IMMUNOSUPPRESSANT DRUGS 31st March, 2017

Revision: READ ON: Cellular mechanisms:host defence (Pharmacology by Rang & Dale-7th Edition, chapter 6) Mediators of inflammation and immune reactions (Pharmacology by Rang & Dale-7th Edition, chapter 17) Understanding the cellular responses and their functions provides an essential basis for understanding the actions of anti-inflammatory and immunosuppressant drugs-a major class of therapeutic agents.

Immunosuppressants Most immunosuppressants act in the induction phase of immunological response, Reducing lymphocyte proliferation; some also inhibit aspects of the effector phase. The drugs used for immunosuppression can roughly be divided into agents that-: Inhibit interleukin-2 (IL-2) production or action, e.g. Ciclosporin, Tacrolimus; Rapamycin ( also known as Sirolimus ).

Immunosuppressants… Inhibit cytokine gene expression e.g. the Corticosteroids. Act by cytotoxic mechanism; e.g. Cyclophosphamide; Chlorambucil Inhibit purine or pyrimidine synthesis, e.g. Azathioprine; Myclophenolate mofetil Block the T cell surface molecules involved in signaling, e.g. Polyclonal and monoclonal antibodies.

Immunosuppressants… Immunosuppressants are used in the therapy of auto-immune disease and to prevent and/or treat transplant rejection. Because they impair immune responses, they carry the hazard of a decrease response to infections and may facilitate the emergence of malignant cells. However; the relationship between these adverse effects and potency in preventing graft rejection varies with different drugs.

Summary: Clinical Use of Immunosuppressants Are used for three main purposes: To suppress rejection of transplanted organs and tissues ( kidneys, bone marrow, heart, liver, etc) To suppress graft-versus-host disease (i.e. the response of lymphocytes in the graft to host antigens) in bone marrow transplants

Summary: Clinical Use of Immunosuppressants… To treat a variety of conditions that, while not completely understood, are believed to have an important autoimmune component in their pathogenesis: -idiopathic thrombocytopenic purpura, -some forms of haemolytic anaemia, -some forms of glomerulonephritis, -myasthenia gravis, -systemic lupus erythematosus, -rheumatoid arthritis, -psoriasis and ulcerative colitis

Summary: Clinical Use of Immunosuppressants… The therapy for this third category often involves a combination of glucocorticoid and cytotoxic agents For transplantation of organs or bone marrow, ciclosporin is usually combined with glucocorticoid, a cytotoxic drug or an antilymphocyte immunoglobin

1. CYCLOSPORIN Is a fungal peptide with potent immunosuppressive activity but no effect on the acute inflammatory reaction per se. It was discovered by Borel and his co-workers in 1976 in the course of screening fungal products for anti-fungal activity and soon revolutionized the field of organ transplantation; significantly reducing the morbidity and the incidence of rejection.

Mechanism of Action Cyclosporin has numerous actions on various cell types; including-: Decreased clonal proliferation of T cells, primarily by inhibiting interleukin-2 release and possibly also by decreasing expression of interleukin-2 receptors. Reduced induction of and clonal proliferation of cytotoxic T cells from CD8+ precursor T cells.

Mechanism of Action… Reduced function of the effector T cells that mediate cell-mediated responses ( e.g. decreased delayed-type hypersensitivity ). Some reduction of T-cell-dependent B cell responses

Pharmacokinetic Aspects Cyclosporin can be given orally or by intravenous infusion. After oral administration, absorption from G.I.T. is rather poor and varies in different individuals but peak plasma concentrations are usually attained in about 3 to 4 hours.

Pharmacokinetic Aspects… Alternative formulations with improved oral absorption are now available. Plasma half-life is approx. 24 hours. Metabolism occurs in the liver and most of metabolite ( of which 14 have been identified ) are excreted in the bile.

Pharmacokinetic Aspects… It accumulates in most tissues at concentrations 3 to 4 times that seen in the plasma. Some of the drug remains in lymphomyeloid tissue and later in fat depots for some time after administration has been stopped. Other drugs (e.g. Ketoconazole ) may increase the plasma concentration of cyclosporin.

Unwanted Effects The commonest and most serious is nephrotoxicity. Hepatotoxicity and hypertension can also occur. Less important unwanted effects are anorexia; lethargy; hirsutism; tremor paraesthesia ( tingling sensation ) gum hypertrophy and G.I.T. disturbances. Cyclosporin has no depressant effects on the bone marrow.

2. TACROLIMUS Originally termed FK506; is a macrolide antibiotic with a very similar mechanism of action to cyclosporin, the main difference being that the internal receptor for this drug is not cyclophilin but FK-binding protein ( FKBP ). The tacrolimus-FKBP complex inhibits calcineurin with the effects described above. Tacrolimus is active at lower concentrations than Cyclosporin.

Pharmacokinetic aspects Tacrolimus can be given orally or by intravenous injection. It is 99 % metabolized by the liver and has a half-life of approximately 7 hours.

Unwanted Effects The unwanted effects of Tacrolimus are similar to those of Cyclosporin but are more severe. Nephrotoxicity and hypertension have so far not been a serious problem. Neurotoxicity; G.I.T. upsets and metabolic disturbances (hyperglycaemia) can occur. Thrombocytopenia and hyperlipidaemia have been reported but are reversible by reducing the dosage.

3. GLUCOCORTICOIDS Immunosuppression by glucocorticoids involves both their effects on the immune response and their inflammatory actions. Glucocorticoids are immunosuppressant mainly because, like Cyclosporin, they restrain the clonal proliferation of the Th cells, through decreasing transcription of the gene for IL-2;

GLUCOCORTICOIDS… However, they also decrease the transcription of many other cytokine genes ( including those for tumour necrosis factor-TNF-, interferon-, IL-1 and many other interleukins ) in both the induction and effector phases of the immune response. These effects on transcription are mediated through inhibition of the action of transcription factors, such as AP-1 and NF-KappaB

4. AZATHIOPRINE It interferes with purine synthesis and is cytotoxic. It is widely used for immunosuppression particularly for control of tissue rejection in transplant surgery. This drug is metabolized to give Mercaptopurine, a purine analogue that inhibits DNA synthesis.

AZATHIOPRINE… Both cell-mediated and antibody-mediated immune reactions are depressed by this drug since it inhibits clonal proliferation in the induction phase of the immune response by a cytotoxic action on dividing cells. The unwanted effect in this drug is depression of bone-marrow. Other toxic effects are nausea and vomiting, skin eruptions and a mild hepatotoxicity.

5. CYCLOPHOSPHAMIDE Is a cytotoxic agent with powerful immunosuppressive effects. It is an alkylating agent with particular action on lymphocytes. As an immunosuppressant it affects the clonal proliferative phase of immune response and reduces both antibody-mediated and cell-mediated immune reactions.

6. CHLORAMBUCIL Another alkylating cytotoxic agent; is also used for immunosuppression and has effects similar to those of Cyclophosphamide.

7. MYCOPHENOLATE MOFETIL Is a semi-synthetic derivative of fungal antibiotic. In the body it is converted to Mycophenolic acid which restrains proliferation of both T and B lymphocytes and reduces production of cytotoxic T cells by inhibiting inosine monophosphate dehydrogenase; an enzyme crucial for de novo purine biosynthesis. T and B cells are particularly dependent on this pathway so the drug has a fairly selective action on these cells.

Pharmacokinetic Aspects & Unwanted Effects It is given orally and well absorbed. Magnesium and Aluminium hydroxides impair absorption and Cholestyramine reduces plasma concentrations. The metabolite Mycophenolic acid, undergoes enterohepatic cycling and is eliminated by the kidney as the inactive glucuronide.

Unwanted G.I.T. effects are common. Used with Cyclosporin and Steroids; it has proved to have effective immunosuppressant action in several clinical trials of kidney transplant rejection.

8. IMMUNOGLOBULINS Antibodies against human lymphocytes have significant immunosuppressant action. These antibodies are raised by immunizing horses or rabbits, or from mice using hybridoma technology.

8. IMMUNOGLOBULINS… Their disadvantage is that the foreign antibodies themselves elicit an immune response. To avoid this, animal immunoglobulin can now be ‘ humanized ‘ by genetic engineering to combine the antigen-binding ( Fab ) site of a mouse monoclonal antibody with human immunoglobulin.

9. POLYCLONAL ANTIBODIES Anti-lymphocyte immunoglobulin (ALG ) and anti-thymocyte immunoglobulin ( ATG ) are obtained by immunizing horses with human lymphocytes or with fetal thymic tissue respectively. These antibodies interact with multiple surface proteins implicated in T cell signal transduction and have indiscriminate action against both useful T cells and those that produce unwanted effects.

POLYCLONAL ANTIBODIES… The immunoglobulin ‘ recognizes ‘ and binds to protein on the lymphocyte surface causing exposure of the complement-binding site on the Fc portion of the immunoglobulin; this activates the complement system leading to the lysis of the lymphocyte.

POLYCLONAL ANTIBODIES… The unwanted effects are mainly those to be expected with injection of foreign protein. Antibodies against the foreign immunoglobulin can be produced; anaphylactic reaction can occur.

10. MONOCLONAL ANTIBODIES Monoclonal antibodies directed against surface component of T cells include immunoglobulin targeting the complex CD3 protein with antigen receptor ( muromonab-CD3; OKT3 ), the CD4 co-receptor and IL-2 receptor. Some are currently being used, some are being assessed. Initial doses of OKT3 can cause fever, hypotension, pulmonary oedema, nephropathy and encephalopathy, which is thought to be due to cytokine release; pretreatment with steroids diminishes these effects.

11. RAPAMYCIN ( SIROLIMUS ) This is a macrolide antibiotic that, like Tacrolimus, binds to the intracellular immunophilin FKBP. However, the complex does not bind calcineurin nor does it affect IL-2 gene transcription; it interferes with the IL-2 signal transduction pathway, blocking the cell cycle of activated T cell in G1 phase . Rapamycin competes with Tacrolimus for FKBP in vitro but their effects are additive in vivo.

RAPAMYCIN ( SIROLIMUS ) This drug also reduces smooth muscle proliferation. Phase I and II clinical trials of Rapamycin supplementation of Cyclosporin regimes are under way. Nephrotoxicity and hypertension have not been serious problems and hyperlipidaemia and thrombocytopenia, have responded to reduction in drug dosage.

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