SATE pronucleotide approaches: an overview

Development of a General Organophosphorus Radical Trap: Deoxyphosphonylation of Alcohols

Here we report the design of a general, redox-switchable organophosphorus alkyl radical trap that enables the synthesis of a broad range of C(sp3)-P(V) modalities. This "plug-and-play" approach relies upon in situ activation of alcohols and O═P(R2)H motifs, two broadly available and inexpensive sources of molecular complexity. The mild, photocatalytic deoxygenative strategy described herein allows for the direct conversion of sugars, nucleosides, and complex pharmaceutical architectures to their organophosphorus analogs. This includes the facile incorporation of medicinally relevant phosphonate ester prodrugs.

Antiviral Activity of Lipophilic Nucleoside Tetraphosphate Compounds

We report on the synthesis and characterization of three types of nucleoside tetraphosphate derivatives 4-9 acting as potential prodrugs of d4T nucleotides: (i) the δ-phosph(on)ate is modified by two hydrolytically stable alkyl residues 4 and 5; (ii) the δ-phosph(on)ate is esterified covalently by one biodegradable acyloxybenzyl moiety and a nonbioreversible moiety 6 and 7; or (iii) the δ-phosphate of nucleoside tetraphosphate is masked by two biodegradable prodrug groups 8 and 9. We were able to prove the efficient release of d4T triphosphate (d4TTP, (i)), δ-monoalkylated d4T tetraphosphates (20 and 24, (ii)), and d4T tetraphosphate (d4T4P, (iii)), respectively, by chemical or enzymatic processes. Surprisingly, δ-dialkylated d4T tetraphosphates, δ-monoalkylated d4T tetraphosphates, and d4T4P were substrates for HIV-RT. Remarkably, the antiviral activity of TetraPPPPro-prodrug 7 was improved by 7700-fold (SI 5700) as compared to the parent d4T in CEM/TK- cells, denoting a successful cell membrane passage of these lipophilic prodrugs and an intracellular delivery of the nucleotide metabolites.

Membrane-permeable tenofovir-di- and monophosphate analogues

The development of new antiviral agents such as nucleoside analogues or acyclic nucleotide analogues (ANPs) and prodrugs thereof is an ongoing task. We report on the synthesis of three types of lipophilic triphosphate analogues of (R)-PMPA and dialkylated diphosphate analogues of (R)-PMPA. A highly selective release of the different nucleotide analogues ((R)-PMPA-DP, (R)-PMPA-MP, and (R)-PMPA) from these compounds was achieved. All dialkylated (R)-PMPA-prodrugs proved to be very stable in PBS as well as in CEM/0 cell extracts and human plasma. In primer extension assays, both the monoalkylated and the dialkylated (R)-PMPA-DP derivatives acted as (R)-PMPA-DP as a substrate for HIV-RT. In contrast, no incorporation events were observed using human polymerase γ. The dialkylated (R)-PMPA-compounds exhibited significant anti-HIV efficacy in HIV-1/2 infected cells (CEM/0 and CEM/TK-). Remarkably, the dialkylated (R)-PMPA-MP derivative 9a showed a 326-fold improved activity as compared to (R)-PMPA in HIV-2 infected CEM/TK- cells as well as a very high SI of 14,000. We are convinced that this study may significantly contribute to advancing antiviral agents developed based on nucleotide analogues in the future.

Lipophilic Nucleoside Triphosphate Prodrugs of Anti-HIV Active Nucleoside Analogs as Potential Antiviral Compounds

Nucleoside analogs require three phosphorylation steps catalyzed by cellular kinases to give their triphosphorylated metabolites. Herein, the synthesis of two types of triphosphate prodrugs of different nucleoside analogs is disclosed. Triphosphates comprising: i) a γ-phosphate or γ-phosphonate bearing a bioreversible acyloxybenzyl group and a long alkyl group and ii) γ-dialkyl phosphate/phosphonate modified nucleoside triphosphate analogs. Almost selective conversion of the former TriPPPro-compounds into the corresponding γ-alkylated nucleoside triphosphate derivatives is demonstrated in CEM/0 cell extracts that proved to be stable toward further hydrolysis. The latter γ-dialkylated triphosphate derivatives lead to the slow formation of the corresponding NDPs. Both types of TriPPPro-compounds are highly potent in wild-type CEM/0 cells and more importantly, they exhibit even better activities against HIV-2 replication in CEM/TK- cell cultures. A finding of major importance is that, in primer extension assays, γ-phosphate-modified-NTPs, γ-mono-alkylated-triphosphates, and NDPs prove to be substrates for HIV-RT but not for cellular DNA-polymerases α,γ.

Pronucleotides of 2',3'-Dideoxy-2',3'-Didehydrothymidine as Potent Anti-HIV Compounds

We report on the synthesis and evaluation of three different nucleotide prodrug systems: (i) nucleoside triphosphate analogues in which the γ-phosph(on)ate has two different lipophilic nonbioreversible alkyl residues with d4TDP as the released nucleotide analogue; (ii) nucleoside diphosphate analogues bearing a bioreversible and a stable β-alkyl group; or (iii) nucleoside diphosphate analogues bearing two nonhydrolysable lipophilic alkyl moieties. The delivery of d4TDP (for the triphosphate precursor) and d4TMP (for the diphosphate precursor) was demonstrated in CD4+ T-lymphocyte CEM cell extracts as well as in phosphate buffer saline (PBS). In primer extension assay, we found that γ-dialkylated d4TTP derivatives and d4TDP were accepted as substrates by HIV-RT. Several of these compounds were observed to be extremely active against HIV-1/2 replication in HIV-infected cells. A more than 45,000-fold increase in the anti-HIV activity was detected for compound 18a as compared to the parent d4T which results in a selectivity index value of 37,000.

NPP-669, a Novel Broad-Spectrum Antiviral Therapeutic with Excellent Cellular Uptake, Antiviral Potency, Oral Bioavailability, Preclinical Efficacy, and a Promising Safety Margin

DNA viruses are responsible for many diseases in humans. Current treatments are often limited by toxicity, as in the case of cidofovir (CDV, Vistide), a compound used against cytomegalovirus (CMV) and adenovirus (AdV) infections. CDV is a polar molecule with poor bioavailability, and its overall clinical utility is limited by the high occurrence of acute nephrotoxicity. To circumvent these disadvantages, we designed nine CDV prodrug analogues. The prodrugs modulate the polarity of CDV with a long sulfonyl alkyl chain attached to one of the phosphono oxygens. We added capping groups to the end of the alkyl chain to minimize β-oxidation and focus the metabolism on the phosphoester hydrolysis, thereby tuning the rate of this reaction by altering the alkyl chain length. With these modifications, the prodrugs have excellent aqueous solubility, optimized metabolic stability, increased cellular permeability, and rapid intracellular conversion to the pharmacologically active diphosphate form (CDV-PP). The prodrugs exhibited significantly enhanced antiviral potency against a wide range of DNA viruses in infected human foreskin fibroblasts. Single-dose intravenous and oral pharmacokinetic experiments showed that the compounds maintained plasma and target tissue levels of CDV well above the EC₅₀ for 24 h. These experiments identified a novel lead candidate, NPP-669. NPP-669 demonstrated efficacy against CMV infections in mice and AdV infections in hamsters following oral (p.o.) dosing at a dose of 1 mg/kg BID and 0.1 mg/kg QD, respectively. We further showed that NPP-669 at 30 mg/kg QD did not exhibit histological signs of toxicity in mice or hamsters. These data suggest that NPP-669 is a promising lead candidate for a broad-spectrum antiviral compound.

Development of Fluorophosphoramidate as a New Biocompatible Transformable Functional Group and its Application as a Phosphate Prodrug for Nucleoside Analogs

Synthetic phosphate-derived functional groups are important for controlling the function of bioactive molecules in vivo . Herein we describe the development of a new type of biocompatible phosphate analog, a fluorophosphoramidate (FPA) functional group that has characteristic P-F and P-N bonds. We found that FPA with a primary amino group was relatively unstable in aqueous solution and was converted to a monophosphate, while FPA with a secondary amino group was stable. Furthermore, by improving the molecular design of FPA, we developed a reaction in which a secondary amino group is converted to a primary amino group in the intracellular environment, and clarified that the FPA group functions as a phosphate prodrug of nucleoside. Various FPA-gemcitabine derivatives were synthesized and their anticancer activities were evaluated. One of the FPA-gemcitabine derivatives showed superior anticancer activity compared with gemcitabine and its ProTide prodrug, which methodology is widely used in various nucleoside analogs, including anti-cancer and anti-virus drugs.

Mononucleoside phosphorodithiolates as mononucleotide prodrugs

The synthesis and in vitro anti-HIV activity of a novel series of pronucleotides are reported. These prodrugs were characterized by a phosphorodithiolate structure, incorporating two O-pivaloyl-2-oxyethyl substituents as biolabile phosphate protections. The compounds were obtained following an original one-pot three-step procedure, involving the formation of a phosphorodithioite intermediate which is in situ oxidized. In vitro, comparative anti-HIV evaluations demonstrate that such original prodrugs are able to allow the efficient intracellular release of the corresponding 5'-mononucleotide. The pronucleotide of 2',3'-dideoxyadenosine (ddA) 3 exhibited a very potent antiretroviral effect with 50% effective concentration (EC₅₀) values in nanomolar concentration range in various cell lines. In primary monocytes/macrophages, this derivative was 500 times more potent in inhibiting HIV replication (EC₅₀ 0.23 pM) than ddA and the selectivity index of the prodrug is fifty times higher than the one of the parent nucleoside.

An original pronucleotide strategy for the simultaneous delivery of two bioactive drugs

The synthesis and in vitro anti-HIV activity of a novel series of phosphoramidate pronucleotides including a S-pivaloyl-2-thioethyl (tBuSATE) group as biolabile phosphate protecting group are reported. Such constructs, obtained through different phosphorus chemistries, are characterized by the association of two different anti-HIV nucleoside analogues linked to the phosphorus atom respectively by the sugar residue and the exocyclic amino function of the nucleobase. In vitro, comparative anti-HIV evaluation demonstrates that such original prodrugs are able to allow the efficient intracellular combination release of a 5'-mononucleotide as well as another nucleoside analogue. In human T4-lymphoblastoid cells, the pronucleotide 1 shows remarkable antiviral activity with an EC₅₀ in the nanomolar range (0.6 ηM) and without additional cytotoxicity. In addition, these two pronucleotide models exhibit higher selectivity index than the equimolar mixture of their constitutive nucleoside analogues opening the way to further studies with regard to the current use of drug combinations.

H-Phosphonate Chemistry in the Synthesis of Electrically Neutral and Charged Antiviral and Anticancer Pronucleotides

In this review a short account of our work on the synthesis and biological activity of electrically neutral and charged anti-HIV and anticancer pronucleotides, presented on the background of the contemporary research in this area, is given.

Lipophilic Triphosphate Prodrugs of Various Nucleoside Analogues

The antiviral efficacy of many nucleoside analogues is strongly dependent on their intracellular activation by host cellular kinases to yield ultimately the bioactive nucleoside analogue triphosphates (NTP). The metabolic conversion of nucleoside analogues into their triphosphates often proceeds insufficiently. We developed a nucleoside triphosphate (NTP) delivery system (the TriPPPro approach), in which the γ-phosphate is covalently modified by two different biodegradable masking units, one is the acyloxybenzyl (AB) moiety and the other is the alkoxycarbonyloxybenzyl (ACB) group. Such compounds formed NTPs with high selectivity by an enzyme-triggered mechanism in human T-lymphocyte CEM cell extracts loosing first the AB moiety, followed by the ACB group. This enables the bypass of all steps of the intracellular phosphorylation. This approach was applied here to convert some modestly active or even inactive nucleoside analogues into powerful biologically active metabolites. Potent antiviral activity profiles were obtained depending on the lipophilicity of the TriPPPro-NTP prodrugs against HIV-1 and HIV-2 replication in cultures of infected wild-type CD4⁺ CEM T-cells and more importantly in thymidine kinase-deficient CD4⁺ T-cells (CEM/TK^-). This TriPPPro strategy offers high potential for future antiviral and antitumoral chemotherapies.

Anti-HIV-Active Nucleoside Triphosphate Prodrugs

We disclose a study on nucleoside triphosphate (NTP) analogues in which the γ-phosphate is covalently modified by two different biodegradable masking units and d4T as nucleoside analogue that enable the delivery of d4TTP with high selectivity in phosphate buffer (pH 7.3) and by enzyme-triggered reactions in human CD4⁺ T-lymphocyte CEM cell extracts. This allows the bypass of all steps normally needed in the intracellular phosphorylation. These TriPPPro-nucleotides comprising an acyloxybenzyl (AB; ester) or an alkoxycarbonyloxybenzyl (ACB; carbonate) in combination with an ACB moiety are described as NTP delivery systems. The introduction of these two different groups led to the selective formation of γ-(ACB)-d4TTPs by chemical hydrolysis and in particular by cell extract enzymes. γ-(AB)-d4TTPs are faster cleaved than γ-(ACB)-d4TTPs. In antiviral assays, the compounds are highly active against HIV-1 and HIV-2 in wild-type CEM/O cells and more importantly in thymidine kinase-deficient CD4⁺ T-cells (CEM/TK^-).

Aryl H-phosphonates. 19. New anti-HIV pronucleotide phosphoramidate diesters containing amino- and hydroxypyridine auxiliaries

We have designed a new type of AZT and ddU phosphoramidate diesters containing various combinations of 2-, 3-, 4-aminopyridine and 2-, 3-, 4-hydroxypyridine moieties attached to the phosphorus center, as potential anti-HIV pronucleotides. Depending on the pK_a values of the aminopyridines and the hydroxypyridines used, alternative synthetic strategies based on H-phosphonate chemistry were developed for their preparation. Synthetic aspects of these transformations and the biological activity of the synthesized compounds are discussed.

S-Acyl-2-Thioethyl: A Convenient Base-Labile Protecting Group for the Synthesis of siRNAs Containing 5′-Vinylphosphonate

We recently reported that (E)-5′-vinylphosphonate (5′-VP) is a metabolically-stable phosphate mimic for siRNA and demonstrated that 5′-VP improves the potency of the fully modified siRNAs in vivo. Here, we report an alternative synthesis of 5′-VP modified guide strand using S-pivaloyl-2-thioethyl (tBu-SATE) protecting group. The tBu-SATE group is readily removed during the final cleavage of the oligonucleotide from the solid support and providing a more convenient route for the synthesis of siRNA guide strand carrying a 5′-vinylphosphonate.

2′-C-Methyl Branched Pyrimidine Ribonucleoside Analogues: Potent Inhibitors of RNA Virus Replication

RNA viruses are the agents of numerous widespread and often severe diseases. Their unique RNA-dependent RNA polymerase (RDRP) is essential for replication and, thus, constitutes a valid target for the development of selective chemotherapeutic agents. In this regard, we have investigated sugar-modified ribonucleoside analogues as potential inhibitors of the RDRP. Title compounds retain ‘natural’ pyrimidine bases, but possess a β-methyl substituent at the 2′-position of the D- or L-ribose moiety. Evaluation against a broad range of RNA viruses, either single-stranded positive (ssRNA), single-stranded negative (ssRNA−) or double-stranded (dsRNA), revealed potent activities for D-2′- C-methyl-cytidine and -uridine against ssRNA+, and dsRNA viruses. None of the L-enantiomers were active. Moreover, the 5′-triphosphates of the active D-enantiomers were found to inhibit the bovine virus diarrhoea virus polymerase. Thus, the 2′-methyl branching of natural pyrimidine ribonucleosides transforms physiological molecules into potent, broad-spectrum antiviral agents that merit further development.

New 3'-O-aromatic acyl-5-fluoro-2'-deoxyuridine derivatives as potential anticancer agents

New aromatic and aliphatic 3'-O-acyl-5-fluoro-2'-deoxyuridine derivatives were synthesized and evaluated as candidates for prodrugs against various cancer cell lines. As the most promising candidate for antimalignant therapeutics was found a dual-acting acyl derivative 7h, which apparently released not only the known anticancer nucleoside, 5-fluoro-2'-deoxyuridine (FdU), but also an additional active metabolite, acetylsalicylic acid, reinforcing thus therapeutic effect of FdU. Promising therapeutic indices showed also some aromatic dicarboxylic acids derivatives decorated with FdU esters (11 and 12).

How can nanomedicines overcome cellular-based anticancer drug resistance?

This review discusses the mechanisms of anticancer drug resistance according to its cellular level of action and outlines the nanomedicine-based strategies adopted to overcome it.

Probing the reactivity of H-phosphonate derivatives for the hydrophosphonylation of various alkenes and alkynes under free-radical conditions

Hydrophosphonylation of various alkenes and alkynes is reported using DPAP as a photoinitiator under UV irradiation.

Design, synthesis and antiviral evaluation of 2'-C-methyl branched guanosine pronucleotides: the discovery of IDX184, a potent liver-targeted HCV polymerase inhibitor

BACKGROUND

Ribonucleoside analogs possessing a β-methyl substituent at the 2'-position of the d-ribose moiety have been previously discovered to be potent and selective inhibitors of hepatitis C virus (HCV) replication, their triphosphates acting as alternative substrate inhibitors of the HCV RdRp NS5B. Results/methodology: In this article, the authors detail the synthesis, anti-HCV evaluation in cell-based replicon assays and structure-activity relationships of several phosphoramidate diester derivatives of 2'-C-methylguanosine (2'-MeG).

CONCLUSION

The most promising compound, namely the O-[S-(hydroxyl)pivaloyl-2-thioethyl]{abbreviated as O-[(HO)tBuSATE)]} N-benzylamine phosphoramidate diester derivative (IDX184), was selected for further in vivo studies, and was the first clinical pronucleotide evaluated for the treatment of chronic hepatitis C up to Phase II trials.

Nucleoside Diphosphate Prodrugs: Nonsymmetric DiPPro-Nucleotides

Nonsymmetric DiPPro-nucleotides are described as nucleoside diphosphate (NDP) delivery systems. The concept is to attach different bis(acyloxybenzyl) moieties at the β-phosphate moiety of a NDP. DiPPro compounds bearing two alkanoylbenzyl residues and DiPPro compounds bearing an alkanoylbenzyl or a benzoylbenzyl group as bioreversible prodrug moieties were studied. Compounds bearing short chain alkanoyl esters led to a fast hydrolysis by chemical or enzymatic means. The ester group in the second prodrug group comprised a long lipophilic aliphatic or an aromatic residue. The lipophilicity of this group enabled the prodrug to penetrate the cell membrane. The introduction of two different groups allowed a controlled stepwise removal of the prodrug moieties to achieve a highly selective delivery of the NDP in CEM cell extracts. The compounds were highly active against HIV even in thymidine kinase-deficient CEM cells. Thus, the compounds, although charged at the α-phosphate group, were taken up by the cells and released NDPs.

Biocatalytic approaches applied to the synthesis of nucleoside prodrugs

Nucleosides are valuable bioactive molecules, which display antiviral and antitumour activities. Diverse types of prodrugs are designed to enhance their therapeutic efficacy, however this strategy faces the troublesome selectivity issues of nucleoside chemistry. In this context, the aim of this review is to give an overview of the opportunities provided by biocatalytic procedures in the preparation of nucleoside prodrugs. The potential of biocatalysis in this research area will be presented through examples covering the different types of nucleoside prodrugs: nucleoside analogues as prodrugs, nucleoside lipophilic prodrugs and nucleoside hydrophilic prodrugs.

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.

Efficient delivery of RNAi prodrugs containing reversible charge-neutralizing phosphotriester backbone modifications

RNA interference (RNAi) has great potential to treat human disease. However, in vivo delivery of short interfering RNAs (siRNAs), which are negatively charged double-stranded RNA macromolecules, remains a major hurdle. Current siRNA delivery has begun to move away from large lipid and synthetic nanoparticles to more defined molecular conjugates. Here we address this issue by synthesis of short interfering ribonucleic neutrals (siRNNs) whose phosphate backbone contains neutral phosphotriester groups, allowing for delivery into cells. Once inside cells, siRNNs are converted by cytoplasmic thioesterases into native, charged phosphodiester-backbone siRNAs, which induce robust RNAi responses. siRNNs have favorable drug-like properties, including high synthetic yields, serum stability and absence of innate immune responses. Unlike siRNAs, siRNNs avidly bind serum albumin to positively influence pharmacokinetic properties. Systemic delivery of siRNNs conjugated to a hepatocyte-specific targeting domain induced extended dose-dependent in vivo RNAi responses in mice. We believe that siRNNs represent a technology that will open new avenues for development of RNAi therapeutics.

Synthesis of novel nucleoside analogue phosphorothioamidate prodrugs and in vitro anticancer evaluation against RKO human colon carcinoma cells

Novel phosphorothioamidates of pyrimidine nucleoside analogues have been prepared and evaluated in vitro against RKO human colon cancer cell by the MTT cytotoxicity assay. The parent nucleoside analogues were inactive in this assay, while the phosphorothioamidate prodrugs were active at low uM levels in some cases. The O-isopropyl phosphorothioamidate of 2 ',3 '-O-isopropylidene-uridine containing the L-phenylalanine ethyl ester 6f was the most active at 148 uM, a 10-fold enhancement in anticancer activity compared with the parent nucleoside 2 with no increase in cytotoxicity.

Evolution of an amino acid based prodrug approach: stay tuned

Certain acyclic nucleoside phosphonates (ANPs) such as (S)-HPMPC (cidofovir, Vistide) and (S)-HPMPA have been shown to be active against a broad spectrum of DNA and retroviruses. However, their poor absorption as well as their toxicity limit the utilization of these therapeutics in the clinic. Nucleoside phosphonates are poorly absorbed primarily due to the presence of the phosphonic acid group, which ionizes at physiological pH. When dosed intravenously they display dose-limiting nephrotoxicity due to their accumulation in the kidney. To overcome these limitations, nucleoside phosphonate prodrug strategies have taken center stage in the development pathway and a number of different approaches are at various stages of development. Our efforts have focused on the development of ANP prodrugs in which a benign amino acid promoiety masks a phosphonate P-OH via a hydroxyl side chain. The design of these prodrugs incorporates multiple chemical groups (the P-X-C linkage, the amino acid stereochemistry, the C-terminal and N-terminal functional groups) that can be tuned to modify absorption, pharmacokinetic and efficacy properties with the goal of improving overall prodrug performance.

5'-Phosphate and 5'-phosphonate ester derivatives of (N)-methanocarba adenosine with in vivo cardioprotective activity

Activation of a cardiac myocyte P2X4 receptor protects against heart failure. 5'-Phosphonate and 5'-phosphate analogues of AMP containing a (N)-methanocarba (bicyclo[3.1.0]hexane) system could protect from heart failure by potentially activating this cardioprotective channel. Phosphoesters and phosphonodiesters were synthesized and administered in vivo via a miniosmotic pump in a mouse ischemic heart failure model and most significantly increased intact heart contractile function (echocardiography) compared to vehicle infusion. Several new thio and deuterated phosphate derivatives were protective in a calsequestrin (CSQ) overexpressing heart failure model. Diethyl (7, MRS4084) and diisopropyl (8, MRS4074) phosphotriesters were highly protective in the ischemic model. Substitution of 2-Cl with iodo reduced protection in the CSQ model. Diisopropyl ester 16 (MRS2978) of (1'S,2'R,3'S,4'R,5'S)-4'-(6-amino-2-chloropurin-9-yl)-2',3'-(dihydroxy)-1'-(phosphonoethylene)bicyclo[3.1.0]hexane was highly efficacious (CSQ), while lower homologue 1'-phosphonomethylene derivative 14 was inactive. Thus, we identified uncharged carbocyclic nucleotide analogues that represent potential candidates for the treatment of heart failure, suggesting this as a viable and structurally broad approach.

Prodrug approaches to reduce hyperexcitation in the CNS

Hyperexcitation in the central nervous system is the root cause of a number of disorders of the brain ranging from acute injury to chronic and progressive diseases. The major limitation to treatment of these ailments is the miniscule, yet formidable blood-brain barrier. To deliver therapeutic agents to the site of desired action, a number of biomedical engineering strategies have been developed including prodrug formulations that allow for either passive diffusion or active transport across this barrier. In the case of prodrugs, once in the brain compartment, the active therapeutic agent is released. In this review, we discuss in some detail a number of factors related to treatment of central nervous system hyperexcitation including molecular targets, disorders, prodrug strategies, and focused case studies of a number of therapeutics that are at a variety of stages of clinical development.

Nucleotide prodrugs for HCV therapy

HCV infection is a significant worldwide health problem and is a major cause of hepatocellular carcinoma. The current standard of care, interferon and ribavirin, is only effective against a proportion of the patient population infected with HCV. To address the shortcomings of existing therapy, the development of direct acting antiviral agents is under investigation. The HCV RNA dependent RNA polymerase is an essential enzyme for viral replication and is therefore a logical target against which to develop novel anti-HCV agents. Nucleosides have been shown to be effective as antiviral agents for other viral diseases and therefore, have been investigated as inhibitors of HCV replication. The development of prodrugs of nucleoside 5'-monophosphates has been pursued to address limitations associated with poor nucleoside phosphorylation. This is required to produce the nucleoside 5'-triphosphate which is the anabolite that is the actual inhibitor of the polymerase enzyme. Prodrugs of nucleoside 5'-monophosphates have been developed that enable their delivery into cells and in vivo into the liver. The implementation of these prodrug strategies has ultimately led to the identification of several prodrugs of nucleoside 5'-monophosphates that are potent inhibitors of HCV replication in vitro. They have progressed into the clinic and the early data demonstrate greatly reduced viral load levels in HCV-infected patients. This review will survey the state of nucleotide prodrugs for the treatment of HCV.

Synthesis and an antiviral activity evaluation of nucleoside 5´-O-(N-acyl) phosphoramidates

BACKGROUND

pyrimidine nucleoside analogues represent an established class of clinically useful antiviral agents. Once inside the cell, they are activated by a series of intracellular phosphorylation steps to produce 5´-triphosphate derivatives. In many cases, nucleoside analogues are poor substrates for the cellular kinases needed for their activation. It is clear that intracellular introduction of nucleoside analogues as phosphorylated metabolites (so called pronucleotides) could circumvent difficulties associated with the use of non-phosphorylated nucleoside analogues.

METHODS

among the current diverse pronucleotide approaches, nucleoside phosphoramidate derivatives appear to be an interesting class of potential antiviral agents because of the known relatively low stability of the P-N bond in cellular media. On the basis of oxathiaphospholane chemistry, a series of novel conjugates of 5´-O-phosphorylated zidovudine (AZT) and stavudine (d4T) with amino acids carboxamidates were obtained. The synthesis was performed using N-(2-thiono-1,3,2-oxathiaphospholane) derivatives of amino acids carboxamides as precursors.

RESULTS

all synthesized compounds were studied against DNA and RNA viruses. Specific antiviral activities were only detected against HIV type-1 and HIV type-2 in MT-4 cell cultures at compound concentrations that were equally active or slightly inferior to the activity of their parent drugs (2- to 20-fold for the AZT prodrugs and 6- to 40-fold for the d4T prodrugs). The compounds were also evaluated for their anti-HIV activity in CEM and in CEM thymidine-kinase-deficient (CEM/TK(-)) cell cultures.

CONCLUSIONS

loss of compound antiviral potency in the CEM/TK(-) cells suggested an eventual conversion of the test compounds to the free nucleosides prior to further phosphorylation to the active 5´-triphosphate metabolite.

3-Acetyloxy-2-cyano-2-(alkylaminocarbamoyl)propyl groups as biodegradable protecting groups of nucleoside 5´-mono-phosphates

Thymidine 5´-bis[3-acetyloxy-2-cyano-2-(2-phenylethylcarbamoyl)propyl]phosphate (1) has been prepared and the removal of phosphate protecting groups by hog liver carboxyesterase (HLE) at pH 7.5 and 37 °C has been followed by HPLC. The first detectable intermediates are the (R_P)- and (S_P)-diastereomers of the monodeacetylated triester 14, which subsequently undergo concurrent retro-aldol condensation to diester 4 and enzyme-catalyzed hydrolysis to the fully deacetylated triester 15. The former pathway predominates, representing 90% of the overall breakdown of 14. The diester 4 undergoes the enzymatic deacetylation 700 times less readily than the triester, but gives finally thymidine 5´-monophosphate as the desired main product. To elucidate the potential toxicity of the electrophilic 2-cyano-N-(2-phenylethyl)acrylamideby-product 17 released upon the deprotection, the hydrolysis of 1 has also been studied in the presence of glutathione (GSH).

Identification of amino acid phosphorodiamidates of antiviral nucleosides using electrospray ionization tandem mass spectrometry

Several amino acid phosphorodiamidate derivatives of d4T as anti-HIV prodrugs were synthesized and investigated using electrospray ionization multistage tandem mass spectrometry (ESI-MS(n)). A novel methyl group migration in gas phase was observed in ESI-MS(2) of the sodium adducts of amino acid methyl ester of phosphorodiamidates of 2',3'-didehydro-2',3'-dideoxythymidine (d4T). The proposed structures of the rearrangement ions were confirmed by high resolution tandem mass spectrometry. A possible mechanism involving the pentacoordinate phosphoric-carboxylic phosphate anhydride was proposed, in which a seven-membered ring intermediate was formed by coordination with the metal ion between the phosphoryl group and carbonyl oxygen atom. Thus, the intrinsic properties of phosphoryl group might be the key factors responsible for this migration.

Diastereoselective synthesis of aryloxy phosphoramidate prodrugs of 3'-deoxy-2',3'-didehydrothymidine monophosphate

The first diastereoselective synthesis of aryloxy phosphoramidate prodrugs of 3'-deoxy-2',3'-didehydrothymidine monophosphate (d4TMP) is reported. In our approach, (S)-4-isopropylthiazolidine-2-thione 1 was used as a chiral auxiliary to introduce the stereochemistry at the phosphorus atom. In the last step of the developed reaction sequence, the nucleoside analogue d4T was introduced to a stereochemically pure phosphordiamidate which led to the formation of the almost diastereomerically pure phosphoramidate prodrugs 8a-d (≥95% de). As expected, the individually prepared diastereomers of the phosphoramidate prodrugs showed significant differences in the antiviral activity. Moreover, the difference was strongly dependent on the aryl substituent attached to the phosphoramidate moiety.

Diastereoselective synthesis of cyclosaligenyl-nucleosyl-phosphotriesters

A diastereoselective synthesis of cycloSal-phosphotriesters (cycloSal=cycloSaligenyl) based on chiral auxiliaries has been developed that allows the synthesis of single diastereomers of the cycloSal-pronucleotides. In previously described synthesis routes, the cycloSal-compounds were always obtained as 1:1 diastereomeric mixtures that could be separated in only rare cases. However, it was shown that the diastereomers have different antiviral activity, toxicity, and hydrolysis stabilities. Here, first a chiral thiazoline derivative was used to prepare nonsubstituted and 5-methyl-cycloSal-phosphotriesters in 48 and ≥95% de (de=diastereomeric excess). However, this approach failed to give the important group of 3-substituted cycloSal-nucleotides. Therefore, two other chiral groups were discovered that allowed the synthesis of (R(P))- and (S(P))-3-methyl-cycloSal-phosphotriesters as well. The antiviral activity was found to be five- to 20-fold different between the two individual diastereomers, which proved the importance of this approach.

Phosphoramidate pronucleotides of cytostatic 6-aryl-7-deazapurine ribonucleosides

A series of O-phenyl methyl-, ethyl- and benzylalanyl phosphoramidate pronucleotides derived from cytostatic 6-aryl-7-deazapurine ribonucleosides were prepared by the cross-coupling reactions of the 2',3'-isopropylidene protected 6-chloro-7-deazapurine ribonucleoside phosphoramidates with (het)arylboronic acids or -stannanes followed by deprotection. Most of the prepared prodrugs exerted in vitro cytostatic effects against both solid tumor and lymphoid cancer cells within low micromolar range of concentrations. These activities were in general weaker or comparable to the activities of the parent nucleosides. Additional testing of selected prodrugs suggests that the lack of activity improvement over parent nucleosides is not due to the lack of permeability or inefficient catabolism of alanyl-ester by intracellular hydrolases. More likely, active efflux of prodrugs may play a role in their weak cytotoxic activity.

CycloSal-phosphate pronucleotides of cytostatic 6-(Het)aryl-7-deazapurine ribonucleosides: Synthesis, cytostatic activity, and inhibition of adenosine kinases

A series of cycloSal-phosphate prodrugs of a recently described new class of nucleoside cytostatics (6-hetaryl-7-deazapurine ribonucleosides) was prepared. The corresponding 2',3'-isopropylidene 6-chloro-7-deazapurine nucleosides were converted into 5-O'-cycloSal-phosphates. These underwent a series of Stille or Suzuki cross-couplings with diverse (het)arylstannanes or -boronic acids to yield the protected 6-(het)aryl-7-deazapurine pronucleotides that were subsequently deprotected to give 12 derivatives of free pronucleotides. The in vitro cytostatic effect of the pronucleotides was compared with parent nucleoside analogues. In most cases, the activity of the pronucleotide was similar to or somewhat lower than that of the corresponding parent nucleosides, with the exception of 7-fluoro pronucleotides 13 a, 13 b, and 13 d, which had exhibited GIC(50) values that were improved by one order of magnitude (to the low nanomolar range). The presence of a cycloSal-phosphate group also influenced selectivity toward various cell lines. Several pronucleotides were found which strongly inhibit human adenosine kinase but only weakly inhibit the MTB adenosine kinase.

Efficient overcoming of drug resistance to anticancer nucleoside analogs by nanodelivery of active phosphorylated drugs

One of the major problems in cancer chemotherapy is the fast development of drug resistance to most anticancer therapeutics. Thus, an important cause of the eventual decline in clinical efficacy of cytotoxic nucleoside analogs was the selection of resistant cancer cells with deficiencies in the expression of nucleoside transporters or nucleoside-activating kinases. Here, we present an efficient strategy of overcoming this type of drug resistance by tumor-specific delivery of nanogel-encapsulated active triphosphates of nucleoside analogs (NATP). The small particles of biodegradable cationic nanogels loaded with anionic NATP efficiently interacted with cancer cells and released active drug compounds into the cytoplasm. The potential of novel drug formulations was evaluated in the nucleoside transport-deficient (CEM/araC/C8) or nucleoside activation-deficient (RL7/G) lymphogenic cancer cells. Compared to nucleoside analogs, NATP-loaded nanogels demonstrated increased cytotoxicity, reducing the drug resistance index 250- to 900-fold in CEM/araC/C8 cells and 70- to 100-fold in RL7/G cells. The strong cytotoxic effect of nanoformulations was accompanied by characteristic cell cycle perturbations, usually observed in drug-treated sensitive cells, and resulted in the induction of apoptosis in all studied drug-resistant cells. Efficient cellular accumulation of nanogels and the consequent increase in intracellular levels of NATP were found to be the major factors determining cytotoxic efficacy of nanoformulations. Decoration of nanogels with multiple molecules of tumor lymphatic-specific peptide (LyP1) enhanced the binding efficacy of nanocarriers with lymphogenic cancer cells. The targeted nanoformulation of activated gemcitabine (LyP1-NG-dFdCTP), when injected in subcutaneous RL7/G xenograft tumor model, demonstrated 2-fold more efficient tumor growth inhibition than gemcitabine at a higher dose. Nanogel-drug formulations exhibited no systemic toxicity during the treatment, hence extending the versatility of nucleoside analogs in the treatment of drug-resistant lymphogenic tumors.

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

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

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.

Cellular pharmacology of multi- and duplex drugs consisting of ethynylcytidine and 5-fluoro-2'-deoxyuridine

Prodrugs can have the advantage over parent drugs in increased activation and cellular uptake. The multidrug ETC-L-FdUrd and the duplex drug ETC-FdUrd are composed of two different monophosphate-nucleosides, 5-fluoro-2′deoxyuridine (FdUrd) and ethynylcytidine (ETC), coupled via a glycerolipid or phosphodiester, respectively. The aim of the study was to determine cytotoxicity levels and mode of drug cleavage. Moreover, we determined whether a liposomal formulation of ETC-L-FdUrd would improve cytotoxic activity and/or cleavage. Drug effects/cleavage were studied with standard radioactivity assays, HPLC and LC-MS/MS in FM3A/0 mammary cancer cells and their FdUrd resistant variants FM3A/TK⁻. ETC-FdUrd was active (IC₅₀ of 2.2 and 79 nM) in FM3A/0 and TK⁻ cells, respectively. ETC-L-FdUrd was less active (IC₅₀: 7 nM in FM3A/0 vs 4500 nM in FM3A/TK⁻). Although the liposomal formulation was less active than ETC-L-FdUrd in FM3A/0 cells (IC₅₀:19.3 nM), resistance due to thymidine kinase (TK) deficiency was greatly reduced. The prodrugs inhibited thymidylate synthase (TS) in FM3A/0 cells (80–90%), but to a lower extent in FM3A/TK⁻ (10–50%). FdUMP was hardly detected in FM3A/TK⁻ cells. Inhibition of the transporters and nucleotidases/phosphatases resulted in a reduction of cytotoxicity of ETC-FdUrd, indicating that this drug was cleaved outside the cells to the monophosphates, which was verified by the presence of FdUrd and ETC in the medium. ETC-L-FdUrd and the liposomal formulation were neither affected by transporter nor nucleotidase/phosphatase inhibition, indicating circumvention of active transporters. In vivo, ETC-FdUrd and ETC-L-FdURd were orally active. ETC nucleotides accumulated in both tumor and liver tissues. These formulations seem to be effective when a lipophilic linker is used combined with a liposomal formulation.

Biodegradable protections for nucleoside 5'-monophosphates: comparative study on the removal of O-acetyl and O-acetyloxymethyl protected 3-hydroxy-2,2-bis(ethoxycarbonyl)propyl groups

The applicability of 3-acetyloxy-2,2-bis(ethoxycarbonyl)propyl and 3-acetyloxymethoxy-2,2-bis(ethoxycarbonyl)propyl groups as biodegradable phosphate protecting groups for nucleoside 5'-monophosphates has been studied in a HEPES buffer at pH 7.5. Enzymatic deacetylation with porcine carboxyesterase triggers the removal of the resulting 3-hydroxy-2,2-bis(ethoxycarbonyl)propyl and 3-hydroxymethoxy-2,2-bis(ethoxycarbonyl)propyl groups by retro-aldol condensation and consecutive half acetal hydrolysis and retro-aldol condensation, respectively. The kinetics of these multistep deprotection reactions have been followed by HPLC, using appropriately protected thymidine 5'-monophosphates as model compounds. The enzymatic deacetylation of the 3-acetyloxymethoxy-2,2-bis(ethoxycarbonyl)propyl 5'-triester (2) is 25-fold faster than the deacetylation of its 3-acetyloxy-2,2-bis(ethoxycarbonyl)propyl-protected counterpart 1, and the difference in the deacetylation rates of the resulting diesters, 12b and 12a, is even greater. With 2, conversion to thymidine 5'-monophosphate (5'-TMP) is quantitative, while conversion of 1 to 5'-TMP is accompanied by formation of thymidine. Consistent with the preceding observations, quantitative release of 5'-TMP from 2 has been shown to take place in a whole cell extract of human prostate cancer cells.

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.

Prodrug approaches to improving the oral absorption of antiviral nucleotide analogues

Nucleotide analogues have been well accepted as therapeutic agents active against a number of viruses. However, their use as antiviral agents is limited by the need for phosphorylation by endogenous enzymes, and if the analogue is orally administered, by low bioavailability due to the presence of an ionizable diacid group. To circumvent these limitations, a number of prodrug approaches have been proposed. The ideal prodrug achieves delivery of a parent drug by attachment of a non-toxic moiety that is stable during transport and delivery, but is readily cleaved to release the parent drug once at the target. Here, a brief overview of several promising prodrug strategies currently under development is given.

Polymeric nanogels containing the triphosphate form of cytotoxic nucleoside analogues show antitumor activity against breast and colorectal cancer cell lines

The therapeutic efficiency of anticancer nucleoside analogues (NA) strongly depends on their intracellular accumulation and conversion into 5'-triphosphates. Because active NATP cannot be directly administrated due to instability, we present here a strategy of nanoencapsulation of these active drugs for efficient delivery to tumors. Stable lyophilized formulations of 5'-triphosphates of cytarabine (araCTP), gemcitabine (dFdCTP), and floxuridine (FdUTP) encapsulated in biodegradable PEG-cl-PEI or F127-cl-PEI nanogel networks (NGC and NGM, respectively) were prepared by a self-assembly procedure. Cellular penetration, in vitro cytotoxicity, and drug-induced cell cycle perturbations of these nanoformulations were analyzed in breast and colorectal cancer cell lines. Cellular accumulation and NATP release from nanogel was studied by confocal microscopy and direct high-performance liquid chromatography analysis of cellular lysates. Antiproliferative effect of dFdCTP nanoformulations was evaluated in human breast carcinoma MCF7 xenograft animal model. Nanoencapsulated araCTP, dFdCTP, and FdUTP showed similar to NA cytotoxicity and cell cycle perturbations. Nanogels without drugs showed very low cytotoxicity, although NGM was more toxic than NGC. Treatment by NATP nanoformulations induced fast increase of free intracellular drug concentration. In human breast carcinoma MCF7 xenograft animal model, i.v. dFdCTP-nanogel was equally effective in inhibiting tumor growth at four times lower administered drug dose compared with free gemcitabine. Active triphosphates of NA encapsulated in nanogels exhibit similar cytotoxicity and cell cycle perturbations in vitro and faster cell accumulation and equal tumor growth-inhibitory activity in vivo at much lower dose compared with parental drugs, illustrating their therapeutic potential for cancer chemotherapy.