Perspectives on the development of acyclic nucleotide analogs as antiviral drugs

Nucleos(t)ide analogs for hepatitis B virus infection differentially regulate the growth factor signaling in hepatocytes

BACKGROUND

Recent clinical studies have suggested that the risk of developing HCC might be lower in patients with chronic hepatitis B receiving tenofovir disoproxil fumarate than in patients receiving entecavir, although there is no difference in biochemical and virological remission between the 2 drugs.

METHODS

The effects of nucleoside analogs (NsAs; lamivudine and entecavir) or nucleotide analogs (NtAs; adefovir disoproxil, tenofovir disoproxil fumarate, and tenofovir alafenamide) on cell growth and the expression of growth signaling molecules in hepatoma cell lines and PXB cells were investigated in vitro. The tumor inhibitory effects of NsAs or NtAs were evaluated using a mouse xenograft model, and protein phosphorylation profiles were investigated. The binding of NsAs or NtAs to the insulin receptor (INSR) was investigated by thermal shift assays.

RESULTS

NtAs, but not NsAs, showed direct growth inhibitory effects on hepatoma cell lines in vitro and a mouse model in vivo. A phosphoprotein array revealed that INSR signaling was impaired and the levels of phosphorylated (p)-INSRβ and downstream molecules phosphorylated (p)-IRS1, p-AKT, p-Gab1, and p-SHP2 were substantially reduced by NtAs. In addition, p-epidermal growth factor receptor and p-AKT levels were substantially reduced by NtAs. Similar findings were also found in PXB cells and nontumor lesions of liver tissues from patients with chronic hepatitis B. Prodrug NtAs, but not their metabolites (adefovir, adefovir monophosphate, adefovir diphosphate, tenofovir, tenofovir monophosphate, and tenofovir diphosphate), had such effects. A thermal shift assay showed the binding of NtAs to INSRβ.

CONCLUSIONS

NtAs (adefovir disoproxil, tenofovir disoproxil fumarate, and tenofovir alafenamide), which are adenine derivative acyclic nucleotide analogs, potentially bind to the ATP-binding site of growth factor receptors and inhibit their autophosphorylation, which might reduce the risk of HCC in patients with chronic hepatitis B.

Chitosan-poly(ethylene oxide) nanofibrous mat as a vaginal platform for tenofovir disoproxyl fumarate - The effect of vaginal pH on drug carrier performance

In the present work, a solution blow spun nanofibrous mat comprised of chitosan (CS) and poly(ethylene oxide) (PEO) was obtained as vaginal platform for tenofovir disoproxil fumarate (TDF) to prevent sexually transmitted infections. Apart from physicochemical and mechanical analysis, the specific steps involved studies on nanofibrous mat mucoadhesive and swelling characteristics upon pH fluctuations over the physiological range. Physicochemical analysis showed uniform drug distribution within the CS/PEO mat volume and pointed toward physical interactions between the drug and polymers. TDF-loaded CS/PEO nanofibrous mat was shown potentially safe when evaluated by the MTT metabolic activity and JC-1 assays in human vaginal epithelial cells VK2-E6/E7. In vitro antiviral studies indicated inhibition efficacy of TDF-CS/PEO nanofibrous mat toward HSV-2 virus and proved the SBS process does not change the microbicidal activity of drug molecule. Fluctuations in the physiological vaginal pH range of 3.8 to 5.0 substantially affected mucoadhesive and swelling behavior of chitosan which in turn impacted drug dissolution rate from polymer carrier. The rate of permeation and accumulation of TDF in vaginal tissue differed in response to vaginal pH. Faster drug permeation assessed at pH 5.0 suggests that an increase in vaginal pH could improve TDF bioavailability at earlier time points.

Improved pharmacokinetics of tenofovir ester prodrugs strengthened the inhibition of HBV replication and the rebalance of hepatocellular metabolism in preclinical models

Tenofovir (TFV) ester prodrugs, a class of nucleotide analogs (NAs), are the first-line clinical anti-hepatitis B virus (HBV) drugs with potent antiviral efficacy, low resistance rate and high safety. In this work, three marketed TFV ester drugs, tenofovir disoproxil fumarate (TDF), tenofovir alafenamide fumarate (TAF) and tenofovir amibufenamide fumarate (TMF), were used as probes to investigate the relationships among prodrug structures, pharmacokinetic characteristics, metabolic activations, pharmacological responses and to reveal the key factors of TFV ester prodrug design. The results indicated that TMF and TAF exhibited significantly stronger inhibition of HBV DNA replication than did TDF in HBV-positive HepG2.2.15 cells. The anti-HBV activity of TMF was slightly stronger than TAF after 9 days of treatment (EC₅₀ 7.29 ± 0.71 nM vs. 12.17 ± 0.56 nM). Similar results were observed in the HBV decline period post drug administration to the HBV transgenic mouse model, although these three TFV prodrugs finally achieved the same anti-HBV effect after 42 days treatments. Furthermore, TFV ester prodrugs showed a correcting effect on disordered host hepatic biochemical metabolism, including TCA cycle, glycolysis, pentose phosphate pathway, purine/pyrimidine metabolism, amino acid metabolism, ketone body metabolism and phospholipid metabolism. The callback effects of the three TFV ester prodrugs were ranked as TMF > TAF > TDF. These advantages of TMF were believed to be attributed to its greater bioavailability in preclinical animals (SD rats, C57BL/6 mice and beagle dogs) and better target loading, especially in terms of the higher hepatic level of the pharmacologically active metabolite TFV-DP, which was tightly related to anti-HBV efficacy. Further analysis indicated that stability in intestinal fluid determined the actual amount of TFV prodrug at the absorption site, and hepatic/intestinal stability determined the maintenance amount of prodrug in circulation, both of which influenced the oral bioavailability of TFV prodrugs. In conclusion, our research revealed that improved pharmacokinetics of TFV ester prodrugs (especially intestinal stability) strengthened the inhibition of HBV replication and the rebalance of hepatocellular metabolism, which provides new insights and a basis for the design, modification and evaluation of new TFV prodrugs in the future.

Tenofovir alafenamide fumarate

Tenofovir alafenamide fumarate is a lipophilic prodrug of tenofovir which is preferentially metabolized in lymphatic tissue resulting in high concentrations of tenofovir (TFV) and its active diphosphate metabolite inside the cells that replicate HIV. Due to its selectivity for these tissues, lower total doses of TAF can be administered relative to tenofovir disoproxil fumarate (TDF) which results in improved bone and renal biomarkers. Tenofovir alafenamide fumarate has become the “backbone” of multiple combination products for the treatment of HIV, combined with emtricitabine for PreP and as a monotherapy for the treatment or HBV.

Retroviral reverse transcriptase: Structure, function and inhibition

Reverse transcriptase (RT) is a multifunctional enzyme that has RNA- and DNA-dependent DNA polymerase activity and ribonuclease H (RNase H) activity, and is responsible for the reverse transcription of retroviral single-stranded RNA into double-stranded DNA. The essential role that RT plays in the human immunodeficiency virus (HIV) life cycle is highlighted by the fact that multiple antiviral drugs-which can be classified into two distinct therapeutic classes-are routinely used to treat and/or prevent HIV infection. This book chapter provides detailed insights into the three-dimensional structure of HIV RT, the biochemical mechanisms of DNA polymerization and RNase H activity, and the mechanisms by which nucleoside/nucleotide and nonnucleoside RT inhibitors block reverse transcription.

Effect of Coformer Selection on In Vitro and In Vivo Performance of Adefovir Dipivoxil Cocrystals

PURPOSE

This study aimed to improve the in vitro dissolution, permeability and oral bioavailability of adefovir dipivoxil (ADD) by cocrystal technology and clarify the important role of coformer selection on the cocrystal's properties.

METHODS

ADD was cocrystallized with three small molecules (i.e., paracetamol (PA), saccharin (SAC) and nicotinamide (NIC)), respectively. The obtained ADD-PA cocrystal was characterized by DSC, TGA, PXRD and FTIR. Comparative study on dissolution rates among the three ADD cocrystals were conducted in water and pH 6.8 phosphate buffer. Besides, effects of coformers on intestinal permeability of ADD were evaluated via in vitro Caco-2 cell model and in situ single-pass intestinal perfusion model in rats. Furthermore, in vivo pharmacokinetic study of ADD cocrystals was also compared.

RESULTS

Dissolution rates of ADD cocrystals were improved with the order of ADD-SAC cocrystal > ADD-PA cocrystal > ADD-NIC cocrystal. The permeability studies on Caco-2 cell model and single-pass intestinal perfusion model indicated that PA could enhance intestinal absorption of ADD by P-gp inhibition, while SAC and NIC did not. Further in vivo pharmacokinetic study showed that ADD-SAC cocrystal exhibited higher C_max (1.4-fold) and AUC_0-t (1.3-fold) of ADD than administration of ADD alone, and C_max and AUC_0-t of ADD-PA cocrystal were significantly enhanced by 2.1-fold and 2.2-fold, respectively, which was attributed to its higher dissolution and improved intestinal permeability.

CONCLUSION

Coformer selection had an important role on cocrystal's properties, and cocrystallization of ADD with a suitable coformer was an effective approach to enhance both dissolution and bioavailability of ADD.

Efficient one-pot, three-component procedure to prepare new α-aminophosphonate and phosphonic acid acyclic nucleosides

An efficient one-pot three-component Kabachnik-Fields reaction of aldehydes (acyclic nucleosides), amines (or amino acid), and triethyl phosphite proceeded for the synthesis of aminophosphonates using natural phosphate coated with iodine (I₂@NP) as a catalyst. The novel α-aminophosphonate and phosphonic acid acyclic nucleosides were tested for their anti-HCV and anti-HIV activities. The molecular docking showed that the non-activity of these compounds could be due to the absence of hydrophobic pharmacophores.

Stabilized oral nanostructured lipid carriers of Adefovir Dipivoxil as a potential liver targeting: Estimation of liver function panel and uptake following intravenous injection of radioiodinated indicator

PURPOSE

Adefovir dipivoxil (AD), a nucleoside reverse transcriptase inhibitor is effective against Hepatitis B virus. Its poor oral bioavailability leads to frequent administration causing severe adverse effects. Thereby the entrapment of AD within lipid nanoparticulate systems is a way of increasing AD oral bioavailability as a result of improving intestinal permeability with efficient liver-targeted delivery together with higher drug stability during storage.

METHODS

AD-loaded nanostructured lipid carriers (AD-NLCs) were prepared via solvent emulsification diffusion technique adopting 2⁴ full factorial design to study the effect of lipid percentage, presence of egg yolk lecithin, surfactant type and percentage on entrapment efficiency (E.E.%), particle size and percent in-vitro drug released after 8 h (Q8hrs).

RESULTS

Formula (F12) showed E.E.% of 90.5 ± 0.2%, vesicle size of 240.2 ± 2.5 nm and Q8hrs of 58.55 ± 9.4% was selected as the optimum formula with desirability value of 0.757 based on highest EE%, lowest P.S. and Q8hrs. Further evaluation of the optimized formula using radioiodinated rose bengal (RIRB) in thioacetamide induced liver damage in Swiss Albino mice revealed a higher liver uptake of 22 ± 0.01% ID/g (percent injected dose/g organ) and liver uptake/Blood (T/B) ratio of 2.22 ± 0.067 post 2 h of I.V injection of RIRB compared to 9 ± 0.01% ID/g and 0.64 ± 0.017 in untreated group, respectively.

CONCLUSION

NLCs could be successfully used as oral drug delivery carriers of the antiviral drug Adefovir Dipivoxil to the liver with higher stability and oral bioavailability. Graphical abstract.

CVB3 Nonstructural 2A Protein Modulates SREBP1a Signaling via the MEK/ERK Pathway

Coxsackievirus B3 (CVB3) is the predominant pathogen of viral myocarditis. In our previous study, we found that CVB3 caused abnormal lipid accumulation in host cells. However, the underlying mechanisms by which CVB3 disrupts and exploits the host lipid metabolism are not well understood. Sterol regulatory element binding protein 1 (SREBP1) is the major transcriptional factor in lipogenic genes expression. In this study, we demonstrated that CVB3 infection and nonstructural 2A protein upregulated and activated SREBP1a at the transcriptional level. Deletion analysis of SREBP1a promoter revealed that two regions, -1821/-1490 and -312/+217, in this promoter were both required for its activation by 2A. These promoter regions possessed several binding motifs for transcription factor SP1. Next, we used SP1-specific small interfering RNAs (siRNAs) to confirm that SP1 might be the essential factor in SREBP1a upregulation by 2A. Furthermore, we showed that MEK/ERK pathway was involved in the activation of SREBP1a by 2A and that blocking this signaling pathway with the specific inhibitor U0126 attenuated SREBP1a activation and lipid accumulation by 2A. Finally, we showed that inhibition of SREBP1 with siRNAs attenuated lipid accumulation induced by CVB3 infection and reduced virus replication. Moreover, inhibition of the MEK/ERK pathway also led to reduction of SREBP1a activation, lipid accumulation, and virus replication during CVB3 infection. Taken together, these data demonstrate that CVB3 nonstructural 2A protein activates SREBP1a at the transcription level through a mechanism involving MEK/ERK signaling pathway and SP1 transcription factor, which promotes cellular lipid accumulation and benefits virus replication.IMPORTANCE Coxsackievirus B3 (CVB3) infection is the leading cause of viral myocarditis, but effective vaccines and antiviral therapies against CVB3 infection are still lacking. It is important to understand the precise interactions between host and virus for the rational design of effective therapies. During infection, CVB3 disrupts and exploits host lipid metabolism to promote excessive lipid accumulation, which benefits virus replication. SREBP1 is the master regulator of cellular lipid metabolism. Here, we report that one of the viral nonstructural proteins, 2A, upregulates and activates SREBP1a. Furthermore, we find that inhibition of SREBP1 decreases CVB3 virus replication. These results reveal the regulation of SREBP1a expression by 2A and the roles of SREBP1 in lipid accumulation and viral replication during CVB3 infection. Our findings provide a new insight into CVB3 host interactions and inform a potential novel therapeutic target for this important pathogen.

Inhibition of adenovirus serotype 14 infection by octadecyloxyethyl esters of (S)-[(3-hydroxy-2-phosphonomethoxy)propyl]- nucleosides in vitro

On September 22, 2008, a physician on Prince of Wales Island, Alaska, notified the Alaska Department of Health and Social Services (ADHSS) of an unusually high number of adult patients with recently diagnosed pneumonia (n = 10), including three persons who required hospitalization and one who died. ADHSS and CDC conducted an investigation to determine the cause and distribution of the outbreak, identify risk factors for hospitalization, and implement control measures. This report summarizes the results of that investigation, which found that the outbreak was caused by adenovirus 14 (Ad14), an emerging adenovirus serotype in the United States that is associated with a higher rate of severe illness compared with other adenoviruses. Among the 46 cases identified in the outbreak from September 1 through October 27, 2008, the most frequently observed characteristics included the following: male (70%), Alaska Native (61%), underlying pulmonary disease (44%), aged > or = 65 years (26%), and current smoker (48%). Patients aged > or = 65 years had a fivefold increased risk for hospitalization. The most commonly reported symptoms were cough (100%), shortness of breath (87%), and fever (74%). Of the 11 hospitalized patients, three required intensive care, and one required mechanical ventilation. One death was reported. Ad14 isolates obtained during the outbreak were identical genetically to those in recent community-acquired outbreaks in the United States which suggests the emergence of a new, and possibly more virulent Ad14 variant. Clinicians should consider Ad14 infection in the differential diagnosis for patients with community-acquired pneumonia, particularly when unexplained clusters of severe respiratory infections are detected.

Adefovir dipivoxil loaded proliposomal powders with improved hepatoprotective activity: formulation, optimization, pharmacokinetic, and biodistribution studies

The present study aimed to prepare proliposomal formulae for improving the oral bioavailability of adefovir dipivoxil (AD), a nucleoside reverse transcriptase inhibitor effective against hepatitis B virus (HBV). The prepared proliposomal formulae were characterized for entrapment efficiency (E.E.%), vesicle size and in vitro drug release after reconstitution to conventional liposomes. The optimized formula (F9) with a maximum desirability value of 0.858 was selected having E.E.% of 71 ± 3.3% with an average vesicle size of 164.6 ± 5 nm. Moreover, the crystallization of AD within the optimized formula investigated via powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirmed the presence of the drug in an amorphous state within the lipid vesicles with enhanced stability over a storage period of 12 months. Thioacetamide-induced liver damage in rats evidenced by elevated liver enzymes was significantly improved after treatment with the optimum formula. Pharmacokinetic and biodistribution studies of formula F9 showed a higher accumulation of AD in the liver with enhanced bioavailability compared to AD suspension which highlights its potential advantage for an effective treatment of chronic HBV. Hence, proliposomal drug delivery is considered as a better choice for the oral delivery of AD.

Viability of primary osteoblasts after treatment with tenofovir alafenamide: Lack of cytotoxicity at clinically relevant drug concentrations

Tenofovir alafenamide (TAF) is a phosphonoamidate prodrug of the nucleotide HIV reverse transcriptase inhibitor tenofovir (TFV). TAF is approved for the treatment of HIV-1 infection as part of the single-tablet regimen containing elvitegravir, cobicistat, emtricitabine, and TAF. When dosed once-daily, TAF results in approximately 90% lower levels of plasma TFV and a 4-fold increase in intracellular TFV-diphosphate (TFV-DP) in PBMCs compared with the TFV prodrug tenofovir disoproxil fumarate (TDF). Several antiretrovirals, including TDF, have been associated with bone mineral density decreases in patients; the effect of clinically relevant TAF concentrations on primary osteoblast viability was therefore assessed in vitro. Studies in PBMCs determined that a 2-hour TAF exposure at concentrations similar to human plasma Cmax achieved intracellular TFV-DP levels comparable to those observed after the maximum recommended human dose of 25 mg TAF. Comparable intracellular TFV-DP levels were achieved in primary osteoblasts with 2-hour TAF exposure daily for 3 days at concentrations similar to those used for PBMCs (100-400 nM). No change in cell viability was observed in either primary osteoblasts or PBMCs. The mean TAF CC50 in primary osteoblasts after 3 days of daily 2-hour pulses was >500 μM, which is >1033 times higher than the TAF maximum recommended human dose plasma Cmax. In summary, primary osteoblasts were not preferentially loaded by TAF compared with PBMCs, with comparable TFV-DP levels achieved in both cell types. Furthermore, there was no impact on osteoblast cell viability at clinically relevant TAF concentrations.

A new motif for inhibitors of geranylgeranyl diphosphate synthase

The enzyme geranylgeranyl diphosphate synthase (GGDPS) is believed to receive the substrate farnesyl diphosphate through one lipophilic channel and release the product geranylgeranyl diphosphate through another. Bisphosphonates with two isoprenoid chains positioned on the α-carbon have proven to be effective inhibitors of this enzyme. Now a new motif has been prepared with one isoprenoid chain on the α-carbon, a second included as a phosphonate ester, and the potential for a third at the α-carbon. The pivaloyloxymethyl prodrugs of several compounds based on this motif have been prepared and the resulting compounds have been tested for their ability to disrupt protein geranylgeranylation and induce cytotoxicity in myeloma cells. The initial biological studies reveal activity consistent with GGDPS inhibition, and demonstrate a structure-function relationship which is dependent on the nature of the alkyl group at the α-carbon.

Evaluation of a 7-Methoxycoumarin-3-carboxylic Acid Ester Derivative as a Fluorescent, Cell-Cleavable, Phosphonate Protecting Group

Cell-cleavable protecting groups often enhance cellular delivery of species that are charged at physiological pH. Although several phosphonate protecting groups have achieved clinical success, it remains difficult to use these prodrugs in live cells to clarify biological mechanisms. Here, we present a strategy that uses a 7-methoxycoumarin-3-carboxylic acid ester as a fluorescent protecting group. This strategy was applied to synthesis of an (E)-4-hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP) analogue to assess cellular uptake and human Vγ9Vδ2 T cell activation. The fluorescent ester displayed low cellular toxicity (IC50 >100 μm) and strong T cell activation (EC50 =0.018 μm) relative to the unprotected anion (EC50 =23 μm). The coumarin-derived analogue allowed no-wash analysis of biological deprotection, which revealed rapid internalization of the prodrug. These results demonstrate that fluorescent groups can be applied both as functional drug delivery tools and useful biological probes of drug uptake.

In Vitro Virology Profile of Tenofovir Alafenamide, a Novel Oral Prodrug of Tenofovir with Improved Antiviral Activity Compared to That of Tenofovir Disoproxil Fumarate

Tenofovir alafenamide (TAF) is an investigational oral prodrug of the HIV-1 nucleotide reverse transcriptase inhibitor tenofovir (TFV). Tenofovir disoproxil fumarate (TDF) is another TFV prodrug, widely used for the treatment of HIV-1 infection. TAF is converted mostly intracellularly to TFV and, in comparison to TDF, achieves higher tenofovir diphosphate (TFV-DP) levels in peripheral blood mononuclear cells. As a result, TAF has demonstrated potent anti-HIV-1 activity at lower doses than TDF in monotherapy studies. Here, the in vitro virology profile of TAF was evaluated and compared to that of TDF. TAF displayed potent antiviral activity against all HIV-1 groups/subtypes, as well as HIV-2. TAF exhibited minimal changes in the drug concentration needed to inhibit 50% of viral spread (EC50) upon removal of the prodrug, similar to TDF, demonstrating intracellular antiviral persistence. While TAF and TDF exhibited comparable potencies in the absence of serum pretreatment, TAF maintained activity in the presence of human serum, whereas TDF activity was significantly reduced. This result demonstrates TAF's improved plasma stability over TDF, which is driven by the different metabolic pathways of the two prodrugs and is key to TAF's improved in vivo antiviral activity. The activity of TAF is specific for HIV, as TAF lacked activity against a large panel of human viruses, with the exception of herpes simplex virus 2, where weak TAF antiviral activity was observed, as previously observed with TFV. Finally, in vitro combination studies with antiretroviral drugs from different classes showed additive to synergistic interactions with TAF, consistent with ongoing clinical studies with TAF in fixed-dose combinations with multiple other antiretroviral drugs for the treatment of HIV.

Stealing the Keys to the Kitchen: Viral Manipulation of the Host Cell Metabolic Network

Host cells possess the metabolic assets required for viral infection. Recent studies indicate that control of the host's metabolic resources is a core host–pathogen interaction. Viruses have evolved mechanisms to usurp the host's metabolic resources, funneling them towards the production of virion components as well as the organization of specialized compartments for replication, maturation, and dissemination. Consequently, hosts have developed a variety of metabolic countermeasures to sense and resist these viral changes. The complex interplay between virus and host over metabolic control has only just begun to be deconvoluted. However, it is clear that virally induced metabolic reprogramming can substantially impact infectious outcomes, highlighting the promise of targeting these processes for antiviral therapeutic development.

Numerous viruses modulate host-cell metabolic processes to ensure successful infection.

The host-cell metabolic network contributes the energy, precursors, and specialized components necessary to produce infectious virions.

Viruses deploy host-cell metabolic activities to organize viral maturation compartments.

Metabolic control is a host–pathogen interaction that can sway the outcome of viral infection.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

Cytomegalovirus-mediated activation of pyrimidine biosynthesis drives UDP-sugar synthesis to support viral protein glycosylation

Human cytomegalovirus (HCMV) induces numerous changes to the host metabolic network that are critical for high-titer viral replication. We find that HCMV infection substantially induces de novo pyrimidine biosynthetic flux. This activation is important for HCMV replication because inhibition of pyrimidine biosynthetic enzymes substantially decreases the production of infectious virus, which can be rescued through medium supplementation with pyrimidine biosynthetic intermediates. Metabolomic analysis revealed that pyrimidine biosynthetic inhibition considerably reduces the levels of various UDP-sugar metabolites in HCMV-infected, but not mock-infected, cells. Further, UDP-sugar biosynthesis, which provides the sugar substrates required for glycosylation reactions, was found to be induced during HCMV infection. Pyrimidine biosynthetic inhibition also attenuated the glycosylation of the envelope glycoprotein B (gB). Both glycosylation of gB and viral growth were restored by medium supplementation with either UDP-sugar metabolites or pyrimidine precursors. These results indicate that HCMV drives de novo-synthesized pyrimidines to UDP-sugar biosynthesis to support virion protein glycosylation. The importance of this link between pyrimidine biosynthesis and UDP-sugars appears to be partially shared among diverse virus families, because UDP-sugar metabolites rescued the growth attenuation associated with pyrimidine biosynthetic inhibition during influenza A and vesicular stomatitis virus infection, but not murine hepatitis virus infection. In total, our results indicate that viruses can specifically modulate pyrimidine metabolic flux to provide the glycosyl subunits required for protein glycosylation and production of high titers of infectious progeny.

KSHV targeted therapy: an update on inhibitors of viral lytic replication

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi’s sarcoma, primary effusion lymphoma and multicentric Castleman’s disease. Since the discovery of KSHV 20 years ago, there is still no standard treatment and the management of virus-associated malignancies remains toxic and incompletely efficacious. As the majority of tumor cells are latently infected with KSHV, currently marketed antivirals that target the virus lytic cycle have shown inconsistent results in clinic. Nevertheless, lytic replication plays a major role in disease progression and virus dissemination. Case reports and retrospective studies have pointed out the benefit of antiviral therapy in the treatment and prevention of KSHV-associated diseases. As a consequence, potent and selective antivirals are needed. This review focuses on the anti-KSHV activity, mode of action and current status of antiviral drugs targeting KSHV lytic cycle. Among these drugs, different subclasses of viral DNA polymerase inhibitors and compounds that do not target the viral DNA polymerase are being discussed. We also cover molecules that target cellular kinases, as well as the potential of new drug targets and animal models for antiviral testing.

Solution-phase parallel synthesis of acyclic nucleoside libraries of purine, pyrimidine, and triazole acetamides

Molecular diversity plays a pivotal role in modern drug discovery against phenotypic or enzyme-based targets using high throughput screening technology. Under the auspices of the Pilot Scale Library Program of the NIH Roadmap Initiative, we produced and report herein a diverse library of 181 purine, pyrimidine, and 1,2,4-triazole-N-acetamide analogues which were prepared in a parallel high throughput solution-phase reaction format. A set of assorted amines were reacted with several nucleic acid N-acetic acids utilizing HATU as the coupling reagent to produce diverse acyclic nucleoside N-acetamide analogues. These reactions were performed using 24 well reaction blocks and an automatic reagent-dispensing platform under inert atmosphere. The targeted compounds were purified on an automated purification system using solid sample loading prepacked cartridges and prepacked silica gel columns. All compounds were characterized by NMR and HRMS, and were analyzed for purity by HPLC before submission to the Molecular Libraries Small Molecule Repository (MLSMR) at NIH. Initial screening through the Molecular Libraries Probe Production Centers Network (MLPCN) program, indicates that several analogues showed diverse and interesting biological activities.

Metabolism and antiretroviral activity of tenofovir alafenamide in CD4+ T-cells and macrophages from demographically diverse donors

BACKGROUND

Tenofovir alafenamide (TAF) is a novel investigational prodrug of tenofovir (TFV) that permits enhanced delivery of TFV into peripheral blood mononuclear cells (PBMCs) and lymphatic tissues. A critical step in the intracellular metabolic activation of TAF is mediated by the lysosomal protease cathepsin A (CatA). Here, we investigated CatA levels together with intracellular metabolism and antiretroviral activity of TAF in primary CD4+ T-lymphocytes (CD4s) and monocyte-derived macrophages (MDMs) isolated from a demographically diverse group of blood donors.

METHODS

CD4s and MDMs were prepared from fresh PBMCs. CatA levels were quantified in cell extracts by monitoring TAF hydrolysis using HPLC. Intracellular TAF metabolites were quantified by HPLC combined with mass spectrometry. Antiviral activities in activated CD4s and MDMs were determined using HIV-1 single-cycle reporter and p24 antigen production assays, respectively.

RESULTS

The levels of CatA and intracellular TAF metabolites differed minimally in CD4s and MDMs among 13 tested donors. TAF was >600-fold and 80-fold more potent than parent TFV in CD4s and MDMs, respectively, and its relative range of antiviral activity across all tested donors was comparable to that of other HIV-1 reverse transcriptase inhibitors, with mean ±sd (range) EC50 values of 11.0 ±3.4 (6.6-19.9) nM and 9.7 ±4.6 (2.5-15.7) nM in CD4s and MDMs, respectively.

CONCLUSIONS

These results indicate consistent intracellular metabolism and antiretroviral potency of TAF in relevant target cells of HIV-1 infection across multiple donors of variable gender, age and ethnicity, supporting further clinical investigation of TAF.

Advances in Nucleotide Antiviral Development from Scientific Discovery to Clinical Applications: Tenofovir Disoproxil Fumarate for Hepatitis B

Exploration of naturally occurring chemical structures for medicinal uses has received significant interest in drug discovery and development research in the past few decades. None have had more success or products of greater clinical efficacy than synthetic analogs of nucleosides and nucleotides, especially as antiviral drugs. Nucleos(t)ide antivirals are synthetic analogs of the natural building blocks of DNA or RNA. This review focuses on the developmental path of tenofovir disoproxil fumarate (TDF), a prodrug of a nucleotide analog and its clinical applications as a first-line antiviral for chronic hepatitis B (CHB). Tenofovir is a potent antiviral compound, but has poor oral availability. The disoproxil fumarate (DF) prodrug moiety greatly enhances intestinal absorption allowing it to become an oral medication. Tenofovir is activated intracellularly, and the incorporation into HBV DNA prevents further elongation thus terminating replication. In patients with CHB, TDF has demonstrated broad, potent and sustained virologic response. Maintenance of viral suppression for up to 5 years resulted in regression of fibrosis and cirrhosis. No tenofovir-resistant HBV variants have been detected in patients after long-term use. The efficacy and safety profiles reported from cohort studies of clinical practices were consistent with those observed in registration trials. Continuous development includes a new oral prodrug, tenofovir alafenamide fumarate (TAF), which has enhanced delivery of tenofovir to target cells compared to TDF.

Inhibition of Mycoplasma pneumoniae growth by FDA-approved anticancer and antiviral nucleoside and nucleobase analogs

BACKGROUND

Mycoplasma pneumoniae (Mpn) is a human pathogen that causes acute and chronic respiratory diseases and has been linked to many extrapulmonary diseases. Due to the lack of cell wall, Mpn is resistant to antibiotics targeting cell wall synthesis such as penicillin. During the last 10 years macrolide-resistant Mpn strains have been frequently reported in Asian countries and have been spreading to Europe and the United States. Therefore, new antibiotics are needed. In this study, 30 FDA-approved anticancer or antiviral drugs were screened for inhibitory effects on Mpn growth and selected analogs were further characterized by inhibition of target enzymes and metabolism of radiolabeled substrates.

RESULTS

Sixteen drugs showed varying inhibitory effects and seven showed strong inhibition of Mpn growth. The anticancer drug 6-thioguanine had a MIC (minimum inhibitory concentration required to cause 90% of growth inhibition) value of 0.20 μg ml(-1), whereas trifluorothymidine, gemcitabine and dipyridamole had MIC values of approximately 2 μg ml(-1). In wild type Mpn culture the presence of 6-thioguanine and dipyridamole strongly inhibited the uptake and metabolism of hypoxanthine and guanine while gemcitabine inhibited the uptake and metabolism of all nucleobases and thymidine. Trifluorothymidine and 5-fluorodeoxyuridine, however, stimulated the uptake and incorporation of radiolabeled thymidine and this stimulation was due to induction of thymidine kinase activity. Furthermore, Mpn hypoxanthine guanine phosphoribosyl transferase (HPRT) was cloned, expressed, and characterized. The 6-thioguanine, but not other purine analogs, strongly inhibited HPRT, which may in part explain the observed growth inhibition. Trifluorothymidine and 5-fluorodeoxyuridine were shown to be good substrates and inhibitors for thymidine kinase from human and Mycoplasma sources.

CONCLUSION

We have shown that several anticancer and antiviral nucleoside and nucleobase analogs are potent inhibitors of Mpn growth and that the mechanism of inhibition are most likely due to inhibition of enzymes in the nucleotide biosynthesis pathway and nucleoside transporter. Our results suggest that enzymes in Mycoplasma nucleotide biosynthesis are potential targets for future design of antibiotics against Mycoplasma infection.

Evaluation of 2'-α-fluorine modified nucleoside phosphonates as potential inhibitors of HCV polymerase

Ribonucleoside phosphonate analogues containing 2'-α-fluoro modifications were synthesized and their potency evaluated against HCV RNA polymerase. The diphosphophosphonate (triphosphate equivalent) adenine and cytidine analogues displayed potent inhibition of the HCV polymerase in the range of 1.9-2.1 μM, but only modest cell-based activity in the HCV replicon. Pro-drugs of the parent nucleoside phosphonates improved the cell-based activity.

Borononucleotides as substrates/binders for human NMP kinases: enzymatic and spectroscopic evaluation

Borononucleotides are a family of natural nucleotide monophosphate analogues with a 5'-boronic acid function. As B-O-P linkages are known to be unstable in solution, we evaluated the ability of borononucleotides to be recognized by nucleoside monophosphate kinases and eventually foil the phosphorylation process. In this context, and with the idea of probing the influence of their size, shape, and flexibility, a library of borononucleotides were synthetized starting from the borononucleotide analogue of thymidine, which was shown to behave as a slow substrate of human TMP kinase. This study thus constitutes a good starting point for the development of new monophosphate mimics as potential substrates or ligands for NMP kinases.

Solid lipid nanoparticles and nanosuspension of adefovir dipivoxil for bioavailability improvement: formulation, characterization, pharmacokinetic and biodistribution studies

The present study was aimed at developing colloidal formulations like solid lipid nanoparticles (SLN) and nanosuspension (NS) for improving bioavailability of adefovir dipivoxil (AD), a nucleoside reverse transcriptase inhibitor which displays poor oral bioavailability. SLNs were prepared by solvent injection method while NS was prepared by pearl milling method. The prepared formulations were characterized for physicochemical parameters such as particle size, ζ potential, drug content, X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC). Pharmacokinetic and biodistribution studies were performed in mice to evaluate in vivo fate of the formulations. The SLNs showed particle size of 267 ± 18 nm and entrapment efficiency of 73.5 ± 2.12%. The particle size obtained for NS was 393 ± 13 nm against 710 ± 70 μm for bulk drug, which led to significant improvement in saturation solubility. DSC and XRD studies of NS and SLN showed reduction in crystallinity while in vitro studies showed improved dissolution rate in both cases. Pharmacokinetics studies of orally administered formulations in mice exhibited higher plasma concentration compared to plain drug. Biodistribution studies showed higher accumulation of drug in liver, kidneys, intestine and stomach. The higher concentration of AD in liver after 24 hr highlights its potential advantage for effective treatment of chronic hepatitis infection. The relative bioavailability for adefovir NS and SLN were 52.46% and 78.23% respectively compared to 34.34% bioavailability obtained after administration of adefovir micro suspension (AMS), indicating suitability of both nanoparticulate formulations for improving bioavailability. SLNs were found to performed better as compared to NS for improving the bioavailability of AD.

Application of kinase bypass strategies to nucleoside antivirals

Nucleoside and nucleotide analogs have served as the cornerstones of antiviral therapy for many viruses. However, the requirement for intracellular activation and side-effects caused by distribution to off-target sites of toxicity still limit the efficacy of the current generation of drugs. Kinase bypass strategies, where phosphorylated nucleosides are delivered directly into cells, thereby, removing the requirement for enzyme catalyzed phosphorylation steps, have already changed the face of antiviral therapy in the form of the acyclic nucleoside phosphonates, cidofovir, adefovir (given orally as its dipivoxil prodrug) and tenofovir (given orally as its disoproxil prodrug), currently used clinically. These strategies hold further promise to advance the field of antiviral therapy with at least 10 kinase bypass and tissue targeted prodrugs, representing seven distinct prodrug classes, currently in clinical trials. This article reviews the history of kinase bypass strategies applied to nucleoside antivirals and the evolution of different tissue targeted prodrug strategies, highlighting clinically relevant examples.

Acyclic nucleoside phosphonates with a branched 2-(2-phosphonoethoxy)ethyl chain: efficient synthesis and antiviral activity

Series of novel acyclic nucleoside phosphonates (ANPs) with various nucleobases and 2-(2-phosphonoethoxy)ethyl (PEE) chain bearing various substituents in β-position to the phosphonate moiety were prepared. The influence of structural alternations on antiviral activity was studied. Several derivatives exhibit antiviral activity against HIV and vaccinia virus (middle micromolar range), HSV-1 and HSV-2 (lower micromolar range) and VZV and CMV (nanomolar range), although the parent unbranched PEE-ANPs are inactive. Adenine as a nucleobase and the methyl group attached to the PEE chain proved to be a prerequisite to afford pronounced antiviral activity.

New prodrugs of Adefovir and Cidofovir

New Adefovir (PMEA) prodrugs with a pro-moiety consisting of decyl or decyloxyethyl chain bearing hydroxyl function(s), hexaethyleneglycol or a (5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl unit were prepared starting from the tetrabutylammonium salt of the phosphonate drug and an appropriate alkyl bromide or tosylate. Analogously, two esters of Cidofovir [(S)-HPMPC] bearing a hexaethyleneglycol moiety were prepared. The activity of the prodrugs was evaluated in vitro against different virus families. A loss in the antiviral activities of the hydroxylated decyl or decyloxyethyl esters and hexaethyleneglycol esters of PMEA against human immunodeficiency virus (HIV) and herpesviruses [including herpes simplex virus (HSV), varicella-zoster virus (VZV), and human cytomegalovirus (CMV)] occurred in comparison with the parent compound. On the other hand, the (5-methyl-2-oxo-1,3-dioxolen-4-yl)methyl ester of PMEA showed significant activities against HIV and herpesviruses. (S)-HPMPC prodrugs exhibited anti-cytomegalovirus activities in the same range as the parent drug, whereas the anti-HSV and anti-VZV activities were one- to seven-fold lower than that of Cidofovir.

Human cytomegalovirus induces the activity and expression of acetyl-coenzyme A carboxylase, a fatty acid biosynthetic enzyme whose inhibition attenuates viral replication

We have previously reported that human cytomegalovirus (HCMV) infection induces large-scale changes to host cell glycolytic, nucleic acid, and phospholipid metabolism. Here we explore the viral mechanisms involved in fatty acid biosynthetic activation. Our results indicate that HCMV targets ACC1, the rate-limiting enzyme of fatty acid biosynthesis, through multiple mechanisms. HCMV infection was found to activate ACC1 expression, increasing the abundance of both ACC1 mRNA and protein. Viral gene expression but not viral DNA replication was found to be necessary for HCMV-mediated induction of ACC1 levels. HCMV infection was also found to increase the proteolytic processing of SREBP-2, a transcription factor whose proteolytic cleavage is known to activate a variety of phospholipid metabolic genes. Processing of SREBP-2 was found to be dependent on mTOR activity; pharmaceutical inhibition of mTOR blocked HCMV-induced SREBP-2 processing and prevented the induction of fatty acid biosynthesis and ACC1 expression. Independent of the increases in ACC1 expression, HCMV infection also induced ACC1's enzymatic activity. Inhibition of ACC1 through either RNA interference (RNAi) or inhibitor treatment was found to attenuate HCMV replication, and HCMV replication was sensitive to ACC1 inhibition even at the later stages of infection, suggesting a late role for fatty acid biosynthesis during HCMV replication. These findings indicate that HCMV infection actively modulates numerous functional aspects of a key metabolic regulatory enzyme that is important for high-titer viral replication.

Pharmacological considerations for tenofovir and emtricitabine to prevent HIV infection

The use of antiretroviral medications in HIV-negative individuals as pre-exposure prophylaxis (PrEP) is a promising approach to prevent HIV infection. Tenofovir disoproxil fumarate (TDF) and emtricitabine exhibit desirable properties for PrEP including: favourable pharmacokinetics that support infrequent dosing; few major drug-drug or drug-food interactions; an excellent clinical safety record; and pre-clinical evidence for efficacy. Several large, randomized, controlled clinical trials are evaluating the safety and efficacy of TDF and emtricitabine for this new indication. A thorough understanding of variability in drug response will help determine future investigations in the field and/or implementation into clinical care. Because tenofovir and emtricitabine are nucleos(t)ide analogues, the HIV prevention and toxicity effects depend on the triphosphate analogue formed intracellularly. This review identifies important cellular pharmacology considerations for tenofovir and emtricitabine, which include drug penetration into relevant tissues and cell types, race/ethnicity/pharmacogenetics, gender, cellular activation state and appropriate episodic or alternative dosing strategies based on pharmacokinetic principles. The current state of knowledge in these areas is summarized and the future utility of intracellular pharmacokinetics/pharmacodynamics for the PrEP field is discussed.

Inhibition of calmodulin-dependent kinase kinase blocks human cytomegalovirus-induced glycolytic activation and severely attenuates production of viral progeny

Viruses depend on the host cell to provide the energy and biomolecular subunits necessary for production of viral progeny. We have previously reported that human cytomegalovirus (HCMV) infection induces dramatic changes to central carbon metabolism, including glycolysis, the tricarboxylic acid (TCA) cycle, fatty acid biosynthesis, and nucleotide biosynthesis. Here, we explore the mechanisms involved in HCMV-mediated glycolytic activation. We find that HCMV virion binding and tegument protein delivery are insufficient for HCMV-mediated activation of glycolysis. Viral DNA replication and late-gene expression, however, are not required. To narrow down the list of cellular pathways important for HCMV-mediated [corrected] activation of glycolysis, we utilized pharmaceutical inhibitors to block pathways reported to be both involved in metabolic control and activated by HCMV infection. We find that inhibition of calmodulin-dependent kinase kinase (CaMKK), but not calmodulin-dependent kinase II (CaMKII) or protein kinase A (PKA), blocks HCMV-mediated activation of glycolysis. HCMV infection was also found to target calmodulin-dependent kinase kinase 1 (CaMKK1) expression, increasing the levels of CaMKK1 mRNA and protein. Our results indicate that inhibition of CaMKK has a negligible impact on immediate-early-protein accumulation yet severely attenuates production of HCMV viral progeny, reduces expression of at least one early gene, and blocks viral DNA replication. Inhibition of CaMKK did not affect the glycolytic activation induced by another herpes virus, herpes simplex virus type 1 (HSV-1). Furthermore, inhibition of CaMKK had a much smaller impact on HSV-1 replication than on that of HCMV. These data suggest that the role of CaMKK during the viral life cycle is, in this regard, HCMV specific. Taken together, our results suggest that CaMKK is an important factor for HCMV replication and HCMV-mediated glycolytic activation.

Straightforward synthesis of triazoloacyclonucleotide phosphonates as potential HCV inhibitors

Preparation of several triazoloacyclic nucleoside phosphonates is described. The key step of the synthesis involves a copper(I)-catalysed azide-alkyne 1,3-dipolar cycloaddition between azidoalkylphosphonates and propargylated nucleobases. The antiviral properties of these new analogues have been evaluated and revealed interesting potencies.

The development of antiretroviral therapy and its impact on the HIV-1/AIDS pandemic

In the last 25 years, HIV-1, the retrovirus responsible for the acquired immunodeficiency syndrome (AIDS), has gone from being an "inherently untreatable" infectious agent to one eminently susceptible to a range of approved therapies. During a five-year period, starting in the mid-1980s, my group at the National Cancer Institute played a role in the discovery and development of the first generation of antiretroviral agents, starting in 1985 with Retrovir (zidovudine, AZT) in a collaboration with scientists at the Burroughs-Wellcome Company (now GlaxoSmithKline). We focused on AZT and related congeners in the dideoxynucleoside family of nucleoside reverse transcriptase inhibitors (NRTIs), taking them from the laboratory to the clinic in response to the pandemic of AIDS, then a terrifying and lethal disease. These drugs proved, above all else, that HIV-1 infection is treatable, and such proof provided momentum for new therapies from many sources, directed at a range of viral targets, at a pace that has rarely if ever been matched in modern drug development. Antiretroviral therapy has brought about a substantial decrease in the death rate due to HIV-1 infection, changing it from a rapidly lethal disease into a chronic manageable condition, compatible with very long survival. This has special implications within the classic boundaries of public health around the world, but at the same time in certain regions may also affect a cycle of economic and civil instability in which HIV-1/AIDS is both cause and consequence. Many challenges remain, including (1) the life-long duration of therapy; (2) the ultimate role of pre-exposure prophylaxis (PrEP); (3) the cardiometabolic side-effects or other toxicities of long-term therapy; (4) the emergence of drug-resistance and viral genetic diversity (non-B subtypes); (5) the specter of new cross-species transmissions from established retroviral reservoirs in apes and Old World monkeys; and (6) the continued pace of new HIV-1 infections in many parts of the world. All of these factors make refining current therapies and developing new therapeutic paradigms essential priorities, topics covered in articles within this special issue of Antiviral Research. Fortunately, there are exciting new insights into the biology of HIV-1, its interaction with cellular resistance factors, and novel points of attack for future therapies. Moreover, it is a short journey from basic research to public health benefit around the world. The current science will lead to new therapeutic strategies with far-reaching implications in the HIV-1/AIDS pandemic. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol. 85, issue 1, 2010.

Nucleotide analogue prodrug tenofovir disoproxil enhances lymphoid cell loading following oral administration in monkeys

The antiviral drug tenofovir (TFV) is orally administered as the fumarate salt of its disoproxil prodrug (TFV disoproxil fumarate (TDF)). TFV is a dianion at physiological pH and, as a result, has poor lipid membrane permeability. Administration of the lipophilic and cell-permeable prodrug, TFV disoproxil, enhances the oral absorption of TFV. In order to determine whether oral administration of TDF also increases distribution to sites of viral infection, the plasma and circulating lymphoid cell pharmacokinetics of TFV and its phosphorylated metabolites were assessed following a single oral TDF or subcutaneous TFV administration at doses yielding equivalent plasma exposures to TFV in macaques. Despite TFV disoproxil's lack of plasma stability and undetectable levels in the first plasma samples taken, oral administration of TDF resulted in 7.9-fold higher peripheral blood mononuclear cell exposures to the active metabolite, TFV-diphosphate. The apparent plasma terminal half-life (t(1/2)) of TFV was also longer following oral TDF relative to subcutaneous TFV administration (median t(1/2) of 15.3 and 3.9 h, respectively), suggesting broader distribution to cells and tissues outside of the central plasma compartment. In conclusion, the disoproxil pro-moiety enhances not only the oral absorption of TFV but also tissue and lymphoid cell loading. These results illustrate that administration of even a fleeting prodrug can increase target tissue loading and give valuable insight for future prodrug development.

Synthesis of fluorophosphonylated acyclic nucleotide analogues via copper(I)-catalyzed Huisgen 1-3 dipolar cycloaddition

Preparation of several acyclonucleosides containing both a difluoromethylphosphonate group and a triazole moiety is described starting from a difluorophosphonosulfide. The key step of the synthesis involves a copper(I)-catalyzed Huisgen 1-3 dipolar cycloaddition between difluorophosphonylated azides and propargylated nucleobases derived from thymine and 2-amino-6-chloropurine.

Synthesis and Antimicrobial Activity of Bisphosphonates

Dimethyl [(substitutedphenyl)(6-oxo6λ5dibenzo[ d,f][1,3,2]dioxaphophepin-6-yl)methyl]phosphonates (5a-j) were synthesised through a three step process involving preparation of dimethyl hydroxy(substitutedphenyl)methyl-phosphonates (4a-j) and their reaction with 6-bromodibenzo[ d,f][1,3,2]dioxaphosphepine (2) in dry toluene in the presence of triethylamine at 50–60°C. Tetramethylguanidine (TMG) as a catalyst was found to increase the yields and purity of the products. These compounds were characterised by IR, 1H, 13C, 31P NMR and mass spectral data found to possess higher antimicrobial activity then the standards.

Serine peptide phosphoester prodrugs of cyclic cidofovir: synthesis, transport, and antiviral activity

Cidofovir (HPMPC, 1), a broad-spectrum antiviral agent, is currently used to treat AIDS-related human cytomegalovirus (HCMV) retinitis and has recognized therapeutic potential for orthopox virus infections, but is limited by its low oral bioavailability. Cyclic cidofovir (2) displays decreased nephrotoxicity compared to 1, while also exhibiting potent antiviral activity. Here we describe in detail the synthesis and evaluation as prodrugs of four cHPMPC dipeptide conjugates in which the free POH of 2 is esterified by the Ser side chain alcohol group of an X-L-Ser(OMe) dipeptide: 3 (X=L-Ala), 4 (X=L-Val), 5 (X=L-Leu), and 6 (X=L-Phe). Perfusion studies in the rat establish that the mesenteric permeability to 4 is more than 20-fold greater than to 1, and the bioavailability of 4 is increased 6-fold relative to 1 in an in vivo murine model. In gastrointestinal and liver homogenates, the cHPMPC prodrugs are rapidly hydrolyzed to 2. Prodrugs 3, 4, and 5 are nontoxic at 100 microM in HFF and KB cells and in cell-based plaque reduction assays had IC 50 values of 0.1-0.5 microM for HCMV and 10 microM for two orthopox viruses (vaccinia and cowpox). The enhanced transport properties of 3-6, conferred by incorporation of a biologically benign dipeptide moiety, and the facile cleavage of the Ser-O-P linkage suggest that these prodrugs represent a promising new approach to enhancing the bioavailability of 2.

Some new acyclic nucleotide analogues as antiviral prodrugs: synthesis and bioactivities in vitro

A series of ester analogues of acyclic nucleotide PMPA and PMEA were synthesized as potent antiviral agents. The antiviral evaluation results indicated that bis benzyl ester prodrug of PMPA 5f and bis allyl ester prodrug of PMEA 5g exhibited potent antiviral activities. The IC(50) of 5f for HBV was 2.15 microM, and the IC(50) of 5g for HIV-1 was 1.61 microM.