Decrease in thymidylate kinase activity in peripheral blood mononuclear cells from HIV-infected individuals

Nucleosides and nucleoside analogs are anabolised to their triphosphates by intracellular kinases. The anti-HIV analogue zidovudine (AZT) is phosphorylated by cytosolic thymidine kinase 1 (TK1), thymidylate kinase (dTMPK), and nucleoside diphosphate kinase. It is known that dTMPK is one of the rate-limiting steps in the activation of zidovudine. The activities of TK1, dTMPK, and deoxycytidine kinase (dCK) were determined in extracts of in vitro activated peripheral blood mononuclear cells from HIV-infected patients and healthy noninfected individuals. dTMPK activity was 10-fold lower and TK1 activity was five-fold lower in extracts from infected as compared to uninfected persons. Deoxycytidine kinase activities in the extracts from both groups were very similar. Differences in in vitro activation, as determined by flow cytometry, of the peripheral lymphocytes were not responsible for the decreased TK1 and dTMPK activities. A reduced level of intracellular azido-dideoxythymidinetriphosphate in activated mononuclear cells from HIV-infected patients was also observed. The low levels of TK1 and dTMPK in lymphocytes from HIV-infected patients may be related to the anergy phenomenon observed as a result of HIV infection. This effect should also be considered in the development of new anti-HIV drugs.

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'-triphosphate (AZTTP) is considered to be the metabolite responsible for both the anti-HIV effect of AZT, via inhibition of reverse transcriptase, and cytoxicity by interference with cellular DNA polymerases. During the characterization of AZT metabolism in cultured human T-lymphoblastoid CEM cells, a spontaneously occurring variant cell line, CEM/Ag-1, was found that showed approximately 10-fold resistance to AZT growth inhibition as compared to wild type (wt) cells (EC50 = 2 mM as compared to 350 microM for wt cells). CEM/Ag-1 cells had a 3-fold reduced capacity to accumulate azidothymidine monophosphate (AZTMP) compared to wt cells whereas similar levels of AZTTP were found in both cell lines. The 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 thymidine kinase was observed in CEM/Ag-1 extracts as compared to wt. The 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 microM) as were HIV-1 and HIV-2 infected wt cells. Thus, minor decreases in cellular thymidine kinase levels may markedly affect the cytoxicity of AZT but have no major effect on the antiviral activity of AZT. Our results strongly suggest that AZTMP is responsible for a major part of the growth inhibitor effects, while AZTTP mainly mediates the antiviral activity of AZT.

Intracellular metabolism of 3'-azidothymidine in isolated human peripheral blood mononuclear cells

Azidothymidine (AZT) inhibits the replication of human immunodeficiency virus and it is the only drug so far licensed for treatment of acquired immunodeficiency syndrome (AIDS). A prerequisite for its antiviral activity is phosphorylation by cellular nucleoside kinases to the mono-, di-, and triphosphate levels. This study determined the capacity of isolated peripheral blood mononuclear cells (PBMC), resting or mitogen stimulated, from 36 different people of whom 5 were HIV+, to phosphorylate and accumulate intracellular AZT nucleotides. We found a large variation in the amount of products formed between PBMC treated at different times from the same individual as well as between the PBMC from different individuals. Resting PBMC showed the largest interindividual variation and their levels of AZT nucleotides were about 60-150-fold lower than in activated PBMC. The intracellular half lives of azidothymidine mono-, di-, and triphosphates, constituting, on the average, 96-99.2, 0.7-1.8, and 0.4-2.7% of total nucleotides at 0.08-1.6 microM AZT, respectively, were also determined. In mitogen-stimulated PBMC it was approximately 2.5 +/- 0.6 h for all the azidothymidine metabolites. The half-life for intracellular azidothymidine monophosphate in resting PBMC from two individuals was determined to 1.5 +/- 0.2 h. There appeared to be no significant difference in the AZT metabolism in PBMC from HIV-positive or-negative persons. A relative decrease in the intracellular formation of AZTDP and AZTTP from AZTMP was observed at concentrations of AZTMP above 1 microM. This fact may explain why lowering the doses of AZT still gives therapeutically efficient levels of the active metabolite AZTTP.

2',3'-Dideoxycytidine toxicity in cultured human CEM T lymphoblasts: effects of combination with 3'-azido-3'-deoxythymidine and thymidine

2',3'-Dideoxycytidine (ddCyd), a potent inhibitor of human immunodeficiency virus DNA replication, requires phosphorylation by cellular nucleoside kinases for antiviral activity. Deoxycytidine kinase (NTP:deoxycytidine 5'-phosphotransferase, EC 2.7.1.74) is responsible for the formation of dideoxycytidine monophosphate and this enzyme is controlled by feedback regulation by the natural endproduct, dCTP. We have examined whether a decrease in intracellular dCTP levels affects the growth inhibition caused by ddCyd, as well as the capacity to accumulate dideoxycytidine triphosphate (ddCTP), using human T lymphoblast (CEM) cells in culture. Subtoxic concentrations of thymidine were used to decrease the dCTP pool. The effects of 3'-azido-3'-deoxythymidine (AZT), alone or in combination with ddCyd, on cell growth, DNA precursor pools, and accumulation of ddCTP were also studied. The combination of ddCyd and thymidine led to growth inhibition of CEM cells that was twice what would be expected from addition, whereas the combination of AZT and ddCyd showed an additive effect. CEM cells accumulated ddCTP efficiently, so that with 10 microM ddCyd (corresponding to the EC50 value) and a 6-hr incubation the ddCTP pool was 3-fold higher than the dCTP pool. Simultaneous addition of thymidine (10 microM) increased the dTTP pool 2-fold and gave a 50% reduction in the dCTP level but only a 10% increase in ddCTP accumulation. The presence of AZT (300 microM, corresponding to the EC50 value) led, in contrast, to an elevation of dCTP and no significant change in the other DNA precursor pools. With this high concentration of AZT, the accumulation of ddCTP decreased 42%. It was also found that ddCyd is metabolized into two additional compounds, besides the dideoxycytidine mono-, di-, and triphosphate, i.e., the liponucleotides dideoxycytidine diphosphate-ethanolamine and dideoxycytidine diphosphate-choline, constituting 45 and 6% of the total phosphorylated ddCyd metabolites, respectively, whereas the mono-, di-, and triphosphate corresponded to 3, 21, and 25% of the phosphorylated dideoxynucleotides. These results indicate that the formation of dideoxycytidine monophosphate is not rate limiting in the synthesis of ddCTP in human lymphoblasts, which clearly differs from what was observed earlier in mouse cells (Mol Pharmacol 32:798-806 1988). Furthermore, growth inhibition by ddCyd seems to be related to the ratio between dCTP and ddCTP. There was no direct interference between ddCyd and AZT metabolism in clinically relevant concentrations, which may encourage the use of combination of these compounds for anti-human immunodeficiency virus treatment.