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

HIV nucleoside reverse transcriptase inhibitors

More than 40 years into the pandemic, HIV remains a global burden and as of now, there is no cure in sight. Fortunately, highly active antiretroviral therapy (HAART) has been developed to manage and suppress HIV infection. Combinations of two to three drugs targeting key viral proteins, including compounds inhibiting HIV reverse transcriptase (RT), have become the cornerstone of HIV treatment. This review discusses nucleoside reverse transcriptase inhibitors (NRTIs), including chain terminators, delayed chain terminators, nucleoside reverse transcriptase translocation inhibitors (NRTTIs), and nucleotide competing RT inhibitors (NcRTIs); focusing on their history, mechanism of action, resistance, and current clinical application, including long-acting regimens.

Pharmacology of Antiretrovirals in the Female Genital Tract for HIV Prevention

Preexposure prophylaxis (PrEP) is a powerful tool that, as part of a comprehensive prevention package, has potential to significantly impact the HIV epidemic. PrEP effectiveness is believed to be dependent on the exposure and efficacy of antiretrovirals at the site of HIV transmission. Clinical trial results as well as modeling and simulation indicate the threshold of adherence required for PrEP efficacy of emtricitabine/tenofovir disoproxil fumarate may differ between sites of HIV transmission with less forgiveness for missed doses in women exposed through genital tissue compared to people exposed through colorectal tissue. This suggests a role for local and host factors to influence mucosal pharmacology. Here we review the mucosal pharmacology of antiretrovirals in the female genital tract and explore potential determinants of PrEP efficacy. Host factors such as inflammation, coinfections, hormonal status, and the vaginal microbiome will be explored as well as the role of drug-metabolizing enzymes and transporters in regulating local drug exposure. The use of preclinical and early clinical models to predict clinical effectiveness is also discussed.

Novel mechanisms to inhibit HIV reservoir seeding using Jak inhibitors

Despite advances in the treatment of HIV infection with ART, elucidating strategies to overcome HIV persistence, including blockade of viral reservoir establishment, maintenance, and expansion, remains a challenge. T cell homeostasis is a major driver of HIV persistence. Cytokines involved in regulating homeostasis of memory T cells, the major hub of the HIV reservoir, trigger the Jak-STAT pathway. We evaluated the ability of tofacitinib and ruxolitinib, two FDA-approved Jak inhibitors, to block seeding and maintenance of the HIV reservoir in vitro. We provide direct demonstration for involvement of the Jak-STAT pathway in HIV persistence in vivo, ex vivo, and in vitro; pSTAT5 strongly correlates with increased levels of integrated viral DNA in vivo, and in vitro Jak inhibitors reduce the frequency of CD4+ T cells harboring integrated HIV DNA. We show that Jak inhibitors block viral production from infected cells, inhibit γ-C receptor cytokine (IL-15)-induced viral reactivation from latent stores thereby preventing transmission of infectious particles to bystander activated T cells. These results show that dysregulation of the Jak-STAT pathway is associated with viral persistence in vivo, and that Jak inhibitors target key events downstream of γ-C cytokine (IL-2, IL-7 and IL-15) ligation to their receptors, impacting the magnitude of the HIV reservoir in all memory CD4 T cell subsets in vitro and ex vivo. Jak inhibitors represent a therapeutic modality to prevent key events of T cell activation that regulate HIV persistence and together, specific, potent blockade of these events may be integrated to future curative strategies.

Aureonitol, a Fungi-Derived Tetrahydrofuran, Inhibits Influenza Replication by Targeting Its Surface Glycoprotein Hemagglutinin

The influenza virus causes acute respiratory infections, leading to high morbidity and mortality in groups of patients at higher risk. Antiviral drugs represent the first line of defense against influenza, both for seasonal infections and pandemic outbreaks. Two main classes of drugs against influenza are in clinical use: M2-channel blockers and neuraminidase inhibitors. Nevertheless, because influenza strains that are resistant to these antivirals have been described, the search for novel compounds with different mechanisms of action is necessary. Here, we investigated the anti-influenza activity of a fungi-derived natural product, aureonitol. This compound inhibited influenza A and B virus replication. This compound was more effective against influenza A(H3N2), with an EC₅₀ of 100 nM. Aureonitol cytoxicity was also very low, with a CC₅₀ value of 1426 μM. Aureonitol inhibited influenza hemagglutination and, consequently, significantly impaired virus adsorption. Molecular modeling studies revealed that aureonitol docked in the sialic acid binding site of hemagglutinin, forming hydrogen bonds with highly conserved residues. Altogether, our results indicate that the chemical structure of aureonitol is promising for future anti-influenza drug design.

SAMHD1 has differential impact on the efficacies of HIV nucleoside reverse transcriptase inhibitors

Sterile alpha motif- and histidine/aspartic acid domain-containing protein 1 (SAMHD1) limits HIV-1 replication by hydrolyzing deoxynucleoside triphosphates (dNTPs) necessary for reverse transcription. Nucleoside reverse transcriptase inhibitors (NRTIs) are components of anti-HIV therapies. We report here that SAMHD1 cleaves NRTI triphosphates (TPs) at significantly lower rates than dNTPs and that SAMHD1 depletion from monocytic cells affects the susceptibility of HIV-1 infections to NRTIs in complex ways that depend not only on the relative changes in dNTP and NRTI-TP concentrations but also on the NRTI activation pathways.

4'-Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) inhibits HIV-1 reverse transcriptase with multiple mechanisms

4'-Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) is a nucleoside analog that, unlike approved anti-human immunodeficiency virus type 1 (HIV-1) nucleoside reverse transcriptase inhibitors, has a 3'-OH and exhibits remarkable potency against wild-type and drug-resistant HIVs. EFdA triphosphate (EFdA-TP) is unique among nucleoside reverse transcriptase inhibitors because it inhibits HIV-1 reverse transcriptase (RT) with multiple mechanisms. (a) EFdA-TP can block RT as a translocation-defective RT inhibitor that dramatically slows DNA synthesis, acting as a de facto immediate chain terminator. Although non-translocated EFdA-MP-terminated primers can be unblocked, they can be efficiently converted back to the EFdA-MP-terminated form. (b) EFdA-TP can function as a delayed chain terminator, allowing incorporation of an additional dNTP before blocking DNA synthesis. In such cases, EFdA-MP-terminated primers are protected from excision. (c) EFdA-MP can be efficiently misincorporated by RT, leading to mismatched primers that are extremely hard to extend and are also protected from excision. The context of template sequence defines the relative contribution of each mechanism and affects the affinity of EFdA-MP for potential incorporation sites, explaining in part the lack of antagonism between EFdA and tenofovir. Changes in the type of nucleotide before EFdA-MP incorporation can alter its mechanism of inhibition from delayed chain terminator to immediate chain terminator. The versatility of EFdA in inhibiting HIV replication by multiple mechanisms may explain why resistance to EFdA is more difficult to emerge.

Prodrug strategies for improved efficacy of nucleoside antiviral inhibitors

PURPOSE OF REVIEW

This review focuses on the chemical and pharmacological rationale behind the development of nucleoside antiviral prodrugs (NAPs).

RECENT FINDINGS

Highly efficacious NAPs have been developed that extend and improve the quality of lives of individuals infected with HIV and hepatitis B virus (HBV), herpes viruses, and adenovirus infection in immunocompromised individuals. A very high rate of hepatitis C virus (HCV) cure is now possible using NAPs combined with other direct acting antiviral agents (DAAs).

SUMMARY

Prodrug strategies can address the issues of poor oral bioavailability and delivery of active metabolites to the targeted cells. Additionally, NAPs demonstrate potential for improving deficiencies in oral absorption, metabolism, tissue distribution, cellular accumulation, phosphorylation, and overall potency, in addition to diminishing potential for in-vivo selection of resistant viruses. NAPs continue to be the backbone for the treatment of HIV and HBV, herpesviruses, and adenovirus infections because their active forms are potent, have long intracellular half-lives and are relatively safe with high barrier to resistance.

Hypersusceptibility mechanism of Tenofovir-resistant HIV to EFdA

Background

The K65R substitution in human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) is the major resistance mutation selected in patients treated with first-line antiretroviral tenofovir disoproxil fumarate (TDF). 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA), is the most potent nucleoside analog RT inhibitor (NRTI) that unlike all approved NRTIs retains a 3'-hydroxyl group and has remarkable potency against wild-type (WT) and drug-resistant HIVs. EFdA acts primarily as a chain terminator by blocking translocation following its incorporation into the nascent DNA chain. EFdA is in preclinical development and its effect on clinically relevant drug resistant HIV strains is critically important for the design of optimal regimens prior to initiation of clinical trials.

Results

Here we report that the K65R RT mutation causes hypersusceptibility to EFdA. Specifically, in single replication cycle experiments we found that EFdA blocks WT HIV ten times more efficiently than TDF. Under the same conditions K65R HIV was inhibited over 70 times more efficiently by EFdA than TDF. We determined the molecular mechanism of this hypersensitivity using enzymatic studies with WT and K65R RT. This substitution causes minor changes in the efficiency of EFdA incorporation with respect to the natural dATP substrate and also in the efficiency of RT translocation following incorporation of the inhibitor into the nascent DNA. However, a significant decrease in the excision efficiency of EFdA-MP from the 3’ primer terminus appears to be the primary cause of increased susceptibility to the inhibitor. Notably, the effects of the mutation are DNA-sequence dependent.

Conclusion

We have elucidated the mechanism of K65R HIV hypersusceptibility to EFdA. Our findings highlight the potential of EFdA to improve combination strategies against TDF-resistant HIV-1 strains.

Cellular pharmacology and potency of HIV-1 nucleoside analogs in primary human macrophages

Understanding the cellular pharmacology of antiretroviral agents in macrophages and subsequent correlation with antiviral potency provides a sentinel foundation for definition of the dynamics between antiretroviral agents and viral reservoirs across multiple cell types, with the goal of eradication of HIV-1 from these cells. Various clinically relevant nucleoside antiviral agents, and the integrase inhibitor raltegravir, were selected for this study. The intracellular concentrations of the active metabolites of the nucleoside analogs were found to be 5- to 140-fold lower in macrophages than in lymphocytes, and their antiviral potency was significantly lower in macrophages constitutively activated with macrophage colony-stimulating factor (M-CSF) during acute infection than in resting macrophages (EC(50), 0.4 to 9.42 μM versus 0.03 to 0.4 μM, respectively). Although tenofovir-treated cells displayed significantly lower intracellular drug levels than cells treated with its prodrug, tenofovir disoproxil fumarate, the levels of tenofovir-diphosphate for tenofovir-treated cells were similar in lymphocytes and macrophages. Raltegravir also displayed significantly lower intracellular concentrations in macrophages than in lymphocytes, independent of the activation state, but had similar potencies in resting and activated macrophages. These data underscore the importance of delivering adequate levels of drug to macrophages to reduce and eradicate HIV-1 infection.

Practical Considerations For Developing Nucleoside Reverse Transcriptase Inhibitors

Nucleoside reverse transcriptase inhibitors (NRTI) remain a cornerstone of current antiretroviral regimens in combinations usually with a non-nucleoside reverse transcriptase inhibitor (NNRTI), a protease inhibitor (PI), or an integrase inhibitor (INI). The antiretroviral efficacy and relative safety of current NRTI results from a tight and relatively specific binding of their phosphorylated nucleoside triphosphates (NRTI-TP) with the HIV-1 reverse transcriptase which is essential for replication. The intracellular stability of NRTI-TP produces a sustained antiviral response, which makes convenient dosing feasible. Lessons learned regarding NRTI pharmacology screening, development, and use are discussed. NRTI and prodrugs currently under clinical development are outlined.

In silico study supports the efficacy of a reduced dose regimen for stavudine

Stavudine (d4T) is used extensively as part of HAART in resource poor settings, despite its toxicities. The revised WHO guidelines specify replacement of d4T with less toxic but more expensive drugs when feasible, and that d4T doses be standardized to 30 mg twice daily (bid) (irrespective of body-weight), from the approved 40 mg bid in adults (body-weight ≥60 kg). Therefore, an in silico population pharmacokinetic and biochemical model was utilized to compare relative efficacies of the two doses in humans. Assessment of predicted quartile ranges of simulated concentrations of the triphosphate of d4T suggested sufficient trough concentrations to inhibit wild type HIV-1 reverse transcriptase at the reduced dose, lending support to the revised WHO recommendations.

The sugar ring conformation of 4'-ethynyl-2-fluoro-2'-deoxyadenosine and its recognition by the polymerase active site of HIV reverse transcriptase

4' Ethynyl-2-fluoro-2'-deoxyadenosine (EFdA) is the most potent inhibitor of HIV reverse transcriptase (RT). We have recently named EFdA a Translocation Defective RT Inhibitor (TDRTI) because after its incorporation in the nucleic acid it blocks DNA polymerization, primarily by preventing translocation of RT on the template/primer that has EFdA at the 3'-primer end (T/PEFdA). The sugar ring conformation of EFdA may also influence RT inhibition by a) affecting the binding of EFdA triphosphate (EFdATP) at the RT active site and/or b) by preventing proper positioning of the 3'-OH of EFdA in T/PEFdA that is required for efficient DNA synthesis. Specifically, the North (C2'-exo/C3'-endo), but not the South (C2'-endo/C3'-exo) nucleotide sugar ring conformation is required for efficient binding at the primer-binding and polymerase active sites of RT. In this study we use nuclear magnetic resonance (NMR) spectroscopy experiments to determine the sugar ring conformation of EFdA. We find that unlike adenosine nucleosides unsubstituted at the 4'-position, the sugar ring of EFdA is primarily in the North conformation. This difference in sugar ring puckering likely contributes to the more efficient incorporation of EFdATP by RT than dATP. In addition, it suggests that the 3'-OH of EFdA in T/PEFdA is not likely to prevent incorporation of additional nucleotides and thus it does not contribute to the mechanism of RT inhibition. This study provides the first insights into how structural attributes of EFdA affect its antiviral potency through interactions with its RT target.

The genetic toxicity effects of lamivudine and stavudine antiretroviral agents

IMPORTANCE OF THE FIELD

The nucleoside reverse transcriptase inhibitors (NRTIs) are used in antiretroviral therapy worldwide for the treatment of HIV infections. These drugs act by blocking reverse transcriptase enzyme activity, causing pro-viral DNA chain termination. As a consequence, NRTIs could cause genomic instability and loss of heterozygosity.

AREAS COVERED IN THIS REVIEW

This review highlights the toxic and genotoxic effects of NRTIs, particularly lamivudine (3TC) and stavudine (d4T) analogues. In addition, a battery of short-term in vitro and in vivo systems are described to explain the potential genotoxic effects of these NRTIs as a single drug or a complexity of highly active antiretroviral therapy.

WHAT THE READER WILL GAIN

The readers will gain an understanding of a secondary effect that could be induced by 3TC and d4T treatments.

TAKE HOME MESSAGE

Considering that AIDS has become a chronic disease, more comprehensive toxic genetic studies are needed, with particular attention to the genetic alterations induced by NRTIs. These alterations play a primary role in carcinogenesis and are also involved in secondary and subsequent steps of carcinogenesis.

Mechanisms involved in the selection of HIV-1 reverse transcriptase thumb subdomain polymorphisms associated with nucleoside analogue therapy failure

Previous studies showed an increased prevalence of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) thumb subdomain polymorphisms Pro272, Arg277, and Thr286 in patients failing therapy with nucleoside analogue combinations. Interestingly, wild-type HIV-1(BH10) RT contains Pro272, Arg277, and Thr286. Here, we demonstrate that in the presence of zidovudine, HIV-1(BH10) RT mutations P272A/R277K/T286A produce a significant reduction of the viral replication capacity in peripheral blood mononuclear cells in both the absence and presence of M41L/T215Y. In studies carried out with recombinant enzymes, we show that RT thumb subdomain mutations decrease primer-unblocking activity on RNA/DNA complexes, but not on DNA/DNA template-primers. These effects were observed with primers terminated with thymidine analogues (i.e., zidovudine and stavudine) and carbovir (the relevant derivative of abacavir) and were more pronounced when mutations were introduced in the wild-type HIV-1(BH10) RT sequence context. RT thumb subdomain mutations increased by 2-fold the apparent dissociation equilibrium constant (K(d)) for RNA/DNA without affecting the K(d) for DNA/DNA substrates. RNase H assays carried out with RNA/DNA complexes did not reveal an increase in the reaction rate or in secondary cleavage events that could account for the decreased excision activity. The interaction of Arg277 with the phosphate backbone of the RNA template in HIV-1 RT bound to RNA/DNA and the location of Thr286 close to the RNA strand are consistent with thumb polymorphisms playing a role in decreasing nucleoside RT inhibitor excision activity on RNA/DNA template-primers by affecting interactions with the template-primer duplex without involvement of the RNase H activity of the enzyme.

Simultaneous quantification of intracellular natural and antiretroviral nucleosides and nucleotides by liquid chromatography-tandem mass spectrometry

Nucleoside reverse transcriptase inhibitors (NRTI) require intracellular phosphorylation, which involves multiple enzymatic steps to inhibit the human immunodeficiency virus type 1 (HIV-1). NRTI-triphosphates (NRTI-TP) compete with endogenous 2'-deoxyribonucleosides-5'-triphosphates (dNTP) for incorporation by the HIV-1 reverse transcriptase (RT). Thus, a highly sensitive analytical methodology capable of quantifying at the low femtomoles/10(6) cells level was necessary to understand the intracellular metabolism and antiviral activity of NRTIs in human peripheral blood mononuclear (PBM) cells and in macrophages. A novel, rapid, and a reproducible ion-pair chromatography-tandem mass spectrometry (MS/MS) method was developed to simultaneously quantify the intracellular phosphorylated metabolites of abacavir, emtricitabine, tenofovir disoproxil fumarate, amdoxovir, and zidovudine, as well as four natural endogenous dNTP. Positive or negative electrospray ionization was chosen with specific MS/MS transitions for improved selectivity on all the compounds studied. The sample preparation, the ion-pair reagent concentration, and buffer composition were optimized, resulting in the simultaneous quantification of 13 different nucleotides in a total run time of 30 min. This novel method demonstrated optimal sensitivity (limit of detection 1-10 nM for various analytes), specificity, and reproducibility to successfully measure NRTI-TP and dNTP in human PBM cells and macrophages.

Raltegravir is a potent inhibitor of XMRV, a virus implicated in prostate cancer and chronic fatigue syndrome

Background

Xenotropic murine leukemia-related retrovirus (XMRV) is a recently discovered retrovirus that has been linked to human prostate cancer and chronic fatigue syndrome (CFS). Both diseases affect a large fraction of the world population, with prostate cancer affecting one in six men, and CFS affecting an estimated 0.4 to 1% of the population.

Principal Findings

Forty-five compounds, including twenty-eight drugs approved for use in humans, were evaluated against XMRV replication in vitro. We found that the retroviral integrase inhibitor, raltegravir, was potent and selective against XMRV at submicromolar concentrations, in MCF-7 and LNCaP cells, a breast cancer and prostate cancer cell line, respectively. Another integrase inhibitor, L-000870812, and two nucleoside reverse transcriptase inhibitors, zidovudine (ZDV), and tenofovir disoproxil fumarate (TDF) also inhibited XMRV replication. When combined, these drugs displayed mostly synergistic effects against this virus, suggesting that combination therapy may delay or prevent the selection of resistant viruses.

Conclusions

If XMRV proves to be a causal factor in prostate cancer or CFS, these discoveries may allow for rational design of clinical trials.

Mechanism of inhibition of HIV-1 reverse transcriptase by 4'-Ethynyl-2-fluoro-2'-deoxyadenosine triphosphate, a translocation-defective reverse transcriptase inhibitor

Nucleoside reverse transcriptase inhibitors (NRTIs) are employed in first line therapies for the treatment of human immunodeficiency virus (HIV) infection. They generally lack a 3'-hydroxyl group, and thus when incorporated into the nascent DNA they prevent further elongation. In this report we show that 4'-ethynyl-2-fluoro-2'-deoxyadenosine (EFdA), a nucleoside analog that retains a 3'-hydroxyl moiety, inhibited HIV-1 replication in activated peripheral blood mononuclear cells with an EC(50) of 0.05 nm, a potency several orders of magnitude better than any of the current clinically used NRTIs. This exceptional antiviral activity stems in part from a mechanism of action that is different from approved NRTIs. Reverse transcriptase (RT) can use EFdA-5'-triphosphate (EFdA-TP) as a substrate more efficiently than the natural substrate, dATP. Importantly, despite the presence of a 3'-hydroxyl, the incorporated EFdA monophosphate (EFdA-MP) acted mainly as a de facto terminator of further RT-catalyzed DNA synthesis because of the difficulty of RT translocation on the nucleic acid primer possessing 3'-terminal EFdA-MP. EFdA-TP is thus a translocation-defective RT inhibitor (TDRTI). This diminished translocation kept the primer 3'-terminal EFdA-MP ideally located to undergo phosphorolytic excision. However, net phosphorolysis was not substantially increased, because of the apparently facile reincorporation of the newly excised EFdA-TP. Our molecular modeling studies suggest that the 4'-ethynyl fits into a hydrophobic pocket defined by RT residues Ala-114, Tyr-115, Phe-160, and Met-184 and the aliphatic chain of Asp-185. These interactions, which contribute to both enhanced RT utilization of EFdA-TP and difficulty in the translocation of 3'-terminal EFdA-MP primers, underlie the mechanism of action of this potent antiviral nucleoside.

Lack of pharmacokinetic interaction between amdoxovir and reduced- and standard-dose zidovudine in HIV-1-infected individuals

Amdoxovir (AMDX) inhibits HIV-1 containing the M184V/I mutation and is rapidly absorbed and deaminated to its active metabolite, beta-D-dioxolane guanosine (DXG). DXG is synergistic with zidovudine (ZDV) in HIV-1-infected primary human lymphocytes. A recent in silico pharmacokinetic (PK)/enzyme kinetic study suggested that ZDV at 200 mg twice a day (b.i.d.) may reduce toxicity without compromising efficacy relative to the standard 300-mg b.i.d. dose. Therefore, an intense PK clinical study was conducted using AMDX/placebo, with or without ZDV, in 24 subjects randomized to receive oral AMDX at 500 mg b.i.d., AMDX at 500 mg plus ZDV at 200 or 300 mg b.i.d., or ZDV at 200 or 300 mg b.i.d. for 10 days. Full plasma PK profiles were collected on days 1 and 10, and complete urine sampling was performed on day 9. Plasma and urine concentrations of AMDX, DXG, ZDV, and ZDV-5'-O-glucuronide (GZDV) were measured using a validated liquid chromatography-tandem mass spectrometry method. Data were analyzed using noncompartmental methods, and multiple comparisons were performed on the log-transformed parameters, at steady state. Coadministration of AMDX with ZDV did not significantly change either of the plasma PK parameters or percent recovery in the urine of AMDX, DXG, or ZDV/GZDV. Larger studies with AMDX/ZDV, with a longer duration, are warranted.

Antiretroviral therapy in macrophages: implication for HIV eradication

HIV type-1 (HIV-1) accounts for more than 25 million deaths and nearly 40 million people are infected worldwide. A significant obstacle in clearing virus from infected individuals is latently infected viral reservoirs. Latent HIV-1 can emerge with recrudescence as a productive infection later in disease progression and could provide a source for the emergence of resistant HIV-1. It is widely recognized that macrophages represent a latently infected viral reservoir and are a significant and critical HIV-1 target cell in vivo. Macrophages can be divided into multiple subsets of macrophage-like cells, all of which are susceptible to HIV-1 infection, including dendritic cells, Langerhans cells, alveolar macrophages, mucosal macrophages and microglial cells. Current antiretroviral therapy (ART) often displays differential antiviral activity in macrophages relative to CD4(+) T-lymphocytes. Significant work has been performed to establish antiviral activity of many clinically approved ART in macrophages; however, a direct link between antiviral activity and specific mechanisms responsible for these antiviral effects are incompletely understood. This review identifies many understudied areas of research, along with topics for further research in the field of HIV therapy and eradication. Discussion focuses upon the known cellular pharmacology and antiviral activity of antiretroviral agents in macrophages and its relationship to latency, chronic HIV-1 infection and therapeutic strategies to eradicate systemic HIV-1 infection.

Thymidine analogue resistance suppression by V75I of HIV-1 reverse transcriptase: effects of substituting valine 75 on stavudine excision and discrimination

Val(75) of HIV-1 reverse transcriptase (RT) plays a role in positioning the template nucleotide +1 during the formation of the ternary complex. Mutations, such as V75M and V75A, emerge in patients infected with HIV-1 group M subtype B and group O variants, after failing treatment with stavudine (d4T) and other nucleoside RT inhibitors. V75I is an accessory mutation of the Q151M multidrug resistance complex of HIV-1 RT and is rarely associated with thymidine analogue resistance mutations (TAMs). In vitro, it confers resistance to acyclovir. TAMs confer resistance to zidovudine (AZT) and d4T by increasing the rate of ATP-mediated excision of the terminal nucleotide monophosphate (primer unblocking). In a wild-type HIV-1 group O RT sequence context, V75A and V75M conferred increased excision activity on d4T-terminated primers, in the presence of PP(i). In contrast, V75I decreased the PP(i)-mediated unblocking efficiency on AZT and d4T-terminated primers, in different sequence contexts (i.e. wild-type group M subtype B or group O RTs). Interestingly, in the sequence context of an excision-proficient RT (i.e. M41L/A62V/T69SSS/K70R/T215Y), the introduction of V75I led to a significant decrease of its ATP-dependent excision activity on AZT-, d4T-, and acyclovir-terminated primers. The excision rate of d4T-monophosphate in the presence of ATP (3.2 mm) was about 10 times higher for M41L/A62V/T69SSS/K70R/T215Y than for the mutant M41L/A62V/T69SSS/K70R/V75I/T215Y RT. The antagonistic effect of V75I with TAMs was further demonstrated in phenotypic assays. Recombinant HIV-1 containing the M41L/A62V/T69SSS/K70R/V75I/T215Y RT showed 18.3- and 1.5-fold increased susceptibility to AZT and d4T, respectively, in comparison with virus containing the M41L/A62V/T69SSS/K70R/T215Y RT.

Synthesis and anti-human immunodeficiency virus type 1 activity of (E)-N-phenylstyryl-N-alkylacetamide derivatives

A series of (E)-N-phenylstyryl-N-alkylacetamides, 5, were synthesized by direct reduction-acetylation of β-arylnitroolefins, followed by N-alkylation. The title compounds were characterized by ¹H-NMR, EIMS and IR analysis. All the synthesized compounds were assayed as HIV-1 non-nucleoside reverse transcriptase inhibitors. A SAR study revealed that when group R¹ in 5 was ortho-substituted, the resulting compounds showed better inhibitory activities against HIV-1 RT. Among the tested compounds, 5i (R¹ = 2-Br, R² = 3,5-difluorobenzyl) exhibited the highest enzyme activity, with a 88.89% inhibitory ratio against HIV-1 reverse transcriptase at the tested concentration. Further cell-based anti-HIV-1 assays showed that compound 5i exhibited a SI value of 29 with an EC₅₀ value of 4 μM in C8166 cells.

Impact of novel human immunodeficiency virus type 1 reverse transcriptase mutations P119S and T165A on 4'-ethynylthymidine analog resistance profile

2',3'-Didehydro-3'-deoxy-4'-ethynylthymidine (4'-Ed4T), a derivative of stavudine (d4T), has potent activity against human immunodeficiency virus and is much less inhibitory to mitochondrial DNA synthesis and cell growth than its progenitor, d4T. 4'-Ed4T triphosphate was a better reverse transcriptase (RT) inhibitor than d4T triphosphate, due to the additional binding of the 4'-ethynyl group at a presumed hydrophobic pocket in the RT active site. Previous in vitro selection for 4'-Ed4T-resistant viral strains revealed M184V and P119S/T165A/M184V mutations on days 26 and 81, respectively; M184V and P119S/T165A/M184V conferred 3- and 130-fold resistance to 4'-Ed4T, respectively. We investigated the relative contributions of these mutations, engineered into the strain NL4-3 background, to drug resistance, RT activity, and viral growth. Viral variants with single RT mutations (P119S or T165A) did not show resistance to 4'-Ed4T; however, M184V and P119S/T165A/M184V conferred three- and fivefold resistance, respectively, compared with that of the wild-type virus. The P119S/M184V and T165A/M184V variants showed about fourfold resistance to 4'-Ed4T. The differences in the growth kinetics of the variants were not more than threefold. The purified RT of mutants with the P119S/M184V and T165A/M184V mutations were inhibited by 4'-Ed4TTP with 8- to 13-fold less efficiency than wild-type RT. M184V may be the primary resistance-associated mutation of 4'-Ed4T, and P119S and T165A are secondary mutations. On the basis of our findings and the results of structural modeling, a virus with a high degree of resistance to 4'-Ed4T (e.g., more than 50-fold resistance) will be difficult to develop. The previously observed 130-fold resistance of the virus with P119S/T165A/M184V to 4'-Ed4T may be partly due to mutations both in the RT sequence and outside the RT sequence.

Anti-human immunodeficiency virus activity, cross-resistance, cytotoxicity, and intracellular pharmacology of the 3'-azido-2',3'-dideoxypurine nucleosides

Although the approved nucleoside reverse transcriptase (RT) inhibitors (NRTI) are integral components of therapy for human immunodeficiency virus type 1 (HIV-1) infection, they can have significant limitations, including the selection of NRTI-resistant HIV-1 and cellular toxicity. Accordingly, there is a critical need to develop new NRTI that have excellent activity and safety profiles and exhibit little or no cross-resistance with existing drugs. In this study, we report that the 3'-azido-2',3'-dideoxypurine nucleosides (ADPNs) 3'-azido-2',3'-dideoxyadenosine (3'-azido-ddA) and 3'-azido-2',3'-dideoxyguanosine (3'-azido-ddG) exert potent antiviral activity in primary human lymphocytes and HeLa and T-cell lines (50% inhibitory concentrations [IC50s] range from 0.19 to 2.1 microM for 3'-azido-ddG and from 0.36 to 10 microM for 3'-azido-ddA) and that their triphosphate forms are incorporated as efficiently as the natural dGTP or dATP substrates by HIV-1 RT. Importantly, both 3'-azido-ddA and 3'-azido-ddG retain activity against viruses containing K65R, L74V, or M184V (IC50 change of <2.0-fold) and against those containing three or more thymidine analog mutations (IC50 change of <3.5-fold). In addition, 3'-azido-ddG does not exhibit cytotoxicity in primary lymphocytes or epithelial or T-cell lines and does not decrease the mitochondrial DNA content of HepG2 cells. Furthermore, 3'-azido-ddG is efficiently phosphorylated to 3'-azido-ddGTP in human lymphocytes, with an intracellular half-life of the nucleoside triphosphate of 9 h. The present data suggest that additional preclinical studies are warranted to assess the potential of ADPNs for treatment of HIV-1 infection.

The chemokine system and CCR5 antagonists: potential in HIV treatment and other novel therapies

Since the recognition of human acquired immune deficiency syndrome, numerous classes of pharmacologic therapeutics have been developed to manage the disease. Current therapy includes co-administration of combinations of drugs classified by their mechanism of action as 'transcriptase inhibitors', 'protease inhibitors', 'integrase inhibitors' and the more recent 'fusion inhibitors'. This review focuses on the chemokine system and the recognition of chemokine receptors as targets for anti-human immunodeficiency virus (HIV) therapy. The FDA-approved chemokine (C-C motif) receptor 5 (CCR5) antagonist maraviroc (Selzentry) is discussed in detail, along with another compound vicriviroc, currently in clinical trials. The mechanism of action, pharmacokinetics, toxicity and current status of research on CCR5 antagonists is described. Further, potential therapeutic uses of these agents other than anti-HIV therapy are discussed.

Comparative study of the persistence of anti-HIV activity of deoxynucleoside HIV reverse transcriptase inhibitors after removal from culture

Background

Most in vitro assays of drug potency may not adequately predict the performance in vivo. Methods to assess the persistence of antiviral activity of deoxynucleoside analogs, which require intracellular activation to the active metabolites that can persist in cells, will be important for designing dosages, combination regimens, and assessing treatment compliance. Using an HIV-IIIB/TZM-bl indicator cell culture system, we assessed the ability of an inhibitor to protect cells from infection and to delay viral rebound after removal of inhibitor from culture.

Results

The order of protection of cells from HIV-infection was 4'-Ed4T > LFD4C > DDI > D4T > 3TC > AZT > FTC > NVP. The fold-increase in EC₅₀ to delay viral rebound was DDI < 4'-Ed4T < LFD4C < FTC < D4T < 3TC < NVP < AZT. The ranking of persistence of anti-HIV activity of the inhibitors based on the two-component assay was DDI > 4'-Ed4T > LFD4C > FTC = D4T > 3TC > NVP > AZT.

Conclusion

The persistence ranking was derived from assays based on measures of single viral replication-cycle and cumulative inhibition at multiple time-points. Therefore, a better indicator of the pharmacodynamic property of an inhibitor. The persistence of anti-HIV activity assay may complement in vitro potency assays to better predict in vivo performance of nucleoside analogs.

Pharmacotherapy of chronic viral hepatitis and hepatocellular carcinoma

BACKGROUND

Hepatitis B virus (HBV) is a major etiologic agent of chronic liver disease (CLD) and hepatocellular carcinoma. Drugs have been developed and shown to be effective against HBV replication. These treatments are often associated with the resolution of CLD. However, they are too expensive, not well tolerated, and result in the development of resistance when given as mono or salvage therapies. In addition, most of these drugs target only the virus polymerase.

OBJECTIVE

To revitalize the field, drugs with other targets and combination therapies need to be developed.

METHODS

Major advances in HBV and liver cancer drug development over the past decade, focusing on Phase III trials and FDA-approved compounds, are presented.

RESULTS/DISCUSSION

A number of potent nucleoside/nucleotide analogs are now available for treatment, but for the long-term management of CLD, the development of combination therapies will probably be required. Development of compounds with new virus targets will enhance the utility of combination therapies. Development of compounds to host targets altered prior to or after the development of liver cancer, as demonstrated by sorafenib, need to be developed. The goal is to devise drug cocktails that will yield sustained virus responses and halt disease progression and tumor development.

Development of an optimized dose for coformulation of zidovudine with drugs that select for the K65R mutation using a population pharmacokinetic and enzyme kinetic simulation model

In vitro selection studies and data from large genotype databases from clinical studies have demonstrated that tenofovir disoproxil fumarate and abacavir sulfate select for the K65R mutation in the human immunodeficiency virus type 1 polymerase region. Furthermore, other novel non-thymine nucleoside reverse transcriptase (RT) inhibitors also select for this mutation in vitro. Studies performed in vitro and in humans suggest that viruses containing the K65R mutation remained susceptible to zidovudine (ZDV) and other thymine nucleoside antiretroviral agents. Therefore, ZDV could be coformulated with these agents as a "resistance repellent" agent for the K65R mutation. The approved ZDV oral dose is 300 mg twice a day (b.i.d.) and is commonly associated with bone marrow toxicity thought to be secondary to ZDV-5'-monophosphate (ZDV-MP) accumulation. A simulation study was performed in silico to optimize the ZDV dose for b.i.d. administration with K65R-selecting antiretroviral agents in virtual subjects using the population pharmacokinetic and cellular enzyme kinetic parameters of ZDV. These simulations predicted that a reduction in the ZDV dose from 300 to 200 mg b.i.d. should produce similar amounts of ZDV-5'-triphosphate (ZDV-TP) associated with antiviral efficacy (>97% overlap) and reduced plasma ZDV and cellular amounts of ZDV-MP associated with toxicity. The simulations also predicted reduced peak and trough amounts of cellular ZDV-TP after treatment with 600 mg ZDV once a day (q.d.) rather than 300 or 200 mg ZDV b.i.d., indicating that q.d. dosing with ZDV should be avoided. These in silico predictions suggest that 200 mg ZDV b.i.d. is an efficacious and safe dose that could delay the emergence of the K65R mutation.

Development of a population simulation model for HIV monotherapy virological outcomes using lamivudine

Current highly active antiretroviral therapy (HAART) requires the use of combinations of three drugs to minimize the early emergence of drug-resistant HIV strains. Therefore, long-term monotherapy data with new agents are unavailable. However, the development of computer models for Monte-Carlo-type simulations of antiviral monotherapy, which incorporate HIV infection dynamic distributions from previously studied populations, together with pharmacokinetics and pharmacodynamic parameters of the new agent, could serve as an important tool. The nucleoside lamivudine (3TC) was used as a representative drug to standardize an improved pharmacodynamic and infection dynamic monotherapy model. 3TC plasma concentration versus time profiles was used to drive the cellular accumulation of 3TC-triphosphate (TP) in primary human lymphocytes in the model, over a 16 week period. The fraction of HIV reverse transcription inhibited was calculated using the median inhibitory concentration and intracellular 3TC-TP levels. Virus loads and activated CD4+ T-cell counts were generated for 2,200 theoretical individuals and compared with the outcomes of an actual 3TC monotherapy trial at the same dose. Pharmacokinetic variance alone did not account for the interindividual HIV-load variability. However, selection of appropriate distributions of the various pharmacokinetic and infection dynamics parameters produced a similar range of virus load reductions to actual observations. Therefore, once parameter and variance distributions are standardized, this modelling approach could be helpful in planning clinical trials and predicting the antiviral contribution of each agent in a HAART modality.

Mechanisms of resistance to nucleoside analogue inhibitors of HIV-1 reverse transcriptase

Human immunodeficiency virus (HIV) reverse transcriptase (RT) inhibitors can be classified into nucleoside and nonnucleoside RT inhibitors. Nucleoside RT inhibitors are converted to active triphosphate analogues and incorporated into the DNA in RT-catalyzed reactions. They act as chain terminators blocking DNA synthesis, since they lack the 3'-OH group required for the phosphodiester bond formation. Unfortunately, available therapies do not completely suppress viral replication, and the emergence of drug-resistant HIV variants is facilitated by the high adaptation capacity of the virus. Mutations in the RT-coding region selected during treatment with nucleoside analogues confer resistance through different mechanisms: (i) altering discrimination between nucleoside RT inhibitors and natural substrates (dNTPs) (e.g. Q151M, M184V, etc.), or (ii) increasing the RT's phosphorolytic activity (e.g. M41L, T215Y and other thymidine analogue resistance mutations), which in the presence of a pyrophosphate donor (usually ATP) allow the removal of chain-terminating inhibitors from the 3' end of the primer. Both mechanisms are implicated in multi-drug resistance. The excision reaction can be modulated by mutations conferring resistance to nucleoside or nonnucleoside RT inhibitors, and by amino acid substitutions that interfere with the proper binding of the template-primer, including mutations that affect RNase H activity. New developments in the field should contribute towards improving the efficacy of current therapies.

Rationally designed dual inhibitors of HIV reverse transcriptase and integrase

Bifunctional inhibitors were designed and synthesized based on 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT)a1 non-nucleoside reverse transcriptase (RT) inhibitors and diketoacid (DKA) integrase (IN) inhibitors. Biochemical studies revealed activity against RT and IN at low nanomolar and low micromolar concentrations, respectively. Exceptionally low IC50 values from a cell-based assay were achieved along with remarkably high therapeutic indices. Compound 7 was identified as the best compound of the series (IC50: 24 nM against RT, 4.4 microM against IN, and 10 nM against HIV-1).

Phosphonated Carbocyclic 2‘-Oxa-3‘-azanucleosides as New Antiretroviral Agents

Phosphonated carbocyclic 2'-oxa-3'-azanucleosides have been synthesized and tested for their antiretroviral activity. The obtained results have shown that some of the compounds were as powerful as azydothymidine in inhibiting the reverse transcriptase activity of the human retrovirus T-cell leukemia/lymphotropic virus type 1 and in protecting human peripheral blood mononuclear cells against human retrovirus T-cell leukemia/lymphotropic virus type 1 transmission in vitro. These data indicate that phosphonated carbocyclic 2'-oxa-3'-azanucleosides possess the necessary requirements to efficiently counteract infections caused by human retroviruses.