Synthesis, structure-activity relationships, and mechanism of drug resistance of D- and L-β-3′-fluoro-2′,3′-unsaturated-4′-thionucleosides as anti-HIV agents

Various D- and L-2',3'-unsaturated 3'-fluoro-4'-thionucleosides (D- and L-3'F-4'Sd4Ns) were synthesized for the studies of structure-activity relationships. The synthesized D-2',3'-unsaturated 3'-fluoro-4'-thionucleosides did not show any significant antiviral activity against HIV-1, while unnatural L-nucleosides such as cytosine 34 (EC50 = 0.13 muM; EC90 = 1.7 muM) and 5-fluorocytosine 35 (EC50 = 0.031 muM; EC90 = 0.35 muM) derivatives exhibited potent antiHIV activity without significant toxicity. Molecular modeling study shows that the X-fluorine atom of the D-2',3'-unsaturated cytidine triphosphate (D-3'F-4'Sd4CTP) experiences unfavorable electrostatic interaction with its own triphosphate moiety, resulting in the decreased binding affinity to wild-type HIV-1 reverse transcriptase (RT), which may be one of the reasons for the insensitivity of HIV-1 RT to these compounds. On the other hand, L-3'F-4'Sd4CTP binds to the active site of wild-type HIV-1 RT without steric hindrance and there is a possible hydrogen bonding between the X-fluorine atom and Asp185, which correlates with its potent anti-HIV activity. However, L-3'F-4'Sd4C 34 and L-3'F-4'Sd4FC 35 showed high cross-resistance to 3TC-resistant mutant (M184V) RT. Like other unnatural L-nucleosides, the unfavorable steric hindrance of the sugar moiety Of L-3'F-4'Sd4CTP with the side chain of Val184 explains its significant cross-resistance to the M184V mutant.