Atrial natriuretic peptide in the prevention of acute renal dysfunction after heart transplantation—a randomized placebo‐controlled double‐blind trial

Acute kidney injury (AKI) and renal dysfunction after heart transplantation are common and serious complications. Atrial natriuretic peptide (ANP) has been shown to increase glomerular filtration rate (GFR) and exert renoprotective effects when used for the prevention/treatment of AKI in cardiac surgery. We tested the hypothesis that intraoperative and postoperative administration of ANP could prevent a postoperative decrease in renal function early after heart transplantation.


Editorial Comment
In this randomized placebo-controlled trial of heart transplantation recipients, prolonged infusion of atrial natriuretic peptide (ANP) was not found to statistically increase the glomerular filtration rate nor reduce the chance of acute kidney injury (AKI) or need for postoperative dialysis. The results suggest that even though ANP might be beneficial in the management of AKI after cardiac surgery on a caseby case basis, its usage for prevention of adverse kidney outcomes is not supported.

| INTRODUCTION
Acute kidney injury (AKI) and renal dysfunction are common complications after heart transplantation (HTx) which are associated with increased morbidity and mortality (1,2). The reported incidences of postoperative AKI and need for renal replacement therapy are 15%-40% and 5%-6%, respectively. 1,2 Patient-and procedure-related risk factors have previously been described. [1][2][3] The pathogenesis of AKI after HTx is multifactorial, but perioperative renal hypoperfusion with impaired renal oxygenation is considered to be important for the development of post-transplant AKI and renal dysfunction. Impaired renal oxygenation has been demonstrated during cardiopulmonary bypass (CPB) due to impaired renal oxygen delivery, in turn, caused by hemodilution and renal vasoconstriction, combined with increased renal oxygen consumption (RVO 2 ) post-CPB. 4 Renal perfusion may be further compromised postoperatively due to hemodynamic instability induced by bleeding and/or graft failure. Furthermore, postoperative initiation of calcineurin inhibitors (CNI), as immunosuppressants, may further impair renal oxygen delivery by renal vasoconstriction 5 triggered by various mechanisms. 6 The natriuretic peptide system is involved in the fluid and blood pressure homeostasis. 7 In the treatment of AKI after cardiac surgery and in cyclosporine-induced acute renal dysfunction after HTx, it has been shown that infusion of atrial natriuretic peptide (ANP) increases both renal blood flow (RBF) and glomerular filtration rate (GFR). 8,9 Furthermore, in a randomized, placebo-controlled trial, it was shown that treatment with ANP improved renal outcome. 10 ANP has also been shown to be renoprotective when used for prevention of acute renal dysfunction after cardiac surgery with CPB. [11][12][13][14] However, there is a lack of studies on the effects of ANP when used for prevention of postoperative AKI in patients undergoing HTx. The aim of this investigation was, therefore, to evaluate the effects of ANP on renal function after HTx. Our hypothesis was that ANP, when compared to placebo, would attenuate renal dysfunction (i.e., a fall in GFR) early after HTx.

| Randomization and treatment
Prior to the start of the transplantation procedure, a dedicated anesthetist nurse, not involved in the patient care, randomized the patients to receive treatment with either ANP or placebo (saline) (1:1 allocation) by a web-based computerized random-number generator. This nurse also prepared the treatment agents for the caring anesthesia and intensive care staffs for the whole treatment period. The anesthesia/intensive care staff and physicians were blinded to the allocated treatment throughout the entire study. Treatment agents, as well as, syringes and infusion lines were identical with respect to color, appearance, size and shape. Data on patient management and outcome variables were stored electronically (eCRF) by the services of dSharp Consulting (www.dSharp.se).
After induction of anesthesia, a continuous intravenous infusion of either ANP (HANP1000 ® , Daichii Sankyo Propharma, Japan) at a dose of 50 ng/kg/min or the equivalent volume of placebo (saline) was started and continued for four postoperative days, or until treatment with dialysis was started. At the fifth postoperative day, the infusion of the study drug was gradually tapered off for another 24 h.

| Anesthesia, surgery and cardiopulmonary bypass
Anesthesia was maintained with sevoflurane before and after CPB and with propofol during CPB. Sternotomy was followed by cannulation of

| Postoperative management
The postoperative management of the patients was at the discretion of the attending anesthesiologists/intensive care physicians. given as antibiotic prophylaxis.

| Immunosuppression
The patients received induction therapy with antithymocyte globulin The adherence to the required procedures of this trial was verified by an independent monitor, which also confirmed that the data were accurately collected according to GCP. No interim analysis was planned.

| Secondary outcomes
The first secondary end-point was the mean preoperative to postoperative change in mGFR. Other secondary end-points were: the incidence

| RESULTS
From September 2016 to June 2020, 107 patients were assessed for eligibility. Twenty-one patients were found noneligible for reasons as shown in Figure 1. After enrollment, 13 patients were excluded for reasons described in Figure 1. Seventy-three patients were randomized with an allocation ratio 1:1 to treatment with either ANP or placebo. One intraoperative death occurred in the ANP group, resulting in 70 patients fulfilling the study protocol, 33 in the ANP group, and 37 in the placebo group.

| Baseline characteristics
The two groups were well balanced with respect to demographic data and preoperative morbidity. There were no obvious differences in preoperative mGFR or serum creatinine between the two groups. The proportion of a preoperative mGFR < 60 mL/min/1.72 m 2 was 49% in the placebo group and 65% in the ANP group (Tables 1 and 2).

| Intraoperative characteristics
There were no differences between the groups with respect to CPB time, the intraoperative use of blood products, urine volume intraoperatively, intraoperative fluid balance, lowest hematocrit or MAP during CPB, and allograft total ischemic time (Table 3).

| Hemodynamic variables at ICU arrival and clinical outcome
There were no clinically relevant between-group differences in MAP, CVP, CO and SvO 2 on arrival to the ICU. Urine volume the first and second days and fluid balance after the procedure did not differ between the groups The incidence of CNI toxicity during the first postoperative week was low and did not differ between the two  Table S1 (Table 4).  difference between groups ( p = .764). The median percentage change in mGFR from baseline was À4.5% (IQR: 83%) and À 21.4% (IQR:

| Postoperative AKI
The incidence of AKI was 76.5% in the placebo group and 63.6% in the ANP group ( p = .616). The incidence of AKI stage 1 was 32.4% in the placebo group and 21.7% in the ANP group ( p = .420) and the incidence of AKI stage 2 or 3 was 37.8% in the placebo group and 42.4% in the ANP group ( p = .808). The incidence of dialysis during the first 4 postoperative days was 21.6% in the placebo group and 9.1% in the ANP group ( p = .197). The median absolute change in serum creatinine was 26.0 ± 74 and 55.5 ± 133 for placebo and ANP group, respectively ( p = .439). Data on serum creatinine the first four postoperative days for the two groups in patients not requiring dialysis are shown in Figure 2. The time trends in serum creatinine levels did not differ between groups ( p = .932) ( Table 5).
Data on the adverse events coded by the preferred terms of the MedDRA terminology are presented in Table S2. There were no significant differences between groups with respect to adverse events.

| DISCUSSION
The major finding of the present placebo-controlled double-blinded randomized trial, was that this study failed to detect that ANP infusion could prevent an attenuation of renal dysfunction early after HTx.
In studies involving the treatment of AKI after cardiac surgery and in cyclosporine-induced acute renal dysfunction after HTx, it has been shown that infusion of ANP, at a rate of 50-100 ng/kg/min, increases both RBF (40%-50%) and GFR (40%-70%). 810,11 This suggests that ANP, at the infusion rates used in those studies, causes a predominant dilation of the afferent arterioles increasing both RBF and GFR, which could explain why ANP is renoprotective when used for treatment of AKI. 10,13,14,16 The number of studies on the effects of natriuretic peptides for

S-creatinine ( mol/L)
Control ANP F I G U R E 2 Shows the evolution of serum creatinine (s-creatinine) after heart transplantation for the placebo and ANP groups. Data are presented as means ± SD. ANP; atrial natriuretic peptide.
CO were above 5 L/min (Table 4), which was 35%-45% higher than the preoperative CO and postoperative mGFR increased by over 20% in one-third of the patients.
One could only speculate why ANP did not exert a renoprotective effect in the present study for patients undergoing Htx. It is possible that the dose of ANP was too low to induce a preglomerular renal vasodilation. It has previously been shown, however, that ANP, at a dose of 50 ng/kg/min, induces a 40%-50% increase in GFR and RBF with a minor (<10%) fall in MAP in cyclosporine-induced acute renal dysfunction after Htx. 8 There was a high incidence of pretransplant chronic renal failure, particularly in the ANP group (65%). In patients with chronic renal failure, renal oxygenation is impaired 19 with chronic renal hypoxia, which stimulates fibrogenesis and the development of renal fibrosis. 20 This, in turn, will cause a reduction in renal vascular density (rarefaction) 20 with a consequent increase in renal vascular resistance and a lower RBF 19,21 Such structural changes of the renal vascular bed may not be amenable/accessible enough for vasodilator treatment.
Another reason could be that ANP, by inducing a dose-dependent systemic vasodilation and hypotension, 7-9,22 may jeopardize renal perfusion, particularly in acute renal failure, which is characterized by a loss of RBF autoregulation. 23 This may explain why high doses of ANP (≈200 ng/kg/ min), in contrast to low-dose ANP, when used for prevention or treatment of AKI are associated with hypotension with no renoprotective effect. 24,25 Still, in the present study, there were no differences between the ANP and placebo groups with respect to intraoperative MAP, MAP on arrival to the ICU or the incidence of postoperative hypotension (Table S2), suggesting that a potential renoprotective effect of ANP was not blunted/counteracted by ANPinduced hypotension.
In vitro studies on renal tubular cells have shown that ANP induces a cGMP-mediated inhibition of tubular sodium reabsorption accompanied by a decrease in tubular cell O 2 -consumption. 26 Such an ANP-induced decrease in tubular O 2 -consumption would protect the renal tubular cells from renal hypoxia caused by an O 2 -supply/demand mismatch. The renal medulla is particularly sensitive to hypoxia as the medullary tissue pO 2 levels are low, even under normal conditions, due to a high renal oxygen demand in combination with low blood flow. 27 Previous studies on uncomplicated postcardiac surgery patients with no renal dysfunction have shown that ANP increases GFR, and tubular sodium reabsorption with a consequent increase in RVO 2 . 22 This increase in RVO 2 was, however, not accompanied by a proportional increase in RBF, as also indicated by an increase in renal filtration fraction (GFR/RPF). 22 Such an increase in filtration fraction has previously been shown to be directly associated with impaired renal oxygenation 28 This could be explained by a vasodilation of afferent arterioles and a vasoconstriction of efferent arterioles, as previously described. 7,29 These findings suggest that ANP when used for prevention of AKI has a potential to impair the renal oxygen supply/ demand relationship. On the other hand, when used for treatment of AKI after cardiac surgery and after heart transplantation, ANP causes a proportional increase in GFR and RBF, as indicated by a maintained renal filtration fraction. 9 Thus, it seems that ANP may have differential effects on renal oxygenation, depending on whether it is used for the prevention or treatment of AKI.
This study is a single-center study, including a relatively small number of patients, which is a major limitation. The major strength of the study is that GFR was actually measured and not estimated postoperatively. Furthermore, it is a randomized double-blinded trial, including blinding of outcome assessment and conducted in compliance with Good Clinical Practice.

| CONCLUSION
We conclude that the present study failed to detect that ANP infusion, in the dose used, attenuates renal dysfunction or decreases the incidence of AKI after heart transplantation.