Synthesis of regioselectively acylated quercetin analogues with improved antiplatelet activity

Metabolic, toxicological, chemical, and commercial perspectives on esterification of dietary polyphenols: a review

Molecular modifications have been practiced for more than a century and nowadays they are widely applied in food, pharmaceutical, or other industries to manipulate the physicochemical, bioactivity, metabolic/catabolic, and pharmacokinetic properties. Among various structural modifications, the esterification/O-acylation has been well-established in altering lipophilicity and bioactivity of parent bioactive compounds, especially natural polyphenolics, while maintaining their high biocompatibility. Meanwhile, various classic chemical and enzymatic protocols and other recently emerged cell factory technology are being employed as viable esterification strategies. In this contribution, the main motivations of phenolic esterification, including the tendency to replace synthetic alkyl phenolics with safer alternatives in the food industry to improve the bioavailability of phenolics as dietary supplements/pharmaceuticals, are discussed. In addition, the toxicity, metabolism, and commercial application of synthetic and natural phenolics are briefly introduced. Under these contexts, the mechanisms and reaction features of several most prevalent chemical and enzymatic esterification pathways are demonstrated. In addition, insights into the studies of esterification modification of natural phenolic compounds and specific pros/cons of various reaction systems with regard to their practical application are provided.

Strategy for the Enzymatic Acylation of the Apple Flavonoid Phloretin Based on Prior α-Glucosylation

The acylation of flavonoids serves as a means to alter their physicochemical properties, enhance their stability, and improve their bioactivity. Compared with natural flavonoid glycosides, the acylation of nonglycosylated flavonoids presents greater challenges since they contain fewer reactive sites. In this work, we propose an efficient strategy to solve this problem based on a first α-glucosylation step catalyzed by a sucrose phosphorylase, followed by acylation using a lipase. The method was applied to phloretin, a bioactive dihydrochalcone mainly present in apples. Phloretin underwent initial glucosylation at the 4'-OH position, followed by subsequent (and quantitative) acylation with C8, C12, and C16 acyl chains employing an immobilized lipase from Thermomyces lanuginosus. Electrospray ionization-mass spectrometry (ESI-MS) and two-dimensional nuclear magnetic resonance spectroscopy (2D-NMR) confirmed that the acylation took place at 6-OH of glucose. The water solubility of C8 acyl glucoside closely resembled that of aglycone, but for C12 and C16 derivatives, it was approximately 3 times lower. Compared with phloretin, the radical scavenging capacity of the new derivatives slightly decreased with 2,2-diphenyl-1-picrylhydrazyl (DPPH) and was similar to 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS•+). Interestingly, C12 acyl-α-glucoside displayed an enhanced (3-fold) transdermal absorption (using pig skin biopsies) compared to phloretin and its α-glucoside.

Hydroxyl Group Acetylation of Quercetin Enhances Intracellular Absorption and Persistence to Upregulate Anticancer Activity in HepG2 Cells

Quercetin, a flavonoid compound widely distributed in many plants, is known to have potent antitumor effects on several cancer cells. Our previous study revealed that the acetylation of quercetin enhanced its antitumor effect. However, the mechanisms remain unknown. This study aimed to elucidate the bioavailability of acylated quercetin in the HepG2 cell model based on its antitumor effect. The positions of quercetin 3,7,3',4'-OH were acetylated as 3,7,3',4'-O-tetraacetylquercetin (4Ac-Q). The inhibitory effect of 4Ac-Q on HepG2 cell proliferation was assessed by measuring cell viability. The apoptosis was characterized by apoptotic proteins and mitochondrial membrane potential shifts, as well as mitochondrial reactive oxygen species (ROS) levels. The bioavailability of 4Ac-Q was analyzed by measuring the uptake and metabolites in HepG2 cells with high performance liquid chromatography (HPLC)-photodiode array detector (PDA) and-ultraviolet/visible detector (UV/Vis). The results revealed that 4Ac-Q enhanced the inhibitory effect on HepG2 cell proliferation and induced its apoptosis significantly higher than quercetin. Protein array analysis of apoptosis-related protein indicated that 4Ac-Q increased the activation or expression of pro-apoptotic proteins, including caspase-3, -9, as well as second mitochondria-derived activator of caspases (SMAC), and suppressed the expression of apoptosis inhibiting proteins such as cellular inhibitor of apoptosis (cIAP)-1, -2, Livin, Survivin, and X-linked inhibitor of apoptosis (XIAP). Furthermore, 4Ac-Q stimulated mitochondrial cytochrome c release into the cytosol by enhancing ROS level and depolarizing the mitochondrial membrane. Finally, the analysis of uptake and metabolites of 4Ac-Q in HpG2 cells with HPLC-PDA and -UV/Vis revealed that 4Ac-Q was metabolized to quercetin and several different acetylated quercetins which caused 2.5-fold higher quercetin present in HepG2 cells than parent quercetin. These data demonstrated that acetylation of the quercetin hydroxyl group significantly increased its intracellular absorption. Taken together, our findings provide the first evidence that acetyl modification of quercetin not only substantially augments the intracellular absorption of quercetin but also bolsters its metabolic stability to elongate its intracellular persistence. Therefore, acetylation could serve as a strategic approach to enhance the ability of quercetin and analogous flavonoids to suppress cancer cell proliferation.

Synthesis and physicochemical characterization of acyl myricetins as potential anti-neuroexocytotic agents

Acyl myricetins (monopropionyl-, dipropionyl-, and monooctanoyl-myricetin, termed as MP₁, MP₂, and MO₁, respectively) were synthesized through enzymatic or non-enzymatic esterification reaction of myricetin aglycone. Structure study indicated the hydroxyl group at C4' in B-ring was highly susceptible to acylation. Over its parental myricetin, acylated compounds showed enhanced lipophilicity (from 7.4- to 26.3-fold) and oxidative stability (from 1.9- to 3.1-fold) on the basis of logP and decay rate, respectively. MO₁, presenting the physicochemical superiority compared to the others, provided lowest EC₅₀ value of 2.51 μM on inhibition of neutrotransmitter release and CC₅₀ value of 59.0 μM, leading to widest therapeutic window. All myricetin esters did not show any irritation toxicity when assessed with a chicken embryo assay. This study describes information on acylation of myricetin that has not yet been explored, and suggests that MO₁ has membrane fusion-arresting and anti-neuroexocytotic potential for industrial application due to its enhanced biological properties.

Development of a Novel Apigenin Dosage form as a Substitute for the Modern Triple Antithrombotic Regimen

The simultaneous administration of three antiplatelet agents has been proposed as an efficient strategy for the secondary prevention of atherothrombotic events and is included in the European guidelines. However, this strategy presented an increased risk of bleeding; therefore, the identification of new antiplatelet agents, with improved efficacy and diminished side effects, is of great importance. In silico studies, UPLC/MS Q-TOF plasma stability, in vitro platelet aggregation experiments, and pharmacokinetic studies were exploited. In the present study, it has been predicted that the flavonoid apigenin could target different platelet activation pathways, including P2Y12, protease-activated receptor-1 (PAR-1), and cyclooxygenase 1 (COX-1). To enhance apigenin's potency, hybridization with docosahexaenoic acid (DHA) was performed, as fatty acids have illustrated potent efficacy against cardiovascular diseases (CVDs). The new molecular hybrid, termed 4'-DHA-apigenin, demonstrated enhanced inhibitory activity against platelet aggregation induced by thrombin receptor activator peptide-6 (TRAP-6), adenosine diphosphate (ADP), and arachidonic acid (AA), with respect to the parent apigenin. The 4'-DHA-apigenin hybrid illustrated an almost 2-fold enhanced inhibitory activity, with respect to apigenin, and an almost 3-fold enhanced inhibitory activity, with respect to DHA, for the ADP-induced platelet aggregation. Additionally, the hybrid presented a more than 12-fold enhanced inhibitory activity with respect to DHA for the TRAP-6 induced platelet aggregation. Furthermore, a 2-fold enhanced inhibitory activity was recorded for the 4'-DHA-apigenin hybrid for the AA-induced platelet aggregation with respect to apigenin. To surmount the reduced LC-MS based plasma stability, a novel dosage form in olive oil has been developed. The 4'-DHA-apigenin olive oil-based formulation presented an enhanced antiplatelet inhibitory effect in three activation pathways. To further explore the pharmacokinetic profile of 4'-DHA-apigenin in olive oil formulations, a UPLC/MS Q-TOF protocol has been established to quantify the serum levels of apigenin after oral administration to C57BL/6J wild type mice. The olive oil-based formulation of 4'-DHA-apigenin demonstrated an increase in apigenin bioavailability of 262 %. This study may offer a new therapeutic strategy tailored to improve the treatment of CVDs.

Preparation of a Regioselective Quercetin-3-palmitate and Its Using for Boosting Cooking Oil Stability

Due to over worldwide use of frying oil, and due to its oxidation and deterioration after the usage for short time, huge oil amounts are wasted. So, most attempts are aimed to increase oil stability. Quercetin is a common name for the lipophobic strong natural phenolic antioxidant 2-(3,4-Dihydroxyphenyl)-5,7-dihydroxy-4H-1-benzopyran-4-one. Thus, its solubility had been improved by converting it to a lipophilic compound via its appending to a fatty acid residue. So, regioselectively 2-(3,4-Dihydroxyphenyl)-5,7-dihydroxy-3-[Hexadecanyl]oxychromen-4-one commonly named as (Quercetin-3-palmitate) was synthesized. The formed compound was confirmed based on its elemental analysis and spectral data (IR, ¹H NMR and MS). The partition coefficient of Quercetin-3-palmitate in octanol/water (k) was determined and compared to that of palmitic acid and quercetin individually to prove its solubility enhancement. Its radical scavenging activity was then tested. The effect of this new antioxidant compound on the oil stability was studied through the frying process. All physical and chemical parameters of this oil were considered before and after the frying process compared to another reference antioxidant (TBHQ) and control sample. The safety of this compound was determined by acute oral toxicity using albino mice. The liver and kidney functions of these mice were also examined. The results showed non-significant change. A sensory evaluation of the fried potato chips has been done. The results showed that the properties of the potato chips were improved by adding Quercetin-3-palmitate to the oil. Thus, good protection against frying oils oxidation was achieved via the addition of Quercetin-3-palmitate. The Quercetin-3-palmitate effectiveness is mainly attributed to its stability at high temperatures. Moreover, Quercetin-3-palmitate was found to be a safe compound according to an acute lethal toxicity test. Consequently, it can be used as a food additive.

Effects of Walnut Bark Extract on the Human Platelet Aggregation, Adhesion, and Plasmatic Coagulation In Vitro

Thrombosis is the formation of a clot within a blood vessel. Antithrombotic drugs are used for treating thrombosis, which can be the cause of hemorrhage. Currently, there is a need to discover novel antithrombotic drugs. Walnut is widely used to treat a wide range of health complaints. In this study, walnut bark extract was tested in hemostasis parameters: platelets adhesion, aggregation, and plasmatic coagulation in human blood. The crude aqueous extract of walnut bark was prepared by infusion and tested in vitro on hemostasis. Through blood collection from healthy volunteer donors, we studied different parameters of the primary hemostasis: platelet adhesion on the collagen-coated surface under flow, ADP, collagen, thrombin, and arachidonic acid-induced platelet aggregation, and of the secondary hemostasis by measuring prothrombin time (PT) and activated partial thromboplastin (APTT) parameters. All experiments are realized in the absence and presence of the extract and repeated at least twice. The obtained data showed that the extract (1 and 2 mg/mL) significantly (p < 0.001) reduced the activated platelet adhesion on the collagen-coated surface. In the same way, the effect of the extract on platelet aggregation seems to depend on its concentration and on the nature of the agonist. The strongest inhibition of aggregation was observed in the case of collagen at 1 mg/mL, while there was no observed effect on arachidonic acid-induced aggregation. Moreover, the extract (1 mg/mL) affects the extrinsic, intrinsic, and common pathways of the human blood coagulation cascade by extending significantly (p < 0.001), both PT and APTT times. This study provides evidence that walnut bark extract, by its antiadhesive, antiaggregant, and anticoagulant activities, could be considered as a serious source of biological compounds for the prevention and treatment of thrombosis.

Quercetin Fatty Acid Monoesters (C2:0-C18:0): Enzymatic Preparation and Antioxidant Activity

Quercetin monoesters were prepared via a one-step enzymatic transesterification. The main acylation products were eight quercetin ester derivatives, respectively, consisting of varying acyl groups ranging from 2 to 18 carbon atoms (acetate, butyrate, caproate, caprylate, caprate, laurate, myristate, and stearate). The purified quercetin esters were structurally characterized by LC-ESI-ToF and NMR HSQC. Meanwhile, several classical chemical (DPPH, ABTS, FRAP, and Fe²⁺ chelation assays), food (β-carotene bleaching assay), and biological (LDL and DNA oxidation assays) models were constructed to evaluate and systematically compare their antioxidant efficacy. O-Acylation increased the lipophilicity of quercetin derivatives, and lipophilicity increased with the increasing chain length of the acyl group. The dual effect of the acyl chain length on biasing quercetin monoesters' antioxidant efficacies has been summarized and verified. Overall, the results imply that the acylated quercetin have great potential as functional/health-beneficial ingredients for use in lipid-based matrices of cosmetics, supplements, and nutraceuticals.

Unveiling Natural and Semisynthetic Acylated Flavonoids: Chemistry and Biological Actions in the Context of Molecular Docking

Acylated flavonoids are widely distributed natural metabolites in medicinal plants and foods with several health attributes. A large diversity of chemical structures of acylated flavonoids with interesting biological effects was reported from several plant species. Of these, 123 compounds with potential antimicrobial, antiparasitic, anti-inflammatory, anti-nociceptive, analgesic, and anti-complementary effects were selected from several databases including SCI-Finder, Scopus, Google Scholar, Science Direct, PubMed, and others. Some selected reported biologically active flavonoids were docked in the active binding sites of some natural enzymes, namely acetylcholinesterase, butyrylcholinesterase, α-amylase, α-glucosidase, aldose reductase, and HIV integrase, in an attempt to underline the key interactions that might be responsible for their biological activities.

Modification of flavonoids: methods and influences on biological activities

Flavonoids are important active ingredients in plant-based food, which have many beneficial effects on health. But the low solubility, poor oral bioavailability, and inferior stability of many flavonoids may limit their applications in the food, cosmetics, and pharmaceutical industries. Structural modification can overcome these shortcomings to improve and extend the application of flavonoids. The study of how to modify flavonoids and the influence of various modifications on biological activity have drawn great interest in the current literature. In this review, the working principles and operating conditions of modification methods were summarized along with their potential and limitations in terms of operational safety, cost, and productivity. The influence of various modifications on biological activities and the structure-activity relationships of flavonoids derivatives were discussed and highlighted, which may give guidance for the synthesis of highly effective active agents. In addition, the safety of flavonoids derivatives is reviewed, and future research directions of flavonoid modification research are discussed.

Flavonoids: Antiplatelet Effect as Inhibitors of COX-1

Flavonoids are compounds with a benzopyranic structure that exhibits multiple pharmacological activities. They are known for their venotonic activity, but their mechanism of action remains unclear. It is thought that, as this mechanism is mediated by prostaglandins, these compounds may interfere with the arachidonic acid (AA) cascade. These assays are designed to measure the antiplatelet aggregation capacity of quercetin, rutin, diosmetin, diosmin, and hidrosmin, as well as to evaluate a potential structure−activity ratio. In this paper, several studies on platelet aggregation at different concentrations (from 0.33 mM to 1.5 mM) of different flavone compounds are conducted, measuring platelet aggregation by impedance aggregometry, and the cyclooxygenase (COX) activity by metabolites generated, including the activity of the pure recombinant enzyme in the presence of these polyphenols. The results obtained showed that quercetin and diosmetin aglycones have a greater antiplatelet effect and inhibit the COX enzyme activity to a greater extent than their heterosides; however, the fact that greater inhibition of the pure recombinant enzyme was achieved by heterosides suggests that these compounds may have difficulty in crossing biological membranes. In any case, in view of the results obtained, it can be concluded that flavonoids could be useful as coadjuvants in the treatment of cardiovascular pathologies.

Human Sirtuin Regulators: The "Success" Stories

The human sirtuins are a group of NAD⁺-dependent protein deacylases. They “erase” acyl modifications from lysine residues in various cellular targets including histones, transcription factors, and metabolic enzymes. Through these far-reaching activities, sirtuins regulate a diverse array of biological processes ranging from gene transcription to energy metabolism. Human sirtuins have been intensely pursued by both academia and industry as therapeutic targets for a broad spectrum of diseases such as cancer, neurodegenerative diseases, and metabolic disorders. The last two decades have witnessed a flood of small molecule sirtuin regulators. However, there remain relatively few compounds targeting human sirtuins in clinical development. This reflects the inherent issues concerning the development of isoform-selective and potent molecules with good drug-like properties. In this article, small molecule sirtuin regulators that have advanced into clinical trials will be discussed in details as “successful” examples for future drug development. Special attention is given to the discovery of these compounds, the mechanism of action, pharmacokinetics analysis, formulation, as well as the clinical outcomes observed in the trials.

Binding and antiplatelet activity of quercetin, rutin, diosmetin, and diosmin flavonoids

Platelets exert an essential role in vascular inflammation and thrombosis. Flavonoids are natural compounds employed for the clinical management of vascular disorders preventing capillary permeability, working as phlebotonics and improving the blood rheology, although their mechanism of action remains partially unknown. The effects of quercetin, rutin, diosmetin and diosmin were investigated in platelet activation utilizing blood from healthy and non-treated volunteers. The arrangement of the different activation states of platelets and GPIIb/IIIa receptor occupation was computed by flow cytometry working with calcium ionophore as pro-aggregant to provoke platelet activation and aggregation. The flavonoids studied demonstrated relevant antiplatelet activity through the blocked of GPIIb/IIIa receptors, the suppression of the platelet activation, as well as the pro-aggregate effect of calcium ionophore. Therefore, whichever of the active ingredients examined could be beneficious in the prevention of cardiovascular disease and this article also contributes to elucidate a new mechanism of action for these drugs.

Cocrystal of Apixaban-Quercetin: Improving Solubility and Bioavailability of Drug Combination of Two Poorly Soluble Drugs

Drug combinations have been the hotspot of the pharmaceutical industry, but the promising applications are limited by the unmet solubility and low bioavailability. In this work, novel cocrystals, consisting of two antithrombotic drugs with poor solubility and low bioavailability in vivo, namely, apixaban (Apx) and quercetin (Que), were developed to discover a potential method to improve the poor solubility and internal absorption of the drug combination. Compared with Apx, the dissolution behavior of Apx–Que (1:1) and Apx–Que–2ACN (1:1:2) was enhanced significantly, while the physical mixture of the chemicals failed to exhibit the advantages. The dissolution improvements of Apx–Que–2ACN could be explained by the fact that the solid dispersion-like structure and column-shaped cage of Que accelerated the access of the solvent to the inner layer of Apx. The fracture of the hydrogen bonds of Apx, which was the joint of the adjacent Que chains, facilitated the break-up of the structures. Besides, the bioavailability of Apx–Que was increased compared with the physical mixture and Apx, and Apx–Que remained stable in high temperature and illumination conditions. Therefore, a drug–drug cocrystal of two antithrombotic agents with poor solubility was developed, which exhibited greatly improved solubility, bioavailability and superior stability, indicating a novel method to overcome the shortages of drug combination.

Current methodologies to refine bioavailability, delivery, and therapeutic efficacy of plant flavonoids in cancer treatment

Over the last two decades, several advancements have been made to improve the therapeutic efficacy of plant flavonoids, especially in cancer treatment. Factors such as low bioavailability, poor flavonoid stability and solubility, ineffective targeted delivery, and chemo-resistance hinder the application of flavonoids in anti-cancer therapy. Many anti-cancer compounds failed in the clinical trials because of unexpected altered clearance of flavonoids, poor absorption after administration, low efficacy, and/or adverse effects. Hence, the current research strategies are focused on improving the therapeutic efficacy of plant flavonoids, especially by enhancing their bioavailability through combination therapy, engineering gut microbiota, regulating flavonoids interaction with adenosine triphosphate binding cassette efflux transporters, and efficient delivery using nanocrystal and encapsulation technologies. This review aims to discuss different methodologies with examples from reported dietary flavonoids that showed an enhanced anti-cancer efficacy in both in vitro and in vivo models. Further, the review discusses the recent progress in biochemical modifications of flavonoids to improve bioavailability, solubility, and therapeutic efficacy.

Synthetic Flavonoids as Novel Modulators of Platelet Function and Thrombosis

Cardiovascular diseases represent a major cause of mortality and morbidity in the world, and specifically, thrombotic conditions such as heart attacks and strokes are caused by unwarranted activation of platelets and subsequent formation of blood clots (thrombi) within the blood vessels during pathological circumstances. Therefore, platelets act as a primary therapeutic target to treat and prevent thrombotic conditions. Current treatments are limited due to intolerance, and they are associated with severe side effects such as bleeding complications. Hence, the development of novel therapeutic strategies for thrombotic diseases is an urgent priority. Flavonoids are naturally occurring plant-derived molecules that exert numerous beneficial effects in humans through modulating the functions of distinct cell types. However, naturally occurring flavonoids suffer from several issues such as poor solubility, lipophilicity, and bioavailability, which hinder their efficacy and potency. Despite these, flavonoids act as versatile templates for the design and synthesis of novel molecules for various therapeutic targets. Indeed, several synthetic flavonoids have recently been developed to improve their stability, bioavailability, and efficacy, including for the modulation of platelet function. Here, we provide insight into the actions of certain natural flavonoids along with the advantages of synthetic flavonoids in the modulation of platelet function, haemostasis, and thrombosis.