Mode of Action
Sibutramine produces its therapeutic effects by norepinephrine, serotonin and dopamine reuptake inhibition. Sibutramine and its major pharmacologically active metabolites (M1 and M2) do not act via release of monoamines.
Sibutramine exerts its pharmacological actions predominantly via its secondary (M1) and primary (M2) amine metabolites. The parent compound, sibutramine, is a potent inhibitor of serotonin (5-hydroxytryptamine, 5-HT) and norepinephrine reuptake in vivo, but not in vitro. However, metabolites M1 and M2 inhibit the reuptake of these neurotransmitters both in vitro and in vivo.
In human brain tissue, M1 and M2 also inhibit dopamine reuptake in vitro, but with ~3-fold lower potency than for the reuptake inhibition of serotonin or norepinephrine.
Potencies of Sibutramine, M1 and M2 as In Vitro Inhibitors of Monoamine Reuptake in Human Brain Potency to Inhibit Monoamine Reuptake (Ki;nM)
| Serotonin || Norepinephrine || Dopamine |
A study using plasma samples taken from sibutramine-treated volunteers showed monoamine reuptake inhibition of norepinephrine > serotonin > dopamine; maximum inhibitions were norepinephrine = 73%, serotonin = 54% and dopamine = 16%.
Sibutramine and its metabolites (M1 and M2) are not serotonin, norepinephrine or dopamine releasing agents. Following chronic administration of sibutramine to rats, no depletion of brain monoamines has been observed.
Sibutramine, M1 and M2 exhibit no evidence of anticholinergic or antihistaminergic actions. In addition, receptor binding profiles show that sibutramine, M1 and M2 have low affinity for serotonin (5-HT1, 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2C), norepinephrine (β,β1, β3, α1 andα2), dopamine (D1 and D2), benzodiazepine, and glutamate (NMDA) receptors. These compounds also lack monoamine oxidase inhibitory activity in vitro and in vivo.
Sibutramine is rapidly absorbed from the GI tract (Tmax of 1.2 hours) following oral administration and undergoes extensive first-pass metabolism in the liver (oral clearance of 1750 L/h and half-life of 1.1 h) to form the pharmacologically active mono- and di-desmethyl metabolites M1 and M2. Peak plasma concentrations of M1 and M2 are reached within 3 to 4 hours. On the basis of mass balance studies, on average, at least 77% of a single oral dose of sibutramine is absorbed. The absolute bioavailability of sibutramine has not been determined.
Radiolabeled studies in animals indicated rapid and extensive distribution into tissues: highest concentrations of radiolabeled material were found in the eliminating organs, liver and kidney. In vitro, sibutramine, M1 and M2 are extensively bound (97%, 94% and 94%, respectively) to human plasma proteins at plasma concentrations seen following therapeutic doses.
Sibutramine is metabolized in the liver principally by the cytochrome P450 (3A4) isoenzyme, to desmethyl metabolites, M1 and M2. These active metabolites are further metabolized by hydroxylation and conjugation to pharmacologically inactive metabolites, M5 and M6. Following oral administration of radiolabeled sibutramine, essentially all of the peak radiolabeled material in plasma was accounted for by unchanged sibutramine (3%), M1 (6%), M2 (12%), M5 (52%), and M6 (27%).
M1 and M2 plasma concentrations reached steady-state within four days of dosing and were approximately two-fold higher than following a single dose. The elimination half-lives of M1 and M2, 14 and 16 hours, respectively, were unchanged following repeated dosing.
Approximately 85% (range 68-95%) of a single orally administered radiolabeled dose was excreted in urine and feces over a 15-day collection period with the majority of the dose (77%) excreted in the urine. Major metabolites in urine were M5 and M6; unchanged sibutramine, M1, and M2 were not detected. The primary route of excretion for M1 and M2 is hepatic metabolism and for M5 and M6 is renal excretion.
Summary of Pharmacokinetic Parameters
| Mean (% CV) and 95% Confidence Intervals of Pharmacokinetic Parameters (Dose = 15 mg) |
† Calculated only up to 24 hr for M1.
| Study |
| Cmax |
| Tmax |
| AUC† |
| T½ |
| Metabolite M1 |
|Obese Subjects (n = 18)||4.0 (42)|
3.2 - 4.8
3.1 - 4.1
18.1 - 32.9
|Moderate Hepatic Impairment (n = 12)||2.2 (36)|
1.8 - 2.7
2.7 - 3.9
11.9 - 25.5
| Metabolite M2 |
(n = 18)
5.6 - 7.2
3.2 - 3.8
81.2 - 103
12.5 - 21.8
Impairment (n = 12)
3.4 - 5.2
3.1 - 4.5
76.9 - 104
18.9 - 26.5
Effect of Food
Administration of a single 20 mg dose of sibutramine with a standard breakfast resulted in reduced peak M1 and M2 concentrations (by 27% and 32%, respectively) and delayed the time to peak by approximately three hours. However, the AUCs of M1 and M2 were not significantly altered.
Plasma concentrations of M1 and M2 were similar between elderly (ages 61 to 77 yr) and young (ages 19 to 30 yr) subjects following a single 15-mg oral sibutramine dose. Plasma concentrations of the inactive metabolites M5 and M6 were higher in the elderly; these differences are not likely to be of clinical significance. In general, dose selection for an elderly patient should be cautious, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.
The safety and effectiveness of sibutramine in pediatric patients under 16 years old have not been established.
Pooled pharmacokinetic parameters from 54 young, healthy volunteers (37 males and 17 females) receiving a 15-mg oral dose of sibutramine showed the mean Cmax and AUC of M1 and M2 to be slightly (≤ 19% and ≤ 36%, respectively) higher in females than males. Somewhat higher steady-state trough plasma levels were observed in female obese patients from a large clinical efficacy trial. However, these differences are not likely to be of clinical significance. Dosage adjustment based upon the gender of a patient is not necessary (see DOSAGE AND ADMINISTRATION).
The relationship between race and steady-state trough M1 and M2 plasma concentrations was examined in a clinical trial in obese patients. A trend towards higher concentrations in Black patients over Caucasian patients was noted for M1 and M2. However, these differences are not considered to be of clinical significance.
The disposition of sibutramine metabolites (M1, M2, M5 and M6) following a single oral dose of sibutramine was studied in patients with varying degrees of renal function. Sibutramine itself was not measurable.
In patients with moderate and severe renal impairment, the AUC values of the active metabolite M1 were 24 to 46% higher and the AUC values of M2 were similar as compared to healthy subjects. Cross- study comparison showed that the patients with end - stage renal disease on dialysis had similar AUC values of M1 but approximately half of the AUC values of M2 measured in healthy subjects (CLcr ≥ 80 mL/ min). The AUC values of inactive metabolites M5 and M6 increased 2 - 3 fold (range 1 - to 7 - fold) in patients with moderate impairment (30 mL/ min < CLcr = 60 mL/ min) and 8 - 11 fold (range 5 - to 15 - fold) in patients with severe impairment (CLcr ≤ 30 mL/ min) as compared to healthy subjects. Cross - study comparison showed that the AUC values of M5 and M6 increased 22 - 33 fold in patients with end - stage renal disease on dialysis as compared to healthy subjects. Approximately 1% of the oral dose was recovered in the dialysate as a combination of M5 and M6 during the hemodialysis process, while M1 and M2 were not measurable in the dialysate.
Sibutramine should not be used in patients with severe renal impairment, including those with end-stage renal disease on dialysis.
In 12 patients with moderate hepatic impairment receiving a single 15-mg oral dose of sibutramine, the combined AUCs of M1 and M2 were increased by 24% compared to healthy subjects while M5 and M6 plasma concentrations were unchanged. The observed differences in M1 and M2 concentrations do not warrant dosage adjustment in patients with mild to moderate hepatic impairment. Sibutramine should not be used in patients with severe hepatic dysfunction.
In vitro studies indicated that the cytochrome P450 (3A4)-mediated metabolism of sibutramine was inhibited by ketoconazole and to a lesser extent by erythromycin. Phase 1 clinical trials were conducted to assess the interactions of sibutramine with drugs that are substrates and/or inhibitors of various cytochrome P450 isozymes. The potential for studied interactions is described below.
Concomitant administration of 200 mg doses of ketoconazole twice daily and 20 mg sibutramine once daily for 7 days in 12 uncomplicated obese subjects resulted in moderate increases in AUC and Cmax of 58% and 36% for M1 and of 20% and 19% for M2, respectively.
The steady-state pharmacokinetics of sibutramine and metabolites M1 and M2 were evaluated in 12 uncomplicated obese subjects following concomitant administration of 500 mg of erythromycin three times daily and 20 mg of sibutramine once daily for 7 days. Concomitant erythromycin resulted in small increases in the AUC (less than 14%) for M1 and M2. A small reduction in Cmax for M1 (11%) and a slight increase in Cmax for M2 (10%) were observed.
Concomitant administration of cimetidine 400 mg twice daily and sibutramine 15 mg once daily for 7 days in 12 volunteers resulted in small increases in combined (M1 and M2) plasma Cmax (3.4%) and AUC (7.3%).
Steady-state pharmacokinetics of sibutramine andmetabolites M1 and M2 were evaluated in 27 healthy volunteers after the administration of simvastatin 20 mg once daily in the evening and sibutramine 15 mg once daily in the morning for 7 days. Simvastatin had no significant effect on plasma Cmax and AUC of M2 or M1 and M2 combined. The Cmax (16%) and AUC (12%) of M1 were slightly decreased. Simvastatin slightly decreased sibutramine Cmax (14%) and AUC (21%). Sibutramine increased the AUC (7%) of the pharmacologically active moiety, simvastatin acid and reduced the Cmax (25%) and AUC (15%) of inactive simvastatin.
Steady-state pharmacokinetics of sibutramine and metabolites M1 and M2 were evaluated in 26 healthy volunteers after the co-administration of omeprazole 20 mg once daily and sibutramine 15 mg once daily for 7 days. Omeprazole slightly increased plasma Cmax and AUC of M1 and M2 combined (approximately 15%). M2 Cmax and AUC were not significantly affected whereas M1 Cmax (30%) and AUC (40%) were modestly increased. Plasma Cmax (57%) and AUC (67%) of unchanged sibutramine were moderately increased. Sibutramine had no significant effect on omeprazole pharmacokinetics.
Steady-state pharmacokinetics of sibutramine and metabolites M1 and M2 were evaluated in 24 healthy volunteers after the co-administration of sibutramine 15 mg once daily with olanzapine 5 mg twice daily for 3 days and 10 mg once daily thereafter for 7 days. Olanzapine had no significant effect on plasma Cmax and AUC of M2 and M1 and M2 combined, or the AUC of M1. Olanzapine slightly increased M1 Cmax (19%), and moderately increased sibutramine Cmax (47%) and AUC (63%). Sibutramine had no significant effect on olanzapine pharmacokinetics.
Steady-state pharmacokinetics of sibutramine and metabolites M1 and M2 after sibutramine 15 mg once daily for 11 days were compared in 25 healthy volunteers in the presence or absence of lorazepam 2 mg twice daily for 3 days plus one morning dose. Lorazepam had no significant effect on the pharmacokinetics of sibutramine metabolites M1 and M2. Sibutramine had no significant effect on lorazepam pharmacokinetics.
Drugs Highly Bound to Plasma Proteins
Although sibutramine and its active metabolites M1 and M2 are extensively bound to plasma proteins (≥94%), the low therapeutic concentrations and basic characteristics of these compounds make them unlikely to result in clinically significant protein binding interactions with other highly protein bound drugs such as warfarin and phenytoin. In vitro protein binding interaction studies have not been conducted.