Design and synthesis of lipophilic phosphoramidate d4T-MP prodrugs expressing high potency against HIV in cell culture: structural determinants for in vitro activity and QSAR

Phosphoramidate Prodrugs Continue to Deliver, The Journey of Remdesivir (GS-5734) from RSV to SARS-CoV-2

Remdesivir (GS-5734) is a monophenol, 2-ethylbutylalanine phosphoramidate prodrug of a 1′-cyano-4-aza-7,9-dideazaadenosine C-nucleoside (GS-441524) that is FDA approved for the treatment of hospitalized patients with COVID-19. The prodrug, initially invented for respiratory syncytial virus, was later found to have activity toward emerging RNA viruses, including Ebola and coronaviruses. Remdesivir is among the first examples of a phosphoramidate prodrug aimed at delivering a nucleoside monophosphate into lung cells to efficiently generate the nucleoside triphosphate inhibitor of viral RNA polymerases. With remdesivir as the central case study, the present work describes the antiviral potency and in vitro metabolism evidence for lung cell activation of phosphoramidates, together with their in vivo pharmacokinetics, lung distribution, and antiviral efficacy toward respiratory viruses. The lung delivery of nucleoside monophosphate analogs using prodrugs warrants further investigation toward the development of novel respiratory antivirals.

New Heights for ProTides?

In this issue of the Journal of Medicinal Chemistry, Janeba et al. found that the in vitro antiviral activity and selectivity index of the aryloxy phosphoramidate (ProTide) prodrug of the acyclic nucleoside phosphonate tenofovir (tenofovir alafenamide) can be dramatically improved by replacing the aryloxy pro-moiety with an appropriate tyrosine derivative. This Viewpoint highlights the possible impact and ramifications that these findings may have in the development of new ProTides.

Discovery of Modified Amidate (ProTide) Prodrugs of Tenofovir with Enhanced Antiviral Properties

This study describes the discovery of novel prodrugs bearing tyrosine derivatives instead of the phenol moiety present in FDA-approved tenofovir alafenamide fumarate (TAF). The synthesis was optimized to afford diastereomeric mixtures of novel prodrugs in one pot (yields up to 86%), and the epimers were resolved using a chiral HPLC column into fast-eluting and slow-eluting epimers. In human lymphocytes, the most efficient tyrosine-based prodrug reached a single-digit picomolar EC₅₀ value against HIV-1 and nearly 300-fold higher selectivity index (SI) compared to TAF. In human hepatocytes, the most efficient prodrugs exhibited subnanomolar EC₅₀ values for HBV and up to 26-fold higher SI compared to TAF. Metabolic studies demonstrated markedly higher cellular uptake of the prodrugs and substantially higher levels of released tenofovir inside the cells compared to TAF. These promising results provide a strong foundation for further evaluation of the reported prodrugs and their potential utility in the development of highly potent antivirals.

A Chemical Strategy for Intracellular Arming of an Endogenous Broad-Spectrum Antiviral Nucleotide

The naturally occurring nucleotide 3'-deoxy-3',4'-didehydro-cytidine-5'-triphosphate (ddhCTP) was recently found to exert potent and broad-spectrum antiviral activity. However, nucleoside 5'-triphosphates in general are not cell-permeable, which precludes the direct use of ddhCTP as a therapeutic. To harness the therapeutic potential of this endogenous antiviral nucleotide, we synthesized phosphoramidate prodrug HLB-0532247 (1) and found it to result in dramatically elevated levels of ddhCTP in cells. We compared 1 and 3'-deoxy-3',4'-didehydro-cytidine (ddhC) and found that 1 more effectively reduces titers of Zika and West Nile viruses in cell culture with minimal nonspecific toxicity to host cells. We conclude that 1 is a promising antiviral agent based on a novel strategy of facilitating elevated levels of the endogenous ddhCTP antiviral nucleotide.

An overview of ProTide technology and its implications to drug discovery

Introduction: The ProTide technology is a phosphate (or phosphonate) prodrug method devised to deliver nucleoside monophosphate (or monophosphonate) intracellularly bypassing the key challenges of antiviral and anticancer nucleoside analogs. Three new antiviral drugs, exploiting this technology, have been approved by the FDA while others are in clinical studies as anticancer agents.Areas covered: The authors describe the origin and development of this technology and its incredible success in transforming the drug discovery of antiviral and anticancer nucleoside analogues. As evidence, discussion on the antiviral ProTides on the market, and those currently in clinical development are included. The authors focus on how the proven capacity of this technology to generate new drug candidates has stimulated its application to non-nucleoside-based molecules.Expert opinion: The ProTide approach has been extremely successful in delivering blockbuster antiviral medicines and it seems highly promising in oncology. Its application to non-nucleoside-based small molecules is recently emerging and proving effective in other therapeutic areas. However, investigations to explain the lack of activity of certain ProTide series and comprehensive structure activity relationship studies to identify the appropriate phosphoramidate motifs depending on the parent molecule are in our opinion mandatory for the future development of these compounds.

Metabolic Efficacy of Phosphate Prodrugs and the Remdesivir Paradigm

Drugs that contain phosphates (and phosphonates or phosphinates) have intrinsic absorption issues and are therefore often delivered in prodrug forms to promote their uptake. Effective prodrug forms distribute their payload to the site of the intended target and release it efficiently with minimal byproduct toxicity. The ability to balance unwanted payload release during transit with desired release at the site of action is critical to prodrug efficacy. Despite decades of research on prodrug forms, choosing the ideal prodrug form remains a challenge which is often solved empirically. The recent emergency use authorization of the antiviral remdesivir for COVID-19 exemplifies a new approach for delivery of phosphate prodrugs by parenteral dosing, which minimizes payload release during transit and maximizes tissue payload distribution. This review focuses on the role of metabolic activation in efficacy during oral and parenteral dosing of phosphate, phosphonate, and phosphinate prodrugs. Through examining prior structure–activity studies on prodrug forms and the choices that led to development of remdesivir and other clinical drugs and drug candidates, a better understanding of their ability to distribute to the planned site of action, such as the liver, plasma, PBMCs, or peripheral tissues, can be gained. The structure–activity relationships described here will facilitate the rational design of future prodrugs.

Phosphoramidates and phosphonamidates (ProTides) with antiviral activity

Following the first report on the nucleoside phosphoramidate (ProTide) prodrug approach in 1990 by Chris McGuigan, the extensive investigation of ProTide technology has begun in many laboratories. Designed with aim to overcome limitations and the key resistance mechanisms associated with nucleoside analogues used in the clinic (poor cellular uptake, poor conversion to the 5'-monophosphate form), the ProTide approach has been successfully applied to a vast number of nucleoside analogues with antiviral and anticancer activity. ProTides consist of a 5'-nucleoside monophosphate in which the two hydroxyl groups are masked with an amino acid ester and an aryloxy component which once in the cell is enzymatically metabolized to deliver free 5'-monophosphate, which is further transformed to the active 5'-triphosphate form of the nucleoside analogue. In this review, the seminal contribution of Chris McGuigan's research to this field is presented. His technology proved to be extremely successful in drug discovery and has led to two Food and Drug Administration-approved antiviral agents.

The ProTide Prodrug Technology: From the Concept to the Clinic

The ProTide technology is a prodrug approach developed for the efficient intracellular delivery of nucleoside analogue monophosphates and monophosphonates. In this approach, the hydroxyls of the monophosphate or monophosphonate groups are masked by an aromatic group and an amino acid ester moiety, which are enzymatically cleaved-off inside cells to release the free nucleoside monophosphate and monophosphonate species. Structurally, this represents the current end-point of an extensive medicinal chemistry endeavor that spans almost three decades. It started from the masking of nucleoside monophosphate and monophosphonate groups by simple alkyl groups and evolved into the sophisticated ProTide system as known today. This technology has been extensively employed in drug discovery, and it has already led to the discovery of two FDA-approved (antiviral) ProTides. In this work, we will review the development of the ProTide technology, its application in drug discovery, and its role in the improvement of drug delivery and efficacy.

Aryl H-Phosphonates 18. Synthesis, properties, and biological activity of 2',3'-dideoxynucleoside (N-heteroaryl)phosphoramidates of increased lipophilicity

Recently, AZT (N-pyridyl)phosphoramidates were reported as a new type of potential anti-HIV therapeutics. In continuation of that work, here we present new (N-heteroaryl)phosphoramidate derivatives of antiviral 2',3'-dideoxynucleosides containing other types of N-heteroaryl moieties, particularly those with higher lipophilicity. The present studies comprise mechanistic investigations using (31)P NMR correlation analysis, which permitted improvements in the synthetic procedures. The obtained compounds were tested in biological systems to establish their cytotoxicity and anti-HIV activity. The results were analyzed with respect to possible correlations between biological and physico-chemical properties of the phosphoramidates studied, to get some insight into their antiviral mode of action.

A novel synthesis of antiviral nucleoside phosphoramidate and thiophosphoramidate prodrugs via nucleoside H-phosphonamidates

A novel and efficient method for the preparation of antiviral nucleoside 5'-H-phosphonamidates has been developed. The oxidization of the H-phosphonamidate intermediates with iodine and sulfur afforded nucleoside 5'-phosphoramidates and 5'-thiophosphoramidates in high yields.

Aryloxy phosphoramidate triesters: a technology for delivering monophosphorylated nucleosides and sugars into cells

Prodrug technologies aimed at delivering nucleoside monophosphates into cells (protides) have proved to be effective in improving the therapeutic potential of antiviral and anticancer nucleosides. In these cases, the nucleoside monophosphates are delivered into the cell, where they may then be further converted (phosphorylated) to their active species. Herein, we describe one of these technologies developed in our laboratories, known as the phosphoramidate protide method. In this approach, the charges of the phosphate group are fully masked to provide efficient passive cell-membrane penetration. Upon entering the cell, the masking groups are enzymatically cleaved to release the phosphorylated biomolecule. The application of this technology to various therapeutic nucleosides has resulted in improved antiviral and anticancer activities, and in some cases it has transformed inactive nucleosides to active ones. Additionally, the phosphoramidate technology has also been applied to numerous antiviral nucleoside phosphonates, and has resulted in at least three phosphoramidate-based nucleotides progressing to clinical investigations. Furthermore, the phosphoramidate technology has been recently applied to sugars (mainly glucosamine) in order to improve their therapeutic potential. The development of the phosphoramidate technology, mechanism of action and the application of the technology to various monophosphorylated nucleosides and sugars will be reviewed.

Cathepsin A is the major hydrolase catalyzing the intracellular hydrolysis of the antiretroviral nucleotide phosphonoamidate prodrugs GS-7340 and GS-9131

GS-7340 and GS-9131 {9-[(R)-2-[[(S)-[[(S)-1-(isopropoxycarbonyl)ethyl]amino]phenoxyphosphinyl]methoxy]-propyl]adenine and 9-(R)-4'-(R)-[[[(S)-1-[(ethoxycarbonyl)ethyl]amino]phenoxyphosphinyl]methoxy]-2'-fluoro-1'-furanyladenine, respectively} are novel alkylalaninyl phenyl ester prodrugs of tenofovir {9-R-[(2-phosphonomethoxy)propyl]adenine} (TFV) and a cyclic nucleotide analog, GS-9148 (phosphonomethoxy-2'-fluoro-2', 3'-dideoxydidehydroadenosine), respectively. Both prodrugs exhibit potent antiretroviral activity against both wild-type and drug-resistant human immunodeficiency virus type 1 strains and excellent in vivo pharmacokinetic properties. In this study, the main enzymatic activity responsible for the initial step in the intracellular activation of GS-7340 and GS-9131 was isolated from human peripheral blood mononuclear cells and identified as lysosomal carboxypeptidase A (cathepsin A [CatA]; EC 3.4.16.5). Biochemical properties of the purified hydrolase (native complex and catalytic subunit molecular masses of 100 and 29 kDa, respectively; isoelectric point [pI] of 5.5) matched those of CatA. Recombinant CatA and the isolated prodrug hydrolase displayed identical susceptibilities to inhibitors and identical substrate preferences towards a panel of tenofovir phosphonoamidate prodrugs. Incubation of both enzymes with 14C-labeled GS-7340 or [3H]difluorophosphonate resulted in the covalent labeling of identical 29-kDa catalytic subunits. Finally, following a 4-h incubation with GS-7340 and GS-9131, the intracellular concentrations of prodrug metabolites detected in CatA-negative fibroblasts were approximately 7.5- and 3-fold lower, respectively, than those detected in normal control fibroblasts. Collectively, these data demonstrate the key role of CatA in the intracellular activation of nucleotide phosphonoamidate prodrugs and open new possibilities for further improvement of this important class of antiviral prodrugs.

Rational design of polymerase inhibitors as antiviral drugs

Almost all viruses have polymerases which are suitable targets for antiviral drugs. The development of selective polymerase inhibitors started with screening of compounds in virus-infected cell cultures and the mechanism of action was investigated once an inhibitor had been found. Especially nucleoside analogs were screened as their triphosphates were potential substrates for polymerases. However, the stepwise phosphorylation by cellular, and sometimes viral, kinases to the active triphosphate prevented a truly rational design of polymerase inhibitors. Nucleotide analogs offers a type of compounds which could be designed in a more rational way than nucleoside analogs since the first, most selective, phosphorylation step is eliminated in the path to the active inhibitor. The development of pyrophosphate analogs made rational design possible since these compounds act directly on the viral enzyme, but the room for structural variation was limited. The non-nucleoside HIV reverse transcriptase inhibitors are direct inhibitors and can thus be designed in a truly rational way by use of structure information on the enzyme-inhibitor complex by use of X-ray and NMR. This rational design of allosteric inhibitors is also being used in the development of inhibitors to other viral polymerases.

Synthesis and antiviral activity of aryl phosphoramidate derivatives of beta-D- and beta-L-C-5-substituted 2',3'-didehydro-2',3'-dideoxy-uridine bearing linker arms

We have previously reported the synthesis and evaluation of potent anti-human immunodeficiency virus compounds based on beta-D-d4T analogues bearing a tether attached at the C-5 position and their beta-L-counterparts. Initial study revealed a requirement for an alkyl side-chain with an optimal length of 12 carbons for a weak antiviral activity. As a continuation of that work, we have now prepared the corresponding phosphoramidate derivatives as possible membrane-permeable prodrugs. Phosphorochloridate chemistry gave the target phosphoramidates which were tested for anti-human immunodeficiency virus type 1 activity; unfortunately, they were devoid of anti-HIV activity.

Enzymatic hydrolysis of stampidine and other stavudine phosphoramidates in the presence of mammalian proteases

Mammalian proteases have not been implicated in the metabolism of any nucleoside phosphoramidate prodrug. The results presented herein provide unprecedented and conclusive experimental evidence that mammalian proteases are capable of hydrolyzing stavudine phosphoramidates. Specifically, cathepsin B and Proteinase K are able to metabolize stampidine and other phosphoramidate derivatives of stavudine. Additionally, cathepsin B exhibits chiral selectivity at the phosphorus center. The elucidation of the metabolic pathways leading to activation of stampidine may provide the basis for pharmacologic interventions aimed at modulating the metabolism and thereby improving the therapeutic window of stampidine as an anti-HIV agent.

Phosphoramidate protides of carbocyclic 2',3'-dideoxy-2',3'-didehydro-7-deazaadenosine with potent activity against HIV and HBV

Synthesis of phosphoramidate protides of carbocyclic D- and L-2',3'-dideoxy-2',3'-didehydro-7-deazaadenosine by treatment of the nucleoside with phosphorochloridates in the presence of pyridine and t-BuMgCl is described. Several of these protides showed significantly improved antiviral potency over the parent nucleosides against both HIV and HBV.

Synthesis of Nucleoside Boranophosphoramidate Prodrugs Conjugated with Amino Acids

[structure: see text] Nucleoside boranophosphates and nucleoside amino acid phosphoramidates have been shown to be potent antiviral and anticancer agents with the potential to act as nucleoside prodrugs. A combination of these two types of compounds results in a boranophosphoramidate linkage between the nucleoside and amino acid. This new class of potential prodrugs is expected to possess advantages conferred by both types of parent compounds. Two approaches, specifically the H-phosphonate and oxathiaphospholane approaches, are described here to synthesize nucleoside boranophosphoramidate prodrugs conjugated with amino acids. The H-phosphonate approach involves a key intermediate, silylated nucleoside amino acid phosphoramidite 6, prepared from a series of reactions starting from nucleoside H-phosphonate in the presence of condensing reagent DPCP. Due to the lengthy procedure and the difficulties in removing DPCP from the final products, we switched to the oxathiaphospholane approach in which the DBU-assisted oxathiaphospholane ring-opening process constituted a key step for the generation of nucleoside amino acid boranophosphoramidates 24. We demonstrate that this key step did not cause any measurable C-racemization of boranophosphorylated amino acids 22. Diastereomers of compounds 24a-f were separated by RP-HPLC. An "adjacent"-type mechanism is proposed to explain the diastereomer ratio in the final products obtained via the oxathiaphospholane approach. A tentative assignment of configuration for the diastereomers was carried out based on the mechanism, molecular modeling, and (1)H NMR. Conclusively, the oxathiaphospholane methodology proved to be more facile and efficient than H-phosphonate chemistry in the preparation of the nucleoside amino acid boranophosphoramidate analogues that are promising as a new type of antiviral prodrugs.

Lipase-mediated stereoselective hydrolysis of stampidine and other phosphoramidate derivatives of stavudine

Enzymatic hydrolysis of stampidine and other aryl phosphate derivatives of stavudine were investigated using the Candida Antarctica Type B lipase. Modeling studies and comparison of the hydrolysis rate constants revealed a chiral preference of the lipase active site for the putative S-stereoisomer. The in vitro anti-HIV activity of these compounds correlated with their susceptibility to lipase- (but not esterase-) mediated hydrolysis. We propose that stampidine undergoes rapid enzymatic hydrolysis in the presence of lipase according to the following biochemical pathway: During the first step, hydrolysis of the ester group results in the formation of carboxylic acid. Subsequent step involves an intramolecular cyclization at the phosphorous center with simultaneous elimination of the phenoxy group to form a cyclic intermediate. In the presence of water, this intermediate is converted into the active metabolite Ala-d4T-MP. We postulate that the lipase hydrolyzes the methyl ester group of the l-alanine side chain to form the cyclic intermediate in a stereoselective fashion. This hypothesis was supported by experimental data showing that chloroethyl substituted derivatives of stampidine, which possess a chloroethyl linker unit instead of a methyl ester side chain, were resistant to lipase-mediated hydrolysis, which excludes the possibility of a direct hydrolysis of stampidine at the phosphorous center. Thus, our model implies that the lipase-mediated formation of the cyclic intermediate is a key step in metabolism of stampidine and relies on the initial configuration of the stereoisomers.

Quantitative structure-activity relationship study on some azidopyridinyl neonicotinoid insecticides for their selective affinity towards the drosophila nicotinic receptor over mammalian alpha4beta2 receptor using electrotopological state atom index

Neonicotinoids are the most important class of synthetic insecticides increasingly used in agriculture and veterinary medicine. Fundamental differences between the nicotinic acetylcholine receptors (nAChRs) of insects and mammals confer remarkable selectivity of the neonicotinoids at insect nAChR over mammalian nAChR. To identify pharmacophoric requirements of azidopyridinyl neonicotinoids for their efficacy and selectivity towards the insect nAChR over the mammalian one, quantitative structure-activity relationship (QSAR) study was performed using electrotopological state atom (ETSA) indices. This study clearly showed that nitroimines, nitromethylenes, and cyanoimines are more selective to Drosophila nAChR and safe for human being, whereas N-substituted imines have affinity to mammalian receptor. Pharmacophore mapping for both the activities was done.

Stereoselective and Concentration-Dependent Polarized Epithelial Permeability of a Series of Phosphoramidate Triester Prodrugs of d4T: An in Vitro Study in Caco-2 and Madin-Darby Canine Kidney Cell Monolayers

Nucleoside analogs are successful, widely used antiviral and anticancer therapeutics. Nucleotide prodrugs (i.e., pronucleotide) have increasingly been used to improve in vivo efficacy of nucleoside analogs. In this study, we evaluated the permeability of a series of phosphoramidate triester prodrugs of the anti-HIV drug 2',3'-didehydro-2',3'-dideoxythymidine across monolayers of Caco-2, Madin-Darby canine kidney (MDCKII) epithelial cell line, and its recombinant clone containing the human MDR1/P-gp gene (MDR1-MDCKII). Transport was studied in the apical-to-basolateral (A-B) and the basolateral-to-apical directions (B-A). The impact upon transport of differences in stereochemistry at the chiral phosphate center was evaluated. In the Caco-2 and MDCK models the A-B permeability was lower than expected based on the lipophilicity of the compounds, suggesting the involvement of a polarized efflux system and/or metabolic degradation in limiting the absorption of these ester-based prodrugs. Average permeability values through cell monolayers obtained in the A-B direction were lower than in the B-A direction. The inclusion of the P-glycoprotein (P-gp) inhibitor verapamil in the transport medium markedly increased the permeability in the A-B direction, whereas decreasing it in the opposite direction, suggesting an efflux mechanism mainly mediated by P-gp. Stereoselective permeability was significant for the most lipophilic compounds, where the diastereoisomer possessing the slower eluting time on a reverse-phase high-performance liquid chromatography column was transported through Caco-2 and MDCK monolayers at higher rate.

New anti-HIV agents and targets

Virtually all the compounds that are currently used or are subject of advanced clinical trials for the treatment of HIV infections, belong to one of the following classes: (i) nucleoside reverse transcriptase inhibitors (NRTIs): i.e., zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, emtricitabine and nucleotide reverse transcriptase inhibitors (NtRTIs) (i.e., tenofovir disoproxil fumarate); (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e., nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e., saquinavir, ritonavir, indinavir, nelfinavir, amprenavir, and lopinavir. In addition to the reverse transcriptase and protease reaction, various other events in the HIV replicative cycle can be considered as potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulfates, polysulfonates, polycarboxylates, polyoxometalates, polynucleotides, and negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 (i.e., bicyclam (AMD3100) derivatives) and CCR5 (i.e., TAK-779 derivatives); (iii) virus-cell fusion, through binding to the viral envelope glycoprotein gp41 (T-20, T-1249); (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as 4-aryl-2,4-dioxobutanoic acid derivatives; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (flavopiridol, fluoroquinolones). Also, various new NRTIs, NNRTIs, and PIs have been developed that possess, respectively: (i) improved metabolic characteristics (i.e., phosphoramidate and cyclosaligenyl pronucleotides by-passing the first phosphorylation step of the NRTIs), (ii) increased activity ["second" or "third" generation NNRTIs ( i.e., TMC-125, DPC-083)] against those HIV strains that are resistant to the "first" generation NNRTIs, or (iii), as in the case of PIs, a different, modified peptidic (i.e., azapeptidic (atazanavir)) or non-peptidic scaffold (i.e., cyclic urea (mozenavir), 4-hydroxy-2-pyrone (tipranavir)). Non-peptidic PIs may be expected to inhibit HIV mutant strains that have become resistant to peptidomimetic PIs.

New developments in anti-HIV chemotherapy

Virtually all the compounds that are currently used, or are subject of advanced clinical trials, for the treatment of human immunodeficiency virus (HIV) infections, belong to one of the following classes: (i) nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs): i.e. zidovudine (AZT), didanosine (ddI), zalcitabine (ddC), stavudine (d4T), lamivudine (3TC), abacavir (ABC), emtricitabine [(-)FTC], tenofovir disoproxil fumarate; (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e. nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e. saquinavir, ritonavir, indinavir, nelfinavir, amprenavir and lopinavir. In addition to the reverse transcriptase (RT) and protease reaction, various other events in the HIV replicative cycle can be considered as potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulfates, polysulfonates, polycarboxylates, polyoxometalates, polynucleotides, and negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 [bicyclam (AMD3100) derivatives] and CCR5 (TAK-779 derivatives); (iii) virus-cell fusion, through binding to the viral envelope glycoprotein gp41 (T-20, T-1249); (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as 4-aryl-2,4-dioxobutanoic acid derivatives; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (flavopiridol, fluoroquinolones). Also, various new NRTIs, NNRTIs and PIs have been developed that possess, respectively: (i) improved metabolic characteristics (i.e. phosphoramidate and cyclosaligenyl pronucleotides by-passing the first phosphorylation step of the NRTIs), (ii) increased activity ["second" or "third" generation NNRTIs (i.e. TMC-125, DPC-083)] against those HIV strains that are resistant to the "first" generation NNRTIs, or (iii) as in the case of PIs, a different, nonpeptidic scaffold [i.e. cyclic urea (mozenavir), 4-hydroxy-2-pyrone (tipranavir)]. Nonpeptidic PIs may be expected to inhibit HIV mutant strains that have become resistant to peptidomimetic PIs. Given the multitude of molecular targets with which anti-HIV agents can interact, one should be cautious in extrapolating the mode of action of these agents from cell-free enzymatic assays to intact cells. Two examples in point are L-chicoric acid and the nonapeptoid CGP64222, which were initially described as an integrase inhibitor or Tat antagonist, respectively, but later shown to primarily act as virus adsorption/entry inhibitors, the latter through blockade of CXCR4.

Advances in QSAR studies of HIV-1 reverse transcriptase inhibitors

A review is presented of the recent advances in quantitative structure-activity relationship (QSAR) studies of HIV-1 reverse transcriptase (RT) inhibitors. These inhibitors have been put into two classes: nucleoside RT inhibitors (NRTIs), which are 2',3'-dideoxynucleoside analogues (ddNs), and non-nucleoside RT inhibitors (NNRTIs). For NRTIs (ddNs), which act as competitive inhibitors or alternate substrates of RT and hence interact at the substrate binding site of the enzyme, QSARs have pointed out the major role of the electronic factors governing their activity. For NNRTIs, which bind to a site entirely distinct from the substrate binding site, the activity has been shown to be largely dependent upon the hydrophobic nature of the compounds or substituents. The hydrophobic nature of the active site in the receptor with which the NNRTIs interact provides relatively few possibilities for the molecules to have polar interactions or hydrogen bondings, but QSARs have indicated that NNRTIs do involve some polar interactions and hydrogen bondings with some pockets of the enzyme. QSARs also indicate the significant roles of steric interactions and conformational shape of the molecule.

Effects of aryl substituents on the anti-HIV activity of the arylphosphoramidate derivatives of stavudine

We compared the anti-HIV activity of 13 phenyl phosphate derivatives of stavudine (2',3'-didehydro-2',3'-dideoxythymidine/d4T) by examining their ability to inhibit HIV-1 replication in human peripheral blood mononuclear cells. Our results show that the introduction of electron-withdrawing substituents enhances the activity of these phosphoramidate derivatives. The rate of chemical hydrolysis under alkaline conditions (but not the lipophilicity) predicted the potency of the compounds.

Strategies in the design of antiviral drugs

A decade ago, just five drugs were licensed for the treatment of viral infections. Since then, greater understanding of viral life cycles, prompted in particular by the need to combat human immunodeficiency virus, has resulted in the discovery and validation of several targets for therapeutic intervention. Consequently, the current antiviral repertoire now includes more than 30 drugs. But we still lack effective therapies for several viral infections, and established treatments are not always effective or well tolerated, highlighting the need for further refinement of antiviral drug design and development. Here, I describe the rationale behind current and future drug-based strategies for combating viral infections.

Hamao Umezawa Memorial Award Lecture: "An Odyssey in the Viral Chemotherapy Field"

In the search of effective and selective chemotherapeutic agents for the treatment of viral infections, my "Odyssey" brought me to explore a variety of approaches, encompassing interferon and interferon inducers, suramin and other polyanionic substances, S-adenosylhomocysteine hydrolase inhibitors, inosine 5'-monophosphate dehydrogenase inhibitors, 5-substituted 2'-deoxyuridines such as (E)-5-(2-bromovinyl)-2'-deoxyuridine, acyclovir (esters) and other acyclic guanosine analogues, 2',3'-dideoxynucleoside analogues, non-nucleoside reverse transcriptase inhibitors (NNRTIs), bicyclams, and acyclic nucleoside phosphonates. This had led to the identification of a number of compounds, efficacious against such important viral pathogens as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes simplex virus (HSV), varicella-zoster virus (VZV), cytomegalovirus (CMV), and other herpesviruses, pox-, adeno-, polyoma-, and papillomaviruses, and hemorrhagic fever viruses.

New developments in anti-HIV chemotherapy

Virtually all the compounds that are currently used, or under advanced clinical trial, for the treatment of HIV infections, belong to one of the following classes: (i) nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs): i.e. zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, emtricitabine, tenofovir (PMPA) disoproxil fumarate; (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e. nevirapine, delavirdine, efavirenz, emivirine; and (iii) protease inhibitors (PIs): i.e. saquinavir, ritonavir, indinavir, nelfinavir and amprenavir. In addition, various other events in the HIV replicative cycle are potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120; (ii) viral entry, through blockade of the viral coreceptors CXCR4 and CCR5; (iii) virus-cell fusion; (iv) viral assembly and disassembly; (v) proviral DNA integration; (vi) viral mRNA transcription. Also, new NRTIs, NNRTIs and PIs have been developed that possess respectively improved metabolic characteristics, or increased activity against NNRTI-resistant HIV strains or, as in the case of PIs, a different, non-peptidic scaffold. Given the multitude of molecular targets with which anti-HIV agents can interact, one should be cautious in extrapolating from cell-free enzymatic assays to the mode of action of these agents in intact cells.

Synthesis and antiviral evaluation of phosphoramidate derivatives of (E)-5-(2-bromovinyl)-2'-deoxyuridine

We report the design, synthesis and antiviral evaluation of a number of lipophilic, masked phosphoramidate derivatives of the antiherpetic agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), designed to act as membrane soluble prodrugs of the free nucleotide. The phosphoramidate derivatives of BVDU that contain L-alanine exhibited potent anti herpes simplex virus type 1 and varicella-zoster virus activity but lost marked activity against thymidine kinase-deficient virus strains. The phosphoramidate derivative bearing the amino acid alpha,alpha-dimethylglycine showed poor activity in all cell lines tested. It appears that successful kinase bypass by phosphoramidates is highly dependent on the nucleoside analogue, amino acid and ester structure, as well as the cell line to which the drugs are exposed.

Synthesis of new homo and heterodinucleosides containing the 2',3'-dideoxynucleosides AZT and D4T

The synthesis of new dinucleosides of AZT and D4T is described.

Practical synthesis, separation, and stereochemical assignment of the PMPA pro-drug GS-7340

The practical synthesis of a mixed phenoxy-amidate derivative of PMPA with high oral bioavailability and favorable pharmacokinetics is described. The non-stereoselective synthetic route produces a 1:1 mixture of two diastereomers at phosphorous. Simulated moving bed chromatography using Chiralpak AS enabled kilo-scale isolation of the more potent diastereomer (GS-7340). The GS-7340 phosphorous chiral center was found to be (S) by X-ray crystallography.

Novel compounds in preclinical/early clinical development for the treatment of HIV infections

Virtually all the compounds that are currently used, or under advanced clinical trial, for the treatment of HIV infections, belong to one of the following classes: (i) nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs), (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs) and (iii) protease inhibitors (PIs). In addition to the reverse transcriptase and protease step, various other events in the HIV replicative cycle are potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120 (polysulphates, polysulphonates, polyoxometalates, zintevir, negatively charged albumins); (ii) viral entry, through blockade of the viral coreceptors CXCR4 and CCR5 [bicyclams (AMD3100), polyphemusins (T22), TAK-779]; (iii) virus-cell fusion, through binding to the viral glycoprotein gp41 [T-20 (DP-178), siamycins, betulinic acid derivatives]; (iv) viral assembly and disassembly, through NCp7 zinc finger-targeted agents [2,2'-dithiobisbenzamides (DIBAs), azadicarbonamide (ADA)]; (v) proviral DNA integration, through integrase inhibitors such as L-chicoric acid; (vi) viral mRNA transcription, through inhibitors of the transcription (transactivation) process (peptoid CGP64222, fluoroquinolone K-12, Streptomyces product EM2487). Also, in recent years new NRTIs, NNRTIs and PIs have been developed that possess, respectively, improved metabolic characteristics (i.e. phosphoramidate and cyclosaligenyl pronucleotides of d4T), or increased activity against NNRTI-resistant HIV strains, or, in the case of PIs, a different, non-peptidic scaffold. Given the multitude of molecular targets with which anti-HIV agents can interact, one should be cautious in extrapolating from cell-free enzymatic assays to the mode of action of these agents in intact cells. A number of compounds (i.e. zintevir and L-chicoric acid, on the one hand; and CGP64222 on the other hand) have recently been found to interact with virus-cell binding and viral entry in contrast to their proposed modes of action targeted at the integrase and transactivation process, respectively.

Enhancing the aqueous solubility of d4T-based phosphoramidate prodrugs

A range of polyether para-substituted phosphoramidates were synthesised and found to have substantially elevated aqueous solubilities compared to the underivatised parent prodrug. A 30-fold increase in aqueous solubility could be achieved without a substantial decrease of in vitro activity against HIV-1. Replacement of the aryl (i.e. phenolic) moiety by tyrosine led to a substantial enhancement in aqueous solubility but also to a decrease in antiviral potency. A previously unobserved trend was identified, relating increased aryl substituent steric bulk to decreased antiviral activity.