Mednotes

L09 Clinical Pharmacology Antiplatelets and anticoagulants

Properties2
tagsclinicalpharmacology, lecture
aliasesL09 Clinical Pharmacology Antiplatelets and anticoagulants

21 min read

Overview

Anti-haemostatic pharmacology for cardiovascular disease: how normal haemostasis and the endogenous antithrombotic mechanisms work, how a thrombus forms after intimal damage, and where each drug class intervenes. Drugs prevent haemostasis and reduce thrombosis in three ways: by reducing platelet aggregation (antiplatelets: low dose aspirin, clopidogrel), by reducing blood coagulation and therefore fibrin formation (anticoagulants: UF heparin, LMWH such as enoxaparin, the VKA warfarin, and the DOACs dabigatran and rivaroxaban), and by increasing fibrin breakdown (fibrinolytics: the rt-PAs alteplase and tenecteplase). Tranexamic acid sits at the opposite end as an antifibrinolytic haemostatic. For every agent the lecture asks for site and mechanism of action, current cardiovascular indications, important adverse effects and contraindications, and relevant pharmacokinetics. All of these drugs carry a risk of bleeding.

Why we intervene: thromboembolic disease

Haemostasis is the arrest of blood loss from damaged vessels by formation of a clot, and is essential to life. It has two limbs: platelet activation and adhesion, which forms the clot, and blood coagulation, which forms fibrin and reinforces it. Platelets (thrombocytes) and the clotting factors (I, II, V, VII, IX, X, XI, XII, XIII) both converge on fibrin formation and thence on the thrombus.

Intravascular thrombi are the manifestation of many conditions, framed by Virchow’s triad:

  • Causes: atherosclerotic plaque rupture, coronary artery disease, atrial fibrillation, valvular incompetence, MI, hypokinetic ventricles, trauma.
  • Consequences: unstable angina and MI, deep vein thrombosis (DVT), pulmonary embolism (PE), transient ischaemic attacks (TIAs) and stroke.

Anticoagulants are given to patients at increased thrombotic risk: prolonged immobility, acute MI, CABG, angioplasty, and AF (risk of thrombotic stroke).

Endogenous antithrombotic mechanisms

What normally stops us forming thrombi, the endogenous players:

  1. Prostacyclin (PGI2), a prostaglandin synthesised and secreted by endothelial cells, inhibits platelet aggregation and secretion. It increases platelet cAMP release and decreases platelet cyclo-oxygenase (COX) activity, which decreases production of pro-thrombotic thromboxane (TXA2).
  2. Antithrombin, a circulating plasma protein, inhibits coagulation.
  3. Thrombomodulin and protein C: antithrombotic proteins expressed on endothelial cells that bind and inactivate coagulation factors.

The endothelium itself switches phenotype. Normal (anticoagulant) endothelium expresses glycosaminoglycans (heparin-like), protein S, thrombomodulin (activates protein C), endothelial protein C receptor (enhances protein C activation), tissue factor pathway inhibitor, and prostacyclin plus nitric oxide (vasodilate and inhibit platelet aggregation). Activated (procoagulant) endothelium expresses tissue factor, von Willebrand factor, intracellular adhesion molecules such as P-selectin, and thrombospondin (activates platelets).

Thrombus formation after intimal damage

Four steps in order:

  1. Platelet adhesion. PGI2 synthesis and secretion by endothelial cells falls, allowing platelets to adhere to exposed extracellular matrix proteins such as von Willebrand factor and collagen via platelet surface glycoproteins; the aggregated platelets then generate thrombin.
  2. Platelet shape change. Thrombin potently activates platelets; Ca2+ influx drives the change from smooth to spiky activated morphology.
  3. Granule release, aggregation and consolidation. Thrombin makes platelets release active compounds from their granules, including TXA2 and ADP, which activate further platelets. TXA2 also provokes vasoconstriction at the site of injury and decreases platelet cAMP.
  4. Fibrin stabilisation. Thrombin converts fibrinogen to fibrin, which polymerises into a fibrous matrix and stabilises the aggregated platelets: the platelet-rich thrombus.

Platelet agonists released on degranulation include ADP, ATP, serotonin, calcium and magnesium; other activators are thrombin, serotonin, epinephrine and collagen.

Purinergic signalling in the platelet

Two parallel routes, both ending in fibrinogen receptor activation:

  • ADP binds the P2Y (ADP) receptor coupled to Gi; the associated Gi-alpha/GTP subunit inhibits adenylyl cyclase, so less ATP is converted to cAMP (cAMP is also broken down to AMP by phosphodiesterase). Decreased PKA activity leads to platelet activation.
  • Thrombin binds its own receptor coupled to PLC, and ADP binds the P2Y1 receptor with beta/gamma/alpha-q subunits. Increased PLC activity leads to platelet activation.

Warning

The transcript flags that several small receptor-subunit labels on this diagram (for example the Gi and Gq subscripts) were only partly legible in the source and were transcribed as far as they could be read.

Coagulation cascade and the clotting factors

  • Contact activation (intrinsic) pathway: damaged surface activates XII to XIIa, XI to XIa, IX to IXa, which with VIIIa as cofactor activates X.
  • Tissue factor (extrinsic) pathway: trauma exposes tissue factor, VII becomes VIIa (inhibited by tissue factor pathway inhibitor), which activates X.
  • Common pathway: X becomes Xa (inactivated by antithrombin); V becomes the cofactor Va; Xa with Va converts prothrombin (II) to thrombin (IIa); thrombin converts fibrinogen (I) to fibrin (Ia); XIII becomes XIIIa and cross-links the fibrin clot.
  • Feedback inhibition: thrombin plus thrombomodulin generates activated protein C, which with protein S inhibits factors V and VIII.

Factor functions:

  • Fibrinogen (I): source of fibrin (Ia), the major protein component of the clot.
  • Prothrombin (II)*: source of thrombin (IIa), which converts fibrinogen to fibrin.
  • Factor V: source of Va, cofactor for Xa.
  • Factor VII*: source of VIIa, activates factors VII, IX and X.
  • Factor VIII: source of VIIIa, cofactor for IXa.
  • Factor IX*: source of IXa, activates factor X.
  • Factor X*: source of Xa, activates prothrombin.
  • Factor XI: source of XIa, activates factor IX.
  • Factor XII: source of XIIa, activates factor XI and prekallikrein.
  • Factor XIII: source of XIIIa, stabilises the fibrin clot.

The suffix a means activated; * marks the vitamin K dependent factors (II, VII, IX, X), which are the warfarin-sensitive ones. The missing numbers (III, IV, VI) turned out to be calcium, errors of purification, and phospholipids.

Antiplatelet agents

Low dose aspirin

  • Dose 75 to 150 mg/day, oral. Aspirin (acetylsalicylate) is a non-selective COX inhibitor that inhibits platelet aggregation and reduces the risk of thrombus formation, especially of platelet-rich “white” thrombi.
  • Mechanism: irreversibly acetylates platelet COX, reducing TXA2 production for the lifetime of the platelet (about 7 to 9 days). Platelets cannot replace COX activity because they have no transcription capability.
  • Selectivity for the platelet comes from first-pass metabolism: low dose aspirin acts on platelet COX in the portal circulation, then hepatic esterase deactivates acetylsalicylate to salicylate (which has no significant COX effect and is broken down by hepatic and renal CYP2C9 and by glucuronidation). Endothelial COX-2 is also spared because the endothelium rapidly turns over and replaces COX.
  • Net effect: platelet COX-1 to TXA2 to aggregation, vasoconstriction and thrombus formation is blocked, while endothelial COX-2 to PGI2, which inhibits aggregation and promotes vasodilation, continues.
  • Evidence: the Antithrombotic Trialists (ATT) Collaboration concluded that low dose aspirin is of definite and substantial net benefit for many people who already have occlusive vascular disease (Lancet, May 2009). In primary prevention its use remains uncertain because of increased major bleeds, and current opinion is that risks generally outweigh benefits (Nature Reviews Cardiology, 2010).
  • Risks: haemorrhage (surgical and otherwise); gastric ulceration and bleeding; Reye’s syndrome, whose risk is increased by aspirin or salicylates taken during a viral illness, aspirin inhibiting hepatic fatty acid metabolism (oxidative phosphorylation and beta-oxidation).

COX inhibition elsewhere explains the NSAID adverse-effect profile. In GI mucosa, COX-1 derived PGE2 gives gastric protection (increased mucus secretion, increased bicarbonate, increased mucosal blood flow), so COX-1 inhibition causes peptic ulcers and GI bleeding. In kidney, COX-1 and COX-2 derived PGE2 and PGI2 cause afferent arteriolar vasodilation (raising GFR) and increased sodium and water excretion, so COX inhibition causes sodium and water retention, hypertension and haemodynamic acute kidney injury. In the cardiovascular system, vascular COX-2 makes PGI2 (vasodilation, inhibits aggregation) and platelet COX-1 makes TXA2 (aggregation, vasoconstriction), so when the vascular effect exceeds the platelet effect the result is stroke and myocardial infarction. Low dose aspirin irreversibly inhibits platelet COX-1.

Clopidogrel

  • ADP activates platelets to aid aggregation and promote fibrinogen binding, by producing a conformational change in the platelet and by inducing presentation of the GPIIb/IIIa complex (the fibrinogen receptor).
  • Clopidogrel is a non-competitive blocker of ADP (purinergic P2Y) receptors, preventing activation of the GPIIb/IIIa receptor and reducing platelet activation. It is synergistic with aspirin.
  • It is a prodrug: CYP2C19, CYP3A4, CYP1A2, CYP2C9 and CYP2B6 convert clopidogrel to its active thiol metabolite.
  • Valuable as monotherapy and in combination with aspirin, and used when a patient cannot tolerate aspirin.

Other antiplatelet agents named

  • Newer agents: monoclonal antibody fibrinogen receptor blockers, for example abciximab, which blocks GPIIb/IIIa directly.
  • Ticlopidine is shown alongside clopidogrel as blocking ADP-driven activation.
  • Dipyridamole (given for note only): a platelet phosphodiesterase inhibitor that increases cAMP in platelets and blocks ADP-mediated platelet activation; used in patients with a history of stroke.

Anticoagulants

Anticoagulants prevent the formation of fibrin-rich thrombi.

Unfractionated heparin

  • A mixture of mucopolysaccharides; the older mainstay.
  • Mechanism: reduces fibrin formation by increasing the action of antithrombin, which binds and inactivates both Xa and thrombin (IIa).
  • Given i.v. by infusion; T½ 0.5 to 1 h.
  • Danger of haemorrhage. Antagonist: protamine sulphate, given i.v., binds heparin to form an inactive complex.
  • Adverse effect to know: heparin-induced thrombocytopenia (HIT), which can progress to limb ischaemia, illustrated by a case of diffuse black discolouration with gangrene involving all the toes and the front portion of both feet. Haemorrhage is the other headline risk, from a bleeding finger to intracranial haemorrhage on CT.

Low molecular weight heparin (enoxaparin)

  • Enoxaparin (Clexane), one of the “parins”, is usually given s.c.
  • Mechanism: potentiates the action of AT III on factor Xa and similar factors, with less effect on thrombin, the key contrast with UF heparin.
  • Pharmacokinetics: longer duration of action (T½ 4 to 6 h versus 0.5 to 1 h) and increased bioavailability compared with UF heparin; cleared by elimination in urine, so elimination is affected by renal impairment. Dose is adjusted to body weight. Action is assessed using anti-factor Xa activity, not INR.
  • Route: generally s.c., not normally i.v., and definitely not i.m.
  • Indications: DVT prophylaxis (for example post-operative and immobile patients); treatment of venous thrombosis / DVT, initially as bridging therapy with warfarin; sometimes coronary syndromes such as MI.
  • Risks: haemorrhage, bruising, elevated AST and ALT, hyperkalaemia. Fewer adverse effects than UF heparin and thrombocytopenia is not a major consideration, but LMWH is not as readily reversed with protamine sulphate as UF heparin is.

Warfarin (vitamin K antagonist)

  • Warfarin is coumarin derived and given orally. Vitamin K is present in the diet and in a number of vitamin replacement products.
  • Mechanism: warfarin competitively blocks hepatic vitamin K epoxide reductase (VKORC1), preventing recycling of vitamin K from its oxidised to its active reduced form. Reduced vitamin K is the cofactor for gamma-glutamyl carboxylase, which converts non-functional prozymogens to functional zymogens, so warfarin prevents activation of prothrombin (II) and factors VII, IX and X, and of the antithrombotic proteins C and S. Overall it reduces fibrin formation.
  • Onset: takes 24 to 48 hours to build effect and weeks to stabilise. Peak blood concentration is reached in about 1 to 8 h but does not coincide with the therapeutic effect.
  • Monitoring: measure INR, not the warfarin level. Very narrow therapeutic index; maintain INR 2 to 3. Major haemorrhagic risk at high INR.
  • Metabolism and interactions: 100% hepatic elimination, 0% renal. S-warfarin, the more important enantiomer, is metabolised primarily by CYP2C9; R-warfarin by CYP1A2 and CYP3A4. Interactions are numerous, mainly through CYP2C9 and less so CYP1A2 and CYP3A4, and all of them affect the anticoagulant response.
  • Adverse effects: haemorrhage is the most common, including haemorrhagic stroke; GI tract loss; bruising; teratogenicity; skin necrosis from paradoxical microvascular thrombosis, because warfarin also reduces formation of proteins C and S, particularly in protein C or S deficient patients.

Important

Reversal of warfarin, in order, when bleeding or INR greater than 4: withdraw warfarin therapy (remembering its persistent action), then give vitamin K1 orally or i.v. (slow acting). In a bleeding crisis give i.v. either Prothrombinex-VF, a freeze-dried prothrombin complex concentrate (fast acting, small volume), or fresh frozen plasma containing factors II, VII, IX and X (fast acting, large volume).

DOACs

  • Dabigatran (oral) is a direct thrombin inhibitor. Dabigatran has fewer adverse effects than warfarin and its effect is directly proportional to dose, so INR monitoring is not needed.
  • Indications: DVT; PE prophylaxis and treatment; venous thromboembolism after arthroplasty; CAD and PAD. Not as effective in valvular AF or in mechanical valve thrombus formation.
  • Emergency reversal: the monoclonal antibody idarucizumab.
  • Pharmacokinetics: predominantly renal clearance (about 80% renal, 20% hepatic), so renal function must be assessed in all patients before initiation and rechecked annually. Affected by interactions at the P-gp drug transporter.
  • Rivaroxaban (oral) is a direct factor Xa inhibitor, affected by interactions through CYP3A4 and P-gp drug transport. [slide does not elaborate further on its indications or adverse effects]

Fibrinolytics

For a thrombus that has already formed. Tissue plasminogen activators are serine proteases normally synthesised by endothelial cells; they promote thrombolysis by converting plasminogen to plasmin, which degrades fibrin as well as fibrinogen and other proteins involved in fibrin formation. The recombinant agents alteplase and tenecteplase are manufactured using rDNA technology, increase survival post-MI and after stroke, and must be given acutely for optimal outcome (i.v. within 6 h post-MI).

Sites of action at the clot: recombinant t-PA binds to fibrin in the thrombus, converts entrapped plasminogen to plasmin, and that plasmin initiates local fibrinolysis, dissolving fibrin and producing fibrin degradation products.

Alteplase

  • Alteplase is given acutely i.v. in MI and ischaemic stroke.
  • Dosing is very strictly controlled and depends on all safety criteria being met.
  • Adverse effect: bleeding.
  • Pharmacokinetics: metabolised by the liver to amino acids; initial half-life 5 minutes (free, unbound form), terminal half-life 72 minutes.
  • Interactions: direct pharmacodynamic interaction with prothrombin complex concentrate; contraindicated with anticoagulants on board.

Tenecteplase

  • Tenecteplase is an engineered variant of alteplase with a longer plasma half-life, enhanced fibrin specificity, and increased resistance to inactivation by plasminogen activator inhibitor 1 (PAI-1).
  • Indication: thrombosis in MI.
  • Dosing: 30 to 50 mg i.v. bolus over 5 to 10 seconds, once, based on weight at 0.53 mg/kg bodyweight.
  • Interactions: never use with PCC; contraindicated with anticoagulants.
  • Pharmacokinetics: rapid onset with peak at 30 min; terminal half-life 90 to 130 min; hepatic metabolism to amino acid degradation products; plasma clearance 99 to 119 mL/min (the drug may be subject to protein binding).

Haemostatics: tranexamic acid

Tranexamic acid (TXA) is an antifibrinolytic and a lysine derivative. It competitively inhibits plasminogen activation and, at much higher concentrations, is a non-competitive inhibitor of plasmin. It is used to arrest major blood loss in major trauma and uncontrolled bleeding, and in other conditions such as menorrhagia. On the cascade diagram it sits exactly where rt-PA does, at the plasminogen to plasmin step, but inhibiting it.

Putting the targets on one cascade

  • LMWH (enoxaparin) potentiates antithrombin’s action on factor Xa; UF heparin potentiates antithrombin against both Xa and thrombin.
  • Warfarin inhibits the vitamin K reductase step, reducing prothrombin and factors VII, IX and X, and anticoagulant proteins C and S.
  • Dabigatran inhibits thrombin (IIa) directly; rivaroxaban inhibits factor Xa directly.
  • Aspirin blocks platelet COX and therefore TXA2; clopidogrel blocks the platelet P2Y receptor; abciximab blocks GPIIb/IIIa.
  • rt-PAs (alteplase, tenecteplase) drive plasminogen to plasmin, which digests fibrin to degradation products; tranexamic acid blocks that same step.

The whole lecture in one slide

  • Low dose aspirin (oral): reduces platelet aggregation by irreversibly inhibiting platelet COX.
  • Clopidogrel (oral): P2Y receptor antagonist, also an antiplatelet agent.
  • Enoxaparin, LMWH (s.c.): almost the same as heparin, but less effect on thrombin, and safer.
  • Warfarin (oral): prevents vitamin K activation of clotting factors II, VII, IX and X; monitor effect with INR.
  • Dabigatran: direct thrombin inhibitor.
  • Alteplase, rt-PA (i.v.): releases plasmin to break down a thrombus that has already formed.
  • All carry a risk of bleeding.

Abbreviations

i.v. intravenous; i.m. intramuscular; s.c. subcutaneous; p.o. per oral; LMWH low molecular weight heparins; VKAs vitamin K antagonists; NOACs novel oral anticoagulants (non-VKA); PCC prothrombin complex concentrate; FFP fresh frozen plasma; INR International Normalised Ratio.

Self-test

  1. Define haemostasis and name its two limbs, saying what each contributes to the clot.
  2. List the three endogenous mechanisms that normally stop us forming thrombi, and explain how prostacyclin does it.
  3. Distinguish the anticoagulant products of normal endothelium from the procoagulant products of activated endothelium, giving at least three of each.
  4. Describe the four steps of thrombus formation following intimal damage.
  5. Explain how ADP binding to the P2Y receptor leads to platelet activation, and how the thrombin/PLC route differs.
  6. Describe the common pathway of the coagulation cascade from factor X to a cross-linked clot.
  7. Which coagulation factors are vitamin K dependent, and why does that matter pharmacologically?
  8. Explain how protein C and protein S provide feedback inhibition of coagulation.
  9. Describe the mechanism of low dose aspirin, and explain why its effect lasts about 7 to 9 days.
  10. Explain why low dose aspirin inhibits platelet TXA2 production but largely spares endothelial PGI2 production.
  11. What is the evidence position on aspirin in secondary versus primary prevention of occlusive vascular disease?
  12. List the risks of aspirin, and explain the mechanism proposed for Reye’s syndrome.
  13. Predict the consequences of COX inhibition in the GI mucosa and in the kidney.
  14. Describe the mechanism of clopidogrel and explain why its effect depends on hepatic metabolism.
  15. Distinguish unfractionated heparin from enoxaparin on mechanism, route, half-life, monitoring and reversibility.
  16. List the indications and the risks of enoxaparin.
  17. Describe the mechanism of warfarin, starting from the vitamin K cycle.
  18. What INR range is targeted for warfarin, why is INR measured rather than the drug level, and what happens at high INR?
  19. Explain why warfarin can paradoxically cause microvascular thrombosis and skin necrosis.
  20. A patient on warfarin presents bleeding with an INR of 6. Describe the reversal steps in order.
  21. Distinguish dabigatran from warfarin on mechanism, monitoring, reversal agent and elimination, and state where dabigatran is less effective.
  22. Describe how alteplase acts at the thrombus, and state its two half-lives and its key contraindication.
  23. Distinguish tenecteplase from alteplase.
  24. Explain the mechanism of tranexamic acid and its indications.
  25. Integrative: a patient has an acute MI and then goes into atrial fibrillation. Using the cascade, explain where an antiplatelet, an anticoagulant and a fibrinolytic each act, and what single adverse effect they share.

Answers

Graph View

Backlinks