IC3- Antiplatelets Flashcards
What is the general MOA of antiplatelets?
Block platelet aggregation and PRIMARY haemostasis
*Recall- primary haemostasis: platelets adhere → secrete contents (ADP, Thromboxane A2 (Tbx A2) → recruits more platelets → platelet aggregation and plug
What are the 4 different types of antiplatelets and the name of the agent under each type?
(Only the ones we are focusing on)
- Adenosine uptake & PDE3 inhibitor (Dipyridamole)
- Irreversible COX-1 inhibitor (Aspirin)
- ADP receptor (P2Y12 inhibitors) (Clopidogrel, Ticagrelor)
MOA of Dipyridamole? (Adenosine uptake & PDE3 inhibitor)
- Inhibits platelet activation and aggregation by ↑ cAMP within platelets:
- Adenosine reuptake inhibitor → ↑ plasma (extracellular) adenosine activation of A2 receptors on platelets
- PDE3 inhibitor → ↓ cAMP degradation into AMP → ↑ cAMP in platelets → inhibits platelet activation and aggregation - Vasodilator as also inhibits adenosine reuptake and PDEs in vascular SM
Is dipyridamole used as monoTx or as a adjunctive antiplatelet? Why?
Dose-limiting effects due to its additional vasodilatory effects.
Hence used as adjunctive agent to other antiplatelets.
What is another indication of dipyridamole due to its vasodilatory effects?
Infused IV as alternative to exercise for myocardial perfusion imaging
What is the onset, peak effect and DOA of dipyridamole?
What do we say about the reversibility of dipyridamole?
- Onset: 20-30 min (fast); peak effect 2-2.5h
- DOA: ~3h (short- rapid reversal) → hence usually modified release
ADEs of dipyridamole? (5)
- *HEADACHE
- *HYPOTENSION
- Dizziness
- Flushing
- GI disturbance (d, n/v)
Contraindications for dipyridamole? (4)
- Hypersensitivity
- Caution in hypotension
- Caution in severe coronary artery disease
- Caution for bleeding when combined with heparin or other anticoagulants/ antiplatelets
DDIs of dipyridamole?
- ↑ adenosine: ↑ cardiac adenosine levels and effects
- ↓ cholinesterase inhibitors: may aggravate myasthenia gravis
What are the functions of COX-1 (found in platelets) and COX-2 (produced by endothelial cells) enzymes?
- COX-1 (found in platelets): synthesis of TXA2 (platelet agonist) which promotes aggregation and further vasoconstriction. Can only be restored by formation of new platelets (7-10d)
- COX-2 (produced by endothelial cells): production of PGI2 → which inhibit platelet aggregation (endogenous antiplatelet effect). Restored by synthesis of new COX enzyme (3-4h)
What is the MOA of aspirin?
Does it act equally on both COX-1 or COX-2?
- Non-selective, non-steroidal IRREVERSIBLE inhibitor of both COX-1 and COX-2
- Inhibits COX-1 more → inhibits platelet production of TXA2
Is OD dosing or Q4-6H dosing for aspirin more effective? Explain why
OD dosing more effective.
Aspirin given less frequently → allows endothelial production of PGI2 to recover while irreversible inhibition of platelets persist
Is low dose or high dose of aspirin more effective?
What are the doses?
Low dose (75-325 mg loading dose OR 40-160 mg daily) more effective than high dose (500 mg-1 g)
ADEs of aspirin? (2)
- Upper GI effects (UGI) → gastric ulcers, bleeding (inhibits COX-1 production of protective PG in stomach) (that’s why low-dose aspirin for cardioprotective effects is a/w 2-4x increase in UGI events)
- ↑ risk of bleeding and bruising
Caution for aspirin?
Caution in pts with platelet and bleeding disorders