A systematic review on the research status of quercetagetin: resources, biosynthesis, bioavailability, bioactive-delivery systems, health effects, marketed functional products and patents

Quercetagetin, 3, 5, 6, 7, 3 ', 4 '-Hexahydroxy-flavone (Que), is a yellow flavonol with an extra OH at C-6 in ring A related to the common 5,7-OH-type ring A flavonol quercetin. The related research about Que over the last 20 years has been obviously increased. This study delves into the research progress of Que: occurrence, chemical and biosynthesis pathways, physicochemical properties, delivery systems, pharmacological effects, clinical trial and human studies, toxicology and safety. Additionally, an overview of its marketed products, as well as patents is presented in this review highlighting future needed work to capitalize upon its biological merits. Que is found enriched in flowers of genus Tagetes and Citrus peel, and the enzyme flavonol 6-hydroxylase plays an indispensable role in the biosynthesis pathway of Que in Tagetes species via adding hydroxyl group in the C6 position of quercetin. With an increasing understanding of Que biosynthesis pathway and related genes being gradually annotated, biotechnology may be a potential way to obtain Que. Que exhibits various health-beneficial effects including antioxidant, anticancer, anti-inflammation, antivirus, antidiabetic and antilipemic. Some delivery systems are reported to improve its poor bioavailability and ultimate efficacy. The market products and patents of Que in pharmaceutical and food industries mostly used a mixture containing Que other than individual Que. The pharmacokinetics, metabolism, and pharmacological actions of individual Que are not fully studied, especially lack of clinical trial result. Therefore, deeper investigation about Que is needed, especially toxicology and safety, to enhance its clinical application, market product development as potential nutraceutical.