On the biochemical modulation of 6-mercaptopurine by methotrexate in murine WEHI-3b leukemia cells in vitro

Understanding thiopurine methyltransferase polymorphisms for the targeted treatment of hematologic malignancies

INTRODUCTION

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurines (mercaptopurine (MP) and tioguanine (TG)), chemotherapeutic agents used in the treatment of acute lymphoblastic leukemia (ALL). Polymorphisms in TPMT gene encode diminished activity enzyme, enhancing accumulation of active metabolites, and partially explaining the inter-individual differences in patients' clinical response.

AREAS COVERED

This review gives an overview on TPMT gene and function, and discusses the pharmacogenomic implications of TPMT variants in the prevention of severe thiopurine-induced hematological toxicities and the less known implication on TG-induced sinusoidal obstruction syndrome. Additional genetic and non-genetic factors impairing TPMT activity are considered. Literature search was done in PubMed for English articles published since1990, and on PharmGKB.

EXPERT OPINION

To titrate thiopurines safely and effectively, achieve the right degree of lymphotoxic effect and avoid excessive myelosuppression, the optimal management will combine a preemptive TPMT genotyping to establish a safe initial dose with a close phenotypic monitoring of TPMT activity and/or of active metabolites during long-term treatment. Compared to current ALL protocols, replacement of TG by MP during reinduction phase in TPMT heterozygotes and novel individualized TG regimens in maintenance for TPMT wild-type subjects could be investigated to improve outcomes while avoiding risk of severe hepatotoxicity.

In vitro metabolism of 6-mercaptopurine by human liver cytosol

1. The aim of this study was to investigate 6-mercaptopurine (6MP) metabolism by human liver cytosol in vitro. 2. Cytosol was prepared from seven human livers (A-G). A single cytosol (C) was used to optimize incubation conditions. 3. Cytosols A-G were incubated with 6MP at 2, 10 and 500 microM for two fixed times (5 and 48 h). Parent drug, thiopurine and thionucleotide metabolites were quantitated by high performance liquid chromatography at all time points. 4. At 5 and 48 h the 2 microM and 10 microM 6MP incubations contained both 6MP and its initial nucleotide metabolite, thioinosine 5'-monophosphate (TIMP). In addition, the 10 microM 6MP 48 h incubates contained small amounts of 6-thioguanine (6TG, median 0.12 microM). At 500 microM 6MP all seven liver incubates produced a range of metabolites. At 48 h these included thiouric acid, 8-hydroxy-6-mercaptopurine and 6-methylmercaptopurine (median 31, 19.5 and 8.8 microM respectively), with TIMP, 6TG, thioxanthine and thioxanthine nucleotide at median concentrations of 61, 0.79, 2.11 and 0.80 microM respectively. Thioguanine nucleotides, major metabolites measured in vivo, were not detected. 5. These results indicate that the human liver 6MP metabolic profile is dependent upon drug concentration.

Increased concentrations of methylated 6-mercaptopurine metabolites and 6-thioguanine nucleotides in human leukemic cells in vitro by methotrexate

The effect of methotrexate (MTX) on 6-mercaptopurine (6-MP) metabolism was studied in four human leukemic cell lines in vitro. CCRF-CEM, WI-L2, TBJ, and HL-60 all expressed thiopurine methyltransferase (TPMT) activity. The cells were grown in horse serum-supplemented RPMI 1640 medium to which was added 4 microM of 6-MP or 4 microM of 6-MP and 20 nM of MTX. The presence of MTX resulted in a 2.1-, 1.7-, 2.4- and 8-fold increase in the concentrations of methylmercaptopurine ribonucleotides (MMPRP) in CEM, WI-L2, TBJ, and HL-60 cells, respectively (P < 0.0008). The concentrations of 6-thioguanine nucleotides (6 TGN) increased 1.9-, 1.4-, 2.4- and 1.9-fold in the same cell lines (P < 0.02). The four cell lines differed with respect to the effect of MTX on the consumption of 6-MP from the medium; CEM consumed more 6-MP and WI-L2 less 6-MP from media containing MTX than from media containing 6-MP only (P = 0.005 and 0.02, respectively). MTX did not affect the consumption of 6-MP by TBJ cells (P = 0.17). Media in which HL-60 cells had been grown did not contain detectable amounts of 6-MP at the end of the experiment. The simultaneous increase in methylated 6-MP metabolites and 6-TGN represents a possible explanation for the synergism of MTX and 6-MP; however, the clinical importance of increased MMPRP remains to be elucidated.

Amidophosphoribosyltransferase limits the rate of cell growth-linked de novo purine biosynthesis in the presence of constant capacity of salvage purine biosynthesis

Factors controlling relative flux rates of the de novo and salvage pathways of purine nucleotide biosynthesis during animal cell growth are not fully understood. To examine the relative role of each pathway for cell growth, three cell lines including CHO K1 (a wild-type Chinese hamster ovary fibroblast cell line), CHO ade -A (an auxotrophic cell line deficient of amidophosphoribosyltransferase (ATase), a presumed rate-limiting enzyme of the de novo pathway), and CHO ade -A transfected with human ATase cDNA (-A+hATase) resulting in 30-350% of the ATase activity of CHO K1, were cultured in purine-rich or purine-free media. Based on the enzyme activities of ATase and hypoxanthine phosphoribosyltransferase, the metabolic rate of the de novo and salvage pathways, the rate of cell growth (growth rate) in three cell lines under various culture conditions, and the effect of hypoxanthine infusion on the metabolic rate of the de novo pathway in rat liver, we concluded the following. 1) In -A+hATase transfectants, ATase activity limits the rate of the de novo pathway, which is closely linked with the growth rate. 2) Purine nucleotides are synthesized preferentially by the salvage pathway as long as hypoxanthine, the most essential source of purine salvage, can be utilized, which was confirmed in rat liver in vivo by hypoxanthine infusion. The preferential usage of the salvage pathway results in sparing the energy expenditure required for de novo synthesis. 3) The regulatory capacity of the de novo pathway (about 200%) was larger than that of the salvage pathway (about 20%) with constant hypoxanthine phosphoribosyltransferase activity.