Kateřina Purchartová

Antioxidant and antiviral activities of silybin fatty acid conjugates

Two selective acylation methods for silybin esterification with long-chain fatty acids were developed, yielding a series of silybin 7-O- and 23-O-acyl-derivatives of varying acyl chain lengths. These compounds were tested for their antioxidant (inhibition of lipid peroxidation and DPPH-scavenging) and anti-influenza virus activities. The acyl chain length is an important prerequisite for both biological activities, as they improved with increasing length of the acyl moiety.

Bioavailability of silymarin flavonolignans: drug formulations and biotransformation

Over the past years, great advances have been made on the development of novel delivery systems for bioactive natural compounds, in parallel to their structural modification via chemical, chemo-enzymatic and enzymatic methodologies. These approaches give rise to novel formulations and derivatives that often display advantages over the parental molecule, such as enhanced bioavailability and pharmacological activity, due to improved dissolution and stability. Silymarin components suffer from poor solubility in water and lipid media and their resorption in the intestine is rather limited. Moreover, silybin undergoes intensive Phase II metabolism and is rapidly excreted in bile and urine, leading to low therapeutic efficacy. This work aims to present the current status of available silymarin formulations, and to highlight successful efforts for the biotransformation of its constituent flavonolignans towards the synthesis of novel derivatives. Herein, various pharmaceutical formulations that aim at the bioavailability improvement of these fascinating phytochemicals, i.e., liposomes, phytosomes, self-microemulsifying drug delivery systems, solid dispersions systems, dripping pills, nanosuspensions, floating tablets, and micronization, are reviewed. Silybin (semi)synthetic derivatives prepared by chemical or enzymatic methods, such as fatty acid conjugates, silybin bishemisuccinate, silybin glycosides, silybin sulfates, silybinic acid, and 2,3-dehydrosilybin, are also discussed in detail. Additionally, this work attempts to direct the attention towards the pharmacological implications of optically pure silybin A and silybin B and their biotransformation reactions, both Phase I and II, in relation to bioavailability.

Enzymatic preparation of acylated derivatives of silybin in organic and ionic liquid media and evaluation of their antitumor proliferative activity

Biocatalytic preparation of acylated derivatives of silybin using several acyl donors (free fatty acids or their esters), catalyzed by immobilized Candida antarctica lipase B, was performed in various organic solvents as well as in imidazolium-based ionic liquids containing either or anions. Total conversion of silybin was achieved in acetone using short-chain acyl donors (vinyl butanoate) at a 10:1 molar ratio of acyl donor to silybin. The enzymatic process in ionic liquids depended strongly on the alkyl chain length of the cation as well the anion used. Higher conversion yields (up to 75.8%) and reaction rates (up to 0.31 mmol h−1 g−1 biocatalyst) were obtained in the as compared with - containing ionic liquids. The amounts of silybin ester formed in a one-step biocatalytic process were high (>40 g L−1) in both reaction media. The antiproliferative effect of acylated silybin derivatives on the growth of K562 human leukemia cells was estimated and compared with that of silybin.

Prokaryotic and Eukaryotic Aryl Sulfotransferases: Sulfation of Quercetin and Its Derivatives

Two types of sulfotransferases, namely recombinant rat liver aryl sulfotransferase AstIV and bacterial aryl sulfotransferase from Desulfitobacterium hafniense, were used for the sulfation of quercetin, its glycosylated derivatives (isoquercitrin and rutin), and dihydroquercetin ((+)‐taxifolin). The rat liver enzyme was able to sulfate only quercetin and taxifolin, whereas the quercetin glycosides remained intact. The D. hafniense enzyme sulfated isoquercitrin and rutin selectively at the C‐4′ position of the catechol moiety with very good yields. Taxifolin was sulfated at the C‐4′ position and a minor amount of the C‐3′ isomer was formed. Sulfation of quercetin proceeded preferentially at the C‐3′ position, but a lower proportion of the C‐4′ isomer was formed as well. A detailed analysis of the kinetics of this reaction is provided and a full structural analysis of all products is presented.

Regioselective alcoholysis of silychristin acetates catalyzed by lipases.

A panel of lipases was screened for the selective acetylation and alcoholysis of silychristin and silychristin peracetate, respectively. Acetylation at primary alcoholic group (C-22) of silychristin was accomplished by lipase PS (Pseudomonas cepacia) immobilized on diatomite using vinyl acetate as an acetyl donor, whereas selective deacetylation of 22-O-acetyl silychristin was accomplished by Novozym 435 in methyl tert-butyl ether/n-butanol. Both of these reactions occurred without diastereomeric discrimination of silychristin A and B. Both of these enzymes were found to be capable to regioselective deacetylation of hexaacetyl silychristin to afford penta-, tetra- and tri-acetyl derivatives, which could be obtained as pure synthons for further selective modifications of the parent molecule.

Sulfated Metabolites of Flavonolignans and 2,3-Dehydroflavonolignans: Preparation and Properties

Silymarin, an extract from milk thistle (Silybum marianum) fruits, is consumed in various food supplements. The metabolism of silymarin flavonolignans in mammals is complex, the exact structure of their metabolites still remains partly unclear and standards are not commercially available. This work is focused on the preparation of sulfated metabolites of silymarin flavonolignans. Sulfated flavonolignans were prepared using aryl sulfotransferase from Desulfitobacterium hafniense and p-nitrophenyl sulfate as a sulfate donor and characterized by high-resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR). Their 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and N,N-dimethyl-p-phenylenediamine (DMPD) radical scavenging; ferric (FRAP) and Folin–Ciocalteu reagent (FCR) reducing activity; anti-lipoperoxidant potential; and effect on the nuclear erythroid 2-related factor 2 (Nrf2) signaling pathway were examined. Pure silybin A 20-O-sulfate, silybin B 20-O-sulfate, 2,3-dehydrosilybin-20-O-sulfate, 2,3-dehydrosilybin-7,20-di-O-sulfate, silychristin-19-O-sulfate, 2,3-dehydrosilychristin-19-O-sulfate, and silydianin-19-O-sulfate were prepared and fully characterized. Sulfated 2,3-dehydroderivatives were more active in FCR and FRAP assays than the parent compounds, and remaining sulfates were less active chemoprotectants. The sulfated flavonolignans obtained can be now used as authentic standards for in vivo metabolic experiments and for further research on their biological activity.