Dextromethorphan-Bupropion

Indications

• FDA-approved in August 2022 for the treatment of major depressive disorder in adults, as a fixed-dose oral combination tablet.
• Positioned as an alternative to standard monoamine antidepressants when a faster onset or a nonserotonergic mechanism is desired.
• Registration trials showed separation from placebo as early as week 1, faster than typical for SSRI or SNRI therapy.
• Off-label interest exists for treatment-resistant depression, but the drug was studied and approved in general adult MDD populations.
• Not approved for smoking cessation, seasonal affective disorder, or agitation in dementia despite the bupropion component.
• Safety and effectiveness in pediatric patients have not been established and use under 18 years is not recommended.

Mechanism of action

• Dextromethorphan is an uncompetitive NMDA receptor antagonist, producing rapid glutamatergic modulation analogous to ketamine.
• It is also a sigma-1 receptor agonist and inhibits serotonin and norepinephrine reuptake, adding monoaminergic and neuroplastic effects.
• Bupropion is a norepinephrine and dopamine reuptake inhibitor that contributes independent antidepressant activity to the combination.
• Bupropion is a strong CYP2D6 inhibitor, and this is the pharmacokinetic reason it is present: it blocks first-pass clearance of dextromethorphan.
• Without bupropion, oral dextromethorphan is metabolized so rapidly that plasma concentrations never reach the therapeutic range.

Pharmacokinetics

• Dextromethorphan peak concentrations occur about 3 hours after dosing, with bupropion peaking around 3 hours as well.
• CYP2D6 inhibition by bupropion raises the dextromethorphan half-life to roughly 22 hours, compared with about 4 hours when given alone.
• Bupropion has a half-life near 15 hours and is metabolized by CYP2B6 to hydroxybupropion, an active metabolite with a longer half-life.
• Steady state for both components is reached in approximately 8 days of twice-daily dosing with the extended-release tablet.
• In known CYP2D6 poor metabolizers dextromethorphan exposure rises substantially, so the maximum is one tablet once daily.

Dosing

• Start one tablet (45 mg/105 mg) PO once daily in the morning for 3 days, then increase to one tablet twice daily.
• Separate the two daily doses by at least 8 hours; the maximum dose is two tablets (90 mg/210 mg) per day.
• Swallow tablets whole and do not crush, chew, or split them, since the extended-release matrix controls bupropion exposure.
• In moderate renal impairment with eGFR 30-59 mL/min/1.73 m2, limit to one tablet once daily; avoid if eGFR is below 30.
• In moderate hepatic impairment give one tablet once daily, and avoid the product entirely in severe hepatic impairment.
• Taper is not formally required, but gradual reduction is prudent given the serotonergic and noradrenergic components.

Adverse effects

• Dizziness affects roughly 16 percent of patients, headache about 8 percent, and diarrhea and somnolence about 7 percent each.
• Dry mouth, hyperhidrosis, and sexual dysfunction occur but are less frequent than with serotonergic antidepressants.
• Seizure risk is dose dependent from the bupropion component, which is why the product is contraindicated in seizure disorders.
• Blood pressure and heart rate can rise, so hypertension should be controlled before starting and monitored during treatment.
• Activation, insomnia, anxiety, and manic switch in undiagnosed bipolar disorder are all recognized with this combination.
• Angle-closure glaucoma, hepatotoxicity, and rare serious hypersensitivity reactions are described in the product labeling.

Monitoring

• Check blood pressure and heart rate at baseline and periodically thereafter, since bupropion raises both in some patients.
• Screen every patient for a personal or family history of seizures, eating disorders, and bipolar disorder before the first dose.
• Monitor for emergent suicidality, agitation, and behavioral activation during the first weeks and after any dose change.
• Verify renal and hepatic function before starting, since both moderate impairments require reduction to once-daily dosing.
• Ask about concurrent CYP2D6 substrates at each visit, because bupropion can raise their levels substantially and unexpectedly.

Interactions

• Contraindicated with monoamine oxidase inhibitors and within 14 days of stopping one because of hypertensive reactions and serotonin syndrome.
• Contraindicated in seizure disorder, current or prior bulimia or anorexia nervosa, and abrupt withdrawal from alcohol or sedatives.
• Do not combine with other bupropion-containing products or with additional dextromethorphan-containing cough preparations.
• As a strong CYP2D6 inhibitor it raises levels of tricyclics, risperidone, metoprolol, and tamoxifen, reducing tamoxifen efficacy.
• CYP2B6 inducers such as carbamazepine, phenytoin, ritonavir, and efavirenz can lower bupropion exposure and blunt response.

Special populations

• Pregnancy data are limited; bupropion exposure registries have not shown a consistent teratogenic signal but data remain sparse.
• Both components enter breast milk, and seizures have been reported in a breastfed infant exposed to bupropion, so caution applies.
• Not approved under 18 years, and adolescents carry the class suicidality warning if the drug is prescribed off-label.
• In older adults use standard dosing but monitor blood pressure closely and consider reduced renal clearance of bupropion metabolites.
• Limit to one tablet daily in moderate renal or hepatic impairment, and avoid entirely with severe impairment of either organ.

Clinical pearls

• Bupropion is here as a CYP2D6 inhibitor; without it oral dextromethorphan never reaches useful levels.
• Never add another bupropion product; the seizure risk is additive and dose dependent.
• CYP2D6 poor metabolizers should receive no more than one tablet daily.