Dose-Dependent Blockade of the Angiotensin II Type 1 Receptor with Losartan in Normal Volunteers

Losartan, an angiotensin II type 1 receptor (AT1) antagonist, was developed as a more specific alternative to angiotensin-converting enzyme (ACE) inhibitors. At a daily dose of 50 mg, losartan is currently evaluated in large outcome trials involving patients with hypertension and postmyocardial infarction. The current study evaluated the level and duration of blockade of a pressor response to angiotensin II by 50 and 150 mg of losartan, compared with 32 mg of candesartan. Eight normotensive volunteers were randomly assigned to a single dose of losartan 50 or 150 mg, candesartan 32 mg, or placebo. Subjects were re-randomized after a 2-week washout period to complete all four study arms. Radial artery systolic pressure response to exogenous angiotensin II was measured at 2, 6, 12, and 24 h after administration of drug. Losartan 50 mg reduced the pressure response to exogenous angiotensin II significantly only at 6 h. In contrast, candesartan and losartan 150 mg produced a greater reduction in the pressure response to angiotensin II throughout the 24-h period. This suppression was not paralleled by a reduction in resting systemic arterial pressure. Higher doses than 50 mg of losartan might be evaluated to elicit optimal clinical effects.

Losartan, an angiotensin II type 1 (AT1) receptor antagonist, was developed as a more specific alternative to angiotensin-converting enzyme (ACE) inhibitors in the treatment of hypertension ⁽¹⁾. In patients with hypertension, losartan appears to be as effective as ACE inhibitors in lowering blood pressure ^(2,3). Because the dose response was fairly flat above 50 mg, and higher doses of losartan offered little additional benefit over 50 mg in comparative studies ⁽⁴⁾, this is the dose that is most often recommended.

By selectively inhibiting AT1 receptors, losartan attenuates or even reverses the adverse effects that increased angiotensin II levels have on vascular function and structure ^(5-7). Several large trials have recently been completed or are on-going that evaluate the role of losartan in patients with hypertension, myocardial infarction, or chronic heart failure ^(8-10). In these trials, a dose of 50 mg of losartan was selected as the efficacious dose to examine clinical outcomes. Likewise, a dose of 50 mg was used to evaluate the effects of losartan on vascular endothelial dysfunction ⁽¹¹⁾, urine albumin excretion ⁽¹²⁾, arterial compliance ⁽¹³⁾, and left ventricular mass ⁽¹⁴⁾. For the treatment of patients with chronic heart failure, a dose of 50 mg of losartan is also recommended, because higher doses result in greater neurohormonal stimulation without further hemodynamic improvement ⁽¹⁵⁾.

The optimal therapeutic dose of losartan is unclear. Intuitively, one would select a dose of losartan that achieves complete and sustained blockade of AT1 receptors and does not excessively reduce blood pressure. Early pharmacodynamic studies reported that 40-50 mg of losartan achieved only partial AT1 receptor blockade, as evidenced by an incomplete suppression of the pressure response to exogenous angiotensin II ^(16,17). Other data on AT1 receptor blockade involve mostly animal and in vitro studies, and few comparisons between different doses of an AT1 receptor antagonist have been studied in human subjects. Accordingly, the current study was undertaken to (a) determine the level of AT1 receptor blockade achieved by 50 and 150 mg of losartan in normal volunteers and (b) compare 150 mg of losartan to 32 mg of candesartan, the highest dose of an AT1 receptor antagonist currently investigated in chronic heart failure.

METHODS

Subjects

Eight normotensive volunteers were enrolled in the study. Subjects ranged in age from 26 to 55 years; the mean and median ages were 34 and 31 years, respectively. Five subjects were women. All subjects underwent history and physical examination before the study and remained on unrestricted sodium diets. Subjects were excluded from the study if they were taking any medication or if the use of an AT1 receptor antagonist was contraindicated. The purpose and design of the study were discussed and informed consent was obtained. The institutional review board approved the protocol.

Study design

At the start of the study, subjects were asked to lie in a supine position and an intravenous catheter was inserted in the right antecubital vein. After 30 min of rest, baseline blood pressure and heart rate were recorded. The radial artery systolic pressure (RASP) response to incremental doses of exogenous angiotensin II was assessed, and a dose-response curve was established for each subject. Bolus intravenous administration of angiotensin II was started at a dose of 5 ng/kg and increased by increments of 5 ng/kg until RASP rose by 15-20 mm Hg. The dose of exogenous angiotensin II that raised RASP by 15-20 mm Hg averaged 12.5 ng/kg and ranged from 10 to 20 ng/kg in the eight volunteers. The same dose of exogenous angiotensin II found to increase RASP by 15-20 mm Hg was administered to each subject at the onset of each of the sessions. The resulting increase in RASP was reproducible with a variation of less than 2 mm Hg. Subjects were randomly assigned to a single dose of losartan 50 mg, losartan 150 mg, candesartan 32 mg, or placebo. The AT1 receptor antagonist or placebo was given at time 0 and the dose of exogenous angiotensin II that raised RASP 15-20 mm Hg was then administered at 2, 6, 12, and 24 h. At each time point, exogenous angiotensin II administration was repeated and RASP responses were averaged. After a 2-week washout period, subjects were re-randomized to another arm in the study until all four arms were completed. The changes in RASP were measured and analyzed by an investigator blinded to the drug, dose, and study design. The level of AT1 receptor blockade after administration of AT1 receptor antagonist or placebo was assessed by dividing the difference between the baseline RASP response (time 0) and the RASP response to exogenous angiotensin II at each time point, by the baseline RASP response.

Technique

The radial artery waveform allows continuous noninvasive monitoring of the blood pressure with a Colin Pilot Monitor 9200 (Colin Instruments, San Antonio, TX, U.S.A.). Data were stored on a notebook computer using TDA program version 2 from Colin Instruments. Angiotensin II was obtained from CLINALFA (Laufelfingen, Switzerland) and dissolved in 0.9% NaCl to obtain a concentration of 1 μg/ml.

Statistical analysis

Values are expressed as mean ± SEM. Percent blockade was defined as the percentage of attenuation of the RASP response to angiotensin II (the difference between the baseline RASP response and the RASP response to exogenous angiotensin II at each time point, by the baseline RASP response.) Repeated-measures ANOVA and the Scheffé test were used for multiple comparisons between groups. Significance was accepted at p < 0.05.

RESULTS

Systolic and diastolic blood pressures were monitored throughout the 24-h study period after administration of placebo, losartan 50 mg, losartan 150 mg, and candesartan 32 mg. Systolic and diastolic blood pressure tended to be lower from baseline at 6 h for each dose of losartan and candesartan, although the decrease did not reach statistical significance. Heart rate remained unchanged.

The levels of AT1 receptor blockade achieved throughout the 24-h period by placebo, 50 and 150 mg of losartan, and 32 mg of candesartan are shown in Figure 1. Fifty milligrams of losartan partially and transiently blocked the AT1 receptor. The level of blockade was 40% at peak effect at 6 h (p = 0.0152 vs. placebo), 33% at 12 h (p = 0.1039 vs. placebo), and 18% at 24 h (p = 0.1655 vs. placebo). The level of AT1 receptor blockade was significantly different from that of placebo only at 6 h. In contrast, 150 mg of losartan and 32 mg of candesartan achieved more complete AT1 receptor blockade throughout the 24-h period. The level of blockade for 150 mg of losartan was 73% at peak effect at 6 h (p < 0.0001 vs. placebo), 65% at 12 h (p = 0.0001 vs. placebo), and 62% at 24 h (p = 0.0001 vs. placebo). The level of blockade for 32 mg of candesartan was 79% at peak effect at 6 h (p < 0.0001 vs. placebo) and 72% at both 12 h (p < 0.0001 vs. placebo) and 24 h (p < 0.0001 vs. placebo). There was no statistical difference in the level of blockade between losartan 150 mg and candesartan 32 mg at all time points.

Fig. 1

DISCUSSION

A 50-mg dose of losartan reduces the pressure response to exogenous angiotensin II for 6 h in normotensive volunteers. Doses of 150 mg of losartan and 32 mg of candesartan provide a greater reduction of the pressure response to exogenous angiotensin II, which lasts throughout a 24-h period. Of note, 50 and 150 mg of losartan do not produce a statistically significant reduction in systemic arterial pressure and there is no fall in blood pressure to parallel the dose-dependent suppression of the pressure response to exogenous angiotensin II. Thus AT1 receptor blockade is not reflected in a fall in resting systemic arterial pressure.

The incomplete and transient AT1 receptor blockade achieved by losartan 50 mg is in agreement with previous studies. Initially, 40 mg of losartan at peak effect was reported to reduce systolic blood pressure response to angiotensin II by 60-65% for 24 h ⁽¹⁶⁾. When the 20% attenuation of placebo in this initial study was corrected for, the response was similar to the 35-45% reductions in systolic blood pressure response to angiotensin II recently reported with 50 mg of losartan ⁽¹⁷⁾. A 150-mg dose of losartan achieved more complete AT1 receptor blockade throughout the 24-h period.

The lack of a significant reduction in blood pressure to losartan 50 or 150 mg is also consistent with previous reports. Plasma concentrations of losartan and its active metabolite increase proportionally with doses up to 200 mg, but the blood pressure response does not differ significantly between higher and lower doses of losartan after a single dose in normal volunteers ^(16,18). The modest decreases in blood pressure reported in one study may be attributed to sodium restrictions in the study population ⁽¹⁹⁾.

Candesartan 32 mg also suppresses the response to exogenous angiotensin II more effectively than losartan 50 mg. Candesartan is comparable to losartan in the treatment of hypertension ⁽²⁰⁾. Although it shares a common mechanism of action, candesartan has a higher affinity for AT1 receptors, is less surmountable, and has a longer duration of receptor antagonism than losartan ^(21-23). At its highest recommended dose, candesartan blocks the response to exogenous angiotensin II similarly to 150 mg of losartan and may have longer-acting effects.

The role AT1 receptor blockade in the management of heart failure and atherosclerosis and the long-term management of hypertension is evolving. Angiotensin II is involved in the pathophysiology of endothelial dysfunction, vascular smooth muscle hypertrophy, leukocyte adhesion, and generation of superoxide anion ^(24-26). Conversely, AT1 receptor blockade reduces vascular small vessel hypertrophy and improves endothelium-dependent vasodilation abnormalities in patients with coronary artery disease ^(27,28). Finally, although maximally recommended doses of ACE inhibitors do not fully inhibit ACE in patients with chronic heart failure, addition of losartan improves symptoms and peak exercise capacity ^(29,30).

Several large clinical trials have examined the role of an AT1 receptor antagonist in the treatment of heart failure and endothelial dysfunction. The ELITE II trial failed to prove the superiority of 50 mg of losartan over 150 mg of captopril in patients with chronic heart failure ⁽¹⁰⁾. In the BANFF trial, 50 mg of losartan compared unfavorably to ACE inhibition on flow-mediated vasodilation, a marker of endothelial dysfunction, in patients with coronary artery disease ⁽¹¹⁾. Our data raise the possibility that a higher dose of losartan that achieves more AT1 receptor blockade could result in greater clinical benefits in patients with heart failure or endothelial dysfunction ⁽³¹⁾. Fifty milligrams was presumably selected as the optimal dose of losartan, because several studies demonstrated a plateauing of the clinical effects at higher doses ^(2,16,18,19). Currently, several ongoing prospective trials are investigating the role of various AT1 receptor antagonists in heart failure, diabetes, myocardial infarction, and left ventricular hypertrophy ⁽³²⁾. Whether higher doses of losartan or an alternative AT1 receptor antagonist may lead to different outcomes remains to be demonstrated.

The present study has several important limitations. First, we assessed AT1 receptor blockade after only one dose in normotensive volunteers. Second, we only assessed changes in the pressure response to exogenous angiotensin II and do not provide data regarding the behavior of the AT1 receptor in other nonvascular tissues. Third, we did not measure the level of endogenous angiotensin II or other components of the renin-angiotensin-aldosterone system. Plasma concentrations of angiotensin II and renin increase after short-term administration of losartan ⁽¹⁶⁾. They tend to decrease during prolonged administration of losartan ^(19,33). The impact of increased plasma concentration of angiotensin II and renin on the pressure response to exogenous angiotensin II is unclear in adult normal subjects who, in the absence of vascular remodeling, are unlikely to express AT2 receptors.

In conclusion, losartan 50 mg only partially and transiently blocks the systolic blood pressure response to exogenous angiotensin II in healthy volunteers. In contrast, both losartan 150 mg and candesartan 32 mg almost completely suppress the response to exogenous angiotensin II for the entire 24-h period. These differences are not paralleled by different effects on resting blood pressure. It is suggested that higher doses of losartan that achieve more effective blockade of responses to angiotensin II should be investigated to guide the choice of dose for optimal therapeutic efficacy.