A practical non-extraction direct liquid chromatography method for determination of thiopurine S-methyltransferase activity in inflammatory bowel disease

Thiopurine drugs remain pivotal therapies for the wide varieties of diseases such as inflammatory bowel disease (IBD). Here, thiopurine S-methyltransferase (TPMT) phenotype, the main metabolizing enzyme of thiopurine-drugs, was studied. This is for the first time that TPMT activity is measured in Iranian IBD patients. We used an improved direct liquid chromatography assay without need for solvent extraction and minimize excess labor handling making it ideal for use in routine referral medical centers. TPMT activity in whole blood was determined by a non-extraction HPLC method. We evaluated 427 individuals including 215 IBD patients and 212 unrelated healthy individuals as control group from Iran’s western population. TPMT phenotyping of this study demonstrated no frequency for deficient, 2.8 % for low and 97.2% for normal activity, which is different with results of other studies. There was a significant negative correlation between TPMT activities as calculated based on nmol/grHb/h and the Hb-levels in IBD and control groups (r= -0.54, P <0.001 and r= -0.27, P <0.001), respectively. Interestingly a significant positive correlation between Hb levels and TPMT-activities were seen when the activity calculated in mU/L in IBD patients and control subjects (r=0.14, P =0.05 and r=0.43, P <0.001), respectively. We strongly suggest the use of international unit (mU/L) is more appropriate than nmol6MTG/grHb/h for expressing TPMT-activity in IBD patients. In addition, in comparison with other providers of TPMT test activity and centers around the world the risk of toxicity is much lower after utilizing thiopurine drugs for IBD patients in this region.