Azathioprine: Wikis


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Systematic (IUPAC) name
6-[(1-methyl-4-nitro-1H-imidazol-5-yl)sulfanyl]-7 H-purine
CAS number 446-86-6
ATC code L04AX01
PubChem 2265
DrugBank APRD00811
ChemSpider 2178
Chemical data
Formula C 9H7N7O2S 
Mol. mass 277.263 g/mol
SMILES eMolecules & PubChem
Pharmacokinetic data
Bioavailability Well absorbed
Metabolism By xanthine oxidase
Half life 3 hours
Excretion Renal, minimally
Therapeutic considerations
Pregnancy cat. D
Legal status Prescription only
Routes oral
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Azathioprine is a drug that suppresses the immune system.

Azathioprine is used in organ transplantation and autoimmune disease. Some of the autoimmune diseases are rheumatoid arthritis, pemphigus, Inflammatory Bowel Disease (such as Crohn's disease and Ulcerative Colitis), multiple sclerosis, autoimmune hepatitis and restrictive lung disease.

Azathioprine interferes with the synthesis of purines (adenine and guanine), which is required for DNA synthesis. Fast-growing cells, including T-cells and B-cells, are particularly affected by the inhibition of purine synthesis.

It is a pro-drug, converted in the body to the active metabolites 6-mercaptopurine (6-MP) 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, Azamun in Finland and Imurel in Scandinavia).



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 cardiac transplantation. 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 is a purine synthesis inhibitor, inhibiting the proliferation of cells, especially leukocytes. It is an effective drug used alone in certain autoimmune diseases, or in combination with other immunosuppressants in organ transplantation.

Immediate or short-term side-effects

Side-effects are uncommon, but include nausea, fatigue, hair loss, and rash. Because azathioprine suppresses the bone marrow, patients will be more susceptible 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.[1]

Despite being 15 times more expensive[2], 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.[3 ] However azathioprine certainly still has a major role.

Long-term side-effects

Metabolic pathway for azathioprine

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.[4] The risks involved seem to be related both to the duration and to the dosage used. People that 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,[5] although the methodology of past studies and the possible underlying mechanisms are questioned.[6] 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.[7]

Azathioprine is not thought to cause fetal malformation (teratogenesis) and any risk to the offspring of treated women is small.[8] 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 state 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.[8] As for all cytotoxic drugs, the manufacturer advises not to breastfeed whilst taking azathioprine. The Lactation Risk Category (LAC) reported by Thomas Hale in "Medications and Mothers' Milk" lists azathioprine as "L3", termed "moderately safe".

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


  1. ^ Konstantopoulou M, Belgi A, Griffiths K, Seale J, Macfarlane A (2005). "Azathioprine-induced pancytopenia in a patient with pompholyx and deficiency of erythrocyte thiopurine methyltransferase". BMJ 330 (7487): 350–1. doi:10.1136/bmj.330.7487.350. PMID 15705694.  
  2. ^ Remuzzi G. et al. Mycophenolate mofetil versus azathioprine for prevention of acute rejection in renal transplantation (MYSS): a randomized trial. The Lancet, Vol. 364, August 7, 2004; 503:12
  3. ^ Woodroffe R, Yao G, Meads C, Bayliss S, Ready A, Raftery J, Taylor R (2005). "Clinical and cost-effectiveness of newer immunosuppressive regimens in renal transplantation: a systematic review and modelling study". Health Technol Assess 9 (21): 1–194. PMID 15899149.  
  4. ^ National Toxicology Program. "Azathioprine". Substance Profiles, Report on Ccarcinogens (Eleventh ed.). U.S. Department of Health and Human Services.  
  5. ^ International Agency for Research on Cancer (IARC) (1987). "Azathioprine - 5. Summary of Data Reported and Evaluation". Summaries & Evaluations. World Health Organization. pp. VOL.: 26 (1981) (p. 47).  
  6. ^ Gombar V, Enslein K, Blake B, Einstein K (1993). "Carcinogenicity of azathioprine: an S-AR investigation". Mutat Res 302 (1): 7–12. doi:10.1016/0165-7992(93)90083-8. PMID 7683109.  
  7. ^ International Agency for Research on Cancer (IARC) (1987). "Azathioprine - Evidence for carcinogenicity to humans (sufficient)". Summaries & Evaluations. World Health Organization. pp. Supplement 7: (1987) (p. 119).  
  8. ^ a b British National Formulary 45 March 2003

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