Toxicity of 2,3-dialkyl-1,4-naphthoquinones in rats: comparison with cytotoxicity in vitro

The short-term toxicities of 2-methyl-1,4-naphthoquinone and a series of 2,3-dialkyl-1,4-naphthoquinones have been determined in rats and compared with their ability to cause oxidative damage to erythrocytes in vitro. In accord with previous results, 2-methyl-1,4-naphthoquinone caused marked oxidative damage to erythrocytes in vitro and haemolytic anaemia in rats. The dialkylnaphthoquinones were also haemolytic agents in vivo, with 2,3-dimethyl-1,4-naphthoquinone being particularly active. Unlike the monoalkyl derivative, however, these substances caused little or no damage to red cells in vitro. The in vivo toxicity of dialkylnaphthoquinones cannot, therefore, be predicted on the basis of in vitro cytotoxicity tests.

[Studies on 1,4-naphthoquinones. 23. 2-3-Halogen-2-3-nitrophenyljuglone derivatives, synthesis and cytotoxic properties]

Radical arylation of 1,4-naphthoquinones is possible by oxidative decarboxylation of 2- and 4-nitrobenzoic acid. With 2- and 3-halogenojuglone derivatives the 4-nitrophenyl compounds 10-13 are prepared in good, the 2-nitrophenyl analogues 14-17 only in poor yields. All compounds are inactive against the virus types HSV and HIV and their marked cytotoxicity in part depends on the substitution pattern. The 3-aryl and o-nitro derivatives are more active than their 2-aryl and p-nitro analogues and the influence of the chloro or bromo substitution is only remarkable in the case of 12/13.

Comparative toxicity of alkyl-1,4-naphthoquinones in rats: relationship to free radical production in vitro

2-Methyl-1,4-naphthoquinone causes haemolysis in vivo. This toxic effect is believed to result from oxidative damage to erythrocytes by "active oxygen" species formed via one-electron reduction of the naphthoquinone by oxyhaemoglobin. In the present investigation, seven 2-alkyl-1,4-naphtoquinones have been studied with regard to their haemolytic activity in rats, their ability to cause oxidative damage in erythrocytes in vitro, and their reactivity toward oxyhaemoglobin. A close correlation was observed between the in vivo and in vitro parameters, suggesting that the proposed mechanism of toxicity of 2-methyl-1,4-naphthoquinone is correct and is also applicable to other alkylnaphthoquinones.

In vitro cytotoxicity of 1,4-naphthoquinone derivatives to replicating cells

The acute cytotoxicities of a series of alkyl-1,4-naphthoquinones (NQ) and of 2-hydroxy-3-alkyl-1,4-NQs, as well as some amino derivatives, were evaluated with the neutral red cytotoxicity assay, using BALB/c mouse 3T3 fibroblasts. As compared to the unsubstituted 1,4-NQ: (i) Substitution at the 2 position reduced toxicity, with the extent of reduction following the sequence, hydroxyl >> dimethylamino >> C1-C5 alkyl group. (ii) Substitution with C2-C5 alkyl groups at position 3 enhanced toxicity. As noted with the n-alkyl-1,4-NQs, increasing the chain length of the 2-hydroxy-3-alkyl-1,4-NQs did not appreciably change potency of the test agent. (iii) Substitution with amino groups at position 3 had little effect on cytotoxicity. Some differences in cytotoxicity of specific test agent were noted between the 3T3 fibroblasts and isolated rat hepatocytes, as reported in the literature.

Effects of 1,4-naphthoquinone derivatives on red blood cell metabolism

The effect on red blood cell metabolism of a series of substituted 1,4-naphthoquinones has been investigated. 2-Methoxy-1,4-naphthoquinone was found to be a potent oxidative compound, generating hydrogen peroxide in erythrocytes and causing both methemoglobin formation and glutathione depletion in the absence of glucose. Flux of glucose through both glycolysis and the hexose monophosphate shunt was stimulated. 2-Hydroxy- and 2,3-dihydroxy-1,4-naphthoquinone were less oxidative. Both compounds caused oxidation of glutathione and formation of hydrogen peroxide with corresponding stimulation of the hexose monophosphate shunt, but did not cause methemoglobin formation. 2-Hydroxy-3-alkyl-1,4-naphthoquinones were not oxidative but did increase the flux of glucose through glycolysis, possibly reflecting membranal damage. The in vitro oxidative effects of these substances do not correlate with their hemolytic activity in rats, indicating that factors other than oxidative damage are important in mediating the in vivo toxicity of these substances.

The role of oxidative processes in the cytotoxicity of substituted 1,4-naphthoquinones in isolated hepatocytes

In order to clarify the role of oxidative processes in cytotoxicity we have studied the metabolism and toxicity of 2-methyl-1,4-naphthoquinone (menadione) and its 2,3 dimethyl (DMNQ) and 2,3 diethyl (DENQ) analogs in isolated rat hepatocytes. The two analogs, unlike menadione, cannot alkylate nucleophiles directly and were considerably less toxic than menadione. This decreased toxicity was consistent with the inability of DMNQ and DENQ to alkylate but we also found them to undergo lower rates of redox cycling in hepatocytes and a higher ratio of two electron as opposed to one electron reduction relative to menadione. Thus, facile analysis of the respective roles of alkylation and oxidation in cytotoxicity was not possible using these compounds. In hepatocytes pretreated with bischloroethyl-nitrosourea (BCNU) to inhibit glutathione reductase, all three naphthoquinones caused a potentiation of reduced glutathione (GSH) removal/oxidized glutathione (GSSG) generation and cytotoxicity relative to that observed in control cells. These data show that inhibition of hepatocyte glutathione reductase by BCNU results in enhanced naphthoquinone-induced oxidative challenge and subsequent cellular toxicity. That DMNQ and DENQ are cytotoxic, albeit at high concentrations, and that this cytotoxicity is potentiated by BCNU pretreatment suggest that oxidative processes alone can be a determinant of cytotoxicity.