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Sirolimus as primary immunosuppression is associated with improved coronary vasomotor function compared with calcineurin inhibitors in stable cardiac transplant recipients

Eugenia Raichlin, Abhiram Prasad, Walter K. Kremers, Brooks S. Edwards, Charanjit S. Rihal, Amir Lerman, Sudhir S. Kushwaha
DOI: http://dx.doi.org/10.1093/eurheartj/ehp123 1356-1363 First published online: 21 April 2009

Abstract

Aims The aim of this study was to evaluate coronary vasomotor function in cardiac transplant recipients maintained on sirolimus (SRL)- or cyclosporin (CyA)-based immunosuppression.

Methods and results Endothelium-independent response to intracoronary nitroglycerin and adenosine and endothelium-dependent response to intracoronary acetylcholine (Ach) were assessed in 15 SRL- and 21 CyA- treated subjects with angiographically normal coronary arteries. Baseline mean blood pressure was lower in the SRL group (85.6 ± 10.3 vs. 105.2 ± 8.7 mmHg, P = 0.002). There was no difference between the groups in coronary flow reserve after adenosine administration in multivariable analysis (P = 0.34). Nitroglycerin administration resulted in increase in coronary artery diameter in the SRL compared with the CyA groups (2.79 ± 0.54 vs. 2.57 ± 0.61, P = 0.0036). In 13 SRL-treated subjects without evidence of cardiac allograft vasculopathy (CAV), Ach administration resulted in less epicardial vasoconstriction compared with CyA-treated subjects (2.7 ± 17.7 vs. −15.6 ± 17.2%, P = 0.005). Two SRL-treated subjects with three-dimensional intravascular ultrasound evidence of CAV developed coronary spasm in response to Ach 10−4. Microvascular endothelial function did not differ between the groups.

Conclusion Sirolimus immunosuppression is associated with less pronounced coronary epicardial endothelial dysfunction compared with CyA immunosuppression. Improvement of coronary vasomotor function with SRL may be an important mechanism for the prevention of CAV.

Keywords
  • Atherosclerosis
  • Blood flow
  • Endothelium
  • Transplantation
  • Sirolimus

Introduction

Although survival and outcomes following cardiac transplantation have improved over the past two decades,1,2 cardiac allograft vasculopathy (CAV) remains the leading cause of late morbidity and mortality in heart transplant recipients3 and accounts for 30% of all mortality at 5 years.1,4 Immunosuppression following cardiac transplantation has traditionally comprised a calcineurin inhibitor in combination with mycophenolate mofetil or azathioprine and corticosteroids. This combination has improved short-term survival following heart transplantation, but does not prevent the development of CAV.

Endothelial dysfunction is an early marker of CAV,5 contributes to CAV progression,6 and is associated with an increase in the risk of cardiovascular events.6,7 Cyclosporin (CyA) is thought to impair endothelial function8,9 by the increase of release and sensitivity to vasoconstrictors, impairing synthesis of nitric oxide (NO) and the generation of free radicals.10 It may also result in increased endothelin (ET) levels,11 which has been postulated to have a role in the development of CAV,12 or an impaired vascular response to NO.13

Sirolimus (SRL), a proliferation signal inhibitor (PSI), is a powerful immunosuppressant with important antiproliferative effects outside the immune system.1416 We have previously reported that SRL is effective as a primary immunosuppressant in cardiac allograft recipients,17,18 and after complete CyA withdrawal, it attenuates the progression of CAV.19 Few experimental ex vivo and animal studies have evaluated the role of SRL on the development of endothelial dysfunction and presented conflicting data.2023 To date, the effect of SRL on cardiac allograft coronary vasomotor function has not been investigated.

We hypothesized that replacement of CyA-based with SRL-based immunosuppression is associated with better coronary vasomotor and endothelial function. The aim of the present study was to evaluate and compare cardiac allograft coronary endothelial and vasomotor function in heart transplant recipients treated with SRL- or CyA-based immunosuppression.

Methods

The study protocol was approved by the Mayo Clinic Institutional Review Board and written informed consent was obtained from all subjects.

Patient population

From November 2007 to March 2008, a total of 36 stable consecutive cardiac transplant recipients without signs of CAV (i.e. stenosis ≥40% in any major branch and/or distal pruning of secondary side branches) during routine annual coronary angiography were offered and accepted enrolment in the study and subsequently underwent testing for coronary vasomotor function. Five other subjects were offered but declined enrolment and were not included in the subsequent analysis. Of this cohort, 15 subjects were treated with SRL-based immunosuppression (SRL group, median 9.6, range 2.5–42 months, interquartile range 5.2–36.3 months after conversion) and 21 subjects were maintained on standard primary immunosuppression with the CyA group. As described previously,17,18 cardiac transplant recipients with impaired renal function secondary to CyA nephrotoxicity and/or CAV had been converted from CyA- to SRL-based immunosuppression in our institution since 2004. Secondary immunosuppression comprised mycophenolate mofetil or azathioprine and prednisone. Subjects with ejection fraction <55%, angiographic evidence of CAV (as defined earlier), previous myocardial infarction, significant systemic disease, or clinical signs of infection were excluded.

Study protocol

After diagnostic angiography and right ventricular endomyocardial biopsy, a 6 or 7 F guiding catheter was placed into the left main coronary artery. Coronary vasoreactivity was assessed as described previously.2426 Briefly, 5000 U of heparin was given intravenously, and a Doppler guidewire (FloWire, Volcano Corp., Rancho Cordova, CA, USA) was positioned within a coronary infusion catheter (Ultrafuse, SciMed Life Systems, Minneapolis, MN, USA) in the mid-portion of the left anterior descending coronary artery (LAD). Velocity signals were obtained instantaneously from the Doppler wire by an online fast Fourier transform, and average peak velocity (APV) was determined.27 This method has been validated previously,26 and analysis of data from our laboratory demonstrates that the variation in repeated measurements is 8 ± 3%.

First, intracoronary bolus injections of incremental doses (18–60 µg) of adenosine were administered until maximal hyperaemia was achieved or the largest dose was given to evaluate endothelium-independent microvascular coronary flow reserve (CFR). Coronary flow reserve was calculated by dividing the APV after adenosine injection by the APV at baseline.28 A normal coronary microvascular endothelium-independent function was defined as a flow velocity ratio of >2.0 to adenosine.

Secondly, to assess endothelium-dependent vasoreactivity, acetylcholine (Ach) was selectively infused at increasing concentrations (10−6, 10−5, and 10−4 mol/L) for 3 min at each concentration into the LAD. Coronary artery diameter (Cad) and APV were measured, and coronary blood flow (CBF) was calculated after each infusion of Ach.

Coronary artery diameter was measured offline in the mid-LAD in the segment 5 mm distal to the tip of the Doppler wire and in the distal LAD with a quantitative coronary angiography programme (Medis Corp., Leiden, The Netherlands), as described previously.26 Coronary blood flow was calculated from the Doppler-derived time velocity integral and vessel diameter as π(Cad/2)2(APV/2).24,25

Thirdly, endothelium-independent epicardial coronary artery function was determined by the change in Cad and CBF in response to intracoronary nitroglycerin (NTG) bolus (100 µg; Abbott Laboratories, Abbott Park, IL, USA).

Finally, three-dimensional intravascular ultrasound (3D IVUS) was performed as described previously.29 Proximal and mid-LAD regions were defined for the interrogated artery. Starting with the first complete vascular ring distal to the bifurcation with the left circumflex artery lumen, plaque and vessel volume were analysed. Each measured volume was normalized to the examined segment length (mm3/mm) to compensate for the differences in examined vessel segment length. A plaque index was calculated as: (plaque volume/vessel volume)×100%.

Trough levels of CyA and SRL were measured using high-performance liquid chromatography with tandem mass spectroscopy (API 4000, Applied Biosystems). Therapeutic levels were 100–150 ng/mL for CyA and 10–15 ng/mL for SRL.

Statistical analysis

Data were summarized using mean ± standard deviations for continuous variables and per cents and counts for categorical variables. The linearity assumption was assessed with plots of residuals for each continuous variable. Variables with heavily skewed distribution are reported as medians with first and third quartiles in parenthesis. Differences in the distribution of demographic, clinical, and laboratory parameters between the SRL and CyA groups were assessed by Wilcoxon and Student's t-test for continuous data and χ2/Fisher's test for categorical data, as appropriate. Haemodynamic values between the groups were compared using analysis of covariance, including baseline value of the term for analysis as a covariate. Multivariable linear regression analysis was performed: donor age, cold ischaemic time, mean blood pressure, creatinine, and baseline characteristic of coronary vasomotor function were included in the multivariable model. The covariates were chosen on the basis of their clinical relevance. No adjustment was made for multiple testing. A two-sided P-value <0.05 was considered statistically significant.

Results

Subject characteristics (Table 1)

All subjects were free from acute rejection (International Society for Heart and Lung Transplantation 2004 R grade 0 was shown on biopsy that was obtained at the time of each study). Donor age (37.1 ± 14.2 vs. 26.4 ± 11.0 years, P = 0.03) and cold ischaemic time (191.5 ± 26.6 vs. 155.8 ± 39.3 min, P = 0.0047) were significantly higher in the SRL group. The SRL-treated subjects had lower uric acid levels 5.3 ± 1.4 vs. 6.7 ± 1.7. There were no differences between the groups in recipient age, sex, underlying pre-transplant cause of CHF, cytomegalovirus infection, previous biopsy results, conventional atherosclerosis risk factors, medical therapy and secondary immunosuppression. Trough levels of CyA and SRL were within the therapeutic levels.

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Table 1

Demographics and clinical characteristics

VariableSRL (n = 15)CyA (n = 21)P-value
Recipient age, years51.9 ± 11.048.2 ± 14.80.39
Gender—male, n (%)11 (73)18 (86)0.36
Time from Tx (years)3.31 ± 1.913.76 ± 2.240.51
ICMP, n (%)5 (33)3 (14)0.29
DCMP, n (%)8 (53)12 (57)
Congenital, n (%)0 (0)2 (10)
Other, n (%)2 (13)4 (19)
Donor age (years)37.1 ± 14.226.4 ± 11.00.03
Ischaemic time (min)191.5 ± 26.6155.8 ± 39.30.0047
BMI (kg/m2)26.4 ± 4.727.3 ± 5.10.43
Hyperlipidaemia, n (%)13 (87)13 (62)0.10
Haemoglobin (mg/dL)13.2 ± 1.413.0 ± 1.70.62
Glucose (mg/dL)106.7 ± 24.7111.7 ± 41.80.38
Uric acid (mg/dL)5.3 ± 1.46.7 ± 1.70.02
Creatinine (mg/dL)1.4 ± 0.31.3 ± 0.40.76
Triglycerides (mg/dL)158.0 ± 76.8126.5 ± 80.00.14
Median (IQR)149.0 (99.0–202.0)99.0 (76.5–154.5)
HDL cholesterol (mg/dL)53.8 ± 16.150.5 ± 14.40.53
LDL cholesterol (mg/dL)99.2 ± 21.2103.8 ± 28.70.67
Hypertension, n (%)13 (87)16 (76)0.43
Diabetes, n (%)3 (13)8 (38)0.24
History of CMV viraemia, n (%)2 (13)6 (29)0.27
Total rejection score0.3 ± 0.30.3 ± 0.20.76
ACE-inhibitor, n (%)5 (33)7 (33)1.0
CCB, n (%)6 (40)9 (43)0.86
Statin, n (%)15 (100)19 (90)0.13
Vitamin E, n (%)2 (6)1 (5)0.16
Multivitamins, n (%)7 (47)14 (67)0.23
Azathioprine/MMF, n (%)5 (33)/10 (67)8 (38)/13 (62)0.36
Prednisone, n (%)4 (27)11 (55)0.16
Ejection fraction (%)62.0 ± 5.962.8 ± 3.80.65
  • ICMP, ischaemic cardiomyopathy; DCMP, dilated cardiomyopathy; CMV, cytomegalovirus; ACE, angiotensin-converting enzyme; CCB, calcium channel blocker; MMF, mycophenolate mofetil; IQR, interquartile range.

Three-dimensional IVUS showed no difference in LAD plaque burden between the SRL and CyA groups (Table 2); however, significant CAV (with plaque indexes of 61 and 51%, respectively) was demonstrated in two SRL-treated subjects, although coronary angiography was normal. There was no significant CAV in any of the CyA-treated subjects.

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Table 2

IVUS data

SRL (n = 15)CyA (n = 21)P-value
VV/SL (mm2/mm)15.59 ± 4.8715.3 ± 3.40.87
LV/SL (mm2/mm)10.59 ± 3.4710.5 ± 2.80.92
PV/SL (mm2/mm)5.00 ± 2.144.9 ± 2.10.88
PI (%)33 ± 1230 ± 90.55
Number subjects with PI > 50%20
  • VV/SL, vessel volume/segment length; PV/SL, plaque volume/segment length; LV/SL, lumen volume/segment length; PI, plaque index, Per cent plaque volume %.

Systemic haemodynamics

There was no difference in baseline heart rate. Mean blood pressure at baseline was lower in the SRL group (85.6 ± 10.3 vs. 105.2 ± 8.7 mmHg, P = 0.002).

There were no significant changes in heart rate and blood pressure following intracoronary administration of Ach (1.4 ± 4 vs. −1.2 ± 3.9 b.p.m., P = 0.37 and −1.7 ± 8.1 vs. −0.77 ± 7.04 mmHg, P = 0.73) and nitrates (1.3 ± 7.7 vs. 1.6 ± 8.4 b.p.m., P = 0.91 and −5.6 ± 12.6 vs. −9.7 ± 10.8 mmHg, P = 0.33) in the SRL and CyA groups.

Endothelial-dependent responses

Coronary epicardial endothelial function

Resting baseline epicardial diameters did not differ between the groups. In the CyA group, intracoronary infusion of Ach was associated with dose-dependent reductions in Cad (Table 3). There was a vasodilator response to low doses of Ach (10−6 and 10−5 M) in SRL-treated subjects; however, the higher dose of Ach (10−4 M) caused significant coronary spasm in two SRL-treated subjects. As a result, there was no difference in Cad response to Ach 10−4 M between the SRL and CSA groups (8.6 ± 34.6 vs. −15.7 ± 17.2%, P = 0.11) (Table 3 and Figure 1A). Importantly, 3D-IVUS examination demonstrated significant coronary plaque burden in these two subjects. When these two subjects were excluded from the analysis, the vasodilator response to Ach 10−4 M was significant in the SRL group compared with the CyA group (2.7 ± 17.7 vs. −15.6 ± 17.2%, P = 0.005).

Figure 1

Changes in coronary epicardial endothelial function. (A) Per cent changes in coronary artery diameter to acetylcholine 10−4: in the SRL group, −8.6 ± 34.6%; in the CyA group, −15.7 ± 17.2% (P = 0.11). Cad, coronary artery diameter; Ach, acetylcholine. (B) Per cent changes in coronary artery diameter to acetylcholine 10−4: in 13 subjects of the SRL group without three-dimensional intravascular ultrasound evidence of cardiac allograft vasculopathy 2.7 ± 17.7%; in the CyA group −15.7 ± 17.2% (P = 0.0048). For all figures, the data are expressed as box-and-whisker plots. The centreline depicts the median. The box depicts the interquartile range and whiskers extend from the box to the outer-most data point that falls within one and a half times the interquartile range of the box. Points beyond that are displayed individually. Mean ± SD information is also displayed below.

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Table 3

Haemodynamic data

SRL (n = 15)CyA (n = 21)P-value
Mean blood pressure (mmHg)93.6 ± 10.2105.2 ± 8.70.002
Heart rate (b.p.m.)85.6 ± 10.382.3 ± 13.00.41
Baseline Cad (mm)2.33 ± 0.392.45 ± 0.570.46
Baseline CBF (mL/min)59.1 ± 25.855.8 ± 44.70.78
Mean (IQR)61.5 (38.4–81.7)49.3 (32.7–62.3)
Adenosine dose57.8 ± 12.943.8 ± 12.80.009
CFR to adenosine2.7 ± 0.63.5 ± 0.70.0006*
CFR < 2.02 (13%)1 (5%)0.34
Cad to Ach (10−6)2.4 ± 0.62.3 ± 0.50.017*
Cad to Ach (10−5)2.4 ± 0.62.1 ± 0.60.002*
Cad to Ach (10−4)2.2 ± 0.92.1 ± 0.70.11*
CBF to Ach (10−6)66.6 ± 28.654.4 ± 39.20.07*
Mean (IQR)60.8 (48.1–92.2)45.2 (36.9–58.6)
CBF to Ach (10−5)89.2 ± 28.666.5 ± 53.90.12*
Mean (IQR)80.8 (55.7–115.8)56.7 (40.3–75.7)
CBF to Ach (10−4)99.8 ± 61.085.4 ± 69.50.54*
Mean (IQR)108 (54.0–138.5)70.7 (37.4–117.3)
Cad to nitroglycerin2.8 ± 0.52.6 ± 0.60.0036*
CBF to nitroglycerin75.1 ± 30.745.0 ± 23.80.002*
  • CBF, coronary blood flow; CFR, coronary flow reserve; Cad, coronary artery diameter; Ach, acetylcholine; IQR, interquartile range.

  • *Analysis of covariance test, baseline value is a covariate.

After adjusting for donor age, ischaemic time, uric acid level, baseline mean aortic blood pressure and baseline Cad, the difference in Cad (P = 0.013) between the SRL and CyA groups remained significant.

Microvascular endothelial function

Baseline CBF (59.1 ± 25.8 vs. 55.8 ± 44.7 mL/min, P = 0.78) did not differ between the groups. Response of CBF to the peak concentration of Ach (104 M) was similar in both groups (Table 3). Abnormal coronary endothelium-dependent function was demonstrated in 7 (48%) subjects of the SRL group and 10 (47%) subjects of the CyA group (P = 0.95). After adjusting for donor age, ischaemic time, uric acid level, baseline mean aortic blood pressure, creatinine, and baseline CBF in the multivariable analysis, there was no difference in CBF between the SRL and CyA groups (P = 0.58).

Endothelial-independent responses

Coronary epicardial endothelium-independent function

The change of epicardial endothelium-independent vasodilation (20.3 ± 14.6 vs. 5.7 ± 11.9, P = 0.0043) after intracoronary administration of NTG was significantly higher in the SRL group compared with the CyA group. In three cases of CyA-treated patients, there was diminution in Cad (Table 3 and Figure 2).

Figure 2

Changes in coronary epicardial endothelium-independent function. Per cent changes in coronary artery diameter to acetylcholine: in the SRL group, 20.3 ± 14.6%; in the CyA group, −5.7 ± 11.9% (P = 0.0036). Cad, coronary artery diameter; Ach, acetylcholine.

After adjusting for donor age, ischaemic time, uric acid level, baseline mean aortic blood pressure, creatinine, and baseline Cad in the multivariable analysis, the differences in Cad (P = 0.042) in response to NTG remained significant between the SRL and CyA groups.

Microvascular endothelium-independent function

Despite a higher maximal dose of intracoronary adenosine in the SRL group (57.8 ± 12.9 vs. 43.8 ± 12.8 µg, P = 0.009), CFR to adenosine was significantly lower in the SRL group compared with the CyA group (2.7 ± 0.6 vs.3.5 ± 0.7, P = 0.0006). However, there were no differences in the number of subjects with coronary endothelium-independent dysfunction between the groups [2 (13%) of the SRL group and 1 (5%) of the CyA group, P = 0.32] (Table 3).

In univariable analysis, mean aortic blood pressure (r = 0.30, P = 0.05) and ischaemic time (r = −0.38, P = 0.034) correlated with CFR. After multivariable analysis adjusted for donor age, ischaemic time, uric acid level, creatinine, and baseline mean aortic blood pressure, there were no difference in CFR in response to adenosine between the SRL and CyA groups (P = 0.38).

Change in CBF (8.6 ± 34.6 vs. −15.7 ± 17.2, P = 0.017) after intracoronary administration of NTG was significantly higher in the SRL group compared with the CyA group (Table 3 and Figure 2A). After adjusting for donor age, ischaemic time, uric acid level, creatinine, baseline mean aortic blood pressure, and baseline CBF in the multivariable analysis, the difference between the groups remained significant (P = 0.045).

Discussion

The present study demonstrates that in recipients without significant CAV, SRL-based immunosuppression results in less pronounced coronary epicardial endothelial dysfunction compared with immunosuppression with CyA. In addition, SRL treatment was associated with preserved endothelium-independent function. Moreover, the lower systemic blood pressure in the SRL group suggests that the beneficial vascular effects of SRL may extend beyond the coronary circulation. The present study suggests that SRL is less deleterious to the cardiac allograft coronary artery vasculature than CyA.

Epicardial function

It was previously demonstrated in animal studies that CyA treatment results in vascular dysfunction characterized by the impairment of endothelium-dependent vasorelaxation and enhanced sensitivity to vasospasm.8,23 Potential mechanisms resulting in vasospasm include the increased release or increased sensitivity to vasoconstrictors such as ET-1, decrease in the expression of endothelial NO synthase (eNOS), altered NO production,13 and generation of free radicals.10,30,31 In a clinical study, we have observed that CyA-treated cardiac transplant recipients when compared with age, gender, and cardiovascular risk factor-matched non-transplant controls demonstrated greater endothelial dysfunction and reduced NO bioavailability, supporting the present experimental findings. In addition, we have demonstrated that there is impairment of the eNOS pathway and a decrease in the basal NO activity and/or synthesis in the CyA-treated transplant recipients.32

Proliferation signal inhibitors (such as SRL and everolimus) have recently been shown to be effective in attenuating the development of CAV following cardiac transplantation.3335 Data on the effects of SRL on endothelial function, however, are limited and controversial. Ex vivo studies suggested that SRL results in worse endothelial-dependent vasorelaxation than CyA22 and may inhibit mTORC236 causing endothelial damage.20 In the setting of acute myocardial infarction and upon mechanical injury such as after angioplasty and stenting, SRL causes endothelial dysfunction of native coronary arteries.37 In contrast, short exposure to SRL results in increased production of prostacyclin by cultured endothelial cells38 and vasomotor relaxation of rat aortic rings in a dose-dependent fashion.21 In a rodent non-transplant model, SRL therapy did not decrease eNOS protein expression as seen after CyA treatment and also did not result in increased sensitivity to ET-1. In addition, SRL treatment appears to cause less oxidative injury than CyA.23

The present clinical study suggests that SRL therapy is associated with better coronary endothelial function in cardiac allografts compared with CyA. Two SRL-treated subjects developed severe coronary spasm in response to higher doses of Ach. However, both had significant CAV diagnosed by 3D IVUS, despite having a normal coronary angiogram. Previous studies have shown that although endothelial dysfunction may represent an early and potentially reversible stage of graft vasculopathy,39,40 the sustained alternations result in permanent vascular injury with structural abnormalities and the development of CAV.6,7 Therefore, in the present study, severe endothelial dysfunction in two SRL-treated subjects may represent non-reversible endothelial damage.

The current study showed that CyA treatment significantly affects NTG-induced vasorelaxation and even caused diminution in the diameter of coronary vessel in three subjects. The mechanism for this observation is not clear, but animal studies have demonstrated that exposure to NTG may be associated with the increased vascular content of ET-1 as well as increased superoxide anion production by the endothelium.41,42 Therefore, it is possible that during NTG therapy, biochemical changes may have made the coronary vasculature more sensitive to vasoconstrictors. Experimental studies have demonstrated that SRL may improve function23 and contractility of vascular smooth muscle cells.43,44 The present study extended these previous observations demonstrating preserved, and perhaps superior, endothelium-independent relaxation in SRL-treated subjects and suggests a beneficial effect of SRL on vasorelaxing property of the cardiac allograft coronary smooth muscle cells. The direct effects on smooth muscle function have been proposed as one mechanism by which SRL inhibits CAV.45

One of the major complications associated with CyA use is hypertension, which along with other mechanisms may be related to systemic vasoconstriction. Baseline blood pressure was significantly lower in the SRL group, suggesting an improvement of systemic vasculature smooth muscle function caused by SRL.

Microvascular function

In line with previous studies,46,47 the present study showed that coronary microvascular responses to adenosine, in the absence of graft atherosclerosis, are relatively preserved in SRL- and CyA-treated transplant recipients. Although CFR was higher in CyA-treated subjects by univariable analysis, after adjustment for differences in baseline characteristics between the groups, the difference was no longer significant.

Microvascular endothelial dysfunction often occurs after heart transplantation.4650 The present study demonstrated no difference in microvascular endothelial function between the CyA- and SRL-treated groups, suggesting that there is a differential impact of immunosuppressive therapy on the epicardial arteries and microvessels. Of note, we have previously observed pronounced coronary epicardial endothelial dysfunction in cardiac transplant recipients and no difference in the severity of microvascular endothelial dysfunction between the CyA-treated cardiac transplant recipients and age, gender, and traditional cardiovascular risk factor-matched non-transplant subjects.32 It is possible that other influences may mediate cardiac allograft endothelial dysfunction at the epicardial and microcirculatory levels.51 Since experimental studies have shown that oxidative stress has a pronounced effect on brachial flow-mediated dilation, with no effect on hyperaemic flow,52 increased oxidative stress in CyA-treated subjects may be one of the mechanisms for more severe epicardial dysfunction in the setting of similar microvascular function in the CyA group compared with the SRL group and placebo.

Although there was no difference in antihypertensive treatment, blood pressure was better controlled in the SRL group. Hypertension has been shown to impair coronary vasodilator response53 caused by reduction of NO bioavailability, inflammation, and oxidative stress in the vascular wall.54 In the present study, the impact of hypertension cannot be excluded; however, the difference in endothelial and vasomotor function between the groups occurred independent of any impact CyA and SRL might have on blood pressure and may reflect the direct action of the immunosuppressant on endothelium and smooth muscle cells.

Although patients with impaired renal function have been previously converted to SRL-based immunosuppression, at time of the study there was no difference in the creatinine level between the groups, and the difference in endothelial and vasomotor function between the SRL- and CyA-treated patients occurred independent of the baseline creatinine level.

Hyperuricaemia is a well-known complication of CyA. Uric acid may impair endothelial function by inhibition of eNOS activity55 and an increase in xanthine oxidase activation with a subsequent generation of oxygen free radicals.56 The level of uric acid was significantly lower during SRL immunosuppression; this may be another mechanism by which SRL results in improved endothelial function.

We recognize that a main limitation to this study is that it is an open-label, neither a blinded nor a randomized, study, but offline measurements of Cad and IVUS data were performed by experienced operators who were unaware of treatment assignment. Older donor age and longer cold ischaemic time in SRL-treated patients may have biased the sample. However, we were able to demonstrate less pronounced coronary epicardial endothelial dysfunction and greater endothelium-independent vasodilation even in this high-risk group of patients.

Conclusions

We have previously demonstrated attenuation in the development of CAV in cardiac transplant recipients treated with SRL-based immunosuppression. The present study suggests that this approach results in less pronounced coronary epicardial endothelial dysfunction and improvement in vasorelaxing properties of the smooth muscle cells of the cardiac allograft compared with CyA-based immunosuppression. This may be a contributory mechanism in the attenuation of CAV by PSIs.

Funding

This study was supported by a CR 20 grant from Mayo Foundation awarded to S.S.K.

Conflict of interest: none declared.

Acknowledgements

We would like to thank Rebecca Nelson for her valuable contributions in making this study possible and also Jean Wagner for her involvement in subject recruitment.

References

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