Intestinal deglycosylation activates saikosaponins as potent, selective UGT2B7/2B15 inhibitors: structural basis and implications for herb-drug interactions.

Abstract

Radix Bupleuri (RB)-induced hepatotoxicity and herb-drug interactions have raised clinical concerns, yet the role of its intestinal metabolites, saikogenins (SGs), in modulating human UDP-glucuronosyltransferase (UGT) activity remains poorly understood.Using 11 recombinant human UGT isoforms and 4-methylumbelliferone as the probe substrate, we demonstrate that the parent saikosaponins (SSA, SSC, SSD) produced no potent, isoform-selective inhibition of any of the tested UGT isoforms at 100 μM, whereas the deglycosylated aglycones SGA and SGD selectively and potently inhibited UGT2B7 and UGT2B15 (IC₅₀: 0.24-1.88 μM; K_i: 0.14-1.34 μM) through competitive mechanisms.Molecular docking revealed that deglycosylation removes steric hindrance from the sugar moiety, enabling the aglycone backbone to penetrate the catalytic pocket, where the most potent metabolite SGD anchored the conserved catalytic histidine of UGT2B7 (HIS481); inhibition of UGT2B15 instead arose from deep occupancy of the hydrophobic pocket.In vitro-in vivo extrapolation yielded an R-value of 1.36 for SGD-UGT2B7, exceeding the regulatory threshold (R ≥ 1.1), indicating a clinically significant risk when RB preparations are co-administered with narrow-therapeutic-index UGT2B7 substrates such as morphine or zidovudine.