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. Valsartan
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 pathways for angiotensin II synthesis.
There is also an AT2 receptor found in many tissues, but
AT2 is not known to be associated with cardiovascular homeostasis. Valsartan
has much greater affinity (about 20000-fold) for the AT1 receptor than for the
AT2 receptor. The primary metabolite of valsartan is essentially inactive with
an affinity for the AT1 receptor about one 200th that of valsartan itself.
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 also catalyzed by ACE. Because
valsartan does not inhibit ACE (kininase II) it does not affect the response to
bradykinin. Whether this difference has clinical relevance is not yet known.
Valsartan does not bind to or block other hormone 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 valsartan on blood pressure.
Hydrochlorothiazide is a thiazide diuretic. Thiazides
affect the renal tubular mechanisms of electrolyte reabsorption, directly
increasing excretion of sodium and chloride in approximately equivalent
amounts. Indirectly, the diuretic action of hydrochlorothiazide reduces plasma
volume, with consequent increases in plasma renin activity, increases in
aldosterone secretion, increases in urinary potassium loss, and decreases in
serum potassium. The renin-aldosterone link is mediated by angiotensin II, so
coadministration of an angiotensin II receptor antagonist tends to reverse the
potassium loss associated with these diuretics.
The mechanism of the antihypertensive effect of thiazides
is unknown.
Pharmacodynamics
Valsartan: Valsartan inhibits the pressor effect
of angiotensin II infusions. An oral dose of 80 mg inhibits the pressor effect
by about 80% at peak with approximately 30% inhibition persisting for 24 hours.
No information on the effect of larger doses is available.
Removal of the negative feedback of angiotensin II causes
a 2- to 3-fold rise in plasma renin and consequent rise in angiotensin II
plasma concentration in hypertensive patients. Minimal decreases in plasma
aldosterone were observed after administration of valsartan; very little effect
on serum potassium was observed.
Hydrochlorothiazide: After oral
administration of hydrochlorothiazide, diuresis begins within 2 hours, peaks in
about 4 hours and lasts about 6 to 12 hours.
Drug Interactions
Hydrochlorothiazide
Alcohol, barbiturates, or narcotics: Potentiation
of orthostatic hypotension may occur.
Skeletal muscle relaxants: Possible increased
responsiveness to muscle relaxants such as curare derivatives.
Digitalis glycosides: Thiazide-induced hypokalemia
or hypomagnesemia may predispose the patient to digoxin toxicity.
Pharmacokinetics
Valsartan: Valsartan peak plasma concentration is
reached 2 to 4 hours after dosing. Valsartan shows bi-exponential decay
kinetics following intravenous administration, with an average elimination
half-life of about 6 hours. Absolute bioavailability for the capsule
formulation is about 25% (range 10% to 35%). Food decreases the exposure (as
measured by AUC) to valsartan by about 40% and peak plasma concentration (Cmax)
by about 50%. AUC and Cmax values of valsartan increase approximately linearly
with increasing dose over the clinical dosing range. Valsartan does not
accumulate appreciably in plasma following repeated administration.
Hydrochlorothiazide: The estimated absolute
bioavailability of hydrochlorothiazide after oral administration is about 70%.
Peak plasma hydrochlorothiazide concentrations (Cmax) are reached within 2 to 5
hours after oral administration. There is no clinically significant effect of
food on the bioavailability of hydrochlorothiazide.
Hydrochlorothiazide binds to albumin (40% to 70%) and
distributes into erythrocytes. Following oral administration, plasma
hydrochlorothiazide concentrations decline bi-exponentially, with a mean
distribution half-life of about 2 hours and an elimination half-life of about
10 hours.
Diovan HCT: Diovan HCT may be administered with or
without food.
Distribution
Valsartan: The steady state volume of distribution
of valsartan after intravenous administration is small (17 L), indicating that
valsartan does not distribute into tissues extensively. Valsartan is highly
bound to serum proteins (95%), mainly serum albumin.
Metabolism
Valsartan: The primary metabolite, accounting for
about 9% of dose, is valeryl 4-hydroxy valsartan. In vitro metabolism studies
involving recombinant CYP 450 enzymes indicated that the CYP 2C9 isoenzyme is
responsible for the formation of valeryl-4-hydroxy valsartan. Valsartan does
not inhibit CYP 450 isozymes at clinically relevant concentrations. CYP 450
mediated drug interaction between valsartan and coadministered drugs are
unlikely because of the low extent of metabolism.
Hydrochlorothiazide: Is not metabolized.
Excretion
Valsartan: Valsartan, when administered as an oral
solution, is primarily recovered in feces (about 83% of dose) and urine (about
13% of dose). The recovery is mainly as unchanged drug, with only about 20% of
dose recovered as metabolites.
Following intravenous administration, plasma clearance of
valsartan is about 2 L/h and its renal clearance is 0.62 L/h (about 30% of
total clearance).
Hydrochlorothiazide: About 70% of an orally
administered dose of hydrochlorothiazide is eliminated in the urine as
unchanged drug.
Special Populations
Geriatric
Exposure (measured by AUC) to valsartan
is higher by 70% and the half-life is longer by 35% in the elderly than in the
young. A limited amount of data suggest that the systemic clearance of
hydrochlorothiazide is reduced in both healthy and hypertensive elderly
subjects compared to young healthy volunteers.
Gender
Pharmacokinetics of valsartan do not
differ significantly between males and females.
Race
Pharmacokinetic differences due to race have
not been studied.
Renal Insufficiency
There is no apparent
correlation between renal function (measured by creatinine clearance) and
exposure (measured by AUC) to valsartan in patients with different degrees of
renal impairment. Valsartan has not been studied in patients with severe
impairment of renal function (creatinine clearance < 10 mL/min). Valsartan is
not removed from the plasma by hemodialysis.
In a study in individuals with impaired renal function,
the mean elimination half-life of hydrochlorothiazide was doubled in
individuals with mild/moderate renal impairment (30 < CrCl < 90 mL/min) and
tripled in severe renal impairment (CrCl ≤ 30 mL/min), compared to
individuals with normal renal function (CrCl > 90 mL/min) [see Use in
Specific Populations].
Hepatic Insufficiency
On average, patients with
mild-to-moderate chronic liver disease have twice the exposure (measured by AUC
values) to valsartan of healthy volunteers (matched by age, sex, and weight) [see
Use in Specific Populations].
Drug Interactions
Hydrochlorothiazide
Drugs that alter gastrointestinal motility: The
bioavailability of thiazide-type diuretics may be increased by anticholinergic
agents (e.g., atropine, biperiden), apparently due to a decrease in
gastrointestinal motility and the stomach emptying rate. Conversely, pro-kinetic
drugs may decrease the bioavailability of thiazide diuretics.
Cholestyramine: In a dedicated drug interaction
study, administration of cholestyramine 2 hours before hydrochlorothiazide
resulted in a 70% reduction in exposure to hydrochlorothiazide. Further,
administration of hydrochlorothiazide 2 hours before cholestyramine resulted in
35% reduction in exposure to hydrochlorothiazide.
Antineoplastic agents (e.g., cyclophosphamide,
methotrexate): Concomitant use of thiazide diuretics may reduce renal
excretion of cytotoxic agents and enhance their myelosuppressive effects.
Developmental Toxicity Studies
Valsartan-Hydrochlorothiazide: There was no
evidence of teratogenicity in mice, rats, or rabbits treated orally with
valsartan at doses up to 600, 100, and 10 mg/kg/day, respectively, in
combination with hydrochlorothiazide at doses up to 188, 31, and 3 mg/kg/day.
These non-teratogenic doses in mice, rats and rabbits, respectively, represent
9, 3.5, and 0.5 times the MRHD of valsartan and 38, 13, and 2 times the MRHD of
hydrochlorothiazide on a mg/m² basis. (Calculations assume an oral dose of 320
mg/day valsartan in combination with 25 mg/day hydrochlorothiazide and a 60-kg
patient.)
Fetotoxicity was observed in association with maternal
toxicity in rats and rabbits at valsartan doses of ≥ 200 and 10
mg/kg/day, respectively, in combination with hydrochlorothiazide doses of
≥ 63 and 3 mg/kg/day. Fetotoxicity in rats was considered to be related
to decreased fetal weights and included fetal variations of sternebrae,
vertebrae, ribs and/or renal papillae. Fetotoxicity in rabbits included
increased numbers of late resorptions with resultant increases in total
resorptions, postimplantation losses, and decreased number of live fetuses. The
no observed adverse effect doses in mice, rats and rabbits for valsartan were
600, 100, and 3 mg/kg/day, respectively, in combination with
hydrochlorothiazide doses of 188, 31, and 1 mg/kg/day. These no adverse effect
doses in mice, rats, and rabbits, respectively, represent 9, 3, and 0.18 times
the MRHD of valsartan and 38, 13, and 0.5 times the MRHD of hydrochlorothiazide
on a mg/m² basis. (Calculations assume an oral dose of 320 mg/day valsartan in
combination with 25 mg/day hydrochlorothiazide and a 60-kg patient.)
Valsartan: No teratogenic effects were observed
when valsartan was administered to pregnant mice and rats at oral doses up to
600 mg/kg/day and to pregnant rabbits at oral doses up to 10 mg/kg/day.
However, significant decreases in fetal weight, pup birth weight, pup survival
rate, and slight delays in developmental milestones were observed in studies in
which parental rats were treated with valsartan at oral, maternally toxic
(reduction in body weight gain and food consumption) doses of 600 mg/kg/day
during organogenesis or late gestation and lactation. In rabbits, fetotoxicity
(i.e., resorptions, litter loss, abortions, and low body weight) associated
with maternal toxicity (mortality) was observed at doses of 5 and 10 mg/kg/day.
The no observed adverse effect doses of 600, 200, and 2 mg/kg/day in mice,
rats, and rabbits represent 9, 6, and 0.1 times, respectively, the MRHD on a
mg/m² basis. (Calculations assume an oral dose of 320 mg/day and a 60-kg
patient.)
Hydrochlorothiazide: Under the auspices of the
National Toxicology Program, pregnant mice and rats that received
hydrochlorothiazide via gavage at doses up to 3000 and 1000 mg/kg/day,
respectively, on gestation days 6 through 15 showed no evidence of
teratogenicity. These doses of hydrochlorothiazide in mice and rats represent
608 and 405 times, respectively, the MRHD on a mg/m² basis. (Calculations
assume an oral dose of 25 mg/day and a 60-kg patient.)
Clinical Studies
Hypertension
Valsartan-Hydrochlorothiazide: In controlled
clinical trials including over 7600 patients, 4372 patients were exposed to
valsartan (80, 160, and 320 mg) and concomitant hydrochlorothiazide (12.5 and
25 mg). Two factorial trials compared various combinations of 80/12.5 mg, 80/25
mg, 160/12.5 mg, 160/25 mg, 320/12.5 mg, and 320/25 mg with their respective
components and placebo. The combination of valsartan and hydrochlorothiazide
resulted in additive placebo-adjusted decreases in systolic and diastolic blood
pressure at trough of 14-21/8-11 mmHg at 80/12.5 mg to 320/25 mg, compared to
7-10/4-5 mmHg for valsartan 80 mg to 320 mg, and 5-11/2-5 mmHg for
hydrochlorothiazide 12.5 mg to 25 mg alone.
Three other controlled trials investigated the addition
of hydrochlorothiazide to patients who did not respond adequately to valsartan
80 mg to valsartan 320 mg, resulted in the additional lowering of systolic and
diastolic blood pressure by approximately 4-12/2-5 mmHg.
The maximal antihypertensive effect was attained 4 weeks
after the initiation of therapy, the first time point at which blood pressure
was measured in these trials.
In long-term follow-up studies (without placebo control)
the effect of the combination of valsartan and hydrochlorothiazide appeared to
be maintained for up to 2 years. The antihypertensive effect is independent of
age or gender. The overall response to the combination was similar for black
and non-black patients.
There was essentially no change in heart rate in patients
treated with the combination of valsartan and hydrochlorothiazide in controlled
trials.
There are no trials of the Diovan HCT combination tablet
demonstrating reductions in cardiovascular risk in patients with hypertension,
but the hydrochlorothiazide component and several ARBs, which are the same
pharmacological class as the valsartan component, have demonstrated such
benefits.
Valsartan: The antihypertensive effects of
valsartan were demonstrated principally in 7 placebo-controlled, 4- to 12-week
trials (1 in patients over 65 years) of dosages from 10 to 320 mg/day in
patients with baseline diastolic blood pressures of 95-115 mmHg. The studies
allowed comparison of once-daily and twice-daily regimens of 160 mg/day;
comparison of peak and trough effects; comparison (in pooled data) of response
by gender, age, and race; and evaluation of incremental effects of
hydrochlorothiazide.
Administration of valsartan to patients with essential
hypertension results in a significant reduction of sitting, supine, and
standing systolic and diastolic blood pressure, usually with little or no
orthostatic change.
In most patients, after administration of a single oral
dose, onset of antihypertensive activity occurs at approximately 2 hours, and
maximum reduction of blood pressure is achieved within 6 hours. The
antihypertensive effect persists for 24 hours after dosing, but there is a
decrease from peak effect at lower doses (40 mg) presumably reflecting loss of inhibition
of angiotensin II. At higher doses, however (160 mg), there is little
difference in peak and trough effect. During repeated dosing, the reduction in
blood pressure with any dose is substantially present within 2 weeks, and
maximal reduction is generally attained after 4 weeks. In long-term follow-up
studies (without placebo control) the effect of valsartan appeared to be
maintained for up to 2 years. The antihypertensive effect is independent of
age, gender or race. The latter finding regarding race is based on pooled data
and should be viewed with caution, because antihypertensive drugs that affect
the renin-angiotensin system (that is, ACE inhibitors and angiotensin II
blockers) have generally been found to be less effective in low-renin
hypertensives (frequently blacks) than in high-renin hypertensives (frequently
whites). In pooled, randomized, controlled trials of Diovan that included a
total of 140 blacks and 830 whites, valsartan and an ACE-inhibitor control were
generally at least as effective in blacks as whites. The explanation for this
difference from previous findings is unclear.
Abrupt withdrawal of valsartan has not been associated
with a rapid increase in blood pressure.
The 7 studies of valsartan monotherapy included over 2000
patients randomized to various doses of valsartan and about 800 patients
randomized to placebo. Doses below 80 mg were not consistently distinguished
from those of placebo at trough, but doses of 80, 160 and 320 mg produced
dose-related decreases in systolic and diastolic blood pressure, with the
difference from placebo of approximately 6-9/3-5 mmHg at 80 to 160 mg and 9/6
mmHg at 320 mg.
Patients with an inadequate response to 80 mg once daily
were titrated to either 160 mg once daily or 80 mg twice daily, which resulted
in a comparable response in both groups.
In another 4-week study, 1876 patients randomized to
valsartan 320 mg once daily had an incremental blood pressure reduction 3/1
mmHg lower than did 1900 patients randomized to valsartan 160 mg once daily.
In controlled trials, the antihypertensive effect of once
daily valsartan 80 mg was similar to that of once daily enalapril 20 mg or once
daily lisinopril 10 mg.
There was essentially no change in heart rate in
valsartan-treated patients in controlled trials.
Initial Therapy - Hypertension
The safety and efficacy of Diovan HCT as initial therapy
for patients with severe hypertension (defined as a sitting diastolic blood
pressure ≥ 110 mmHg and systolic blood pressure ≥ 140 mmHg off all
antihypertensive therapy) was studied in a 6-week multicenter, randomized,
double-blind study. Patients were randomized to either Diovan HCT (valsartan
and hydrochlorothiazide 160/12.5 mg once daily) or to valsartan (160 mg once
daily) and followed for blood pressure response. Patients were force-titrated
at 2-week intervals. Patients on combination therapy were subsequently titrated
to 160/25 mg followed by 320/25 mg valsartan/hydrochlorothiazide. Patients on
monotherapy were subsequently titrated to 320 mg valsartan followed by a
titration to 320 mg valsartan to maintain the blind.
The study randomized 608 patients, including 261 (43%)
females, 147 (24%) blacks, and 75 (12%) ≥ 65 years of age. The mean blood
pressure at baseline for the total population was 168/112 mmHg. The mean age
was 52 years. After 4 weeks of therapy, reductions in systolic and diastolic
blood pressure were 9/5 mmHg greater in the group treated with Diovan HCT
compared to valsartan. Similar trends were seen when the patients were grouped
according to gender, race, or age.