Mechanism of Action of Bupropion
Bupropion works primarily by blocking the neuronal reuptake of dopamine and norepinephrine, and additionally acts as a noncompetitive antagonist of α4β2 nicotinic acetylcholine receptors. 1
Primary Mechanisms
Monoamine Reuptake Inhibition
Bupropion is a relatively weak inhibitor of the neuronal uptake of norepinephrine and dopamine and does not inhibit monoamine oxidase or the reuptake of serotonin 2. This dual norepinephrine and dopamine reuptake inhibition distinguishes bupropion from other antidepressants and is devoid of clinically significant serotonergic effects 3.
The drug acts as a sympathomimetic, blocking neuronal uptake of dopamine and norepinephrine, thereby simulating some of the desirable effects of nicotine when used for smoking cessation 1.
Nicotinic Receptor Antagonism
Bupropion has antagonistic activity at the α4β2 nicotinic cholinergic receptors, which may blunt the rewarding effects of nicotine from cigarettes 1, 4. This noncompetitive antagonism occurs through a sequential mechanism: (1) bupropion binds to receptors in the resting state, decreasing the probability of ion channel opening, (2) it accelerates the desensitization process of remaining open channels, and (3) it interacts with a binding domain located between specific amino acid rings shared with other compounds 5.
Active Metabolites
Bupropion is extensively metabolized into three active metabolites: hydroxybupropion (formed via CYP2B6), threohydrobupropion, and erythrohydrobupropion 2. These metabolites contribute to the drug's clinical effects:
- Hydroxybupropion is approximately one-half as potent as bupropion
- The other metabolites are approximately 5-fold less potent than the parent compound
- At steady state, hydroxybupropion reaches peak plasma concentrations approximately 7 times higher than bupropion, with an AUC about 13 times that of the parent drug 2
Clinical Implications
The mechanism of action remains presumed to be mediated by noradrenergic and/or dopaminergic mechanisms 2. This unique pharmacological profile results in:
- Efficacy in treating depression comparable to SSRIs
- Effectiveness as a smoking cessation aid
- Reduced incidence of common antidepressant side effects such as sexual dysfunction, weight gain, and sedation 3
- Potential benefits in patients with comorbid depression and nicotine dependence 1
Important Caveat
At clinical doses (150 mg), bupropion achieves relatively low dopamine transporter occupancy and does not significantly increase extracellular dopamine levels in human striatum, despite its dopamine reuptake inhibition properties 6. This suggests the therapeutic mechanism may be more complex than simple dopamine elevation.