Strong inhibition of celastrol towards UDP-glucuronosyl transferase (UGT) 1A6 and 2B7 indicating potential risk of UGT-based herb-drug interaction

Celastrol, a quinone methide triterpene isolated from Tripterygium wilfordii Hook F., has various biochemical and pharmacological activities, and is now being developed as a promising anti-tumor agent. Inhibitory activity of compounds towards UDP-glucuronosyltransferase (UGT) is an important cause of clinical drug-drug interactions and herb-drug interactions. The aim of the present study is to investigate the inhibition of celastrol towards two important UDP-glucuronosyltransferase (UGT) isoforms UGT1A6 and UGT2B7. Recombinant UGT isoforms and non-specific substrate 4-methylumbelliferone (4-MU) were used. The results showed that celastrol strongly inhibited the UGT1A6 and 2B7-mediated 4-MU glucuronidation reaction, with 0.9 ± 0.1% and 1.8 ± 0.2% residual 4-MU glucuronidation activity at 100 μM of celastrol, respectively. Furthermore, inhibition kinetic study (Dixon plot and Lineweaver-Burk plot) demonstrated that celastrol noncompetitively inhibited the UGT1A1-mediated 4-MU glucuronidation, and competitively inhibited UGT2B7-catalyzed 4-MU glucuronidation. The inhibition kinetic parameters (Ki) were calculated to be 0.49 μM and 0.045 μM for UGT1A6 and UGT2B7, respectively. At the therapeutic concentration of celastrol for anti-tumor utilization, the possibility of celastrol-drug interaction and celastrol-containing herbs-drug interaction were strongly indicated. However, given the complicated nature of herbs, these results should be viewed with more caution.