Differing contribution of thiopurine methyltransferase to mercaptopurine versus thioguanine effects in human leukemic cells

Thioguanine and mercaptopurine are prodrugs requiring conversion into thiop urine nucleotides to exert cytotoxicity. Thiopurine S-methyltransferase (TP MT), an enzyme subject to genetic polymorphism, catabolizes thiopurines int o inactive methylated bases, but also produces methylthioguanine nucleotide s and methylmercaptopurine nucleotides from thioguanine and mercaptopurine nucleotides, respectively. To study the effect of TPMT on activation versus inactivation of mercaptopurine and thioguanine, we used a retroviral gene transfer technique to develop human CCRF-CEM cell lines that did (TPMT+) an d did not (MOCK) overexpress TPMT. After transduction, TPMT activities were 14-fold higher in the TPMT+ versus the MOCK cell lines (P < 0.001). TPMTcells were less sensitive to thioguanine than MOCK cells (IC50 = 1.10 +/- 0 .12 muM versus 0.55 +/- 0.19 muM; P = 0.02); in contrast, TPMT+ cells were more sensitive to mercaptopurine than MOCK cells (IC50 = 0.52 +/- 0.20 muM versus 1.50 +/- 0.23 muM; P < 0.01). The lower sensitivity of TPMT+ versus MOCK cells to thioguanine was associated with lower thioguanine nucleotide concentrations (917 +/- 282 versus 1515 +/- 183 pmol/5 X 10(6) cells; P = 0 .01), higher methylthioguanine nucleotide concentrations (252 +/- 34 versus 27 +/- 10 pmol/5 X 106 cells; P = 0.01), less inhibition of de novo purine synthesis (13 versus 95%; P < 0.01), and lower deoxythioguanosine incorpor ation into DNA (2.0 +/- 0.6% versus 7.2 +/- 2.0%; P < 0.001). The higher se nsitivity of TPMT+ cells to mercaptopurine was associated with higher conce ntrations of methylmercaptopurine nucleotide (2601 +/- 1055 versus 174 +/- 77 pmol/5 X 106 Cells; P = 0.01) and greater inhibition of de novo purine s ynthesis (> 99% versus 74%; P < 0.01) compared with MOCK cells. We conclude that methylation of mercaptopurine contributes to the antiproliferative pr operties of the drug, probably through inhibition of de novo purine synthes is by methylmercaptopurine nucleotides, whereas thioguanine is inactivated primarily by TPMT.