Clinical studies of thiopurine metabolism in inflammatory bowel disease.

Inflammatory bowel disease (IBD, i.e. primarily Crohn's disease and ulcerative colitis) is characterised by a chronic or relapsing inflammation of the digestive tract. The thiopurine drugs 6-mercaptopurine (6-MP) and azathioprine (AZA, an imidazol derivative and pro-drug of 6-MP) are currently used to an increased extent in IBD, particularly in Crohn's disease. Metabolism of thiopurines is complex and individually variable. Thus, the formation of biologically active thioguanine nucleotides (TGN) and methylated metabolites may vary considerably. Thiopurine methyltrasferase (TPMT) is a key enzyme in this metabolism and exhibits a genetic variability due to a number of variant alleles coding for an inactive enzyme when occurring in the homozygous form. Postulating that pharmacological monitoring and TPMT determinations may improve clinical efficacy and reduce side effects of thiopurine therapy we examined TMPT variation and metabolite levels in relation to clinical findings in patients with IBD. TPMT status was of clinical importance in the toxicity observed during thiopurine therapy. Patients with decreased TPMT activity were more prone to develop adverse events and less likely to tolerate a standard thiopurine dose due to toxicity. The formation of metabolites after dose escalation was influenced by TPMT status. Subjects with normal TPMT activity shifted the metabolism towards production of methylated metabolites, while subjects with intermediate TPMT activity exhibit pronounced elevations of TGN metabolites even with small dose escalations. There was no general induction of TPMT activity after standardized initiation of thiopurine therapy. We found no correlation between AZA dose and TGN levels in two independent patient populations, but there was a linear relationship between AZA dose and methylated metabolites. The level of TGN metabolites were related to disease activity, with higher levels in disease remission. It is thus clinically useful to monitor TGN metabolites as an evaluation of treatment intensity. The total number of adverse events was higher in patients both with high TGN and high meTIMP metabolite concentrations and high levels of methylated metabolites (meTIMP) were associated with the development of myelotoxicity. In conclusion, these results suggest that TPMT activity measurements and pharmacological monitoring of thiopurine metabolites are useful in the clinical setting