Potential Beneficial Effects of Naringin and Naringenin on Long COVID-A Review of the Literature

Coronavirus disease 2019 (COVID-19) caused a severe epidemic due to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Recent studies have found that patients do not completely recover from acute infections, but instead, suffer from a variety of post-acute sequelae of SARS-CoV-2 infection, known as long COVID. The effects of long COVID can be far-reaching, with a duration of up to six months and a range of symptoms such as cognitive dysfunction, immune dysregulation, microbiota dysbiosis, myalgic encephalomyelitis/chronic fatigue syndrome, myocarditis, pulmonary fibrosis, cough, diabetes, pain, reproductive dysfunction, and thrombus formation. However, recent studies have shown that naringenin and naringin have palliative effects on various COVID-19 sequelae. Flavonoids such as naringin and naringenin, commonly found in fruits and vegetables, have various positive effects, including reducing inflammation, preventing viral infections, and providing antioxidants. This article discusses the molecular mechanisms and clinical effects of naringin and naringenin on treating the above diseases. It proposes them as potential drugs for the treatment of long COVID, and it can be inferred that naringin and naringenin exhibit potential as extended long COVID medications, in the future likely serving as nutraceuticals or clinical supplements for the comprehensive alleviation of the various manifestations of COVID-19 complications.

Coumarin Derivatives Inhibit ADP-Induced Platelet Activation and Aggregation

Coumarin was first discovered in Tonka bean and then widely in other plants. Coumarin has an anticoagulant effect, and its derivative, warfarin, is a vitamin K analogue that inhibits the synthesis of clotting factors and is more widely used in the clinical treatment of endovascular embolism. At present, many artificial chemical synthesis methods can be used to modify the structure of coumarin to develop many effective drugs with low toxicity. In this study, we investigated the effects of six coumarin derivatives on the platelet aggregation induced by adenosine diphosphate (ADP). We found that the six coumarin derivatives inhibited the active form of GPIIb/IIIa on platelets and hence inhibit platelet aggregation. We found that 7-hydroxy-3-phenyl 4H-chromen-4-one (7-hydroxyflavone) had the most severe effect. In addition, we further analyzed the downstream signal transduction of the ADP receptor, including the release of calcium ions and the regulation of cAMP, which were inhibited by the six coumarin derivatives selected in this study. These results suggest that coumarin derivatives inhibit coagulation by inhibiting the synthesis of coagulation factors and they may also inhibit platelet aggregation.

Development of a new nano arginase HPLC capillary column for the fast screening of arginase inhibitors and evaluation of their binding affinity

A simple and rapid Nano LC method has been developed for the screening of arginase inhibitors. The method is based on the immobilization of biotinylated arginase on a neutravidin functionalized nano HPLC capillary column. The arginase immobilization step performed by frontal analysis is very fast and only takes a few minutes. The miniaturized capillary column of 170 nL (length 5 cm, internal diameter 75 μm) significantly decreased the required amount of used enzyme (25 pmol). This was of significance importance when working with less available or expensive purified enzyme. Non-selective adsorption of the organic monolith matrix was reduced (<6%) and the arginase efficient yield was high (92%). The resultant affinity capillary columns showed excellent repeatability and long lifetime. The arginase reaction product was achieved within 60 s and the immobilized arginase retained 97% of the initial activity beyond 90 days. This novel approach can thus be used for the fast evaluation of recognition assay induced bya series of inhibitor molecules (caffeic acid phenylamide, chlorogenic acid, piceatannol, nor-NOHA acetate) and plant extracts.

Naringenin Inhibits Platelet Activation and Arterial Thrombosis Through Inhibition of Phosphoinositide 3-Kinase and Cyclic Nucleotide Signaling

Citrus flavanoids intake can reduce the risk of cardiovascular diseases. Naringenin, a natural predominant flavonoid abundant in citrus fruits, possesses protective effects against atherothrombotic diseases. As platelet activation plays central roles in atherothrombogenesis, we studied the effects of naringenin on platelet activation, signaling, thrombosis and hemostasis. Naringenin dose-dependently inhibited agonist-induced platelet aggregation in vitro, and exhibited more-potent efficacy on ADP-induced platelet aggregation. It also suppressed platelet aggregation stimulated by ADP ex vivo. Naringenin inhibited ADP-induced platelet α-granule secretion, fibrinogen binding, intracellular calcium mobilization and platelet adhesion on collagen-coated surface. Naringenin also inhibited platelet spreading on fibrinogen and clot retraction, processes mediated by outside-in integrin signaling. Mechanism studies indicated that naringenin suppressed PI3K-mediated signaling and phosphodiesterase activity in platelets, in addition to increasing cGMP levels and VASP phosphorylation at Ser239. Furthermore, naringenin-induced VASP phosphorylation and inhibition of platelet aggregation were reversed by a PKA inhibitor treatment. Interestingly, naringenin inhibited thrombus formation in the (FeCl₃)-induced rat carotid arterial thrombus model, but not cause a prolonged bleeding time in mice. This study suggests that naringenin may represent a potential antiplatelet agent targeting PI3K and cyclic nucleotide signaling, with a low bleeding risk.

The role of phytochemicals in sepsis: A mechanistic and therapeutic perspective

Sepsis and septic shock are still a leading cause of mortality and morbidity in intensive care units worldwide. Sepsis is an uncontrolled and excessive response of the innate immune system toward the invading infectious microbes, characterized by the hyper-production of pro-inflammatory mediators such as interleukin (IL)-1β, IL-6, tumor-necrosis factor (TNF)-α, and high-mobility group box 1 (HMGB1). In severe sepsis, the overwhelming production of pro-inflammatory cytokines and reactive oxygen species may compromise organ function and lead to the induction of abnormal apoptosis in different organs, resulting in multiple organ dysfunction syndrome and death. Hence, compounds that are able to attenuate inflammatory responses may have therapeutic potential for sepsis treatment. Understanding the pathophysiology and underlying molecular mechanisms of sepsis may provide useful insights in the discovery and development of new effective therapeutics. Therefore, numerous studies have invested much effort into elucidating the mechanisms involved with the onset and development of sepsis. The present review mainly focuses on the molecules and signaling pathways involved in the pathogenicity of sepsis. Additionally, several well-known natural bioactive herbal compounds and phytochemicals, which have shown protective and therapeutic effects with regard to sepsis, as well as their mechanisms of action, are presented. This review suggests that these phytochemicals are able to attenuate the overwhelming inflammatory responses developed during sepsis by modulating different signaling pathways. Moreover, the anti-inflammatory and cytoprotective activities of phytochemicals make them potent compounds to be included as complementary therapeutic agents in the diets of patients suffering from sepsis in an effort to alleviate sepsis and its life-threatening complications, such as multi-organ failure.

Anticancer activities of phytoconstituents and their liposomal targeting strategies against tumor cells and the microenvironment

Various bioactive ingredients have been extracted from Chinese herbal medicines (CHMs) that affect tumor progression and metastasis. To further understand the mechanisms of CHMs in cancer therapy, this article summarizes the effects of five categories of CHMs and their active ingredients on tumor cells and the tumor microenvironment. Despite their treatment potential, the undesirable physicochemical properties (poor permeability, instability, high hydrophilicity or hydrophobicity, toxicity) and unwanted pharmacokinetic profiles (short half-life in blood and low bioavailability) restrict clinical studies of CHMs. Therefore, development of liposomes through relevant surface modifying techniques to achieve targeted CHM delivery for cancer cells, i.e., extracellular and intracellular targets and targets in tumor microenvironment or vasculature, have been reviewed. Current challenges of liposomal targeting of these phytoconstituents and future perspective of CHM applications are discussed to provide an informative reference for interested readers.

Chemical Fingerprint of Free Polyphenols and Antioxidant Activity in Dietary Fruits and Vegetables Using a Non-Targeted Approach Based on QuEChERS Ultrasound-Assisted Extraction Combined with UHPLC-PDA

Fruits and vegetables are considered a good source of antioxidants, which are beneficial in protecting the human body against damage induced by free radicals and other reactive oxygen (ROS) and nitrogen (RNS) species. In this work, we aimed to evaluate the integral antioxidant activity (AOA) and determine individual polyphenols in fruits and vegetables of frequent consumption. For this purpose, an innovative and high throughput analytical approach based on original QuEChERS assisted by ultrasound extraction (USAE), instead of the manual agitation used in the classical procedure, was optimized and implemented for the isolation of polyphenols. The total phenolic content (TPC), flavonoids, anthocyanins, and betalains were evaluated using different spectrophotometric assays. In addition, free radical scavenging by methods 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) and 2,2'-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) were used to estimate the AOA of the investigated fruit and vegetable extracts. Red onion, tamarillo, and beetroot were the samples with the highest AOA. The quantification and identification of free low molecular weight polyphenols from QuEChERS-USAE extracts was carried out by ultra-high-pressure liquid chromatography equipped with a photodiode array detection system (UHPLC-PDA). Catechin was the most abundant polyphenol, followed by gentisic and ferulic acids, mainly in the watercress sample. In relation to flavonols, quercetin and kaempferol were found mostly in onion samples, and in small quantities in tomato and watercress. The improved analytical approach, QuEChERS-USAE/UHPLC-PDA, offers an attractive alternative for the analysis of polyphenols from fruit and vegetable samples, providing several advantages over traditional extraction techniques, in terms of reproducibility, simplicity, low cost, analysis speed, and analytical performance.

Molecular Mechanisms of Leonurus Cardiaca L. Extract Activity in Prevention of Staphylococcal Endocarditis-Study on in Vitro and ex Vivo Models

Better understanding the mechanisms of Leonurus cardiaca L. extract (LCE) activity is necessary to prepare recommendations for the use of LCE-based herbal products for preventive/supportive purposes in case of infective endocarditis (IE) and other staphylococcal invasive infections. The aim of the study was to analyze molecular mechanisms of LCE effect on Staphylococcus aureus and blood platelets in the context of their interactions playing a pivotal role in such disorders. Using atomic force microscopy, we demonstrated that adhesion forces of S. aureus were markedly reduced after exposure to LCE at subinhibitory concentrations. The effect resulted from the impact of LCE on S. aureus cell morphology and the composition of phospholipids and fatty acids in bacterial membranes (assessed by HPLC), which modulated their stabilization, hydrophobicity, and charge. Moreover, using FACS we showed also that LCE significantly reduced GP IIb/IIIa expression on blood platelets, thus the disruption of platelet-fibrinogen interactions seems to explain antiplatelet effect of LCE. The obtained results prove the usefulness of LCE in the prevention of S. aureus adhesion, platelet activation, and vegetations development, however, also pointed out the necessity of excluding the cationic antibiotics from the treatment of S. aureus-associated IE and other invasive diseases, when motherwort herb is used simultaneously as an addition to the daily diet.

Binding of Plant Polyphenols to Serum Albumin and LDL: Healthy Implications for Heart Disease

Cardiovascular disease (CVD) is the leading cause of death in industrialized nations. The initiating event in atherosclerosis, or hardening of the arteries, is oxidation of low density lipoprotein (LDL). Binding with serum albumin and LDL of 41 polyphenols (major antioxidants in plant foods) constituting four classes of flavonoids, three types of phenolic acids, and seven polyphenol conjugate metabolites was investigated indirectly by fluorescence quenching and directly by affinity separation/high-performance liquid chromatography (four of the polyphenols). Stern-Volmer plots yielded K values for the two proteins. Polyphenol binding was significantly stronger for albumin than with LDL. K values were highly correlated with the lipophilicity of the polyphenols. The number of polyphenol molecules determined by quenching was ∼1 for both proteins. Direct analysis under saturation conditions yielded from 2 to 13 molecules of polyphenols/LDL particle. Multiple substituent effects on binding were analyzed. Evidence was put forward that binding of polyphenols to these proteins is protective for CVD by multiple mechanisms.

Inhibition of Human Platelet Aggregation and Low-Density Lipoprotein Oxidation by Premna foetida Extract and Its Major Compounds

Many Premna species have been used in traditional medicine to treat hypertension and cardiac insufficiency, and as a tonic for cardiac-related problems. Some have been reported to possess cardiovascular protective activity through several possible mechanisms, but not Premna foetida. In the present study, the methanol extract of P. foetida leaves (PFM) and its isolated compounds were evaluated for their ability to inhibit copper-mediated human low-density lipoprotein (LDL) oxidation and arachidonic acid (AA)- and adenosine diphosphate (ADP)-induced platelet aggregation. Six flavonoids, three triterpenoids, vanillic acid and stigmasterol were successfully isolated from PFM. Of the isolated compounds, quercetin was the most active against LDL oxidation (IC₅₀ 4.25 µM). The flavonols were more active than the flavones against LDL oxidation, suggesting that hydroxyl group at C-3 and the catechol moiety at B-ring may play important roles in protecting LDL from oxidation. Most tested flavonoids showed stronger inhibition towards AA-induced than the ADP-induced platelet aggregation with apigenin exhibiting the strongest effect (IC₅₀ 52.3 and 127.4 µM, respectively) while quercetin and kaempferol showed moderate activity. The results suggested that flavonoids, especially quercetin, apigenin and kaempferol were among the major constituents of P. foetida responsible for anti-LDL oxidation and anti-platelet aggregation.

Tailoring naringenin conjugates with amplified and triple antiplatelet activity profile: Rational design, synthesis, human plasma stability and in vitro evaluation

BACKGROUND

The current standard-of-care antiplatelet therapy in cardiovascular disease patients is consisted of cyclooxygenase-1 (COX-1) inhibitor aspirin, along with a platelet receptor P2Y₁₂ antagonist. Recently, the triple antiplatelet therapy with aspirin, a P2Y₁₂ receptor antagonist and a protease activated receptor-1 (PAR-1) antagonist, has been suggested for the secondary prevention of atherothrombotic events, however presented an increased risk of bleeding. Therefore, the quest for novel antiplatelet agents simultaneously targeting the three pathways with improved efficacy/safety profile is of immense importance. Flavonoids as pre-validated ligands for numerous targets could serve as scaffolds targeting the three platelet activation pathways.

METHODS

Computational methods, Ultra High Performance Liquid Chromatography-Tandem Mass Spectrometry (UHPLC-MS/MS) plasma stability and in vitro platelet aggregation experiments were used to establish the antiplatelet activity of the flavonoid naringenin and its conjugates.

RESULTS

In silico studies indicated that naringenin could bear a potent triple antiplatelet activity by inhibiting different platelet aggregation mechanisms. However, we found that in human platelets naringenin has diminished activity. We rationally designed and synthesized different naringenin conjugates aiming to amplify the antiplatelet activity of the parent compound. UHPLC-MS/MS revealed a slow degradation rate for a docosahexaenoic acid (DHA) - naringenin conjugate in human plasma. The antiplatelet profile of the new analogues was evaluated against in vitro platelet aggregation induced by several platelet agonists.

CONCLUSIONS

The DHA - naringenin hybrid presented triple antiplatelet activity simultaneously targeting PAR-1, P2Y₁₂ and COX-1 platelet activation pathways.

GENERAL SIGNIFICANCE

Natural products could offer a rich source for novel bioactives as a powerful alternative to the current combinatorial use of three different antiplatelet drugs.

Metformin-like antidiabetic, cardio-protective and non-glycemic effects of naringenin: Molecular and pharmacological insights

Metformin is a widely used drug for the treatment of type 2 diabetes (T2D). Its blood glucose-lowering effects are initially due to inhibition of hepatic glucose production and increased peripheral glucose utilization. Metformin has also been shown to have several beneficial effects on cardiovascular risk factors and it is the only oral antihyperglycaemic agent thus far associated with decreased macrovascular complications in patients with diabetes. Adenosine Monophosphate Activated-Protein Kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Recent evidence shows that pharmacological activation of AMPK improves blood glucose homeostasis, lipid profiles, blood pressure and insulin-resistance making it a novel therapeutic target in the treatment of T2D. Naringenin a flavonoid found in high concentrations as its glycone naringin in citrus fruits, has been reported to have antioxidant, antiatherogenic, anti- dyslipidemic and anti-diabetic effects. It has been shown that naringenin exerts its anti-diabetic effects by inhibition of gluconeogenesis through upregulations of AMPK hence metformin-like effects. Naringin has further been shown to have non-glycemic affects like metformin that mitigate inflammation and cell proliferation. This review evaluates the potential of naringenin as anti-diabetic, anti-dyslipidemic anti-inflammatory and antineoplastic agent similar to metformin and proposes its further development for therapeutic use in clinical practice.

Dietary flavonoids and modulation of natural killer cells: implications in malignant and viral diseases

Flavonoids are a large group of secondary plant metabolites present in the diet with numerous potentially health-beneficial biological activities. In addition to antioxidant, anti-inflammatory, cholesterol-lowering, and many other biological functions reported in the literature, flavonoids appear to inhibit cancer cell proliferation and stimulate immune function. Although the immunomodulatory potential of flavonoids has been intensively investigated, only little is known about their impact on natural killer (NK) cells. Enhancing NK cell activity, however, would have strong implications for a possible clinical use of flavonoids, especially in the treatment and prevention of diseases like cancer and viral infections. Therefore, the purpose of this review is to summarize the currently available information on NK cell modulation by flavonoids. Many of the structurally diverse flavonoids stimulate NK cell activity and have thus great potential as diet-derived immune-modulatory chemopreventive agents and may even serve as therapeutic compounds or lead structures for the development of novel drugs for the treatment of both malignant and viral diseases.

Anti-platelet and anti-thrombogenic effects of shikimic acid in sedentary population

This ex vivo study was performed to evaluate the anti-platelet and anti-thrombogenic potential of shikimic acid (SA), a plant phenolic metabolite. Fasting blood samples were collected from 22 sedentary participants to analyse the effect of varying concentrations of SA (0.1 mM, 0.2 mM, 0.5 mM, 1 mM and 2 mM) on platelet surface-marker expression, platelet aggregation and biomarkers of thrombogenesis. Monocyte-platelet aggregates (CD14/CD42b) and platelet endothelial cell adhesion molecule-1 (PECAM-1 or CD31), effective indicators of thrombus formation were evaluated. Procaspase-activating compound 1 (PAC-1) and P-selectin or CD62P were used to assess platelet activation-related thrombogenesis. Adenosine diphosphate (ADP) was used to stimulate the P2Y1/P2Y12 pathway of platelet activation to mimic the in vivo thrombogenic pathway. Platelet aggregation studies utilised both ADP and collagen as exogenous platelet agonists to target both P2Y1/P2Y12 and GPVI pathways of thrombus formation. It was observed with flow cytometry that SA produced a significant antiplatelet effect on PAC-1 (p = 0.03 at 2 mM) and CD62P (p = 0.017, p = 0.036 at 1 mM and 2 mM respectively) expression in addition to lowering monocyte-platelet aggregate formation (p = 0.013, p < 0.01 and p < 0.01 at 0.5 mM, 1 mM and 2 mM respectively). SA at 1 mM concentration reduced PECAM-1 expression (p = 0.035), signifying a reduction to endothelial leucocyte migration during thrombus growth. SA did not demonstrate a platelet aggregation inhibitory effect by targeting the GPVI collagen pathway but reduced ADP induced platelet aggregation at 2 mM concentration (p < 0.01 at 2 mM). The results suggest that SA, an active metabolite of polyphenol-rich food intake, could play an important role in reducing platelet activation, aggregation related thrombus formation and biomarkers of thrombogenesis in sedentary individuals.

Pentamethylquercetin (PMQ) reduces thrombus formation by inhibiting platelet function

Flavonoids exert both anti-oxidant and anti-platelet activities in vitro and in vivo. Pentamethylquercetin (PMQ), a polymethoxylated flavone derivative, has been screened for anti-carcinogenic and cardioprotective effects. However, it is unclear whether PMQ has anti-thrombotic effects. In the present study, PMQ (20 mg/kg) significantly inhibited thrombus formation in the collagen- epinephrine- induced acute pulmonary thrombosis mouse model and the ferric chloride-induced carotid injury model. To explore the mechanism, we evaluated the effects of PMQ on platelet function. We found that PMQ inhibited platelet aggregation and granule secretion induced by low dose agonists, including ADP, collagen, thrombin and U46619. Biochemical analysis revealed that PMQ inhibited collagen-, thrombin- and U46619-induced activation of Syk, PLCγ2, Akt, GSK3β and Erk1/2. Therefore, we provide the first report to show that PMQ possesses anti-thrombotic activity in vivo and inhibited platelet function in vitro, suggesting that PMQ may represent a potential therapeutic candidate for the prevention or treatment of thrombotic disorders.

Apigenin Attenuates Atherogenesis through Inducing Macrophage Apoptosis via Inhibition of AKT Ser473 Phosphorylation and Downregulation of Plasminogen Activator Inhibitor-2

Macrophage survival is believed to be a contributing factor in the development of early atherosclerotic lesions. Dysregulated apoptosis of macrophages is involved in the inflammatory process of atherogenesis. Apigenin is a flavonoid that possesses various clinically relevant properties such as anti-inflammatory, antiplatelet, and antitumor activities. Here we showed that apigenin attenuated atherogenesis in apoE (-/-) mice in an in vivo test. In vitro experiments suggested that apigenin induced apoptosis of oxidized low density lipoprotein- (OxLDL-) loaded murine peritoneal macrophages (MPMs). Proteomic analysis showed that apigenin reduced the expression of plasminogen activator inhibitor 2 (PAI-2). PAI-2 has antiapoptotic effects in OxLDL-loaded MPMs. Enhancing PAI-2 expression significantly reduced the proapoptosis effects of apigenin. Molecular docking assay with AutoDock software predicted that residue Ser473 of Akt1 is a potential binding site for apigenin. Lentiviral-mediated overexpression of Akt1 wild type weakened the proapoptosis effect of apigenin in OxLDL-loaded MPMs. Collectively, apigenin executes its anti-atherogenic effects through inducing OxLDL-loaded MPMs apoptosis. The proapoptotic effects of apigenin were at least partly attributed to downregulation of PAI-2 through suppressing phosphorylation of AKT at Ser473.

Endogenous and exogenous mediators of quercetin bioavailability

Quercetin is a dietary flavonol that has poor and highly variable bioavailability. Epidemiological studies suggest that higher dietary intakes of quercetin decease cardiovascular disease (CVD) risk. However, experimental findings examining its cardioprotective activities are inconsistent, thereby precluding a full understanding of its health benefits. Bioavailability of dietary constituents is a critical mediator of their health benefits. Thus, a better understanding of the factors regulating quercetin bioavailability is expected to support its potential role in managing CVD risk. This review provides an update on the evidence describing endogenous and exogenous factors responsible for the limited and highly variable bioavailability of quercetin. It focuses on pharmacokinetics studies in clinical and animal models, while also describing strategies aimed at improving quercetin bioavailability to better realize its cardioprotective activities in vivo that are routinely observed in vitro. Although significant advances have been made in understanding determinants of quercetin bioavailability, additional research in controlled trials is needed to more comprehensively examine dose-response effects, whether its cardioprotective activities improve in response to its greater bioavailability, and if the putative health benefits of quercetin are mediated directly or indirectly from one or more of its metabolites generated during xenobiotic metabolism.

Dietary manipulation of platelet function

Activated platelets contribute to plaque formation within blood vessels in the early and late stages of atherogenesis, and therefore they have been proposed as risk factor for cardiovascular disease. Anti-platelet drugs, such as aspirin, are now the most prescribed pharmacological treatment in Europe. Certain dietary bioactives also beneficially affect platelet function, and with less side effects, albeit that effects are generally more subtle. Therefore, consumption of dietary bioactives could play a role in the prevention of atherothrombotic vascular disease. Here we review the efficacy of dietary treatment strategies, especially those involving certain dietary fatty acids and polyphenols, to modulate platelet function in healthy subjects or in patients with cardiovascular disease. Variation in study populations, small study sizes and lack of comparability between methods to assess platelet function currently limit robust evidence on the efficacy of dietary bioactives in healthy subjects or specific patient groups. Also, limited knowledge of the metabolism of dietary bioactives, and therefore of the bioavailability of bioactive ingredients, restricts our ability to identify the most effective dietary regimes to improve platelet function. Implementation of uniform point-of-care tests to assess platelet function, and enhanced knowledge of the efficacy by which specific dietary compounds and their metabolites affect platelet function, may enable the identification of functional anti-platelet ingredients that are eligible for a health claim, or combined treatment strategies, including both pharmacological anti-platelet treatment as well as dietary intervention, to tackle atherothrombotic vascular disease.

Quercetin-containing self-nanoemulsifying drug delivery system for improving oral bioavailability

Quercetin is a dietary flavonoid with potential chemoprotective effects, but has low bioavailability because of poor aqueous solubility and low intestinal absorption. A quercetin-containing self-nanoemulsifying drug delivery system (Q-SNEDDS) was developed to form oil-in-water nanoemulsions in situ for improving quercetin oral bioavailability. On the basis of the quercetin solubility, emulsifying ability, and stability after dispersion in an aqueous phase, an optimal SNEDDS consisting of castor oil, Tween® 80, Cremophor® RH 40, and PEG 400 (20:16:34:30, w/w) was identified. Upon mixing with water, Q-SNEDDS formed a nanoemulsion having a droplet size of 208.8 ± 4.5 nm and zeta potential of -26.3 ± 1.2 mV. The presence of Tween® 80 and PEG 400 increased quercetin solubility and maintained supersaturated quercetin concentrations (5 mg/mL) for >1 month. The optimized Q-SNEDDS significantly improved quercetin transport across a human colon carcinoma (Caco-2) cell monolayer. Fluorescence imaging demonstrated rapid absorption of the Q-SNEDDS within 40 min of oral ingestion. Following oral administration of Q-SNEDDS in rats (15 mg/kg), the area under the concentration curve and maximum concentration of plasma quercetin after 24 h increased by approximately twofold and threefold compared with the quercetin control suspension. These data suggest that this Q-SNEDDS formulation can enhance the solubility and oral bioavailability of quercetin for appropriate clinical application.