Multiple Myeloma Inhibitory Activity of Plant Natural Products

SIMPLE SUMMARY: Multiple myeloma is the second most common hematological cancer and is still incurable. Although enhanced understanding of the disease background and the development of novel therapeutics during the last decade resulted in a significant increase of overall survival time, almost all patients relapse and finally succumb to their disease. Therefore, novel medications are urgently needed. Nature-derived compounds still account for the majority of new therapeutics and especially for the treatment of cancer often serve as lead compounds in drug development. The present review summarizes the data on plant natural products with in vitro and in vivo activity against multiple myeloma until the end of 2020, focusing on their structure–activity relationship as well as the investigated pathways and involved molecules. ABSTRACT: A literature search on plant natural products with antimyeloma activity until the end of 2020 resulted in 92 compounds with effects on at least one human myeloma cell line. Compounds were divided in different compound classes and both their structure–activity-relationships as well as eventual correlations with the pathways described for Multiple Myeloma were discussed. Each of the major compound classes in this review (alkaloids, phenolics, terpenes) revealed interesting candidates, such as dioncophyllines, a group of naphtylisoquinoline alkaloids, which showed pronounced and selective induction of apoptosis when substituted in position 7 of the isoquinoline moiety. Interestingly, out of the phenolic compound class, two of the most noteworthy constituents belong to the relatively small subclass of xanthones, rendering this group a good starting point for possible further drug development. The class of terpenoids also provides noteworthy constituents, such as the highly oxygenated diterpenoid oridonin, which exhibited antiproliferative effects equal to those of bortezomib on RPMI8226 cells. Moreover, triterpenoids containing a lactone ring and/or quinone-like substructures, e.g., bruceantin, whitaferin A, withanolide F, celastrol, and pristimerin, displayed remarkable activity, with the latter two compounds acting as inhibitors of both NF-κB and proteasome chymotrypsin-like activity.