Thiopurine methyl transferase

The number of drugs susceptible to S-methylation is still limited but greater than the number turned over by COMT. Thiopurine methyl transferase (TPMT) is an important enzyme responsible for detoxifying mercaptopurine—a drug used to treat leukemia— as well as azathioprine —a prodrug that is metabolized to mercaptopurine (Fig. 7.12). This enzyme is polymorphic and patients who are homozygous for the deficient enzyme experience severe toxicity when given usual doses of mercaptopurine (19). Similar aromatic and heterocyclic sulfhydryls can also be substrates for TPMT. The similar thiol... 

Fig. 13.1 Pathways of thiopurine metabolism. The positions of two polymorphically expressed enzymes, TPMT (thiopurine methyl transferase) and ITPA (inosine triphosphate pyrophosphatase), are shown. HGPRT, hypoxanthine guanine phosphoribosyl transferase 6-TIDP, 6-thioi-nosine diphosphate 6-TIMP, 6-thioinosine monophosphate 6-TITP, 6-thio inosine trinophosphate...

Fig. 13.2 Thiopurine methyl transferase (TPMT) methylator genotypes in Crohn s disease during azathioprine/6-mercaptopurine (AZA/6-MP) treatment influences the time in months to development of severe myelosuppression. However, only 27% of patients with Crohn s disease and myelo-suppression during AZA therapy had mutant alleles of the TPMT gene associated with enzyme deficiency. Myelosuppression is more often caused by other factors. Continued monitoring of blood cell counts remains mandatory in patients treated with AZA. (Reproduced from ref 35.)...

Fig. 13.3 Clinical response, adverse effects, and hematological parameters were determined and correlated with thiopurine methyl transferase (TPMT) enzyme activity and genotype in 106 patients with inflammatory bowel disease. The odds of achieving complete remission (CR) to azathioprine is approx, five times lower if TPMT is greater than 14 units/mL red blood cells (RBCs). (Reproduced from ref 39.)...

Weinshilboum R. Thiopurine pharmacogenetics clinical and molecular studies of thiopurine methyl-transferase. DrugMetab Dispos 2001 29 601-605. 

Thiopurine methyl-transferase Mercaptopurines (antileukemic) Myelotoxicity... 

In a retrospective analysis of 106 patients with inflammatory bowel disease, to evaluate the importance of thiopurine methyl transferase (TPMT) activity in the management of azathioprine therapy in inflammatory bowel disease, the relation between inherited variations in TPMT enzyme activity and azathioprine toxicity was confirmed (91). 

Soria-Royer C, Legendre C, Mircheva J, Premel S, Beaune P, Kreis H. Thiopurine-methyl-transferase activity to assess azathioprine myelotoxicity in renal transplant recipients. Lancet 1993 341(8860) 1593. ... 

Holme SA, Duley JA, Sanderson J, Routledge PA, Anstey AV. Erythrocyte thiopurine methyl transferase assessment prior to azathioprine use in the UK. QJM 2002 95(7) 439 4. 

Mutations that cause a disease are rare and often lead to early death. Genetic poly-tnorphism are more common and are officially defined as occurring in more than l%of the population. Some of the clinically important polymorphisms associated with metabolism enzymes include CYP2C9, CYP2D6, CYP2C19, FMO, plasma pseudocholinesterase, N-acetyltransferase, thiopurine methyl-transferase, and UDP-glucuronysyltrans-ferase (107-113). 

TPMT thiopurine methyl transferase UGT lAl uridine disphosphate glucuro-nosyl transferase lAl VGDS voluntary genomic data submission... 

UGT lAl uridine diphosphate glucuronosyl transferase lAl TPMT thiopurine methyl transferase SN-38 an active metabolite of ironotecan SN-38G a glucuronide metabolite of SN-38. 

Thiopurines are metabolized by S-methylation via the polymorphic enzyme thiopurine methyl transferase (TPMT) with S-adenosylmethionine serving as cofactor. The methylated thiopurine bases cannot react with HGPRT and, therefore, cannot form the active false ribonucleotides. Drug manufacturers take this into... 

Lee EJD, Kalow W. Thiopurine S-methyl-transferase activity in a Chinese population. Clin Pharmacol Ther 1993 54 28-33. 

Hon YY, Fessing MY, Pui CH, Reeling MV, Krynetski EY, Evans WE. Polymorphism of the thiopurine S-methyl-transferase (TPMT) gene in African Americans. Hum Mol Genet 1999 8 371-376. 

Deficiency of thiopurine S-methyl transferase (TPMT) is another phenotype that exhibits inter-ethnic differences in frequency. TPMT is an enzyme that catalyzes methylation of therapeutic agents used in the treatment of acute lymphoblastic leukemia, rheumatoid arthritis, and autoimmune/inflammatory diseases, as well as in organ transplantation. Patients who have TPMT deficiency experience less efficient methylation and are at greater risk of fatal toxicity when treated with standard doses of fhiopurines. TPMT phenotype is defined by erythrocyte 6-mercapto-purine methylation. African American populations exhibit a 20% lower erythrocyte TPMT than Caucasian Americans, and persons of Chinese descent tend to exhibit greater activity than either of these other American subpopulations. 

Sebbag, L., Boucher, P., Davelu, P., Boissonnat, P., Champsaur, G., Ninet, J., Dureau, G., obadia, J.F., Vallon, J.J. and Delaye, J. (2000) Thiopurine s-methyl-transferase gene polymorphism is predictive of azathioprine-induced myelosuppression in heart transplant recipients. Transplantation, 69,1524—1527. 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

Key Words Thiopurines mercaptopurine thioguanine thiopurine 5-methyl-transferase TPMT leukemia aeute lymphoblastic leukemia childhood treatment genetie variation genetie polymorphism... 

Evans WE, Hon YY, Bomgaars L et al. Preponderance of thiopurine S -methyl-transferase deficiency and heterozygosity among patients intolerant to mercaptopurine or azathioprine. J Clin Oncol 2001 19 2293-2301. 

Krynetski EY Evans WE. Pharmacogenetics as a molecular basis of individualized drug therapy the thiopurine S-methyl-transferase paradigm. Pharm Res 1999 16 342-349. 

Evans WE (2004) Pharmacogenetics of thiopurine S-methyl-transferase and thiopurine therapy. Therapeutic Drug Monitoring 26 186-191... 

FIGURE 13.7 Thiopurine metabolic pathways. TPMT, ThiopLirine methyl transferase XO, xantlaine oxidase HPRX, hypoxanthine guanine phosphoribosyltransferase IMPDH, inosine monophosphate dehydrogenase. 

As discussed in Chapter 14/ genetic polymorphisms for the Phase I enzymes (CYP2D6 and CYP2C19) and the Phase II enzymes (N-acetyltransferase and the methyltransferases thiopurine methyltransferase/ catechol O-methyl transferase/ and thiol methyltransferase) may significantly alter exposure to relevant drug substrates. Evaluation of the frequency... 

McLeod HL, Krynetski EY, Wdimas JA, Evans WE. Higher activity of polymorphic thiopurine S-methyl-transferase in erythrocytes from neonates compared to adults. Pharmacogenetics 1995 5 281-6. 

A number of methyltransferases are able tc methylate small molecules (46,47). Thus, re. actions of methylation fulfill only two of the three criteria defined above, because the methyl group is small compared with the substrate. The main enzyme responsible for O-methylation is catechol 0-methyltransferas (EC 2.1.1.6 COMT), which is mainly cytosolic but also exists in membrane-bound form. Several enzymes catalyze reactions of xenobiotic N-methylation with different substrate specificities, e.g., nicotinamide iV-methyltrans-ferase (EC 2.1.1.1), histamine methyltrans-ferase (EC 2.1.1.8), phenylethanolamine N-methy 1 transferase (noradrenal ine A-meth-yltransferase EC 2.1.1.28), and nonspecific amine N-methyltransferase (arylamine N-methyltransferase, tryptamine JV-methyl-transferase EC 2.1.1.49) of which some isozymes have been characterized. Reactions of xenobiotic S-methylation are mediated by the membrane-bound thiol methyltransferase (EC 2.1.1.9) and the cytosolic thiopurine methyltransferase (EC 2.1.1.67) (3). 

Although the pathway has not been established, relatively high yields of trimethyltin from inorganic tin have been observed in yeast concomitant with the degradation of butyltin compounds (Errecalde et al. 1995). Exceptionally, methionine transferase may carry out the methylation of Hg in Neurospora crassa (Landner 1971) and thiopurine methyltransferase the methylation of inorganic Se in Escherichia coli (Ranjard et al. 2003). 

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