Tannins in cancer prevention and therapy

Tannins are the heterogenous class of polyphenolic natural products with promising cancer chemopreventive and therapeutic potential. Studies undertaken over the last 30 years demonstrated their capacity to target many cellular pathways and molecules important in the development of cancer. Recently, new mechanisms which might be important in anticarcinogenic activity, such as inhibition of epithelial‐to‐mesenchymal transition, reduction of cancer stem cell creation, and modulation of cancer cells metabolism were described. Besides the mechanisms of tannins' activity, this review focuses on their possible application as chemosensitizers in adjuvant therapy and countering multidrug resistance. Furthermore, characteristic physicochemical properties of some tannins, particularly tannic acid, are found useful in the formation of nano‐vehicles for anti‐cancer drugs or the isolation of circulating cancer cells. These new potential applications of tannins deserve further studies. Well‐designed clinical trials, which are scarce, are necessary to assess the therapeutic effects of tannins themselves or as adjuvants in cancer treatment.