Interpretation
Azathioprine and 6-mercaptopurine (6-MP) are thiopurine drugs used to treat some types of leukaemia and some rheumatological, gastroenterological, dermatological and neurological diseases where there is thought to be an immune basis.
They are metabolised to 6-thioguanine nucleotides (6-TGN) which are the active metabolites responsible for the therapeutic action and also toxicity when present in excess. They are also inactivated in part by S-methylation, by the action of the cytosolic enzyme thiopurine methyl transferase (TPMT).
Low TPMT activity reduces the conversion of 6MP to 6MMP (an inactive metabolite), and increases the relative conversion of 6MP to 6TGNs, the active metabolites. 6-TGNs are also metabolised by TPMT enzyme, which additionally leads to higher active 6TGN levels in TPMT deficient patients.
Almost all TPMT activity in blood is present in red cells and must be released by lysing the cells and measuring enzyme activity prior to analysis of the product (6MMP).
TPMT enzyme activity is genetically determined. Loss-of-function variants in TPMT gene cause low TPMT activity. This is inherited as a monogenic, autosomal co-dominant trait.
Testing for TPMT activity is recommended in all patients prior to starting a thiopurine drug. The most well established and important reason, and the primary benefit recognised in international guidelines, is to identify individuals who are homozygous or compound heterozygous carriers of inactivating/low activity TPMT alleles (i.e. carry two inactivating alleles). Such patients are classified to be poor metabolisers. Their prevalence is estimated to be 1 in 300. These patients have an extremely high concentration of 6-TGN if given a standard dose of thiopurine drugs and are at risk of rapid and potentially fatal bone marrow toxicity with leukopaenia. Alternative non-thiopurine therapy should be considered in this group of patients. If thiopurine drugs are used, they should be started at less than 10% of target dose.
Individuals who are heterozygous (carriers) for an inactivating TPMT allele (i.e. carry one inactivating allele plus one normal wild type allele) are classified as intermediate metabolisers. Approximately 10% of patients are in this category and some dose reduction in thiopurine drugs may also be appropriate in this group.
TPMT phenotype testing does not replace the need for clinical monitoring (including regular full blood counts) of patients treated with thiopurine drugs. TPMT genotyping is only routinely undertaken in children less than 16 years of age and for confirmation of suspected homozygous (or compound heterozygous) deficiency where TPMT activity is less than 50 ug/L RBC. Phenotype testing should not be requested for patients currently treated with thiopurine drugs.
Given that TPMT activity is measured in red blood cells, if a patient has had a blood transfusion within 30 to 60 days of testing, the patient’s true enzyme activity may not be accurately reflected.
