• Azathioprine (AZA) is a pro drug which undergoes conversion to 6- mercaptopurine (6-MP).  The conversion is done non-enzymatically by glutathione present in red blood cells and other tissues.
• 6-MP is then metabolized in the liver and gut by one of three enzymes; 6-thiopurine methyl transferase (TPMT), Xanthine oxidase, and Hypoxanthine-guanine-phosphoribosyl transferase (HGPRT).
• HGPRT converts 6-MP to active metabolite 6-thioguanine (6-TG) nucleotides whereas the other two enzymes metabolises 6-MP to inactive metabolites. Deficiency of TPMT may lead to increased active metabolite i.e. 6-TG and thus increased risk of toxicity
• Toxicity of AZA or 6-MP is largely related to the activity of TPMT. Deficiency of TPMT leads to preferential metabolisation of 6-MP and AZA to thioguanine nucleotides responsible for much of the drug toxicity.
• Low TPMT activity has been observed in up to 10 percent (heterozygous) of the population, with 0.3 percent (homozygous) having negligible activity.
• Either TPMT genotype or TPMT enzyme activity can be measured. TPMT enzyme activity is often measured in clinical practice in UK
• Low TPMT enzyme activity may lead to increased risk of myelosuppression.  However, the majority of patients who develop myelosuppression while taking AZA do not have detectable TPMT gene mutations. Thus a normal TPMT screening test does not preclude bone marrow and/or liver toxicity.  Thus, even when TPMT testing is performed, regular FBC and liver function tests must still be obtained. Experts vary in their use of TPMT.
• However, in current clinical practice TPMT levels are often measured before initiating treatment. Those with absent TPMT enzyme activity should not receive AZA or 6-MP. Patients with normal TPMT enzyme activity can be treated either by beginning with a low dose and increasing incrementally to the target dose or by beginning with the target dose at the outset.
• In non responding patients 6-TG levels may be obtained to check compliance.

5-10ml of whole blood (EDTA) is needed for the test

Precaution

• Recent blood transfusions will confuse the enzyme phenotype.
• TPMT is inducible so the analysis should be done before commencing therapy with thiopurines