Immunosuppressive drugs or immunosuppressive agents or antirejection medications are drugs that inhibit or prevent activity of the immune system.
Classification
Immunosuppressive drugs can be classified into five groups:
Glucocorticoids
In pharmacologic (supraphysiologic) doses, glucocorticoids, such as prednisone, dexamethasone, and hydrocortisone 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 mechanism
Glucocorticoids suppress the cell-mediated immunity. They act by inhibiting genes that code for the cytokines Interleukin 1 (IL-1), IL-2, IL-3, IL-4, IL-5, IL-6, IL-8, and TNF-alpha, the most important of which is 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 IL-2 receptors. This diminishes both B cell clone expansion and antibody synthesis.
Anti-inflammatory effects
Glucocorticoids influence all types of inflammatory events, no matter 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.
Cytostatics
Cytostatics inhibit cell division.
In immunotherapy, they are used in smaller doses than in the treatment
of malignant diseases. They affect the proliferation of both T cells and
B cells. Due to their highest effectiveness, purine analogs are most frequently administered.
Alkylating agents
The alkylating agents used in immunotherapy are nitrogen mustards (cyclophosphamide), nitrosoureas, platinum
compounds, and others. Cyclophosphamide (Baxter's Cytoxan) is probably
the most potent immunosuppressive compound. In small doses, it is very
efficient in the therapy of systemic lupus erythematosus, autoimmune hemolytic anemias, granulomatosis with polyangiitis, and other immune diseases. High doses cause pancytopenia and hemorrhagic cystitis.
Antimetabolites
Antimetabolites interfere with the synthesis of nucleic acids. These include:
- folic acid analogues, such as methotrexate
- purine analogues, such as azathioprine and mercaptopurine
- pyrimidine analogues, such as fluorouracil
- protein synthesis inhibitors.
Methotrexate
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 or Behcet's Disease) and in transplantations.
Azathioprine and mercaptopurine
Azathioprine
(Prometheus' Imuran), is the main immunosuppressive cytotoxic
substance. It is extensively used to control transplant rejection
reactions. It is nonenzymatically cleaved to mercaptopurine, that acts as a purine analogue and an inhibitor of DNA synthesis. Mercaptopurine itself can also be administered directly.
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.
Cytotoxic antibiotics
Among these, dactinomycin is the most important. It is used in kidney transplantations. Other cytotoxic antibiotics are anthracyclines, mitomycin C, bleomycin, mithramycin.
Antibodies
Antibodies
are sometimes used as a quick and potent immunosuppressive therapy to
prevent the acute rejection reactions as well as a targeted treatment of
lymphoproliferative or autoimmune disorders (e.g., anti-CD20
monoclonals).
Polyclonal antibodies
Heterologous polyclonal antibodies are obtained from the serum of animals (e.g., rabbit, horse), and injected with the 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 added primarily to other
immunosuppressives to diminish their dosage and toxicity. They also
allow transition to cyclosporin therapy.
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.
As of March 2005, there are two preparations available to the market: Atgam, obtained from horse serum, and Thymoglobuline,
obtained from 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 reduce these risks, treatment is provided in a hospital, where
adequate isolation from infection is available. They are usually
administered for five days intravenously in the appropriate quantity.
Patients stay in the hospital as long as three weeks to give the immune
system time to recover to a point where there is no longer a risk of serum sickness.
Because of a high immunogenicity of polyclonal antibodies, almost all patients have an acute reaction to the treatment. 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. 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 use
antibodies and ciclosporin simultaneously in order to prevent patients
from gradually developing a strong immune response to these drugs,
reducing or eliminating their effectiveness.
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.
T-cell receptor directed antibodies
Muromonab-CD3 is a murine 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 administered to control the
steroid- and/or polyclonal antibodies-resistant acute rejection
episodes. As it acts more specifically than polyclonal antibodies it is
also used prophylactically in transplantations.
The muromonab's mechanism of action is only partially understood.
It is known that the molecule binds TCR/CD3 receptor complex. In 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 arthralgia.
Sometimes it develops in a life-threatening reaction of the
cardiovascular system and the central nervous system, requiring a
lengthy therapy. Past this period CD3 blocks the TCR-antigen binding and
causes conformational change
or the removal of the entire TCR3/CD3 complex from the T-cell surface.
This lowers the number of available T-cells, perhaps by sensitizing them
for the uptake by the epithelial reticular cells.
The cross-binding of CD3 molecules as well activates an intracellular
signal causing the T cell anergy or apoptosis, unless the cells receive
another signal through a co-stimulatory molecule. CD3 antibodies shift the balance from Th1 to Th2 cells.
The patient may develop neutralizing antibodies
reducing the effectiveness of muromonab-CD3.
Muromonab-CD3 can cause excessive immunosuppression. Although CD3
antibodies act more specifically than polyclonal antibodies, they lower
the cell-mediated immunity significantly, predisposing the patient to opportunistic infections and malignancies.
IL-2 receptor directed antibodies
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 research has been focused on the
development of effective and safe anti-IL-2 antibodies. By the use of recombinant gene technology,
the mouse anti-Tac antibodies have been modified, leading to the
presentation of two chimeric mouse/human anti-Tac antibodies in the year
1998: basiliximab (Simulect) and daclizumab
(Zenapax). 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 prophylaxis of the
acute organ rejection after bilateral kidney transplantation, both being similarly effective and with only few side-effects.
Drugs acting on immunophilins
Ciclosporin
Like tacrolimus, ciclosporin (Novartis' Sandimmune) is a calcineurin
inhibitor (CNI). It has been in use since 1983 and is one of the most
widely used immunosuppressive drugs. It is a cyclic fungal peptide,
composed of 11 amino acids.
Ciclosporin is thought to bind to the cytosolic protein cyclophilin (an immunophilin) of immunocompetent lymphocytes, especially T-lymphocytes.
This complex of ciclosporin and cyclophilin inhibits the phosphatase
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.
Ciclosporin is used in the treatment of acute rejection reactions, but has been increasingly substituted with newer, and less nephrotoxic, immunosuppressants.
Calcineurin inhibitors and azathioprine have been linked with post-transplant malignancies and skin cancers in organ transplant recipients. Non-melanoma skin cancer
(NMSC) after kidney transplantation is common and can result in
significant morbidity and mortality. The results of several studies
suggest that calcineurin inhibitors have oncogenic properties mainly
linked to the production of cytokines that promote tumor growth,
metastasis and angiogenesis.
This drug has been reported to reduce the frequency of regulatory T cells (T-Reg) and after converting from a CNI monotherapy to a mycophenolate monotherapy, patients were found to have increased graft success and T-Reg frequency.
Tacrolimus
Tacrolimus (trade names Prograf, Astagraf XL, Envarsus XR) is a product of the bacterium Streptomyces tsukubaensis. It is a macrolide lactone and acts by inhibiting calcineurin.
The drug is used primarily in liver and kidney transplantations,
although in some clinics it is used in heart, lung, and heart/lung
transplantations. It binds to the immunophilin FKBP1A, followed by the binding of the complex to calcineurin and the inhibition of its phosphatase activity. In this way, it prevents the cell from transitioning from the G0 into G1 phase of the cell cycle. Tacrolimus is more potent than ciclosporin and has less pronounced side-effects.
Sirolimus
Sirolimus (rapamycin, trade name Rapamune) is a macrolide lactone, produced by the actinomycete bacterium 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 ciclosporin and tacrolimus, drugs that affect the
first phase of T lymphocyte activation, sirolimus affects the second
phase, namely signal transduction and lymphocyte clonal proliferation.
It binds to FKBP1A like tacrolimus, however the complex does not inhibit
calcineurin but another protein, mTOR.
Therefore, sirolimus acts synergistically with ciclosporin and, in
combination with other immunosuppressants, has few side effects. Also,
it indirectly inhibits several T lymphocyte-specific kinases and
phosphatases, hence preventing their transition from G1 to S
phase of the cell cycle. In a similar manner, Sirolimus prevents B cell
differentiation into plasma cells, reducing production of IgM, IgG, and
IgA antibodies.
It is also active against tumors that are PI3K/AKT/mTOR-dependent.
Everolimus
Everolimus is an analog of sirolimus and also is an mTOR inhibitor.
Other drugs
Interferons
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.
Opioids
Prolonged use of opioids may cause immunosuppression of both innate and adaptive immunity.
Decrease in proliferation as well as immune function has been observed
in macrophages, as well as lymphocytes. It is thought that these effects
are mediated by opioid receptors expressed on the surface of these
immune cells.
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, preventing 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.
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.
TNF or the effects of TNF are also suppressed by various natural compounds, including curcumin (an ingredient in turmeric) and catechins (in green tea).
Mycophenolate
Mycophenolic acid acts as a non-competitive, selective, and reversible inhibitor of Inosine-5′-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. Mycophenolate
mofetil is used in combination with ciclosporin or tacrolimus in
transplant patients.
Small biological agents
Fingolimod
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.
Myriocin has been reported being 10 to 100 times more potent than Ciclosporin.
Therapy
Immunosuppressive drugs are used in immunosuppressive therapy to:
- Prevent the rejection of transplanted organs and tissues (e.g., bone marrow, heart, kidney, liver)
- Treat autoimmune diseases or diseases that are most likely of autoimmune origin (e.g., rheumatoid arthritis, multiple sclerosis, myasthenia gravis, psoriasis, vitiligo, granulomatosis with polyangiitis, systemic lupus erythematosus, systemic sclerosis/scleroderma, sarcoidosis, focal segmental glomerulosclerosis, Crohn's disease, Behcet's Disease, pemphigus, and ulcerative colitis).
- Treat some other non-autoimmune inflammatory diseases (e.g., long term allergic asthma control), ankylosing spondylitis.
Side effects
A common side-effect of many immunosuppressive drugs is immunodeficiency, because the majority of them act non-selectively, resulting in increased susceptibility to infections and decreased cancer immunosurveillance. There are also other side-effects, such as hypertension, dyslipidemia, hyperglycemia, peptic ulcers, lipodystrophy, moon face, liver and kidney injury. The immunosuppressive drugs also interact with other medicines and affect their metabolism and action. Actual or suspected immunosuppressive agents can be evaluated in terms of their effects on lymphocyte subpopulations in tissues using immunohistochemistry.
