Unsaturated Keto and Exomethylene Pyranonucleoside Analogues of Thymine and Uracil Exhibit Potent Antioxidant Properties

Nucleoside analogues play an important role in the development of antitumor and antiviral agents. Specific sugar modified pyranonucleosides, like the keto and exocyclic methylene nucleosides, have been studied for their biological properties, but there is little information regarding their antioxidant activity. The present study reports the antioxidant activity of a series of α,β-unsaturated 2’or 4’keto and exomethylene 5’-hydroxymethyl-lacking pyranonucleosides. The antioxidant activity was evaluated using an in vitro assay which is based on the capacity to protect DNA strand scission induced by peroxyl radicals (ROO). The majority of the tested nucleoside analogues exhibit potent antioxidant properties against ROO radicals. We conclude that the presence of a carbon-carbon double bond at α,β-disposition to exomethylene group at position 2 of the sugar moiety and the substitution of thymine with uracil improves the antioxidant capacity of these analogues.

In order to investigate the antioxidant properties of nucleoside analogues we have previously synthesized a new class of unsaturated 3'-fluoro-4'-ketonucleosides, that of N 4 -benzoyl cytosine and N 6 -benzoyl adenine, respectively [19].Most of the aforementioned com-pounds showed significant ability to protect DNA from the strand breaking activity of ROO • radicals.Furthermore, those nucleoside analogues containing an α,β-unsaturated keto system were the most potent against the activity of ROO • radicals.
In extending these studies the antioxidant activity of a series of α,β-unsaturated 2'-or 4'-keto and exomethylene 5'-hydroxymethyl-lacking pyranonucleoside analogues was investigated.Specifically, the present study is the first attempt to correlate structural modifications of the aforementioned nucleoside analogues with the ability to inhibit ROO • radicals-induced DNA damage.

Results
Five out of the eight tested nucleoside analogues inhibited the DNA damage induced by ROO • radicals (Table 1).Compound 4 was the most potent as it exhibited 22% inhibition of radical-induced DNA damage at the concentration of 20 μΜ.It should be noted that all compounds had no effect on plasmid conformation when they were tested alone at the highest concentration.The presence of an exomethylene group at 2' position compared to the 4' position of the sugar moiety seems to be important for the antioxidant properties of the nucleoside analogues.Specifically, compounds 1 and 3 which contain the exomethylene group at 4' position of the sugar moiety had no activity, while their corresponding compounds 2 and 4 which contain the exomethylene group at 2' position exhibited potent antioxidant activity (Table 1).Compound 2 inhibited ROO • radicals to 16% and 36% at 50 and 100 μM respectively and compound 4 was a potent inhibitor even at 20 μΜ (Table 1 and Figure 1).The differences in the potency observed between compounds 2 and 4 can be attributed to the different nucleobase (thymine and uracil respectively).
Similar to the exomethylene group the effect of the position of an unsaturated keto system in these nucleoside analogues was examined.Compound 5 had no inhibitory activity at any concentration, while compound 6 had antioxidant activity only at the highest tested concentration (100 μΜ).Both compounds have thymine as nucleobase but differ at the position of the unsaturated keto system.It is possible that the translocation of the keto group at position 2' favors the antioxidant activity of the compounds.However, this is not the case when thymine is replaced by uracil, as exhibited by the similar antioxidant capacities of compounds 7 and 8 (22% and 19% inhibition at 100 μΜ respectively).

Discussions
The present study reports the antioxidant properties of α,β-unsaturated keto and exomethylene D-arabino-and D-lyxo-pyranonucleoside analogues with thymine and uracil as heterocyclic base.Specifically, these nucleoside analogues were evaluated for the ability to inhibit ROO • radicals-induced DNA damage.
Our results demonstrate that the α,β-unsaturated 2'exomethylene nucleosides exhibit potent antioxidant activities.This property is further reinforced when the nucleobase thymine is replaced by uracil.The potent antioxidant properties of these compounds can be explained by a radical stabilization resonance effect, which can be attributed to their structural properties.It should be mentioned that the α,β-unsaturated 4'-exomethylene nucleosides had no antioxidant properties.
In contrast to the exomethylene group the influence of the keto group in the antioxidant properties of these compounds is less efficient.The α,β-unsaturated 2'-keto nucleosides exhibit antioxidant properties only at concentrations of 100 μM.The 2'-keto and the 4'-keto uracil nucleoside analogues showed similar antioxidant properties which does not apply to the 2'-keto and the 4'-keto thymine analogues.These results point to a nucleobase preference since the substitution of thymine with the smaller uracil leads to compounds with increased antioxidant abilities.It seems that uracil might be beneficial for the interaction of these compounds with the specific radicals.

Conclusions
The results of this study demonstrate that the presence of a carbon-carbon double bond at α,β-disposition to exomethylene group at 2'-position of the sugar moiety and uracil as nucleobase improves the antioxidant capacity of the nucleoside analogues.These might be necessary structural modifications that favor the interaction of these nucleosides with the radicals.ROO • radicals are involved as a major initiating factor in lipid peroxidation chain reactions [22].Thus, the ability of the tested compounds to protect DNA strand breakage by scavenging peroxyl radicals could suggest that these compounds may also prevent lipid peroxidation.Based on the above findings it would be interesting to further investigate the potential effectiveness of these nucleoside analogues in the prevention and probably the treatment of diseases caused by overproduction of free radicals.Further in vitro studies are required to elucidate the exact mechanisms involved in the antioxidant activity of these compounds.thesized [18] and analogues 2, 4, 6 and 8 were also previously synthesized [23].All analogues were freshly prepared in DMSO.

Peroxyl Radical-Induced DNA Strand Scission Assay
The assay was performed using the method described by Chang et al. [24].Peroxyl radicals were generated from thermal decomposition of AAPH.The reaction mixture (10 μL) containing 1 μg Bluescript-SK + plasmid DNA, 2.5 mM AAPH in phosphate-buffered saline (PBS: 137 mM NaCl, 2.7 mM KCl, 8.1 mM Na 2 HPO 4 , 1.5 mM KH 2 PO 4 ) and the tested product at different concentrations (5,10,20, 50, 100 μM) was incubated in darkness for 45 min at 37˚C.AAPH was added last right before incubation.The reaction was terminated by the addition of 3 μL loading buffer (0.25% bromophenol blue and 30% glycerol) and analyzed in 0.8% agarose gel electrophoresis at 70 V for 1 h.The gels were stained with ethidium bromide (0.5 μg/mL), destained with water, photographed by UV translumination using the Vilber Lourmat photodocumentation system (DP-001.FDC) (Torcy, France) and analyzed with Gel-Pro Analyzer version 3.0 (MediaCybernetics, Silver Spring, USA).Each experiment was carried out in triplicate.The use of DMSO at the tested concentrations did not affect the results of the assay.

Inhibition of Free Radical-Induced DNA Damage
The induction of DNA strand breaks by peroxyl (ROO where S o is the percentage of supercoiled conforma-tion in the negative control sample (plasmid DNA alone), S p is the percentage of supercoiled conformation in the positive control sample (plasmid DNA with the radical initiating factor) and S is the percentage of supercoiled conformation in the sample containing plasmid DNA, the tested compound and the radical initiating factor.It should be noted that prior to treatment Bluescript-SK + plasmid DNA contained approximately 10% -20% open circular DNA.

Statistical Analysis
All results are expressed as mean ±SD (n = 3).Statistical computations were carried out using the SPSS 13.0 software.For statistical analysis, one-way ANOVA was applied followed by Dunnett's test for multiple pair-wise comparisons.Dose response relationships were examined by Spearman's correlation analysis.Differences were considered significant at p < 0.05.
‡ NI: no significant inhibition.† Values are the means ± SΕ of the percent inhibition from three independent experiments.* p < 0.05 when compared with control (plasmid DNA plus AAPH).Thy: Thymine, U: Uracil.Unsaturated Keto and Exomethylene Pyranonucleoside Analogues of Thymine and Uracil Exhibit Potent Antioxidant Properties 125