Ebselen inhibits contractile responses of guinea-pig parenchymal lung strips

Inhibition by ebselen on nitric oxide mediated relaxations in the rat anococcygeus muscle

The effect of 2-phenyl-1,2-benzisoselenazol-3(2H)-one (ebselen) on nitric oxide (NO) mediated responses and NO generation from NO donors were studied in vitro. In precontracted rat isolated anococcygeus muscles, relaxations induced by NO donors, electrical field stimulation and 5-[1-(phenylmethyl)-1H-indazole-3-yl]-2-furanmethanol (YC-1) were significantly inhibited by ebselen (100 microM), whereas responses elicited by papaverine and theophylline were not affected; those by 8-bromo-cyclic-guanosine-monophosphate (8-Br-cGMP) were slightly enhanced. NO generation from NO gas aqueous solution or acidified nitrite was not affected, but that from S-nitroso-N-acetyl-penicillamine (SNAP) was attenuated by ebselen, and the attenuation was reserved by glutathione. Both glutathione and cupric sulphate altered the ultraviolet spectrum of ebselen. These findings suggest that ebselen at high concentrations nonselectively inhibited NO-mediated responses, possibly through inhibiting soluble guanylate cyclase. Ebselen does not appear to directly interact with NO, but it may inhibit NO release from nitrosothiols by a thiol- and/or copper-dependent mechanism.

The chemistry and tumoricidal activity of nitric oxide/hydrogen peroxide and the implications to cell resistance/susceptibility

The mechanism of cytotoxicity of the NO donor 3-morpholino-sydnonimine toward a human ovarian cancer cell line (OVCAR) was examined. It was found that the NO-mediated loss of cell viability was dependent on both NO and hydrogen peroxide (H2O2). Somewhat surprisingly, superoxide (O2) and its reaction product with NO, peroxynitrite (-OONO), did not appear to be di- rectly involved in the observed NO-mediated cytotoxicity against this cancer cell line. The toxicity of NO/H2O2 may be due to the production of a potent oxidant formed via a trace metal-, H202-, and NO-dependent process. Because the combination of NO and H2O2 was found to be particularly cytotoxic, the effect of NO on cellular defense mechanisms involving H2O2 degradation was investigated. It was found that NO was able to inhibit catalase activity but had no effect on the activity of the glutathione peroxidase (GSHPx)-glutathione reductase system. It might therefore be expected that cells that utilize primarily the GSHPx-glutathione reductase system for degrading H2O2 would be somewhat resistant to the cytotoxic effects of NO. Consistent with this idea, it was found that ebselen, a compound with GSHPx-like activity, was able to protect cells against NO toxicity. Also, lowering endogenous GSHPx activity via selenium depletion resulted in an increased susceptibility of the target cells to NO-mediated toxicity. Thus, a possible NO/H2O2/metal-mediated mechanism for cellular toxicity is presented as well as a possible explanation for cell resistance/susceptibility to this NO-initiated process.

Molecular actions of ebselen--an antiinflammatory antioxidant

1. Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one) is a non-toxic seleno-organic drug with antiinflammatory, antiatherosclerotic and cytoprotective properties. 2. Ebselen and some of its metabolites are effective reductants of hydroperoxides including those arising in biomembranes and lipoproteins. 3. By reactions with hydroperoxides and thiols several interconversion cycles are formed which include ebselen metabolites with varying oxidation number of the selenium. 4. In the presence of thiols ebselen mimics the catalytic activities of phospholipid hydroperoxide glutathione peroxidase. 5. Ebselen inhibits at low concentrations a number of enzymes involved in inflammation such as lipoxygenases, NO synthases, NADPH, oxidase, protein kinase C and H+/K(+)-ATPase. The inhibitions are manifested on the cellular level and may contribute to the antiinflammatory potential of ebselen.

Ebselen, a selenoorganic compound as glutathione peroxidase mimic

The selenoorganic compound ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)-one, exhibits activity as an enzyme mimic. The reaction catalyzed is that of a glutathione (GSH) peroxidase (i.e., the reduction of a hydroperoxide at the expense of thiol). The specificity for substrates ranges from hydrogen peroxide and smaller organic hydroperoxides to membrane-bound phospholipid and cholesterol hydroperoxides. In addition to glutathione, the thiol reductant cosubstrate can be dithioerythritol, N-acetylcysteine or dihydrolipoate, or other suitable thiol compounds. Ebselen also has properties such as free radical and singlet oxygen quenching. Model experiments in vitro with liposomes, microsomes, isolated cells, and organs show that the protection against oxidative challenge afforded by ebselen can be explained largely by the activity as GSH peroxidase mimic. Whether this also explains the known preliminary results in clinical settings is yet open. The metabolism and disposition of ebselen is presented in this review. The main point is that the selenium is not bioavailable, explaining the extremely low toxicity observed in animal studies. The occurrence of natural GPx mimics, ovothiol and related compounds, is briefly mentioned.

Ebselen affects calcium homeostasis in human platelets

Ebselen (PZ 51, 2-phenyl-1,2-benzoisoselenazol-3-(2H)-one) is a selenoorganic compound with anti-inflammatory properties. Its pharmacological action is thought to originate from its peroxidase activity which could lower the peroxide tonus required for cyclooxygenase and lipoxygenase activations. From experiments with aspirin-treated human platelets we now present evidence that ebselen also affects intracellular calcium homeostasis by inhibiting the agonist-triggered increase in intracellular calcium. Using Mn2+ entry to quench the fura-2 fluorescence after cell stimulation, we could exclude an interaction of ebselen with receptor-operated calcium channels and therefore an inhibition of extracellular calcium influx. It became evident from whole cell experiments and by using isolated platelet microsomal vesicles that ebselen inhibits the inositol 1,4,5-trisphosphate (IP3) induced calcium release. Besides this inhibitory effect of ebselen on the calcium release higher concentrations of the compound (greater than or equal to 5 microM) induced a calcium release from our microsomal vesicles which also could be reversed by dithiothreitol. An activation of inflammatory cells is usually associated with increased cytosolic calcium concentrations. An inhibition of such calcium movements by ebselen may account for an up to now unidentified anti-inflammatory mechanism of ebselen action which is linked to a direct effect of this compound rather than to its peroxidase-like activity.