Of silybin

Fig. 2.50. Profiles of calendula (a), milk-thistle (b) and passion flower (c) tinctures (each 60 per cent v/v, ethanol) with the HPLC-MS attributions of the components detected, lc = quercetin-3-O-ruti-nosylrhamnoside 2c = rutin 3c = isorhamnetin-3-O-ruti-nosylrhamnoside 4c = isoquercitrin 5c = isorhamnetin-3-O-gluco-sylglucoside 6c = narcissin 7c = isorhamnetin-3-O-glucoside lm = taxifolin 2m = siliydianin 3m = silychristin 4m = oxy derivative of silybin/isosilybin isomers 5m and 6m = 2,3-dehydro derivatives of silybin/isosilybin isomers 7m = silybin 8m = isosilybin lp = 6,8-diC-glucosylapigenin 2p = isoschaftoside 3p = shaftoside 4p = homoorientin 5p = isovetexin-2"-0-glucoside 6p = vitexin. Reprinted with permission from A. R. Biha et al. [150].

Intravenous silymarin has been demonstrated to lower mortality from Amanita mushroom poisonings, but this formulation is available only in Europe. Animal studies have demonstrated hepatic protection against alcohol, acetaminophen, and mushroom toxins and protection against hepatic fibrosis with bile duct occlusion. There is also evidence of silybin protecting against cis-platin-induced nephrotoxicity in rats. It is not yet clear whether milk thistle extract offers any renal protection to humans. 

Silymarin is considered the active constituent of the milk thistle seed, but it is not a single compound but a descriptive term for several flavonoli-gnans. Constituent analysis of five milk thistle products identified six constituents (representative amount) taxifolin (3.3%), silichristin (23.6 /o), silidianin (5.3 /o), silybin A (20%i), silybin B (30.7%i), and isosilybin (17.3 /o). The total amounts of silybin A and B in the five different products analyzed ranged from 45.7 /o to 61%i. The biological effect of each constituent is not known hence, spectrophotometric analysis would not provide sufficient information for a critical comparison of these products. 

Milk thistle (Silybum marianum) is an annual to biennial plant of the Aster-aceae family. It is native principally to southern Europe and northern Africa. The crude drug consists of the ripe fruits from which the pappus has been removed. Milk thistle fruits contain 15% to 30% proteins. The main active compounds constitute only about 2% to 3% of the dried fruits. The active principle is a mixture of flavolignans called silymarin. Silymarin, a polyphenolic extract isolated from the seeds of milk thistle, is composed mainly of silybin (50 70%), with small amounts of other silybin structural isomers, namely isosilybin, silydianin, and silychristin (Fig. 11) (123-125). The highest concentration of silymarin is found in the ripe fruits (126). Silibinin is the main compound, also considered to be the most active one in several paradigms (127). 

Schandalik R, Gatti G, Perucca E. Pharmacokinetics of silybin in bile following administration of silipide and silymarin in cholecystectomy patients. Arzneim Eorsch/Drug Res 1992 42 664—668. 

Orlando R, Eragasso A, Lampertico M, et al. Pharmacokinetic study of silybin-phosphatidylcholin complex in liver cirrhosis after multiple doses. Med Sci Res 1990 19 827-828. 

Silybin (52) is a naturally occurring flavanolignan from milk thistle. De-hydrosilybin (53) and isoprenoid dehydrosilybins were also recognized as potent inhibitors of P-gp [211], These compounds were tested for their binding affinity toward domain NBD2 of P-gp. Oxidation of silybin to flavonol dehydrosilybin (53) increased the affinity three times. Prenylation and ger-anylation led to a further increase in affinity [207]. C-isoprenylated deriva-... 

Higher conversion yields were observed for all dicarboxylic acids used in the case of silybin, as compared to rutin, due to the presence of primary hydroxyls in this flavonolignan for which the lipase shows higher specificity. Analysis of the chemical structures of purified products by NMR revealed that the acylation of rutin occurred at the 4""-OH of the rhamnose moiety, while the aylation of silybin occurred at its unique primary hydroxyl at position C23. It must be noted that, in the case of silybin, both monoester and diester formation was observed and identi-... 

As can been seen in Table 9.1 the use of tert-butyl alcohol led to lower conversion yields both for rutin and silybin as compared to tert-amyl alcohoL In the case of acetone, higher conversion yields were obtained for C12 and C16 dicarboxylic acids. The solubility capacity of acetone for silybin is about 10 times higher ( 40g/l) than tert-amyl alcohol and tert-butyl alcohoL The higher solubility of silybin in acetone is advantageous, as the amount of esters obtained was high, reaching values of 4.5, 3.4, and 6.6g/l for hexanedioic, dodecanedioic, and he a-decanedioic acid, respectively, after 96 h of incubation. 

In order to investigate the effect of the substrates molar ratio on the reaction performance, the concentrations of the enzyme and antioxidant were kept constant, whereas that of the acyl donor was increased. In the case of rutin, independently of the molar ratio, used the acylation proceeded with low yields, while only monoesters were formed in all reaction systems used. The same behavior was observed in the case of silybin using tert-amyl alcohol and tert-butyl alcohol as reaction media (data not shown). In contrast, when acetone was used as medium the conversion yield for the acylation of silybin increased as the molar ratio was increased (Table 9.2). 

Table 9.2 Effect of the molar ratio of the acyl donor to flavonolignan on the conversion yield and reaction rate after 96h of incubation for the acylation of silybin (30mM) with dicarboxylic acids catalyzed by Immobilized CALB in acetone...

Table 9.4 Effect of the acyl donor chain length on the enzymatic acylation of silybin (30 mM) catalyzed by immobilized CALB in the ionic liquids [bmim]BF4 and [bmimJPFs at 60°C and in acetone at 50°C after 72h of incubation.

In the early years after the discovery of silybin (1), the structure determination of this compound class was mainly achieved through extensive chemical conversions and degradative reactions (15, 95), which often led to structures that later needed revision. Analysis of comprehensive spectroscopic data along with classical chemical reactions and synthesis approaches has led to the unequivocal determination of the absolute stmctures of flavonolignans (102,135). The complete structure elucidation of flavonolignans is summarized below. 

The stmcture of silybin (1), C25H22O10, mp 158-160°C, [aJo +12.6°cm g the principal biologically active constituent of S. marianum, was first settled by Pelter... 

Pelter A, Hansel R (1968) The Structure of Silybin (Silybum Substance E6). The First Flavonolignan. Tetrahedron Lett 25 2911... 

Amone A, Merlini L, Zanarotti A (1979) Constituents of Silybum marianum. Structure of Isosilybin and Stereochemistry of Silybin. J Chem Soc Chem Commun 696... 

Kim NC, Graf TN, Sparacino CM, Wani MC, Wall ME (2003) Complete Isolation and Characterisation of Silybins and Isosilybins from Milk Thistle Silybum marianum). Org Biomol Chem 1 1684... 

Lee DYW, Liu Y (2003) Molecular Structure and Stereochemistry of Silybin A, Silybin B, Isosilybin A and IsosilybinB, IsolaloAfmmSilybummarianum (Milk Thistle). J Nat Prod 66 1171... 

Tanaka H, Shibata M, Ohira K, Ito K (1985) Total Synthesis of ( )-Silybin, an Antihepa-totoxic Flavonolignan. Chem. Pharm. Bull. 33 1419... 

Gazak R, Svobodova A, Psotova J, Sedmera P, Prikrylova V, Walterova D, Kren V (2004) Oxidized Derivatives of Silybin and their Antiradical and Antioxidant Activity. Bioorg Med Chem 12 5677... 

Kren V, Kubisch J, Sedmera P, Halada P, Prikrylova V, Jegorov A, Cvak L, Gebhardt R, Ulrichova J, Simanek V (1997) Glycosylation of Silybin. J Chem Soc Perkin Trans I 2467... 

Kubisch J, Sedmera P, Halada P, Gazak R, Skottova N, Simanek V, Kren V (2001) Chemoenzymatic Preparation of Gligoglycosides of Silybin, the Flavonolignan from Silybum marianum. Heterocycles 53 901... 

Intramolecular oxidative coupling of phenols (6, 516). A biomimetic synthesis of natural silybin (3), an interesting antihepatotoxic agent, involves oxidation of a mixture of optically active 1 and coniferyl alcohol (2) with AgO in a benzene-acetone mixture at 55°. This reaction affords a mixture of silybin (3) and isosilybin (4) in 78% yield. The products 3 and 4 are diastereomeric mixtures at C2 and Cs- relative to C2 and... 

Gyorgy, L, Antus, S., Blazovics, A., Foldiak, G. Substituent effects in the free radical reactions of silybin radiation-induced oxidation of the flavonoid at neutral pH. Int. J. Radiat. Biol. 1992 61 603 - 609... 

Buzzelll, G., Moscarella, S., Giusti, A., Duchini, A., Marena, C., Lam-pertfco, M. A pilot study on the liver protective effect of silybin-phosphatidylcholine complex (IdB 1016) in chronic active hepatitis. Int. J. Clin. Pharmacol. Ther. Toxicol. 1993 31 456-460... 

Schandalik, R., Perrucca, E. Pharmacokinetics of silybin following oral administration of silipide in patients with extrahepatic biliary obstruction. Drugs Exp. Clin. Res. 1994 20 37-42... 

Silymarin is a flavonol- lignan mixture obtained from seeds of Silybum marianum. Silymarin is a mixture of silybin, isosilybin, silychristin and silydianin. Silybin A and B are collectively known as silibinin. Silymarin is a reputed hepatoprotective drug. 

---