Divergent anti-human immunodeficiency virus activity and anabolic phosphorylation of 2‘,3‘-dideoxynucleoside analogs in resting and activated human cells

Activity against human immunodeficiency virus type 1, intracellular metabolism, and effects on human DNA polymerases of 4'-ethynyl-2-fluoro-2'-deoxyadenosine

We examined the intracytoplasmic anabolism and kinetics of antiviral activity against human immunodeficiency virus type 1 (HIV-1) of a nucleoside reverse transcriptase inhibitor, 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA), which has potent activity against wild-type and multidrug-resistant HIV-1 strains. When CEM cells were exposed to 0.1 microM [(3)H]EFdA or [(3)H]3'-azido-2',3'-dideoxythymidine (AZT) for 6 h, the intracellular EFdA-triphosphate (TP) level was 91.6 pmol/10(9) cells, while that of AZT was 396.5 pmol/10(9) cells. When CEM cells were exposed to 10 microM [(3)H]EFdA, the amount of EFdA-TP increased by 22-fold (2,090 pmol/10(9) cells), while the amount of [(3)H]AZT-TP increased only moderately by 2.4-fold (970 pmol/10(9) cells). The intracellular half-life values of EFdA-TP and AZT-TP were approximately 17 and approximately 3 h, respectively. When MT-4 cells were cultured with 0.01 microM EFdA for 24 h, thoroughly washed to remove EFdA, further cultured without EFdA for various periods of time, exposed to HIV-1(NL4-3), and cultured for an additional 5 days, the protection values were 75 and 47%, respectively, after 24 and 48 h with no drug incubation, while those with 1 microM AZT were 55 and 9.2%, respectively. The 50% inhibitory concentration values of EFdA-TP against human polymerases alpha, beta, and gamma were >100 microM, >100 microM, and 10 microM, respectively, while those of ddA-TP were >100 microM, 0.2 microM, and 0.2 microM, respectively. These data warrant further development of EFdA as a potential therapeutic agent for those patients who harbor wild-type HIV-1 and/or multidrug-resistant variants.

In vitro interactions between apricitabine and other deoxycytidine analogues

Apricitabine is a novel deoxycytidine analogue reverse transcriptase inhibitor that is under development for the treatment of human immunodeficiency virus type 1 (HIV-1) infection. Apricitabine is phosphorylated to its active triphosphate by deoxycytidine kinase, which is also responsible for the intracellular phosphorylation of lamivudine (3TC) and emtricitabine (FTC); hence, in vitro studies were performed to investigate possible interactions between apricitabine and these agents. Human peripheral blood mononuclear cells (PBMC) were incubated for 24 h with various concentrations of (3)H-labeled or unlabeled apricitabine, 3TC, or FTC. Intracellular concentrations of parent compounds and their phosphorylated derivatives were measured by high-performance liquid chromatography. In other experiments, viral reverse transcriptase activity was measured in PBMC infected with HIV-1 bearing M184V in the presence of various concentrations of apricitabine and 3TC. [(3)H]apricitabine and [(3)H]3TC were metabolized intracellularly to form mono-, di-, and triphosphates. 3TC and FTC (1 to 10 microM) produced concentration-dependent decreases in apricitabine phosphorylation; in contrast, apricitabine at concentrations of up to 30 muM had no effect on the phosphorylation of 3TC or FTC. The combination of apricitabine and 3TC reduced the antiviral activity of apricitabine against HIV-1: apricitabine concentrations producing 50% inhibition of viral reverse transcriptase were increased two- to fivefold in the presence of 3TC. These findings suggest that nucleoside reverse transcriptase inhibitors with similar modes of action may show biochemical interactions that affect their antiviral efficacy. It is therefore essential that potential interactions between combinations of new and existing agents be thoroughly investigated before such combinations are introduced into clinical practice.

Inhibition of HIV-1 replication in macrophages by a heterodinucleotide of lamivudine and tenofovir

OBJECTIVES

(i) To generate a new heterodinucleotide (3TCpPMPA) comprising the drugs lamivudine and tenofovir which have been shown to act synergistically and (ii) to protect macrophages from 'de novo' HIV-1-infection through its administration.

METHODS

3TCpPMPA was obtained by coupling the morpholidate derivative of tenofovir with the mono n-tri-butylammonium salt of lamivudine 5'-monophosphate. Stability and metabolism were evaluated in vitro and in vivo in mice. 3TCpPMPA was encapsulated into autologous erythrocytes by a procedure of hypotonic dialysis, isotonic resealing and reannealing. 3TCpPMPA-loaded erythrocytes were modified to increase their phagocytosis by human macrophages. Macrophages were infected by HIV-1(Ba-L) and inhibition of HIV-1 replication was assessed by HIV p24(gag) quantification.

RESULTS

Pharmacokinetic studies in mice revealed a rapid disappearance of the heterodinucleotide from circulation (t(1/2)=15 min) without any advantage compared with the administration of single drugs. Adding free 3TCpPMPA to macrophages (18 h), a 90% inhibition of viral replication up to 35 days post-treatment was achieved, while only a 60% inhibition was obtained by the combined treatment 3TC and (R)PMPA. When 3TCpPMPA was selectively targeted to the macrophage compartment by a single addition of loaded erythrocytes, the protection of macrophages from 'de novo' infection (99% protection 3 weeks post-treatment) was nearly complete.

CONCLUSIONS

Erythrocytes loaded with 3TCpPMPA and modified to increase their phagocytosis are able to protect macrophages from 'de novo' HIV-1 infection. 3TCpPMPA acts as an efficient antiviral pro-drug that, once inside macrophages, can be slowly converted into 3TCMP and (R)PMPA protecting these cells for a longer period of time.

The role of mitochondrial dNTP levels in cells with reduced TK2 activity

Both the nuclear and mitochondrial DNA (mtDNA) depend on separate balanced pools of dNTPs for correct function of DNA replication and repair of DNA damage. Import of dNTPs from the cytosolic compartment to the mitochondria has been suggested to have the potential of rectifying a mitochondrial dNTP imbalance. Reduced TK2 activity has been demonstrated to result in mitochondrial dNTP imbalance and consequently mutations of mtDNA in non-dividing cells. In this study, the consequences of a reduced thymidine kinase 2 (TK2) activity were measured in proliferating HeLa cells, on both whole-cell as well as mitochondrial dNTP levels. With the exception of increased mitochondrial dCTP level no significant difference was found in cells with reduced TK2 activity. Our results suggest that import of cytosolic dNTPs in mitochondria of proliferating cells can compensate a TK2 induced imbalance of the mitochondrial dNTP pool.

Potential inhibition of somatic hypermutation by nucleoside analogues

Somatic hypermutation, which occurs in antigen-activated germinal centre B lymphocytes, diversifies the genes that encode immunoglobulin variable regions and leads to the 'affinity maturation' of the humoral immune response. Hypermutation affects dC/dG and dA/dT pairs with approximately equal frequency in vivo. DNA polymerase-theta contributes to hypermutagenesis at dC/dG pairs and DNA polymerase-eta is substantially involved in the generation of hypermutations at dA/dT pairs. The biochemical properties of polymerases-theta and -eta indicate that their DNA synthetic activities are potentially susceptible to inhibition by nucleoside analogues, so it is feasible that nucleoside analogues reduce the accumulation of dC/dG- and dA/dT-targeted hypermutations in vivo. Nucleoside analogues could hence impair the humoral adaptive immune response of HIV-infected patients who are prescribed these chemotherapeutic agents.

Simultaneous measurement of intracellular triphosphate metabolites of zidovudine, lamivudine and abacavir (carbovir) in human peripheral blood mononuclear cells by combined anion exchange solid phase extraction and LC-MS/MS

All nucleoside reverse transcriptase inhibitors (NRTI) must first be metabolized to their triphosphate forms in order to be active against HIV. Zidovudine (ZDV), abacavir (ABC) and lamivudine (3TC) have proven to be an efficacious combination. In order simultaneously to measure intracellular levels of the triphosphates (-TP) of ZDV, ABC (carbovir, CBV) and 3TC, either together or individually, we have developed a cartridge-LC-MS/MS method. The quantitation range was 2.5-250 pg/microl for 3TC-TP, 0.1-10.0 pg/microl for ZDV-TP and 0.05-5.00 pg/microl for CBV-TP. This corresponds to 0.1-11.0 pmol 3TC-TP per million cells, 4-375 fmol ZDV-TP per million cells and 2-200 fmol CBV-TP per million cells, extracted from 10 million cells. Patient samples demonstrated measured levels in the middle regions of our standard curves both at pre-dose and 4h post-dose times.

Model for intracellular Lamivudine metabolism in peripheral blood mononuclear cells ex vivo and in human immunodeficiency virus type 1-infected adolescents

The pharmacologic variability of nucleoside reverse transcriptase inhibitors such as lamivudine (3TC) includes not only systemic pharmacokinetic variability but also interindividual differences in cellular transport and metabolism. A modeling strategy linking laboratory studies of intracellular 3TC disposition with clinical studies in adolescent patients is described. Data from ex vivo laboratory experiments using peripheral blood mononuclear cells (PBMCs) from uninfected human subjects were first used to determine a model and population parameter estimates for 3TC cellular metabolism. Clinical study data from human immunodeficiency virus type 1-infected adolescents were then used in a Bayesian population analysis, together with the prior information from the ex vivo analysis, to develop a population model for 3TC systemic kinetics and cellular kinetics in PBMCs from patients during chronic therapy. The laboratory results demonstrate that the phosphorylation of 3TC is saturable under clinically relevant concentrations, that there is a rapid equilibrium between 3TC monophosphate and diphosphate and between 3TC diphosphate and triphosphate, and that 3TC triphosphate is recycled to 3TC monophosphate through a 3TC metabolite that remains to be definitively characterized. The resulting population model shows substantial interindividual variability in the cellular kinetics of 3TC with population coefficients of variation for model parameters ranging from 47 to 87%. This two-step ex vivo/clinical modeling approach using Bayesian population modeling of 3TC that links laboratory and clinical data has potential application for other drugs whose intracellular pharmacology is a major determinant of activity and/or toxicity.

Current status of antiretroviral therapy

At present, there are 22 FDA-approved antiretroviral agents, which are categorised into four classes of drugs. Several others are in various stages of basic and clinical development. The authors of this paper review the general characteristics of each class of antiretrovirals, as well as individual investigational agents that are in advanced clinical development. A brief synopsis of US and WHO antiretroviral treatment guidelines is also provided.

Pharmacology of current and promising nucleosides for the treatment of human immunodeficiency viruses

Nucleoside antiretroviral agents are chiral small molecules that have distinct advantages compared to other classes including long intracellular half-lives, low protein binding, sustained antiviral response when a dose is missed, and ease of chemical manufacture. They mimic natural nucleosides and target a unique but complex viral polymerase that is essential for viral replication. They remain the cornerstone of highly active antiretroviral therapy (HAART) and are usually combined with non-nucleoside reverse [corrected] transcriptase and protease inhibitors to provide powerful antiviral responses to prevent or delay the emergence of drug-resistant human immunodeficiency virus (HIV). The pharmacological and virological properties of a selected group of nucleoside analogs are described. Some of the newer nucleoside analogs have a high genetic barrier to resistance development. The lessons learned are that each nucleoside analog should be treated as a unique molecule since any structural modification, including a change in the enantiomeric form, can affect metabolism, pharmacokinetics, efficacy, toxicity and resistance profile.

R5 variants of human immunodeficiency virus type 1 preferentially infect CD62L- CD4+ T cells and are potentially resistant to nucleoside reverse transcriptase inhibitors

The persistence of human immunodeficiency virus type 1 (HIV-1) in memory CD4+ T cells is a major obstacle to the eradication of the virus with current antiretroviral therapy. Here, we investigated the effect of the activation status of CD4+ T cells on the predominance of R5 and X4 HIV-1 variants in different subsets of CD4+ T cells in ex vivo-infected human lymphoid tissues and peripheral blood mononuclear cells (PBMCs). In these cell systems, we examined the sensitivity of HIV replication to reverse transcriptase inhibitors. We demonstrate that R5 HIV-1 variants preferentially produced productive infection in HLA-DR- CD62L- CD4+ T cells. These cells were mostly in the G1b phase of the cell cycle, divided slowly, and expressed high levels of CCR5. In contrast, X4 HIV-1 variants preferentially produced productive infection in activated HLA-DR+ CD62L+ CD4+ T cells, which expressed high levels of CXCR4. The abilities of the nucleoside reverse transcriptase inhibitors (NRTI) zidovudine and lamivudine to stop HIV-1 replication were 20 times greater in activated T cells than in slowly dividing HLA-DR- CD62L- CD4+ T cells. This result, demonstrated both in a highly physiologically relevant ex vivo lymphoid tissue model and in PBMCs, correlated with higher levels of thymidine kinase mRNA in activated than in slowly dividing HLA-DR- CD62L- CD4+ T cells. The non-NRTI nevirapine was equally efficient in both cell subsets. The lymphoid tissue and PBMC-derived cell systems represent well-defined models which could be used as new tools for the study of the mechanism of resistance to HIV-1 inhibitors in HLA-DR- CD62L- CD4+ T cells.

Pharmacokinetic properties of nucleoside/nucleotide reverse transcriptase inhibitors

Options for antiretroviral therapy in patients infected with HIV continue to expand as new drugs are integrated into treatment regimens. Nonetheless, nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs/NtRTIs) remain the backbone of highly active antiretroviral therapy (HAART). With the approval of emtricitabine in 2003, there are now 8 Food and Drug Administration (FDA)-approved NRTIs/NtRTIs. Several of these agents are effective as once-daily therapy, including didanosine, lamivudine, extended-release stavudine (FDA approved, but not currently available), tenofovir DF, and emtricitabine. Recent results from pharmacokinetic and clinical trials indicate that another NRTI, abacavir, may also be effective as a once-daily therapy, and FDA approval of once-daily dosing is anticipated. NRTIs are inactive as administered, requiring anabolic phosphorylation within target cells to achieve their antiretroviral effects. All NRTIs are converted to nucleoside triphosphates, which serve as the active metabolites (the NtRTI, tenofovir DF, only requires conversion to the diphosphate form). Frequency of drug administration is closely related to the pharmacokinetic properties of a drug. The key parameter is the half-life; however, the plasma elimination half-life of the NRTIs/NtRTIs as administered is of little use in developing a dosing schedule. Rather, the intracellular half-life of the nucleoside triphosphate is the relevant parameter. This article reviews the pharmacokinetic properties, particularly those of the various phosphorylation steps, of the NRTIs/NtRTIs.

The pharmacology of antiretroviral nucleoside and nucleotide reverse transcriptase inhibitors: implications for once-daily dosing

The trend toward once-daily dosing in HIV antiretroviral therapy is based on the association between adherence, treatment outcome, and patient preferences. Patients prefer simpler treatments, fewer pills, less frequent dosing, and no food restrictions. When a regimen meets a patient's preferences, the patient is more likely to be adherent, and with good adherence, the regimen is more likely to be effective. Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) have been a prime focus for developing once-daily therapies primarily because they form the backbone of most current regimens. Within the NRTI class, however, drugs differ in their pharmacokinetic properties, such as plasma and intracellular half-lives, and thus in their suitability for once-daily dosing. For example, newer NRTIs, such as tenofovir and emtricitabine, combine longer plasma half-lives with longer intracellular half-lives, prolonging exposure and the period of pharmacologic activity. Of equal importance, the clinical impact of systemic and intracellular interactions between concomitant drugs defines which once-daily drugs may be combined in once-daily regimens. To construct simplified and effective therapies for individual patients, clinicians require an understanding of the plasma and intracellular pharmacokinetic properties of NRTIs and how these properties determine a drug's appropriateness for once-daily dosing and placement within a once-daily regimen.

Triple nucleoside analogue antiretroviral therapy: expanding the treatment approaches for management of HIV infection

The expanding range of drug options for the management of persons with HIV infection and concerns about the long-term safety of some treatment approaches has precipitated the HIV treatment community to examine a wider range of treatment approaches. One new therapeutic approach is to consider using three nucleoside analogue reverse transcriptase inhibitors, particularly as initial therapy. Recent data comparing triple nucleoside analogue therapy with established 'standard of care' triple therapy regimens have demonstrated similar antiviral effects and tolerability for such regimens. Efficacy and tolerability of triple nucleoside therapy has been demonstrated up to 96 weeks of therapy. This approach has advantages over conventional two nucleoside plus either a protease inhibitor or non-nucleoside reverse transcriptase inhibitor in terms of drug interactions and maintenance of multiple class options. It creates regimens which are, for the most part, compact, twice daily and non-food dependent. Specifically, this approach has already become widely established for individuals for whom drug interactions exclude the use of protease or non-nucleoside antiretrovirals. Triple nucleoside analogue therapy is likely to represent an attractive option for both physicians and patients; its use in clinical practice is likely to further expand.

To sequence or not to sequence?

There is now incontrovertible evidence that a combination of two nucleoside analogues is superior to AZT monotherapy for the treatment of HIV infection. There is also evidence that a range of antiretroviral combinations can produce profound drops in plasma HIV viral load, often below the detectable limits of the assay. A lower rate of viral replication is associated with delays in the selection of viral mutants resistant to drugs; the principal reason for therapeutic failure. These highly effective combinations are likely to delay clinical events and considerably improve the prognosis in HIV-infected individuals. Using mathematical modelling techniques to assess the half-life of plasma virus infectious cells and proviral DNA, one group has gone so far as to suggest it may be possible to stop therapy after several years, as the virus will have been eliminated. More sanguine investigators, however, believe that this is unlikely, as some infected cellular compartments may have long half-lives and the virus may persist, or even continue to replicate slowly, in sanctuary sites, such as the CNS and genital tract, poorly penetrated by drugs. Thus, some virologists and clinicians believe that it is crucially important to give initial therapy with the strongest possible combination of drugs to achieve an 'undetectable' plasma viral load and that, providing compliance is good, this will lead to a prolonged therapeutic effect. In these circumstances no sequencing issues arise. Others, however, continue to believe that, in due course, the virus is likely to evade drug pressure. In these circumstances, consideration of a sequence of drugs, to avoid initial and subsequent therapy from squandering future therapy options through, principally, cross-resistance, becomes an important issue. None of the issues discussed in this review has been tested by controlled clinical trials but they generate hypotheses requiring urgent evaluation.

Differential effects of R5 and X4 human immunodeficiency virus type 1 infection on CD4+ cell proliferation and activation

Human immunodeficiency virus type 1 (HIV-1) isolates can be distinguished by their chemokine coreceptor usage. Non-syncytium-inducing (NSI), macrophage-tropic viruses utilize CCR5 and are called R5 viruses; syncytium-inducing (SI) isolates use CXCR4 and are known as X4 viruses. R5 and X4 HIV isolates are both transmitted but, in most cases, R5 viruses predominate in the blood prior to the development of AIDS-related pathogenesis. The reason for the selective growth of the R5 strain is not known, but could reflect a replication advantage of R5 viruses over X4 viruses in CD4+ cells. To explore this possibility, eight phenotypically distinct viruses were used to infect CD4+ cells and cellular proliferation and activation were evaluated. In unstimulated CD4+ cells, R5 virus isolates increased the level of cell activation compared with X4 virus isolates and uninfected control cells. In CD4+ cells that were stimulated with interleukin 2, both R5 and X4 viruses were found to decrease the level of cell proliferation and reduce the majority of the activation markers studied when compared with uninfected control CD4+ cells from the same donors. However, although equal amounts of CD4+ cells were infected, R5 virus-infected CD4+ cells showed a two- to fourfold increase in cellular proliferation over X4 viruses, as measured by [3H]thymidine incorporation (P=0.001) and nuclear expression of Ki67 (P=0.001). In addition, a larger proportion of CD4+ T cells infected with R5 viruses had significantly higher levels of activation-marker expression (e.g. CD25, CD71 and HLA-DR) than CD4+ T lymphocytes infected with X4 viruses (P<0.02). Taken together, these results indicate that CD4+ cells infected with R5 virus isolates may have a selective advantage over X4 virus-infected CD4+ T cells for survival and, hence, virus spread.

Thymidine kinase and deoxycytidine kinase activity in mononuclear cells from antiretroviral-naive HIV-infected patients

OBJECTIVE

To evaluate whether an inter-individual variability in the activity of thymidine kinase (TK) and deoxycytidine kinase (dCK), which are involved in the first step of phosphorylation of some nucleoside analogues, exists in antiretroviral-naive, HIV-seropositive patients.

DESIGN

Forty-five randomly selected antiretroviral-naive HIV-infected patients were recruited, together with 26 healthy volunteers with no concurrent infection and under no pharmacological treatment.

METHODS

Peripheral blood mononuclear cells (PBMC) were isolated from venous blood and their TK and dCK activities evaluated. CD4 T cells and HIV-RNA were measured in HIV-infected patients, too.

RESULTS

There was a broad range of variability in TK activity in HIV-infected individuals. Furthermore, the activity in PBMC was significantly higher in HIV-infected individuals than in healthy volunteers. dCK activity in seropositive patients was significantly lower than in healthy volunteers. A marked inter-individual variability in dCK levels was observed in the HIV-infected group. No correlations were found between TK or dCK activities and plasma viral load, CD4 cell count, sex or age of patients.

CONCLUSIONS

A marked range of inter-individual variability of TK and dCK activities in PBMC exists in HIV-infected individuals but not in healthy volunteers, indicating that the activity of enzymes with key roles in drug activation could vary greatly from one patient to another. Furthermore, TK expression is greater in HIV-infected individuals than in healthy volunteers. Better understanding of the viral or cellular factors that contribute to this variability, as well as their effect on responses to antiretroviral treatment, may aid optimization of the management of HIV-infected patients.

Viral resistance patterns selected by antiretroviral drugs and their potential to guide treatment choice

Massive viral turnover and reverse transcriptase's high error rate create the potential for drug-resistant viral variants to appear rapidly under the selective pressure of antiretroviral therapy. Loss of antiviral effect in treatment-adherent persons is most commonly coincident with the appearance of viral mutants with reduced drug sensitivity. Thus, detection of viral resistance may represent an early marker of therapy failure. Similarly, control of viral replication in the plasma compartment, as defined by plasma viral load below the levels of assay quantification, is associated with a sustained therapeutic response and delayed development of viral resistance. Information on patterns of resistance to and cross-resistance between antiretroviral agents is increasingly well characterised and represents an important consideration when deciding how to combine and/or sequence antiretrovirals to achieve optimal antiviral effects. Given the limited number of antiretrovirals presently available or in advanced development, it is important not to limit future therapeutic options by using therapies early in the treatment sequence which may select for cross-resistant viral variants and hence potentially reduce the magnitude of therapeutic response when treatment is changed to another member of that drug class. However, no studies using resistance to guide clinical decision making have been reported to date and available sequencing studies have focused largely on switching or adding therapies to patients experienced with zidovudine monotherapy. Thus, no resistance driven treatment algorithm is currently available.

Pharmacokinetic and pharmacodynamic characteristics of emtricitabine support its once daily dosing for the treatment of HIV infection

Emtricitabine (FTC) is a potent deoxycytidine nucleoside analogue that was recently approved for the treatment of HIV infection. Emtricitabine is activated by intracellular phosphorylation to its 5'-triphosphate (FTC5'-TP), a competitive inhibitor of the HIV reverse transcriptase (RT). Early clinical studies incorporating pharmacokinetic-pharmacodynamic (PK-PD) analyses provided a sound rationale for developing FTC as a once daily drug. A short-term open-label monotherapy trial in therapy naive HIV-infected subjects evaluated various dosage regimens of FTC, i.e., 25, 100, and 200 mg qd and/or bid, with serial measurements of plasma HIV RNA, plasma FTC, and intracellular (PBMC) FTC-5'-TP levels over the 14 days of treatment. PK data were augmented by other steady-state studies, one in healthy volunteers and the other in HIV-infected patients receiving 200 mg FTC qd, with measurements of plasma FTC and/or intracellular FTC-5'-TP levels. Correlation between anti-HIV activity and FTC-5'-TP levels was examined with dose- and concentration-response relationships determined. The once daily dosing schedule is supported by the relatively long half-lives of plasma FTC (8-10 hr) and PBMC FTC-TP (39 hr) and the high plasma FTC and PBMC FTC-5'-TP concentrations. HIV RNA suppression (PD) correlates well with PBMC FTC-5'-TP levels (PK), both reaching a plateau at doses > or = 200 mg/day. The PK and PD characteristics of FTC demonstrate that it is a once daily nucleoside RT inhibitor.

Zidovudine, lamivudine, and abacavir have different effects on resting cells infected with human immunodeficiency virus in vitro

We have previously described an in vitro model for the evaluation of the effects of different immunomodulatory agents and immunotoxins (ITs) on cells latently infected with human immunodeficiency virus (HIV). We demonstrated that latently infected, replication-competent cells can be generated in vitro after eliminating CD25+ cells with an IT. Thus, by selectively killing the productively infected cells with an anti-CD25 IT we can generate a population of latently infected cells. CD25- cells generated in this manner were treated with nucleoside analog reverse transcriptase inhibitors and subsequently activated with phytohemagglutinin in the presence of the drugs. The antiviral activities of zidovudine (ZDV), lamivudine (3TC), and abacavir (ABC) were evaluated by using this model. 3TC and ABC demonstrated significant activity in decreasing HIV production from recently infected resting cells following their activation, whereas the effect of ZDV was more modest. These results suggest that the differences in antiviral activity of nucleoside analogs on resting cells should be considered when designing drug combinations for the treatment of HIV infection. The model presented here offers a convenient alternative for evaluating the mechanism of action of new antiretroviral agents (J. Saavedra, C. Johnson, J. Koester, M. St. Claire, E. Vitteta, O. Ramilo, 37th Intersci. Conf. Antimicrob. Agents Chemother., abstr. I-59, 1997).

A loss of viral replicative capacity correlates with altered DNA polymerization kinetics by the human immunodeficiency virus reverse transcriptase bearing the K65R and L74V dideoxynucleoside resistance substitutions

Mechanisms governing viral replicative capacity are poorly understood at the biochemical level. Human immunodeficiency virus, type 1 reverse transcriptase (HIV-1 RT) K65R or L74V substitutions confer viral resistance to 2',3'-dideoxyinosine (ddI) in vivo. The two substitutions never occur together, and L74V is frequently found in patients receiving ddI, while K65R is not. Here we show that recombinant viruses carrying K65R and K65R/L74V display the same resistance level to ddI (about 9.5-fold) relative to wild type. Consistent with this result, purified HIV-1 RT carrying K65R RT or K65R/L74V substitutions exhibits an 8-fold resistance to ddATP as judged by pre-steady state kinetics of incorporation of a single nucleotide into DNA. Resistance is due to a selective decrease of the catalytic rate constant k(pol): 22-fold (from 7.2 to 0.33 s(-1)) for K65R RT and 84-fold (from 7.2 to 0.086 s(-1)) for K65R/L74V RT. However, the K65R/L74V virus replication capacity is severely impaired relative to that of wild-type virus. This loss of viral fitness is correlated to a poor ability of K65R/L74V RT to use natural nucleotides relative to wild-type RT: 15% that of wild-type RT for dATP, 36% for dGTP, 50% for dTTP, and 25% for dCTP. The order of incorporation efficiency is wild-type RT > L74V RT > K65R RT > K65R/L74V RT. Processivity of DNA synthesis remains unaffected. These results explain why the two mutations do not combine in the clinic and might give a mechanism for a decreased viral fitness at the molecular level.

Determination of ddATP levels in human immunodeficiency virus-infected patients treated with dideoxyinosine

Clinical failures of the highly active antiretroviral therapy could result from inefficient intracellular concentrations of antiviral drugs. The determination of drug contents in target cells of each patient would be useful in clinical investigations and trials. The purpose of this work was to quantify the intracellular concentration of ddATP, the active metabolite of dideoxyinosine (ddI), in peripheral blood mononuclear cells (PBMCs) of human immunodeficiency virus (HIV)-infected patients treated with ddI. We have raised antibodies against ddA-citrate, a stable isostere of ddATP selected on the basis of its structural and electronic analogies with ddATP. The anti-ddA-citrate antibodies recognized ddATP and ddA with nanomolar affinities and cross-reacted neither with any of the nucleotide reverse transcriptase inhibitors used in HIV therapy nor with their phosphorylated metabolites. The three phosphorylated metabolites of ddI (ddAMP, ddADP, and ddATP) were purified by anion exchange chromatography and the amount of each metabolite was determined by radioimmunoassay with or without prior phosphatase treatment. The intracellular levels of the three ddI metabolites were measured both in an in vitro model and in PBMCs of HIV-infected patients under ddI treatment. The possibility to measure intracellular levels of ddATP from small blood samples of HIV-infected patients treated with ddI could be exploited to develop individual therapeutic monitoring.

The cellular pharmacology of nucleoside- and nucleotide-analogue reverse-transcriptase inhibitors and its relationship to clinical toxicities

Nucleoside- and nucleotide-analogue reverse-transcriptase inhibitors (NRTIs) require intracellular phosphorylation for anti-human immunodeficiency virus (HIV) activity and toxicity. Long-term toxicities associated with NRTIs may be related to overactivation of this process. In vitro experiments have shown increased rates of NRTI and endogenous nucleoside phosphorylation to be associated with cellular activation. Patients with advanced HIV disease often have overexpression of cytokines, which corresponds to an elevated cellular activation state. These patients also have higher rates of NRTI phosphorylation and NRTI toxicity, suggesting an interaction between a proinflammatory biological state, NRTI phosphorylation, and toxicity. Studies suggest that women may have higher rates of NRTI phosphorylation than do men, as well as increased risk for NRTI-induced toxicity. Future research is needed to understand the NRTI activation process and improve the long-term toxicity profile of NRTIs. Such research should include comparisons of NRTI phosphorylation according to sex and cellular activation state (i.e., elevated vs. low).

Pharmacodynamics and clinical use of anti-HIV drugs

Antiretroviral drug exposure has been linked to both antiviral efficacy and the development of toxicity and further research in this area is ongoing and necessary. Use of these data may have important implications for TDM of HAART regimens in clinical practice. TDM, in conjunction with an assessment of the patient's viral resistance in the form of an IQ, needs to be examined and validated in large clinical trials.

Antiviral dynamics and sex differences of zidovudine and lamivudine triphosphate concentrations in HIV-infected individuals

OBJECTIVES

Nucleoside analog reverse transcriptase inhibitors (NRTI) are used in virtually all anti-HIV regimens. Clinical response depends on the intracellular formation of the pharmacologically active triphosphate moiety. Our objective was to quantify the pharmacological characteristics of zidovudine and lamivudine triphosphate in HIV-infected individuals.

METHODS

Peripheral blood mononuclear cells were obtained at multiple planned intervals from antiretroviral-naive adults participating in a study of zidovudine, lamivudine and indinavir, and triphosphate levels were determined by immunoassay and high-performance liquid chromatography/mass spectrometry. Plasma HIV-RNA, CD4 cell counts, and plasma drug concentrations were collected over 18 months. Data were analysed using non-parametric, regression and time-to-event methods.

RESULTS

Thirty-three subjects were evaluated. The estimated half-lives of zidovudine and lamivudine triphosphate were 7 and 22 h, respectively. Triphosphate concentrations were elevated in individuals with low baseline CD4 cell counts. Triphosphate concentrations in women were higher than in men by 2.3 and 1.6-fold for zidovudine and lamivudine, respectively. Women reached an HIV-RNA level under 50 copies/ml twice as fast as men. Zidovudine triphosphate above 30 fmol/10(6) cells was independently predictive of the time to under 50 copies/ml. Lamivudine triphosphate above 7017 fmol/10(6) cells was independently predictive of a longer virological response. Indinavir concentrations were related to antiviral responses in univariate analyses.

CONCLUSION

Zidovudine and lamivudine triphosphate concentration thresholds were independently associated with the antiviral activity of zidovudine, lamivudine, and indinavir. The significantly elevated triphosphate concentrations in women and individuals with low baseline CD4 cell counts, groups that historically experience high rates of serious NRTI toxicities, provide a hypothesis for the pathogenesis of these events.

Mechanistic basis for reduced viral and enzymatic fitness of HIV-1 reverse transcriptase containing both K65R and M184V mutations

HIV-1 drug resistance mutations are often inversely correlated with viral fitness, which remains poorly described at the molecular level. Some resistance mutations can also suppress resistance caused by other resistance mutations. We report the molecular mechanisms by which a virus resistant to lamivudine with the M184V reverse transcriptase mutation shows increased susceptibility to tenofovir and can suppress the effects of the tenofovir resistance mutation K65R. Additionally, we report how the decreased viral replication capacity of resistant viruses is directly linked to their decreased ability to use natural nucleotide substrates and that combination of the K65R and M184V resistance mutations leads to greater decreases in viral replication capacity. All together, these results define at the molecular level how nucleoside-resistant viruses can be driven to reduced viral fitness.

Update on didanosine

Didanosine (ddl) has been a cornerstone of HIV management since it was made available in October 1991. Didanosine was originally introduced as an alternative to zidovudine (ZDV) for patients who were intolerant of ZDV or experienced disease progression during ZDV monotherapy. Didanosine is now used extensively as an integral component of multidrug combination regimens in both adults and children with HIV infection, and is now available for once-daily administration in the United States, Canada, and Europe. The recently approved Videx EC is an enteric-coated didanosine capsule dosed as one capsule, once daily. This paper provides an update of recently published studies on the use of ddl in combination anti-HIV therapy. In particular, these studies examine the rationale for the use of ddl as first-line anti-HIV therapy, and describe newer findings concerning its long-term efficacy, side effects, compliance, resistance, and once-daily use. The increased survival of HIV-infected patients is largely attributed to the introduction of the triple combination drug therapy but is probably also due to the long-term clinical efficacy of ddl.

Metabolism of tenofovir and didanosine in quiescent or stimulated human peripheral blood mononuclear cells

OBJECTIVE

As tenofovir disoproxil fumarate substantially increases plasma concentrations of didanosine in patients with human immunodeficiency virus-1 infection, we sought to determine whether tenofovir and didanosine showed a similar intracellular interaction in human peripheral blood mononuclear cells (PBMCs).

DESIGN

Comparative in vitro incubation of two antiretrovirals in lymphocytes.

SETTING

Clinical research laboratory.

MATERIAL

Radiolabeled tenofovir and didanosine in human PBMCs.

MEASUREMENTS AND MAIN RESULTS

Phosphorylation of 2 and 20 microM didanosine to dideoxyadenosine triphosphate (ddATP) was determined in quiescent and stimulated PBMCs in the presence or absence of 5 microM tenofovir. Similarly, phosphorylation of 5 microM tenofovir to tenofovir diphosphate (TFVpp) was examined in the presence or absence of 2 and 20 microM didanosine. Intracellular amounts of ddATP and TFVpp were determined by incubating PBMCs with radiolabeled tenofovir or didanosine alone and together for up to 16 hours and then separating the anabolites by high-performance liquid chromatography for quantitation. The presence of tenofovir did not affect the amount of ddATP in quiescent or stimulated PBMCs with 2 or 20 microM didanosine. In addition, didanosine did not alter the amount of TFVpp that formed. The amount of ddATP was modestly (1.5-3-fold) but consistently higher in stimulated than in quiescent PBMCs, but the amount of TFVpp did not differ.

CONCLUSION

There is no significant interaction between tenofovir and didanosine in human PBMCs as determined by the extent of formation of the phosphorylated anabolites. This suggests that adjusting didanosine dosage, when given with tenofovir, to achieve similar didanosine plasma concentrations, may be sufficient to accommodate the systemic drug interaction.

Phosphorylation of 3'-azidothymidine in maternal and fetal peripheral blood mononuclear cells during gestation and at term

The present study compared the phosphorylation rate of 3'-azidothymidine (AZT) in isolated maternal and fetal peripheral blood mononuclear cells (PBMCs) with that in amniocytes obtained during gestation and at term. Maternal PBMCs were isolated from venous blood samples obtained from HIV-seronegative pregnant women during delivery. Immediately after delivery, cord blood specimens were collected, and fetal PBMCs were isolated. In a separate set of experiments, maternal and fetal PBMCs and amniocytes were obtained at 17-21 weeks of gestation. The fresh isolated PBMCs and amniocytes were maintained in RPMI 1640 medium until incubation with 10 microM tritiated AZT (10 microCi/mL). Thereafter, methanolic cell extracts were prepared for determination of AZT phosphates by high-performance liquid chromatography. Fetal PBMCs can efficiently convert AZT to its antivirally active metabolite. There were no significant differences after 6 or 12 hours of incubation with AZT between AZT phosphate levels in maternal and fetal PBMCs isolated at term or at 17-21 weeks of gestation: AZT monophosphate was found to be the major metabolite (about 95%). AZT phosphate levels in the amniocytes were up to sevenfold higher than those in the maternal or fetal PBMCs. These results show that during pregnancy and at term, fetal PBMCs-like maternal PBMCs-are able to take up AZT and to efficiently generate the active metabolite AZT triphosphate. These results are of major significance both in enabling efficient treatment of the fetuses of HIV-infected women and in the prediction and understanding of the efficacy and toxicity of AZT in pregnant women and their fetuses.

Effects of dipeptide insertions between codons 69 and 70 of human immunodeficiency virus type 1 reverse transcriptase on primer unblocking, deoxynucleoside triphosphate inhibition, and DNA chain elongation

Finger insertion mutations of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) (T69S mutations followed by various dipeptide insertions) have a multinucleoside resistance phenotype that can be explained by decreased sensitivity to deoxynucleoside triphosphate (dNTP) inhibition of the nucleotide-dependent unblocking activity of RT. We show that RTs with SG or AG (but not SS) insertions have three- to fourfold-increased unblocking activity and that all three finger insertion mutations have threefold-decreased sensitivity to dNTP inhibition. The additional presence of M41L and T215Y mutations increased unblocking activity for all three insertions, greatly reduced the sensitivity to dNTP inhibition, and resulted in defects in in vitro DNA chain elongation. The DNA chain elongation defects were partially repaired by additional mutations at positions 210, 211, and 214. These results suggest that structural communication between the regions of RT defined by these mutations plays a role in the multinucleoside resistance phenotype.

Lamivudine can exert a modest antiviral effect against human immunodeficiency virus type 1 containing the M184V mutation

The M184V mutation in human immunodeficiency virus (HIV) reverse transcriptase is associated with high-level resistance to both (-)2',3'-dideoxy-3'-thiacytidine (3TC) and (-)2',3'-dideoxy-5-fluoro-3'-thiacytidine as well as low-level resistance to 2',3'-dideoxyinosine, 2',3'-dideoxycytidine, and abacavir. This mutation is also associated with diminished HIV replicative fitness as well as several functional changes in enzyme activity, including diminutions in polymerase processivity, pyrophosphorylysis, and nucleotide primer unblocking. Despite the fact that M184V encodes up to 1,000-fold resistance to 3TC, we asked whether this drug might still display some antiviral effect in regard to viruses containing this mutation. Cell-free assays revealed that high concentrations of 3TC triphosphate (i.e., >100 micro M) could affect chain termination and/or inhibit purified reverse transcriptase containing the M184V substitution. This effect became more pronounced with elongation of reverse transcriptase products. In newly infected cells (i.e., peripheral blood mononuclear cells), we found that the amount of full-length reverse transcriptase product was diminished in the presence of 2 to 10 micro M 3TC, although no decrease in the first product of the reverse transcriptase reaction, i.e., minus strong-stop DNA, was observed. In the presence of two other HIV inhibitors, e.g., nevirapine and indinavir, 3TC exerted additive effects in tissue culture at concentrations only marginally higher than the 50% inhibitory concentration (IC(50)). Reverse transcriptases cloned from clinical isolates harboring M184V in the context of multidrug resistance had similar IC(50) values for 3TC triphosphate compared to reverse transcriptase containing only the M184V mutation. These results suggest that viruses containing M184V can retain a higher degree of sensitivity to 3TC than previously assumed.

HIV-1 resistance profile of the novel nucleoside reverse transcriptase inhibitor beta-D-2',3'-dideoxy-2',3'-didehydro-5-fluorocytidine (Reverset)

Nucleoside reverse transcriptase inhibitors (NRTIs) represent the cornerstone of highly active antiretroviral therapy when combined with non-nucleoside reverse transcriptase inhibitors (NNRTIs) or HIV-1 protease inhibitors (PIs). Unlike the NNRTIs and PIs, NRTIs must be successively phosphorylated by cellular kinases to a triphosphate form, which represents the active metabolite possessing antiviral activity. Emergence of viral resistance to NRTIs has severely hampered treatment options for persons infected with HIV-1. As such, there is an urgent need to develop NRTIs capable of suppressing NRTI-resistant strains of HIV-1. We have recently reported that the cytidine analogue D-d4FC (DPC817, Reverset) effectively inhibits clinically prevalent resistant strains of HIV-1. In this report, we have extended these findings and now describe a detailed resistance profile for this novel NRTI. By examining a panel of 50 viruses carrying RTs derived from HIV-1 clinical isolates displaying a wide range of NRTI resistance mutations, we report that the median fold increase in effective antiviral concentration for such a panel of viruses is 3.2, which is comparable to tenofovir (2.8-fold) and didanosine (2.4-fold). D-d4FC is highly effective at inhibiting subsets of lamivudine- and zidovudine-resistant variants but, like other NRTIs, seems less potent against multi-NRTI-resistant viruses, particularly those carrying the Q151M complex of mutations. Finally, in vitro selections for HIV-1 mutants capable of replicating in the presence of D-d4FC yielded a mutant carrying the RT K65R mutation. This mutation confers 5.3- to 8.7-fold resistance to D-d4FC in vitro. These findings suggest that D-d4FC may represent an alternative NRTI for the treatment of individuals infected with lamivudine- and zidovudine-resistant strains of HIV-1.

Cellular pharmacology of D-d4FC, a nucleoside analogue active against drug-resistant HIV

The backbone of effective highly active antiretroviral therapy regimens for the treatment of HIV infections currently contains at least two nucleosides. Among the features that influence the potency of each component of a regimen and the overall efficacy of the combination are the cellular uptake and bioconversion of nucleoside analogues to their active triphosphate form, and the extent of possible interactions in these steps that might occur when more than one nucleoside is used in a regimen. D-d4FC (Reverset), a new cytidine analogue with the ability to inhibit many nucleoside-resistant viral variants, was examined for these parameters. In phytohemaglutinin-stimulated human peripheral blood mononuclear cells, D-d4FC was taken up in a rapid (8 h to 50% maximal value), saturable (plateau above 10 microM parent nucleoside concentration) process, resulting in levels of D-d4FC triphosphate that should provide potent antiviral activity against a variety of virus genotypes. Based on measurement of antiviral effects in cell culture, additive and in some cases, synergistic interactions were observed with protease inhibitors, non-nucleoside reverse transcriptase inhibitors or other nucleosides, including cytidine analogues.

Time-dependent changes in HIV nucleoside analogue phosphorylation and the effect of hydroxyurea

BACKGROUND

Nucleoside analogues are activated to their triphosphates, which compete with endogenous deoxynucleoside triphosphate (dNTP) pools to inhibit HIV reverse transcriptase. Hydroxyurea has been administered with nucleoside analogues to modulate intracellular dNTP pools and thus the ratio of drug triphosphate:endogenous triphosphate.

OBJECTIVES

To examine changes in drug activation over time and investigate the effects of hydroxyurea on intracellular phosphorylation of antiretroviral nucleoside analogues.

PATIENTS

A total of 229 HIV-infected individuals receiving abacavir, lamivudine and zidovudine were randomly assigned to receive or not nevirapine and hydroxyurea. Twenty-four patients were recruited to an observational substudy measuring intracellular drug triphosphate and dNTP concentrations at 0, 2, 6, 12, 24 and 48 weeks.

METHODS

Drugs were extracted from isolated peripheral blood mononuclear cells before analysis of endogenous dNTP and drug triphosphates by primer extension assays.

RESULTS

Twenty-two out of 24 patients were followed to completion of the substudy. Hydroxyurea had no demonstrable effect on endogenous dNTP or drug triphosphate levels at any timepoint. However, the ratio of zidovudine triphosphate to endogenous deoxythymidine triphosphate was significantly increased with hydroxyurea. A significant decrease in lamivudine triphosphate (3TCTP) and the 3TCTP:endogenous deoxycytidine triphosphate ratio was seen over 48 weeks. In five patients who failed therapy in the first 24 weeks, significantly reduced 3TCTP was seen.

CONCLUSION

Hydroxyurea does not affect measurable pools of endogenous nucleosides in vivo. Decreased lamivudine phosphorylation over time may provide a novel pharmacological explanation for the mechanism of resistance to this drug.

[Creation of new supramolecular chemistry based on multiple interaction in aqueous solution]

Novel supramolecular chemistry of multinuclear zinc(II) complexes in aqueous solution has been created by utilizing multiple interactions with polyanions. We have established reliable and convenient synthetic methods of multidentate macrocyclic polyamines and their zinc(II) complexes and thereby undertook a focused investigation of four topics: 1) efficient inhibition of photo[2 + 2] cycloaddition of thymidine dimer by dimeric zinc(II) complexes; 2) selective extraction and transport of thymidine nucleotide derivatives from the aqueous phase to the organic phase by lipophilic zinc(II) complexes and selective recognition of thymidine nucleotides by ditopic zinc(II) complexes in aqueous solution; 3) supramolecular polyhedrons formed by self-assembly of a trimeric zinc(II) complex with cyanuric acid or trithiocyanuric acid; and 4) a selective fluorescent probe for lanthanide ions such as Y3+ and La3+ based on a double-functionalized ligand with carbamoyl and dansyl groups. This knowledge should afford new methodology for supramolecular chemistry in aqueous solution.

[Pharmacological and clinical properties of didanosine (VIDEX), a nucleoside reverse transcriptase inhibitor]

An active metabolite, ddATP, of didanosine that is an analogue of purine-nucleoside (a component of nucleic acid) was known to inhibit the activity of DNA polymerase for E. coli. In 1985, Dr. Michiya et al. of NCI reported that didanosine and ddA inhibited replication of the human immunodeficiency virus (HIV). This discovery led to the clinical application of both the compounds. Didanosine, after being uptaken into a cell, becomes an active metabolite, ddATP, to inhibit a reverse transcriptase of HIV. Compared with zidovudine, didanosine has weak cytotoxicity both in vitro and in vivo. Didanosine, which is recommended as a first-line therapy drug in the Japanese Guideline on an anti-HIV Infection Therapy, was approved as twice-daily Videx Tablet and Dry Syrup formulations for launch in June 1992. In March 2001, a once-daily Videx EC Capsule formulation was approved and launched, having expected adherence improvements in HIV/AIDS patients.

DPC 817: a cytidine nucleoside analog with activity against zidovudine- and lamivudine-resistant viral variants

Highly active antiretroviral therapy (HAART) is the standard treatment for infection with the human immunodeficiency virus (HIV). HAART regimens consist of protease inhibitors or nonnucleoside reverse transcriptase inhibitors combined with two or more nucleoside reverse transcriptase inhibitors (NRTIs). DPC 817, 2',3'-didehydro-2',3'-dideoxy-5-fluorocytidine (PSI 5582 D-D4FC) is a potent inhibitor of HIV type 1 replication in vitro. Importantly, DPC 817 retains activity against isolates harboring mutations in the reverse transcriptase gene that confer resistance to lamivudine (3TC) and zidovudine (AZT), which are frequent components of initial HAART regimens. DPC 817 combines this favorable resistance profile with rapid uptake and conversion to the active metabolite DPC 817-triphosphate, which has an intracellular half-life of 13 to 17 h. Pharmacokinetics in the rhesus monkey suggest low clearance of parent DPC 817 and a plasma half-life longer than that of either AZT or 3TC. Together, these properties suggest that DPC 817 may be useful as a component of HAART regimens in individuals with resistance to older NRTI agents.

Selection of a T-Cell Line Resistant to Stavudine and Zidovudine by Prolonged Treatment with Stavudine

It has been demonstrated that prolonged treatment with nucleoside analogues, such as 3′-azido-3′-deoxythymi-dine (zidovudine), 2’,3′-dideoxycytidine (zalcitabine) and 9-(2-phosphonylmethoxyethyl) adenine (PMEA), may cause selection of cells that are resistant to their anti-HIV activity. A human T-lymphoblastoid cell line that is resistant to the antiviral and cytotoxic activity of 2’,3′-didehydro-3′-deoxythymidine (stavudine) has developed as a result of prolonged treatment. These cells, called CEMstavudine, are also less sensitive to zidovudine. The cellular/pharmacological resistance acquired by the CEMstavudine cells is relatively low and appears to correlate with a reduction in thymidine kinase (TK) activity, rather than with a decreased expression of TK mRNA.

S-acyl-2-thioethyl (SATE) pronucleotides are potent inhibitors of HIV-1 replication in T-lymphoid cells cross-resistant to deoxycytidine and thymidine analogs

The biological evaluation of mononucleotide prodrugs (pronucleotides) of various nucleoside reverse transcriptase inhibitors (NRTIs) such as zidovudine (AZT), zalcitabine (ddC) and lamivudine (3TC) was reported in human T-lymphoid MOLT-4/8 cells which were grown continuously for more than 1 year in a medium containing cytarabine (Ara-C). In this cell line, expression of deoxycytidine kinase (dCK) and thymidine kinase 1 (TK1) was decreased in comparison to parental cells (3.8 and 2.9-fold, respectively). The lower mRNA level of TK1 correlated significantly with lower enzyme activity, whereas no dCK activity was detectable. In Ara-C-resistant cells, anti-HIV-1 effects of ddC, 3TC and AZT were more than 100-fold lower compared with parental cells. In contrast, the corresponding mononucleoside phosphotriesters bearing S-acyl-2-thioethyl (SATE) groups as biolabile phosphate protection retained anti-HIV-1 activity due to their ability to bypass the first monophosphorylation step catalyzed by dCK or TK1. The results demonstrate that in vitro selection of T-lymphoid cells in the presence of Ara-C results in cross-resistance to deoxycytidine (ddC, 3TC) and thymidine (AZT) analogs and that these cellular resistance mechanisms can be bypassed by the use of bis(SATE) pronucleotides.

Improved method for the simultaneous determination of d4T, 3TC and ddl intracellular phosphorylated anabolites in human peripheral-blood mononuclear cells using high-performance liquid chromatography/tandem mass spectrometry

There is still a need for direct determination (i.e. without dephosphorylation) of nucleoside reverse transcriptase inhibitor (NRTI) triphosphorylated nucleotides in the peripheral-blood mononuclear cells (PBMCs) of HIV-positive patients. The objective of this paper was first to improve our previously described direct liquid chromatography/tandem mass spectrometry (LC/MS/MS) assay for stavudine triphosphate (d4T-TP). Preparation of PBMCs was modified to reduce degradation of d4T-TP during cell preparation and to simplify this step for routine use in clinical units. The performance of several HPLC columns was compared in order to improve the stability of peak shape over time. The SMT C(18) column was replaced by a Supelcogel ODP-50, thereby reducing two-fold the concentration of the first standard. Various internal standards were compared to optimize peak shape and remove an interfering peak in LC. 2-Chloroadenosine 5prime prime or minute-triphosphate was chosen as the most appropriate internal standard. Substitution of the narrowbore column by a microbore column (150 x 0.32 mm) is also presented and discussed. Secondly, this improved method was successfully applied to the simultaneous determination of d4T-TP, dideoxyadenosine triphosphate (ddA-TP) and lamivudine triphosphate (3TC-TP) in PBMCs, which is useful in view of the common use of NRTI combinations. The method was subsequently applied to clinical samples from HIV-positive patients receiving antiretroviral therapy containing d4T, ddl and/or 3TC. This method can be used simply and routinely on approximately 200 samples per week, using commercially available instruments and with a simple cell lysis as sample treatment.

Mitochondrial Toxicity Associated with Nucleoside Reverse Transcriptase Inhibitor Therapy

Nucleoside reverse transcriptase inhibitors result in a wide range of toxic side effects. These include lactic acidosis syndrome, myopathy, cardiomyopathy, pancreatitis, peripheral neuropathy, and possibly lipodystrophy. Despite the seemingly diverse nature of these side effects, all of these toxicities may be mediated by a common pathophysiologic mechanism, namely, mitochondrial toxicity resulting from nucleoside reverse transcriptase inhibitor-induced inhibition of DNA polymerase g. This article reviews the relevant mitochondrial biology and mechanism underlying nucleoside reverse transcriptase inhibitor-induced mitochondrial toxicity. Clinical manifestations of this toxicity are reviewed followed by a discussion of clinical management.

Testing the reverse transcriptase model of somatic mutation

Somatic hypermutation of the variable (V) regions of rearranged immunoglobulin genes leads to antibody affinity maturation. Although this process has been extensively studied, the mechanisms responsible for these multiple point mutations are still elusive. One mechanism that was proposed over 10 years ago by Steele and Pollard was that an intrinsic reverse transcriptase (RT) copies the nascent mRNA creating the large number of observed point mutations due to its high error rate. A cDNA copy of the mutated V region would then replace the endogenous DNA through a gene conversion-like event, thus integrating these point mutations into the genome. This model of hypermutation would account for the very high mutation rate, the presence of hotspots, strand bias, the requirement for transcription and localization of mutation within the immunoglobulin V region. Using AZT and ddC to inhibit endogenous RTs, we have assayed for somatic mutation using a murine in vivo model. Somatic mutation occurred at similar frequencies and with the same characteristics with or without treatment of RT inhibitors, suggesting that standard reverse transcription is not required for antibody V region hypermutation in the mouse.

Current status of antiretroviral therapies

The availability of potent antiretroviral drugs and their use in combination regimens has led to a dramatic decline in the morbidity and mortality associated with HIV infection. Currently, there are 15 FDA-approved antiretrovirals categorised into three classes of drugs. Several others are in various stages of basic and clinical development.

New bis(SATE) prodrug of AZT 5'-monophosphate: in vitro anti-HIV activity, stability, and potential oral absorption

The in vitro anti-HIV activity, stability, and potential for oral absorption of a phosphotriester derivative of AZT (zidovudine; 3'-azido-2',3'-deoxythymidine) bearing a new esterase-labile S-acyl-2-thioethyl (SATE) group as transient phosphate protection are reported. The biolabile protection is characterized by the presence of a hydroxyl function in the acyl chain. In accordance with previously reported data in the bis(SATE) prodrug series, the present results demonstrate that the studied bis(hydroxytBuSATE)phosphotriester exerts its biological effects via intracellular delivery of the 5'-monophosphate of AZT. The hydroxyl function confers a high resistance against esterase hydrolysis, and the studied prodrug is able to cross the Caco-2 cell monolayers in intact form, suggesting that its further development as a possible anti-HIV pronucleotide candidate is warranted.

Zidovudine (AZT) resistance in H9 cells due to decreased TK expression is associated with hypermethylation of TK gene

AZT resistant human T-lymphoid H9 cells, deficient in TK gene expression, re-expressed TK mRNA and regained the ability to metabolize AZT by exposure to the demethylation agent azacytidine (AzaCd). Cytotoxic and anti-HIV-1 effects of AZT were increased in H9 AZT resistant cells treated with AzaCd when compared to untreated cells. This leads to the assumption that drug induced DNA hypermethylation was involved in the TK gene-silencing mechanism. Our results suggest approaches using modulation of gene methylation for increasing antiviral efficiency of drugs.

Influence of prior exposure to zidovudine on stavudine phosphorylation in vivo and ex vivo

Intracellular phosphorylation of stavudine (d4T) and zidovudine (ZDV) was investigated in peripheral blood mononuclear cells (PBMCs) isolated from ZDV-naive and ZDV-experienced human immunodeficiency virus (HIV)-positive patients. An in vivo study measured the amount of d4T triphosphate (d4TTP), while an ex vivo study assessed the capacity of cells to phosphorylate added d4T. Endogenous dTTP was also measured. d4TTP and dTTP were determined in vivo using a reverse transcriptase chain termination assay. In ex vivo studies, d4T (1 microM) was incubated in resting and phytohemagglutinin-stimulated (10 microg ml(-1); 72 h) PBMCs for 24 h. After washing and methanol extraction, radiolabeled anabolites were detected by high-performance liquid chromatography. d4TTP reached its highest level 2 to 4 h after dosing (0.21 +/- 0.14 pmol/10(6) cells; n = 27 [mean +/- standard deviation]). Comparison of ZDV-naive and ZDV-experienced individuals showed no significant difference in levels of d4TTP (ZDV naive, 0.23 +/- 0.17 pmol/10(6) cells [n = 7] versus ZDV experienced, 0.20 +/- 0.14 pmol/10(6) cells [n = 20]; P = 0.473) or the d4TTP/dTTP ratio (0.14 +/- 0.12 [n = 7] and 0.10 +/- 0.08 [n = 20], respectively; p = 0.391). Ex vivo data demonstrated no significant difference in the formation of d4TTP or total d4T phosphates in naive and experienced patients (0.086 +/- 0.055 pmol/10(6) cells in ZDV-naive patients [n = 17] versus 0.081 +/- 0.038 pmol/10(6) cells in ZDV-experienced patients [n = 22]; P = 0.767). The ability of HIV-infected patients to phosphorylate d4T in vivo and ex vivo was unchanged with increasing exposure to ZDV.

Phase I/II dose escalation and randomized withdrawal study with add-on azodicarbonamide in patients failing on current antiretroviral therapy

BACKGROUND

Azodicarbonamide (ADA), a HIV-1 zinc finger inhibitor, targets a new step in viral replication and cell infectivity.

OBJECTIVE

A first phase I/II clinical study of ADA.

METHODS

ADA was administered at escalating doses concomitantly with current antiviral therapy during a 3-month open-label period in patients with advanced AIDS and documented virological failure. After 3 months, patients were randomized in a double-blind placebo-controlled withdrawal, ADA being given at the highest tolerated dosage.

RESULTS

Fifteen patients with advanced disease failing on combined antiretroviral therapy, 75% of them with proven phenotypic resistance, had a median baseline CD4 cell count of 85 x 10(6) cells/l, CD4/CD8 cell ratio of 0.09 and median plasma RNA viral load of 4.2 log10 copies/ml. Tolerance to ADA was dose dependent and some patients developed nephrolithiasis, glucose intolerance or showed an ADA-related cytotoxicity towards CD4 cells at higher dosages. No patient died during the study period. ADA increased CD4 cell percentage, increased the CD4/CD8 cell ratio and decreased plasma RNA viral load from baseline. At the end of the double-blind period, the ADA group, but not the placebo group, showed a significant response (P < 0.05). No phenotypic resistance to ADA was observed. Overall, 3/11 patients (27%) had consistent viral load reductions > 0.5 log10 copies/ml compared with baseline and 5/ 11 (45%) showed a CD4 cell recovery from baseline > 33%. In responders, ADA induced a median peak increase in CD4 cell percentage change from baseline of 65% (range 47-243%), and viral load decrease of 1.04 log10 copies/ml (range 0.52-1.23).

CONCLUSIONS

The maximal tolerated dosage of ADA appears to be 2 g (three times daily). This study provides safety results that will allow larger clinical trials to confirm the preliminary efficacy data.

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.

Fluorometric determination of 2'-beta-fluoro-2',3'-dideoxyadenosine 5'-triphosphate, the active metabolite of a new anti-human immunodeficiency virus drug, in human lymphocytes

A sensitive precolumn derivatization method has been developed to measure the 5'-triphosphate of 2'-beta-fluoro-2',3'-dideoxyadenosine (F-ddA, lodenosine), a new anti-HIV drug, in human lymphocytes by HPLC using fluorescence detection. Reaction of chloroacetaldehyde with F-ddA triphosphate in extracts from human lymphocytes produces a highly fluorescent etheno adduct. This derivative is then separated and quantitated by reverse-phase paired-ion chromatography. Degradation of natural nucleic acid ribosides, such as ATP, using periodate oxidation simplifies the chromatogram and minimizes interference with detection of the target analyte. This method, modeled using cultured MOLT-4 T-lymphocytes, achieves a linear detector response for peak area measurements over the range 2.5 to 22.5 pmol (50-450 nM using 50 microl sample). Analyte recovery is greater than 90%, and the method achieves a limit of detection and limit of quantitation of 1.4 and 2.5 pmol per HPLC injection (50 microl sample containing cellular extract from 2.5 x 10(6) cells), respectively. Application of this method to measure F-ddATP in peripheral blood mononuclear cells from HIV-infected patients treated with F-ddA at 3.2 mg/kg twice daily for 22 days shows F-ddATP levels which range from 1.5 to 3.5 pmol/10(6) cells.