Author + information
- Received November 10, 2000
- Revision received August 3, 2001
- Accepted October 11, 2001
- Published online January 2, 2002.
- W.H Wilson Tang, MD∗,
- Randall H Vagelos, MD∗,
- Yin-Gail Yee, BS∗,
- Claude R Benedict, MD†,
- Kathy Willson, RN∗,
- Charles L Liss, MS‡,
- Patrice LaBelle, MD‡ and
- Michael B Fowler, MBBS, FRCP, FACC∗,* ()
- ↵*Reprint requests and correspondence:
Dr. Michael B. Fowler, Falk-CVRC 295, Stanford University Medical Center, Stanford, California 94305 USA.
Objectives We sought to compare the neurohormonal responses and clinical effects of long-term, high-dose versus low-dose enalapril in patients with chronic heart failure (CHF).
Background Examination of neurohormonal and clinical responses in patients receiving different doses of angiotensin-converting enzyme (ACE) inhibitors may provide insight into the potential for additional suppression with angiotensin II (AT-II) or aldosterone antagonists.
Methods Seventy-five patients with CHF were randomized to receive either high-dose (40 mg/day, n = 37) or low-dose (5 mg/day, n = 38) enalapril over six months. The results from exercise testing, echocardiography, tissue-specific ACE activity and monthly pre- and post-enalapril neurohormonal levels were compared.
Results Despite greater intra-group improvements in plasma renin activity and serum aldosterone levels in the high-dose group, no statistically significant differences were observed between the two groups in all variables, except for serum ACE activity at the end of study. Elevated serum aldosterone and plasma AT-II levels were observed in 35% and 85% of patients, respectively, at 34 weeks, an inter-group difference that was not statistically significant. A trend toward higher levels of tissue-specific ACE activity in the high-dose group compared with the low-dose group at the end of study was observed (p = 0.054). A predefined composite end point of clinical events had a trend toward better improvement in the high-dose group.
Conclusions This study could not demonstrate a difference between high- and low-dose enalapril in terms of serum aldosterone and plasma AT-II suppression, despite a dose-dependent reduction in serum ACE activity. Even at maximal doses of enalapril, elevated serum aldosterone and plasma AT-II levels were frequently observed.
Over the past decade, treatment with angiotensin-converting enzyme (ACE) inhibitors has improved the prognosis of patients with chronic heart failure (CHF). As the precise mechanisms of angiotensin II (AT-II) and aldosterone generation in serum and tissue become increasingly understood, it is apparent that ACE inhibitors may provide inadequate long-term suppression of the renin-angiotensin-aldosterone (RAA) system. This phenomenon, often known as “AT-II reactivation” (1,2)or “aldosterone escape” (3,4), may contribute to the persistently high morbidity and mortality rates seen in patients with CHF, despite ACE inhibitor therapy.
The dose-response characteristics of ACE inhibitors have been explored as a potential factor in the development of AT-II and aldosterone escape. Current strategies to counteract this “escape” involve maximizing the ACE inhibitor dose. However, two large-scale clinical trials have reported inconclusive results on the potential benefits of high-dose ACE inhibition as compared with lower doses (5,6). Mechanistically, few studies have carefully quantified the direct, long-term impact of ACE inhibitor dosage on the degree of neurohormonal suppression in advanced heart failure. Therefore, we performed a prospective, randomized study to describe the relationship between neurohormonal responses and the dose and timing of long-term enalapril therapy in patients with CHF.
Patients enrolled in this study had advanced heart failure of any etiology, with a clinical diagnosis of New York Heart Association (NYHA) functional class II–IV. Only those patients who had CHF (>3 months duration), with systolic dysfunction (left ventricular ejection fraction <40%), already receiving treatment with digoxin and diuretics were considered for the study. Subjects recruited into the study were either not receiving ACE inhibitors at the time of study entry or receiving ACE inhibitors at very low doses (equivalent to enalapril ≤2.5 mg/day). Patients who would have required a reduction in ACE inhibitor dose, those who had contraindications to ACE inhibitors or who experienced a myocardial infarction within the previous three months, were excluded.
This was a prospective, single-center, randomized, double-blind, parallel study comparing the neurohormonal response and effects of high- and low-dose enalapril regimens in 84 patients with advanced heart failure (Fig. 1). After providing informed, written consent, all patients had their history taken and underwent physical examination, chest X-ray, electrocardiography, radionuclide ventriculography and/or echocardiography, a chemistry panel, a complete blood count and urinalysis for enrollment. Patients enrolled in the study were started on or switched to oral enalapril, 2.5 mg/day, and titrated to a steady dose of 2.5 mg twice daily for at least two weeks, in addition to stable dosages of digoxin and diuretics. At the end of this phase (defined as the “start” at week 4), repeated laboratory studies and cardiopulmonary exercise testing were performed and plasma neurohormone levels were obtained.
Patients were then randomized into two groups. For patients randomized to high-dose enalapril, weekly dose increments of 5 mg, 7 mg, 12.5 mg, 15 mg and 20 mg twice daily of oral enalapril were given if there were no adverse effects (e.g., hypotension, lightheadedness, dizziness) at each increase in dose. For patients randomized to low-dose enalapril, oral enalapril at 2.5 mg twice daily was continued for six to eight weeks; the patients were subjected to a false upward titration. During this phase, patients were clinically evaluated weekly, with close monitoring of electrolytes and kidney function, and repeated cardiopulmonary exercise testing was performed and plasma neurohormone levels obtained at the end of the upward titration (week 10). Patients continued on their assigned enalapril doses for the next six months. Their dosing schedules were converted from twice daily to once daily. Throughout this phase, patients were clinically evaluated weekly during the six-week initial treatment phase, followed by a monthly clinical evaluation, with close monitoring of electrolytes and kidney function and measurement of serial plasma neurohormone levels. At the end of the study (defined as the “end” at week 34), a complete physical examination, follow-up laboratory tests, echocardiography and cardiopulmonary exercise testing were performed and plasma neurohormone levels were obtained.
Two-dimensional echocardiography was performed by an experienced sonographer using a Hewlett-Packard ULS machine (Hewlett-Packard Co., Medical Products Group, Palo Alto, California), at 2.5 or 3.5 MHz, combining imaging and Doppler transducer. Each patient was studied in the left lateral decubitus position after 15 min of recumbency. Cardiopulmonary exercise testing was performed in the sitting position using an electrically braked bicycle ergometer. The protocol started with a workload of 50 W and was increased in steps of 10 W; each step was maintained for 1 min. Heart rate and blood pressure were measured before and throughout the study, and standard 12-lead electrocardiograms were monitored continuously.
Plasma and serum neurohormone levels
Samples were taken at baseline (week 10), as well as during monthly follow-up visits at weeks 14, 18, 22, 26, 30 and 34 (Fig. 1). All samples were taken from an 18-gauge angiocatheter in an antecubital vein just before and 3 h after the morning dose of enalapril. The patients were instructed to hold the dose before the test and to rest in a supine position in a quiet room for 30 min after venous cannulation and before blood samples were taken. A total of 25 ml of blood was collected in pyrogen-free, vacuum, blood-collection tubes (Becton Dickinson, San Jose, California), with no additives (serum) or with EDTA as an anticoagulant (plasma) or with EDTA/glutathione plus a reducing agent (plasma). The patients took their enalapril dose after completion of the blood draw. The patients then returned in 3 h to repeat the procedure.
All tubes were immediately immersed in melting ice and centrifuged within 15 min before centrifugation at 1,000 gand 4°C for 10 min. All samples were then stored at −80°C in multiple aliquots until analysis. All samples were thawed only once. Plasma renin activity was determined with an immunoradiometric assay of the rate of angiotensin I generation by plasma renin (Rianen Assay System angiotensin iodine-125 radioimmunoassay kit, NEA-104 or NEA-105, DuPont, Boston, Massachusetts). Serum ACE activity was determined by fluorometric measurement of the rate of histidyl-leucine generation from a C-terminus cleavage of hippuryl-histidyl-leucine, a model substrate (modified from Cushman and Cheung ). Plasma AT-II levels were measured by solid-phase extraction and subsequent radioimmunoassay (AT-II iodine-125 radioimmunoassay kit, American Laboratory Products Co., Windham, New Hampshire). Serum aldosterone levels were determined by solid-phase radioimmunoassay (Coat-A-Count aldosterone kit, Diagnostic Products Corp., Los Angeles, California). Plasma norepinephrine and epinephrine levels were determined by simultaneous radioenzyme assay of radioactive O-methylated derivatives of norepinephrine and epinephrine (8). All normal values of neurohormonal levels were reported from healthy subjects in a supine position for at least 30 min.
Tissue ACE measurement
A subset of 17 patients (9 in the high-dose group and 8 in the low-dose group) underwent right ventricular endomyocardial biopsy where an assay of tissue-specific ACE activity was performed at the end of the study (week 34). Activity of ACE was determined in a grinded biopsy specimen, using the same methods as described earlier for measuring plasma ACE activity of tissue homogenates.
An intent-to-treat analysis of the two groups of patients was performed after randomization. Analysis of variance with repeated measures was used to compare inter-treatment differences in the change from baseline to the end of the study for the pre-dose, post-dose and change measurements. Paired ttests were used to compare these measurements within treatments. For the composite end point, the Fisher exact test was used to compare treatments. Differences in baseline characteristics were evaluated by the chi-square test (categorical variables) or the unpaired ttest (continuous variables). A p value <0.05 was considered statistically significant.
Eighty-six patients were enrolled in the study, and 75 patients remained in the study after the open-label baseline treatment phase at the time of randomization (Table 1). Thirty-eight patients were randomized to the low-dose group, and 37 patients were randomized to the high-dose group. At the end of the study (week 34), 24 patients in the low-dose group and 26 patients in the high-dose group remained in the study for data analysis. As shown in Figure 1, there was no statistically significant difference in the rates of drug discontinuation or adverse events between the two groups.
Plasma/serum and tissue neurohormones
Pre-dose (“trough” drug levels) and post-dose (“peak” drug levels) responses of plasma and serum neurohormones after six months of treatment with low-dose versus high-dose enalapril are summarized in Table 2. Baseline (week 4) neurohormonal levels were similar between the two groups before and after oral administration of enalapril. After 34 weeks of once-daily, oral enalapril therapy, only serum ACE activity demonstrated statistically significant reductions (both pre- and post-dose levels) between the low- and high-dose groups (Fig. 2). Although there was adequate neurohormonal suppression after enalapril therapy (as indicated by a large proportion of patients with serum ACE activity in the normal control ranges), there was no statistically significant difference in the level of post-dose plasma AT-II (Fig. 3) or serum aldosterone (Fig. 4) levels between the two groups. Similar findings were observed in the pre-dose analyses, as well as in neurohormonal levels drawn at weeks 14, 18, 22, 26 and 30. In addition, a trend toward higher levels of “tissue” ACE activity in the high-dose versus low-dose group at the end of study was observed in a subgroup of 17 patients (0.498 ± 0.27 vs. 0.330 ± 0.1 nM/ml per m, p = 0.054).
Post-dose serum ACE activity levels were lower than pre-dose levels in both groups. Only in the high-dose group was there a significant fall in pre-dose serum ACE activity at the end of the study. Similarly, a statistically significant decrease in post-dose serum aldosterone levels (Fig. 4) and increase in post-dose plasma renin levels (Fig. 5) were seen only in the high-dose group between baseline and end of the study. In both groups, at the end of the study, post-dose serum aldosterone and AT-II levels were lower than the pre-dose levels, whereas post-dose plasma renin activity was higher than the pre-dose levels. There were no statistically significant differences between plasma norepinephrine and epinephrine levels in both the intra-group and inter-group comparisons (Table 2). No correlation was observed between serum AT-II and aldosterone levels and serum potassium levels throughout the study.
Clinical and functional outcomes
Overall, there was no statistically significant difference between the two groups in terms of changes in NYHA class and cardiopulmonary exercise test results (Table 3). There were nearly twice the number of predetermined patient events (e.g., emergency room visits, hospital admissions, deaths, sustained increase in diuretics dosage) in the low-dose group as compared with the high-dose group, but this finding did not reach statistical significance (p = 0.061).
Echocardiographic data analysis
Over six months of enalapril therapy, there was a trend toward a greater reduction in left ventricular end-diastolic dimensions in the high-dose group, but no significant changes were seen in the low-dose group (p=0.08) (Table 4).
Neurohormonal modulation by high- versus low-dose enalapril
The ability of ACE inhibitors to suppress neurohormonal activation is considered to be the principal mechanism of their therapeutic benefits in patients with CHF. This is the first report of a rigorously performed, serial, pre-dose and post-dose measurement study of plasma neurohormones after long-term enalapril therapy in patients with CHF. As shown in previous studies (9–11), a dose-dependent relationship in the suppression of serum ACE activity was observed. The lack of catecholamine suppression was also consistent with previous studies (9,12,13), although a reduction in plasma norepinephrine levels had previously been observed at high-dose ACE inhibition (14). The finding of a lack of suppression in pre-dose serum ACE activity emphasized two important aspects of measuring plasma neurohormones in relation to drug therapy. First, this shows the importance of meticulous control of the timing of neurohormonal sampling in defining an accurate observation; despite improvement seen in some post-dose measurements, pre-dose measurements could be unaffected. Second, the relatively small differences in pre-dose neurohormone levels between the two groups and the inadequate long-term suppression of aldosterone and AT-II, despite high-dose enalapril therapy, may provide an explanation for the relatively minor impact of enalapril dosing on clinical variables observed in our study, as well as in other major studies (5,6).
Like most previous neurohormonal studies (10–12,15,16), we found a wide variation of neurohormonal levels both between different patients and between different time intervals. It has been well known that collection times, activity levels and presumably clinical stability have major influences on subsequent neurohormonal levels. However, even in stable patients with heart failure, Masson et al. (17)observed coefficients of variations in neurohormone measurements of up to 30%.
Previous clinical studies used a twice daily dosing of enalapril (5,12,14,18). Once daily dosing of enalapril has been examined in small clinical studies and has been suggested to be as effective as twice-daily dosing (19–21). Mechanistic studies also showed that neurohormone levels after once-daily enalapril dosing remain markedly suppressed after 24 h (1,22), suggesting that once-daily dosing has the potential to provide an effective 24-h impact on neurohormone levels. Using a once-daily regimen, our study demonstrated a statistically significant reduction in serum ACE activity within the high-dose group after six months of enalapril therapy, even at pre-dose (trough) levels. A 3-h waiting interval was chosen for logistical reasons; however, with the addition of the 30-min supine rest period before the blood draw (total 3.5 h), this was close to the peak drug effects seen 4 h after taking oral enalapril (23).
Suppression of tissue-specific ACE activity has been suggested as a potential mechanism of reverse remodeling by ACE inhibitors and has been observed in animal studies (24). Surprisingly, a trend toward higher myocardial tissue-specific ACE activity in the setting of lower serum ACE activity was observed in the high-dose group as compared with the low-dose group. Although these data were limited by the small sample size, a discrepancy may exist between ACE activity measured at the tissue level and that at the serum level. The significance of this finding is unclear and needs further validation. The inability to suppress tissue-specific ACE activity by enalapril may provide an additional mechanism to explain the persistently elevated serum aldosterone and plasma AT-II levels, despite long-term enalapril therapy. The observed influence of ACE inhibitors (especially peak effects at the high dose) on increasing circulating plasma renin activity may also reduce the influence of ACE inhibitors on tissue components of the RAA system. Further neurohormonal and clinical comparison studies are needed to determine the significance of tissue-specific ACE activity and to see whether tissue-specific ACE inhibitors (such as ramipril and quinipril) would have a larger impact on tissue-specific ACE suppression.
Aldosterone escape and AT-II reactivation
In both the high- and low-dose groups, a significant degree of AT-II reactivation and aldosterone escape was observed in the pre-dose measurements at the end of the study. In an open-label observation series of 81 patients, MacFadyen et al. (25)found that post-dose AT-II reactivation occurred in 15% and failure of aldosterone suppression occurred in 38% of patients with heart failure treated with ACE inhibitors for no less than four months. Under similar circumstances, we determined a similar rate of aldosterone escape (35%) in our study group (39% in the low-dose group vs. 30% in the high-dose group, p = NS by the chi-square test), whereas significantly more patients (85%) demonstrated AT-II reactivation (88% in the low-dose group vs. 81% in the high-dose group, p = NS by the chi-square test) (Fig. 6). The cut-off values of neurohormone levels were determined from samples of normal control subjects and were similar to those reported in previous studies (25). Our patient group had a longer period of serum ACE inhibition (≥6 months) and possibly more advanced disease, which may explain the high percentage of AT-II reactivation and aldosterone escape, even in the high-dose group. The relative lack of plasma AT-II and the suppression of serum aldosterone in both groups suggest that possible alternative pathways for AT-II and aldosterone production have clinical relevance in CHF. Our data provide a rationale for the results of the Randomized ALdactone Evaluation Study (RALES) (26), owing to the high proportion of patients who showed a pre-dose elevation of serum aldosterone levels. These findings also provide support for the theoretical role of combination therapy of ACE inhibitors with specific angiotensin receptor blockers (ARBs), and this combination therapy was recently supported by the clinical benefits seen in the Valsartan in Heart Failure Trial (Val-HeFT) in patients randomized to receive the addition of an ARB, although a mortality benefit was not observed in the combined ACE inhibitor/ARB group (27). The upcoming Valsartan in Acute Myocardial Infarction Trial (VALIANT) should provide further insights into the clinical impact of using combinational neurohormonal suppression strategies (28).
Clinical effects of high- versus low-dose enalapril
Enalapril was well tolerated, even at the 40-mg once-daily dose, as compared with the 5-mg once-daily dose. In fact, there were more reported adverse events and deaths (requiring withdrawal from the trial) in the low-dose group than in the high-dose group. As seen in previous studies, a large proportion of patients with advanced CHF could receive up to very high doses of enalapril without significantly more adverse effects (29,30).
We did observe a trend toward improvement of ventricular remodeling, although our study may have been underpowered to detect a dose-dependent effect on ventricular structural and functional changes after enalapril therapy. The predefined composite clinical end point favored the group randomized to the high dose and was very close to statistical significance (p = 0.06). In the light of the conflicting results from previous ACE inhibitor dosing studies (5,6,18), equivalence in some clinical variables between the high-dose and low-dose groups may not be too surprising. However, as long-term follow-up of the Assessment of Treatment with Lisinopril And Survival (ATLAS) study has already indicated a statistically significant reduction in combined mortality and hospital admission rates, when comparing high-dose with low-dose lisinopril (6), the notion that high-dose ACE inhibition is superior to low-dose therapy is likely to be valid, even though the degree of benefit may be uncertain.
Our study was limited by a large withdrawal rate within a six-month period, which may be inevitable in patients with advanced heart failure, and may be underpowered to detect statistically significant differences in neurohormone levels. Despite a total of 22 patients who withdrew during the maintenance phase, randomization to higher drug dosing of enalapril tended to be associated with fewer withdrawals due to adverse events. The wide variability in neurohormonal measurements may have limited our abilities to detect other statistically significant differences within and between the two dosing groups. Also once-daily dosing of enalapril may not provide adequate neurohormonal suppression, as previously expected. Patients in this study were not taking other neurohormonal drugs, such as beta-blockers and spironolactone. Our results might not be observed with different dosing regimens or with other ACE inhibitor drugs with different pharmacologic properties and different degrees of tissue affinity. Nevertheless, our results confirmed the observation that in this group of patients with advanced heart failure, high concentrations of plasma AT-II and serum aldosterone levels frequently exist, despite high-dose ACE inhibition.
This study could not demonstrate a difference between high-dose and low-dose enalapril in terms of serum aldosterone and plasma AT-II suppression, despite a dose-dependent reduction in serum ACE activity. Trends toward improved composite clinical end points and improved cardiac structure and function were observed in the high-dose group. Similar improvements in heart failure symptoms, exercise capacity and adverse effects were seen in both groups. Persistently elevated pre-dose serum ACE activity and high degrees of aldosterone escape and AT-II reactivation were observed in both groups, suggesting the limitations of only once-daily enalapril therapy at completely suppressing these neurohormones, even at maximal doses.
☆ This study was supported by Merck Sharp and Dohme AS, West Point, Pennsylvania.
- angiotensin-converting enzyme
- angiotensin II
- angiotensin II receptor blocker
- chronic heart failure
- New York Heart Association
- Received November 10, 2000.
- Revision received August 3, 2001.
- Accepted October 11, 2001.
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