Platelets and Primary Hemostasis
What is hemostasis?
-Arrest of blood loss from damaged vessels and is essential to life. It is the whole process of stopping bleeding while coagulation includes just the clotting of plasma with the formation of fibrin.
What is thrombosis?
-In contrast, thrombosis is the pathological formation of a hemostatic plug within the vasculature in the absence of bleeding.
- Both hemostasis and thrombosis involve same mechanisms of platelet aggregation and blood coagulation.
- Clot: Only used to refer to blood outside of the vasculature (i.e. should use thrombus in this case, within vessels)
Hemostasis: Divided into primary and secondary hemostasis
1. Primary hemostasis (concerns with the function of vascular endothelium and platelets)
-In a nutshell, platelets adhere to damaged areas of the circulation (which release collagen) by attaching to Von Willebrand Factor (VWF) and become activated, GP 1b receptors used to bind to VWF.
-Platelets change shape, exposing phospholipids and GP IIb/IIIa receptors. This produces thromboxane A2 and ADP to stimulate other platelets to aggregate.
-Calcium ions needed to calcify primary hemostatic plug. Fibrinogen trapped in the middle but requires secondary hemostasis to convert into fibrin.
Summary of Platelet aggregation (Primary hemostasis)
-Link to entire video lecture: http://www.youtube.com/watch?v=UiPP_ccFX3E
-Can be measured by Complete Blood Count (CBC)
2. Secondary Hemostasis (Concerns coagulation cascade)
Intrinsic Pathway 12–>11–>9–(8)–>10–>(5)–>2 (Prothrombin)–>1 (Fibrinogen)
Extrinsic Pathway (Requires Vitamin K)
-Intrinsic Pathway can be measured by PT (Prothrombin Time)
-Extrinsic Pathway can be measured by PTT/INR (Partial Thromboplastin Time/International Normalised Ratio)
What are platelets? [Not true cells but are circulating fragments of a cell]
-Platelets are small, non-nucleated, biconvex discs (1.5-3.5 microns in diameter)
-Normal lifespan is 7-10 days, made in bone marrow
-Forms plugs to occlude sites of vascular damage, promote clot formation by providing a surface for the activation of coagulation factors and secrete factors involved in vascular repair.
-Contains four granule types:
(a) Alpha granules: Contains host of adhesion molecules, coagulation factors and growth factors
(b) Dense granules: Very electron dense and contains serotonin
(c) Lysosomes: Contain lysosomal enzymes
(d) Microperoxisomes: Present in small numbers and contains catalase
- They also contain a well developed cytoskeleton with a marginal band of microtubules which depolymerises at the onset of platelet aggregation.
- Located deep to the marginal band is the dense tubular system (DTS) consisting of narrow membranous tubules (probably the site of prostaglandin synthesis)
- Platelets also contain interconnected membrane channels continuous with the external environment via external pits. This is the open canalicular system (OCS) and is a channel for the secretion of alpha-granule contents.
- Also contains glycoproteins (GP) on the platelet surface or inserted into alpha-granule membrane.
-They play a key role in normal platelet adhesion or aggregation.
-GP IIb/IIIa and GP 1b are examples.
Clinical deficiencies of GP:
1. Bernard-Soulier Syndrome-Congenital lack of GP 1b. Thus hemostatic plug/thrombus is poorly formed or absent in these patients.
2. Glanzmann’s thrombasthenia-GP IIb/IIIa is missing (cannot aggregate platelets)
Platelet activation and aggregation
-In response to a variety of agonists, platelets undergo a series of changes resulting in aggregation.
(a) Coagulation system: Thrombin. Generated via coagulation pathways.
-Thrombin is a powerful platelet agonist. Platelets have specific receptors for thrombin.
(b) Extravascular proteins: When the vasculature is damaged, the removal of the endothelial layer exposes vascular basement membrane rich in collagen.
(c) Also exposes large adhesive protein Von Willebrand Factor (VWF) which binds to GP 1b/GPIX (9)/GPV (5) complex
(d) Platelets themselves: Contains stores of ADP in the dense granules. However, ADP is a weak agonist of platelets.
-Platelets have specific receptors for ADP.
When one of these agonist has bound to the platelet, internal signal are generated. Once activated:
1. Altered expression of surface glycoproteins and high affinity to glycoprotein.
-Leads to an altered expression of already constitutively expressed surface glycoproteins (GP IIb/IIIa complexes).
-Increased GP IIb/IIIa complexes and reduced number of GPIIb-IX complexes -This is due to bi-directional trafficking of these glycoproteins between the cell surface, the surface connected canlicular system and intracellular storage.
-The ‘inside-out’ signalling leads to conformational changes in GPIIb/IIIa complexes and they develop high affinity for their ligands (esp. fibrinogen)
–Fibrinogen acts as a glue that ‘sticks’ platelets together during aggregation.
2. Platelets change shape
-Platelets normally circulate as flattened discs. However when activated, they undergo shape change.
-Cells become irregularly spherical with multiple pseudopodia
-At this time, the platelets become loosely associated with each other.
-This stage is called primary aggregation and is reversible.
-If the agonist is strong enough (e.g. if thrombin was present), the shape change continues and pseudopodia becomes more apparent/pronounced.
3. Platelet secretion
-At this stage (after pseudopodia formed), platelet secretion is observed.
-Platelet secretion involves:
(a) Biosynthesis of labile mediators (Platelet activating factors (PAF) and thromboxane A2-From the Prostaglandin pathway)
-PAF causes platelet aggregation and dilation of blood vessels.
(b) Exposure of acidic phospholipid
-This occurs on the platelet plasma membrane and acts as a site of assembly for certain coagulation factor complexes.
-The result is enhanced thrombin formation and further platelet activation as well as fibrin formation.
(c) Secretion of platelet granules
–Dense granules are released first (containing ADP, calcium and serotonin)
-Alpha granules (containing adhesion molecules e.g. fibrinogen, fibronectin and thrombospondin, coagulation factors) released second
-Lysososomal granules (containing lysosomal enzymes) released last
Function of these secretory substances:
-Accelerates aggregation and cements permanent attachments between platelets.
-The most important link between platelets is formed by reaction of GP IIb/IIIa and fibrinogen.
Question: Where does fibrinogen come from?
Answer: Derived from plasma and from platelet internal stores.
.-This aggregation is now irreversible and is know as seccondary aggregation.
After primary aggregation and secondary aggregation, the individual platelets lose their integrity and fuse with each other.
Drugs that inhibit platelet aggregation (Anti-platelet drugs):
1. Aspirin (Metabolism)
-Aspirin (acetylsalicyliate) is a prodrug which is converted to salicylic acid (salicylate) in many tissues. Primarily in GIT mucosa and the liver.
-Salicylate is a highly protein bound (approx 90%)
- Metabolism of salicylate normally follows first order kinetics.
- However, when large doses of aspirin taken, metabolism pathways become saturated (zero-order kinetics) and small dosage increments result in large increases in aspirin levels.
-Salicylates are excreted predominantly by the kidneys.
–Plasma aspirin, rather than salicylate is required for anti-platelet effects.
-Plasma elimination half-life is approx 30 mins. Therapeutic half life 7 days!!!
-Alters the balance between Thromboxane A2 which promotes aggregation and PGI2 which inhibits it.
Mechanism of Aspirin:
- Alters the balance between Thromboxane A2 which promotes aggregation and PGI2 which inhibits it.
- Aspirin inactivates COX-1 in platelets by irreversibly acetylating a serine residue in its active site.
- Reduces Thromboxane A2 in platelets and PGI2 in endothelium (platelets do not possess prostacyclin synthase to produce PGI2)
2. NSAIDS (Refer to Pharmacology B lecture notes: NSAIDs)
-COX-1 enzyme inhibited by NSAIDs (COX-1 Functions–>Produces molecules that affect Platelet aggregation, GI-mucosa [Protective], kidneys and lungs.
-COX-2 enzyme also inhibited by NSAIDs (COX-2 Functions–>Produce PgE2 (inflmmation)
3. Paracetamol (Refer to Pharmacology B lecture notes: NSAIDs)
-Inhibits COX-3 enzyme which is has reversible inhibition. It has central anti-pyretic and analgesic effects.
-However, it has very little anti-inflammatory effect as COX-3 not primarily responsible for producing inflammation prostaglandins.
Mechanism of action: Phosphodiesterase inhibitor
-Phosphodiesterase convert cAMP to AMP. cAMP inhibits platelet function hence this inhibitor (dipyridamole) increases cAMP and induces platelet inhibition.
-Beneficial effects of dipyridamole and aspirin are additive.
Side effects: Headache
5. Prostacyclin (PgI2) Analogue
-Since prostacyclin (PgI2) inhibits all platelet activation pathways (by increasing cAMP and thus decreasing cytosolic calcium), the analogues can be used.
-However, prostacyclin itself has a short half life.
-Stable analogues such as Iioprost may be useful.
Side effects associated with Iioprost: Headache, flushing (due to vasodilation)
6. Thienopyridine derivatives
Mechanism of action: Blocks ADP binding to platelet P2Y12 receptor. This reduces ADP mediated platelet activation of glycoprotein IIb/IIIa sites. Thus reducing platelet aggregation.
-t1/2: 8 hours, hepatic metabolism, extensive protein binding (96%)
-Clopidogrel is a prodrug, thus needs CYP2C19 metabolism for active to be formed.
-Bleeding, intracranial hemorrhage, gastric ulcers
Mechanism of action: Active metabolite irreversibly binds to and antagonises platelet P2Y12 receptor for the life of the platelet. Prevents ADP from binding and activatoin of glycoprotein IIb/IIIa complex
- t1/2-7 hours
- 98% plasma protein bound
- Inactive Prodrug (thus must be converted by CYP3A4, CYP2B6 and to a lesser extent CYP2C9 and CYP2C19. Needs hydrolysis by intestinal carboxyl-esterases and hepatic conversion to become active.
Mechanism of action: Blocks platelet-fibrinogen binding by inhibiting ADP
-Has active metabolites which are 10x more potent than the parent drug
98% plasma protein bound
Mechanism of action: Ticagrelor and major metabolites reversibly bind to platelet P2Y12 ADP receptor. Antagonises ADP and prevents platelet activation
-Ticagrelor and active metabolite are 99% plasma protein bound
-Ticagrelor is a substrate and inhibitor of CYP3A4
7. Glycoprotein IIb/IIIa complex inhibitors
(A) Abciximab (humanised monoclonal antibody)
-Initial phase t1/2 of less than ten minutes and a second phase half life of about 30 mins (related to rapid binding to the platelet IIb/IIIa receptors)
-Platelets recover in 2 days.
Adverse effects: Allergic reaction against monoclonal antibodies, increasing risk of thrombocytopenia.
(B) Eptifibatide (cyclic heptapeptide blocking agent)
-Protein binding 25%
-No major metabolites detected in blood, but deamination takes place in the urine
-In patients with coronary artery disease, the mean clearance of eptifibatide is 55-80 ml/kg/hour
-t1/2 is 2.5 hours
Adverse effects: Bleeding/ thrombocytopenia, care in renal and hepatic dysfunction
(C) Torofiban (non-peptide antagonist)
-Bind/block glycoprotein IIb/IIIa receptor and so prevent fibrinogen binding to
platelet. This leads to decreased platelet agggregation.
-Urinary and biliary excretion
-Half life-1.5 hours
Adverse effects: Bleeding/ Thrombocytopenia