CYTOTOXICITY OF 3'-AZIDO-3'-DEOXYTHYMIDINE CORRELATES WITH 3'-AZIDOTHYMIDINE-5'-MONOPHOSPHATE (AZTMP) LEVELS, WHEREAS ANTI-HUMAN-IMMUNODEFICIENCY-VIRUS (HIV) ACTIVITY CORRELATES WITH 3'-AZIDOTHYMIDINE-5'-TRIPHOSPHATE (AZTTP) LEVELS IN CULTURED CEM T-LYMPHOBLASTOID CELLS

Activation of the anti-human immunodeficiency virus (HIV) compound 3'- azido-3'deoxythymidine (AZT) is dependent on its 5'-phosphorylation by cellular nucleoside and nucleotide kinases. Azidothymidine 5'-triphos phate (AZTTP) is considered to be the metabolite responsible for both the anti-HIV effect of AZT, via inhibition of reverse transcriptase, a nd cytoxicity by interference with cellular DNA polymerases. During th e characterization of AZT metabolism in cultured human T-lymphoblastoi d CEM cells, a spontaneously occurring variant cell line, CEM/Ag-1, wa s found that showed approximately 10-fold resistance to AZT growth inh ibition as compared to wild type (wt) cells (EC(50) = 2 mM as compared to 350 mu M for wt cells). CEM/Ag-1 cells had a 3-fold reduced capaci ty to accumulate azidothymidine monophosphate (AZTMP) compared to wt c ells whereas similar levels of AZTTP were found in both cell lines. Th e intracellular half-life of AZTMP was approximately 70 min in both wt and CEM/Ag-1 cells. A 3-fold lower specific activity of cytoplasmic t hymidine kinase was observed in CEM/Ag-1 extracts as compared to wt. T he reduced thymidine kinase activity was not correlated to a decreased level of thymidine kinase mRNA. Syncytium formation of CEM/Ag-1 cells infected with HIV-2 as well as HIV-1 antigen production was inhibited at the same concentrations of AZT (approx. 0.01 mu M) as were HIV-1 a nd HIV-2 infected wt cells. Thus, minor decreases in cellular thymidin e kinase levels may markedly affect the cytoxicity of AZT but have no major effect on the antiviral activity of AZT. Our results strongly su ggest that AZTMP is responsible for a major part of the growth inhibit or effects, while AZTTP mainly mediates the antiviral activity of AZT.