Question
A 70-year-old man with no significant past illness presents with crushing chest discomfort. ECG reveals ST elevation in anterior leads. He is treated with aspirin and a second antiplatelet drug before urgent angioplasty. What is the primary action of this second drug (ticagrelor)?
a. Blocks ATP interaction with platelet receptors
b. Prevents fibrinogen binding to GP IIb/IIIa receptors
c. Inhibits phosphodiesterase type 5
d. Inhibits ADP attachment to platelet receptors
e. Acts as a non-specific phosphodiesterase blocker
Answer
Inhibits ADP attachment to platelet receptors
Detailed Explanation
Ticagrelor belongs to the class of P2Y12 receptor antagonists (ADP receptor inhibitors), similar to clopidogrel and prasugrel.
- Platelet activation is mediated significantly by ADP binding to P2Y12 receptors.
- Ticagrelor reversibly blocks this receptor.
- This prevents activation of glycoprotein IIb/IIIa receptors, thereby inhibiting platelet aggregation.
Key clinical relevance:
- Used in acute coronary syndrome (ACS) including STEMI
- Given with aspirin → dual antiplatelet therapy (DAPT)
- Reduces risk of thrombosis after PCI
Why others are wrong:
- ATP inhibition → no such clinical mechanism
- GP IIb/IIIa inhibitors → drugs like abciximab, not ticagrelor
- PDE5 inhibitors → sildenafil group
- Non-selective PDE inhibitors → theophylline
Important nuance:
- Ticagrelor is reversible (unlike clopidogrel, which is irreversible)
- Can cause dyspnoea due to ↑ adenosine levels
Cheat Sheet (Exam Gold)
Ticagrelor
- Class: P2Y12 inhibitor
- Mechanism: Blocks ADP receptor → ↓ platelet aggregation
- Reversibility: Reversible
- Use: ACS + PCI (with aspirin)
- Dose: 90 mg BD
- Side effects: Dyspnoea, bleeding
- Advantage: Faster, more consistent than clopidogrel
Compare
- Clopidogrel → irreversible, prodrug
- Prasugrel → stronger, higher bleeding risk
Flash Cards
Q1: What receptor does ticagrelor block?
A: P2Y12 ADP receptor → prevents platelet activation
Q2: Is ticagrelor reversible or irreversible?
A: Reversible
Q3: Why is ticagrelor used with aspirin?
A: Different pathways → synergistic platelet inhibition
Q4: Key side effect of ticagrelor?
A: Dyspnoea (due to adenosine accumulation)
Q5: Which drug class inhibits GP IIb/IIIa?
A: Abciximab, eptifibatide
MCQs (High-Level)
MCQ 1
Which of the following best explains the antiplatelet action of ticagrelor?
a. Inhibition of thromboxane A2 synthesis
b. Blockade of ADP-induced platelet activation
c. Direct thrombin inhibition
d. Inhibition of vitamin K-dependent factors
Answer: b
Explanation: Ticagrelor blocks P2Y12 receptor → prevents ADP-mediated platelet aggregation. Others represent aspirin, DOACs, and warfarin respectively.
MCQ 2
Which statement about ticagrelor is FALSE?
a. It binds reversibly to platelet receptors
b. It requires hepatic activation
c. It may cause dyspnoea
d. It is used in ACS
Answer: b
Explanation: Ticagrelor is NOT a prodrug (unlike clopidogrel). It acts directly.
MCQ 3
Which drug acts at the final common pathway of platelet aggregation?
a. Ticagrelor
b. Aspirin
c. Abciximab
d. Clopidogrel
Answer: c
Explanation: GP IIb/IIIa inhibitors block fibrinogen binding → final pathway.
MCQ 4
Which of the following combinations provides synergistic antiplatelet action in ACS?
a. Warfarin + heparin
b. Aspirin + ticagrelor
c. Clopidogrel + prasugrel
d. Dabigatran + aspirin
Answer: b
Explanation: Aspirin (TXA2 inhibition) + P2Y12 inhibitor → dual pathway blockade.
MCQ 5
A patient develops breathlessness after starting an antiplatelet drug post-PCI. Which mechanism explains this?
a. Histamine release
b. Adenosine accumulation
c. Bradykinin increase
d. Nitric oxide excess
Answer: b
Explanation: Ticagrelor inhibits adenosine uptake → ↑ adenosine → dyspnoea.
MCQ 6
Which of the following is TRUE regarding clopidogrel but NOT ticagrelor?
a. Causes dyspnoea
b. Requires metabolic activation
c. Used in ACS
d. Inhibits ADP receptor
Answer: b
Explanation: Clopidogrel is a prodrug requiring CYP activation.
Summary for Quick Exam Revision
Ticagrelor is a reversible P2Y12 receptor antagonist that inhibits ADP-mediated platelet activation and aggregation, making it a cornerstone drug in acute coronary syndrome management alongside aspirin as part of dual antiplatelet therapy. Unlike clopidogrel, ticagrelor is not a prodrug and has faster, more predictable action. It prevents activation of glycoprotein IIb/IIIa receptors, thereby reducing thrombus formation following plaque rupture or PCI. A unique adverse effect is dyspnoea due to increased adenosine levels. GP IIb/IIIa inhibitors act downstream at the final common pathway, while aspirin acts via thromboxane A2 inhibition—hence the synergy in combined therapy. Ticagrelor is contraindicated in high bleeding risk and prior intracranial hemorrhage. Understanding the platelet activation cascade and drug targets is critical for mastering ACS pharmacology.