In vitro selective inhibition of human UDP-glucuronosyltransferase (UGT) 1A4 by finasteride, and prediction of in vivo drug-drug interactions.

In the present study, we evaluated the inhibitory potentials of finasteride for the major human hepatic UDP-glucuronosyltransferases (UGTs) (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7, and UGT2B15) in vitro using LC-MS/MS by specific marker reactions in human liver microsomes (except for UGT2B15) or recombinant supersomes (UGT2B15). Of the seven tested UGTs, finasteride potently, selectively, and competitively inhibited UGT1A4-mediated trifluoperazine-N-glucuronidation in human liver microsomes with an IC₅₀ value of 11.5 ± 1.78 μM and Ki value of 6.03 ± 0.291 μM. This inhibitory potency was similar to that of hecogenin, a well-known inhibitor of UGT1A4. However, finasteride did not seem to inhibit any of the other six UGTs: UGT1A1, UGT1A3, UGT1A6, UGT1A9, UGT2B7, or UGT2B15. Similarly, finasteride markedly inhibited UGT1A4 activity in recombinant human UGT1A4 supersomes, with a Ki value of 6.05 ± 0.410 μM. In addition, finasteride strongly inhibited UGT1A4-catalyzed imipramine-N-β-D-glucuronidation. However, on the basis of an in vitro-in vivo extrapolation, our data strongly suggested that finasteride is unlikely to cause clinically significant drug-drug interactions mediated via inhibition of the hepatic UGT enzymes involved in drug metabolism in vivo.