Drug Guide
Dextromethorphan Hydrobromide and Quinidine Sulfate
Classification
Therapeutic: Cough suppressant and cognitive enhancer (for pseudobulbar affect)
Pharmacological: NMDA receptor antagonist and sigma-1 receptor agonist; Quinidine is an antiarrhythmic agent and inhibitor of CYP2D6
FDA Approved Indications
- Treatment of pseudobulbar affect (PBA) in patients with Alzheimer’s disease, stroke, multiple sclerosis, or traumatic brain injury
Mechanism of Action
Dextromethorphan acts on sigma-1 receptors and NMDA receptors to modulate neuronal excitability, reducing PBA episodes; Quinidine inhibits the CYP2D6 enzyme, increasing plasma levels of dextromethorphan to enhance its therapeutic effect.
Dosage and Administration
Adult: Typically, 20 mg of dextromethorphan combined with 10 mg of quinidine twice daily; dosage may vary based on clinical response.
Pediatric: Not approved for pediatric use.
Geriatric: Use with caution due to potential for increased adverse effects and drug interactions.
Renal Impairment: Adjust dose as needed; see prescribing information.
Hepatic Impairment: Use caution; monitor closely, dose adjustments may be necessary.
Pharmacokinetics
Absorption: Well absorbed orally.
Distribution: Wide distribution; crosses blood-brain barrier.
Metabolism: Dextromethorphan is extensively metabolized in the liver via CYP2D6; quinidine inhibits CYP2D6, increasing dextromethorphan levels.
Excretion: Primarily renal.
Half Life: Approximately 3-6 hours for dextromethorphan; quinidine’s half-life varies.
Contraindications
- Known hypersensitivity to dextromethorphan or quinidine.
- Concomitant use with MAO inhibitors.
- History of QT prolongation or risk factors for arrhythmia.
Precautions
- Use with caution in patients with hepatic impairment, cardiac disease, or electrolyte abnormalities; monitor cardiac rhythm and ECG.
Adverse Reactions - Common
- Dizziness (Common)
- Drowsiness (Common)
- Gastrointestinal upset (Common)
Adverse Reactions - Serious
- QT prolongation, Torsades de Pointes (Rare)
- Serious hypersensitivity reactions including anaphylaxis (Rare)
Drug-Drug Interactions
- Other medications prolonging QT interval (e.g., certain antidepressants, antipsychotics).
- MAO inhibitors (contraindicated).
- Other serotonergic drugs (risk of serotonin syndrome).
Drug-Food Interactions
- Avoid foods or beverages that could interact with QT prolongation risk.
Drug-Herb Interactions
- St. John’s Wort, SSRIs, SNRIs could increase serotonergic effects.
Nursing Implications
Assessment: Monitor cardiac status, ECG for QT interval prolongation, hepatic function.
Diagnoses:
- Risk for cardiac arrhythmia.
- Risk for adverse drug reactions.
Implementation: Administer as prescribed; monitor for adverse effects, especially cardiac and neurological.
Evaluation: Assess symptom control and adverse effects regularly.
Patient/Family Teaching
- Instruct on the importance of adherence to dosing schedule.
- Warn about potential side effects like dizziness, drowsiness.
- Advise to report symptoms of arrhythmia (e.g., palpitations, dizziness).
- Caution about QT prolongation and avoiding concomitant use of contraindicated drugs.
Special Considerations
Black Box Warnings:
- QT prolongation and risk of serious cardiac arrhythmias.
- Serious interactions with MAO inhibitors leading to hypertensive crises or serotonin syndrome.
Genetic Factors: CYP2D6 ultra-rapid metabolizers may have altered drug levels.
Lab Test Interference: May interfere with some laboratory tests due to QT prolongation effects.
Overdose Management
Signs/Symptoms: Serious cardiac arrhythmias, dizziness, hallucinations, serotonin syndrome.
Treatment: Supportive care, cardiac monitoring, activated charcoal if ingestion was recent, and specific interventions for arrhythmias.
Storage and Handling
Storage: Store at room temperature, protected from moisture and light.
Stability: Stable for the duration of the shelf life specified in the package insert.