Quinidine (Cinchona Quinoline Alkaloid · Class Ia Antiarrhythmic · Informational)
| Compound | Quinidine (Conquinine) |
| Chemical class | Alkaloid — Quinoline (Cinchona bark diastereomer; Class Ia antiarrhythmic) |
| CAS | 56-54-2 |
| Primary source | Cinchona species (bark) — minor alkaloid; also produced by quinine epimerisation |
| Key applications | Pharmaceutical Class Ia antiarrhythmic; antimalarial (second-line); informational reference |
| Claim strength | High (as pharmaceutical); Informational only |
| Typical form | Pharmaceutical tablet (quinidine sulfate, quinidine gluconate); not a dietary supplement |
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Name origin: Quinidine is the C-8, C-9 diastereoisomer of quinine — same molecular formula (C20H24N2O2) and same connectivity, differing only in stereochemistry at two chiral centres. Despite this minor structural difference, their pharmacological profiles diverge significantly: quinine is primarily antimalarial; quinidine is primarily antiarrhythmic. Pharmaceutical history: Quinidine was introduced as an antiarrhythmic in 1918 (Walter Frey) after observation that quinine-treated malaria patients with concurrent atrial fibrillation showed AF conversion. This established quinidine as the first pharmacological treatment for AF. Quinidine (Class Ia) was the dominant antiarrhythmic for decades but has been largely displaced by safer agents (beta-blockers, amiodarone, flecainide, dronedarone). Current pharmaceutical use: Quinidine retains FDA approval for: (1) atrial fibrillation and atrial flutter (conversion to sinus rhythm, maintenance); (2) life-threatening ventricular arrhythmias; (3) Brugada syndrome (sodium channel gain-of-function, treated with quinidine sodium channel blockade); (4) as an adjunct in severe malaria where IV quinine is unavailable; (5) quinidine + darifenacin for Wolff-Parkinson-White syndrome. Supplement status: Quinidine is a regulated pharmaceutical drug with significant cardiac toxicity potential; not a dietary supplement ingredient.
Quinidine — Pharmaceutical Context
Class Ia antiarrhythmic mechanism: Quinidine blocks voltage-gated sodium channels (Phase 0 of cardiac action potential), slowing conduction velocity, and blocks multiple potassium channels (IKr, IKur, IKs, IKAch), prolonging the action potential duration and refractory period. QT interval prolongation is the primary toxicity risk — predisposing to torsades de pointes, a potentially fatal ventricular arrhythmia. The pro-arrhythmic risk has substantially reduced quinidine’s clinical use despite its efficacy. Pharmaceutical pharmacology reference.
Brugada syndrome — niche quinidine application: Brugada syndrome is a genetic sodium channelopathy (SCN5A loss-of-function or related) causing ventricular fibrillation risk in young adults. Quinidine’s sodium channel-blocking action, counterintuitively, suppresses the Brugada arrhythmia by normalising the voltage gradient across the epicardial-endocardial activation map. This is one of the few remaining niche quinidine applications where it is pharmacologically difficult to replace. Pharmaceutical evidence: Moderate (observational and small controlled studies).
CYP2D6 inhibition — pharmacokinetic probe: Quinidine is one of the most potent inhibitors of CYP2D6 known (Ki ~0.06 µM). In pharmacokinetic research, small sub-therapeutic doses of quinidine (50 mg) are used as a CYP2D6 phenotyping probe — converting extensive metabolisers to phenotypic poor metabolisers and revealing CYP2D6-mediated drug metabolism. Drug combination Nuedexta (dextromethorphan + quinidine 20 mg) uses this mechanism to prevent dextromethorphan CYP2D6 metabolism, allowing therapeutic dextromethorphan plasma levels for pseudobulbar affect. Pharmacokinetic research reference.
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Frequently Asked Questions — Quinidine
Why is quinidine used in Nuedexta if it causes arrhythmias?
Nuedexta contains quinidine at 20 mg — far below the antiarrhythmic dose (300–600 mg). At 20 mg, quinidine provides near-complete CYP2D6 inhibition (converting extensive metabolisers to phenotypic poor metabolisers) without meaningful antiarrhythmic or QT-prolonging effects. This allows co-administered dextromethorphan (a CYP2D6 substrate) to reach therapeutic CNS levels for pseudobulbar affect treatment. The Nuedexta approval demonstrates how sub-therapeutic doses can be used to exploit the drug interaction mechanism without the adverse effects seen at therapeutic doses.
What is Brugada syndrome and why does sodium channel blockade help?
Brugada syndrome is caused by reduced sodium channel function (SCN5A loss-of-function or related mutations), creating an imbalance between Ito (transient outward potassium current) and INa (sodium inward current) in the epicardium during early repolarisation. This produces a characteristic ECG pattern and creates conditions for ventricular fibrillation. Quinidine blocks Ito more than it further reduces INa at the doses used, restoring the balance and suppressing the arrhythmic substrate — a mechanistically counterintuitive application for a sodium channel blocker drug.
Is tonic water a relevant source of quinidine?
No. Commercial tonic water contains quinine, not quinidine. The two are structurally similar diastereoisomers but quinidine is not naturally produced in significant quantities by Cinchona trees (only minor amounts). Industrial quinidine is produced by epimerisation of quinine under acidic conditions. Tonic water has no clinically relevant quinidine content.
What replaced quinidine as the standard AF treatment?
Quinidine was largely displaced by: amiodarone (Class III, more effective with lower pro-arrhythmic risk); flecainide and propafenone (Class Ic, for AF without structural heart disease); dronedarone (Class III with better safety profile); sotalol (Class II/III). For rate control rather than rhythm control in AF, beta-blockers (metoprolol, bisoprolol) and calcium channel blockers (diltiazem, verapamil) are standard first-line agents without the QT risks of quinidine.
Related compounds: Quinine, Cinchonine, Sparteine, Colchicine
Claim-strength scale – High = multiple human RCTs; Moderate = limited trials or strong preclinical convergence; Emerging = early-stage lab or animal data.
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