Dextromethorphan-Bupropion-Esketamine Triadic Pharmacology: Molecular Targets, Enzymatic Interactions, and Risk-Benefit Assessment of a Three-Agent Combination for Treatment-Resistant Depression
Andrea Fagiolini, Alessandro Cuomo, Giulio Corrivetti, Mario Pinzi, Pietro Carmellini, Alessandra Marenna, Francesco Monaco, Annarita Vignapiano, Eva Frigola-Capell, The Opade Group
International Journal of Molecular Sciences November 9, 2026 DOI: 10.3390/ijms272110036 (opens in new tab)
Study at a glance
AI-extracted from the abstract| Characteristics | Review Peer reviewed |
|---|---|
| Interventions | Dextromethorphan Dextrorphan Quinidine Bupropion Esketamine |
| Topics | Depression Esketamine |
| Key findings | Argues that CYP2D6 activity determines which pharmacology of dextromethorphan is expressed rather than scaling exposure to a fixed pharmacology, because dextrorphan is the more potent NMDA channel blocker while dextromethorphan more potently inhibits the serotonin transporter. In the authors' model, CYP2D6 inhibition raises the serotonergic fractional-target-engagement index approximately fourfold while reducing the NMDA channel index; the direction was robust to one-at-a-time 0.5- to 2-fold perturbations but not to all joint extreme assumptions. |
Abstract
Dextromethorphan is extensively O-demethylated by cytochrome P450 2D6 (CYP2D6) to dextrorphan, and the two molecules do not share a target profile. Dextrorphan is the more potent N-methyl-D-aspartate (NMDA) channel blocker; the parent compound is the more potent serotonin transporter inhibitor; their sigma-1 affinities are comparable, so differences in sigma-1 engagement depend on exposure rather than intrinsic affinity. CYP2D6 activity therefore does not scale exposure to a fixed pharmacology; it determines which pharmacology is expressed. This review develops that principle and extends it to a three-agent system. From published unbound concentrations and functional potencies, we calculate a model-derived fractional-target-engagement index. Under the base-case inputs, CYP2D6 inhibition raises the serotonergic index approximately fourfold while reducing the NMDA channel index; deterministic sensitivity analysis shows that the direction is robust to one-at-a-time 0.5-fold to 2-fold perturbations but not to all joint extreme assumptions. Quinidine and bupropion are analyzed as mechanistically distinct proofs of concept, and quinidine-derived exposure estimates are not treated as quantitative surrogates for the dextromethorphan–bupropion steady state. The framework is then applied to the untested question of whether a boosted dextromethorphan combination and esketamine are redundant or complementary. The agents share an NMDA channel mechanism, while CYP2B6 pathway overlap remains a hypothesis-generating observation rather than evidence of clinically relevant competition. No controlled data on the three-agent combination exist, and no therapeutic recommendation is made.
Comparable studies
Other narrative reviews on esketamine for depression, most cited first.
| Study | Year | Design | Participants |
|---|---|---|---|
| Synthesizing the Evidence for Ketamine and Esketamine in Treatment-Resistant Depression: An International Expert Opinion on the Available Evidence and Implementation Adults with treatment-resistant depression | 2021 | Review | |
| Long-term safety of ketamine and esketamine in treatment of depression | 2022 | Review | |
| Ketamine, Esketamine, and Arketamine: Their Mechanisms of Action and Applications in the Treatment of Depression and Alleviation of Depressive Symptoms. | 2024 | Review | |
| Variations in BDNF and Their Role in the Neurotrophic Antidepressant Mechanisms of Ketamine and Esketamine: A Review. Patients with depression | 2024 | Narrative review | |
| The Possible Application of Ketamine in the Treatment of Depression in Alzheimer’s Disease | 2022 | Review |
Citations in the library
Cites 13
- NMDAR inhibition-independent antidepressant actions of ketamine metabolites Nature April 24, 2016
- Ketamine and Ketamine Metabolite Pharmacology: Insights into Therapeutic Mechanisms Pharmacological Reviews June 26, 2018
- Efficacy and Safety of Flexibly Dosed Esketamine Nasal Spray Combined With a Newly Initiated Oral Antidepressant in Treatment-Resistant Depression: A Randomized Double-Blind Active-Controlled Study American Journal of Psychiatry May 21, 2019
- Efficacy of Esketamine Nasal Spray Plus Oral Antidepressant Treatment for Relapse Prevention in Patients With Treatment-Resistant Depression JAMA Psychiatry June 5, 2019
- Ketamine: A tale of two enantiomers Journal of Psychopharmacology November 6, 2020
and 8 more in the library