|Wellbutrin SR 150 mg|
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Wellbutrin SR (Bupropion)
Wellbutrin (Bupropion) is a selective inhibitor of catecholamine reuptake (noradrenaline and dopamine) with little effect on indoleamine (serotonin) reuptake, it does not inhibit monoamine oxidase.
Although the mechanism of action of bupropion, like other antidepressants, is currently unknown, it is assumed that its antidepressant effect is carried out by influencing the noradrenergic and/or dopaminergic mediator system.
The efficacy and tolerability of bupropion in sustained release tablets have been investigated in 7 double-blind studies; of these 7 studies, 3 were conducted in Europe in the dose range of interest (up to 300 mg/day). The next 4 studies were conducted in the United States with a flexible dose range up to 450 mg/day. Evidence of long-term effect support was provided by the 1-year study of exacerbation prevention with bupropion; 816 patients received bupropion in the single-blind phase of this study, of whom 423 were randomized to a follow-up double-blind phase (bupropion, 210; placebo, 213).
Wellbutrin (Bupropion) was effective in patients with mild to moderate depression, as well as in patients with severe depression. In pooled European comparative studies, similar degrees of superiority of the drug over placebo were observed in both patients with mild to moderate depression (-1.8, p=0.032) and patients with severe depression (-2.3, p=0.041).
After oral administration of bupropion in sustained-release tablets in healthy volunteers, the time to reach maximum plasma concentration was approximately 5 hours.
Food intake has no significant effect on absorption of Wellbutrin (Bupropion) in sustained release tablets.
The kinetics of bupropion and its metabolites are linear with chronic therapy at doses ranging from 150 to 300 mg/day.
Bupropion penetrates tissues well; its apparent volume of distribution is approximately 2,000 L.
Bupropion and hydroxybupropion are moderately bound to plasma proteins (84% and 77%, respectively). The degree of binding to proteins of the trio-hydrobupropion metabolite is about 50% of that of bupropion.
Wellbutrin (Bupropion) undergoes intensive metabolism. Three active metabolites are detected in plasma: hydroxybupropion and aminopyrate isomers - treo-hydrobupropion and erythro-hydrobupropion. This fact may be of clinical significance, since plasma concentrations of the metabolites are high or even exceed plasma concentrations of bupropion. Erythro-hydrobupropion is not quantified in plasma after a single dose of the drug. Active metabolites are further metabolized to inactive metabolites and excreted by the kidneys.
In in vitro studies, it was shown that bupropion is metabolized to its main metabolite hydroxybupropion primarily through the CYP2B6 isoenzyme system, while the cytochrome P450 isoenzyme system is not involved in the formation of trio-hydrobupropion
In vitro, bupropion and hydroxybupropion are relatively weak competitive inhibitors of the CYP2D6 isoenzyme system (Ki - inhibition constant is 21 and 13.3 μmol, respectively). In volunteers who were highly metabolized by the CYP2D6 isoenzyme, the simultaneous administration of bupropion and desipramine resulted in a two- to fivefold increase in Cmax and AUC of desipramine This effect was maintained for at least 7 days after the last dose of bupropion.
Because bupropion is not metabolized by the CYP2D6 isoenzyme, desipramine should not be expected to affect the pharmacokinetics of bupropion. Caution is advised when prescribing bupropion concomitantly with substrate drugs for the CYP2D6 isoenzyme enzyme system.
Studies involving volunteers or patients receiving recommended doses of bupropion for 10-45 days have not shown induction of enzyme systems by bupropion or hydroxybupropion
The Cmax of hydroxybupropion in plasma is approximately 10 times the maximum concentration of the parent substance in equilibrium.
The time to reach Cmax for erythro-hydrobupropion and trio-hydrobupropion is approximately the same as for hydroxybupropion.
In a study in healthy volunteers, concomitant administration of ritonavir at a dose of 100 mg twice daily resulted in a 22% and 21% reduction in AUC and Cmax of bupropion, respectively. The AUC and Cmax of bupropion metabolites were reduced by 0 to 44%. In another study in healthy volunteers, ritonavir at a dose of 600 mg twice daily reduced the AUC and Cmax of bupropion by 66% and 62%, respectively. The AUC and Cmax of bupropion metabolites were reduced by 42% and 78%, respectively.
In a study in healthy volunteers, combined administration of the combination drug loinavir 400 mg/ritonavir 100 mg twice daily decreased the AUC and Cmax of bupropion by 57%. AUC and Cmax of hydroxybupropion were decreased by 50% and 31%, respectively.
The Cmax of hydroxybupropion in plasma is reached 7 hours after administration of bupropion in sustained release tablets.
After ingestion of 200 mg of carbon isotope C14-labeled bupropion, 87% and 10% of the radioactive dose were detected in the urine and feces, respectively. The bupropion dose fraction excreted unchanged was only 0.5% due to active metabolism of bupropion. Less than 10% of the isotope-labeled dose was detected in the urine as an active metabolite
The clearance of bupropion when taken orally is approximately 200 l/h and the elimination half-life is 20 h.
The T1/2 of hydroxybupropion is approximately 20 h, the AUC in equilibrium is approximately 17 times that of bupropion. The T1/2 of trio-hydrobupronion and erythro-hydropropion are longer (37 and 33 h, respectively), with AUCs 8 and 1.6 times higher than those of bupropion. Time to reach equilibrium state of bupropion and its metabolites is within 8 days.