Thiopurine S-methyltransferase as a target for drug interactions

Objective: The present study was undertaken to investigate the possible effects of various agents on thiopurine methyltransferase (TPMT) activity in red blood cells (RBCs) from patients with chronic inflammatory bowel disease (IBD). Methods: In three groups of patients with very high, normal and intermediate TPMT activity (each n=6), the inhibitory potential of furosemide, piretanide, azathioprine (AZA) and testosterone was assessed by ex vivo measurements of TPMT activity in RBCs. From individual concentration-response curves, IC50 values have been determined. Results: Independent of the basal TPMT activity, lowest IC50 values were calculated for furosemide (15-19 mu M), followed by testosterone (30-72 mu M), piretanide (300-313 mu M) and AZA (430-532 mu M). Compared with reported plasma concentration achieved during treatment, only furosemide would have the potential to inhibit TPMT also in vivo, whereas the IC50 values of the other agents are far above the corresponding plasma levels. Conclusions: Our ex vivo study revealed that only furosemide has the potential to inhibit TPMT activity in patients with IBD. This possibility should be taken into consideration if the diuretic and AZA or 6-mercaptopurine are coadministered. However, the extrapolation to the clinical setting remains open.