Week 8 part 2 - Haemostasis Flashcards
Haemostasis definition
the stopping of blood loss from damaged vessels and protect against haemorrhage. A highly complex, regulated process
(basically a plug formed)
Mechanisms involved in haemostasis following a wound include:
- Vasoconstriction
- Platelet adhesion to the exposed tissue
- Platelet activation to form a haemostatic plug
- Reinforcement of plug by fibrin
There are times we want to promote haemostasis
- Haemophilia
- Haemorrhage (after surgery or trauma etc)
There are more times though that we want to inhibit
haemostasis (thrombotic disease)
- Myocardial infarct (heart-attack), stroke (arterial thrombosis-emboli)
- Deep vein thrombosis (venous thrombosis)
facts and terms about blot clots
- Myocardial infarction = heart attack where blood flow is stopped to the heart musclke
- stroke = clot that blocks blood flow to the brain
- deep vein thrombosis = a clot that forms in one of the deep veins of the body which is usually in the leg (calf/thigh).
- pulmonary embelism = when a clot (usually from a dvt) breaks free and could travel in the blood stream to block an important organ e.g. heart
what is haemostasis?
- Body’s response to blood vessel injury and bleeding.
- Involves a coordinated effort between platelets and numerous blood clotting proteins (or factors).
- Results in the formation of a blood clot and subsequent stopping of the bleed.
- Interactions between platelet activation and the coagulation cascade
- Thrombin is a critical mediator
Thrombosis
‘haemostasis in the wrong place’; the formation of a haemostatic plug within the vasculature in the absence
of bleeding
Predisposing factors (Virchow’s triad) include:
- Injury to the vessel wall (eg rupture of atherosclerotic plaque)
- Altered blood flow (eg veins in legs during prolonged sitting, turbulence)
- Increased coagulability of the blood (thrombophilia)
Drugs affecting haemostasis/thrombosis
antiplatelet agents
anticoagulants (heparin, LMWHs, hirudin, warfarin)
fibrinolytic agents (anistreplase, alteplase, reteplase, streptokinase, urokinase)
Haemostasis: formation of a platelet plug
- platelets adhere to and are activated by exposed collagen at the site of vessel injury
- activated platelets release ADP and thromboxane A2.
- These chemical messengers work together to activate other platelets passing by
- Newly activated platelets aggregate onto growing platelet plug and release even more platelet-attracting chemicals
- normal(uninjured) endothelium releases prostacyclin and nitric-oxide, which inhibit platelet aggregation, so platelet plug is confined to site of injury.
Platelet activation
Platelets are activated through an enzyme called cyclooxygenase-1 (COX-1). When blood vessel injury occurs, COX-1 produces thromboxane A2, which promotes platelet aggregation and blood clot formation. Inhibiting COX-1, such as with aspirin, reduces thromboxane A2 production, preventing excessive platelet activation and clotting.
Antiplatelet agents
Antiplatelet agents reduce platelet aggregation
Used for prophylaxis(to treat disease) of arterial thrombosis (platelet-rich clots), including acute MI; pts at high risk of MI; after cardiac surgery. Reduce the risk of vascular events
Combining different classes of antiplatelet agent often leads to a synergistic effect, but increases the risk of bleeding
Arachidonic acid metabolism
Arachidonic acid is released from membrane phospholipids by phospholipase A2
AA can then be catalytically converted into a series of biologically active metabolites (eicosanoids).
The three major pathways for eicosanoid production involve the actions of cyclooxygenases, lipoxygenases and epoxygenases
PGs as mediators of platelet function
Arachidonate
> (Cyclooxygenase) (COX) >
Endoperoxides (PGG2, PGH2)
> [1] PGI2 (prostacyclin)
= INHIBITS AGGREGATION
> [2] TXA2 (thromboxane A2)
= PROMOTES AGGREGATION
Antiplatelet drugs: aspirin
- Aspirin irreversibly acetylates the active site of COX-1, preventing the formation of endoperoxides.
- Major products downstream of COX include prostacyclin (inhibits aggregation) and thromboxane A2 (promotes aggregation)
- intereferes with haemostasis by inhibiting the production of thromboxane A2 (which promotes platelet aggregation and vasoconstriction)
- suppressing thromboxane A2 = impedes platelet aggregation and reduce clot formation
- thins the blood less likely to clot
- If aspirin reduces prostacyclin production, how does it act as an antiplatelet drug?
- Aspirin irreversibly inhibits COX1, thus reducing TXA2 synthesis for the
lifespan of the platelet - PERMANENT inactivation of platelet COX-1 (in circulation) and reduction in clotting mechanisms (TXA2 from platelets no longer available). Platelets do not have a nucleus- so cannot make new COX-1
- In contrast, the inhibition of prostacyclin formation is reversible,
because the endothelium is capable of re-synthesising cyclooxygenase
(have a nucleus) - So, aspirin produces a very long coagulation time (clotting time)
- A single (low) dose of aspirin doubles clotting time, and this effect lasts for a week- not coincidentally, the time it takes platelets to regenerate
- Low dosing allows for reduced gastrointestinal side-effects of blocking PGs
Antiplatelet drugs: aspirin MOA
- COX-1 activated when platelets become activated –> generates TXA2.
- TXA2 binds to TP receptors on platelets thereby increasing expression of GPIIb/IIIa on platelet cell membranes (see later slides)
- Aspirin: inhibits TXA2 production
permanently from platelets –> chronically reduces platelet aggregation
Aspirin Side Effects
AEs include:
Increased bleeding time (common);
GI irritation, haemorrhage/ulcers (see above) - these are rare, due to low dosing frequency – once/twice weekly, low dosages (e.g. 100-600 mg), and enteric coatings)
blood thinning!
Antiplatelet drugs: purinergic antagonists
- ADP induces platelet aggregation by
activating P2Y12 receptors - Thienopyridines irreversibly inhibit P2Y12 receptors to inhibit platelet aggregation
- Clopidogrel is well absorbed orally; prodrug is converted to active metabolite by CYP enzymes (including CYP2C19) in the liver
- Variant alleles or CYP2C19 inhibitors may decrease effectiveness (25 SNPs identified to date, many of which are associated with alterations in activity)
- Used for prevention of ischaemic events in patients with symptomatic atherosclerosis; can be used in combination with aspirin
- AEs include diarrhoea, GI ulcer, increased risk of bleeding
Antiplatelet agents (2)
GP IIb/IIIa inhibitors (eg abiciximab, Arofiban) prevent fibrinogen crosslinking of platelets
Haemostasis: the role of thrombin
thrombin is a component of the clotting cascade and plays multiple roles in haemostasis
- stimulated conversion of fibrinogen to fibrin
- activates factor stabilising fibrin meshwork of clot
- enhances activation of more prothrombin into thrombin through positive feedback
- enhances platelet aggregation
through positive feedback, aggregated platelets secrete PF3, which stimulates clotting cascade that results in thrombin activation
Blood coagulation
Fibrinogen (soluble) > (thrombin)> fibrin (insoluble)
- Activation initiates the coagulation cascade, amplifying the signal
- Thrombin (factor IIa) cleaves fibrinogen=fibrin; also activates factor XIII fibrinoligase, induces platelet aggregation and affects VSM tone
- Cascade requires control by endogenous inhibitors: eg antithrombin inhibits critical clotting factors (IXa, Xa, XIa, XIIa, thrombin).
anticoagulants
Drugs affecting haemostasis/thrombosis
- heparin
- LMWHs
- hirudin
- warfarin
Anticoagulants: direct thrombin inhibitors
- Hirudin was one of the first anticoagulants, and is a direct thrombin inhibitor
- Direct thrombin inhibitors do not rely on endogenous anticoagulant systems (compare with heparin and LMWH)
- Dabigatran is an orally-available DTI that reversibly inhibits thrombin (free and clot-bound) and thrombin-mediated platelet aggregation.
- Fixed dose produces predictable anticoagulant effect (promising alternative to warfarin)
- Rivaroxaban is an orally-available drug that inhibits factor Xa to reduce thrombin production. Similar indications/usage as dabigatran.
Anticoagulants: heparins
- Heparin is family of large sulfated polysaccharides; not absorbed from the gut (charge, high Mw), administered IV or SC.
- Indirectly inhibits thrombin by enhancing anti-thrombin (AT) mediated inactivation of factor Xa and thrombin (increases affinity of AT for clotting factors)
- Inactivation of thrombin requires long polysaccharide chains that can complex with both AT and thrombin; these are absent in LMWH (eg enoxaparin).
- Limited efficacy on platelet/fibrin bound factors
- Main risk is haemorrhage: can be treated with protamine sulphate
(complexes with and inactivates heparin). AEs include bruising,
thrombosis, osteoporosis (long-term), hypersensitivity
Anticoagulants: heparin vs LMWH
?
Anticoagulants: vitamin K antagonists
- Vitamin K is a fat-soluble vitamin that is required for synthesis of factors II, VII, IX and X.
- Obtained from plants and from gut microflora; given routinely at birth in Australia
- Acts as a co-factor for carboxylase
Anticoagulants: warfarin
- Warfarin is the most important oral anticoagulant*, however there are several issues associated with its use
- It competes with vitamin K for binding at vitamin K reductase (VKORC1)
- Orally acIve and rapidly absorbed
- Small distribution volume, strongly albumin-bound
- Peak concentration in blood within a few hours, yet peak pharmacological effects takes ~ 48h to develop
- The response of individual patients to warfarin can vary dramatically
- The VKORC1 gene is polymorphic
- Warfarin is metabolized by CYP2C9 (also polymorphic)
- Low margin of safety; requires
frequent monitoring and dose
individualization - Multiple drug interactions (other
albumin-bound drugs; antibiotics;
P450) - Crosses placenta
Fibrinolysis
?
Drugs affecting Fibrinolysis
?
Fibrinolytics
- Endogenous plasminogen activators (serine proteases) present in blood
- At a thrombus, they cleave plasminogen (deposited on fibrin strands) to release plasmin
- Plasmin digests fibrin, fibrinogen, ECM proteins and some clotting factors
- Action is localized to the clot as plasminogen activators are mainly effective on plasminogen that is bound to fibrin; escaping plasmin is inactivated by circulating
plasmin inhibitors
*Fibrinolytic drugs are used to dissolve clots and reopen occluded arteries in patients with acute MI within 12 h of onset. Small window of opportunity for administration (time is muscle/brain)
* Initial fibrinolytic drugs derived from haemolytic streptococci;
current versions utilise recombinant DNA technology.
* Alteplase is a recombinant tissue plasminogen activator. Greater activity on plasminogen bound to fibrin, which increases ‘clot selectivity’. Non-antigenic. Modified version of tPA including tenecteplase.
* Administered IV, major risk of bleeding (including GI
haemorrhage and haemorrhagic stroke).
