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

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.