COMPARISON OF THE PHARMACOKINETICS AND PHARMACODYNAMICS OF PERINDOPRIL, CILAZAPRIL AND ENALAPRIL

1. The pharmacokinetic and pharmacodynamic responses to enalapril, perindopril and cilazapril have been studied in essential hypertensives (2, 4 and 8 mg perindopril and 2.5 mg cilazapril, single dose and steady state) and normotensive volunteers (10 mg enalapril, single dose).
2. Plasma levels of the active diacid compounds reached similar peaks after single dose administration of the drugs. However, perindoprilat levels persisted for 5 days whereas cilazaprilat levels were not detectable beyond 12 h.
3. The higher levels of perindoprilat were associated with a greater inhibition of plasma angiotensin-converting enzyme (ACE) activity in both acute and steady state studies.
4. The potency of the active diacids in inhibiting plasma ACE activity was perindoprilat > cilazaprilat > enalaprilat.
5. There was a close relationship between plasma concentration, ACE inhibition and blood pressure decrease. Although both cilazapril and perindopril administration reduced blood pressure in hypertensive subjects, only perindopril exerted 24 h blood pressure control at the doses used.     

ACE inhibition with cilazapril improves myocardial perfusion to the ischemic regions during exercise: a pilot study.

This study was performed to examine whether cilazapril, a novel nonsulfhydril angiotensin-converting enzyme (ACE) inhibitor, may improve regional myocardial perfusion at exercise in patients with coronary heart disease (CHD). In a single-blind, nonrandomized trial, 5 mg cilazapril or placebo was administered to eight patients with documented CHD and stable exertional angina. Multistage bicycle exercise tests were performed and each patient served as his own control. At peak exercise, as well as at rest, [99mTc]hexakis-2-methoxy-2-isobutylisonitrile myocardial images were obtained. Percent myocardial activity differences (%AD) between exercise and resting images were compared (cilazapril versus placebo). Heart rate and blood pressure were not significantly different between trials. However, after cilazapril administration, %AD was higher than after placebo, with relative differences between trials of greater than 30%, along with alleviation of clinical symptoms in seven of eight patients. These data suggest that in patients with stable-effort angina, ACE inhibition with cilazapril is able to redistribute myocardial blood flow and to improve regional oxygen supply to the ischemic myocardium.     

Antihypertensive effects and pharmacokinetics of single and consecutive doses of cilazapril in hypertensive patients with normal and impaired renal function

The antihypertensive effects and pharmacokinetic properties of cilazapril, a long-acting angiotensin-converting enzyme (ACE) inhibitor, were investigated in hypertensive patients with normal renal function (NRF; n = 5) and those with impaired renal function (IRF; n = 7). A 1.25-mg dose of cilazapril was administered orally once a day for 5 or 8 days. Measurement of blood pressure and sampling of blood specimens were done on the first and last days of treatment. Cilazapril induced significant falls in systolic and diastolic blood pressures as early as 1 h after administration. The antihypertensive effects were still present at 24 h postdose. Serum ACE activity was markedly suppressed over 24 h, with the enzyme inhibition greater in the IRF group. Plasma levels of the active diacid in the IRF group were higher than those in the NRF group, with significant differences in the peak levels and areas under the curve (AUC). A significant inverse correlation was found between the creatinine clearance and the AUC for the diacid. Cilazapril was well tolerated by all the patients, and no adverse reactions were observed. These results suggest that cilazapril has a long-lasting action and that it is a useful antihypertensive agent for controlling blood pressure in patients with either NRF or IRF.     

A comparison of the pharmacokinetics and pharmacodynamics of cilazapril between Chinese and Caucasian healthy,normotensive volunteers

Methods: The pharmacokinetics and pharmacodynamics of the angiotensin converting enzyme (ACE) inhibitor cilazapril were studied in 12 Chinese and 13 Caucasian, healthy, normotensive volunteers on their normal diet. Cilazapril was given orally as a single 2.5 mg capsule. Plasma was sampled for assay of the active metabolite, cilazaprilat, plasma renin activity (PRA), aldosterone, angiotensin I (AI) and ACE-activity. Plasma concentrations of the active drug were measured by radioimmunoassay. Blood pressure and heart rate were measured at regular intervals. Results: The pharmacokinetic parameters of cilazaprilat were similar in the two ethnic groups. No significant difference in plasma concentrations was found at any of the time points. However, the weight-adjusted plasma clearance was significantly higher in the Chinese group, which is compatible with their lower body weight. The effects on plasma hormones were also comparable, although there was a somewhat greater rise in PRA and greater fall in aldosterone levels in Chinese than in Caucasians. The effect of cilazapril on blood pressure and heart rate was greater than was previously reported in healthy volunteers. Systolic (SBP) and diastolic (DBP) blood pressure were significantly reduced in both groups, but there was a more prolonged reduction in DBP in Caucasians. In addition, heart rate (HR) was significantly increased from baseline from 5 h onwards in Chinese subjects and significantly higher in comparison with Caucasians at most time points from 1.5 h onwards. The pharmacokinetic parameters of cilazapril were essentially the same in healthy, normotensive Chinese and Caucasians. Cilazapril reduced blood pressure acutely in both groups, with good tolerance. The inhibition of ACE in relationship to time and the plasma concentrations of cilazaprilat were similar in the two groups, although the changes in PRA and aldosterone suggest an ethnic difference in the responses of the renin-angiotensin-aldosterone system. Received: 24 January 1995/Accepted in revised form: 18 September 1995     

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.     

Effects of intravenous S-9780, an angiotensin-converting enzyme inhibitor, in normotensive subjects

S-9780 is the active diacid metabolite of the new angiotensin-converting enzyme (ACE) inhibitor perindopril. In a double-blind, randomised, crossover study, the effects of 1, 2, and 4 mg of S-9780 administered intravenously (i.v.) were compared with placebo in eight normotensive subjects. All active doses caused immediate, maximal, and similar inhibition of plasma ACE with 40% inhibition persisting after 48 h. Plasma renin activity was elevated 4 and 8 h after dosing, but no effect on plasma aldosterone, adrenaline or noradrenaline levels was detected. Diastolic blood pressure was lowered by 4 mg of S-9780 until 24 h after dosing. Heart rate did not change. The pharmacokinetics of S-9780 fitted a three-compartment model with a terminal half-life (t1/2) of 31 h. Inhibition of plasma ACE was closely related to observed drug concentration, with 1.8 +/- 0.9 ng/ml (mean +/- S.D.) producing 50% inhibition of the enzyme. S-9780 caused predictable effects on the cardiovascular and renin angiotensin systems.     

Comparison of the actions of the angiotensin-converting enzyme inhibitors enalapril and S-9490-3 in sodium-deplete and sodium-replete spontaneously hypertensive rats

The hypotensive action of the angiotensin-converting enzyme (ACE) inhibitors enalapril and S-9490-3 was examined in conscious, chronically cannulated Na+-replete and Na+-deplete spontaneously hypertensive rats (SHR) of the Okamoto strain. Blood pressure, plasma ACE activity, plasma renin activity (PRA), and pressor responses to intravenous bolus injections of angiotensin I (AI) were measured over a 24-h period following a single oral dose of ACE inhibitor (0.3, 1.0, and 3.0 mg/kg) or vehicle. S-9490-3 caused a significantly greater hypotensive response and inhibition of plasma ACE and AI pressor responses than enalapril for each dose in both diet groups. Single oral doses of both drugs (3 mg/kg) caused slow, progressive falls in blood pressure which were maximal at 12 h. In contrast, inhibition of plasma ACE was maximal 1 h following the oral dose and returned to control levels over the 24-h period. The inhibition of the pressor response to intravenous AI paralleled, and was significantly correlated with, the inhibition of plasma ACE. There was no correlation between the maximal fall in blood pressure with PRA or with inhibition of plasma ACE activity in either diet group. The hypotensive response to both drugs at the 3-mg/kg dose was greater in Na+-deplete SHR than in Na+-replete animals. Both drugs caused large rises in PRA. The ACE inhibitor S-9490-3 is a significantly more potent hypotensive agent than enalapril in the SHR and a significantly more potent ACE inhibitor in vivo. The hypotensive response to both drugs was dissociated in onset and duration from the inhibition of plasma ACE and AI pressor responses.     

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.     

Steady-state pharmacokinetics and pharmacodynamics of cilazapril in the presence and absence of cyclopenthiazide.

Twenty-two patients with essential hypertension received a single dose of 2.5 mg cilazapril and were then randomised into a double-blind parallel group study to receive either placebo, 1.25 mg cilazapril + 0.5 mg cyclopenthiazide (CPTZ), 2.5 mg cilazapril + 0.5 mg CPTZ, or 2.5 mg cilazapril alone for 1 month. After oral administration of a single dose of 2.5 mg cilazapril, the active diacid cilazaprilat appeared rapidly in the plasma (Tmax 2.0 +/- 0.2 h). With the radioinhibitor assay used in this study, a single elimination phase of cilazaprilat was evident, with a half-life (t1/2) of 2-3 h. At steady state, the pharmacokinetics of cilazaprilat were similar to single-dose administration and were not altered by CPTZ. The Cmax and area under the curve (AUC) of cilazaprilat were directly proportional to dose. Cilazapril administration in the dose range of 1.25-2.5 mg produced a dose-proportional inhibition of angiotensin-converting enzyme (ACE) activity that was maximum 2 h after drug administration. The degree of ACE inhibition correlated with the plasma concentration-time profile of cilazaprilat and the maximum decrease in blood pressure (BP). The EC50 for ACE inhibition by cilazaprilat was 7.7 ng/ml after acute treatment and was not significantly altered during chronic administration or by concomitant administration of CPTZ. There was no evidence of a dose-related antihypertensive effect of cilazapril at steady state and, with the small numbers of subjects used in this study, there was no evidence of 24-h BP control with monotherapy.     

Comparison of the effects of felodipine and cilazapril on exercise performance in patients with mild to moderate hypertension. A crossover study

This double-blind crossover study was designed to compare the effects of felodipine and cilazapril on exercise performance in hypertensive patients. After a 2-week placebo run-in period, 40 patients with mild to moderate hypertension were randomized into two parallel groups to receive either felodipine (10 mg) or cilazapril (5 mg) for 4 weeks. After another 2-week washout period, treatments were then crossed over for a further 4-week study period. All patients were given an extensive rest and exercise evaluation at the end of the placebo period. Extensive rest and exercise evaluations were repeated after a 4-week treatment period and again after the second washout period and after the second 4-week treatment period. Before each exercise test, epinephrine, norepinephrine and dopamine plasma levels and plasma renin activity were measured. Two groups were similar at baseline for systolic and diastolic blood pressure and heart rate as well as for laboratory and hormonal variables and duration of exercise test. At the end of treatment diastolic blood pressure was significantly reduced in the felodipine group (p = 0.019). Duration of exercise test was longer than at baseline (p = 0.031) in the felodipine group. Plasma dopamine levels were significantly increased in the cilazapril group. Plasma renin activity significantly increased in the felodipine group. In conclusion, our data show that the two drugs have the same effectiveness in resting conditions but that felodipine is more effective in lowering maximum exercise diastolic blood pressure and in improving exercise time with an double product increase (not significant); it has no statistically significant effect on maximal exercise systolic blood pressure.     

Antihypertensive, enzymatic, and hormonal activity of cilazapril, a new angiotensin-converting enzyme inhibitor in patients with mild to moderate essential hypertension

The antihypertensive activity of cilazapril, a new nonsulfhydryl angiotensin-converting enzyme (ACE) inhibitor was evaluated in 20 outpatients (13 women, 7 men; mean age, 49 +/- 2.4 years) with mild to moderate essential hypertension, by means of an open dose-finding study of 10 weeks' duration. Cilazapril, 0.5 mg/day, was given, and the dose increased up to 10 mg/day if sitting diastolic blood pressure (SDBP) was not normalized (less than or equal to 90 mm Hg). Blood pressure measurements were carried out every 2 weeks before and 2 h after dosing. Predose and 2-h postdose measurements of plasma renin activity (PRA), angiotensin II (AII), plasma aldosterone (PA), and enzyme converting activity (ECA) were performed on the 1st day of active treatment and after 2 weeks of therapy. The SDBP decreased from 107.6 +/- 2 to 97.2 +/- 3 mm Hg 2 h after the initial dose (p less than 0.01). At the same time, ECA was inhibited 84.2 +/- 5% (p less than 0.01), AII decreased from 21.2 +/- 3 to 13.6 +/- 2 pg/ml (p less than 0.05), and PA from 208 +/- 29 to 119 +/- 14 pg/ml (p less than 0.01). After 2 weeks of therapy, ECA remained markedly reduced, by 68 +/- 6%, 24 h after the preceding cilazapril dose (p less than 0.01). The mean SDBP decreased from baseline to the end of treatment by 14.6 +/- 3 mm Hg (p less than 0.01). Cilazapril seems to be an effective antihypertensive drug which exerts potent and long-lasting ACE inhibition.     

Interactions between cilazapril and propranolol in man; plasma drug concentrations, hormone and enzyme responses, haemodynamics, agonist dose-effect curves and baroreceptor reflex.

1. The pharmacokinetics, hormonal and haemodynamic responses at rest and during challenges with angiotensin I (blood pressure), isoprenaline (heart rate), and noradrenaline (blood pressure) were investigated in six healthy male volunteers following a 1 week treatment with placebo, propranolol (120 mg day-1), cilazapril (2, 5 mg day-1), and a combination of both in a double-blind cross-over design. 2. Both drugs reduced systolic and diastolic blood pressure by about 7 mm Hg as compared with placebo. After coadministration, this drop in blood pressure was doubled and lasted longer than after the administration of the individual components. 3. Following cilazapril, a pronounced increase in plasma renin activity (PRA) was found (factor approximately 10 at drug peak concentrations). Coadministration of both drugs resulted only in a moderate increase in the PRA (factor approximately 3). Significant changes in plasma catecholamines were not observed. 4. Propranolol shifted the isoprenaline dose-effect curve to the right, and cilazapril that of angiotensin I, irrespective of the presence of the other drug. Cilazapril tended to shift the noradrenaline dose-effect curve somewhat to the right. 5. The gain of the baroreceptor reflex (angiotensin-stimulation) was not influenced by cilazapril but was lowered by propranolol, irrespective of the presence of the ACE inhibitor. 6. Except for a statistically not significant decrease in the peak concentrations of each drug during the combined therapy, a pharmacokinetic interaction between the two drugs was not found.     

Bopindolol: a new long acting beta-receptor antagonist; its effects on haemodynamics, and on the renin response to tilting.

Oral administration of single doses of bopindolol (1-4mg) caused significant reductions in the rises of systolic blood pressure and heart rate produced by exercise; only the reduction in the rise of heart rate was significantly dose-related. Resting heart rate was reduced by bopindolol. There were small effects on resting blood pressure. Bopindolol caused a significant attenuation of the rise in plasma renin activity produced by passive head-up tilting to 75-85 degrees. Bopindolol produced a dose-related attenuation of the increase in pulse rate evoked by passive tilting. All effects 1-4 were maintained for at least 24 h. There was no measurable effect on plasma potassium concentration, peak flow rate or forced expiratory volume (FEV1).     

Haemodynamic and humoral effects of oral perindopril, an angiotensin converting enzyme inhibitor, in man.

The tolerance to and dynamic effects of 1 week's oral treatment with the angiotensin converting enzyme inhibitor, perindopril, were assessed in a placebo controlled, parallel group study in 36 normotensive males. The daily dose of perindopril was 1, 2, 4, 8 or 16 mg. The drug was well tolerated and produced no change in routine haematology or serum biochemistry tests. Dose related inhibition of plasma angiotensin converting enzyme was observed. Perindopril 16 mg produced 90% inhibition 4 h after dosing and 60% after 24 h. A dose related rise in plasma renin activity followed doses of 4 mg and over. The renin remained above the normal range for 24 h. Perindopril caused a modest lowering of plasma aldosterone levels but had no effect on plasma adrenaline or noradrenaline levels. Standing diastolic blood pressure was lowered, particularly with 16 mg daily of perindopril but only a slight rise in heart rate occurred. Perindopril appears to be a well tolerated inhibitor of plasma angiotensin converting enzyme, with predictable effects on the renin angiotensin system and blood pressure. An appropriate dose range for further study would appear to be 4 to 16 mg daily.     

Dose-finding study of imidapril,a novel angiotensin converting enzyme inhibitor,in patients with stable chronic heart failure

Objective: To study the haemodynamic profile and tolerability of imidapril, a new long-acting ACE inhibitor, and to investigate the effect of inhibition of circulating ACE on blood pressure in patients with stable chronic heart failure. Methods: Twenty-four patients with stable, chronic heart failure (New York Heart Association (NYHA) functional Class II–III) were randomised to receive either 2.5 mg or 5 mg imidapril. Other vasodilators were withheld for ≥ 5 half-lives. Blood pressure and ACE activity were carefully monitored for 24 h after dosing. Results: Both 2.5 mg and 5 mg imidapril decreased systolic blood pressure, while diastolic blood pressure fell only after 5 mg imidapril. The two doses produced a significant and similar inhibition of circulating (serum) ACE. No serious adverse effects were observed, although symptomatic hypotension occurred in 1 patient (5 mg). The decrease in blood pressure was not related to baseline ACE activity, serum sodium or serum creatinine concentration. Conclusions: Imidapril significantly lowered systolic blood pressure and was well tolerated. The difference in the first dose response to the two doses with respect to diastolic blood pressure suggests that this haemodynamic effect of ACE-inhibition is not related to inhibition of circulating ACE. Received: 21 April 1995/Accepted in revised form: 15 December 1995     

Haemodynamic response and pharmacokinetics after the first dose of quinapril in patients with congestive heart failure.

1. Twenty-four elderly patients with stable, chronic congestive heart failure, NYHA II-IV, requiring addition of an ACE inhibitor to their existing therapy were randomised to receive double-blind a single dose of quinapril 2.5 mg p.o. or matching placebo after 24-48 h supervised diuretic withdrawal. 2. The effect of treatment on resting supine blood pressure, heart rate, plasma angiotensin converting enzyme (ACE) and circulating plasma renin activity was compared between groups over the first 24 h after dosing. The pharmacokinetic profiles of quinapril and the active metabolite quinaprilat were determined. 3. Compared with placebo, quinapril caused a statistically significant but modest fall in blood pressure from 3 to 10 h post dose. The maximum fall of 12 mm Hg (95% C.I. 5.4-18.5) was seen at approximately 5 h. Circulating ACE activity was 40% inhibited within 1 h. Maximum ACE inhibition (83.6%, 95% C.I. 76.7-90.5) was observed at 3 h. ACE remained 60% inhibited at 24 h post dose. tmax for quinapril was seen at 2.6 +/- 1.2 h. while tmax for quinaprilat was at 3.6, +/- 0.8 h. 4. Treatment with quinapril was associated with a significant rise in plasma renin activity (PRA) of 8.83 ng AI ml-1 h-1 (95% C.I. 0.30-17.96) compared with placebo. 5. Compared with placebo, quinapril 2.5 mg inhibits plasma ACE by over 60% for 24 h and reduces blood pressure for at least 10 h in patients with stable, chronic congestive heart failure. The blood pressure fall, although moderate and well tolerated, is more sustained than previously described for quinapril in heart failure.     

Angiotensin-converting enzyme responses following enalapril in the sodium deficient rat

The relationship between blood pressure lowering activity and inhibition of plasma and tissue angiotensin-converting enzyme (ACE) has been studied in the sodium deficient normotensive rat at 24, 48 and 96 h after the administration for 21 days of enalapril (MK-421, 10 mg/kg per day p.o.). Blood pressure was reduced and plasma ACE activity inhibited at 24 and 48, but not 96 h, after cessation of dosing. Tissue ACE activity (aorta, lung, mesenteric bed) was inhibited up to 96 h post dose when blood pressure had returned to control values. ACE production (activity following removal of inhibitor) was increased in plasma at 24, 48 and 96 h post dose but in tissues (adrenal glands, renal arteries and mesenteric bed) only at 48 h post dose. There was no tendency for ACE production to increase in the lung, the largest source of the enzyme in the rat. Thus it appears that inhibition of ACE activity in both plasma and tissue contributes to the blood pressure lowering activity of enalapril in the sodium deficient normotensive rat.     

Safety and tolerance of single oral doses of trandolapril (RU 44.570), a new angiotensin converting enzyme inhibitor

Summary The safety and tolerance of single oral doses of a new angiotensin converting enzyme (ACE) inhibitor, trandolapril have been examined in 90 healthy male volunteers, in a randomised, double blind, placebo-controlled study. The subjects were divided into 10 groups, each of 9 subjects and treatments (6 subjects on trandolapril and 3 on placebo per group) were allocated by unbalanced randomisation. Ten single, increasing oral doses were tested: 0.125, 0.25, 0.5, 1, 2, 4, 8, 16, 24 and 32 mg. The assessment criteria were clinical (monitoring of blood pressure, heart and respiratory rate, electrocardiogram, temperature and evaluation of behaviour and side effects) and routine laboratory tests.Blood pressure did not fall except for a slight drop in diastolic pressure during the first 4 h following the 32-mg dose. However, although an effect of the compound cannot be excluded, the reduction in blood pressure may have reflected intersubject variability. No orthostatic hypotension was observed. There was no change in the other vital signs, and in particular no increase in heart rate was observed. No serious adverse effect was encountered.The pharmacological activity of the compound was studied by assaying plasma ACE activity. Inhibition of ACE was linearly dose-dependant from 0 (placebo) to 2 mg, and above that dose, the inhibition was nearly total. ACE activity was markedly reduced within 30 min after administration of trandolapril, and maximal inhibition was observed from 2–4 h onwards, lasting for up to 24 h after dosing. For doses above 2 mg, inhibition was still 40% of the basal activity on Day 8 after dosing.     

A method for estimating the potency of angiotensin-converting enzyme inhibitors in man.

Dose-response curves of the effect of angiotensin I (A-I) infusion on diastolic blood pressure were constructed before and 3 h following single oral doses of the angiotensin-converting enzyme (ACE) inhibitor cilazapril (1.25 to 30 mg) in six normal male subjects. Cilazapril shifted the A-I dose-response curves dose dependently rightward; Schild-plot analysis indicated a competitive antagonism by cilazapril with an apparent Ki-dose of about 0.6 mg.     

Pharmacodynamics of converting enzyme inhibition: The cardiovascular, endocrine and autonomic effects of MK421 (enalapril) and MK521

1 Two new orally active inhibitors of angiotensin converting enzyme MK421 (enalapril maleate) and its lysine analogue MK521 were given to nine salt-replete normal subjects in a randomised placebo-controlled study.

2 Supine and erect blood pressure and heart rate were measured before (0) and 1, 2, 4, 6, 8, 12 and 24 h after drug administration. Plasma converting enzyme, renin, renin substrate and noradrenaline levels were measured at 0, 2, 4, 6, 8, 12 and 24 h with measurement of plasma drug levels from 4 h in addition. Blood pressure and heart rate responses to Valsalva's manoeuvre, isometric exercise and the cold pressor test were measured at 0, 6 and 24 h and to dynamic exercise at 6 and 24 h.

3 Both MK421 and MK521 significantly inhibited plasma converting enzyme activity, but the inhibition was more prolonged after MK521. Plasma drug concentrations were correspondingly higher after MK521 at the later time intervals. Significant decreases in supine and erect blood pressure without a reflex increase in heart rate were observed after active drug treatment and plasma renin activity was increased. Renin substrate was decreased significantly after MK421. Maximum changes in blood pressure, renin and renin substrate occurred 4-6 h after drug ingestion, corresponding to peak plasma drug levels and maximal inhibition of converting enzyme. Hormonal changes were also more prolonged after MK521. Plasma noradrenaline was unchanged. No significant effects of active drug treatment on any test of autonomic function were seen.

4 MK421 and MK521 are potent inhibitors of converting enzyme which decrease blood pressure in normal subjects without reflex tachycardia. Pharmacodynamic effects on blood pressure and renin occur in parallel with plasma drug levels and converting enzyme inhibition. These studies suggest that the hypotensive effect of MK421 and MK521 is due to inhibition of the renin-angiotensin system secondary to converting enzyme inhibition. Absence of tachycardia during converting enzyme inhibition is not due to alterations in autonomic reflexes.