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Azathioprine and mercaptopurine

Authors: Hon A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand, 2001. Updated: Hana Numan, Medical Writer, New Zealand. Copy edited by Gus Mitchell. October 2021


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What is azathioprine and mercaptopurine?

Azathioprine is an antimetabolite thiopurine analogue drug that interferes with DNA synthesis and suppresses the immune system response.

Azathioprine is metabolised in the liver to mercaptopurine, also known as 6-mercaptopurine (6-MP) which is also commercially available as a drug. Mercaptopurine is then converted into thioguanine nucleotides which inhibit cell growth.

Who uses azathioprine and mercaptopurine?

Azathioprine is more often used in practice than mercaptopurine for the treatment of skin conditions, however they are closely related.

Approved azathioprine indications may include:

Azathioprine may be approved to treat these skin conditions

Off-label uses in dermatology with good evidence include:

Limited evidence is available for many other dermatologic conditions.

What are the contraindications and precautions with azathioprine and mercaptopurine?

Contraindications

  • Drug or excipient allergy
  • Pregnancy (relative contraindication — see below)
  • Very low thiopurine methyltransferase (TPMT) activity (relative contraindication — see below)    
  • Concomitant use of allopurinol or febuxostat (relative contraindication)

Precautions

Pregnancy and breastfeeding

Treatment with azathioprine and mercaptopurine during all stages of pregnancy should be avoided except where benefits outweigh risks.

Females of childbearing potential should be advised of the risks involved and use effective contraception where appropriate. Teratogenicity is unclear in men taking azathioprine or mercaptopurine and adequate contraception may also be advised in this case.

Although low concentrations of 6-mercaptopurine may be found in breastmilk, there has been no evidence to suggest harm. Potential benefits should outweigh risks; for further information, see Lactation and medications used in dermatology.

Tell me more about azathioprine and mercaptopurine

Mechanism of action

Azathioprine has been widely prescribed since the 1960s, however its mechanism of action as well as that of its metabolite 6-MP is not fully understood. There are a number of proposed mechanisms including:

  • Reduction in cell division through self-incorporation into DNA and RNA as a false nucleotide
  • Immunosuppression occurs after mercaptopurine crosses cell membranes and becomes intracellularly activated.

Drug forms and dosing

Azathioprine and mercaptopurine may be available as an oral tablet, oral liquid, or intravenous injection (azathioprine only).

The dose of azathioprine is generally 1–3 mg/kg/day, however the individualised dose is dependent on several factors including indication, age, response, and TPMT activity (see below). The bioavailability may vary between different formulations. Dose reduction may be required in patients with hepatic and renal impairment.

Effects of azathioprine and mercaptopurine may take up to several months to be seen — this should be considered when reviewing treatment.

What are the benefits of azathioprine and mercaptopurine?

  • Potential steroid-sparing effect
  • Multiple dosage forms and strengths, allowing for easy dose titration
  • Oral formulation available

What are the disadvantages of azathioprine and mercaptopurine?

  • May take several months for the full benefits to be seen
  • Metabolism varies between individuals (see TPMT monitoring below)
  • Regular blood test monitoring is required

What are the side effects and risks of azathioprine and mercaptopurine?

Side effects

Azathioprine and mercaptopurine usually cause mild side effects but may occasionally be severe enough to stop treatment. These include:

Mucocutaneous side effects that may be caused by azathioprine

  • Common side effects
    • nausea and dose-related bone marrow suppression
  • Less common side effects
    • diarrhoea, vomiting, infections, liver impairment, and pancreatitis.

Drug interactions

If the use of an interacting drug combination is unavoidable, dose adjustment may be required and blood counts should be monitored carefully. Interactions include:

  • Warfarin dose requirement may increase 3-4 fold when given with azathioprine [see Anticoagulants and antiplatelet agents]
  • Trimethoprim + sulfamethoxazole
  • Angiotensin-converting enzymes inhibitors
  • Neuromuscular blocking agents eg, rocuronium, atracurium
  • Indomethacin
  • Sulfasalazine and other benzoic acid derivatives

Monitoring during treatment

Before starting treatment:

During treatment:

  • Regular full blood counts are checked weekly at first for the first 4–8 weeks, then at longer intervals.
  • Liver and kidney function tests — baseline and periodically
  • Regular skin checks — patients should practice self skin examination.

Thiopurine methyltransferase (TPMT)

Azathioprine and mercaptopurine is primarily metabolised by the enzyme TPMT. Some people have a genetic mutation causing them to have either low enzyme activity (approximately 11% of the population) or a lack of enzyme activity (1 in 300 people). Those with both genes are at severe risk of bone marrow suppression. Conversely, some other individuals have high levels of enzyme activity and may require a higher dose than normal.

TPMT levels should be measured to determine a patient's level of risk before starting treatment. Low levels are < 5 U/mL, intermediate levels are 5–13.7 U/mL, and high levels are > 13.8 U/mL.

Discontinuation of treatment

Azathioprine should be stopped and promptly managed jointly with a haematologist if:

  • Neutrophil count < 1.0x109/L
  • Lymphocyte count < 0.5x109/L
  • Platelet count < 50x109/L.

New Zealand approved datasheets are the official source of information for prescription medicines, including approved uses and risk information. Check the individual New Zealand datasheet on the Medsafe website.

If you are not based in New Zealand, we suggest you refer to your national drug approval agency for further information about medicines (eg, the Australian Therapeutic Goods Administration and the US Food and Drug Administration) or a national or state-approved formulary (eg, the New Zealand Formulary and New Zealand Formulary for Children and the British National Formulary and British National Formulary for Children).

 

Bibliography

  • Gisbert JP, Luna M, Maté J, González-Guijarro L, Cara C, Pajares JM. Choice of azathioprine or 6-mercaptopurine dose based on thiopurine methyltransferase (TPMT) activity to avoid myelosuppression. A prospective study. Hepatogastroenterology. 2006;53(69):399–404. PubMed
  • Lee JH, Kim TJ, Kim ER, et al. Measurements of 6-thioguanine nucleotide levels with TPMT and NUDT15 genotyping in patients with Crohn's disease. PLoS One. 2017;12(12):e0188925. doi:10.1371/journal.pone.0188925. Journal
  • Lennard L. Implementation of TPMT testing. Br J Clin Pharmacol. 2014;77(4):704–14. doi:10.1111/bcp.12226. Journal
  • Meggitt SJ, Anstey AV, Mohd Mustapa MF, Reynolds NJ, Wakelin S. British Association of Dermatologists' guidelines for the safe and effective prescribing of azathioprine 2011. Br J Dermatol. 2011;165(4):711–34. doi:10.1111/j.1365-2133.2011.10575.x. Journal

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