Pharmacogenetic studies of thiopurine methyltransferase genotype-phenotype concordance and effect of methotrexate on thiopurine metabolism

Sammanfattning: The thiopurines (6-mercaptopurine [6-MP], 6-thioguanine and azathioprine) are cytotoxic drugs used in the treatment of acute lymphoblastic leukemia (ALL), inflammatory bowel diseases, certain autoimmune diseases and after transplantation. The metabolism of thiopurines is complex with several enzymes involved in the conversion into active drug metabolites. One of the enzymes, thiopurine methyltransferase (TPMT), is one of the best examples of implemented pharmacogenetics so far. Due to lowered TPMT enzyme activity caused by genetic polymorphism, carriers of heterozygous or homozygous defective TPMT alleles need dose reduction to avoid cytotoxic adverse reactions like myelosuppression or hepatotoxicity if treated with thiopurines.To determine TPMT status before the start of treatment, genotyping (for the three most occurring TPMT alleles) and/or phenotyping (TPMT enzyme activity measurements) are used in the clinical setting. In the focus of this thesis, concordance of these methods was investigated in a large cohort of unique samples (n=12,663) collected in the routine analysis service of TPMT status determinations in Linköping. By sequencing all exons in samples where the results of the two methods differed, rare or novel TPMT alleles were discovered. Four TPMT alleles (TPMT'41, '42, '43, '44), not previously described, were characterized in terms of clinical in vivo data as well as protein structure and stability data obtained from recombinant human TPMT (rTPMT) produced by E. Coli and biophysical methods.The clinical cohort was also used in the search for other factors (except genetic factors) that influence TPMT enzyme activity, and both age and gender turned out to affect TPMT enzyme activity level. In addition, TPMT enzyme activity in the early treatment of ALL was investigated and shown to be significantly lower at time of ALL diagnosis.In the treatment protocol of ALL, the combined treatment using 6-MP and methotrexate (MTX) has increased the positive outcomes since the start in the 1950s. Despite this, the synergistic effect of these drugs is not yet fully understood. To evaluate the effect of MTX on thiopurine metabolism specifically, TPMT enzyme activity, TPMT gene expression, and thiopurine metabolite levels were determined before and after MTX infusions in vivo and after cotreatment in lymphoblasts in vitro. In the presence of MTX, TPMT enzyme activity and metabolite levels decreased, both in vivo and in vitro, although dose- and time-dependent. In addition, MTX bound to rTPMT and caused inhibition of rTPMT enzyme activity.The results found in the scope of this thesis may be used for further individualization of thiopurine treatment.

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