Thiopurine methyltransferase: a review and a clinical pilot study

Abstract

Thiopurine methyltransferase (TPMT) is an important enzyme in the metabolism of 6-mercaptoptirine (6MP), which is used in the treat men t of acute lymphoblastic leukemia (ALL). TPMT catalyzes the formation of methylthioi nosine monophosphate (MetlMP), which is cytotoxic for cultured cell lines, and it plays a role in detoxification of 6MP, Population studies show a genetic polymorphism for TPMT with both high and low activity alleles. About 1 of 300 subjects is homozygous for the low activity. The function TPMT plays in detoxification or therapeutic efficacy of 6MP in vivo is not clear. In this article the genetic polymorphism of TPMT is reviewed and the contribution of TPMT to the cytotoxic action, or detoxification, of 6MP in children with ALL is discussed. Induction of TPMT activity has been described during the treatment for ALL. We performed a pilot study on the influence of higlvdose 6MP infusions (1300 m g/m 2 in 24 h) on TPMT activity of peripheral blood mononuclear cells (pMNC) of eleven patients with ALL. The TPMT activities were in, or, above the normal range. There was no statistically significant difference between the TPMT activities before and after the 6MP infusions. MetlMP levels in pMNC increased during successive courses. This might be explained by TPMT induction, but other explanations are plausible as well. Twenty five percent of the TPMT assays failed, because less than the necessary 5*i0f' pMNC could be isolated from the blood of leukopenic patients. Red blood cells can not be used for TPMT measurements, since transfusions are frequently required during the treatment with 6MP infusions. Therefore, the influence of high-dose 6MP infusions on TPMT activity can only be investigated further when a TPMT assay which requires less pMNC has been developed.

Effects on transmethylation by high-dose 6-mercaptopurine and methotrexate infusions during consolidation treatment of acute lymphoblastic leukemia

“…1, 2). TPMT ac tivities in peripheral mononuclear cells ranged from 4.6-16.1 IU/109 cells in the present group, demonstrating in termediate and high activity [25]. The genetic polymor phism of TPMT may be partially responsible for the interpatient variability.…”

“…The genetic polymor phism of TPMT may be partially responsible for the interpatient variability. We could not demonstrate a cor relation between TPMT activity and (methyl)thiopurines, because a high percentage of TPMT assays in mononuclear cells fail due to the lack of cells in leukopenic patients as well as other enzymes involved in thiopurine metabolism (Scheme 1) [25]. Measurement of TPMT in RBC would not be informative because RBC are transfused during the in duction and consolidation treatment.…”

“…Leucovorin rescue (Scheme 2) was started at 36 hr after the MTX infusion was begun and continued every 6 hr until MTX plasma levels were less than 0.25 |xM. Five of the 10 patients were randomized for oral 6MP (25 • m 2 daily for 8 weeks, starting 7 days before the first MTX course) and the other 5 received 6MP intravenously (1300 mg • m"2 in 24 hr, from 24 until 48 hr after MTX infusion, 4 courses in 8 weeks). Patient or parental approval was obtained according to the ethical guidelines of our hos- Figure 1 shows the mean levels of the methylated m etabo lites of 6MP in plasma of the intravenous 6MP group.…”

Measurement of thiopurine S-methyltransferase activity in human blood samples based on high-performance liquid chromatography: reference values in erythrocytes from children

“…In an earlier study, 23 using a radiochemical TLC procedure, we could not measure pMNC TPMT activity in nine out of 36 cases because these patients were leucopenic and we were unable to collect the necessary 5610 6 cells for the assay. However, with the present HPLC -based method, we can perform the assays with only 2610 6 cells.…”

Thiopurine Methyltransferase Activity in Spain: A Study of 14,545 Patients

“…On the other hand, it has recently been shown that uremia in renal transplant is a significant inducer of TPMT activity [26]. Only one study to date has addressed the issue of TMPT activity in healthy controls following the administration of 6-MP, and reported no induction of TPMT activity [32]. Finally, several pharmacologic studies, performed both in vitro and in vivo, have reported inhibition of this enzyme and an increase of 6-thioguanine nucleotides owing to administration of sulfasalazyne or 5-aminosalicylates, both drugs frequently prescribed in patients with IBD [10,[33][34][35][36].…”