Lecture 17 Platelet Pharmacology Flashcards
What is the main target of platelet drugs
Arterial thrombosis
What is the problem with targeting thrombosis
Blood clots that stop bleeding are essential for life yet thrombus formation is a problem
Describe the shape and structure of a platelet
Platelets are much smaller than red and white blood cells. They are flat but with channels that increase their surface area
What is different about the shape of an activated platelet
Upon activation there is a change in the shape of the platelet which transitions from smooth and discoid to spiculated and with pseudopodia. This increases the surface area of the platelet and also increases the efficiency with which they interact
Which receptor on the surface of the platelets binds to fibrinogen to trigger aggregation
Glycoprotein IIb/IIIa
What can be said about the number of receptors that trigger aggregation in response to platelet activation. What is the effect of this on the platelet
At rest there are 50000 to 100000 copies of the receptor on each resting platelet however following platelet activation there is an upregulation of glycoprotein IIb/IIIa receptors. This results in an increased affinity of the receptor for fibrinogen which acts to link the receptors on several platelets together causing aggregation
What is the other name for gp IIb/IIIa
Integrin αiibβ3
How can the gp IIb/IIIa receptor be used as a therapeutic target
Antagonists of gp IIb/IIIa can be used therapeutically to stop clot formation
Give some examples of gp IIb/IIIa antagonists
Abciximab Tirofiban Eptifibatide
What are the downsides of gp IIb/IIIa antagonists
They have a narrow therapeutic window as they aren’t effective at low doses and high doses cause to greater risk of bleeding. The increased risk of major bleeding actually offsets their benefit in reducing ischaemic events
How are gp IIb/IIIa antagonists administered
Intravenous
Aspirin is an effective and strong antiplatelet drug T of F
F – it is a weak APT
Which two isoforms of its enzyme target does aspirin inhibit
Constitutive COX1 (housekeeping gene) and inducible COX2 (inflammatory gene)
Which COX isoform is important for aspirins action as an antiplatelet drug
COX1 is expressed in platelets and is involved in aggregation
At low doses aspirin selectively inhibits which COX isoform
COX1 – higher concentrations inhibit COX2 as well
What is the main role of COX2
Inflammation
What is the role of COX
Phospholipids are metabolised by phospholipase A2 to arachidonic acid. Arachidonic acid is then metabolised by COX to produce prostaglandins H
What is the other name for COX enzymes
Prostaglandin H synthases
Describe the subunit composition of COX
COX is made up of two identical subunits each with two catalytic sites; a peroxidase site and a cyclooxygenase site
What is the molecular mechanism of action of aspirin
The acetyl group from aspirin forms a covalent bond with a serine residue in the COX enzyme. This prevents the arachidonic acid that was made at the plasma membrane from entering the COX channel and reaching the cyclooxygenase site
What is meant when aspirin is referred to as a suicide inhibitor
Because it forms a covalent bond aspirin binds permanently to the enzyme and hence the duration of aspirin’s effects depends on ability of the body to synthesise new COX enzymes. In the case of platelets which have no nucleus this isn’t possible and hence aspirin lasts for the lifetime of the platelet itself
How long does aspirins effect on platelets last
7 to 10 days
What is the specific effect of inhibiting COX1 on platelet function
By preventing the conversion of arachidonic acid to prostaglandin H aspirin blocks the pathway that leads to platelet thromboxane A2 release. Thromboxane A2 activates platelets via binding to the TPα surface receptor
What does this data show about the efficacy of aspirin
This data shows that aspirin reduced the number of vascular deaths compared to placebo. This reduction in vascular mortality was comparable to the clotbuster drug streptokinase
What is meant by aspirin resistance
The continued secretion of thromboxane A2 by platelets in response to appropriate agonist stimulation (such as arachidonic acid and collagen) despite therapy with aspirin at a standard dose
High platelet reactivity despite aspirin therapy signifies aspirin resistance T or F
F
True aspirin resistance is rate T or F
T
Explain the interaction of aspirin with ibuprofen
Unlike aspirin ibuprofen reversibly binds to the COX1 enzyme. Therefore if ibuprofen is taken before an aspirin the inhibition of COX1 activity (TxB2 levels) is lessened. This is because aspirin is metabolised within hours so by the time its able to exert its effect and displace ibuprofen its begun to be metabolised. Hence this is a negative interaction that decreases the efficacy of aspirin
Why is aspirin only a weak antiplatelet drug
Overall aspirin has excellent efficacy in inhibiting platelet thromboxane A2 release but this process plays a limited role in platelet reactivity
What is the most effective target in the treatment of arterial thrombosis
Most effective treatment is targeting the P2Y purinergic receptor on the platelet
There are three classes of purinergic receptors activated by nucleotides such as ADP and ATP what are they
P2Y1 P2Y12 and P2X1
Describe the role of P2Y1
P2Y1 is a GαQ linked GPCR whose activation leads to the activation of PLC-β. This receptor is responsible for the initiation of platelet aggregation and the subsequent shape change
Describe the role of P2Y12
P2Y12 is a GαI linked GPCR whose activation leads to the activation of PI3K+ and an inhibition of adenylate cyclase. This receptor is responsible for the amplification/sustenance of platelet aggregation. It is also involved in the amplification of granule release and procoagulant activity
Describe the role of P2X1
P2X1 is a Ca2+ channel and a weak activator of platelets. It has a minor role in the shape change of the activated platelet as well as an amplification of activation
Which nucleotide is the main activator of platelets and which receptor(s) does it act on
ADP is the main activator of platelets acting on the P2Y1 and P2Y12 receptors
Explain what happens as a result of ADP binding to receptors on the platelet
ADP binds to the P2Y1 receptor to activate the platelet. It also binds to the P2Y12 receptor which serves as an amplification step to increase the activation by P2Y1 receptor activation. Activation of these purinergic receptors leads to the upregulation of integrin αIIbβ3. Activation of integrin αIIbβ3 leads to its binding to fibrinogen and cross linking of platelets hence platelet aggregation
What is the origin of the ADP responsible for activating platelets
ADP comes from the platelet itself where it is stored in dense granules in the cytosol. Hence upon activation of the platelet it degranulates releasing its stored ADP
Outline the positive feedback loop that exists in platelet activation
ADP released from the activated platelet feeds back to further activate more P2Y1 receptors hence amplifying the activation
Explain how the coagulation cascade can interact with platelets to cause activation
Thrombin can also activate platelets by binding to the platelet receptors PAR-1 and PAR-4. In addition activated platelets also catalyse the generation of thrombin which feeds back to activate the platelet receptors and cause aggregation
Other than ADP what factors can activate platelets
Thromboxane A2 binding to the TPα receptor collagen binding to CPVI and 5HT binding to 5HT2A
What evidence from knockout mice supports the role of P2Y12 in the activation of platelets
P2Y12 knockout mice have a drastically weakened thrombus formation
What type of drug is clopidogrel
A thienopyridine prodrug
Clopidogrel has to be converted to its active metabolite before it can exert its effects on P2Y12. Which enzymes are responsible for this metabolism
CYP2C19 and CYP34A
How does the active metabolite of clopidogrel interact with the P2Y12 receptor
R-130964 binds to and inhibits the P2Y12 receptor via its thiol group (-SH) to prevent activation of platelets
Why is clopidogrel in ineffective prodrug
Approximately 85% of absorbed clopidogrel is hydrolysed by hepatic carboxylesterase (CES1) to an inactive metabolite
The CURE study investigated the effects of clopidogrel plus aspirin compared to aspirin alone in patients with ACS. What were the results of this study
Clopidogrel plus aspirin was effective in reducing the incidence of CV death/MI/stroke
What does the data below show about the key issue with clopidogrel
This data shows that there is massive variation between individuals in their response to clopidogrel. Some patients had a complete inhibition of platelet aggregation whilst in others it had almost no effect
Give an example of a drug interaction that changes the response of a patient to clopidogrel
CYP2C19 metabolises many other drugs such as proton-pump inhibitors. Omeprazole is often given with aspirin and clopidogrel to counteract the increased risk of gastrointestinal bleeding that is likely with these drugs. However omeprazole has a negative impact on the effect of clopidogrel by impairing its conversion to its active metabolite which in turn decreases the inhibitory effect of clopidogrel on platelet activation
Give an example of a drug interaction that increases the responsiveness of a patient to clopidogrel
Rifampicin is a drug used for treating tuberculosis that ramps up the response of the liver to clopidogrel. Pretreatment of patients with rifampicin increases clopidogrel active metabolite production (4-fold increase). This leads to a much more effective blockade of P2Y12 receptors which leads to a greater inhibition of platelet aggregation
What is the genetic basis for some patients decreased ability to respond to clopidogrel
Around a quarter of people have a loss of function allele for the gene CYP2C19 that is involved in the formation of the active metabolite of clopidogrel. The patients often have one of two single nucleotide polymorphisms in the CYP2C19 gene. Homozygotes for the *2/*2 and *3/*3 variant are poor metabolisers of clopidogrel and are unlikely to respond to treatment
What is the relationship between CYP2C19 loss of function genotype patients and the risk of complications following PCI
Patients with loss of function alleles in CYP2C19 carry a greater risk of thrombosis following stenting
How can we measure a patients’ response to clopidogrel
The VerifyNow test measures platelet reactivity in response to anti-platelet therapy. This test measures the transmission of light through a blood sample taken from a patient on clopidogrel in order to determine the ability of the blood to coagulate. This can be used to determine whether or not the patient is responding to P2Y12 antagonism
Other than genetics and drug interactions what factors influence a patients’ response to clopidogrel
Dose age weight disease states including diabetes mellitus and chronic kidney disease
Why is clopidogrel less often used now in the clinic
The complexity of factors influencing the response to clopidogrel makes the response of each patient impossible to predict accurately
Why is prasugrel as more effective thienopyridine drug than clopidogrel
The thiol group of prasugrel require for its action on P2Y12 is effectively more protected due to its location in the structure of the molecule. This means that it is less susceptible to degradation by esterases. In fact esterases convert prasugrel to its intermediate prior to conversion to the active metabolite by CYP enzymes.
What does the data below show about the response of patients to prasugrel compared to clopidogrel
Patients treated with 60mg of prasugrel had a much more consistent inhibition of platelet aggregation than patients treated with clopidogrel
Outline the results of the TRITON-TIMI trial that compared the incidence of CV death MI and stroke in patients receiving prasugrel compared to clopidogrel
There was a decreased incidence of CV death MI and stroke in patients with prasugrel compared to clopidogrel
What is the downside of using prasugrel compared to clopidogrel
TRITON-TIMI found that prasugrel did result in an increased incidence of major bleeds life-threatening bleeds and fatal bleeds. Hence due to the significantly increased chance of bleeding prasugrel is often limited to the most high-risk patients of MI stroke etc.
What is different about ticagrelor compared to other P2Y12 antagonists
Ticagrelor is the first oral reversibly binding platelet P2Y12 antagonist. It is from a different class of drug to clopidogrel and prasugrel and not a prodrug so doesn’t require conversion to an active metabolite. Ticagrelor reversibly binds to a different site on the P2Y12 receptor than the thienopyridines
To what class of drugs does ticagrelor belong
Ticagrelor is a cyclo-pentyl-triazolo-pyrimidine (CPTP) drug
What evidence from mice models supports the efficacy of ticagrelor in prevents platelet aggregation
Ticagrelor effectively inhibits platelet aggregation in wild type mice to the levels seen in P2Y12 knockout mice models whose platelets can’t aggregate
What is the benefit of ticagrelor over clopidogrel in terms of onset and offset
Ticagrelor has a faster onset of effects compared to clopidogrel. In addition the effects of ticagrelor wear off much quicker than clopidogrel. Clopidogrel takes 7-10days for the inhibition to wear off whereas ticagrelor inhibition is alleviated over 72hours.
What is the main benefit of ticagrelors faster offset of effects
This is advantageous in MI patients who may need to go to surgery for bypass operations as it allows for the increased chance of bleeding associated with the drug to wear off before surgery is conducted
The PLATO trial compared aspirin and clopidogrel to aspirin and ticagrelor. What did the results of this trial show on the incidence of CV death/MI/stroke
Aspirin + ticagrelor group has decreased incidence of CV death/MI/stroke
The PLATO trial compared aspirin and clopidogrel to aspirin and ticagrelor. What did the results of this trial show on the risk of bleeding associated to surgery
Bleeding related to bypass surgery was decreased with ticagrelor compared to clopidogrel reflecting the rapid offset of action
Ticagrelor is cost effective compared to clopidogrel T or F
T – NICE verified its cost-effectiveness
What is the first line treatment for STEMI and NSTEMI patients in South Yorkshire
DAPT – aspirin + ticagrelor
Currently how long are patients on anti-platelet drugs following MI
Anti-platelet drugs tend to be used for the first year after a heart attack before the drug is stopped
What evidence is there for the long-term use of ticagrelor in preventing thrombotic events
Ticagrelor at two different doses on a background of aspirin reduced thrombotic events (CV death/MI/stroke) when given for several years. This was significantly better than placebo for all endpoints
What are the current ESC guidelines on the management of ACS
Dual antiplatelet therapy (DAPT) is the first line treatment for patients with acute coronary syndromes. This consists of aspirin + ticagrelor or aspirin + prasugrel
