Recent advances in phytochemistry and pharmacology of C₂₁ steroid constituents from Cynanchum plants

Cynanchum is one of the most important genera in Asclepiadaceae family, which has long been known for its therapeutic effects. In this genus, 16 species are of high medicinal value. The extracts of the root and/or rhizome parts have been applied in traditional Chinese medicines (TCM) for the prevention and treatment of various illnesses for centuries. C21 steroids, as the typical constituents of Cynanchum species, possess a variety of structures and pharmacological activities. This review summarizes the comprehensive information on phytochemistry and pharmacology of C21 steroid constituents from Cynanchum plants, based on reports published between 2007 and 2015. Our aim is to provide a rationale for their therapeutic application, and to discuss the future trends in research and development of these compounds. A total of 172 newly identified compounds are reviewed according to their structural classifications. Their in vitro and in vivo pharmacological studies are also reviewed and discussed, focusing on antitumor, antidepressant, antifungal, antitaging, Na+/K+-ATPase inhibitory, appetite suppressing and antiviral activities. Future research efforts should concentrate on in vitro and in vivo biological studies and structure activity relationship of various C21 steroid constituents.