IUPAC: (RS)-2-(tert-butylamino)-1-(3-chlorophenyl)propan-1-one · MW 239.74 g/mol · CAS 34911-55-2 · C13H18ClNO
Bupropion (Wellbutrin, Zyban, Aplenzin, Forfivo). Aminoketone antidepressant on a substituted-cathinone (β-keto amphetamine) scaffold · NDRI — norepinephrine–dopamine reuptake inhibitor · plus non-competitive nicotinic ACh receptor antagonist. Also a component of Contrave (bupropion + naltrexone, obesity) and Auvelity (bupropion + dextromethorphan, MDD).
Bupropion is mechanistically two drugs in one molecule. It is a weak, competitive reuptake inhibitor at the dopamine (DAT) and norepinephrine (NET) transporters — it plugs the S1 central pocket the way cocaine and the SSRIs do, but with modest affinity — and it is simultaneously a non-competitive antagonist of neuronal nicotinic acetylcholine receptors (α3β4 > α4β2 > α7). Unlike amphetamine it is not a substrate-releaser; it does not run the transporters in reverse and it does not touch VMAT2. Critically, it has essentially no serotonergic activity (SERT Ki > 10 μM).
Occupies the DAT S1 orthosteric site (Ki ≈ 440 nM; DA-uptake IC50 ≈ 0.55–1.6 μM), blocking dopamine clearance in mesocortical/striatal terminals. This is the dopaminergic backbone of the antidepressant and pro-motivational effect.
Inhibits norepinephrine reuptake (IC50 ≈ 1.45 μM; Ki ≈ 7 μM), 2–4× weaker than at DAT. Noradrenergic tone drives the activating, arousal-promoting quality and the mild off-label ADHD utility.
Blocks nicotine-evoked currents at α3β4 (IC50 ≈ 1.8 μM) and α4β2 (≈ 8–12 μM) receptors by an open-channel / allosteric mechanism, not by competing with ACh. This — not the monoamine effect — is why Zyban works for smoking cessation.
CYP2B6 converts bupropion to (2S,3S)-hydroxybupropion (radafaxine), which circulates at ~10× the parent concentration with a long half-life. It carries much of the real-world NET/DAT inhibition and α3β4 antagonism in vivo.
SERT Ki ≈ 13 μM / IC50 ≈ 47 μM — pharmacologically silent at serotonin transport. No 5-HT release, no 5-HT2 agonism. This is the mechanistic root of its distinct side-effect profile.
Bupropion does not enter the terminal as a substrate and does not collapse vesicular gradients. Its ceiling on synaptic DA is therefore set by reuptake blockade — bounded, unlike the runaway efflux of releasers — which caps the euphoria and the abuse potential.
The clinical logic of the smoking-cessation indication is a two-front attack on nicotine dependence: bupropion partially substitutes for nicotine's dopaminergic reward (via DAT blockade) while simultaneously blunting nicotine's own signal at the α3β4/α4β2 nicotinic receptors it would otherwise activate. Craving and withdrawal fall on both axes.
Bupropion undergoes extensive first-pass metabolism; the parent compound is largely a delivery vehicle for its active metabolites. CYP2B6 hydroxylates it to hydroxybupropion, while carbonyl-reductase enzymes (including 11β-HSD1) generate threo- and erythro-hydrobupropion. At steady state, hydroxybupropion plasma levels run roughly an order of magnitude above the parent and its long half-life means it, not bupropion, dominates chronic exposure. This is why CYP2B6 polymorphisms and inducers/inhibitors (e.g. carbamazepine, ritonavir, ticlopidine) meaningfully shift response and seizure risk.
Metabolism cascade: One oxidative and one reductive branch, both yielding pharmacologically active species that outlast and outnumber the parent drug.
Hydroxybupropion (marked ★) is the therapeutically important metabolite — it retains NET/DAT reuptake inhibition and α3β4 nicotinic antagonism and, because of its exposure and persistence, is arguably the species you are actually treating with. Independently, bupropion and hydroxybupropion are potent inhibitors of CYP2D6. That makes bupropion a major perpetrator of drug interactions: it raises levels of CYP2D6 substrates (many antidepressants, antipsychotics, atomoxetine, metoprolol, opioids like tramadol/codeine whose activation it can blunt, and it lowers active tamoxifen). This is a prescribing hazard independent of any effect on bupropion itself.
Because bupropion skips serotonin entirely, its clinical signature is the photographic negative of an SSRI. The two most common reasons patients quit SSRIs — sexual dysfunction and weight gain — are largely absent, and it is frequently added to an SSRI specifically to rescue SSRI-induced anorgasmia. The trade-off is that it is activating: it can drive insomnia, jitteriness and anxiety, and it lowers the seizure threshold in a dose-dependent way.
Sustained DAT blockade in mesocortical and striatal circuits lifts hedonic capacity and motivation — bupropion is a preferred agent for depression with prominent anhedonia, fatigue, and psychomotor slowing. The same dopaminergic action underlies its mild pro-cognitive/attention effect and its off-label use in ADHD.
NET inhibition raises central noradrenergic drive, producing the characteristic energizing, alerting quality. Clinically this means dose it in the morning, watch for insomnia and anxiety in the first weeks, and expect a modest rise in heart rate and blood pressure — real but usually manageable, and a reason for caution in uncontrolled hypertension.
Non-competitive antagonism at α3β4 and α4β2 nicotinic receptors dampens the reinforcing hit of inhaled nicotine and softens withdrawal. Marketed as Zyban for this purpose, bupropion roughly doubles quit rates versus placebo — an effect that is mechanistically distinct from, and additive to, its monoamine action (and to nicotine replacement, with which it is often combined).
With SERT effectively untouched, bupropion produces little of the anorgasmia, libido loss, or weight gain that define serotonergic antidepressants — it is weight-neutral to weight-reducing (hence its pairing with naltrexone in Contrave for obesity). It also carries essentially no serotonin-syndrome risk on its own, though a monoamine oxidase interaction is a separate, serious hazard (see Harm Reduction).
Reinforcement/abuse liability from oral therapeutic dosing is low — the reuptake-block ceiling on synaptic dopamine and the slow XL kinetics keep the dopamine curve shallow. That picture changes entirely with non-oral, supratherapeutic misuse, where the dominant emergent risk is not euphoria but seizure.
Evidence-based, non-moralistic. Bupropion is not serotonergic and is not a classic euphoriant — its defining hazard is that it is proconvulsant, and that risk scales with dose, peak plasma level, and route.
Bupropion is a useful stress-test for the FlexAID∆S thesis that binding free energy is dominated by a conformational-entropy (−T∆S) collapse, not enthalpy alone. Its DAT affinity is modest (Ki ≈ 440 nM, ∆G ≈ −8.7 kcal·mol⁻¹) despite excellent ligand-efficiency metrics (LE ≈ 0.5–0.55, BEI ≈ 25–27). A small, rigid aryl-ketone with a bulky tert-butylamino group pays a small entropic price on binding — few rotatable bonds (3) to freeze — but it also makes few high-quality enthalpic contacts in the roomy S1 pocket. The result is a shallow, reversible occupancy: exactly the bounded, non-euphoric dopamine ceiling seen clinically.
The nicotinic side of the molecule is a different entropy story. As a non-competitive (open-channel / allosteric) nAChR antagonist, bupropion does not compete for the ACh orthosteric site; it stabilizes a low-conductance channel state. That distinction matters for docking: an orthosteric ∆Sconf decomposition against the ACh pocket would mis-score the real pharmacology. Honest modeling has to place the ligand in the pore/allosteric ensemble and account for the entropy of the channel's gating landscape — the tENCoM vibrational-entropy term (∆Svib) becomes the load-bearing variable, not orthosteric shape complementarity.
The 3D pose at right is the honest structural anchor: a Drosophila DAT (dDAT) — the best experimentally-tractable proxy for the human transporter S1 site — captured with the tricyclic nortriptyline. No bupropion co-crystal exists; the caption says so plainly.
| Target | Affinity | Rel. | Mechanism |
|---|---|---|---|
|
DAT
Dopamine transporter (SLC6A3)
|
Ki = 441 nM
IC₅₀ 0.55–1.6 μM (uptake)
|
Reuptake inhib. | |
|
NET
Noradrenaline transporter (SLC6A2)
|
IC50 = 1,450 nM
Ki ≈ 6,970 nM (binding)
|
Reuptake inhib. | |
|
α3β4 nAChR
Nicotinic ACh receptor
|
IC50 ≈ 1.8 μM
functional (non-competitive)
|
NC antagonist | |
|
α4β2 nAChR
Nicotinic ACh receptor
|
IC50 ≈ 8–12 μM
|
NC antagonist | |
|
α7 nAChR
Nicotinic ACh receptor
|
IC50 ≈ 7–90 μM
|
Antagonist | |
|
SERT
Serotonin transporter (SLC6A4)
|
Ki > 10,000 nM
IC₅₀ ≈ 47 μM — inactive
|
Negligible |