Alemtuzumab induction therapy in solid organ transplantation

Alemtuzumab (Campath) is a monoclonal antibody that has a profound lymphocyte-depleting effect, targeting the CD52 antigen that is present on all lymphocytes. Alemtuzumab has been used for the treatment of chronic lymphocytic leukaemia and various autoimmune disorders, and has also shown potential as an induction agent in the prevention of rejection following solid organ transplantation. Alemtuzumab has been studied in randomised controlled trials and has demonstrated low levels of rejection in renal transplant recipients compared with other induction agents, albeit mainly in the early months following transplantation. Studies have shown that alemtuzumab enables the use of lower calcineurin inhibitor (CNI) maintenance drugs; however, this reduction in nephrotoxic immunosuppression has not consistently been matched by an improvement in renal function. The hypothesis has been suggested that alemtuzumab might allow the development of immunosuppressive regimens that avoid CNIs completely; studies have investigated the combination of alemtuzumab with mammalian target of rapamycin-inhibitor maintenance therapy, and, in particular, sirolimus. Initial studies with this combination showed that regimens of sirolimus alone and of sirolimus with mycophenolate mofetil were unsuccessful, with a high rate of rejection and complications. Subsequent studies have targeted the combination of alemtuzumab induction with a short course of a CNI, before switching to maintenance therapy with sirolimus. This regimen might combine good protection from acute cellular rejection and chronic nephrotoxicity. A randomised controlled trial has been established to study this regimen, with results pending.


Introduction
Th e design of an immunosuppressive strategy must take into account the three major causes of morbidity following organ transplantation: ischaemia-reperfusion injury (particularly in marginal donor organs); rejection; and complications of drug treatment (toxicity, infection and cancer). Induction immunosuppression is now used in the majority of transplant programmes in solid organ transplantation.
Induction therapy is intended to protect the graft from the immediate postoperative period of high rejection risk. However, there is increasing interest in the eff ects of induction agents in mitigating the eff ects of ischaemiareperfusion injury. With respect to mainte nance therapy, the priority in the early post operative weeks is to prevent rejection and minimise postsurgical infections and other surgical complications. Subse quently, drug toxicity, opportunistic infection and cancer risk become the primary concerns.
Despite several decades of experience, there is no consensus regarding many aspects of induction therapy. Both polyclonal antibodies and monoclonal antibodies are used with no clear evidence as to which is preferable. Similarly, there is little agreement as to whether depleting or nondepleting antibodies are preferable. Th ere is a range of targets from the highly selective anti-CD25 antibodies (basiliximab) that target an antigen (the interleukin-2 receptor) only present on activated lymphocytes to antibodies that recognise all peripheral blood lymphocytes (for example, antithymocyte globulin (ATG), alemtu zumab). Although anti-CD25 antibodies are very widely used globally, there is now increasing interest in the potential benefi ts of depleting agents with a broader spectrum. Th e advantages that have been hypothesised for these agents include reduction of ischaemia-reperfusion injury, a more profound protection from early acute rejection and, fi nally, a long-lasting eff ect in relation to donor-specifi c hyporesponsiveness.

Alemtuzumab
Alemtuzumab (Campath) is a monoclonal antibody that recognises the CD52 antigen, present on all peripheral blood lymphocytes. Campath antibodies have been tested experimentally and clinically for 30 years [1].
Initially an IgM rat-derived antibody (Campath-1M) was used for the removal of T cells from bone marrow for transplantation; this was carried out as a means to prevent graft versus host disease. It was noted at the time that an antibody of profound lymphocyte-depleting capabilities would have potential application in solid organ transplantation. Campath-1M was used in clinical renal transplant trials and was found to have a profound eff ect on peripheral lymphocyte numbers, although with rapid recovery between doses [2], and was also shown to reduce the incidence of acute rejection [3]. Smaller studies with the IgG2b isotype (Campath-1G) demonstrated a more profound and long-lasting depletion of lymphocytes [4].
Th e humanised antibody (Campath-1H) was developed with the potential advantages of greater effi cacy and, second, a lower risk of sensitisation. Th is antibody was used for (and subsequently licensed for) the treatment of chronic lymphocytic leukaemia [5]. Campath-1H was also used in a number of autoimmune conditions [6][7][8] and shown to be eff ective in treating multiple sclerosis [9]. Th e fi rst report of the use of the humanised antibody in solid organ transplantation was in the treatment of acute cellular rejection of kidney transplants [10], which was associated with a high incidence of infections.

Randomised trials of alemtuzumab in solid organ transplantation
A number of randomised trials of alemtuzumab in solid organ transplantation have been published. In 2008 Margreiter and colleagues studied 131 patients randomised to receive induction therapy with alemtuzumab (2 × 20 mg) versus no induction [11]. All patients were subsequently treated with tacrolimus and patients who had not received induction therapy also received mycophenolate. Th is trial demonstrated a signifi cant reduction in the incidence of acute rejection at 6 months (29% vs. 15%, P = 0.05). However, by 12 months the incidence of rejection was no longer signifi cantly diff er ent (32% vs. 30%). Th ere was no diff erence in renal function between the two groups and, apart from an increase in the incidence of cytomegalovirus infection, there was no other adverse consequence of induction therapy.
In 2011 Chan and colleagues reported a randomised controlled trial of 82 alemtuzumab-treated patients (with tacrolimus monotherapy) versus 42 controls (receiving dacluzimab, tacrolimus, mycophenolate), all patients having a rapid steroid taper [12]. Th is study showed a lower incidence of rejection in the alemtuzumab-treated group at 6 months but not 12 months and no diff erence in graft survival or function. However, the study confi rmed that alemtu zu mab therapy does enable simple and eff ective tacrolimus monotherapy with a very low rate of rejection.
Also in 2011, Hanaway and colleagues carried out a randomised trial involving 474 kidney transplant recipients [13]. Th ese were divided into high-risk patients (defi ned as panel reactive antibody >20%, black race, retransplant; n = 139) and low-risk patients (all others). Th e high-risk group was treated with either ATG (1.5 mg/ kg × 4, n = 70) or alemtuzumab (30 mg × 1, n = 69). Th e low-risk group of patients was treated with basiliximab (20 mg × 2, n = 171) or alemtuzumab (30 mg × 1, n = 164). All patients received tacrolimus 8 to 14 ng/ml, mycophenolate 2 g/day, and steroids for 5 days. Patients were followed up for 3 years. Th is trial demonstrated that patients treated with alemtuzumab had a signifi cantly lower incidence of cellular rejection at 6, 12 and 36 months (3% vs. 15%, 5% vs. 17% and 13% vs. 20% respectively). When divided into the high-risk and lowrisk subgroups, it was clear that the reduction in rejection was entirely within the group of patients at low risk of rejection (alemtuzumab vs. basiliximab) and that the incidence of rejection in the high-risk patients was similar between the alemtuzumab and ATG groups. Th ere was no signifi cant diff erence in graft or patient survival. Analysis of lymphocyte populations demonstrated that alemtuzumab and rabbit ATG had similar eff ects with respect to lymphocyte depletion and the rate of recovery of lymphocyte populations. Th is was contrasted in the low-risk group, in which the lymphocyte population was profoundly suppressed by alemtuzumab but very little altered by treatment with basiliximab. Th ere was no eff ect on renal function. Th is trial did show that alemtuzumab-treated patients are more liable to late rejection episodes (after 12 months), a fi nding also noted by others including Watson and colleagues [14]. What is also notable from this and other large series of alemtuzumab-treated transplant patients is the relative absence of autoimmune complications, a problem that has been seen more commonly in patients treated for multiple sclerosis [15].
When the potency and duration of lymphocyte depletion induced by alemtuzumab was recognised, a number of investigators considered whether this may lead to the induction of donor-specifi c hyporesponsive ness. Th is followed the important observations of Knechtle and colleagues in demonstrating that profound lymphocyte depletion, using an anti-CD3 monoclonal antibody conjugated to diphtheria toxin, can lead to long-lasting Friend Transplantation Research 2013, 2(Suppl 1):S5 http://www.transplantation research.com/supplements/2/S1/S5 donor-specifi c hyporesponsiveness in a non human primate experimental model of kidney trans plan tation [16]. Calne and colleagues tested the use of alemtuzumab induction (2 × 20 mg) followed by low-dose cyclosporin monotherapy (75 to 125 mg/ml) [17]. Th is study, although without a control group, demonstrated the capability of a low-dose mono therapy immunosup pression regimen to achieve low levels of rejection and good levels of graft survival and function. Th e same group of patients was reviewed 5 years after transplantation [14] and was compared with a group of matched controls treated with conventional therapy (cyclosporin, prednisolone, azathioprine). Lympho cyte counts were signifi cantly lower in the alemtuzumab group only in the fi rst 3 months post operatively. Cyclosporin levels were significantly lower in the alemtuzumab group for approximately 2 years. However, despite lower cyclosporin levels, there was no signifi cant diff erence in renal function. An early benefi t with respect to acute rejection in the alemtuzumab-treated patients was matched by an increased level of rejection after 6 months, culminating in a similar overall level of rejection between the two groups. Th ere was no signifi cant diff erence in patient or graft survival or in the incidence of infection or other serious adverse events.

Alemtuzumab and mammalian target of rapamycin inhibitors
Th e ability to lower calcineurin inhibitor (CNI) doses was regarded as one of the primary advantages of alemtu zumab therapy, particularly because of the desire to avoid chronic nephrotoxicity [18]. However, despite the ability of alemtuzumab to enable lower levels of CNI therapy, none of the above studies demonstrated improvement of renal function in patients treated with alemtuzumab. Th e profound lympho-depletion caused by alemtuzumab led Kirk and colleagues to question whether the antibody alone would enable tolerance and avoid the need for main tenance therapy. Seven living donor recipients were treated with three or four doses of alemtuzumab preoperatively; despite profound depletion of lymphocytes in the peripheral blood and lymph nodes, all patients developed rejection (reversible) and required conversion to maintenance sirolimus [19]. Th e availability of mammalian target of rapamycin inhibitors, however, provided the opportunity for long-term maintenance of patients without the use of nephrotoxic immunosuppression; the combination of alemtuzumab induction with sirolimus monotherapy was a theoretically attractive proposition.
In the fi rst study of an immunosuppression strategy of this design, 29 patients were treated with alemtuzumab (20 mg × 2) followed by low-dose maintenance sirolimus therapy [20]. Eight patients required treatment for rejection and one graft was lost. Th e investigators concluded that sirolimus monotherapy was inadequate in this context. Other investigators tested the combination of sirolimus with mycophenolate following alemtuzumab induction [21]. Twenty-two patients were treated with alemtuzumab induction (30 mg × 2) and maintenance therapy sirolimus (8 to 12 mg/ml) and mycophenolate (500 mg/twice daily). Th ere was an acute rejection incidence of 36%, leuko paenia in 27% of patients and acute respiratory distress syndrome in two patients. Th e investigators concluded that the combination of alem tuzumab, sirolimus and mycophenolate, when started at the time of trans plan tation, was associated with a high rejection rate and a high incidence of other complications.
Attention then moved to the use of alemtuzumab induction followed by a short course of CNI therapy, before switching to sirolimus maintenance therapy for the longer term. In a study of 30 patients, alemtuzumab (30 mg × 2) induction was followed by mycophenolate (500 mg/twice daily for 12 months), tacrolimus (5 to 8 ng/ml for 6 months) and, after tacrolimus withdrawal, sirolimus (5 to 8 ng/ml) (data submitted for publication). A number of rejection episodes occurred following mycophenolate withdrawal at 12 months (all reversible with steroids) and the protocol was therefore amended to continue mycopheno late 250 mg twice daily thereafter. Th is protocol was associated with a low incidence of rejection (6.6% at 12 months). Following the change of protocol to maintain low-dose mycophenolate after 12 months, there were no acute rejection episodes after 12 months. Two patients were withdrawn from the protocol because of respiratory side eff ects of sirolimus. Eighty-fi ve per cent of patients remained steroid and CNI free 5 years postoperatively.
Th is trial demonstrated the potential utility of alemtuzumab in enabling patients to be established safely on CNI-free therapy. Th e real benefi t of this strategy is in the potential absence of chronic nephrotoxicity and the consequent long-term improvement in graft half-life. Clearly this hypothesis required formal testing in an adequately powered randomised control clinical trial. Such a trial was established in 2010 with the intention of testing the eff ects of alemtuzumab versus basiliximab as an induction agent and testing the eff ect of switching to sirolimus from tacrolimus at 6 months. Th e end points for this study will fi rst be the incidence of acute cellular rejection and then medium-term graft function. Th e results of this study (Campath, calcineurin inhibitor reduction, chronic allograft nephropathy) are awaited.