Service de Transplantation, Hôpital Necker, Paris, France
Keywords: immunosuppressive drugs; rapamycin; renal transplantation; sirolimus
Introduction
For about 30 years, maintenance immunosuppression in the field of organ transplantation has been limited by the availability of only three drugs: corticosteroids, azathioprine, and cyclosporin. For the last 10 years, however, new immunosuppressive agents such as tacrolimus and mycophenolate mofetil (MMF) have been developed and are now available for transplant patients. Sirolimus (rapamycin), the latest one, is still being developed. It seems a very promising immunosuppressive drug which could in future replace calcineurin inhibitors such as cyclosporin.
Originally described in 1975 as an antibiotic of the macrolide family [1] the immunological activity of sirolimus was reported in 1977 in a rodent model of autoimmune encephalomyelitis [2]. However, it is only recently that its effect on the immune system has generated great interest, because its structure resembles that of tacrolimus, another potent immunosuppressive macrolide [3]. Both in vitro and preclinical studies have confirmed the immunosuppressive properties of sirolimus, and have shown how it blocks the immune response.
This review aims at defining the rationale for including sirolimus in therapeutic regimens after renal transplantation, on the basis of its two main characteristics: it has an original mechanism of action and it does not induce nephrotoxicity.
Mechanisms of action
T-cell activation comprises two major phases. The first, which follows triggering of the T-cell receptor, results in the transcriptional activation of cytokine genes, and leads from the quiescent T-cell state (G0) to the competent state (G1). The second phase involves the response of the competent T cells, i.e. their secretion of growth-promoting interleukins (IL) such as IL2 or IL4, in an autocrine or paracrine fashion. It also drives competent T-cell entry into the proliferation cycle via progression from the G1 to the S phase, with subsequent clonal expansion and the acquisition of T-cell effector functions.
The mechanism of action of sirolimus is distinct from that of cyclosporin and tacrolimus [4]. The latter drugs inhibit the first phase of T-cell activation. They interrupt the signal from the T-cell receptor by blocking calcineurin, a serine-threonine phosphatase required for transcriptional activation of the IL2 gene in response to T-cell antigen receptor engagement, thereby inhibiting IL2 production and subsequent T-cell stimulation. In contrast, sirolimus interferes with the second phase of T-cell activation. It interrupts the signal from the IL-2 receptor and the receptors for other cytokines and growth factors. It blocks the signal transduction pathway required for the progression of cytokine-stimulated T cells from the G1 into the S phase, thus suppressing interleukin-driven T-cell proliferation.
Note that tacrolimus and sirolimus have the same intracellular target, namely the FKBP12 protein, whereas cyclosporin binds to another intracellular receptor, cyclophylline P [4].
At first it appeared that the effect of sirolimus might be complementary to that of calcineurin inhibitors and that it would be best to combine it with cyclosporin, to avoid competition with intracellular drug-binding proteins.
Absence of nephrotoxic effects
Animal studies in Wistar rats and pigs have demonstrated that unlike treatment with calcineurin inhibitors, sirolimus treatment alters neither renal blood flow nor glomerular filtration rate [5,6]. This absence of nephrotoxicity has been confirmed in humans. At the end of a 12-week treatment with sirolimus at three different doses, psoriasis patients exhibited normal renal function, as did control group patients given a placebo only (Wyeth-Ayerst, personal communication). In addition, the results of two phase-II randomized trials involving renal transplant recipients, in which sirolimus and cyclosporin were compared as base immunosuppressive therapy, showed that 2 years after transplantation, serum creatinine levels were always lower in sirolimus-treated patients [7,8]. These results also showed that unlike calcineurin inhibitors, sirolimus does not induce nephrotoxicity.
Use of sirolimus in human renal transplantation
Combination of sirolimus and cyclosporin
Because in vitro and animal studies have confirmed the synergistic immunosuppressive effect of cyclosporin and sirolimus [9,10], these drugs were initially combined in human renal transplantation. Two large phase-III and randomized studies showed that the cyclosporinsirolimus combination reduced the number of acute rejection episodes significantly, more than either a placebo or azathioprine [11]. Nevertheless, despite this reduction, the apparent absence of intrinsic nephrotoxicity, and similar cyclosporin trough levels in the groups compared, sirolimus patients had worse renal function at 1 year than those treated with placebo or azathioprine (Wyeth-Ayerth, personal communication). These results suggest that for still unknown reasons, sirolimus might increase the renal toxicity of cyclosporin. Therefore the combined use of sirolimus and cyclosporin should be avoided on a long-term basis.
One possibility was to use this combination as induction therapy for a few months, and then to discontinue cyclosporin. Two such randomized multicentre trials are still in progress. Preliminary interim results seem satisfactory, with an acceptable overall number of acute rejections, and only a small increase in the cyclosporin cessation group. More relevant is the significant improvement in renal function in this group after only 6 months of follow-up. Thus, a 23-month treatment with the cyclosporinsirolimus combination seems safe and effective, and the higher sirolimus trough levels after cyclosporin cessation give better results than when cyclosporin is maintained.
Sirolimus as base immunosuppressive therapy
Because sirolimus has potent immunosuppressive properties and does not cause intrinsic nephrotoxicity, it has also been used as base immunosuppressive therapy together with steroids and either azathioprine or MMF. Two prospective phase-II randomized European studies using therapeutic dose monitoring centred on the comparative effects of combining either cyclosporin or sirolimus with steroids and azathioprine [7] or with steroids and MMF [8]. In one study, the incidence of acute rejection episodes in the sirolimus and cyclosporin groups was comparable (41 vs 38%) [7]. In the other, patients given sirolimus, steroids, and MMF had slightly, but not significantly, more acute rejection episodes than those given cyclosporin, steroids, and MMF (27.5 vs 18.5%) [8]. Note that in both studies, renal function at 1 year was significantly better in the sirolimus than the cyclosporin group. These results suggest that in a triple-drug immunosuppressive regimen, sirolimus has almost the same efficacy as cyclosporin and that in contrast to cyclosporin, it is unlikely to cause chronic nephrotoxicity. They also imply that sirolimus is a non-nephrotoxic agent capable of competing with cyclosporin for the prophylaxis of acute rejection in renal transplantation. However, it should not be combined with cyclosporin for more than 23 months.
Adverse events
The clinical side-effects of sirolimus treatment are mainly asthenia, headache, epistaxis, diarrhoea, and arthalgia, and common biological adverse events are thrombocytopenia, hyperlipidaemia, and occasionally anaemia and leukopenia. Hypertriglyceridaemia and hypercholesterolaemia are reversible and can be managed by dose reduction and/or the addition of antihyperlipidaemic agents. In the two European studies referred to above [7,8], comparison of sirolimus and cyclosporin as base immunosuppressive therapy showed that the percentage of hypertriglyceridaemia and hypercholesterolaemia during the first year of treatment was higher in the sirolimus than the cyclosporin group (51 vs 12% and 73 vs 50% respectively for hypertriglyceridaemia, and 44 vs 14% and 65 vs 45% for hypercholesterolaemia). However, the differences were maximal at month 2, and diminished considerably by month 6. At month 12, serum cholesterol and triglyceride levels were not significantly different in the two groups [8]. More patients were given either statins or fibrates in the sirolimus than in the cyclosporin group.
Sirolimus-induced thrombocytopenia, anaemia, and leukopenia are usually mild. They are dose-dependent and not clinically relevant [7,8]. A slight increase in lactate dehydrogenase is also commonly associated with sirolimus therapy.
Infection with herpes simplex virus and pneumonia are more frequently reported with sirolimus than cyclosporin [7,8], but there is no significant difference in the incidence rates for moderate or severe opportunistic or common transplant-related infections.
We and others recently reported cases of interstitial pneumonitis which had no identified infectious aetiology and may have been due be to sirolimus treatment [12,13]. The mechanisms and clinical relevance of this possibly drug-induced pneumonitis are still unknown.
Conclusions and prospects
For the last 4 years, sirolimus has proved a potent immunosuppressive drug in humans. It has been successfully used with and without cyclosporin, and might be a good alternative to calcineurin inhibitors [7,8]. Its original mechanisms of action and toxicity profile make it a true immunosuppressive agent. Given these characteristics, other combinations need testing in randomized studies. Preliminary data in humans showed that tacrolimus could be an efficient and safe partner for sirolimus [14,15].
Many other properties of sirolimus will have to be further investigated. Animal studies suggest that its mechanism of action might make sirolimus a good immunosuppressive agent in induction tolerance protocols [16]. In vitro and animal studies using a sirolimus analogue have suggested that sirolimus has an antiproliferative effect on EpsteinBarr virus (EBV)-infected B cells, and a therapeutic effect on EBV-induced lymphoma [17]. Finally, the fact that sirolimus is not nephrotoxic, and that in animal models it inhibited growth factor signal transduction [18] and vascular smooth-muscle cell proliferation [19] makes it a good candidate for the treatment or prevention of chronic allograft nephropathy.
Note added in proof
Data on sirolimus and azathioprine treatment have been recently published by Barry D. Kahan in the paper entitled Efficacy of sirolimus compared with azathioprine for reduction of acute renal allograft rejection: a randomized multicentre study,' Lancet 2000; 356: 194202.
Notes
Correspondence and offprint requests to: Emmanuel Morelon, Service de Transplantation, 149 rue de Sèvres, F-75743 Paris Cedex 15, France.
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