Thiopurine methyltransferase: should it be measured before commencing thiopurine drug therapy?

Abstract

Thiopurines [azathioprine (AZA), 6-mercaptopurine (6-MP) and thioguanine (6-TG)] have a well-established role as immunosuppressive agents in a variety of chronic inflammatory conditions, haematological neoplasia and in transplant rejection. Despite good overall clinical response rates, particularly when used as steroid sparing agents, adverse effects are a limiting problem leading to withdrawal in up to a quarter of patients. Severe myelosuppression is the most serious toxicity occurring early or occasionally later during treatment.An understanding of the competing pathways involved in the metabolism of thiopurines has important implications for predicting some of the more severe toxicity seen with these drugs. Thiopurine methyl transferase (TPMT) is an enzyme catalysing the methylation of 6-MP, competing with xanthine oxidase (XO) and hypoxanthine guanine phosphoribosyl transferase (HGPRT) to determine the amount of 6-MP metabolised to cytotoxic thioguanine nucleotides. Allelic polymorphisms in the TPMT gene predict the activity of the enzyme such that 1 in 10 of the population are heterozygous and have approximately 50% of normal activity, whilst 1 in 300 are completely deficient. As a result, these individuals are at high risk of severe myelosuppression. Conversely, individuals with very high levels of TPMT activity are hyper-methylators in whom clinical response is less likely.Prior knowledge of TPMT status avoids exposure of individuals with zero TPMT to potentially fatal treatment with AZA or 6-MP and provides one of the best examples of predictive pharmacogenetics in therapeutics. This article reviews literature on the role of TPMT measurement prior to treatment with thiopurines and provides some guidance to the use of TPMT as a guide to tailoring thiopurine therapy.

Determination of thiopurine S -methyltransferase activity by hydrophilic interaction liquid chromatography hyphenated with mass spectrometry

“…The evaluation of TPMT activity is based on the determination of the actual enzymatic activity in red blood cells (RBC) by measuring amount of 6-MMP or 6-methylthioguanine (6-MTG) produced by the TPMT-mediated methylation of 6-MP or 6-TG in presence of the methyl donor, most commonly S-adenosyl-L-methionine (SAM) [13,14].…”

“…Moreover, the radiochemical method is more time-consuming than currently preferred HPLC methods [16]. Over the years more suitable methods for TPMT phenotyping were developed, namely mass spectrometry [14], capillary electrophoresis combined with UV detection [17] or, most commonly, HPLC coupled with UV/DAD [16,[18][19][20][21][22][23][24][25][26][27] or fluorescence detection [28]. Although separations with non-selective UV detection have been preferred due to their simplicity and low cost, increasing demands on the reliability of data and more detail sample profiles stimulated development of more advanced methods.…”

“…In addition, high TPMT activity leads to accumulation of the hepatotoxic methylated metabolites [11]. Therefore, some scientific committees advise to determine the TPMT activity before the start of thiopurine treatment [12] in order to predict myelo-and hepatotoxicity and adjust the dose accordingly.The evaluation of TPMT activity is based on the determination of the actual enzymatic activity in red blood cells (RBC) by measuring amount of 6-MMP or 6-methylthioguanine (6-MTG) produced by the TPMT-mediated methylation of 6-MP or 6-TG in presence of the methyl donor, most commonly S-adenosyl-L-methionine (SAM) [13,14]. …”

Whole-blood thiopurine S-methyltransferase activity with genotype concordance: a new, simplified phenotyping assay

“…9 Patients with very high TPMT activity, who are at risk of hepatotoxicity caused by the increased production of methylatedthiopurine metabolites, may also bene¢t from dose modi¢cation. 7,10,11 A potential further role for testing may be in monitoring patients during treatment, allowing adjustment of thiopurine drug dosage in response to variation of TPMT enzyme activity. 12 There are two strategies for determining patient TPMT status --phenotyping and genotyping --and for routine clinical use phenotyping is more ¢rmly established.…”

Measurement of Erythrocyte Inosine Triphosphate Pyrophosphohydrolase (ITPA) Activity by HPLC and Correlation of ITPA Genotype-Phenotype in a Caucasian Population

“…Individuals with low or undetectable TPMT activity are at high risk for developing life-threatening myelosuppression at standard doses of thiopurine drugs (7,8 ). Patients with high TPMT activity may experience treatment failure (9,10 ) or hepatotoxicity (11 ), but the situation is much less clear than for the TPMT deficiencies (12)(13)(14). In addition, an estimated 10% to 30% of patients cannot tolerate AZA therapy because of nonmyelosuppression adverse reactions, such as influenzalike symptoms, nausea, vomiting, hepatotoxicity, pancreatitis, fever, or rash (1,12,15,16 ).…”