CLINICAL PHARMACOLOGY
Mechanism Of Action
Angiotensin II is formed from angiotensin I in a reaction
catalyzed by angiotensin converting enzyme (ACE, kininase II). Angiotensin II
is the principal pressor agent of the renin-angiotensin system, with effects
that include vasoconstriction, stimulation of synthesis and release of
aldosterone, cardiac stimulation and renal reabsorption of sodium. Olmesartan
blocks the vasoconstrictor effects of angiotensin II by selectively blocking
the binding of angiotensin II to the AT1 receptor in vascular smooth
muscle. Its action is, therefore, independent of the pathways for angiotensin
II synthesis.
An AT2 receptor is found also in many tissues,
but this receptor is not known to be associated with cardiovascular
homeostasis. Olmesartan has more than a 12,500-fold greater affinity for the AT1
receptor than for the AT2 receptor.
Blockade of the renin-angiotensin system with ACE
inhibitors, which inhibit the biosynthesis of angiotensin II from angiotensin
I, is a mechanism of many drugs used to treat hypertension. ACE inhibitors also
inhibit the degradation of bradykinin, a reaction also catalyzed by ACE.
Because olmesartan medoxomil does not inhibit ACE (kininase II), it does not
affect the response to bradykinin. Whether this difference has clinical
relevance is not yet known.
Blockade of the angiotensin II receptor inhibits the
negative regulatory feedback of angiotensin II on renin secretion, but the
resulting increased plasma renin activity and circulating angiotensin II levels
do not overcome the effect of olmesartan on blood pressure.
Pharmacodynamics
Benicar doses of 2.5 mg to 40 mg inhibit the pressor
effects of angiotensin I infusion. The duration of the inhibitory effect was
related to dose, with doses of Benicar > 40 mg giving > 90% inhibition at
24 hours.
Plasma concentrations of angiotensin I and angiotensin II
and plasma renin activity (PRA) increase after single and repeated
administration of Benicar to healthy subjects and hypertensive patients.
Repeated administration of up to 80 mg Benicar had minimal influence on
aldosterone levels and no effect on serum potassium.
Pharmacokinetics
Absorption
Olmesartan medoxomil is rapidly and completely
bioactivated by ester hydrolysis to olmesartan during absorption from the
gastrointestinal tract.
Benicar tablets and the suspension formulation prepared
from Benicar tablets are bioequivalent [see DOSAGE AND ADMINISTRATION].
The absolute bioavailability of olmesartan is
approximately 26%. After oral administration, the peak plasma concentration (Cmax)
of olmesartan is reached after 1 to 2 hours. Food does not affect the bioavailability
of olmesartan.
Distribution
The volume of distribution of olmesartan is approximately
17 L. Olmesartan is highly bound to plasma proteins (99%) and does not
penetrate red blood cells. The protein binding is constant at plasma olmesartan
concentrations well above the range achieved with recommended doses.
In rats, olmesartan crossed the blood-brain barrier
poorly, if at all. Olmesartan passed across the placental barrier in rats and
was distributed to the fetus. Olmesartan was distributed to milk at low levels in
rats.
Metabolism And Excretion
Following the rapid and complete conversion of olmesartan
medoxomil to olmesartan during absorption, there is virtually no further
metabolism of olmesartan. Total plasma clearance of olmesartan is 1.3 L/h, with
a renal clearance of 0.6 L/h. Approximately 35% to 50% of the absorbed dose is
recovered in urine while the remainder is eliminated in feces via the bile.
Olmesartan appears to be eliminated in a biphasic manner
with a terminal elimination half-life of approximately 13 hours. Olmesartan
shows linear pharmacokinetics following single oral doses of up to 320 mg and
multiple oral doses of up to 80 mg. Steady-state levels of olmesartan are
achieved within 3 to 5 days and no accumulation in plasma occurs with
once-daily dosing.
Geriatric
The pharmacokinetics of olmesartan were studied in the
elderly ( ≥ 65 years). Overall, maximum plasma concentrations of olmesartan
were similar in young adults and the elderly. Modest accumulation of olmesartan
was observed in the elderly with repeated dosing; AUCss,τ, was 33% higher in elderly patients, corresponding to an
approximate 30% reduction in CLR [see DOSAGE AND ADMINISTRATION and Use
in Specific Populations].
Pediatric
The pharmacokinetics of olmesartan were studied in
pediatric hypertensive patients aged 1 to16 years. The clearance of olmesartan
in pediatric patients was similar to that in adult patients when adjusted by the
body weight [see Use in Specific Populations].
Olmesartan pharmacokinetics have not been investigated in
pediatric patients less than 1 year of age [see WARNINGS AND PRECAUTIONS
and Use In Specific Populations].
Gender
Minor differences were observed in the pharmacokinetics
of olmesartan in women compared to men. AUC and Cmax were 10-15% higher in
women than in men.
Hepatic Insufficiency
Increases in AUC0-∞ and Cmax were observed in
patients with moderate hepatic impairment compared to those in matched
controls, with an increase in AUC of about 60% [see DOSAGE AND
ADMINISTRATION Â and Use In Specific Populations].
Renal Insufficiency
In patients with renal insufficiency, serum
concentrations of olmesartan were elevated compared to subjects with normal
renal function. After repeated dosing, the AUC was approximately tripled in patients
with severe renal impairment (creatinine clearance < 20 mL/min). The
pharmacokinetics of olmesartan in patients undergoing hemodialysis has not been
studied [see DOSAGE AND ADMINISTRATION, WARNINGS AND PRECAUTIONS
and Use In Specific Populations].
Drug Interactions
Bile Acid Sequestering Agent Colesevelam
Concomitant administration of 40 mg olmesartan medoxomil
and 3750 mg colesevelam hydrochloride in healthy subjects resulted in 28%
reduction in Cmax and 39% reduction in AUC of olmesartan. Lesser effects, 4%
and 15% reduction in Cmax and AUC respectively, were observed when olmesartan medoxomil
was administered 4 hours prior to colesevelam hydrochloride [see DRUG
INTERACTIONS].
Animal Toxicology And/Or Pharmacology
Reproductive Toxicology Studies
No teratogenic effects were observed when olmesartan
medoxomil was administered to pregnant rats at oral doses up to 1000 mg/kg/day
(240 times the maximum recommended human dose [MRHD] of olmesartan medoxomil on
a mg/m² basis) or pregnant rabbits at oral doses up to 1 mg/kg/day (half the MRHD
on a mg/m² basis; higher doses could not be evaluated for effects on fetal
development as they were lethal to the does). In rats, significant decreases in
pup birth weight and weight gain were observed at doses ≥ 1.6 mg/kg/day,
and delays in developmental milestones (delayed separation of ear auricula,
eruption of lower incisors, appearance of abdominal hair, descent of testes,
and separation of eyelids) and dose-dependent increases in the incidence of
dilation of the renal pelvis were observed at doses ≥ 8 mg/kg/day. The no
observed effect dose for developmental toxicity in rats is 0.3 mg/kg/day, about
one-tenth the MRHD of 40 mg/day.
Clinical Studies
Adult Hypertension
The antihypertensive effects of Benicar have been
demonstrated in seven placebo controlled studies at doses ranging from 2.5 mg
to 80 mg for 6 to 12 weeks, each showing statistically significant reductions in
peak and trough blood pressure. A total of 2693 patients (2145 Benicar; 548 placebo)
with essential hypertension were studied. Benicar once daily lowered diastolic
and systolic blood pressure. The response was dose-related, as shown in the
following graph. A Benicar dose of 20 mg daily produces a trough sitting BP
reduction over placebo of about 10/6 mmHg and a dose of 40 mg daily produces a trough
sitting BP reduction over placebo of about 12/7 mmHg. Benicar doses greater
than 40 mg had little additional effect. The onset of the antihypertensive
effect occurred within 1 week and was largely manifest after 2 weeks.
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Data above are from seven placebo-controlled studies
(2145 Benicar patients, 548 placebo patients). The blood pressure lowering
effect was maintained throughout the 24-hour period with Benicar once daily,
with trough-to-peak ratios for systolic and diastolic response between 60 and
80%.
The blood pressure lowering effect of Benicar, with and
without hydrochlorothiazide, was maintained in patients treated for up to 1
year. There was no evidence of tachyphylaxis during long-term treatment with
Benicar or rebound effect following abrupt withdrawal of olmesartan medoxomil
after 1 year of treatment.
The antihypertensive effect of Benicar was similar in men
and women and in patients older and younger than 65 years. The effect was
smaller in black patients (usually a low renin population), as has been seen with
ACE inhibitors, beta-blockers and other angiotensin receptor blockers. Benicar
had an additional blood pressure lowering effect when added to
hydrochlorothiazide.
There are no trials of Benicar demonstrating reductions
in cardiovascular risk in patients with hypertension, but at least one
pharmacologically similar drug has demonstrated such benefits.
Pediatric Hypertension
The antihypertensive effects of Benicar in the pediatric
population were evaluated in a randomized, double-blind study involving 302
hypertensive patients aged 6 to 16 years. The study population consisted of an
all black cohort of 112 patients and a mixed racial cohort of 190 patients,
including 38 blacks. The etiology of the hypertension was predominantly
essential hypertension (87% of the black cohort and 67% of the mixed cohort).
Patients who weighed 20 to < 35 kg were randomized to 2.5 or 20 mg of Benicar
once daily and patients who weighed ≥ 35 kg were randomized to 5 or 40 mg
of Benicar once daily. At the end of 3 weeks, patients were re-randomized to
continuing Benicar or to taking placebo for up to 2 weeks. During the initial
dose-response phase, Benicar significantly reduced both systolic and diastolic
blood pressure in a weight-adjusted dose-dependent manner. Overall, the two dose
levels of Benicar (low and high) significantly reduced systolic blood pressure
by 6.6 and 11.9 mmHg from the baseline, respectively. These reductions in
systolic blood pressure included both drug and placebo effect. During the
randomized withdrawal to placebo phase, mean systolic blood pressure at trough
was 3.2 mmHg lower and mean diastolic blood pressure at trough was 2.8 mmHg
lower in patients continuing Benicar than in patients withdrawn to placebo.
These differences were statistically different. As observed in adult
populations, the blood pressure reductions were smaller in black patients.
In the same study, 59 patients aged 1 to 5 years who
weighed ≥ 5 kg received 0.3 mg/kg of Benicar once daily for three weeks in
an open label phase and then were randomized to receiving Benicar or placebo in
a double-blind phase. At the end of the second week of withdrawal, the mean
systolic/diastolic blood pressure at trough was 3/3 mmHg lower in the group
randomized to Benicar; this difference in blood pressure was not statistically
significant (95% C.I. -2 to 7/-1 to 7).