CLINICAL PHARMACOLOGY
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.
Pharmacodynamics
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 (5Âhydroxytryptamine, 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 |
Sibutramine |
298 |
5451 |
943 |
M1 |
15 |
20 |
49 |
M2 |
20 |
15 |
45 |
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, 5ÂHT1A, 5-HT1B, 5-HT2A,
5-HT2C), norepinephrine (β, β1, β3, α1 and α2),
dopamine (D1 and D2), benzodiazepine, and glutamate (NMDA) receptors. These
compounds also lack monoamine oxidase inhibitory activity in vitro and
in vivo.
Pharmacokinetics
Absorption
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.
Distribution
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.
Metabolism
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.
Excretion
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
= 15mg)
Study Population |
Cmax
(ng/mL) |
Tmax
(h) |
AUC†
(ng*h/mL) |
T½
(h) |
Metabolite M1 |
Target Population: |
Obese Subjects (n = 18) |
4.0 (42) |
3.6 (28) |
25.5 (63) |
– – |
|
3.2 - 4.8 |
3.1 - 4.1 |
18.1 - 32.9 |
|
Special Population: |
Moderate Hepatic Impairment (n = 12) |
2.2 (36) |
3.3 (33) |
18.7 (65) |
– – |
|
1.8 - 2.7 |
2.7 - 3.9 |
11.9 - 25.5 |
|
Metabolite M2 |
Target Population: |
Obese Subjects (n = 18) |
6.4 (28) |
3.5 (17) |
92.1 (26) |
17.2 (58) |
5.6 - 7.2 |
3.2 - 3.8 |
81.2 - 103 |
12.5 - 21.8 |
Special Population: |
Moderate Hepatic Impairment (n = 12) |
4.3 (37) |
3.8 (34) |
90.5 (27) |
22.7 (30) |
3.4 - 5.2 |
3.1 - 4.5 |
76.9 - 104 |
18.9 - 26.5 |
† Calculated only up to 24 hr for
M1. |
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.
Special Populations
Geriatric
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. Sibutramine is contraindicated
in patients over 65 years of age (see CONTRAINDICATIONS).
Pediatric
The safety and effectiveness of sibutramine in pediatric patients under 16
years old have not been established.
Gender
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).
Race
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.
Renal Insufficiency
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 M 1 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 M6during 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.
Hepatic Insufficiency
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.
Drug-Drug Interactions
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.
Ketoconazole
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.
Erythromycin
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.
Cimetidine
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%).
Simvastatin
Steady-state pharmacokinetics of sibutramine and metabolites 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.
Omeprazole
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 pharmacokineti cs.
Olanzapine
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. Olanza pine 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.
Lorazepam
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 themunlikely 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.
Clinical Studies
Observational epidemiologic studies have established a relationship between
obesity and the risks for cardiovascular disease, non-insulin depende nt diabetes
mellitus (NIDDM), certain forms of cancer, gallstones, certain respiratory disorders,
and an increase in overall mortality. These studies suggest that weight loss,
if maintained, may produce health benefits for some patients with chronic obesity
who may also be at risk for other diseases.
The long-term effects of sibutramine on the morbidity and mortality associated
with obesity have not been established. Weight loss was examined in 11 double-blind,
placebo-controlled obesity trials (BMI range across all studies 27-43) with
study durations of 12 to 52 weeks and doses ranging from 1 to 30 mg once daily.
Weight was significantly reduced in a dose-related manner in sibutramineÂtreated
patients compared to placebo over the dose range of 5 to 20 mg once daily. In
two 12-month studies, maximal weight loss was achieved by 6 months and statistically
significant weight loss was maintained over 12 months. The amount of placebo-subtracted
weight loss achieved on sibutramine was consistent across studies.
Analysis of the data in three long-term ( ≥ 6 months) obesity trials indicates
that patients who lose at least 4 pounds in the first 4 weeks of therapy with
a given dose of sibutramine are most likely to achieve significant long-term
weight loss on that dose of sibutramine. Approximately 60% of such patients
went on to achieve a placebo-subtracted weight loss of ≥ 5% of their initial
body weight by month 6. Conversely, of those patients on a given dose of sibutramine
who did not lose at least 4 pounds in the first 4 weeks of therapy, approximately
80% did not go on to achieve a placebo-subtracted weight loss of ≥ 5% of
their initial body weight on that dose by month 6.
Significant dose-related reductions in waist circumference, an indicator of
intra-abdominal fat, have also been observed over 6 and 12 months in placebo-controlled
clinical trials. In a 12-week placebo-controlled study of non-insulin dependent
diabetes mellitus patients randomized to placebo or 15 mg per day of sibutramine,
Dual Energy X-Ray Absorptiometry (DEXA) assessment of changes in body composition
showed that total body fat mass decreased by 1.8 kg in the sibutramine group
versus 0.2 kg in the placebo group (p < 0.001). Similarly, truncal (android)
fat mass decreased by 0.6 kg in the sibutramine group versus 0.1 kg in the placebo
group (p < 0.01). The changes in lean mass, fasting blood sugar, and HbA1
were not statistically significantly different between the two groups.
Eleven double-blind, placebo-controlled obesity trials with study durations
of 12 to 52 weeks have provided evidence that sibutramine does not adversely
affect glycemia, serum lipid profiles, or se rum uric acid in obese patients.
Treatment with sibutramine (5 to 20 mg once daily) is associated with mean increases
in blood pressure of 1 to 3 mm Hg and with mean increases in pulse rate of 4
to 5 beats per minute relative to placebo. These findings are similar in normotensives
and in patients with hypertension controlled with medication. Those patients
who lose significant ( ≥ 5% weight loss ) amounts of weight on sibutramine
tend to have smaller increases in blood pressure and pulse rate (see WARNINGS).
In Study 1, a 6-month, double-blind, placebo-controlled study in obese patients,
Study 2, a 1-year, double-blind, placebo-controll ed study in obese patients,
and Study 3, a 1-year, double-blind, placebo-controlled study in obese patients
who lost at least 6 kg on a 4-week very low calorie diet (VLCD), sibutramine
produced significant reductions in weight, as shown below . In the two 1-year
studies, maximal weight loss was achieved by 6 months and statistically significant
weight loss was maintained over 12 months.
Mean Weight Loss (lbs) in the Six-Month and One-Year Trials
Study/Patient Group |
Placebo (n) |
S ibutramine(mg) |
5 (n) |
10 (n) |
15 (n) |
20 (n) |
Study 1 |
All patients* |
2.0 |
6.6 |
9.7 |
12.1 |
13.6 |
(142) |
(148) |
(148) |
(150) |
(145) |
Completers** |
2.9 |
8.1 |
12.1 |
15.4 |
18.0 |
(84) |
(103) |
(95) |
(94) |
(89) |
Early responders*** |
8.5 |
13.0 |
16.0 |
18.2 |
20.1 |
(17) |
(60) |
(64) |
(73) |
(76) |
Study 2 |
All patients* |
3.5 |
|
9.8 |
14.0 |
|
(157) |
|
(154) |
(152) |
|
Completers** |
4.8 |
|
13.6 |
15.2 |
|
(76) |
|
(80) |
(93) |
|
Early responders*** |
10.7 |
|
18.2 |
18.8 |
|
(24) |
|
(57) |
(76) |
|
Study 3**** |
All patients* |
15.2 |
|
28.4 |
|
|
(78) |
|
(81) |
|
|
Completers** |
16.7 |
|
29.7 |
|
|
(48) |
|
(60) |
|
|
Early responders*** |
21.5 |
|
33.0 |
|
|
(22) |
|
(46) |
|
|
* Data for all patients who received study
drug and who had any post-baseline measurement (last observation carried
forward analysis).
** Data for patients who completed the entire 6-month (Study 1) or one-year
period of dosing and have data recorded for the month 6 (Study 1) or month
12 visit.
*** Data for patients who lost at least 4 lbs in the first 4 weeks of
treatment and completed the study.
**** Weight loss data shown describe changes in weight from the pre-VLCD;
mean weight loss during the 4-week VLCD was 16.9 lbs for sibutramine and
16.3 lbs for placebo. |
Maintenance of weight loss with sibutramine was examined in a 2-year, double-blind,
placebo-controlled trial. After a 6-month run-in phase in which all patients
received sibutramine 10 mg (mean weight loss, 26 lbs.), patients were randomized
to sibutramine (10 to 20 mg, 352 patients) or placebo (115 patients). The mean
weight loss from initial body weight to endpoint was 21 lbs. and 12 lbs. for
sibutramine and placebo patients, respectively. A statistically significantly
(p < 0.001) greater proportion of sibutramine treated patients, 75%, 62%,
and 43%, maintained at least 80% of their initial weight loss at 12, 18, and
24 months, respectively, compared with the placebo group (38%, 23%, and 16%).
Also 67%, 37%, 17%, and 9% of sibutramine treated patients compared with 49%,
19%, 5%, and 3% of placebo patients lost ≥ 5%, ≥ 10%, ≥ 15%, and ≥
20%, respectively, of their initial body weight at endpoint. From endpoint to
the post-study follow-up visit (about 1 month), weight regain was approximately
4 lbs for the sibutramine patients and approximately 2 lbs for the placebo patients.
Sibutramine induced weight loss has been accompanied by beneficial changes
in serum lipids that are similar to those seen with nonpharmacologically-mediated
weight loss. A combined, weighted analysis of the changes in serum lipids in
11 placebo-controlled obesity studies ranging in length from 12 to 52 weeks
is shown below for the last observation carried forward (LOCF) analysis.
Combined Analysis (11 Studies) of Changes in Serum Lipids
– LOCF
Category |
TG
% (n) |
CHOL
% (n) |
LDL-C
% (n) |
HDL-C
% (n) |
All Placebo |
0.53 (475) |
-1.53 (475) |
-0.09 (233) |
-0.56 (248) |
< 5% Weight Loss |
4.52 (382) |
-0.42 (382) |
-0.70 (205) |
-0.71 (217) |
≥ 5% Weight Loss |
-15.30 (92) |
-6.23 (92) |
-6.19 (27) |
0.94 (30 ) |
All Sibutramine |
-8.75 (1164) |
-2.21 (1165) |
-1.85 (642) |
4.13 (664) |
< 5% Weight Loss |
-0.54 (547) |
0.17 (548) |
-0.37 (320) |
3.19 (331) |
≥ 5% Weight Loss |
-16.59 (612) |
-4.87 (612) |
-4.56 (317) |
4.68 (328) |
Baseline mean values:
Placebo: TG 187 mg/dL; CHOL 221 mg/dL; LDL-C 140 mg/dL; HDL-C 47 mg/dL Sibutramine:
TG 172 mg/dL; CHOL 215 mg/dL; LDL-C 140 mg/dL; HDL-C 47 mg/dL TG: Triglycerides,
CHOL: Cholesterol, LDL-C Low Density Lipoprotein-Cholesterol HDL-C: High Density
Lipoprotein-Cholesterol
Sibutramine induced weight loss has been accompanied by reductions in serum
uric acid. Certain centrally-acting weight loss agents that cause release of
serotonin from nerve terminals have been associated with cardiac valve dysfunction.
The possible occurrence of cardiac valve disease w as specifically investigated
in two studies. In one study 2-D and color Doppler echocardiography we re performed
on 210 patients (mean age, 54 years) receiving sibutramine 15 mg or placebo
daily for periods of 2 weeks to 16 months (mean duration of treatment, 7.6 months).
In patients without a prior history of valvular heart disease, the incidence
of valvular heart disease was 3/132 (2.3%) in the sibutramine treatment group
(all three cases were mild aortic insufficiency) and 2/77 (2.6%) in the placebo
treatment group (one case of mild aortic insufficiency and one case of severe
aortic insufficiency). In another study, 25 patients underwent 2-D and color
Doppler echocardiography before treatment with sibutramine and again after treatment
with sibutramine 5 to 30 mg daily for three months; there were no cases of valvular
heart disease.
The effect of sibutramine 15 mg once daily on measures of 24-hour blood pressure
was evaluated in 12-week placebo-controlled study. Twenty-six male and female,
primarily Cauasian individuals ac with an average BMI of 34 kg/m² and an
average age of 39 years underwent 24-hour ambulatory blood pressure monitoring
(ABPM). The mean changes from baseline to Week 12 in various measures of ABPM
are shown in the following table.
Parameter mm Hg |
Systolic |
Diastolic |
Placebo
n=12 |
Sibutramine |
Placebo |
Sibutramine |
15 mg
n=14 |
20 mg
n=16 |
15 mg
n=12 |
20 mg
n=16 |
Daytime |
0.2 |
3.9 |
4.4 |
0.5 |
5.0 |
5.7 |
Nighttime |
-0.3 |
4.1 |
6.4 |
-1.0 |
4.3 |
5.4 |
Early am |
-0.9 |
9.4 |
5.3 |
-3.0 |
6.7 |
5.8 |
24-hour mean |
-0.1 |
4.0 |
4.7 |
0.1 |
5.0 |
5.6 |
Normal diurnal variation of blood pressure was maintained.