One of the formidable challenges in
therapy of
infections by human immunodeficiency virus (HIV) is the emergence of drug-resistant variants that attenuate the efficacy of
highly active antiretroviral therapy (
HAART). We have recently introduced 4'-ethynyl-nucleoside analogs as
nucleoside reverse transcriptase inhibitors (NRTIs) that could be developed as
therapeutics for treatment of
HIV infections. In this study, we present
2'-deoxy-4'-C-ethynyl-2-fluoroadenosine (
EFdA), a second generation 4'-ethynyl inhibitor that exerted highly potent activity against wild-type HIV-1 (EC50 approximately 0.07 nM).
EFdA retains potency toward many HIV-1 resistant strains, including the multi-drug resistant clone HIV-1A62V/V75I/F77L/F116Y/Q151M. The selectivity index of
EFdA (cytotoxicity/inhibitory activity) is more favorable than all approved NRTIs used in HIV
therapy. Furthermore,
EFdA efficiently inhibited clinical isolates from patients heavily treated with multiple anti-HIV-1 drugs.
EFdA appears to be primarily phosphorylated by the cellular 2'-deoxycytidine
kinase (dCK) because: (a) the
antiviral activity of
EFdA was reduced by the addition of dC, which competes
nucleosides phosphorylated by the dCK pathway, (b) the
antiviral activity of
EFdA was significantly reduced in dCK-deficient HT-1080/Ara-Cr cells, but restored after dCK transduction. Further, unlike other dA analogs,
EFdA is completely resistant to degradation by
adenosine deaminase. Moderate decrease in susceptibility to
EFdA is conferred by a combination of three RT mutations (I142V, T165R, and M184V) that result in a significant decrease of viral fitness. Molecular modeling analysis suggests that the M184V/I substitutions may reduce anti-HIV activity of
EFdA through steric hindrance between its 4'-ethynyl moiety and the V/I184 beta-branched side chains. The present data suggest that
EFdA, is a promising candidate for developing as a therapeutic agent for the treatment of individuals harboring multi-drug resistant HIV variants.