Synthesis, antiviral activity, and mechanism of drug resistance of D- and L-2',3'-didehydro-2',3'-dideoxy-2'-fluorocarbocyclic nucleosides.

Carbocyclic nucleosides have received much attention due to their interesting biological activity and metabolic stability. Among nucleoside analogues, a fluorine substitution on the carbohydrate moiety or introduction of a 2',3'-unsaturated structure motif has been proven to be successful in producing effective antiviral agents. By combining these structural features, both D- and L-2',3'-dideoxy-2',3'-didehydro-2'-fluoro-carbocyclic nucleosides (D- and L-2'F-C-d4Ns) were synthesized as potential anti-HIV agents. The target D- and L-carbocyclic nucleosides were both stereospecifically synthesized from D-ribose. The structure-activity relationships of synthesized compounds against HIV-1 in activated human peripheral blood mononuclear (PBM) cells were studied, from which we found that the L-2',3'-dideoxy-2'3'-didehydro-2'-fluoroadenosine analogue (L-2'F-C-d4A) 46 showed potent anti-HIV activity (EC50 = 0.77 microM), although it is cross-resistant to the lamivudine-resistant variant (HIV-1M184V). Modeling studies demonstrated a good correlation between calculated relative binding energies and activity/resistance data. The modeling study also indicated that an additional hydrogen bond and a favorable van der Waals interaction contribute to the higher antiviral activity of L-2'F-C-d4A in comparison to its D-counterpart. Also, like other L-nucleosides, the unfavorable steric hindrance of the sugar moiety of L-2'F-C-d4A and the side chain of Val184 could explain the cross-resistance of L-2'F-C-d4A with the M184V mutant. The significant difference of antiviral activity between carbovir and its analogue L-2'F-C-d4G 25 may be due to distortion of the phenyl ring of Tyr115 in the L-2'F-C-d4G-TP/HIV-RT complex, which resulted in a poor pi-pi interaction.