15. Immunosuppressants, immunomodulators, treatment of rheumatoid arthritis

Last updated on June 15, 2019 at 17:54

Introduction

Immunosuppression refers to decreasing the activity of the immune system through cytotoxic effects or through bone marrow suppression. In other words, the activity of the immune system is decreased because there are fewer white bloods cells.

In contrast, immunomodulation refers to altering the immune response, without any bone marrow suppression or cytotoxic activity. It often involves inhibiting the function of certain WBCs or inhibiting certain cytokines, but it can also involve using drugs to stimulate the immune system in some way.

Immunosuppression

Immunosuppressants are used in the therapy of autoimmune disease and also to prevent transplant rejection. We can distinguish four types:

  • Corticosteroids
  • Drugs that inhibit lymphocyte signalling
  • Cytotoxic drugs
  • Antibodies with immunosuppressing effect
Corticosteroids

These drugs are glucocorticoid analogues and act on glucocorticoid receptors. They have immunosuppressive effect through many mechanisms.

The important drugs here are prednisone and dexamethasone.

Indications:

Autoimmune disorders like autoimmune haemolytic anaemia, inflammatory bowel disease, SLE.

Bronchial asthma (by local administration)

Organ transplant recipients.

Substitution therapy in adrenal insufficiency.

Mechanism of action:

Glucocorticoid drugs are lipophilic steroid drugs that diffuse into the nucleus of cells. Here it binds to the glucocorticoid receptor, a nuclear receptor. This regulates the expression of many genes. This has several effects on the immune system:

  • Decreased transcription of IL-2, which inhibits the proliferation of T-helper cells
  • Decreases transcription of many pro-inflammatory cytokines like TNF-α, IL-1
  • Inhibition of monocyte and neutrophil chemotaxis
  • Decreases the lymphocyte count (lymphopaenia)

Dosing:

Oral or IV.

Side effects:

Iatrogenic Cushing syndrome, adrenal suppression (through negative feedback on ACTH), serious bacterial and viral infections.

Drugs that inhibit lymphocyte signalling

By inhibiting the signalling between lymphocytes, we can decrease their proliferation and efficacy. By binding IL-2 to their IL-2 receptors T-cells are stimulated to proliferate. These drugs inhibit this mechanism.

The important drugs here are cyclosporin, tacrolimus and rapamycin (also known as sirolimus).

Indications:

Cyclosporin is used in organ transplantation, rheumatoid arthritis, psoriasis and certain autoimmune diseases.

Tacrolimus and rapamycin are used in organ transplantation.

Mechanism of action:

Cyclosporine and tacrolimus inhibit a protein called calcineurin. Calcineurin activates the transcription of IL-2 and IL-2 receptors on T-cells. By inhibiting calcineurin will these two drugs indirectly decrease the IL-2-mediated T-cell proliferation.

Rapamycin also interferes with IL-2 signalling, but by a different mechanism. It’s also a macrolide antibiotic, but it’s mechanism here is that it binds to an intracellular protein kinase called “molecular target of rapamycin”, or mTOR, which you might remember from biochemistry.

mTOR is involved in the intracellular signalling that occurs inside T-cells when their IL-2 receptor is activated. By inhibiting mTOR the signal from the IL-2 receptor is stopped, effectively blocking IL-2-mediated T-cell proliferation.

Side effects:

Cyclosporine is nephrotoxic and it may cause transient liver dysfunction.

Cytotoxic drugs

These drugs are cytotoxic, so they are immunosuppressive by killing WBCs.

The important drugs here are azathioprine, cyclophosphamide and mycophenolate.

Indications:

Azathioprine is used in organ transplantation, acute glomerulonephritis and autoimmune diseases where glucocorticoids alone aren’t sufficient.

Cyclophosphamide and mycophenolate are used in organ transplantation and autoimmune disorders.

Mechanism of action:

Azathioprine is a prodrug of mercaptopurine, a cytotoxic anticancer drug we saw in topic 13. This drug interferes with purine synthesis and is cytotoxic to all rapidly proliferating cells, but especially lymphocytes during the initial phase of the immune response.

Cyclophosphamide is also a cytotoxic anticancer drug from topic 13. It’s an alkylating agent that damages DNA by alkylating it. The result is the same as for azathioprine.

Mycophenolate inhibits an enzyme that is crucial for the de novo biosynthesis of purines. Only lymphocytes are largely dependent on de novo synthesis of purines as other cells can produce purines by the salvage pathway. This causes mycophenolate to be fairly selective to lymphocytes.

Contraindications:

Azathioprine is broken down by xanthine oxidase, so xanthine oxidase inhibitors like allopurinol should not be used together with azathioprine.

Side effects:

Azathioprine can cause bone marrow depression.

Antibodies with immunosuppressing effect

Antibodies can bind anything so there’s no surprise that they can be useful in immunosuppression as well.

We can group these drugs into two types:

  • Polyclonal antibodies
    • Hyperimmunoglobulin
    • Rh(D) or anti-D immunoglobulin
  • Monoclonal antibodies
    • Monoclonal antibodies against TNF-α: infliximab, adalimumab
    • Monoclonal antibodies against IL-12 and IL-23: ustekinumab
    • Monoclonal antibodies against CD20: rituximab
    • Monoclonal antibodies against IL-2 receptor: basiliximab

Hyperimmunoglobulin is the basis of passive immunization against diseases like rabies and tetanus. They are simply antibodies prepared from the plasma of people who have been exposed to the toxin or pathogen.

Indications:

Hyperimmunoglobulin is used for passive immunization in conditions like rabies and tetanus.

Rh(D) immunoglobulin is given to Rh- mothers who are pregnant with their second Rh+ child to prevent erythroblastosis foetalis.

Infliximab treats rheumatoid arthritis, inflammatory bowel disease and psoriasis.

Adalimumab treats rheumatoid arthritis.

Rituximab treats therapy-resistant rheumatoid arthritis.

Mechanism of action:

Hyperimmunoglobulins bind to and inactivate whatever their target is, like tetanus toxin or the rabies virus.

Rh(D) immunoglobulin bind to the D-antigen on the Rh+ foetus’ RBCs. This prevents the mother’s antibodies from binding to them.

Infliximab and adalimumab bind to the pro-inflammatory cytokine TNF-α and prevents it from activating the immune system.

Ustekinumab binds to a region found on both IL-12 and IL-23, so it binds these cytokines and prevent their action.

Rituximab binds to CD20, a cell surface receptor found on B-cells. This triggers the immune system to kill B-cells, causing B-cell depletion.

Basiliximab binds to the IL-2 receptor, preventing IL-2 from binding to and activating T-cells.

Immunomodulators

In this group we have both drugs that inhibit some parts of the immune system and drugs that stimulate the immune system:

  • Immunostimulants
  • Immune modulating interferons
Immunostimulants

Drugs like levamisole and isoprinosine may be useful in immunodeficiency disorders like AIDS.

BCG, the vaccine against tuberculosis, has immunostimulatory effects as well. More specifically it is injected into bladders with cancer to treat the cancer. The vaccine enhances the immune system’s response against the bladder cancer somehow. The mechanism is unknown.

Immune modulating interferons

Interferons are cytokines that have many immune-stimulatory functions. There exist multiple types of interferon. Interferon alpha (IFN-α) is the most pharmacologically used.

Indications:

Interferon alpha is used to treat chronic hepatitis B and C and certain leukaemias and lymphomas.

Treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory joint disease that causes deformation of small joints. It is not curable, but pharmacological therapy can slow the progression, alleviate the symptoms and improve the physical condition of the patient.

Many drugs can be used in the treatment of RA. We can divide these drugs into two groups:

  • Symptom-improving drugs
    • NSAIDs
    • Corticosteroids
  • Disease-modifying antirheumatic drugs (DMARDs)
    • Synthetic
      • Methotrexate
      • Cyclophosphamide
      • Cyclosporine
    • Biological
      • Adalimumab
      • Infliximab
      • Rituximab

Like the name implies, the symptom-improving drugs don’t actually affect the progression of the disease; they only improve the symptoms. They’re nevertheless an important part of RA treatment.

The disease-modifying antirheumatic drugs (the DMARDs) actually slow down the progression and induce clinical remission. Of the DMARDs, methotrexate is the first choice. Cyclophosphamide and cyclosporine are also included in this group. Methotrexate is here used in much lower doses than when used in cancer chemotherapy.

If standard DMARD therapy is not effective, biological DMARDs (monoclonal antibodies) can be used. This includes adalimumab, infliximab and rituximab.


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