Bioactive Ingredients of Helleborus niger L. (Christmas Rose): The Renaissance of an Old Medicinal Herb—A Review

The medicinal plant Helleborus nig er L. (Ranunculaceae) found in the European Alps has been valued as an effective remedy for numerous diseases for centuries. Recent preclinical evaluations showed pharmacologic potential of the plant extract for cancer treatment. Furthermore, several compounds extracted from H niger exhibited potent pharmacological effects. Thus, the renaissance of this old and proven medicinal herb requires better knowledge of its bioactive ingredients. A key ingredient of H niger is protoanemonin which is responsible for its burning hot taste and vesicant effect. Protoanemonin possesses antibacterial, antifungal, cytotoxic, and antimutagenic activity, and its isolation, synthesis, and preparation as a therapeutically valuable medicinal product are well elaborated. Anemonin, the dimer of protoanemonin, has antimicrobial, anti-inflammatory, and antimalarial activity, whereas (−)-ranunculin, a protoanemonin glycoside, possesses cytotoxicity and antimutagenic activity. (+)-Ranuncoside, another protoanemonin glycoside, has a spiro acetal molecular scaffold, a feature that is responsible for the biological activity of a multitude of natural products. Furthermore, the flavonoids kaempferol glycoside, quercetin glycoside, and glucosyl-phenyllactic acid have been isolated, and other phenolics with complex structures were detected as well. The main active ingredient of H niger rhizomes is the bufadienolide hellebrin. It is a steroidal cardiac diglycoside with utility for heart failure treatment. Hellebrin and its aglycon hellebrigenin are patented as lead compounds to treat cancer. Recently, additional bufadienolides as well as ecdysteroids were isolated from H niger whole plants and showed potent cytotoxicity on human cancer cell lines. Finally, various saponins have been determined by mass spectrometry, of which 4 compounds isolated from the rhizome are awaiting pharmacological assessment. This review gives an overview of the current knowledge of the isolation, synthesis, structure elucidation, derivatization, and biological activities of known H niger constituents. Furthermore, recent advances concerning the development of pharmaceuticals from these constituents and their derivatives are described.