Enalapril in essential hypertension: a comparative study with propranolol. Enalapril in Hypertension Study Group (UK)

We report the first comparative study on enalapril maleate, a new angiotensin converting enzyme inhibitor, in patients with uncomplicated mild to moderate essential hypertension. Fifty-four patients were randomly assigned to treatment with enalapril or propranolol for 16 weeks following a placebo run-in-phase. The study was double-blind. Enalapril and propranolol both reduced blood pressure, though the changes were significantly treated with enalapril were normotensive at the end of the study. Enalapril treatment was associated with a significant reduction in weight. Both drugs raised plasma potassium and urea. No haematological abnormalities occurred with enalapril and there were no reports of rash, taste disturbance or proteinuria. At the end of the trial the mean daily dose of enalapril was 20 mg and that of propranolol was 180 mg.

Systolic blood pressure responses to enalapril maleate (MK 421, an angiotensin converting enzyme inhibitor) and hydrochlorothiazide in conscious Dahl salt-sensitive and salt-resistant rats

Systolic blood pressure responses to enalapril maleate (MK 421, a new angiotensin converting enzyme inhibitor (CEI] and hydrochlorothiazide (HTZ) were studied in conscious Dahl salt-sensitive (DS) and salt-resistant (DR) rats maintained on a high salt (8.0% NaCl) and a normal salt (0.4% NaCl) diet. The DS rats were severely hypertensive after 3 weeks on the high salt diet whereas the systolic blood pressure (SBP) of the DR rats were normotensive. Oral treatment with enalapril (15-100 mg X kg-1 X day-1) and HTZ (60-400 mg X kg-1 X day-1) caused a significant reduction of SBP in the DS rats with the high salt diet (P less than 0.001); however, this was not observed until after 4 weeks of treatment when the dosage was 30 and 150 mg X kg-1 X day-1, respectively. Furthermore, enalapril therapy alone significantly reduced the SBP of all groups of rats regardless of diet or Dahl strain (P less than 0.001), but this was not observed until the end of the 7th week of therapy in DR rats on 8.0% NaCl and the end of the 3rd week of therapy for DR and DS rats on 0.4% NaCl. These results suggest that enalapril may lower SBP by mechanisms other than those related to an action as a CEI.

Pharmacokinetics of repeated single oral doses of enalapril maleate (MK-421) in normal volunteers

Enalapril, the ethyl ester of a potent angiotensin converting enzyme inhibitor, enalaprilat, was administered to healthy volunteers as a capsule containing 10 mg of the maleate salt, every 24h for eight doses. Serum profiles show little accumulation of enalaprilat following eight daily doses of enalapril maleate. An average effective half-life for accumulation of approximately 11h was calculated from urine data. Comparison of observed 24-h urinary recoveries of enalaprilat to predicted steady-state recovery indicates that an 'average' steady state for enalaprilat is attained by the third or fourth dose of enalapril maleate. Statistical comparison of daily urinary recoveries, as well as Cmin values for enalaprilat, confirm this. Observed fluctuations in serum and urine data during apparent steady state suggest some day-to-day variability in the absorption of enalapril maleate and/or its hydrolysis to enalaprilat. An accumulation ratio of 1.3 for enalaprilat was calculated from the predicted steady-state urinary recovery and observed urinary recovery for dose one.

Angiotensin converting enzyme during acute and chronic enalapril therapy in essential hypertension

The acute and chronic effects of enalapril (MK421) were assessed in a double-blind randomized trial in subjects with essential hypertension. In acute studies, twelve subjects received enalapril (10 mg p.o.), following which there was a fall in blood pressure, maximal at 6 h and lasting for 24 h. Serum MK422 (enalaprilic acid, the bioactive form of enalapril) and serum angiotensin converting enzyme (ACE) inhibition had a similar time course with good correlation between drug levels and ACE inhibition (P less than 0.001, r = 0.98, n = 16) and between ACE inhibition and the hypotensive effect (P less than 0.001, r = 0.84, n = 16). In chronic studies enalapril was titrated from 5 mg to 20 mg twice a day in eleven hypertensive patients to achieve a diastolic blood pressure less than 90 mmHg. Treatment continued for 3-12 months. Increasing serum MK422 was correlated with reducing serum ACE activity (P less than 0.001, r = 0.8, n = 104). The fall in blood pressure correlated both with serum MK422 level (P less than 0.05, r = 0.37, n = 39) as well as ACE inhibition (P less than 0.01, r = 0.45, n = 42). The drug level:ACE inhibition curve was shifted to the right during chronic enalapril treatment. ID50 for serum ACE was 32 ng MK422/ml following the single 10 mg dose of MK421 and 70 ng MK422/ml during chronic treatment. Blood pressure falls during acute and chronic treatment were similar over the range of serum MK422 levels achieved. The shift in the drug level:ACE inhibition curve suggests induction of ACE during chronic treatment with enalapril in man.

Hemodynamic effects of vasodilators and long-term response in heart failure

Hemodynamic responses to vasodilators are commonly assessed when starting long-term vasodilator treatment in patients with chronic left ventricular failure, although the relation between short- and long-term responses is not established. Thus, short- and long-term hemodynamic responses to placebo and vasodilators (isosorbide dinitrate, minoxidil and enalapril or captopril) were measured and long-term clinical efficacy was assessed by changes in exercise capacity after 1 to 5 months of vasodilator administration (plus digitalis and diuretic agents) in 46 patients with New York Heart Association functional class II to IV heart failure caused by cardiomyopathy. There were no significant changes in hemodynamics or exercise capacity during placebo treatment. After initial doses and during long-term administration of vasodilator drugs, hemodynamics were significantly improved. After long-term vasodilator treatment, maximal oxygen uptake during exercise increased by 2.9 +/- 5.7 ml/min per kg from a control value of 14.1 +/- 5.6 ml/min per kg (p less than 0.01), and exercise duration also increased by 1.8 +/- 3.5 minutes (p less than 0.01). Changes in maximal oxygen uptake, however, did not correlate with short-term changes in pulmonary wedge pressure (correlation coefficient [r] = -0.14), cardiac index (r = -0.01) or systemic vascular resistance (r = -0.20). Long-term hemodynamic changes also failed to correlate with changes in exercise capacity. Baseline hemodynamics, cardiac dimensions and left ventricular ejection fraction before vasodilator administration all failed to correlate with baseline exercise capacity or with long-term changes in exercise capacity. Thus, hemodynamic measurements at initiation or during follow-up of vasodilator therapy do not relate to long-term clinical efficacy assessed by exercise capacity in patients with chronic left ventricular failure. Therefore, the rationale for making invasive hemodynamic measurements before initiating long-term vasodilator therapy for heart failure is questioned.

Inhibitor binding assay for angiotensin-converting enzyme

We describe a new principle for the determination of enzymes, here applied to angiotensin-converting enzyme (ACE, EC 3.4.15.1) in human serum. The enzyme inhibitor binding assay is based on specific binding of labeled inhibitor to the active center of the enzyme. Serum (10-15 microL) is incubated with 125I-labeled ACE inhibitor (" 351A ," a p- hydroxybenzamidine derivative of N-(1-carboxy-3-phenylpropyl)-L-lysyl-L-proline) at pH 7.0 at 37 degrees C for 2 h in a non-equilibrated system. Inhibitor bound to ACE is separated by adsorption to coated charcoal, the radioactivity remaining in the supernate is counted, and the ACE value is calculated from a standard curve. Sensitivity for ACE in serum is 200 U/L, corresponding to 5.0 ng of ACE purified from human lung. The coefficient of variation was 3.9% within assay, and 6.4% between assays for normal ACE activities. Correlation with a comparison spectrophotometric method (Am J Med 59: 363-372, 1975) for ACE assay was excellent (r = 0.98) in 59 samples from healthy subjects and from patients with various diseases including active sarcoidosis. The novel assay principle presented here is simple and specific, and can be extended to use with various biological fluids and tissues, and to other enzymes as well.

Duration of action and short-term hormonal responses to enalapril (MK 421) in normal subjects

Enalapril is a converting enzyme inhibitor with a prolonged duration of action. We investigated the arterial pressure and hormonal responses to angiotensin I infusion(s) in eight normotensive human volunteers at various intervals after administration of 10 mg enalapril to assess more precisely its duration of action, particularly in relationship to angiotensin II's influence on aldosterone secretion and renal vasoconstriction. In normotensive sodium-restricted subjects, there was significant attenuation (p less than 0.025) of diastolic blood pressure response to angiotensin I infusion even as long as 28 h after administration of enalapril. This was accompanied by a significant (p less than 0.02) accumulation of angiotensin I and reduction of the angiotensin II increment in response to angiotensin I infusion. In contrast to the persistent efficacy of enalapril, as assessed by angiotensin I infusion, basal levels of angiotension II had returned to control levels by 22-24 h postdrug. Yet there was a persistent reduction of diastolic blood pressure even as long as 28 h postdrug. The persistent hypotension could not be explained on the basis of changes in other potential vasoactive factors (epinephrine, norepinephrine, bradykinin, or prostaglandin), as none of these was significantly modified by enalapril administration. In summary, enalapril in a dose of 10 mg p.o. produced a significant reduction in blood pressure in this study for up to 28 h. Unlike captopril, enalapril does not modify circulating prostaglandins and kinins. Although the level of activation of the renin-angiotensin system had returned to control values 24 h after enalapril administration, there was evidence from the angiotensin I infusions of continued blockade of the angiotensin-converting enzyme for as long as 28 h.

Disposition of enalapril and its diacid metabolite, enalaprilat, in a perfused rat liver preparation. Presence of a diffusional barrier for enalaprilat into hepatocytes

Enalaprilat (MK-422), a new and potent angiotensin- converting enzyme inhibitor, and its monoethyl ester precursor, enalapril, were studied in a single pass perfused rat liver preparation under constant perfusate flow (10 ml/min) at concentrations of 0.29-0.41 microM for 14C-enalapril and 0.01-0.015 microM for 3H- enalaprilat . During their simultaneous delivery to the same rat liver preparation, the steady state hepatic extraction ratio of 14C-enalapril was high (0.861 +/- 0.02) and 14C- enalaprilat appeared rapidly in effluent perfusate plasma. Of the enalapril dose, 22.7 +/- 6.9% appeared in bile. 14C- Enalaprilat accounted for 79% of the total radioactivity in bile (18% of dose) whereas 14C-enalapril was present only as 10% of the total (2.3% of dose). By contrast, the steady state hepatic extraction of 3H- enalaprilat was very low (0.053) and the disappearance was virtually identical to the appearance of 3H- enalaprilat in bile. These findings suggest that diffusional barrier exists for enalaprilat as the preformed metabolite, which hinders penetration into hepatocytes, and therefore, elimination. The precursor, enalapril, effectively brings enalaprilat into hepatocytes were more extensive biliary excretion of the generated metabolite takes place. This account adds to our further understanding of metabolite kinetics; in addition to the uneven distribution of enzyme system and the intrinsic clearance for metabolite formation and elimination, the presence of a diffusional barrier is another important determinant which may cause deviations between the kinetics of a generated and performed metabolite.

Kinetics of inhibition of angiotensin converting enzyme by captopril and by enalapril diacid

Using a continuous spectrophotometric assay, inhibition of angiotensin converting enzyme by captopril and by the active diacid derivative of enalapril was reinvestigated. The onset of inhibition was comparatively slow, but the inhibition achieved was stronger than previous estimates: approximate Ki-values were 0.3 nM for captopril and 0.06 nM for enalapril diacid. The rate-constants for association and dissociation of these enzyme-inhibitor complexes were estimated, and half-times of approximately 12 min for the captopril complex and 60 min for the enalapril diacid complex were calculated. The rate of dissociation of the captopril-enzyme complex was measured directly by reacting the thiol group in free captopril with 5,5'-dithiobis(2-nitrobenzoic acid) and observing the reactivation of the enzyme; a half-time of approximately 30 min was obtained. Therefore the release of these inhibitors from the enzyme may be slow enough to affect the duration of their hypotensive action.

Regression of left ventricular hypertrophy from systemic hypertension by enalapril

Reversal of left ventricular (LV) hypertrophy with medical therapy has been studied increasingly in patients with systemic hypertension. However, serial changes of LV function are not found during reversal of LV hypertrophy in hypertensive patients. Seven patients with LV hypertension were studied to evaluate serial changes of LV mass and function after the initiation of the new converting enzyme inhibitor MK-421. LV mass and function were determined serially at the end of a placebo period and at 5 days, 1 month, 3 months and 7 months after the initiation of MK-421, using both 2-dimensional (2-D) guided M-mode echocardiography and radionuclide techniques. All patients except 1 had LV hypertrophy and all had normal LV function (ejection fraction derived from gated blood pool method greater than 49%). There was an inverse relation between LV fractional shortening (percent FS) and end-systolic stress before medication (r = -0.81, p less than 0.05). LV mass decreased significantly at 3 months and at 7 months (-10%, p less than 0.05, and -12%, p less than 0.01, respectively) accompanied with persistent decrease of mean blood pressure, which occurred as early as 5 days after start of therapy (133 +/- 5 mm Hg at control, to 112 +/- 4 mm Hg at day 5). During reversal of LV hypertrophy, the inverse correlation between FS and end-systolic stress remained significant (r = -0.80 to -0.95, p less than 0.025 for all), with no difference from the placebo period and from this relation in the normal group. Moreover, percent FS, ejection fraction and stroke index remained unchanged. Thus, LV hypertrophy in patients with systemic hypertension can be reversed without deterioration of LV function. Moreover, overall LV function is likely to be determined by afterload even after reversal of LV hypertrophy.

Acute antihypertensive synergism of angiotensin-converting enzyme inhibitors and diuretics

In spontaneously hypertensive rats (SHR), after 1 day of dosing with an angiotensin-converting enzyme (ACE) inhibitor (captopril or enalapril) plus a diuretic (hydrochlorothiazide), a synergistic antihypertensive effect was observed when a second dose of the combination or ACE inhibitor alone but not the diuretic alone was given the next day. Bilateral ureteral ligation did not prevent the synergism, which indicates that diuresis per se was not the mechanism. Vascular responses to various agonists did not differ in SHR given ACE inhibitor or ACE inhibitor plus diuretic. SHR given combination treatment had higher and more prolonged increases in plasma renin activity. Aprotinin or indomethacin did not alter the synergism, which suggests that endogenous kinins and prostaglandins did not play a role. These data suggest that the mechanism for the synergistic antihypertensive effect resulted from the combination treatment's shifting the blood pressure regulation system to be renin dependent and responding more to drugs affecting the renin-angiotensin system (RAS). Evidence was presented that the RAS can be shifted rapidly to assume a greater role in blood pressure regulation in SHR as well as in normotensive and two-kidney, one-clip Goldblatt renal hypertensive dogs by restricting sodium intake. The data may partly explain the various degrees of antihypertensive responsiveness of essential hypertensive patients to ACE inhibitors.

Effects of captopril and enalapril on sodium excretion and blood pressure in sodium-deficient dogs

Inhibition of angiotensin I-converting enzyme (ACE) (kininase II) provides a powerful new method for evaluating the role of the renin-angiotensin-aldosterone and kallikrein-kinin systems in the control of aldosterone secretion, renal function, and arterial blood pressure. This study compares the effects of long-term administration of a sulfhydryl inhibitor, captopril, with a nonsulfhydryl inhibitor, enalapril (1-[N-[1-(ethoxycarbonyl-3-phenylpropyl]-L-alanyl]-L-proline), in conscious sodium-deficient dogs. Plasma aldosterone concentration (PAC), plasma renin activity (PRA), urinary sodium excretion (UNaV), arterial pressure (AP), blood kinins (BK), urinary kinins (UK), and urinary kallikrein activity (UKA) were determined during long-term inhibition of ACE in sodium-deficient dogs. In response to captopril administration (20 mg/(kg . day], PAC decreased from 38.9 +/- 6.7 to 14.3 +/- 2.3 ng/dl, PRA increased from 3.58 +/- 0.53 to 13.7 +/- 1.6 ng/(ml . h), UNaV increased from 0.65 +/- 0.27 to 6.4 +/- 1.2 meq/day, AP decreased from 102 +/- 3 to 65 +/- 2 mm Hg, BK increased from 0.17 +/- 0.02 to 0.41 +/- 0.04 ng/ml, UK increased from 7.2 +/- 1.5 to 31.4 +/- 3.2 micrograms/day, and UKA decreased from 23.6 +/- 3.1 to 5.3 +/- 1.2 EU/day. Quantitatively similar changes in AP, UNaV, and PAC were observed in sodium-deficient dogs in response to long-term enalapril administration (4 mg/(kg X day]. In sodium-deficient dogs maintained on captopril or enalapril for several days, angiotensin II (AngII) infusion (3 ng/(kg X min] restored PAC, UNaV, and AP to levels observed in untreated sodium-deficient dogs. These data indicate that the long-term hypotensive and natriuretic actions of inhibitors of ACE are mediated by inhibition of AngII formation and that the renin-angiotensin system plays an essential role in regulating aldosterone secretion, renal function, and AP during sodium deficiency.

Overview: the role of angiotensin-converting enzyme inhibitors in cardiovascular therapy

Angiotensin-converting enzyme (ACE) inhibitors favorably modify control mechanisms that are disturbed in hypertension and congestive heart failure, principally, but perhaps not exclusively, through reduction in angiotensin II levels. Pharmacodynamic actions are vasodilation, increased sodium excretion, and lowering of blood pressure. Investigations with captopril and enalapril in the treatment of hypertension indicate efficacies comparable to each other and to current step 1 and 2 agents. Enalapril is more potent than captopril and has a longer duration of action. The hemodynamic mechanism of action is reduction in peripheral vascular resistance. Addition of a diuretic potentiates blood pressure lowering and proportion of patients responding. When used in congestive heart failure, ACE inhibitors exert a balanced vasodilator effect on arterial and venous beds and do not induce tachycardia or fluid retention. Cardiac output is increased whereas systemic vascular resistance, central pressures, and systemic blood pressure are reduced acutely and chronically. Although captopril is associated with certain side effects, possibly resulting from the sulfhydryl group in its structure, this profile has not been encountered thus far in clinical investigations with enalapril. The effects of ACE inhibitors on the natural histories of hypertension (independent of blood pressure lowering) and congestive heart failure are yet to be determined.

Antihypertensive profile of the angiotensin-converting enzyme inhibitors CI-906 and CI-907

CI-906 and CI-907, new orally active nonsulfhydryl angiotensin-converting enzyme inhibitors, were examined for antihypertensive effects in unanesthetized hypertensive rats and dogs. In two-kidney, one-clip Goldblatt hypertensive rats, single oral daily doses (0.03-30 mg/kg) produced dose-dependent decreases in blood pressure; a single 3 mg/kg oral dose lowered blood pressure to normotensive levels. In spontaneously hypertensive rats, 30 mg/(kg X day) orally administered for 5 consecutive days achieved the same blood pressure decrease as that obtained on the first day in the renal hypertensive rats. In diuretic-pretreated renal hypertensive dogs, a 10 mg/kg oral dose decreased blood pressure by 25%. No adverse side effects were observed with CI-906 and CI-907 in any of the conscious animals. These studies indicate that CI-906 and CI-907 are potent, orally active antihypertensive agents without any apparent limiting side effects.

CI-906 and CI-907: new orally active nonsulfhydryl angiotensin-converting enzyme inhibitors

CI-906, [3S-[2[R*(R*)]], 3R*]-2-[2-[[1-(ethoxycarbonyl)-3-phenylpropyl]-amino]-1-oxopropyl] 1,2,3,4-tetrahydro-3-isoquinolinecarboxylic acid, monohydrochloride, and CI-907, [2S-[1[R*(R*)]], 2 alpha, 3a beta, 7a 7a beta]-1-[2-[[1-(ethoxycarbonyl)-3-phenylpropyl]amino] 1-oxopropyl]octahydro-1H-indole-2-carboxylic acid, monohydrochloride, are two new nonsulfhydryl-type angiotensin-converting enzyme (ACE) inhibitors. Monoester (prodrug) and diacid forms produced concentration-related ACE inhibition in guinea pig serum (IC50 for CI-906 = 8.3 X 10(-9) M, diacid = 2.8 X 10(-9) M; CI-907 = 1.0 X 10(-7) M, diacid = 2.6 X 10(-9) M). In isolated rabbit aortic rings and in in vivo rat and dog autonomic studies, both compounds were highly specific in suppressing the contractile or pressor responses to angiotensin I. In two-kidney, one-clip Goldblatt (renin-dependent) hypertensive rats there was a good correlation between the inhibition of vascular converting enzyme and blood pressure lowering and a poor correlation between blood pressure lowering and plasma and brain converting enzyme inhibition. Cardiovascular, pulmonary, and central nervous system performance evaluations showed no side effects or gross toxicity. The preclinical profile shows CI-906 and CI-907 to be specific, potent, orally active ACE inhibitors. They are expected to have therapeutic utility in hypertension and in any other condition where converting enzyme inhibition would be useful.

Comparison of oral MK 421 and propranolol in mild to moderate essential hypertension and their effects on arterial and venous vessels of the forearm

The efficacy of MK 421 and propranolol was compared in 48 patients with mild to moderate hypertension. Each patient was randomly assigned to receive 1 of the drugs for 12 weeks. Additionally, a subgroup of 28 patients underwent studies of forearm arterial and venous circulation by means of pulsed Doppler and mercury-in-silastic plethysmography. Both drugs reduced supine and standing blood pressure (BP) (p less than 0.001). Propranolol reduced heart rate (p less than 0.001), while MK 421 did not change it. Brachial artery diameter, blood velocity and flow increased after MK 421 (p less than 0.001), but were not changed after propranolol therapy. Forearm vascular resistance decreased after MK 421 (p less than 0.001) and after propranolol (p less than 0.05). Forearm venous tone was unaffected on MK 421, but increased after propranolol (p less than 0.01). Thus, in moderate hypertension, 3 months of treatment with MK 421 or propranolol similarly decrease BP, but affect the forearm circulation differently: MK 421 dilates both the brachial artery and the arterioles of the forearm, but does not affect the venous vessels, and propranolol causes little arterial change but increases the forearm venous tone.

Central and peripheral indices of sympathetic activity after blood pressure lowering with enalapril (MK-421) or hydralazine in normotensive rats

Various antihypertensive drugs have different effects on vasoactive mechanisms. In normotensive Wistar rats, we investigated the effects on plasma and tissue catecholamines of chronic treatment with two agents: MK-421, an angiotensin converting-enzyme inhibitor (CEI), and hydralazine, a vascular smooth muscle relaxant. Both agents lowered blood pressure via arteriolar dilation, to the same final level. However, hydralazine stimulated the renin-angiotensin system and elevated the plasma norepinephrine (NE) and epinephrine (E) levels, whereas MK-421 did not. In MK-421-treated animals, NE, E, and the ratio of E/NE were decreased in the brain stem, and this ratio was increased in the heart. In hydralazine-treated rats, the catecholamine levels were unchanged in the brain stem and heart. The turnover rate of NE was significantly reduced in the brain stem and heart of MK-421-treated rats, whereas, after hydralazine, the turnover rate in the heart was increased (decreasing the half-life of NE by about 50%), indicating increased sympathetic activity. Thus, elimination of angiotensin II (AII) by CEI is associated with decreased sympathetic activity in both the brain stem and heart, whereas an equipotent antihypertensive action by smooth muscle relaxation leads to stimulation of both the renin-angiotensin and the sympathetic systems. These differences are more readily apparent by measurement of catecholamine turnover rates in tissues than of catecholamine levels, and they may account for the different hemodynamic effects of the two agents, even though both drugs act through arteriolar dilation.

Hemodynamic and antihypertensive effects of the new oral angiotensin-converting-enzyme inhibitor MK-421 (enalapril)

The antihypertensive, hemodynamic, and humoral effects of the new converting-enzyme inhibitor enalapril (MK-421) were assessed by sequential studies during 3 months of uninterrupted treatment (20 mg twice daily) in 10 hypertensive patients. Six achieved good blood pressure (mean arterial pressure) control with enalapril alone (from 126 +/- 7.0 mm Hg pretreatment to 105 +/- 1.6 mm Hg at 3 months, p less than 0.05). The other four required the addition of diuretics (hydrochlorothiazide 25 mg orally twice daily) at different stages of follow-up, with resultant blood pressure control (128 +/- 9.6 mm Hg pretreatment to 113 +/- 1.9 mm Hg at 2 months after the addition of diuretics). Neither the acute nor long-term blood pressure response could be predicted from the pretreatment levels of plasma renin activity. The blood pressure reduction during enalapril therapy was characterized by a decrease in total peripheral resistance (53 +/- 2.5 U X M2 pretreatment to 38 +/- 3.0 U X M2 at 3 months, p less than 0.05) with no significant change in cardiac output or heart rate. This lack of reflex tachycardia could not be ascribed to baroceptor dysfunction since the response to head-up tilt (the increase in diastolic blood pressure, in heart rate, and in plasma catecholamines) was normal and not significantly different from pretreatment response. Average blood volume did not change (91% +/- 4.3% of normal in the pretreatment period to 93% +/- 2.9% after 3 months of therapy, p = NS) despite the significant lowering of arterial pressure with enalapril alone (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of deterioration in renal function in patients with renovascular hypertension treated with enalapril

Azotemia produced by converting enzyme inhibition in renovascular hypertension was studied in six patients by measurement of glomerular filtration rate (GFR) and effective renal plasma flow (ERPF). Patients with unilateral renovascular hypertension lowered GFR acutely to 40% of control values at 4 hours, but after 4 days it only returned to 75% of control even though ERPF exceeded control values. Calculated filtration fraction (GFR/ERPF) decreased from 0.31 to 0.19 in 4 hours (p less than 0.05) and remained at 0.20 after 4 days of treatment despite increases in GFR and ERPF. In bilateral renovascular hypertension, neither the decreased GFR nor lowered ERPF induced by enalapril showed any tendency to return toward normal with continued treatment. These data are consistent with selective glomerular efferent arteriolar dilation in response to enalapril and suggest that angiotensin converting-enzyme inhibition interferes with the autoregulatory capacity of the kidney in the presence of severe renovascular hypertension.

Enalapril (MK421) and its lysine analogue (MK521): a comparison of acute and chronic effects on blood pressure, renin-angiotensin system and sodium excretion in normal man

The immediate and long-term effects of enalapril (MK421) and its lysine analogue (MK521) in once-daily dosage, were compared in a study of 12 normal subjects. Both compounds lowered blood pressure equally throughout 24 h without causing tachycardia. The biochemical changes with MK521 were more sustained than with MK421, but this did not affect the magnitude of blood pressure reduction. Twenty-four hours after the previous dose, with both active drugs, plasma renin concentration was significantly higher on day 8 than on day 1, though angiotensin I did not increase in proportion; this probably reflects a fall in renin-substrate with prolonged converting enzyme inhibition. There was an early natriuresis with each compound but this effect was no longer apparent after 8 days of continuous therapy. Both MK421 and MK521 were well tolerated with no serious side effects.

Antihypertensive effect of MK-421 in rats. Role of the renal kallikrein-kinin system

To study the hypotensive mechanism of the new oral converting-enzyme inhibitor, MK-421, we evaluated the antihypertensive effect of MK-421 in rats with hypertension induced by chronic administration of norepinephrine (NE) or vasopressin and measured urinary kallikrein and kinin excretions as indices of the renal kallikrein-kinin system. When 6 mg/kg/day of MK-421 was administered simultaneously with 1.8 mg/kg/day of NE, the systolic blood pressure of conscious rats rose on Day 1 to only 122.6 +/- 3.4 mm Hg compared with the rise to 146.3 +/- 1.6 mm Hg when NE alone was infused (p less than 0.001). Similarly, when the same dose of MK-421 was administered simultaneously with 7.2 U/kg/day of vasopressin, the systolic blood pressure of conscious rats rose on Day 1 to only 117.4 +/- 3.8 mm Hg compared with the rise to 141.6 +/- 3.4 mm Hg when vasopressin alone was infused (p less than 0.01). The antihypertensive effect of MK-421 was sustained for 6 days in rats infused with NE or vasopressin. Infusion of NE alone resulted in a small but significant increase in urinary kallikrein excretion and no change in urinary kinin excretion. The combined administration of NE with MK-421 induced additional increases in urinary kallikrein and kinin excretions. Vasopressin alone resulted in marked decreases in urinary kallikrein and kinin excretions. The combined administration of vasopressin with MK-421 induced no additional changes in urinary kallikrein and kinin excretion. These results indicate that the hypotensive effect of MK-421 may depend on a reduced sensitivity of the peripheral arteries to vasoconstrictor substances.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic and clinical response to enalapril, a long-acting converting-enzyme inhibitor, in patients with congestive heart failure

Enalapril, a new oral angiotensin converting-enzyme inhibitor, was administered to nine patients with severe congestive heart failure. Short-term hemodynamic response was noted within 2 hr and persisted for up to 24 hr. At peak effect mean arterial pressure fell from 83.4 +/- 10(SD) to 72.1 +/- 16.2 mm Hg (p less than .01), right atrial pressure from 13.6 +/- 6.0 to 10.4 +/- 7.5 mm Hg (p less than .01), pulmonary arterial pressure from 38.9 +/- 4.8 to 31.9 +/- 4.8 mm Hg (p less than .01), pulmonary capillary wedge pressure from 28.2 +/- 3.5 to 22.1 +/- 5.1 mm Hg (p less than .01), and total pulmonary resistance from 875 +/- 304 to 697 +/- 291 dynes-sec-cm-5 (p less than .05). Cardiac index was not changed, but there was a significant redistribution of regional blood flow with an increase of renal blood flow after enalapril. Plasma renin activity rose significantly from 6.2 to 28.6 ng/ml/hr, whereas plasma norepinephrine did not change after enalapril. Seven patients were treated with enalapril for 4 weeks. Five patients reported symptomatic improvement. Five of six patients tested had an increase in both exercise time (NS) and maximum oxygen consumption (NS). Repeat hemodynamic evaluation in six patients after long-term enalapril therapy showed a persistent effect with significant reductions in right atrial pressure from 13.8 +/- 7.2 to 7.1 +/- 4.7 mm Hg and in mean arterial pressure from 82.5 +/- 10.4 to 76.6 +/- 5.3 mm Hg and a significant increase in cardiac index from 2.1 +/- 0.5 to 2.5 +/- 0.5 l/min/m2 (all p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Enalapril, a new nonsulfhydryl angiotensin converting enzyme inhibitor, does not potentiate morphine analgesia

Sprague-Dawley rats received oral doses of enalapril maleate (5 mg/kg), a potent, nonsulfhydryl, angiotensin converting enzyme inhibitor, or saline. Sixty min later, morphine sulfate, 5 mg/kg, or saline was injected subcutaneously. Response to a thermal stimulus was monitored before and up to 5 h ter morphine injection using the rat tail flick test. Serum ACE activity was greater than 90% inhibited by enalapril throughout the experiment. Enalapril did not exhibit analgesic activity nor did it potentiate morphine analgesia.

Responses of the kallikrein-kinin system to angiotensin converting enzyme inhibitors in the rat

Three structurally different drugs, MK421, SA446 and captopril, are angiotensin converting enzyme inhibitors. They induced a significantly greater fall in systolic blood pressure in sodium depleted rats than in sodium repleted ones. The combined administration of vasopressin or norepinephrine with MK421 eliminated the hypertensive effect of vasopressin or norepinephrine. Urinary kallikrein and kinin excretion were increased by MK421, SA446 or captopril in the sodium depleted rats whereas any significant changes in them were not observed in the sodium repleted rats. The combined administration of norepinephrine with MK421 induced further increases in urinary kallikrein and kinin excretion when compared to rats infused with norepinephrine alone, whereas the combined administration of vasopressin with MK421 did not induce any changes in them when compared to rats infused with vasopressin alone. Chronic infusion of captopril for up to 6 days in the rats induced a reduction of the vascular response to exogenous bradykinin; the blood pressure fall was less than that of the controls by 17% on Day 2 and by 18% on Day 6. These results indicate that the hypotensive response to angiotensin converting enzyme inhibitors was in part associated with the enhanced renal and vascular smooth muscle kallikrein-kinin system.

Antihypertensive effects of CI-907 (indolapril): a novel nonsulfhydryl angiotensin converting enzyme inhibitor

CI-907 is a new orally active nonsulfhydryl angiotensin converting enzyme (ACE) inhibitor. Monoester (prodrug) and diacid forms produced concentration related ACE inhibition in guinea-pig serum (IC50 for monoester, 1.7 X 10(-7) M and for diacid, 2.6 X 10(-9) M). In isolated rabbit aortic rings and in rat and dog autonomic studies, CI-907 is highly specific in suppressing the contractile or pressor responses to angiotensin I. In two-kidney, one-clip Goldblatt hypertensive rats, single daily doses (0.03-30 mg/kg p.o.) produced dose-dependent decreases in blood pressure; 3 mg/kg lowered blood pressure to normotensive levels. In the spontaneously hypertensive rat, subacute administration of CI-907 (30 mg/kg/day for 5 days) produced the same decrease in blood pressure as that obtained in the renal hypertensive rat. In diuretic-pretreated renal hypertensive dogs, 10 mg/kg normalized blood pressure. For equivalent drops in blood pressure, heart rate increases were less in CI-907 than in enalapril-treated renal hypertensive dogs. No side effects were observed with CI-907 in any of the conscious animals. The antihypertensive response to CI-907 (0.03-1.0 mg/kg p.o.) was found to correlate with inhibition of vascular tissue ACE, but not plasma or brain ACE in two-kidney, one-clip renal hypertensive rats. These studies indicate that CI-907 is a potent antihypertensive agent with a heart rate profile different from enalapril.

Tissue distribution of angiotensin converting enzyme in the rat: effect of captopril treatment

Effect on different tissues with regard to angiotensin converting enzyme during captopril treatment in the rat was studied. Male Wistar-Kyoto rats (n = 9) were treated during four weeks with captopril dissolved into the drinking water at the dose 30 mg/kg/day. Control rats (n = 9) had water only. Serum angiotensin converting enzyme (ACE) activity increased three-fold during captopril treatment, and ACE of purified pulmonary plasma membranes increased about 64% (P less than 0.001) compared to untreated rats. ACE activity of membrane fractions of other tissues studied i.e. testicles, epididymes, kidneys, and small intestine brush border did not increase similarly during captopril treatment. The highest amounts of ACE was demonstrated in epididymes, but captopril did not produce increased amounts of ACE in the epididymes. The main source of increased serum ACE activity during captopril treatment appeared to be in the pulmonary tissue.

Coronary vascular actions of the converting enzyme inhibitor, enalapril

Enalapril, a potent angiotensin converting enzyme inhibitor, effectively blocked the constrictor actions of angiotensin I in isolated perfused cat coronary arteries. Enalapril, at concentrations of 25 to 100 micrograms/ml, inhibited angiotensin I by 65 to 80%. Moreover, enalapril at 100 to 200 micrograms/ml, markedly antagonized the coronary vasoconstrictor effects of angiotensin II. At 150 micrograms/ml, enalapril blocked the angiotensin II response by 80 +/- 5%, and at 200 micrograms/ml, it was blocked by 95 +/- 4%. Enalaprilic acid at 0.5 to 1.0 microgram/ml also blocked the angiotensin II response by 94 +/- 5%. Captopril, up to 250 micrograms/ml, failed to significantly antagonize angiotensin II, although it readily blocked angiotensin I in this preparation. The duration of this angiotensin II blockade lasted about 60-90 min. This angiotensin II antagonism may help explain the beneficial effects of enalapril in situations such as acute myocardial ischemia.

Measurement of low angiotensin concentrations after ethanol and Dowex extraction procedures

Current radioimmunoassays do not demonstrate total absence of angiotensin II during converting enzyme inhibition. To assess the meaning of plasma angiotensin II determinations during converting enzyme inhibition, plasma angiotensin I and II levels of normotensive humans during maximal converting enzyme inhibition by single oral doses of CGS 13945, MK 421, or MK 521 were compared with those of anephric rats (18 hr after nephrectomy) after intravenous administration of MK 422 (1 mg/kg). Prior to radioimmunoassay, plasma was extracted with Dowex for angiotensin II and blood extracted with ethanol for angiotensin I. During converting enzyme inhibition, in the 20 normotensive subjects plasma angiotensin II was 6.3 +/- 2.3 pg/ml (mean +/- S.D.) and blood angiotensin I was 65 +/- 59 pg/ml. In the nephrectomized rats, plasma angiotensin II was 8.9 +/- 2.3 pg/ml without converting enzyme inhibitor (n = 15) and 7.6 +/- 2.8 with MK 422 (n = 14), and blood angiotensin I was 9.8 +/- 2.4 pg/ml and 8.2 +/- 0.7, respectively. Dowex extraction of Tris buffer containing no angiotensin II provided blank values ranging from 5.0 to 7.8 pg/ml (n = 5). Thus plasma angiotensin II of normotensive humans treated with converting enzyme inhibitors fell to blank levels even in the presence of markedly elevated plasma angiotensin I. Angiotensin II concentrations in anephric rats with or without converting enzyme inhibition were the same. We therefore conclude that plasma levels of angiotensin II below 8 pg/ml measured after Dowex extraction probably reflect complete converting enzyme inhibition and virtual absence of angiotensin II generation.

New potent inhibitors of angiotensin converting enzyme

Using an earlier model of the favoured orientation of binding functions of angiotensin converting enzyme (ACE) inhibitors, it has been possible to postulate a new, 7,6-bicyclic system, based on hexahydropyridazine, which might be expected to have high potency. Some members of this system which have been synthesised have been shown to be very active ACE inhibitors, in vitro and in vivo.

[Reproducibility of acute captopril-induced renal insufficiency with enalapril]

In this case we are reporting on a patient suffering from malignant renovascular hypertension and chronic renal failure due to occlusion of both renal arteries. The acute renal insufficiency after Captopril and later on after Enalapril treatment was fully reversible. We believe that the acute reversible renal insufficiency was caused by the blockage of glomerular autoregulation depending on Angiotensin II.

Plasma enalapril levels and hormonal effects after short- and long-term administration in essential hypertension

Enalapril lowers blood pressure both acutely and during long-term therapy in patients with essential hypertension. After a single 10 mg dose of enalapril a close relationship between plasma enalaprilic acid (MK-422) levels, angiotensin converting enzyme (ACE) inhibition and the acute hypotensive and hormonal effects was demonstrated. During long-term administration of enalapril, a similar relationship between the plasma enalaprilic acid level, ACE inhibition and the hypotensive effect was shown, although the dose-response curve for plasma enalaprilic acid to ACE inhibition was displaced to the right compared to the acute dose-response curve. Several weeks' administration of enalapril was needed to reach stable plateau levels of plasma enalaprilic acid and ACE inhibition. During long-term treatment with enalapril in essential hypertension, there was sustained inhibition of ACE and the associated hormonal changes.

Hemodynamic and humoral responses to enalapril and nifedipine in the rat

The hemodynamic and humoral effects of enalapril, an angiotensin converting enzyme (ACE) inhibitor, and nifedipine, a calcium-entry blocker, were evaluated in conscious spontaneously hypertensive rats (SHR). Rats received enalapril (5 mg/kg, po) or vehicle, followed in one hour by nifedipine (2.5 mg/kg, ip) or its vehicle. After treatment with enalapril alone, systolic blood pressure (BP) declined over a 3 hour period from 204 +/- 4 (mean +/- SE) to 168 +/- 6 mm Hg and remained suppressed for an additional 4 hours. Heart rate (HR) did not change. Plasma renin activity (PRA) increased approximately 9-fold and serum ACE was maximally inhibited. BP response to nifedipine was more rapid, greater in magnitude (-59 +/- 6 mm Hg) and shorter in duration; heart rate increased and remained elevated for 2.5 hours. PRA only rose two-fold. After the drugs in combination, BP declined as rapidly and to the same degree as after nifedipine and remained reduced for a longer duration. Treatment with enalapril attenuated the reflex tachycardia observed after nifedipine. These data suggest that coadministration of an ACE inhibitor and calcium-entry blocker may provide better blood pressure control than either drug class alone and at the same time prevent the reflex tachycardia frequently observed after nifedipine.

Enalapril treatment in a patient with impaired renal function and intolerance to captopril

In a 36-year-old woman with malignant hypertension and moderate renal insufficiency from nephrosclerosis normotension was not achieved by the combination of a beta-blocker, a vasodilator, and a loop-diuretic. The angiotensin-converting enzyme (ACE) inhibitor captopril was then added to the therapy. The blood pressure control was good. However, due to adverse reactions, captopril had to be withdrawn. Later on, the patient was successfully treated with enalapril, another ACE inhibitor, without the relapse of any adverse reactions.

Acute and subacute haemodynamic effects of enalapril. Non-invasive studies of a new converting enzyme inhibitor for antihypertensive treatment

After a run-in period on placebo, 15 patients with primary hypertension got antihypertensive treatment with enalapril in single therapy during six weeks. Before and four hours after drug administration simultaneous non-invasive recordings of ECG, phonocardiogram and carotid pulse tracing or apexcardiogram or impedance cardiogram were made. After six weeks' therapy the recordings were repeated 12 hours and four hours after dose intake. The results showed enalapril to be a potent antihypertensive agent with considerable effect already four hours after the first dose intake. The blood pressure reduction was explained by a decrease in total peripheral resistance, semiquantitatively measured with impedance cardiography. Cardiac output was significantly higher during long-term treatment than before therapy started, due to a significant increase in stroke volume. Lef ventricular (LV) diastolic function was not altered but a considerable improvement in LV systolic function was achieved as judged from the systolic time intervals. Open questioning regarding side effects indicated good tolerance.

Comparative effects of enalapril, enalaprilic acid and captopril in blocking angiotensin I-induced pressor and dipsogenic responses in spontaneously hypertensive rats

The role of central angiotensin converting enzyme (ACE), in the maintenance of high blood pressure, was examined in unanesthetized spontaneously hypertensive rats (SHR). Pressor and dipsogenic responses induced by intracerebroventricular (ICV) injections of angiotensin I (AI) were elicited before and 30 min after either captopril (120-800 nanomoles ICV), enalapril (66-460 nanomoles ICV) and enalaprilic acid (70-280 nanomoles ICV). Enalapril was 1.6 (0.7-3.9) and 1.7 (0.9-2.9) times more potent than captopril in inhibiting AI-induced pressor and dipsogenic responses, respectively. Enalaprilic acid was 2.7 (1.1-7.1) and 2.9 (1.9-4.8) times more potent than captopril in inhibiting AI- (ICV administration) induced pressor and dipsogenic responses, respectively. None of the ACE inhibitors, in contrast, reduced the central actions of AII. Basal mean arterial pressure was not reduced by these ACE inhibitors after ICV administration. Administered orally at doses which produced similar hypotensive responses, neither captopril (30 mg/kg) nor enalapril (3 mg/kg) blocked the responses induced by AI given ICV (10 ng). These findings indicate that ACE inhibitors given acutely do not penetrate into the central nervous system sufficiently to block the dipsogenic and pressor responses induced by AI given ICV, and suggest that inhibition of central ACE may not be important to the acute antihypertensive activity of the ACE inhibitors tested.

Cardioprotective effects of enalapril in acute myocardial ischemia

Enalapril, a new potent orally active angiotensin-converting enzyme inhibitor, was studied in cats subjected to acute myocardial ischemia. Enalapril, administered intravenously (2 mg/kg, plus 2 mg/kg/h) 30 min after ligation of the left coronary artery, significantly reduced the pressure-rate index, an indicator of myocardial oxygen demand. This was confirmed in isolated cat papillary muscles where enalapril reduced contractile force by 5-10%. During myocardial ischemia, enalapril reversed the elevated S-T segment of the electrocardiogram toward normal 2 h after the onset of ischemia. Moreover, enalapril significantly blunted the increases in circulating creatine kinase (CK) activity, as well as significantly prevented the loss in myocardial CK activity. These changes correlated with reduced myocardial loss of compounds containing free amino-nitrogen. Enalapril effectively acted as a converting enzyme inhibitor over the 5-hour course of the observation period. However, enalapril also acted as an angiotensin antagonist in isolated coronary arteries, a finding that may help explain its efficacy in myocardial ischemia. Enalapril did not appear to stabilize the membranes of cat liver lysosomes, and thus probably does not protect the ischemic myocardium by lysosomal stabilization.

Acute and long-term response to enalapril in congestive failure

Enalapril, a novel long acting angiotensin converting enzyme (ACE) inhibitor, was given orally to 12 patients with chronic heart failure (NYHA functional class III and IV) and cardiomegaly. The optimal dose averaged 17 mg given once-daily. Heart rate, systemic arterial blood pressure, pulmonary arterial pressure, right and left ventricular filling pressures and cardiac index were monitored during dose efficacy titration. Eleven patients were recatheterised 3 months later. After stabilisation of cardiac filling pressures, all patients had left ventricular filling pressures in excess of 20 mmHg. Enalapril increased cardiac index acutely by 34% but at 12 weeks follow-up, cardiac index was not different from control levels. Left ventricular filling pressure was reduced acutely by 36% and by 41% at 3 months. Heart rate, systemic arterial and right atrial pressures and plasma concentrations of aldosterone were reduced during the observation period. ACE activity was inhibited at the time of peak haemodynamic effect from 25.3 +/- 9.8 to 4.9 +/- 3.4 U/ml (P less than 0.01). Renin was markedly elevated. These changes were accompanied by marked and sustained clinical improvement and subjective well-being.