Pharmacokinetics, Excretion, and Metabolite Profiling of Leonurine in Rats: Evidence for Extensive Phase II...

Leonurine, a bioactive alkaloid from Leonurus japonicus, has attracted considerable pharmacological interest, yet its in vivo disposition remains insufficiently defined. In the present study, the pharmacokinetics, excretion, and metabolic profile of leonurine were systematically investigated in rats after intravenous (IV), oral (PO), and intraperitoneal (IP) administration. A validated LC-MS/MS method was used to quantify leonurine in plasma, urine, and feces, and high-resolution MS was applied for metabolite profiling. Following IV administration, leonurine exhibited rapid systemic disposition, with a half-life of 2.48 h and a clearance of 152 mL/min/kg. Oral exposure was negligible, with an absolute bioavailability of 0.14%, whereas IP administration produced markedly higher systemic exposure (66.6%). Recovery of unchanged leonurine in urine and feces remained low across all dosing routes, with total excretory recovery below 6% of doses. The results indicated that metabolic conversion, rather than parent drug excretion, was the dominant elimination pathway. A total of 30 leonurine-related components were characterized in vivo, including 24 previously unreported metabolites. The metabolic profile was dominated by phase II conjugation, comprising 12 glucuronidated and 12 sulfated metabolites, together with hydrolysis, methylation/demethylation, and other transformation products. Notably, ester bond cleavage was identified as one of the major primary biotransformation routes, and several glucuronide and sulfate conjugates were also formed on hydrolysis-derived fragments. These findings provide a more comprehensive view of leonurine disposition in rats and offer a mechanistic basis for its rapid clearance and limited systemic availability after oral administration.