Immunomodulatory Drugs | Toxicology

This article provides an overview on immunomodulatory drugs.

Introduction to Immunomodulatory Drugs:

The body provides an ideal environment to the microorganisms and hence is need of a defence mechanism in the body to deal with the foreign invaders. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. This system has amazing capacity to deal with a complex array of antigens.

Lymph nodes, thymus, spleen, bone marrow and lymphocytes are the major players of immune system. Immune system has a unique ability to distinguish between self and non-self by recognising specific cell surface markers called Major Histocompatibility Complex (MHC).

Immune system has two components:

1. Innate immunity:

It is also called natural immunity. It is inborn, non-specific and broadly reactive. It is most active during early stage of immune response. Major effectors of innate immunity are complement, granulocytes, macrophages, natural killer cells, mast cells and basophils.

2. Adaptive immunity:

It is also called acquired immunity. It is antigen specific and it occurs in response to infection. It can be of high affinity and is predominantly active in later phase of immune response. Major effectors of this type of immunity are B and T lymphocytes. B lymphocytes make antibodies and T lymphocytes work as helper, cytotoxic and suppressor cells.

Immune system is a highly synchronized and vigilant machinery of the body and a precise regulation of this system is of utmost importance to maintain body’s homeostasis. A low or high activity of immune system leads to a number of diseases like immunodeficiency disorders, autoimmune disorders, allergic disorders and cancer. Immuno-suppressants and immunostimulans are the major group of drugs used to deal with such immunity related diseases.

Immunosuppressant’s:

These are drugs that inhibit or prevent activity of the immune system. They are used to control severe manifestations of allergic, auto-immune and transplant related diseases.

Most commonly used immunosuppressive drugs are:

1. Glucocorticoids

2. Calcineurin inhibitors- eg. Cyclosporin, Tacrolimus

3. Anti-proliferative/anti-metabolic agents- eg., Sirolimus, Everolimus, Azathioprine, Mycophenolate mofetil and other anti-proliferative and cytotoxic agents

4. Antibodies- e.g., (a) Anti-thymocyte globulin, (b) Anti CD3 monoclonal antibody-Muromonab (c) Anti interleukin-2 (TL-2) receptor antibody – Daclizumab, basiliximab, (d) Anti tumor necrosis factor alpha (TNF-α) antibody infliximab, etanercept.

Glucocorticoids:

Glucocorticoids e.g., prednisone, prednisolone, dexamethasone etc. are commonly used either alone or in combination with other immunosuppressive agents in several immune disorders such as graft rejection, autoimmune disorders and neoplasms.

Structure:

Glucocorticoids have a cyclopentanoperhydro- phenanthrene nucleus. They are synthesized in adrenal cortex from cholesterol. Cortisol (prototype glucocorticoid) structure is given in fig. 51.1. Several new compounds have been made by chemical alteration in its structure. 4, 5 double bond and 3-keto group is essential for glucocorticoid activity. 17a-hydroxyl group on ring D gives optimal potency. Additional double bond in the 1, 2 position of ring A selectively increases glucocorticoid activity eg., prednisone, prednisolone.

Pharmacokinetics:

Corticosteroids are effective by oral route. They are well absorbed systemically also from sites of local administration such as synovial, spaces, conjunctival sac, skin, respiratory tract. Prolonged absorption leads to systemic effects including suppression of hypothalamus-pituitary-adrenal (HPA) axis. More than 90% of drug bind to plasma proteins specially transcortin/corticosteroid binding globulin and albumin.

Biotransformation occurs by hepatic microsomal enzymes and sulfate and glucuronide conjugation which are excreted mainly in urine. Cortisone and prednisone are inactive until converted in vivo to hydrocortisone and prednisolone respectively.

Mechanism of Action:

Multiple mechanisms are involved in immunosuppressive action of glucocorticoids. They bind to intracellular glucocorticoid receptor which is then translocated to nucleus. The liganded glucocorticoid receptor dimer binds to glucocorticoid response element (GKE) within the promoter sequence of the gene and up-regulate the transcription of a number of genes. Glucocorticoids induce redistribution of lymphocytes, causing a rapid and transient decrease in peripheral lymphocyte counts.

They inhibit NF-kB, a critical transcription factor involved in the synthesis of many mediators (i.e., cytokines) and proteins (i.e., adhesion proteins) that promote the immune response. In addition, they also down regulate key pro-inflammatory cytokines, such as IL-1, IL-2, IL-3, IL-4, IL-5, IL-6 and TNF-a. Activation of cytotoxic T lymphocytes is inhibited.

Neutrophil and monocyte chemo taxis is inhibited and release of lysosomal enzymes is also decreased. Thus they affect multiple components of cell mediated immunity but have little effect on humoral immunity.

Therapeutic Uses:

Glucocorticoids are indicated in many clinical states like:

1. Prevention and treatment of graft-versus-host disease following organ or bone marrow transplant.

2. Autoimmune disorders such as rheumatoid arthritis, systemic lupus erythematous, psoriasis etc.

3. Replacement therapy for patients with adrenal failure (Addison’s disease).

4. Hypersensitivity states like severe allergic reactions.

5. Along with cytotoxic drugs in neoplastic diseases (e.g., Hodgkin’s disease, acute lymphocytic leukemia) or to reduce cerebral oedema in patients of metastatic brain tumors.

Adverse Effects:

Growth retardation in children, avascular necrosis of bone, osteoporosis, increased risk of infection, poor wound healing, hyperglycemia and hypertension are common adverse effects associated with glucocorticoids. Adrenal insufficiency occurs if it is stopped suddenly after prolonged use. Combined use with calcineurin inhibitors reduces adverse effects of glucocorticoids.

Calcineurin Inhibitors:

They are one of the most effective immunosuppressive agents. Cyclosporin and tacrolimus are routinely used which have different structures but similar mechanism of action.

(i) Cyclosporin:

It is a naturally occurring compound first found in fungus Beauveria nivea.

Structure:

It is a cyclic polypeptide of 11 amino acids (Fig.51.2).

Pharmacokinetics:

Cyclosporin is lipophilic and highly hydrophobic. It can be administered intravenously or orally. Administration with food delays and decreases absorption. Widely distributed in body. It is extensively metabolised in liver and to some extent in GI tract and kidneys.

Excretion mainly occurs through bile into feaces and only 0.1% cyclosporin is excreted unchanged in urine. Drugs such as verapamil, nicardipine, ketoconazole, erythromycin, indinavir that inhibit CYP3A can decrease cyclosporin metabolism and increase its blood concentration.

Mechanism of Action:

Cyclosporin inhibits antigen-triggered signal transduction in T lymphocytes, suppresses expression of many lymphokines, including IL-2, and anti- apoptotic proteins.

It also increases expression of TGF-b. Cyclosporin forms a complex with cyclophilin, a cytoplasniic receptor protein of immunophilin family, present in target cells. This complex binds and inhibits calcineurin. This prevents gene transcription and T lymphocytes fail to respond to specific antigen stimulation. Cylosporin also increases expression of TGF-ᵦ, a potent inhibitor of IL-2 stimulated T-cell proliferation.

Therapeutic Uses:

Cyclosporin is used in organ transplant, rheumatoid arthritis and psoriasis. It is usually combined with other agents especially glucocorticoids, azathioprine, mycophenolate and sirolimus.

Adverse Effects:

The most common adverse effect of cyclosporin is renal dysfunction. Hypertension, tremor, hirsutism, hyperlipidemia and gut hyperplasia are also encountered. Combined use of calcineurin inhibitors and glucocorticoids is diabetogenic.

(ii) Tacrolimus:

It is a macrolide antibiotic produced by Streptomyes tsukubaensis. It has similar mechanism of action but higher potency than cyclosporin.

Pharmacokinetics:

Tacrolimus is given both orally and intravenously. GI absorption is incomplete and variable. Food decreases rate and extent of absorption. Plasma protein binding is 75-99% involving primarily albumin and a-acid glycoprotein. Its 1/2 is approximately 12 hours and is 99% metabolised in liver by CYP3A. CYP3A inhibitors potentiate the action of tacrolimus and CYP3A inducers inhibit its action. Excretion mainly occurs through faeces and less than 1% is excreted through urine.

Mechanism of Action:

Tacrolimus is also a calcineurin inhibitor like cyclosporin but is 10-100 times more effective. Tacrolimus binds to an immunophilin, FK506 binding protein (FKBP). This complex inhibits calcineurin phosphatase which inhibits signal transduction pathway leading to T-cell activation and IL-2 transcription. The drug inhibits calcium-dependent events, such as interleukin-2 gene transcription, nitric oxide synthase activation, cell degranulation, and apoptosis.

Therapeutic Uses:

Tacrolimus is indicated for prophylaxis of solid organ allograft rejection. Also used in bone marrow transplant and severe atopic eczema.

Adverse Effects:

Nephrotoxicity, neurotoxicity, G1 complaints, hyperkalemia, hyperglycemia and diabetes are commonly associated with tacrolimus. Co-administration with cyclosporin causes additive nephrotoxicity.

Anti-proliferative and Anti-metabolic Drugs:

(i) Sirolimus:

Also known as rapamycin. It is a macro cyclic lactone produced by Streptomyces hygroscopicus. Originally developed as an antifungal agent but now used as a potent immunosuppressive and anti-proliferative agent.

Pharmacokinetics:

Sirolimus has poor oral absorption. It distributes widely in tissues. Metabolised by CYP3A4 enzyme and has long elimination half life. Excretion mainly occurs through faeces and to some extent through urine.

Mechanism of Action:

Sirolimus bind the cytosolic protein FK-binding protein 12 (FKBP12) in a manner similar to tacrolimus. Unlike the tacrolimus-FKBP12 complex which inhibits calcineurin/ Protein phosphatase 2B (PP2B). Sirolimus-FKBP12 complex inhibits the mammalian target of rapamycin (mTOR) pathway and produce cytostatic effect. It also inhibits the response to interleukin-2 (IL-2) in activation of T- and B-cells.

Therapeutic Uses:

In cases of patients with kidney transplants for hemolytic-uremic syndrome sirolimus is preferred over cyclosporin or tacrolimus. Also used for coronary stent coating in coronary artery after angioplasty.

Adverse Effects:

Lung toxicity is a serious complication associated with sirolimus therapy. Diabetes likes symptoms and risk of cancer proliferation are the other adverse effects of sirolimus. As compared to cyclosporin and tacrolimus it has low nephrotoxic potential.

(ii) Everolimus:

Everolimus is the 40-0-(2-hydroxyethyl) derivative of sirolimus and works similarly to sirolimus as an inhibitor of mTOR. It is used as an immunosuppressant to prevent rejection of organ transplants and treatment of renal cell cancer and other tumors.

(iii) Azathioprine:

It is a purine analogue which was first introduced as an immunosuppressive agent in 1961.

Structure:

Azathioprine is a thiopurine linked to a second heterocycle (an imidazole derivative) via athioether (Fig. 51.5).

Pharmacokinetics:

Azathioprine has good oral absorption. Bioavailability varies greatly among individuals due to its partial inactivation in liver. Plasma protein binding is about 25%.

Azathioprine is a pro-drug, it is metabolized to 6-Mercaptopurine (6-MP) through reduction by glutathione and other sulphydryl-containing compounds and then enzymatically converted into 6-thiouric acid, 6-methyl-mercaptopurine, and 6-thioguanine (6-TG). Thiouric acid is the end product of azathioprine metabolism, which is excreted via urine. Half life is about 5 hours.

Mechanism of Action:

Azathioprine is a purine analogue. It works at the level of DNA.

The active metabolites of azathroprine get incorporated into replicating DNA and can also block the de novo pathway of purine synthesis. Thus it blocks DNA, RNA and protein synthesis. This drug has relative specificity to fast growing cells like lymphocytes.

Therapeutic Uses:

It is indicated for prevention of organ transplant rejection and severe rheumatoid arthritis.

Adverse Effects:

Hypersensitivity, bone marrow suppression, anaemia, diarrhoea and fatigue are the common adverse effects. Azathioprine is listed as a human carcinogen.

(iv) Mycophenolate mofetil (MMF):

MMF is a less toxic alternative to azathioprine. It is 2,4-morpholinoethyl ester of mycophenolic acid, an antibiotic derived from Penicillium stoloniferum.

Pharmacokinetics:

Absorption is rapid after oral administration. Oral bioavailability is 94%. Food do not interfere with absorption. Plasma protein binding is approximately 98%. MMF is completely metabolised to mycophenolic acid (MPA). MP A is metabolized principally by glucuronyl transferase to form the phenolic glucuronide which is not pharmacologically active. Mean elimination half life of active metabolite is 13-17 hour.

Mechanism of Action:

Mycophenolate mofetil is hydrolyzed to form mycophenolic acid, the active metabolite. MPA is a potent, selective, non-competitive and reversible inhibitor of inosine monophosphate dehydrogenase. Therefore it inhibits de novo guanosine nucleotide synthesis. T and B lymphocytes are highly dependent on this pathway thus lymphocyte proliferation and functions are selectively inhibited.

Therapeutic Uses:

It is indicated for prophylaxis of transplant rejection. Combined treatment with glucocorticoids and calcineurin inhibitors is done.

Adverse effects:

Gastrointestinal and hematologic toxicities, are common. Diarrhoea and vomiting.

(v) Other Anti-Proliferative and Cytotoxic Agents:

Methotrexate, cyclophosphamide, thalidomide and chlorambucil are some examples of anticancer drugs possessing immunosuppressive action due to their action on lymphocytes and other cells of the immune system.

Methotrexate is used in treatment of graft rejection, rheumatoid arthritis, psoriasis and some cancers. Cyclophosphamide and chlorambucil are used in leukemia and lymphomas. Leflunomide is a pyrimidine synthesis inhibitor indicated for treatment of rheumatoid arthritis.

Antibodies:

Presently a wide variety of both monoclonal and polyclonal antibodies are being used for prevention and treatment of transplant rejection.

(i) Anti-thymocyte Globulin:

It is purified gamma globulin from the serum of rabbits immunized with human thymocytes. It produces cytotoxicity by binding with different CD proteins and block lymphocyte function. It is indicated in treatment of acute renal transplant rejection in combination with other immunosuppressive agents. Polyclonal antibodies are xenogenic proteins that can produce major side effects like leucopenia and thrombocytopenia. Also produce fever, chills and hypotension.

(ii) Monoclonal Antibodies:

(a) Anti-CD3 monoclonal antibodies- e.g. muronomab. It binds to CD3 of T cell receptor complex and prevents antigen recognition, cell signalling and proliferation. Indicated for treatment of acute graft rejection.

(b) Anti-IL-2 receptor/anti-CD-25 antibodies- e.g. daclizumab, basiliximab., produced by recombinant DNA technology.

Daclizumab has longer half-life than basiliximab. They are used for prophylaxis of acute organ rejection.

(c) Anti-TNF reagents- e.g. Infliximab. It is an anti-TNF-a monoclonal antibody used commonly in patients of rheumatoid arthritis with elevated level of TNF-a in joint.

(d) Anti IL-1 reagents: e.g. Anakinra is an IL-1 receptor antagonist used in treatment of rheumatoid arthritis.

Immunostimulants:

Immunostimulants are substances that modulate the immune system by stimulating the function of one or more of the system’s components. They are of two types:

1. Specific immunostimulants such as vaccines, stimulate an immune response to one or more specific antigenic types.

2. Non-specific immunostimulants they do not have any antigenic specificity but can act as general stimulants that enhance the function of certain types of immune cells, e.g., Levamisole, thalidomide, Bacillus calme the- Guerin(BCG) Recombinant Cytokines : Interferons, interleukin-2.

Levamisole:

It was synthesized originally as an anthelmintic but later found to restore depressed immune function by activating function of B lymphocytes, T lymphocytes, monocytes and macrophages.

Levamisole is rapidly absorbed from GI tract and extensively metabolised in liver and excreted mainly through kidney. Plasma elimination half life is 3-4 hours.

It is now withdrawn from market because of its risk for fatal agranulocytosis. Its only indication was as adjuvant therapy in rheumatoid arthritis and colorectal cancer.

Thalidomide:

Thalidomide was synthesized in Germany, in 1954 from glutamic acid. It was withdrawn from market because of its production of congenital malformations.

It is indicated in multiple myeloma. Its mechanism of action is not clear although it has reported to decrease level of circulating TNF-a and inhibit de novo synthesis of antibodies.

Bacillus Calmette-Guerin (BCG):

It is attenuated, live culture of bacillus of Calmette and Guerin strain of Mycobacterium bovis. It has activity against tumors but mechanism is unknown. It is indicated for prophylaxis and treatment of carcinoma of urinary bladder. Hypersensitivity, shock, fever, chill, malaise are associated side effects.

Recombinant Cytokines:

(a) Interferons:

a, b and g interferon’s were initially identified by their antiviral activity, later recognized to have immunomodulatory activities. They bind to specific cell surface receptors that initiate a series of intracellular events like induction of enzymes, inhibition of cell proliferation, stimulation of phagocytosis by macrophages and cytotoxicity by T lymphocytes.

Recombinant IFN-a-2b obtained from E. coli. It is indicated in treatment of a variety of tumors like malignant melanoma, follicular lymphoma and also used in case of chronic hepatitis.

IFN-g-lb is recombinant polypeptide that activates phagocytes and induces generation of oxygen metabolites that are toxic to a number of microorganisms. It is indicated in granulomatous disease.

IFN-b-la is a recombinant glycoprotein, has antiviral and immunomodulatory properties.

(b) Interleukin-2:

Human IL-2 is produced by recombinant DNA technology in E.coli. It enhances cellular immunity with lymphocytosis, eosinophilia, thrombocytopenia and release of multiple cytokines. Indicated for the treatment of melanoma and metastatic renal cell carcinoma.