Tomáš Filipský

Cardiovascular effects of flavonoids are not caused only by direct antioxidant activity.

Epidemiological, as well as most in vivo, studies suggest that flavonoids have a positive influence on various cardiovascular diseases. Traditionally, these effects were only attributed to their antioxidant activity, which has been extensively studied. Apart from the direct antioxidant properties, which include direct reactive oxygen species scavenging activity and transient metal chelation, this review reports on many other effects that in pharmacologically achievable concentrations may also be responsible for their positive cardiovascular influence. These include direct inhibition of some radical-forming enzymes (xanthine oxidase, NADPH oxidase, and lipoxygenases), decreased platelet aggregation and leukocyte adhesion, and vasodilatory properties. For each of the aforementioned effects different structural features are necessary. Briefly, a catecholic B-ring is necessary for scavenging activity; hydroxyl groups in an ortho position, the 3-hydroxy-4-keto group, or the 5-hydroxy-4-keto group enable iron chelation; planar conformation with the 4-keto group and 2,3-double bond is essential for inhibition of leukocyte adhesion and platelet aggregation; specific hydroxy-methoxy ortho conformation in ring B is necessary for the inhibition of NADPH oxidase; and the 4-keto group is a requisite for vasodilatory action. This review shows that positive cardiovascular effects of flavonoids are achieved by various flavonoids via the interaction with different targets.

In vitro analysis of iron chelating activity of flavonoids

Flavonoids have been demonstrated to possess miscellaneous health benefits which are, at least partly, associated with iron chelation. In this in vitro study, 26 flavonoids from different subclasses were analyzed for their iron chelating activity and stability of the formed complexes in four patho/physiologically relevant pH conditions (4.5, 5.5, 6.8, and 7.5) and compared with clinically used iron chelator deferoxamine. The study demonstrated that the most effective iron binding site of flavonoids represents 6,7-dihydroxy structure. This site is incorporated in baicalein structure which formed, similarly to deferoxamine, the complexes with iron in the stoichiometry 1:1 and was not inferior in all tested pH to deferoxamine. The 3-hydroxy-4-keto conformation together with 2,3-double bond and the catecholic B ring were associated with a substantial iron chelation although the latter did not play an essential role at more acidic conditions. In agreement, quercetin and myricetin possessing all three structural requirements were similarly active to baicalein or deferoxamine at the neutral conditions, but were clearly less active in lower pH. The 5-hydroxy-4-keto site was less efficient and the complexes of iron in this site were not stable at the acidic conditions. Isolated keto, hydroxyl, methoxyl groups or an ortho methoxy-hydroxy groups were not associated with iron chelation at all.

Iron reduction potentiates hydroxyl radical formation only in flavonols

Flavonoids, substantial components of the human diet, are generally considered to be beneficial. However, they may possess possible pro-oxidative effects, which could be based on their reducing potential. The aims of this study were to evaluate the ability of 26 flavonoids to reduce ferric ions at relevant pH conditions and to find a possible relationship with potentiation of hydroxyl radical production. A substantial ferric ions reduction was achieved under acidic conditions, particularly by flavonols and flavanols with the catecholic ring B. Apparently corresponding bell-shaped curves displaying the pro-oxidant effect of flavonols quercetin and kaempferol on iron-based Fenton reaction were documented. Several flavonoids were efficient antioxidants at very low concentrations but rather inefficient or pro-oxidative at higher concentrations. Flavonols, morin and rutin were progressively pro-oxidant, while 7-hydroxyflavone and hesperetin were the only flavonoids with dose-dependent inhibition of hydroxyl radical production. Conclusively, administration of flavonoids may lead to unpredictable consequences with few exceptions.

In vitro evaluation of copper-chelating properties of flavonoids

Copper is an essential trace element involved in plenty of redox reactions in living systems, however, unbound copper ions cause damage to various biomolecules via excessive generation of reactive oxygen species. Flavonoids, ubiquitous plant secondary metabolites, possess complex effects on human health and chelation of transient metal ions is one of their proposed mechanisms of action. In this in vitro study, 26 flavonoids from various subclasses were screened for their interactions with both copper oxidation states at four (patho)physiologically relevant pH conditions (4.5, 5.5, 6.8 and 7.5) by two spectrophotometric approaches and compared with the clinically used copper chelator trientine. In a slightly competitive environment, the majority of flavonoids were able to chelate cupric ions, however, under more competitive conditions, only flavones and flavonols were able to chelate both cupric and cuprous ions. Apparently, the 2,3-double bond was essential for stable copper chelation. The most efficient copper chelation sites were the 3-hydroxy-4-keto group in flavonols and the 5,6,7-trihydroxyl group in flavones. On the other hand, the 3′,4′-dihydroxyl group was associated only with a weak activity. 3-Hydroxyflavone, kaempferol and partly baicalein were even more potent than trientine in the acidic environment, however, none of the tested flavonoids was able to surpass it at physiological pH or slightly acidic conditions. In conclusion, flavonoids possessing appropriate structural characteristics were efficient copper chelators and some of them were even more potent than trientine under acidic conditions.

Protection of Cells against Oxidative Stress by Nanomolar Levels of Hydroxyflavones Indicates a New Type of Intracellular Antioxidant Mechanism

Natural polyphenol compounds are often good antioxidants, but they also cause damage to cells through more or less specific interactions with proteins. To distinguish antioxidant activity from cytotoxic effects we have tested four structurally related hydroxyflavones (baicalein, mosloflavone, negletein, and 5,6-dihydroxyflavone) at very low and physiologically relevant levels, using two different cell lines, L-6 myoblasts and THP-1 monocytes. Measurements using intracellular fluorescent probes and electron paramagnetic resonance spectroscopy in combination with cytotoxicity assays showed strong antioxidant activities for baicalein and 5,6-dihydroxyflavone at picomolar concentrations, while 10 nM partially protected monocytes against the strong oxidative stress induced by 200 µM cumene hydroperoxide. Wide range dose-dependence curves were introduced to characterize and distinguish the mechanism and targets of different flavone antioxidants, and identify cytotoxic effects which only became detectable at micromolar concentrations. Analysis of these dose-dependence curves made it possible to exclude a protein-mediated antioxidant response, as well as a mechanism based on the simple stoichiometric scavenging of radicals. The results demonstrate that these flavones do not act on the same radicals as the flavonol quercetin. Considering the normal concentrations of all the endogenous antioxidants in cells, the addition of picomolar or nanomolar levels of these flavones should not be expected to produce any detectable increase in the total cellular antioxidant capacity. The significant intracellular antioxidant activity observed with 1 pM baicalein means that it must be scavenging radicals that for some reason are not eliminated by the endogenous antioxidants. The strong antioxidant effects found suggest these flavones, as well as quercetin and similar polyphenolic antioxidants, at physiologically relevant concentrations act as redox mediators to enable endogenous antioxidants to reach and scavenge different pools of otherwise inaccessible radicals.

Novel method for rapid copper chelation assessment confirmed low affinity of D-penicillamine for copper in comparison with trientine and 8-hydroxyquinolines

Copper is an essential trace element involved in many physiological processes. Since disorder of copper homeostasis is observed in various pathologies, copper chelators may represent a promising therapeutic tool. This study was aimed at: 1) formation of an in vitro methodology for screening of copper chelators, and 2) detailed analysis of the interaction of copper with clinically used D-penicillamine (D-PEN), triethylenetetramine (trientine), experimentally tested 8-hydroxyquinolines, and the disodium salt of EDTA as a standard chelator. Methodology based on bathocuproinedisulfonic acid disodium salt (BCS), usable at (patho)physiologically relevant pHs (4.5-7.5), enabled assessment of both cuprous and cupric ions chelation and comparison of the relative affinities of the tested compounds for copper. In the case of potent chelators, the stoichiometry could be estimated too. Clioquinol, chloroxine and EDTA formed very stable complexes with Cu(+)/Cu(2+) at all tested pHs, while copper complexes with trientine were stable only under neutral or slightly acidic conditions. Non-substituted 8-hydroxyquinoline was a less efficient copper chelator, but still unequivocally more potent than D-PEN. Both 8-hydroxyquinoline and D-PEN chelation potencies, similarly to that of trientine, were pH-dependent and decreased with pH. Moreover, only D-PEN was able to reduce cupric ions. Conclusively, BCS assay represents a rapid, simple and precise method for copper chelation measurement. In addition, lower binding affinity of D-PEN compared with 8-hydroxyquinolines and trientine was demonstrated.

Amino acid derivatives as transdermal permeation enhancers

Transdermal permeation enhancers are compounds that temporarily decrease skin barrier properties to promote drug flux. In this study, we investigated enhancers with amino acids (proline, sarcosine, alanine, β-alanine, and glycine) attached to hydrophobic chain(s) via a biodegradable ester link. The double-chain lipid-like substances displayed no enhancing effect, whereas single-chain substances significantly increased skin permeability. The proline derivative l-Pro2 reached enhancement ratios of up to 40 at 1% concentration, which is higher than that of the well-established and standard enhancers Azone, DDAIP, DDAK, and Transkarbam 12. No stereoselectivity was observed. l-Pro2 acted synergistically with propylene glycol. Infrared studies revealed that l-Pro2 forms a separate liquid ordered phase in the stratum corneum lipids and has no significant effect on proteins. l-Pro2 action was at least partially reversible as measured by skin electrical impedance. Toxicity in keratinocyte (HaCaT) and fibroblast (3T3) cell lines showed IC(50) values ranging from tens to hundreds of μM, which is comparable with standard enhancers. Furthermore, l-Pro2 was rapidly decomposed in plasma. In vivo transdermal absorption studies in rats confirmed the enhancing activity of l-Pro2 and suggested its negligible skin toxicity and minimal effect on transepidermal water loss. These properties make l-Pro2 a promising candidate for potential clinical use.

Mathematical calculations of iron complex stoichiometry by direct UV-Vis spectrophotometry.

The effects of iron-chelating agents on miscellaneous pathologies are currently largely tested. Due to various indications, different properties for chelators are required. A stoichiometry of the complex in relation to pH is one of the crucial factors. Moreover, the published data on the stoichiometry, especially concerning flavonoids, are equivocal. In this study, a new complementary approach was employed for the determination of stoichiometry in 10 iron-chelating agents, including clinically used drugs, by UV-Vis spectrophotometry at relevant pH conditions and compared with the standard Jobs method. This study showed that the simple approach based on absorbance at the wavelength of complex absorption maximum was sufficient when the difference between absorption maximum of substance and complex was high. However, in majority of substances this difference was much lower (9-73 nm). The novel complementary approach was able to determine the stoichiometry in all tested cases. The major benefit of this method compared to the standard Jobs approach seems to be its capability to reveal a reaction stoichiometry in chelators with moderate affinity to iron. In conclusion, using this complementary method may explain several previous contradictory data and lead to a better understanding of the underlying mechanisms of chelators action.

In vitro platelet antiaggregatory properties of 4-methylcoumarins.

Platelets play a crucial role in physiological haemostasis. However, in coronary arteries damaged by atherosclerosis, enhanced platelet aggregation, with subsequent thrombus formation, is a precipitating factor in acute myocardial infarction. Current therapeutic approaches are able to reduce approximately one quarter of cardiovascular events, but they are associated with an increased risk of bleeding and in some resistant patients are not efficient. Some coumarins possess antiplatelet activity and, due to their additional antioxidant effects, may be promising drugs for use in combination with the present therapeutic agents. The aim of this study was to analyse a series of simple 4-methylcoumarins for their antiplatelet activity. Human plasma platelet suspensions were treated with different aggregation inducers [arachidonic acid (AA), collagen and ADP] in the presence of the 4-methylcoumarins. Complementary experiments were performed to explain the mechanism of action. 5,7-Dihydroxy-4-methylcoumarins, in particular those containing a lipophilic side chain at C-3, reached the activity of acetylsalicylic acid on AA-induced aggregation. Other tested coumarins were less active. Some of the tested compounds mildly inhibited either collagen- or ADP-induced aggregation. 5,7-Dihydroxy-4-methylcoumarins did not interfere with the function of thromboxane synthase, but were competitive antagonists of thromboxane A(2) receptors and inhibited cyclooxygenase-1 as well. 5,7-Dihydroxy-4-methylcoumarins appear to be promising candidates for the extension of the current spectrum of antiplatelet drugs.

Simultaneous determination of the novel thiosemicarbazone anti‐cancer agent, Bp4eT, and its main phase I metabolites in plasma: Application to a pilot pharmacokinetic study in rats

Novel thiosemicarbazone metal chelators are extensively studied anti-cancer agents with marked and selective activity against a wide variety of cancer cells, as well as human tumor xenografts in mice. This study describes the first validated LC-MS/MS method for the simultaneous quantification of 2-benzoylpyridine 4-ethyl-3-thiosemicarbazone (Bp4eT) and its main metabolites (E/Z isomers of the semicarbazone structure, M1-E and M1-Z, and the amidrazone metabolite, M2) in plasma. Separation was achieved using a C18 column with ammonium formate/acetonitrile mixture as the mobile phase. Plasma samples were treated using solid-phase extraction on 96-well plates. This method was validated over the concentration range of 0.18-2.80 μM for Bp4eT, 0.02-0.37 μM for both M1-E and M1-Z, and 0.10-1.60 μM for M2. This methodology was applied to the analysis of samples from in vivo experiments, allowing for the concentration-time profile to be simultaneously assessed for the parent drug and its metabolites. The current study addresses the lack of knowledge regarding the quantitative analysis of thiosemicarbazone anti-cancer drugs and their metabolites in plasma and provides the first pharmacokinetic data on a lead compound of this class.