Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Immunosuppressive Biologic Agents
Historical Perspective
The development of potent immunosuppressive therapies transformed liver transplantation from the experimental procedure pioneered by Thomas Starzl and Sir Roy Calne into the preferred clinical treatment for end-stage liver disease. The greatest incremental improvements in clinical outcomes have tracked closely with the introduction of improved pharmacological agents and the subsequent development of guidelines for their proper use. Today the 1-year and 5-year survival following liver transplant is estimated to be greater than 85% and 70%, respectively. These outcomes reflect improved perioperative management, organ preservation, operative technique, and most importantly, the use of various combinations of calcineurin inhibitors, corticosteroids, and antimetabolite treatments that have converted acute rejection, once the greatest barrier to successful outcome, into a relatively uncommon, or at least manageable, event. However, the adverse effects of these immunosuppression medications on cardiovascular, metabolic, endocrine, and renal function are now the most common causes of long-term morbidity and mortality after transplantation. Thus the current paradigm for improved results involves significant attention to the efficient use of available immunosuppressive agents.
There has been an exponential growth in the development of biologic therapies (defined later) whose effects are mediated through precise interactions with defined immunological targets with the consequent potential for eliminating off-target side effects. Many agents are now available, and their use is increasing throughout other fields of transplantation. The development and clinical application of biologics in liver transplantation are limited at present, largely as a function of the relatively low incidence of refractory rejection, and will be reviewed here. It is hoped that future advances in these agents will improve the specificity of the immunosuppression, allowing for more personalized and conscribed immunosuppressive regimens, reducing long-term side effects and improving transplant outcomes.
Biology of Biologics
Immunosuppressive agents can be broadly characterized as either small molecule inhibitors of intracellular pathways or biologic agents composed of larger protein molecules, such as antibodies or fusion proteins, that target extracellular surface molecules. The small molecule inhibitors (e.g., calcineurin inhibitors, mycophenolate mofetil, and prednisone) impair shared motifs and domains that are used in multiple pathways and thus have rather broad effects on multiple cellular functions. Biologics, on the other hand, are highly specific for cell surface receptors, and the associated intracellular pathways downstream of those receptors determine their effects. By interacting with receptors with limited distributions, these agents can be used to precisely alter immunological function without impairing other cellular functions, thus eliminating off-target side effects ( Figs. 96-1 and 96-2 ). Biologic agents may be classified as polyclonal antibodies, monoclonal antibodies, or fusion proteins.
The use of biologics dates back to the 1960s, when increasing awareness of lymphocytes as critical players in transplant rejection prompted a search for lymphocyte-directed therapy. By the mid-1960s, several studies highlighted the phenomenon of animals being able to produce antibodies when injected with human lymphocytes. These antibodies, when harvested from the sera of immunized mice and injected into nonimmunized animals, were found to cause a dramatic reduction in lymphocytes. Further improvements in the techniques of purification, monoclonal antibody production, and genetic manipulation of protein structure have led to an exponential rise in the number of biologic agents available for use. Although transplantation served as the initial indication for biologic therapies, most biologics are now being developed for use in oncology and autoimmune indications.
Polyclonal Antibodies
The first biologics developed were antibody preparations derived from the sera of immunized animals, termed polyclonal antibodies . The concentrated serum contains numerous antibodies exhibiting an array of binding specificities to a variety of epitopes. The administration of these polyclonal antibodies produces a potent and broad immunosuppressive effect, and preparations have been employed in liver transplantation for a number of purposes, including induction therapy and in treatment of steroid-refractory rejection. The clinical application of polyclonal antibodies is limited by their heterogeneity and immunogenicity. The true binding-affinity profile of any given polyclonal preparation is by its nature variable and not precisely definable; this corresponds to an inherent variation in efficacy and side effect profile from agent to agent and batch to batch. The diversity of antibodies in each preparation reduces product specificity and increases adverse effects, including pancytopenia when used clinically. The preparations used clinically are purified to obtain only IgG isotypes and adsorbed against platelets, erythrocytes, and selected proteins to reduce these undesirable effects. Despite this processing, adverse reactions remain common. In addition, these agents cannot be used as maintenance therapy because their animal protein components elicit persistent and strong immune responses. This is manifested clinically as serum sickness, or in severe cases anaphylaxis, and limits the dose and timing of rabbit and horse preparations. These limitations of polyclonal antibodies stimulated the development of a more specific class of biologic immunosuppression, monoclonal antibodies.
Monoclonal Antibodies
The development of hybridoma technology was pioneered by Kohler and Milstein in the 1960s and led to the successful fusion of immortalized tumor cell lines with splenic cells from an immunized animal. These hybridoma-produced antibody molecules identical in composition exhibited the same reliable antigen-binding affinity profile clinically and thus produced consistently from batch to batch. This technology was first used clinically in transplantation. Specifically, Kung et al developed OKT3, an agent specific for T cells and a logical agent to apply for use in a T cell–mediated disease such as acute rejection following transplant. However, similar to polyclonal antibodies, early clinical use of monoclonal antibodies remained limited by their immunogenicity, since they were prepared in mice and were composed of heterologous proteins and consequently elicited a strong mouse-specific antibody response from the recipient’s immune system, termed human antimurine antibody (HAMA) responses . Similar to polyclonal antibodies, this reaction limited the duration of clinical therapy, making monoclonal antibodies raised in animals difficult to incorporate into maintenance transplant immunosuppression protocols.
Several genetic engineering techniques have been applied to produce molecules that are structurally more similar to human proteins (in some cases identical) and therefore avoid many of the immunogenic issues of heterologous preparations. Today, engineered biologic agents include chimeric monoclonal antibodies, recombinant humanized monoclonal antibodies, and fully human monoclonal antibodies. Chimeric monoclonal antibodies are specialized antibodies containing a human constant region or FC fragment and a mouse variable (Fab) region (an example is basiliximab). Composed mostly of human DNA sequences, these antibodies are less likely to elicit an immune response from a human host. However, responses to the mouse Fab sections can develop and produce responses capable of neutralizing the effectiveness of the agent over time.
Riechmann et al created the first humanized monoclonal antibody, Campath-1H. This molecule contained a “reshaped” heavy chain domain that enabled the antibody to bind to the Campath-1 antigen with precisely the same affinity as its chimeric and rat counterparts. In addition, it was noted that this new “reshaped” or “humanized” antibody generated much less of a syndromic immune response during infusion when compared to its less humanized chimeric and fully murine rat counterparts. However, there are limitations to the clinical use of fully human antibodies. First, every manipulation of the human DNA portion of these constructs, irrespective of how minor, holds the potential of altering the amino acid sequence of the final protein and may ultimately lower the clinical binding affinity of the active antibody. Moreover, such extensive genetic engineering can result in either improper or nonexistent glycosylation of the protein product, potentially hindering the binding affinity of Fc-associated functions of the final antibody.
Fusion Proteins
An additional subclass of biologic immunosuppression agents is fusion proteins. Fusion proteins are specialized molecules composed most commonly of a single known specific receptor fused together with another protein that offers a specific property of interest. These compounds are genetically engineered much like murine antibodies. The union enables the molecule to exhibit a property that neither component is able to express individually. Depending on the Fc subtype being used (e.g., IgG1 versus IgG4), fusion proteins can fix complement, be bound by Fc receptors, and engage in a number of antibody-like effector or opsonization properties. They offer the advantage of increased epitope-directed drug delivery. The most contemporary example of a fusion protein is belatacept. Multiple potential uses are still actively being investigated for fusion proteins.
Clinical Use of Biologics in Liver Transplantation
Biologics have become widely used within standard immunosuppression protocols for all solid organ transplants except for liver transplantation. Despite recent United Network for Organ Sharing (UNOS)-derived data that demonstrate benefit from induction, only 20% to 25% of liver transplant patients receive biologic agents ( Fig. 96-3 ). The reasons for this low utilization are multifactorial and include (1) excellent short-term outcomes from liver transplant with current regimens, (2) widely accepted opinion that the liver is resistant to rejection, (3) decreased incidence of acute rejection, and (4) concern that liver transplant recipients are immunosuppressed by their disease and that additional treatments increase the risk for infection and cancer, especially because cancer (hepatocellular carcinoma) and chronic infection (hepatitis C virus [HCV]) are common indications for transplantation.
Numerous practical matters limit the use of biological therapies. Biologics require administration in a clinical setting, necessitating visits to the transplant center. Patients must be under direct clinical supervision for anticipated adverse reactions such as the critical flash pulmonary edema and profound hypotension associated with the initial doses of lymphocyte-depleting products. Along similar lines, there continues to be an element of broad reactivity manifesting often as pancytopenia, and the humoral immune response generated against antibodies themselves may result in serum sickness and anaphylaxis. The long half-lives for antibody preparations also add a level of complexity to posttransplant management, and once administered, the immune potency of biologic agents cannot simply be stopped or rapidly “reversed.” Furthermore, biological immunosuppression agents are among the most expensive treatment protocols, and the costs of routinely incorporating biological immunosuppression into existing induction and maintenance protocols is anticipated to likely drive these annual drug costs of transplant immunosuppression even higher, raising questions on their cost-effectiveness. There are also concerns, based on data from extrahepatic organ transplantation, for impaired protective immunity associated with the enhanced immunosuppressive effects of additive biologics. Prominent examples include increased susceptibility to primary and reactivated herpesviruses like Cytomegalovirus (CMV) and Epstein-Barr virus, the latter leading to increased posttransplantation lymphoproliferative disorder (PTLD) with its associated increased risk for death from malignancy. Previous data investigating the risk for PTLD after kidney transplantation found incidence as high as 0.85% following OKT3 induction and 0.81% with polyclonal induction. The risk was 0.5% in patients who did not receive antibody induction therapy. There are limited data in liver transplant patients from small single-center studies and registry analyses addressing the malignancy risk with the use of polyclonal antibody or interleukin-2 receptor antagonist (IL-2RA) treatments. There has been no evidence of additional risk for de novo or recurrent hepatocellular carcinoma when biologics have been used in tacrolimus or steroid-sparing regimens.
Employment of biologic agents in liver transplantation harbors its own unique levels of complexities and considerations. Early intense induction and treatment regimens in hepatitis C patients were associated with higher posttransplant viral load and more aggressive disease recurrence; however, more contemporary data have failed to find significant increases using rabbit antithymocyte globulin (rATG; Thymoglobulin) or daclizumab as compared with steroid for induction. Similarly, outcome studies from the 1990s on patients transplanted for hepatocellular cancer recurrence suggested worse outcome when induction therapy was used. Recently reported randomized controlled and retrospective studies addressing this issue with current rATG and IL-2RA induction protocols found similar recurrence rates of hepatocellular cancer with and without biologics. The effect of biologics use on infection risk is difficult to interpret from the limited published data and the diversity of agents, protocols, and prophylactic treatments used. When used within a steroid-sparing regimen, one small, single-center, randomized controlled trial found that ATG was associated with fewer CMV infections after transplantation. IL-2RA induction did not result in a significant increase in infections when compared with standard therapy in a recent meta-analysis; however, there were differences in treatment, reporting, and protocols within each study. Alemtuzumab has been reported to increase the incidence of viral infection after transplantation. In general, the increased intensity of immunosuppression should be assumed to have a negative impact on protective immune competence, because this has been the case for all situations (all extrahepatic) in which an adequately powered trial can be performed. However, explicit data in this regard are lacking in liver transplantation.
Induction Therapy
Induction consists of short-term intense perioperative treatment to provide prophylaxis against acute rejection and may also facilitate strategies to reduce steroid or calcineurin inhibitor treatments after transplant. This is of particular importance considering that one in five liver transplant patients will develop chronic renal failure long term, especially if renal function is marginal before transplant. Induction agents are either lymphocyte depleting (polyclonal antibodies, alemtuzumab) or nondepleting (basiliximab, daclizumab). rATG induces T-cell apoptosis and lymphopenia in a dose-dependent fashion. Alemtuzumab depletes T cells for prolonged periods up to 1 year and also depletes B cells. Basiliximab and daclizumab have specificity for activated T cells by blocking CD25, although this includes not only activated effector T cells, but also potential interactions with regulatory cells that constitutively express IL-2 receptor. An additional theoretical benefit of induction therapy is the possibility of promoting intrinsic tolerogenic properties of the liver and skewing the balance of T cells toward memory and regulatory T cells after lymphocyte depletion. Murine and human studies have shown favorable expansion in regulatory populations in response to alemtuzumab and rATG. However, IL-2RA induction appears to decrease Treg expression. rATG may also lessen the amount of ischemia-reperfusion injury through its interaction with adhesion molecules and other molecules upregulated with ischemic injury.
Cai and Terasaki recently published an analysis of UNOS data that demonstrates superior 5-year graft and patient survival following liver transplantation with the use of induction treatment. In addition, published data by Moonka et al suggest that both HCV and non-HCV recipients derive benefit from induction. According to the Scientific Registry of Transplant Recipients, induction therapy continues to be infrequently used in liver transplantation, with approximately 30% of liver transplant recipients receiving induction from 2008 to 2009 compared to the 50% to 70% of renal transplant recipients. Use of these biologics as induction agents in renal transplantation has been associated with a significantly decreased incidence of acute rejection in the first 6 months after renal transplantation, an effect that is documented as being strongest in sensitized individuals. Importantly, most data fail to show improvements in long-term graft or patient survival associated with biologic induction, even in populations, like kidney transplantation, where the reduced incidence of acute rejection is clear. Unlike the data in kidney transplantation, the data supporting induction therapy for liver transplantation consist largely of retrospective or single-arm studies. However, there have been three small randomized controlled trials that suggest less rejection in treated patients. Both hepatitis C–infected recipients and those who are not infected with hepatitis C show diminished acute rejection, although there has been no demonstrated impact on patient or graft survival. A number of subgroups appeared to benefit, particularly recipients with existing renal impairment and those admitted to the intensive care unit preoperatively. When induction therapy is used in liver transplantation, the agent of choice tends to be the IL-2RA basiliximab, likely due to its low toxicity for a patient population that is already immunologically impaired as a result of hepatic failure. Infrequently, lymphocyte-depleting agents such as polyclonal antibodies are employed on an off-label basis.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here