3-Acetyl-11-keto-β-boswellic acid (AKBA) and 11-keto-β-boswellic acid (KBA) are widely used in the clinic as anti-inflammatory drugs. However, these drugs have the poor bioavailability, which may be caused by their extensive metabolism. In this study, we systemically characterized both phase I and II metabolism of AKBA and KBA in vitro. In total, four major metabolites were firstly biosynthesized and identified using 1D and 2D NMR spectroscopy. Among them, three metabolites were novel. The kinetic parameters (K m , V max , CL int, and K i ) were also analyzed systematically in various biological samples. Finally, the deacetylation of AKBA and hydroxylation of KBA were confirmed to be the major metabolic pathways based on their large CL int and the high amounts of KBA (46.7%) and hydroxylated KBA (50.8%) along with a low amount of AKBA (2.50%) in human primary hepatocytes. Carboxylesterase 2 (CE2) selectively catalyzed the deacetylation of AKBA to form KBA. Although CYP3A4, CYP3A5, and CYP3A7 catalyzed the metabolism of KBA, CYP3A4 played a predominant role in the hydroxylation reaction of KBA in human. Notably, deacetylation and regioselective hydroxylation exhibited considerable species differences. Deacetylation was only observed in human liver microsomes and primary human hepatocytes; 21- and 20-mono-hydroxylation of KBA were primarily observed in human, monkey, and dog; and 16- and 30-mono-hydroxylation were observed in other species. More importantly, all four mono-hydroxylation metabolites exhibited a moderate anti-inflammatory activity. The 21- and 20-hydroxylation metabolites inhibited the expression of iNOS, the LPS-induced activation of IkBα and p65 phosphorylation, and suppressed p65 nuclear translocation in RAW264.7 cells.
Keywords: CYP3A4; acetyl-11-keto-β-boswellic acid; anti-inflammatory; metabolism profiles; species difference.