2. Polyphenols
Polyphenols are secondary metabolites that are produced by plants, and they are characterized by the presence of numerous phenolic rings
[10]. The main sources of polyphenols are blueberries, grapes, olive oil, cocoa, nuts, peanuts, and other fruits and vegetables that contain up to 200–300 mg of polyphenols per 100 g fresh weight
[11].
Figure 1 shows the classification of the main groups of polyphenols.
Figure 1. Classification of polyphenols and flavonoids. Examples of each subgroup with antitumor activity are listed.
Polyphenols can be extracted by simple and ecological techniques, including ultrasonically assisted extraction. T After extraction, polyphenols retain most of their properties. This characteristic facilitates research into the use of these compounds as potential anticancer drugs
[12][13].
2.1. Flavonoids
The most important group of polyphenols the family. Flavonoids consist of over 6000 molecules that have been identified and isolated. Flavonoids are found in abundance in colorful vegetables and fruits such as blueberries, apples, grapes, oranges, strawberries, plums, and in some common foods and drinks, including dark chocolate, nuts, red wine, tea, soybeans and soybean derivatives, spinach
[14].
2.2. Phenolic Acids
Another subgroup of polyphenols that can be found in several plants, especially in dried fruit, is phenolic acids. These compounds are characterized by the presence of a phenolic ring and have the function of an organic carboxylic acid
[15].
Phenolic acid (p-coumaric acid) has shown medicinal properties that make it a likely candidate for the treatment of cancer.
P-coumaric acid (or 4-hydroxycinnamic acid) is an organic compound that is derived from cinnamic acid, which can be found in many different edible plants (tomatoes, carrots, garlic, mushrooms, white beans, and other plants). Moreover, p-coumaric acid that is contained in pollen is a component of honey
[16].
In the last decade, several studies have been published that confirm the antitumor activity of p-coumaric acid in breast and gastric cancer cells
[17][18][19].
2.3. Lignans
Lignans are diphenolic compounds found in a wide variety of plants, including broccoli, beans, soybeans, rye, sesame seeds, pumpkin seeds, linseed, and some berries in very small amounts
[20]. Lignans are one of the two main groups of phytoestrogens that are known for their own good antioxidant properties
[21]. Numerous lignans can be considered possible anticancer drugs. Among them are etoposide, arctigenin, and magnolol, the main lignans that are studied in medicine. In addition, etoposide is a commercial lignan, belonging to the podophyllotoxin subfamily, that is used to treat various types of cancer such as lung cancer and breast cancer
[22][23]. However, etoposide chemotherapy has several side effects, including low blood cell counts, vomiting, diarrhea, fever, loss of appetite, and alopecia.
Arctigenin Some plants produce arctigenin, especially in the seeds of the greater burdock (
Arctium lappa). Current studies have shown that arctigenin inhibits the growth of various cancer cells, including cells in the stomach, lung, liver, and colon, as well as leukocytes
[24]. At the same time, the addition of arctigenin enhances the action of caspase-3, a protein that plays a key role in the death of cancer cells. Huang et al. showed that the treatment of OVCAR3 and SKOV3 ovarian cancers with arctygenin causes apoptosis of neoplastic cells in vitro
[25].
Lee et al. investigated the effect of arctigenin (ATG) on doxorubicin-induced cell death (DOX) using human breast cancer cells MDA-MB-231. The results showed that DOX-induced cell death was enhanced by concurrent treatment with ATG/DOX in a concentration-dependent manner and that this was associated with increased DOX uptake and the suppression of multidrug-resistance-associated protein 1 (MRP1) gene expression in MDA-MB cells-231
[26].
2.4. Resveratrol
Resveratrol is a natural polyphenol from the stilbene family. Resveratrol is produced by several plants (grapes, almonds, beans, blueberries, raspberries, mulberries, peanuts, etc.) in response to infections and injuries, or as a defense against various attacks by pathogens such as fungi and bacteria
[27]. In addition, red wine contains significant amounts of resveratrol.
In 1997, Jang et al. were the first investigators to report the inhibition of skin cancer development in mice with resveratrol
[28]. Since then, many studies have suggested that resveratrol is able to prevent or delay the onset of cancer
[29][30][31].
In fact, studies have shown that resveratrol is active in vitro against many human cancers, including cancers of the breast, skin, ovary, stomach, prostate, colon, liver, pancreas, and cervix, as well as thyroid cancer cells, lymphoid carcinoma cells, and myeloid carcinoma cells
[32].
Resveratrol has been shown to be beneficial at various stages of neoplastic disease (tumor initiation, promotion, and progression). For example, resveratrol protects DNA against reactive oxygen species (ROS) and traps the hydroxyls, superoxides, and free radicals produced in cells (events that are usually associated with tumor initiation)
[33].
In addition, human clinical trials with resveratrol have been conducted with satisfactory results
[34][35][36].
2.5. Curcuminoids
Curcuminoids are natural polyphenols that contain two phenolic units linked by a linear diarylheptanoid. Among them, curcumin is one of the best known and researched structures, with high potential as a drug. Nevertheless, the poor solubility of curcumin in water of acidic and physiological pH requires a variety of alternatives to avoid losing the efficacy of curcumin as a drug
[37].
Curcumin has been used in anticancer therapies for various types of cancer: lung, cervical, prostate, breast, bone, and liver
[38]. Nevertheless, the administration of free curcumin has some drawbacks, including poor water solubility, instability under water conditions, and low bioavailability
[39].
Many different clinical trials have been conducted on the use of curcumin as an anticancer drug. Recently, various research groups have reported that the combination of curcumin with gemcitabine-based chemotherapy is safe and that its use is possible in pancreatic cancer patients
[40][41][42].
Overall, gemcitabine adjuvant therapy with curcumin phytosome complex is not only safe, but it also effectively translates into a good first-line response rate for advanced pancreatic cancer.
Table 1 shows the most important studies on the molecular mechanisms of anticancer activity of the main polyphenols and flavonoids.
Table 1. Mechanisms of anticancer activity of selected polyphenols and flavonoids.