Question

Which of the following is NOT a Class 1C anti-arrhythmic drug:

• A. \( \text{Propafenone} \)
• B. \( \text{Mexiletine} \)
• C. \( \text{Flecainide} \)
• D. \( \text{Moricizine} \)

Hint:

Anti-arrhythmic drugs are classified into four main classes based on the Vaughan-Williams classification. Class 1 drugs (sodium channel blockers) are further divided into: 
- 1A: Moderate sodium blockade (e.g., Quinidine). 
- 1B: Weak sodium blockade (e.g., Mexiletine). 
- 1C: Strong sodium blockade (e.g., Flecainide, Propafenone).

Answer 1

The Correct Option is B

The Vaughan Williams Class 1 anti-arrhythmics are split into subgroups 1A, 1B, and 1C mainly based on how quickly the drug binds to and releases from cardiac sodium channels. Comparing that kinetic profile across the four options settles this question.

1. Propafenone: Binds sodium channels and releases from them slowly, the hallmark of the Class 1C subgroup, giving it a strong overall conduction-slowing effect.
2. Mexiletine: Binds and releases from sodium channels quickly, which is the defining kinetic behavior of Class 1B rather than Class 1C, and this fast on-off action is why it is grouped separately from the other three drugs here.
3. Flecainide: Shares the slow channel release kinetics of propafenone, placing it firmly in Class 1C as well.
4. Moricizine: Also shows the slow-release channel binding typical of Class 1C, grouped with this subclass in standard pharmacology references.

Three of the four drugs share the slow sodium-channel release kinetics that define Class 1C, while mexiletine's fast-binding kinetics set it apart as the Class 1B outlier. The correct answer is Mexiletine.