Relationship between antiviral activity and host toxicity: comparison of the incorporation efficiencies of 2',3'-dideoxy-5-fluoro-3'-thiacytidine-triphosphate analogs by human immunodeficiency virus type 1 reverse transcriptase and human mitochondrial DNA polymerase

Antimicrob Agents Chemother. 2004 Apr;48(4):1300-6. doi: 10.1128/aac.48.4.1300-1306.2004.


Emtricitabine [(-)FTC; (-)-beta-L-2'-3'-dideoxy-5-fluoro-3'-thiacytidine] is an oxathiolane nucleoside analog recently approved by the Food and Drug Administration for the treatment of human immunodeficiency virus (HIV). Structurally, (-)FTC closely resembles lamivudine [(-)3TC] except that the former is 5-fluorinated on the cytosine ring. In HIV-1 reverse transcriptase (RT) enzymatic assays, the triphosphate of (-)FTC [(-)FTC-TP] was incorporated into both DNA-DNA and DNA-RNA primer-templates nearly 3- and 10-fold more efficiently than (-)3TC-TP. Animal studies and clinical trial studies have demonstrated a favorable safety profile for (-)FTC. However, a detailed study of the incorporation of (-)FTC-TP by human mitochondrial DNA polymerase gamma, a host enzyme associated with nucleoside toxicity, is required for complete understanding of the molecular mechanisms of inhibition and toxicity. We studied the incorporation of (-)FTC-TP and its enantiomer (+)FTC-TP into a DNA-DNA primer-template by recombinant human mitochondrial DNA polymerase in a pre-steady-state kinetic analysis. (-)FTC-TP was incorporated 2.9 x 10(5)-, 1.1 x 10(5)-, 1.6 x 10(3)-, 7.9 x 10(3)-, and 100-fold less efficiently than dCTP, ddCTP, (+)3TC-TP, (+)FTC-TP, and (-)3TC-TP, respectively. The rate of removal of (-)FTC-MP from the corresponding chain-terminated 24-mer DNA by polymerase gamma's 3'-->5' exonuclease activity was equal to the removal of (+)FTC-MP, 2-fold slower than the removal of (-)3TC-MP and (+)3TC-MP, and 4.6-fold slower than the excision of dCMP. These results demonstrate that there are clear differences between HIV-1 RT and polymerase gamma in terms of preferences for substrate structure.

Publication types

  • Comparative Study
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Antiviral Agents / pharmacology*
  • Antiviral Agents / toxicity*
  • DNA / metabolism
  • DNA Polymerase gamma
  • DNA-Directed DNA Polymerase / metabolism*
  • Deoxycytosine Nucleotides / pharmacology*
  • Deoxycytosine Nucleotides / toxicity*
  • Dideoxynucleotides
  • HIV Reverse Transcriptase / isolation & purification
  • HIV Reverse Transcriptase / metabolism*
  • Humans
  • Isoenzymes / isolation & purification
  • Isoenzymes / metabolism
  • Kinetics
  • Mitochondria / enzymology*
  • Reverse Transcriptase Inhibitors / pharmacology*
  • Reverse Transcriptase Inhibitors / toxicity*
  • Stereoisomerism
  • Structure-Activity Relationship
  • Substrate Specificity


  • 2',3'-dideoxy-5-fluoro-3'-thiacytidine triphosphate
  • Antiviral Agents
  • Deoxycytosine Nucleotides
  • Dideoxynucleotides
  • Isoenzymes
  • Reverse Transcriptase Inhibitors
  • DNA
  • HIV Reverse Transcriptase
  • DNA Polymerase gamma
  • DNA-Directed DNA Polymerase