Identification and characterization of the structure–activity relationships involved in UGT1A1 inhibition by anthraquinone and dianthrone constituents of Polygonum multiflorum

The adverse effects of Polygonum (P.) multiflorum, including abnormal bilirubin metabolism, are a serious public health issue. As uridine diphosphate (UDP)-glucuronosyltransferase 1A1 (UGT1A1) is the only enzyme responsible for bilirubin metabolism, we investigated the inhibitory effect of a P. multiflorum extract and 10 anthraquinone and dianthrone compounds on UGT1A1 in rat liver microsomes in vitro. The P. multiflorum extract exhibited the strongest inhibitory effect on UGT1A1 activity (inhibition constant [Ki] = 0.3257 μM, 1422 μg of material/mL), followed by cis-emodin dianthrones (Ki = 0.8630 μM), trans-emodin dianthrones (Ki = 1.083 μM), emodin-8-O-glc (Ki = 3.425 μM), and polygonumnolide C2 (Ki = 4.291 μM). Analysis of the structure–activity relationships of these compounds suggested that the spatial orientation of the molecules and the presence of particular functional groups affect UGT1A1 inhibition. A mechanistic analysis showed that all the tested compounds docked into two of the nine active sites of UGT1A1 and suggested that hydrophobic interactions and hydrogen bonds are important for the affinity of the tested compounds for UGT1A1; moreover, their interaction energies were generally in agreement with the Ki values. These findings provide insight into adverse reactions to P. multiflorum and identify the pharmacophores involved in inhibition of UGT1A1.