Covalent protein modification by multiple reactive dialdehyde metabolites of catalpol via intestinal bioactivation

Catalpol, an iridoid glycoside, exhibits potent and versatile pharmacological effects. It is a promising drug candidate for treating ischemic stroke. However, its drug metabolism and disposition remain poorly understood, which hinders a better understanding of its mode of action. Here, we elucidated the intriguing metabolic characteristics of catalpol in rats. Catalpol underwent sequential metabolism mainly in the intestine, resulting in 31 stable metabolites and 8 unstable dialdehyde metabolites. Twelve glucosyl-containing metabolites were generated through the direct metabolism of catalpol. Eleven deglycosylated metabolites were primarily derived from catalpol aglycone metabolism. Seven N-heterocyclic metabolites originated from aglycone metabolites. Eight unstable and reactive dialdehyde metabolites were formed by the ring-opening of the hemiacetal reaction in aglycone metabolites. Eleven metabolic pathways were involved in catalpol metabolism. All eight dialdehyde metabolites were produced in the intestine through the sequential metabolism of catalpol. Notably, five dialdehyde metabolites could covalently bind to the proteins in the intestine. The dialdehyde metabolites were primarily derived from didehydroxylated and acetylated aglycone. Catalpol could improve the levels of gut bacterial metabolites, short-chain fatty acids. In conclusion, catalpol underwent extensive and sequential metabolism, generating 31 stable metabolites and 8 reactive dialdehyde metabolites. Five dialdehyde metabolites enable covalent protein modification in the intestine, which may be vital to the potent and versatile pharmacological effects of catalpol.