Many viruses within the Flavivirus genus cause significant disease in humans; 
however, effective antivirals against these viruses are not currently available. 
We have previously shown that a thiopurine drug, 6-methylmercaptopurine riboside 
(6MMPr), inhibits replication of distantly related viruses within the 
Flaviviridae family in cell culture, including bovine viral diarrhea virus and 
hepatitis C virus replicon. Here we further examined the potential antiviral 
effect of 6MMPr on several diverse flaviviruses. In cell culture, 6MMPr 
inhibited virus production of yellow fever virus, dengue virus-2 (DENV-2) and 
West Nile virus (WNV) in a dose-dependent manner, and DENV-2 was significantly 
more sensitive to 6MMPr treatment than WNV. We then explored the use of 6MMPr as 
an antiviral against WNV in an immunocompetent mouse model. Once a day treatment 
of mice with 0.5 mg 6MMPr was just below the toxic dose in our mouse model, and 
this dose was used in subsequent studies. Mice were treated with 6MMPr 
immediately after subcutaneous inoculation with WNV for eight consecutive days. 
Treatment with 6MMPr exacerbated weight loss in WNV-inoculated mice and did not 
significantly affect mortality. We hypothesized that 6MMPr has low 
bioavailability in the central nervous system (CNS) and examined the effect of 
pre-treatment with 6MMPr on viral loads in the periphery and CNS. Pre-treatment 
with 6MMPr had no significant effect on viremia or viral titers in the 
periphery, but resulted in significantly higher viral loads in the brain, 
suggesting that the effect of 6MMPr is tissue-dependent. In conclusion, despite 
being a potent inhibitor of flaviviruses in cell culture, 6MMPr was not 
effective against West Nile disease in mice; however, further studies are 
warranted to reduce the toxicity and/or improve the bioavailability of this 
potential antiviral drug.