Vascular Pharmacology Phil Copeman and Alex Hammant

Haemostasis drugs Fibrinolytics Antiplatelets Anticoagulants Aspirin Alteplase Clopidogrel Abciximab, tirofiban and eptifibatide Streptokinase activator complex / anistreplase Anticoagulants Heparin Unfractionated LMWH Fondaparinux Warfarin Rivaroxaban Dabigatran Bivalirudin

What are the two main stages of clotting? Primary Haemostasis – haemostatic plug formation (Platelet aggregation) Secondary Haemostasis - clot formation (Fibrin cleavage)

Primary Haemostatsis - Explain how the haemostatic plug forms? (6) Endothelial damage exposes collagen vWF binds to collagen. Platelets bind to vWF Platelets are activated: they change shape and secrete granules Thromboxane A2 causes vasoconstriction and aids platelet aggregation (MoA of aspirin) ADP (MoA of clopidogrel) ADP causes conformational change in GPIIb/IIIa platelet receptors, inducing binding to fibrinogen. Fibrinogen joins the platelets together, causing platelet aggregation. The coagulation cascade causes an eruption of thrombin, which converts fibrinogen to fibrin, consolidating the haemostatic plug. Platelets adhere to the damaged vascular endothelium via linkage of glycoprotein (GP) Ia receptors with exposed collagen and via linkage of Ib receptors with von Willebrand’s factor, a circulating factor that is similar to clotting factor VIII. The adherence of platelets to vascular endothelium activates the platelets and leads to the synthesis and release (degranulation) of various mediators of platelet aggregation, including thromboxane A2 (TXA2), adenosine diphosphate (ADP), and 5-hydroxytryptamine (serotonin). These mediators increase the expression of GP IIb/IIIa receptors that bind to fibrinogen and thereby cause platelet aggregation. Aspirin and clopidogrel inhibit the synthesis or activity of specific mediators of platelet aggregation, whereas abciximab, tirofiban, and eptifibatide block GP IIb/IIIa receptors. Thromboxane A2 is produced from arachidonic acid by the COX-1 enzyme and aids platelet aggregation. Aspirin irreversibly inhibits COX-1 enzyme, preventing formation of TXA2 and aggregation of platelets. Binding of ADP to P2Y12 receptors causes the conformational change of GPIIb/IIIa, allowing them to bind fibrinogen and cause platelet aggregation. Clopidogrel irreversibly inhibits the P2Y12 receptor.

Antiplatelet drugs All antiplatelet drugs prevent platelet aggregation

Aspirin Thromboxane A2 is produced from arachidonic acid by the COX-1 enzyme and aids platelet aggregation. Aspirin irreversibly inhibits COX-1 enzyme, preventing formation of TXA2 and aggregation of platelets.

Aspirin Thromboxane A2 is produced from arachidonic acid by the COX-1 enzyme and aids platelet aggregation. Aspirin irreversibly inhibits COX-1 enzyme, preventing formation of TXA2 and aggregation of platelets.

Clopidogrel Normally - Binding of ADP to P2Y12 receptors causes the conformational change of GPIIb/IIIa receptors, allowing them to bind fibrinogen and cause platelet aggregation. Clopidogrel irreversibly inhibits the P2Y12 receptor, inhibiting the effects of ADP and thereby preventing platelet aggregation. Basically – “Clopidogrel reduces platelet aggregation by irreversibly blocking the effects of ADP on platelets” Medical Pharmacology At A Glance, M.J. Neal It is often used by patients in whom aspirin is contraindicated.

Clopidogrel http://www.ataglanceseries.com/pharmacology/fig11-1.asp

Should probably call it Vitamin K Epoxide Reductase Found in liver http://www.handwrittentutorials.com/videos.php?id=35 The importance of the conversion of fibrinogen to fibrin is that: Fibrinogen is soluble Fibrin is insoluble

So…which pathway does warfarin effect? Extrinsic pathway of coagulation

What are the contraindications of warfarin therapy? Haemorrhagic stroke Significant bleeding 48 hrs post partum First trimester of pregnancy Severe hepatic and renal impairment

Which cytochrome P450 isoform is responsible for the reduction (and thus deactivation) of warfarin? CYP2A6

What are the side effects of warfarin? Nausea Pyrexia Vomiting Alopecia Haemorrhage Purpura Diarrhoea Rash Jaundice Hepatic dysfunction Pancreatitis

If warfarin affects the extrinsic pathway, how would you measure its effect on coagulation? Prothrombin Time OR INR

What is the target INR for most patients taking warfarin? 2.5 Can you think of any conditions where the target INR might be 3.5? Recurrence of venous thromboembolism whilst on warfarin therapy Antiphospholipid syndrome Mechanical prosthetic heart valve Cornary artery graft thrombosis

How long does it take for warfarin to take its therapeutic effect? Why? 2-4 days Because it takes time for the clotting factors in the blood to be metabolised/depleted

Anticoagulant drugs - Heparin Inhibits coagulation by activating antithrombin III Antithrombin III inhibits thrombin, Xa and other coagulation factors Acts on the intrinsic pathway Increases aPTT time Reversed with protamine sulphate Other factors inhibited by heparin include VIIa, IXa, XIa and XIIa; its actions on thrombin and Xa seem to be the most important Acts on intrinsic pathway: PET A PITBULL

Types of heparin Low-molecular weight heparin (LMWH) Administered subcutaneously More predictable – can be given to outpatients Fewer side effects Increase the effect of antithrombin III on Factor Xa Less reversible with protamine sulphate than unfractionated Unfractionated heparin Inhibits thrombin and Factor Xa Administered I.V. Unpredictable anticoagulant activity so can only be given in hospital because its effects need to be monitored Can cause heparin-induced thrombocytopenia

http://www.handwrittentutorials.com/videos.php?id=35 The importance of the conversion of fibrinogen to fibrin is that: Fibrinogen is soluble Fibrin is insoluble

Fibrinolytic drugs MoA: Activate plasminogen to form plasmin, which breaks down clots Types of recombinant t-PA: Alteplase, Reteplase, Urokinase Streptokinase activator complex / anistreplase Streptokinase bound to plasminogen in a complex Slowly activated in plasma Not clot-selective Can only be used once because the body forms antibodies against it

What stops clots forming normally? Endothelium is normally antithrombotic Antiplatelet Even if the platelets are activated, there’s nothing for them to bind to on the endothelial surface Platelets also produce NO, and Prostacyclin (PGI2) which stop platelet adhesion Anticoagulant Antithrombins (e.g. antithrombin III), a type of endogenous anticoagulants, inhibit the activity of thrombin and other coagulation factors Heparin-like molecules activate antithrombin III Proteins C and S Tissue Factor Pathway Inhibitor Fibrinolysis Endothelial cells synthesise t-PA, which degrades thrombi Proteins C and S, which are activated by thrombomodulin, act in a complex to inhibit clotting by inactivating factors Va and VIIIa Tissue Factor Pathway Inhibitor, an endothelial cell surface protein, inhibits the tissue factor-factor VIIa and factor Xa