Immunosuppressants are all designed to inhibit T-cell activation, a two-part process consisting of the sensitization and effector stages. The sensitization stage involves an interaction between antigen presenting cells (APC, e.g. dendritic cells, which load their antigens onto the MHC molecules for display) and host T-cells. The effector stage involves stimulation by co-stimulatory receptors (e.g. CD28, CD40). Activated T-cells secrete cytokines (e.g. IL-2) which amplify T-cell activation through a positive feedback loop, and ultimately play a major role in graft rejection
Phases of Rejection
Phases of Rejection
- Hyperacute: within 24 hours
- Acute: within the first few weeks
- Chronic: months to years
History of Immunosuppressants
Azathioprine, the first immunosuppressant, was made available in 1962. The introduction of cyclosporine in 1983 significantly improved outcomes for non-renal organs such as heart, lung, liver and pancreas transplants. The modern approach to immunosuppression takes into account the organ being transplanted (which have different pharmacologic requirements), characteristics of the recipient (e.g. whether or not they are pre-sensitized, or whether or not they have received a blood-compatible organ), and blend different immunosuppressants in an effort to produce synergy while minimizing harmful side effects. While great strides have been made in reducing the incidence of acute rejection, relatively little progress has been made with regards to long term graft survival. Immunosuppression is lifelong and typically consists of two (or three) phases:
Phases of Immunosuppression
- Induction: initial immunosuppression regimen, initiated prior to transplantation (may be followed by sequential therapy)
- Maintenance: compromise between long term efficacy and minimization of side effects. Usually a steroid, calcineurin inhibitor, +/- anti-proliferative (e.g. sirolimus)
- Anti-Rejection: high doses (e.g. steroids) or new agents (e.g. tacrolimus, RATG) added. Not required by all patients
With induction the focus is on complete paralysis of the cellular immune system, with relatively little regard for medication side effects. As time goes on, side effects become more important and the level of the immune suppression diminishes. Most maintenance regiments consists of steroid plus a calcineurin inhibitor and possibly an antiproliferative agent (e.g. sirolimus). In the setting of rejection, the anti-rejection regimens usually consist of high dose steroids, or anti-lymphocyte antibodies, or both.
A relatively new concept in transplantation biology is that of sequential therapy – transplantologists who adhere to sequential therapy will follow the induction phase with antibody therapy for 1-2 weeks in an effort to delay the initiation of calcineurin inhibitors (cyclosporine or tacrolimus) in an attempt to reduce early post-operative renal dysfunction
Key Concepts of Immunosuppression
As acute rejection rates have diminished, the focus of immunosuppression has shifted from preventing acute rejection to maximizing long-term survival while minimizing the comorbidities associated with long-term immunosuppression (see below). Because of the risks associated with underdosing, as well as the side effect profile of these drugs, monitoring drug levels is critical for several commonly used immunosuppressants. Additionally, many of the immunosuppressants activate CYP 3A4 and P-glycoprotein, further complicating management
Side Effects of Immunosuppression
Infection, malignancy, and organ dysfunction (e.g. renal or CNS toxicity) are the major concerns with long term immunosuppression.
Major component of induction, maintenance, and anti-rejection phases. Decrease IL-2 production, reduce capillary permeability, inhibit histamine release, reduce leukocyte trafficking, reduce circulating immunoglobulin levels, reduce neutrophils, reduce eosinophils, inhibit leukocyte adhesion. Doses are not standardized and are generally institution-specific, although given the side effects (see below), most institutions strive to reduce these drugs to the equivalent of ~ 5 mg/day or less of prednisone.
Adverse effects include poor wound healing a clear increase in the risk of serious infections, and glucose intolerance.
Cyclosporine inhibits calcineurin, which reduces the production of IL-2 via inhibition of nuclear factor of activated T cells (NF-AT). Metabolized by CYP 3A4. Monitoring is essential although target levels are institution specific. Reasonable serum/plasma levels are 150-250 ng/mL at transplantation and 50-100 ng/mL at 3-6 months (or 100-300 and 80-200 ng/mL as measured in whole blood). Daily IV dosing is usually ~ 5 mg/kg in two divided doses, with PO dosing increased about 3-fold. Hypertension and renal injury are major side effects of this drug, neurotoxicity is also problematic.
Tacrolimus also inhibits calcineurin, which reduces the production of IL-2 via binding of FK-binding protein 12 (FKBP12). Unlike cyclosporine, tacrolimus inhibits the expression of TNF-β. Metabolized by CYP 3A4. Monitoring is essential. Target levels are 0.5-2 ng/mL in serum/plasma and 5-20 ng/mL in whole blood. Daily IV dosing is usually ~ 0.05-0.1 mg/kg slowly over 24 hours, with PO dosing increased about 4-fold. A major advantage of tacrolimus over cyclosporine is its ability to reverse ongoing acute rejection. Renal injury is a major side effect of this drug, and neurotoxicity may also be problematic.
Azathioprine is a purine analog which is toxic to proliferating cells – it is useful for maintenance regimens (3-5 mg/kg PO qday) but has no utility for the treatment of acute rejection. Myelosuppression is a major side effect, develops in 1-2 weeks, and is often dose-limiting. Other major concerns include hepatotoxicity and pancreatitis, both of which have limited its use
Mycophenolate mofetil is an inosine monophosphate dehydrogenase inhibitor which also impacts purine synthesis (like azathioprine). Like azathioprine, mycophenolate can cause myelosuppression, however mycophenolate is more selective for B and T cells than azathioprine, and thus has a superior side effect profile. The most common side effects of mycophenolate are gastrointestinal in origin (e.g. nausea, vomiting, constipation, diarrhea). Maintenance dose is 2 g/day for most organ systems (3 g/day for hearts)
Sirolimus binds to FKBP12 (like tacrolimus), however, it does not affect calcineurin but rather inhibits mammalian targets of rapamycin (mTOR) which causes cell cycle arrest. Because sirolimus is metabolized by CYP3A4 enzymes (and a substrate for P-glycoprotein pump), it must be separated from cyclosporine by about four hours. The half-life of sirolimus is 60 hours. Sirolimus rarely causes nephrotoxicity or neurotoxicity, its major side effects are myelosuppression and hyperlipidemia. Sirolimus can be used for maintenance therapy and is also useful for minimizing both steroids or nephrotoxic drugs (e.g. cyclosporine)
Antilymphocyte antibodies alter lymphocyte activity via multiple mechanisms. Equine antithymocyte globulin (polyclonal) has recently been supplanted by the polyclonal rabbit antithymocyte globulin (RATG), which has less of an effect on B-cells (and thus partially preserves humoral immunity). Monocytes, thymocytes, leukocytes, dendritic cells, and NK cells are all affected by RATG. RATG is useful in both induction therapy as well as in treatment of acute rejection. Myelosuppression is the major side effect although fever is also common
OKT3 is a monoclonal murine antibody directed at the CD3 receptor of T-cells. It has largely been replaced by RATG because of the high incidence of side effects (fever, chills, tachycardia, hypotension, bronchospasm, pulmonary edema, etc.) and immunogenicity (which leads to rapid development of resistance)
Anti-Interleukin-2 monoclonal antibodies include daclizumab (hybrid monoclonal) and basiliximab (chimeric antibody with less murine contributions than daclizumab). These drugs are often given immediately before transplantation and are not useful for treating acute rejection. They may increase the time to needing calcineurin inhibitors
Alemtuzumab is a monoclonal Ab against CD52 which may offer equivalent T-cell depression with less overall immunosuppression. Rituximab is a CD20 antibody which affects B cells and may be used to treat post-transplantation lymphoproliferative disease
Immunosuppression for Specific Organs
The efficacy of induction therapy in heart transplantation is a matter of debate. Most programs now induce with anti-IL-2 antibodies although a recent trial suggested that daclizumab reduces both allograft rejection and survival [Hershberger RE et al. NEJM 352: 2705, 2005], mostly due to infectious complications. ~ 50% of patients receive anti-lymphocyte therapy at implantation [Taylor DO et al. J Heart Lung Trans 25: 869 2006]. Maintenance is often accomplished with steroids, a calcineurin inhibitor, and a cell cycle inhibitor (mycophenolate may be safer than azathioprine [Hosenpud JD. NEJM 352: 2749, 2005]) – when to start calcineurin inhibitors may depend on the post-operative creatinine.
Aggressive induction therapy usually includes at the very least steroids plus anti-lymphocyte antibodies, with many centers adding IL-2 antibodies as well
Sample Lung Transplantation Induction Regimen
- Tacrolimus (calcineurin inhibitor) prior to roll-back to OR
- Basiliximab (Anti-IL-2 mAb) post-induction
- Mycophenolate (cell cycle inhibitor) post-induction
- IVIG (Ab) post-induction
- Methylprednisolone (steroids) pre-reperfusion
The classic lung transplantation maintenance regimen consists of steroids, a calcineurin-inhibitor (cyclosporine troughs 250-300 ng/mL, tacrolimus trougs 8-12 ng/mL), and azathioprine