Platelet Aggregation Inhibitors

Professor. Dr. MAHMOUD KHATTAB,

The components of a platelet

Platelets and the Mechanisms of Arterial

Thrombosis Platelet adhesion and aggregation occur at the site of plaque

rupture at luminal exposed subendothelium ADHESION occurs in response to collagen or vWf followed by a

cascade of further platelets recruitment Activated platelets exert pro-coagulant effects and the soluble

coagulation cascade is activated Fibrin strands and erythrocytes predominate within the lumen of

the vessel and downstream in the body and tail of the thrombus

Platelet Aggregation Activated platelets undergo three consecutive

processes (a) shape change, (b) secretion of platelet granular contents (ADP, fibrinogen & 5-HT) and finally (c) platelet aggregation

The final common pathway in platelet aggregation is cross-linking of the activated GP IIb/IIIa receptoractivated GP IIb/IIIa receptor (integrin αIIbβ3) with circulating adhesive arginine-arginine-glycine-asparagine (R-G-D) sequenceglycine-asparagine (R-G-D) sequence macromolecules, predominantly fibrinogen and von Willebrand factor

There is ~50,000-80,000 GP IIb/IIIa receptors on the surface of each platelet

Platelet Aggregation GP IIb/IIIa receptors undergo inside-out (low-high

affinity) signalling in order to bind to vWf/fibrinogen Main stimuli for full platelet aggregation include:

Collagen, ADP, thromboxane A2 (TXA2), & thrombin

All except for collagen act through G-protein coupled receptors activating two transductions

• Phospholipase C

• Phospholipase A2

Stored ADP & de novo synthesized TXA2 act as positive feedback mediators

The stimulatory pathway for platelet secretion

TXA2 is produced from AA via 3-steps enzymatic process; PLA2-COX-1/TXA2 synthase

ADP activates Gi-coupled P2Y12

receptors TXA2 & thrombin activate Gq-

PLC-coupled TXA2 & thrombin receptors respectively

Ultimately, GP IIb/IIIa receptors undergo inside-out signalling bringing platelet aggregation

Platelet Activation Activation of membrane-bound G proteins catalyzes the

intracellular signaling system, predominantly inositol biphosphate (IP3) and diacyl glycerol (DAG)

IP3 causes the release of intracellular calcium from the SR & dense granules, phosphorylation of the myosin light chain, and allows the platelet contractile apparatus to perform platelet shape changes

DAG activates PKC and in turn the activation of the surface integrin glycoprotein (GP) IIb/IIIa receptor occurs

Activated receptor undergoes a conformational change and binds to the arginine-glycine-asparagine (R-G-D) sequence on circulating macromolecules mainly fibrinogen & vWf, and is responsible for cross-linking platelets together

TXA2 provides amplification of the aggregation response & a significant vasoconstrictor effect

Increased platelet activity implicated in post-MI reocclusion and ischemia

Several lines of evidence implicate increased platelet activity in reocclusion and recurrent ischemia following acute MI

A, Autopsy data indicate that patients who died after acute MI are several times more likely to have a platelet-rich thrombus occluding the coronary infarct-related coronary artery if they had received thrombolytic therapy than if they had been treated without it

B, The production of thromboxane B2 (TXB2), the stable metabolite of TXA2, is markedly increased, indicating that platelet activation is stimulated in the early minutes after thrombolytic therapy is given (The administration of oral aspirin is able to block this reaction)

C, Platelet aggregability following thrombolysis with either streptokinase or tissue-type plasminogen activator is increased, as measured by responses to either thrombin or ADP added to platelet-rich plasma

ASPIRIN IN ACUTE ISCHEMIC SYNDROMESASPIRIN IN ACUTE ISCHEMIC SYNDROMES

Low dose of aspirin inhibit TXA2 production by inhibiting irreversibly COX-1 acetylation (at serine-530)

After oral intake, platelets are exposed to relatively high concentration of ASA in the portal circulation

Antiplatelet activity is achieved at low dose of ASA than that required for analgesia or endothelial PGI2 production?

Aspirin Antiplatelet EfficacyAspirin Antiplatelet Efficacy1- Dose1- Dose

Inhibition of 90% to 95% of TXA2 levels is needed to abolish in vitro platelet aggregation

Given over a period of days, this level of inhibition can be achieved with doses as low as 20 mg

In volunteer subjects, a dose of 80 mg can inhibit aggregation as soon as 15 minutes after ingestion; in the unstable patient, however, in whom absorption is more likely to be a problem and in whom circulating platelets are more likely to be activated

Most authorities recommend initial therapy with a dose of 160 mg (one half-tablet) to 325 mg (one adult tablet)160 mg (one half-tablet) to 325 mg (one adult tablet)

The aspirin should be crushed/chewed to facilitate absorption

The frequency of gastrointestinal complications appears to be dose-dependent

Aspirin prolongs bleeding time, risk of hemorrhage

Aspirin Antiplatelet Efficacy

A, Efficacy of aspirin in patients with unstable angina Studies have demonstrated reductions in morbid ischemic events

in patients with unstable angina following the ingestion of aspirin using varying doses of aspirinvarying doses of aspirin and varying periodsvarying periods between the onset of symptoms and treatment with aspirin

B. Efficacy of aspirin in patients following acute MI Analysis of 10 trials of antiplatelet agents (mainly aspirin) for secondary

prophylaxis of vascular events following acute MI, the Antiplatelet Trialists Collaboration reported striking reductions in nonfatal MI and nonfatal stroke in patients treated chronically with antiplatelet therapy (n=18,441) as well as a highly significant reduction in cardiovascular mortality

Similar reductions were seen for patients receiving antiplatelet therapy following stroke. There were no differences in effect between varying doses of aspirin

II- Glycoprotein IIb/IIIa Receptor Antagonists1- Glycoprotein IIb/IIIa murine-derived 7E3

Fab monoclonal antibody (Abciximab) Abciximab is composed of 7E3 Fab fragments a murine-

derived (m) monoclonal antibody directed against glycoprotein GPIIb/IIIa.

Mechanism: The m7E3 Fab binds selectively to the glycoprotein GPIIb/IIIa receptors inhibiting platelets binding to fibrinogen and von Willebrand factor, and consequently inhibiting platelet aggregation

Clinical Efficacy: In acute MI patients, when the m7E3 Fab antibody was administered 3, 6, or 15 hours after beginning treatment with rt-PA, the incidence of recurrent ischemia was lower than expected

II- Glycoprotein IIb/IIIa Receptor Antagonists1- Glycoprotein IIb/IIIa murine-derived 7E3

Fab monoclonal antibody (Abciximab) Administration and therapeutic use: Abciximab is administered

intravenously as an adjuvant to angioplasty surgery for the prevention of ischemic complications of angioplasty

o Heparin or aspirin is given with abciximab Blockade of glycoprotein IIb/IIIa receptors following a bolus

injection of c7E3 Fab (0.25 mg/kg): Although the plasma half-life of this monoclonal antibody is only several

minutes, a potent biologic effect can be measured 2 hours after the injection

In patients undergoing elective angioplasty, at this point, more than 80% of receptors are bound by the antibody (and therefore unable to participate in aggregation). At 6 hours, between 60% and 70% of receptors are bound, and at 24 hours, 50% are bound

In patients undergoing thrombolysis for acute MI, the return toward baseline occurs more rapidly. A continuous infusion leads to sustained blockade of the receptor

II- Glycoprotein IIb/IIIa Receptor Antagonists

2- Synthetic arginine-glycine-aspartic acid (R-G-D) sequence mimetics

Eptifibatide (peptidergic) Eptifibatide (peptidergic) andand tirofiban (non- tirofiban (non-peptiic)peptiic) are synthetic mimetics of the R-G-D sequence of fibrinogen

Hence, they block the binding of fibrinogen to glycoprotein GPIIb/IIIa receptors

They are given intravenously for the reduction of thrombotic complications during coronary angioplasty

Clinical trials showed reductions in incidence of death and non-fatal MI in response to the use of both agents

III- Thromboxane III- Thromboxane AntagonistsAntagonists

Ridogrel is a combined thromboxane synthasethromboxane synthase inhibitor and thromboxane A2 (TXA2) receptor antagonist, orally active

It has no effect on the vascular production of prostacyclinprostacyclin but cyclic endoperoxides (PGH2) may increase

In the Ridogrel Aspirin Perfusion Trial (RAPT), patients received either ridogrel oror aspirin (initially given IV then orally) + IV streptokinase for acute MI

In both groups of patients inhibition of TXA2 was markedly inhibited, but recurrent ischemic events (angina, reinfarction, and ischemic stroke) were observed less frequently among the ridogrel-treated patients

IV- Platelet ADP Receptor Antagonists (Thienopyridines)

Ticlopidine & Clopidogrel They inhibit irreversibly ADP binding to receptors Prevent ADP-mediated activation of the

glycoprotein GPIIb/IIIa & ultimately inhibiting platelet aggregation

No effect on PGs synthesis In the small percentage of patients who are truly

aspirin-intolerant, ticlopidine should be considered as an alternative form of therapy

Ticlopidine & ClopidogrelAdverse Effects

Ticlopidine is associated with more side effects than Clopidogrel

Nausea, dyspepsia, diarrhea (20% of patients) Hemorrahge (5%) Leukopenia Leukopenia in 1% of patients (most serious),

check WBC in the first 3 months of treatment Ticlopidine, but not, clopidogrel can lead to fatal

neutopenia Thrombotic thrombocytopenic purpura may occur

with both agents

Other Antiplatelet Agents

DipyridamoleDipyridamole, inhibits phosphodiesterase activity and inhibit adenosine uptake

It is weak antiplatelet vasodilator If used, should be combined with aspirin CliostazolCliostazol is another PD inhibitor used for

intermittent claudication

Antiplatelet DrugsAspirin Irreversibly inhibits

production of TXA2 80-325 mg/d Minutes

to h Up to 1 wk

Ticlopidine Inhibits and antagonizes ADP receptor and may inhibit interactions of GP IIb/IIIa receptor with fibrinogen

250 mg three times daily

3-5 d Up to 1 wk

Dipyridamole Phosphodiesterase inhibitor

25-75 mg three times daily

Hours -

c7E3 Fab (Abciximab)

Monoclonal antibody antagonist of GP IIb/IIIa-ligand binding

0.25 mg/kg bolus, 0.1 mg/min infusion, over 12 h

Minutes 12-24 h

Investigational

Clopidogrel Similar to ticlopidine 75 mg/d - Up to 1 wk

Ridogrel Thromboxane synthetase and thromboxane receptor antagonist

300 mg twice daily

- -

Synthetic R-G-D sequence mimetics

Antagonist of GP IIb/IIIa-ligand binding

Under investigation

Minutes Approximately 4-6 h (longer for

oral compounds)

Overview of Antiplatelets