Calcineurin Inhibitor Sparing in Renal Transplantation

Abstract

Although calcineurin inhibitors (CNIs) are effective at preventing acute rejection, their long-term use is associated with nephrotoxicity that may compromise long-term renal allograft survival. Consequently, there is considerable interest in identifying immunosuppressive regimens that permit reduced exposure to CNIs while maintaining adequate immunosuppression. Introducing such strategies early after transplantation may mean that the development of CNI-associated nephrotoxicity could be minimized or prevented. Several CNI-sparing regimens have shown at least comparable efficacy with standard-dose CNI regimens. In particular, a regimen of mycophenolate mofetil (MMF), corticosteroids, interleukin-2 receptor antagonist induction, and low-dose tacrolimus from the time of transplantation provided superior renal function and a lower acute rejection rate than the same regimen but with low-dose cyclosporine or low-dose sirolimus, or standard-dose cyclosporine, MMF, and corticosteroids. The use of low-dose cyclosporine does not seem to eliminate nephrotoxicity in de novo renal transplant recipients. The early withdrawal of CNIs from MMF-based regimens generally improves renal function but has been associated with an increased risk of acute rejection, in particular when the levels of mycophenolic acid were not adjusted to maintain the same total level of immunosuppression. Research aiming to achieve the “best” balance of efficacy and toxicity of available immunosuppressive regimens continues.

Calcineurin inhibitors (CNIs) have been a component of immunosuppressive regimens in renal transplantation for more than 25 years (¹). Although cyclosporine A (CsA) and tacrolimus are effective at preventing acute rejection and improving short-term graft survival, their long-term use can be associated with serious toxicities, including hypertension, dyslipidemia, and adverse effects on glucose homeostasis (^{2, 3}). In addition, CNIs cause nephrotoxicity, which may affect long-term renal graft survival (²).

Calcineurin inhibitors cause microvascular and glomerular damage in the kidney and are associated with arteriolar hyalinosis and striped fibrosis (²). Calcineurin inhibitor-associated nephrotoxicity becomes increasingly common late after transplantation. In one study more than 60% of CNI recipients had chronic allograft nephropathy (CAN) after 2 years, and this was significantly correlated with the occurrence of nephrotoxicity during the first year (⁴). After 10 years, evidence of CsA toxicity was found in virtually all renal allografts (²). Calcineurin inhibitor-related renal changes occurring soon after transplantation may be reversible; however, the nephrotoxicity that develops during chronic treatment seems to be largely irreversible (²).

Consequently, there is considerable interest in immunosuppressive regimens that permit reduction of CNIs and their associated toxicity while maintaining adequate immunosuppression and graft survival. This article provides an overview of the development of CNI-sparing (or avoidance) strategies for patients undergoing renal transplantation. It is not intended to be “all-encompassing,” but rather to focus on pertinent examples of the early initiation of strategies to prevent or minimize CNI-associated nephrotoxicity.

“Standard” Immunosuppressive Therapy

Currently, “standard” immunosuppressive protocols typically use drugs from three groups: antiproliferative agents (e.g., azathioprine and mycophenolate mofetil [MMF]), CNIs (CsA and tacrolimus), and corticosteroids. Sirolimus, a mammalian target of rapamycin inhibitor, licensed for use in combination with CsA and/or corticosteroids, is rarely used as part of standard immunosuppressive regimens (⁵). In addition, antibodies (e.g., interleukin-2 receptor antagonists) may be given as induction therapy with the aim of increasing the efficacy of the regimen in high-risk patients or facilitating the minimization of immunosuppression in normal-risk patients (³).

Cyclosporine A has been used for many years in standard regimens with excellent clinical success; however, in recent years tacrolimus has been increasingly used in many immunosuppressive regimens in renal transplantation (⁵). Regimens using tacrolimus have been shown to be at least as effective as regimens using CsA for patient and graft survival (^{3, 6}), and more effective than CsA in acute rejection (^{3, 6}). Acute rejection and graft survival were also shown to improve with tacrolimus compared with cyclosporine in a meta-analysis of 30 trials (⁷). The toxicity profiles of the two CNIs partly overlap. Hypertension and dyslipidemia are more common with CsA and diabetes mellitus and neurotoxicity more frequent with tacrolimus (^{3, 7}). Both drugs are associated with nephrotoxicity (²).

Mycophenolate mofetil has proven efficacy at preventing acute rejection or other treatment failure after renal transplantation (^{8, 9}). Adverse events associated with MMF include gastrointestinal effects and leukopenia. Like other immunosuppressive agents, treatment with MMF may be associated with an increased risk of opportunistic infection, although in the pivotal efficacy and safety trials of MMF this was observed only with higher doses (3 g/day) of the drug (⁸). It is not associated with nephrotoxicity (^{10, 11}). Quadruple therapy with interleukin-2 receptor antagonist induction in addition to MMF, standard-dose CsA, and steroids provides effective immunosuppression without adversely affecting the pharmacokinetics of MMF or the incidence of adverse events (¹²).

Sirolimus is more effective than azathioprine at reducing the risk of treatment failure (defined as acute rejection, graft loss, or death) when both drugs are administered with CsA and steroids (¹³). However, combinations of sirolimus and CNIs seem inferior to MMF plus CNIs in terms of graft survival (^{14, 15}). A particular concern with sirolimus is the increased risk of nephrotoxicity when administered in combination with CsA (^{13, 16}). Sirolimus is also associated with impaired wound healing, lymphocele formation, myelotoxicity, and dyslipidemia (¹⁷).

Calcineurin Inhibitor Avoidance

One strategic approach is to avoid CNI exposure altogether in de novo renal transplant recipients. With this approach, immunosuppressive cover is provided in the early posttransplant period by induction therapy plus non-CNI agents and in the maintenance immunosuppression period with non-CNI drugs. The CNI-avoidance regimens first investigated in noncomparative studies used induction (^{18, 19}), MMF, and corticosteroids, but these regimens have generally not been adopted because they were associated with a high risk of acute rejection. Additional immunosuppressive cover is required, and several studies discussed here investigated the addition of agents, such as cyclosporine, tacrolimus, or sirolimus, at least in a low dose, or belatacept to a CNI-free regimen with MMF (Table 1).

TABLE 1: Overview of CNI sparing trials reviewed

TABLE 1: Continued

Some initial studies with sirolimus plus MMF and corticosteroids, with or without antibody induction suggested that this CNI-avoidance strategy might be effective in terms of transplant outcomes and improved renal function (^{20, 21}). However, the benefit of replacing CsA with sirolimus in de novo patients remains a subject for debate. More recently, the Symphony study (²²), which was designed to find a regimen of low toxicity and high efficacy, compared CNI avoidance with low-dose sirolimus to low-dose cyclosporine, low-dose tacrolimus, and standard cyclosporine-based regimens (see the “CNI Minimization and/or Withdrawal” section). The choice of low-dose sirolimus and CNI trough levels was based on commonly used long-term maintenance levels (in this study used from the day of transplantation) and hypothesized to be equipotent. The results showed that they were not—the sirolimus-based regimen was less effective but also had the most toxicity and premature withdrawals.

In contrast, preliminary results with belatacept, a selective costimulation blocker, are more promising (²³). In 218 renal transplant patients (89% low risk) randomized to receive belatacept or CsA in addition to basiliximab, MMF, and corticosteroids, the incidence of acute rejection (defined as increase in serum creatinine of more than or equal to 44.2 μmol/L above prerejection level in the absence of confounding factors and biopsy consistent with acute rejection) after 6 months was similar for patients receiving an intensive regimen of belatacept, a less intensive regimen of belatacept, or CsA (7% vs. 6% vs. 8%). However, measured glomerular filtration rate (GFR) was significantly higher with both intensive- and less-intensive belatacept treatment than with CsA after 1 year (66.3 vs. 62.1 vs. 53.5 mL/min/1.73 m²) and the incidence of CAN was lower with both belatacept regimens than with CsA (29% vs. 20% vs. 44%) (²³). A note of caution is, however, necessary; altogether, only 44% of patients were evaluated for GFR and 69% for CAN.

Calcineurin Inhibitor Minimization and/or Withdrawal

With minimization and withdrawal strategies it is important to ensure that the doses of the remaining immunosuppressive agents provide optimal exposure to achieve adequate total immunosuppression.

Sirolimus-Based Regimens

Cyclosporine A can be withdrawn in patients receiving a sirolimus-based regimen (^24–26). In the Rapamune Maintenance Regimen Study, 430 recipients of renal allografts, selected out of 525 enrolled patients, initially treated with CsA, sirolimus, and corticosteroids, were randomized 3 months after transplantation to continue triple therapy or to have CsA withdrawn (sirolimus trough level 20–30 ng/mL) (²⁴). After 12 months, there was no significant difference between the groups for graft survival (primary endpoint) or patient survival, although the biopsy-proven acute rejection (BPAR) rate was higher in the CsA-withdrawal group than in the CsA-continuation group (3–12 months; 9.8% vs. 4.2%; P=0.035). Serum creatinine levels and calculated GFR at 12 months were significantly better in the CsA-withdrawal than the CsA-continuation group (142 vs. 158 μmol/L and 63 vs. 57 mL/min; both P<0.001) (²⁴). After 4 years, graft survival was significantly better in the CsA-withdrawal group (91.5% vs. 84.2%; P=0.024). Calculated GFR was 14.5 mL/min higher in this group at this time point (P<0.001) (²⁵). Similar benefit in renal function was reported with lower doses of sirolimus (trough levels 8–16 ng/mL) in a smaller (n=87) comparative study (²⁶).

It is not clear whether the superior graft function in the CsA-withdrawal (sirolimus) group compared with those continuing on a combination of CsA and sirolimus relates to the avoidance of CsA nephrotoxicity in the former group or to synergistic nephrotoxicity from CsA combined with sirolimus in the latter group.

Mycophenolate Mofetil-Based Regimens

Mycophenolate mofetil-based regimens allow for CNI withdrawal late (>12 months posttransplant) after transplantation in patients with renal dysfunction (²⁷) and for CNI dose minimization earlier (<12 months posttransplant) after transplantation (^{22, 28}). Although studies of early CNI withdrawal suggest a possible trend toward improved renal function, there seems to be an increased risk of acute rejection (^{28, 29}). Similarly, studies on CNI withdrawal in stable renal transplant recipients late after transplantation suggest that, although a modest improvement in renal function may be achieved, it could be at the risk of an increased incidence of acute rejection (^{30, 31}). This is in contrast to removal of CsA early after transplantation from a sirolimus-based regimen which seems to improve renal function and reduce rejection rates (²⁴).

In the “Creeping Creatinine” study, CNI withdrawal, when conducted in the presence of MMF and corticosteroids, improved renal function without increasing the risk of acute rejection in patients with deteriorating renal function secondary to chronic allograft dysfunction (²⁷). In this study, the mean time since transplantation was 6 years and patients had had deteriorating renal function for 3 to 18 months. A possible key factor was that no patient had subclinical acute rejection; a biopsy was conducted within 12 months before study entry (²⁷). Mycophenolate mofetil was started and increased to a dosage of 2 g/day before CsA was discontinued, and no rejections were reported during the course of this study.

In another study, 187 stable renal transplant recipients who were between 12 and 30 months after transplantation received CsA, MMF (2 g/day), and corticosteroids for 3 months before randomization to either CsA withdrawal (completed within 3 months) or CsA continuation (³⁰). In the intent-to-treat analysis, there was no significant between-group difference for creatinine clearance 6 months after withdrawal was completed, but in the per-protocol population (patients with acute rejection excluded), creatinine clearance improved significantly after CsA withdrawal (by 7.5 mL/min vs. the CsA-continuation group; P=0.02). However, CsA withdrawal was associated with an increased incidence of reversible acute rejection during the 1-year interventional period (10.6% vs. 2.4% in the CsA-continuation group; P=0.03) (³⁰). Moreover, although improved creatinine clearance was maintained in the CsA-withdrawal group through an additional 4 years of follow-up (67.4 vs. 61.7 mL/min; P=0.05), there was an increased risk of late acute rejection (10% vs. 1%; P=0.03) and a combined endpoint of acute rejection or graft loss resulting from rejection (19% vs. 5.5%; P=0.01) (³¹). There were no significant differences in the rates of graft loss or death. The authors of this study suggested that using mycophenolic acid (MPA) monitoring to ensure optimal MMF levels may have helped to avoid insufficient immunosuppression and rejection resulting from CNI withdrawal.

It seems that the level of the remaining immunosuppression after CNI withdrawal is another key factor. Indeed, it was recently shown in 172 stable renal transplant recipients (median time since transplantation 3.5 years) that CNI withdrawal from triple maintenance immunosuppression including MMF (2 g/day) and steroids can safely be achieved using the concentration-controlled approach for MPA (target area under the curve [AUC]-MPA 75 μg·hr/mL after CsA withdrawal), with the added benefit of improved renal function (³²).

Sufficient immunosuppression may be even more important early after transplantation. In a study of de novo renal transplantation, the incidence of acute rejection was compared in low-risk renal transplant recipients initially maintained on a triple drug regimen (CsA, MMF and prednisone) who were randomized to withdrawal of CsA (MMF-maintained group; n=54) or MMF (CsA-maintained group; n=54) at 3 months after transplantation (²⁹). Although renal function was improved in the MMF-maintained group compared with CsA-maintained group at 12 months posttransplant (Cockcroft calculated clearance 64.7±18.7 vs. 56.5±18.0 mL/min; P=0.023), acute rejection was higher in the MMF-maintained group (18.5% vs. 5.6%; P=0.045). Interestingly, the patients who developed acute rejection after CsA withdrawal (MMF-maintained group) had significantly lower MPA exposure (43±9 vs. 58±22 mg·hr/L; P=0.045) before CsA withdrawal. Multivariate analysis confirmed that along with borderline renal histologic changes as defined by the Banff classification scheme at 3 months, MPA exposure was a significant risk factor for acute rejection after CsA discontinuation, and a cut-off level for MPA AUC of 50 mg·hr/L was suggested before CsA withdrawal.

Insufficient MPA exposure after CsA withdrawal may also have contributed to the increased risk of acute rejection in the CAESAR study (²⁸). This study evaluated whether low-dose CsA (target trough levels 50–100 ng/mL; either administered continuously or withdrawn between months 4 and 6) administered as part of a regimen including interleukin-2 receptor antagonist induction, MMF (2 g/day), and corticosteroids was as effective as a regimen of standard-dose CsA, MMF, and corticosteroids in recipients of primary renal allografts (n=536). There was no benefit in renal function with reduced exposure to CsA. After 12 months, mean GFR (primary endpoint) in the CsA-withdrawal group was the same as that in the low-dose CsA group (50.9 mL/min/1.73 m²) and was not significantly greater than in the standard-dose CsA group (48.6 mL/min/1.73 m²). The incidence of BPAR was similar at 6 months (∼25%) but increased with CsA withdrawal and was significantly higher in the CsA-withdrawal group than in the low- or standard-dose CsA groups at 12 months (38% vs. 25.4% vs. 27.5%, respectively; P<0.05) (²⁸). A post hoc analysis found patients who experienced BPAR generally received lower MMF doses than rejection-free patients (²⁸). In addition to showing that CsA withdrawal was associated with increased acute rejection rates, the CAESAR study showed that low-dose CsA (with interleukin-2 receptor antagonist induction) produced similar rates of acute rejection to standard-dose CsA (without induction), but did not improve renal function (²⁸). This means that CsA concentrations in the low-dose CsA group may not have been low enough for differences in renal function to be apparent compared with the standard-CsA group. Alternatively, it may mean that reduced doses of CsA are still nephrotoxic.

The Symphony study was a further development of strategies for reducing the burden of CNI-associated adverse effects, and included both a CNI-free strategy incorporating sirolimus and two CNI-sparing strategies (²²). This group of investigators, having previously assessed the CNI-free strategy of daclizumab, MMF, and corticosteroids mentioned above (¹⁸) in a study that showed the necessity for an additional immunosuppressive agent, decided to add low doses of CsA, tacrolimus, or sirolimus. The Symphony study was also a further development of the CAESAR study (²⁸) that included the same low-dose CsA arm and the same control arm. Patients in the Symphony study were randomized to receive one of the three quadruple therapy regimens incorporating daclizumab induction, MMF 2 g/day, corticosteroids, and low-dose CsA, low-dose tacrolimus, or low-dose sirolimus. In the control group, patients received standard-dose CsA plus MMF and corticosteroids. After 12 months, patients in the low-dose tacrolimus group had a significantly higher mean calculated GFR (primary endpoint), lower rate of BPAR, and higher graft survival rate than the low-dose CsA, standard-dose CsA, and low-dose sirolimus groups (except for graft survival rate vs. low-dose CsA) (Table 1). The proportions of patients reporting treatment-emergent adverse events were similar in all groups (86.3%–90.5%), although serious adverse events were more frequent with sirolimus (²²). Moreover, reduced CsA dose did not result in significant improvement in graft function, nor in the CAESAR study either. Thus, a regimen of MMF, low-dose tacrolimus with interleukin-2 receptor antagonist, induction and corticosteroids was the optimum among the regimens evaluated.

CONCLUSIONS

Long-term use of CNIs is associated with serious toxicities, in particular, nephrotoxicity. Chronic CNI-associated nephrotoxicity is most probably irreversible and plays a part in limiting the long-term effectiveness of CNIs for renal graft function and survival. Various strategies aimed at reducing this effect have been evaluated, including CNI avoidance, dose reduction, and withdrawal. The aim of such approaches is to provide adequate immunosuppression usually with MMF-based regimens while reducing CNI-associated toxicity. The introduction of alternative regimens early after transplantation may mean that the development of CNI-associated nephrotoxicity could be avoided. Overall, strategies based on complete avoidance of CNIs have been disappointing. In contrast, several CNI-sparing regimens have shown at least comparable efficacy with standard CNI-dose regimens, and renal function may be improved. Late withdrawal of CNIs several years after transplantation may be feasible if adequate MMF doses/levels are ensured. However, the benefits of late CNI withdrawal in patients with stable renal function may come at a cost of increased acute rejection compared with those with a creeping creatinine—it is possible that the poor acute rejection rates in these patients may be due, in part, to insufficient immunosuppression in the absence of the CNI. Another issue of importance is to exclude patients with subclinical rejection from CNI withdrawal—or perhaps any minimization of immunosuppressive therapy. Early withdrawal of CNIs following their initial use after transplantation may improve renal function, but has also been associated with an increased risk of acute rejection. In low-risk patients, that is, those with no subclinical rejections at the time of withdrawal, it is possible that CNI withdrawal could be feasible within the first year, provided an MPA AUC of more than 70 mg·hr/L is achieved after CNIs are withdrawn, although this needs to be confirmed.

Calcineurin inhibitor-sparing in the format of reduced-dose tacrolimus has been shown effective and safe. Thus, in the Symphony study, a regimen of MMF 2 g/day, interleukin-2 receptor antagonist induction, corticosteroids, and low-dose tacrolimus (target trough 3–7 ng/mL) from the time of transplantation provided superior renal function and a significantly lower rate of BPAR than a standard-dose CsA regimen. It is likely that low-dose tacrolimus is less nephrotoxic than standard-dose CsA while still sufficiently immunosuppressive to prevent acute rejection and possibly also subclinical rejection. Moreover, the Symphony study confirmed the findings of the CAESAR study that lower doses of CsA did not improve renal function compared with standard-dose CsA, indicating that even lower doses may be required to avoid nephrotoxicity, but efficacy at preventing rejection would then more than likely be compromised.

It is important that research in this area continues, with the aim of achieving further improvements in the future. At present, CNI-sparing strategies may provide a means of maintaining immunosuppressive efficacy while reducing CNI-associated chronic nephrotoxicity in renal allograft recipients.

ACKNOWLEDGMENT

The author thanks Richard Glover for editorial support in the preparation of the manuscript.