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