2',3'-Dideoxycytidine induced drug resistance in human cells

The Inhibitory Effect of ddC on Human Immunodeficiency Virus Replication Diminishes in Cells that are Chronically Exposed to the Drug

One possible explanation for the failure of human immunodeficiency virus type 1 (HIV-1) antiretroviral inhibitors to block the clinical progression of the infection may be a failure to maintain adequate drug levels at the site of viral replication. We have previously found that exposure of human monoblastoid cells (U937) for several months to a therapeutically relevant concentration (0.1 μM) of 2′,3′-dideoxycytidine (zalcitabine, ddC) allowed the isolation of a drug-resistant cell line characterized by a normal drug transport but a reduced ability to accumulate 2′,3′-dideoxycytidine 5′-triphosphate (the active antiretroviral form of the drug). In this paper we show that the drug-resistant cells were indistinguishable from normal cells in terms of surface CD4 receptors. The susceptibility of parental and ddC-resistant U937 cells to infection by HIV-1 was similar, as measured by proviral DNA formation. However, HIV-1 p24 production and the number of infectious virus particles produced were significantly lower in the drug-resistant compared to control cells. Addition of 0.1 μM ddC inhibited viral production by up to 92% in the control cells but had no effect on ddC-resistant cells. Thus, human cells exposed to therapeutically relevant ddC concentrations for several months show a reduced ddC anabolism and allow ddC-sensitive HIV-1 to replicate in the presence of inhibitory ddC concentrations.

Direct measurement of nucleoside monophosphate delivery from a phosphoramidate pronucleotide by stable isotope labeling and LC-ESI(-)-MS/MS

Amino acid phosphoramidates of nucleosides have been shown to be potent antiviral and anticancer agents with the potential to act as nucleoside monophosphate prodrugs. To access their ability to deliver 3'-azido-3'-deoxythymidine (AZT) 5'-monophosphate to cells, the decomposition pathway of an 18O-labeled AZT amino acid phosphoramidate was investigated by capillary reverse-phase high-performance liquid chromatography interfaced with negative ion electrospray ionization mass spectrometry (LC-ESI(-)-MS/MS). 18O-labeled L-AZT tryptophan phosphoramidate methyl ester ([18O]2) was synthesized with an 18O/16O relative ratio of 1.22 +/- 0.18. For CEM cells, a human T-lymphoblast leukemia cell line, incubated with [18O]2, values of 1.55 +/- 0.37, 0.34, and 0.13 were found for the 18O/16O relative ratio of intracellular AZT-MP for time intervals of 0.5, 4, and 20 h, respectively. The decrease in the level of labeled AZT-MP in CEM cells corresponded to a rapid increase in the amount of intracellular AZT presumably by dephosphorylation of AZT-MP. In contrast, for peripheral blood mononuclear cells (PBMCs), the 18O/16O relative ratio values of intracellular AZT-MP were 1.43, 1.06, and 0.61 for time intervals of 0.5, 4, and 20 h, respectively. Intracellular AZT in PBMCs was nearly undetectable for each time interval. Taken together, these results are consistent with the detection of direct P-N bond cleavage by CEM cells and PBMCs. However, AZT phosphoramidates are able to more effectively deliver AZT-MP to PBMCs than to CEM cells. Differential expression of 5'-nucleotidase in CEM cells relative to PBMCs is likely the reason for this discrepancy. Although applied to a phosphoramidate pronucleotide, the judicious use of 18O labeling and LC-MS is a general approach that could be applied to the investigation of the intracellular fate of other pronucleotides.

Synthesis of Nucleoside Boranophosphoramidate Prodrugs Conjugated with Amino Acids

[structure: see text] Nucleoside boranophosphates and nucleoside amino acid phosphoramidates have been shown to be potent antiviral and anticancer agents with the potential to act as nucleoside prodrugs. A combination of these two types of compounds results in a boranophosphoramidate linkage between the nucleoside and amino acid. This new class of potential prodrugs is expected to possess advantages conferred by both types of parent compounds. Two approaches, specifically the H-phosphonate and oxathiaphospholane approaches, are described here to synthesize nucleoside boranophosphoramidate prodrugs conjugated with amino acids. The H-phosphonate approach involves a key intermediate, silylated nucleoside amino acid phosphoramidite 6, prepared from a series of reactions starting from nucleoside H-phosphonate in the presence of condensing reagent DPCP. Due to the lengthy procedure and the difficulties in removing DPCP from the final products, we switched to the oxathiaphospholane approach in which the DBU-assisted oxathiaphospholane ring-opening process constituted a key step for the generation of nucleoside amino acid boranophosphoramidates 24. We demonstrate that this key step did not cause any measurable C-racemization of boranophosphorylated amino acids 22. Diastereomers of compounds 24a-f were separated by RP-HPLC. An "adjacent"-type mechanism is proposed to explain the diastereomer ratio in the final products obtained via the oxathiaphospholane approach. A tentative assignment of configuration for the diastereomers was carried out based on the mechanism, molecular modeling, and (1)H NMR. Conclusively, the oxathiaphospholane methodology proved to be more facile and efficient than H-phosphonate chemistry in the preparation of the nucleoside amino acid boranophosphoramidate analogues that are promising as a new type of antiviral prodrugs.

Molecular basis of 2',3'-dideoxycytidine-induced drug resistance in human cells

Human monoblastoid cells (U937) grown in the presence of therapeutically relevant dideoxycytidine concentrations (0.1 microM) become resistant to the drug thanks to an altered deoxycytidine kinase. In this paper we show that deoxycytidine kinase mRNA is significantly reduced in drug-resistant U937 cells (U937-R) although the deoxycytidine kinase promoter is normal. Anumber of nucleotide deletions, insertions and substitutions was found in the coding region of deoxycytidine kinase gene. Several identified mutations result in truncated forms of the enzyme or in the introduction of stop codons: in one case a complete lack of exon 4 was found. Thus, the deoxycytidine kinase gene accumulates mutations at a very high rate, as already reported for other cytidine analogues (i.e. Ara C) suggesting that the design of new antiviral or anticancer drugs of the cytidine family should take into account the potential development of cell resistance as a critical factor in drug failure.

Laboratory Guidelines for the Practical Use of HIV Drug Resistance Tests in Patient Follow-Up

HIV drug resistance is one of the major limitations in the successful treatment of HIV-infected patients using currently available antiretroviral combination therapies. When appropriate, drug susceptibility profiles should be taken into consideration in the choice of a specific combination therapy. Guidelines recommending resistance testing in certain circumstances have been issued. Many clinicians have access to resistance testing and will increasingly use these results in their treatment decisions. In this document, we comment on the different methods available, and the relevant issues relating to the clinical application of these tests. Specifically, the following recommendations can be made: (i) genotypic and phenotypic HIV-1 drug resistance analyses can yield complementary information for the clinician. However, insufficient information currently exists as to which approach is preferable in any particular clinical setting; (ii) when HIV-1 drug resistance testing is required, it is recommended that testing be performed on plasma samples obtained before starting, stopping or changing therapy, on samples that have a viral load above the detection limit of the resistance test; (iii) the panel recommends that genotypic and phenotypic HIV-1 drug resistance testing for clinical purposes be performed in a certified laboratory under strict quality control and quality assurance standards; and (iv) the panel recommends that resistance testing laboratories provide clinicians with resistance reports that include a list of drug-related resistance mutations (genotype) and/or a list of drug-related fold resistance values (phenotype), with interpretations of each by an experienced virologist. The interpretation of genotypic and phenotypic analysis is a complex and developing science, and in order to understand HIV-1 drug resistance reports, communication between the requesting clinician and the expert that interpreted the resistance report is recommended.

Programmed cell death in 2',3'-dideoxycytidine-resistant human monoblastoid U937 cells

2',3'-Dideoxycytidine is a powerful in vitro inhibitor of human immunodeficiency virus and is currently used in the treatment of acquired immunodeficiency syndrome. A long-term exposure of U937 monoblastoid cells to dideoxycytidine induces the selection of drug-resistant cells (U937-R). In previous studies, we investigated some important biochemical properties and functional activities, such as basal respiration, protein kinase C activity, superoxide anion release, and the level of reduced glutathione, which were found to be higher in the drug-resistant cell line, compared to the parental one. In the present study, we evaluated the response of the two cell lines to the induction of apoptosis by treatment with staurosporine and okadaic acid, which interfere with the protein kinase and phosphatase pathways, respectively. Moreover, knowing that GSH plays a crucial role in the regulation of nitric oxide-dependent apoptosis, U937-R and parental lines have been treated with SIN-1, which is known to generate significant amounts of O2 and nitric oxide. Resistant and parental cells have been analysed by light and electron microscopy and agarose gel electrophoresis of isolated DNA has been performed. The obtained results demonstrate a different susceptibility of U937-R cell line to apoptosis induced with the three triggers. U937-R cells show more advanced apoptotic features if compared with parental cells, after staurosporine treatment. Differently, the okadaic acid does not induce a different behaviour in the two models. On the contrary, the agent SIN-1 determines an increased number of apoptotic cells in the U937 line. The results suggest that a higher level of protein kinase C and glutathione could prevent programmed cell death in U937-R.

Cross-resistance of dideoxycytidine-resistant cell lines to azidothymidine

2',3'-Dideoxycytidine (ddC) and azidothymidine (AZT) inhibit HIV-1 replication and currently are used in AIDS therapy. Long-term use of the drugs is associated with the selection of drug-resistant HIV strains, thus limiting their effectiveness. Another mechanism, associated with their altered metabolism in host cells, also can cause "cellular" drug resistance. Human lymphocytic H9 cell lines (H9-ddC0.5w and H9-ddC5.0w) selected for ddC resistance by exposure to 0.5 and 5.0 microM ddC were found to be cross-resistant to AZT. Compared with controls, the thymidine kinase (TK) activities in H9-ddC0.5w and H9-ddC5.0w cells were 56.7 and 51.4% (with thymidine as a substrate) and 50.3 and 42% (with AZT as a substrate). Consequently the cellular incorporation of AZT and thymidine (24-hr incubation) also was reduced to 51.3 and 70.0% in H9-ddC0.5w cells and to 12.1 and 17.3% in H9-ddC5.0w cells. A 3-hr incubation with 25 microM AZT and ddC decreased their cellular incorporation to 50.5 and 76.15% in H9-ddC0.5w cells and to 12.95 and 47.8% in H9-ddC5.0w cells compared with H9 cells. Thus, the change in AZT accumulation did not correlate exactly with the decrease in TK activity and far exceeded the effect on ddC accumulation. Evidence is presented that ddC, in addition to deoxycytidine kinase, affected TK1 activity. The involvement of multidrug resistance proteins in the mechanism of the resistance was ruled out by the failure of trifluoperazine and verapamil to alter cellular accumulations of AZT, ddC, daunorubicin, and rhodamine-123. Development of cellular ddC and AZT cross-resistance may affect the therapeutic efficacy of these antiviral agents.

Collateral resistance of a dideoxycytidine-resistant cell line to 5-fluoro-2'-deoxyuridine

Exposure of a human lymphocytic cell line, H9 cells, to 0.5 microM and 5.0 microM dideoxycytidine (ddC) resulted in isolation of ddC-resistant H9-ddC0.5w and H9-ddC5.0w cell lines. In addition, these cell lines were also resistant to azidothymidine and had reduced deoxycytidine kinase and thymidine kinase activities. We now show that these cell lines are 4-fold and 2000-fold collaterally resistant to 5-fluoro-2'-deoxyuridine (FdUR), respectively, but not to 5-fluorouracil (FU). Biochemical evaluations show that, compared to the parental cells, the FdUR phosphorylation was reduced to 36.3% and 9.2% and the FdUMP levels were decreased to 48.1% and 1.2% in these cell lines. Taken together, the data suggest that ddC, an antiviral agent, is capable of inducing resistance to FdUR-a drug that is not its analog and which has a different metabolism, target site, and mechanism of action.

Managing resistance to anti-HIV drugs: an important consideration for effective disease management

Current recommendations for the treatment of HIV-infected patients advise highly active antiretroviral therapy (HAART) consisting of combinations of 3 or more drugs to provide long-term clinical benefit. This is because only a complete suppression of virus replication will be able to prevent virus drug resistance, the main cause of drug failure. Virus drug resistance may remain a cause of concern in patients who have already received suboptimal mono- or bitherapy, or for patients who do not experience complete shut-down of virus replication under HAART. For these patients, replacement of one combination therapy regimen by another at drug failure, taking into account the existing resistance profile, will be needed. The development of new drugs will remain necessary for those patients who have failed to respond to all currently available drugs, as will be the institution of more effective and less toxic HAART regimens.

2',3'-Dideoxycytidine cytotoxicity in human macrophages

Human macrophages when cultured for several weeks in the presence of therapeutically relevant 2',3'-dideoxycytidine (ddC) concentrations show a time-dependent decay in mitochondrial DNA content. This decay is associated with a reduction of Rhodamine 123 fluorescence, a marker for mitochondrial membrane potential suggesting that impairment of mitochondrial functions occurs. Mitochondrial metabolic impairment was confirmed by direct evaluation of lactate production, which is markedly increased in cells treated with ddC. The activity of protein kinase C and intracellular free Ca2+ upon addition of phorbol 12-myristate 13-acetate (PMA) were lower in the drug-treated cells compared to controls. A 50% reduction in O2-release was also found upon PMA stimulation. Fluorescent latex beads, yeast and bacteria phagocytosis were normal, but intracellular bacteria killing was markedly impaired in ddC-exposed macrophages. Thus, ddC exerts a delayed mitochondrial toxicity also on differentiated macrophages with impairment of several metabolic properties and O2 production causing a reduced ability of these phagocytic cells to kill phagocytosed bacteria.

Depletion of mitochondrial DNA by ddC in untransformed human cell lines

In order to study the interaction between the nuclear and mitochondrial genomes we have developed a non-transformed cell system. It is based upon the complete removal of mtDNA from fibroblasts by treatment with a nucleoside analogue, 2',3' dideoxycytidine (ddC). After exposure to ddC we were able to generate viable fibroblasts devoid of mtDNA and to successfully repopulate them with exogenous mitochondria. This model system will be useful in characterizing nuclear mitochondrial interactions and in studying the effects of different nuclear backgrounds on the expression of different primary defects of mtDNA associated with human disease.

Increased microbicidal activity of human monoblastoid cells upon long-term exposure to dideoxycytidine

2',3'-Dideoxycytidine (ddC) is a nucleoside analogue currently used in AIDS therapy. We had previously found that long term exposure of U937 human monoblastoid cells to ddC induces the selection of drug-resistant cells (U937-R). In the present work we investigated some important biochemical properties and functional activities of these resistant cells. The results obtained show that U937-R maintained the properties of cell aggregation, adhesion and differentiation. Basal respiration, protein kinase C activity, superoxide anion release and intracellular free calcium were all increased in the drug-resistant line. Phagocytosis of fungi (Candida albicans) and bacteria (Staphylococcus aureus and Salmonella anatum) were similar in U937 and U937-R cells. Killing of C. albicans was significantly higher in drug-resistant cells (29.07 +/- 2.23% of killing vs 19.07 +/- 2.01 in the control; p < 0.001). Similarly, the bacterial killing was enhanced in U937-R cells (34.07 +/- 8.06% vs 22.60 +/- 4.41% in the control; p < 0.05). Thus, the results presented in this paper provide evidence of an increased microbicidal activity of human monocytic cells upon long term exposure to ddC, most likely due to an increased oxidative metabolism.