Differential phosphorylation of azidothymidine, dideoxycytidine, and dideoxyinosine in resting and activated peripheral blood mononuclear cells

The Phenomenon of Antiretroviral Drug Resistance in the Context of Human Immunodeficiency Virus Treatment: Dynamic and Ever Evolving Subject Matter

Human immunodeficiency virus (HIV) is a significant global health issue that affects a substantial number of individuals across the globe, with a total of 39 million individuals living with HIV/AIDS. ART has resulted in a reduction in HIV-related mortality. Nevertheless, the issue of medication resistance is a significant obstacle in the management of HIV/AIDS. The unique genetic composition of HIV enables it to undergo rapid mutations and adapt, leading to the emergence of drug-resistant forms. The development of drug resistance can be attributed to various circumstances, including noncompliance with treatment regimens, insufficient dosage, interactions between drugs, viral mutations, preexposure prophylactics, and transmission from mother to child. It is therefore essential to comprehend the molecular components of HIV and the mechanisms of antiretroviral medications to devise efficacious treatment options for HIV/AIDS.

Identification of SARS-CoV-2 Mpro inhibitors containing P1' 4-fluorobenzothiazole moiety highly active against SARS-CoV-2

COVID-19 caused by SARS-CoV-2 has continually been serious threat to public health worldwide. While a few anti-SARS-CoV-2 therapeutics are currently available, their antiviral potency is not sufficient. Here, we identify two orally available 4-fluoro-benzothiazole-containing small molecules, TKB245 and TKB248, which specifically inhibit the enzymatic activity of main protease (M^pro) of SARS-CoV-2 and significantly more potently block the infectivity and replication of various SARS-CoV-2 strains than nirmatrelvir, molnupiravir, and ensitrelvir in cell-based assays employing various target cells. Both compounds also block the replication of Delta and Omicron variants in human-ACE2-knocked-in mice. Native mass spectrometric analysis reveals that both compounds bind to dimer M^pro, apparently promoting M^pro dimerization. X-ray crystallographic analysis shows that both compounds bind to M^pro's active-site cavity, forming a covalent bond with the catalytic amino acid Cys-145 with the 4-fluorine of the benzothiazole moiety pointed to solvent. The data suggest that TKB245 and TKB248 might serve as potential therapeutics for COVID-19 and shed light upon further optimization to develop more potent and safer anti-SARS-CoV-2 therapeutics.

Virological failure of antiretroviral therapy and associated social and clinical factors in children and adolescents living with HIV

   According to the World Health Organization, sustained virological suppression of 90 % should be achieved among children and adolescents living with HIV / AIDS, which makes it important to assess the prevalence of virological failure of antiretroviral therapy.   The aim of this study was to determine the prevalence of virological failure and the clinical factors associated with it, as well as therapeutic drug monitoring in groups divided by the viral load level among children and adolescents with HIV.   Materials and Methods: A retrospective analysis of the medical records of 184 children and adolescents receiving antiretroviral therapy and registered at the Irkutsk Regional Center for the Prevention and Control of AIDS and Infectious Diseases, Irkutsk, was carried out. The study included 172 children aged 1-18 years with perinatal HIV infection. Patients were divided into groups depending on the level of viral load: group 1 – 21 patients with viral load > 1000 copies/ml of plasma, group 2 – 42 patients with viral load 50– 1000 copies/ml of plasma, group 3 – 109 patients with undetectable viral load (< 50 copies/ml). All patients underwent standard tests in accordance with clinical guidelines for the treatment of HIV infection in children, as well as therapeutic drug monitoring.   Results. Against the background of ongoing antiretroviral therapy, a significant number of patients 21 / 172 (12,2 %) experienced virological failure. The proportion of children and adolescents with incomplete suppression of HIV replication is 42 / 172 (24,4 %). Statistically significant differences were obtained by changing the ART regimen (p = 0,031). In the first group, the proportion of patients who changed the therapy regimen is 7 / 21 (33,3 %), which is two times less than in the group with a zero viral load of 70 / 109 (64,2 %). There are differences in the proportion of children and adolescents with zero concentrations of ritonavir and lopinavir (p = 0,020 and p = 0,012) in the three compared groups. The distribution of patients with zero concentrations was as follows: for ritonavir in the first group 3 / 17 (17,6 %), in the second – 8/37 (21,6 %), in the third group – 4/80 (5 %); for lopinavir – 4/17 (23,5 %), 6/36 (16,7 %), 3/80 (3,8 %), respectively.   Conclusion. This study demonstrates that the prevalence of virological failure among children and adolescents receiving ART remains high. To achieve sustained virological suppression in children and adolescents taking a protease inhibitor regimen, adherence to therapy must be increased. As one of the methods for assessing adherence, therapeutic drug monitoring can be used.

Genital Immune Cell Activation and Tenofovir Gel Efficacy: A Case-Control Study

Genital inflammation (GI) undermines topical human immunodeficiency virus (HIV) pre-exposure prophylaxis (PrEP) efficacy through unknown mechanisms. Here, associations between activated endocervical CD4 + T-cell numbers and higher deoxyadenosine triphosphate (dATP) concentrations suggest that competition for intracellular metabolites within HIV target cells may reduce the efficacy of antiretroviral-based PrEP in women with GI.

Non-structural protein 5 (NS5) as a target for antiviral development against established and emergent flaviviruses

Flaviviruses are among the most critical pathogens in tropical regions and cause a growing number of severe diseases in developing countries. The development of antiviral therapeutics is crucial for managing flavivirus outbreaks. Among the ten proteins encoded in the flavivirus RNA, non-structural protein 5, NS5, is a promising drug target. NS5 plays a fundamental role in flavivirus replication, viral RNA methylation, RNA polymerization, and host immune system evasion. Most of the NS5 inhibitor candidates target NS5 active sites. However, the similarity of NS5 activity sites with human enzymes can cause side effects. Identifying new allosteric sites in NS5 can contribute enormously to antiviral development. The NS5 structural characterization enabled exploring new regions, such as the residues involved in MTase-RdRp interaction, NS5 oligomerization, and NS5 interaction with other viral and host-cell proteins. Targeting NS5 critical interactions might lead to new compounds and overcomes the toxicity of current NS5-inhibitor candidates.

Intracellular Tenofovir and Emtricitabine Concentrations in Younger and Older Women with HIV Receiving Tenofovir Disoproxil Fumarate/Emtricitabine

The altered immune states of aging and HIV infection may affect intracellular metabolism of tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC); increased cellular senescence decreases FTC-triphosphate (FTCtp) concentrations. The effects of age and inflammation on the ratio of intracellular metabolites (IMs; tenofovir diphosphate [TFVdp] and FTCtp) to their endogenous nucleotides (ENs; dATP and dCTP), a potential treatment efficacy marker, were assessed among participants of the Women's Interagency HIV Study (WIHS), who ranged from 25 to 75 years. Samples from women receiving TDF-FTC with viral loads of <200 copies/ml were dichotomized by age at collection into two groups (≤45 years and ≥60 years). IM/EN concentrations were measured in peripheral blood mononuclear cell (PBMC) pellets; interleukin-6 (IL-6) and sCD163 were measured in plasma; senescent CD8⁺ T cells were measured in viable PBMCs. The TFVdp:dATP and FTCtp:dCTP ratios had statistically significantly different distributions in older and younger women (log-rank test, P = 0.0023 and P = 0.032, respectively); in general, IM and EN concentrations were higher in the older women. After adjusting for potential confounders, these findings were not significant. In women aged ≤45 years, TFVdp was negatively associated with IL-6 and sCD163, while FTCtp was positively associated with sCD163 and IL-6 in women aged ≥60 years. Body mass index (BMI) was positively associated with IL-6 in both age groups and negatively associated with TFVdp in women aged ≤45 years. After adjustment, age remained significant for sCD163, while black race, BMI, and renal function remained significant for several IMs and ENs, suggesting that factors associated with aging, but not age itself, govern intracellular TDF-FTC pharmacology.

Nucleotide Reverse Transcriptase Inhibitors: A Thorough Review, Present Status and Future Perspective as HIV Therapeutics

BACKGROUND

Human immunodeficiency virus type-1 (HIV-1) infection leads to acquired immunodeficiency syndrome (AIDS), a severe viral infection that has claimed approximately 658,507 lives in the US between the years 2010-2014. Antiretroviral (ARV) therapy has proven to inhibit HIV-1, but unlike other viral illness, not cure the infection.

OBJECTIVE

Among various Food and Drug Administration (FDA)-approved ARVs, nucleoside/ nucleotide reverse transcriptase inhibitors (NRTIs) are most effective in limiting HIV-1 infection. This review focuses on NRTIs mechanism of action and metabolism.

METHODS

A search of PubMed (1982-2016) was performed to capture relevant articles regarding NRTI pharmacology.

RESULTS

The current classical NRTIs pharmacology for HIV-1 prevention and treatment are presented. Finally, various novel strategies are proposed to improve the efficacy of NRTIs, which will increase therapeutic efficiency of present-day HIV-1 prevention/treatment regimen.

CONCLUSION

Use of NRTIs will continue to be critical for successful treatment and prevention of HIV-1.

Chalcogenozidovudine Derivatives With Antitumor Activity: Comparative Toxicities in Cultured Human Mononuclear Cells

Considering a novel series of zidovudine (AZT) derivatives encompassing selenoaryl moieties promising candidates as therapeutics, we examined the toxicities elicited by AZT and derivatives 5'-(4-Chlorophenylseleno)zidovudine (SZ1); 5'-(Phenylseleno)zidovudine (SZ2); and 5'-(4-Methylphenylseleno)zidovudine (SZ3) in healthy cells and in mice. Resting and stimulated cultured human peripheral blood mononuclear cells (PBMCs) were treated with the compounds at concentrations ranging from 10 to 200 µM for 24 and/or 72 h. Adult mice received a single injection of compounds (100 µmol/kg, s.c.) and 72 h after administration, hepatic/renal biomarkers were analyzed. Resting and stimulated PBMCs exposed to SZ1 displayed loss of viability, increased reactive species production, disruption in cell cycle, apoptosis and increased transcript levels and production of pro-inflammatory cytokines. In a mild way, most of these effects were also induced by SZ2. AZT and SZ3 did not cause significant toxicity towards resting PBMCs. Differently, both compounds elicited apoptosis and S phase arrest in stimulated cells. AZT and derivatives administration did not change the body weight and plasma biochemical markers in mice. However, the absolute weight and organ-to-body weight ratio of liver, kidneys and spleen were altered in AZT, SZ1-, and SZ2-treated mice. Our results highlighted the involvement of derivatives SZ1 and SZ2 in redox and immunological dyshomeostasis leading to activation of apoptotic signaling pathways in healthy cells under different division phases. On the other hand, the derivative SZ3 emerged as a promising candidate for further viral infection/antitumor studies as a new effective therapy with low toxicity for immune cells and after acute in vivo treatment.

Arbidol (Umifenovir): A Broad-Spectrum Antiviral Drug That Inhibits Medically Important Arthropod-Borne Flaviviruses

Arthropod-borne flaviviruses are human pathogens of global medical importance, against which no effective small molecule-based antiviral therapy has currently been reported. Arbidol (umifenovir) is a broad-spectrum antiviral compound approved in Russia and China for prophylaxis and treatment of influenza. This compound shows activities against numerous DNA and RNA viruses. The mode of action is based predominantly on impairment of critical steps in virus-cell interactions. Here we demonstrate that arbidol possesses micromolar-level anti-viral effects (EC₅₀ values ranging from 10.57 ± 0.74 to 19.16 ± 0.29 µM) in Vero cells infected with Zika virus, West Nile virus, and tick-borne encephalitis virus, three medically important representatives of the arthropod-borne flaviviruses. Interestingly, no antiviral effects of arbidol are observed in virus infected porcine stable kidney cells (PS), human neuroblastoma cells (UKF-NB-4), and human hepatoma cells (Huh-7 cells) indicating that the antiviral effect of arbidol is strongly cell-type dependent. Arbidol shows increasing cytotoxicity when tested in various cell lines, in the order: Huh-7 < HBCA < PS < UKF-NB-4 < Vero with CC₅₀ values ranging from 18.69 ± 0.1 to 89.72 ± 0.19 µM. Antiviral activities and acceptable cytotoxicity profiles suggest that arbidol could be a promising candidate for further investigation as a potential therapeutic agent in selective treatment of flaviviral infections.

Nucleoside analogs as a rich source of antiviral agents active against arthropod-borne flaviviruses

Nucleoside analogs represent the largest class of small molecule-based antivirals, which currently form the backbone of chemotherapy of chronic infections caused by HIV, hepatitis B or C viruses, and herpes viruses. High antiviral potency and favorable pharmacokinetics parameters make some nucleoside analogs suitable also for the treatment of acute infections caused by other medically important RNA and DNA viruses. This review summarizes available information on antiviral research of nucleoside analogs against arthropod-borne members of the genus Flavivirus within the family Flaviviridae, being primarily focused on description of nucleoside inhibitors of flaviviral RNA-dependent RNA polymerase, methyltransferase, and helicase/NTPase. Inhibitors of intracellular nucleoside synthesis and newly discovered nucleoside derivatives with high antiflavivirus potency, whose modes of action are currently not completely understood, have drawn attention. Moreover, this review highlights important challenges and complications in nucleoside analog development and suggests possible strategies to overcome these limitations.

Broad-spectrum agents for flaviviral infections: dengue, Zika and beyond

Infections with flaviviruses, such as dengue, West Nile virus and the recently re-emerging Zika virus, are an increasing and probably lasting global risk. This Review summarizes and comments on the opportunities for broad-spectrum agents that are active against multiple flaviviruses. Broad-spectrum activity is particularly desirable to prepare for the next flaviviral epidemic, which could emerge from as-yet unknown or neglected viruses. Potential molecular targets for broad-spectrum antiflaviviral compounds include viral proteins, such as the viral protease or polymerase, and host targets that are exploited by these viruses during entry and replication, including α-glucosidase and proteins involved in nucleoside biosynthesis. Numerous compounds with broad-spectrum antiviral activity have already been identified by target-specific or phenotypic assays. For other compounds, broad-spectrum activity can be anticipated because of their mode of action and molecular targets.

Exposure to zidovudine adversely affects mitochondrial turnover in primary T cells

Zidovudine (ZDV) is a widely used component of antiretroviral therapy (ART) in resource-limited settings, despite its known adverse effects, which include mitochondrial toxicity in muscle, liver and adipose tissue. It has also been associated with impaired immunological recovery. We hypothesised that ZDV might impair mitochondrial health and survival of primary T cells. We performed a cross-sectional analysis of mitochondrial function, mitophagy and susceptibility to apoptosis in healthy donor primary T cells after exposure to ZDV in vitro, together with T cells from patients who were virologically suppressed on ZDV-containing ART regimens for ≥1 year and age-matched subjects receiving non-ZDV ART regimens. The proportion of T cells expressing mitochondrial reactive oxygen species (mtROS) was significantly higher after in vitro (CD4(+) T cells and CD8(+) T cells) and in vivo (CD4(+) T cells) exposure to ZDV than other antiretroviral agents. We did not detect any effect of ZDV on mitophagy, as indicated by change in autophagic flux. However, spontaneous apoptosis, indicated by upregulation of caspase-3 was greater in ZDV-exposed T cells. In conclusion, ZDV exposure was associated with impaired mitochondrial turnover and increased susceptibility to apoptosis in T cells. These mechanisms could contribute to sub-optimal immune reconstitution.

A Translational Pharmacology Approach to Predicting Outcomes of Preexposure Prophylaxis Against HIV in Men and Women Using Tenofovir Disoproxil Fumarate With or Without Emtricitabine

BACKGROUND

A novel translational pharmacology investigation was conducted by combining an in vitro efficacy target with mucosal tissue pharmacokinetic (PK) data and mathematical modeling to determine the number of doses required for effective human immunodeficiency virus (HIV) preexposure prophylaxis (PrEP).

METHODS

A PK/pharmacodynamic (PD) model was developed by measuring mucosal tissue concentrations of tenofovir, emtricitabine, their active metabolites (tenofovir diphosphate [TFVdp] and emtricitabine triphosphate [FTCtp], respectively), and competing endogenous nucleotides (dATP and dCTP) in 47 healthy women. TZM-bl and CD4(+) T cells were used to identify 90% effective concentration (EC90) ratios of TFVdp to dATP and FTCtp to dCTP (alone and in combination) for protection against HIV. Monte-Carlo simulations were then performed to identify minimally effective dosing strategies to protect lower female genital tract and colorectal tissues.

RESULTS

The colorectal TFVdp concentration was 10 times higher than that in the lower female genital tract, whereas concentrations of endogenous nucleotides were 7-11 times lower. Our model predicted that ≥98% of the population achieved protective mucosal tissue exposure by the third daily dose of tenofovir disoproxil fumarate plus emtricitabine. However, a minimum adherence to 6 of 7 doses/week (85%) was required to protect lower female genital tract tissue from HIV, while adherence to 2 of 7 doses/week (28%) was required to protect colorectal tissue.

CONCLUSIONS

This model is predictive of recent PrEP trial results in which 2-3 doses/week was 75%-90% effective in men but ineffective in women. These data provide a novel approach for future PrEP investigations that can optimize clinical trial dosing strategies.

Host Cell Dependence of Human Immunodeficiency Virus Type-1 Drug Resistance Profiles and Tissue Culture Selection Patterns

Clinical isolates of the human immunodeficiency virus type 1 (HIV-1) displayed differential sensitivity to antiviral nucleosides depending on the type of host cell employed for viral propagation. Viruses derived from the peripheral blood mononuclear cells (PBMC) of subjects on prolonged 3′-azido-3′-deoxythymidine (AZT) therapy behaved as AZT-resistant when tested in either cord blood mononuclear cells or MT-4 cells but as relatively drug-sensitive in the U-937 monocytic cell line. Viruses derived from monocytes/ macrophages of the same individuals behaved as drug-sensitive in all cells tested. It was also shown that cloned recombinant viruses, which contained defined resistance-conferring mutations at either position 65 or 184 in the HIV pol gene, were generally less susceptible to each of 2′-3′-dideoxyinosine (ddl), 2′,3′-dideoxycytidine (ddC) and the (-)enantiomer of 2′,3′-dideoxy-3′thiacytidine (3TC) in MT-4 cells than in any of PBMC, cord blood mononuclear cells (CBMC) or Jurkat cells. Finally, resistance against each of AZT, ddl and ddC could be selected for more easily using MT-4 cells than CBMC or Jurkat lymphocytes and not at all with the U-937 monocytic cell line.

Intracellular Metabolism of 3′-Azido-3′-Deoxythymidine in the Presence of Ganciclovir or Foscarnet

Comparisons were made between the intracellular phosphorylation of 3′-Azido-3′-deoxythymidine (AZT) alone and in combination with ganciclovir (GCV) or foscarnet (PFA) in lymphocytes, uninfected fibroblasts and CMV-infected fibroblasts. The effects of AZT and the combinations of AZT with GCV or PFA on cellular dNTP pools were also examined. The phosphorylation of AZT was not altered by the presence of GCV or PFA in lymphocytes, and neither AZT nor the combinations of AZT with GCV or PFA changed the levels of cellular dNTP pools in these cells. AZT was phosphorylated to a greater extent in lymphocytes when compared to fibroblasts, but the proportion of AZT di- and triphosphates was greater in fibroblasts. The infection of fibroblasts with CMV enhanced AZT phosphorylation and increased the levels of cellular dNTP pools. GCV caused a specific reduction in AZT phosphorylation in CMV-infected fibroblasts, with a seven-fold drop in AZT triphosphate compared to AZT alone. GCV did not affect AZT phosphorylation in uninfected fibroblasts, nor did GCV reduce the dTTP pool compared to AZT alone. The effects of GCV upon AZT phosphorylation in CMV-infected cells may shed light on the antagonistic effects of GCV upon the anti-HIV activity of AZT, and are of importance for the development of effective combination therapies for the treatment of AIDS patients infected with CMV.

2′-Fluoro-2′,3′-Dideoxyarabinosyladenine (F-ddA): Activity against Drug-Resistant Human Immunodeficiency Virus Strains and Clades A-E

2′-Fluoro-2′,3′-dideoxyarabinosyladenine (F-ddA), an anti-human immunodeficiency virus (HIV) drug currently in clinical trial, was compared with zidovudine (AZT), ddl and ddC for anti-HIV activity and potency in HIV-1 strains both sensitive and resistant to zidovudine, ddl and non-nucleoside reverse transcriptase inhibitors. A variety of host cell systems [MT-2, MT-4, phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC)] was used. F-ddA was effective against each of the drug-resistant isolates, including the strain resistant to ddl, the other purine dideoxynucleoside evaluated in this study. The anti-HIV-1 activities of F-ddA and zidovudine were also determined against clades A-E in PHA-PBMCs. Although activities were similar, zidovudine was significantly more potent than F-ddA in the PHA-PBMC system.

Intracellular Metabolism of 3′-azido-3′-deoxythymidine and 2′,3′-dideoxyinosine in Combination in the Absence and Presence of Ribavirin

We examined the intracellular phosphorylation of 3′-azido-3′-deoxythymidine (AZT) and 2′,3′-dideoxyinosine (ddl) and the effects on rNTP and dNTP pools when AZT and ddl were incubated separately and in combination in lymphocytes. We also compared the effect of adding ribavirin (RBV) to the two-drug combination of AZT + ddl. AZT and ddl, used singly or in combination, had no effect on the dNTP pools of CEM lymphoblastoid cells. Neither did the combination of AZT + ddl have any effect on the rNTP pools. RBV, a known inhibitor of IMP dehydrogenase, caused a decrease in GTP and an increase in dTTP whether incubated alone or with the drug combination of AZT + ddl. The addition of AZT + ddl therefore did not alter the effects of RBV upon cellular nucleotide pools. AZT was phosphorylated to a much greater extent than ddl. The activation of ddl to ddA-TP was increased 2-fold in the presence of AZT, whereas AZT phosphorylation was unchanged when combined with ddl. This increase in ddl activation may explain in part the synergistic antiviral activity of the combination of AZT + ddl. The increased activation was not due to increased phosphorylation of ddl resulting from IMP dehydrogenase inhibition. The addition of 10 μm RBV to the two-drug combination of AZT + ddl did not change the intracellular phosphorylation of AZT or ddl. The activation of ddl to ddA-TP, when combined with AZT, appeared to be maximal and could not be further increased by addition of RBV to this combination.

Zalcitabine (ddC) Phosphorylation and Drug Interactions

Zalcitabine (2′,3′-dideoxycytidine; ddC) is an inhibitor of HIV reverse transcriptase. The intracellular metabolism of ddC in peripheral blood mononuclear cells (PBMCs), U937 cells and Molt 4 cells were investigated, and phosphate metabolites were determined by on-line radiometric HPLC. Comparable levels of all three ddC phosphate metabolites were formed in PHA-stimulated PBMCs, U937 cells and Molt 4 cells.

Zidovudine (ZDV), didanosine (ddl) and stavudine (d4T) had no significant effect on ddC (0.06μM) phosphorylation in PBMCs whereas the endogenous nucleoside, cytidine decreased phosphorylation in a concentration-dependent manner ( e.g. 41 % inhibition of total phosphate formation at 6μM cytidine, 85% inhibition at 60μM).

The cytotoxic anticancer drug doxorubicin caused a decrease in ddC phosphorylation in U937/Molt 4 cells (e.g. 56% inhibition of total phosphate formation in U937 cells; 55% in Molt 4 cells at a doxorubicin concentration of 60μM), whilst the antiviral agent ribavirin exhibited no inhibitory effects.

The Medicinal Chemistry of Dengue Virus

The dengue virus and related flaviviruses are an increasing global health threat. In this perspective, we comment on and review medicinal chemistry efforts aimed at the prevention or treatment of dengue infections. We include target-based approaches aimed at viral or host factors and results from phenotypic screenings in cellular assay systems for viral replication. This perspective is limited to the discussion of results that provide explicit chemistry or structure-activity relationship (SAR), or appear to be of particular interest to the medicinal chemist for other reasons. The discovery and development efforts discussed here may at least partially be extrapolated toward other emerging flaviviral infections, such as West Nile virus. Therefore, this perspective, although not aimed at flaviviruses in general, should also be able to provide an overview of the medicinal chemistry of these closely related infectious agents.

The search for nucleoside/nucleotide analog inhibitors of dengue virus

Nucleoside analogs represent the largest class of antiviral agents and have been actively pursued for potential therapy of dengue virus (DENV) infection. Early success in the treatment of human immunodeficiency virus (HIV) infection and the recent approval of sofosbuvir for chronic hepatitis C have provided proof of concept for this class of compounds in clinics. Here we review (i) nucleoside analogs with known anti-DENV activity; (ii) challenges of the nucleoside antiviral approach for dengue; and (iii) potential strategies to overcome these challenges. This article forms part of a symposium in Antiviral Research on flavivirus drug discovery.

Impact of HIV-1 infection pathways on susceptibility to antiviral drugs and on virus spread

The infection routes of HIV-1 can affect several viral properties, including dissemination, pathogenesis, and immune evasion. In this study, we evaluated the inhibitory activity of a wide variety of anti-HIV drugs, focusing on the impact that different infection pathways have on their efficacy. Compared to cell-free infection, inhibitory activities were reduced in cell-to-cell productive transmission for all drugs tested. We detected weak reporter-expressing target cells after cell-to-cell transmission in the presence of integrase strand transfer inhibitors (INSTIs). Further analysis revealed that this expression was mainly due to unintegrated circular HIV (cHIV) DNAs, consisting of 1-LTR and 2-LTR circles. When in vitro-constructed cHIV DNAs were introduced into cells, the production of infectious and intercellular transmittable virions was observed, suggesting that cHIV DNA could be a source of infectious virus. These results highlight some advantages of the cell-to-cell infection mode for viral expansion, particularly in the presence of anti-retroviral drugs.

Pharmacology lessons from chemoprophylaxis studies

Pharmacological studies in the context of preexposure prophylaxis (PrEP) for human immunodeficiency virus (HIV) are fundamental to inform on different drug pharmacokinetics, pharmacodynamics, and pharmacogenetics in view of the absence of easily measurable surrogate markers of efficacy. Although the combination of tenofovir and emtricitabine is the only PrEP agent that was studied and showed efficacy in preventing HIV transmission, prospective randomized clinical trials have reported varying efficacy due to poor adherence to the drug. Importantly, this could be overcome by the introduction of long-acting injectable PrEP agents, which may be administered monthly and ensure optimal and prolonged drug exposure in HIV target tissues. Notably, clinical pharmacology studies play a central role in interpreting drug concentration-responses and optimal drug exposure achievement.

Depot-medroxyprogesterone acetate does not reduce the prophylactic efficacy of emtricitabine and tenofovir disoproxil fumarate in macaques

Concerns that the injectable contraceptive depot-medroxyprogesterone acetate (DMPA) may increase the risk of HIV acquisition in women led to questions on whether DMPA could reduce efficacy of pre-exposure prophylaxis (PrEP) for HIV prevention. We used a macaque model to investigate the impact of prolonged DMPA exposure on PrEP with emtricitabine/tenofovir disoproxil fumarate. Twelve pigtail macaques treated with DMPA were exposed vaginally to simian HIV once a week for up to 5 months and received either placebo (n = 6) or emtricitabine/tenofovir disoproxil fumarate (n = 6). All control macaques were infected, whereas the PrEP-treated animals remained protected (P = 0.0007). This model suggests that women using DMPA will fully benefit from PrEP.

SAMHD1 specifically affects the antiviral potency of thymidine analog HIV reverse transcriptase inhibitors

Sterile alpha motif and histidine-aspartic domain-containing protein 1 (SAMHD1) is a deoxynucleoside triphosphate (dNTP) triphosphohydrolase recently recognized as an antiviral factor that acts by depleting dNTP availability for viral reverse transcriptase (RT). SAMHD1 restriction is counteracted by the human immunodeficiency virus type 2 (HIV-2) accessory protein Vpx, which targets SAMHD1 for proteosomal degradation, resulting in an increased availability of dNTPs and consequently enhanced viral replication. Nucleoside reverse transcriptase inhibitors (NRTI), one of the most common agents used in antiretroviral therapy, compete with intracellular dNTPs as the substrate for viral RT. Consequently, SAMHD1 activity may be influencing NRTI efficacy in inhibiting viral replication. Here, a panel of different RT inhibitors was analyzed for their different antiviral efficacy depending on SAMHD1. Antiviral potency was measured for all the inhibitors in transformed cell lines and primary monocyte-derived macrophages and CD4(+) T cells infected with HIV-1 with or without Vpx. No changes in sensitivity to non-NRTI or the integrase inhibitor raltegravir were observed, but for NRTI, sensitivity significantly changed only in the case of the thymidine analogs (AZT and d4T). The addition of exogenous thymidine mimicked the change in viral sensitivity observed after Vpx-mediated SAMHD1 degradation, pointing toward a differential effect of SAMHD1 activity on thymidine. Accordingly, sensitivity to AZT was also reduced in CD4(+) T cells infected with HIV-2 compared to infection with the HIV-2ΔVpx strain. In conclusion, reduction of SAMHD1 levels significantly decreases HIV sensitivity to thymidine but not other nucleotide RT analog inhibitors in both macrophages and lymphocytes.

Activation of peripheral blood mononuclear cells by dengue virus infection depotentiates balapiravir

In a recent clinical trial, balapiravir, a prodrug of a cytidine analog (R1479), failed to achieve efficacy (reducing viremia after treatment) in dengue patients, although the plasma trough concentration of R1479 remained above the 50% effective concentration (EC(50)). Here, we report experimental evidence to explain the discrepancy between the in vitro and in vivo results and its implication for drug development. R1479 lost its potency by 125-fold when balapiravir was used to treat primary human peripheral blood mononuclear cells (PBMCs; one of the major cells targeted for viral replication) that were preinfected with dengue virus. The elevated EC(50) was greater than the plasma trough concentration of R1479 observed in dengue patients treated with balapiravir and could possibly explain the efficacy failure. Mechanistically, dengue virus infection triggered PBMCs to generate cytokines, which decreased their efficiency of conversion of R1479 to its triphosphate form (the active antiviral ingredient), resulting in decreased antiviral potency. In contrast to the cytidine-based compound R1479, the potency of an adenosine-based inhibitor of dengue virus (NITD008) was much less affected. Taken together, our results demonstrate that viral infection in patients before treatment could significantly affect the conversion of the prodrug to its active form; such an effect should be calculated when estimating the dose efficacious for humans.

Ten years of dengue drug discovery: progress and prospects

To combat neglected diseases, the Novartis Institute of Tropical Diseases (NITD) was founded in 2002 through private-public funding from Novartis and the Singapore Economic Development Board. One of NITD's missions is to develop antivirals for dengue virus (DENV), the most prevalent mosquito-borne viral pathogen. Neither vaccine nor antiviral is currently available for DENV. Here we review the progress in dengue drug discovery made at NITD as well as the major discoveries made by academia and other companies. Four strategies have been pursued to identify inhibitors of DENV through targeting both viral and host proteins: (i) HTS (high-throughput screening) using virus replication assays; (ii) HTS using viral enzyme assays; (iii) structure-based in silico docking and rational design; (iv) repurposing hepatitis C virus inhibitors for DENV. Along the developmental process from hit finding to clinical candidate, many inhibitors did not advance beyond the stage of hit-to-lead optimization, due to their poor selectivity, physiochemical or pharmacokinetic properties. Only a few compounds showed efficacy in the AG129 DENV mouse model. Two nucleoside analogs, NITD-008 and Balapiravir, entered preclinical animal safety study and clinic trial, but both were terminated due to toxicity and lack of potency, respectively. Celgosivir, a host alpha-glucosidase inhibitor, is currently under clinical trial; its clinical efficacy remains to be determined. The knowledge accumulated during the past decade has provided a better rationale for ongoing dengue drug discovery. Though challenging, we are optimistic that this continuous, concerted effort will lead to an effective dengue therapy.

Tenofovir, emtricitabine intracellular and plasma, and efavirenz plasma concentration decay following drug intake cessation: implications for HIV treatment and prevention

BACKGROUND

In vivo data on tenofovir (TFV), emtricitabine (FTC), and efavirenz (EFV) concentration decay after intake cessation are limited; determinations of "true" elimination half-lives (t½) have often been based on suboptimal sampling windows. Understanding these parameters is important in managing missed doses and planning HIV pre-exposure prophylaxis (PrEP). This study investigated the pharmacokinetics (PK) of plasma TFV/FTC, their intracellular (IC) anabolites, TFV-diphosphate (DP) and FTC-triphosphate (TP), and plasma EFV over 10 days after intake cessation in HIV-negative volunteers.

METHODS

Volunteers received an Atripla (TFV/FTV/EFV) tablet daily for 14 days. PK sampling occurred before final dose and up to 228 hours after stopping. Peripheral blood mononuclear cells for [IC](drug) and [plasma](drug) were isolated, with analysis by tandem mass spectrometry.

RESULTS

Sixteen participants completed the study. Geometric mean plasma (t½)(228h) of TFV and FTC were 31 and 37 hours. These were longer than the previous reports (TFV 12-18 hours, FTC 10 hours).Geometric mean (t½)(228h) of IC TFV-DP and FTC-TP were 164 and 39 hours, whereas for EFV in plasma was 92 hours. [EFV](plasma) in 5/16 participants were below the suggested MEC of 1000 ng/mL within 48 hours postdose; however, 50% of the participants maintained concentrations above this level after 84 hours.

CONCLUSIONS

These data fully characterize the PK of TFV and TFV-DP, FTC and FTC-TP, and EFV after stopping the drug combination. Although decay in concentrations can be related to a target for EFV, this is more difficult for the IC phosphates. Consensus on 'target' triphosphate/diphosphate concentrations will further our understanding of missed/delayed doses in treatment and prevention strategies.

Pharmacokinetics of abacavir and its anabolite carbovir triphosphate without and with darunavir/ritonavir or raltegravir in HIV-infected subjects

BACKGROUND

Here, we aimed to investigate the pharmacokinetics of abacavir and carbovir triphosphate (CBV-TP) with darunavir/ritonavir 900/100 mg once daily or raltegravir 400 mg twice daily.

METHODS

HIV-infected subjects on abacavir (600 mg once daily) underwent steady-state pharmacokinetic assessments without and with darunavir/ritonavir or raltegravir. Within-subject changes in plasma and intracellular pharmacokinetic parameters were evaluated by geometric mean ratios (GMRs) and 90% CIs.

RESULTS

A total of 19 patients completed the study. With darunavir/ritonavir (versus abacavir alone), abacavir GMRs (90% CI) were 0.73 (0.66, 0.80), 0.62 (0.50, 0.77) and 0.78 (0.69, 0.87) for area under the curve (AUC), trough concentration (C(trough)) and maximum concentration (C(max)), respectively. With raltegravir, they were 1.03 (0.97, 1.10), 0.83 (0.62, 1.11) and 1.06 (0.95, 1.18), respectively. Intracellular CBV-TP GMRs (90% CI) were 0.88 (0.72, 1.07), 0.68 (0.48, 0.95) and 0.98 (0.79, 1.23) for AUC, C(trough) and C(max), respectively, with darunavir/ritonavir, and 0.96 (0.76, 1.20), 0.57 (0.33, 1.00) and 1.07 (0.85, 1.35), respectively, with raltegravir.

CONCLUSIONS

There was a 27% decrease in abacavir plasma exposure with darunavir/ritonavir and no changes with raltegravir. CBV-TP C(trough) was significantly decreased with darunavir/ritonavir (32%) and showed a high inter-individual variability with raltegravir.

Effect of HIV-1 infection and sex on the cellular pharmacology of the antiretroviral drugs zidovudine and lamivudine

The cellular pharmacology of zidovudine (ZDV) and lamivudine (3TC) in vivo is not completely understood. This prospective longitudinal study investigated the relationship between HIV-1 serostatus, sex, race, and time on therapy with intracellular and plasma ZDV and 3TC concentrations. Of 20 HIV-seronegative and 23 HIV-seropositive volunteers enrolled, 16 (8 women) and 21 (5 women) completed all 12 study days, respectively. Volunteers began ZDV-3TC therapy (plus a third active drug in HIV-seropositive volunteers), and steady-state concentrations (C(ss)) were determined after days 1, 3, 7, and 12. A repeated-measures mixed model was utilized. HIV-seronegative status was associated with 22% (95% confidence interval [CI], 0%, 50%) and 37% (15%, 67%) higher C(ss) estimates compared to those of HIV-seropositive individuals for intracellular ZDV-TP and 3TC-TP levels, respectively. African-Americans had 36% (8%, 72%) higher ZDV-TP estimates than non-African-Americans. Sex was not associated with ZDV-TP or 3TC-TP (P > 0.19). Women had 36% (4%, 78%) higher plasma ZDV, but the effect was lessened when normalized by lean body weight (5% [-19%, 38%]; P = 0.68). Plasma 3TC was 19% (0%, 41%) higher in HIV-seropositive volunteers and 22% (0%, 48%) higher in African American volunteers, but these effects were not significant when corrected for creatinine clearance (7% [-9%, 20%] and -5% [-26%, 12%] for HIV serostatus and race, respectively; P > 0.35). These results suggest that HIV-seropositive status decreases and African American race elevates the cellular triphosphates of ZDV and 3TC. This information extends knowledge of ZDV and 3TC cellular pharmacology in vivo and provides new leads for future cellular pharmacology studies aimed at optimizing HIV prevention/treatment with these agents.

Iatrogenic mitochondriopathies: a recent lesson from nucleoside/nucleotide reverse transcriptase inhibitors

The use of nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) has revolutionized the treatment of infection by human immunodeficiency virus (HIV) and hepatitis-B virus. NRTIs can suppress viral replication in the long-term, but possess significant toxicity that can seriously compromise treatment effectiveness. The major toxicity of NRTIs is mitochondrial toxicity. This manifests as serious side effects such as myopathy, peripheral neuropathy and lactic acidosis. In general, it is believed that the mitochondrial pathogenesis is closely related to the effect of NRTIs on mitochondrial DNA polymerase-γ. Depletion and mutation of mitochondrial DNA during chronic NRTI therapy may lead to cellular respiratory dysfunction and release of reactive oxidative species, resulting in cellular damage. It is now apparent that the etiology is far more complex than originally thought. It appears to involve multiple mechanisms as well as host factors such as HIV per se, inborn mitochondrial mutation, and sex. Management of mitochondrial toxicity during NRTI therapy remains a challenge. Interruption of NRTI therapy and substitution of the causative agents with alternative better-tolerated NRTIs represents the mainstay of management for mitochondrial toxicity and its clinical manifestations. A range of pharmacological approaches has been proposed as treatments and prophylaxes.

The role of therapeutic drug monitoring in the management of patients with human immunodeficiency virus infection

Therapeutic drug monitoring (TDM) is a well-established method to optimize dosing regimens in individual patients for drugs that are characterized by a narrow therapeutic range and large interindividual pharmacokinetic variability. For some antiretroviral drugs, mainly nonnucleoside reverse transcriptase inhibitors and protease inhibitors, TDM has been proposed as a means to improve the response in human immunodeficiency virus-infected patients. In contrast, nucleoside reverse transcriptase inhibitors do not show a predictable plasma concentration-response (toxicity, efficacy) relationship, and intracellular analyses are expensive. Therefore, TDM is generally not recommended for this class of drugs. TDM has been successfully applied in the clinical practice for certain antiretroviral drugs, but there are ongoing research efforts on the use and refinement of TDM for human immunodeficiency virus treatment, and convincing data from randomized trials are still needed. The best pharmacokinetic measures of drug exposure such as trough and peak concentrations or concentration ratios have not been unambiguously established.

Natural substrate concentrations can modulate the prophylactic efficacy of nucleotide HIV reverse transcriptase inhibitors

Preexposure prophylaxis (PrEP) with antiretroviral drugs is a novel human immunodeficiency virus (HIV) prevention strategy. It is generally thought that high systemic and mucosal drug levels are sufficient for protection. We investigated whether GS7340, a next-generation tenofovir (TFV) prodrug that effectively delivers tenofovir diphosphate (TFV-DP) to lymphoid cells and tissues, could protect macaques against repeated weekly rectal simian-human immunodeficiency virus (SHIV) exposures. Macaques received prophylactic GS7340 treatment 3 days prior to each virus exposure. At 3 days postdosing, TFV-DP concentrations in peripheral blood mononuclear cells (PBMCs) were about 50-fold higher than those seen with TFV disoproxil fumarate (TDF), and they remained above 1,000 fmol/10(6) cells for as long as 7 days. TFV-DP accumulated in lymphoid and rectal tissues, with concentrations at 3 days exceeding 500 fmol/10(6) mononuclear cells. Despite high mucosal and systemic TFV levels, GS7340 was not protective. Since TFV-DP blocks reverse transcription by competing with the natural dATP substrate, we measured dATP contents in peripheral lymphocytes, lymphoid tissue, and rectal mononuclear cells. Compared to those in circulating lymphocytes and lymphoid tissue, rectal lymphocytes had 100-fold higher dATP concentrations and dATP/TFV-DP ratios, likely reflecting the activated status of the cells and suggesting that TFV-DP may be less active at the rectal mucosa. Our results identify dATP/TFV-DP ratios as a possible correlate of protection by TFV and suggest that natural substrate concentrations at the mucosa will likely modulate the prophylactic efficacy of nucleotide reverse transcriptase inhibitors.

Effect of nucleoside and nucleotide analog reverse transcriptase inhibitors on cell-mediated immune functions

Nucleoside analog reverse transcriptase inhibitors (NRTIs) constitute the most commonly used drugs in antiretroviral therapy. NRTIs differ with respect to their host cell toxicity. We compared the in vitro effect of zidovudine (AZT; 2 μg/ml), lamivudine (3TC; 5 μg/ml), stavudine (d4T; 1 μg/ml), and tenofovir (TFV; 1 μg/ml) on Candida cell-mediated immunity (CMI) of peripheral blood mononuclear cells (PBMCs). The concentrations of the active derivative AZT-triphosphate were 4-fold higher in Candida-stimulated compared with unstimulated PBMCs (p = 0.01), but those of 3TC-triphosphate and TFV-diphosphate did not differ significantly. AZT treatment decreased proliferation of unstimulated and Candida-stimulated PBMCs and IFN-γ ELISPOT responses; 3TC decreased proliferation of unstimulated PBMCs only; d4T and TFV decreased proliferation of Candida-stimulated PBMCs only. AZT, but not the other NRTIs, increased unstimulated PBMC apoptosis measured by caspase 3 activity. All NRTIs increased annexin-V-measured apoptosis of Candida-stimulated PBMCs. The effect of d4T on apoptosis of Candida-stimulated PBMCs strongly correlated with its inhibitory effect on mitochondrial DNA synthesis (r² = 0.94; p = 0.007). The other NRTIs did not significantly decrease the mitochondrial:nuclear DNA ratios in Candida-stimulated or unstimulated cultures, suggesting that other mechanisms mediated their effect on apoptosis and CMI. In conclusion, AZT had the most pronounced inhibitory effect on CMI. Further studies are warranted to determine the clinical significance of this observation.

Pre-steady-state kinetic analysis of the incorporation of anti-HIV nucleotide analogs catalyzed by human X- and Y-family DNA polymerases

Nucleoside reverse transcriptase inhibitors (NRTIs) are an important class of antiviral drugs used to manage infections by human immunodeficiency virus, which causes AIDS. Unfortunately, these drugs cause unwanted side effects, and the molecular basis of NRTI toxicity is not fully understood. Putative routes of NRTI toxicity include the inhibition of human nuclear and mitochondrial DNA polymerases. A strong correlation between mitochondrial toxicity and NRTI incorporation catalyzed by human mitochondrial DNA polymerase has been established both in vitro and in vivo. However, it remains to be determined whether NRTIs are substrates for the recently discovered human X- and Y-family DNA polymerases, which participate in DNA repair and DNA lesion bypass in vivo. Using pre-steady-state kinetic techniques, we measured the substrate specificity constants for human DNA polymerases β, λ, η, ι, κ, and Rev1 incorporating the active, 5'-phosphorylated forms of tenofovir, lamivudine, emtricitabine, and zidovudine. For the six enzymes, all of the drug analogs were incorporated less efficiently (40- to >110,000-fold) than the corresponding natural nucleotides, usually due to a weaker binding affinity and a slower rate of incorporation for the incoming nucleotide analog. In general, the 5'-triphosphate forms of lamivudine and zidovudine were better substrates than emtricitabine and tenofovir for the six human enzymes, although the substrate specificity profile depended on the DNA polymerase. Our kinetic results suggest NRTI insertion catalyzed by human X- and Y-family DNA polymerases is a potential mechanism of NRTI drug toxicity, and we have established a structure-function relationship for designing improved NRTIs.

Development of a stochastic model for the efficacy of NRTIs using known mechanisms of action

We analyze the mechanisms by which nucleoside-analogue reverse transcriptase inhibitors, the most common class of drugs used in the treatment of HIV-1, exert their antiviral effects. We then seek to identify ways in which those known mechanisms can be employed to generate mathematical models for drug efficacy in terms of measurable physical values. We demonstrate that the probability a NRTI instead of a natural nucleotide is included can be expressed in terms of intracellular drug concentrations, natural nucleotide concentrations, and relevant rate constants derived from reverse transcriptase's mechanism of nucleotide addition. In order to determine the ultimate effect, the resistance of the NRTI to removal from the genome must be considered, which is achieved via stochastic modeling. We employ this model to determine the relationship between efficacy and drug concentration, as well as other drug characteristics like half life. We also investigate the effect of drug administration time on the overall efficacy. The model is employed for four different drugs and a sensitivity analysis on mutation and resistance is performed.

Decomposition of 3'-azido-2',3'-dideoxythymidine 5'-monophosphate (AZTMP) prodrugs in biological media studied by on-line solid-phase extraction coupled to liquid chromatography mass spectrometry

Using a column-switching HPLC method previously described, we studied the behavior of some mononucleotide prodrugs (pronucleotides) of 3'-azido-2',3'-dideoxythymidine in various biological media. From UV data, this method allowed quantification of transient metabolites resulting from prodrug bioconversion. The kinetic data related to the successive steps were calculated according to pseudo-first-order kinetic models and optimized using mono- or poly-exponential regressions. Various metabolites were identified by co-injection with authentic samples and/or ESI-MS coupling. The results led us to propose, for each considered pronucleotide, a global decomposition pathway ending in the selective delivery of the corresponding mononucleotide. Associated to the determination of other parameters (lipophilicity, aqueous solubility), the present study contributes to the search of suitable pharmacological properties for further in vivo evaluations.

Recent developments in the clinical pharmacology of anti-HIV nucleoside analogs

PURPOSE OF REVIEW

This review summarizes recent developments regarding the unique clinical pharmacologic profile of nucleoside analog reverse transcriptase inhibitors for management of HIV.

RECENT FINDINGS

First, intracellular data in patients suggest that nucleoside reverse transcriptase inhibitor-triphosphates are compartmentalized in different cell types. Additionally, intracellular drug-drug interactions were identified, which were undetectable in plasma. Second, extracellular data illustrate multiple bidirectional plasma drug-drug interactions between renally eliminated tenofovir and liver-metabolized drugs. Definitive mechanistic details for these interactions are lacking but they appear to involve renal and/or enteric drug transporters. Furthermore, the plasma versus female genital tract disposition of these agents was recently elucidated, which is important for currently investigated indications for pre-exposure and post-exposure prophylaxis. Finally, tenofovir/emtricitabine and abacavir (using a promising human leukocyte antigen-B*5701 genetic test for hypersensitivity)/lamivudine have emerged as common first-line nucleoside analog reverse transcriptase inhibitors because of co-formulations, once-daily dosing, and favorable tolerability and adverse effect profiles. Nevertheless, elucidating the long-term safety profile for all nucleoside analog reverse transcriptase inhibitors remains a priority.

SUMMARY

Knowledge of nucleoside analog reverse transcriptase inhibitor disposition intracellularly and extracellularly has expanded. This provides a basis for rational use of these agents clinically and adds new perspectives for future research.

Nucleoside and nucleotide HIV reverse transcriptase inhibitors: 25 years after zidovudine

Twenty-five years ago, nucleoside analog 3'-azidothymidine (AZT) was shown to efficiently block the replication of HIV in cell culture. Subsequent studies demonstrated that AZT acts via the selective inhibition of HIV reverse transcriptase (RT) by its triphosphate metabolite. These discoveries have established the first class of antiretroviral agents: nucleoside and nucleotide reverse transcriptase inhibitors (NRTIs). Over the years that followed, NRTIs evolved into the main component of antiretroviral drug combinations that are now used for the treatment of all populations of HIV infected patients. A total of thirteen NRTI drug products are now available for clinical application: eight individual NRTIs, four fixed-dose combinations of two or three NRTIs, and one complete fixed-dose regimen containing two NRTIs and one non-nucleoside RT inhibitor. Multiple NRTIs or their prodrugs are in various stages of clinical development and new potent NRTIs are still being identified through drug discovery efforts. This article will review basic principles of the in vitro and in vivo pharmacology of NRTIs, discuss their clinical use including limitations associated with long-term NRTI therapy, and describe newly identified NRTIs with promising pharmacological profiles highlighting those in the development pipeline. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, volume 85, issue 1, 2010.

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.

Effector caspase activation, in the absence of a conspicuous apoptosis induction, in mononuclear cells treated with azidothymidine

In the present study we focused our attention on the effect of AZT, at pharmacological and suprapharmacological concentrations, on some apoptosis-related key events and, particularly, on caspase activation in fresh human peripheral blood mononuclear cells (PBMCs). The main results can be summarized as follows: (i) AZT induced a strong, dose-dependent antiproliferative effect in mitogen-stimulated PBMCs, but low levels of cytotoxicity. in comparison with 5FU; (ii) low levels of cytotoxicity were coupled with a poor increase of apoptosis after AZT treatment in PBMCs; (iii) despite low levels of apoptosis, remarkable signs of both initiator and effector caspase enhanced expression with respect to control were detected by immunoblot analysis in AZT-treated PBMCs; (iv) enhanced caspase expression was associated with an increased expression of both anti-apoptotic Bcl-2 and pro-apoptotic Fas and p53 proteins, as detected by flow cytometry analysis; (v) combination treatment in vitro with AZT and anti-Fas significantly increased apoptosis in PBMCs with respect to single treatments. Overall, these results suggest that AZT treatment activates a complex, and apparently contrasting apoptosis-related signaling activity in PBMCs and that additional events are necessary to disrupt the balance induced by AZT towards apoptosis, on these cells.

Simultaneous determination of endogenous deoxynucleotides and phosphorylated nucleoside reverse transcriptase inhibitors in peripheral blood mononuclear cells using ion-pair liquid chromatography coupled to mass spectrometry

Nucleoside reverse transcriptase inhibitors (NRTIs) are activated intracellularly to their triphosphate (TP) form, which compete with endogenous deoxynucleotide-triphosphates (dNTP) as substrate for HIV reverse transcriptase. The activity of NRTIs is thus described by the NRTI-TP-to-dNTP ratio in relevant cell types. Therefore, we developed an ion-pair (IP) LC-MS method for the simultaneous analysis of the mono-, di-, and TP forms of NRTIs and endogenous deoxynucleosides in peripheral blood mononuclear cells (PBMC). The IP-LC method was applied on an IT mass spectrometer using the MS-mode as well as on a triple quadrupole mass spectrometer using the MS/MS mode. The MS/MS approach on the triple quadrupole mass spectrometer demonstrated the best clinical applicability due to its higher sensitivity. The LOD (minimum amount on column) were 25 fmol for the TP forms of zidovudine, lamivudine, and stavudine, as well as for their endogenous dNTP counterparts. The linearity (R(2) ) of the calibration curves were>0.99. The obtained LOD readily allow for clinical applications using just one million PBMC obtained from HIV-infected patients under therapy.

DNA polymerases as therapeutic targets

Numerous pathological states, including cancer, autoimmune diseases, and viral/bacterial infections, are often attributed to uncontrollable DNA replication. Inhibiting this essential biological process provides an obvious therapeutic target against these diseases. A logical target is the DNA polymerase, the enzyme responsible for catalyzing the addition of mononucleotides to a growing polymer using a DNA or RNA template as a guide for directing each incorporation event. This review provides a summary of therapeutic agents that target polymerase activity. A discussion of the biological function and mechanism of polymerases is first provided to illustrate the strategy for therapeutic intervention as well as the rational design of various nucleoside analogues that inhibit various polymerases associated with viral infections and cancer. The development of nucleoside and non-nucleoside inhibitors as antiviral agents is discussed with particular emphasis on their mechanism of action, structure-activity relationships, toxicity, and mechanism of resistance. In addition, commonly used anticancer agents are described to illustrate the similarities and differences associated with various nucleoside analogues as therapeutic agents. Finally, new therapeutic approaches that include the inhibition of selective polymerases involved in DNA repair and/or translesion DNA synthesis as anticancer agents are discussed.

Differential extracellular and intracellular concentrations of zidovudine and lamivudine in semen and plasma of HIV-1-infected men

OBJECTIVES

To quantitate extracellular and intracellular zidovudine (ZDV) and lamivudine (3TC) concentrations in blood and semen of HIV-1-infected men.

DESIGN

: Nonblind, single-center, open-label pharmacokinetic (PK) study in 14 subjects receiving ZDV plus 3TC.

METHODS

Paired blood and semen samples were obtained during 1 intensive visit and 3 single time point visits over 2 weeks. Extracellular ZDV and 3TC concentrations were measured in blood plasma (BP) and seminal plasma (SP), and intracellular ZDV and 3TC triphosphate (TP) concentrations were measured in isolated mononuclear cells using validated methods. HIV-1 RNA was measured in blood and semen. PK parameters were estimated using noncompartmental analysis.

RESULTS

Median (interquartile range [IQR]) SP/BP area under the time-concentration curve over the 12-hour dosing interval (AUC0-12h) ratios for ZDV and 3TC were 2.28 (1.48 to 2.97) and 6.67 (4.10 to 9.14), respectively, whereas individual SP/BP concentration ratios ranged from 1.9 to 91.4. Intracellular median (IQR) SP/BP AUC0-12h ratios for ZDV-TP and 3TC-TP were 0.36 (0.30 to 0.37) and 1.0 (0.62 to 1.30), respectively, whereas individual SP/BP concentration ratios ranged from 0.11 to 2.9. HIV-1 RNA was undetectable in both compartments.

CONCLUSIONS

ZDV and 3TC SP exposures are 2- to 6-fold greater than BP exposures. Seminal ZDV-TP exposures are approximately 40% of those found in peripheral blood mononuclear cells (PBMCs), whereas 3TC-TP exposures are similar to PBMC exposures. PK variability makes individual SP/BP ratios a suboptimal surrogate for genital tract exposure.

Cytokine and sex hormone effects on zidovudine- and lamivudine-triphosphate concentrations in vitro

INTRODUCTION

Elevated zidovudine- and lamivudine-triphosphates have been observed in peripheral blood mononuclear cells (PBMCs) from females versus males and in patients with high inflammatory states versus lower inflammatory states. Consistent with high triphosphate exposures, these same patient groups also experience elevated rates of toxicity, including lipoatrophy. The purpose of this study was to evaluate the effects of oestradiol, progesterone and testosterone as well as tumour necrosis factor (TNF)-alpha and interferon (IFN)-alpha on zidovudine- and lamivudine-triphosphates in PBMCs and, for the cytokines, in 3T3-L1 adipocytes.

METHODS

Multiple replicates of adipocytes and human PBMCs were incubated with experimental versus control conditions using several cytokine and sex hormone doses. Zidovudine- and lamivudine-triphosphate concentrations were determined with validated LC-MS-MS assays. A mixed effects, cell means model that accounted for experiment number was used to evaluate the effects of experimental conditions relative to control.

RESULTS

In adipocytes, TNF-alpha doses below 2 ng/mL increased zidovudine-triphosphate by 18% (5-31%). Lamivudine-triphosphate was not detectable in adipocytes. In PBMCs, pooled IFN-alpha doses (0.1-10 U/mL) decreased zidovudine-triphosphate 26% (10-42%); 100 and 1000 ng/mL of progesterone decreased lamivudine-triphosphate by 22% (1-43%) and 47% (25-68%), respectively. Pooled testosterone doses (10-1000 ng/mL) decreased lamivudine-triphosphate by 24% (7-41%). No other statistically significant effects were observed.

CONCLUSIONS

We found evidence that sex hormones and cytokines influence zidovudine-triphosphate and lamivudine-triphosphate slightly in PBMCs and adipocytes in vitro. These findings provide insight and scientific direction to address inflammation-dependent and sex-dependent phosphorylation and responses in patients.

Concentrations of zidovudine- and lamivudine-triphosphate according to cell type in HIV-seronegative adults

INTRODUCTION

Concentrations of zidovudine (ZDV)- and lamivudine (3TC)-triphosphates (TP) have been quantified in unfractionated peripheral blood mononuclear cells (PBMC) from HIV+ patients. The objective of this study was to determine whether concentrations of ZDV-TP and 3TC-TP in PBMC reflect the concentrations within CD4 T cells in HIV-seronegative adults.

METHODS

Volunteers had taken 300 mg of ZDV plus 150 mg of 3TC twice daily for > or = 7 days. Blood (60 mL) was collected 2 or 5 h post observed dose. PBMC were processed into three cell fractions using CD4 magnetic immunobeads: CD4-purified cells; unfractionated PBMC; and CD4-depleted PBMC. TP were determined in each cell fraction with liquid chromatography-mass spectrometry and compared across cell types by non-parametric analyses.

RESULTS

Six males and two females participated. The median (range) percentage of CD4 T cells (CD4%) in each fraction were: CD4-purified, 99%; unfractionated, 63% (range, 53-70); and CD4-depleted, 14% (range, 4-29). Corresponding median (range) ZDV-TP concentrations were 8.0 (5.3-10.3), 26.5 (12.9-42.2), and 34.2 (16.4-52.2) fmol/1 x 10 cells (Friedman P = 0.0008). The 3TC-TP values were 4.6 (2.3-6.7), 4.8 (3.5-8.8), and 6.8 (4.0-13.1) pmol//1 x 10 cells (Friedman P = 0.01). In mixed model analyses: ZDV-TP (fmol/1 x 10 cells) = 42-0.32 (CD4%); P < 0.001 and 3TC-TP (pmol/1 x 10 cells) = 7.3-0.03(CD4%); P = 0.003.

CONCLUSIONS

In HIV-seronegative volunteers, 3TC-TP concentrations in PBMC reflected the concentrations within CD4 T cells, but ZDV-TP concentrations were more than 70% lower in CD4 T cells than in PBMC. Thus, TP concentrations differ according to cell type in vivo with corresponding efficacy and toxicity implications for cells with low or high triphosphates.