Pharmacogenetics of thiopurines: can posology be guided by laboratory data?

Abstract

Background. The purpose of this study was to investigate the relationships between the presence of mutations in the TPMT gene, the consequent reduced enzymatic activity, and the clinical toxicity of the treatment with thiopurine antimetabolite drugs.

Materials and methods. The study was performed on 44 patients with inflammatory bowel disease treated with AZA. DNA was extracted from blood samples collected from each patient, and genotyping was performed using specific polymerase chain reaction assays in order to detect the three more frequent mutations of the gene. Enzymatic activity was measured on red blood cell lysates by HPLC.

Results. Among the subjects, 4 (9.0%) were heterozygous for mutations in the TPMT gene; no subject was homozygous for mutations in the TPMT gene. A complete concordance between TPMT mutated genotype and reduced enzymatic activity could be determined. The incidence of toxicity in the subjects with a mutated genotype was not different from that observed in the patients with a normal TPMT gene.

Conclusion. Genotyping methods provide a simple and reliable DNA-based strategy to identify TPMT homozygotes that should avoid thiopurines administration. However, it seems that the most common, less dangerous forms of thiopurine toxicity could be caused by factors different from TPMT gene mutations examined.