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Circulation. 1995;91:330-338

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(Circulation. 1995;91:330-338.)
© 1995 American Heart Association, Inc.

Articles

Dose-Dependent Effects of the Renin Inhibitor Zankiren HCl After a Single Oral Dose in Mildly Sodium-Depleted Normotensive Subjects

Joël Ménard, MD; Robert S. Boger, MD; Denise M. Moyse, MS; Tam T. Guyene, MD; Harriet N. Glassman, MS; Hollis D. Kleinert, PhD

From the INSERM U367, Hospital Broussais, Paris, France (J.M., T.T.G.) and Abbott Laboratories, Abbott Park, Ill (R.S.B., D.M.M., H.N.G., H.D.K.).

Correspondence to Hollis D. Kleinert, PhD, Abbott Laboratories, Department 48G, Bldg AP9, 100 Abbott Park Rd, Abbott Park, IL 60064-3500.


*    Abstract
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 *Abstract
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down arrowResults
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Background Zankiren HCl (A-72517) is a potent renin inhibitor shown to have substantial bioavailability in several animal species and to produce dose-related reductions in blood pressure, plasma renin activity, and angiotensin II (Ang II) in salt-depleted dogs. The present study was designed to evaluate the hemodynamic effects of oral zankiren HCl administration in healthy volunteers and to characterize the response of the renin-angiotensin system (RAS) to specific blockade by this new renin inhibitor.

Methods and Results Twenty-four male volunteers participated in a double-blind randomized, placebo-controlled in-hospital study to evaluate the effects of zankiren HCl (10 to 250 mg). All subjects were pretreated with 40 mg furosemide 12 hours before study drug administration. Blood pressure and heart rate were monitored by an automated oscillometric device, and blood samples were obtained for active renin, total renin, plasma renin activity, angiotensin I (Ang I), Ang II, aldosterone, and plasma zankiren concentration. Satisfactory absorption of zankiren HCl was demonstrated by the results of plasma drug concentration determinations, and renin inhibitory activity was confirmed by dose-related suppression of plasma renin activity, Ang I, Ang II, and aldosterone and increases in plasma active renin concentration. Furthermore, hypotensive activity was readily observed in these normotensive subjects, as evidenced by statistically significant dose-related blood pressure reductions (P<.01).

Conclusions Results from this study demonstrate for the first time that oral administration of a renin inhibitor can dose-dependently decrease blood pressure and circulating components of the RAS in normotensive volunteers as a result of documented absorption.


Key Words: angiotensin • antihypertensive agents • blood pressure • hypertension


*    Introduction
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 up arrowAbstract
 *Introduction
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down arrowResults
down arrowDiscussion
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Most of the investigations performed with renin inhibitors in human beings have been limited by the very low bioavailability of the compounds tested so far: enalkiren (A-64662),1 2 3 CGP 38560 A,4 5 6 and remikiren (Ro 42-5892).7 8 However, evaluation of these compounds given intravenously has permitted demonstration of the antihypertensive effect of renin inhibition, the expected sodium-dependency of the blood pressure fall,2 and the reactive rise in active renin (AR) that accompany the fall in plasma angiotensin II (Ang II)4 5 6 9 observed with all renin inhibitors except ES-8891.10

Zankiren HCl (A-72517) is a transition-state analogue inhibitor of renin that consists of a dipeptide core, a dipeptide-glycol fragment at the C-terminus, an alanine residue substituted with a heterocyclic moiety at the P2 site, a (phenylmethyl) propionyl group at the P3 site, and a (methylpiperazine) sulfonyl group at its N-terminus that improves proteolytic stability and potency. Zankiren HCl is a potent renin inhibitor. The IC50 of zankiren HCl measured at pH 7.4 is 1.1 nmol/L in human plasma, and species specificity is evident by the IC50s in other species: 0.24 nmol/L in monkeys, 9.4 nmol/L in guinea pigs, 110 nmol/L in dogs, and 1400 nmol/L in rats. Oral bioavailability is 8%, 24%, 32%, and 53% in the monkey, rat, ferret, and dog, respectively. Pharmacodynamic activity after oral administration of zankiren HCl to conscious, salt-depleted dogs has been demonstrated by the observation of dose-related reductions in blood pressure, plasma renin activity (PRA), and Ang II in conjunction with ascending peak plasma drug levels.11

The objectives of this first clinical trial with orally administered zankiren HCl were to confirm the animal findings of dose-related absorption in normotensive volunteers. Studies with previous renin inhibitors have uniformly shown low oral bioavailability (<2%). To assess the pharmacodynamic activity of zankiren HCl in these normotensive subjects, a single oral dose of 40 mg furosemide was administered 12 hours before study drug administration to produce mild stimulation of the renin-angiotensin system (RAS). Subjects were studied while in the sitting position for 6 hours and again at 24 hours after ingestion of a single dose of zankiren HCl or placebo in a double-blind, placebo-controlled study. Results show that zankiren HCl is orally active and produced dose-dependent decreases in blood pressure, PRA, and plasma angiotensins. Plasma AR increased in a dose-related manner and, at the highest dose, was accompanied by a rise in plasma prorenin (PR). Zankiren HCl was well tolerated. There were three episodes of hypotension and no clinically important trends in laboratory data suggestive of drug-related toxicity.


*    Methods
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 up arrowAbstract
 up arrowIntroduction
 *Methods
down arrowResults
down arrowDiscussion
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Subjects
Twenty-four healthy, male Caucasian subjects participated in this double-blind, randomized, placebo-controlled study to evaluate the pharmacodynamic effects of selected oral doses of zankiren HCl (10-, 25-, 50-, 125-, and 250-mg tablets). The Berlin Ethics Committee reviewed and approved the study. Informed consent was obtained, and subjects were instructed to follow a diet approximating 100 mEq sodium/d for at least 1 week before confinement for study procedures. Subjects were assigned to one of four dosing groups, each composed of six subjects. Within each dosing group, four subjects were randomly assigned to receive tablets containing zankiren HCl, and two subjects were randomly assigned to receive placebo. Dosing groups were studied in ascending dose order, with the low-dose group also receiving the high dose 96 hours after the first administration. All subjects received a 40-mg oral dose of furosemide 12 hours before study drug administration and were studied while seated under fasting conditions for the first 7 hours of the investigation. Blood samples for AR, total renin (TR), PRA, angiotensin I (Ang I), Ang II, aldosterone, and zankiren concentration were obtained before furosemide administration; 30 minutes before study drug administration; and at 0.5, 0.75, 1, 2, 4, 6, and 24 hours after study drug administration. Subjects were seated for at least 1 hour before sample collection.

Blood pressure and heart rate were measured with subjects in the seated position every 5 minutes by an automated oscillometric device (Dinamap model 1846SX/P, Critikon) during the initial 7-hour observation period and for 1 hour on the day after dosing. Efficacy was evaluated primarily on changes from predosing in systolic, diastolic, and mean arterial blood pressures (MAP). For each subject, the predosing value was taken to be the average of Dinamap measures over the hour preceding dosing, and hourly averages of Dinamap measurements over each of the first 6 hours and the 24th hour after dosing were calculated.

Laboratory Methods
Blood samples were collected in heparinized vacuum tubes for measurement of PRA, AR, TR, plasma aldosterone, and drug levels. For measurement of plasma Ang I and Ang II, 10 mL of blood was taken in EDTA-K3 vacuum tubes, and a mixture of inhibitors was immediately added to prevent in vitro generation and degradation of angiotensins. The solution included 62.5 mmol/L EDTA, 100 µmol/L zankiren HCl, and 100 µmol/L enalaprilat.12 The tubes were centrifuged at 4°C, and 2.5-mL plasma aliquots were frozen at -80°C. A 2.2-mL aliquot was prepared for either Ang I or Ang II measurement and immediately extracted on phenylsilylsilica columns (Bondelut PH, Analytichem) according to Nussberger et al.13 The dried extracts containing angiotensins were diluted in 220 µmol/L of a 0.1 mol/L, pH 7.5 Tris HCl buffer containing 1 mmol/L EDTA and 2 g/L BSA and were measured immediately or kept frozen at -30°C. Recoveries of angiotensins were 98.5±3.5%. For the Ang I radioimmunoassay, we used a polyclonal antibody that cross-reacts 100% with des-Asp Ang I and <1% with Ang II and angiotensin III. The assay can detect 0.5 pg Ang I/tube, and the 50% displacement of the tracer varies between 4 and 8 pg/tube.14 For the Ang II radioimmunoassay, we used a monoclonal antibody that cross-reacts 190% with des-Asp Ang II and 1% with Ang I. The assay can detect 0.5 pg Ang I/tube, and 50% of the tracer is displaced by 10 pg/tube.9

PRA was measured at pH 7.4 by the methods of Sealey et al15 and Poulsen and Jorgensen16 as previously described.14 AR was measured by a radioimmunometric assay, using the two monoclonal antibodies 3E8 and 125I-4G1,17 in a commercially available kit (Pasteur Diagnostics); TR was measured with a different pair of monoclonal antibodies, 3E8 and 125I-3-36-16.18 Ang I production rate (APR) is the PRA/AR ratio and is expressed in picograms of Ang I per picogram of AR per hour. PR was measured by the difference between total TR and AR. Plasma aldosterone levels were determined in plasma by use of a commercially available kit (Behring Diagnostics). Circulating levels of zankiren and its active metabolites were determined by a radio inhibitor binding assay,19 with 3H-remikiren as a tracer. The lower level of sensitivity of this assay is 0.4 ng/mL.

Statistical Analysis
Results are presented as mean±SEM. Values of P<=.05 were considered statistically significant.

To incorporate the monotonic behavior of zankiren over these dose ranges, the hemodynamic and neurohormonal responses were characterized through dose-response models across all dose levels administered. These analyses pooled information from each dose level of zankiren (and the corresponding placebo-dosed subjects); the resulting dose-response curves provided estimates of the effects of zankiren at any of the doses studied. Specifically, the changes in blood pressures were analyzed across all dose levels by use of a mixed-effects model with fixed effects for treatment group (defined by dose level and drug), period, and treatment group by period interaction, with subject as a random effect. A dose-response curve was then estimated with a contrast constructed from the difference between zankiren HCl and placebo at each level. The contrast corresponding to a quadratic term in dose was constructed to detect any nonlinearity in the dose-response curve. Because no significant nonlinearity was detected, the contrast corresponding to a linear dose effect (with zero intercept) was used to estimate the slope of the linear dose-response curve. A test of drug effect and estimates of the effect of each dose were obtained from the slope estimate of the dose-response curve.20 Changes from predosing in pulse rate were analyzed in the same manner. Dose-response analyses of change from baseline in AR, Ang I, and Ang II were performed by use of the same mixed-effects modeling as described above for blood pressure.

A similar approach was used in the dose-response analysis of change from baseline in aldosterone; however, a fixed-effects model was used because assays were performed in the zankiren HCl– and placebo-treated subjects at the 25-, 50-, 125-, and 250-mg dose levels only.

Baseline values of the RAS components were examined for significant correlations by use of Pearson correlation coefficients. The relation between components was examined on the baseline values before and after furosemide administration.


*    Results
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*Results
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The healthy volunteers were between 23 and 39 years of age, with a mean age of 29 years. Average body weight was 73 kg (range, 60 to 90 kg).

Furosemide administration 12 hours before study drug ingestion increased all parameters of the plasma RAS. AR increased from 30±4 to 61±5 pg/mL, PRA (conventional assay) from 1.17±0.24 to 2.45±0.26 ng Ang I · mL-1 · h-1, and PRA (trapping assay) from 1.40±0.27 to 2.95±0.29 ng Ang I · mL-1 · h-1. The APR assessed as the ratio of PRA (trapping assay) to AR was unchanged. Plasma Ang I increased from 11.4±2.9 to 40.0±6.6 pg/mL, plasma Ang II increased from 6.6±1.0 to 19.6±2.7 pg/mL, and plasma aldosterone increased from 125±14 to 207±22 pg/mL. These results demonstrate that the mild stimulation of the RAS expected 12 hours after a single 40-mg furosemide dose was successfully achieved.

Blood Pressure and Pulse Rate
Mean systolic/diastolic blood pressure in 24 subjects before dosing was 109/71 (±1.7/1.3). Statistical analysis across all doses showed statistically significant placebo-adjusted, dose-related reductions in systolic and diastolic pressures and MAP after zankiren HCl administration (Table 1Down). Fig 1Down shows the treatment effects of placebo and 50, 125, and 250 mg zankiren on blood pressure. The peak effect was determined to occur from 0 to 2 hours after oral administration of zankiren HCl. Fig 2Down shows the actual change from baseline data for MAP (open squares) and the results of the statistical model used to characterize the linear dose-response relation (solid line). The MAP analysis demonstrated statistically significant reductions throughout the 6-hour postdosing observation period. Maximum reductions in blood pressure (placebo-adjusted) were observed between 1 and 2 hours after, for example, the 250-mg tablet intake: systolic, 10.0±3.1 mm Hg; diastolic, 10.3±3.3 mm Hg; MAP, 12.5±3.1 mm Hg. There was no significant change in pulse rate.


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  		Table 1. Estimated Placebo-Adjusted Treatment Effect of Zankiren HCl on Mean Arterial Pressure



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  		Figure 1. Graph showing hourly average blood pressure after placebo or 50-, 125-, or 250-mg doses of zankiren 50. Blood pressures were recorded for each subject every 5 minutes from 1 hour before dosing through 6 hours after dosing, and again from 23 to 24 hours after dosing. Values shown are the averages of the subjects' hourly averages.



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  		Figure 2. Graphs showing dose-response relation of mean arterial pressure change from baseline. Estimated effect is shown as the solid line; the actual observed data points are shown as open squares. Statistically significant (*P<.05) dose-related reductions were observed at peak and throughout the 6-hour postdosing observation period.

Renin-Angiotensin System
RAS Status 12 Hours After 40 mg Oral Furosemide
The expected mild stimulation of the RAS induced by a single oral dose of 40 mg furosemide was characterized by a twofold to fourfold increase in all plasma parameters of the RAS. Pearson correlation coefficients were calculated to evaluate correlations between the values of the RAS components both before and after furosemide stimulation. All biochemical parameters except aldosterone were very significantly correlated, with coefficient correlations before and after furosemide from r=.47 (Ang I versus Ang II, P<.05) to r=.93 (PRA versus Ang I, P<.001). Aldosterone may correlate to Ang II before furosemide treatment (r=.77, P<.05), but the correlation was not significant after treatment (r=.32). This may be due to a significant fall in plasma potassium, from 4.38±0.06 before furosemide to 4.12±0.08 after furosemide (P=.017).

Dose-Response Analysis of Ang I, Ang II, AR, and Aldosterone
Fig 3Down shows the dose-dependent suppression and duration of effect on Ang I and II after zankiren HCl administration. Dose-related peak suppression of Ang I occurred 1 hour after dosing, with statistically significant dose-related suppression for at least 4 hours (Table 2Down). Of note was the >95% suppression after the 250-mg dose. Ang II also decreased rapidly after administration of zankiren HCl, with peak dose-related reductions at 45 minutes as demonstrated by >95% suppression after the 250-mg dose, with statistically significant reductions for at least 6 hours.



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  		Figure 3. Graphs showing time course of effect (mean percent change from baseline) after zankiren and placebo administration for components of the renin-angiotensin system. Baseline samples were obtained 30 minutes before each dose. PRA indicates plasma renin activity.


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  		Table 2. Estimated Placebo-Adjusted Treatment Effect of Zankiren HCl

AR showed peak dose-related increases 2 hours after dosing, as demonstrated by an increase of >250% after the 250-mg dose, with statistically significant increases for at least 6 hours (Table 2Up).

Analysis across doses showed a statistically significant dose-related reduction in plasma aldosterone at the time of maximum reduction (2 hours, P<.001). The reduction in plasma aldosterone observed after the highest zankiren HCl dose administration (250 mg) illustrates treatment-related suppression of aldosterone, with peak reduction at 2 hours (mean of 423 pg/mL at baseline, decreasing to a mean of 267 pg/mL at 2 hours, and persisting effect lasting at least 6 hours). In contrast, the subjects receiving placebo showed a slight increase in plasma aldosterone during the 6 hours after dosing.

Other Assessments of RAS
PRA measured by the conventional method of in vitro Ang I generation15 showed a rapid suppression of PRA after oral administration of zankiren HCl. Suppression was already near maximal at 30 minutes, with only a slight further suppression evident at 45 minutes after dosing. The duration of suppression was dose-related; with the higher doses, treatment effect was still clearly apparent at 24 hours (Fig 3Up).

PRA measured by the antibody trapping method16 showed a similar pattern, with brisk suppression at 30 minutes and maximal suppression by 45 minutes. Comparison of the activity-versus-time curves for the conventional and trapping assay results shows a more rapid return toward baseline with the trapping method than with the conventional method. Nevertheless, even with the trapping method, there is evidence of continued treatment-related suppression of PRA 24 hours after dosing with 125- and 250-mg tablets.

The APR (using the PRA trapping assay results) decreased dose-dependently (Fig 3Up). After the 250-mg dose, the APR decreased to nearly zero 30 minutes after dosing and was still decreased 24 hours after drug intake.

No change in plasma PR could be detected for the 24 hours after 10-, 25-, 50-, and 125-mg dose intake. After the 250-mg dose, PR (baseline, 333±14 pg/mL) showed a rapid increase from baseline that was evident at 30 minutes (115±55 pg/mL increase). PR continued to rise to 842±301 pg/mL 6 hours after zankiren HCl administration and was still increased by 164±75 pg/mL (P>.05) 24 hours after drug intake, at a time when AR was increased by only 7±60 pg/mL.

Zankiren Concentration
Plasma levels of zankiren and its active metabolites could be detected after the 50-mg dose. Peak levels were obtained 1 hour after drug intake and were 29±15, 47±25, and 407±154 ng/mL after 50-, 125-, and 250-mg tablets, respectively. Six hours after drug intake, plasma levels were 1.6±1.4, 4.8±1.7, and 62±50 ng/mL; 24 hours after intake, they were 0.4±0.4, 2.2±1.6, and 4.5±2.7 ng/mL.

Tolerability
Asymptomatic orthostatic hypotension was detected at the end of the 6-hour study period in two subjects, when the subjects (one treated with 50 mg and the other treated with 250 mg) assumed the upright position. No data were excluded from these two subjects. One subject experienced hypotension during the 6-hour monitoring period within 30 minutes of dosing (250 mg). However, this subject had also become hypotensive 30 minutes before drug intake in association with blood sampling and exhibited further evidence of blood pressure instability after dosing that necessitated exclusion from analyses of a significant portion of data collected during the 6-hour postdose monitoring period.


*    Discussion
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
up arrowResults
*Discussion
down arrowReferences
 
In this paper, we describe for the first time the dose-response curves in humans of an orally absorbed renin inhibitor, zankiren HCl, whose pharmacodynamic effects had been investigated previously after oral administration to sodium-depleted dogs.11 Attempts to assess the efficacy of candidate antihypertensive drugs, especially those that block the RAS, in normotensive volunteers have met with mixed results. While some investigators have successfully demonstrated blood pressure reduction in a placebo-controlled study in normotensive subjects on a normal sodium intake using angiotensin-converting enzyme (ACE) inhibitors,21 others have not observed blood pressure reductions in this setting.22 Even if blood pressure changes are detectable, the reductions are not likely to be large under these conditions, which makes obtaining a dose-response curve difficult. One would expect that construction of a dose-response curve would be facilitated by measures that would provide a wider range of effect. In healthy subjects, enhanced blood pressure responsiveness has been demonstrated after vigorous sodium depletion for both an ACE inhibitor23 and an Ang II receptor antagonist,24 but the diuretic-induced hypovolemia exposed the volunteers to a risk of orthostatic hypotension and functional renal insufficiency. In the present report, the activation of the RAS observed 12 hours after ingestion of a single dose of 40 mg furosemide corresponds to a twofold increase in AR, a fourfold increase in plasma Ang I, a threefold increase in plasma Ang II, and a twofold increase in plasma aldosterone, instead of a fivefold increase in PRA and aldosterone after multiple doses of furosemide over 3 days.23 24

Dose-related falls in blood pressure were indeed observed in these salt-depleted healthy subjects. Analysis of the pharmacodynamic effects across doses in this setting provides an estimation of the change in blood pressure from baseline that results from oral administration of zankiren HCl. The estimated treatment effect at peak after the high dose of 250 mg removing placebo effect was 10.0±3.7 mm Hg, with no effect at 24 hours.

Analysis of the falls in plasma Ang I and plasma Ang II indicates a parallelism between the changes in their plasma levels and blood pressure after a single oral dose of zankiren HCl, as already reported after intravenous administration of CGP 38560 A.5 After 125 mg zankiren HCl, plasma Ang II is significantly reduced by 14 to 29 pg/mL for 6 hours after drug intake, but no significant fall is still present 24 hours after drug intake. Parallel changes are observed for blood pressure. The 250-mg dose of zankiren induces the maximal decrease in plasma Ang II 1 to 2 hours after drug intake, but the circulating levels of Ang II are still approximately 10% of their initial value and return toward their initial value 24 hours after drug intake, when no blood pressure effect is detected. This suggests that a higher dose could be more effective for a longer period of time. A dose-related effect of the renin inhibitor on plasma aldosterone was also observed.

Measurement of PRA by two different methods has confirmed previous results, showing that the duration of plasma renin inhibition is longer when assessed by a conventional method than when assessed by the trapping assay.6 25 We found an IC50 of zankiren HCl of 2 nmol/L in our conventional PRA assay versus 9 nmol/L in the PRA trapping assay. If we consider that PRA is inhibited 24 hours after drug ingestion, as best shown by the in vitro APR (PRA/AR) in the subjects treated with 125 and 250 mg zankiren HCl, our results show that despite this plasma inhibition, plasma angiotensin and blood pressure have both come back toward their initial levels. This is not surprising because zankiren, like other renin inhibitors, is protein-bound. It has been shown that most of the plasma Ang I comes from the interstitial space, where a sufficient amount of renin inhibitor should be present to inhibit the enzymatic reaction between hepatic angiotensinogen and renal renin that have been trapped by the peripheral tissues.14 26 The persistence of some residual inhibitory activity in peripheral compartments provides a reasonable explanation of the progressive increase in the blood pressure–lowering effect of the renin inhibitor enalkiren after repeated administration of intravenous doses to hypertensive patients.1 The zankiren levels, measured by a radioinhibitor binding assay, confirm that this renin inhibitor is indeed the first available orally. By comparison, oral administration of 600 mg remikiren, which was initially thought to be active on blood pressure in hypertensive patients,7 was able to achieve a peak level only between 10 and 20 pmol/mL and could not be detected in plasma after 3 hours.27 28 The interruption of the feedback between Ang II and renin explains the release of AR, whose magnitude and duration are both dose-dependent.9 The values of AR are surprisingly high, but very likely this is not characteristic of renin inhibition because these results are within the range observed during a previous investigation performed with captopril.29 A single oral dose of 0.9 mg/kg of captopril administered to normotensive volunteers on a 0.3-mg/kg daily sodium intake for 7 days increased AR to peak values ranging from 700 to 1950 pg/mL, which represented an increase of more than sixfold in each subject. The same pair of monoclonal antibodies has been used for all the investigations performed so far to measure the circulating levels of renin after administration of a renin inhibitor,17 which makes it possible to compare our results with those previously published. The oral administration of CGP 38560 A (250 mg) did not increase AR, and it was concluded that it was not orally active in human beings.4 The 600-mg oral dose of remikiren was reported to slightly decrease blood pressure in six hypertensive patients.7 However, it increased AR only from 14.2±1.4 (mean±SEM) to 37.5±2.5 pg/mL in normotensive volunteers on a normal sodium intake.27 The same dose of 600 mg remikiren was also compared with 50 mg captopril in normotensive volunteers.30 Because of the fall in blood pressure measured after captopril and the absence of blood pressure effect after the renin inhibitor, the authors concluded that the blood pressure effect of ACE inhibition in normotensive subjects on a normal sodium intake was more dependent on a bradykinin effect than on the blockade of the RAS.30 31 In reality, the rise in AR was more marked with captopril than with remikiren, which suggests that the oral absorption of this renin inhibitor was extremely low, as calculated by Kleinbloesem et al.28 The estimated treatment effect (±SEM) of 25 mg zankiren HCl on AR varies from 21±5 pg/mL at 30 minutes to 48±15 pg/mL at 2 hours. It is still 41±12 pg/mL at 6 hours and is not significant (4±3 pg/mL) at 24 hours. After the 250-mg dose, the estimated placebo-adjusted rise in AR after 24 hours is 41±32 pg/mL, which in addition to the persistence of renin inhibition in plasma suggests that a biological effect does exist 24 hours after oral administration of the highest dose of this inhibitor, even if no fall in blood pressure is detected in normotensive subjects 24 hours after this first dose.

The 250-mg single oral dose of zankiren HCl could also acutely increase PR. This observation was confirmed when 250 and 500 mg zankiren HCl solution was administered (J.M., unpublished results, 1992). Acute rises in plasma concentration of AR generally occur in the absence of a detectable change in the level of PR, suggesting that under these conditions, the secretion originates as preformed active enzyme stored in secretory granules.32 33 On the contrary, 24-hour and longer stimulations of renin secretion cause a parallel change in the release of both PR and AR, presumably reflecting an overall increase in the rate of hormone biosynthesis and an augmented secretion by both the constitutive and regulated pathways.29 33 The rise in PR after acute administration of the higher dose of this renin inhibitor may be characteristic of renin inhibition. More probably, it is dependent on the experimental conditions selected. A first renin stimulation by furosemide is likely to have induced within the JG cells a signal to increase renin biosynthesis,34 and acute depletion of the stored AR after a second and major stimulus, the oral administration of the highest dose of the renin inhibitor, was accompanied by the release of de novo synthesized PR, as a consequence of the previous furosemide signal.

In conclusion, the model of mild stimulation of the RAS using a single 40-mg oral dose of furosemide provided appropriate conditions for assessing the blood pressure–lowering activity of zankiren HCl in healthy volunteers. Oral administration of zankiren HCl confirms the preclinical data obtained in dogs. It is the first orally available renin inhibitor that dose-proportionally (1) is measurably absorbed, (2) decreases blood pressure and plasma angiotensins for 6 hours, (3) inhibits PRA for 24 hours, and (4) stimulates renin release as ACE inhibitors and Ang II antagonists. The most complete renin inhibition also induces a PR release.

These results suggest that renin inhibition, a specific method of inhibiting the RAS, has the potential to produce blood pressure reductions comparable in magnitude to those seen with ACE inhibition. Further studies will be helpful in assessing the role of this new approach to the treatment of hypertension and congestive heart failure.


*    Acknowledgments
 
This work was supported in part by a grant from Abbott Laboratories to Association Claude Bernard (Paris). The study was conducted at the Clinical Pharmacology Research Unit of Arzneimittelforschung GmbH in Berlin, Germany, under the direction of Prof Dr med H.-P. Breuel.

Received May 2, 1994; accepted August 29, 1994.


*    References
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
up arrowResults
up arrowDiscussion
*References
 

  1. Boger RS, Glassman HN, Cavanaugh JH, Schmitz PJ, Lamm J, Moyse D, Cohen A, Kleinert HD, Luther RR. Prolonged duration of a blood pressure response to enalkiren, the novel dipeptide renin inhibitor in essential hypertension. Hypertension. 1990;15(6, pt 2):835-840.
  2. Weber MA, Neutel JM, Essinger I, Glassman HN, Boger RS, Luther RR. Assessment of renin dependency of hypertension with a dipeptide renin inhibitor. Circulation. 1990;81:1768-1774. [Abstract/Free Full Text]
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  5. Nussberger J, Delabays A, De Gasparo M, Cumin F, Waeber B, Brunner HR, Menard J. Hemodynamic and biochemical consequences of renin inhibition by infusion of CGP 38560 A in normal volunteers. Hypertension. 1989;13:948-953. [Abstract/Free Full Text]
  6. Jeunemaitre X, Menard J, Nussberger J, Guyene TT, Brunner HR, Corvol P. Plasma angiotensins, renin, and blood pressure during acute renin inhibition by CGP 38560 A in hypertensive patients. Am J Hypertens. 1989;2:819-827. [Medline] [Order article via Infotrieve]
  7. Van den Meiracker AH, Admiraal PJJ, Man in't Veld AJ, Derkx FHM, Ritsema Van Eck HJ, Mulder P, van Brummelen P, Schalekamp MADH. Prolonged blood pressure reduction by orally active renin inhibitor Ro 42-5892 in essential hypertension. BMJ. 1990;301:205-210.
  8. Kobrin I, Viskoper RJ, Laszt A, Bock J, Weber C, Charlon V. Effects of an orally active renin inhibitor, Ro 42-5892, in patients with essential hypertension. Am J Hypertens. 1993;6:349-358. [Medline] [Order article via Infotrieve]
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