B9 - anti-coagulant and anti-platelet drugs

Question 1

Anticoagulants

Answer 1

• Classical anti coagulants
  ○ Warfarin (oral)
  ○ Heparin (SC/IV)
  
  • New direct oral anti-coagulants (NOACs/DOACs)
    ○ Direct anti-Xa (rivaroxaban, apixaban, edoxaban)
    ○ Direct anti-Iia (dabigatran)

Question 2

Warfarin

Answer 2

• Drug derived from coumarin, a toxin naturally occurring in plants
  
  • Is the active component or common rat poisons
  • Inhibits vitamin K epoxide reductase
    ○ Inhibits formation of vitamin K
    ○ Vitamin k antagonist
    ○ Leads to reduced formation of vitamin k dependant coagulation proteins
    § E.g. factor II, VII, IX, and X
  • Only active in vivo
  • Delayed onset of activity
    ○ Doesn’t affect already formed coagulation factors
  • Orally active
  • Readily absorbed
  • Long half life (dosage 1/day)
  • Becomes strongly bound to plasma protein
  • Interacts with many different other drugs and is affected by diet (e.g. vit K dietary intake)
    ○ Makes dosage difficult - must be monitored

Question 3

warfarin dosing

Answer 3

○ Increased activity in 
			§ Vit K deficiency 
			§ Hepatic disease (already impaired synthesis of coagulation factors)
			§ Hypermetabolic states 
			§ Drug interactions 
				□ Other anti-haemostatic drugs (e.g. aspirin)
		○ Decreased activity in 
			§ Pregnancy 
			§ High vitamin k diet 
			§ Drug interaction 
				□ Barituates and alcohol stimulate the liver to increase clearance 
				□ Vitamin k supplements

Question 4

monitoring warfarin

Answer 4

○ Prothrombin time
§ Time for coagulation of plasma after stimulation of the extrinsic factor pathway by addition of tissue factor and calcium
○ International normalised ratio (INR)
§ The PT of the patient divided by a normal PT adjusted for the batch of reagents

Question 5

warfarin adverse effects

Answer 5

○ Haemorrhage 
			§ Difficult to reverse 
				□ Oral vit K (no immediate effect)
				□ Fresh frozen plasma (FFP)
			§ Dosage must be carefully titrated based on INR

Question 6

Heparin

Answer 6

• Highly sulphated glycosaminoglycan derived from pig or cow mucosa
  
  • Very strong negative charge
  • Highly variable molecular weight (60-100kDa) - making dosage difficult
  • Mimics the effect of human heparan sulphate
  • Active part of the molecule is a repeating pentasaccharide unit
  • Binds to and increases the activity of endogenous antithrombin (AT)
  • Increases the ability of antithrombin to bind activated factor II (thrombin) by 1000 fold
    ○ Also binds other coagulation proteins
  • Once a heparin AT complex binds to a thrombin molecule, the heparin is released leaving the inactive T-AT complex behind, while the heparin can go and bind another antithrombin molecule in circulation
    ○ Potential for use of heparin to desensitize the patient to a subsequent heparin injection by consuming AT

Question 7

heparin action

Answer 7

• Increases the ability of antithrombin to bind activated factor II (thrombin) by 1000 fold
  ○ Also binds other coagulation proteins

Question 8

heparin - Pharmacodynamics and monitoring

Answer 8

○ Not orally available - IV or sub cutaneous only
○ Effect is immediate - inactivated activated coagulation proteins
○ Monitored by activated partial thromboplastin time
§ Time for clot formation after stimulation by intrinsic coagulation by addition of calcium and phospholipid
§ Effect is reversible by administration of protamine sulphate
§ Adverse effects
□ Haemorrhage
® Administer protamine
□ Thrombocytopaenia
□ Osteoporosis (through inhibition of vitamin D in the kidneys)

Question 9

• Low molecular weight heparin

Answer 9

○ Heparin which has been titrated to remove many of the non-active parts of the protein
○ Molecular weight is more consistent
○ Pharmacokinetics are more predictable
○ Molecule is far more active and potent
○ Cannot be reverse
○ Pentasaccharides now also on the market - more potent, should be used with caution

Question 10

Summary of classical anticoagulants

Answer 10

• Warfarin for chronic thrombo-embolic risk
  
  • Heparin for acute anticoagulation
    ○ E.g. percutaneous coronary intervention (blockage in coronary artery that is scrubbed out leading to fragments circulating), cardiopulmonary bypass
    ○ Bridge to warfarin
  • Relatively ineffective alone for cardiovascular disease

Question 11

New anticoagulants

Answer 11

• Direct thrombin inhibitors
  ○ Ximelagatran withdrawn, dabigatran approved
  ○ Do not require antithrombin, do not require routine monitoring
  ○ Overcomes dietary and drug interactions of warfarin
  ○ Reversible with idarucizumab (TGA approved 2016)
  
  • Oral factor Xa inhibitor
    ○ Rivaroxaban, apixaban TGA approved
    ○ Alternative to heparin for certain procedures and in AF
    ○ Irreversible in australia
  • Andexanet alfa (inactivated recombinant Xa) FDA approved in May 2018

Question 12

• Direct thrombin inhibitors

Answer 12

○ Ximelagatran withdrawn, dabigatran approved
○ Do not require antithrombin, do not require routine monitoring
○ Overcomes dietary and drug interactions of warfarin
○ Reversible with idarucizumab (TGA approved 2016)

Question 13

Oral factor Xa inhibitor

Answer 13

○ Rivaroxaban, apixaban TGA approved
○ Alternative to heparin for certain procedures and in AF
○ Irreversible in australia

Question 14

Profibrinolytics - breaking down already formed clots

Answer 14

• Streptokinase/urokinase
  ○ Derived from bacteria
  ○ Inexpensive and highly effective
  ○ Antigenic, single use (streptokinase only)
  
  • Alteplase (hrtPA)
    ○ Human recombinant protein
    ○ Extremely expensive
    ○ ‘clot selective’ - more active on fibrin bound plasminogen
  • Can be used after the event to rapidly break down a thrombus or embolism which is causing the infarct
    ○ Effective treatment for myocardial infarction, stroke
  • Adverse effects
    ○ Inflammation, antigenic
    ○ Overdose leading to haemorrhage
    § May treat aminocapraoic acid

Question 15

• Streptokinase/urokinase

Answer 15

○ Derived from bacteria
○ Inexpensive and highly effective
○ Antigenic, single use (streptokinase only)

Question 16

Alteplase (hrtPA)

Answer 16

○ Human recombinant protein
○ Extremely expensive
○ ‘clot selective’ - more active on fibrin bound plasminogen

Question 17

Atherothrombosis

Answer 17

• Atherothrombosis is characterised by a sudden (unpredictable) atherosclerotic plaque disruption (rupture or erosion) leading to platelet activation and thrombus formation
  
  • Atherothrombosis is the underlying condition that results in event leading to myocardial infarction, ischaemic stroke, and vascular death

Question 18

when there is disruption of endothelium

Answer 18

Endothelial cells produce mediators than inhibit platelet activation eg. prostacyclin
When there is disruption of this, subendothelial collagen is exposed and recognised by platelets or through VWF
Platelets adhere and are activated and release their own mediators eg. Thrombin, ADP, thromboxane - pathways that cause further platelet activation
Cross linking of fibrinogen to fibrin

Question 19

nitric oxide produce by sub endothelial matrix

Answer 19

In a normal situation, subendothelial matrix is producing nitric oxide which directly inhibits signal transduction through upregulating cyclic GMP

Question 20

prostacyclin produced by endothelial cells

Answer 20

prostacyclin is produced by endothelial cells and through a receptor, increases cyclic AMP which inhibits signal transduction in the platelets

Question 21

CD39

Answer 21

CD39 is an ADPase, inhibits ADP in the blood which would otherwise be binding receptors P2I1 and P2I12 on the platelet surface which cause signal transduction
by inhibiting ADP it inhibits signal transduction

Question 22

collagen

Answer 22

Collagen can directly activate platelets through glycoprotein 6 as well as alpha 2b beta 1
Or it can work with VWF which is normally found in plasma but is only active once bound to collagen
Through VWF binding to glycoprotein 1b95 you get signal transduction
Once the platelets receive signal transduction signal they produce their own signal transduction
When the platelets are activated by collagen through VWF or directly - it will release granules which contain ADP and 5HT (serotonin)

Question 23

VWF is only active once bound to

Answer 23

collagen

Question 24

VWF

Answer 24

Through VWF binding to glycoprotein 1b95 you get signal transduction
Once the platelets receive signal transduction signal they produce their own signal transduction

Question 25

When the platelets are activated by collagen through VWF or directly

Answer 25

it will release granules which contain ADP and 5HT (serotonin)
Receptors for serotonin on the platelet
The platelets release alpha granules which have adhesion molecules on the surfaces which makes the platelets sticky
Undergoes inside-out signalling which changes the receptor from a low affinity to a high affinity so it can bind it’s ligand - fibrinogen

Question 26

When lots of platelets bind to the same fibrinogen molecule

Answer 26

When lots of platelets bind to the same fibrinogen molecule this creates platelet aggregation

Question 27

thromboxane

Answer 27

Also produced thromboxane by liberating aracodonic acid from the membrane of the platelet which is converted to thromboxane through a process mediated by COX1 - cyclooxegenase 1
Self perpetuating cycle, platelet activates itself through production of thromboxane
Thrombin has receptors on the surface of the platelet called protease activating receptors which are cleaved by the thrombin - activates the platelet - outside activation

Question 28

COX1 - cyclooxegenase 1

Answer 28

converts arcadonic acid into thromboxane

Question 29

aspirin

Answer 29

• Most commonly used drug in the world - cheap and protects from cardio-vascular disease
  
  • Inhibits cyclo-oxygenase
    ○ Irreversible
    ○ Prevents one self perpetuating pathway for activation of platelets
  • Reduced production of thromboxane
    ○ Cox-1 in platelets
    ○ Platelet permanently affect (no nuclear material to produce more)
    ○ Blocks production of prostaglandin
    ○ Platelets don’t have a nucleolus so they cant produce more once the aspirin has had it’s effect - platelet is irreversibly switched off
    ○ Will take 9 days to produce new platelets
  • Also inhibits PGI2 production in endothelial cells
    ○ Cox-2 in endothelial cells
    ○ Endothelial cells rapidly restore PGI2 by synthesising new cox-2
  • Thus aspirin prevents one of the mechanisms of platelet amplification following initial activation.
    ○ Tips the balance away from thrombosis
  • Very short circulating half life, but the effect on platelets is irreversible
    ○ Must wait for new platelets to be produced (9 days)
    ○ Transfuse platelets
  • Adverse effects
    ○ Bleeding (low risk)
    ○ GI bleeds
  • Drug interactions
    ○ Other NSAIDs affecting COX
  • Monitoring and resistance
    ○ Monitored with arachidonic acid stimulated platelets aggregometry
    ○ Resistance is an issue, especially in diabetes

Question 30

Most commonly used drug in the world

Answer 30

aspirin

Question 31

Fibrinogen Receptor inhibitors

Answer 31

• Antibodies or small molecules which bind to and block the GPIIb-IIIa integrin
  ○ Abciximab, eptifibatide, tirofiban
  
  • Prevents binding of fibrinogen to the platelets
    ○ Therefore prevents platelet aggregation
  • Platelet activation and granule exocytosis occurs normally
  • Only effective if injected acutely
    ○ Long term receptor blockade causes more problems than it prevents
  • Used in PCI

Question 32

thienopyridines

Answer 32

• P2Y12 (ADP receptor) antagonists
  ○ Block affect of ADP on p[platelets
  ○ Does not affect ADP working in other processes
  
  • Inhibits amplification of the platelet response by ADP release by degranulation
    ○ Does not affect other pathways of platelet activation (e.g. collagen, thrombin)
  • Ticlopidine, clopidogrel, prasugrel, ticagrelor, cangrelor (IV)
  • Metabolised in the liver
    ○ Interaction with other liver metabolised drugs
    ○ Response variability
    ○ Irreversible
  • Monitoring
    ○ Functional monitoring with ADP stimulated platelet aggregometry
    ○ Resistance associated with CYP450 mutations
  • Effect of aspirin + thienopyridine is extremely effective for cardiovascular disease
    ○ Some bleeding risk, especially with 3rd and 4th generation thienopyridines