Pharmacology 5

Question 1

What is haemostasis?

Answer 1

Arrest of blood loss from a damaged vessel at the site of injury

Question 2

Initiation of haemostasis?

Answer 2

Vascular wall damage exposing collagen and tissue factor (TF - also known as thromboplastin)

Question 3

Steps of primary haemostasis? (4)

Answer 3

• local vasoconstriction
• platelet adhesion
• activation and aggregation (by fibrinogen)
• activation of blood clotting (coagulation) and the formation of a stable clot (by fibrin enmeshing platelets)

Question 4

What is TF also known as? What do platelets aggregate together form? What is the purpose of local vasoconstriction in haemostasis?

Answer 4

• Thromboplastin
• Soft plug to arrest blood loss
• Local constriction restricts blood loss

Question 5

What do platelets bind to? (2)

What do activated platelets do? (2)

Answer 5

• Von Willebrand Factor and collagen
• Extend pseudopodia to aggregate
• Synthesise and release thromboxane A2 (TXA2)

Question 6

What is the purpose of TXA2? (2)

Answer 6

• Binds to platelet GPCR TXA2 receptors (TP receptors) which causes mediator release – 5-hydroxytryptamine (5-HT – aka serotonin) and adenosine diphosphate (ADP)
• Act on vascular smooth muscle cell TXA2 receptors causing vasoconstriction (5-HT also does the same thing by binding to smooth muscle GPCR 5-HT receptors)

Question 7

What mediators are released by TXA2 binding to TP receptors? (2)

Answer 7

• 5-hydroxytryptamine (5-HT, Serotonin)

* Adenosine diphosphate (ADP)

Question 8

What receptors do ADP bind to? What is the purpose of ADP by TXA2 binding to TP receptors? (3)

Answer 8

ADP binds to platelet GPCR purine receptors (P2Y12) that:

• act locally to activate further platelets
• aggregate platelets into a ‘soft plug’ at site of injury via increased expression of platelet glycoprotein (GP IIb/IIIa) receptors that bind fibrinogen
• expose acidic phospholipids on platelet surface that initiate coagulation of blood and solid clot formation

Question 9

What does increased expression of GP receptors by ADP allow? What also increases expression of GP receptors?

Answer 9

• Allows plasma fibrinogen to bind to receptors to form physical bridge between the platelets as part of aggregation process
• Thomboxane A2

Question 10

What is the key event in the coagulation cascade?

Answer 10

Production of the protease thrombin (factor IIa) that cleaves fibrinogen to fibrin to form a solid clot

Question 11

Explain coagulation cascade (3)

Answer 11

• Inactive factor X converted to Xa by tenase (IXa, VIIIa)
• Inactive factor II converted to IIa by prothrombinase (Xa, Va)
• Fibrinogen converted into fibrin by thrombin to form solid clot

Question 12

What coverts X to Xa? What is this? What converts II (prothrombin) to IIa (thombin)? What is this?

Answer 12

• Tenase - combination of IXa and VIIIa

* Prothrombinase - combination of Xa and Va

Question 13

What is the purpose of thrombin? What are points of drug intervention to inhibit coagulation cascade?

Answer 13

• Thrombin converts fibrinogen into fibrin causing formation of solid clot
• Inhibit action of Xa or IIa (thrombin)

Question 14

What is thrombosis? What are predisposing factors for thrombosis?

Answer 14

• Pathological haemostasis - innaproapriate activation of coagulation cascade
• Virchow’s triad - endothelial injury, turbulent flow, hypercoaguability of blood

Question 15

What are features of an arterial thrombus? (2)

What are arterial thrombi treated with?

Answer 15

• WHITE thrombus - mainly platelets in fibrin mesh
• Forms embolus if it detaches from site of origin and often lodges in artery in the brain (stroke)
• Antiplatelet drugs

Question 16

What are features of a venous thrombus? (2)

What are venous thrombi treated with?

Answer 16

• RED thrombus - white head, red tail, fibrin rich due to high % of erythrocytes
• Forms an embolus that usually lodges in the lung (PE)
• Anticoagulants

Question 17

What is the site of action of Warfarin? Rivaroxiban? Heparin? Dabigatran?
What are these drugs classed as?

Answer 17

• Warfarin - blocks modification of factors X and II essential for their function
• Rivaroxiban - directly inhibits factor Xa
• Heparin - inactivates factor Xa and IIa via antithrombin III
• Dabigatran - directly inhibits factor IIa

Anticoagulant drugs

Question 18

What are clotting factors II, VII, IX and X? How do they produce active factors?

Answer 18

• Glycoprotein precursors of the active factors IIa (thrombin), VIIa, IXa and Xa that act as SERINE PROTEASES
• Precursors are post-translationally modified (gamma-carboxylation of glutamate residues) to produce active factors

Question 19

Why is vitamin K essential for coagulation cascade?

Answer 19

The carboxylase enzyme that mediates y-carboxylation requires vitamin K from diet (K1) and intestinal flora (K2) in its REDUCED form as an ESSENTIAL COFACTOR

Question 20

What is vitamin K in its oxidised form? Reduced form? Which form is req for y-carboxylation? What enzyme is required for conversion from oxidised form to reduced form?

Answer 20

• Epoxide
• Hydroquinone
• Reduced form
• Vitamin K reductase

Question 21

How does Warfarin block modification of factors X and II?

Answer 21

• Warfarin nhibits vitamin K reductase, which means hydroquinone cannot be formed
• Hydroquinone required for y-carboxylation of precursors to form active factors

Question 22

What are anticoagulants used to treat/prevent? Examples? What is the risk of using anticoagulants?

Answer 22

• VENOUS (not arterial) thrombosis and embolism
• deep vein thrombosis (DVT)
• prevention of post-operative thrombosis
• patients with artificial heart valves
• atrial fibrillation
• All carry a significant risk of haemorrhage

Question 23

What is the effect of Warfarin? Can it be used both in vivo and in vitro? How is it administered?

Answer 23

• renders factors II, VII, IX and X inactive
• Blocks coagulation in vivo but not in vitro
• Administered orally + is v well absorbed

Question 24

What is Warfarin’s onset of action? What can be added for a more rapid anticoagulant effect? What is Warfarin’s half-life? How long must Warfarin administration be suspended before an operation?

Answer 24

• 2-3 days as inactive factos replace the y-carboxylated factors that are slowly cleared from the plasma
• Heparin may be added for rapid anticoagulant effect
• Has a long half-life (about 40 hr)
• Around 4-5 days as takes about 5 half-lives to be cleared from body

Question 25

Why is it difficult to strike balance between anticoagulant effect and haemorrhage with Warfarin? How is the effect of Warfarin monitored? How is overdosage of Warfarin treated? (2)

Answer 25

* Warfarin has low therapeutic index * Must be monitored on reg basis as international normalised ratio (INR) * Overdosage treated with administration of Vit K1 (phytomenadione) OR IV administration of plasma clotting factors

Question 26

What do factors that potentiate warfarin action do? Examples? (5)

Answer 26

Increase risk of haemorrhage * Liver disease (decreased clotting factors) * High metabolic rate (increased clearance of clotting factors) * Drugs that inhibit hepatic metabolism of warfarin by CYP2C9 * Drugs that inhibit platelet function (e.g. aspirin, other NSAIDs) * Drugs that inhibit reduction, or decrease availability, of vitamin K

Question 27

What do factors that lessen warfarin action do? Examples? (4)

Answer 27

Increase risk of thrombosis * Pregnancy (increased clotting factor synthesis) * Hypothyroidism (decreased degradation of clotting factors) * vitamin K consumption * drugs that increase hepatic metabolism of warfarin

Question 28

What is antithrombin III? What does Heparin do?

Answer 28

* Important inhibitor of coagulation - neutralises all serine protease factors in coagulation cascade by binding to their active site in 1 to 1 ratio * Heparin binds to antithrombin III, increasing its affinity for serine protease clotting factors Xa and IIa to increase their rate of inactivation

Question 29

How does heparin inhibit IIa? Xa? What do LMWHs do?

Answer 29

* Must bind to both AT III and IIa to inhibit IIa * Need bind only to AT III to inhibit Xa * Will accelerate inactivation of Xa but will not affect rate of inactivation of IIa (thrombin)

Question 30

What is heparin? What is it extracted from?

Answer 30

* Naturally occurring sulphated glycosaminoglycan of variable molecular size * Extracted from animal offal

Question 31

What are generally preferred over heparin? Examples? When can they not be used?

Answer 31

* LMWHs * e.g. enoxaparin and dalteparin * In renal failure as they can only be excreted from body via kidneys (whereas Heparin can be metabolised by liver)

Question 32

What agents act similarly to LMWHs? How is heparin administered? (2) How are LMWHs administered?

Answer 32

* Fondaparinux and idabriotaparinux * IV for immediate onset or SC (delayd 1 hr onset) * LMWHs given SC

Question 33

How is optimum dosage of heparin (NOT LMWHs) determined? What is the elimation of heparin? HMWHs?

Answer 33

* In vitro clotting test * Heparin - zero order * HMWHs - first order

Question 34

What are adverse effects of heparin and LMHWs? (4)

Answer 34

* haemorrhage – discontinue drug, if necessary administer protamine sulfate IV (inactivates heparin) * osteoporosis (long term treatment) * hypoaldosteronism * hypersensitivity reactions

Question 35

What are orally active agents that act as direct inhibitors of thrombin? Xa? What are the advantages of these agents? (2) Major disadvantage??

Answer 35

* Thrombin inhibitors - dabigatran etexilate * Xa - rivaroxaban * Convenience of administration * Predictable degree of coagulation * Major disadvantage - NO agent available to reduce haemorrhage in overdose D:

Question 36

What are rivaroxiban and dabigatran etexilate used for?

Answer 36

To prevent venous thrombosis in patients undergoing hip and knee replacements

Question 37

What are actions of anti-platelet drugs? (4)

Answer 37

* Clopidogrel - blocks ADP irreversibly * Tirofiban - blocks GP IIb/IIIa receptor on platelets * Aspirin - blocks cyclo-oxygenase-1 (COX-1) irreversibly (prevents conversion of arachidonic acid to cyclic endoperoxides and thus synthesis of more TXA2) * Ifetroban blocks thromboxane synthase (prevents conversion of cyclic endoperoxides to TXA2) Note: arachidonic acid produced by platelet-synthesised TXA2

Question 38

What are anti-platelet drugs used for? What is aspirin? What does it do? (2)

Answer 38

* Used for ARTERIAL thrombosis * Main antiplatelet agent * Irreversibly blocks cycoxygenase (COX) in platelets preventing TXA2 synthesis * Also blocks COX in endothelial cells inhibiting production of antithrombotic prostaglandin I2 (PGI2)

Question 39

Why does TXA2 synthesis not recover for 7-10 days once treatment with aspirin is stopped?

Answer 39

* Endothelial cells can synthesise new COX enzyme whereas platelets cannot * TXA2 synthesis does not recover until affected platelets are replaced (7-10 days)

Question 40

How is aspirin administered? What patients is it used in? Adverse effects? (2)

Answer 40

* Orally * Thromboprophylaxis in patients at high cardiovascular risk * GI bleeding and ulceration

Question 41

What does clopidogrel do? What patients used for? Administered how? What happens when used in combo with apsirin?

Answer 41

* Links to P2Y12 receptor by a disulphide bond producing irreversible inhibition * Patients intolerant to aspirin * Orally * When combined with aspirin has a synergistic action

Question 42

How is tirofiban administered? Used in what patients? With what other drugs?

Answer 42

* IV * Prevent MI in high risk patients with unstable angina * Used with heparin and aspirin

Question 43

What cascade opposes coagulation cascade? Explain cascade (2)

Answer 43

Fibrinolytic cascade * Plasminogen activated to form plasmin by endogenous tissue plasminogen activator (tPA) * Plasmin breaks down fibrin into fibrin fragments resulting in clot lysis

Question 44

What are fibinolytics used for? (2) Administered? What can give an additive beneficial effect?

Answer 44

* To reopen occluded arteries in acute MI or stroke * Less frequently in PE or venous thrombosis * IV * If combined with aspirin

Question 45

What are examples of fibrinolytic drugs? (3) What do they do?

Answer 45

* Streptokinase, alteplase and duteplase | * Activate plasminogen

Question 46

What is streptokinase? What is it used for? Administered? What are the problems with streptokinase?

Answer 46

* Don't be fooled by its name - it is not an enzyme, it's a protein extracted from strep cultures * Used to reduce mortality in acute MI * IV or intracoronary Problems * Action blocked after 4 days by generation of antibodies * May cause allergic reactions (so can't be given to patients with recent strep infections)

Question 47

What are alteplase and duteplase? Administered? Advantage over streptokinase? Adverse effects??

Answer 47

* Recombinant tissue plasminogen activators (rt-PA) * IV since short half-life * Does not cause allergic reactions like streptokinase * Adverse effect - haemorrhage (controlled by oral tranexamic acid that inhibits plasminogen activation)