J C/in Pharmacol. 1983; 23:227-233.

ComparativeAntibypertensiveEffectsof EnalaprilMaleateandHydrochiorothiazide,Aloneandin Combination

PETER H. VLASSES, Pharm.D., HESCHI H. ROTMENSCH, M.D.,BRIAN N. SWANSON, Ph.D., JOHN D. IRVIN, M.D., Ph.D., ROBYN B. LEE, M.S.,

JANICE R. KOPLIN, R.N., and ROGER K. FERGUSON, M.D.Philadelphia, Penn., and West Point, Penn.

Abstract: Enalapril maleate isan investigational oral prodrug whose hydrolyzed diacidmetabolite is a potent angiotensin-converting enzyme inhibitor. Fourteen patientswith mild to moderate hypertension were evaluated after receiving placebo, and twoweeks of treatment with each of the following: enalapril maleate (20 mg b.i.d.), hy-drochlorothiazide (25 mg b.i.d.), and the two in combination. In comparison toplacebo, the magnitudes of the blood pressure reduction after enalapril and hydro-

chlorothiazide alone were comparable. The reduction in blood pressure following enala-pril was evident throughout the 12-hour dosing interval. The combination of enalapril

and hydrochlorothiazide resulted in a marked further reduction in blood pressure thatwas greater than that predicted from the responses to the individual drugs (P <0.05).Biochemical parameters confirmed inhibition of angiotensmn-converting enzyme dur-

ing enalapril treatment: serum angiotensmn-converting enzyme activity proved anexcellent monitor of compliance. Enalapril was generally well tolerated. Adverseeffects included symptomatic hypotension in three patients when enalapril was firstadded to hydrochlorothiazide and hyperesthesia of the oral mucosa without a loss oftaste in one patient on enalapril. Enalapril maleate alone and especially in combinationwith hydrochlorothiazide appears to be an effective, well-tolerated converting enzyme

inhibitor with at least a 12-hour duration of action.

NALAPRIL maleate, N-(1-S-1-carbeth- chemical structure of captopril may ac-

.J__4 oxy-3-phenylpropyl)-S-alanyl-S-pro- count for some of its adverse effects such as

line (MK-421), is an investigational oral skin rash, loss of taste, proteinuria, and

prodrug whose hydrolyzed diacid form is a leukopenia. Enalapril, which lacks this moi-

potent angiotensin-converting enzyme ety, may have a more favorable benefit-to-

(ACE) inhibitor. The efficacy of this new risk ratio.

class of agents in the treatment of hyper- In a previous pilot study,2 we compared

tension and congestive heart failure has the antihypertensive and humoral effects of

been widely documented with use of capto- 5-and 10-mg once-daily doses of enalapril ma-

pril, the first marketed oral ACE inhibitor.’ leate with those of hydrochlorothiazide in

However, the sulfhydryl moiety of the small groups of patients. The purpose of the

From the Division of Clinical Pharmacology, Depart- present study was to evaluate the effects ofment of Medicine, Jefferson Medical College, Thomas a larger dose of enalapril maleate (20 mgJefferson University, Philadelphia, Penn., and Merck .

Sharp and Dohme Research Laboratories, West Point, b:1.d.), hydrochiorothiazide, and their com-Penn. (Dr. Irvin and Ms. Lee). bination.

May-June 1983 227

VLASSES, ROTMENSCH. SWANSON, ETAL.

228

Methods

Patients

Fourteen patients with mild to moderate

hypertension (untreated, sitting diastolic

blood pressure between 95 and 120 mm Hg)

entered and completed the study. The pa-

tients (12 men, two women, aged 35 to 69

years) were judged to have essential hyper-

tension on the basis of a thorough history,

physical examination and laboratory eval-

uation. No patient had severe end-organ

involvement. No other medications were

consumed during the study period. The pa-

tients were instructed to follow a daily 100

mEq sodium, 80 mEq potassium diet which

they were asked to maintain throughout

the study.

Experimental Design

After at least a two-week no-treatment

washout period, all patients were admitted

to the Clinical Pharmacology Unit and re-

ceived a single dose of placebo. Supine and

seated blood pressures and pulse rates were

measured at hourly intervals for 12 hours

by an automatic device using the Doppler

principle (DINAMAP). Blood was collected

from a forearm intravenous catheter for de-

termination of serum electrolytes, plasma

renin activity (PRA), serum ACE activity,

and plasma aldosterone concentration

(PAC) before and at 4,12, and 24 hours after

placebo administration. Urine was col-

lected for 24 hours for electrolyte and pro-

tein determinations.

In the previous pilot study,2 the treat-

ments were allocated in a double-blind,

randomized crossover manner. In this

study, a single-blind fixed-sequence, con-

secutive treatment design was employed to

allow for a careful assessment of the acute

and chronic responses of enalapril maleate

as well as an evaluation of the safety of

instituting enalapril therapy in a patient

previously treated with hydrochlorothia-

zide. Thus, subsequent to the baseline

placebo evaluation, each patient received

consecutively 14 days of each of the follow-

ing treatments: 20 mg enalapril maleate

twice daily, 25 mg hydrochlorothiazide

twice daily, and the combination of en ala-

pril and hydrochlorothiazide in these doses.

All drug supplies were provided by Merck

Sharp & Dohme Research Laboratories.

The patients were admitted to the Unit on

days 1 and 14 of each treatment, and study

procedures were performed exactly as on

the placebo day. Patients were carefully

evaluated for side effects. Medication diar-

ies, pill counts, and ACE determinations

(on enalapril treatments) were used to

assess outpatient compliance.

Laboratory Analysis

PRA, at pH 7.4, and PAC were measured

by radioimmunoassay methods as pre-

viously described.34 ACE activity was meas-

ured by a modified radioenzymatic method

which assesses the percentage of hippuryl-glycylglycine substrate metabolized per

minute per milliliter of serum.5 In our

laboratory, normal values (mean ± S.D.) for

PRA, PAC, and ACE are 1.6 ± 0.4 ng/ml/hr,

11 ± 4 ng/dl, and 5.4 ± 0.3% metabolized!

min/ml, respectively.

Statistical Analysis

The mean seated diastolic blood pressure

(SDBP) for the group was calculated at the

various time points. In addition, the overall

treatment effect for the group was charac-

terized by the mean of the average SDBP

over 12 hours for each patient (1-12-hour

SDBP). Mean seated pulse rate was eval-

uated in a similar manner. The 1-12-hour

time period was selected to minimize the

effect of variability in the 0-hour (predrug)

values which were determined upon an in-

dividual’s arrival to the Clinical Pharma-

cology Unit; this 1-12-hour time period al-

lows a more precise comparison of drug

effects versus placebo response under com-

parable conditions.

Analysis of variance using a general lin-

ear models approach was used to test for

treatment effects and interactions after

The Journal of Clinical Pharmacology

0SO 105

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0.

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‘I’

z

85

75

PLACEBO

HCTZ

ENALAPRIL

ENA/HCTZ

0 4 8 10 12

TIME (hours)

ENALAPRIL ANTIHYPERTENSIVE EFFECTS

May-June 1983 229

removing subject effects. The presence of a

significant interaction indicates that the

evaluated effect during combination treat-

ment differs from the additive sum of the

effects of the individual treatments. Dun-

can’s multiple range test was used for com-

parison of individual treatments. All tests

were two-tailed and performed at the a =

0.05 level of significance.

Results

Blood Pressure and Pulse Rate

The time course of the mean SDBP for the

group after placebo and on the last day (day

14) of each treatment is depicted in Fig. 1.In

comparison to placebo, both enalapril ma-

leate and hydrochlorothiazide alone de-

creased mean SDBP significantly at most

time points. Mean (±S.D.) 1-12-hour SDBP

was 101 ± 5 mm Hg on placebo, 98 ± 8mm

Hg on enalapril, and 97 ± 8 mm Hg on

hydrochlorothiazide. The enalapril and

hydrochlorothiazide responses, though

small, differed significantly (P< 0.05) from

placebo but not from each other. Reduction

in mean SDBP with enalapril was evident

at 12 hours after dosing, and the days 1 and

14 responses were not significantly dif-

ferent.

In contrast, the mean SDBP on the com-

bination of enalapril and hydrochlorothia-

zide was significantly (P< 0.01) lower than

those on all other treatments at all time

points and persisted for 12 hours (Fig. 1).

The mean SDBP response on day 1 of com-

bination therapy (when enalapril was in-

itially added to hydrochlorothiazide) was

82± 7 mm Hg, which was not significantly

different from that on day 14 of this treat-

ment. Mean (±S.D.) 1-12-hour SDBP on day

14 was 85 ± 8 mm Hg, which was signifi-

cantly (P< 0.05) lower than those of all other

treatment. Blood pressure control (individ-

ual average 1-12-hour SDBP �90 mm Hg)

was achieved in two patients on enalapril,

in one on hydrochlorothiazide, and in 11

patients on the combination. Statistical

1 1 5-

ANTIHYPERTENSIVE EFFECTS OF ENALAPRIL,

HCTZ AND THEIR COMBINATION (n=14)

e

Fig. 1. Time course of the group (N = 14) mean seated diastolic blood

pressure (SDBP) after placebo (0 - - - 0) and on day 14 of treatment with

enalapril maleate (o-o), hydrochlorothiazide (s-s), and their combination(.-e) (refer to text for dosage).

8.

ENC

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VLASSES, ROTMENSCH, SWANSON, ETAL.

230

BIOCHEMICAL EFFECTS OF ENALAPRIL,

HCTZ AND THEIR COMBINATION (n14)

TIME (hours)

Different from Placebo(s), Enalapril(t), All Treatments(#{149}) p’(0.05

Fig. 2. Changes in the group (N = 14) mean serum angiotensin-converting

enzyme (ACE) activity, plasma aldosterone concentration (PAC), and

plasma renin activity (PRA) at various times after treatment with placebo

(0 - - - 0) and on day 14 of enalapril maleate (0-0), hydrochlorothiazide

(s-.), and their combination (,-e) (refer to text for dosage).

analysis revealed a positive interaction (P

<0.01) between enalapril and hydrochioro-

thiazide when used in combination, indi-

cating an enhancement of the effect by the

combination. Similar treatment effects were

observed for mean supine diastolic as well

as mean sitting and supine systolic blood

pressures.

Mean (±S.D.) 1-12-hour seated pulse rate

(beats!min) was 78±9 on placebo and was

unchanged after enalapril either on day 1

(78 ± 10) orday 14(80 ± 12) of treatment. On

day 14 of hydrochiorothiazide, mean sit-

ting pulse rate was 84 ± 14, which was sig-

nificantly different from values on placebo.

On day 1 on the combination, though blood

pressure decreased markedly, no change in

pulse rate was evident (87 ± 14) in compari-

son to values on hydrochlorothiazide the

previous day. Interestingly, on day 14 of the

combination, though blood pressure re-

mained markedly decreased, the mean

pulse rate (80 ± 12) actually decreased in

relation to the value on hydrochlorothia-

zide and did not differ significantly from

placebo values.

Humoral and Electrolyte Effects

Mean ACE, PRA, and PAC before and at

various times after dosing on the last day of

each treatment are depicted in Fig. 2. Sig-

nificant marked decreases in mean ACE

(Fig. 2, left panel) at all time points in com-

parison to placebo values were seen on day

14 of treatment with both enalapril alone

and in combination with hydrochiorothia-

zide (P < 0.01). Mean ACE values during

hydrochlorothiazide treatment were com-

parable to those after placebo. In compari-

son to placebo, enalapril alone tended to

decrease mean PAC, while hydrochlorothi-

azide alone significantly increased this pa-

rameter (Fig. 2, middle panel). Mean PAC

on the combination returned toward, though

did not differ significantly from, placebo

values. Although increases in mean PRA

(Fig. 2, right panel) from placebo values

The Journal of Clinical Pharmacology

ENALAPRIL ANTIHYPERTENSIVE EFFECTS

May-June 1983 231

were seen with all treatments, only those

after combination therapy achieved statis-

tical significance. Humoral responses on

day 1 of treatment with enalapril, alone or

with hydrochiorothiazide, were similar to

those on day 14 of each treatment. No corre-

lation existed between the pretreatment

PRA and the change from placebo in 1-12-

hour SDBP after enalapril alone.

Mean serum electrolyte, uric acid, and

creatinine values on the morning of the last

day of each treatment are depicted in Table

I. In comparison to enalapril, hydrochioro-

thiazide resulted in decreases in serum pot-

assium and chloride concentrations and an

increase in serum uric acid concentrations.

The addition of enalapril to hydrochioro-

thiazide tended to reverse the changes in

serum potassium and chloride.

Adverse Effects

When enalapril was added to hydrochioro-

thiazide, three of the 14 patients ex-

perienced marked reductions in SDBP ac-

companied by dizziness. In one patient, the

symptoms resolved over the first 24 hours.

In the other two patients, because symp-

toms persisted, the daily dosage was de-

creased (20 mg enalapril and 25 mg hy-

drochlorothiazide) with subsequent im-

provement. The study data (Fig. 1) include

the responses of these patients at the lower

dosage, the character of which was similar

to that of other patients. Whether the dos-

age reduction in fact contributed to the im-

proved tolerance is unclear. One patient

experienced hyperesthesia (“scalded

mouth”) of the oral mucosa during treat-

ment with enalapril, both alone and in

combination with hydrochiorothiazide,

which resolved with discontinuation of

treatment.6 Two patients complained of de-

creased libido on all active treatments. No

skin rash, leukopenia, or proteinuria was

noted.

Discussion

In these patients, two weeks of treatment

with enalapril maleate or hydrochlorothia-

zide alone lowered SDBP comparably in

comparison to placebo. The mean reduc-

tions in SDBP from placebo values, though

significant, were modest. These responses,

especially with hydrochlorothiazide, may

be related to recruitment of an inherently

more resistant group of patients, the dura-

tion of treatment, or to the fact that treat-

ment effects were compared to the placebo

response (Fig. 1) rather than to the un-

treated SDBP. The mean blood pressure re-

sponses on days 1 and 14 of enalapril were

comparable, and the reduction in blood

pressure was evident throughout the 12-

hour dosing interval. In comparison to our

TABLE I

Mean Serum Electrolyte, Uric Acid, and Creatinine Concentrationson the Last Day of Each Treatments

TreatmentSodium

(mEq/liter)Potassium(mEq/liter)

Chloride(mEq/liter)

Uric acid(mg/dl)

Creatinine(mg/dl)

Placebo 144 3.9 109 5.5 1.1

Enalapril (20mg b.i.d.) 142 4.0 108 6.3 1.0

Hydrochlorothiazide

(25mg b.i.d.) 143 3.7” 103bb.,8...bb

Combination 143 3.9 106b 765*.bb

* a,aa = Different from enalapril, P< 0.05 and 0.01, respectively; b,bb = different from placebo, P <0.05 and 0.01,

respectively; c = different from combination, P < 0.05.

VLASSES, ROTMENSCH, SWANSON, ETAL.

232 The Journal of Clinical Pharmacology

experience with captopril at similar doses,

enalapril had a slower onset and a longer

duration of action.

Enalapril alone significantly lowered ACE

and tended to decrease PAC and increase

PRA. Measurement of serum ACE was an

excellent predictor of compliance with enal-

april treatment, either alone or in combi-

nation with hydrochlorothiazide. This

differs from captopril which chemically de-

grades rapidly after sample collection,

making measurement and interpretation of

serum ACE values difficult.9 Though the

biochemical effects observed are consistent

with angiotensin-converting enzyme inhi-

bition, the SDBP response to enalapril

alone was not related to the pretreatment

PRA. In combination, enalapril tended to

lower the increased PAC observed after hy-

drochlorothiazide alone. Interestingly, the

response to the enalapril!hydrochlorothia-

zide combination was of a greater magni-

tude than that predicted statistically from

the individual responses.

Theoretically, in the fixed-sequence, con-

secutive treatment design employed in this

study, the duration of treatment could have

contributed to an enhanced antihyperten-

sive response in the last treatment period.

Likewise, if the SDBP response to hydro-

chlorothiazide alone was not maximal after

two weeks of treatment, then the subse-

quent response to the combination with

enalapril may have appeared more than

additive. However, these factors do not

adequately explain the enhanced SDBP re-

sponse to enalapril/hydrochiorothiazide as

the blood pressure response was at least as

good on the first as on the last day of treat-

ment with the combination. Furthermore,

an enhanced effect of the combination of

enalapril and hydrochiorothiazide was

suggested in our previous double-blind,

randomized order of treatment pilot study

with lower doses of enalapril (5 and 10 mg

daily)2 indicating that the fixed-treatment

sequence of the present study is not the rea-

son for the enhanced effect.

One possible explanation for the enhanced

effect of the combination was that after in-

itial treatment with hydrochlorothiazide,

the blood pressure of these patients was

more dependent on the renin-angioten-

sin-aldosterone axis, as evidenced by the

increases in PRA and PAC. Thus, inhibi-

tion of this system with enalapril during

combination treatment might account for

the additional antihypertensive effect. It is

interesting to note, however, that the anti-

hypertensive effects of the combination of

captopril and hydrochiorothiazide, in our

previous experience7 and consistent with

the findings of others8’9 are additive and do

not suggest the enhanced response evident

for the combination of enalapril and hy-

drochiorothiazide in the present study. The

reason for this apparent discrepancy is un-

clear and warrants further investigation.

Several interesting differences were noted

between the effects of enalapril maleate

and hydrochlorothiazide. In comparison to

placebo values, sitting pulse rate increased

after hydrochiorothiazide, as commonly

seen after diuretic treatment, but was un-

changed after enalapril. More importantly,

when enalapril was added to hydrochloro-

thiazide, though blood pressure fell

markedly, heart rate did not increase

further. In fact, by day 14 of the combina-

tion treatment, although the blood pressure

reduction was maintained, heart rate had

actually decreased toward placebo values.

Whether this decrease in heart rate would

have accompanied the continued treatment

with hydrochiorothiazide or was caused by

enalapril is unclear. Of note, however, is

that the lack of effect of captopril on pulse

rate has been attributed in part to para-

sympathomimetic and/or sympatholytic

effects of this agent.1”#{176}

As expected, hydrochlorothiazide de-

creased serum potassium and chloride con-

centrations, presumably due in part to the

concomitant increase in serum aldosterone

concentrations. Enalapril alone had no ef-

fect on serum potassium and, in combina-

tion with hydrochlorothiazide, tended to

correct the electrolyte changes while

ENALAPRIL ANTIHYPERTENSIVE EFFECTS

May-June 1983 233

decreasing plasma aldosterone concentra-

tions. Hydrochlorothiazide, but not enala-

pril, increased serum uric acid concen-

trations.

During this short-term study, enalapril

maleate and hydrochiorothiazide were gen-

erally well tolerated; no skin rash, leukope-

nia, or proteinuria occurred. When enalapril

was added to hydrochlorothiazide, marked

symptomatic falls in blood pressure oc-

curred in three patients. Because of this,

careful monitoring is indicated when enal-

april is added to the regimen of diuretic

treated patients. However, in our previous

pilot study, when hydrochlorothiazide was

added to enalapril, a comparable SDBP re-

sponse to the combination resulted after

chronic treatment but the reduction was

more gradual and asymptomatic.2 Revers-

ible hyperesthesia of the oral mucosa oc-

curred in one patient during enalapril

treatment; this side effect has also been de-

scribed in captopril-treated patients in our

unit.2

References

1. Vlasses PH, Ferguson RK, Chatterjee

K. Captopril: clinical pharmacology

and benefit-to-risk ratio in hyperten-

sion and congestive heart failure.

Pharmacotherapy. 1982; 2:1-17.

2. Ferguson RK, Vlasses PH, Irvin JD,

Swanson BN, Lee RB. A comparative

pilot study of enalapril, a new con-

verting enzyme inhibitor, and hy-

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22:281-289.

3. Haber E, Koerner D, Page LB, Kliman

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493-495.

9. Swartz SL, Williams GH, Hollenberg

NK, Crantz FR, Moore TJ, Levine L,

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10. Imai Y, Abe K, Sato M, Haruyama T,

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Requests for reprints to: Peter H. Vlasses, Pharm.D.,

Clinical Pharmacology (M-502), Thomas Jefferson

University, 11th and Walnut Streets, Philadelphia,Penn. 19107.