Dose-Ranging Study of the Angiotensin II Receptor Antagonist Irbesartan (SR 47436/BMS-186295) on Blood Pressure and Neurohormonal Effects in Salt-Deplete Men

Abstract

Summary 

We characterised the blood pressure (BP) and hormonal responses to the oral angiotensin II (Ang II) receptor antagonist irbesartan (SR47436/BMS-186295) or placebo in normal men with an activated renin-angiotensin system (RAS) during salt depletion. We also evaluated safety and tolerability. Twelve healthy, normotensive male volunteers followed a standardised salt-depletion regimen for 3 days before each study day. Six different single oral doses of irbesartan (1, 5, 10, 25, 50, and 100 mg) were administered double-blind in a three-panel, dose escalation with placebo randomised in each panel. Supine and erect BP and heart rate (HR), serum and urinary electrolytes; plasma renin activity (PRA), and Ang II were measured at intervals. Urinary electrolytes were measured for the 24-h period before dosing (to confirm salt depletion) and for 24 h afterward. No drug-related side effects were noted. There was a dose-related decrease in supine and erect systolic and diastolic BP (SBP, DBP) with irbesartan from 10 mg and beyond, with no change in HR. Supine mean arterial pressure (MAP) decreased by 18.8 mm Hg. There was a dose-related reactive increase in PRA (to 35 ng/ml/h) and Ang II (to 450 pg/ml) with irbesartan. Irbesartan is an orally active AT₁ receptor antagonist. In salt-deplete normal men, it has a dose-related haemodynamic, hormonal, and electrolyte profile characteristic of AT₁ antagonists. The dose range studied did not show a plateau or maximum effect.

Selective blockers of the receptors for angiotensin II (Ang II) were developed many years ago in the form of peptide analogues of Ang II, most notably saralasin ⁽¹⁾. These drugs were useful in physiological and pharmacological studies of the renin-angiotensin system (RAS), but had partial agonist properties and were not active as oral preparations. Angiotensin receptor blockade has been further developed as a specific and selective means of blocking RAS with the recent appearance of orally active, nonpeptide Ang II receptor antagonist (ARA) drugs ^(2,3). Those under clinical development are active at the AT₁-receptor subtype responsible for all of the haemodynamic and endocrine responses of Ang II ⁽⁴⁾. A wide variety of agents is currently under development and the first agent of this class, losartan, was recently approved in several countries for clinical use in the treatment of uncomplicated hypertension ⁽⁵⁾.

Irbesartan (SR 47436/BMS 186295) is a novel, orally active ARA with selectivity for the AT₁-receptor subtype ⁽⁶⁾. In this study, we characterised the dose-response relation both for blood pressure (BP) and RAS hormones, for irbesartan, which is currently undergoing clinical assessment in humans. We have used a setting of controlled activation of the RAS based on dietary sodium restriction and prestudy diuretic treatment with frusemide in normal subjects. This has previously been characterised both in terms of the degree of activation of the RAS produced and also for the response to drugs blocking the RAS such as an angiotensin-converting enzyme (ACE) inhibitor ⁽⁷⁾, a renin inhibitor ⁽⁸⁾, or an ARA ⁽⁹⁾. This protocol can be safely used for dose-ranging studies of drugs blocking the RAS in men.

SUBJECTS AND METHODS

Twelve normotensive male volunteers aged 24.9 ± 3.72 years (±SD) weighing 82.1 ± 12.4 kg (±SD), and free of concomitant illness or drug therapy gave their written and informed consent to participate in the study. Normal health was confirmed by clinical history, physical examination, biochemical and haematological screen, 12-lead ECG and 24-h urinary sodium excretion (100-250 mmol) on an unrestricted diet. The protocol was approved by the local research and ethical review committee.

Controlled manipulation of salt status was implemented with a previously described protocol ^(7-9) based on dietary sodium restriction and oral diuretic treatment for 3 days before each of 4 study days with a 7-day interval between each study day. During all phases, volunteers were provided with a complete diet containing 40 mmol sodium/day (≈60-80 mmol potassium). Dietary instruction by a qualified dietitian, and an accompanying diet sheet, defined allowed or disallowed supplemental foods as required. Free fluid intake was encouraged, but alcohol intake was prohibited during the preparatory phases or the study day (24 h). Subjects continued the dietary regimen during the study day and until the return visit 24 h after drug administration. In addition, during the 3-day pretreatment schedule, subjects received frusemide 40 mg (Antigen Pharmaceuticals, NI), twice daily at 0900 and 1200 h. The last dose of diuretic was administered ≈20 h before drug administration. After subjects received instructions on technique, 24-h urinary collections were performed at the screening visit, for the 24 h preceding each of the study days (day 3), and for the 24 h of each study day (day 4). During the study day, fractional collections of urine at 2-h intervals were performed for the first 10 h after dosing.

Volunteers attended the research unit after an overnight fast (10 h) on four separate occasions, 7 days apart, to receive irbesartan or placebo after salt depletion. Because the study was undertaken in the early stages of clinical investigation of irbesartan, treatment was administered in a dose-escalation panel design (Table 1). Four subjects were studied in each of three panels. Treatment was administered in a double-blind fashion. For safety reasons, the study was not fully randomised. Active treatment with irbesartan was always administered in a progressive series of incremental doses for each individual subject in the panel. However, treatment with placebo was administered in a randomised, double-blind fashion in the panel. At the end of each phase of treatment, raw data, blinded to treatment, were reviewed without formal statistical analyses by the investigators and the sponsors of the study (Sanofi Recherche, Montpellier, France) to reach consensus on safety issues with respect to progression to the next dose panel. Efficacy was not under consideration at this point.

Procedure

On attendance (0700 h), subjects rested supine for at least 1 h after placement of a heparinised venous cannula in an antecubital fossa vein for the purpose of blood sampling. Basal blood samples and BP readings were collected in supine subjects before measurement of erect BP and HR and before bladder voiding. BP and HR were determined in triplicate in supine position, and at 1, 3, and 5 min of standing with a semiautomatic device (Critikon Vital Signs Monitor, Tampa, FL, U.S.A.) at frequent intervals during the study day. Blood samples were drawn at intervals for the determination of serum drug concentration, serum electrolytes, plasma renin activity (PRA), and Ang II. All subjects had been supine for at least 50 min before having their blood sampled for hormones. Subjects received 200 ml fluid with oral dosing (0 h), 200 ml more with a light breakfast (+2 h), 400 ml with lunch (+4 h), and 400 ml with an evening snack (+9 h). All meals were taken after the appropriate sampling was completed.

Analyses

Serum and urinary electrolytes were determined with a routine autoanalyser by the hospital biochemistry laboratory. Creatinine clearance was calculated with the standard equation. PRA was determined in the presence of phenylmethyl sulfonyl fluoride (PMSF) and EDTA as inhibitors; it was analysed with a standard assay ⁽¹⁰⁾; both the intra- and interassay coefficients of variation were between 5.6% and 7.6%. Plasma Ang II levels were analysed with the Sanofi Diagnostic Kit at Biotec Centre (Orleans, France). Both the intra- and interassay coefficients of variation were <15%. Plasma drug concentrations were assayed by the radioimmunoassay (RIA) of Sanofi Recherche. Both the intra- and the interassay coefficients of variation were in the range of 6.2-8.3%.

BP and HR data were compared by repeated-measures analysis of variance (ANOVA) with Bonferroni correction adjusted for multiple treatment comparisons between groups. Electrolyte and creatinine clearance data were compared by a paired t test. Data are mean ± 1 SD.

RESULTS

Baseline effects of prestudy preparation

All volunteers completed study preparation phases without difficulty. No spontaneous symptoms were reported by volunteers. The effects of prestudy salt depletion are summarised in Table 2. Study day body weights were reduced slightly but significantly as compared with that at screening (salt replete) values. Salt depletion was associated with slight but significant decreases in serum sodium and potassium with a similar slight but significant increase in urea as compared with screening, salt-replete, values. Salt depletion had no significant effect on study day creatinine as compared with screening value. On the third day of preparation, urinary volume was significantly increased as compared with the screening value, in keeping with diuretic administration. Twenty-four-hour urinary sodium excretion was significantly reduced during all collections on the third day of dietary modification as compared with the screening value, in keeping with salt depletion. Supine PRA pretreatment was significantly increased (≈fourfold) before all study treatments as compared with screening value.

Treatment effects

General. All study phases were completed without difficulty. There were no biochemical or haematological adverse events or symptom side effects attributable to active treatment.

BP and HR. The supine BP and HR responses to irbesartan and placebo are summarised in Fig. 1. Corresponding 1-min erect measurements are summarised in Fig. 2. There was a dose-related decrease in both supine and erect SBP and DBP. At higher doses, several timepoints differed significantly from placebo (p < 0.05) by ANOVA (Table 3). Despite the visibly greater BP reduction induced by irbesartan 100 mg, none of the timepoints at that dose reached statistical significance, most likely due to the small number of subjects studied at that dose (n = 4). The baseline corrected mean maximal decreases in supine mean arterial pressure (MAP) after placebo and irbesartan are summarised in Table 4. There was no significant change in supine or erect HR attributable to irbesartan as compared with placebo.

Irbesartan produced little change in BP at an oral dose <10 mg. Above that dose, the magnitude and duration of response tended to increase. No dose of irbesartan used produced a significant decrease in BP, as compared with placebo, 24 h after dosing.

PRA and Ang II. The change in PRA is shown in Fig. 3a. Placebo treatment caused little change in PRA from the already increased baseline values due to diuretic and dietary pretreatment. The continuing low-salt diet sustained a constant baseline hyperreninaemia. Treatment with irbesartan produced a dose-related increase in PRA at all doses studied. Timepoints and doses of irbesartan that differed significantly from placebo are shown in Table 3. No dose of irbesartan increased PRA significantly above placebo at 24 h postdosing. However, there was a trend toward increased PRA, which was greater at the higher doses.

Figure 3b shows the changes in Ang II during the study. The predicted reactive increase in Ang II was not evident at <10-mg doses. With doses >10 mg, there was a dose-related increase in Ang II. Timepoints and doses of irbesartan that differed significantly from placebo are shown in Table 3.

Plasma concentrations of irbesartan. Plasma drug levels for irbesartan treatments were measured in samples collected at intervals (pretreatment and 1, 2, 4, 6, 8, 10, and 24 h postdosing). These data are insufficient to describe the pharmacokinetics of irbesartan in detail. Although the number of samples at each timepoint was small, there was a dose-related increase in peak plasma drug concentration (C_max), with time to C_max (T_max) occurring between 1.49 and 2.62 h (Table 5).

Serum electrolytes. Samples were collected at 0, 2, 4, 6, 8, 10, and 24 h. There was no significant change in serum sodium, potassium, urea, creatinine, or urate with any dose of irbesartan as compared with placebo (data not shown).

Urinary electrolytes and creatinine clearance. Twenty-four-hour urinary sodium excretion was reduced during all treatment days, in keeping with an expected phase of antinatriuresis. There were no significant differences between placebo and any dose of irbesartan (data not shown). Neither calculated creatinine clearance during the first 10 h post-dosing using 2-h fractional collections nor the cumulative 24-h figure was significantly different after placebo or after any dose of irbesartan (data not shown).

DISCUSSION

We demonstrated a dose-related BP-lowering effect of the oral ARA irbesartan in salt-deplete subjects with activation of the RAS. There was no appreciable effect on HR. The strategy of subject preparation was previously used successfully to demonstrate the response to an ACE ⁽⁷⁾, a renin inhibitor ⁽⁸⁾, and other ARA ^(9,11) in single oral dose studies. Because ARA is a selective therapy, and in keeping with previous experience with losartan, we suggest that the dose that lowers BP in such normal subjects as these will be similar to the effective dose in hypertensive subjects. The level of activation of the RAS in the present study was similar to that in previous reports and comparable to certain forms of renovascular hypertension and diuretic-treated heart failure ^(12,13).

Irbesartan was well tolerated, and no symptomatic or biochemical adverse effects were noted. There were no episodes of profound hypotension after irbesartan treatment. Such first-dose responses are well documented after ACE inhibitor (ACEI) treatment ⁽¹⁴⁾ in states associated with activation of the RAS such as diuretic treatment, electrolyte imbalance, or heart failure. Their exact origin is not clear. Whether ARA will share these properties is still unclear. No reports have yet been published documenting such a response for losartan ⁽⁵⁾, but this possibility will be better assessed during widespread clinical use of ARA. The first-dose hypotensive response documented widely for ACEI may relate to the non-angiotensin-mediated effects of ACEI and may therefore be less evident or indeed absent with ARA. This may also apply to the cough associated with ACEI ⁽¹⁵⁾. Our results show a dose-related decrease in BP with increasing duration of response for irbesartan, with no evidence of postural hypotension or the bradycardia reported with ACEI ⁽¹⁶⁾.

The study design, with relatively small numbers in the lowest and highest dose groups (n = 4) and baseline differences between groups makes it difficult to define the dose-response relation for irbesartan accurately in this setting. However, there was no indication that a maximum response had been achieved at the higher doses studied.

The hormone measurements confirm the expected activation of the RAS, in keeping with salt depletion and the results of previous studies. Treatment with irbesartan produced a reactive increase in PRA at all doses studied, although it was slight and transient after the lowest dose. This increased reactive release of PRA after ARA was observed in our previous studies with losartan ^(9,17) and ACEI ⁽¹⁸⁾. In the present study, we noted that the reactive increases in Ang II equated to doses of ARA that decrease BP. Because Ang II increases after ARA and decreases after ACEI, much of the analytical controversies surrounding this measurement as an index of ACEI effect are circumvented. Peptide separation, with preparative high-performance liquid chromatography ⁽¹⁹⁾ was not necessary in this study. A clear dose-response curve was generated with standard sample collection, storage, and assay procedures.

Irbesartan had no effect on calculated creatinine clearance in the salt-deplete volunteers. Previously, we documented a dose-related decrease in calculated creatinine clearance, particularly after losartan 100 mg and with an identical protocol of subject preparation ⁽¹⁷⁾. Whether these results reflect any difference between the response of irbesartan or losartan or whether it is merely a consequence of a relatively lower dose is not clear. Initial results in hypertensive patients with ARA are reassuring when combinations of losartan and thiazide diuretics ⁽²⁰⁾ are used or when losartan is used as monotherapy in patients with stable chronic renal impairment ⁽²¹⁾, because renal function does not decrease in chronic dosing studies. The use of loop diuretics in combination with ARA has not been widely described in men, however ⁽²²⁾.

We demonstrated significant BP-lowering effects of irbesartan at a ≥10-mg oral dose in salt-deplete men. The expected profile of hormone changes was confirmed. Irbesartan had no effect on serum urate or on calculated creatinine clearance. In men, irbesartan is an effective and well-tolerated orally active ARA that is currently undergoing further study at doses of 75-150 mg in patients with essential hypertension.

Acknowledgment: We thank Drs. R. Creek and J. Dickinson of Sanofi Winthrop (Manchester, U.K.) for logistical support and for providing supplies of irbesartan, J. Seymour for preparing diets and dietary instruction of subjects, and J. Fenton, K. Shields, and S. MacKie for assistance during study days.

FIG. 1.: Change in supine systolic (SBP), diastolic blood pressure (DBP), and heart rate (HR) with time postdose for placebo (n = 12), irbesartan 10 mg (n = 8), 50 mg (n = 8), and 100 mg (n = 4). The curves for irbesartan 1, 5, and 25 mg and SD have been omitted for clarity.

FIG. 2.: Change in erect systolic (SBP), diastolic blood pressure (DBP), and heart rate (HR) with time postdose for placebo (n = 12), irbesartan 10 mg (n = 8), 50 mg (n = 8), and 100 mg (n = 4). The curves for irbesartan 1, 5, and 25 mg and SD have been omitted for clarity.

FIG. 3.: Change in plasma renin activity (P.R.A.) and angiotensin II (AngII) with time postdose for placebo (n = 12), irbesartan 1 mg (n = 4), 5 mg (n = 4), 10 mg (n = 8), 25 mg (n = 8), 50 mg (n = 8), and 100 mg (n = 4), irbesartan 1, 5, 10, 25, 50, and 100 mg and SD have been omitted for clarity.