Synthesis of 1-(2'-O-Methyl-ß-D-ribofuranosyl)-1H-imidazo[4,5-d]pyridazine-4,7(5H,6H)-dione: An Attractive Building Block for Antisense and Triple-helical Applications

Nucleotide triphosphatase/helicase of hepatitis C virus as a target for antiviral therapy

The RNA nucleoside triphosphatase (NTPase)/helicases represent a large family of proteins that are detected in almost all biological systems where RNA plays a central role. The enzymes are capable of enzymatically unwinding duplex RNA structures by disrupting the hydrogen bonds that keep the two strands together. The strand separating activity is associated with hydrolysis of nucleoside triphosphate (NTP). Because of this, potential specific inhibitors of NTPase/helicases could act by one or more of the following mechanisms: (i) inhibition of NTPase activity by interference with NTP binding, (ii) inhibition of NTPase activity by an allosteric mechanism and (iii) inhibition of the coupling of NTP hydrolysis at the unwinding reaction. There are also other inhibitory mechanisms conceivable, which may involve a modulation of the interaction of the enzyme with its RNA substrate, for example, (iv) the competitive inhibition of RNA binding and (v) the inhibition of the unwinding by sterical blockade of the translocation of the NTPase/helicase along the polynucleotide chain. NTPase/helicase has also been identified in the viral genome of hepatitis C virus (HCV) which is a member of the Flaviviridae family. It is conceivable that the inhibition of the unwinding activity of the enzyme leads to the inhibition of virus replication and this may represent a novel antiviral strategy. This review updates the current spectrum of inhibitors targeting different mechanisms by which the NTPase and/or helicase activities of the HCV NTPase/helicase are inhibited. Consequently, some of the compounds might be important as antiviral agents against HCV.

Characterization of imidazo[4,5-d]pyridazine nucleosides as modulators of unwinding reaction mediated by West Nile virus nucleoside triphosphatase/helicase: evidence for activity on the level of substrate and/or enzyme

Compounds that interact with DNA or RNA generally act as inhibitors of enzymes that unwind DNA or RNA. In the present study we describe the synthesis and properties of some nucleoside analogues that interact with double-stranded DNA but that, in contrast, facilitate the unwinding reaction mediated by West Nile (WN) virus nucleoside triphosphatase (NTPase)/helicase. The nucleoside analogues described, 1-(2'-O-methyl-beta-D-ribofuranosyl)imidazo[4,5-d]pyridazine-4,7(5H,6H)-dione (HMC-HO4), 1-(beta-D-ribofuranosyl)imidazo[4,5-d]pyridazine-4,7(5H,6H)-dione, and 1-(2'-deoxy-alpha-D-ribofuranosyl)imidazo[4,5-d]pyridazine-4,7(5H,6H)dione, all contain the imidazo[4,5-d]pyridazine ring system. The extent of the enhancing effect on helicase activity was found to be dependent on the time of exposure of the DNA substrate to the compounds and their concentrations. The nucleoside analogues were nevertheless found to be capable of uncoupling the ATPase and helicase activities of the enzyme by a mechanism operating on the level of the enzyme. Thus, in the case of HMC-HO4, the direct interaction with the enzyme caused inhibition of its helicase activity, with a half-maximal inhibitory concentration of 30 microM. The similar potency of the compound against replication of WN virus in cell culture suggests that inhibition of the helicase activity of the viral enzyme is responsible for the observed antiviral activity of HMC-HO4 and may indeed represent an important mode of action of antiviral drugs in general. Comparative studies performed with the related NTPase/helicase from hepatitis C virus revealed that the extent of the effects mediated by imidazo[4,5-d]pyridazine nucleosides is enzyme specific. The substances described may represent a starting point for the development of a new class of helicase-specific antivirals.