Mercaptopurine therapy intolerance and heterozygosity at the thiopurine S-methyltransferase gene locus

J Natl Cancer Inst. 1999 Dec 1;91(23):2001-8. doi: 10.1093/jnci/91.23.2001.


Background: Patients with acute lymphoblastic leukemia are often treated with 6-mercaptopurine, and those with homozygous deficiency in thiopurine S-methyltransferase (TPMT) enzyme activity have an extreme sensitivity to this drug as a result of the accumulation of higher cellular concentrations of thioguanine nucleotides. We studied the metabolism, dose requirements, and tolerance of 6-mercaptopurine among patients with different TPMT phenotypes.

Methods: We compared, by use of statistical modeling, 6-mercaptopurine pharmacology and tolerance in 180 patients who achieved remission on St. Jude Children's Research Hospital Protocol Total XII composed of weekly methotrexate (40 mg/m(2)) and daily oral 6-mercaptopurine (75 mg/m(2)) given for 2.5 years, interrupted every 6 weeks during the first year for treatment with either high-dose methotrexate or teniposide plus cytarabine. Statistical tests were two-sided.

Results: Erythrocyte concentrations of thioguanine nucleotides (pmol/8 x 10(8) erythrocytes) were inversely related to TPMT enzyme activity (P<.01), with averages (+/- standard deviations) of 417 (+/-179), 963 (+/-752), and 3565 (+/-1282) in TPMT homozygous wild-type (n = 161), heterozygous (n = 17), and homozygous-deficient (n = 2) patients, respectively. There was complete concordance between TPMT genotype and phenotype in a subset of 28 patients for whom TPMT genotype was determined. There were no sex differences in thioguanine nucleotide concentrations (P =.24), TPMT enzyme activity (P =.22), or average weekly prescribed dose of 6-mercaptopurine (P=.49). The cumulative incidence of 6-mercaptopurine dose reductions due to toxicity was highest among patients homozygous for mutant TPMT (100%), intermediate among heterozygous patients (35%), and lowest among wild-type patients (7%) (P<.001), with average (+/- standard deviation) final weekly 6-mercaptopurine doses of 72 (+/-60), 449 (+/-160), and 528 (+/-90) mg/m(2), respectively. Lowering doses of 6-mercaptopurine in TPMT heterozygotes and in deficient patients allowed administration of full protocol doses of other chemotherapy while maintaining high thioguanine nucleotide concentrations.

Conclusion: We conclude that genetic polymorphism in TPMT is an important determinant of mercaptopurine toxicity, even among patients who are heterozygous for this trait.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Antimetabolites, Antineoplastic / administration & dosage*
  • Antimetabolites, Antineoplastic / adverse effects
  • Antimetabolites, Antineoplastic / pharmacokinetics*
  • Antineoplastic Combined Chemotherapy Protocols / therapeutic use*
  • Area Under Curve
  • Child
  • Erythrocytes / metabolism
  • Female
  • Guanine Nucleotides / metabolism
  • Heterozygote
  • Humans
  • Male
  • Mercaptopurine / administration & dosage*
  • Mercaptopurine / adverse effects
  • Mercaptopurine / pharmacokinetics*
  • Methylation
  • Methyltransferases / genetics*
  • Methyltransferases / metabolism
  • Phenotype
  • Polymorphism, Genetic
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / drug therapy*
  • Precursor Cell Lymphoblastic Leukemia-Lymphoma / genetics
  • Prodrugs / administration & dosage*
  • Prodrugs / adverse effects
  • Prodrugs / pharmacokinetics*
  • Thionucleotides / metabolism


  • Antimetabolites, Antineoplastic
  • Guanine Nucleotides
  • Prodrugs
  • Thionucleotides
  • Mercaptopurine
  • Methyltransferases
  • thiopurine methyltransferase