New Silybin Scaffold for Chemical Diversification: Synthesis of Novel 23-Phosphodiester Silybin Conjugates

Phosphodiester Silybin Dimers Powerful Radical Scavengers: A Antiproliferative Activity on Different Cancer Cell Lines

Silibinin is the main biologically active component of silymarin extract and consists of a mixture 1:1 of two diastereoisomeric flavonolignans, namely silybin A (1a) and silybin B (1b), which we call here silybins. Despite the high interest in the activity of this flavonolignan, there are still few studies that give due attention to the role of its stereochemistry and, there is still today a strong need to investigate in this area. In this regard, here we report a study concerning the radical scavenger ability and the antiproliferative activity on different cell lines, both of silybins and phosphodiester-linked silybin dimers. An efficient synthetic strategy to obtain silybin dimers in an optical pure form (6aa, 6ab and 6bb) starting from a suitable building block of silybin A and silybin B, obtained by us from natural extract silibinin, was proposed. New dimers show strong antioxidant properties, determined through hydroxyl radical (HO^●) scavenging ability, comparable to the value reported for known potent antioxidants such as quercetin. A preliminary screening was performed by treating cells with 10 and 50 μM concentrations for 48 h to identify the most sensitive cell lines. The results show that silibinin compounds were active on Jurkat, A375, WM266, and HeLa, but at the tested concentrations, they did not interfere with the growth of PANC, MCF-7, HDF or U87. In particular, both monomers (1a and 1b) and dimers (6aa, 6ab and 6bb) present selective anti-proliferative activity towards leukemia cells in the mid-micromolar range and are poorly active on normal cells. They exhibit different mechanisms of action in fact all the cells treated with the 1a and 1b go completely into apoptosis, whereas only part of the cells treated with 6aa and 6ab were found to be in apoptosis.

Cold-Active Lipase-Based Biocatalysts for Silymarin Valorization through Biocatalytic Acylation of Silybin

Extremophilic biocatalysts represent an enhanced solution in various industrial applications. Integrating enzymes with high catalytic potential at low temperatures into production schemes such as cold-pressed silymarin processing not only brings value to the silymarin recovery from biomass residues, but also improves its solubility properties for biocatalytic modification. Therefore, a cold-active lipase-mediated biocatalytic system has been developed for silybin acylation with methyl fatty acid esters based on the extracellular protein fractions produced by the psychrophilic bacterial strain Psychrobacter SC65A.3 isolated from Scarisoara Ice Cave (Romania). The extracellular production of the lipase fraction was enhanced by 1% olive-oil-enriched culture media. Through multiple immobilization approaches of the cold-active putative lipases (using carbodiimide, aldehyde-hydrazine, or glutaraldehyde coupling), bio-composites (S1–5) with similar or even higher catalytic activity under cold-active conditions (25 °C) have been synthesized by covalent attachment to nano-/micro-sized magnetic or polymeric resin beads. Characterization methods (e.g., FTIR DRIFT, SEM, enzyme activity) strengthen the biocatalysts’ settlement and potential. Thus, the developed immobilized biocatalysts exhibited between 80 and 128% recovery of the catalytic activity for protein loading in the range 90–99% and this led to an immobilization yield up to 89%. The biocatalytic acylation performance reached a maximum of 67% silybin conversion with methyl decanoate acylating agent and nano-support immobilized lipase biocatalyst.

Investigation on the solid-phase synthesis of silybin prodrugs and their timed-release

Prodrugs are ingenious derivatives of therapeutic agents designed to improve the pharmacokinetic profile of the drug. Here, we report an efficient and regioselective solid phase approach for obtaining new prodrugs of 9″-silybins conjugated with 3'-ribonucleotide units (uridine and adenosine) as pro-moieties. Uridine and adenosine conjugates were obtained in good yields (41-50%), beginning with silibinin and its diastereomers (silybin A and silybin B), using a NovaSyn® support functionalized with an ad hoc linker, which allowed selective detachment of only the desired products. As expected, the solubility of both uridine and adenosine conjugates was higher than that of the parental natural product (5 mg/mL and 3 mg/mL for uridine and adenosine, respectively). Our investigations revealed that uridine conjugates were quickly cleaved by RNase A, releasing silybin drugs, even at low enzyme concentrations. No toxic effects were found for any ribonucleotide conjugate on differentiated neuroblastoma SH-SY5Y cells when tested at increasing concentrations. All results strongly encourage further investigations of uridine-silybin prodrugs as potential therapeutic agents for both oral and intravenous administration. The present synthetic approach represents a valuable strategy to the future design of new prodrugs with modified nucleoside pro-moieties to modulate the pharmacokinetics of silybins or different natural products with strong pharmacological activities but poor bioavailability.

Trehalose Conjugates of Silybin as Prodrugs for Targeting Toxic Aβ Aggregates

Alzheimer's disease (AD) is linked to the abnormal accumulation of amyloid β peptide (Aβ) aggregates in the brain. Silybin B, a natural compound extracted from milk thistle (Silybum marianum), has been shown to significantly inhibit Aβ aggregation in vitro and to exert neuroprotective properties in vivo. However, further explorations of silybin B's clinical potential are currently limited by three main factors: (a) poor solubility, (b) instability in blood serum, and (c) only partial knowledge of silybin's mechanism of action. Here, we address these three limitations. We demonstrate that conjugation of a trehalose moiety to silybin significantly increases both water solubility and stability in blood serum without significantly compromising its antiaggregation properties. Furthermore, using a combination of biophysical techniques with different spatial resolution, that is, TEM, ThT fluorescence, CD, and NMR spectroscopy, we profile the interactions of the trehalose conjugate with both Aβ monomers and oligomers and evidence that silybin may shield the "toxic" surfaces formed by the N-terminal and central hydrophobic regions of Aβ. Finally, comparative analysis with silybin A, a less active diastereoisomer of silybin B, revealed how even subtle differences in chemical structure may entail different effects on amyloid inhibition. The resulting insight on the mechanism of action of silybins as aggregation inhibitors is anticipated to facilitate the future investigation of silybin's therapeutic potential.

A New Class of Synthetic Flavonolignan-Like Dimers: Still Few Molecules, but with Attractive Properties

In recent years, there has been increasing interest in dimeric molecules due to reports of their promising therapeutic value in the treatment of numerous diseases (such as cancer, HIV, Alzheimer's and, malaria). Many reports in the literature have highlighted the ability of these molecules to interact not only with specific biologic receptors but also to induce a biological response that more than doubles the results of the corresponding monomeric counterpart. In this regard, flavonolignan dimers or simply bi-flavonolignans are an emerging class of dimeric compounds that unlike bi-flavonoids, which are very widespread in nature, consist of synthetic dimers of some flavonolignans isolated from the milk thistle Silybum marianum [L. Gaertn. (Asteraceae)]. This mini-review will discuss recent developments in the synthesis, characterization and antioxidant activity of new families of flavonolignan dimers, in light of emerging medicinal chemistry strategies.

Selective Synthesis of 3-O-Palmitoyl-Silybin, a New-to-Nature Flavonolignan with Increased Protective Action against Oxidative Damages in Lipophilic Media

A selective acylation protocol using cerium chloride (CeCl₃) as catalyst was applied to functionalize silybinin (1), a natural antioxidant flavonolignan from milk thistle fruit, in order to increase its solubility in lipophilic media while retaining its strong antioxidant activity. The selective esterification of 1 at the position 3-OH with a palmitate acyl chain leading to the formation of the 3-O-palmitoyl-silybin (2) was confirmed by both mass spectroscopy (MS) and nuclear magnetic resonance (NMR) analyses. The antioxidant activity of 1 was at least retained and even increased with the CUPRAC assay designed to estimate the antioxidant activity of both hydrophilic and lipophilic compounds. Finally, the 3-O-palmitoylation of 1, resulting in the formation of 2, also increased its anti-lipoperoxidant activity (i.e., inhibition of conjugated diene production) in two different lipophilic media (bulk oil and o/w emulsion) subjected to accelerated storage test.

Silibinin phosphodiester glyco-conjugates: Synthesis, redox behaviour and biological investigations

New silibinin phosphodiester glyco-conjugates were synthesized by efficient phosphoramidite chemistry and were fully characterized by 2D-NMR. A wide-ranging study focused on the determination of their pKa and E° values as well as on their radical scavenging activities by different assays (DPPH, ABTS⁺ and HRSA) was conducted. The new glyco-conjugates are more water-soluble than silibinin, and their radical scavenging activities are higher than those of silibinin. The conjugation therefore improves both the water solubilities and antioxidant activities of the flavonolignan moieties. The serum stability was evaluated under physiological conditions, and the glyco-conjugates degraded with half-lives of 40-70 h, making them useful in pro-drug approaches. We started by treating androgen-dependent prostate cancer (PCa) LNCaP cells and then expanded our studies to androgen-independent PCa PC3 and DU145 cells. In most cases, the new derivatives significantly reduced both total and live cell numbers, albeit at different levels. Anti-HIV activities were evaluated and the glucosamine-phosphate silibinin derivative showed higher activity (IC₅₀ = 73 μM) than silibinin.

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.