Immunosuppressive Therapy-Related Cardiovascular Risk Factors in Renal Transplantation: A Narrative Review Open Access

Kidney transplantation is the best treatment for patients with end-stage kidney disease as it improves their life expectancy and decreases the risk of death from all causes. Initially, the risk of death is higher as a consequence of perioperative complications and immunosuppressive therapy administered at the time of transplantation. However, from day 244 posttransplant onward, there is a cumulative survival benefit, with an average reduction in the risk of death of 66% when compared to dialyzed patients on the waiting list (patients matched for age, sex, race, and causes of end-stage kidney disease), resulting in an increased total average life expectancy of 10 years [1].

The leading cause of death in transplant patients, as in the general population, is cardiovascular (CV) events. Indeed, patients with chronic renal disease enter renal transplantation with a preexisting higher CV risk compared to the general population, as a consequence of progressively declining kidney function and its complications (heart failure, chronic kidney disease [CKD]-mineral bone disease, hypertension), administered therapies (renal replacement therapy, steroids) as well as lifestyle habits (smoking, obesity, ethyl habit) [2]. All these elements are worsened by the side effects of immunosuppressive therapy, which amplify both traditional and nontraditional CV risk factors, such as proteinuria, anemia, and chronic transplant glomerulopathy (Table 1) [3, 4].

This is a narrative review, which aims to delve as deeply as possible into the topic of immunosuppressive drugs and their impact on CV risk in kidney transplant patients. To this end, an extensive literature search of the literature was conducted of the studies that had investigated the topic of interest in depth. English-language articles were identified.

Inclusion criteria were the English language, randomized, nonrandomized, and observational studies; narrative and systematic literature reviews; and meta-analyses related to the topic of interest. Doubles, case studies, editorials, and commentaries were excluded (Fig. 1).

The articles of our interest were searched on the major databases (PubMed, EMBASE, Cochrane Library, Google Scholar). The keywords included in the search string were: “immunosuppressants,” “immunosuppressive therapy,” “immunosuppressive drugs,” “tacrolimus,” “FK 506,” “cyclosporine,” “steroids,” “steroid therapy,” “everolimus,” “sirolimus,” “mTOR inhibitors,” “renal transplantation,” “kidney transplant,” “renal transplant,” “kidney transplantation,” “cardiovascular risk,” “cardiovascular disease.” The literature search was updated to October 2023.

Hypertension is the most common CV risk factor in transplant patients [5, 6]. The prevalence of hypertension in kidney recipients has generally increased over time, approaching 90% and being significantly correlated to recipient gender (male), delayed graft function, immunosuppressive therapy (cyclosporine), serum creatinine, and year of transplantation [2, 7‒10].

Glucocorticoids are known for inducing or worsening hypertension through several mechanisms: they can activate the renin-angiotensin-aldosterone axis and potentiate vasoactive responses to catecholamines, resulting in hydrosaline retention and increased renal vascular resistance [11]. The effect of steroid sparing protocols (SSPs) on hypertension is still controversial. A recent study on pediatric liver transplantation has shown how steroid withdrawal resulted in normalization of mean blood pressure values, especially at night and restored normal circadian rhythm [12]. Furthermore, Ahmad et al. [13] demonstrated that the use of SSP led to a reduction in body weight gain and a normalization of mean blood pressure levels. In all, 68.2% of SSPs were normotensive, while only 57.2% of steroid-treated patients had normal BP.

Calcineurin inhibitors (CNIs) are also known to cause the onset of ex novo hypertension [14] in as much as prior to the introduction in transplant protocols of cyclosporine A (CsA), and the prevalence of posttransplant hypertension was 20% [10]. CsA appears to cause hypertension through a vasoconstriction mechanism caused by an increased release of endothelin [15]. Furthermore, calcineurin inhibition appears to interfere with renal (vasoconstriction), vascular (inhibition of nitric oxide-induced vasodilation), and neural (increased glutamate release resulting in increased intracellular calcium levels) pressor control mechanisms and its mediation [16]. Nephrotoxicity has also been proposed as a hypertensive mechanism, although no correlation between blood pressure values and worsening renal function has been demonstrated [15].

Early studies showed that the prevalence of hypertension is similar in subjects treated with CsA or tacrolimus (also known as FK506) and subsequent studies have since confirmed these early data [17]. The American College of Cardiology Foundation and American Heart Association 2017 Guidelines on Blood Pressure Management recommends the use of calcium antagonists for the treatment of hypertension in transplant patients [18]. These drugs have indeed been associated with an increase in glomerular filtration rate (GFR) and a reduction in allograft loss [19]. The hypothesis is that these drugs are able to counteract the increase in intrarenal vasoconstriction and peripheral vascular resistance that occurs in kidney transplant recipients under CNI therapy [20]. In contrast, angiotensin-converting enzyme inhibitors (also known as ACE inhibitors) have not shown significant reduction of the CV risk, all-cause mortality, and transplant survival, but conversely seems to increase the risk of more severe adverse effects than other antihypertensive drugs (such as hyperkalemia) [21, 22].

In reference to mammalian target of rapamycin (mTOR) inhibitors, the RMR study has demonstrated that early discontinuation (at 3 months posttransplantation) of CsA in transplant patients maintained on mTOR inhibitors (also known as mTORi) therapy was correlated with a significant reduction in blood pressure values [23]. For the time being, the prevalence of hypertension in patients treated with belatacept seems to be similar to that of patients with CsA [24]. However, some studies (including the BENEFIT and BENEFIT-EXT trials) show that patients treated with belatacept have better pressure control than patients treated with CsA [25, 26].

There are currently no data, suggesting that azathioprine (AZA) and mycophenolate inhibitors may affect blood pressure; thus, they appear to be well-tolerated drugs and not to have significant hemodynamic repercussions [16].

Dyslipidemia is one of the main adverse effects of immunosuppressant drugs, usually manifesting within 6 months with a peak starting at 6 weeks posttransplant. Its prevalence in transplant patients is 50–60%, with total cholesterol values 27% higher than baseline [16, 27]. The development of hypercholesterolemia in transplant patients is a risk factor for the development or worsening of atherosclerosis involving coronary, cerebral, and peripheral arteries; therefore, the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines recommend the treatment of transplant patients with statins, regardless of blood cholesterol level [28]. Generally, the maximum dose administered in transplant patients is lower than that used in the general population, mainly because of the known interactions they may have with CNIs [29]. Interferences with these and other drugs taken by transplant patients warrant close monitoring of the main side effects of statins. In addition, dose adjustment of immunosuppressants could help reduce their impact on the lipid profile [30, 31]. However, it should be considered that high-dose statins may cause significant adverse effects, such as myopathy and graft dysfunction. Consequently, the combination of ezetimibe and low-dose statins could be considered in reducing possible adverse effects in patients with severe hypercholesterolemia [32].

Several classes of immunosuppressants are responsible for the development or worsening of posttransplant dyslipidemia, causing both hypercholesterolemic and hypertriglyceridemic conditions. Steroids cause dyslipidemia by inducing an increase in the enzymatic activity of acetyl coenzyme convertase, resulting in an increase in hepatic synthesis of very low density lipoprotein (VLDL), a downregulation of low-density lipoprotein (LDL) receptor activity, an inhibition of lipoprotein lipase, and an increase in β-hydroxy β-methylglutaryl-CoA reductase (also known as HMG-CoA reductase) activity. In addition, steroids lead to weight gain, resulting in an insulin resistance, increased hepatic secretion of VLDL, and an increase in total cholesterol and triglyceride levels [33].

Several studies have shown that dyslipidemia is also a common adverse effect of CNIs, although cholesterol and triglyceride levels tend to be more severe in patients treated with CsA than with tacrolimus [34‒37]. Rayyes et al. [38] demonstrated that CsA reduces the synthesis of the hepatic receptor for LDL, thereby interfering with its cellular uptake and degradation, thus causing dyslipidemia. The hypothesis that hypercholesterolemia could result from an increase in VLDL, a precursor to LDL, had been ruled out by previous studies, which have shown reduced secretion of apolipoprotein B-100, a constituent of VLDL, in patients treated with CsA [39].

Other studies have also shown how a shift from CsA to tacrolimus results in a significant reduction in serum lipid levels [40, 41]. Specifically, Baid-Agrawal et al. [42] demonstrated an improvement in LDL cholesterol levels, but not in triglycerides, detectable as early as 3 months and persistent at 6 months after the shift and without a reduction in high-density lipoprotein cholesterol levels. The hypothesis is that tacrolimus restores LDL receptor synthesis in the liver, which is inhibited by CSA, thus leading to an increase in the hepatic uptake and catabolism of these molecules.

The hypothesis that CsA is internalized into cells by binding to the LDL receptor through CsA-containing LDL particles is also noteworthy. It appears that CsA makes bound LDL susceptible to oxidation, increasing the risk of coronary artery disease, including the accelerated atherosclerosis seen in transplant recipients [43].

One of the classes of immunosuppressants most correlated with the development of dyslipidemia is the mTORi. Tedesco Silva and Pascual demonstrated a higher prevalence of hypercholesterolemia in transplanted patients on mTORi plus low-dose CsA therapy compared to patients maintained on standard immunosuppressive therapies (mycophenolic acid [MPA] plus standard‐exposure CsA) [44, 45]. These findings were confirmed by the ASCERTAIN trial [46] and other studies [47], which showed lower levels of total cholesterol and triglyceride levels in patients on CNI therapy compared with those treated with mTORi.

However, a recent study [48] has shown that mTORi are not correlated with worsened CV outcomes compared with other immunosuppressants, pointing out that these events may be related to the worsening renal function (which is often the indication for therapeutic switch to rapamycin inhibitors from CNIs) or to the worsened clinical conditions of patients given mTORi. Sirolimus and everolimus seem to affect to the same degree the prevalence and severity of dyslipidemia [49], although this needs further investigation. In addition, the combination of rapamycin and calcineurin inhibitors appears to result in a further increase in cholesterol and triglyceridemia levels, with a greater need for the introduction of statins to control the resulting dyslipidemia [50, 51].

The results of several studies appear to demonstrate a better metabolic control in patients treated with belatacept compared with CsA, with lower cholesterol and triglyceride levels in the former compared to the latter [25, 26]. There are currently no data, suggesting that AZA and mycophenolate inhibitors cause clinically significant increases in any lipid fraction.

One of the main factors of CV risk in posttransplantation is the development of posttransplant diabetes mellitus (PTDM), which occurs in 16% at 1 year and 24% at 3 years, or worsening of preexisting diabetes [2, 52].

The use of steroids poses a strong risk in the development of PTDM, being implicated in weight gain due to increased appetite, reducing glucose uptake in myocytes and adipocytes, causing leptin resistance, impairing insulin sensitivity, and indirectly inhibiting insulin secretion, where the latter is further compromised when the glucocorticoid is administered together with CNIs (particularly with FK506) [53‒57]. Steroid-sparing therapies, either as steroid-free or as early steroid withdrawal therapy, may lead to a decrease in the incidence of PTDM and an improvement in the glycemic profile [58]. However, the adoption of steroid-sparing therapies can lead to an increased risk of rejection as well as patient and graft loss, thus illustrating the need for risk-benefit analysis [59].

CNIs are known to have strong diabetogenic power. Several studies have shown that the risk of developing PTDM is greater in patients treated with tacrolimus than in patients treated with CsA [60, 61], especially when it is used as induction therapy or with early conversion [61, 62]. The DIRECT trial, in which patients were randomized to an immunosuppressive regimen with either CsA or tacrolimus combined with mycophenolate, steroids, and basiliximab, showed that the 6-month posttransplant incidence of PTDM, impaired fasting glucose, and treated diabetes was significantly lower in patients treated with CsA compared with tacrolimus [56]. Baid-Agrawal et al. [42] demonstrated that a late shift from CsA to tacrolimus did not lead to the onset of diabetes in nondiabetic patients and did not decompensate glycemic levels in diabetic patients, without leading to the need for changes in the hypoglycemic regimen. The hypothesis is that, after the first year posttransplant, stable patients require lower doses of tacrolimus, as well as steroids, thus reducing the diabetogenic risk of these drugs. A recent randomized controlled trial showed that the therapeutic switch from tacrolimus to ciclosporin in patients who had developed posttransplant diabetes resulted in greater resolution of diabetes than in patients who were maintained on tacrolimus therapy (10% vs. 34%, respectively) [63].

Tacrolimus diabetogenic risk appears to be dose-dependent and reducible by keeping therapeutic dosage and basal plasma levels of the drug low [37, 64, 65]. The main risk factors for the development of diabetes after transplantation when on tacrolimus therapy are the drug dosage and its blood concentrations, race (with African-American at highest risk of developing PTDM [66]), concomitant steroid administration, age of the recipient, and a preexisting condition of metabolic syndrome and/or diabetes [67].

Inhibitors of mTOR can also result in a diabetic condition by reducing beta-pancreatic cell proliferation and insulin sensitivity [68]. In a randomized controlled trial of 150 patients, the incidence of PTDM in patients treated with tacrolimus and sirolimus, tacrolimus and mycophenolate mofetil (MMF), or CsA and sirolimus was 17, 14, and 33%, respectively; suggesting a possible diabetogenic effect of sirolimus [69, 70]. Furthermore, Johnston et al. [71] found that immunosuppressive therapy maintenance combined with the use of sirolimus was at an increased risk for PTDM compared with treatment with ciclosporin plus MMF or AZA. This association was consistent regardless of its combination with CsA, tacrolimus, or MMF/AZA.

The mechanism by which mTORi result in a hyperglycemic condition is still unclear. The most widely accepted hypotheses claim that mTOR inactivation interferes with the insulin receptor transduction signal, as shown by Di Paolo et al. [72]. Contrarily, sirolimus is also hypothesized to be responsible for ectopic triglyceride deposition [73, 74], resulting in the development of insulin resistance. Other possible mechanisms include impaired insulin-mediated inhibition of hepatic gluconeogenesis [75] or a direct toxic effect on pancreatic cells [76, 77]. Belatacept appears to have less diabetogenic power than CsA, with less development of PTDM in patients treated with this drug compared to CsA [25, 26].

Along with appropriate lifestyle modifications, the onset of posttransplant diabetes may require pharmacological intervention to achieve an adequate glycemic target. Metformin has been suggested as a potential first-line therapy in the event of the onset of PTDM, where renal function permits its administration; however, insulin appears to be the currently most widely used therapy to reduce the postprandial hyperglycemia that typically follows steroid therapy. The use of dipeptidyl peptidase 4 inhibitors and glucagon-like peptide-1 analogs in kidney transplant recipients has demonstrated only modest hypoglycemic efficacy, and although sodium-glucose cotransporter 2 inhibitors have been shown to be effective in reducing glycemia and body weight in kidney-transplanted patients with stable renal function, the true efficacy and long-term cardio- and nephroprotective power still need to be investigated further [78, 79].

Both morphological and functional vascular alterations remain after transplantation [80]. The endothelium regulates essential functions, such as vascular tone, blood cells circulation, leukocyte adhesion, inflammation, and platelet activity. Endothelial dysfunction represents an early manifestation of atherosclerosis and, since most CV diseases are either related to or are a direct consequence of atherosclerosis, it is considered a predictor of CV risk [81, 82]. It has been shown that advanced endothelial dysfunction correlates with an impending CV event in patients at high CV risk [83].

Several studies have compared the vascular impact of different available CNIs. Studies confirmed the presence of endothelial dysfunction in asymptomatic kidney transplant patients treated with CNIs and without evidence of atherosclerosis; Ovuworie et al. [84] have also demonstrated that cyclosporine- and tacrolimus-induced endothelial dysfunctions are superimposable [85]. In contrast, Oflaz et al. [86] reported that endothelial dysfunction was significantly worse (p < 0.001) in patients treated with CsA than in patients treated with tacrolimus despite the higher incidence of diabetes (a well-known cause of endothelial dysfunction) in the former group.

Several studies have shown that patients treated with CsA exhibit reduced basal and stimulated nitric oxide endothelial production, as well as increased levels of endothelin, highlighting the presence of endothelial dysfunction. As previously mentioned, such alterations are likely to result in the development of hypertension in renal transplant recipients on immunosuppressive treatment with cyclosporine A [15, 16, 87].

In addition, Eguchi et al. [88] demonstrated how tacrolimus also induces endothelial dysfunction through the attenuation of protein kinase B (also known as Akt) and extracellular signal-regulated kinases 1/2 independently of calcineurin inhibition and the caspase pathway, resulting in tube breakdown and endothelial cell death. Rapamycin inhibitors appear to have favorable effects on vascular damage, having demonstrated an ability to slow the progression of atherosclerosis. Such drugs, by inhibiting mTOR complex 1 (mTORC1), improve endothelial dysfunction, reduce the proliferation of smooth muscle cells in the vessels, and reduce cholesterol deposition at the atherosclerotic plaques level [89].

A study by Joannides et al. [90] demonstrated how a CNI-free immunosuppressive regimen based on sirolimus prevents endothelial dysfunction compared with CsA in kidney recipients. A study from Baetta et al. [91] strengthened the hypothesis of an antiatherosclerotic effect of mTORi, demonstrating that the administration of everolimus to rabbits fed a high-cholesterol diet resulted in a drastic reduction in macrophage accumulation in carotid atheromatous plaques. Subsequently, this resulted in a reduction of intimal thickening in the absence of changes in plasma cholesterol levels. However, the study did not confirm a reduction in vascular smooth muscle cell proliferation.

The mechanism by which mTORi determine a reduction of macrophages at the plaque level is not yet fully elucidated, but seems to be related to a reduced viability and an increased local macrophage autophagy, which is induced by these drugs [92, 93]. Further studies have also supported the hypothesis that the administration of everolimus results in a reduction not only in macrophage survival but also in the activation and migration of these cells [94]. Additional studies have also shown that mTORi reduce local atherosclerotic plaque inflammation, thus increasing its stability regardless of serum lipid levels.

If the early stages of atherosclerosis are characterized by endothelial dysfunction, the advanced stages of atherosclerosis result in arterial stiffness [95]. Large artery stiffening is the most crucial pathophysiological determinant of isolated systolic hypertension and age-dependent increase in pulse pressure and is a predictor of CV events and mortality in the general population. It can be evaluated through noninvasive methods with the measurement of pulse wave velocity (PWV), which is considered the gold standard index for measuring aortic stiffness. A worsening in PWV is related to an increased risk of CV events, CV mortality, and all-cause mortality almost twice as high when compared with the risk of subjects with lower aortic PWV in the general population. Vlachopoulos et al. [96] demonstrated that with an increase in aortic PWV of 1 m/s or of 1 SD, the risk of CV events increases by more than 10% or 40%, respectively. Currently, the European guidelines for arterial hypertension suggest aortic PWV as a tool for the assessment of subclinical target organ damage and/or hypertension-mediated organ damage [97].

CNIs are in particular associated with increased systemic and renal arterial resistance with a negative impact on arterial compliance. Several studies have delved into the impact of CNIs on arterial stiffness, with often conflicting results. Indeed, several studies have shown a protective effect of CNIs on stiffness [98, 99], while other studies have documented a worsening of stiffness in CNI-treated patients, with a tendency to worsen in patients treated with CsA compared with tacrolimus [100, 101]. However, Cohen et al. [102] showed that tacrolimus had a greater impact on arterial compliance, resulting in a greater increase in PWV than in patients treated with CsA.

Immunosuppressive therapies based on mTORi do not seem to worsen arterial stiffness and have instead been associated with an improvement in PWV. As a matter of fact, most studies to date have shown stable or improved arterial stiffness in transplant recipients treated with mTORi [103, 104]. Joannides et al. [105] demonstrated that a CNI-free immunosuppressive regimen of mTOR inhibitors (sirolimus) decreased aortic stiffness in renal recipients as compared with CsA, resulting in an improvement in CV coupling and a decrease in central blood pressure, supporting the hypothesis that mTOR inhibitors can act by preserving arterial distensibility. These improvements were simultaneous with a parallel reduction in plasma endothelin-1 and oxidative stress markers, suggesting that improvement in endothelial function may influence arterial stiffness [105]. On the contrary, in a cross-sectional study, Gungor et al. [106] compared patients on mTOR inhibitor treatment (everolimus or sirolimus for at least 6 months) with patients on immunosuppressive treatment with CNIs (CsA or FK506), without finding evidence of a correlation between the immunosuppressive regimen and augmentation index and arterial stiffness values, which are more affected by conventional CV risk factors such as blood pressure and proteinuria.

Left ventricular hypertrophy (LVH) is highly associated with an increased risk of CV morbidity and mortality. In fact, LVH represents a strong, independent risk factor for a predictor of sudden death, coronary artery disease, acute cerebrovascular events, and congestive heart failure [107‒109], the latter representing one of the greatest risk factors for adverse outcomes among renal transplant recipients [110].

An adequate blood pressure control was shown to be correlated with the regression of LVH. It is noteworthy that some studies have correlated such regression with ACE inhibitor treatment, while they have not shown the same results in patients maintained on antihypertensive therapy not including renin-angiotensin system blockers. This finding could be due to greater pressure control in patients treated with the first drugs [111, 112].

In the setting of kidney transplantation, several studies correlated mTOR inhibitors with a reduction in LVH. Paoletti et al. [113] described a significant reduction in LVH in kidney transplant recipients at 12 months posttransplantation. However, more pronounced results were observed in patients who underwent early discontinuation of CNI treatment and were converted to mTORi than in patients who maintained a standard CNI therapy. The substantial overlap in hemodynamic control in the two groups, with pressor values maintained <130/80 mm Hg, led to the hypothesis that a greater reduction in LVH in patients on mTORi therapy was due not only to blood pressure control but also to nonhemodynamic mechanisms [113]. In particular, the reduction in LVH appeared to result from a reduction in ventricular wall thickness. Studies have shown that sirolimus-based protocols are associated with an improvement in LVH, reduction in posterior ventricular wall thickness, and a regression of cardiac fibrosis [114]. Similarly, a more recent trial by Paoletti et al. [115] showed that an immunosuppressive regimen consisting of everolimus plus low-dose CsA was associated with a reduction in LVH, which was not seen in patients maintained on standard CNI therapy. This consequently argues that this effect does not result from discontinuation of CNIs, but from a likely antiproliferative effect of mTORi [115].

More recently, Anthony et al. [116] demonstrated that an immunosuppressive regimen based on low-dose tacrolimus and everolimus was associated with lower LVH than in patients treated with standard doses of tacrolimus, reduction in CV magnetic resonance imaging-measured fibrosis, and improvement in myocardial strain. Finally, the EVITA study investigated the reduction in LVH 24 months after transplantation in patients who were switched from tacrolimus + MPA to everolimus + MPA compared to patients who continued to be treated with tacrolimus + MPA. The study results showed a reduction in LVH in both arms, with a more intense reduction of concentric LVH after conversion from tacrolimus to everolimus and a positive impact of everolimus on additional CV risk factors, such as reduced nocturnal SBP and preservation of renal function. It also showed a reduction in the values of PINP (procollagen type I N-terminal propeptide) and NT-proBNP (N-terminal pro-B-type natriuretic peptide), suggesting lower collagen production and a decrease in cardiac fibrosis and remodeling with mTORi [117].

Proteinuria is prevalent in 20–40% of kidney transplant recipients [118, 119], and it is an independent CV risk factor for the development of ischemic heart disease in both diabetic and nondiabetic patients [120]. Epidemiological studies have shown that abnormal levels of proteinuria are a stronger predictor of CV risk than hypertension, hyperlipidemia, and male gender [120]. Proteinuric kidney transplant recipients were found to have a 10 years posttransplant CV disease risk of 39.4%, compared with 20.9% of nonproteinuric kidney transplant recipients at 10 years posttransplant (p < 0.001) [121].

Cyclosporine A appears to have antiproteinuric effects independent of an antirejection effect and secondary to a stabilization of the podocyte actin cytoskeleton. Indeed, Faul et al. [122] demonstrated how CsA is able to inhibit calcineurin-mediated dephosphorylation and subsequent degradation of synaptopodin, contributing to the maintenance, integrity, and function of the kidney filtration barrier. Further studies have also shown that activation of intracellular signaling of nuclear factor of activated T-cells, resulting from its dephosphorylation by calcineurin (upon activation by increased intracellular Ca²⁺), can result in podocyte damage, glomerulosclerosis, and increased proteinuria. Inhibition of calcineurin by CsA interferes with this pathway, thereby reducing proteinuria [123, 124].

Unlike CsA, mTORi are related to worsening proteinuria, the etiopathogenesis of which is currently not fully known. In 2003, Morelon and Kreis et al. [125] first described how, as a consequence of discontinuation of CNIs with the introduction of sirolimus in 50 transplant patients, 32 of them developed proteinuria. Fifteen biopsies were taken, 5 of which showed a picture of focal segmental glomerulosclerosis (FSGS) not found in biopsies taken before the switch, while the others revealed no informative changes. This evidence was confirmed by Izzedine et al. [126], who demonstrated how, in a transplanted patient with Kaposi sarcoma switched to sirolimus with the removal of CsA and MMF, the occurrence of proteinuria was correlated with a histological picture of collapsing FSGS. These data supported the hypothesis that the aforementioned could be induced by sirolimus and be the cause of proteinuria in mTORi-treated subjects [126]. Moreover, they also evidenced an increase in plasma vascular endothelial growth factor (VEGF) levels, the upregulation of which had been previously correlated with the development of proteinuria [126]. Similarly, Letavernier et al. [127] confirmed the association between a histological picture of FSGS in patients treated with mTORi and the subsequent development of proteinuria; at odds with previous studies, VEGF levels were reduced [127]. The role of VEGF is still debated: several studies suggest that podocytes have a functional autocrine VEGF-A system that promotes podocyte survival and differentiation, protecting podocytes from apoptosis via vascular endothelial growth factor receptor 2 (VEGFR2) and upregulating podocin in vitro [128].

In a single-center cohort study, Stallone et al. [129] showed that sirolimus was seemingly associated with alterations of the podocyte cytoskeleton and slit diaphragm, caused by a reduction in synaptopodin, nephrin, podocin, and CD2-associated protein (CD2ap) expressions. Additionally, it was shown to induce a dose-dependent reduction in Wilms’ tumor gene 1 (WT1), a transcription factor essential for maintaining podocyte integrity, thus causing a subsequent derangement of the proteins that form the glomerular filtration barrier. These findings support the hypothesis that high-dose sirolimus therapy may induce de novo FSGS, a glomerular disease characterized by significant podocyte alterations.

Moreover, several studies have shown defects in tubular reabsorption in patients treated with mTORi (sirolimus) as well as demonstrating evidence of increased urinary excretion of markers of tubular damage and biopsy-related tubular injury. All of this supports the idea of a possible mechanism of tubular toxicity responsible for tubular proteinuria [130, 131].

Contrary to previous evidence, other studies have shown that sirolimus appears to be protective in proteinuric nephropathy when administered at doses lower than those maintained as antirejection therapy [132]. Such evidence suggests that adverse effects of the drug may be dose-dependent, although there are currently no studies suggesting adequate drug dosages to minimize the risk of adverse effects and still prevent the development of acute rejection in transplant recipients.

At the time of the onset of proteinuria, all necessary measures should be applied to reduce the extent of proteinuria. Use of ACE-i and Sartans has been shown in several trials to effectively reduce the extent of proteinuria in kidney transplant patients, although the cardio- and nephroprotective effects long confirmed in the general population are yet to be established. In addition, adequate blood pressure control, dyslipidemia control, smoking cessation, and weight loss are essential. Finally, dietary protein restriction has also been shown to be effective in bringing about a reduction in the magnitude of proteinuria, underscoring the importance of adopting appropriate lifestyle and dietary habits [133, 134].

Although it is essential to prevent and limit the development of organ rejection, the introduction of maintenance immunosuppressive therapy is unable to prevent progressive organ failure. Kidney allograft dysfunction has its clinical manifestation in a decline of renal function, usually associated with development or worsening of proteinuria, hypertension, or appearance of histological changes in the transplanted organ. Reduced renal function is an independent risk factor for CV events. As a matter of fact, a decline in eGFR of 10 mL/min/1.73 m² is associated with a 6–10% increased risk of CV events [135, 136].

Depending on the timing of transplantation, kidney allograft dysfunction can be distinguished into early onset (within 6 months after transplantation) or late onset (over 6 months after transplantation) and can be due to both immunological and nonimmunological factors, with cyclosporine inhibitors among the latter [137]. The nephrotoxicity associated with CNIs is the major limitation to their use in clinical practice. In particular, CNIs have both acute and chronic nephrotoxic effects. Acute calcineurin nephrotoxicity is primarily due to hemodynamic effects, which in turn result from the release of vasoconstrictor factors (endothelin, thromboxane, renin-angiotensin system activation), the reduction of vasodilator factors (nitric oxide, prostacyclin, prostaglandin E2, etc.), the release of free radicals, and the activation of the sympathetic nervous system. These alterations are usually reversible and not accompanied by detectable histological changes. Acute calcineurin toxicity has also been associated with isomeric vacuolization of the tubular cytoplasm, which can be detected histologically. This alteration is nonspecific and has also been found in patients receiving CNI-free immunosuppressive therapy. A further cause of acute CNI-related kidney allograft dysfunction is the development of thrombotic microangiopathy [138].

CNIs also cause chronic, irreversible histological lesions in all renal districts. At the vascular level, the typical lesion is arteriolar hyalinosis, characterized by the replacement of necrotic muscle cells by hyaline deposits. At the tubular level, tubular atrophy and interstitial fibrosis are typically found. This alteration is probably a consequence of hypoxic and ischemic damage, resulting from vascular hemodynamic alterations and the release of free radicals and reactive oxygen species. Finally, at the glomerular level, the use of CNIs results in the development of global glomerulosclerosis, atubular glomeruli, and FSGS [138].

To attempt and contain CNI-related injuries, several studies have investigated the effect of CNI-minimization protocols compared with standard-dose protocols on renal outcome. Silva et al. [45] compared renal function and graft and patient survival in subjects receiving everolimus and reduced doses of calcineurin compared with patients treated with standard CNI therapy, finding no significant differences. These results were confirmed by the ASSET study and the TRANSFORM trial [44, 139]; the latter study also demonstrated a similar incidence of acute rejection at the control biopsy in patients treated with everolimus and low-dose CNIs compared with patients on MPA and standard-dose CNI therapy.

Several studies have also investigated the effect of CNI-free regimens on renal outcome. In the Rapamune Maintenance Regimen Study, eGFR levels were significantly higher in patients randomized to mTORi treatment compared with patients treated with CsA standard therapy [23]; these findings were confirmed by the HERAKLES study and the CENTRAL study [140‒143]. Similarly, the ZEUS study showed that patients treated with everolimus had better eGFR levels than patients treated with CNI, with no significant difference in the incidence of acute rejection at the control biopsy [144]. The ASCERTAIN study demonstrated how discontinuation of CsA 6 months after transplantation resulted in increased eGFR levels in patients with creatinine clearance greater than 50 mL/mL [46]. Finally, the SCHEDULE study also examined the outcome on renal function in cardiac transplant patients randomized to receive everolimus, low-dose CsA, MMF, and steroids followed by CNI withdrawal at weeks 7–11 posttransplant compared with patients randomized to receive standard-dose CsA, MMF, and corticosteroids. The study results showed that renal function at 5–7 years posttransplantation was significantly better in patients treated with everolimus (mean eGFR 74.7 mL/min) than in patients treated with CsA (mean eGFR 62.4 mL/min), despite a higher incidence of biopsy-proven acute rejection in the everolimus group. In addition, coronary allograft vasculopathy rates were lower in the everolimus group, with overlapping late adverse events between the two groups [145].

However, these findings were not confirmed by further studies in which sirolimus was found to be associated with lower transplant survival and greater adverse effects without positive effects on graft function [146‒148]. Similarly, the SOCRATES study showed that early switch to everolimus with CNI and MPA withdrawal compared with standard immunosuppressive therapy produced noninferior eGFR at 12 months compared with standard immunosuppressive therapy, with, however, higher rates of biopsy-proven acute rejection and treatment discontinuation [149]. More recently, the ELEVATE study failed to demonstrate a nephroprotective effect of rapamycin inhibitors [150]. Again, the C3 study showed that elective conversion to sirolimus at about 6 months after kidney transplantation does not improve the graft function compared with patients maintained on tacrolimus treatment, also showing higher incidence of biopsy-proven acute rejection and severe infections (by opportunistic germs or requiring hospitalization) in patients randomized to mTORi treatment [151]. One of the possible explanations is that tacrolimus is less nephrotoxic than cyclosporine; this hypothesis seems to find support in the Elite-SYMPHONY study, which reported better renal function in patients treated with low-dose tacrolimus compared with patients treated with standard- or low-dose cyclosporine 1 year after transplantation [148]. A recent report also examined samples of kidney biopsies from kidney-pancreas transplant patients taken between 1999 and 2012, showing that biopsies taken after 1999 (tacrolimus era) exhibited less early CNI nephrotoxicity than biopsies taken before 2000 (cyclosporine era) [152].

However, several studies have proven that tacrolimus induces nephrotoxicity in 17–44% of transplanted patients on maintenance therapy with this drug, and it appears to have nephrotoxic properties similar to those of cyclosporine and to induce similar histological injury [153]. These studies also raised concerns about the safety of a CNI-free regimen because of the higher incidence of acute rejection in the transplanted kidney [146, 149, 150].

De novo belatacept-based immunosuppression has been associated with better renal function and a lower nephrotoxicity on renal tissue, with higher eGFR values in belatacept-treated patients compared with patients on immunosuppressive treatment with CNIs [154, 155]. Notably, the BENEFIT study demonstrated how patients treated with belatacept, despite having a higher incidence of early acute rejection than patients treated with CsA, had a stable renal function at 3 years, while function declined in the cyclosporine group [26, 156]. Similarly, the BENEFIT-EXT study confirmed higher GFR levels in belatacept-treated versus cyclosporine-treated patients, with less decline in renal function over time; in addition, the acute rejection rates in this study were comparable between the two groups throughout the duration of the study [157, 158].

Immunosuppressive drugs are essential for transplant survival, but they are associated with the occurrence of hematologic complications. As a matter of fact, cytopenia occurs at least once in 20–60% of transplant recipients, in most cases within the first 3 months after transplantation [159].

Anemia, defined as hemoglobin levels <12 g/dL in women and <13 g/dL in men in accordance with World Health Organization (WHO) criteria [160], is an independent risk factor for the development of CV events. Several studies report a prevalence of anemia between 30 and 40% in kidney transplant recipients at various times post renal transplantation [161‒164]. Early posttransplant anemia is usually defined as anemia, which develops up to 6 months after transplantation, and late posttransplant anemia is defined as anemia, which develops after 6 months [165]. CV events at 6 months after transplantation were found to be 35% less likely in diabetic recipients with a hematocrit of more than 30% compared to diabetic recipients with lower hematocrit values [166].

Anemia in the kidney transplant patient has multifactorial etiology: female gender, immunosuppressive drugs, decline in renal graft function, age, acute rejection, iron status, vitamin B12, and folate deficiency and infections [162, 164, 167, 168]. Among immunosuppressive drugs, those most associated with the onset of anemia are tacrolimus, AZA, and MMF, while CsA appears to have a therapeutic effect.

Tacrolimus is an immunosuppressive drug that inhibits the development and proliferation of T lymphocytes by reducing interleukin-2 production, without significant immunosuppressive effects. However, rare cases of pure red cell aplasia (PRCA) associated with tacrolimus have been reported, although the pathogenesis is not yet known [169‒174].

CsA has been shown to be effective in the treatment of PRCA, with efficacy ranging from 65% to 87% (CsA levels of 12 mg/kg/day) [175]. Recently, a study by Yucel et al. [173] confirmed how the discontinuation of tacrolimus and the introduction of CsA in a PRCA patient resulted in a dramatic resolution of the problem. However, it remains to be clarified whether the clinical regression is solely attributed to the discontinuation of tacrolimus or, rather, to a therapeutic effect of CsA [173].

AZA-induced erythroid aplasia has been described as a rare complication, occurring at various times following kidney transplantation. McGrath et al. [176] and DeClerck et al. [177] reported several patients presenting anemia, which resolved upon discontinuation of AZA with its subsequent reintroduction at lower dosages. Similarly, Old et al. [178] and Hogge et al. [179] described several patients whose clinical picture resolved with discontinuation of AZA and a change in the immunosuppressive regimen (introduction of cyclophosphamide).

In vitro experiments have suggested that the etiopathogenesis may be attributable to the presence of autoreactive T lymphocytes. This mechanism may explain the therapeutic efficacy of CsA, which is able to control such cell clones when AZA withdrawal alone appears to be ineffective [180, 181]. The AZA-driven myelosuppression appears to be secondary to a deficiency in the enzyme thiopurine S-methyltransferase, the absence of which leads to the accumulation of 6-thioguanine nucleotides, an active metabolite of 6-mercaptopurine that in turn is derived from the degradation of AZA. The enzymatic activity of thiopurine S-methyltransferase is genetically determined and dictated by a genetic polymorphism inherited as an autosomal codominant trait [182].

Several studies have also supported an association between MMF or its active metabolite MPA and the development of anemia in transplant patients. In particular, MPA has been associated with both the development of bone marrow aplasia and pure red cell aplasia [183‒185]. The exact pathogenetic mechanism is still poorly understood, but it seems likely to be attributed to the antiproliferative effect of this drug. As a matter of fact, in an in vitro study Pile et al. [186] demonstrated how MPA inhibits the proliferation of human megakaryoblastic leukemia cell line of erythroid lineage (UT-7) and erythropoiesis in murine bone marrow cells, thus stimulating apoptosis of these cell lines. These effects are reversible with the addition of guanosine, indicating that the likely mechanism is through IMPDH inhibition, which is the same mechanism at the base of the immunosuppressive action of mycophenolate [186]. In addition, both Engelen et al. [183] and Elimelakh et al. [187] demonstrated how discontinuation of MMF in patients presenting red cell aplasia led to the resolution of the clinical picture, supporting the hypothesis that these drugs may have an impact on the etiology of anemia in transplanted patients.

The administration of hemotransfusions in transplanted patients is an eventuality to be avoided as much as possible because of the possibility of immunization of the recipient; in the presence of anemia, patients should therefore be screened for possible viral infections and for iron or vitamin deficiencies in order to initiate specific etiologic therapy as soon as possible. Erythropoietin therapy has also been shown to be effective in treating anemia despite the prolonged use of myelosuppressive therapies in the posttransplant period and in the presence of impaired renal function [188].

Few studies have investigated the impact of immunosuppressive drugs on electrolyte balance. Recently, Gordan et al. [189, 190] showed that CsA, inhibiting the mitochondrial permeability transition pores, is able to reduce mitochondrial calcium efflux and consequently the incidence of cardiac arrhythmias. CsA was also effective in attenuating downregulation of L-type calcium channel alpha1c subunit expression and prolonging the atrial effective refractory period, thus improving atrial electrophysiological remodeling, in a canine model of atrial fibrillation [191]. Further, animal studies have shown that administration of CsA in dogs with chronic atrioventricular block and biventricular hypertrophy results in structural and electrophysiological changes. In fact, CsA-treated dogs showed a reduction in cardiac hypertrophy and were no longer susceptible to almokalant-induced polymorphic ventricular tachycardia, suggesting that hypertrophy provides the substrate for continuation of the arrhythmia and its reduction may result in a protective effect on the onset of cardiac rhythm alterations [192].

However, conflicting studies have reported a proarrhythmic effect of CsA. A study in mice showed that high doses of CsA (30 mg/kg/day) were able to induce tachycardia and electrocardiographic changes (QT prolongation and an increase in QTc) in the absence of electrolyte disturbances [193].

As with CsA, an ex vivo study in animal models have shown tacrolimus-induced QT prolongation at FK506 whole blood trough concentration of 5–20 ng/mL, suggesting that it may induce QT prolongation within the therapeutic range when used clinically [194]. In addition, rare case reports have described the association between tacrolimus administration and cardiac arrhythmias in patients undergoing renal or liver transplantation [195‒198]. In contrast, further ex vivo studies in a rat model of myocardial ischemia and/or reperfusion injury showed that tacrolimus significantly alleviated arrhythmias and impaired cardiac function and inhibited oxidative stress and apoptosis in cardiomyocytes [199]. It is also noteworthy that both CNIs, and particularly FK506 [200], may contribute to the development of hyperkalemia through several mechanisms such as inhibition of mineralocorticoid production, mineralocorticoid resistance, and the activation of the thiazide-sensitive sodium-chloride cotransporter in the distal convoluted tubule [201].

Another possible cause of hyperkalemia during CNI treatment is the development of renal tubular acidosis type IV (RTA), which is characterized by a normal anion gap and hyperkalemic metabolic acidosis. Although the exact mechanism by which these drugs may cause the development of RTA is not yet fully understood, both structural (direct tubular toxic effect) and functional (alteration of tubular electrolyte transport) mechanisms have been considered. Animal studies have shown that CNI treatments lead to a reduction of the chloride-bicarbonate anion exchanger Cl−/HCO₃− at the basolateral membrane and to the (ATP6V0A4) subunit of the vacuolar H+-ATPase at the apical membrane. In turn, these effects lead to renal tubular acidosis and the development of hyperkalemia as a response mechanism. Moreover, the blocking of peptidyl-prolyl cis-trans isomerase activity by a cyclophilin-dependent mechanism and the increased activity of sodium transporters (especially the Na-Cl cotransporter) induced by CsA seem to be responsible for renal tubular acidosis. In addition, CsA prevents the conversion of beta-intercalated tubule cells (which secrete bicarbonate) into alpha-intercalated cells (which secrete hydrogen), thus increasing the risk of RTA development. On the other hand, tacrolimus administration appears to affect key transport proteins involved in acid-base homeostasis in the proximal and distal tubules, such as endothelin 1 and the H+-ATPase transport protein. Finally, CNIs have been shown to increase blood potassium levels by downregulating mineral corticoid receptors and reducing the activity of medullary K+ channels, also known as ROMK [202‒204].

Hyperkalemia can be controlled in several ways: correction of metabolic acidosis by administration of sodium bicarbonate is the first indication, followed by reduction in potassium intake, expansion of intravascular volume, and increase in total sodium intake. In addition, a reduction in medication – when feasible – may reverse the acidosis [202‒204]. When necessary, the use of potassium binders – nonabsorbable resins that act through cation exchange mechanisms – is also indicated. Among them, a typical choice is sodium polystyrene sulfate, which causes intestinal sodium-potassium exchanges. However, this drug is not selective for potassium exchanges and may lead to further electrolyte imbalances. Besides, its unpalatability results in poor therapeutic compliance. Finally, it may cause severe complications, including intestinal necrosis [205]. Two relatively new binders approved in recent years for the treatment of hyperkalemia are patiromer (a calcium-potassium exchanger) and sodium zirconium cyclosilicate (ZS9, a hydrogen/sodium-potassium exchanger). Their effectiveness in correcting hyperkalemia in the kidney transplant population has been proven by several studies. Their administration has not led to significant side effects, particularly in the gastrointestinal tract, although the possibility of increased tacrolimus blood trough levels during patiromer administration has been reported [205‒208]. Since studies on this subject are still scarce and are typically performed on small samples of patients, further research is needed in order to rule out rare or unknown adverse events and to investigate possible pharmacological interferences with immunosuppressive drugs.

AZA also appears to be associated with an increased incidence of arrhythmias. In a 3-year randomized controlled trial, comparing administration of MMF vs AZA in heart transplant recipients, patients on AZA therapy were found to have a higher rate of atrial arrhythmias compared with patients on MMF. The pathogenesis for this occurrence is currently unknown [209]. In addition, several case reports have described an association between atrial fibrillation and AZA [210‒213].

According to the most recent European guidelines on CV disease prevention, patients with established atherosclerotic CV disease and/or diabetes and/or moderate-to-severe renal disease and/or genetic/rarer lipid or blood pressure disorders are to be considered at high or very high CV risk [214]. Kidney transplantation represent the best treatment for patients who reach end-stage kidney renal disease, allowing an improvement in CV risk compared to dialyzed patients on the waiting list, which, however, remains higher than the general population [1, 3]. As a matter of fact, immunosuppressive therapy in kidney transplant patients is essential to ensure optimal organ survival; however, all currently used drugs have variable adverse effects that may affect the patient’s overall outcome and the functionality of the graft itself, including the patient’s CV risk.

On these assumptions, traditional therapeutic strategies to reduce CV risk factors, such as smoking cessation, alcohol restriction, and increased physical activity, should be implemented in all patients affected by chronic renal failure and undergoing dialysis and/or renal transplantation. In addition, in kidney transplant patients, the immunosuppressive regimen should be selected carefully, taking into account the general health status as well as the individual risk of each patient, in order to outweigh the risks and benefits of the administered therapy. In our opinion, assessment of the CV risk of each CKD patient before and after kidney transplantation may be an effective tool to best guide the choice of immunosuppressive therapy, trying to best balance its risks with the benefits of adequate immunosuppression.

We suggest that patients with higher CV and lower immunological risk should be elected for steroid-free immunosuppressive regimens and may be evaluated for an immunosuppressive regimen based on mTORi versus CNIs, the former being associated with the development of fewer CV side effects. In contrast, patients with high risk of organ rejection (hyperimmune patients, previous transplant recipients) might benefit most from immunosuppressive therapy based on CNIs.

The most recent European guidelines on CV disease prevention suggest that the use of coronary artery calcification score to risk stratify patients with CKD might be a promising tool to improve risk prediction for CV disease, myocardial infarction, and heart failure over use of established and novel risk factors among patients with CKD after adjusting for important risk factors for CV disease [214]. Likewise, the evaluation of arterial stiffness through PWV aortic measurement may also be useful in assessing the CV risk of each patient, providing data that could assist in the choice of the immunosuppressive regimen.

We emphasize the importance of studying not only the antirejection properties but also the CV impact of new immunosuppressive drugs, in order to reduce the burden of CV comorbidities on transplant patients as much as possible. Similarly, we underscore the need to improve our knowledge about cardiovascular and renal protective properties of novel drugs such as glucagon-like peptide-1 analogs and sodium-glucose cotransporter 2 inhibitors. As a matter of fact, while being well studied in the general population, these drugs lack studies in the transplant fields. Their use in the transplant recipients’ population may allow to at least partially curb the side effects of immunosuppressive drugs, increasing the expectations and quality of life of transplanted patients. Therefore, more in-depth studies are essential. In conclusion, it is clear that the different CV impact of immunosuppressive drugs and the appropriate scores to predict the CV risk in CKD patients and kidney transplant recipients still need to be further investigated, in order to be able to tailor therapy by reducing adverse effects as much as possible while allowing an adequate immunosuppressive status.

The authors have no conflicts of interest to declare.

No specific financial support was obtained for preparation of this article.

Conceptualization and methodology of the work, and writing original draft: C.P. and C.C.; writing review and editing: A.G., G.G., and S.A; and supervision: G.G. All authors have read and agreed to the published version of the manuscript.