Pharmacokinetic Studies On The Total Sesquiterpene Lactones From Inulae Radix

Total sesquiterpene lactones(TSL)prepared from Inula helenium L(IHL),mainly consisting of alantolactone(AL)and isoalantolactone(IAL),are useful in the treatment of rheumatoid arthritis(RA).Previous studies on the pharmacokinetic parameters of IAL and AL after intravenous and oral administration suggested that these two compounds were poorly absorbed and could be eliminated rapidly in rat plasma.For the low plasma concentration and the aera under concentration-time curve(AUC),the oral bioavailability and exposure will be underestimated if the parent drugs are tested alone,which impedes preclinical safety evaluation.Thus,the observation of metabolites is useful in the pharmacokinetic study.In this paper,the metabolism of AL and IAL was studied,which played an important role in the pharmacokinetic studies of TSL-IHL,which could expedite the discovery and development of Radix Inulae extract and the studies of pharmacokinetics in human.Firstly,the metabolic stabilities of AL and IAL were examined,and GSH and Cys conjugations were observed as their predominant metabolic pathway.The metabolites identified in vitro were also found in vivo in rats.Secondly,four metabolites(AL-GSH,AL-Cys,IAL-GSH,IAL-Cys)were isolated and identified by NMR.The results showed that the thiol of GSH or Cys reacted with the exomethylene carbon atoms of α-methylene-γ-lactones of AL and IAL.This addition was nucleophilic,also known as Michael addition.Finally,a LC-MS/MS method was established and validated for the simultaneous determination of AL,AL-GSH,AL-Cys,IAL,IAL-GSH and IAL-Cys in rat plasma.After intravenous administration in rats,AL and IAL were extensively metabolized,and the exposure,as measured by AUC,for AL-GSH,AL-Cys,IAL-GSH,and IAL-Cys was approximately 1.538-,0.959-,1.497-,and 0.908-fold that of the parent drug,respectively.The AUC ratio of metabolites to parent compounds of oral administration was 3.655-,9.193-,12.971-,and 9.923-fold that of the parent drug for the above metabolites,respectively.These findings provide useful information for preclinical safety evaluation,and for understanding the mechanism of action of AL and IAL,and for predicting AL and IAL metabolism in humans.