Novel 10-substituted antipsoriatic anthrones as inhibitors of epidermal 12-lipoxygenase and lipid peroxidation in membranes

2-Arylalkyl-substituted anthracenones as inhibitors of 12-lipoxygenase enzymes. 2. Structure-activity relationships of the linker chain

A series of 2-arylalkyl-substituted anthracenones were tested as inhibitors of three types of 12-lipoxygenase isoforms in epidermal homogenate of mice, bovine platelets and porcine leukocytes. Their inhibitory activities were compared with those to inhibit the 5-lipoxygenase enzyme in bovine leukocytes. The compounds were synthesised by Marschalk, Wittig or Horner-Emmons reaction at the anthracenedione stage and then reduced to the anthracenones. Structure-activity relationship for the chain linking the anthracenone nucleus and the phenyl ring terminus was investigated. The 2-phenylethyl analogues were among the most potent inhibitors, and 3,4-dimethoxy-substituted 10f was identified as a selective inhibitor of the 12-LO enzymes over 5-LO. Selectivity for 12-LO isoforms was observed with an increase in the overall lipophilicity of the inhibitors. However, none of the linker chains of the 2-substituted anthracenones provided inhibitors that were able to discriminate between the 12-LO isoforms.

Lichen metabolites. 1. Inhibitory action against leukotriene B4 biosynthesis by a non-redox mechanism

Of several lichen metabolites isolated from Parmelia nepalensis and Parmelia tinctorum, the didepsides atranorin (4) and diffractaic acid (5), as well as (+)-protolichesterinic acid (7), inhibited LTB4 biosynthesis in polymorphonuclear leukocytes. Ethyl hematommate (3) and (+)-usnic acid (1) were only weak inhibitors, while methyl beta-orcinolcarboxylate (2) and gyrophoric acid (6) were inactive at concentrations up to 60 microM. Redox properties of the compounds were evaluated in terms of inhibition of nonenzymatic lipid peroxidation in model membranes, reactivity against the stable free radical 2,2-diphenyl-1-picrylhydrazyl, and deoxyribose degradation as a measure of hydroxyl-radical generation. The results revealed that lichen metabolites neither acted as antioxidants against the peroxidation process in model membranes nor did they scavenge or produce free radicals, suggesting that the inhibitory effects on LTB4 biosynthesis was due to specific enzyme interaction rather than a nonspecific redox mechanism.

Antipsoriatic and proinflammatory action of anthralin. Implications for the role of oxygen radicals

Anthralin is among the most effective agents for the topical treatment of psoriasis. However, this drug causes unpleasant side-effects such as inflammation and staining of the nonaffected skin surrounding a psoriatic lesion. The biochemical basis for the induction of an inflammatory response in the skin and the antipsoriatic effectiveness are uncertain, although several cellular targets of anthralin action have been identified. Because no single mechanism is operative, the view was taken that all the effects exerted by anthralin are caused by its redox activity leading to the generation of anthralin free radicals and oxygen radicals. Clear relationships between oxygen-radical production by anthralin and biological response are evident with respect to chemical lesions in cellular macromolecules such as DNA, lipid membranes, and enzymes, indicating that these species account for the antipsoriatic and proinflammatory effects elicited by anthralin. This poses new challenges for the medicinal chemist and provides impetus for identifying novel compounds having potential for an improved therapeutic index.

Effects of reactive oxygen species on the biosynthesis of 12 (S)-hydroxyeicosatetraenoic acid in mouse epidermal homogenate

Arachidonic acid is converted to 12-hydroxyeicosatetraenoic acid (12-HETE) in a homogenate of mouse epidermal cells. When the epidermal homogenate was preincubated with scavengers of reactive oxygen species (ROS), catalase or superoxide dismutase, significantly larger amounts of 12-HETE were produced as compared to untreated controls, suggesting that 12-lipoxygenase is quite prone to inactivation by ROS and peroxides. Mouse epidermal homogenate was then exposed to nine different ROS-generating systems to study the effects of superoxide, hydrogen peroxide, singlet oxygen, hypochlorite, peroxyl radicals, and alkyl hydroperoxides on the enzyme activity. Analysis by chiral phase high performance liquid chromatography demonstrated that the 12-HETE biosynthesized from arachidonic acid by mouse epidermal homogenate was the 12 (S)-enantiomer and excludes oxidation of arachidonic acid by ROS in a nonspecific free radical mechanism which leads to racemic 12-HETE. ROS generated by the interaction of xanthine with xanthine oxidase strongly inhibited epidermal 12 (S)-HETE biosynthesis. A flux of 0.7 nmol of superoxide/min/ml of reaction medium resulted in more than 50% inhibition of epidermal 12-lipoxygenase activity. The decrease in 12 (S)-HETE biosynthesis appeared to involve both superoxide and hydrogen peroxide. The efficacy of the latter species was also documented by exposure of mouse epidermal 12-lipoxygenase to glucose and glucose oxidase, which resulted in similar inhibitory effects on 12 (S)-HETE biosynthesis. The presence of the iron chelator diethylenetriaminepentaacetic acid during incubation of epidermal 12-lipoxygenase with both the xanthine/xanthine oxidase or the glucose/glucose oxidase systems partially protected the enzyme against inhibition, indicating that hydroxyl radical contributes to the overall inhibitory effect. Also, organic hydroperoxides inhibited epidermal 12-lipoxygenase, whereas singlet oxygen, hypochlorite, and peroxyl radicals were not effective. The results of this study lead to the proposal that 12-lipoxygenase activity may be regulated by ROS such as hydrogen peroxides, superoxide, and hydroxyl radicals.

Antipsoriatic anthrones: aspects of oxygen radical formation, challenges and prospects

1. Antipsoriatic anthrones are among the most commonly used topical agents for the treatment of psoriasis. 2. These drugs generate reactive oxygen species during their auto-oxidation under physiological conditions. 3. Recent studies have indicated that activation of molecular oxygen by anthrones may play a critical role in their mechanism of action at the molecular level. 4. This article summarizes the evidence pointing toward the significance of oxygen activation and radical formation in the antipsoriatic action and induction of skin inflammation of anthrones. 5. Also, the role of oxygen radical formation in the development of novel anthrones devoid of inflammatory and staining properties is discussed.

Antipsoriatic anthrones with modulated redox properties. 3. 10-thio-substituted 1,8-dihydroxy-9(10H)-anthracenones as inhibitors of keratinocyte growth, 5-lipoxygenase, and the formation of 12(S)-HETE in mouse epidermis

The synthesis of a series of 1,8-dihydroxy-9(10H)-anthracenones bearing sulfur-linked substituents in the 10-position is described. These compounds were evaluated for their ability to inhibit the growth of the human keratinocyte cell line HaCaT and the 5- and 12-lipoxygenase enzymes in bovine polymorphonuclear leukocytes and mouse epidermal homogenate, respectively. In addition, the following redox properties of the compounds were determined: reactivity against 2,2-diphenyl-1-picrylhydrazyl, generation of hydroxyl radicals as measured by deoxyribose degradation, and inhibition of lipid peroxidation in model membranes. Compounds 4e and 4h of this series compare favorably in the cellular assays with the antipsoriatic anthralin. They have the combined inhibitory action against leukotriene B4 and 12(S)-HETE formation and are highly potent antiproliferative agents against keratinocyte growth. In contrast to anthralin, 4h, 1,8-dihydroxy-10-[(4-hydroxyphenyl)thio]-9(10H)-anthracenone, is not cytotoxic as documented by the LDH activity released from cytoplasm of keratinocytes and does not enhance lipid peroxidation in model membranes.

Inactivation of mouse epidermal 12-lipoxygenase by anthralin--implications for the role of oxygen radicals

In activation of 12-lipoxygenase (12-LO) in mouse epidermal homogenate by the antipsoriatic drug anthralin has been studied in detail. In view of the chemical instability of anthralin in a physiological buffer, the biological effects ascribed to the molecule itself may be related to some of its breakdown products. However, the inhibitory activity could not be attributed to the known stable oxidation product of anthralin, danthron, which did not decrease (12-LO activity. Addition of the antioxidants 2,6-di-tert-butyl-4-methylphenol (BHT) or beta-carotene, or the hydroxyl radical scavenger sodium benzoate, protected against anthralin-mediated 12-LO inactivation, suggesting that pro-oxidant species derived from anthralin play a key role in the inhibitory action. Even though inhibitory effects of anthralin against catalase and superoxide dismutase (SOD) have been observed under the conditions applied in this study, these antioxidant enzymes also partially prevented the inhibition of 12-LO by anthralin when added to the incubation mixtures. Control experiments without anthralin revealed that the oxygen radical scavengers and antioxidant enzymes, themselves, did not appreciably influence epidermal 12-LO activity. A mechanism underlying the inactivation of epidermal 12-LO by anthralin is proposed, which involves active oxygen species formed during the auto-oxidation of the drug.