Isorhamnetin and Quercetin Derivatives as Anti-Acetylcholinesterase Principles of Marigold (Calendula officinalis) Flowers and Preparations

Marigold (<i>Calendula officinalis</i> L.) is one of the most common and widespread plants used medicinally all over the world. The present study aimed to evaluate the anti-acetylcholinesterase activity of marigold flowers, detect the compounds responsible and perform chemical analysis of marigold commercial products. Analysis of 23 varieties of <i>C. officinalis</i> flowers introduced into Siberia allowed us to select the Greenheart Orange variety due to the superior content of flavonoids (46.87 mg/g) and the highest inhibitory activity against acetylcholinesterase (IC₅₀ 63.52 µg/mL). Flavonoids, isorhamnetin and quercetin derivatives were revealed as potential inhibitors with the application of high-performance liquid chromatography (HPLC) activity-based profiling. Investigation of the inhibitory activity of isorhamnetin glycosides demonstrated the maximal potency for isorhamnetin-3-<i>О</i>-(2'',6''-di-acetyl)-glucoside (IC₅₀ 51.26 μM) and minimal potency for typhaneoside (isorhamnetin-3-<i>O</i>-(2'',6''-di-rhamnosyl)-glucoside; IC₅₀ 94.92 µM). Among quercetin derivatives, the most active compound was quercetin-3-<i>О</i>-(2'',6''-di-acetyl)-glucoside (IC₅₀ 36.47 µM), and the least active component was manghaslin (quercetin-3-<i>O</i>-(2'',6''-di-rhamnosyl)-glucoside; IC₅₀ 94.92 µM). Some structure-activity relationships were discussed. Analysis of commercial marigold formulations revealed a reduced flavonoid content (from 7.18-19.85 mg/g) compared with introduced varieties. Liquid extract was the most enriched preparation, characterized by 3.10 mg/mL of total flavonoid content, and infusion was the least enriched formulation (0.41 mg/mL). The presented results suggest that isorhamnetin and quercetin and its glycosides can be considered as potential anti-acetylcholinesterase agents.