Studies on the interactions between some flavonols and cyclodextrins

Long-Lived States Provide Insights from NMR into the β-Cyclodextrin Drug Assemblies

In the last two decades, extending spin memory in NMR has been used for several purposes. Long-lived states (LLS) or singlet states are one of the first spin memory enhancement techniques used. LLS have the potential to extract structural information and intra- and intermolecular interactions of complex systems other than studying slow phenomenon. The motional regime of β-cyclodextrin (β-CD) drug inclusion complexes generally lies in the intermediate region, where ωτ_c ≈ 1, and the standard methods of studying these interactions, i.e., NOE and chemical shift monitoring, suffer from insufficient output information. The sensitivity of LLS toward the environmental changes is utilized here to gain insights into the drug assemblies formed by β-CD. One can use change in relaxation of LLS to study the structural changes during complexation. The examples of β-CD with the drugs indomethacin, paracetamol, gliclazide, and CI-933 (a precursor 4-methoxybenzamide) were studied. Indomethacin, paracetamol, and 4-methoxybenzamide show strong interaction through the para-substituted benzene ring, unlike gliclazide. Relaxation of LLS in β-CD-drug complexes is modeled using standard Redfield Relaxation Theory. Computational studies performed support the experimental observations. Docking and molecular dynamics simulation provided the explanation of the relaxation properties of these drug molecules.

Mass Spectrometry as a Complementary Approach for Noncovalently Bound Complexes Based on Cyclodextrins

An important and well-designed solution to overcome some of the problems associated with new drugs is provided by the molecular encapsulation of the drugs in the cyclodextrins (CDs) cavity, yielding corresponding inclusion complexes (ICs). These types of non-covalent complexes are of current interest to the pharmaceutical industry, as they improve the solubility, stability and bioavailability of the guest molecules. This review highlights several methods for cyclodextrin ICs preparation and characterization, focusing mostly on the mass spectrometry (MS) studies that have been used for the detection of noncovalent interactions of CDs inclusion complexes and binding selectivity of guest molecules with CDs. Furthermore, the MS investigations of several ICs of the CD with antifungal, antioxidants or fluorescent dyes are presented in greater details, pointing out the difficulties overcome in the analysis of this type of compounds.

Quercetin and 3-O-methylquercetin in vitro skin layers permeation/retention from hydrogels: why only a methoxy group difference determines different behaviors?

OBJECTIVES

The present study was designed to verify if quercetin (QCT), a flavonoid with antioxidant and antiviral activity, and 3-O-methylquercetin (3OMQ), a quercetin C3-methoxylated derivative, present differences in their behavior against complexation with β-cyclodextrin (β-CD) and the corresponding permeation/retention trhough porcine ear skin, when incorporated into hydroxypropyl methylcellulose (HPMC) or chitosan (CS) hydrogels.

METHODS

The influence of β-CD on the skin permeation/retention of QCT and 3OMQ from hydrogels is comparatively evaluated for both flavonoids using porcine ear skin in Franz cells model. The properties of the two flavonoids using the semi-empirical method Recife Model was studied.

KEY FINDINGS

Quercetin presented higher skin retention compared with its C3-methoxy derivative 3OMQ. The best permeation/retention of QCT was observed when it was incorporated into CS hydrogel containing 5% β-CD, whereas, for 3OMQ, the HPMC hydrogel containing 5% β-CD was the best formulation. The flavonoids complexation with β-CD in water occurred preferentially with the insertion of the B ring through the secondary OH rim.

CONCLUSIONS

The dynamic molecular modeling revealed that the methyl group at C3 in 3OMQ molecule determined significant difference in its complexation with β-CD, in comparison to its analogous QCT and that difference is coincident with the permeation behavior of these flavonoids, denoting a possible relationship with their molecular dynamics.

Complexation of phytochemicals with cyclodextrin derivatives - An insight

Natural compounds have been attracting huge attention because of their broad therapeutic properties with specificity in their action in human health care as functional foods, pharmaceuticals and nutraceuticals. However poor bioavailability and reduced bioactivity attributed to poor solubility and instability is the major drawback hindering the incorporation of these therapeutically potential molecules in novel drug delivery systems. Based on the findings of reported research investigations; complexation of poorly water soluble phytochemicals with cyclodextrins has emerged to be a promising approach to improve their aqueous solubility, stability, rate of dissolution and bioavailability. The present article summarizes the encapsulation of natural compounds ranging from various flavonoids, phenolic derivatives, coumestans to triterpenes, with cyclodextrin and their derivatives. Also the article highlights the method of complexation, complexation ability, drug solubility, stability, bioavailability and safety aspects of reported natural compounds. Additionally we present the glimpses of patents published in recent 10-15 years to highlight the significance of inclusion of phytochemicals in cyclodextrins. In patents narrated, improvement in stability and solubility of curcumin by complexation with alkyl ether derivative of gamma-cyclodextrin is claimed. Another patent mentioned, complexation of artemisinins with β-cyclodextrin, improved the stability and integrity of peroxide part of artemisinins for long period. On the other hand the complex of dihydromyricetin with γ-CD has shown improved solubility, stability and bioavailability. Thus it can be concluded that phytochemicals have multiple biological activities with broader safety index and improvement of their solubility will be truly beneficial to aid their effective delivery in healthcare.

Gold nanoparticles interacting with β-cyclodextrin-phenylethylamine inclusion complex: a ternary system for photothermal drug release

We report the synthesis of a 1:1 β-cyclodextrin-phenylethylamine (βCD-PhEA) inclusion complex (IC) and the adhesion of gold nanoparticles (AuNPs) onto microcrystals of this complex, which forms a ternary system. The formation of the IC was confirmed by powder X-ray diffraction and NMR analyses ((1)H and ROESY). The stability constant of the IC (760 M(-1)) was determined using the phase solubility method. The adhesion of AuNPs was obtained using the magnetron sputtering technique, and the presence of AuNPs was confirmed using UV-vis spectroscopy (surface plasmon resonance effect), which showed an absorbance at 533 nm. The powder X-ray diffractograms of βCD-PhEA were similar to those of the crystals decorated with AuNPs. A comparison of the one- and two-dimensional NMR spectra of the IC with and without AuNPs suggests partial displacement of the guest to the outside of the βCD due to attraction toward AuNPs, a characteristic tropism effect. The size, morphology, and distribution of the AuNPs were analyzed using TEM and SEM. The average size of the AuNPs was 14 nm. Changes in the IR and Raman spectra were attributed to the formation of the complex and to the specific interactions of this group with the AuNPs. Laser irradiation assays show that the ternary system βCD-PhEA-AuNPs in solution enables the release of the guest.

Improved blood-brain barrier distribution: effect of borneol on the brain pharmacokinetics of kaempferol in rats by in vivo microdialysis sampling

ETHNOPHARMACOLOGICAL RELEVANCE

Kaempferol (KA) exists in a variety of herbal medicines. In vitro and in vivo studies have focused on the anti-Alzheimer effect of KA. However, little is known about its brain pharmacokinetic profile. The accumulated amount of KA in brain is very low because of the protection of blood-brain barrier (BBB). Borneol (BO) is a classical aromatic refreshing traditional Chinese medicine and commonly used as an adjuvant component of traditional Chinese medicines (e.g. compound Danshen dropping pills) in the treatment of cardiovascular and cerebrovascular diseases. According to the basic theories of traditional Chinese medicine, BO is called an "upper guiding drug", which can guide other components to the targeting tissues or organs in the upper part of the body, especially in the brain.

MATERIALS AND METHODS

The probes for blood and brain sampling were implanted within the jugular vein/right atrium and right hippocampus of SD rats, respectively. Rats were intravenous administered of KA (25 mg/kg) alone or combined with BO (15, 30 mg/kg) via caudal vein. The blood and brain microdialysates were collected every 15 min for 180 min and every 30 min for 180-300 min. A selective and sensitive high performance liquid chromatography-chemiluminescence method was developed for the determination of unbound KA in rat blood and brain microdialysates, which can be converted to their actual free-form concentrations based on the in vivo relative recoveries of KA across microdialysis probes.

RESULTS

KA quickly crossed the BBB to enter the extracellular fluid of hippocampus and reached the maximum concentration of 0.11 μg/mL within 30 min. The brain bioavailability and brain delivery of KA evidently increased with the co-administration of 15 and 30 mg/kg of BO. The AUC0-inf of KA in brain increased 1.84 and 2.19 times, and the Cmax of KA in brain increased 2.09 and 3.18 times than that without BO, respectively. In addition, the brain-to-blood distribution ratio of KA increased by 48.68% and 57.97% compared with that without BO. However, no significant difference in the T1/2 of unbound KA in blood aserved between three groups.

CONCLUSIONS

BO can enhance the BBB permeability and improve the transportation of KA to brain. The dose-dependent effect of BO on the brain pharmacokinetic parameters of KA was observed. This co-administration strategy can be designed to enhance the brain accumulation of other neuropsychiatric medications.

Effect of pH on the complexation of kaempferol-4'-glucoside with three β-cyclodextrin derivatives: isothermal titration calorimetry and spectroscopy study

The utilization of kaempferol and its glycosides in food and pharmaceutical industries could be improved by the formation of inclusion complexes with cyclodextrins at different pH. This study explores the complexation of kaempferol-4'-glucoside with sulfobutyl ether-β-cyclodextrin (SBE-β-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD), and methylated-β-cyclodextrin (M-β-CD) in phosphate buffer solutions of different pH using isothermal titration calorimetry, UV-vis absorption and proton nuclear magnetic resonance spectroscopy at 298.2 K. Experimental results showed that kaempferol-4'-glucoside binds with the three β- cyclodextrins in the same 1:1 stoichiometry. The rank order of stability constants is SBE-β-CD > HP-β-CD > M-β-CD at the same pH level and pH 6.0 > pH 7.4 > pH 9.0 for the same cyclodextrin. The binding of kaempferol-4'-glucoside with the three β-cyclodextrin derivatives is synergistically driven by enthalpy and entropy at pH 6.0 and enthalpy-driven at pH 7.4 and 9.0. The possible inclusion mode was that in the cavity of β-CD is included the planar benzopyranic-4-one part of the kaempferol-4'-glucoside.

The influence of the host–guest interaction on the oxidation of natural flavonoid dyes

The influence of the molecular cavity protection on degradation processes of bioorganic compounds quercetin and luteolin used as the original dyes in old tapestries was studied. The degradation processes were studied by electrochemical methods in aqueous media. The products of the exhaustive electrolysis were separated and identified by GC-MS analysis. Cyclic voltammetry characteristics indicate that the inclusion complex is formed. The inclusion affects the redox potentials of both oxidation waves related to the different dissociation forms of the flavonoid molecule. It was shown that decomposition products formed by the oxidation of quercetin are stabilized in the cavity of β-cyclodextrin, including the main oxidation product 2(3′,4′-dihydroxybenzoyl)-2,4,6-trihydroxybenzofuran-3(2H)-one. The formation of the 1:1 inclusion complex of luteolin with β-cyclodextrin is supported by the enhancement of fluorescence intensity. In the case of quercetin, a decrease of fluorescence intensity occurs when 1:1 inclusion complex with β-cyclodextrin is formed.

Spectroscopic study and antioxidant properties of the inclusion complexes of rosmarinic acid with natural and derivative cyclodextrins

Measurement of total antioxidant activity/capacity of polyphenols in various solvent media necessitates the use of cyclodextrins to solubilize lipophilic antioxidants of poor aqueous solubility. The inclusion complexes of the slightly water soluble antioxidant, rosmarinic acid (RA), with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), 2-hydroxyethyl-β-cyclodextrin (HE-β-CD), and methyl-β-cyclodextrin (M-β-CD) were investigated for the first time. The effect of cyclodextrins (CDs) on the spectral features of RA was measured in aqueous medium using UV-vis and steady-state fluorescence techniques by varying the concentrations of CDs. The molar stoichiometry of RA-CD inclusion complexes was verified as 1:1, and the formation constants of the complexes were determined from Benesi-Hildebrand equation using fluorescence spectroscopic data. Among the CDs, maximum inclusion ability was measured in the case of M-β-CD followed by HP-β-CD, HE-β-CD, β-CD and α-CD. Solid inclusion complexes were prepared by freeze drying, and their functional groups were analyzed by IR spectroscopy. Antioxidant capacity of CD-complexed rosmarinic acid was measured to be higher than that of the lone hydroxycinnamic acid by the CUPric Reducing Antioxidant Capacity (CUPRAC) method. The mechanism of the TAC increase was interpreted as the stabilization of the 1-e oxidized o-catechol moiety of RA by enhanced intramolecular H-bonding in a hydrophobic environment provided by CDs, mostly by M-β-CD.

Kaempferol complexation in cyclodextrins at basic pH

The complexation of kaempferol with cyclodextrins (CDs) (beta-, G(2)-beta-, and HP-beta-CDs) in basic medium was studied, and the complexation constants (K(c)) were calculated by using enzymatic, solubility, and fluorometric methods. This is the first time that a decrease in fluorescence has been observed as result of the analyte complexation by CDs. The highest K(c) value for kaempferol complexation was obtained for HP-beta-CDs. To establish the validity of the fluorometric method for determining the K(c) between kaempferol and CDs, the same parameters were also determined by enzymatic and solubility methods. The enzymatic method was carried out by using horseradish peroxidase as oxidative enzyme, and the K(c) values obtained were similar to those obtained by using the solubility method. However, the fluorometric method underestimated the K(c) value by about 1.2-fold with respect to the other methods used. In all cases HP-beta-CDs showed the highest K(c) value, indicating that they are more efficient in the formation of inclusion complexes with kaempferol.

Quercetin/beta-cyclodextrin solid complexes prepared in aqueous solution followed by spray-drying or by physical mixture

The present study was designed to investigate the influence of operating conditions (temperature, stirring time, and excess amount of quercetin) on the complexation of quercetin with beta-cyclodextrin using a 2(3) factorial design. The highest aqueous solubility of quercetin was reached under the conditions 37 degrees C/24 h/6 mM of quercetin. The stoichiometric ratio (1:1) and the apparent stability constant (Ks = 230 M(-1)) of the quercetin/beta-cyclodextrin complex were determined using phase-solubility diagrams. The semi-industrial production of a 1:1 quercetin/beta-cyclodextrin solid complex was carried out in aqueous solution followed by spray-drying. Although the yield of the spray-drying process was adequate (77%), the solid complex presented low concentration of quercetin (0.14%, w/w) and, thus, low complexation efficiency. The enhancement of aqueous solubility of quercetin using this method was limited to 4.6-fold in the presence of 15 mM of beta-cyclodextrin. Subsequently, an inclusion complex was prepared via physical mixture of quercetin with beta-cyclodextrin (molar ratio of 1:1 and quercetin concentration of 23% (w/w)) and characterized using infrared spectroscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy, and scanning electron microscopy analyses. The enhancement of aqueous solubility of quercetin using this method was 2.2-fold, similar to that found in the complex prepared in aqueous solution before the spray-drying process (2.5-fold at a molar ratio of 1:1, i.e., 6 mM of quercetin and 6 mM of beta-cyclodextrin).

1968–2008: 40 Years of Franco F. Vincieri's Natural Products Research

This paper presents an overview of Prof. Vincieri's accomplishments in his career as a researcher in the field of pharmacognosy (pharmaceutical biology), analytical phytochemistry and pharmaceutical technology applied to herbal drug preparations at the Department of Pharmaceutical Sciences of the University of Florence. This article is a recognition of his valuable contributions to these research fields, especially for his outstanding and innovative interdisciplinary studies on the quality control of herbal drugs, herbal drug preparations, herbal medicinal products, botanical food supplements, and some “special foods” such as grapes, wines, olives and olive oil.

Studies on coumestrol/beta-cyclodextrin association: Inclusion complex characterization

Coumestrol is an estrogenic and antioxidant agent, characterized by its low solubility in aqueous and lipophilic media, once in the aglicone form. In order to improve its solubility in water, coumestrol was associated with beta-cyclodextrin in aqueous media followed by freeze-drying and characterized by SEM, (1)H NMR and molecular modeling. The analysis proved the existence of an inclusion complex, with higher probability of inclusion of the coumestrol B-ring into the wider rim of the beta-cyclodextrin molecule.