Nebivolol Effects on Nitric Oxide Levels, Blood Pressure, and Renal Function in Kidney Transplant Patients

Abstract

In hypertensive kidney transplant recipients, the effects of nebivolol vs metoprolol on nitric oxide (NO) blood level, estimated glomerular filtration rate (eGFR), and blood pressure (BP) have not been previously reported. In a 12‐month prospective, randomized, open‐label, active‐comparator trial, hypertensive kidney transplant recipients were treated with nebivolol (n=15) or metoprolol (n=15). Twenty‐nine patients (nebivolol [n=14], metoprolol [n=15]) completed the trial. The primary endpoint was change in blood NO level after 12 months of treatment. Secondary endpoints were changes in eGFR, BP, and number of antihypertensive drug classes used. After 12 months of treatment, least squares mean change in plasma NO level in the nebivolol kidney transplant recipient group younger than 50 years was higher by 68.19% (99.17% confidence interval [CI], 13.02–123.36), 69.54% (99.17% CI, 12.71–126.37), and 66.80% (99.17% CI, 12.95–120.64) compared with the metoprolol group younger than 50 years, the metoprolol group 50 years and older, and the nebivolol group 50 years and older, respectively. The baseline to month 12 change in mean arterial BP, eGFR, and number of antihypertensive drug classes used was not significantly different between the treatment groups. In hypertensive kidney transplant recipients, nebivolol use in patients younger than 50 years increased blood NO.

The success of modern immunosuppression in improving short‐term graft outcome after kidney transplantation primarily through reduction of acute rejection rates has not been matched with as much progress in prolonging long‐term graft or patient survival.1, 2 The most common cause of kidney transplant loss is death with a functioning graft primarily caused by cardiovascular diseases.3, 4 Therefore, in conjunction with prevention of rejection through immunosuppression, risk reduction for and treatment of cardiovascular diseases is paramount in ensuring prolonged overall graft survival.

Hypertension occurs in up to 80% to 90% of kidney transplant recipients and is identified as a significant risk factor for allograft vasculopathy, peripheral artery disease, and decreased survival and allograft.4, 5, 6, 7, 8, 9, 10, 11 Endothelial dysfunction with deficiency of nitric oxide (NO) is a common pathophysiologic mechanism underlying aging, atherosclerosis, diabetes mellitus, renal disease, vascular disease, and hypertension.12, 13, 14, 15 The progression of chronic kidney disease has been associated with NO deficiency.16 After successful kidney transplantation, endothelium‐dependent vasorelaxation increases with the reversal of the NO deficiency that characterizes end‐stage renal disease.12, 13 Unfortunately, calcineurin inhibitors (CNIs), the cornerstone of current antirejection immunosuppression, can negate these beneficial effects through various mechanisms including elaboration of vasoconstrictor cytokines and reduction in vasodilator prostaglandins and NO.3, 5, 17, 18, 19, 20 CNIs are also implicated in the pathogenesis of post‐transplant hypertension through activation of the sympathetic nervous and renin‐angiotensin‐aldosterone systems, intense afferent arteriolar vasoconstriction, and sodium retention.21, 22, 23

In the absence of a robust, evidence‐based guideline on the management of post‐transplant hypertension,11, 24 clinicians have a wide latitude in selecting drugs for treating hypertension in transplant recipients. Thus, the recognition that a certain agent can control blood pressure (BP) and concomitantly improve vascular endothelial NO generation would make it the agent of choice for renal transplantation, once its benefits in improving graft and patient outcomes are proven.

Nebivolol, a third‐generation β‐blocker with BP‐lowering effects comparable with other β‐blockers, has been effective in treating hypertension, preventing vascular thrombosis, decreasing endothelin secretion, and increasing NO formation in human endothelial cells.25, 26, 27 Animal studies have demonstrated that nebivolol can cause a dose‐dependent reduction in renal perfusion pressure and increase NO release with vasodilation of the renal vasculature.28, 29 Models of partial renal ablation have shown that nebivolol decreases levels of collagen type 1 expression resulting in reduced glomerular and interstitial fibrosis.30

We sought to answer the question of whether nebivolol's benefits in treating hypertension and improving NO levels shown in previous studies would also apply in renal transplant patients given that most of them are taking a CNI‐containing immunosuppression regimen.31 We also sought to determine whether exposure to high or low tacrolimus (FK‐506) levels has different effects on the change in plasma NO level of the treatment groups. For the purpose of this study, we used 10 ng/mL, the midpoint of the reference range for FK‐506 (5–15 ng/mL) in categorizing exposure to tacrolimus; although, in clinical practice, a target FK‐506 level is determined based on certain factors such as age of the transplant.32

Previous studies have shown that the age‐related decline in NO level is evident by the third decade of life and progressively worsens from the fourth through the sixth decades of life.16, 33, 34 The Scientific Registry of Transplant Recipients report in 2009 (the year preceding recruitment of patients for the study), showed that the ages of adult kidney transplant recipients in the United States were close to being evenly distributed between two ranges: 18–49 years and 50+ years.35 Thus, we applied these ranges in categorizing age and using it as a predictor variable on the relevant linear models of the study.

Within the setting of our university‐based kidney transplant clinic, we conducted this randomized open‐label trial comparing the effects of nebivolol and a more commonly used β‐blocker, metoprolol, on the plasma NO level, BP, and renal function of recent kidney transplant recipients who were being followed regularly according to our center's post‐transplant care protocol. After 12 months of treatment, we did not find a significant difference in the effects of nebivolol and metoprolol on BP and renal function in kidney transplant recipients. However, we found that nebivolol, but not metoprolol, increased NO levels in kidney transplant recipients younger than 50 years and nebivolol combined with low to moderate trough levels of tacrolimus significantly increased NO plasma levels. Our findings have not been previously reported, and large randomized trials will be needed to confirm our conclusions and determine the clinical implications of increased NO from nebivolol use in kidney transplant recipients.

Patients and Methods

This was a prospective randomized open‐label comparator study. Hypertensive kidney transplant patients were randomly assigned to receive oral treatment with either nebivolol or metoprolol. The trial, which consisted of 14 study visits over a 12‐month period, was undertaken at the University of Florida and Shands Hospital Renal Transplant Clinic. The study protocol was approved by the Western Institutional Review Board (Olympia, WA). The trial is registered at www.clinicaltrials.gov (NCT01157234). The primary endpoint of the study was change in plasma NO levels. Secondary endpoints were changes in renal function based on the Modification of Diet of Renal Disease (MDRD) estimated glomerular filtration rate (eGFR)36; systolic BP (SBP), diastolic DP (DBP), and mean arterial pressure (MAP); quantity of BP medication classes used to control BP; markers of oxidative stress in serum including asymmetric dimethylarginine (ADMA) and dimethylarginine dimethylaminohydrolase (DDAH); and serum arginine levels. Inclusion criteria included men or women at least 18 years of age who were recipients of a solitary kidney or combined kidney‐pancreas transplant within 24 months of enrollment, current diagnosis of hypertension, normal hepatic enzyme levels, and estimated creatinine clearance ≥30 mL/min. Exclusion criteria included uncontrolled hypertension defined as an SBP ≥210 mm Hg or a DBP ≥120 mm Hg, symptomatic hypotension, previous intolerance to β‐blockers, cerebrovascular accident within 3 months of randomization, bradycardia (heart rate <60 beats per minute), greater than first‐degree heart block, decompensated cardiac failure, sick sinus syndrome (unless a permanent pacemaker is in place), severe hepatic impairment (defined as elevation of aspartate aminotransferase, alanine aminotransferase, or bilirubin levels to three times the upper limit of normal reference range), severe peripheral arterial circulatory disorder, history of bronchospasm and/or asthma, regular use of inhaled bronchodilators, or any medical condition that may interfere with the participant's ability to safely complete the protocol.

Patients who signed informed consent were screened based on the above inclusion and exclusion criteria, and those who qualified were enrolled in the study. A computer‐generated list of random numbers was used to allocate participants to study drug. Baseline health information, 12‐lead electrocardiography, heart rate, BP measurements, and 12‐hour fasting laboratory tests including plasma NO concentration and serum creatinine levels were obtained. In each subsequent study visit, heath information, heart rate, BP measurements, and 12‐hour fasting laboratory tests including plasma NO level and serum creatinine were again obtained. Patients were randomly assigned to receive oral treatment with either nebivolol or metoprolol with dose titration to a target BP of at least <140/90 mm Hg. If the target BP of at least <140/90 mm Hg was not achieved following a study drug titration period of up to 10 weeks after randomization, the treating physicians were allowed to add other antihypertensive medications.

Procedures

Measurement of BP

BP measurements were made using an automatic BP monitor (Omron Healthcare, Inc, Lake Forest, IL). Two separate measurements 2 minutes apart were taken with the patient quietly seated in a chair for at least 5 minutes with both feet on the floor and the arm supported at heart level. The average of systolic and diastolic BPs were used for data analysis. MAP was calculated as 1/3 (SBP) + 2/3 (DBP).37

Laboratory Analyses

Measurement of NO

Sample: A 12‐hour overnight fast and 20 to 30 minutes of rest in a sitting position was observed prior to obtaining blood samples. Blood was obtained by phlebotomy and collected into an EDTA tube and the samples centrifuged for 10 minutes at −4°C at 1000 × g and then the plasma stored at −80°C until analyzed.

NO₂ Determination: Standard solution of 100 mM NO₂− was freshly prepared by weighing NaNO₂ and diluting it in nitrite‐free, deionized water. Standards prepared by serial dilution were stored in the dark at 4°C until analysis. After obtaining a stable baseline, the solutions were injected into an NO analyzer (NOA 280i Sievers; GE Analytical Instruments, Boulder, CO) and the amount of NO was calculated on the basis of the peak area from each injection and used to plot the calibration curves (1–100 pmol injected, r ²=0.9999). Plasma samples were thawed on ice, an antifoaming was agent added, the sample was injected as above, and the basis of the peak area was determined.

Measurement of Other Blood Tests

Blood samples were collected for measurements of serum creatinine (mg/dL) and whole blood trough level of tacrolimus (FK‐506 in ng/mL). The eGFR in mL/min was calculated according to the MDRD study group equation. Due to unforeseen technical limitations in the laboratory, markers of oxidative stress in serum including ADMA and DDAH and serum arginine levels were not measured as originally planned.

Safety

Adverse Event and Serious Adverse Event

Patients were monitored for possible adverse events associated with the study drugs. Information about all adverse events were collected, recorded, and followed as appropriate. All serious adverse events were reported to Forest Global Drug Safety.

Statistical Analysis

Statistical analyses were performed in the intention‐to‐treat (ITT) population using general linear models with continuous outcome and concomitant (covariate) variables and categorical predictor variables. Primary and secondary endpoints were analyzed by comparison of least squares (LS) mean change derived from the difference in baseline and last‐visit observations adjusted for the baseline measurement. For missing data, values were imputed using the next‐observation‐carried‐backward or last‐observation‐carried‐forward method. Paired comparisons with Bonferroni‐adjusted P values were used when an interaction of main factor effects was shown in the analyses of primary and secondary outcomes. Exploratory analyses using general linear models were performed to determine: (1) the effect of study drugs and baseline FK‐506 trough level categories (≤10 ng/mL or >10 ng/mL) on the change in NO level; (2) the effect of age and FK‐506 interaction on the change in NO levels; and (3) the correlation of nebivolol or metoprolol dose on the change in NO levels in the age and FK‐506 subgroups described above. Statistical evaluations were performed using an online statistical package (StatsToDo Trading Pty Ltd, Queensland, Australia).38, 39 In all analyses, an actual or adjusted P value equivalent to <.05 was considered significant.

Results

The first patient was randomized on July 8, 2010, and the follow‐up of the last patient ended on July 21, 2014. Recruitment to the trial was stopped before reaching the enrollment target of 50 patients due to changes in the funding entity. In total, 32 patients were screened, of whom 30 met the eligibility criteria and were randomized (15 nebivolol, 15 metoprolol), forming the ITT safety and efficacy population. A total of 29 of 30 patients (96.7%) completed the 12‐month study, with 14 (48.3%) taking nebivolol and 15 (51.7%) taking metoprolol (Figure 1). One patient in the nebivolol group withdrew from the study after the eighth week because of BP normalization. The mean study duration was 11.9 months (standard deviation [SD], 0.73). The baseline characteristics of the study population are shown in Table 1. The mean ages of the patients in the nebivolol and metoprolol groups were 53 years (SD, 15.9) and 46.4 years (SD, 13.8), respectively (P=.23). The mean duration from kidney transplantation to study enrollment was 56.7 days (SD, 111.7) in the nebivolol group and 46.7 days (SD, 62.7) in the metoprolol group. All participants were taking a tacrolimus‐containing immunosuppressant regimen at randomization and the mean 12‐hour trough serum drug levels were not significantly different between groups (nebivolol 10.7 ng/mL [SD, 2.9] vs metoprolol 11.2 ng/mL [SD, 1.9], P=.612).

Figure 1.

Trial profile.

Table 1.

Baseline Characteristics

Baseline Characteristics	Nebivolol Cohort (n=15)	Metoprolol Cohort (n=15)	P Value
Age, y	53 (15.90)	46.40 (13.80)	.24
Sex, No. (%)			.69
Male	10.00 (66.70)	11.00 (73.30)
Female	5.00 (33.30)	4.00 (26.70)
Race, No. (%)			.27
White	6.00 (40.00)	10.00 (66.70)
Black	8.00 (53.30)	5.00 (33.30)
Hispanic	1.0 (6.70)	0 (0)
Days post‐transplant at screening	56.70 (111.70)	46.70 (62.70)	.77
β‐Blockers at enrollment	15.00 (100.00)	15.00 (100.00)
Mean blood pressure medications at enrollment, No.	2.00 (0.84)	1.80 (0.68)	.43
Transplant kidney donor			.78
Type
Living unrelated	3.00 (20.00)	4.00 (26.70)
Living related	2.00 (13.00)	3.00 (20.00)
Deceased	10.00 (66.70)	8.00 (53.00)
Age, y	43.80 (40.0)	34.30 (14.90)
≤50 y	7.00 (46.60)	12.00 (80.00)	.08
Sex
Female	8.00 (53.30)	9.00 (60.00)	.12
Male	7.00 (46.70)	6 (40.00)	.71
Weight, kg	88.40 (22.50)	82.50 (15.80)	.41
Height, cm	167.60 (11.10)	170.10 (9.70)	.69
Systolic average, sitting	132.93 (15.79)	131.33 (18.14)	.79
Diastolic average, sitting	84.07 (8.59)	86.33 (8.62)	.48
Mean arterial pressure, sitting	100.20 (8.06)	101.47 (10.76)	.72
Heart rate average, sitting	72.7 (9.7)	72.6 (9.8)	.97
Nitric oxide, nmol/L	52.90 (21.10)	48.10 (15.4)	.48
Serum creatinine, mg/dL	1.51 (0.45)	1.45 (0.41)	.73
Glomerular filtration rate, mL/minute/1.73 m2	53.13 (14.15)	54.57 (14.65)	.80
Tacrolimus at baseline, µg/L	10.70 (2.90)	11.20 (1.90)	.61

Analysis of Outcomes

Nitric Oxide

The nebivolol and metoprolol groups did not differ in mean plasma NO levels at baseline (52.93 nmol/L [n=15; SD, 21.09] and 48.13 nmol/L [n=15; SD, 15.37], respectively; P=.482). After 12 months of treatment, the LS mean plasma NO level of the nebivolol group was 50.07 nmol/L (standard error [SE] ±3.93) vs 38.13 (SE ±3.93) nmol/L of the metoprolol group (difference, 11.94 nmol/L, P=.04; 95% CI, 0.48–23.40). There was a significant interaction between treatment and age effects on the plasma NO level (F [1, 25], 7.22; P=.013), (Figure 2). Mandatory subgroup analyses showed that the nebivolol younger than 50 years age subgroup had achieved a significantly higher LS mean plasma NO level than any of the metoprolol subgroups and the nebivolol 50 years and older subgroup (Figure 2). None of the patients in the treatment groups were diagnosed with an acute allograft rejection at the time of NO testing.

Figure 2.

Plasma nitric oxide achieved at month 12 of nebivolol (Neb) or metoprolol (Met) treatment corrected for pretreatment level. LS indicates least square.

The 12‐month change in LS mean plasma NO was +11.47% (standard error of the mean [SEM] ±9.20) in the nebivolol group and −17.27% (SEM ±9.20) in the metoprolol group (difference in LS means, 28.73% [SEM ±13.01], P=.038; 95% CI, 1.93–55.53) (Figure 3). Statistically, there was a significant interaction in the effects of treatment and age (categorized <50 years or ≥50 years) on the LS mean percent change in plasma NO level (F [1, 25], 7.66; P=.011). To clarify this interaction, post hoc multiple subgroup comparisons were employed (Figure 3). Among the four subgroups categorized based on drug treatment and age, only the nebivolol younger than 50 years subgroup had a significant increase in plasma NO level from baseline: LS mean +51.55% (95% CI, 23.03–80.07) (Figure 3). Results of mandatory post hoc subgroup comparisons (Figure 3) demonstrated that nebivolol treatment in kidney transplant recipients younger than 50 years resulted in a significant increase in plasma NO levels compared with: (1) metoprolol treatment in kidney transplant recipients younger than 50 years (difference of LS means, +68.19%; 99.17% CI, 13.02–123.36); (2) metoprolol treatment in kidney transplant recipients 50 years and older (difference of LS means, +69.54%; 99.17% CI, 12.71–126.37); and (3) nebivolol treatment in kidney transplant recipients 50 years and older (difference of LS means, +66.80%; 99.17% CI, 12.95–120.64). Exploratory analysis demonstrated that the use of nebivolol with the rejection prophylaxis drug tacrolimus at a baseline trough level ≤10 ng/mL resulted in a significant increase in plasma NO level from baseline (LS mean change, +14.49 nM; 95% CI, 1.94–27.04) (Figure 4). The use of nebivolol or metoprolol with tacrolimus at baseline trough level above 10 ng/mL resulted in a significant decrease in plasma NO level from baseline (Figure 4). The change in plasma NO level from baseline was not affected by the interaction effect of age and FK‐506 level (F [1, 25], 2.73; P=.11). Drug doses did not correlate with the change in NO from baseline in the age subgroups (<50 years: nebivolol, t [6] = 2.78, P=.56; metoprolol, t [6] = 2.45, P=.87) and (≥50 years: nebivolol, t [6] = 0.17, P=.87; metoprolol, t [5] = 0.47, P=.66). Similarly, drug doses did not correlate with the change in NO from baseline in the FK‐506 level subgroups (≤10 ng/mL: nebivolol, t [4] = −1.80, P=.14; metoprolol, t [2] = −1.5, P=.27) and (>10 ng/mL: nebivolol, t [7] = −0.46, P=.66; metoprolol, t [9] = −0.14, P=.89).

Figure 3.

Percent change in plasma nitric oxide level from baseline to month 12 of nebivolol (Neb) or metoprolol (Met) treatment. LS indicates least square; CI, confidence interval.

Figure 4.

Change in plasma nitric oxide (NO) level from baseline to month 12 of treatment: effect of nebivolol (Neb) or metoprolol (Met) with tacrolimus (FK) trough level categories <10 µg/L or >10 µg/L.

Renal Function

The endpoint for renal function is the change from baseline to 12‐month visit eGFR in the nebivolol and metoprolol groups stratified by the kidney donors’ age (<50 or ≥50 years). The baseline eGFR of the nebivolol and metoprolol groups were similar: 53.13 mL/min (SD, 14.15) and 54.47 mL/min (SD, 14.65), respectively (P=.80). After 12 months of treatment, the LS mean GFR of the nebivolol group was 53.10 mL/min (SE ±3.89) vs 58.57 (SE ±3.89) mL/min in the metoprolol group (difference, 5.47 mL/min; 95% CI, −5.87 to 16.81 [P=.33]).

The 12‐month change in LS mean eGFR was not significantly different between the nebivolol and metoprolol groups (8.27% [SE ±10.13]; 95% CI, −12.56 to 29.12 [P=.42]) (Figure 5). Recipients of kidney allografts from donors younger than 50 years had a significant increase in LS mean GFRs from baseline (+14.65%; 95% CI, 2.19–27.11), while recipients of kidney allografts from donors 50 years old and older had trends of decrease in their LS mean GFRs from baseline (−8.13%, 95% CI, −24.52 to 8.26). Between the younger than 50 years and 50 years and older donor age categories, the mean change of LS mean eGFR from baseline differed by 22.77% (95% CI, 1.13–44.41; P=.04) in favor of the younger than 50 years donors.

Figure 5.

Percent change in glomerular filtration rate (GFR) after 12 months of nebivolol (Neb) or metoprolol (Met) treatment. Groups stratified by age of kidney donors (<50 years or >50 years).

BP Control and Medication Requirements

The baseline and change from baseline SBP, DBP, and MAP were compared between treatment groups and no significant differences were found (Tables 1 and 2). Similarly, the average number of BP medication classes used at the time of randomization (Table 1) and at the end of the study were not significantly different between the nebivolol and metoprolol treatment groups (Table 2).

Table 2.

Secondary Endpoint Measures

Endpoint Measures	Nebivolol (n=15)	Metoprolol (n=15)	Differencea	95% CI	P Value
Systolic blood pressure, mm Hg
Baseline, observed mean (SD)	132.93 (15.79)	131.33 (18.14)			.79
12 mo, observed mean (SD)	129.93 (16.18)	125.93 (8.89)			.41
Absolute change, LS mean (SE)	−2.65 (±2.73)	−3.88 (±2.73)	1.23 (±3.87)	−6.74 to 9.20	.75
Diastolic blood pressure, mm Hg
Baseline, observed mean (SD)	84.07 (8.59)	86.33 (8.62)			.48
12 mo, observed mean (SD)	84.47 (7.12)	82.93 (7.04)			.56
Absolute change, LS mean (SE)	−0.66 (±1.91)	−2.35 (±1.91)	1.71 (±2.70)	−3.85 to 7.23	.54
Mean arterial blood pressure, mm Hg
Baseline, observed mean (SD)	100.20 (8.06)	101.47 (10.76)			.72
12 mo, observed mean (SD)	99.67 (8.39)	97.40 (7.05)			.43
Absolute change, LS mean (SE)	−1.07 (±2.11)	−3.19 (±2.11)	2.05 (±2.99)	−4.10 to 8.20	.48
Number of antihypertensive drug classes used
Baseline, observed mean (SD)	2.00 (0.84)	1.80 (0.68)			.80
12 mo, observed mean (SD)	2.27 (0.96)	2.20 (1.21)			.34
Absolute change, LS mean (SE)	−0.26 (±0.22)	−0.01 (±0.22)	0.25 (±0.32)	−0.40 to 0.89	.45
Percent change, LS mean (SE)	−8.14 (±11.58)	+8.70 (±11.58)	16.84 (±16.38)	−16.9 to 50.5	.32

Abbreviations: CI, confidence interval; SD, standard deviation; SE, standard error.

^aIn estimating differences in all outcome categories, change from baseline was analyzed through the general linear regression model evaluating differences in least‐square (LS) means, fitted with treatment and age < or ≥50 years old as categorical independent variables, corrected with the baseline measurement as continuous co‐variate.

Safety

A total of 60% (9 of 15) of patients in the nebivolol group and 86% (13 of 15) in the metoprolol group had at least one mild or moderate adverse event. There were six serious adverse events in four patients (26.7%) in the nebivolol group, consisting of urinary tract infection (1), peritoneal hematoma (1), and graft dysfunction (4). There were seven serious adverse events in five (33.3%) patients in the metoprolol group, consisting of prostate cancer (1), hernia repair (1), graft dysfunction (1), nausea and vomiting (1), and cytomegalovirus infection (1). Each adverse event was investigated and none were found to be related to the study drugs. There were no deaths or graft kidney loss reported during the study.

Discussion

While kidney transplantation provides significant survival benefit over maintenance dialysis, cardiovascular disease is still among the most common causes of death in renal transplant recipients.3, 4, 22 It is important to apply any treatment that can mitigate cardiovascular risk in transplant recipients to improve overall graft and patient survival. Hypertension, diabetes, dyslipidemia, and impaired kidney function––states characterized by endothelial dysfunction and impaired NO generation––are risk factors for increased cardiovascular complications in kidney transplant recipients.4, 12, 22, 23, 40 CNIs seem to aggravate endothelial dysfunction and vasoconstriction caused by the foregoing risk factors.21, 22, 41, 42 Conversely, the potent vasodilator substance, NO, has been shown to provide beneficial protective cardiovascular effects including the prevention of atherosclerosis and its complications.19, 25 Thus, we conducted this randomized clinical trial in order to compare the effects of nebivolol and metoprolol on NO plasma level, BP, and renal function in kidney transplant recipients. We performed an exploratory analysis to study the effect of the study drugs combined with tacrolimus, the CNI used for immunosuppression in all of the study participants. Our results demonstrate that during 12 months, nebivolol and metoprolol treatment did not have significantly different effects on the reduction of BP, change in renal function, or change in drug regimen for hypertension. We found a significant increase from baseline in the plasma NO levels of the nebivolol‐treated transplant recipients younger than 50 years or those receiving tacrolimus at a trough exposure no higher than 10 ng/mL. We did not find any significant improvement in the plasma NO level in patients in the metoprolol‐treated subgroups.

In addition, we did not find a significant difference in the change in BP parameters between the treatment groups after 12 months of observation (Table 2). Although our results are consistent with previous findings of a comparable BP‐lowering effect of nebivolol as with other β‐blockers,25 the reduction in BPs in both treatment groups was not significant. This is likely because the BP of the patients in this study were already at or close to the target of <140/90 mm Hg at the time of randomization.43 The results could also possibly be attributable to the post‐transplant improvement in renal function, volume status, and endothelium‐dependent vasodilation.12, 13, 21, 22, 31, 40, 43 The uniform use of tacrolimus rather than the more prohypertensive CNI, cyclosporine, may have also contributed to the excellent BP achieved in this population of kidney transplant recipients.3, 17, 21, 31, 44, 45

Our analysis did not show a difference in the effect of nebivolol and metoprolol on the renal function of kidney transplant recipients from baseline to the 12th month of the study. This is consistent with the relatively neutral physiologic effect of β‐blockers on overall renal function.46 Our analysis demonstrated that kidney allografts from older donors had achieved lower GFRs at the 12‐month study visit and a decreased GFR relative to the baseline visit as compared with allografts from younger donors. These findings possibly reflect how aging affects the kidney by increasing glomerular, vascular, and tubular senescence resulting in decreased renal blood flow and GFR.33, 47, 48 In a previous study, these factors were implicated in the increased incidence of delayed graft function and graft loss in kidneys from older donors.48

Based on our results, nebivolol increased NO in kidney transplant recipients younger than 50 years but not in older patients. We hypothesize that the effect of nebivolol in increasing NO generation is counterbalanced by the decline in NO synthesis or secretion––a consequence of endothelial dysfunction and increased endothelin‐1 that occurs with aging.16, 33

Tacrolimus, the cornerstone of immunosuppressive regimens after kidney transplantation, significantly reduces graft rejection rates but could have nephrotoxic effects.3, 17 Tacrolimus attenuates NO production, release, and endothelium‐dependent relaxation in a dose‐dependent manner.7, 41 This impairment has been implicated in the causation of kidney allograft vasculopathy.49, 50 In this study, nebivolol used with tacrolimus at a trough level of ≤10 ng/mL (but not >10 ng/mL) increased the plasma NO level. We theorize that the NO‐elevating effect of nebivolol operates when tacrolimus level is low but can be reversed by high tacrolimus levels, resulting in inhibition of NO generation.

Metoprolol or nebivolol used with tacrolimus at a trough level of >10 ng/mL decreased the plasma NO level. These findings may have implications for long‐term renal allograft outcomes, since previous studies have shown that inhibition of NO production by all NO synthase isoforms decreases the survival of the kidney transplant by augmentation of the alloimmune response or by ischemia to the graft.49, 50

Limitations of the Study

This study was limited by its small sample size. We enrolled only 15 participants in each arm instead of 25 as originally planned, and we were not able to measure mediators of oxidative stress (ADMA and DDAH) and arginine levels in the blood. The participants were randomly assigned to the treatment arms but investigators and drug dispensers were not blinded to the treatment allocation of patients. However, follow‐up of participants in our single university‐based transplant clinic using a uniform protocol minimized the variability in patient management.

Conclusions

We report the first randomized study to directly compare the effect of two β‐blockers on NO blood levels in kidney transplant recipients. Nebivolol but not metoprolol increased plasma NO levels in kidney transplant recipients younger than 50 years or with lower tacrolimus exposure. A large randomized trial is needed to confirm our findings and determine the clinical implications of increased plasma NO level from nebivolol treatment in hypertensive transplant recipients.

Disclosures

The authors of this manuscript have no conflicts of interest to disclose.

J Clin Hypertens (Greenwich). 2016;18:741–749. DOI: 10.1111/jch.12745. © 2015 Wiley Periodicals, Inc.