CLINICAL PHARMACOLOGY
Mechanism Of Action
Angiotensin II is formed from angiotensin I in a reaction
catalyzed by angiotensin-converting enzymes (ACE, kinase 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. Azilsartan medoxomil is
an orally administered prodrug that is rapidly converted by esterases during
absorption to the active moiety, azilsartan. Azilsartan blocks the
vasoconstrictor and aldosterone-secreting effects of angiotensin II by
selectively blocking the binding of angiotensin II to the AT1 receptor in many
tissues, such as vascular smooth muscle and the adrenal gland. Its action is,
therefore, independent of the pathway for angiotensin II synthesis.
An AT2 receptor is also found in many tissues, but this
receptor is not known to be associated with cardiovascular homeostasis.
Azilsartan has more than a 10,000-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 widely used in the treatment of hypertension. ACE inhibitors also inhibit
the degradation of bradykinin, a reaction catalyzed by ACE. Because azilsartan
does not inhibit ACE (kinase II), it should not affect bradykinin levels.
Whether this difference has clinical relevance is not yet known. Azilsartan
does not bind to or block other receptors or ion channels known to be important
in cardiovascular regulation.
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 angiotensin II circulating levels
do not overcome the effect of azilsartan on blood pressure.
Pharmacodynamics
Azilsartan inhibits the pressor effects of an angiotensin
II infusion in a dose-related manner. An azilsartan single dose equivalent to
32 mg azilsartan medoxomil inhibited the maximal pressor effect by
approximately 90% at peak, and approximately 60% at 24 hours. Plasma
angiotensin I and II concentrations and plasma renin activity increased while
plasma aldosterone concentrations decreased after single and repeated
administration of Edarbi to healthy subjects; no clinically significant effects
on serum potassium or sodium were observed.
Effect On Cardiac Repolarization
A thorough QT/QTc study was conducted to assess the
potential of azilsartan to prolong the QT/QTc interval in healthy subjects.
There was no evidence of QT/QTc prolongation at a dose of 320 mg of Edarbi.
Pharmacokinetics
Absorption
Azilsartan medoxomil is hydrolyzed to azilsartan, the
active metabolite, in the gastrointestinal tract during absorption. Azilsartan
medoxomil is not detected in plasma after oral administration. Dose
proportionality in exposure was established for azilsartan in the azilsartan
medoxomil dose range of 20 mg to 320 mg after single or multiple dosing.
The estimated absolute bioavailability of azilsartan
following administration of azilsartan medoxomil is approximately 60%. After
oral administration of azilsartan medoxomil, peak plasma concentrations (Cmax)
of azilsartan are reached within 1.5 to 3 hours. Food does not affect the
bioavailability of azilsartan.
Distribution
The volume of distribution of azilsartan is approximately
16 L. Azilsartan is highly bound to human plasma proteins ( > 99%), mainly
serum albumin. Protein binding is constant at azilsartan plasma concentrations
well above the range achieved with recommended doses.
In rats, minimal azilsartan-associated radioactivity
crossed the blood-brain barrier. Azilsartan passed across the placental barrier
in pregnant rats and was distributed to the fetus.
Metabolism And Elimination
Azilsartan is metabolized to two primary metabolites. The
major metabolite in plasma is formed by O-dealkylation, referred to as
metabolite M-II, and the minor metabolite is formed by decarboxylation,
referred to as metabolite M-I. Systemic exposures to the major and minor metabolites
in humans were approximately 50% and less than 1% of azilsartan, respectively.
M-I and M-II do not contribute to the pharmacologic activity of Edarbi. The
major enzyme responsible for azilsartan metabolism is CYP2C9.
Following an oral dose of 14C-labeled
azilsartan medoxomil, approximately 55% of radioactivity was recovered in feces
and approximately 42% in urine, with 15% of the dose excreted in urine as
azilsartan. The elimination half-life of azilsartan is approximately 11 hours
and renal clearance is approximately 2.3 mL/min. Steady-state levels of
azilsartan are achieved within five days, and no accumulation in plasma occurs
with repeated once-daily dosing.
Special Populations
The effect of demographic and functional factors on the
pharmacokinetics of azilsartan was studied in single and multiple dose studies.
Pharmacokinetic measures indicating the magnitude of the effect on azilsartan
are presented in Figure 1 as change relative to reference (test/reference).
Effects are modest and do not call for dosage adjustment.
Figure 1: Impact of intrinsic factors on the
pharmacokinetics of azilsartan
Animal Toxicology And/Or Pharmacology
Reproductive Toxicology
In peri- and postnatal rat development studies, adverse
effects on pup viability, delayed incisor eruption and dilatation of the renal
pelvis along with hydronephrosis were seen when azilsartan medoxomil was
administered to pregnant and nursing rats at 1.2 times the MRHD on a mg/m² basis.
Reproductive toxicity studies indicated that azilsartan medoxomil was not
teratogenic when administered at oral doses up to 1000 mg azilsartan
medoxomil/kg/day to pregnant rats (122 times the MRHD on a mg/m² basis) or up to
50 mg azilsartan medoxomil/kg/day to pregnant rabbits (12 times the MRHD on a
mg/m² basis). M-II also was not teratogenic in rats or rabbits at doses up to
3000 mg M-II/kg/day. Azilsartan crossed the placenta and was found in the
fetuses of pregnant rats and was excreted into the milk of lactating rats.
Clinical Studies
The antihypertensive effects of Edarbi have been
demonstrated in a total of seven double-blind, randomized studies, which
included five placebo-controlled and four active comparator-controlled studies
(not mutually exclusive). The studies ranged from six weeks to six months in
duration, at doses ranging from 20 mg to 80 mg once daily. A total of 5941
patients (3672 given Edarbi, 801 given placebo, and 1468 given active
comparator) with mild, moderate or severe hypertension were studied. Overall,
51% of patients were male and 26% were 65 years or older; 67% were white and
19% were black.
Two 6-week, randomized, double-blind studies compared the
effect on blood pressure of Edarbi at doses of 40 mg and 80 mg, with placebo
and with active comparators. Blood pressure reductions compared to placebo
based on clinic blood pressure measurements at trough and 24-hour mean blood
pressure by ambulatory blood pressure monitoring (ABPM) are shown in Table 1
for both studies. Edarbi, 80 mg, was statistically superior to placebo and
active comparators for both clinic and 24-hour mean blood pressure
measurements.
Table 1: Placebo Corrected Mean Change from Baseline
in Systolic/Diastolic Blood Pressure at 6 Weeks (mm Hg)
|
Study 1
N=1285 |
Study 2
N=989 |
Clinic Blood Pressure (Mean Baselinem 157.4/92.5) |
24 Hour Mean by ABPM (Mean Baseline 144.9/88.7) |
Clinic Blood Pressure (Mean Baseline 159.0/91.8) |
24 Hour Mean by ABPM (Mean Baseline 146.2/87.6) |
Edarbi 40 mg |
-14.6/-6.2 |
-13.2/-8.6 |
-12.4/-7.1 |
-12.1/-7.7 |
Edarbi 80 mg |
-14.9/-7.5 |
-14.3/-9.4 |
-15.5/-8.6 |
-13.2/-7.9 |
Olmesartan 40 mg |
-11.4/-5.3 |
-11.7/-7.7 |
-12.8/-7.1 |
-11.2/-7.0 |
Valsartan 320 mg |
-9.5/-4.4 |
-10.0/-7.0 |
|
|
In a study comparing Edarbi to valsartan over 24 weeks,
similar results were observed.
Most of the antihypertensive effect occurs within the
first two weeks of dosing.
Figure 2 shows the 24-hour ambulatory systolic and diastolic
blood pressure profiles at endpoint.
Figure 2: Mean Ambulatory Blood Pressure at 6 Weeks by
Dose and Hour
Other studies showed similar 24-hour ambulatory blood
pressure profiles.
Edarbi has a sustained and consistent antihypertensive
effect during long-term treatment, as shown in a study that randomized patients
to placebo or continued Edarbi after 26 weeks. No rebound effect was observed
following the abrupt cessation of Edarbi therapy.
Edarbi was effective in reducing blood pressure
regardless of the age, gender, or race of patients, but the effect, as
monotherapy, was smaller, approximately half, in black patients, who tend to
have low renin levels. This has been generally true for other angiotensin II
antagonists and ACE inhibitors.
Edarbi has about its usual blood pressure lowering effect
size when added to a calcium channel blocker (amlodipine) or a thiazide-type
diuretic (chlorthalidone).
There are no trials of Edarbi demonstrating reductions in
cardiovascular risk in patients with hypertension, but at least one
pharmacologically similar drug has demonstrated such benefits.