Human lymphocyte thiopurine methyltransferase pharmacogenetics: Effect of phenotype on 6-mercaptopurine-induced inhibition of mitogen stimulation

J. A. Van Loon, R. M. Weinshilboum

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Abstract

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of 6-mercaptopurine (6-MP) and other thiopurine drugs. The level of TPMT activity in human tissue is controlled by a common genetic polymorphism. Our experiments were performed to study the relationship between TPMT phenotype and the effect of 6-MP on [3H]thymidine ([3H]TdR) incorporation into mitogen-stimulated human peripheral blood lymphocytes. Lymphocytes were isolated from blood samples obtained from 12 subjects, four each with each of the three classes of TPMT phenotype. The effect of 6-MP concentration on [3H]TdR incorporation was determined in the presence of two mitogens, phytohemagglutinin at concentrations of 1 and 10 μg/ml and concanavalin A at concentrations of 1 and 5 μg/ml. ED50 values for 6-MP inhibition of [3H]TdR incorporation into lymphocytes from subjects who genetically lacked TPMT activity were uniformly higher than were ED50 values for lymphocytes from subjects with intermediate or high enzyme activities. However, in the presence of either mitogen, ED50 values decreased as the level of stimulation increased. Therefore, to make it possible to account for degree of mitogen stimulation, the effect of 6-MP and [3H]TdR incorporation was studied in the presence of a series of phytohemagglutinin concentrations from 1 to 10 μg/ml. Lymphocytes from subjects who genetically lacked TPMT activity had significantly higher K(i) values (1.37 ± 0.340 μM, mean ± SEM, n = 3) for inhibition of [3H]TdR incorporation by 6-MP than did lymphocytes from subjects with intermediate or high enzyme activities (0.529 ± 0.068 and 0.327 ± 0.064 μM, respectively, P < .05 for both comparisons, n = 3 in both cases). Our results are compatible with the conclusion that peripheral blood lymphocytes from subjects with genetically low TPMT activities are less sensitive to 6-MP inhibition of [3H]TdR incorporation into DNA than are lymphocytes from subjects with intermediate or high levels of TPMT activity.

Original languageEnglish (US)
Pages (from-to)21-26
Number of pages6
JournalJournal of Pharmacology and Experimental Therapeutics
Volume242
Issue number1
StatePublished - Jan 1 1987

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ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology

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