PANEL DE DISCUSION
Deparment of Nephrology
Bellvitge Hospital. Barcelona. Spain firstname.lastname@example.org
Dose-related acute and chronic nephrotoxic effects of calcineurin inhibitors have been reported. Mihatsch et al. (1) found that the side effect pattern of these drugs (namely, cyclosporine and tacrolimus) is nearly identical: they were unable to differentiate morphological changes associated with their clinical nephrotoxicity.
Acute nephrotoxicity of calcineurin inhibitors
Acute CNI nephrotoxicity induces renal vasoconstriction which leads to a reduction of renal plasma flow, glomerular filtration rate and natriuresis. Recently, Amore et al. (2) showed that initial vasoconstriction depends upon an imbalance between the various modulators of the renal vascular tone, among which endothelins and nitric oxide seem to be the most powerful. However, these functional, abnormalities are reversible whenever CNI are withdrawn (3).
It was previously thought that the reversibility of CNI vasoconstriction was limited to the early stages of renal transplantation. Interestingly, Ader et al. (4) have recently shown that renal allografts are capable of retaining a good functional reserve in surviving stable CsA-treated kidney transplant recipients beyond the first year after grafting.
Recent body of evidence demonstrates that CsA induces TGF-b1 expression (5) and that TGF-b 1 stimulates endothelin production (6). Interestingly, Dogu et al. (7) showed that prostaglandin E1 inhibits CsA-induced up-regulation of TGF-b1.
Moreover, other strategies can also down-regulate TGF-b1 expression. Thus, Islam et al. (8) neutralized TGF-b 1 effect by employing an anti-TGF-b 1 antibody. They prevented both a creatinine clearance fall and CsA-mediated arteriolar hyalinosis in an experimental model, and they concluded that certain nephrotoxic features were clearly mediated by TGF-b 1.
Strategies to minimize acute CNI nephrotoxicity
In order to properly decrease the long-term effects of acute CNI nephrotoxicity, we have to bear in mind that different inter-related factors may have additive deleterious effects on kidney allograft survival (9). Thus, the confluence of different risk factors such as elder donors or prolonged cold ischemia times will definitely influence preventive therapeutic approaches.
Among several possibilities, the first potential approach would be the use of different drugs with a beneficial effect on ischemia-reperfusion damage (10). The most promising to date are shown in Table 1.
Long-term consequences of early CNI nephrotoxicity
Solez et al. (11) described a higher occurrence of chronic transplant nephropathy (CTN) among those patients who experienced previous episodes of clinical CsA nephrotoxicity when they analyzed 144 protocol biopsies performed 2 years after transplantation. We also have data supporting a potential long-term damaging effect of early CsA exposure (12, 13).
Therefore, it is possible that even seemingly normal levels of CsA might have deleterious effects on graft fibrosis and the use of the lowest CsA dose compatible with efficient immunosuppression should be recommended. The presence or absence of early damage in protocol biopsies also predicts long-term allograft outcome.
Immunosuppressive strategies to minimize early CNI exposure
Some groups initially tried to assess the efficacy of CNI-free induction protocols when managing patients receiving suboptimal grafts, where it would not be wise to expose them to a potential additional damaging factor. Therefore, a new array of non-nephrotoxic immunosuppressive agents have been introduced in clinical practice, including mycophenolate mofetil (MMF) and anti IL-2R monoclonal antibodies. The availability of these new drugs has further allowed the use of these CNI-free protocols in patients with delayed graft function or those who show at a later time clinical or histological evidence of CNI nephrotoxicity. It is well known that allografts from suboptimal donors are associated with a higher incidence of delayed graft function (14, 15) and that these grafts are more susceptible to CNI nephrotoxicity (16). We have recently reported our experience on 17 non-sensitized recipients of a suboptimal graft (17). The immunosuppressive induction protocol was based on five alternate doses of antithymocyte globulin plus MMF and steroids avoiding the use of CNI. As a maintenance treatment, patients were treated with MMF and steroids. In this study, we have shown that this protocol is efficient and can avoid the use of CsA in 70% of patients.
Before the appearance of these new non-nephrotoxic drugs, two strategies were employed to decrease the nephrotoxic effects of CNI during the early post-transplant period: the delayed introduction of CsA in the so-called sequential induction protocols (18-20); or the use of concomitant low doses of CsA (21-23). Both induction regimens are associated with the use of antilymphocytic polyclonal or monoclonal antibodies.
With the advent of new immunosuppressants (MMF, rapamycin and monoclonal antibodies against IL-2R) novel sequential or concomitant protocols may be designed using even lower doses of CNI. Strategies to minimize chronic nephrotoxicity induced by calcineurin inhibitors
There is a quite general agreement that the main risk factors related to chronic graft dysfunction are donor-related risk factors, such as age > 50 years and/or the presence of a reduced nephron mass; ischemia-reperfusion injury secondary to prolonged cold ischemia times; severe acute rejection-mediated renal damage; and acute and/or chronic CNI-induced nephrotoxicity. The main target of these damaging pathways leading to chronic graft dysfunction is the renal vasculature. Myers et al. (24) reported 10 years ago the appearance of CsA-induced nephrotoxicity in a group of 37 cardiac transplant patients treated with high CsA doses for 12-24 months.
They found increased renal vascular resistance, decreased glomerular filtration rate, proteinuria and hypertension. Common pathological findings were obliterative arteriolopathy and tubulointerstitial damage. This chronic CsA damage was rarely reversible and potentially progressive.
a) Maintenance protocols without anticalcineurin agents
Beyond the previously mentioned protocols used for suboptimal donors, some groups have employed new non-nephrotoxic immunosuppressants without CNI in conventional primary cadaveric kidney transplantation. Similarly to anticalcineurin agents, recently developed humanized (Daclizumab, zenapax) and chimeric (basiliximab, simulect) anti IL-2R monoclonal antibodies that interfere with IL-2 synthesis. Its homology to humans explains the very prolonged therapeutic effect of this drug.
On behalf of the DZB/MMF Combination Renal Transplant Study (25), Grinyó reported promising results with the combination of DZB, MMF and steroids, without CNI, in recipients of first renal allografts. Five doses of DZB were given intravenously every 2 weeks and MMF was initially given orally at 3g/day and tapered to 2g/day after 6 months. Acute rejection episodes appeared in 27% of patients who then received CNI. This protocol seems to be an effective and safe non-nephrotoxic immunosuppressive regimen that may avoid the use of CNI in almost 75% of treated patients.
b) Calcineurin inhibitor conversion to mycophenolate mofetil
A different approach is represented by those protocols which use CNI during the early phase post-transplantation, but then switch to a non-nephrotoxic drug (26, 27).
It is noteworthy that the later CNI is switched to MMF, the lower the likelihood of late acute rejection episodes. Houde et al. (28) reported 10 patients with biopsy-proven CsA nephrotoxicity who were switched from CsA to MMF 4 years after transplantation. In this study, all patients increased their creatinine clearance whereas no episodes of acute rejection were observed. On the other hand, Schrama et al. (26) and Van Gelder et al (27) performed an earlier switch, 6 or 12 months post-transplantation, respectively. In both studies, the conversion of CsA to MMF was safe in the vast majority of patients and was followed by an improvement of renal hemodynamics, blood pressure and serum uric acid. Unfortunately, some patients experienced acute rejection episodes but these were either steroid-sensitive or responsive to the reintroduction of CsA.
Long-term functional effects of these early conversions remain a matter of discussion.
c) Maintenance therapies with low doses of calcineurin inhibitors.
In an attempt to minimize chronic CNI nephrotoxicity and analyze its potential reversibility, several authors significantly reduced the previous administered CsA doses in patients with suspected chronic CNI nephrotoxicity. Thus, Mourad et al. (29) assessed 23 renal transplant patients with biopsy-proven CsA nephropathy who were followed for more than 2 years either after a CsA dose reduction (18 patients) or CsA withdrawal (five patient) associated with the introduction of azathioprine. They observed a significant increase in the glomerular filtration rate and renal plasma flow, as well as a significant decrease in serum creatinine. Only one episode of acute reversible rejection was recorded. Nevertheless, chronic rejection developed in three cases.
Other groups have recently reported the use of MMF in this context (30, 31).We have evaluated the impact of CsA reduction associated with MMF introduction, both on TGF-b 1 production and renal function in 18 long-term stable renal allograft recipients with suspected CsA nephrotoxicity. The CsA dose was reduced to reach whole-blood levels between 40 and 60 ng/mL within one month. The introduction of MMF allowed us to decrease significantly the CsA dose and CsA levels, and an increase in both the filtration rate and renal plasma flow was recorded. We also showed for the first time in humans that this manoeuver was followed by a reduction in plasma TGF-b 1 levels (from 4.6 to 2 ng/ml;p =-0.003). Cyclosporine levels were positively correlated with TGF-b 1 levels (R = 0.536, p = 0.0002). We observed no rejection episodes and there was a significant improvement in both systolic and diastolic blood pressure (30). Weir et al. (31) followed a similar approach in a group of patients with histologically proven CTN. They added MMF, halved the dose of CSA and observed an improvement of renal function in 21 out of 28 patients in the absence of acute rejection episodes. Thus, we believe that a CNI dose reduction should not be limited to the patients with a clear-cut biopsy-proven CNI nephrotoxicity or CTN, but it could be also attempted in patients with a progressive deterioration of renal function (namely chronic graft dysfunction), provided that no signs of ongoing acute rejection are present. Nevertheless, in cases with stable suboptimal renal function (i.e. serum creatinine between 140 and 300 m mol/l, proteinuria < 1g/day) we think that a biopsy is not mandatory.
Hong Song et al. (32) have confirmed by immunohistochemical staining of renal allograft biopsies that chronic reduction of CsA may diminish the production of TGF-b 1 and they have also shown that this manoeuver decreases the expression of TGF-b 1 receptor. It results in a decreased collagen synthesis and stabilization or improvement of fibrosis in the tissue of patients with CTN.
d) Interference with molecular signals leading to scarring
Antagonizing TGF-b 1production:
Since direct antagonism of TGF-b 1 effects is not yet clinically available, an indirect strategy has been recently reported. Peters et al. (33) demonstrated that both an angiotensin I converting enzyme inhibitor (enalapril) and the angiotensin II receptor blocker losartan reduced TGF-b 1 overproduction in an experimental rat model of glomerulonephritis. Furthermore,
Campistol et al. (34) have shown that treatment of hypertensive renal transplant patients with losartan not only achieved control of hypertension and a fall in urinary protein excretion but also significantly decreased the levels of TGF-b 1 and ET-1. We do not know yet whether these drugs will help to limit CNI nephrotoxicity by interfering these pathways.
Modulating extracellular matrix metabolism and decresasing interstitial cellular activity: Investigation of the molecular events that lead to renal fibrosis in rodent models indicate that different pathways may be at work, such as up-regulation of TGF-b 1 and disturbances in extracellular matrix turnover, as well as infiltration of cells of the macrophage/monocyte lineage.
It seems that MMF may attenuate macrophage infiltration in a remnant kidney rat model, and this drug largely diminished glomerular and interstitial injury (35). Hence, in addition to its immunosuppressive properties, MMF may have beneficial non-immunological effects that might further attenuate CNI toxicity.
Minimizing transplant vasculopathy:
Many groups have reported the presence of an association between the pre- or post-transplant cholesterol levels and chronic rejection (36-38). Importantly, our group has shown that total serum cholesterol before transplantation was the only independent predictor for the development of early transplant vasculopathy (13). Thus, lipid-mediated damage might also contribute to the appearence and/or progression of chronic rejection. It is possible that renal transplant patients could also benefit of an early treatment with statins (13). Moreover, it has been shown recently that statins may inhibit progressive chronic vascular rejection in rat cardiac allografts, and that they may act by mechanism other than their effects on the lipid profile (39). Thus, statins may also modulate extracellular matrix turnover and block macrophage infiltration.
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Acknowledgements: I would like to thank Mrs. Carmen Fernández for her secretarial assistance.