Formation of Threohydrobupropion from Bupropion Is Dependent on 11β-Hydroxysteroid Dehydrogenase 1

Abstract

Bupropion is widely used for treatment of depression and as a smoking-cessation drug. Despite more than 20 years of therapeutic use, its metabolism is not fully understood. While CYP2B6 is known to form hydroxybupropion, the enzyme(s) generating erythro-and threohydrobupropion have long remained unclear. Previous experiments using microsomal preparations and the nonspecific inhibitor glycyrrhetinic acid suggested a role for 11b-hydroxysteroid dehydrogenase 1 (11b-HSD1) in the formation of both erythro-and threohydrobupropion. 11b-HSD1 catalyzes the conversion of inactive glucocorticoids (cortisone, prednisone) to their active forms (cortisol, prednisolone). Moreover, it accepts several other substrates. Here, we used for the first time recombinant 11b-HSD1 to assess its role in the carbonyl reduction of bupropion. Furthermore, we applied human, rat, and mouse liver microsomes and a selective inhibitor to characterize species-specific differences and to estimate the relative contribution of 11b-HSD1 to bupropion metabolism. The results revealed 11b-HSD1 as the major enzyme responsible for threohydrobupropion formation. The reaction was stereoselective and no erythrohydrobupropion was formed. Human liver microsomes showed 10 and 80 times higher activity than rat and mouse liver microsomes, respectively. The formation of erythrohydrobupropion was not altered in experiments with microsomes from 11b-HSD1-deficient mice or upon incubation with 11b-HSD1 inhibitor, indicating the existence of another carbonyl reductase that generates erythrohydrobupropion. Molecular docking supported the experimental findings and suggested that 11b-HSD1 selectively converts R-bupropion to threohydrobupropion. Enzyme inhibition experiments suggested that exposure to bupropion is not likely to impair 11b-HSD1-dependent glucocorticoid activation but that pharmacological administration of cortisone or prednisone may inhibit 11b-HSD1-dependent bupropion metabolism.

Stereoselective Glucuronidation of Bupropion Metabolites In Vitro and In Vivo

“…Overall, direct description of stereoselective urinary excretion of bupropion and metabolites at 48 hours agreed well with a previous (indirect, nonstereoselective) report of urinary recovery at 24 hours (Benowitz et al, 2013) and with findings after administration of C 14 -labeled bupropion (Johnston et al, 2002). The enzymes responsible for the oxidative and reductive metabolism of bupropion to form hydroxybupropion, erythro-hydrobupropion, and threo-hydrobupropion have been investigated extensively (Faucette et al, 2000(Faucette et al, , 2001Hesse et al, 2000;Bondarev et al, 2003;Damaj et al, 2004;Molnari and Myers, 2012;Zhu et al, 2012;Meyer et al, 2013;Skarydova et al, 2014). Metabolite exposure is presumably dependent on metabolic pathways leading to their formation and elimination; however, the enzymes responsible for glucuronidation and subsequent renal elimination of the pharmacologically important bupropion metabolites have not been fully elucidated.…”

“…Reduction of the keto group by 11b-hydroxysteroid dehydrogenase type 1 and other carbonyl reductases (Molnari and Myers, 2012;Meyer et al, 2013;Skarydova et al, 2014;Connarn et al, 2015) appears to favor formation of threo-hydrobupropion (Benowitz et al, 2013). Although this reduction creates an additional chiral center, potentially generating two distinct threo-hydrobupropion and two erythro-hydrobupropion diastereomers, data describing stereoselective elimination pathways and effect are lacking.…”

Metabolism of Bupropion by Carbonyl Reductases in Liver and Intestine

“…Therefore, on the basis of liver microsome stability assays, it was thought that hydroxybupropion was the major metabolite. Several studies failed to realize that the CR pathway to form threohydrobupropion and erythrohydrobupropion may not occur extensively in liver microsomes since most of the CR enzymes involved are located subcellularly within the cytosol Meyer et al, 2013). Therefore, examining all subcellular fractions-microsome, cytosolic, and S9 fractions-will help explain more broadly which enzymes are responsible for bupropion's metabolism and at what rate its metabolites are formed.…”

Chiral Plasma Pharmacokinetics and Urinary Excretion of Bupropion and Metabolites in Healthy Volunteers

“…Numerous BUP metabolites have been identified (Welch et al, 1987;Petsalo et al, 2007;Gufford et al, 2016), but OHBUP, threohydrobupropion (THRHBUP), and erythrohydrobupropion (ERYHBUP) are of primary interest to understand BUP's effect and DDIs. BUP is 4-hydroxylated by CYP2B6 to OHBUP (Faucette et al, 2000;Hesse et al, 2000;Benowitz et al, 2013), whereas the reduction of BUP by 11b-hydroxysteroid dehydrogenase 1 and other carbonyl reductases (Meyer et al, 2013;Connarn et al, 2015) forms two amino alcohol stereoisomers, THRHBUP and ERYHBUP. OHBUP, THRHBUP, and ERYHBUP exhibit pharmacological activity in preclinical models (Martin et al, 1990;Bondarev et al, 2003;Damaj et al, 2004Damaj et al, , 2010 and may mediate BUP-induced seizures (Silverstone et al, 2008).…”