Year : 2000 | Volume
: 11 | Issue : 2 | Page : 137--144
New Trends in Immunosuppression in Transplant Patients
Michael J Hanaway, Hans W Sollinger
Department of Surgery, University of Wisconsin Medical School, Madison, Wisconsin, USA
Michael J Hanaway
Department of Surgery, University of Wisconsin, 600, Highland Avenue, H4/780-7375, Madison, WI 53792-7375
|How to cite this article:|
Hanaway MJ, Sollinger HW. New Trends in Immunosuppression in Transplant Patients.Saudi J Kidney Dis Transpl 2000;11:137-144
|How to cite this URL:|
Hanaway MJ, Sollinger HW. New Trends in Immunosuppression in Transplant Patients. Saudi J Kidney Dis Transpl [serial online] 2000 [cited 2019 Dec 10 ];11:137-144
Available from: http://www.sjkdt.org/text.asp?2000/11/2/137/36670
The field of clinical immunosuppression is currently experiencing a stage of rapid growth and development. While the introduction of cyclosporine A (CsA) was the first major advance in clinical immunosuppression in decades, the last five years have yielded both pharmacologic and biological immunosuppressants, which hold great promise for the future. Advances in our understanding of the molecular processes involved in the activation of lymphocytes have uncovered many potential sites of intervention, which could allow manipulation and down regulation of the rejection response. The advent of new immunosuppressants not only demands a rigorous test of safety and efficacy but also challenges us to effectively integrate these medications into our current regimens. Furthermore, the development of newer, more selective agents obligates a reevaluation of our current standard of care. This article reviews some of the important recent developments in clinical immunosuppression.
Biological immunosuppressants, such as OKT 3 and anti-lymphocyte sera, have been used for treatment of refractory rejection or as induction therapy for patients at high-risk for early graft failure. Polyclonal antilymphocyte antibodies and OKT 3 appear equipotent for induction therapy as well as treatment of severe or steroid resistant rejection.  The use of OKT 3 is limited by the "cytokine release syndrome" characterized by severe headache, nausea, fever, chills and risk of rapid pulmonary edema. 2 Recent advances in monoclonal antibody technology have enabled the development of biological immunosuppressants, which deplete or selectively block the activation and proliferation of T-lymphocytes without significant side effects. The recent availability of a new, highly potent, rabbit antithymocyte globulin raises new questions about the role of polyclonal antilymphocyte therapy in our current antirejection armamentarium.
Monoclonal Antibody Preparations
The ultimate goal of utilizing monoclonal antibodies as immunosuppressants is to specifically eliminate those cells which will participate in the rejection response while leaving the remaining immune system intact. Monoclonal antibodies targeting antigens only expressed on activated T cells offer many theoretical advantages over polyclonal anti-sera. First, monoclonal antibodies directed at "activation antigens", should have a dose-related response and avoid the variable response seen between different preparations of anti-lymphocyte globulin. Antigen-specific monoclonal antibodies also require a lower immunoglobulin load for a desired effect, and are less likely to induce serum sickness than polyclonal antisera. Monoclonal antibodies exert immunosuppressive effects by deleting or blocking the function of cells necessary for the rejection response. They may also inhibit cell function by blocking a functional antigen or causing the depletion of a particular antigen from the cell surface (either by shedding or internalization of that antigen). Monoclonal antibodies may also cause the deletion of function cells through cytolysis. This cell destruction may occur via complement or antibody-dependent cell mediated cytolysis. Since most monoclonal antibodies are derived from mouse cell lines, the effectiveness of these therapies is limited by the development of human antibodies directed against murine monoclonal antibodies, namely the human antimouse antibodies (HAMA). In an effort to minimize the HAMA response, these murine monoclonal antibodies have been genetically altered to contain human constant regions. The review of monoclonal antibody immunosuppressants will include Interleukin-2 (IL-2) receptor antagonists currently in clinical use (Zenapax and Simulect) and three potential immunosuppressants currently under investigation.
Recently, several monoclonal antibody therapies have been introduced which are designed to selectively inhibit T-cell responses that are thought to initiate allograft rejection. Basiliximab (Simulect) and daclizumab (Zenapax) are murine monoclonal antibodies directed against the alpha chain of the high affinity IL-2 receptor, blocking the stimulation of T-cells by IL-2. Development of monoclonal antibodies directed against the functional receptors of activated T-cells have the potential for more selective immunosuppression and fewer adverse effects.
Simulect and Zenapax are monoclonal antibodies against the alpha chain of the inducible IL-2 receptor. The high affinity IL-2 receptor is composed of three transmembrane protein chains: alpha (CD 25 ), beta (CD122) and gamma (CD132). CD 25 is expressed on activated, but not resting, Tlymphocyte.  The interaction of IL-2 and the high affinity IL-2 receptor is necessary for the clonal expansion and proliferation of activated T-cells. The use of antibodies that disrupt the interaction of IL-2 and its receptor has been shown to delay or inhibit allograft rejection.  The efficacy of murine monoclonal antibodies as immunosuppressants can be limited by the development of neutralizing HAMA. Development of humanized and chimeric monoclonal antibodies has overcome the immunogenicity of murine antibodies and prevents the formation of HAMA.
Simulect is a chimeric monoclonal antibody against CD 25, which has been constructed to contain the entire murine antigen-binding site on a human antibody backbone.  Pre-clinical studies have shown that Simulect provides complete saturation of CD25 and has a half-life of one to two weeks. Induction therapy using 40 mg (2x 20 mg doses) of Simulect provides complete IL-2 receptor blockade for 40 to 50 days.  Zenapax is a humanized (90% human and 10% mouse) monoclonal antibody against the alpha sub-unit of the IL-2 receptor. Although the clinical duration of action of Zenapax is not known, at recommended dosages this agent saturates CD25 for about 120 days. 
Since its introduction in 1997, clinical studies have shown Zenapax to be an effective induction therapy with minimal side effects. Initial prospective, randomized Phase III trials, comparing Zenapax to placebo in primary renal transplant recipients maintained on triple immunosuppression, (prednisone, azathioprine and CyA) showed a 13% decrease in the incidence of acute rejection in the first six months among those patients treated with Zenapax.  A later Phase III trial of Zenapax with CsA and prednisone again demonstrated a significantly lower rate of rejection, prolonged time to first rejection and improved renal function in Zenapax-treated patients.  In addition, both studies found the number of patients with two or more rejection episodes and the number of patients receiving anti-lymphocyte therapy for severe rejection was reduced in those receiving Zenapax. , The results in these two trials were obtained without concomitant increase in infectious complications or malignancies.
Results of early European/Canadian trials examining efficacy of Simulect in prevention of acute rejection in primary cadaveric renal allograft recipients were similar to those seen in recipients on Zenapax. A later US multi-center trial again showed a lower rate of rejection and improved renal function in patients treated with Simulect compared to placebo. No study has shown an increased incidence of side effects or adverse events in patients treated with either Zenapax or Simulect when compared to placebo.
While both of the IL-2 receptor monoclonal antibodies have shown similar efficacy in prevention of acute rejection, no prospective studies have been performed to directly compare Simulect to Zenapax or either monoclonal antibody to a polyclonal anti-thymocyte preparation. We have recently performed a retrospective analysis of primary renal allograft recipients at the University of Wisconsin who received Zenapax, Simulect or anti-thymocyte globulin (ATG) as induction therapy. Rejection rates after four months for patients treated with Simulect or Zenapax were 46% in each group. When examining patients with delayed graft function (DGF), there was a trend towards a lower rejection rate in recipients treated with ATG, a polyclonal anti-lymphocyte preparation. The total cost of treatment with Zenapax ($6270) was greater than that of a course of Simulect ($2448). Given the similar rejection rates for Simulect and Zenapax at 120 days posttransplant, Simulect appears to be the more economic choice. While induction with ATG is more expensive than either Simulect or Zenapax, patients with post-transplant DGF treated with ATG tended to have a lower incidence of rejection.
The use of OKT 3 (Muromonab) for the treatment of severe rejection is limited by the "cytokine release syndrome" which can range from a flu-like syndrome to a severe, life threatening, shock-like reaction. The effectiveness of OKT 3 can be limited by the formation of HAMA, which often necessitates higher dosages for adequate T cell depletion. HuM291, a recently developed humanized OKT 3 , contains an altered CH 2 region of the antibody Fc domain that may limit both the cytokine release syndrome and HAMA formation. Early Phase I/II trials of HuM291 have demonstrated only a mild to moderate cytokine release syndrome which is limited to the first 24 hours of administration with no detectable HAMA formation.  Norman, et al demonstrated only a mild flu-like syndrome (nausea, vomiting, headache) after administration of HuM291 with T cell depletion occurring two hours after administration and lasting for eight days.  No HAMA formation was detected in this study.
Highly selective monoclonal antibodies have been developed with the ability to block T-cell co-stimulation and subsequent activation. 5C 8 is a monoclonal antibody which binds to CD 40 L and is capable of interrupting the CD 40 /CD 40 L T-cell costimulatory pathway. 5C 8 has been tested alone and in conjunction with CTLA4Ig, a monoclonal antibody which blocks the CD 28 /B 7 co-stimulatory interaction. In a pre-clinical primate model, 5C 8 monotherapy was able to prevent rejection for 95-100 days. Mild acute rejection in these subjects was reversed completely with an additional 14-day course of 5C 8 . The efficacy of CTLA4Ig was more transient than that of 5C 8 .  The safety and efficacy of 5C8 for human subjects is currently being tested in a clinical trial at our institution.
Campath-1H is a humanized IgG1 rat monoclonal antibody, which recognizes the CD52 glycolipid present at high density on nearly all human lymphocytes and monocytes. Studies in patients with hematologic malignancy have shown the CD 52 antigen to be a useful target for lymphocytolytic therapy. After infusion of Campath-1H, lymphocyte and monocyte counts fall rapidly over the first hour and a leukopenia ensues for over two years.  While Campath-1H is a powerful lytic agent for both B and T cells, it does not affect bone marrow stem cells.  Knechtle and colleagues have found (using a CD 3 diptheria immunotoxin) that depletion of the T-cell response before renal transplantation produced an operational tolerance in a rhesus monkey model.  Initial investigations of the Campath-1H antibodies were designed to induce tolerance by temporal depletion of lymphocytes around the time of transplantation.
Early primate studies of Campath-1H antibodies were disappointing. Despite effective lymphocyte depletion after treatment with Campath-1H antibodies, animals treated with Campath-1H antibodies rejected allografts which was at times comparable to untreated controls.  It was theorized that while the majority of lymphocytes were depleted, a small but important lymphocyte subset was spared and was sufficient to initiate a rejection response. Further primate studies were not performed because initial human clinical trials had shown Campath-1H antibodies to be effective in preventing and reversing renal allograft rejection when compared to controls.  It is possible that while the Campath-1H antibodies bind to primate lymphocytes, the degree of binding and the overall efficacy of lymphocyte depletion is less in primates than in humans. Campath1H, a human IgG1 antibody incorporating the hypervariable regions of the rat antiCD52 monoclonal antibody, was created to minimize the immunogenicity of earlier Campath-1H antibodies. Calne and colleagues have performed early clinical studies of Campath-1H as an induction agent. Thirteen primary recipients of cadaveric renal allografts were treated with two, 20 mg doses of Campath-1H at the time of transplant and post-operative day one. Low dose Neoral (to achieve a level of 75-125 pgram/mL) was started 48 hours after the second dose of Campath-1H. All 13 patients had good renal function at six to 11 months after surgery. Only one patient had an episode of rejection, which necessitated maintenance immunosuppression with prednisone. The remaining 12 patients were maintained on low dose Neoral monotherapy. 
Polyclonal Antibody Preparations
Thymoglobulin is a new, potent, rabbit anti-thymocyte globulin, which has been approved in the US for the treatment of severe renal allograft rejection. Thymoglobulin is a polyclonal, gamma globulin preparation containing antibodies against multiple T-cell markers including CD 2 , CD 3 , CD 4 , CD 8 , CD 11a , CD 18 , CD 25 , CD 44 , CD 45 , HLA-DR, HLA Class I and beta-2 microglobulin. Thymoglobulin's mechanism of action is thought to be through T-cell depletion and modulation of T-cell homing, activation and proliferation. Thymoglobulin also acts to prevent B-cell proliferation and differentiation. ,,
Until recently, ATG was the only polyclonal anti-lymphocyte preparation approved for treatment of acute rejection in the United States. Thymoglobulin, used in Europe for more than a decade, has been found to be safe, effective and nearly equivalent to OKT 3 in single center trials.  Initial studies by Tesi, et al showed a 7-14 day course of thymoglobulin had twice the efficacy of ATG in reversing steroid resistant rejection and preserving graft function in renal transplant patients. This trial revealed that thymoglobulin was associated with a higher rate of leukopenia without any increase in the rate of malignancy or infection.  A 12-month follow-up study of the US multicenter trial confirmed that the overall incidence of rejection was lower and graft survival significantly improved in patients treated with thymoglobulin. No difference in morbidity and mortality was noted in thymoglobulin recipients in either study. 
While thymoglobulin is not approved in the US for primary induction therapy, there is clinical experience using thymoglobulin to prevent acute rejection episodes following renal transplantation. Single center Canadian trials have suggested benefit of thymoglobulin as an induction agent in both low-risk  and high-risk patient populations.  Brennan, et al recently reported a four-fold reduction in the incidence of biopsy proven rejection in renal transplant patients treated with thymoglobulin over ATG for induction. Patients in this trial treated with ATG experienced significantly more episodes of recurrent and steroid resistant rejection and a shorter graft survival time. Greater and, more persistent T-cell depletion as well as more frequent leukopenia (56% for thymoglobulin vs. 4% for ATG) were seen in thymoglobulin treated patients, as compared to ATG. However, this persistent leukopenia was not associated with a higher rate of adverse events.  Based on early clinical experience, thymoglobulin appears to be a highly potent, safe alternative to ATG for induction therapy as well as treatment of acute rejection.
Rapamycin is a macrolide antibiotic isolated from Streptomyces Hygroscopicus which acts as an anti-metabolite to prevent the proliferation of activated T-cells. Whereas both CsA and FK506 achieve their effects principally by blocking calcineurin and preventing IL-2 production, rapamycin has no effect on calcineurin and exerts its effects by inhibiting the IL-2 mediated signal transduction pathway. Rapamycin acts by binding to FKBP 12 and inhibiting P 70 S 6 kinase activation, which leads to cell cycle arrest at the late G1 phase.  Rapamycin has also been shown to inhibit vascular and non-vascular smooth muscle cell, fibroblast and keratinocyte proliferation in-vivo and in-vitro.  Tilney and colleagues have hypothesized that allografts may be lost due to acute and chronic rejection because of the eventual occlusion of graft vasculature from myointimal hyperplasia.  The ideal immunosuppressant to combine with CsA would be an agent, which is synergistic in action with CsA and also inhibits smooth muscle proliferation without causing nephrotoxicity. Rapamycin, which inhibits Tcell proliferation independent of calcineurin inhibition, has been shown to potentiate the effects of CsA in early clinical trials.  Phase II trials indicated that use of either rapamycin or CsA showed similar efficacy in reducing the incidence of acute rejection.  However, when rapamycin was used as a component of a CsA based regimen, there was a reduction in the incidence of rejection from 35 to 7% in living related and living unrelated renal transplants and from 40 to 10% in cadaveric renal transplants. 
Rapamycin's synergy with calcineurin inhibitors and lack of nephrotoxicity creates several potential roles for this drug in our armamentarium of immunosuppressive agents. One major limitation of calcineurin inhibitors is the nephrotoxicity in patients with either delayed graft function or marginal renal allograft function. The use of rapamycin may allow a great reduction or elimimation of calcineurin inhibitors in such a patient population while maintaining adequate immunosuppression. Lower doses of tacrolimus/CsA would likely translate into lower rates of post-transplant diabetes and hypertension. Minimization of dependence on calcineurin inhibitors may also help prevent intrarenal myointimal hyperplasia and vascular changes often seen in chronic rejection.
Pilot studies comparing CsA to rapamycin showed similar efficacy but different side effect profiles. The incidence of acute rejection, time to first rejection and severity of rejection (as measured by need for OKT 3 or polyclonal antibody therapy) were comparable between these two drugs. However, hypercholesterolemia and hypertriglyceridemia were much more common in the rapamycin group. Early work in this study indicated that lowering the rapamycin dose and treatment with fibrates were effective in lowering serum lipid levels. Although patients treated with rapamycin also had higher rates of leukopenia and thrombocytopenia, this did not equate with higher risk of bleeding or opportunistic infection.  While the FDA has recently approved rapamycin for use in the United States, its precise role as an immunosuppressant and the significance of its side effects will require further elucidation by phase III clinical trials.
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