Methoxylated flavones

Methoxylated Flavones

1. Identity, Chemistry, and Natural Sources

The flavone scaffold — 2-phenylchromen-4-one — plays multifaceted roles in biological responses and is abundantly present in natural sources. Methoxylated flavones (MFs) are a structural subclass of this scaffold in which one or more of the hydroxyl (–OH) groups normally present on the flavone ring system have been replaced by methoxy (–OCH₃) groups through a process of O-methylation. Polymethoxylated flavones (PMFs) are characterised by multiple methoxy (–OCH₃) groups attached to the flavone structure. When four or more methoxy substituents are present, these compounds are commonly referred to as polymethoxylated flavones (PMFs) or polymethoxyflavones, and they represent the most extensively studied subgroup within the broader methoxylated flavone category.

Flavonoids are divided into groups based on slight differences in chemical structure; flavones are one of those groups, and methoxylated flavones are a subdivision of that group. Lipophilic flavones with several methoxyl residues occur in various clades of land plants, from liverworts to core eudicots, and their chemodiversity is mediated by the manifold combinations of oxygenation and methoxylation patterns.

1.1 Key Individual Compounds

The most prominent citrus PMFs with proven pharmacological effects are tangeretin, nobiletin, 5′-demethylnobiletin, tetramethyl-O-scutellarein, pentamethoxyflavone, tetramethyl-O-isoscutellarein, and sinensetin. Additional well-studied methoxylated flavones from other botanical families include acacetin (5,7-dihydroxy-4′-methoxyflavone), eupatorin, cirsiliol, and xanthomicrol. Acacetin (5,7-dihydroxy-4′-methoxyflavone) is a naturally occurring O-methylated flavone found in plants.

• Nobiletin — 5,6,7,8,3′,4′-hexamethoxyflavone; among the most studied PMFs.
• Tangeretin — 5,6,7,8,4′-pentamethoxyflavone; a major PMF in citrus peel.
• Sinensetin — 5,6,7,3′,4′-pentamethoxyflavone; found in both citrus and Orthosiphon species.
• Heptamethoxyflavone (3,5,6,7,8,3′,4′-HMF) — a fully methylated compound present in aged citrus peel.

1.2 Principal Botanical Sources

Methoxylated flavones are found in especially large amounts in citrus fruits. Relatively common in citrus are two polymethoxylated flavones, tangeretin and nobiletin, both present in tangerines, sweet orange peel (Citrus sinensis), and in bitter orange peel (Citrus aurantium). The majority of PMFs are found in citrus plants, with nobiletin, tangeretin, and sinensetin being the most abundant and widely studied PMFs in citrus peel.

Beyond the genus Citrus (family Rutaceae), methoxylated flavones are widely distributed. In the Lamiaceae, Asteraceae, and Rutaceae, (poly)methoxylated flavones are thought to be produced by secretory tissues and stored externally or in oil cavities. The biosynthesis of (poly)methoxylated flavones in sweet basil (Ocimum basilicum L.) has been largely elucidated in the past few years. Sinensetin, a plant-derived polymethoxylated flavonoid, is found in Orthosiphon aristatus var. aristatus and several citrus fruits, and has been found to possess strong anticancer activities and a variety of other pharmacological benefits. Casimiroa edulis La Llave (family Rutaceae) is known to contain unusual 5,6-dimethoxyflavones, and phytochemical investigation of its leaves and roots has yielded new methoxylated flavones named casedulones A and B, together with 12 known analogues.

1.3 Common Forms and Preparations

Over 4,000 different flavonoids have been identified from various plant sources, with common dietary sources including red wine, stems, flowers, fruits, vegetables, nuts, seeds, herbs, spices, coffee, and teas. In commercial dietary supplement contexts, methoxylated flavones and PMFs are typically presented as standardized extracts from citrus peel — particularly from the dried peel of mandarin, sweet orange, and bitter orange — and may be sold as isolated compounds (e.g., nobiletin or tangeretin), as proprietary blends, or as whole dried-peel powders. Citrus fruits are a rich source of PMFs and hydroxylated PMFs, particularly in their peels, which have been used as commonly available herbal medicines for thousands of years.

2. Traditional and Historical Use

In China, the dried mandarin peel called "Chenpi" (Citri Reticulatae Pericarpium) has been used for disease treatment for two thousand years, dating back to the Han dynasty, according to the earliest work on Chinese medicine, the Shennong Bencao Jing. Since thousands of years in China, Citri Reticulatae Pericarpium (CRP) has been used widely in clinical practice to treat nausea, vomiting, indigestion, anepithymia, diarrhea, cough, and expectoration. Currently, CRP is listed in the Pharmacopoeia of the People's Republic of China.

The dried mature peels of Citrus reticulata cv. Chachiensis, named "Guangchenpi," have been used as a traditional Chinese medicine to treat cough, indigestion, and lung diseases for several hundred years. Flavonoids are one of the main medicinal ingredients of "Guangchenpi," and polymethoxylated flavonoids (PMFs) with four or more methoxy groups are the key component that can distinguish "Guangchenpi" significantly from the broader "Chenpi" category.

Citrus reticulata Blanco has been cultivated for more than 3,000 years and is mainly distributed in Guangdong, Guangxi, Sichuan, Fujian, and Zhejiang Provinces of China; approximately 140 chemical components have been isolated and identified from CRP, including alkaloids, flavonoids, and essential oils, among which flavonoids were considered to be the primary bioactive constituents of the herbal medicine.

In traditional Chinese medicine, CRP has been used for the treatment of diseases pertaining to the digestive and respiratory systems; modern pharmacological studies have found that CRP and its ingredients also have effects on diseases of the cardiovascular system.

Dried young fruits of Citrus aurantium and "Chenpi" (Citri Reticulatae Pericarpium) are used as traditional Chinese medicines and are ingredients of the lung-cleansing and detoxifying decoction (LCDD). Beyond Chinese medicine, in 1936, some scientists suggested that flavonoids be recognised as vitamins, believing that they were necessary to protect the health of capillaries — the smallest blood vessels — but there was not enough evidence to justify classifying flavonoids as vitamins.

3. Chemical Properties and Bioavailability

The high level of methoxylation (5 or 6 methoxy groups) is responsible for a significant hydrophobic character, which favours transport and cellular uptake. The chemical nature of polymethoxylated flavones makes them more lipophilic than hydroxyl flavones. This difference in physical chemistry has important consequences for absorption and tissue distribution. Methoxylated flavonoids represent a relatively highly bioavailable group; compared to hydroxyl flavonoids, a 100-fold higher plasma concentration has been obtained for methoxylated derivatives.

The lipophilicity of PMFs affects their bioavailability by facilitating their transport through the blood–brain barrier, thereby enabling multidirectional pharmacological effects. Most unmethylated dietary flavonoids have low bioavailability, but methylated dietary flavonoids not only retain the anticancer activity of unmethylated dietary flavonoids, but also have higher water solubility, metabolic stability, bioavailability, and lower toxicity.

Methoxylated flavones were shown to be promising cancer chemopreventive agents, and their high metabolic stability compared with the hydroxylated analogues was demonstrated using the human hepatic S9 fraction with cofactors for glucuronidation, sulfation, and oxidation. In studies investigating the resistance of methoxylated flavones toward oxidative metabolism with human liver microsomes and recombinant cytochrome P450 isoforms, among 15 methoxylated flavones investigated, the two partially methylated compounds, tectochrysin and kaempferide, were among the most susceptible to microsomal oxidation. Of the fully methylated compounds, 5,7-dimethoxyflavone and 5-methoxyflavone were the most stable, whereas 4′-methoxyflavone, 3′-methoxyflavone, 5,4′-dimethoxyflavone, and 7,3′-dimethoxyflavone were the least stable, emphasising the importance of the positions of the methoxy substituents in the flavone ring system.

Among the five P450 isoforms tested in metabolic studies, CYP1A1 showed the highest rate of metabolism of fully methylated compounds, followed by CYP1A2 and CYP3A4.

4. Key Active Constituents and Mechanisms of Action

4.1 Antioxidant Activity: Direct and Indirect

Certain flavonoids confer direct antioxidant protection to cells, others induce enzymes that protect cells against oxidative and other insults ("indirect antioxidants"), and others appear to be protective by both mechanisms. Hydroxylated flavones manifest substantial direct antioxidant activity but do not effectively induce cytoprotective enzymes, whereas methoxylated flavones that potently induce cytoprotective enzymes were evaluated to elucidate the structural prerequisites for effective chemoprotective agents. Flavones and flavanones methoxylated at the 5-position of the A-ring were among the most potent inducers of the cytoprotective NAD(P)H:quinone-oxidoreductase 1 (NQO1) in three different cell lines.

In electronic excited states, demethylation of methoxylated flavones is exothermic due to electron excitation energies larger than O–C bond dissociation enthalpies, indicating that the demethylation of methoxy groups present in PMFs and naturally occurring polyphenols may contribute to their antioxidant and photoprotective effect.

4.2 Anti-Inflammatory Mechanisms

Nobiletin significantly inhibited LPS-induced PGE₂ production and IL-1α, IL-1β, TNF-α, and IL-6 in mouse J774A.1 macrophages, and selectively downregulated the expression level of COX-2 instead of COX-1. It has been established that the anti-inflammatory activity of citrus flavonoids is closely associated with the structural properties of different flavonoids, and in particular, the strong inhibitory capacities of tangeretin and nobiletin against IL-1β and COX-2 are speculated to be correlated with the methoxy substituents at positions 5 and 8.

In the body, in addition to any potential action against pathogens, certain of these compounds potentiate enzymes which detoxify carcinogenic hydrocarbons, exhibit anti-inflammatory activity, exert antiadhesive action on blood cells, and show antithrombogenic activity.

4.3 Lipid Metabolism

Reduction of plasma cholesterol by citrus flavonoids is associated with effects on specific liver functions related to lipid handling; in previous in vivo studies, polymethoxylated flavones reduced plasma cholesterol levels at lower doses than required for flavanones, and HepG2 cells were used to quantitate effects on expression of the LDL receptor (LDLR) gene. Nobiletin, a PMF found at the highest concentration in oranges and tangerines, achieved maximal stimulation of LDLR expression 1.5- to 1.6-fold of control at only 5 μmol/L.

4.4 Anticancer Mechanisms

The methoxy group in naturally occurring flavones promotes cytotoxic activity in various cancer cell lines by targeting protein markers, facilitating ligand–protein binding mechanisms and activating cascading downstream signalling pathways leading to cell death. The most effective methoxyflavones inhibiting cancer cell growth both in vitro and in vivo are tangeretin (5,6,7,8,4′-pentamethoxyflavone) and nobiletin (5,6,7,8,3′,4′-hexamethoxyflavone); both compounds were verified to block cell cycle progression, although apoptosis did not occur.

Evidence indicates that tangeretin acts through several mechanisms including growth inhibition, induction of apoptosis, autophagy, antiangiogenesis, and estrogenic-like effects. The potential anti-cancer chemopreventive effects of PMFs as inhibitors of P-glycoprotein (P-gp) have been discussed; P-gp is an ATP-binding transmembrane efflux pump protein that prevents cellular exposure to xenobiotics by transporting compounds out of the cell, and it is overexpressed in tumor cell membranes.

4.5 Neuroprotective Mechanisms

Apart from their antioxidant and anti-inflammatory effects, nobiletin and tangeretin have been shown to attenuate cholinergic deficits, reduce the abnormal accumulation of neurotoxic amyloid-beta peptides, reverse N-methyl-D-aspartate (NMDA) receptor hypofunction, ameliorate ischemic injury, inhibit hyperphosphorylation of tau protein, enhance neprilysin levels, modulate several signalling cascades, and protect against MPP⁺ and MPTP toxicity.

4.6 Blood Rheology

The antiadhesive action of methoxylated flavones on cells in blood of hospitalised patients was significantly associated with calcium, probably at membrane interfaces; this action is consistent with the role of calcium in cell aggregation and disaggregation. These and other findings suggest the body has a capacity for an environmentally related, dietary-conditioned resistance to disease, based on evidence that phenylbenzo-gamma-pyrone derivatives and methoxylated flavonoids synthesised in plants with antiviral, antifungal, and bacteriostatic action may be absorbed into the body and attach reversibly to blood cells.

5. Scientific Evidence by Area of Use

5.1 Cardiovascular Disease and Lipid Metabolism

Of the more than 4,000 flavonoids thus identified, citrus fruit-derived flavonoids are suggested to have an inverse association with the occurrence of coronary heart disease via their ability to reduce plasma cholesterol concentrations. Strong in vivo and in vitro evidence now exists to indicate that citrus flavonoids could reduce the occurrence of cardiovascular disease through their ability to reduce hepatic production of cholesterol-containing lipoproteins, and hence reduce total plasma cholesterol concentration.

Preclinical evidence: In in vivo studies using dietary supplementation in hypercholesterolaemic hamsters, citrus PMFs were more potent than citrus flavanones at reducing serum cholesterol and triglycerides; the total concentration of PMFs in the livers of these animals reached approximately 16–67 μmol/L. In previous in vivo studies, polymethoxylated flavones reduced plasma cholesterol levels at lower doses than required for flavanones.

In a cell-culture study examining acLDL-metabolism and scavenger receptor class A (SR-A) expression by cultured murine J774A.1 macrophages following 24-hour pretreatment at 100 μM with naringenin, hesperetin, and tangeretin and nobiletin, only nobiletin inhibited (50–72%) acLDL metabolism as measured by both cellular cholesterol ester mass and [³H]oleate incorporation into cholesterol esters.

Studies indicate that daily supplementation with PMFs inhibits the early stages of local inflammation, reduces the overproduction of proinflammatory biomolecules (cytokines and nitric oxide), decreases systemic inflammation, and maintains healthy blood cholesterol and triglyceride levels.

Evidentiary strength: The cardiovascular evidence base for PMFs rests predominantly on animal models and cell culture systems. Human clinical trial data specifically on isolated PMFs are limited; the evidence remains at the preclinical stage and cannot yet be translated directly into therapeutic claims for humans.

5.2 Cancer Chemoprevention and Oncology

In the Lamiaceae, Asteraceae, and Rutaceae, (poly)methoxylated flavones may constitute part of the plants' chemical defence mechanisms and represent promising natural lead molecules for the development of potent antiproliferative, antidiabetic, or anti-inflammatory drugs.

In vitro and animal studies: Studies have found that PMFs such as nobiletin and tangeretin exhibit dose- and time-dependent anticancer activities against different subtypes of breast cancer cell lines. Tangeretin was shown to inhibit CDK2 and CDK4 with associated increased p21 and p27 levels in the colon COLO205 cell line, while nobiletin was associated with interference of metastasis through the downregulation of MMP-7 levels in colon HT29 cells. Sinensetin is another PMF identified in citrus peels that is also reported to present antiproliferative effects in several cancer cell lines and anti-angiogenesis activity.

Oral administration of nobiletin effectively suppressed tumour formation and metastasis in xenograft mice and lowered the levels of NF-κB in the isolated tumours. Tangeretin is known to exhibit considerable selective toxicity to many types of cancer cell proliferation, including ovarian, brain, blood, and skin cancer.

The main mechanisms of eupatorin antitumour activity include the inhibition of cancer cell growth and proliferation, induction of apoptosis, cell differentiation, cell cycle arrest, mitochondrial membrane potential reduction, alteration of cytoskeletal organisation, and modulation of cancer cell signalling pathways.

Flavonoids have often been associated with cancer prevention and activity of the human cytochrome P450 enzymes CYP1A1 and CYP1B1 with the occurrence of cancer; the flavones eupatorin and cirsiliol enhanced CYP1 enzyme activity in a concentration-dependent manner in MCF7 human breast adenocarcinoma cells.

Translation to clinical evidence: Pre-clinically, tangeretin has enjoyed extensive scientific scrutiny; however, it was noted that the quantum of pre-clinical data on tangeretin could not translate into a strong rationale for clinical trials on tangeretin or other PMFs either as monotherapy or as a combination with conventional chemotherapeutics. The anticancer evidence for methoxylated flavones remains overwhelmingly in vitro and animal-based. No robust human clinical trials have established efficacy for any PMF against cancer in humans.

5.3 Neurological and Cognitive Health

Nobiletin and tangeretin are important citrus flavonoids derived from the peel and other parts of the Citrus L. genus, and have been shown to exhibit neuroprotective effects in several in vitro and in vivo studies. Apart from their antioxidant and anti-inflammatory effects, nobiletin and tangeretin have been shown to attenuate cholinergic deficits, reduce the abnormal accumulation of neurotoxic amyloid-beta peptides, reverse NMDA receptor hypofunction, ameliorate ischemic injury, inhibit hyperphosphorylation of tau protein, and enhance neprilysin levels.

Molecular docking, network pharmacology, and molecular dynamics simulation have been used to explore pathways, mechanisms of action, and potential targets of methoxylated flavonoids in managing Parkinson's disease. Drug-likeness and ADMET profiling of ten MFs revealed favourable pharmacokinetics, including good oral bioavailability and low predicted toxicity.

Taken together, these naturally occurring phytochemicals may represent beneficial drug candidates for the treatment and prevention of Alzheimer's and Parkinson's disease.

Evidentiary strength: Despite extensive preclinical and clinical research in neurodegenerative disorders, therapeutic strategies aimed at the prevention and chronic treatment of neurodegenerative conditions have not been successfully translated to the clinic. The neuroprotective evidence for methoxylated flavones is currently limited to in vitro cell models, animal models, and computational studies. No human clinical trials confirming neuroprotective efficacy have been published.

5.4 Anti-Obesity and Metabolic Effects

The results of in vivo and in vitro preclinical studies of PMFs have highlighted molecular mechanisms and signalling pathways for anti-inflammatory, anti-cancer, chemopreventive, antidiabetic, anti-obesity, hepatoprotective, and neuroprotective effects in neurodegenerative diseases.

In a rodent model, body weight gain of obese mice was significantly reduced after administration of a PMF-rich fraction for 5 weeks. The PMF sudachitin (5,7,4′-trihydroxy-6,8,3′-trimethoxyflavone) was extracted from the peel of Citrus sudachi and evaluated for its anti-obesity and anti-diabetic effects in two distinct models of metabolic dysfunction.

Polymethoxylated flavones (PMFs), which are compounds characteristic of citrus plants, possess a wide range of biological properties, particularly affecting glucose and lipid metabolism. One study investigated whether PMFs are able to induce insulin secretion by pancreatic β-cells and observed that different concentrations (12.5, 25, 37.5, and 50 μg/ml) of PMFs exerted no effect on insulin synthesis and secretion in INS-1 cells regardless of glucose levels, demonstrating that the regulation of glucose and lipid metabolism by PMFs is not mediated by directly affecting insulin synthesis or secretion.

Evidentiary strength: A 2021 systematic review screened 1,615 records; 16 studies met criteria, and the range of dosage of PMFs across selected animal and in vitro studies was varied from 10 to 200 mg/kg (5–26 weeks) and 1–100 μmol (2 hours–8 days). The anti-obesity evidence is essentially confined to animal and cell-based models; no robust human trials have confirmed these effects.

5.5 Blood Rheology and Vascular Effects

Early human-derived evidence (characterised as preliminary) relates to the effects of methoxylated flavones on blood cell behaviour. The antiadhesive action of methoxylated flavones on cells in blood of hospitalised patients was significantly associated with calcium, probably at membrane interfaces, consistent with the role of calcium in cell aggregation and disaggregation. These observations, made by R.C. Robbins in studies published in the 1970s in the Journal of Clinical Pharmacology and International Journal of Vitamin and Nutrition Research, were among the first to suggest a direct effect of dietary methoxylated flavones in human blood samples. The studies were small, and the findings have not been reproduced in modern randomised controlled trials.

5.6 Skin and Melanogenesis

A polymethoxyflavone mixture extracted from orange peels, mainly containing nobiletin, 3,3′,4′,5,6,7,8-heptamethoxyflavone, and tangeretin, suppresses melanogenesis through the acidification of cell organelles, including melanosomes. This finding derives from in vitro research and has not been confirmed in clinical studies.

6. Body Systems and Health Areas Associated With Methoxylated Flavones

• Cardiovascular system: lipid metabolism, LDL receptor regulation, anti-atherosclerotic activity, blood cell anti-adhesion.
• Immune and inflammatory system: suppression of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α), COX-2 downregulation, NF-κB inhibition.
• Central nervous system: amyloid-beta reduction, tau dephosphorylation, cholinergic modulation, ischemia protection.
• Metabolic system: effects on glucose and lipid metabolism, adipogenesis inhibition, mitochondrial biogenesis.
• Oncology / chemopreventive: cell cycle arrest, apoptosis (compound-dependent), P-gp inhibition, NQO1 induction.
• Digestive / respiratory system: traditional indications for cough, indigestion, nausea (via Chenpi use in TCM).
• Skin: melanogenesis suppression in vitro.

PMFs possess a wide range of bioactivities, including potential anticancer, neuroprotective, anti-inflammatory, anti-obesity, antioxidant, and antiatherosclerotic effects.

7. Dosage Forms and Dosages Reported in Research

Methoxylated flavones are available as dietary supplements in capsule, tablet, and powder form, typically as standardized citrus peel extracts or as isolated PMF compounds. The following dosages appear in primary research and systematic reviews; they are reported here as observed in sources and do not constitute recommendations.

• Animal anti-obesity studies: The range of dosage of PMFs across selected animal studies was 10 to 200 mg/kg body weight over 5–26 weeks, and 1–100 μmol in in vitro conditions over 2 hours to 8 days.
• Rodent anti-obesity model (PMF-rich fraction, Citrus sunki): HFD-induced obese mice were administered a PMF-rich fraction at 50, 100, and 200 mg/kg of body weight per day.
• Nobiletin LDL receptor study (in vitro, HepG2 cells): Nobiletin achieved maximal stimulation of LDLR expression at only 5 μmol/L in HepG2 cells.
• Nobiletin macrophage study (in vitro): Flavonoid pretreatment of macrophages was examined at 100 μM for 24 hours.
• Acacetin neuroprotection study (in vitro): Acacetin at 50–200 nM caused dose-dependent inhibition of MPP⁺-induced inflammatory factors in primary mesencephalic culture.
• PMF insulin secretion study: Concentrations of 12.5, 25, 37.5, and 50 μg/ml were investigated for effects on insulin biosynthesis and secretion in INS-1 cells.
• Tangeretin acute toxicity (murine): Oral administration of tangeretin up to 3,000 mg/kg in an acute toxicity study did not induce mortality in mice.

Notably, the vast majority of dosage data originate from cell culture or animal studies. No standardized human clinical dose has been established for methoxylated flavones as a class.

8. Safety Considerations and Interactions

8.1 General Safety Profile

Methoxylated flavones are a normal part of the diet when taken by mouth. No signs of toxicity (including liver, kidney, or pancreas) were observed for several weeks after tangeretin or nobiletin consumption in animal studies. Many studies have reported the safety of tangeretin in experimental animals.

The lipophilic nature of methoxylated flavone analogues is a key concern as it impacts drug membrane transfer; while lipophilicity is crucial for drug efficacy, the excessive lipophilic effects in flavonoids can reduce water solubility and hinder drug transport to target sites. Recent in vitro studies suggest that the incorporation of polar hydroxyl groups may help overcome the challenges associated with methoxy groups while maintaining essential lipophilic properties.

8.2 Interaction with Tamoxifen — A Specific, Evidence-Backed Concern

The most extensively documented and clinically significant safety signal for PMFs involves an interaction with the breast cancer drug tamoxifen. Treatment with tangeretin did not inhibit tumour growth in an animal model, and addition of this compound to drinking water alongside tamoxifen completely neutralised tamoxifen's inhibitory effect. This in vivo model demonstrated potential interference of dietary compounds such as flavonoids with tamoxifen, which could lead to reduced efficacy of adjuvant therapy; in that study, the tumour growth-inhibiting effect of oral tamoxifen was reversed upon addition of tangeretin to the diet.

Tangeretin is an effective tumour growth and invasion inhibitor in vitro of human MCF-7/6 breast cancer cells; however, when added to the drinking water of MCF-7/6 tumour-bearing mice it neutralised the beneficial tumour-suppressing effect of tamoxifen. Tangeretin reduces the number of natural killer cells, which may explain why the beneficial suppressive effect of tangeretin on MCF-7/6 cell proliferation in vitro is completely counteracted in vivo.

In combination with the chemotherapeutic agent tamoxifen, tangeretin could reduce the antitumour efficacy of this drug in some patients; therefore, combination regimens should be validated in vivo before clinical application. This evidence reports adverse effects that may occur through the interaction of certain herbal or plant products with tamoxifen that doctors and patients should pay attention to during treatment of oestrogen receptor-positive breast cancer. It must be noted that this interaction has been demonstrated in animal models; confirmation in human clinical settings has not been reported in the available literature.

8.3 Cytochrome P450 Interactions

In incubations with hepatic S9 fraction with cofactors for oxidation and both conjugation reactions, partially methylated flavones were much less metabolically stable than fully methylated flavones, confirming that oxidative demethylation is the rate-limiting metabolic reaction for fully methylated flavones only. The rate of oxidative metabolism of methoxylated flavones, mainly involving CYP1A1 and CYP1A2, varied widely even between compounds with very similar structures.

Flavonoids have often been associated with cancer prevention and activity of the human cytochrome P450 enzymes CYP1A1 and CYP1B1; the flavones eupatorin and cirsiliol enhanced CYP1 enzyme activity in a concentration-dependent manner in MCF7 human breast adenocarcinoma cells. These CYP1-modulating properties mean that co-administration with drugs metabolised by CYP1A1, CYP1A2, or CYP3A4 carries theoretical interaction potential.

8.4 Multidrug Resistance Modulation

The potential anti-cancer chemopreventive effects of PMFs as inhibitors of P-glycoprotein (P-gp) have been discussed; P-gp is an ATP-binding transmembrane efflux pump protein which prevents cellular exposure to xenobiotics by transporting compounds out of the cell. P-gp inhibition by PMFs theoretically could alter the tissue distribution and efficacy of numerous P-gp–substrate drugs, though this has not been studied in clinical pharmacokinetic trials involving humans.

8.5 Limitations of the Evidence Base

Although advanced phytochemical isolation and structural identification techniques have revealed more than 80 citrus PMFs, studies to date have tended to focus on just two of these — nobiletin and tangeretin — whereas relatively little information is available regarding other PMFs. Various PMFs showed differential effects on anti-inflammatory and anti-cancer activity, making it difficult to identify which activity was responsible for their protective effects.

Methoxylated flavones are used for poor circulation in the legs (venous insufficiency), varicose veins, heart disease, high cholesterol, cataracts, cancer, and other conditions, but there is no good scientific research to support any of these uses. Methoxylated flavones are natural antioxidants and might reduce inflammation. They might also affect the way the liver processes cholesterol and other blood fats, and scientists think methoxylated flavones might also reduce the spread of cancer cells, but more information is needed.

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