Azathioprine

Overview
Azathioprine is an immunosupressant used in organ transplantation, autoimmune disease such as rheumatoid arthritis or inflammatory bowel disease such as Crohn's disease and ulcerative colitis. It is a pro-drug, converted in the body to the active metabolites 6-mercaptopurine and 6-thioinosinic acid.

Azathioprine is produced by a number of generic manufacturers and as branded names (Azasan by Salix in the U.S., Imuran by GlaxoSmithKline in Canada and the U.S., Australia and UK and Azamun in Finland).

History
Azathioprine was first introduced into clinical practice by Sir Roy Calne, the British pioneer in transplantation. Following the work done by Sir Peter Medawar in discovering the immunological basis of rejection of transplanted tissues and organs, Calne introduced 6-mercaptopurine as an experimental immunosuppressant for kidney transplants. When azathioprine was discovered, he then introduced it as a less toxic replacement for 6-mercaptopurine. For many years, dual therapy with azathioprine and steroids was the standard anti-rejection regime, until cyclosporine was introduced into clinical practice (also by Calne) in 1978.

Mechanism of action
Azathioprine acts to inhibit purine synthesis necessary for the proliferation of cells, especially leukocytes and lymphocytes. It is an effective drug used alone in certain autoimmune diseases, or in combination with other immunosuppressants in organ transplantation. Side effects are uncommon, but include nausea, HA and rash. Because azathioprine suppresses the bone marrow, patients will be more susceptable to infection. Caution should be exercised when it is used in conjunction with purine analogues such as allopurinol. The enzyme thiopurine S-methyltransferase (TPMT) deactivates 6-mercaptopurine. Genetic polymorphisms of TPMT can lead to excessive drug toxicity, thus assay of serum TPMT may be useful to prevent this complication.

Mycophenolate mofetil is increasingly being used in place of azathioprine in organ transplantation as it is associated with less bone marrow suppression, fewer opportunistic infections and a lower incidence of acute rejection. However azathioprine certainly still has a major role.

Long term side effects
It is listed as a human carcinogen in the 11th Report on Carcinogens of the U.S. Department of Health and Human Services, although they note that the International Agency for Research on Cancer (IARC) considered some of the animal studies to be inconclusive because of limitations in the study design and inadequate reporting. The risks involved seem to be related both to the duration and dosage used. People who have previously been treated with an alkylating agent may have an excessive risk of cancers if treated with azathioprine. Epidemiological studies have provided "sufficient" evidence of Azathioprine carcinogenicity in humans, although the methodology of past studies and the possible underlying mechanisms are questioned. The various diseases requiring transplantation, and thus azathioprine, may in themselves increase the risks of non-Hodgkin's lymphoma, squamous cell carcinomas of the skin, hepatobiliary carcinomas and mesenchymal tumours to which azathioprine may add additional risks. Those receiving azathioprine for rheumatoid arthritis may have a lesser risk than those following transplantation.

Azathioprine is not thought to cause fetal malformation (teratogenesis) and any risk to the offspring of treated men is small. A more recent product monograph produced by Glaxo Smith Kline and dated June 2005 does note that IMURAN® can cause fetal harm when given to a pregnant woman. Their document also states that the drug should not be given during pregnancy or in patients of reproductive potential without careful weighing of benefit versus the risks and should be avoided whenever possible in pregnant women. It goes on to say that when used in pregnancy the patient should be apprised of the potential hazard to the fetus. While stating that no adequate and well-controlled studies have taken place in humans, it notes that when given to animals in doses equivalent to human dosages teratogenesis was observed. Transplant patients already on this drug should not discontinue on becoming pregnant. This contrasts to the later developed drugs tacrolimus and myophenolate which are contra-indicated by the manufacturers during pregnancy. As for all cytotoxic drugs, it is advised not to breastfeed whilst taking azathioprine.

Under FDA rules, this drug, like many others, excludes eligibility for blood donation.