DESCRIPTION (provided by applicant): We have recently discovered a novel ring-expanded nucleoside (REN) that potently inhibits both the human immunodeficiency virus (HIV) as well as the hepatitis C virus (HCV) in vitro with IC50 values in the micromolar and nanomolar ranges, respectively, for the two viruses, with little toxicity, if any, to the host cells. Since HCV is a major co-infection in the HIV-infected individuals, the discovery of a compound capable of dual viral inhibition of HIV and HCV, the two dreaded viruses eliciting current national and international health concern, is biomedically significant and merits further exploration. This proposal specifically focuses on investigation of the mechanism of anti-HIV activity of the compound. Since nothing is known on this front, we intend to use this R21 application to quickly acquire some important preliminary data concerning the stage of the viral life cycle that is being affected by the REN. These preliminary data would serve as the basis for a subsequent R01 application for an in-depth investigation of the mechanism of anti-HIV activity specific to the stage of the viral life cycle, and in turn, for the extensive SAR studies. The important questions to be addressed in here include: (a) whether the REN exerts its effect on the virus production or virus inhibition, (b) whether it inhibits the early events in the viral life cycle, including the viral entry, reverse transcription, nuclear localization and integration, or later events, including transcription of the integrated provirus, nuclear export, assembly and particle production, (c) whether in vivo phosphorylation of the REN is necessary for it to be active, considering that the active forms of many, if not all, nucleoside analogues are known to be their corresponding mono-, di-, or triphosphate derivatives, and (d) whether the mechanism of activity of REN involves incorporation into nucleic acids via their 5'-triphosphate derivatives and subsequent chain termination, considering that many antiviral nucleoside analogues are known to operate by this mechanism. In addition, we will synthesize a few selected analogues of the lead compound to see if the antiviral activity could be further enhanced. This includes (a) synthesis of the ribose analogue of the lead REN, and (b) extending the alkyl chain at position-6 by a few more carbon atoms. Finally, we will investigate if the leading REN as well as its analogues are effective against the resistant HIV mutants.