Inhibitory Effects of Baicalein Derived from Japanese Traditional Herbal Medicine on SN-38 Glucuronidation

Takashi Satoh1, Ayaka Igarashi2, Misaki Tanno3, Koki Yamada1, Natsuko Takahashi-Suzuki1, Kazuhiro Watanabe1

1Department of Pharmaceutics, Faculty of Pharmaceutical Sciences, Hokkaido University of Science.
2Department of Pharmaceutics, Hokkaido Pharmaceutical University School of Pharmacy. Department of Pharmacy, NTT East Japan Sapporo Hospital.
3Department of Pharmaceutics, Hokkaido Pharmaceutical University School of Pharmacy. Department of Pharmacy, Tomakomai Ohji General Hospital.

Abstract


Purpose: The chemotherapeutic agent irinotecan is hydrolyzed to its active form SN-38 by human carboxyesterases, but SN-38 is converted into the inactive form SN-38G by hepatic UDP-glucuronosyltransferases (UGTs). The aim of the present study was to evaluate the inhibitory effects of two b-glucuronidase-treated Japanese traditional herbal medicines (kampo), Hange-Shashin-To (TJ-14) and Sairei-To (TJ-114) on SN-38 glucuronidation, and the deglycosylation of baicalin (BG) and glycyrrhizic acid (GL) derived from TJ-14 and TJ-114 to form their respective aglycones, baicalein (BA) and glycyrrhetinic acid (GA). Methods: The inhibitory effects of b-glucuronidase-treated TJ-14 and TJ-114 on SN-38 glucuronidation by human liver microsomes were examined. BA and GA, which were enzymatically converted from BG and GL present in TJ-14 and TJ-114, were examined in the same manner. Furthermore, the enzymatic activities were measured by using recombinant UGT1A1 and UGT1A9 isoforms instead of human liver microsomes. BA, GA, SN-38, and their glycosides/glucuronides were analyzed with an LC-MS system. Results: As regards the linear initial reaction rate, SN-38 glucuronidation by human liver microsomes was significantly inhibited by the addition of b-glucuronidase-untreated TJ-14 and TJ-114, but was more strongly inhibited by the addition of b-glucuronidase-treated TJ-14 and TJ-114. The results of LC-MS analysis and pharmacokinetic studies suggested that BA is the main inhibitor of SN-38 glucuronidation. In the Dixon plot, BA showed competitive inhibition of SN-38 glucuronidation, and the inhibition constant was 8.70 ± 3.24 mM. Previous reports, studies of recombinant UGT isoforms indicated that SN-38 glucuronidation was mainly catalyzed by UGT1A1. Conclusions: These findings strongly suggested that SN-38 glucuronidation is inhibited by BA. BA could act as a pharmacokinetic regulating factor associated with SN-38 glucuronidation.

 

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J Pharm Pharm Sci, 21 (1): 195-206, 2018

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DOI: http://dx.doi.org/10.18433/jpps29783