Quinidine is a class IA antiarrhythmic agent used to treat heart rhythm disturbances. It is a diastereomer of antimalarial agent quinine, originally derived from the bark of the cinchona tree. The drug causes increased action potential duration, as well a reduction in phase 0 upstroke velocity through blockade of cardiac fast inward sodium current INa and inhibition of several potassium currents including IKr and IKs, resulting in prolonged QRS and QT intervals on the electrocardiogram. Quinidine also exhibits vagolytic and peripheral alpha adrenergic blocking activity that can produce reflex tachycardia and hypotension, and it has modest negative inotropic effects at higher plasma concentrations. Pharmacokinetically, quinidine is well absorbed orally, undergoes hepatic metabolism primarily via cytochrome P450 enzymes including CYP3A4, is subject to enterohepatic recirculation, and is highly protein bound; therapeutic monitoring is often used because of a narrow therapeutic index and concentration related toxicity such as cinchonism and proarrhythmia.
Parent: Quinidine / Quinidine Sulfate
Typical quality control specifications for quinidine active pharmaceutical ingredient include limits for related compounds and specified impurities with acceptance criteria set to control safety and efficacy. Representative limits used in regulatory and compendial specifications are: quinine not more than 0.5% w w, epiquinidine not more than 0.2% w w, dihydroquinidine not more than 0.2% w w, quinidine N oxide not more than 0.3% w w, desethylquinidine not more than 0.2% w w, with any individual unspecified impurity commonly limited to 0.1% w w and total impurities typically limited to 1.0% w w. Identification and quantitation are routinely performed by validated HPLC with orthogonal confirmation by LC MS or NMR for critical impurities, and impurity control follows ICH Q3A guidance for threshold levels relative to maximum daily dose where applicable.
Quinidine is used to manage certain atrial and ventricular arrhythmias by slowing conduction and prolonging refractoriness to suppress abnormal automaticity and reentrant circuits; clinical use today is limited to selected indications such as maintenance of sinus rhythm in atrial fibrillation or control of ventricular arrhythmias when alternative agents are unsuitable.
No, Quinidine is not classified as a calcium channel blocker; its primary electrophysiological actions are blockade of fast sodium channels and inhibition of potassium currents, although at high concentrations it can exert weak effects on calcium conductance.
Widespread use of quinidine declined because of safety concerns including a relatively high incidence of proarrhythmia such as torsades de pointes, systemic toxicities including cinchonism, frequent drug interactions, and the availability of alternative antiarrhythmic and rate control therapies with more favorable safety and efficacy profiles.
Quinidine slows conduction velocity in atrial and ventricular myocardium, prolongs action potential duration and effective refractory period, widens the QRS complex and prolongs the QT interval, can reduce contractile force at high concentrations, and through autonomic effects may produce tachycardia or hypotension; these electrophysiologic actions underlie both its therapeutic antiarrhythmic effects and its proarrhythmic risks.
