Determination of Thiopurine Methyltransferase Activity in Isolated Human Erythrocytes Does Not Reflect Putative in Vivo Enzyme Inhibition by Sulfasalazine

Authors:
Open Access
Publisher website
Google Scholar
Abstract
Determination of thiopurine S -methyltransferase (TPMT; EC 2.1.1.67) activity is intended to screen patients before therapy with thiopurine drugs (6-mercaptopurine, 6-thioguanine, and azathioprine) to rule out a deficiency in this enzyme. The enzyme catalyzes the S-methylation of these medicinal agents, a metabolic pathway that competes with the formation of pharmacologically active 6-thioguanine nucleotides (6-TGNs), thereby modulating their therapeutic and toxic effects (1)(2). Individuals with low TPMT activity are known to be at high risk for severe thiopurine-induced myelodepression. In addition to a genetic polymorphism of TPMT alleles, which is responsible for the wide interindividual differences in TPMT activity, cotherapy with various drugs, including aminosalicylates, has been shown to influence enzyme activity (1)(2). In vitro kinetic studies with recombinant human TPMT demonstrated that sulfasalazine, 5-aminosalicylic acid, balsalazide, and olsalazine (3)(4)(5) inhibit the enzyme activity in a noncompetitive manner. Therefore, simultaneous therapy with both thiopurines and aminosalicylates was postulated to create a “phenocopy” of individuals with a low TPMT activity genotype (1). In line with this hypothesis, Lewis et al. (3) described a case of a patient with Crohn disease and TPMT activity in the low normal range who had severe bone marrow dysfunction while receiving treatment with 6-mercaptopurine and olsalazine. The authors speculated that the TPMT activity determined ex vivo represented the patient’s baseline TPMT activity in vivo because the inhibitory effect of olsalazine would be removed by dilution in the assay. Subsequently, several groups (6)(7)(8) investigated this drug–drug interaction in clinical studies with more cases. As indicators of an inhibitory effect, ex vivo determination of erythrocyte TPMT activity and erythrocyte 6-TGNs were chosen. No changes in TPMT activity, but significantly increased 6-TGN concentrations, were observed under combined thiopurine/aminosalicylate therapy (6)(8). Whereas Lowry et al. (6) speculated …

This publication has 7 references indexed in Scilit: