In this study, we demonstrate that N(6)-isopentenyladenosine, which essentially is a plant cytokinin-like compound, exerts a potent and selective antiviral effect on the replication of human enterovirus 71 with an EC50 of 1.0 ± 0.2 μM and a selectivity index (SI) of 5.7. The synthesis of analogs with modification of the N(6)-position did not result in a lower EC50 value. However, in particular with the synthesis of N(6)-(5-hexene-2-yne-1-yl)adenosine (EC50 = 4.3 ± 1.5 μM), the selectivity index was significantly increased: because of a reduction in the adverse effect of this compound on the host cells, an SI > 101 could be calculated. With this study, we for the first time provide proof that a compound class that is based on the plant cytokinin skeleton offers an interesting starting point for the development of novel antivirals against mammalian viruses, in the present context in particular against enterovirus 71.

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A series of N6-adenosine derivatives were synthesized by alkylation of N6-acetyl-2′,3′,5′-tri-O-acetyladenosine (1) with alkyl halides and alcohols. It was shown that propargyl derivative 2a is a good substrate for copper(I) catalyzed Huisgen [3+2] cycloaddition with azides. This click-reaction can be used for preparation of the libraries of 1,2,3-triazolyl modified adenosines. Biological activities of N6-adenosines were studied in two plant and six human cancer cell assays. The remarkable parallel between cytokinin and cytotoxic activities was found. The most cytokinin active compounds 3c–3e at the same time appeared to be the most potent cytotoxic agents.

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