Ethylenediamines Ethanolamines Alkylamines Piperazines Tricyclics Common structural features of classical antihistamine 2 Aromatic rings Connected to a central Carbon, Nitrogen or CO Spacer between the central X and the amine Usually 2-3 carbons in length Linear, ring, branched, saturated or unsaturated Amine is substituted with small alkyl groups eg CH3

14. Ethylenediamines These were the first group of clinically effective H 1 -antihistamines Mepyramine (Pyrilamine) http://en.wikipedia.org/wiki/Mepyramine

15. Ethanolamines This class has significant anticholinergic side effects and sedation, however reduced the gastroinestnal side effects Diphenhydramine (Benedryl) Oldest and most effective antihistamine on the market Available over the counter Because it induces sedation, it’s used in nonprescription sleep aids such as Tylenol PM Also inhibits the reuptake of serotonin, which led to the search for viable antidepressants with similar structures (prozac) http://en.wikipedia.org/wiki/Image:Diphenhydramine_Structure.png

16. Ethanolamines Carbinoxamine(Clistine)Doxylamine succinate 2nd in effectiveness of anti-allergy activity only to Benadryl Active ingredient in NyQuil Potent anti-cholinergic effects http://en.wikipedia.org/wiki/Image:Doxylamine.png Is used to treat Hay fever and is especially popular to children due its its mild taste After 21 reported deaths in children under 2, its now only marketed to children above 3 (FDA, June 2006)

17. Ethanolamines Clemastine (Tavist) Dimenhydrinate (Dramamine) Exhibits fewer side effects than most antihistamines Widely used as an antiprurtic (stops itching) http://redpoll.pharmacy.ualberta.ca/drugbank/ cgi-bin/getCard.cgi?CARD=APRD00875.txt Anti-emetic (anti nausea) Also causes strong sedation Readily crosses the BBB http://redpoll.pharmacy.ualberta.ca/drugbank/cgi-bin/getCard.cgi?CARD=APRD00924.txt

18. Alkylamines Isomerism is an important factor in this class of drugs, which is due to the positioning and fit of the molecules in the H1-receptor binding site These drugs have fewer sedative and GI adverse effects, but a greater incidence of CNS stimulation These drugs lack the “spacer molecule” (which is usually a nitrogen or oxygen) between the two aromatic rings and at least one of the rings has nitrogen included in the aromatic system

19. Akylamines Chlorphenamine Brompheniramine (Dimetapp) Originally used to prevent allergic conditions Shown to have antidepressant properties and inhibit the reuptake of serotonin The first SSRI was made as a derivative of chlorphenamine Available over the counter Used to treat the common cold by relieving runny nose, itchy, watery eyes and sneezing http://redpoll.pharmacy.ualberta.ca/drugbank/drugBank/PC_IMAGE/APRD00832_ZOOM.gif

20. Akylamines Triprolidine hydrochloridePheniramine (Avil) Used to alleviate the symptoms associated with allergies Can be combined with other cold medicine to relieve “flu-like” symptoms http://en.wikipedia.org/wiki/Image:Triprolidine.svg Used most often to treat hay fever or urticaria (hives) Antihistamine component of Visine-A http://www.chiralpure.com/Figures/pheniramine.jpg

21. Piperazines Structurally related to the ethylenediamines and the ethanolamines and thus produce significant anti-cholinergic effects Used most often to treat motion sickness, vertigo, nausea and vomiting Used to treat the symptoms associated with motion sickness, vertigo and post-op following administration of general anaesthesia and opiods Mechanism of inhibiting motion sickness is not well understood, but it may act on the labyrinthine apparatus and the chemoreceptor trigger zone (area of the brain which receives input and induces vomiting) http://en.wikipedia.org/wiki/Image:Cyclizine.svg Cyclizine

22. Piperazines ChlorcyclizineHydroxyzine http://en.wikipedia.org/wiki/Image:Chlorcyclizine.pnghttp://en.wikipedia.org/wiki/Image:Hydroxyzine.png In addition to treating itches and irritations, its an anitemetic, a weak analgesic and an anxiolytic (treat anxiety) This drug is used to treat motion sickness

23. Piperazines MeclizineCetirizine (Zyrtec) http://redpoll.pharmacy.ualberta.ca/drugbank/drugBank/PC_IMAGE/APRD00354_ZOOM.gif It is most commonly used to inhibit nausea and vomiting as well as vertigo, however it does cause drowsiness This drug treats indoor and outdoor allergies and is safe to use in children as young as 2

24. Tricyclics These drugs are structurally related to tricyclic antidepressants, which explains why they have cholinergic side effects Promethazine (Phenegran) This drug has extremely strong anticholinergic and sedative effects It was originally used as an antipsychotic, however now it is most commonly used as a sedative or antinausea drug (also severe morning sickness) and requires a prescription http://upload.wikimedia.org/wikipedia/commons/c/c9/Promethazine.png

25. Tricyclics CyproheptadineKetotifen (Zaditor) http://en.wikipedia.org/wiki/Image:Cyproheptadine.png This drug both an antihistamine and an antiserotonergic agent It is a 5-HT2 receptor antagonist and also blocks calcium channels Used to treat hay fever and also to stimulate appetite in people with anorexia It is also rarely used to treat SSRI induced sexual dysfunction and also Cushing’s Syndrome (high level of cortisol in the blood) and migraine headaches This drug is available in two forms: an ophthalmic form used to treat allergic conjunctivitis or itchy red eyes and an oral form used to prevent asthma attacks It has several adverse side effects including drowsiness, weight gain, dry mouth, irritability and increased nosebleeds http://en.wikipedia.org/wiki/Image:Ketotifen.png

26. Tricyclics Alimemazine (Vallergan) Azatadine (Optimine or Trinalin) http://www.genome.jp/Fig/compound/C07172.gif This drug is used to treat itchiness and hives that results from allergies Since it causes drowsiness, it is useful for rashes that itch worse at night time It is also used to sedate young children before operations http://www.genome.jp/Fig/compound/C07774.gif This drug is used to treat symptoms of allergies and the common cold such as sneezing, runny nose, itchy watery eyes, itching, hives and rashes

27. Second Generation Antihistamines  These are the newer drugs and they are much more selective for the peripheral H1-receptors involved in allergies as opposed to the H1- receptors in the CNS  Therefore, these drugs provide the same relief with many fewer adverse side effects  The structure of these drugs varies and there are no common structural features associated with them  They are however bulkier and less lipophilic than the first generation drugs, therefore they do not cross the BBB as readily  Recent studies have also showed that these drugs also have anti- inflammatory activity and therefore, would be helpful in the management of inflammation in allergic airways disease (Devalia and Davies).

28. Second generation H 1 -receptor antagonists Acrivastine (Semprex-D) Astemizole (Hismantol) http://en.wikipedia.org/wiki/Image:Acrivastine.svghttp://en.wikipedia.org/wiki/Image:Astemizole.png This drug has a long duration of action It suppresses the formation of edema and puritus It doesn’t cross the BBB It has been taken off the market in most countries because of adverse interactions with erythromycin and grapefruit juice This drug relieves itchy rashes and hives It is non-sedating because it does not cross the BBB

29. Second generation H 1 -receptor antagonists Loratadine (Claritin)Terfenadine (Seldane) It is the only drug of its class available over the counter It has long lasting effects and does not cause drowsiness because it does not cross the BBB http://en.wikipedia.org/wiki/Image:Loratadin.svghttp://scienceblogs.com/moleculeoftheday/images/terfenadine.gif It was formerly used to treat allergic conditions In the 1990’s it was removed from the market due to the increased risk of cardiac arrythmias

30. Second generation H 1 -receptor antagonists Azelastine (Astelin or Optivar) Levocabastine (Livostin) http://en.wikipedia.org/wiki/Image:Azelastine.png It is a mast cell stablilizer Available as a nasal spray (Astelin) or eye drops for pink eye (Optivar) http://en.wikipedia.org/wiki/Image:Levocabastine.png Both of these drugs are used as eye drops to treat allergic conjunctivitis Olopatadine (Patanol)

31. Third generation H 1 -receptor antagonists These drugs are derived from second generation antihistamines They are either the active enantiomer or metabolite of the second generation drug designed to have increased efficacy and fewer side effects Levocetirizine (Zyzal) http://en.wikipedia.org/wiki/Image:Levocetirizine.png This drug is the active enantiomer of cetirizine and is believed to be more effective and have fewer adverse side effects. Also it is not metabolized and is likely to be safer than other drugs due to a lack of possible drug interactions (Tillement). It does not cross the BBB and does not cause significant drowsiness It has been shown to reduce asthma attacks by 70% in children

32. Third generation H 1 -receptor antagonists Deslortadine (Clarinex) Fexofenadine (Allegra) http://en.wikipedia.org/wiki/Image:Desloratadine.png It is the active metabolite of Lortadine Even though it is thought to be more effective, there is no concrete evidence to prove this http://en.wikipedia.org/wiki/Image:Fexofenadine_Structure.png It was developed as an alternative to Terfenadine Fexofenadine was proven to be more effective and safe

33. “Next” Generation Antihistamines Metabolite derivatives or active enantiomers of existing drugs Safer, faster acting or more potent than Second Generation drugs Examples: Fexofenadine Desloratadine Levocetirizine

34. The future of antihistamines  The anti-inflammatory activity of second generation antihistamines, about which little is known, will continue to be researched and possibly lead to an effective alternative to corticosteriods in the treatment of allergic airways conditions.  The action of the H4 receptor will also continue to be researched and will possibly lead to effective treatment of autoimmune dieseases.  Creating antihistamines with higher selectivity and less adverse side effects will continue to be the goal

35. Therapeutic indications of H1 blockers Cough Common cold Allergic disorder Motion sickness Parkinson’s disease Preanaesthetic medication Sedative/ Hypnotic agent Anaphylaxis Allergic conjunctivitis Allergic dermatitis Anxiolytic agent Eczema Allergic reactions due to drug or fruit.

36. Pharmacokinetics of H1 receptor antagonists  Well absorbed (from oral and parenteral routes)  Metabolized in liver and excreted in urine.  Penetrate BBB poorly (In 1 st generation antihistamines)  Duration of action is 4-6 hours except for meclizine, estimazole, loratidine, cetrizine and clemastine (12-24 hours).

37. Adverse Reactions and Side Effects First Generation Drugs: Anticholinergic CNS interactions Gastrointestinal reactions Common side effects: sedation, dizziness, tinnitus, blurred vision, euphoria, lack of coordination, anxiety, insomnia, tremor, nausea and vomiting, constipation, diarrhea, dry mouth, and dry cough Second Generation Drugs: Common side effects: drowsiness, fatigue, headache, nausea and dry mouth Side effects are far less common in Second Generation drugs

38. Therapeutic indications of H2 blockers: Peptic ulcer disease like gastric ulcer, deodenal ulcer, benign gastric ulcer, post operative ulcer etc. GERD (Gastroesophagul reflux disease) Zollinger-ellision syndrome Acute gastritis Prophylaxis of aspiration pneumonia

39. H2 receptor blockers Mechanism of action  They competitively and reversibly block to H2 receptors on the parietal cells thus reduce gastric secretion. They include: - Cimetidine - Ranitidine - Famotidine - Nizatidine

40. Pharmacokinetics Good oral absorption Plasma half life (1-3 h). Duration (4-12 h). First pass metabolism (50% Except Nizatidine 100 % bioavailability). Given before meals. Metabolized by liver. Excreted mainly in urine. Cross placenta & excreted in milk

42. Pharmacological actions: Inhibit histamine, gastrin, cholinergic drug - induced secretions. Reduce basal and food-stimulated gastric secretion. Reduce pepsin activity. Promote mucosal healing & decrease pain

43. USES: Duodenal Ulcer (6-8 weeks). Benign gastric ulcer (8-12 weeks). Reflux esophagitis Zollinger Ellison Syndrome (large doses). Pre-anesthetic medication (To prevent aspiration pneumonitis). Eradication of H. pylori infections.

44. Adverse Effects of H2 blockers: 1. GIT disturbances: nausea, vomiting 2. CNS effects: Headache, dizziness, confusion (elderly – renal or hepatic dysfunction). 3. CVS effects Bradycardia and hypotension (rapid I.V.)

45. Cimetidine has other adverse effects: 4. Endocrine effects  Antiandrogenic actions (gynecomasteia – impotence)  Galactorrhea in women. 5. Cytochrome P450 inhibitor: decrease metabolism of oral anticoagulant, phenytoin, benzodiazepines.

46. Precautions 1. Maintenance dose (Relapse may occur). 2. Dose reduction in severe renal or hepatic failure and elderly.

47. Eicosanoid Synthesis

48. Prostaglandins and related compounds are collectively known as eicosanoids. Most are produced from arachidonic acid, a 20- carbon polyunsaturated fatty acid (5,8,11,14- eicosatetraenoic acid). The eicosanoids are considered "local hormones."  They have specific effects on target cells close to their site of formation.  They are rapidly degraded, so they are not transported to distal sites within the body. But in addition to participating in intercellular signaling, there is evidence for involvement of eicosanoids in intracellular signal cascades.

49.  Examples of eicosanoids: prostaglandins prostacyclins thromboxanes leukotrienes epoxyeicosatrienoic acids.  They have roles in: inflammation fever regulation of blood pressure blood clotting immune system modulation control of reproductive processes & tissue growth regulation of sleep/wake cycle.

50. PGE 2 (prostaglandin E 2 ) is an example of a prostaglandin, produced from arachidonic acid.

51. PGE 2 (prostaglandin E 2 ). Prostaglandins all have a cyclopentane ring.  A letter code is based on ring modifications (e.g., hydroxyl or keto groups).  A subscript refers to the number of double bonds in the two side-chains. Thromboxanes are similar but have instead a 6- member ring.

52. Prostaglandin receptors:  Prostaglandins & related compounds are transported out of the cells that synthesize them.  Most affect other cells by interacting with plasma membrane G-protein coupled receptors. Depending on the cell type, the activated G-protein may stimulate or inhibit formation of cAMP, or may activate a phosphatidylinositol signal pathway leading to intracellular Ca ++ release.  Another prostaglandin receptor, designated PPAR , is related to a family of nuclear receptors with transcription factor activity.

53.  Prostaglandin receptors are specified by the same letter code. E.g., receptors for E-class prostaglandins are EP. Thromboxane receptors are designated TP.  Multiple receptors for a prostaglandin are specified by subscripts (E.g., EP 1, EP 2, EP 3, etc.). Different receptors for a particular prostaglandin may activate different signal cascades. Effects of a particular prostaglandin may vary in different tissues, depending on which receptors are expressed. E.g., in different cells PGE 2 may activate either stimulatory or inhibitory or G-proteins, leading to either increase or decrease in cAMP formation.

54. Arachidonate is released from phospholipids by hydrolysis catalyzed by Phospholipase A 2. This enzyme hydrolyzes the ester linkage between a fatty acid and the OH at C2 of the glycerol backbone, releasing the fatty acid & a lysophospholipid as products. The fatty acid arachidonate is often esterified to OH on C2 of glycerophospho -lipids, especially phosphatidyl inositol.

55. Corticosteroids are anti-inflammatory because they prevent inducible Phospholipase A 2 expression, reducing arachidonate release. There are multiple Phospholipase A 2 enzymes, subject to activation via different signal cascades.  The inflammatory signal platelet activating factor is involved in activating some Phospholipase A 2 variants.  Attempts have been made to develop drugs that inhibit particular isoforms of Phospholipase A2, for treating inflammatory diseases. Success has been limited by the diversity of Phospholipase A2 enzymes, and the fact that arachidonate may give rise to inflammatory or anti-inflammatory eicosanoids in different tissues.

56. After PI is phosphorylated to PIP 2, cleavage via Phospholipase C yields diacylglycerol (and IP 3 ). Arachidonate release from diacylglycerol is then catalyzed by Diacylglycerol Lipase. Phosphatidyl inositol signal cascades may lead to release of arachidonate.

57. Prostaglandin H 2 Synthase catalyzes the committed step in the “cyclic pathway” that leads to production of prostaglandins, prostacyclins, & thromboxanes. Different cell types convert PGH 2 to different compounds. Two major pathways of eicosanoid metabolism Cyclic pathway:

58. PGH 2 Synthase is a heme-containing dioxygenase, bound to ER membranes. (A dioxygenase incorporates O 2 into a substrate). PGH 2 Synthase exhibits 2 activities: cyclooxygenase & peroxidase.

59. PGH 2 Synthase (expressing both cyclooxygenase & peroxidase activities) is sometimes referred to as Cyclooxygenase, abbreviated COX. The interacting cyclooxygenase and peroxidase reaction pathways are complex.

60. A peroxide (such as that generated later in the reaction sequence) oxidizes the heme iron. The oxidized heme accepts an electron from a nearby tyrosine residue (Tyr385). The resulting tyrosine radical is proposed to extract a H atom from arachidonate, generating a radical species that reacts with O 2.

61. The signal molecule ·NO (nitric oxide) may initiate prostaglandin synthesis by reacting with superoxide anion (O 2 ·  ) to produce peroxynitrite, which oxidizes the heme iron enabling electron transfer from the active site tyrosine. Prostaglandin synthesis in response to some inflammatory stimuli is diminished by inhibitors of Nitric Oxide Synthase.

62. Arachidonate, derived from membrane lipids, approaches the heme via a hydrophobic channel extending from the membrane-binding domain. In the image above, the channel is occupied by an inhibitor, an ibuprofen analog. Membrane- binding domain: 4 short amphipathic  -helices that insert into one leaflet of the bilayer, facing the ER lumen.

63. Ibuprofen and related compounds block the hydrophobic channel by which arachidonate enters the cyclooxygenase active site. Non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin and derivatives of ibuprofen, inhibit cyclooxygenase activity of PGH 2 Synthase.  They inhibit formation of prostaglandins involved in fever, pain, & inflammation.  They inhibit blood clotting by blocking thromboxane formation in blood platelets.

64. Aspirin acetylates a serine hydroxyl group near the active site, preventing arachidonate binding. The inhibition by aspirin is irreversible. However, in most body cells re-synthesis of PGH 2 Synthase would restore cyclooxygenase activity.

65. Thromboxane A 2 stimulates blood platelet aggregation, essential to the role of platelets in blood clotting.  Many people take a daily aspirin for its anti-clotting effect, attributed to inhibition of thromboxane formation in blood platelets.  This effect of aspirin is long-lived because platelets lack a nucleus and do not make new enzyme.

66. Two isoforms of PGH 2 Synthase: COX-1 & COX-2 (Cyclooxygenase 1 & 2):  COX-1 is constitutively expressed at low levels in many cell types.  COX-2 expression is highly regulated. Transcription of the gene for COX-2 is stimulated by growth factors, cytokines, and endotoxins. COX-2 expression may be enhanced by cAMP, and in many cells PGE 2 produced as a result of COX-2 activity itself leads to changes in cAMP levels. Both catalyze PGH 2 formation, but differing localization within a cell, & localization of enzymes that convert PGH 2 into particular prostaglandins/ thromboxanes, may result in COX-1 & COX-2 yielding different ultimate products.

67.  COX-1 is essential for thromboxane formation in blood platelets, and for maintaining integrity of the gastrointestinal epithelium.  COX-2 levels increase in inflammatory diseases such as arthritis. Inflammation is associated with up-regulation of COX-2 & increased amounts of particular prostaglandins.  COX-2 expression is increased in some cancer cells. Angiogenesis (blood vessel development), which is essential to tumor growth, requires COX-2. Overexpression of COX-2 leads to increased expression of VEGF (vascular endothelial growth factor). Regular use of NSAIDs has been shown to decrease the risk of developing colorectal cancer.

68. Most NSAIDs inhibit both COX I & COX II. Selective COX-2 inhibitors have been developed, e.g., Celebrex and Vioxx.  COX-2 inhibitors are anti-inflammatory & block pain, but are less likely to cause gastric toxicity associated with chronic use of NSAIDs that block COX-1.  A tendency to develop blood clots when taking some of these drugs has been attributed to: decreased production of an anti-thrombotic (clot blocking) prostaglandin (PGI 2 ) by endothelial cells lining small blood vessels lack of inhibition of COX-1-mediated formation of pro-thrombotic thromboxanes in platelets.

69. Some evidence suggests the existence of a third isoform of PGH 2 Synthase, designated COX-3, with roles in mediating pain and fever, and subject to inhibition by acetaminophen (Tylenol). Acetaminophen has little effect on COX-1 or COX-2, and thus lacks anti-inflammatory activity. Explore the structure of PGH 2 Synthase-1 (COX-1) crystallized with bound iodosuprofen, a derivative of ibuprofen.

70. The 1st step of the Linear Pathway for synthesis of leukotrienes is catalyzed by Lipoxygenase. Mammals have a family of Lipoxygenase enzymes that catalyze oxygenation of various polyunsaturated fatty acid at different sites. Many of the products have signal roles.

71. E.g., 5- Lipoxygenase, found in leukocytes, catalyzes conversion of arachidonate to 5-HPETE (5-hydroperoxy- eicosatetraenoic acid). 5-HPETE is converted to leukotriene-A 4, which in turn may be converted to various other leukotrienes.

72. A non-heme iron is the prosthetic group of Lipoxygenase enzymes. Ligands to the Fe include 3 His N atoms & the C-terminal carboxylate O. The arachidonate substrate binds in a hydrophobic pocket, adjacent to the catalytic iron atom. O 2 is thought to approach from the opposite side of the substrate than the iron, for a stereospecific reaction.

73. Lipoxygenase reaction starts with extraction of H from arachidonate, with transfer of the e  to the iron, reducing it from Fe 3+  Fe 2+. The resulting fatty acid radical reacts with O 2 to form a hydroperoxy group. Which H is extracted, & the position of the hydroperoxy group, varies with different lipoxygenases (e.g., 5-Lipoxgenase shown here, 15-Lipoxygenase, etc.) Additional reactions then yield the various leukotrienes.

74. Leukotrienes have roles in inflammation. They are produced in areas of inflammation in blood vessel walls as part of the pathology of atherosclerosis. Leukotrienes are also implicated in asthmatic constriction of the bronchioles. Some leukotrienes act via specific G-protein coupled receptors (GPCRs) in the plasma membrane. Anti-asthma medications include:  inhibitors of 5-Lipoxygenase, e.g., Zyflo (zileuton)  drugs that block leukotriene-receptor interactions. E.g., Singulair (montelukast) & Accolate (zafirlukast) block binding of leukotrienes to their receptors on the plasma membranes of airway smooth muscle cells.

75. 5-Lipoxygenase requires the membrane protein FLAP (5-lipoxygenase-activating protein). FLAP binds arachidonate, facilitating its interaction with the enzyme. Translocation of 5-Lipoxygenase from the cytoplasm to the nucleus, and formation of a complex including 5-Lipoxygenase, FLAP, & Phospholipase A 2 in association with the nuclear envelope has been observed during activation of leukotriene synthesis in leukocytes.

76. A  -barrel domain at the N-terminus of Lipoxygenase enzymes may have a role in membrane binding. Explore the structure of Lipoxygenase, with a substrate analog present at the active site.

77. Cytochrome P450 epoxygenase pathways: Epoxyeicosatrienoic acids (EETs) and hydroxyeicosatrienoic acids are formed from arachidonate by enzymes of the cytochrome P 450 family. Other members of the cytochrome P 450 family participate in a variety of oxygenation reactions, including hydroxylation of sterols.

78. EETs are modified by additional enzyme-catalyzed reactions to produce many distinct compounds. They may be incorporated into phospholipids, and released by action of phospholipases. EETs have roles in regulating cellular proliferation, inflammation, peptide hormone secretion, & various signal pathways relevant to cardiovascular and renal functions. E.g., EETs inhibit apoptosis in endothelial cells. Shown is an EET produced from arachidonate by activity of a cytochrome P 450 epoxygenase. (14,15-epoxyeicosatrienoic acid)