Stachydrine in Leonurus cardiaca, Leonurus japonicus, Leonotis leonurus: detection and quantification by instrumental HPTLC and ¹H-qNMR analyses

Stachydrine ((2S)-1,1-dimethylpyrrolidinium-2-carboxylic acid) may be regarded as an essential active principle of the aerial parts of Leonurus japonicus Houtt. (Leonuri herba, yimucao; Chin. Ph., DAB) which are used in TCM and Kampo for the treatment of various gynaecological and cardiovascular disorders. Medically and botanically closely related Lamioideae drugs are the fruits of L. japonicus (Leonuri fructus, chongweizi; Chin. Ph.), the aerial parts of European Leonurus cardiaca L. (Leonuri cardiacae herba; Ph. Eur.) as well as the aerial parts of their South African relative Leonotis leonurus (L.) R. Br. (Leonotis leonuri herba). Regarding L. cardiaca, stachydrine might be an exceptionally interesting constituent as Dragendorff-positive substances like stachydrine were found to be enriched in an antiarrhythmic L. cardiaca refined extract, which was most recently developed via bioassay guided fractionation. The few pharmacological publications on this betaine do indeed describe cardiovascular, hypotensive, and tissue protective effects. However, its pharmacopeial analytics poses a severe difficulty, as it does not contain any chromophoric group suitable for customary HPLC-UV detection. For quality control of yimucao according to Chin. Ph. the entirety of its N-containing compounds is photometrically quantified after Reinecke's complexation. Unfortunately, this method suffers from a relatively low reproducibility. Since no reliable quantification method for stachydrine is available up to now, a highly reproducible instrumental HPTLC method was newly developed, using postchromatographic derivatization by Vágújfalvi reagent, thus changing non absorbing stachydrine into a detectable derivative at 517 nm, and an automatic HPTLC system with scanner and analysis software (winCATS). This method was shown to be precise with respect to concentration and yielded highly reproducible data over numerous inter-day repetitions. Not only did the independent evaluation of the scanned HPTLC sheets for stachydrine peak area and height result in almost identical values for all samples, but also the results of a parallel-developed direct quantitative ¹H-NMR procedure using its N-CH₃ singlet δ 3.03 ppm in comparison with the singlet of the two vinylic protons of the internal standard maleic acid at δ 6.18 ppm were always within the standard deviation of the HPTLC data. These measurements of 12 drug samples revealed stachydrine contents (w/w) of 0.2 to 1.0 % for the L. japonicus aerial parts, 0.6 to 1.5 % for the L. cardiaca aerial parts, 6.7 % for the antiarrhythmic refined extract of L. cardiaca, and 0.3 % for the aerial parts of Leonotis leonurus, while both L. japonicus and L. cardiaca fruits contained, on average, 0.2 %. Furthermore, stachydrine was found for the first time as a constituent of L. japonicus and L. cardiaca fruits as well as Leonotis leonurus. Methodically, instrumental HPTLC may be a powerful tool for quality assurance for stachydrine containing plants and herbal drugs, especially for industrial routine protocols.