Total Synthesis of (+)-Silybin A.

We report the first total synthesis of silybin A (1). Key synthetic steps include the construction of the 1,4-benzodioxane neolignan skeleton, a modified Julia-Kocienski olefination reaction between m-nitrophenyltetrazole sulfone (m-NPT sulfone) 10 and aldehyde 21, the formation of the flavanol lignan skeleton 28 via a quinomethide intermediate under acidic conditions, and stepwise oxidation of the benzylic position of flavanol 29.

Phosphate-Linked Silibinin Dimers (PLSd): New Promising Modified Metabolites.

By exploiting the regioselective protection of the hydroxyl groups of silibinin along with the well-known phosphoramidite chemistry, we have developed an efficient strategy for the synthesis of new silibinin-modified species, which we have named Phosphate-Linked Silibinin Dimers (PLSd), in which the monomer units are linked by phosphodiester bonds. The antioxidant abilities of the new PLSd were estimated on HepG2 cells using DPPH free radical scavenging and xanthine/xanthine oxidase assays. The new phosphate-metabolites showed a higher anti-oxidant activity than the silibinin, as well as very low toxicity. The ability to scavenge reactive oxygen species (ROS) such as singlet oxygen () and hydroxyl radical () reveals that the two dimers are able to scavenge about two times more effectively than silibinin. Finally, solubility studies have shown that the PLSd present good water solubility (more than 20 mg·L-1) under circumneutral pH values, whereas the silibinin was found to be very poorly soluble (less than 0.4 mg·L-1) and not stable under alkaline conditions. Together, the above promising results warrant further investigation of the future potential of the PLSd as anti-oxidant metabolites within the large synthetic polyphenols field.

5- or/and 20-O-alkyl-2,3-dehydrosilybins: Synthesis and biological profiles on prostate cancer cell models.

To investigate the effects of alkylation at 5-OH and 20-OH of 2,3-dehydrosilybin on prostate cancer cell proliferation, the synthetic approaches to 5- or/and 20-O-alkyl-2,3-dehydrosilybins, through a multi-step sequence from commercially available silybin, have been successfully developed. The first three reactions in the syntheses were completed through a one-pot procedure by managing anaerobic and aerobic conditions. With these synthetic methods in hand, twenty-one 2,3-dehydrosilybins, including seven 20-O-alkyl, seven 5,20-O-dialkyl, and seven 5-O-alkyl-2,3-dehydrosilybins, have been achieved for the evaluation of their biological profiles. Our WST-1 cell proliferation assay data indicate that nineteen out of the twenty-one 2,3-dehydrosilybins possess significantly improved antiproliferative potency as compared with silybin toward both androgen-sensitive (LNCaP) and androgen-insensitive prostate cancer cell lines (PC-3 and DU145). 5-O-Alkyl-2,3-dehydrosilybins were identified as the optimal subgroup that can consistently inhibit cell proliferation in three prostate cancer cell models with all IC50 values lower than 8µM. Our flow cytometry-based assays also demonstrate that 5-O-heptyl-2,3-dehydrosilybin effectively arrests the cell cycle in the G0/G1 phase and activates PC-3 cell apoptosis.

3-O-Alkyl-2,3-dehydrosilibinins: Two synthetic approaches and in vitro effects toward prostate cancer cells.

Eight 3-O-alkyl-2,3-dehydrosilibinins have been synthesized from commercially available silibinin through two synthetic approaches. A one-pot reaction, starting with aerobic oxidation of silibinin followed by direct alkylation of the phenolic hydroxyl group in the subsequent 2,3-dehydrosilibinin, furnishes the desired derivatives in 11-16% yields. The three-step procedure employing benzyl ether to protect 7-OH in silibinin generates the desired derivatives in 30-46% overall yields. The antiproliferative activity of the 2,3-dehydrosilibinin derivatives against both androgen-sensitive and androgen-insensitive prostate cancer cells have been assessed using a WST-1 cell proliferation assay. All derivatives exhibited greater antiproliferative potency than silibinin, with 2,3-dehydrosilibinins each possessing a three- to five-carbon linear alkyl group to 3-OH (IC50 values in a range of 1.71-3.06µM against PC-3 and LNCaP cells) as the optimal derivatives. The optimal potency was reached with three- to five-carbon alkyl groups. Our findings suggest that 3-O-propyl-2,3-dehydrosilibinin effectively inhibits the growth of PC-3 prostate cancer cells by arresting cell cycle in the G0/G1 phase, but not by activating PC-3 cell apoptosis.

Silibinin derivatives as anti-prostate cancer agents: Synthesis and cell-based evaluations.

This study aims to systematically explore the alkylation effect of 7-OH in silibinin and 2,3-dehydrosilibinin on the antiproliferative potency toward three prostate cancer cell lines. Eight 7-O-alkylsilibinins, eight 7-O-alkyl-2,3-dehydrosilibinins, and eight 3,7-O-dialkyl-2,3-dehydrosilibinins have been synthesized from commercially available silibinin for the in vitro cell-based evaluation. The WST-1 cell proliferation assay indicates that nineteen out of twenty-four silibinin derivatives have significantly improved antiproliferative potency when compared with silibinin. 7-O-Methylsilibinin (2) and 7-O-ethylsilibinin (3) have been identified as the most potent compounds with 98- and 123-fold enhanced potency against LNCaP human androgen-dependent prostate cancer cell line. Among 2,3-dehydrosilibinin derivatives, 7-O-methyl-2,3-dehydrosilibinin (10) and 7-O-ethyl-2,3-dehydrosilibinin (11) have been identified as the optimal compounds with the highest potency towards both androgen-dependent LNCaP and androgen-independent PC-3 prostate cancer cell lines. 7-O-Ethyl-2,3-dehydrosilibinin (11) was demonstrated to arrest PC-3 cell cycle at the G0/G1 phase and to induce PC-3 cell apoptosis. The findings in this study suggest that antiproliferative potency of silibinin and 2,3-dehydrosilibinin can be appreciably enhanced through suitable chemical modifications on the phenolic hydroxyl group at C-7 and that introduction of a chemical moiety with the potential to improve bioavailability through a linker to 7-OH in silibinin and 2,3-dehydrosilibinin would be a feasible strategy for the development of silibinin derivatives as anti-prostate cancer agents.

Synchronous microencapsulation of multiple components in silymarin into PLGA nanoparticles by an emulsification/solvent evaporation method.

The development of polymeric carriers loaded with extracts suffers from the drawback not to be able to incorporate simultaneously various pharmacological compounds into the formulation. The aim of this study was therefore to achieve synchronous microencapsulation of multiple components of silymarin into poly (lactic-co-glycolic acid) nanoparticle, the most commonly used polymeric carrier with biodegradability and safety. The main strategy taken was to improve the overall entrapment efficiency and to reduce the escaping ratio of the components of different physicochemical properties. The optimized nanoparticles were spherical in morphology with a mean particle size of 150 ± 5 nm. Under common preparative conditions, silybin and isosilybin were entrapped in high efficiency, whereas taxifolin, silychristin and silydianin, especially taxifolin, showed less entrapment because they were more hydrophilic. By changing the pH of the outer aqueous phase and saturating it with silymarin, the entrapment efficiency of taxifolin, silychristin and silydianin could be significantly improved to over 90%, the level similar to silybin and isosilybin, thereby achieving synchronous encapsulation. It could be concluded that synchronous encapsulation of multiple components of silymarin was achieved by optimizing the preparative variables.

In vitro study of 2,3-dehydrosilybin and its galloyl esters as potential inhibitors of angiogenesis.

2,3-Dehydrosilybin exhibits substantial anticancer and antiangiogenic effects, which can be potentially improved by semi-synthetic modification such as esterification with gallic acid. The aim of this study was to examine the potential antiangiogenic effect of 2,3-dehydrosilybin and its galloyl esters (3-O-galloyl-2,3-dehydrosilybin; 7-O-galloyl-2,3-dehydrosilybin; 20-O-galloyl-2,3-dehydrosilybin and 23-O-galloyl-2,3-dehydrosilybin) and to determine which molecular mechanism could be responsible for their activity. The effect on cell proliferation, tube formation, signal transduction pathways (PI3K/Akt and ERK) and the cell cycle was studied in human microvascular endothelial cells (HMEC). The results showed that all compounds decreased the growth of HMEC, but the strongest effect was observed for 20-O-galloyl-2,3-dehydrosilybin at 5 µmol/l. In addition, at 5 and 10 µmol/l, this was the only compound that significantly inhibited HMEC tube formation. Based on an assessment of Akt and ERK1/2 expression, we suggest that 20-O-galloyl-2,3-dehydrosilybin influences the angiogenic process through the Akt pathway.

Preparation and performance evaluation of emulsomes as a drug delivery system for silybin.

We developed silybin (SIL) emulsomes and evaluated their physicochemical properties and the in vivo pharmacokinetics of SIL delivered by emulsomes in rats. SIL emulsomes were prepared using the thin film dispersion method. SIL emulsomes were evaluated for their entrapment efficiency, particle size, zeta potential, morphology, in vitro release, and in vivo drug delivery in rats. The entrapment efficiency was above 80 %. The average particle size and zeta potential were 364.1 ± 20 nm and -34 ± 8 mV, respectively. Morphological analysis revealed that the SIL emulsomes were spherical in shape. Compared to an SIL solution, emulsomes produced sustained release of SIL for up to 48 h after an initial burst release in vitro. The pharmacokinetics of SIL emulsomes in rats were evaluated after intravenous injection, and the results were compared with those obtained for the control SIL solution. Following SIL delivery by emulsomes, the area under the curve was 2.2-fold higher and the mean residence time was 2.5-fold higher than the corresponding values recorded using SIL solution. Hence, emulsomes might represent a promising system for improving the bioavailability of lipophilic drugs. Moreover, emulsomes produce sustained drug release, which is advantageous in the clinical setting.

Synthesis and Antiproliferative Activity of Silybin Conjugates with Salinomycin and Monensin.

Aiming at development of multitarget drugs for the anticancer treatment, new silybin (SIL) conjugates with salinomycin (SAL) and monensin (MON) were synthesized, in mild esterification conditions, and their antiproliferative activity was studied. The conjugates obtained exhibit anticancer activity against HepG2, LoVo and LoVo/DX cancer cell lines. Moreover, MON-SIL conjugate exhibits higher anticancer potential and better selectivity than the corresponding SAL-SIL conjugate.

A biomimetic strategy to access the silybins: total synthesis of (-)-isosilybin A.

We report the first asymmetric, total synthesis of (-)-isosilybin A. A late-stage catalytic biomimetic cyclization of a highly functionalized chalcone is employed to form the characteristic benzopyranone ring. A robust and flexible approach to this chalcone provides an entry to the preparation of the entire isomeric family of silybin natural products.

New silibinin glyco-conjugates: synthesis and evaluation of antioxidant properties.

New silibinin glyco-conjugates have been synthesized by efficient method and in short time. Exploiting our solution phase strategy, several structurally diverse silibinin glyco-conjugates (gluco, manno, galacto, and lacto-) were successfully realized in very good yields and in short time. In preliminary study to evaluate their antioxidant and neuroprotective activities new derivatives were subjected to DPPH free radical scavenging assay and the Xanthine oxidase (XO) inhibition models assay. Irrespective of the sugar moiety examined, new glyco-conjugates are more than 50 times water-soluble of silibinin. In the other hand they exhibit a radical scavenging activities slightly higher than to silibinin and XO inhibition at least as silibinin.

Chemo-enzymatic synthesis of silybin and 2,3-dehydrosilybin dimers.

Divalent or multivalent molecules often show enhanced biological activity relative to the simple monomeric units. Here we present enzymatically and chemically prepared dimers of the flavonolignans silybin and 2,3-dehydrosilybin. Their electrochemical behavior was studied by in situ and ex situ square wave voltammetry. The oxidation of monomers and dimers was similar, but adsorption onto the electrode and cell surfaces was different. A 1,1-diphenyl-2-picrylhydrazyl (DPPH) and an inhibition of microsomal lipoperoxidation assay were performed with same trend of results for silybin and 2,3-dehydrosilybin dimers. Silybin dimer showed better activity than the monomer, while on the contrary 2,3-dehydrosilybin dimer presented weaker antioxidant/antilipoperoxidant activity than its monomer. Cytotoxicity was evaluated on human umbilical vein endothelial cells, normal human adult keratinocytes, mouse fibroblasts (BALB/c 3T3) and human liver hepatocellular carcinoma cell line (HepG2). Silybin dimer was more cytotoxic than the parent compound and in the case of 2,3-dehydrosilybin its dimer showed weaker cytotoxicity than the monomer.

Identification of 23-(s)-2-amino-3-phenylpropanoyl-silybin as an antiviral agent for influenza A virus infection in vitro and in vivo.

It has been reported that autophagy is involved in the replication of many viruses. In this study, we screened 89 medicinal plants, using an assay based on the inhibition of the formation of the Atg12-Atg5/Atg16 heterotrimer, an important regulator of autophagy, and selected Silybum marianum L. for further study. An antiviral assay indicated that silybin (S0), the major active compound of S. marianum L., can inhibit influenza A virus (IAV) infection. We later synthesized 5 silybin derivatives (S1 through S5) and found that 23-(S)-2-amino-3-phenylpropanoyl-silybin (S3) had the best activity. When we compared the polarities of the substituent groups, we found that the hydrophobicity of the substituent groups was positively correlated with their activities. We further studied the mechanisms of action of these compounds and determined that S0 and S3 also inhibited both the formation of the Atg12-Atg5/Atg16 heterotrimer and the elevated autophagy induced by IAV infection. In addition, we found that S0 and S3 could inhibit several components induced by IAV infection, including oxidative stress, the activation of extracellular signal-regulated kinase (ERK)/p38 mitogen-activated protein kinase (MAPK) and IκB kinase (IKK) pathways, and the expression of autophagic genes, especially Atg7 and Atg3. All of these components have been reported to be related to the formation of the Atg12-Atg5/Atg16 heterotrimer, which might validate our screening strategy. Finally, we demonstrated that S3 can significantly reduce influenza virus replication and the associated mortality in infected mice. In conclusion, we identified 23-(S)-2-amino-3-phenylpropanoyl-silybin as a promising inhibitor of IAV infection.

Enhanced bioactivity of silybin B methylation products.

Flavonolignans from milk thistle (Silybum marianum) have been investigated for their cellular modulatory properties, including cancer chemoprevention and hepatoprotection, as an extract (silymarin), as partially purified mixtures (silibinin and isosilibinin), and as pure compounds (a series of seven isomers). One challenge with the use of these compounds in vivo is their relatively short half-life due to conjugation, particularly glucuronidation. In an attempt to generate analogues with improved in vivo properties, particularly reduced metabolic liability, a semi-synthetic series was prepared in which the hydroxy groups of silybin B were alkylated. A total of five methylated analogues of silybin B were synthesized using standard alkylation conditions (dimethyl sulfate and potassium carbonate in acetone), purified using preparative HPLC, and elucidated via spectroscopy and spectrometry. Of the five, one was monomethylated (3), one was dimethylated (4), two were trimethylated (2 and 6), and one was tetramethylated (5). The relative potency of all compounds was determined in a 72 h growth-inhibition assay against a panel of three prostate cancer cell lines (DU-145, PC-3, and LNCaP) and a human hepatoma cell line (Huh7.5.1) and compared to natural silybin B. Compounds also were evaluated for inhibition of both cytochrome P450 2C9 (CYP2C9) activity in human liver microsomes and hepatitis C virus infection in Huh7.5.1 cells. The monomethyl and dimethyl analogues were shown to have enhanced activity in terms of cytotoxicity, CYP2C9 inhibitory potency, and antiviral activity (up to 6-fold increased potency) compared to the parent compound, silybin B. In total, these data suggested that methylation of flavonolignans can increase bioactivity.

Synthesis and antiangiogenic activity of new silybin galloyl esters.

The synthesis of various silybin monogalloyl esters was developed, and their antiangiogenic activities were evaluated in a variety of in vitro tests with human umbilical vein endothelial cells (HUVECs). A structure-activity relationship (SAR) study found the regioselectivity of the silybin galloylation to be highly significant. Silybin (as an equimolar mixture of two diastereomers A and B) exhibited quite poor antiangiogenic activities, whereas its B stereoisomer is more active than silybin A. The galloylation of phenolic OH groups of natural silybin (a mixture of both isomers) leads to increases in their antiangiogenic activities, which is more apparent with the 7-OH than the 20-OH. In contrast, gallates at aliphatic OH groups either had a comparable activity to the parent compound or are even worse than silybin, which was observed in the case of 3-O-galloylsilybin. The most effective compound from this series (7-O-galloylsilybin) has also been prepared from stereochemically pure silybins A and B to evaluate the effect of stereochemistry on the activity. As with silybin itself, the B isomer of 7-O-galloylsilybin was more active than the A isomer.

New C-23 modified of silybin and 2,3-dehydrosilybin: synthesis and preliminary evaluation of antioxidant properties.

Silybin is the major flavonolignan of silymarin and it displays a plethora of biological effects, generally ascribed to its antioxidant properties. Herein we shall describe an efficient synthetic strategy to obtain a variety of new and more water-soluble silybin and 2,3-dehydrosilybin (DHS) derivatives in which the 23-hydroxyl group was converted to a sulfate, phosphodiester, or amine group, using a solution-phase approach. Furthermore a new and efficient method for the preparation of DHS from silybin was developed and optimised. The antioxidant properties of the new compounds were evaluated in a cellular model in vivo and they displayed an antioxidant activity comparable to or higher than silybin and DHS, being able to prevent H(2)O(2)-induced generation of intracellular reactive oxygen species (ROS). Most of the derivatives also displayed a better hydrophilicity while retaining the biological activities of silybin and they might broaden the in vivo applications of this class of natural compounds.

Proliposomes for oral delivery of dehydrosilymarin: preparation and evaluation in vitro and in vivo.

AIM: To formulate proliposomes with a polyphase dispersed system composed of soybean phospholipids, cholesterol, isopropyl myristate and sodium cholate to improve the oral bioavailability of dehydrosilymarin, an oxidized form of herbal drug silymarin. METHODS: Dehydrosilymarin was synthesized from air oxidation of silymarin in the presence of pyridine, and proliposomes were prepared by a film dispersion-freeze drying method. Morphological characterization of proliposomes was observed using a transmission electron microscope. Particle size and encapsulation efficiency of proliposomes were measured. The in vitro release of dehydrosilymarin from suspension and proliposomes was evaluated. The oral bioavailability of dehydrosilymarin suspension and proliposomes was investigated in rabbits. RESULTS: The proliposomes prepared under the optimum conditions were spherical and smooth with a mean particle size in the range of 7 to 50 nm. Encapsulation efficiency was 81.59%±0.24%. The in vitro accumulative release percent of dehydrosilymarinloaded proliposomes was stable, which was slow in pH 1.2, and increased continuously in pH 6.8, and finally reached 86.41% at 12 h. After oral administration in rabbits, the relative bioavailability of proliposomes versus suspension in rabbits was 228.85%. CONCLUSION: Proliposomes may be a useful vehicle for oral delivery of dehydrosilymarin, a drug poorly soluble in water.

Biocatalytic synthesis and antitumor activities of novel silybin acylated derivatives with dicarboxylic acids.

Novel silybin acylated derivatives with dicarboxylic acids were prepared in various organic solvents using immobilized Candida antarctica lipase B (Novozym 435(®)). The reaction parameters affecting the silybin conversion, such as the nature of the organic solvent and the acyl donor used were investigated. The antiproliferative effects of silybin monoesters, and their ability to modulate the secretion of vascular endothelial growth factor (VEGF) were estimated using K562 human lymphoblastoma cells and compared to the parental compound. The synthesized esters retained the biological function of silybin and in some cases were more effective, indicating that target biotransformation may generate novel compounds with improved antitumor and antiangiogenic activities.

Identification and initial SAR of silybin: an Hsp90 inhibitor.

Through Hsp90-dependent firefly luciferase refolding and Hsp90-dependent heme-regulated eIF2α kinase (HRI) activation assays, silybin was identified as a novel Hsp90 inhibitor. Subsequently, a library of silybin analogues was designed, synthesized and evaluated. Initial SAR studies identified the essential, non-essential and detrimental functionalities on silybin that contribute to Hsp90 inhibition.

Preparation and effects of 2,3-dehydrosilymarin, a promising and potent antioxidant and free radical scavenger.

OBJECTIVES: Silymarin or silybin has been effectively used for treating liver diseases and acute liver injury partly due to its antioxidant activity. In this study, 2,3-dehydrosilymarin, a compound exhibiting remarkable antiradical/antioxidant activity, was prepared from silymarin for the first time. The solubility, radical scavenging capacity and liver protecting activity of 2,3-dehydrosilymarin were studied and compared with silybin, dehydrosilybin and silymarin. METHODS: The structures of its main components were verified by ultra-performance liquid chromatography/mass spectrometry (UPLC-MS) and other spectral analysis. In addition, a rapid screening method, online high-performance liquid chromatography/1,1-dipheny1-2-picrylhydrazyl (HPLC-DPPH) system, was developed for identifying the individual antioxidants in 2,3-dehydrosilymarin. KEY FINDINGS: Both in-vitro and in-vivo results markedly proved that dehydrosilymarin has decent aqueous solubility and remarkable antiradical/antioxidation capacity. Moreover, 2,3-dehydrosilybin and 2,3-dehydrosilychristin were identified to be the two major active compounds contained in 2,3-dehydrosilymarin. CONCLUSIONS: Our results suggest that 2,3-dehydrosilymarin may be a promising and potent alternative for inhibition of free radical and prevention of oxidation.