Genotoxicity and mitochondrial damage in human lymphocytic cells chronically exposed to 3'-azido-2',3'-dideoxytbymidine

AZT (3'-azido-2',3'-dideoxythymidine), the first nucleoside analog approved for the treatment of AIDS (acquired immunodeficiency syndrome), induces significant toxic effects in humans exposed to therapeutic doses. As an inhibitor of the HIV-1 (human immunodeficiency virus 1) reverse transcriptase, AZT blocks the incorporation of nucleotides into the host's newly synthesized DNA. Incorporation of AZT into mammalian DNA as well as specific localization of the drug into telomeric DNA, has been previously documented by immunohistochemistry. As with other nucleoside analogs, AZT has affinity for polymerase-@c, the enzyme responsible for the replication of mitochondrial DNA. In order to examine the mechanisms of toxic events induced by long-term AZT exposure, human T-lymphocytic H₉ cells were cultured with 25 μM AZT for 7 months. In the resulting H(9-AZT), cells, incorporation of AZT into DNA was demonstrated by radioimmunoassay and immunohistochemistry chromosomal aberrations and micronuclei were scored and intracellular lipid distribution was determined. Two pmol of AZT per microgram of DNA were detected by radioimmunoassay in H(9-AZT) cells. Control cells showed negative values in the radioimmunoassay. Cytogenetic observations on H(9-AZT), cells showed an increase in chromosomal aberrations and nuclear fragmentation when compared with unexposed H₉ cells. Electron microscopy revealed mitochondrial damage and an elevated accumulation of neutral intracellular lipid deposits probably as a consequence of a distortion in the β-oxidation of fatty acids normally carried out by this organelle. The toxicities explored here suggest that the mechanisms of AZT induced cytotoxicity in bone marrow of the patients chronically exposed to the drug in vivo may involve both chromosomal and mitochondrial DNA damage.