title: Insights into the molecular mechanism of mitochondrial toxicity by AIDS drugs.
abstract: Several of the nucleoside analogs used in the treatment of AIDS exhibit a delayed clinical toxicity limiting their usefulness. The toxicity of nucleoside analogs may be related to their effects on the human mitochondrial DNA polymerase (Pol gamma), the polymerase responsible for mitochondrial DNA replication. Among the AIDS drugs approved by the FDA for clinical use, two are modified cytosine analogs, Zalcitabine (2',3'-dideoxycytidine (ddC)) and Lamivudine (beta-d-(+)-2',3'-dideoxy-3'-thiacytidine ((-)3TC])). (-)3TC is the only analog containing an unnatural l(-) nucleoside configuration and is well tolerated by patients even after long term administration. In cell culture (-)3TC is less toxic than its d(+) isomer, (+)3TC, containing the natural nucleoside configuration, and both are considerably less toxic than ddC. We have investigated the mechanistic basis for the differential toxicity of these three cytosine analogs by comparing the effects of dideoxy-CTP), (+)3TC-triphosphate (TP), and (-)3TC-TP on the polymerase and exonuclease activities of recombinant human Pol gamma. This analysis reveals that Pol gamma incorporates (-)3TC-triphosphate 16-fold less efficiently than the corresponding (+)isomer and 1140-fold less efficiently than dideoxy-CTP, showing a good correlation between incorporation rate and toxicity. The rates of excision of the incorporated analogs from the chain-terminated 3'-end of the DNA primer by the 3'-5'-exonuclease activity of Pol gamma were similar (0.01 s(-)1) for both 3TC analogs. In marked contrast, the rate of exonuclease removal of a ddC chain-terminated DNA occurs at least 2 orders of magnitude slower, suggesting that the failure of the exonuclease to remove ddC may play a major role in its greater toxicity. This study demonstrates that direct analysis of the mitochondrial DNA polymerase structure/function relationships may provide valuable insights leading to the design of less toxic inhibitors.
authors.affiliation: Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
authors.forename: J Y
authors.initials: JY
authors.lastname: Feng
authors.forename: A A
authors.initials: AA
authors.lastname: Johnson
authors.forename: K A
authors.initials: KA
authors.lastname: Johnson
authors.forename: K S
authors.initials: KS
authors.lastname: Anderson
mesh_terms: D000163:Acquired Immunodeficiency Syndrome / Q000188:drug therapy; D019380:Anti-HIV Agents / Q000009:adverse effects / Q000378:metabolism / Q000494:pharmacology*; D003570:Cytidine Triphosphate / Q000009:adverse effects / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D004247:DNA / Q000096:biosynthesis; D000074002:DNA Polymerase gamma; D004261:DNA Replication / Q000187:drug effects; D004272:DNA, Mitochondrial / Q000378:metabolism; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism; D003845:Deoxycytosine Nucleotides / Q000009:adverse effects / Q000378:metabolism / Q000494:pharmacology; D054306:Dideoxynucleotides; D005090:Exodeoxyribonucleases / Q000037:antagonists & inhibitors; D006801:Humans; D007700:Kinetics; D019259:Lamivudine / Q000009:adverse effects / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D008928:Mitochondria / Q000187:drug effects*; D019384:Nucleic Acid Synthesis Inhibitors; D018894:Reverse Transcriptase Inhibitors / Q000009:adverse effects / Q000378:metabolism / Q000494:pharmacology*; D016047:Zalcitabine / Q000009:adverse effects / Q000378:metabolism / Q000494:pharmacology
chemical_list: D019380:Anti-HIV Agents; D004272:DNA, Mitochondrial; D003845:Deoxycytosine Nucleotides; D054306:Dideoxynucleotides; D019384:Nucleic Acid Synthesis Inhibitors; D018894:Reverse Transcriptase Inhibitors; C417268:lamivudine triphosphate; C024107:2'-deoxycytidine 5'-triphosphate; D019259:Lamivudine; D003570:Cytidine Triphosphate; D016047:Zalcitabine; D004247:DNA; D000074002:DNA Polymerase gamma; D004259:DNA-Directed DNA Polymerase; D005090:Exodeoxyribonucleases