Flavonoids are a group of over 2000 phenolic compounds with many therapeutic properties. They are based on the flavan (2-phenyl-benzopyran) core and can be found in a free state or as glycosides. Flavonoids are presented in the vegetal world mainly as yellow, but also red, purple, blue, or brown pigments of the petals, leaves, stems, and fruits. This group of bioactive compounds is known for the inhibition of tumors. The chapter summarizes the most important flavonoids with anti-cancer properties, describing their chemical structure, their prevalence among medicinal plants, and their mechanism of action, based on the recent in vivo and in vitro studies.
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Plants and their natural compounds are being used for centuries in traditional herbal medicine, but now they are gaining importance in developing new modern drugs, as they are involved in clinical studies for new approaches in human diseases Flavonoids are a large group of natural polyphenolic compounds with interesting pharmacological activities.
Albert Szent-Gyorgyii reported in 1938 the evidence of a pharmacological action for flavonoids, as preventing capillary bleeding and fragility associated with scurvy. Since then, a wide range of pharmacological activities such as anti-inflammatory, antioxidant, antibacterial, antiviral, anticancer, and protective has been discovered for flavonoids (Zakaryan et al., 2017).
Flavonoids act on cancer through a complex mechanism, and their anti-cancer potential is based on the capacity of scavenging ROS, anti-inflammatory action (through different pathways), inhibition of protein kinase activity, of angiogenesis and of cell growth, induction of apoptosis, and suppression of metastasis process.
Kopustinskiene et al. (2020) have explained in detail the flavonoids’ way of action in cancer. The authors reported thatʺ flavonoids exert a wide variety of anticancer effects: they modulate reactive oxygen species (ROS) - scavenging enzyme activities, participate in arresting the cell cycle, induce apoptosis, autophagy, and suppress cancer cell proliferation and invasivenessʺ. Moreover, flavonoids ʺtarget apoptotic signaling cascade both extrinsic, related to tumor necrosis factor (TNF) superfamily with main signaling protein—caspase 8; and intrinsic—mitochondrial pathway, where Bcl-2 family proteins launch the activation of caspases 9, 3 and 7, stimulating the cell death pathwaysʺ (Kopustinskiene et al., 2020).
Benavente -Garcia & Castillo (2008) have described flavonoids as inhibitors, preventing the formation of cancer cells; as blocking agents involved in reaching critical initiation sites, and as transformation agents, influencing the metabolism of carcinogenic components. Some of the flavonoids may ʺinduce the expression of different tumor suppressor genes that may contribute to decreasing cancer progression and metastasisʺ. (Selvakumar et al., 2020).