PLATELET ACTIVATING FACTORS: MECHANISM OF ACTION AND CLINICAL USES

PLATELET ACTIVATING FACTORS: MECHANISM OF ACTION AND CLINICAL USES, INTRODUCTION

1. ASSIGNMENT PRESENTED TO Dr. TARIQUE MAHMOOD ASSOCIATE PROFESSOR & HEAD Mrs. ARSHIYA SHAMIM ASST. PROFESSOR PHARMACOLOGY - 1 PRY - 206 INTEGRAL UNIVERSITY , LUCKNOW PRESENTED BY VIKAS AAGRAHARI DOCTOR OF PHARMACY 7/30/2021 1

2. TOPIC Platelet activating factor: Mechanism of action & Clinical uses 7/30/2021 2

3. INTRODUCTION Platelet-activating factor (PAF) is a lipid mediator that is well-known for its ability to cause platelet aggregation, inflammation, and allergic response at very low concentrations (approaching picomolar). 7/30/2021 3

4. platelet Platelets  are tiny blood cells that help your body form clots to stop bleeding. If one of your blood vessels gets damaged, it sends out signals that are picked up by platelets . The  platelets  then rush to the site of damage and form a plug, or clot, to repair the damage. 7/30/2021 4

5. FUNCTION OF platelet 7/30/2021 5

6. MECHANISM OF ACTION In pharmacology, the term  mechanism of action  (MOA) refers to the specific biochemical interaction through which a drug substance produces its pharmacological effect. A mechanism of action  usually includes mention of the specific molecular targets to which the drug binds, such as an enzyme or receptor. 7/30/2021 6

7. CLINICAL USE relating to or based on work done with real patients : of or relating to the medical treatment that is given to patients in hospitals, clinics, etc. : requiring treatment as a medical problem. : of or relating to a place where medical treatment is given : of or relating to a clinic. 7/30/2021 7

8. What is platelet activating factor..?? Platelet-activating factor is a potent biological mediator that exerts its effects in a variety of cells and tissues. Platelet-activating factor, also known as PAF, PAF- acether or AGEPC (acetyl- glyceryl -ether- phosphorylcholine ), is a potent phospholipid activator and mediator of many leukocyte functions, including platelet aggregation and degranulation , inflammation, and anaphylaxis. 7/30/2021 8

9. It is also involved in changes to vascular permeability, the oxidative burst, chemotaxis of leukocytes, as well as augmentation of arachidonic acid metabolism in phagocytes. PAF is produced by a variety of cells, but especially those involved in host defense , such as , platelets, endothelial cells, neutrophils,monocytes , and macrophages. 7/30/2021 9

10. Discovery of PAF. In 1970, during a study of immunological mechanisms involving histamine and serotonin release from platelets in immunized rabbits, Henson proposed that 'a soluble factor' was released from leukocytes which stimulated platelets to release vasoactive amines. This observation was confirmed independently by Siraganian and Osler in 1971 7/30/2021 10

11. In 1972, Benveniste , Henson and Cochrane demonstrated that the antibody involved in the immunological reaction described was an IgE class antibody and coined the term 'platelet-activating factor (PAF)' for the soluble factor released from basophils following IgE stimulation. However, it was not until 1979 that Demopoulos, Pinckard and Hanahan demonstrated that a semisynthetic phosphoacylglycerol , l-O-alkyl-2-acetyl-snglycero-3-phosphocholine (AGEPC), had physiochemical as well as biological properties (i.e. aggregation of platelets and secretion of serotonin) indistinguishable from those of naturally-generated PAF 7/30/2021 11

12. Synthesis of PAF The biosynthesis of PAF via remodeling of membrane phospholipid 7/30/2021 12

13. The biosynthesis of PAF via  de novo  synthesis  7/30/2021 13

14. This pathway may contribute to physiological levels of PAF for normal cellular functions. 1-The synthesis of PAF may be stimulated during antigen–antibody reactions or by a variety of agents, including chemotactic peptides, thrombin, collagen, and other autacoids. 2- PAF also can stimulate its own formation. Both the phospholipase and acetyltransferase are Ca2+-dependent enzymes; thus, PAF synthesis is regulated by the availability of Ca2+. 3- The inactivation of PAF also occurs in two steps . 1)Initially, the acetyl group of PAF is removed by PAF acetyl hydrolase to form lyso -PAF 2) Lyso -PAF is then converted to a 1-O-alkyl-2-acyl-glycerophosphocholine by an acyltransferase 7/30/2021 14

15. 1- In addition to these enzymatic routes. PAF-like molecules can be formed from the oxidative fragmentation of membrane phospholipids (oxidized phospholipids). 2- These compounds are increased in settings of oxidant stress such as cigarette smoking. 3-They differ structurally from PAF in that they contain a fatty acid at the sn-1 position of glycerol joined through an ester bond and various short-chain acyl groups at the sn-2 position. 4-Oxidized phospholipids mimic the structure of PAF closely enough to bind to its receptor and elicit the same responses. 7/30/2021 15

16. 5-Unlike the synthesis of PAF, which is highly controlled, oxidized phospholipids production is unregulated; degradation by PAF acetyl hydrolase , therefore, is necessary to suppress the toxicity of oxidized phospholipids. 6-Levels of PAF acetyl hydrolase (also known as lipoprotein-associated phospholipase a2) are increased in colon cancer, cardiovascular disease & stroke. 7-Polymorphisms have been associated with altered risk of cardiovascular events. Remodelling of cell membrane phospholipid denevo 1-0-alkyl-sn-glycero-3-phosphate 7/30/2021 16

17. Mechanism of action PAF Extracellular PAF exerts its actions by stimulating a specific GPCR that is expressed in numerous cell types. The PAF receptor's strict recognition requirements, including a specific head group and specific atypical sn-2 residue, also are met by oxPLs . The PAF receptor couples with Gq to activate the PLC–IP3–Ca2+ pathway and phospholipases A2 and D such that AA is mobilized from diacylglycerol , resulting in the synthesis of PGs, TxA2, or LTs, which may function as extracellular mediators of the effects of PAF. PAF also may exert actions without leaving its cell of origin. For example, PAF is synthesized in a regulated fashion by endothelial cells stimulated by inflammatory mediators. 7/30/2021 17

18. This PAF is presented on the surface of the endothelium, where it activates the PAF receptor on juxtaposed cells, including platelets, polymorphonuclear leukocytes, and monocytes , and acts co-operatively with P- selectin to promote adhesion. Endothelial cells under oxidant stress release oxPLs , which activate leukocytes and platelets and can spread tissue damage. 7/30/2021 18

19. Physiological and pathophysiological effects of PAF 1-Platelet aggregation and secretion, thrombosis. 2-Stimulation of neutrophils and macrophages. 3-Acute inflammation. 4-Asthma and systemic anaphylaxis. 5-Endotoxin and immune factor-induced shocks. 6-Gastrointestinal ulceration. 7-Glycogenolysis and increased portal pressure. 8-Pancreatitis. 9-Cardiac anaphylaxis (negative inotropic effect and increased heart beating rate). 10-Pregnancy and ovo implantation. 11-Ovulation. 12-Acute lung injury. 7/30/2021 19

20. Platelet aggregation 1.Platelet aggregate or clump together using fibrinogen of vWF as a connecting agent. 2.The most abundant platelet aggregation receptor is glycoprotein Iib /3A. Activated platelets will adhere, via glycoprotein Ia to the collagen that is exposed by endothelial damage. 3.Aggregation and adhesion act together to form platelet plug. Platelet aggregation is stimulated by ADP , Thromboxane and a2 receptor activation but inhibited by other inflammatory products like PGI2 and PGD2. Platelet aggregation is enhanced by exogenous administration of anabolic steroids. 7/30/2021 20

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22. Initially, PAF was found to effect aggregation of platelets at concentrations as low as 10^-11 M, and it induced a hypertensive response at very low levels also. More generally, it is now recognised that its primary role is to mediate intercellular interactions. For example, by binding to its specific receptor, PAF activates the cytoplasmic phospholipase A2 and phospholipase C. The result of the latter is an increase in intracellular Ca2+ downstream of the cell and activation of protein kinase C. It is now known to exert effects on many different types of non-inflammatory biological events and functions, including glycogen degradation, reproduction, brain function and blood circulation 7/30/2021 22

23. Thrombus formation The function of platelets is the maintainence of haemostasis,this is achievd by the formation of thrombi when damage to endothelium of blood vessels occurs. Conversely ,thrombus formation must be inhibited at times when there is no damage to the endothelium, Activation-the inner surface of blood vessel is lined with a thin layer of endothelial cells,that in normal haemostasis acts to inhibit platelet activation by producing endothelial ADPase , nor-adrenaline and PGI2. Endothelial ADPase clears away ADP,a platelet activator,from platelet surface receptors. Endothelial cells produce a protein called von Willebrand factor,a cell adhesion ligand,which helps endothelial cells adhere to collagen in the basement membrane. Under physiological condition collgen does not pass into the bloodstream;however vWF is secreted constitutively into the plasma by the Endothelial cells that 7/30/2021 23

24. Produce it or otherwise is stored within the endothelial cells or in platelets. When endothelial damage occurs platelets comes into contact with exposed collagen and vWF , causing a reduction in secretion of endothelium platelet inhibitors the inner surface of blood vessels is lined with a thin layer of endothelial cells. Under this is a layer of collagen. When the endothelial layer is injured the collagen is exposed. When platelets contact collagen they are activated 7/30/2021 24

25. Pharmacological actions CVS 1-PAF is a potent dilator in most vascular beds; when administered intravenously, it causes hypotension in all species studied. 2- PAF-induced vasodilation is independent of effects on sympathetic innervation , the renin–angiotensin system, or arachidonate metabolism and likely results from a combination of direct and indirect actions. 3- PAF induces vasoconstriction or vasodilation depending on the concentration, vascular bed, and involvement of platelets or leukocytes. For example, the intracoronary administration of very low concentrations of PAF increases coronary blood flow by a mechanism that involves the release of a plateletderived vasodilator. 7/30/2021 25

26. 4-Coronary blood flow is decreased at higher doses by the formation of intravascular aggregates of platelets and/or the formation of TXA2. 5-The pulmonary vasculature is also constricted by PAF and a similar mechanism is thought to be involved. 6-Intradermal injection of PAF causes an initial vasoconstriction followed by a typical wheal and flare. 7-PAF increases vascular permeability and edema in the same manner as histamine and bradykinin . but PAF is more potent than histamine or bradykinin by three orders of magnitude. 7/30/2021 26

27. Platelets 1-PAF potently stimulates platelet aggregation in vitro. 2-While this is accompanied by the release of TxA2 and the granular contents of the platelet, PAF does not require the presence of TxA2 or other aggregating agents to produce this effect. 3-The intravenous injection of PAF causes formation of intravascular platelet aggregates and thrombocytopenia 7/30/2021 27

28. Smooth Muscle 1-PAF generally contracts gastrointestinal, uterine, and pulmonary smooth muscle. 2- PAF enhances the amplitude of spontaneous uterine contractions; quiescent muscle contracts rapidly in a phasic fashion. 3- These contractions are inhibited by inhibitors of PG synthesis. 4-PAF does not affect tracheal smooth muscle but contracts airway smooth muscle. Most evidence suggests that another autacoid (e.g., LTC4 or TxA2) mediates this effect of PAF. 5-When given by aerosol, PAF increases airway resistance as well as the responsiveness to other bronchoconstrictors . 6-PAF also increases mucus secretion and the permeability of pulmonary microvessels ; this results in fluid accumulation in the mucosal and submucosal regions of the bronchi and trachea 7/30/2021 28

29. Stomach 1-In addition to contracting the fundus of the stomach, PAF is the most potent known ulcerogen . 2-When given intravenously, it causes hemorrhagic erosions of the gastric mucosa that extend into the submucosa 7/30/2021 29

30. Kidney When infused intrarenally in animals, PAF decreases renal blood flow, glomerular filtration rate, urine volume, and excretion of Na+ without changes in systemic hemodynamics These effects are the result of a direct action on the renal circulation. PAF exerts a receptor-mediated biphasic effect on afferent arterioles, dilating them at low concentrations and constricting them at higher concentrations. The vasoconstrictor effect appears to be mediated, at least in part, by COX products, whereas vasodilation is a consequence of the stimulation of NO production by endothelium. 7/30/2021 30

31. Pathophysiological function of PAF PAF generally is viewed as a mediator of pathological events. Dysregulation of PAF signaling or degradation has been associated with some human diseases, aided by data from genetically modified animals. platelet 1-Since PAF is synthesized by platelets and promotes aggregation, it was proposed as the mediator of cyclooxygenase inhibitor–resistant, thrombin-induced aggregation. 2-However, PAF antagonists fail to block thrombin-induced aggregation, even though they prolong bleeding time and prevent thrombus formation in some experimental models. 3-Thus, PAF does not function as an independent mediator of platelet aggregation but contributes to thrombus formation in a manner analogous to TxA2 and ADP 7/30/2021 31

32. Inflammatory & allergic response 1-The proinflammatory actions of PAF and its elaboration by endothelial cells, leukocytes, and mast cells under inflammatory conditions are well characterized. 2-PAF and PAF-like molecules are thought to contribute to the pathophysiology of inflammatory disorders, including anaphylaxis, bronchial asthma, endotoxic shock, and skin diseases. 3-The plasma concentration of PAF is increased in experimental anaphylactic shock, and the administration of PAF reproduces many of its signs and symptoms, suggesting a role for the autacoid in anaphylactic shock. 4- In addition, mice overexpressing the PAF receptor exhibit bronchial hyperreactivity and increased lethality when treated with endotoxin . 5-PAF receptor knockout mice display milder anaphylactic responses to exogenous antigen challenge, including less cardiac instability, airway constriction, and alveolar edema ; they are, however, still susceptible to endotoxic shock. 7/30/2021 32

33. 6-Despite the broad implications of these observations, the effects of PAF antagonists in the treatment of inflammatory and allergic disorders have been disappointing. 7- Although PAF antagonists reverse the bronchoconstriction of anaphylactic shock and improve survival in animal models, the impact of these agents on animal models of asthma and inflammation is marginal. 8-Similarly, in patients with asthma, PAF antagonists partially inhibit the bronchoconstriction induced by antigen challenge but not by challenges of exercise, or inhalation of cold air. 9-These results may reflect the complexity of these pathological conditions and the likelyhood that other mediators contribute to the inflammation associated with these disorders. 7/30/2021 33

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