Flavonoids are nearly ubiquitous in plants and are recognized as the pigments responsible for the colors of leaves, especially in autumn. They are rich in seeds, citrus fruits, olive oil, tea, and red wine. They are low molecular weight compounds composed of a three-ring structure with various substitutions. This basic structure is shared by tocopherols (vitamin E). Flavonoids can be subdivided according to the presence of an oxy group at position 4, a double bond between carbon atoms 2 and 3, or a hydroxyl group in position 3 of the C (middle) ring. These characteristics appear to also be required for best activity, especially antioxidant and antiproliferative, in the systems studied. The particular hydroxylation pattern of the B ring of the flavonoles increases their activities, especially in inhibition of mast cell secretion. Certain plants and spices containing flavonoids have been used for thousands of years in traditional Eastern medicine. In spite of the voluminous literature available, however, Western medicine has not yet used flavonoids therapeutically, even though their safety record is exceptional. Suggestions are made where such possibilities may be worth pursuing.

The Effects of Plant Flavonoids on Mammalian Cells:Implications for Inflammation, Heart Disease, and Cancer

There has been increasing interest in the research on flavonoids from plant sources because of their versatile health benefits reported in various epidemiological studies. Since flavonoids are directly associated with human dietary ingredients and health, there is need to evaluate structure and function relationship. The bioavailability, metabolism, and biological activity of flavonoids depend upon the configuration, total number of hydroxyl groups, and substitution of functional groups about their nuclear structure. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. Most recent researches have focused on the health aspects of flavonoids for humans. Many flavonoids are shown to have antioxidative activity, free radical scavenging capacity, coronary heart disease prevention, hepatoprotective, anti-inflammatory, and anticancer activities, while some flavonoids exhibit potential antiviral activities. In plant systems, flavonoids help in combating oxidative stress and act as growth regulators. For pharmaceutical purposes cost-effective bulk production of different types of flavonoids has been made possible with the help of microbial biotechnology. This review highlights the structural features of flavonoids, their beneficial roles in human health, and significance in plants as well as their microbial production.

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Chemistry and Biological Activities of Flavonoids: An Overview

Polyphenols are the most abundant antioxidants in the diet and are widespread constituents of fruits, vegetables, cereals, dry legumes, chocolate, and beverages, such as tea, coffee, or wine. Experimental studies on animals or cultured human cell lines support a role of polyphenols in the prevention of cardiovascular diseases, cancers, neurodegenerative diseases, diabetes, or osteoporosis. However, it is very difficult to predict from these results the effects of polyphenol intake on disease prevention in humans. One of the reasons is that these studies have often been conducted at doses or concentrations far beyond those documented in humans. The few clinical studies on biomarkers of oxidative stress, cardiovascular disease risk factors, and tumor or bone resorption biomarkers have often led to contradictory results. Epidemiological studies have repeatedly shown an inverse association between the risk of myocardial infarction and the consumption of tea and wine or the intake level of some particular flavonoids, but no clear associations have been found between cancer risk and polyphenol consumption. More human studies are needed to provide clear evidence of their health protective effects and to better evaluate the risks possibly resulting from too high a polyphenol consumption.

Dietary polyphenols and the prevention of diseases

Plants consumed by humans contain thousands of phenolic compounds. The effects of dietary polyphenols are of great current interest due to their antioxidative and possible anticarcinogenic activities. A popular belief is that dietary polyphenols are anticarcinogens because they are antioxidants, but direct evidence for this supposition is lacking. This chapter reviews the inhibition of tumorigenesis by phenolic acids and derivatives, tea and catechins, isoflavones and soy preparations, quercetin and other flavonoids, resveratrol, and lignans as well as the mechanisms involved based on studies in vivo and in vitro. Polyphenols may inhibit carcinogenesis by affecting the molecular events in the initiation, promotion, and progression stages. Isoflavones and lignans may influence tumor formation by affecting estrogen-related activities. The bioavailability of the dietary polyphenols is discussed extensively, because the tissue levels of the effective compounds determine the biological activity. Understanding the bioavailability and blood and tissue levels of polyphenols is also important in extrapolating results from studies in cell lines to animal models and humans. Epidemiological studies concerning polyphenol consumption and human cancer risk suggest the protective effects of certain food items and polyphenols, but more studies are needed for clear-cut conclusions. Perspectives on the application of dietary polyphenols for the prevention of human cancer and possible concerns on the consumption of excessive amounts of polyphenols are discussed.

Inhibition of carcinogenesis by dietary polyphenolic compounds

Flavonoids are polyphenolic compounds that are ubiquitously in plants. They have been shown to possess a variety of biological activities at nontoxic concentrations in organisms. The role of dietary flavonoids in cancer prevention is widely discussed. Compelling data from laboratory studies, epidemiological investigations, and human clinical trials indicate that flavonoids have important effects on cancer chemoprevention and chemotherapy. Many mechanisms of action have been identified, including carcinogen inactivation, antiproliferation, cell cycle arrest, induction of apoptosis and differentiation, inhibition of angiogenesis, antioxidation and reversal of multidrug resistance or a combination of these mechanisms. Based on these results, flavonoids may be promising anticancer agents.

Flavonoids: Promising anticancer agents

We have performed a Phase I clinical trial with the naturally occurring flavonoid quercetin (3,3',4',5,7-pentahydroxyflavone). Quercetin has antiproliferative activity in vitro and is known to inhibit signal transduction targets including tyrosine kinases, protein kinase C, and phosphatidyl inositol-3 kinase. Quercetin was administered by short i.v. infusion at escalating doses initially at 3-week intervals. The first dose level was 60 mg/m2; at the 10th dose level of 1700 mg/m2, dose-limiting nephrotoxicity was encountered, but no myelosuppression. At the preceding dose level of 1400 mg/m2, five patients were treated at 3-week intervals, and another eight patients were treated on a once-weekly schedule; overall, 2 of 10 evaluable patients had renal toxicity, 1 at grade 2 and 1 at grade 4. We therefore treated other patients at 945 mg/m2 (eight at 3-week intervals and six at weekly intervals); 3 of 14 patients had clinically significant renal toxicity, 2 patients with grade 2 and 1 patient with grade 3. Patients treated on the weekly schedule did not have cumulative renal impairment but did have a fall in the glomerular filtration rate of 19 +/- 8% in the 24 h after drug administration. We recommend 1400 mg/m2 as the bolus dose, which may be given either in 3-week or weekly intervals, for Phase II trials. Quercetin pharmacokinetics were described by a first-order two-compartment model with a median t(1/2)alpha of 6 min and median t(1/2)beta of 43 min. The median estimated clearance was 0.28 liter/min/m2, and median volume of distribution at steady state was 3.7 liter/m2. In 9 of 11 patients, lymphocyte protein tyrosine phosphorylation was inhibited following administration of quercetin at 1 h, which persisted to 16 h. In one patient with ovarian cancer refractory to cisplatin, following two courses of quercetin (420 mg/m2), the CA 125 had fallen from 295 to 55 units/ml, and in another patient with hepatoma, the serum alpha-fetoprotein fell. In conclusion, quercetin can be safely administered by i.v. bolus at a dose injection. The plasma levels achieved inhibited lymphocyte tyrosine kinase activity, and evidence of antitumor activity was seen.

https://clincancerres.aacrjournals.org/content/clincanres/2/4/659.full.pdf

J Baker

Papers

Phase I clinical trial of the flavonoid quercetin: pharmacokinetics and evidence for in vivo tyrosine kinase inhibition.