Monoclonal antibodies in renal transplantation: old and new

Dirk R. J. Kuypers and Yves F. C. Vanrenterghem

Department of Nephrology and Renal Transplantation, University of Leuven, Belgium

Correspondence and offprint requests to: Dirk Kuypers, Department of Nephrology and Renal Transplantation, University Hospitals Leuven, Herestraat 49, B-3000 Leuven, Belgium. Email: dirk.kuypers{at}uz.kuleuven.ac.be

Keywords: Campath1-H; CTLA4Ig; kidney transplantation; monoclonal antibodies

The first monoclonal antibody (MoAb) used for prevention and treatment of allograft rejection was OKT3, a murine antibody directed against the 20 000 Da molecular subunit linked to the T-cell antigen receptor [1]. Although several studies demonstrated the efficacy of OKT3 in the prevention and treatment of acute rejection (AR), its widespread clinical use was hampered by a number of serious side effects that can be summarized in four categories. The first, administration of the drug is often accompanied by a cytokine-release syndrome with high fever, chills, arterial hypertension and even pulmonary oedema as result of capillary leak. Secondly, a substantial number of patients will develop antibodies against the xenogeneic epitope that are responsible for decreased efficacy. Thirdly, cross-reactions with non-target tissue may produce thrombocytopenia and neutropenia or aseptic meningo-encephalitis. Lastly, a higher incidence of infectious complications and malignancies has been reported, suggesting over-immunosuppression.

More recently, through molecular engineering, chimeric and humanized MoAb's have been generated in an attempt to reduce the abovementioned side effects. In chimeric antibodies the variable antigen binding site region is of murine origin, while the constant region is human. In the humanized antibodies the recombinant genes encoding the antibody are a mainly human (90%) composite and only a small part is murine. The reduction in xenogeneic epitopes precludes the formation of anti-xenogenic antibodies and the risk of loss of efficacy. Because of alterations in the constant region, cytokine-release is attenuated while the narrow selection of target antigens on T-lymphocytes makes these MoAb less prone to overall lymphocyte depletion and hence lowers the risk of inducing overwhelming infections.

Intensive research has generated a variety of new MoAb's directed against specific epitopes that play an important role in the activation of T-lymphocytes. Only a few have already been proven efficacious and safe in human studies. The largest experience exists for the MoAb's against the interleukin-2 receptor (IL2-R); for all other MoAb's the experience is still limited. These new MoAb's may not only serve their purpose as induction or anti-rejection therapy, but also as maintenance immunosuppression, prevention of ischaemia-reperfusion injury, and induction of donor-specific hyporesponsiveness or ‘near’-tolerance.

Monoclonal antibodies against the interleukin-2 receptor (anti-IL2-R-MoAb's)

Anti-IL2-R-MoAb's are directed against the {alpha} chain (CD25 or Tac: T-cell activation) of the IL-2-R. Resting T-cells do not express high affinity IL-2-R, but their expression is quickly up-regulated upon activation through binding of the allo-antigen to the T-cell receptor.

Daclizumab (Zenapax®) is a humanized MoAb with an estimated elimination half-life of 20 days in renal transplant patients. Two double-blind multi-centre studies have demonstrated its efficacy when given as a five-dose regimen of 1 mg/kg body weight [2,3]. In the 1-year pooled data of these two trials the incidence of biopsy-proven AR was reduced from 43.3 to 27.7% (P < 0.001). In both studies patients received cyclosporine and steroids; in the US study azathioprine was added. In more recent studies, daclizumab has also been used with cyclosporine or tacrolimus in combination with mycophenolate mofetil (MMF) and steroids [4]. Although most studies examined a five-dose regimen of daclizumab, recent data suggest that a two-dose strategy may be equally effective [5].

Basiliximab (Simulect®) is a human/mouse chimeric MoAb combining the entire variable region of the murine MoAb with human heavy and light chain constant regions. Its efficacy was demonstrated in two double-blind multi-centre trials [6,7]. In these studies basiliximab was given in a two-dose regimen of 20 mg with one administration prior to transplantation and a second on day 4 post-transplantation. In both studies patients also received Neoral and corticosteroids. Similar to the trials with daclizumab, the incidence of AR was significantly reduced from 44 and 46% to 30 and 33%, respectively. The tolerability of both daclizumab and basiliximab was excellent with complete absence of severe first-dose cytokine-release effects and a comparable rate of infectious complications rate in the experimental and the placebo arms. The rather short follow-up precludes too strong statements but until now no increase in the occurrence of malignancies has been reported.

An additional advantage of anti-IL2R MoAb's is the fact that their use allows for substantial reduction or even avoidance of more toxic drugs, especially nephrotoxic calcineurin-inhibitors. The possibility of completely avoiding the use of cyclosporine by giving daclizumab in combination with a high dose of MMF (3 g/day) and steroids was recently tested in a multi-centre controlled trial [8]. The percentage of patients who were spared from calcineurin-inhibitor therapy during the first year was only 38% and biopsy-proven AR were seen in 53% of the patients. A much lower incidence of AR (6.4%) was found with the combination of basiliximab, MMF, steroids and sirolimus [9]. The use of IL-2-R MoAb's also makes calcineurin-inhibitor sparing regimens more feasable. In two trials it was shown that the calcineurin-inhibitor dose could be reduced by 50% without an increased risk of rejection [10,11]. The lower dose of calcineurin-inhibitor resulted in better renal function.

Efalizumab

Efalizumab is a non-lymphocyte depleting, humanized IgG1 MoAb against the CD11a chain of the Leukocyte-Function-Associated Antigen 1 (LFA1). It is capable of blocking binding of LFA1-ICAM, thereby blocking T-cell adhesion, trafficking and activation. The drug was recently tested in a dose-ranging trial in renal transplants recipients [12]. Combined with either cyclosporin, MMF and steroids or with reduced-dose cyclosporin, sirolimus and steroids, the use of efalizumab resulted in a 10.4% incidence of AR at 6 months [12]. However, in the group that received a high dose of the drug (2 mg/kg/week), three patients (three of 10) developed post-transplant lymphoproliferative disease. It therefore seems prudent to await further data on the use of lower dosages of this powerful drug since earlier (pre-)clinical experience with odulimomab, another anti-CD11a MoAb aimed at prevention of ischaemia-reperfusion injury, did not lead to clinical development of the drug because of the disappointing outcomes.

Monoclonal antibodies directed against the co-stimulatory signals

When alloantigens bind to the specific receptor of the naïve T-lymphocyte, a second or co-stimulatory signal is needed to obtain full T-cell activation. At least two co-stimulatory signalling pathways have been recognized as being essential in this respect. The first consists of CD80 and CD86 receptors (also known collectively as B7 molecules) that are expressed on many cells including antigen presenting cells (APC), B-cells and activated endothelial cells. Both molecules bind to either of the T-cell co-stimulatory receptors, CD28 and CD152, leading to stimulatory or down-regulatory signals, respectively. The second signalling pathway consists of the CD40 molecule present on APC that binds the CD154 receptor expressed on activated lymphocytes. MoAb's directed against these co-stimulatory effector molecules have been developed and tested in animal models. Subsequently, humanized MoAb have been derived and tested in non-human primates and humans. A humanized MoAb directed against CD154 (the CD40 ligand), hu5C8, was used in a non-human primate renal allograft model and although results were very promising [13], a subsequent pilot study in human renal allograft recipients was stopped because of the occurrence of severe thrombo-embolic complications.

A phase I study in de novo renal transplant recipients was initiated last year, comparing two different dose-regimens of LEA29Y—a MoAb binding to CD80 and CD86—with a control-group receiving cyclosporine, MMF and steroids. All three treatment groups received induction therapy with basiliximab while patients receiving LEA29Y were maintained on MMF and steroids. Early results of this trial are expected in the beginning of 2004 and may provide more insight in the possible role of blocking one co-stimulatory signal in the prevention of AR and prolongation of allograft survival. Its mechanism of action, the fact that LEA29Y appears to be devoid of serious adverse events and the intermittent dosing strategy, make this drug an attractive candidate to ‘probe’ the path of tolerance in future trials. Achieving specific tolerance has been tried recently with the MoAb Campath-1-H but also with the old polyclonal rATG.

The lymphocyte depleting anti-CD52 MoAb Campath-1H (Alemtuzumab)

The anti-CD52 rat IgM MoAb Campath-1 causes profound T-lymphocyte depletion through a process of complement-dependent lysis. The use of Campath-1 in organ transplantation goes back to 1986 when Calne et al. reported on the preliminary results of a pilot study in renal, liver and pancreas transplant recipients. This pilot study was followed by a randomized controlled trial [14]. Fifty-two renal transplant recipients were randomized to receive either cyclosporine monotherapy or cyclosporine in combination with Campath-1 for 10 days (25 mg twice daily). Although patients receiving Campath-1 experienced significantly less AR, they had more infectious complications and infusion-related side-effects. The latter were most probably related to the xenogenic nature of the rat antibody. A humanized form of Campath-1, alemtuzumab (Campath-1H) has recently been tested by Calne's group in 31 renal allograft recipients in association with a low-dose cyclosporine [15]. Campath-1H was now used in a lower dose of 20 mg (two gifts, on day 0 and day 1 after transplantation). Twenty-nine of the 31 grafts are still functioning and only six episodes of steroid-sensitive AR (19.3%) were seen. Similar results were obtained when Campath-1H was combined with sirolimus monotherapy [16]. Finally, in a small study of non-sensitized recipients of living-donor kidneys, T-cell depletion with Campath-1H monotherapy was not sufficient to prevent acute renal allograft rejection in all seven study patients and consequently lead to the initiation of (mostly sirolimus) maintenance therapy after treatment of the AR [17]. The Campath-1H-associated cytokine-release syndrome could be completely prevented by co-administration of steroids and no serious drug-related adverse events occurred. Opportunistic infections and malignancies were of grave concern with a potent lymphocyte-depleting agent like this but an excessive frequency was not reported in these first clinical trials. A further reassurance is the finding that—as opposed to the rat derived Campath-1G—the humanized form of Campath is far less immunogenic, potentially allowing repeat courses of treatment. This rather extraordinary profile makes Campath-1H an interesting molecule for further clinical development despite the fact that in its present situation it is not a tolerogenic agent. Patients never treated with MoAb's and maintained on a low-dose azathioprine and steroids [18], recipients that received total lymphoid irradiation [19] or more recently, tacrolimus monotherapy [20], are in fact not much different from the patients who became ‘near’-tolerant in the alemtuzumab trials. It is also noteworthy that similar results of ‘near’ specific tolerance have been found by using the old polyclonal rabbit ATG in combination with either an intermittent, very low dosing strategy of tacrolimus monotherapy or with sirolimus monotherapy [21,22].

Conclusion

New monoclonal antibodies have emerged that are potentially valuable candidates for using in innovative low-dosed, drug sparing immunosuppressive combinations. Some of these new compounds might in the future even play a role in the quest for specific tolerance. However, at this moment none of the studied regimens is capable of inducing graft acceptance as one would expect in case of donor-specific tolerance. It remains therefore a challenging task to differentiate whether current minimal immunosuppressive drug regimens will lead to ‘near’ acceptance of the transplanted organ or on the contrary will result in more severe chronic immunological graft damage and early graft loss.

Conflict of interest statement. None declared.

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