Two antihistaminics terfenadine and astemizole were withdrawn from the market following the occurrence of cardiac arrhythmias when they were present in high levels in the blood. These effects were explained by the fact :
First, the core concept here is likely related to drug interactions and their effect on cardiac ion channels. Both terfenadine and astemizole are antihistamines, but they were withdrawn because of their association with arrhythmias. I remember that these drugs inhibit the CYP3A4 enzyme, which is involved in their metabolism. If CYP3A4 is inhibited by other drugs, the levels of terfenadine and astemizole can rise, leading to toxicity.
The correct answer probably involves their ability to block potassium channels, specifically the hERG channels, which are responsible for the rapid repolarization phase of the cardiac action potential. Blocking these channels causes QT prolongation and can lead to torsades de pointes, a type of ventricular arrhythmia. So the mechanism is hERG channel blockade.
Now, the wrong options might include other mechanisms like blocking sodium channels, calcium channels, or other enzymes. For example, an option suggesting they block sodium channels would be incorrect because that's more relevant to antiarrhythmics like sodium channel blockers. Another wrong option could be about inhibiting acetylcholine receptors, which isn't related here. Also, if an option mentions a different enzyme like CYP2D6 instead of CYP3A4, that's incorrect because terfenadine and astemizole are metabolized by CYP3A4.
The clinical pearl here is that drugs causing hERG blockade can lead to QT prolongation and arrhythmias. It's important to remember that CYP3A4 inhibitors (like ketoconazole) can increase the levels of these antihistamines, leading to toxicity. So, the key takeaway is the hERG channel blockade and the role of CYP3A4 metabolism in their toxicity.
**Core Concept**
The question examines the mechanism by which certain antihistamines (terfenadine, astemizole) cause cardiac arrhythmias. Key concepts include **hERG potassium channel blockade**, **QT interval prolongation**, and **CYP3A4 enzyme inhibition**. These drugs were withdrawn due to risks of **torsades de pointes**, a life-threatening arrhythmia.
**Why the Correct Answer is Right**
Terfenadine and astemizole are **hERG (human ether-a-go-go-related gene) potassium channel blockers**, which disrupt the rapid repolarization phase (Phase 3) of the cardiac action potential. This causes **QT interval prolongation** on the ECG, increasing the risk of **torsades de pointes**. Their metabolism via **CYP3A4** means co-administration with CYP3A4 inhibitors (e.g., ketoconazole) raises plasma levels, exacerbating toxicity. High drug concentrations enhance hERG blockade, directly linking their withdrawal to this mechanism.
**Why Each Wrong Option is Incorrect**
**Option A:** Suggests sodium channel blockade. This is incorrect, as sodium channel blockade is characteristic of antiarrhythmics like lidocaine, not terfenadine/astemizole.
**Option B