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

Zidovudine in synergistic combination with fosfomycin: an in vitro and in vivo evaluation against multidrug-resistant Enterobacterales

Multidrug-resistant (MDR) Enterobacterales are a priority health issue with few treatment options. Recently, fosfomycin has been reconsidered for MDR bacterial infections. Zidovudine, licensed for the treatment of human immunodeficiency virus (HIV), has unexploited antibacterial properties and has been considered for drug repurposing. The aim of this study was to assess the effect of the combination of fosfomycin plus zidovudine against clinical MDR Enterobacterales isolates. Minimum inhibitory concentration (MIC) determination and checkerboard assays for 36 MDR Enterobacterales strains were performed. In addition, fosfomycin-resistant strains were evaluated using time-kill assay and in an in vivo Galleria mellonella infection model. Zidovudine and fosfomycin MICs ranged between 0.06 to >64 mg/L and 0.125 to >512 mg/L, respectively. A synergistic effect [fractional inhibitory concentration index (FICI) ≤0.5] was observed in 25 isolates and no antagonistic effect was observed in the remaining isolates. For 7 of 8 fosfomycin-resistant strains (MIC > 32 mg/L), zidovudine combination was able to restore fosfomycin susceptibility. These results were confirmed by time-kill assays. Fosfomycin + zidovudine presented greater larval survival (20-50%) than monotherapy. Synergistic activity was observed for fosfomycin + zidovudine in 69.4% of the tested strains. In vivo experiments confirmed the enhanced effectiveness of the combination. The zidovudine concentrations tested here can be reached in human serum using the actual licensed dosage, therefore this combination deserves further clinical investigation.

Inactivation of thymidine kinase as a cause of resistance to zidovudine in clinical isolates of Escherichia coli: a phenotypic and genomic study

Objectives

The antiviral zidovudine has been recently identified as an active drug against resistant Enterobacteriaceae, but prevalence of resistance to this compound remains unknown. The aim was to estimate the prevalence of clinical Escherichia coli isolates resistant to zidovudine and to decipher the mechanism of zidovudine resistance.

Methods

We screened 537 isolates on zidovudine-containing agar plates and studied their thymidine kinase (tdk) gene sequences, the putative target involved in zidovudine resistance. Moreover, sequence analysis of 633 complete genomes of E. coli was performed to investigate mutation in the tdk gene. A comparative genomic analysis was done on an in vitro zidovudine-resistant mutant.

Results

After screening on our medium containing 2.7 mg/L (10 μM) zidovudine, nine strains had a zidovudine MIC >26.7 mg/L. The gene was absent in three isolates, inactivated by an IS (IS1X2 and ISApl1) in two isolates and mutated in four isolates. A genomic analysis of 633 E. coli genomes showed heterogeneity of the tdk gene sequence, with 27 different sequences. Among them, three genomes showed an inactivation of the gene (IS, stop codon and no tdk gene sequence). The in vitro mutant E. coli had 27 SNPs in eight genes of the core genome compared with the initial strain.

Conclusions

Our study reports zidovudine-resistant clinical isolates of E. coli, presumably related to tdk inactivation. Diversity of Tdk in bacterial genomes can be large. Other mechanisms need to be considered in zidovudine resistance. The use of zidovudine in antibiotic-resistant infections needs to be in combination and should be tested before clinical administration.

Determinants of individual variation in intracellular accumulation of anti-HIV nucleoside analog metabolites

Individual variation in response to antiretroviral therapy is well-known, but it is not clear if demographic characteristics such as gender, age, and ethnicity are responsible for the variation. To optimize anti-HIV therapy and guide antiretroviral drug discovery, determinants that cause variable responses to therapy need to be evaluated. We investigated the determinants of intracellular concentrations of nucleoside analogs using peripheral blood mononuclear cells from 40 healthy donors. We observed individual differences in the concentrations of the intracellular nucleoside analogs; the mean concentrations of the triphosphate metabolite of ethynylstavudine (4'-Ed4T), zidovudine (AZT), and lamivudine (3TC) were 0.71 pmol/10(6) cells (minimum and maximum, 0.10 and 3.00 pmol/10(6) cells, respectively), 0.88 pmol/10(6) cells (minimum and maximum, 0.10 and 15.18 pmol/10(6) cells, respectively), and 1.70 pmol/10(6) cells (minimum and maximum, 0.20 and 7.73 pmol/10(6) cells, respectively). Gender and ethnicity had no effect on the concentration of 4'-Ed4T and 3TC metabolites. There was a trend for moderation of the concentrations of AZT metabolites by gender (P = 0.17 for gender·metabolite concentration). We observed variability in the activity and expression of cellular kinases. There was no statistically significant correlation between thymidine kinase 1 (TK-1) activity or expression and thymidine analog metabolite concentrations. The correlation between the activity of deoxycytidine kinase (dCK) and the 3TC monophosphate metabolite concentration showed a trend toward significance (P = 0.1). We observed an inverse correlation between the multidrug-resistant protein 2 (MRP2) expression index and the concentrations of AZT monophosphate, AZT triphosphate, and total AZT metabolites. Our findings suggest that the observed variation in clinical response to nucleoside analogs may be due partly to the individual differences in the intracellular concentrations, which in turn may be affected by the cellular kinases involved in the phosphorylation pathway and ATP-binding cassette (ABC) transport proteins.

Pharmacokinetics of intracellular zidovudine and its phosphorylated anabolites in the absence and presence of stavudine using an in vitro human peripheral blood mononuclear cell (PBMC) model

Both zidovudine (ZDV) and stavudine (D4T) must be intracellularly converted to their respective active triphosphate anabolites (ZDV-TP and D4T-TP). It is hypothesized that the combination of ZDV and D4T may lead to altered formation of phosphorylated anabolites for either drug. The objective of this study was to investigate the effect of D4T on intracellular ZDV phosphorylation. Human PBMCs were incubated with [(3)H]ZDV in the presence and absence of D4T. Cells were harvested at several time points over 12 h to determine area under the intracellular concentration versus time curve (AUC) of ZDV and its phosphorylated anabolites. Radiolabled ZDV and anabolites were quantified using HPLC and LS. The AUC for ZDV-TP was 0.53 and 0.52 pmol x h/10(6) PBMC in the absence and presence of D4T, respectively. The AUC for ZDV monophosphate was 157.45 and 172.44 pmol x h/10(6) PBMC pre and post D4T. D4T does not appear to affect the formation of intracellular ZDV phosphates in human PBMCs under the conditions studied.

CD4 immunophenotyping in HIV infection

The ability to rapidly identify immune cell subsets such as CD4 cells, which became possible around the same time as the onset of the HIV/AIDS pandemic, was one of the greatest advances in clinical and diagnostic immunology. The evolution of this global pandemic and the subsequent development of treatment strategies to prolong the life of infected individuals mean that it is now more crucial than ever that we develop affordable, reliable and accurate methods for the enumeration of CD4 cells. Here, we provide an overview of the historical developments in CD4 enumeration technologies that are related to HIV infection, and summarize the current technological challenges that must be overcome to meet the needs of those living with HIV infection.

High levels of zidovudine (AZT) and its intracellular phosphate metabolites in AZT- and AZT-lamivudine-treated newborns of human immunodeficiency virus-infected mothers

Newborns from human immunodeficiency virus-infected mothers are given antiretroviral prophylaxis against mother-to-child transmission, including predominantly nucleoside reverse transcriptase inhibitors. Pharmacological monitoring of these drugs in newborns has so far been limited to plasma and cord blood. In this study, samples from newborns (up to 45 days old) treated with zidovudine (AZT) alone (n = 29) or in combination with lamivudine (3TC) (n = 20) were analyzed for both intracellular concentrations of phosphate metabolites in peripheral blood mononuclear cells and levels of parent drugs in plasma. Plasma AZT and intracellular AZT-monophosphate and AZT-triphosphate (TP) concentrations were significantly higher during the first 15 days of life (199 versus 52.7 ng/ml [P < 0.0001], 732 versus 282 fmol/10(6) cells [P < 0.0001], and 170 versus 65.1 fmol/10(6) cells [P < 0.0001], respectively) and then became comparable to those of adults. No difference in intracellular AZT metabolite concentrations was found when AZT- and AZT-3TC-treated groups were compared. Plasma 3TC levels (lower limit of quantification [LLOQ], 1,157 ng/ml; median, 412.5 ng/ml) were not associated with the newborn's age, gender, or weight. Intracellular 3TC-TP concentrations (LLOQ, 40.4 pmol/10(6) cells; median, 18.9 pmol/10(6) cells) determined for newborns receiving the AZT-3TC combination were associated with neither the age nor weight of the newborns. Concentrations in females were significantly higher (1.8-fold [P = 0.0415]) than those in males. Unexpectedly, newborns on AZT monotherapy whose mothers' treatment included 3TC displayed residual plasma 3TC and intracellular 3TC-TP levels up to 1 week after birth.

Improved method to quantify intracellular zidovudine mono- and triphosphate in peripheral blood mononuclear cells by liquid chromatography-tandem mass spectrometry

The determination of intracellular triphosphate metabolites of nucleoside analogs used in anti-HIV therapy is very challenging. Despite the well-known sensitivity and selectivity of LC-MS/MS, the measurement of the triphosphate metabolite of zidovudine (AZT-TP) remains difficult because of the interferences induced by endogenous nucleotides triphosphates. We describe a new approach that allows improved determination of AZT-TP simultaneously with AZT-monophosphate (MP). This was obtained, first, by monitoring a transition from the molecular ion of AZT-TP to a minor but very specific product ion. Then, the spiking of samples with a constant amount of AZT-TP allowed the signal to emerge from background, leading to increased sensitivity. Finally, the analytical run time was reduced to less than 10 min. The low limits of quantification were at 150 and 300 fmol per sample for AZT-TP and AZT-MP, respectively. Recoveries were higher than 85%. Inaccuracy and precision were lower than 10% and 15% (17% at the limit of quantification), respectively. The new method offers the possibility of determining simultaneously other nucleotide phosphates, as shown here for d4T-TP (the triphosphate metabolite of another nucleoside analog, stavudine or d4T) and 2'-deoxythymidine-5'-triphosphate or dTTP (the corresponding natural nucleotide triphosphate).

Lack of reduction of thymidine kinase activity in stavudine-treated HIV-infected patients

It has been demonstrated that prolonged in vivo or in vitro treatment with some nucleosides analogs may favor the selection of cells with a reduced activity of enzymes involved in the phosphorylation of these drugs leading to a reduced sensitivity to their antiretroviral action. The aim of this study was to evaluate the effect, in vivo, of zidovudine and stavudine treatment on thymidine kinase (TK) activity. The results obtained showed that TK levels in PBMC from naive patients and stavudine-treated patients did not significantly differ (naive TK = 4.16 +/- 1.19 U/mg protein; stavudine TK = 3.65 +/- 1.73 U/mg protein; p = 0.42), suggesting that the treatment with this nucleoside analog is not associated to a defect of TK activity. On the contrary, PBMC from zidovudine-treated patients showed a significant reduction in TK activity compared to naive patients (naive TK = 4.16 +/- 1.19 U/mg protein; zidovudine TK = 2.70 +/- 1.54; p = 0.014. Although the clinical significance of these results has to be established, we can speculate that stavudine and zidovudine, which are presumably phosphorylated by the same cellular kinases, might display a different ability to in vivo select cells with a resistant phenotype.

Zidovudine phosphorylation determined sequentially over 12 months in human immunodeficiency virus-infected patients with or without previous exposure to antiretroviral agents

We sought to determine whether the intracellular activation of zidovudine (ZDV) varied over time and with previous antiretroviral exposure in human immunodeficiency virus-infected individuals and to examine whether there is an association between virological responses and intracellular phosphorylation. A total of 23 patients (12 treatment naïve, 11 previously treated with ZDV) who commenced ZDV as part of dual nucleoside therapy were prospectively monitored for 12 months or until withdrawal from the study. No association was observed between virological responses at 2 weeks and 3 months and ZDV phosphorylation. The mean intracellular concentrations of ZDV mono-, di-, and triphosphates did not change significantly over time or with previous ZDV exposure. The rate of formation of total ZDV phosphates was increased in patients with CD4 counts <100 cells/mm(3). Previous reports from in vitro cell culture experiments or cross-sectional cohort studies suggesting alterations of ZDV phosphorylation over time are not confirmed by this longitudinal study.

Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians

Nucleoside analogs (zidovudine, didanosine, zalcitabine, stavudine, abacavir, lamivudine) have been administered as antiretroviral agents for more than a decade. They undergo anabolic phosphorylation by intracellular kinases to form triphosphates, which inhibit human immunodeficiency virus replication by competitively inhibiting viral reverse transcriptase. Numerous methods are used to elucidate the intracellular metabolic pathways of these agents. Intracellular and extracellular factors affect intracellular phosphorylation. Lack of standardization and complexity of methods used to study phosphorylation in patients limit interpretation of study results and comparability of findings across studies. However, in vitro and in vivo studies give important insights into mechanisms of action, metabolic feedback mechanisms, antiviral effects, and mechanisms of toxicity, and have influenced dosing regimens of nucleoside analogs.

Zidovudine and stavudine sequencing in HIV treatment planning: findings from the CHORUS HIV cohort

BACKGROUND

Optimal sequencing of zidovudine and stavudine in antiretroviral therapy has not been elucidated.

OBJECTIVE

To examine the impact of the sequence of therapeutic regimens containing zidovudine and stavudine on HIV-1 RNA and CD4 lymphocyte counts over 12 months.

DESIGN

Observational, multicenter, longitudinal cohort study.

SETTING

Four large outpatient, HIV practices participating in the community-based Collaborations in HIV Outcomes Research-U.S. (CHORUS) cohort study.

PARTICIPANTS

940 HIV-infected patients.

METHODS

Comparison of HIV-1 RNA and CD4 lymphocyte responses in patients sequenced from zidovudine to stavudine or from stavudine to zidovudine using repeated measures regression models fit to outcomes by application of generalized estimating equation (GEE) methodology.

RESULTS

Patients treated with zidovudine prior to stavudine (n = 834) achieved a greater mean drop from baseline HIV-1 RNA (p = .01) and higher proportion of undetectable HIV-1 RNA results (p = .05) over 12 months than those sequenced from stavudine to zidovudine (n = 106). CD4+ lymphocyte increases did not differ between the groups (p = .6).

CONCLUSIONS

Prior zidovudine therapy was not associated with long-term attenuation of HIV-1 RNA or CD4 response to subsequent stavudine-containing regimens. Zidovudine before stavudine may have benefit in a strategic long-term therapeutic plan.

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.

Simultaneous quantitation of the 5'-triphosphate metabolites of zidovudine, lamivudine, and stavudine in peripheral mononuclear blood cells of HIV infected patients by high-performance liquid chromatography tandem mass spectrometry

A high-performance liquid chromatography (HPLC) method utilizing triple quadrupole mass spectrometry (MS) detection was developed and validated for the simultaneous measurement of the intracellular nucleoside 5'-triphosphate anabolites of zidovudine (ZDV-TP), lamivudine (3TC-TP), and stavudine (d4T-TP). These compounds were extracted from patient peripheral blood mononuclear cells (PBMCs) which are the sites of HIV replication and drug action. Ion-exchange solid phase extraction (SPE) followed by enzymatic digestion with alkaline phosphatase was utilized to yield the measurable nucleoside forms of the nucleotides. Reversed phase C-18 SPE with addition of a nucleoside internal standard, 3'-azido-2',3'-dideoxyuridine (AzdU) allowed for the indirect measurement of the original 5'-triphosphate concentration by HPLC/MS/MS. Quantitation was performed from calibration curves generated from authentic 5'-triphosphate standards spiked in PBMCs from healthy volunteers. Analytical range for the three 5'-triphosphates was equivalent to 50-45,000 pg. Mean interassay accuracies for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 99.4%, 100.1%, and 108.0%, respectively. Mean interassay precisions (%C.V.) for 3TC-TP, d4T-TP, and ZDV-TP (n > 90) were 8.8%, 10.4%, and 8.2%, respectively. Recovery of the extraction method was 79.2%, 83.1%, and 98.3% for 3TC-TP, d4T-TP, and ZDV-TP, respectively. This method can be utilized to measure the intracellular 5'-triphosphate levels in HIV infected patients receiving antiretroviral therapy containing the nucleoside reverse transcriptase inhibitors 3TC, d4T, or ZDV.

Pharmacokinetics of zidovudine phosphorylation in human immunodeficiency virus-positive thai patients and healthy volunteers

We assessed the pharmacokinetics of zidovudine (ZDV) in plasma and intracellular ZDV phosphate anabolites in peripheral blood mononuclear cells in Thai human immunodeficiency virus (HIV) type 1-infected patients and healthy volunteers. The plasma ZDV area under the concentration-time curve from 0 to 6 h (AUC(0-6)) was similar in patients and healthy volunteers (32.77 and 22.77 micromol/liter. h, respectively; confidence interval, -3.37 to 19. 92). Although the concentration of ZDV triphosphate (ZDVTP) was similar in the two groups, the ZDV monophosphate (ZDVMP) AUC(0-6) was significantly greater in HIV patients (1.12 pmol/10(6) cells) than in healthy volunteers (0.15 pmol/10(6) cells). In agreement with previously published data obtained with Caucasians, the significant difference in intracellular phosphorylation in Thai volunteers and HIV patients is primarily due to ZDVMP. Comparing the data from this study with the data obtained with Caucasians suggests no marked ethnic differences in ZDV phosphorylation.

The pharmacokinetics of lamivudine phosphorylation in peripheral blood mononuclear cells from patients infected with HIV-1

OBJECTIVE

To assess the pharmacokinetics of lamivudine phosphorylation in peripheral blood mononuclear cells (PBMC) from patients infected with HIV-1.

DESIGN

Single-center, open-label, randomized, two-period, cross-over study in 10 asymptomatic, antiretroviral-experienced, HIV-1-infected patients who had a CD4+ lymphocyte count of 200-500 x 10(6)/l and had received combination treatment with lamivudine 150 mg twice a day plus zidovudine 600 mg a day (divided into two or three doses) for > or = 16 weeks prior to study entry.

METHODS

Patients were randomly assigned to receive lamivudine 150 mg twice a day or lamivudine 300 mg twice a day for 14 days, with at least a 48-h washout period between treatments. Serial blood samples were collected over 36 h for determination of lamivudine serum concentrations using liquid chromatography/mass spectrometry and intracellular phosphate PBMC concentrations using high performance liquid chromatography/radioimmunoassay methods. Pharmacokinetic parameters were calculated based on lamivudine and lamivudine anabolite concentration-time data.

RESULTS

Intracellular pharmacokinetic parameters were highly variable between patients (coefficient of variations approximately 50%). The two regimens produced lamivudine-total phosphate (totP) values of a similar magnitude. Although the 300-mg regimen tended to produce higher lamivudine-monophosphate (MP) and -triphosphate (TP) values, differences from values produced by the 150-mg regimen were not statistically significant. As lamivudine diphosphate (DP) was the predominant anabolite, accounting for 50-55% of lamivudine-totP (compared with 30-35% for lamivudine-MP and 15-20% for lamivudine-TP), the conversion of lamivudine-DP to lamivudine-TP can be regarded as the rate-limiting step. The median lamivudine-TP intracellular half-life (t1/2) for the 150-mg and 300-mg regimens did not differ significantly (15.3 and 16.1 h, respectively). Serum lamivudine pharmacokinetic parameters were consistent with those observed in previous studies in HIV-1-infected patients. No apparent linear relationships were observed between lamivudine intracellular anabolite and serum data.

CONCLUSIONS

The intracellular pharmacokinetics of lamivudine phosphorylation in PBMC from asymptomatic HIV-1-infected patients are highly variable and do not differ statistically between the 150- and 300-mg twice a day regimens. The variations in intracellular lamivudine-TP concentrations following these two lamivudine dosage regimens are unlikely to result in differences in clinical effect. This was confirmed by the results of a large phase III study in HIV-1-infected patients which showed no differences in HIV-1 RNA or CD4+ lymphocyte counts between the 150- and 300-mg lamivudine regimens in combination with zidovudine.

Role of P-glycoprotein in the renal transport of dideoxynucleoside analog drugs

P-glycoprotein (P-gp), the MDR1 multidrug transporter, is known to be expressed in several human organs and tissues, including the apical membrane of the renal proximal tubular cells. It has been reported that human immunodeficiency virus 1 (HIV-1) can trigger the expression of P-gp in cultured cells (i.e., H9, a T-lymphocyte cell line, and U937, a monocyte cell line), which may render the cells resistant to antiretrovirals. Since multiple membrane transport systems (i.e., organic cation, organic anion, and nucleoside systems) can be involved in the renal tubular transport of dideoxynucleoside analog drugs (DADs) (i.e., zidovudine and zalcitabine), we have questioned if P-gp is involved in the renal transport of DADs. Chinese hamster ovary colchicine-resistant cells (CHRC5), a cell line that is well known to highly express P-gp, and continuous renal epithelial cell lines (LLC-PK1 and OK), which have also been shown to express P-gp, were used. The accumulation of [3H]vinblastine (20 nM), an established P-gp substrate, by the monolayer cells was significantly enhanced in the presence of two P-gp inhibitors (i.e., verapamil and cyclosporin A) and nucleoside transport inhibitors (i.e., dipyridamole and dilazep). In contrast, DADs (i.e., zidovudine, lamivudine, didanosine, and zalcitabine) did not significantly affect vinblastine accumulation by these cell lines. These data suggest that P-gp does not play a significant role in the renal tubular transport of DADs. Dipyridamole and dilazep, two nucleoside membrane transport inhibitors, appear to be P-gp inhibitors.Key words: P-glycoprotein, dideoxynucleoside analogs, human immunodeficiency virus 1, transport, renal.

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.

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-human immunodeficiency virus drug combination strategies

It is now generally accepted that mono- and bitherapy for human immunodeficiency virus type 1 (HIV-1) infection are only transiently efficient mainly due to virus drug resistance. To obtain a sustained benefit from antiviral therapy, current guidelines recommend at least triple-drug combinations, or the so-called highly active antiretroviral therapy (HAART). In some patients, HAART can be problematic, either because it is difficult to remain compliant or because previous suboptimum therapies have limited the choice of drugs. For compliant drug-naive patients, HAART should be able to offer long-term virus suppression, when changing from first- to second- to third-line HAART at drug failure. Long-term treatment might ultimately result in multi-drug resistant virus leaving few options for salvage therapy. HIV drug resistance testing to guide this salvage therapy and the development of new drugs to allow new options will therefore remain priorities in anti-HIV drug research.