A 62-year-old woman with acute coronary syndrome (NSTEMI) and a history of aspirin allergy (documented severe rash) is admitted to the coronary care unit. She requires dual antiplatelet therapy. Clopidogrel loading dose has been given. What is the drug of choice for the second antiplatelet agent in this patient?

Explanation

## Dual Antiplatelet Therapy in NSTEMI with Aspirin Allergy **Key Point:** Ticagrelor is the preferred alternative P2Y₁₂ inhibitor when aspirin is contraindicated in acute coronary syndrome. It provides superior efficacy compared to clopidogrel and does not cross-react with aspirin allergy. ### Rationale for Ticagrelor **Mechanism of Action:** - Ticagrelor is a **direct, reversible P2Y₁₂ receptor antagonist** (non-thienopyridine) - Achieves faster and more potent platelet inhibition than clopidogrel - Reversible binding allows platelet function recovery within 3–5 days of cessation **Clinical Advantages in ACS:** 1. **Superior efficacy** — PLATO trial (2009) demonstrated 16% reduction in cardiovascular death, MI, or stroke vs. clopidogrel in ACS 2. **Faster onset** — Median time to peak effect: 2 hours (vs. 3–5 hours for clopidogrel) 3. **No aspirin cross-reactivity** — Safe in aspirin-allergic patients; allergy typically involves salicylate sensitivity, not the P2Y₁₂ pathway 4. **Reversible binding** — Shorter duration of action; useful if urgent surgery is needed ### Comparison of P2Y₁₂ Inhibitors in ACS | Agent | Class | Onset | Peak Effect | Reversibility | Aspirin Allergy Safe | Efficacy vs Clopidogrel | |-------|-------|-------|-------------|----------------|----------------------|------------------------| | **Clopidogrel** | Thienopyridine | Slow | 3–5 hours | Irreversible | No (prodrug, hepatic metabolism) | Baseline | | **Ticagrelor** | Cyclopentyl-triazolo-pyrimidine | Fast | 2 hours | Reversible | **Yes** | **Superior (PLATO)** | | **Prasugrel** | Thienopyridine | Moderate | 3–4 hours | Irreversible | No (thienopyridine class) | Superior in STEMI, not preferred in NSTEMI | | **Dipyridamole** | Phosphodiesterase inhibitor | Slow | 24–72 hours | Reversible | Yes | Weak; not used in acute ACS | **Clinical Pearl:** Ticagrelor is the **preferred P2Y₁₂ inhibitor** in acute coronary syndrome (both STEMI and NSTEMI) according to current ESC and ACC/AHA guidelines. In aspirin-allergic patients, ticagrelor + clopidogrel dual therapy is not ideal (two thienopyridines); ticagrelor monotherapy or ticagrelor + another non-aspirin agent is preferred. **High-Yield:** The PLATO trial (2009) established ticagrelor's superiority in ACS. Ticagrelor is now preferred over clopidogrel in acute coronary syndromes, especially STEMI. In aspirin allergy, ticagrelor is the agent of choice because it does not share the salicylate-related mechanism of aspirin hypersensitivity. **Warning:** Prasugrel, while effective in STEMI, is a thienopyridine and shares the same chemical class as clopidogrel; it is not preferred in NSTEMI and offers no advantage in aspirin-allergic patients. Dipyridamole is too weak for acute ACS and is reserved for secondary stroke prevention. **Mnemonic:** **PLATO** = **P**latelet inhibition and patient **O**utcomes — the landmark trial that established ticagrelor's superiority in ACS. [cite:Harrison 21e Ch 295; ESC Guidelines on ACS 2020]