Plasma Converting Enzyme Research Articles

Effect of a new angiotensin converting enzyme inhibitor MK 421 and its lysine analogue on the components of the renin system in healthy subjects

1 MK 421 and its lysine analogue are two new inhibitors of angiotensin converting enzyme. Ten mg of both compounds were each given p.o. to 12 normotensive volunteers to determine their effect on the various components of the renin angiotensin aldosterone system. 2 Plasma converting enzyme activity decreased to very low levels within 3 to 4 h to recover only slowly over the next 72 h. Plasma angiotensin II and aldosterone also fell but returned to baseline within 24 h, whereas plasma renin activity rose reflecting the low angiotensin II levels. 3 There was a close correlation between both angiotensin II and aldosterone levels and the logarithm of plasma converting enzyme activity demonstrating that angiotensin II and aldosterone fell only when converting enzyme activity was reduced to very low levels. 4 Mean hourly urinary sodium excretion increased markedly 6 to 10 h post-drug, while blood pressure decreased slightly. Both drugs were well tolerated. 5 Thus 10 mg of MK 421 or its lysine analogue given orally are effective and long acting angiotensin converting enzyme inhibitors.

CIRCADIAN RHYTHMS IN THE RENIN-ANGIOTENSIN SYSTEM AND ADRENAL STEROIDS MAY CONTRIBUTE TO THE INVERSE BLOOD PRESSURE RHYTHM IN HYPERTENSIVE TGR(mREN-2)27 RATS

The transgenic TGR(mREN-2)27 rat is not only characterized by fulminant hypertension, but also by a disturbance in circadian blood pressure regulation, resulting in inverse circadian blood pressure profiles. The reasons for these alterations are not very well understood at present. We therefore investigated the circadian rhythms in several hormones participating in blood pressure regulation. From TGR and Sprague-Dawley (SPRD) control rats synchronized to 12h light and 12h dark (LD 12:12) blood was collected at different circadian times (07, 11, 15, 19, 23, 03, and 07 again, 5 rats per strain and time). The activities of plasma renin and converting enzyme, as well as plasma concentrations of corticosterone and aldosterone, were determined by radioimmunoassay (RIA). SPRD rats showed significant circadian rhythms in all variables except plasma renin activity, with maxima occurring during the day. TGR rats showed significant circadian rhythmicity in plasma renin activity and corticosterone and daily variation in aldosterone; angiotensin-converting enzyme (ACE) activity did not reach statistical significance. In TGR rats, 24h means in plasma renin activity and aldosterone were approximately sevenfold and fourfold higher, respectively, than in SPRD rats. Peak concentrations in corticosterone around 15h were more than two times higher in TGR rats than in SPRD rats, whereas no differences were observed during the night. It is concluded that, in TGR rats, the overall increase in plasma renin activity and aldosterone may contribute to the elevated blood pressure. The comparatively high levels in corticosterone and plasma renin activity during daytime may be involved in the inverse circadian blood pressure profiles in the transgenic animals. (Chronobiology International, 17(5), 645–658, 2000)

Refractory Hypotension After Induction of Anesthesia in a Patient Chronically Treated with Angiotensin Receptor Antagonists

Refractory Hypotension After Induction of Anesthesia in a Patient Chronically Treated with Angiotensin Receptor Antagonists

Effect of acute and chronic administration of ceronapril on angiotensin converting enzyme in plasma, kidney, lung, brain regions and cerebrospinal fluid of rats

Angiotensin converting enzyme (ACE) in brain, cerebrospinal fluid (C.S.F.) and peripheral tissues was studied ex vivo after oral administration of ceronapril (SQ 29,852) to male Sprague-Dawley rats. Angiotensin converting enzyme in tissue was measured by in vitro autoradiography or enzymatic assay. In in vitro experiments, ceronapril inhibited ACE in slices of brain with an IC 50, of approximately 34 nM, as measured by in vitro autoradiography. In C.S.F. ACE was inhibited with an IC 50, of approximately 34 nM, as assessed by a fluorimetric enzyme assay. Ceronapril (1OO mg/kg, p.o.) inhibited ACE in plasma, kidney and lung rapidly (3hr) after administration. Inhibition of ACE in kidney lasted up to 48 hr after administration of ceronapril, whereas the activity of ACE in plasma and lung recovered rapidly (8 hr). In lung and plasma ACE was increased at 72 hr after administration. Therefore, induction of ACE in plasma and lung by the drug may partly obscure the acute inhibition and may contribute to the different time-course of inhibition of ACE from kidney. Ceronapril inhibited ACE in vascular organ of the lamina terminals (OVLT) and subfornical organ (SFO) of the brain slowly (onset at 8 hr) but persistently (from 24 to 48 hr). However the drug did not inhibit ACE in structures of the brain within the blood-brain barrier, such as the caudate-putamen, choroid plexus, globus pallidus, supraoptic nucleus and paraventricular nucleus of the hypothalamus. In C.S.F. ACE was not detectably inhibited by ceronapril but was increased at 72 hr. After chronic treatment with ceronapril (100 mg/kg, p.o., twice daily), the results were similar to those seen in the acute experiments, with clear inhibition of ACE in the 2 circumventricular organs-the subfornical organ and the lamina terminals-but no change in other regions of the brain. Also, as in the acute experiments, no inhibition of ACE in C.S.F, was detected. These results show that ceronapril blocked ACE in peripheral sites acutely after a single dose. A slower onset, but more persistent, blockade of ACE occurred in the circumventricular organs of the brain. However, the drug did not appear to accumulate in brain tissue, within the blood-brain barrier or in C.S.F. Reports that very small doses of ceronapril might exert cognitive-enhancing effects in rats seem unlikely to be explained by accumulation of the drug in structures of the brain within the blood-brain barrier.

Determinants of angiotensin II generation during converting enzyme inhibition.

The reaction of the renin-angiotensin system to acute angiotensin converting enzyme inhibition was investigated in a single-blind, crossover study in nine normal volunteers receiving two out of three regimens in random order: the new converting enzyme inhibitor benazepril (20 mg once or 5 mg four times at 6-hour intervals) or enalapril (20 mg). Plasma converting enzyme activity, drug levels, angiotensin I and angiotensin II, active renin, and aldosterone were measured before and 1-4 hours and 14-30 hours after drug intake. Baseline in vitro plasma converting enzyme activity was 97 +/- 15 nmol/ml/min (mean +/- SD) when Hip-Gly-Gly was used as substrate, but with carbobenzoxy-Phe-His-Leu (Z-Phe-His-Leu) or angiotensin I as substrate it was only 20 +/- 4 and 1.7 +/- 0.3 nmol/ml/min, respectively. Discriminating power at peak converting enzyme inhibition was enhanced with the two latter substrates. In vivo converting enzyme activity was estimated by the plasma angiotensin II/angiotensin I ratio, which correlated well with in vitro converting enzyme activity using Z-Phe-His-Leu as substrate (r = 0.76, n = 252). Angiotensin II levels returned to baseline less than 24 hours after drug administration, whereas in vitro and in vivo converting enzyme activity remained considerably inhibited and active renin together with angiotensin I levels were still elevated. A close linear relation was found between plasma angiotensin II and the angiotensin I/drug level ratio (r = 0.91 for benazeprilat and r = 0.88 for enalaprilat, p less than 0.001). Thus, plasma angiotensin II truly reflects the resetting of the renin-angiotensin system at any degree of converting enzyme inhibition. The ratio of plasma angiotensin II to angiotensin I represents converting enzyme inhibition more accurately than in vitro assays, which vary considerably depending on substrates and assay conditions used.

Pharmacodynamic, pharmacokinetic and humoral effects of oral zabicipril, an angiotensin converting enzyme inhibitor in normotensive man.

1. Zabicipril, S9650, a new angiotensin converting enzyme inhibitor, was administered to salt-replete, normotensive males in single doses of up to 10 mg. 2. The safety, tolerance and dose-response relationship with regard to inhibition of plasma ACE activity were characterised initially in an open, pilot, dose-finding study in 12 subjects and further explored in a double-blind, parallel group, placebo controlled study in another 30 subjects. 3. The drug was generally well tolerated and produced no change in routine haematology or serum biochemistry tests. 4. Dose related (0.03 to 10 mg) inhibition of plasma converting-enzyme was observed, with 2.5 mg of zabicipril producing over 90% inhibition at 4 h and 60% at 24 h. 5. There were no significant changes in blood pressure or heart rate in normotensive subjects over the dose range studied. 6. A dose related rise in plasma renin activity and angiotensin I was observed. No dose related reduction in plasma aldosterone was observed. 7. These initial studies suggest that zabicipril is a well tolerated inhibitor of converting enzyme with near maximal inhibitory effect occurring at a dose of 2.5 mg. Further exploration of the dose range after single and multiple doses is indicated.

Non parametric pharmacodynamic modeling of the relationship between plasma concentration of perindoprilat and plasma converting enzyme blockade: E. Bellissant, N.P. Chau, C. Thuillez and J.F. Giudicelli. URBB U263, 2 Place Jussieu, 75005 Paris. Service de Pharmacologie Clinique, Hoˆpital de Biceˆtre, 94270 Le Kremlin Biceˆtre, France

Non parametric pharmacodynamic modeling of the relationship between plasma concentration of perindoprilat and plasma converting enzyme blockade: E. Bellissant, N.P. Chau, C. Thuillez and J.F. Giudicelli. URBB U263, 2 Place Jussieu, 75005 Paris. Service de Pharmacologie Clinique, Hoˆpital de Biceˆtre, 94270 Le Kremlin Biceˆtre, France

The pharmacokinetics and bioavailability of cilazapril in normal man.

1. The pharmacokinetics of cilazapril and its active metabolite, cilazaprilat, were investigated in a three-part crossover study in 12 healthy male volunteers aged 19-38 years, excluding one subject who withdrew from the study. 2. Single 2.5 mg oral doses of cilazapril, and equivalent oral and intravenous doses of cilazaprilat were administered as aqueous solutions to the fasted subjects. There was an interval of 1 week between treatments. Concentrations of cilazapril and cilazaprilat in plasma and urine, and activities of angiotensin converting enzyme (ACE) in plasma were measured by radioenzymatic methods. 3. After 10 min infusion of cilazaprilat, the mean plasma concentration was 194 ng ml-1, and ACE inhibition was almost 100%. The decline in concentrations was polyphasic, with mean half-lives for the periods 1-4 h and 24-168 h of 0.90 and 46 h, respectively. Between 4 and 24 h the decline was non-linear, and ACE inhibition decreased from 91% to 67%. Urinary recovery of cilazaprilat averaged 91% of dose. 4. After oral cilazapril, the parent drug was rapidly absorbed and rapidly eliminated, with a mean maximum plasma concentration of 82 ng ml-1 at 0.83 h and a single elimination half-life of 1.3 h. Cilazaprilat peaked at 36 ng ml-1 about 1.7 h after dosing and the decline in concentrations was biphasic, with half-lives of 1.8 h and 45 h. After oral cilazaprilat, plasma concentrations were considerably lower, and the peak later (2.2 h). 5. Urinary recovery data indicated an absolute bioavailability for cilazaprilat of 57% (range 45-75%) from oral cilazapril, but only 19% (range 8-40%) from oral cilazaprilat.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of angiotensin converting enzyme inhibition and atrial natriuretic factor.

The interaction of angiotensin converting enzyme (ACE) inhibition and atrial natriuretic factor (ANF) was investigated in six supine, sodium-replete, normal volunteers who received captopril (10 mg i.v. bolus followed by 10 mg/hr constant infusion) or vehicle superimposed on background 3-hour, constant, low-dose (1.5 pmol/kg/min) infusions of human ANF (99-126). Plasma converting enzyme activity was significantly inhibited but this had no effect on endogenous plasma ANF concentrations. ANF infusions, with or without captopril, caused similar increases in plasma ANF concentrations, and calculated metabolic clearance rates for ANF were unchanged. Similarly, blood pressure, heart rate, renal blood flow, glomerular filtration rate, and renal electrolyte excretion, including ANF-induced natriuresis, were unaffected by captopril. The combination of ANF plus captopril produced a significant increase in plasma aldosterone (79 +/- 8 vs. 60 +/- 6 pmol/l, p less than 0.05), cortisol (406 +/- 52 vs. 265 +/- 29 nmol/l, p less than 0.01), adrenaline (119 +/- 21 vs. 76 +/- 10 pg/ml, p less than 0.05), and noradrenaline (319 +/- 49 vs. 215 +/- 38 pg/ml, p less than 0.05) compared with time-matched placebo data. Converting enzyme inhibition, in the absence of major changes in blood pressure or renal blood flow, has little effect on ANF metabolism or renal bioactivity. However, ACE inhibition and ANF combined may interact to increase activity of the hypothalamo-pituitary-adrenal axis and sympathetic nervous system by unknown mechanisms.

Angiotensin converting enzyme in human seminal plasma is synthesized by the testis, epididymis and prostate

The activity of angiotensin converting enzyme (ACE) was assessed in human body fluids (serum, seminal plasma, prostatic secretions), in tissue extracts of the testis, epididymis, prostate and skeletal muscle, in split ejaculates and in seminal plasma obtained from patients before and after vasectomy. To ensure the specificity of the results the dependence of ACE activity on specific inhibitors was evaluated. Enzyme activity found in tissues of the male genital tract was considerably higher than that in serum and other tissues. ACE in human seminal plasma is synthesized by the testis, epididymis and prostate in different amounts.

Inhibition of Converting Enzyme in Brain Tissue and Cerebrospinal Fluid of Rats Following Chronic Oral Treatment with the Converting Enzyme Inhibitors Ramipril and Hoe 288

A direct central nervous system (CNS)-related component of the cardiovascular actions of converting enzyme (CE) inhibitors will be governed by the ability of these drugs to gain access to the brain. We investigated the inhibitory effect of the two CE inhibitors ramipril and Hoe 288 on CE activity in different brain regions and in the cerebrospinal fluid of rats. One-week oral gavage treatment with ramipril (10 mg/kg/day) and Hoe 288 (10 mg/kg/day) resulted in a marked inhibition of CE activity in the brain cortex (90 and 91%, respectively), the hypothalamus (78 and 82%, respectively), and in the brainstem (67 and 66%, respectively). The complete blockade of plasma CE activity was paralleled by a 84% inhibition of CE activity in cerebrospinal fluid. Both CE inhibitors failed significantly to inhibit CE activity in the striatum. Our results demonstrate that the CE inhibitors Hoe 288 and ramipril were able to pass the blood-brain barrier (BBB) to inhibit central CE activity. The penetration of CE inhibitors into the CNS appears to depend on the lipophilicity of the drugs and on the mode of drug application. The possibility that an inhibition of CE activity in circumventricular organs outside the BBB such as the subfornical organ and the organum vasculosum of the lamina terminalis may be sufficient to explain the central effects of orally applied CE inhibitors is discussed.

Inhibition of Converting Enzyme in Brain Tissue and Cerebrospinal Fluid of Rats Following Chronic Oral Treatment with the Converting Enzyme Inhibitors Ramipril and Hoe 288

A direct central nervous system (CNS)-related component of the cardiovascular actions of converting enzyme (CE) inhibitors will be governed by the ability of these drugs to gain access to the brain. We investigated the inhibitory effect of the two CE inhibitors ramipril and Hoe 288 on CE activity in different brain regions and in the cerebrospinal fluid of rats. One-week oral gavage treatment with ramipril (10 mg/kg/day) and Hoe 288 (10 mg/kg/day) resulted in a marked inhibition of CE activity in the brain cortex (90 and 91% respectively), the hypothalamus (78 and 82% respectively), and in the brainstem (67 and 66% respectively). The complete blockade of plasma CE activity was paralleled by a 84% inhibition of CE activity in cerebrospinal fluid, Both CE inhibitors failed significantly to inhibit CE activity in the striatum. Our results demonstrate that the CE inhibitors Hoe 288 and ramipril were able to pass the blood-brain barrier (BBB) to inhibit central CE activity. The penetration of CE inhibitors into the CNS appears to depend on the lipophilicity of the drugs and on the mode of drug application. The possibility that an inhibition of CE activity in circumventricular organs outside the BBB such as the subfornical organ and the organum vasculosum of the lamina terminalis may be sufficient to explain the central effects of orally applied CE inhibitors is discussed.

Plasma Angiotensin-(1-8) Octapeptide Measurement to Assess Acute Angiotensin-Converting Enzyme Inhibition with Captopril Administered Parenterally to Normal Subjects

The blood pressure (BP), heart rate (HR), and humoral effects of single intravenous (i.v.) doses of the angiotensin-converting enzyme (ACE) inhibitor captopril was investigated in five normotensive healthy volunteers. Each subject received at 1-week intervals a bolus dose of either captopril (1, 5, and 25 mg) or its vehicle. The study was conducted in a single-blind fashion, and the order of treatment phases was randomized. The different doses of captopril had no acute effect on BP and HR. They induced a dose-dependent decrease in plasma ACE activity and plasma angiotensin II levels. The angiotensin-(1-8) octapeptide was isolated by solid-phase extraction and high-performance liquid chromatography (HPLC) prior to radioimmunoassay (RIA). All three doses of captopril reduced circulating angiotensin II levels within 15 min of drug administration. Only with the 25-mg dose was the angiotensin II concentration below the detection limit at 15 min and still significantly reduced 90 min after drug administration. Simultaneous and progressive decreases in plasma aldosterone levels were observed both with ACE inhibition and during vehicle injection, but the relative fall was more pronounced after captopril administration. No adverse reaction was noticed. These results demonstrate that captopril given parenterally blocks the renin-angiotensin system in a dose-dependent manner. Only with the dose of 25 mg was the inhibition of plasma-converting enzyme activity and the reduction of plasma angiotensin II sustained for at least 1 1/2 h.

Perindopril, Converting Enzyme Blockade, and Peripheral Arterial Hemodynamics in the Healthy Volunteer

The effects of three doses (4, 8, and 16 mg) of perindopril, a new angiotensin I converting enzyme inhibitor, on systemic blood pressure, heart rate, brachial and carotid artery flow and diameter (assessed by the pulsed Doppler technique), forearm vascular resistance, plasma converting enzyme and renin activities, and plasma aldosterone were investigated in the normal volunteer and compared with those of a placebo over a 24-h period following oral drug intake in a double-blind, crossover trial. Perindopril dose-dependently decreased plasma converting enzyme activity, an effect that peaked at 3-4 h and persisted up to at least 48 h. Plasma renin activity increased for 12 h and plasma aldosterone was slightly decreased. Systemic blood pressure and heart rate were not drug-affected but perindopril dose-dependently augmented brachial and carotid artery flow, indicating an increase in peripheral arterial compliance. These vasodilating effects, which lasted up to 10 h after drug intake, affected both large arteries and arterioles, the latter being more sensitive, however, and were more marked in the muscular resistance vessels.

Effects of Single Doses of the Converting Enzyme Inhibitor Cilazapril in Normal Volunteers

The new converting enzyme inhibitor cilazapril, or RO 31-2848, was evaluated in 14 healthy male volunteers. In a pilot study in two subjects, the inhibiting capacity of single oral doses of 5 and 10 mg on the pressure and heart rate response to exogenous angiotensin I was assessed. Both doses reduced the blood pressure response to angiotensin I to 10% of control within 45 min and for the 4 h tested. In the main study, 12 volunteers each received two single oral doses of cilazapril at a 2-week interval, and plasma converting enzyme and renin activity, blood angiotensin I, plasma immunoreactive angiotensin II and aldosterone were measured serially. Single doses of 1.25, 2.5, 5, and 10 mg of cilazapril were tested in groups of six subjects each. All doses inhibited plasma converting enzyme activity by 90% for at least 8 h and induced the expected pattern of changes of the renin-angiotensin-aldosterone system. Only slight dose-dependent variations in the effect were observed. Basic heart rate and blood pressure were not altered by any of the doses, which all were well tolerated. These data suggest that cilazapril is a very potent and long-acting new converting enzyme inhibitor.

Perindopril, Converting Enzyme Blockade, and Peripheral Arterial Hemodynamics in the Healthy Volunteer

Summary: The effects of three doses (4, 8, and 16 mg) of perindopril, a new angiotensin I converting enzyme inhibitor, on systemic blood pressure, heart rate, brachial and carotid artery flow and diameter (assessed by the pulsed Doppler technique), forearm vascular resistance, plasma converting enzyme and renin activities, and plasma aldosterone were investigated in the normal volunteer and compared with those of a placebo over a 24-h period following oral drug intake in a double-blind, crossover trial. Perindopril dose-dependently decreased plasma converting enzyme activity, an effect that peaked at 3–4 h and persisted up to at least 48 h. Plasma renin activity increased for 12 h and plasma aldosterone was slightly decreased. Systemic blood pressure and heart rate were not drug-affected but perindopril dose-dependently augmented brachial and carotid artery flow, indicating an increase in peripheral arterial compliance. These vasodilating effects, which lasted up to 10 h after drug intake, affected both large arteries and arterioles, the latter being more sensitive, however, and were more marked in the muscular resistance vessels.

Repeated Administration of the Converting Enzyme Inhibitor Cilazapril to Normal Volunteers

Summary: Cilazapril 1.25 and 5.0 mg p.o. q.d. was administered in double-blind fashion to two groups of six normal volunteers on 8 consecutive days. Blood pressure, heart rate, and plasma converting enzyme activity were measured each day prior to drug administration and up to 72 h after the last dose. Plasma renin activity, blood angiotensin I, plasma angiotensin II, and aldosterone as well as plasma cilazaprilat levels were determined on the first and the last day of active treatment at times 0, 4, and 24 h. The drug was very well tolerated by all volunteers. At 4 h postdrug, plasma converting enzyme activity was reduced in dose-dependent fashion on the first and the eighth day; plasma cilazaprilat levels were also clearly dose dependent. Nevertheless, 24 h postdrug cilazaprilat levels were similar on the first and last day of drug administration, and plasma converting enzyme activity was also stable throughout the 8 days. The various components of the renin-angiotensin system responded in the usual fashion. These results provide strong evidence that cilazapril is a very potent and highly effective converting enzyme inhibitor. Doses well below 5 mg/day will probably suffice for therapeutic efficacy. These data also confirm the hypothesis formulated in the preceding article, i.e., that there is no accumulation of the drug with repeated administration despite its long pharmacological half-life (t½).

Pharmacokinetics of the Converting Enzyme Inhibitor Cilazapril in Normal Volunteers and the Relationship to Enzyme Inhibition: Development of a Mathematical Model

The pharmacokinetics of the new converting enzyme inhibitor cilazapril were investigated in 12 healthy male volunteers. Single oral doses of 1.25, 2.5, 5, and 10 mg of cilazapril were tested in groups of six subjects, each of whom received two different doses. A 2-week interval was allowed between treatments. Plasma levels of cilazaprilat, the active form of cilazapril, were measured for up to 3 days after drug administration. Peak plasma levels and 24-h areas under the curve (AUCs) were almost directly proportional to dose, and the elimination half-life (t1/2) during the first 8 h after dosing was 1.5 h. From 24 h on, there was a prolonged terminal phase with a t1/2 of approximately 50 h, and there was only slight dose-dependency during this phase. These data suggest that the pharmacokinetics of cilazapril are nonlinear. A physiologically realistic model based on saturable binding to converting enzyme was developed to account both for the drug kinetics and for the relationship of the kinetics to the dynamics of plasma converting enzyme inhibition. A number of conclusions relevant to the therapeutic application of cilazapril in hypertension are drawn from the data and from the pharmacokinetic-pharmacodynamic model.

Pharmacokinetics of the Converting Enzyme Inhibitor Cilazapril in Normal Volunteers and the Relationship to Enzyme Inhibition: Development of a Mathematical Model

Summary: The pharmacokinetics of the new converting enzyme inhibitor cilazapril were investigated in 12 healthy male volunteers. Single oral doses of 1.25, 2.5, 5, and 10 mg of cilazapril were tested in groups of six subjects, each of whom received two different doses. A 2-week interval was allowed between treatments. Plasma levels of cilazaprilat, the active form of cilazapril, were measured for up to 3 days after drug administration. Peak plasma levels and 24-h areas under the curve (AUCs) were almost directly proportional to dose, and the elimination half-life (t½) during the first 8 h after dosing was 1.5 h. From 24 h on, there was a prolonged terminal phase with a t½ of ˜50 h, and there was only slight dosedependency during this phase. These data suggest that the pharmacokinetics of cilazapril are nonlinear. A physiologically realistic model based on saturable binding to converting enzyme was developed to account both for the drug kinetics and for the relationship of the kinetics to the dynamics of plasma converting enzyme inhibition. A number of conclusions relevant to the therapeutic application of cilazapril in hypertension are drawn from the data and from the pharmacokinetic-pharmacodynamic model.

Repeated Administration of the Converting Enzyme Inhibitor Cilazapril to Normal Volunteers

Cilazapril 1.25 and 5.0 mg p.o. q.d. was administered in double-blind fashion to two groups of six normal volunteers on 8 consecutive days. Blood pressure, heart rate, and plasma converting enzyme activity were measured each day prior to drug administration and up to 72 h after the last dose. Plasma renin activity, blood angiotensin I, plasma angiotensin II, and aldosterone as well as plasma cilazaprilat levels were determined on the first and the last day of active treatment at times 0, 4, and 24 h. The drug was very well tolerated by all volunteers. At 4 h postdrug, plasma converting enzyme activity was reduced in dose-dependent fashion on the first and the eighth day; plasma cilazaprilat levels were also clearly dose dependent. Nevertheless, 24 h postdrug cilazaprilat levels were similar on the first and last day of drug administration, and plasma converting enzyme activity was also stable throughout the 8 days. The various components of the renin-angiotensin system responded in the usual fashion. These results provide strong evidence that cilazapril is a very potent and highly effective converting enzyme inhibitor. Doses well below 5 mg/day will probably suffice for therapeutic efficacy. These data also confirm the hypothesis formulated in the preceding article, i.e., that there is no accumulation of the drug with repeated administration despite its long pharmacological half-life (t1/2).