Nobiletin

Nobiletin

1. Identity: Chemical and Botanical Characterization

1.1 Chemical Names and Structure

Nobiletin (NOB) is chemically known as 5,6,7,8,3′,4′-hexamethoxyflavone and is a dietary polymethoxylated flavonoid found in citrus fruits. Its empirical formula is C₂₁H₂₂O₈ and its molecular weight is 402.39; it is classified as one of the nontoxic dietary polymethoxyflavones (PMFs) within the unique class of flavonoids mainly isolated from citrus fruits. Nobiletin is also referred to in the literature by the name 3′,4′,5,6,7,8-hexamethoxyflavone, reflecting the arrangement of its six methoxy substituents.

Polymethoxyflavones such as nobiletin are structurally characterized by a benzo-γ-pyrone (15-C) skeleton consisting of two benzene rings (6-C) linked by a linear three-carbon chain (3-C) with a carbonyl group at the C4-position and presenting several methoxy substituents. Nobiletin specifically contains six methoxy groups on its flavonoid backbone — at positions 5, 6, 7, and 8 on the A-ring and at 3′ and 4′ on the B-ring.

The crystal structure of nobiletin shows that the chromene and the arene rings are almost in the same plane. The C atoms of the two methoxy groups in the arene ring are also in the same plane, while the C atoms of the four methoxy groups linking to the chromene ring are not in parallel. This conformational characteristic of chiral structure is indicated by the covalent bond rotation between the arene and the chromene rings and the conformational alternations of methoxy groups.

1.2 Natural Sources

Citrus species are the most rich dietary sources of diverse PMFs, and their representative PMFs are nobiletin and tangeretin. Overall, citrus species in the Rutaceae family are the most rich dietary sources of diverse PMFs. In addition, several plants in the Compositae, Labiatae, Rosaceae, Solanaceae, Verbenaceae, and Zingiberaceae families can also be good sources of certain PMFs.

Different citrus species have been reported to contain diverse PMFs, mainly existing in the citrus peel and citrus leaves. Nobiletin is found in the peel of fruits of the citrus species — for example, king orange (Citrus nobilis) and Seville orange (Citrus aurantium) — and of the round kumquat (Fortunella japonica). These flavonoids are mainly derived from citrus plants of the Rutaceae family, especially from Citrus sinensis and Citrus reticulata. Nobiletin is also a natural compound derived from Citrus depressa, which is a small citrus native to Okinawa, Japan, Taiwan, and China, known locally as Shikuwasa or Sikwasa.

The concentration of nobiletin within the peel oils of diverse citrus fruits can exhibit variability. For example, nobiletin is found at approximately 0.50 g/L in orange, 0.60 g/L in king tangerine, 0.40 g/L in clementine tangerine, and 1.50 g/L in tangerine peel oil. In citrus fruits, nobiletin content is estimated to range from 7 to 173 mg/kg of dry weight, with higher concentrations found in unripe citrus fruits. Among local Chinese citrus species studied by HPLC, Zaoju (C. subcompressa) was found to have the highest content of nobiletin at 0.59%, a polymethoxylated flavone.

1.3 Common Forms and Preparations

Flavonoid-enriched tissues of citrus such as peel, immature fruit, and flower are consumed as culinary seasonings and tea ingredients in China for centuries. In the contemporary scientific and commercial context, nobiletin is available in purified form as a supplement ingredient — typically in capsule or powder formulations — derived from citrus peel extracts. The nobiletin-containing test food (Nobilex®) investigated in clinical trials comprised high-purity nobiletin powder combined with dried root powder of Kaempferia parviflora and dried leaf powder of Peucedanum japonicum, and was administered to elderly Japanese subjects once a day for 16 weeks.

Researchers have also explored novel delivery systems to overcome nobiletin's solubility limitations. Nanoemulsifying and emulsion-based drug delivery systems are reported to improve the solubility and absorption of poorly water-soluble compounds such as nobiletin. Nobiletin has extremely low water solubility and low oral bioavailability; however, transdermal delivery using ionic liquids such as choline and geranic acid (CAGE) has been investigated as a promising approach.

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2. Traditional and Historical Use

2.1 Traditional Chinese Medicine

Chenpi (dried mature fruit peel of Citrus reticulata Blanco and its cultivated varieties) is one of the most commonly used traditional Chinese medicines and has the described effects of regulating qi, strengthening the spleen, drying dampness, and resolving phlegm. Herbal medicine has been essential in treating diseases in China for thousands of years, and other Asian and European countries have gradually adopted Chinese herbal medicine as part of their disease treatment.

Chenpi (also called chen pi or chimpi) is sun-dried mandarin orange peel used as a traditional seasoning in Chinese cooking and traditional medicine. It is aged by storing the peels dry. The taste is first slightly sweet, but the aftertaste is pungent and bitter. According to Chinese herbology, its attribute is warm. Chenpi contains volatile oils which include the chemical compounds nobiletin, hesperidin, neohesperidin, tangeretin, citromitin, synephrine, carotene, cryptoxanthin, inositol, vitamin B1, and vitamin C. Traditional Chinese herbal medicine uses the alcohol extracts of several citrus peels, including those extracted from mandarin orange and bitter orange.

In traditional Chinese medicine, citrus peel has been used to treat and alleviate a wide range of ailments including skin inflammation and respiratory infections for thousands of years. The antitumor effect of Citrus folium (citrus leaf) is related to its traditional use, mainly in its Chinese medicinal properties of soothing the liver and promoting qi, resolving phlegm, and dispelling stagnation.

Citrus peels have been utilized in traditional Chinese medicine for their therapeutic properties, laying the foundation for contemporary pharmacological research that has validated and expanded these uses.

2.2 Discovery and Isolation

Nobiletin was discovered by Kwong-Fong Tseng in the 1930s and has been isolated from an oily matter from the Chinese drug chen-pi (made from the peel of C. reticulata). In the 1960s, researchers extracted a variety of flavonoids from Chenpi and, based on this discovery, nobiletin was subsequently isolated and further purified as 3′,4′,5,6,7,8-hexamethoxyflavone. Thus, the compound that had been consumed for centuries as part of whole-peel preparations in East Asian medicine was only characterized chemically in the twentieth century.

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3. Key Constituents, Metabolites, and Mechanisms of Action

3.1 Nobiletin as the Principal Active Compound

Nobiletin itself is the primary pharmacologically active compound studied in this line of PMFs. The bioactivities of a compound depend on its structure and its metabolism. Without a glycoside moiety, nobiletin is easily absorbed due to its high lipophilic nature and high permeability. Nobiletin has two or more methoxy groups, has no glycosidic bonds, is more hydrophobic than hydroxyl compounds, and has a higher intestinal absorption rate than other flavonoids.

3.2 Metabolites

After prolonged storage or following consumption, nobiletin is degraded by autolysis or gastric juices into 5-demethylnobiletin (5-DMN). Phase II metabolism of nobiletin takes place in the small intestine, where it undergoes sulfation and glucuronidation processes. By comparing the demethylated metabolite profiles in the urine and feces, gut microbiota has been shown to demonstrate greater biotransformation activity on nobiletin than the host organism itself. Key demethylated metabolites — including 5-demethylnobiletin (5-DMN), 3′-demethylnobiletin, and further demethylated variants — have themselves demonstrated biological activity in experimental models, contributing to the overall pharmacological profile of dietary nobiletin consumption.

3.3 Central Signaling Pathways

Nobiletin modulates a broad range of intracellular signaling networks. The principal pathways consistently reported in the peer-reviewed literature include the following:

• NF-κB pathway: The beneficial anti-inflammatory effects of nobiletin in muscle cells are accompanied by suppression of ROS production and reduction of MAPK activity (especially JNK) and inhibition of the NF-κB signaling pathway.
• AMPK pathway: Nobiletin significantly improved behavioral deficits and ameliorated neuroinflammation induced by LPS in rats. Furthermore, nobiletin promoted autophagy and attenuated NLRP3 inflammasome activation induced by LPS, involving the adenosine monophosphate-activated protein kinase (AMPK) pathway. Neuroprotective and anti-depressant actions of nobiletin relied on its effects of promoting autophagy and suppressing the activation of NLRP3, in which process the AMPK pathway may be involved.
• Nrf2 pathway: Nobiletin protects the liver by exerting antioxidant and anti-inflammatory effects through the Nrf2 and AMPK pathways. Furthermore, Nrf2 is not only a direct target for nobiletin to improve oxidative damage but also indirectly involved in lipid-lowering and anti-inflammatory processes in NAFLD.
• PI3K/Akt and BDNF/TrkB signaling: Nobiletin increases the activities of PI3K/Akt and BDNF/TrkB signaling. In the 5XFAD transgenic mouse model, nobiletin promotes neuronal survival and cognitive improvement by activating the PI3K/Akt–CREB–BDNF signaling pathway.
• ROR nuclear receptors and circadian clock modulation: Filter-binding analysis has revealed that RORs (α and γ subtypes) are direct protein targets of nobiletin. Nobiletin has been shown to affect circadian rhythms via their enhancement, resulting in protection against metabolic syndrome.
• MAPK/ERK signaling: The effects of nobiletin are mediated by modulating intricate pathways such as the TGF-β1/SMAD, PI3K/Akt/mTOR, NOX4-Nrf2, AMPK, NF-κB, and STAT3 signaling pathways.
• TLR4 and NLRP3 inflammasome: Nuclear factor erythroid 2-related factor 2 (Nrf2), NF-κB, AMPK, peroxisome proliferator-activated receptor α (PPAR-α), ERK, AKT, TLR4, and transcription factor EB (TFEB) signaling pathways are important molecular targets for nobiletin in ameliorating liver diseases.
• PDE4B inhibition: Nobiletin has been identified as a direct inhibitor of PDE4B through pharmacophore modeling, molecular docking, molecular dynamics simulation, SPR, and enzyme activity assays.

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4. Scientific Evidence by Area of Use

Important qualification: The overwhelming majority of the evidence for nobiletin's biological effects is derived from in vitro cell studies and in vivo animal models. Human clinical trial data are limited in number, scale, and duration. This distinction is noted explicitly for each area below.

4.1 Neuroprotection and Cognitive Function

Animal / Preclinical Evidence

Previous studies have demonstrated that nobiletin, a polymethoxylated flavone from citrus peels, ameliorates learning and memory impairment in olfactory-bulbectomized mice, amyloid precursor protein transgenic mice, NMDA receptor antagonist-treated mice, and senescence-accelerated mouse prone 8. A triple transgenic mouse model of AD (3XTg-AD) that progressively develops amyloid plaques, neurofibrillary tangles, and cognitive impairments has also been used, and nobiletin improved cognitive impairment and reduced soluble Aβ levels in these animals.

In a 5XFAD transgenic mouse model, nobiletin activated AMPK/SIRT1/PGC-1α and NRF2 pathways, enhancing antioxidant defenses, and promoted PI3K/Akt–CREB–BDNF signaling, increasing PSD95 and synaptophysin. Nobiletin exerted strong neuroprotective and antioxidant effects by targeting multiple signaling cascades, mitigating amyloid pathology and neuroinflammation, and improving synaptic plasticity.

The major effects of nobiletin on the nervous system are: inhibiting inflammatory cytokine expression, reducing neurotoxicity, promoting the survival of nerve cells and axon outgrowth, reducing blood-brain barrier permeability and cerebral edema, and promoting mild depolarization of nerve cell mitochondria and increasing capacity against oxidative stress. Nobiletin has also been shown to have the ability to penetrate the blood-brain barrier (BBB) and plays a role in improving brain function.

Nobiletin has been shown in animal models to counteract Alzheimer's disease through several mechanisms including anti-oxidant, anti-inflammatory effects, enhancing autophagy through the SIRT1/FoxO3a pathway, cognitive and behavioral preservation, halting cholinergic neurodegeneration, and attenuating Aβ pathology and tau hyperphosphorylation.

In a study evaluating the protection effects of nobiletin on neuroinflammation and memory deficit, mice were administered nobiletin by oral gavage every day for 6 weeks (100 mg/kg/day), and were subsequently injected intraperitoneally with lipopolysaccharide (LPS) for 7 days. Results of behavioral tests revealed that nobiletin dramatically ameliorated LPS-triggered memory deficit regarding synaptic dysfunctions and neuronal loss. Nobiletin also suppressed microglial activation and proinflammatory cytokine secretion, such as COX-2, IL-1β, TNF-α, and iNOS.

Human Clinical Evidence

Nobiletin exerts beneficial effects on cognitive function in various animal models of Alzheimer's disease. A clinical study aimed to investigate the benefits and safety of a combination food of nobiletin-rich extract from C. depressa peel for healthy elderly subjects. The nobiletin-containing test food (Nobilex®) comprised high-purity nobiletin powder combined with dried root powder of K. parviflora and dried leaf powder of P. japonicum and was administered to elderly Japanese subjects once a day for 16 weeks. The Japanese version of the Wechsler Memory Scale-Revised (WMS-R) was used as the primary evaluation item for the assessment of global memory.

Data from a protocol-matched population (Per Protocol Set: PPS) (n = 108) were analyzed. The scores of "general memory" or "visual memory" in the indices of WMS-R were significantly higher in the nobiletin-containing test food group than in the placebo group. The difference in the total WMS-R score was significantly higher in the test-food group (9.0 ± 7.20) than in the placebo group (5.9 ± 7.70). An age-stratified analysis of the WMS-R test showed similar changes in subjects aged ≤74 years to those in the overall subject population. In the stratified analysis involving subjects with an MMSE-J score of between 24 and 28, the "figural memory" subscale score in WMS-R was significantly higher in the test food group than in the placebo group.

Limitations: Each capsule of the test food contained the dried powder extract (containing 10.0 mg nobiletin and 5.8 mg tangeretin) of C. depressa peel, dried root powder (126.7 mg) of K. parviflora, and dried leaf powder of P. japonicum (33.3 mg). Because the supplement contained multiple botanical extracts in addition to nobiletin, it is not possible to attribute the observed effects to nobiletin alone. The population comprised healthy elderly individuals without dementia, limiting extrapolation to clinical populations with Alzheimer's disease. The evidence in this area, while promising, remains preliminary in humans.

4.2 Metabolic Health: Obesity, Dyslipidemia, and Insulin Resistance

Animal / Preclinical Evidence

Animal studies have demonstrated that nobiletin regulates lipid metabolism-related genes and reduces proinflammatory cytokine mRNA expression in high-fat diet-induced C57BL/6J mice, alleviating obesity, dyslipidemia, hyperglycemia, and insulin resistance. In HFD-fed mice, nobiletin upregulates CPT1-α and PGC1-α, reducing hepatic lipid accumulation and VLDL-TG secretion while improving glucose tolerance, lowering hyperinsulinemia, and enhancing insulin sensitivity.

Nobiletin administration attenuates weight gain and glucose tolerance in mice fed a high-fat diet. Additionally, nobiletin administration substantially restored lipid metabolic disorder and repressed the level of genes related to lipid metabolism in HFD-induced obese mice. The sequencing of 16S rRNA genes in fecal samples unveiled that nobiletin administration reversed HFD-induced intestinal microbiota composition, particularly in the relative abundances of Bacteroidetes and Firmicutes at the phylum and genus level.

Nobiletin at 17 mg/kg/day for 16 weeks improved glucose tolerance, insulin resistance, and total cholesterol levels in mice fed a high-fat diet, but did not affect food intake, body weight, or adiposity. In a 5-week study of obese diabetic mice, nobiletin 200 mg/kg/day was associated with improved hyperglycemia and insulin resistance, with no significant differences in body weight gain and mean daily food intake observed between vehicle-treated and nobiletin-treated groups.

In rats with streptozotocin-induced diabetes, 4 weeks of nobiletin 10 mg/kg or 25 mg/kg orally daily resulted in improved mean arterial pressure, heart rate, and left ventricular end diastolic pressure. The 25 mg/kg dose also improved maximal left ventricular systolic pressure.

Studies have shown that nobiletin exerts anti-adipogenic effects through modification of the AMPK signaling pathway and prompts a brown adipocyte-like phenotype in 3T3-L1 cells. Nobiletin has also been demonstrated to stimulate lipolysis via activating signal cascades mediated by cAMP/CREB.

Human / Clinical Evidence

Clinical trial data are lacking to support specific dosing recommendations for nobiletin in metabolic disorders. Animal studies and a limited number of clinical trials suggest that nobiletin has multifunctional biological activities such as protection against obesity and obesity-related cardiometabolic disorders, neuroprotection, antidiabetic, anticancer, anti-allergy, antioxidant, anti-inflammatory, and free radical scavenging abilities. The evidence in humans for metabolic effects remains largely indirect or absent; the preclinical data are extensive and internally consistent, but translation to clinical practice has not yet been established through well-powered randomized trials.

4.3 Liver Disease and Non-alcoholic Fatty Liver Disease (NAFLD)

Animal / Preclinical Evidence

Nobiletin improves liver function, reduces inflammation and oxidative stress, remodels gut microflora, ameliorates hepatocellular necrosis, steatosis, and insulin resistance, and modulates biorhythms. Nrf2, NF-κB, AMPK, PPAR-α, ERK, AKT, TLR4, and TFEB signaling pathways are important molecular targets for nobiletin in ameliorating liver diseases.

In both mouse and cell models, nobiletin significantly ameliorated lipid deposition, oxidative stress, and inflammation in NAFLD. Its mechanism may involve the Nrf2, SREBP-1c, and NF-κB signaling pathways. Nobiletin also improves NAFLD by modulating intestinal flora.

Nobiletin can regulate macrophage M1-to-M2 polarization through the NLRP3 pathway. In addition, nobiletin can regulate bile acid metabolism through PPARα.

Nobiletin maintained high cell viability in THLE-2 hepatocytes and cholangiocytes in vitro, confirming its low cytotoxicity at tested concentrations.

Strength of evidence: Evidence in the liver disease domain is currently confined to in vitro and animal models. No completed randomized controlled human trials are available specifically for nobiletin in NAFLD or hepatic disease as of 2024.

4.4 Anticancer Activity

Preclinical Evidence

Nobiletin is a polymethoxyflavone that has been extensively studied in the fight against various types of cancer. At low doses, it shows anti-inflammatory activity by regulating the expression of cytokines and reactive oxygen species. It also acts by inducing cell cycle arrest and is a potent inhibitor of cell proliferation, promoting programmed cell death and blocking angiogenesis.

From the perspective of modern biomedical research, nobiletin has anticancer effects. Its potential molecular mechanisms include inhibition of the cell cycle, induction of apoptosis, and inhibition of angiogenesis, invasion, and migration. Nobiletin and preparations containing it can also reduce the side effects of chemotherapy drugs and reverse multidrug resistance (MDR).

Treatment with nobiletin has resulted in anti-oncogenic effects in a variety of cancer models by affecting major pathways including AKT, MAPK, TGF-β1/SMAD3, ERK, and JNK. In studies where it was shown to affect the AKT pathway, nobiletin was found to decrease tumor viability, weight, and volume in A2780/CP70 ovarian cancer xenograft models. It also reduced cell adhesion, invasion, and migration in the highly metastatic AGS gastric adenocarcinoma cell line.

Nobiletin is a natural compound known to display anticancer effects. Studies involving ROR nuclear receptor activation showed that nobiletin identified several triple-negative breast cancer (TNBC) cell lines as sensitive to nobiletin, by itself or in combination. Cell and xenograft experiments showed that nobiletin significantly inhibited TNBC cell proliferation and motility in vitro and in vivo.

Nobiletin also functions in a combination setting, acting to sensitize a paclitaxel-resistant A2780 ovarian cancer cell line to the chemotherapeutic agent, illustrating a versatile natural compound that warrants further functional and mechanistic investigations.

Strength of evidence: Anticancer evidence for nobiletin is exclusively preclinical — derived from cell culture experiments and animal xenograft models. No human clinical trials evaluating nobiletin as an anticancer agent have been reported. This area of research is considered exploratory and the results cannot be extrapolated to clinical cancer prevention or treatment without further investigation.

4.5 Cardiovascular Health

Preclinical Evidence

In the cardiovascular system, nobiletin ameliorates metabolic syndrome, promotes locomotor activity, and inhibits platelet aggregation. For heart diseases, nobiletin results in neuroprotection of cerebral ischemia-reperfusion and lowers serum LDL/VLDL cholesterol.

Nobiletin attenuates the adverse effects of acute myocardial infarction in rats and protects against pressure overload-induced cardiac hypertrophy by inhibiting NADPH oxidase. Nobiletin may also alleviate myocardial ischemia and reperfusion injury.

At a low dose of 150 nM, nobiletin derived from Citrus depressa significantly prevented phenylephrine-induced cardiomyocyte hypertrophy in screening assays.

Strength of evidence: Cardiovascular effects have been demonstrated in animal models and isolated cell systems. Human data are absent in this domain specifically for nobiletin as a purified compound.

4.6 Anti-inflammatory Effects

PMFs including nobiletin have received increasing attention due to their multiple bioactivities, such as antioxidant, anti-inflammatory, anti-cancer, metabolic regulatory, immunoregulatory, neuroprotective, and skin protective effects.

In vitro and in vivo experiments have investigated the anti-inflammatory effects of nobiletin in the progression of osteoarthritis. Mouse chondrocytes were pretreated with nobiletin (0, 10, 20, 40 μM) for 24 h and then incubated with IL-1β (10 ng/ml, 24 h) in vitro. Induction of proinflammatory and catabolic mediators by IL-1β stimulation of mouse chondrocytes could be partially blocked by treatment with nobiletin or ammonium pyrrolidine dithiocarbamate (an NF-κB inhibitor).

In a mouse model of pulmonary fibrosis, nobiletin treatment significantly ameliorated lung fibrosis by suppressing pathological damage, collagen deposition, and fibroblast activation. Moreover, nobiletin obviously reduced M2 macrophage-related proteins, including CD206, Arg1, and pro-fibrotic mediators such as TGF-β and CTGF, which may contribute to the antifibrosis effect of nobiletin.

Strength of evidence: Anti-inflammatory evidence is robust at the preclinical level, with multiple consistent mechanistic studies across a range of cell types and disease models. No randomized human trials have specifically assessed anti-inflammatory endpoints for isolated nobiletin.

4.7 Circadian Rhythm Modulation and Nocturia

Preclinical Evidence

From high-throughput screenings, nobiletin and tangeretin (NoT) have been identified as clock amplitude enhancers. Studies reveal a novel mode of action for nobiletin; specifically, nobiletin augments the robustness (amplitude) of circadian rhythms, the daily rhythmic processes throughout the body. In metabolic disorders and aging settings where circadian amplitude is attenuated, nobiletin-treated mice showed marked improvement in metabolism and healthy aging, suggesting a key role of nobiletin as a clock modifier to promote fitness over a lifetime.

Human Clinical Evidence

Nobiletin and tangeretin (NoT) are flavonoids derived from the peel of Citrus depressa and have been found to modulate circadian rhythms. Because nocturia can be considered a circadian rhythm disorder, the efficacy of NoT for treating nocturia was investigated in a randomized, placebo-controlled, double-blind, crossover study. Nocturia patients aged ≥50 years who presented nocturia more than two times on a frequency-volume chart were recruited. Participants received NoT or a placebo (50 mg once daily for 6 weeks), followed by a washout period of ≥2 weeks. The placebo and NoT conditions were then switched.

Compared with the placebo, NoT showed no significant change in the primary endpoint (nocturnal bladder capacity). For the secondary endpoints, the variable of mean change in nighttime frequency was significantly decreased with NoT compared with that of the placebo.

Limitations: This was a crossover study using a combination of nobiletin and tangeretin, making it impossible to attribute the effect to nobiletin alone. The primary endpoint was not met, and benefits were observed only for a secondary endpoint. The trial size was modest and conducted in a single-country population. The evidence is therefore considered preliminary.

4.8 Respiratory Health

In an animal model of asthma, nobiletin significantly reduced the levels of inflammatory cells and cytokines in mice and alleviated airway hyperresponsiveness. In work examining pulmonary fibrosis, nobiletin reduced M2 polarization and alleviated lung fibrosis by regulating AMPK-mTOR-mediated autophagy. Nobiletin is considered a promising dietary treatment for lung diseases such as pulmonary fibrosis.

A review has summarized findings regarding the potential therapeutic efficacy and mechanism of nobiletin against respiratory diseases such as lung cancer, COPD, asthma, pulmonary fibrosis, acute lung injury, pulmonary arterial hypertension, and pulmonary infectious diseases. All evidence in this area is preclinical; no human trials have been conducted for nobiletin in respiratory conditions.

4.9 Bone Health

In mouse models, nobiletin has been shown to result in inhibition of bone resorption and maintenance of bone mass. This area remains at the preclinical stage with no human data available.

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5. Body Systems and Health Areas Associated with Nobiletin

Based on the accumulated body of preclinical and limited clinical research, nobiletin has been associated with the following body systems:

• Central nervous system: Neuroprotection, cognitive support, anti-neuroinflammation, antidepressant-like effects in animal models, and potential relevance to Alzheimer's and Parkinson's disease.
• Metabolic / endocrine system: Glucose homeostasis, insulin sensitization, lipid metabolism regulation, anti-adipogenic effects, and management of metabolic syndrome in animal models.
• Hepatic system: Protection against NAFLD, steatosis, oxidative liver injury, and viral hepatitis in preclinical models.
• Cardiovascular system: Anti-atherosclerotic effects, reduction of LDL/VLDL cholesterol, cardioprotection against ischemia-reperfusion injury, and anti-hypertrophic effects in animal models.
• Immune and inflammatory system: Broad suppression of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), inhibition of NF-κB, modulation of macrophage polarization.
• Oncological: Antiproliferative, pro-apoptotic, anti-angiogenic, and anti-metastatic activity in cancer cell lines and animal tumor models.
• Respiratory system: Anti-asthmatic, anti-fibrotic, and anti-inflammatory effects in preclinical respiratory models.
• Circadian / chronobiological: Enhancement of circadian clock amplitude via ROR nuclear receptor activation, with potential implications for metabolic and sleep-related disorders.
• Musculoskeletal system: Anti-inflammatory effects in chondrocytes in the context of osteoarthritis models; inhibition of bone resorption in animal models.

Nobiletin has been reported to yield therapeutic effects in a variety of disease models, including those of neurological, inflammatory, cardiac, and metabolic diseases, in addition to cancer cell lines and tumors.

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6. Pharmacokinetics, Bioavailability, and Dosage Forms

6.1 Absorption and Bioavailability

Despite its poor solubility in water, nobiletin's chemical structure results in high absolute oral bioavailability compared with other flavanones, which has been proved to be over 20% in various experiments. The absolute oral bioavailability of nobiletin in lean rats (22.37% ± 4.52%) and obese rats (18.67% ± 4.80%) shows a negligible statistically significant difference. By comparing the demethylated metabolite profiles in the urine and feces, gut microbiota demonstrated greater biotransformation activity on nobiletin than the host.

After oral intake, nobiletin undergoes complex biotransformation. Nobiletin has low oral bioavailability and absorption due to crystallization and poor water solubility. The low oral bioavailability may limit the pharmacological outcomes. Note that these findings on bioavailability appear to describe the compound's inherent poor aqueous solubility as the limiting factor relative to its actual in vivo absorption rates observed experimentally.

6.2 Blood-Brain Barrier Penetration

Nobiletin has the ability to penetrate the blood-brain barrier (BBB) and plays a role in improving brain function. This property distinguishes it from many other hydrophilic polyphenols that do not readily enter the central nervous system and is considered a significant advantage in the context of neurological applications.

6.3 Concentrations Reported in Animal Studies

The C_max of nobiletin was 1.78 and 4.20 μg/ml in the plasma and brain, respectively, at 1 hour (T_max) after a single oral dose of 50 mg/kg in rat pharmacokinetic studies.

Animal study dosages reported in the peer-reviewed literature span a wide range. Nobiletin at 17 mg/kg/day for 16 weeks improved glucose tolerance, insulin resistance, and total cholesterol levels in mice fed a high-fat diet. In a 5-week study of obese diabetic mice, nobiletin 200 mg/kg/day was associated with improved hyperglycemia and insulin resistance. In neuroinflammation studies, mice were administered nobiletin by oral gavage at 100 mg/kg/day for 6 weeks. In the 3XTg-AD mouse model, chronic treatment with nobiletin at 10 or 30 mg/kg was evaluated.

6.4 Dosages in Human Studies

In the Japanese multicenter cognitive function trial, three capsules of the nobiletin-containing test food or placebo food were administered to each subject once a day for 16 weeks, in a randomized, double-blind, placebo-controlled trial. One capsule contained the dried powder extract providing 10.0 mg nobiletin and 5.8 mg tangeretin from C. depressa peel, along with other botanical extracts. Thus, the daily nobiletin dose in this trial amounted to 30 mg of nobiletin (across three capsules).

In the nocturia crossover clinical trial, nocturia patients aged ≥50 years received NoT (a nobiletin/tangeretin mixture) or a placebo at 50 mg once daily for 6 weeks.

Clinical trial data are lacking to support specific dosing recommendations for most indications, and dosages used in human studies have been confined to investigational supplement preparations tested over short durations.

6.5 Novel Delivery Approaches

New drug delivery systems, dosage forms, and structural modifications are being developed to overcome the limitations of poor absolute bioavailability of nobiletin. These include nanoemulsion formulations, self-nanoemulsifying drug delivery systems (SNEDDS), and transdermal delivery using ionic liquid systems such as CAGE.

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7. Safety Considerations and Drug Interactions

7.1 General Toxicity Profile

Nobiletin (NOB), a hexamethoxyflavonoid derived from the citrus pomace, is described as a non-toxic constituent of dietary phytochemicals approved by the Food and Drug Administration. Nobiletin, as a compound with high fat solubility, high bioavailability, and low toxicity, has been extensively studied. At tested concentrations in the published literature, nobiletin generally does not exhibit significant cytotoxicity in normal cell lines, and animal studies have not identified acute toxicity at tested doses.

In vitro studies with nobiletin at non-toxic concentrations (10 and 25 µM) in THLE-2 human hepatocytes and primary human cholangiocytes confirmed its low cytotoxicity in these hepatic cell types.

7.2 Human Safety Data from Clinical Trials

The human clinical data on nobiletin are still in their early stages. Most studies have been small in scale and of short duration. However, the existing evidence provides some insight into its safety profile in humans. In general, the available human studies suggest that nobiletin is well-tolerated at moderate doses for short-term use. The side effects reported are typically mild and often occur at a similar frequency to the placebo group.

Based on the limited human data available, no specific dosing recommendation is supported, and information regarding safety and efficacy in pregnancy and lactation is lacking.

7.3 Cytochrome P450 Enzyme Interactions

A substantively important safety consideration for nobiletin relates to its potential to modulate drug-metabolizing enzymes. CYP1A2 was profoundly inhibited by nobiletin, sinensetin, and tangeretin in vitro. Nobiletin and sinensetin activated the aryl hydrocarbon receptor (AhR) and thus induced CYP1A1/CYP1A2 expression. Tangeretin was a PXR activator and thus induced CYP3A4 and ABCB1 expression.

The flavonoid fraction of clementine juice provoked induction of several genes and inhibition of both CYP3A4 and CYP1A2, matching effects observed with whole clementine juice. CYP1A2 inhibition and induction can most likely be attributed to nobiletin, sinensetin, and tangeretin.

The interaction between co-administered compounds and nobiletin can be induced by the inhibition of CYP3A4, which should draw special attention in clinical co-administration scenarios. These interactions have been demonstrated in vitro and in animal pharmacokinetic models. The magnitude of these interactions in humans at supplemental doses has not been systematically quantified in dedicated clinical pharmacokinetic interaction studies. Medications metabolized by CYP1A2 and CYP3A4 represent a theoretical concern based on existing in vitro data.

7.4 Pregnancy and Lactation

Information regarding safety and efficacy in pregnancy and lactation is lacking, and use should be avoided in these populations absent adequate human safety data.

7.5 Overall Evidence Characterization

The safety database for nobiletin in humans is small. The two completed human randomized controlled trials (the Japanese cognitive function study and the nocturia crossover study) both reported no notable safety signals at the doses used, but both were of limited size and duration. Long-term safety, effects in vulnerable populations, and the full spectrum of potential drug interactions in humans remain to be established through larger, longer-duration trials.

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8. Summary of Evidence Strength

• Neuroprotection / Cognitive Function: Robust preclinical evidence across multiple animal models of Alzheimer's disease and neuroinflammation; one small human RCT with a multi-ingredient supplement suggesting benefit on memory in healthy elderly subjects. Evidence in humans is preliminary and not specific to isolated nobiletin.
• Metabolic Health / Obesity / Diabetes: Extensive preclinical evidence; human clinical data absent specifically for nobiletin. Evidence characterized as preliminary.
• Liver Disease / NAFLD: Strong preclinical evidence; no human RCT data available. Exploratory.
• Anticancer: Extensive in vitro and animal xenograft data covering multiple cancer types; no human clinical trials. Evidence is exploratory and cannot be applied clinically.
• Cardiovascular: Preclinical evidence; no dedicated human trials. Exploratory.
• Circadian Rhythms / Nocturia: Mechanistic preclinical evidence; one human crossover RCT with nobiletin/tangeretin mixture showing reduction in secondary (not primary) endpoint of nighttime frequency. Evidence is preliminary.
• Anti-inflammatory: Consistent preclinical evidence across many models and cell types; no dedicated human anti-inflammatory trials with isolated nobiletin.
• Safety: Generally regarded as low toxicity in animal and short-term human studies; CYP enzyme interactions are documented in vitro, and long-term human safety data are absent.

References

• The Multifunctional Effects of Nobiletin and Its Metabolites In Vivo and In Vitro — PMC/PubMed
• Natural sources, refined extraction, biosynthesis, metabolism, and bioactivities of dietary polymethoxyflavones (PMFs) — ScienceDirect / PharmaNutrition
• Natural sources, refined extraction, biosynthesis, metabolism, and bioactivities of dietary polymethoxyflavones (PMFs) — Food Science and Human Wellness
• Nobiletin from citrus peel: a promising therapeutic agent for liver disease — Frontiers in Pharmacology / PMC
• Nobiletin from citrus peel: a promising therapeutic agent for liver disease — Frontiers in Pharmacology (Full Text)
• The Citrus Flavone Nobiletin Reduces Pro-Inflammatory and Pro-Labour Mediators in Fetal Membranes and Myometrium — PMC
• The Application of Citrus folium in Breast Cancer and the Mechanism of Its Main Component Nobiletin: A Systematic Review — PMC
• Nobiletin, a citrus polymethoxyflavone, enhances the effects of bicalutamide on prostate cancer cells — PMC
• Comparison of Nobiletin and 5-Demethylnobiletin as Cancer Chemopreventive Agents — European Journal of Cancer Care
• Citrus flavonoids in fruit and traditional Chinese medicinal food ingredients in China — PubMed
• Chenpi — Wikipedia
• Nobiletin, a citrus flavonoid, improves cognitive impairment and reduces soluble Aβ levels in a triple transgenic mouse model of Alzheimer's disease (3XTg-AD) — Behavioural Brain Research / ScienceDirect
• Nobiletin Ameliorates Alzheimer's Disease Pathology via AMPK/SIRT1/PGC-1α and PI3K/Akt–CREB–BDNF Pathways in 5XFAD Mice — Biomedicines / MDPI
• Beneficial effects of a nobiletin-rich formulated supplement of Sikwasa (C. depressa) peel on cognitive function in elderly Japanese subjects; A multicenter, randomized, double-blind, placebo-controlled study — Food Science & Nutrition / PMC
• Nobiletin Ameliorates NLRP3 Inflammasome-Mediated Inflammation Through Promoting Autophagy via the AMPK Pathway — Molecular Neurobiology / Springer
• Nobiletin Protects against Systemic Inflammation-Stimulated Memory Impairment via MAPK and NF-κB Signaling Pathways — Journal of Agricultural and Food Chemistry / ACS
• Nobiletin Ameliorates Hepatic Lipid Deposition, Oxidative Stress, and Inflammation via the Nrf2/NF-κB Axis in NAFLD — Journal of Agricultural and Food Chemistry / ACS
• Nobiletin Inhibits IL-1β-Induced Inflammation in Chondrocytes via Suppression of NF-κB Signaling and Attenuates Osteoarthritis in Mice — PMC
• Effectiveness and Safety of a Mixture of Nobiletin and Tangeretin in Nocturia Patients: A Randomized, Placebo-Controlled, Double-Blind, Crossover Study — PMC
• Nobiletin affects circadian rhythms and oncogenic characteristics in a cell-dependent manner — PMC / PLOS ONE
• ROR activation by Nobiletin enhances antitumor efficacy via suppression of IκB/NF-κB signaling in triple-negative breast cancer — PMC
• Comparative Analyses of Bioavailability, Biotransformation, and Excretion of Nobiletin in Lean and Obese Rats — PubMed
• The pharmacokinetic study on the interaction between nobiletin and anemarsaponin BII in vivo and in vitro — PMC
• Nobiletin, sinensetin, and tangeretin are the main perpetrators in clementines provoking food-drug interactions in vitro — Food Chemistry / PubMed
• A review on recent advances on nobiletin in central and peripheral nervous system diseases — PMC
• Modulation of Fat Deposition–Gut Interactions in Obese Mice by Administrating with Nobiletin — PMC
• Nobiletin Enhances Skeletal Muscle Mass and Modulates Bile Acid Composition in Diet-Induced Obese Mice — PMC
• Recent advances in the therapeutic potential of nobiletin against respiratory diseases — Phytomedicine / ScienceDirect
• Nobiletin, as a Novel PDE4B Inhibitor, Alleviates Asthma Symptoms by Activating the cAMP-PKA-CREB Signaling Pathway — PMC
• Nobiletin, a hexamethoxyflavonoid from citrus pomace, attenuates G1 cell cycle arrest and apoptosis in hypoxia-induced human trophoblast cells — Food & Nutrition Research / PMC
• Efficacy of nobiletin in improving hypercholesterolemia and nonalcoholic fatty liver disease in high-cholesterol diet-fed mice — PMC
• Bioguided Identification of Polymethoxyflavones as Novel Vascular CaV1.2 Channel Blockers from Citrus Peel — PMC
• Nobiletin Attenuates Inflammation and Modulates Lipid Metabolism in an In Vitro Model of Intestinal Failure-Associated Liver Disease — Pharmaceutics / PMC
• Nobiletin Uses, Benefits & Dosage — Drugs.com Natural Products (citing NCBI)