Immunosuppressive drug: Encyclopedia - Immunosuppressive drug

Immunosuppressive drug

Immunosuppressive drugs or immunosuppressants are drugs that are used in immunosuppressive therapy to inhibit or prevent activity of the immune system. Clinically they are used to:

• prevent the rejection of transplanted organs and tissues (e.g. bone marrow, heart, kidney, liver)
• treatment of autoimmune diseases or diseases that are most likely of autoimmune origin (e.g. rheumatoid arthritis, myasthenia gravis, systemic lupus erythematosus, ulcerative colitis).

These drugs are not without side effects and risks. Because the majority of them act non-selectively, the immune system loses its ability to successfully resist infections and spreading of malignant cells. There are also other side effects, like hypertension, dyslipidemia, hyperglycemia, peptic ulcers, liver and kidney injury. The immunosuppressive drugs also interact with other medicines and affect their metabolism and action.

Immunosuppressive drugs can be classified into four groups:

• glucocorticoids
• cytostatics
• antibodies
• drugs acting on immunophilins
• other drugs

Immunosuppressive drug - Glucocorticoids

General information: Glucocorticoid.

In pharmacologic (supraphysiologic) doses, glucocorticoids are used to suppress various allergic, inflammatory, and autoimmune disorders. They are also administered as posttransplantory immunosuppressants to prevent the acute transplant rejection and graft-versus-host disease. Nevertheless, they do not prevent an infection and also inhibit later reparative processes.

Immunosuppressive drug - Immunosuppressive mechanism

Glucocorticoids suppress the cell-mediated immunity. They act by inhibiting genes that code for the cytokines IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8 and TNF-γ, the most important of which is the IL-2. Smaller cytokine production reduces the T cell proliferation.

Glucocorticoids also suppress the humoral immunity, causing B cells to express smaller amounts of IL-2 and of IL-2 receptors. This diminishes both B cell clone expansion and antibody synthesis.

Immunosuppressive drug - Antiinflammatory effects

Glucocorticoids influence all types of inflammatory events, no matter what their cause. They induce the lipocortin-1 (annexin-1) synthesis, which then binds to cell membranes preventing the phospholipase A2 from coming into contact with its substrate arachidonic acid. This leads to diminished eicosanoid production. The cyclooxygenase (both COX-1 and COX-2) expression is also suppressed, potentiating the effect.

Glucocorticoids also stimulate the lipocortin-1 escaping to the extracellular space, where it binds to the leukocyte membrane receptors and inhibits various inflammatory events: epithelial adhesion, emigration, chemotaxis, phagocytosis, respiratory burst and the release of various inflammatory mediators (lysosomal enzymes, cytokines, tissue plasminogen activator, chemokines etc.) from neutrophils, macrophages and mastocytes.

Immunosuppressive drug - Cytostatics

General information: Chemotherapy

Cytostatics inhibit cell division. In immunotherapy, they are used in smaller doses than in the treatment of malign diseases. They affect the proliferation of both T cells and B cells. Due to their highest effectiveness, purine analogs are most frequently administered.

The alkylating agents used in immunotherapy are nitrogen mustards (cyclophosphamide), nitrosoureas, platinum compounds and others. Cyclophosphamide is probably the most potent immunosuppressive compound. In small doses, it is very efficient in the therapy of systemic lupus erythematosus, autoimmune hemolytic anemias, Wegener's granulomatosis and other immune diseases. High doses cause pancytopenia and hemorrhagic cystitis.

Main representatives of antimetabolites are folic acid analogues (methotrexate), purine analogues (azathioprine, mercaptopurine), pyrimidine analogues, protein synthesis inhibitors. They interfere with the synthesis of nucleic acids.

Methotrexate is a folic acid analogue. It binds dihydrofolate reductase and prevents synthesis of tetrahydrofolate. It is used in the treatment of autoimmune diseases (for example rheumatoid arthritis) and in transplantations.

Azathioprine, is the main immunosuppressive cytotoxic substance. It is extensively used to control transplant rejection reactions. It is nonenzymatically cleaved to 6-mercaptopurine that acts as a purine analogue and an inhibitor of DNA synthesis. By preventing the clonal expansion of lymphocytes in the induction phase of the immune response, it affects both the cell and the humoral immunity. It is also efficient in the treatment of autoimmune diseases.

Among these, dactinomycin is the most important. It is used in kidney transplantations. Other cytotoxic antibiotics are anthracyclines, mitomycin C, bleomycin, mitramycin.

Immunosuppressive drug - Antibodies

Antibodies are used as a quick and potent immunosuppression method to prevent the acute rejection reaction.

Immunosuppressive drug - Polyclonal antibodies

Heterologous polyclonal antibodies are obtained from the serum of different animals (e.g. rabbit, horse) injected with patient's thymocytes or lymphocytes. The antilymphocyte (ALG) and antithymocyte antigens (ATG) are being used. They are part of the steroid-resistant acute rejection reaction and grave aplastic anemia treatment. However, they are primarily added to other immunosuppressives to diminish their dosage and toxicity. They also allow transition to cyclosporine therapy. They are usually administered for five days intravenously in the appropriate quantity. Patient stays in the hospital for three weeks so the immune system recovers and there is no risk of serum sickness anymore.

Polyclonal antibodies inhibit T lymphocytes and cause their lysis, which is both complement mediated cytolysis and cell-mediated opsonization followed by removal of reticuloendothelial cells from the circulation in the spleen and liver. In this way, polyclonal antibodies inhibit cell-mediated immune reactions, including graft rejection, delayed hypersensitivity (i.e. tuberculin skin reaction), and the graft-versus-host disease (GVHD), but influence thymus-dependent antibody production.

Currently (March 2005) there are two preparations available to the market: Atgam (R), obtained from the horse serum, and Thymoglobuline (R), obtained from the rabbit serum.

Polyclonal antibodies affect all lymphocytes and cause general immunosuppression possibly leading to post-transplant lymphoproliferative disorders (PTLD) or serious infections, especially by cytomegalovirus. To prevent this, the therapy must obligatorily be performed in a hospital, where adequate isolation from infection is available.

Because of a high immunogenicity of polyclonal antibodies, almost all patients develop an acute reaction develop at first. It is characterized by fever, rigor episodes and even anaphylaxis. Later during the treatment, some patients develop serum sickness or immune complex glomerulonephritis. Serum sickness arises seven to fourteen days after the therapy has begun. The patient suffers from fever, joint pain and erythema that can be soothed with the use of steroids and analgesics. Urticaria (hives) can also be present. Patients also gradually develop a strong immune response against these drugs, so they stop to be effective. It is possible to diminish their toxicity by using highly purified serum fractions and intravenous administration in the combination with other immunosuppressants, for example calcineurin inhibitors, cytostatics and cortisteroids. The most frequent combination is to simultaneously use antibodies and cyclosporine.

Immunosuppressive drug - Monoclonal antibodies

Monoclonal antibodies are directed towards exactly defined antigens. Therefore, they cause fewer side effects. Especially significant are the IL-2 receptor (CD25) and CD3 directed antibodies. They are used to prevent the rejection of transplanted organs, but also to track changes in the lymphocyte subpopulations. It is reasonable to expect similar new drugs in the future.

OKT3 (R) is presently the only approved anti-CD3 antibody. It is a mouse anti-CD3 monoclonal antibody of the IgG2a type that prevents T-cell activation and proliferation by binding the T-cell receptor complex present on all differentiated T cells. As such, it is one of the most potent immunosuppressive substances and is clinically used to control the steroid and/or polyclonal antibodies resistant acute rejection episodes. For acting more specifically than polyclonal antibodies, it is also used preventively in transplantations.

Presently, the OKT3's action mechanism is not yet sufficiently understood. It is known that the molecule binds TCR/CD3, the T-cell receptor complex. During the first few administrations, this binding non-specifically activates T cells, leading to a serious syndrome 30 to 60 minutes later. It is characterized by fever, myalgia, headache and artralgia. In some cases, it progresses to a life-threatening reaction of the cardiovascular system and the central nervous system needing a lengthy therapy. Past this period, CD3 (R) blocks the TCR - antigen binding and causes conformation change or the removal of the entire TCR3/CD3 from the T-cell surface. This lowers the number of T cells, perhaps by sensitising them for the uptake by the reticular epithelial cells. The cross-binding of CD3 molecules also activates an intracellular signal, causing the T cells' anergy or apoptosis, unless they receive another signal through a costimulatory molecule. CD3 antibodies also shift the balance from Th1 to Th2 cells.

Deciding whether to use OKT3(R) in the treatment, it is therefore necessary not only to consider its great effectiveness, but also its toxic side effects: the risk of excessive immunosuppression and the risk that the patient develops neutralizing antibodies against the drug, making it inefficacious. Although CD3(R) antibodies act more specifically than polyclonal antibodies, they lower the cell-mediated immunity significantly, predisposing the patient to opportunistic infections and malignancies.

Interleukin-2 is an important immune system regulator necessary for the clone expansion and survival of activated lymphocytes T. Its effects are mediated by the trimer cell surface receptor IL-2a, consisting of the α, β and γ chains. The IL-2a (CD25, T-cell activation antigen, TAC) is expressed only by the already activated T lymphocytes. Therefore, it is of special significance to the selective immunosuppressive treatment and the research has been focused on the development of effective and safe anti-IL-2 antibodies. By the use of the recombinant gene technology, the mouse anti-Tac antibodies have been modified leading to the presentation of two himeric mouse/human anti-Tac antibodies in the year 1998: basiliximab (Simulect (R)) and daclizumab (Zenapax (R)). These drugs act by binding the IL-2a receptor's α chain, preventing the IL-2 induced clonal expansion of activated lymphocytes and shortening their survival. They are used in the profilaxis of the acute organ rejection after the bilateral kidney transplantation, both being similarly effective and with only few side effects.

Immunosuppressive drug - Drugs acting on immunophilins

Immunosuppressive drug - Cyclosporine

General information:cyclosporine

Together with tacrolimus, cyclosporine is a calcineurin inhibitor. It has been in use since 1983 and is one of the most widely used immunosuppressive drugs. It is a fungal peptide, composed of 11 amino acids.

Cyclosporine is thought to bind to the cytosolic protein cyclophilin (an immunophilin) of immunocompetent lymphocytes, especially T-lymphocytes. This complex of cyclosporin and cyclophylin inhibits calcineurin, which under normal circumstances induces the transcription of interleukin-2. The drug also inhibits lymphokine production and interleukin release, leading to a reduced function of effector T-cells.

Cyclosporine is used in the treatment of acute rejection reactions, but has been increasingly substituted with newer immunosuppressants, as it is nephrotoxic.

Immunosuppressive drug - Tacrolimus PrografTM FK506

Tacrolimus is a fungal product (Streptomyces tsukubaensis) too. It is a macrolide lactone and acts by inhibiting calcineurin.

The drug is used particularly in the liver and kidney transplantations although in some clinics it is used in heart, lung and heart/lung transplants. It binds to an immunophilin, followed by the binding of the complex to calcineurin and the inhibition of its phosphatase activity. In this way, it prevents the passage of G0 into G1 phase. Tacrolimus is more potent than cyclosporine and has less pronounced side effects.

Immunosuppressive drug - Sirolimus Rapamune Tm Rapamicin

Sirolimus is a macrolide lactone, produced by the actinomycetes Streptomyces hygroscopicus. It is used to prevent rejection reactions. Although it is a structural analogue of tacrolimus, it acts somewhat differently and has different side effects.

Contrary to cyclosporine and tacrolimus that affect the first phase of the T lymphocyte activation, sirolimus affects the second one, namely the signal transduction and their clonal proliferation. It binds to the same receptor (immunophilin) as tacrolimus, however the produced complex does not inhibit calcineurin, but an other protein. Therefore, sirolimus acts synergistically with cyclosporine and, in combination with other immunosuppressants, has few side effects. Indirectly it inhibits several T lympohocyte kinases and phosphatases, preventing the transmission of signal into their activity and the transition of the cell cycle from G1 to S phase. Similarly, it prevents the B cell differentiation to the plasma cells, which lowers the quantity of IgM, IgG and IgA antibodies produced. It acts immunoregulatory.

Immunosuppressive drug - Other drugs

Immunosuppressive drug - Interferons

General information:Interferon.

IFN-β suppresses the production of Th1 cytokines and the activation of monocytes. It is used to slow down the progression of multiple sclerosis. IFN-γ is able to trigger lymphocytic apoptosis.

Immunosuppressive drug - Opioids

Prolonged use of opioids may cause immunosuppression by inhibiting the migration of leukocytes.

Immunosuppressive drug - TNF binding proteins

A TNF-α (tumor necrosis factor alpha) binding protein is a monoclonal antibody or a circulating receptor such as infliximab (Remicade®), etanercept (Enbrel®), or adalimumab (Humira®) that binds to TNF-α and prevent it from inducing the synthesis of IL-1 and IL-6 and the adhesion of lymphocyte activating molecules. They are used in the treatment of rheumatoid arthritis, ankylosing spondylitis, Crohn's disease and psoriasis.

TNF or the effects of TNF are also suppressed by various natural compounds, including curcumin (an ingredient in turmeric) and catechins (in green tea).

These drugs may raise the risk of contracting tuberculosis or inducing a latent infection to become active. Infliximab and adalimumab have label warnings stating that patients should be evaluated for latent TB infection and treatment should be initiated prior to starting therapy with them.

Immunosuppressive drug - Mycophenolate mofetil

Mycophenolate mofetil acts as a non-competitive, selective and reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), which is a key enzyme in the de novo guanosine nucleotide synthesis. In contrast to other human cell types, lymphocytes B and T are very dependent on this process.

Immunosuppressive drug - Small biological agents

FTY720 is a new synthetic immunosuppressant, currently in phase 3 of clinical trials. It increases the expression or changes the function of certain adhesion molecules (α4/β7 integrin) in lymphocytes, so they accumulate in the lymphatic tissue (lymphatic nodes) and their number in the circulation is diminished. In this respect, it differs from all other known immunosuppressants.

Adapted from the Wikipedia article "Immunosuppressive drug", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki