Platelets and Anti-platelet Agents

Platelets and Anti-Platelet Agents
Dr. Roger Preston, School of Pharmacy and Biomolecular Sciences, RCSI Royal College of Surgeons in Ireland
Platelet Plug Formation

  • Key Role: Platelet aggregation prevents excessive blood loss at the site of injury.

Clot Formation in Vivo

  • Cite: Falati et al., Nat Med 2002.

  • Process Overview: Exposure of sub-endothelial proteins leads to accumulation of activated platelets at the site of injury, resulting in fibrin deposition and clot formation.

What is a Platelet?

  • Definition: Platelets are anucleate discs derived from megakaryocytes.

  • Characteristics:

    • Size: Very small, 2-4 $\mu m$ .

    • Life-span: Approximately 10 days.

    • Function: Circulate as quiescent cells to survey vascular integrity and undergo explosive activation upon vessel wall damage.

How are Platelets Activated?

  1. Change in Shape:

    • Transition from disc to spherical shape with filopodia.

    • Occurs within seconds of adhesion due to cytoskeletal reorganization (actin and tubulin).

  2. Activation of Cell Surface Integrins: GpIIb/IIIa.

  3. Granule Secretion: Release of chemical messengers.

  4. “Flip/Flop” of Plasma Membrane: Alters membrane properties.

Purpose of Platelet Activation

  • Conversion from a quiescent state to an active hemostatic state.

  • Structural changes to aid clot formation through:

    • Secretion of factors that facilitate clot formation.

    • Activation of cell surface molecules to enhance adhesion to other platelets and coagulation factors.

Release of Platelet Granules

  1. Dense Granules:

    • Appearance: Dense on electron microscopy.

    • Contents: Contains Ca2+Ca^{2+}

      , ADP, ATP, serotonin - these messengers promote further platelet activation and aggregation.

  2. Alpha Granules:

    • Contents: Contain clotting factors essential for coagulation.

Open Canalicular System (OCS)

  • Function: Increases the area for rapid granule content release.

  • Structure: Flat surface with periodic invaginations forming internal membrane tunnels that act as conduits.

Arterial Thrombosis

  • Feature: Arterial clots are platelet-rich.

  • Components Involved: Include collagen, thrombin, and factors inducing high shear flow conditions.

GpIIb/IIIa Activation Mechanism

  • Activate GpIIb/IIIa upon exposure to:

    • Collagen

    • Epinephrine

    • ADP

    • Thrombin

    • Thromboxane A2

  • Function: GpIIb/IIIa is crucial for platelet aggregation through binding to fibrinogen, an essential step in thrombus formation.

Anti-platelet Drug Regulation

  • Anti-platelet drugs can regulate platelet activity, hence influencing thrombus formation.

Major Classes of Anti-Platelet Drugs

  1. Aspirin

  2. Clopidogrel/Prasugrel

  3. Dipyridamole

  4. GPIIb/IIIa Antagonists

Inhibitors of Platelet Activation

  • Purpose: To prevent arterial thrombosis, particularly rich in aggregated platelets.

  • Most Common Example: Aspirin, which inhibits signaling pathways leading to platelet activation.

Anti-platelet Agent 1: Aspirin

  • Mechanism of Action: Aspirin irreversibly inhibits platelet Cyclo-oxygenase (COX), decreasing thromboxane A2 (TxA2) synthesis, which promotes platelet recruitment and activation.

  • Biochemical Pathway:

    • Arachidonic Acid is metabolized by COX to an intermediate prostaglandin PgG2, which is then converted to TXA2 by thromboxane synthetase.

Side Effects of Aspirin

  • Excessive Bleeding: Due to reduced platelet function.

  • Gastrointestinal Symptoms: GI hemorrhage and upper-GI issues resulting from COX inhibition in the gastric mucosa.

Anti-platelet Agent 2: Clopidogrel and Prasugrel

  • Mechanism of Action:

    • Clopidogrel acts as a non-competitive antagonist of ADP (P2Y12) receptors on platelets.

    • It is administered as a ‘pro-drug’ and metabolized to its active form in the liver, covalently binding to the ADP receptor to prevent further platelet recruitment.

  • Obstacles to Therapeutic Efficacy:

    • Low levels of hepatic enzyme CYP2C19 in 2-14% of the US population result in insufficient clopidogrel activation.

Prasugrel

  • Characteristics:

    • A recent addition to ADP receptor inhibitors approved for reducing thrombotic cardiovascular events, including stent thrombosis in acute coronary syndrome.

    • Requires activation by liver enzymes but is activated by different Cytochrome P450s than clopidogrel, resulting in lower incidence of drug resistance.

Anti-platelet Agent 3: Dipyridamole

  • Mechanism of Action: Elevates intracellular cAMP levels which inhibit platelet activation.

  • PGI2 (Prostacyclin): Released from endothelial cells; inhibits platelet activation via binding to PGI2 receptors, increasing cAMP production.

  • PDE Inhibitors: Cause an elevation of cAMP by inhibiting the enzyme phosphodiesterase (PDE), disrupting normal degradation pathways.

Anti-platelet Agent 4: GpIIb/IIIa Antagonists

  • Role of GpIIb/IIIa Complex:

    • Critical for platelet aggregation as a receptor for von Willebrand factor (vWF) and fibrinogen.

    • GpIIb/IIIa activation leads to platelet aggregation through fibrinogen cross-linking.

    • GPIIb/IIIa Inhibitor Abciximab: Monoclonal antibody targeting the fibrinogen binding site on GpIIb/IIIa to inhibit platelet aggregation, especially during acute coronary syndromes.

GPIIb/IIIa Deficiency

  • Consequence of Deficiency: Platelet plug fails to accumulate at the site of injury, leading to conditions such as Glanzmann’s thrombasthenia where platelet aggregation is impaired.