Amino acid phosphoramidate monoesters of 3'-azido-3'-deoxythymidine: relationship between antiviral potency and intracellular metabolism

The Many Faces of Histidine Triad Nucleotide Binding Protein 1 (HINT1)

The histidine triad nucleotide binding protein 1 (HINT1) is a nucleoside phosphoramidase that has garnered interest due to its widespread expression and participation in a broad range of biological processes. Herein, we discuss the role of HINT1 as a regulator of several CNS functions, tumor suppressor, and mast cell activator via its interactions with multiple G-protein-coupled receptors and transcription factors. Importantly, altered HINT1 expression and mutation are connected to the progression of multiple disease states, including several neuropsychiatric disorders, peripheral neuropathy, and tumorigenesis. Additionally, due to its involvement in the activation of several clinically used phosphoramidate prodrugs, tremendous efforts have been made to better understand the interactions behind nucleoside binding and phosphoramidate hydrolysis by HINT1. We detail the substrate specificity and catalytic mechanism of HINT1 hydrolysis, while highlighting the structural biology behind these efforts. The aim of this review is to summarize the multitude of biological and pharmacological functions in which HINT1 participates while addressing the areas of need for future research.

Zinc-Catalyzed Phosphonylation of Alcohols with Alkyl Phosphites

In the presence of a catalytic amount of either Zn(acac)₂ or bis(2,2,6,6-tetramethyl-3,5-heptanedionato)zinc(II) (Zn(TMHD)₂), primary, secondary, and tertiary alcohol substituents on a wide range of substrates, including acyclic and cyclic structures, carbohydrates, steroids, and amino acids, reacted with dimethyl phosphite to afford the corresponding H-phosphonate diesters in high to excellent yields.

Anchimerically Activated ProTides as Inhibitors of Cap-Dependent Translation and Inducers of Chemosensitization in Mantle Cell Lymphoma

The cellular delivery of nucleotides through various pronucleotide strategies has expanded the utility of nucleosides as a therapeutic class. Although highly successful, the highly popular ProTide system relies on a four-step enzymatic and chemical process to liberate the corresponding monophosphate. To broaden the scope and reduce the number of steps required for monophosphate release, we have developed a strategy that depends on initial chemical activation by a sulfur atom of a methylthioalkyl protecting group, followed by enzymatic hydrolysis of the resulting phosphoramidate monoester. We have employed this ProTide strategy for intracellular delivery of a nucleotide antagonist of eIF4E in mantle cell lymphoma (MCL) cells. Furthermore, we demonstrated that chemical inhibition of cap-dependent translation results in suppression of c-Myc expression, increased p27 expression, and enhanced chemosensitization to doxorubicin, dexamethasone, and ibrutinib. In addition, the new ProTide strategy was shown to enhance oral bioavailability of the corresponding monoester phosphoramidate.

Artemisinin-derived dimer phosphate esters as potent anti-cytomegalovirus (anti-CMV) and anti-cancer agents: a structure-activity study

We recently reported the anti-cancer and anti-cytomegalovirus (CMV) activity of artemisinin-derived trioxane diphenylphosphate dimer 838. To probe the relationship between chemical structure and anti-CMV and anti-cancer activities, we now report synthesis and evaluation of a series of eight new dimer phosphate ester analogs of 838. This series of novel molecules was screened against human foreskin fibroblasts (HFFs) infected with CMV and against the human Jurkat T cell acute lymphoblastic leukemia cell line. This SAR study confirms the very high anti-CMV and anti-cancer potencies of dimer diphenyl phosphate ester 838 without its being toxic to normal cells.

Synthesis and anti-herpetic activity of phosphoramidate ProTides

Among the many prodrug approaches aimed at delivering nucleoside monophosphates into cells, the phosphoramidate ProTide approach is one that has shown success, which has made it possible for some of the phosphoramidates to enter into clinical trials. Herein, we report the synthesis and antiviral activity of a series of phosphoramidate ProTides designed to bypass the thymidine kinase (TK) dependence of the parent nucleoside analogues. Phosphoramidate derivatives of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) that contain L-alanine or pivaloyloxymethyl iminodiacetate (IDA-POM) exhibit anti-HSV-1 and anti-VZV activity in cell cultures, but they largely lost antiviral potency against TK-deficient virus strains. Among deazapurine nucleosides and their phosphoramidate derivatives, the 7-deazaadenine containing nucleosides and their phosphoramidate triester derivatives showed weak antiviral activity against VZV. Apparently, intracellular nucleotide delivery with these phosphoramidates is partly successful. However, none of the compound prodrugs showed superior activity to their parent drugs.

Synthesis of Oligonucleotide Conjugates and Phosphorylated Nucleotide Analogues: An Improvement to a Solid Phase Synthetic Approach

An improvement to our solid phase strategy to generate pharmacologically interesting molecule libraries is proposed here. The synthesis of newo-chlorophenol-functionalised solid supports with very high loading (0.18–0.22 meq/g for control pore glass (CPG) and 0.25–0.50 meq/g for TG) is reported. To test the efficiency of these supports, we prepared nucleotide and oligonucleotide models, and their coupling yields and the purity of the crude detached materials were comparable to previously available results. These supports allow the facile and high-yield preparation of highly pure phosphodiester and phosphoramidate monoester nucleosides, conjugated oligonucleotides, and other yet unexplored classes of phosphodiester and phosphoramidate molecules.

Cap-dependent translation initiation factor eIF4E: an emerging anticancer drug target

Cancer cells tend to be more highly dependent on cap‐dependent translation than normal tissues. Thus, proteins involved in the initiation of cap‐dependent translation have emerged as potential anti‐cancer drug targets. Cap‐dependent translation is initiated by the binding of the factor eIF4E to the cap domain of mRNA. Detailed x‐ray crystal and NMR structures are available for eIF4E in association with cap‐analogs, as well as domains of other initiation factors. This review will summarize efforts to design potential antagonist of eIF4E that could be used as new pharmacological tools and anti‐cancer agents and. Insights drawn from these studies should aid in the design of future inhibitors of eIF4E dependent translation initiation. © 2012 Wiley Periodicals, Inc. Med Res Rev., 32, No. 4, 786‐814, 2012

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.

Nontoxic chemical interdiction of the epithelial-to-mesenchymal transition by targeting cap-dependent translation

Normal growth and development depends upon high fidelity regulation of cap-dependent translation initiation, a process that is usurped and redirected in cancer to mediate acquisition of malignant properties. The epithelial-to-mesenchymal transition (EMT) is a key translationally regulated step in the development of epithelial cancers and pathological tissue fibrosis. To date, no compounds targeting EMT have been developed. Here we report the synthesis of a novel class of histidine triad nucleotide binding protein (HINT)-dependent pronucleotides that interdict EMT by negatively regulating the association of eIF4E with the mRNA cap. Compound eIF4E inhibitor-1 potently inhibited cap-dependent translation in a dose-dependent manner in zebrafish embryos without causing developmental abnormalities and prevented eIF4E from triggering EMT in zebrafish ectoderm explants without toxicity. Metabolism studies with whole cell lysates demonstrated that the prodrug was rapidly converted into 7-BnGMP. Thus we have successfully developed the first nontoxic small molecule able to inhibit EMT, a key process in the development of epithelial cancer and tissue fibrosis, by targeting the interaction of eIF4E with the mRNA cap and demonstrated the tractability of zebrafish as a model organism for studying agents that modulate EMT. Our work provides strong motivation for the continued development of compounds designed to normalize cap-dependent translation as novel chemo-preventive agents and therapeutics for cancer and fibrosis.

New AZT analogues having 5'-alkylsulfonyl groups: synthesis and anti-HIV activity

New derivatives of azidothymidine (AZT) substituted by alkyl and alkylsulphonyl groups at N-3 and C-5', respectively, have been synthesized. The new synthesized derivatives showed remarkable anti-HIV-1 and HIV-2 activity in MT-4 cells. Compounds 8 and 10 have IC(50) values of 0.83 and 0.31 microg/mL against HIV-1 with therapeutic index of 83 and 403, respectively, and IC(50) values of 0.93 and 0.29 microg/mL against HIV-2 with therapeutic index of 74 and 431, respectively. This means that compounds 8 and 10 were cytotoxic to MT-4 cells at CC(50) of 69.2 microg/mL and 125 microg/mL, respectively.

Phosphoramidate and phosphate prodrugs of (−)-β-d-(2R,4R)-dioxolane-thymine: Synthesis, anti-HIV activity and stability studies

A series of phosphoramidate and phosphate prodrugs of DOT were synthesized via dichlorophosphate or H-phosphonate chemistry and evaluated for their anti-HIV activity against LAI M184V mutants in PBM cells as well as for their cytotoxicity. The antiviral and cytotoxic profiles of the prodrugs were compared with that of the parent compound (DOT), and it was found that four aryl phosphoramidates 5, 18, 20, and 26 showed a significant enhancement (8- to 12-fold) in anti-HIV activity without cytotoxicity. Chemical stability of these prodrugs was evaluated in phosphate buffer at pH values of biological relevance (i.e., pH 2.0 and 7.4). Enzymatic hydrolysis was also studied in esterase or lipase in buffer solution. Chemical stability studies indicate that the phosphoramidates have good chemical stability at pH 2.0 and at pH 7.4 phosphate buffer. Phosphoramidate prodrugs were hydrolyzed in vitro by esterase or lipase and found to be better substrates for lipases than for esterases. 1,3-Diol cyclic phosphates showed potent anti-HIV activity without increasing the cytotoxicity compared with that of DOT and have good chemical and enzymatic stability. Long-chain lipid phosphates, although showed potent anti-HIV activity, exhibited increased cytotoxicity.

Model synthesis of nucleoside boranophosphoramidate with amino acid for prodrug purpose

A model synthesis of a nucleoside boranophosphoramidate prodrug with (L)-tryptophan methyl ester was accomplished in a one-pot reaction via an H-phosphonate approach. This new type of compound is expected to possess the potent antiviral and anticancer advantages conferred by boranophosphates and normal nucleoside amino acid phosphoramidate.

Synthesis, in vitro anticancer evaluation, and interference with cell cycle progression of N-phosphoamino acid esters of zidovudine and stavudine

A series of N-diisopropylphosphoryl (DIPP) L-amino acid ester prodrugs of zidovudine (AZT) (3a-3e) and stavudine (d4T) (4a-4e) has been prepared. The activity of these compounds against MCF-7 cells (human pleural effusion breast adenocarcinoma cell line) and K562 cells (human chronic myeloid leukemia (CML) cell line) was evaluated. In difference from that of AZT amino acid phosphoramidates, the alophatic amino acid esters of AZT were found to be more cytotoxic than the aromatic analogues toward MCF-7 cell. Two DIPP-L-amino acid esters of d4T 4b (CC50 = 83 microM) and 4c (CC50 = 182 microM) were found to be more cytotoxic than the parent drug toward K562 cells. MCF-7 and K562 cell cycle disturbance was investigated showing detectable blockade in the S phase when exposed to biologically active AZT, 3a, 3b, 3c, 4b and 4c, indicating that they inhibit cell growth by blocking cell cycle progression. Together with previous reports, present findings suggest that anti-breast cancer activity of AZT may be due to hamper DNA synthesis.

An efficient solid phase synthesis of 5'-phosphodiester and phosphoramidate monoester nucleoside analogues

An easy and efficient strategy to obtain libraries of 5'-phosphodiester and 5'-phosphoramidate monoester nucleoside analogues in a highly pure form has been developed, starting from a new nucleoside based solid support. The nucleoside scaffold has been anchored through a 5'-phosphodiester linkage to Tentagel HL resin, functionalized with a 3-chloro-4-hydroxyphenylacetic linker. The solid phase synthesis of small libraries of 5'-phosphodiester and 5'-phosphoramidate monoester thymidine analogues is also reported.

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.

31P NMR and genetic analysis establish hinT as the only Escherchia coli purine nucleoside phosphoramidase and as essential for growth under high salt conditions

Eukaryotic cells encode AMP-lysine (AMP-N-epsilon-(N-alpha-acetyl lysine methyl ester) 5'-phosphoramidate) hydrolases related to the rabbit histidine triad nucleotide-binding protein 1 (Hint1) sequence. Bacterial and archaeal cells have Hint homologs annotated in a variety of ways, but the enzymes have not been characterized, nor have phenotypes been described due to loss of enzymatic activity. We developed a quantitative (31)P NMR assay to determine whether Escherichia coli possesses an adenosine phosphoramidase activity. Indeed, soluble lysates prepared from wild-type laboratory E. coli exhibited activity on the model substrate adenosine 5'-monophosphoramidate (AMP-NH(2)). The E. coli Hint homolog, which had been comprehensively designated ycfF and is here named hinT, was cloned, overexpressed, purified, and characterized with respect to purine nucleoside phosphoramidate substrates. Bacterial hinT was several times more active than human or rabbit Hint1 on five model substrates. In addition, bacterial and mammalian enzymes preferred guanosine versus adenosine phosphoramidates as substrates. Analysis of the lysates from a constructed hinT knock-out strain of E. coli demonstrated that all of the cellular purine nucleoside phosphoramidase activity is due to hinT. Physiological analysis of this mutant revealed that the loss of hinT results in failure to grow in media containing 0.75 m KCl, 0.9 m NaCl, 0.5 m NaOAc, or 10 mm MnCl(2). Thus, cation-resistant bacterial cell growth may be dependent on the hydrolysis of adenylylated and/or guanylylated phosphoramidate substrates by hinT.

Monitoring the intracellular metabolism of nucleoside phosphoramidate pronucleotides by 31P NMR

The intracellular metabolism of 3'-azido-3'-deoxythymidine (AZT)-(L)-tryptophan methyl ester phosphoramidate (L-ATO) and AZT-(L)-phenylalanine methyl ester phosphoramidate (L-APO) by the human T-lymphoblastoid cell line CCRF-CEM (CEM-1.3) and peripheral blood mononuclear cell line (PBMC) was investigated with high field 31P NMR spectroscopy. The AZT amino acid phosphoramidates were shown to accumulate intracellularly and to be readily converted into AZT-MP by both tissues types. Thus, the efficient delivery of nucleoside monophosphates to cells can be facilitated by nucleoside phosphoramidate pronucleotides.

Small-Molecule Anthracene-Induced Cytotoxicity and Induction of Apoptosis through Generation of Reactive Oxygen Species

A series of anthracene derivatives have been synthesized, and their potential individual cytotoxicity was evaluated using Jurkat T cells and peripheral blood mononuclear cells (PBMCs) in vitro. These compounds, except for 2l, showed less cytotoxicity in PBMCs than mitoxantrone. We also analyzed the antiproliferative activity of these derivatives using the annexin V/propidium iodide assay. These synthetic compounds induced apoptosis, thus leading to antitumor effects. Compounds 2b, 2e, 2f, 2g, 2h, 2i, 2j, and mitoxantrone produced dose-dependent cytotoxicity, while the antiproliferative activity of the anthracene pharmacophore was retained in Jurkat T cells base on the detection of DNA degradation and membrane unpacking. These clearly indicate a correlation between cytotoxicity and antitumor activity. Unlike mitoxantrone, cytotoxic properties were observed, as documented by the reactivity of these novel compounds against Jurkat T cells and PBMCs as normal cells, respectively. Various concentrations of 2b, 2e, 2f, 2g, 2h, 2i, and 2j preparations also inhibited Jurkat T cell proliferation and induced apoptosis of Jurkat T cells, potentially confirmed through the detection of DNA degradation and membrane unpacking. In the present report we also investigated the antiinflammatory activity against phorbol-12-myristate-13-acetate induced superoxide anion production, a marker for an inflammatory mediator produced by neutrophils, with IC(50) (microM) values of 2b, 2h, 2l, and 2o of 4.28+/-0.89, 3.31+/-0.88, 4.38+/-0.25, and 5.45+/-1.78, respectively. These results suggest that, in addition to the specific chromosomal aberrations and cell death, elevated apoptosis could also be a marker for exposure to anthracene derivatives.

Disposition and oral bioavailability in rats of an antiviral and antitumor amino acid phosphoramidate prodrug of AZT-monophosphate

PURPOSE

The purpose of this study was to characterize the in vivo disposition of 3'-azido-2'-deoxythymidine-5'-methylamino-L-tryptophanylphosphoramidate (NMe-Trp-AZT), a potential pronucleotide of 3'-azido-2'-deoxythymidine monophosphate (AZT-MP).

METHODS

The in vitro metabolic stability of NMe-Trp-AZT was evaluated in a wide variety of tissue homogenates. NMe-Trp-AZT was administered orally (n = 3) to female Sprague-Dawley rats. Its biliary excretion and intestinal permeability were also studied.

RESULTS

Renal excretion of unchanged prodrug (16.4 +/- 5.6% of the total dose administered intravenously), its conversion to AZT (12.1 +/- 5.4% of total dose administered intravenously), and its biliary excretion (54.3 +/- 4.9% of the total dose up to 4 h after intravenous administration) accounted for most of the elimination of NMe-Trp-AZT. Significant amounts of AZT were found in both plasma and urine after oral administration of the prodrug. The prodrug itself was not permeable through the small intestinal wall but was slowly converted to AZT-MP in gastric fluids at low pH.

CONCLUSIONS

The NMe-Trp-AZT prodrug itself was not orally bioavailable because of poor intestinal permeability; however, AZT was readily available in the systemic circulation after the oral administration of the prodrug. Modification of the phosphoramidate to promote intestinal uptake should lead to enhanced oral bioavailability of this and other nucleoside phosphoramidate monoesters.

Synthesis of prodrug candidates: conjugates of amino acid with nucleoside boranophosphate

[structure: see text] Preparation of antiviral and anticancer prodrug candidates, P-tyrosinyl(P-O)-5'-P-nucleosidyl boranophosphates, is described. One-pot synthesis via a phosphoramidite method resulted in the title compounds with good yields. The P-boranophosphate diastereomers were separated by RP-HPLC, and their structures were confirmed by 1H and 31P NMR spectroscopy and MS analysis.

Pharmacokinetics of amino acid phosphoramidate monoesters of zidovudine in rats

In vitro studies have demonstrated that water-soluble, nontoxic phosphoramidates of azidothymidine (zidovudine [AZT]) have significant and specific anti-human immunodeficiency virus and anticancer activity. Although polar, these compounds are internalized and processed to the corresponding nucleoside monophosphates. Eight methyl amide and methyl ester phosphoramidate monoesters composed of D- or L-phenylalanine or tryptophan and AZT were synthesized. The plasma stability and protein binding studies were carried out in vitro. Then in vivo pharmacokinetic evaluations of six of the compounds were conducted. Sprague-Dawley rats received each compound by intravenous bolus dose, and serial blood and urine samples were collected. AZT and phosphoramidate concentrations in plasma and urine were quantitated by high-performance liquid chromatography with UV or fluorescence detection. Pharmacokinetic parameters were calculated by standard noncompartmental means. The plasma half-lives of the phosphoramidates were 10- to 20-fold longer than the half-life of AZT. Although the renal clearances of the phosphoramidates were similar to AZT, their total body clearances were significantly greater than that of AZT. The 3- to 15-fold-larger volume of distribution (Vss) for the phosphoramidates relative to AZT appeared to be dependent on the stereochemistry of the amino acid, with the largest values being associated with the L-amino acids. The increased Vss indicates a much greater tissue distribution of the phosphoramidate prodrugs than of AZT. Amino acid phosphoramidate monoesters of AZT have improved pharmacokinetic properties over AZT and significant potential as in vivo pronucleotides.

Amino acid phosphoramidate nucleosides: potential ADEPT/GDEPT substrates

A series of aromatic, serum-stable, water soluble and nontoxic amino acid phosphoramidate monoesters of 5-fluoro-2'-deoxyuridine (FUdR) and 1-beta-arabinofuranosylcytosine (Ara-C) was shown to inhibit the cellular growth of the human leukemia cell line CCRF-CEM in the presence of human prostatic acid phosphatase (hPAP).

Rational design of a new series of pronucleotide

A new pronucleotide series is described involving a two-step degradation process mediated by, respectively, carboxylesterase and phosphoramidase. Taking AZT as nucleosidyl moiety, it is shown that most of the compounds inhibit HIV replication in TK(-) cell line, which proves 5'-AZTMP delivery.