Pharmacology 5 - Anti-platelet, Anticoagulant and Thrombolytic Drugs Flashcards
What is the process of haemostasis?
A series of mechanisms by which a damaged blood vessel will limit blood loss
What are the three main stages in the haemostasis sequence?
- Vascular wall damage - exposes collagen, von Willebrand factor and tissue factor within the subendothelial matrix
- Primary haemostasis - local vasoconstriction occurs immediately, platelets adhere forming a soft plug, fibrinogen allows for activation and aggregation of platelets
- Activation of clotting mechnisms - fibrinogen is converted to fibrin due to thrombin producing a solid clot
What are the three key events in primary haemostasis?
- Adhesion
- Activation
- Aggregation
How do platelets adhere to the subendothelial matrix?
Platelets bind to collagen in the subendothelial matrix via von Willebrand factor - to which they bind by GPIb receptors
During the activation step in primary haemostasis, how can platelets bind to the subendothelial matrix more strongly?
Utilising their integrin (α2β1) anf GPVI receptors for a direct bind to collagen
How do platelets become activated? (2)
- By binding directly to collagen via integrin and GPVI
- Action of thrombin
What morphological change do platelets undergo during activation?
Disc shaped to star-like with many projections
What are the projections from activated platelets called, and what is their function?
Pseudopodia
(allows connections between platelets)
What do platelets do once activated, and what is the outcome of this?
Produce thromboxane A2 from arachidonic acid due to enzyme action of COX-1
Thromboxane A2 acts on adjacent platelets causing activation
Platelets release 5-HT and ADP from dense granules
von Willebrand factor and factor V are released from α-granules
What is 5-HT?
5-hydroxytryptamine
(serotonin)
Why is ADP released from activated platelets useful to coagulation?
ADP binds to GPCR P2Y12 receptors on platelets allowing for:
- Activation of more platelets
- Increased expression of platelet GP IIb/IIIa receptors to bind to fibrinogen
- Exposure of more acidic phospholipids on the surface of platelets for clot formation
In the final stage of primary haemostasis, aggregation, what are the three sub-stages?
- Initiation
- Propagation
- Amplification
What is the overarching aim of the three stages of aggregation?
Form a tight mesh of fibrin stands
This is acieved by creating large amounts of thrombin which allows fibrin strands to be formed
What does the initiation phase require and why?
Cells associted with tissue factor such as:
- Fibroblasts
- Monocytes
- Damaged epithelial cells
- Cell fragments associated with TF
These cells are present in the subendothelial matric and are exposed to factor VIIa forming a complex TF:VIIa with tissue factor
What does the TF:VIIa complex proceed to do after its formation?
Activate factor X to Xa
How is prothrombin converted to thrombin?
Factor Xa, in combination with Va (cofactor) convertes prothrombin to thrombin
The activation of which factors requires calcium?
Factor X and prothrombin
After the initial production of thrombin via this extrinsic pathway, why is only a small amount of thrombin produced?
Producing large amounts of thrombin at this stage is not the goal
The aim is to produce enough to allow for amplification of the process to occur
How is clot formation usually supressed?
Tissue factor pathway inhibitor (TFPI)
This inhibts factor Xa by forming a complex with it
This complex subsequently blocks TF:VIIa preventing the extrinsic coagulation pathway completely
How is the tissue factor pathway inhibitor counteracted when a clot if formed?
The action of this inhibitor is overwhelmed when the influx of plasma factors and endothelial damage is too great
Where does the process of amplification occur?
On the surface of platelets
How does thrombin produced during the initiation phase aid in the amplification phase?
Thrombin activates more platelets and factor V which is released from α-granules upon activation
Upon binding to the platelet membrane, thrombin cleaves factor VIII from vWF which normally keeps the factor from degrading
Factor VIIIa is now active
In the propagation phase, how can factor IX become activated? (2)
- Factor XIa (which is itself activated by thrombin)
- Via TF:VIIa
When activated, what happens to factor IXa?
It will complex with factor VIIIa produced in the amplification phase
This complex is called tenase
What is the function of tenase?
Activate factor X
It is better at this than TF:VIIa meaning more factor Xa is produced
What happens after more factor Xa has been produced in the propagation phase?
Like before, factors Xa and cofactor Va are produced and bind to form prothrombinase on the platelet membrane
This difference now, is that this happens on a much larger scale due to increases factor Xa production because of the action of tenase
During the propagation phase, how much larger is the prothrombinase mediated production of thrombin than in the initiation phase?
1000 times
After thrombin is produced what effect does it have?
It can cleave small fibrinopeptides from fibrinogen which creates small fibrin monomers
These monomers will form polymers - strands of fibrin
These strands are crosslinked with factor XIIIa which is a factor activated in the presence of thrombin and calcium
This allows a stable clot to be formed
How is the stable clot compressed and made tougher/denser?
Platelets trapped in the fibrin mesh will retract
This aids in toughening up the clot and bringing the edges of the wound together
What is Virchow’s triad?
- Injury to the vessel wall
- Abnormal blood flow (stasis)
- Increased coaguability of blood
Describes the factors likely to contribute to thrombus formation
What are the two types of thrombus?
- Red thrombi
- White thrombi
Describe white thrombi
They are formed primarily of platelets and a few red blood cells
A fibrin mesh holds this together
Where are white thrombi likely to develop?
In the coronary circulation
They can detach forming emboli affecting the brain or another organ
Which drug class is utilised to treat white thrombi?
Antiplatelet drugs
(white thrombi contain mostly platelets)
Describe red thrombi
Red thrombi have a higher concentration of red blood cells than white thrombi
They also have less platelets and fibrin and are more jelly-like
Where are red thrombi likely to originate from?
Deep veins such as those in the lower limbs
Upon forming emboli they will tend to lodge in the lungs
Which drug class is used to treat red thrombi?
Anticoagulants
Upon activation, factor X will associate with what?
Cofactor Va
Where does cofactor Va originate and how is it activated?
α-granules from platelets
Activated by thrombin
Together, what are factor Xa and Va called?
Prothrombinase
Which drugs can act on the production or action of prothrombinase thereby preventing conversion of prothrombin to thrombin?
- Warfarin
- Rivaroxiban
- Heparin
- Dabigatran
What does the drug warfarin do?
Block modifications of factors X and II preventing activation
This is called gamma carboxylation
What does the drug rivaroxiban do?
Direct inhibition of factor Xa causing its inactivation
What does the drug heparin do?
Heparin, LMWH and fondaparinux can directly inhibit factor Xa via antithrombin III
Heparin also acts on thrombin (factor IIa) via antithrombin II to inactive thrombin
What does the drug dabigatran do?
Directly inhibits thrombin
After roughly how many half lives is a drug considered eliminated from the body?
5
What must happen to clotting factors II, VII, IX and X before they can be activated?
They require a specific conformational change to allow them to become available for activation
This change is γ carboxylation of glutamate residues which involves the addition of oxygen and carbon dioxide to allow for glutamic acid residues to be carboxylated
What acts as a cofactor in the γ carboxylation of glutamate residues?
Vitamin K in its reduced form - hydroquinone
Vitamin K reductase provides a constant supply
How does warfarin affect vitamin K reductase?
It inhibits it
This prevents the clotting factors II, VII, IX and X from being made available for activation because γ carboxylation of glutamate residues cannot occur without hydroquinone, which now cannot be reduced
Why does wafarin take a while to work?
γ carboxylated factors must be cleared from the body
Which drug is used for a quick anticoagulant effect?
Heparin
In which instances would an anticoagulant be used?
- DVT
- PE
- AF
- MI
- Congestive heart failure
- Thromboembolic stroke
When warfarin is used what is mandatory?
Regular labratory monitoring
(pro-thrombin time)
Which factors will potentiate the action of warfarin?
- Liver disease (less clotting factors produced)
- High metabolic rate
- Drug interactions - drugs that inhibit vitamin K reduction, platelet activation
- Drug interactions - drugs that inhibit hepatic metabolism of warfarin
Which factors lessen warfarin action?
- Physiological state - pregancy (increased clotting factor synthesis)
- Vitamin K consumption
- Drug interactions - agents that increase hepatic metabolism of warfarin
How is warfarin overdose treated?
- Vitamin K administration (an antidote)
- Withdrawal of warfarin
- Blood transfusion of clotting factors
Where is heparin naturally extracted from?
Beef lung or hog intestine
Why is heparin measured in units of activity?
Due to the non-synthetic origin of heparin, there is significant variation found between batches
Hence, a certain weight is not guranteed a uniform efficacy between batches
In every case except what are LMWH preferred over heparins and why?
Renal failure
LMWHs require renal excretion, heparins do not
How does heparin work?
Speeds up the process at which antithrombin III functions by binding to the antithrombin/factor complex
Give two examples of LMWHs
- Enoxaparin
- Dalteparin
How do LMWHs function?
They inhibit factor X but not factor II (prothrombin)
(this is due to their low molecular weight, but this is all that is required to inhibit the active factor)
Which drugs are related to LMWHs, yet act in a simpler fashion?
- Fondaparinux
- Idrabiotaparinux
How do fondaparinux and idrabiotaparinux differ?
Idrabiotaparinux - longer duration of action (does not need to be administer subcutaneously very frequently)
Idrabiotaparinux has an antedote
How are:
a) Heparins administered
b) LMWHs administered
a) IV or SC
b) SC
By which order of kinetics are heparins and LMWHs processed?
Heparins - zero order kinetics
LMWHs - first order (more predictable)
Why is something such as protamine sulfate an effective antidote against heparin?
Heparin is highly charged, so molecules with opposing charges can act to inhibit heparin
What is the main downside to LMWHs?
They have no antidote
What are the adverse effects of heparin and LMWHs?
- Haemorrhage
- Osteoporosis
- Hypoaldosteronism
- Hypersensitivity reactions
- Thrombocytopenia - depleted platelet count (caused primarily by heparins vs LMWHs
Dabigatran etexilate is a prodrug of dabigatran. Once metabolised to dabigatran, what is the function of this oral drug?
Direct inhibition of thrombin
Molecularly, what is significant about dabigatran that allows it to avoid absorption by the small intestine?
It is a zwitterion
It has both positive and negative charges meaning it is not absorbed.
Rivaroxiban can directly inhibits what?
Factor Xa
Do orally active inhibitors of coagulation have an antidote?
No
When thromboxane is produced during the activation of platelets, what does it do?
Acts on G protein coupled thromboaxane A2 receptors
When ADP s produced by activated platelets, what does it then do?
Binds to P2Y12 receptors
What happens when P2Y12 and thromboxane A2 receptors are bound?
Intracellular calcium is increased
This increases expression of GP IIb and IIIa receptors on platelet surfaces
This allows for the binding of fibrinogenwhich brings the platelets together and under the action of thrombinthe fibrin is converted to fibrinogen
How is the process of increasing intracellular calcium in platelets inhibited?
- Blocking P2Y12 receptors - Clopidogrel
- Block the binding of fibrinogen to GP IIb and IIIa receptors preventing platelet aggregation - Tirofiban
- Block cyclooxygenase 1 preventing synthesis of thromboxane A2 hindering fibriongen binding - Aspirin
What are anti-platelet drugs primarily used for?
Arterial thrombi
What are anticoagulant drugs primarily used for?
Venous thrombi
How does aspirin work?
Irreversibly blocks COX-1 in platelets
This prevents synthesis of thromboxane A2
(also, COX in endothelial cells is blocked so the production of antithrombotic prostaglandin I2 is inhibited - this is not useful, but the overall effect is the desired effect)
Why is the effect of aspirin on platelets relatively long lasting?
Platelets do not have a nucleus so cannot synthesise new enzyme
What are the main adverse effects of aspirin?
GI bleeding
Ulceration
In order to function what does clopidogrel require?
Hepatic metabolism
How does clopidogrel function?
Links to P2Y12 receptors on platelets by disulphide bonds
This produces irrevrsible inhibition
This means ADP cannot bind and intracellular calcium cannot be raised - so GP IIb/IIIa do not incease in expression and fibrinogen cannot bind as easily
When would clopidogrel be administered?
In patients intolerant to aspirin
When _________ is administed with aspirin, there is a synergistic effect
Clopidogrel
When would tirofiban be administered?
Alongside aspirin and heparin to prevent MI in high risk patients with unstable angina
How do fibrinolytic drugs work?
They activate tissue plasminogen which can produce plasmin
Plasmin dissolves clots
How can plasminogen be activated endogenously?
- Tissue plasminogen activators (tPA)
- Plasminogen is converted to plasmin
- Plasmin acts on fibrin to break it into fragments
What is the purpose of fibrinolytic drugs?
To reopen occuluded arteries in acute MI or stroke
(also potentially in life threatening venous thrombosis or PE)
What is the treatment of choice for acute MI?
Primary percutaneous coronary intervention (PCI)
This involves using catherisation to visulaise the occulsed artery and reopen it via balloon catheterisation
Fibrinolytics have a synergistic effect when used with what other drug?
Aspirin
What is streptokinase and what is its origin?
Fibrinolytic drug
It is extracted from streptococci cultures so the body produces antibodies against it after 4 days
It should not be used if the patietn has recently had a streptococcal infection
What are alteplase and duteplase?
Tissue plasminogen activators
Alteplase and duteplase are more effective where?
Fibrin bound plasminogen - they show selectivity for clots
How can fibrinolytics be controlled?
Oral tranexamic acid
Can inhibit plasminogen activation
