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

Phosphorylation of deoxycytidine analog monophosphates by UMP-CMP kinase: molecular characterization of the human enzyme

Phosphorylation of deoxycytidine analogs by cellular enzymes is a prerequisite for the activity of these compounds. We have investigated the kinetic parameters for the phosphorylation of 1-beta-D-arabinofuranosylcytosine (araC) and 2', 2'-difluorodeoxycytidine (dFdC) to their diphosphate forms catalyzed by human UMP-CMP kinase. We cloned the cDNA of this enzyme to enable characterization of the recombinant protein, determine its expression in different tissues, and determine the chromosome location of the gene. We showed that the recombinant UMP-CMP kinase phosphorylated CMP, dCMP, and UMP with highest efficiency and dUMP, AMP, and dAMP with lower efficiency. The monophosphates of araC and dFdC were shown to be phosphorylated with similar efficiency as dCMP and CMP. We further showed, in a combined enzymatic assay, that human deoxycytidine kinase and UMP-CMP kinase together phosphorylated araC, dFdC, and 2',3'-dideoxycytidine to their diphosphate forms. Northern blot analysis showed that the UMP-CMP kinase mRNA was ubiquitously present in human tissues as a 3.9-kb transcript with highest levels in pancreas, skeletal muscle, and liver. The human UMP-CMP kinase gene was localized to chromosome 1p34.1-1p33 by radiation hybrid analysis. We further expressed the UMP-CMP kinase as a fusion protein to the green fluorescent protein in Chinese hamster ovary cells, and showed that the fusion protein was located in the cytosol and nucleus.

A new class of anti-HIV agents: synthesis and activity of conjugates of HIV protease inhibitors with a reverse transcriptase inhibitor

Conjugates of HIV protease inhibitors with a reverse transcriptase inhibitor were synthesized, which expressed excellent antiviral activity compared with that of the individual components. The remarkable antiviral activity of the conjugated compounds may be due to their penetration into the cell and later splitting into two different classes of anti-HIV agents.

3'-Azidothymidine significantly alters glycosphingolipid synthesis in melanoma cells and decreases the shedding of gangliosides

In this report, we establish that 3'-azido-3'-deoxythymidine (AZT) treatment of melanoma cells greatly alters the pattern of glycosphingolipid biosynthesis. In SK-MEL-30 cells, synthesis of the gangliosides GM3 and GD3 was significantly inhibited (60% and 50% of control, respectively) and the production of their precursor, lactosylceramide, was stimulated by 2.5-fold. Control experiments established that phospholipid synthesis was not affected by AZT treatment, consistent with AZT treatment only affecting lipid biosynthetic reactions that involve glycosylation. Likely as a consequence of decreased rates of ganglioside synthesis, AZT treatment of SK-MEL-30 cells also significantly suppressed the amount of gangliosides shed from the membranes of these cells. Since shedding of gangliosides has been proposed to allow melanoma cells to avoid destruction by the immune system and alterations of glycosphingolipid levels are likely important for the malignant cell phenotype, these results may have important implications regarding the potential use of AZT or related glycosylation inhibitors as cancer chemotherapeutics.

Effect of ribavirin on zidovudine efficacy and toxicity in vitro: a concentration-dependent interaction

Zidovudine (ZDV) is converted to its active triphosphate (ZDVTP) by intracellular kinases. The intermediate ZDV monophosphate (ZDVMP) is believed to play a major role in ZDV toxicity. Manipulation of ZDV phosphorylation is a possible therapeutic strategy for altering the risk-benefit ratio. Here we investigate whether combining RBV with ZDV is able to modulate efficacy and toxicity of ZDV. We have measured the intracellular activation of ZDV (0.3 microM) in the absence and presence of ribavirin (RBV; 2 and 20 microM) in Molt 4 and U937 cells. MTT cytotoxicity of ZDV (10-1000 microM) was also measured with and without RBV (2 microM) in Molt 4 and U937 cells. Measurement of endogenous deoxythymidine triphosphate (dTTP) allowed investigation of the dTTP/ZDVTP ratio. The antiviral efficacy of ZDV in combination with RBV (2 microM) was assessed by HIV p24 antigen measurements. In the presence of RBV (2 and 20 microM) a decrease in total ZDV phosphates was observed, owing mainly to an effect primarily on ZDVMP rather than the active ZDVTP. RBV also increased endogenous dTTP pools in both cell types, resulting in an increase in the dTTP/ZDVTP ratio. ZDV alone significantly reduced p24 antigen production, with an IC50 of 0.34 microM. Addition of RBV increased the IC50 approximately fivefold (1.52 microM). However, at higher concentrations of ZDV (10 and 100 microM) the antagonistic effect of RBV (2 microM) on ZDV was lost. The RBV-mediated decrease in ZDVMP may explain the reduction in ZDV toxicity when combined with RBV (2 microM). Cytotoxicity of ZDV was reduced in the presence of RBV (2 microM) at all concentrations in both cell lines, probably owing to saturation of ZDVTP formation. The interaction of ZDV and RBV is concentration dependent.

Intracellular nucleotides of (-)-2',3'-deoxy-3'-thiacytidine in peripheral blood mononuclear cells of a patient infected with human immunodeficiency virus

An analytical methodology was developed to quantitate the intracellular nucleotides including mono-, di-, and triphosphates and the diphosphocholine derivative of (-)-2', 3'-deoxy-3'-thiacytidine (3TC) in human peripheral blood mononuclear cells (PBMCs). The procedure includes the resolution of 3TC nucleotides by solid-phase extraction (SPE) on an anion-exchange cartridge, with subsequent enzyme digestion of the resulting phosphates to the parent drug that is ultimately quantitated by high-performance liquid chromatography with UV detection (HPLC-UV). Validation was performed with PBMCs from healthy donors exposed to [3H]3TC, leading to the formation of intracellular nucleotides that were quantitated by anion-exchange HPLC with radioactive detection (HPLC-RA). These nucleotide levels served as reference values and were used for cross-validation with data obtained by HPLC-UV. An excellent correlation was established between the results obtained by HPLC-RA and those obtained by HPLC-UV, with a slope of the regression lines close to unity and intercepts near nullity as well as a correlation coefficient close to unity for all 3TC phosphates. The assay was characterized by a limit of quantitation below 1 ng (amount on column) with a precision (percentage of coefficient of variation of repeated measurement) ranging from 0.8 to 18.1% and an accuracy (deviation of the amount determined by HPLC-UV from the nominal reference value) varying from -14.8 to 19.4%. This methodology was successfully applied to determine the quantity of 3TC nucleotides in PBMCs of a patient infected with human immunodeficiency virus after oral administration of 3TC and stavudine.

An Escherichia coli system expressing human deoxyribonucleoside salvage enzymes for evaluation of potential antiproliferative nucleoside analogs

Deoxyribonucleoside salvage in animal cells is mainly dependent on two cytosolic enzymes, thymidine kinase (TK1) and deoxycytidine kinase (dCK), while Escherichia coli expresses only one type of deoxynucleoside kinase, i.e., TK. A bacterial whole-cell system based on genetically modified E. coli was developed in which the relevant bacterial deoxypyrimidine metabolic enzymes were mutated, and the cDNA for human dCK or TK1 under the control of the lac promoter was introduced. The TK level in extract from induced bacteria with cDNA for human TK1 was found to be 20,000-fold higher than that in the parental strain, and for the strain with human dCK, the enzyme activity was 160-fold higher. The in vivo incorporation of deoxythymidine (Thd) and deoxycytidine (dCyd) into bacterial DNA by the two recombinant strains was 20 and 40 times higher, respectively, than that of the parental cells. A number of nucleoside analogs, including cytosine arabinoside, 5-fluoro-dCyd, difluoro-dCyd, and several 5-halogenated deoxyuridine analogs, were tested with the bacterial system, as well as with human T-lymphoblast CEM cells. The results showed a close correlation between the inhibitory effects of several important cytostatic and antiviral analogs on the recombinant bacteria and the cellular system. Thus, E. coli expressing human salvage kinases is a rapid and convenient model system which may complement other screening methods in drug discovery projects.

Intracellular phosphorylation of zidovudine (ZDV) and other nucleoside reverse transcriptase inhibitors (RTI) used for human immunodeficiency virus (HIV) infection

Dramatic reductions of viral load and increased survival have been achieved in patients infected with the Human Immunodeficiency Virus (HIV) with the introduction of combination antiretroviral therapy. Currently 11 agents including nucleoside reverse transcriptase inhibitors (RTI), non-nucleoside RTI and protease inhibitors are available for the use for treatment of HIV infection. Recent studies have demonstrated that certain combinations of these drugs are advantageous over their individual use as monotherapy with an even more sustained viral suppression. Much emphasis has therefore been put on studies evaluating the interactions of these different compounds. Especially the intracellular metabolism of nucleoside RTI has been evaluated to some extent, by both in vitro and in vivo studies. These compounds need to undergo phosphorylation to their active 5'-triphoshates involving several enzymatic steps and the nucleoside concentration in the plasma may not correlate with intracellular concentrations of active drug. It is therefore of great importance to study these drugs at an intracellular level in order to evaluate their efficacy. This review summarizes the intracellular phosphorylation of Zidovudine and other nucleoside analogs investigated by in vitro experiments and the efforts of measuring the active anabolites in vivo in cells isolated from HIV infected patients on nucleoside therapy.

Anti-HIV Antiviral Activity of Stavudine in a Thymidine Kinase-Deficient Cellular Line

Stavudine (d4T) is a potent inhibitor of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase. It is known that stavudine is metabolized in cells to the mono-, di- and triphosphate nucleotides but the enzymes responsible for its phosphorylation are as yet unidentified. In particular, there are conflicting results concerning the role of thymidine kinase 1 (TK1) in stavudine metabolism. To gain new insights into this phenomenon we analysed the antiviral activity of stavudine in a TK1-deficient, resistant cell line. The results indicate that TK1 is responsible for the phosphorylation of stavudine but it is not the only enzyme involved in its activation. The other enzyme(s) that might be involved in the metabolism of stavudine, however, are not able to phosphorylate stavudine with the same efficiency as TK1. Since it has been shown that prolonged treatment with zidovudine may induce an in vivo defect in TK1 activity, it is tempting to speculate that patients treated for a long time with zidovudine could be resistant to further treatment with stavudine.

Intracellular metabolism of 3'-azido-3'-deoxythymidine (AZT): a nuclear magnetic resonance study on T-lymphoblastoid cell lines with different resistance to AZT

This paper reports the results of 31P and 1H nuclear magnetic resonance (NMR) studies on the uptake and phosphorylation of 3'-azido-3'-deoxythymidine (AZT) in the human CD4+ T-lymphoblastoid cell line CCRF-CEM (CEM-1.3) and in its AZT-resistant cell variant MT-500, isolated by prolonged culturing of CEM cells in the presence of increasing AZT concentrations. After 3 hr of incubation in the presence of 0.5 mM AZT, both AZT and its monophosphorylated form (AZT-MP) could be detected in the sensitive cell line in concentrations above the NMR detection levels. In another cell line, MOLT-4, which is less sensitive to AZT effects, the intracellular level of AZT-MP was much lower and was only slightly raised by increasing the concentration of AZT in the extracellular and intracellular compartments. In the AZT-resistant clone MT-500, characterized by a very low thymidine kinase (TK, EC 2.7.1.21) activity with respect to the parental clone, the intracellular AZT-MP concentration was below detection (<0.02 nmol/10(6) cells). Since, however, not only AZT-MP but also AZT signals failed to be detected in MT-500 extracts following cell incubation with AZT, it was concluded that a TK deficiency cannot be the exclusive mechanism of AZT resistance in these cells. The possible effects of additional mechanisms of drug resistance, such as specific AZT cell extrusion and limited permeation, are discussed, together with the new prospects offered by NMR spectroscopy to further evaluate the limiting steps for the utilization of antiretroviral nucleoside analogues.

Probing the mechanism of action and decomposition of amino acid phosphomonoester amidates of antiviral nucleoside prodrugs

The decomposition pathways in peripheral blood mononuclear cells (PBMCs) and the in vitro anti-HIV-1 activity of the structurally similar 3'-azido-3'-deoxythymidine (AZT) phosphoramidates 1-6 and 3'-fluoro-3'-deoxythymidine (FLT) phosphoramidates 7-10 are reported. The AZT phosphoramidates exhibited no cytotoxicity toward CEM cells at concentrations as high as 100 microM, whereas the FLT phosphoramidates 9 and 10 had CC50 values of 95.6 and 35.1 microM, respectively. All 10 compounds exhibited no cytotoxicity toward PBMCs at concentrations as high as 100 microM and were effective at inhibiting viral replication. In particular, the AZT phosphomonoester amidate 4 displayed comparable antiviral activity to the parent nucleoside analog AZT. Mechanistic studies on the amino acid carbomethoxy ester phosphomonoester amidates revealed that their decomposition pathway differs from that of amino acid carbomethoxy ester aryl phosphodiester amidates of nucleotide prodrugs. AZT phosphomonoester amidates are internalized by lymphocytes to the same extent as AZT by a nonsaturable process. In lymphocytes, the amino acid carbomethoxy ester phosphomonoester amidates of AZT are not significantly metabolized to either AZT or the mono-, di-, or triphosphate of AZT. The amount of active anabolite, AZT-5'-triphosphate, formed in PBMCs incubated with the AZT phosphomonoester amidates 3 and 4 was 2- and 3-fold less than that observed after treatment with AZT, respectively. In contrast, FLT phosphomonoester amidates are rapidly converted to FLT-5'-monophosphate by a process that is antagonized by the corresponding AZT derivative 4. These results suggest that the metabolism of aromatic amino acid carbomethoxy ester phosphomonoester amidate nucleotide prodrugs by PBMCs does not require prior conversion to the corresponding carboxylic acid before proceeding to P-N bond cleavage.

Metabolism and activities of 3'-azido-2',3'-dideoxythymidine and 2',3'-didehydro-2',3'-dideoxythymidine in herpesvirus thymidine kinase transduced T-lymphocytes

T-lymphocytes transduced with the conditionally toxic herpesvirus thymidine kinase gene (HSV-1 TK) are increasingly becoming important tools in genetic therapy approaches for treating viral infections and cancers. Therefore, the effects of different antiviral nucleoside drugs on the growth inhibition of parental and HSV-1 TK-transduced human T-lymphocyte cell lines (H9 and CEM TK-) were examined. As expected, both transduced cell lines were most sensitive to growth inhibition by ganciclovir (GCV). While the presence of HSV-1 TK did not potentiate 3'-azido-2',3'-dideoxythymidine (AZT) growth inhibition of H9 cells containing cellular TK; transduction of HSV-1 TK into the cellular TK-deficient CEM cells (CEM TK-) restored sensitivity to AZT. In both transduced cell lines, an HSV-1 TK-dependent growth inhibition with 2',3'-didehydro-2',3'-dideoxythymidine (d4T) was observed and a Km of 143 microM for d4T and HSV-1 TK was determined. Metabolic labeling analysis showed that drug metabolism correlated with the observed effects on cell growth. The effects of HIV-1 replication in the CEM TK- cell lines in the presence of AZT or d4T was evaluated. CEM TK- cells are largely resistant to AZT or d4T inhibition of HIV-1 replication, however, transduction of HSV-1 TK into the CEM TK- cells completely restored AZT and d4T inhibition of HIV-1 replication. These studies confirm the requirement for a thymidine kinase activity for the anti-HIV activities of d4T and suggest that AZT, but not d4T, could be potentially administered to patients receiving HSV-1 TK-transduced lymphocytes.

The bottleneck in AZT activation

Nucleoside-based inhibitors of reverse transcriptase were the first drugs to be used in the chemotherapy of AIDS. After entering the cell, these substances are activated to their triphosphate form by cellular kinases, after which they are potent chain terminators for the growing viral DNA. The two main factors limiting their efficacy are probably interrelated. These are the insufficient degree of reduction of viral load at the commencement of treatment and the emergence of resistant variants of the virus. The reason for the relatively poor suppression of viral replication appears to be inefficient metabolic activation. Thus, for the most extensively used drug, 3'-azido-3'-deoxythymidine (AZT), whereas phosphorylation to the monophosphate is facile, the product is a very poor substrate for the next kinase in the cascade, thymidylate kinase. Because of this, although high concentrations of the monophosphate can be reached in the cell, the achievable concentration of the active triphosphate is several orders of magnitude lower. Determination of the structure of thymidylate kinase as a complex with AZT monophosphate (AZTMP) together with studies on the kinetics of its phosphorylation have now led to a detailed understanding of the reasons for and consequences of the poor substrate properties.

Quantitation of intracellular zidovudine phosphates by use of combined cartridge-radioimmunoassay methodology

This report describes the development of a potentially clinical method to measure the cellular metabolites of zidovudine (ZDV) in patients receiving the drug. This new method combines the use of Sep-Pak cartridges to separate ZDV phosphates with radioimmunoassaying to quantitate ZDV. The detection limit is 0.02 pmol/10(6) cells, and this assay can measure a wide range of intracellular drug concentrations. The use of the cartridge-radioimmunoassay methodology should prove very useful for in vivo cellular pharmacokinetic studies of ZDV.

Zidovudine phosphorylation after short-term and long-term therapy with zidovudine in patients infected with the human immunodeficiency virus

BACKGROUND

Resistance of human immunodeficiency virus (HIV) to zidovudine (AZT) has been associated with mutations in the viral reverse transcriptase gene. However, recent studies suggest that host cellular factors such as a decreased thymidine kinase activity or an increased cellular P-glycoprotein expression may be important. This study compared concentrations of zidovudine monophosphate, zidovudine diphosphate, and zidovudine triphosphate with P-glycoprotein expression in peripheral blood mononuclear cells from patients receiving long-term (> 18 months) and short-term (< 2 months) zidovudine treatment.

METHODS

Ten subjects in the short-term group and 11 subjects in the long-term group with CD4 counts between 300 and 500 received a single oral dose of zidovudine (200 mg) after a 24-hour washout period. Blood samples were collected at 0, 1, 2, 4, and 6 hours. Intracellular nucleotide concentrations were measured by a combined HPLC-radioimmunoassay method, and P-glycoprotein expression was determined by fluorescence activated cell sorting (FACS) analysis with use of the monoclonal mouse antibody MRK-16.

RESULTS

Zidovudine monophosphate was the predominant compound, accounting for 73.4% +/- 7.1% (SD) of the total phosphates in the long-term treatment group and 74.2% +/- 15.0% (SD) in the short-term group. Zidovudine diphosphate accounted for 13.3% +/- 3.3% (SD) in the long-term group and 12.5% +/- 6.6% (SD) in the short-term group. Zidovudine triphosphate accounted for 13.4% +/- 4.1% (SD) in the long-term group and 13.5% +/- 8.3% (SD) in the short-term group. Mean peak concentrations for the active zidovudine triphosphate were 0.04 +/- 0.02 (SD) pmol/10(6) cells in both groups. Comparison of the individual zidovudine phosphate concentrations and P-glycoprotein expression revealed no significant difference in the two patient populations.

CONCLUSIONS

These data suggest that intracellular phosphorylation does not change over time and that zidovudine does not select for P-glycoprotein expressing cells.

Cellular phosphorylation of anti-HIV nucleosides. Role of nucleoside diphosphate kinase

Nucleotide analogs are widely used in antiviral therapy and particularly against AIDS. Delivered to the cell as nucleosides, they are phosphorylated into their active triphospho derivative form by cellular kinases from the host. The last step in this series of phosphorylations is performed by nucleoside diphosphate (NDP) kinase, an enzyme that can use both purine or pyrimidine and oxy- or deoxynucleotides as substrates. Using pure recombinant human NDP kinase type B (product of the gene nm23-H2), we have characterized the kinetic parameters of several nucleotide analogs for this enzyme. Contrary to what is generally assumed, diphospho- and triphospho- derivatives of azidothymidine as well as of dideoxyadenosine and dideoxythymidine are very poor substrates for NDP kinase. The rate of phosphorylation of these analogs varies between 0.05% and 0.5%, as compared to the corresponding natural nucleotide, a result that is not due to the inability of the analogs to bind to the enzyme. Using the data from the high resolution crystal structure of NDP kinase, we provide an interpretation of these results based on the crucial role played by the 3'-OH moiety of the nucleotide in catalysis.