Steady-State Plasma Concentration of Wellbutrin XL 150mg
At steady state following Wellbutrin XL 150 mg once daily, peak plasma concentration (Cmax) of bupropion occurs approximately 5 hours after dosing, with hydroxybupropion (the major active metabolite) reaching peak levels approximately 7 hours post-dose. The hydroxybupropion peak concentration is approximately 7 times higher than the parent drug, and its area under the curve (AUC) at steady state is about 13 times that of bupropion 1.
Pharmacokinetic Parameters at Steady State
For bupropion parent compound:
- Steady state is achieved after approximately 5-8 days of daily dosing (5 half-lives)
- Peak plasma concentration occurs ~5 hours after administration
- The elimination half-life is approximately 21 hours 1
For the major metabolites at steady state:
Hydroxybupropion (most clinically significant):
- Peak occurs ~7 hours post-dose
- Elimination half-life: ~20 hours
- AUC is approximately 13 times that of bupropion
- Peak concentration is ~7 times higher than parent drug 1
Threohydrobupropion:
- Elimination half-life: ~37 hours
- Steady-state AUC is ~7 times that of bupropion 1
Erythrohydrobupropion:
- Elimination half-life: ~33 hours
- Steady-state AUC is ~1.4 times that of bupropion 1
Clinical Significance
The metabolites, particularly hydroxybupropion, are pharmacologically active and likely contribute substantially to therapeutic effects. Hydroxybupropion has been demonstrated to be approximately one-half as potent as bupropion in antidepressant screening tests, while the other metabolites are approximately 5-fold less potent 1. Given that hydroxybupropion plasma concentrations are significantly higher and more sustained than the parent compound, this metabolite probably has key pharmacodynamic effects 2.
Specific Concentration Values
In Chinese healthy volunteers receiving 150 mg once daily for 5 days, the geometric mean bupropion AUC0-24 was 498.2 h·ng/mL and Cmax was 49.9 ng/mL 3. When the dose was increased to 300 mg daily (days 6-14), steady-state values increased approximately dose-proportionally: AUC0-24 of 1,165.7 h·ng/mL and Css_max of 111.9 ng/mL 3.
For optimal antidepressant response, trough plasma bupropion concentrations should be maintained between 25-100 ng/mL, with maximum response observed at 50-100 ng/mL and virtually no response below 25 ng/mL 4.
Important Clinical Considerations
Bupropion and its metabolites exhibit linear kinetics at therapeutic doses (150-450 mg/day), meaning concentrations increase proportionally with dose 1. The extended-release formulation provides more sustained plasma levels with lower peak-to-trough fluctuation compared to immediate-release formulations 2.
Blood sampling for therapeutic drug monitoring should occur at steady state (after at least 5-7 days of stable dosing) and should be drawn as a trough level immediately before the morning dose, approximately 24 hours after the previous dose 5.
Factors that can affect steady-state concentrations include hepatic impairment (which can increase bupropion levels 3-fold in severe cirrhosis), renal impairment (which increases metabolite levels 2-3 fold), and concomitant medications that inhibit or induce CYP2B6 1.