**Identification of Catalposide Metabolites in Human Liver and Intestinal Preparations and Characterization of the Relevant Sulfotransferase, UDP-glucuronosyltransferase, and Carboxylesterase Enzymes**

**Deok-Kyu Hwang 1,**† **, Ju-Hyun Kim 2,**† **, Yongho Shin <sup>1</sup> , Won-Gu Choi <sup>1</sup> , Sunjoo Kim <sup>1</sup> , Yong-Yeon Cho <sup>1</sup> , Joo Young Lee <sup>1</sup> , Han Chang Kang <sup>1</sup> and Hye Suk Lee 1,\***


Received: 24 June 2019; Accepted: 19 July 2019; Published: 22 July 2019

**Abstract:** Catalposide, an active component of *Veronica* species such as *Catalpa ovata* and *Pseudolysimachion lingifolium*, exhibits anti-inflammatory, antinociceptic, anti-oxidant, hepatoprotective, and cytostatic activities. We characterized the in vitro metabolic pathways of catalposide to predict its pharmacokinetics. Catalposide was metabolized to catalposide sulfate (M1), 4-hydroxybenzoic acid (M2), 4-hydroxybenzoic acid glucuronide (M3), and catalposide glucuronide (M4) by human hepatocytes, liver S9 fractions, and intestinal microsomes. M1 formation from catalposide was catalyzed by sulfotransferases (SULTs) 1C4, SULT1A1\*1, SULT1A1\*2, and SULT1E1. Catalposide glucuronidation to M4 was catalyzed by gastrointestine-specific UDP-glucuronosyltransferases (UGTs) 1A8 and UGT1A10; M4 was not detected after incubation of catalposide with human liver preparations. Hydrolysis of catalposide to M2 was catalyzed by carboxylesterases (CESs) 1 and 2, and M2 was further metabolized to M3 by UGT1A6 and UGT1A9 enzymes. Catalposide was also metabolized in extrahepatic tissues; genetic polymorphisms of the carboxylesterase (CES), UDP-glucuronosyltransferase (UGT), and sulfotransferase (SULT) enzymes responsible for catalposide metabolism may cause inter-individual variability in terms of catalposide pharmacokinetics.

**Keywords:** catalposide; in vitro human metabolism; UDP-glucuronosyltransferase; sulfotransferase; carboxylesterase
