Probing the mechanism of action and decomposition of amino acid phosphomonoester amidates of antiviral nucleoside prodrugs

Impregnate Carbonation: CO2-Guided In Situ Growth of Robust Superhydrophobic Structures on Concrete Surfaces

Superhydrophobic surfaces applying on concrete can greatly improve the durability of concrete by preventing the damage from water. However, traditional design of superhydrophobic concrete surfaces by external coating encounters to problems of flaking and poor surface robustness, while that by adding hydrophobic agents or particles faces the challenges of strength damage of concrete. Drawing inspiration from the carbonation phenomenon of concrete, here a new design of in situ growing superhydrophobic structures on concrete is proposed: The concrete sample is impregnated into Mg2+-containing silane-water system with continuous CO2 injection. The contact angle of the concrete surface achieves 171.9° without obvious strength decrease after 120 min, which are mainly attributed to the formation of CaxMg1-xCO3 crystals with micro-nano-structures and the reduction of carbonates surface energy by silane. This superhydrophobic concrete structure can be divided into a superhydrophobic-hydrophobic-hydrophilic three layers structure, providing the stable water-proof protection under mechanical fatigue, capillary water absorption, UV aging, sulfate attack, and impurity water impact tests due to the in situ growing robust superhydrophobic structures. Furthermore, it captures 29.80 g m-2 CO2 during the reaction process, providing new insights for the design and preparation of eco-friendly superhydrophobic concrete.

Regiochemical Analysis of the ProTide Activation Mechanism

ProTides are nucleotide analogues used for the treatment of specific viral infections. These compounds consist of a masked nucleotide that undergoes in vivo enzymatic and spontaneous chemical transformations to generate a free mononucleotide that is ultimately transformed to the pharmaceutically active triphosphorylated drug. The three FDA approved ProTides are composed of a phosphoramidate (P-N) core coupled with a nucleoside analogue, phenol, and an l-alanyl carboxylate ester. The previously proposed mechanism of activation postulates the existence of an unstable 5-membered mixed anhydride cyclic intermediate formed from the direct attack of the carboxylate group of the l-alanyl moiety with expulsion of phenol. The mixed anhydride cyclic intermediate is further postulated to undergo spontaneous hydrolysis to form a linear l-alanyl phosphoramidate product. In the proposed mechanism of activation, the 5-membered mixed anhydride intermediate has been detected previously using mass spectrometry, but the specific site of nucleophilic attack by water (P-O versus C-O) has not been determined. To further interrogate the mechanism for hydrolysis of the putative 5-membered cyclic intermediate formed during ProTide activation, the reaction was conducted in 18O-labeled water using a ProTide analogue that could be activated by carboxypeptidase Y. Mass spectrometry and 31P NMR spectroscopy were used to demonstrate that the hydrolysis of the mixed anhydride 5-membered intermediate occurs with exclusive attack at the phosphorus center.

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.

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.

In Vitro and In Vivo Characterization of the Anti-Zika Virus Activity of ProTides of 2'-C-β-Methylguanosine

The ProTide approach has emerged as a powerful tool to improve the intracellular delivery of nucleotide analogs with antiviral and anticancer activity. Here, we characterized the anti-ZIKV (ZIKV, Zika virus) activity of two ProTides of 2'-C-β-methylguanosine. ProTide UMN-1001 is a 2'-C-β-methylguanosine tryptamine phosphoramidate monoester, and ProTide UMN-1002 is a 2-(methylthio)-ethyl-2'-C-β-methylguanosine tryptamine phosphoramidate diester. UMN-1002 undergoes stepwise intracellular activation to the corresponding nucleotide monophosphate followed by P-N bond cleavage by intracellular histidine triad nucleotide binding protein 1 (Hint1). UMN-1001 is activated by Hint1 but is less cell-permeable than UMN-1002. UMN-1001 and UMN-1002 were found to be more potent than 2'-C-β-methylguanosine against ZIKV in human-derived microvascular endothelial and neuroblastoma cells and in reducing ZIKV RNA replication. Studies with a newborn mouse model of ZIKV infection demonstrated that, while treatment with 2'-C-β-methylguanosine and UMN-1001 was lethal, treatment with UMN-1002 was nontoxic and significantly reduced ZIKV infection. Our data suggests that anchimeric activated ProTides of 2'-C-β-methyl nucleosides should be further investigated for their potential as anti-ZIKV therapeutics.

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.

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.

Toward a physical interpretation of substituent effects: the case of fluorine and trifluoromethyl groups

The application of ab initio and DFT computational methods at six different levels of theory (MP2/cc-pVDZ, MP2/aug-cc-pVTZ, B3LYP/cc-pVDZ, B3LYP/aug-cc-pVTZ, M06/cc-pVDZ, and M06/aug-cc-pVTZ) to meta- and para-substituted fluoro- and trifluoromethylbenzene derivatives and to 1-fluoro- and 1-trifluoromethyl-2-substituted trans-ethenes allowed the study of changes in the electronic and geometric properties of F- and CF3-substituted systems under the impact of other substituents (BeH, BF2, BH2, Br, CFO, CHO, Cl, CN, F, Li, NH2, NMe2, NO, NO2, OH, H, CF3, and CH3). Various parameters of these systems have been investigated, including homodesmotic reactions in terms of the substituent effect stabilization energy (SESE), the π and σ electron donor-acceptor indexes (pEDA and sEDA, respectively), the charge on the substituent active region (cSAR, known earlier as qSAR), and bond lengths, which have been regressed against Hammett constants, resulting mostly in an accurate correspondence except in the case of p-fluorobenzene derivatives. Moreover, changes in the characteristics of the ability of the substituent to attract or donate electrons under the impact of the kind of moiety to which the substituent is attached have been considered as the indirect substituent effect and investigated by means of the cSAR model. Regressions of cSAR(X) versus cSAR(Y) for any systems X and Y allow final results to be obtained on the same scale of magnitude.

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.

Treatment of breast and lung cancer cells with a N-7 benzyl guanosine monophosphate tryptamine phosphoramidate pronucleotide (4Ei-1) results in chemosensitization to gemcitabine and induced eIF4E proteasomal degradation

The development of cancer and fibrotic diseases has been shown to be highly dependent on disregulation of cap-dependent translation. Binding protein eIF4E to N(7)-methylated guanosine capped mRNA has been found to be the rate-limiting step governing translation initiation, and therefore represents an attractive target for drug discovery. Our group has found that 7-benzyl guanosine monophosphate (7Bn-GMP) is a potent antagonist of eIF4E cap binding (K(d) = 0.8 μM). Recent X-ray crystallographic studies have revealed that the cap-dependent pocket undergoes a unique structural change in order to accommodate the benzyl group. Unfortunately, 7Bn-GMP is not cell permeable. Recently, we have prepared a tryptamine phosphoramidate prodrug of 7Bn-GMP, 4Ei-1, and shown that it is a substrate for human histidine triad nucleotide binding protein (hHINT1) and inhibits eIF4E initiated epithelial-mesenchymal transition (EMT) by Zebra fish embryo cells. To assess the intracellular uptake of 4Ei-1 and conversion to 7Bn-GMP by cancer cells, we developed a sensitive assay using LC-ESI-MS/MS for the intracellular quantitation of 4Ei-1 and 7Bn-GMP. When incubated with the breast cancer cell line MDA-231 or lung cancer cell lines H460, H383 and H2009, 4Ei-1 was found to be rapidly internalized and converted to 7Bn-GMP. Since oncogenic mRNAs are predicted to have the highest eIF4E requirement for translation, we carried out chemosensitization studies with 4Ei-1. The prodrug was found to chemosensitize both breast and lung cancer cells to nontoxic levels of gemcitabine. Further mechanistic studies revealed that the expressed levels of eIF4E were substantially reduced in cells treated with 4Ei-1 in a dose-dependent manner. The levels of eI4E could be restored by treatment with the proteasome inhibitor MG-132. Taken together, our results demonstrate that 4Ei-1 is likely to inhibit translation initiation by eIF4E cap binding by both antagonizing eIF4E cap binding and initiating eIF4E proteasomal degradation.

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.

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.

Negative ion electrospray ionization mass spectrometry of nucleoside phosphoramidate monoesters: elucidation of novel rearrangement mechanisms by multistage mass spectrometry incorporating in-source deuterium labelling

Several O-2',3'-isopropylideneuridine-O-5'-phosphoramidate monoesters were synthesized and analyzed by negative ion electrospray ionization tandem mass spectrometry (ESI-MS(n)). Two kinds of novel rearrangement reactions were observed due to the difference in the amino acid in the nucleoside phosphoramidate monoesters, and possible mechanisms were proposed. One involves a five-membered cyclic transition state. The other is formation of a stable five-membered ring intermediate by Michael addition. Results were confirmed by tandem mass spectrometry and isotopically labeled hydrogen atoms. Furthermore, the internal hydrogen exchange between active hydrogen and methyl acrylate in the heated capillary of the mass spectrometer was found. The characteristic fragmentation behavior in ESI-MS may be used to monitor this kind of compounds in the biological metabolism.

Phosphoramidate pronucleotides: a comparison of the phosphoramidase substrate specificity of human and Escherichia coli histidine triad nucleotide binding proteins

To facilitate the delivery of nucleotide-based therapeutics to cells and tissues, a variety of pronucleotide approaches have been developed. Our laboratory and others have demonstrated that nucleoside phosphoramidates can be activated intracellularly to the corresponding 5'-monophosphate nucleotide and that histidine triad nucleotide binding proteins (Hints) are potentially responsible for their bioactivation. Hints are conserved and ubiquitous enzymes that hydrolyze phosphoramidate bonds between nucleoside 5'-monophosphate and an amine leaving group. On the basis of the ability of nucleosides to quench the fluorescence of covalently linked amines containing indole, a sensitive, continuous fluorescence-based assay was developed. A series of substrates linking the naturally fluorogenic indole derivatives to nucleoside 5'-monophosphates were synthesized, and their steady state kinetic parameters of hydrolysis by human Hint1 and Escherichia coli hinT were evaluated. To characterize the elemental and stereochemical effect on the reaction, two P-diastereoisomers of adenosine or guanosine phosphoramidothioates were synthesized and studied to reveal a 15-200-fold decrease in the specificity constant (kcat/Km) when the phosphoryl oxygen is replaced with sulfur. While a stereochemical preference was not observed for E. coli hinT, hHint1 exhibited a 300-fold preference for d-tryptophan phosphoramidates over l-isomers. The most efficient substrates evaluated to date are those that contain the less sterically hindering amine leaving group, tryptamine, with kcat and Km values comparable to those found for adenosine kinase. The apparent second-order rate constants (kcat/Km) for adenosine tryptamine phosphoramidate monoester were found to be 107 M-1 s-1 for hHint1 and 106 M-1 s-1 for E. coli hinT. Both the human and E. coli enzymes preferred purine over pyrimidine analogues. Consistent with observed hydrogen bonding between the 2'-OH group of adenosine monophosphate and the active site residue, Asp43, the second-order rate constant (kcat/Km) for thymidine tryptamine phosphoramidate was found to be 3-4 orders of magnitude smaller than that for uridine tryptamine phosphoramidate for hHint1 and 2 orders of magnitude smaller than that for E. coli hinT. Ara-A tryptamine phosphoramidate was, however, shown to be a good substrate with a specificity constant (kcat/Km) only 10-fold lower than the value for adenosine tryptamine phosphoramidate. Consequently, nucleoside phosphoramidates containing unhindered primary amines and either an alpha or beta 2'-OH group should be easily bioactivated by Hints with efficiencies rivaling those for the 5'-monophosphorylation of nucleosides by nucleoside kinases. The differential substrate specificity observed for human and E. coli enzymes represents a potential therapeutic rationale for the development of selective antibiotic phosphoramidate pronucleotides.

Stereochemistry of rHint1 hydrolase assisted cleavage of P–N bond in nucleoside 5′-O-phosphoramidothioates

The Hint-1 hydrolase assisted cleavage of the P-N bond in adenosine-5'-O-[N-(tryptophanylamide)]phosphoramidothioate proceeds with retention of configuration at the phosphorus atom which is consistent with the formation of a covalent enzyme-substrate complex.

AZT and AZT-monophosphate prodrugs incorporating HIV-protease substrate fragment: synthesis and evaluation as specific drug delivery systems

With the view to deliver anti-HIV nucleoside and nucleoside-monophosphate (MP) analogues specifically into HIV-infected cells, we synthesized a series of ester and phosphoramidate peptide conjugates of zidovudine (AZT) and of AZT-MP, respectively, wherein the peptide sequences derive from a HIV-protease (PR) hydrolysable substrate. Their in vitro stability with respect to hydrolysis, anti-HIV activity and cytotoxicity, and ability to inhibit the HIV-PR activity were investigated. Concerning the ester AZT-peptide conjugates, their antiviral activity level in thymidine kinase-expressing (TK+) CEM-SS and MT-4 cells was in most cases closely correlated to their hydrolysis rate: the faster the hydrolysis, the closer the anti-HIV activity to that of AZT. None of them was a HIV-PR substrate, indicating that their antiviral activity was not related to their intracellular hydrolysis by this enzyme. None of them inhibited HIV in TK-deficient (TK-) CEM cells, demonstrating that they probably act as prodrugs of AZT. Most of the phosphoramidate peptide conjugates of AZT-MP were rapidly degraded in a physiological buffer into several metabolites including AZT. Their anti-HIV activity in TK+ CEM-SS and MT-4 cells was much lower than that of AZT, indicating that only low amounts of AZT or AZT-MP were released into cells during incubation. Antiviral activities measured on TK- CEM cells for some phosphoramidates suggest that low amounts of AZT-MP could be released intracellularly. However, this AZT-MP release was not initiated by a HIV-PR hydrolysis, as no evidence for peptide cleavage was obtained by HPLC analysis of one representative compound after incubation with HIV-PR.

Synthesis and metabolism of naphthyl substituted phosphoramidate derivatives of stavudine

The synthesis of naphthylphosphoramidate derivatives of stavudine was achieved using a four-step procedure. The derivatives were subjected to several different enzymes including lipase, esterase, Subtilisin Carlsberg, and Carica papaya, and their hydrolysis rates were determined. Based on the rates of hydrolysis, we were able to differentiate between the chiralities at the phosphorus center of the phosphoramidate compounds. In addition, lipase was found to distinguish between both alpha and beta forms of the compounds. The superior chiral selectivity shown by lipase toward the naphthyl substituted phosphoramidate derivatives is attributed to the restrictive binding pocket of the lipase.

Comparison of nanogel drug carriers and their formulations with nucleoside 5'-triphosphates

PURPOSE

The aim of the study is to synthesize and characterize nanogel carriers composed of amphiphilic polymers and cationic polyethylenimine for encapsulation and delivery of cytotoxic nucleoside analogs 5'-triphosphates (NTPs) into cancer cells.

METHODS

Nanogels were synthesized by a novel micellar approach and compared with carriers prepared by the emulsification/evaporation method. Complexes of nanogels with NTP were prepared; particle size and in vitro drug release were characterized. Resistance of the nanogel-encapsulated NTP to enzymatic hydrolysis was analyzed by ion-pair high-performance liquid chromatography. Binding to isolated cellular membranes, cellular accumulation and cytotoxicity were compared using breast carcinoma cell lines CL-66, MCF-7, and MDA-MB-231. In vivo biodistribution of the 3H-labeled NTP encapsulated in different types of nanogels was evaluated in comparison to the injected NTP alone.

RESULTS

Nanogels with a particle size of 100-300 nm in the unloaded form and less than 140 nm in the NTP-loaded form were prepared. An in vitro release of NTP was >50% during the first 24 h. Nanogel formulations ensured increased NTP drug stability against enzymatic hydrolysis as compared to the drug alone. Pluronic-based nanogels NG(F68), NG(F127), NG(P85), and NGM(P123) demonstrated 2-2.5 times enhanced interaction with cellular membranes and association with various cancer cells compared to NG(PEG). Among them, NG(F68) and NG(F127) exhibited the lowest cytotoxicity. Injection of nanogel-formulated NTP significantly modulated the drug accumulation in different mouse organs.

CONCLUSIONS

Nanogels composed of Pluronic F68 and P123 were shown to display certain advanced properties compared to NG(PEG) as a drug delivery system for NTP analogs. Formulations of nucleoside analogs in active NTP form with these nanogels will improve the delivery of these cytotoxic drugs to cancer cells and the therapeutic potential of this anticancer chemotherapy.

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.

A method for quantitating the intracellular metabolism of AZT amino acid phosphoramidate pronucleotides by capillary high-performance liquid chromatography-electrospray ionization mass spectrometry

A methodology has been developed for the analysis of the intracellular metabolism of 3'-azido-3'-deoxythymidine (AZT) amino acid phosphoramidates utilizing reverse-phase high-performance liquid chromatography interfaced with negative ion electrospray ionization mass spectrometry (LC/ESI(-) -MS). The presented work demonstrates the potential of capillary LC/MS and LC/MS/MS to identify and quantitate the cellular uptake and metabolism of nucleoside phosphoramidate. Significant intracellular amounts of D- and L-phenylalanine methyl ester or D- and L-tryptophan methyl ester AZT phosphoramidates were observed for human T-lymphoblastoid leukemia (CEM) cells incubated for 2 and 4 h with the prodrugs. AZT-MP was the primary metabolite observed for human T-lymphoblastoid leukemia (CEM) cells. In this paper, the details of using LC/MS to analyze AZT amino acid phosphoramidates in biological samples are discussed. LC/MS is an efficient method for analyzing multiple samples containing several analytes in a short period of time. The method also provides high selectivity and sensitivity, and requires minimal sample preparation. This approach should be broadly applicable for the analysis of the intracellular metabolism of nucleoside prodrugs and pronucleotides.

Selective intracellular activation of a novel prodrug of the human immunodeficiency virus reverse transcriptase inhibitor tenofovir leads to preferential distribution and accumulation in lymphatic tissue

An isopropylalaninyl monoamidate phenyl monoester prodrug of tenofovir (GS 7340) was prepared, and its in vitro antiviral activity, metabolism, and pharmacokinetics in dogs were determined. The 50% effective concentration (EC(50)) of GS 7340 against human immunodeficiency virus type 1 in MT-2 cells was 0.005 microM compared to an EC(50) of 5 microM for the parent drug, tenofovir. The (L)-alaninyl analog (GS 7340) was >1,000-fold more active than the (D)-alaninyl analog. GS 7340 has a half-life of 90 min in human plasma at 37 degrees C and a half-life of 28.3 min in an MT-2 cell extract at 37 degrees C. The antiviral activity (>10 x the EC(50)) and the metabolic stability in MT-2 cell extracts (>35 x) and plasma (>2.5 x) were also sensitive to the stereochemistry at the phosphorus. After a single oral dose of GS 7340 (10 mg-eq/kg tenofovir) to male beagle dogs, the plasma bioavailability of tenofovir compared to an intravenous dose of tenofovir was 17%. The total intracellular concentration of all tenofovir species in isolated peripheral blood mononuclear cells at 24 h was 63 microg-eq/ml compared to 0.2 microg-eq/ml in plasma. A radiolabeled distribution study with dogs resulted in an increased distribution of tenofovir to tissues of lymphatic origin compared to the commercially available prodrug tenofovir DF (Viread).

Direct measurement of nucleoside monophosphate delivery from a phosphoramidate pronucleotide by stable isotope labeling and LC-ESI(-)-MS/MS

Amino acid phosphoramidates of nucleosides have been shown to be potent antiviral and anticancer agents with the potential to act as nucleoside monophosphate prodrugs. To access their ability to deliver 3'-azido-3'-deoxythymidine (AZT) 5'-monophosphate to cells, the decomposition pathway of an 18O-labeled AZT amino acid phosphoramidate was investigated by capillary reverse-phase high-performance liquid chromatography interfaced with negative ion electrospray ionization mass spectrometry (LC-ESI(-)-MS/MS). 18O-labeled L-AZT tryptophan phosphoramidate methyl ester ([18O]2) was synthesized with an 18O/16O relative ratio of 1.22 +/- 0.18. For CEM cells, a human T-lymphoblast leukemia cell line, incubated with [18O]2, values of 1.55 +/- 0.37, 0.34, and 0.13 were found for the 18O/16O relative ratio of intracellular AZT-MP for time intervals of 0.5, 4, and 20 h, respectively. The decrease in the level of labeled AZT-MP in CEM cells corresponded to a rapid increase in the amount of intracellular AZT presumably by dephosphorylation of AZT-MP. In contrast, for peripheral blood mononuclear cells (PBMCs), the 18O/16O relative ratio values of intracellular AZT-MP were 1.43, 1.06, and 0.61 for time intervals of 0.5, 4, and 20 h, respectively. Intracellular AZT in PBMCs was nearly undetectable for each time interval. Taken together, these results are consistent with the detection of direct P-N bond cleavage by CEM cells and PBMCs. However, AZT phosphoramidates are able to more effectively deliver AZT-MP to PBMCs than to CEM cells. Differential expression of 5'-nucleotidase in CEM cells relative to PBMCs is likely the reason for this discrepancy. Although applied to a phosphoramidate pronucleotide, the judicious use of 18O labeling and LC-MS is a general approach that could be applied to the investigation of the intracellular fate of other pronucleotides.

Stampidine: a selective oculo-genital microbicide

Adenoviruses (ADVs) are causative agents of severe and extremely contagious ocular and genital infections associated with conjunctivitis, genital ulcers and urethritis. Yet, no functional antiviral compounds are currently available against adenoviral infections. We discovered halogen-substituted phenyl phosphoramidate derivatives of stavudine (STV/d4T) as a new class of dual-function anti-human immunodeficiency virus (HIV) agents with potent and selective anti-ADV activity. The lead compound, stampidine [5'-(4-bromophenyl methoxyalaninylphosphate)-2',3'-didehydro-3'-deoxythymidine], was the most potent non-toxic dual-function antiviral agent. Stampidine displayed remarkable in vitro and in vivo anti-HIV activity against drug-sensitive and drug-resistant HIV strains. Stampidine was non-cytotoxic and nonirritating to mucosal epithelial cells. Several preclinical studies conducted thus far, suggest that stampidine has clinical potential as a dual-function topical agent for the prevention and/or effective treatment of oculo-genital ADV/HIV infections.

Protease-mediated enzymatic hydrolysis and activation of aryl phosphoramidate derivatives of stavudine

Several proteases are capable of hydrolyzing the aryl substituted phosphoramidate derivatives of stavudine resulting in the formation of the active metabolite, alaninyl d4T monophosphate. Subtilisin Protease A, Subtilisin Griseus, Subtilisin Carlsberg, Papaya, Bacillus were amongst the most effective proteases in hydrolyzing stavudine derivatives and specificity of their activity was confirmed using several protease inhibitors to block the hydrolysis of these phosphoramidate derivatives. We found that these proteases exhibit chiral selectivity at the phosphorus center of stavudine derivatives. Our results indicate that cellular proteases may be responsible for the activation of these phosphoramidate derivatives. In addition, we show that the enzymatic hydrolysis takes place at the carboxymethyl ester side chain of these pro-drugs and the direct attack on the phosphorus center by these enzymes does not occur. Finally, we describe a novel activation pathway hitherto unknown for the activation and viral inhibitory characteristic shown by these phosphoramidate derivatives of stavudine.

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.

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.

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.

Phenyl phosphoramidate derivatives of stavudine as anti-HIV agents with potent and selective in-vitro antiviral activity against Adenovirus

Adenoviruses are responsible for a broad range of clinical diseases that may be associated with high mortality, including pneumonia, hepatitis, encephalitis, hemorrhagic cystitis, nephritis, and gastroenteritis in immunocompromised patients, including HIV-infected individuals. Here we report the identification of halo-substituted stavudine phenyl phosphoramidate derivatives as a new class of dual-function anti-HIV agents with potent and selective anti-adenovirus (ADV) activity. We examined the investigational stavudine phenyl phosphoramidate derivative stampidine and 12 structurally similar stavudine derivatives for anti-ADV activity. All 13 derivatives of stavudine, including stampidine, were substantially more potent than stavudine and inhibited ADV-induced plaque formation at nanomolar IC(50) values. Compounds with halo substitutions in the phenyl ring as well as the unsubstituted compound 607 were more potent than compounds with methoxy, methyl, or cyano substitutions. Compound 113 (stampidine) with a 4-Br substitution and compound 609 with a 4-Cl substitution were identified as the most potent lead anti-ADV agents. Compound 113/Stampidine inhibited ADV-induced plaque formation in skin fibroblasts in a concentration-dependent fashion with a mean (+/-S.E.M.) IC(50) value of 17 +/- 2 nM without any evidence of cytotoxicity even at 100 microM. Similarly, compound 609 inhibited ADV-induced plaque formation with an IC(50) value of 27 +/- 3 nM. We next sought to determine if the lead compounds 113 and 609 can also inhibit other viruses. Both compounds exhibited potent anti-HIV activity at nanomolar concentrations. However, neither compound exhibited any antiviral activity against non-HIV viruses, including Cytomegalovirus (CMV), Type I or Type II herpes simplex viruses (HSV-1, HSV-2), enterovirus ECHO 30, or respiratory syncytial virus (RSV) (IC(50) > 100 microM). The remarkable anti-ADV potency of the lead compounds stampidine and compound 609 warrants the further development of these promising new antiviral agents for possible clinical use in ADV infected patients.

S-Acyl-2-thioethyl Phosphoramidate Diester Derivatives as Mononucleotide Prodrugs

The synthesis and in vitro anti-HIV activity of phosphoramidate diester derivatives of 3'-azido-2',3'-dideoxythymidine (AZT) bearing one S-pivaloyl-2-thioethyl (tBuSATE) group and various amino residues are reported. These compounds were obtained from an H-phosphonate strategy using an amidative oxidation step. Most of these derivatives appeared to inhibit HIV-1 replication, with EC(50) values at micromolar concentration in thymidine kinase-deficient (TK-) cells, revealing a less restrictive intracellular decomposition process than previously reported for other phosphoramidate prodrugs. The proposed decomposition pathway of this new series of mixed pronucleotides may successively involve an esterase and a phosphoramidase hydrolysis.

Are 5'-O-carbamate-2',3'-dideoxythiacytidine new anti-HIV and anti-HBV nucleoside drugs or prodrugs?

In contrast to 5'-O-carbonate 3TC derivatives (23, 24), which are clearly 3TC prodrugs, the corresponding 3TC carbamates (15-21 and 25), found to be very stable compounds with respect to enzymatic hydrolysis (cellular lysates and culture cell media) and still active on both HIV-1 and HBV infected cells, may not be 3TC prodrugs. The antiviral properties as well as the mechanism of action of 3TC analogues have been studied and evaluated.

Synthesis and hydrolysis of a phenylalanyl adenylate pentacoordinated phosphorane

Amino acid-nucleotide conjugates have important biological functions and therapeutic applications. For example, aminoacyl adenylates are key intermediates in aminoacyl tRNA synthetase reactions. They may also be involved in the prebiotic synthesis of polypeptides. Finally, various amino acid carbomethoxy aryl phosphoramidates of nucleotide prodrugs may be activated through a mechanism involving a pentacoordinated phosphorane intermediates. In order to understand better the chemistry of these compounds, a phenylalanyl adenylate pentacoodinated phosphorane has been synthesized in 72% yield and its decomposition in aqueous solution studied. Hydrolysis gave 2('),3(')-O-isopropylidene adenosine 5(')-monophosphate, 2('),3(')-O-isopropylidene adenosine, and phenylalanine. The results provide model chemistry for the enzymatic degradation mechanism of antiviral aryl amino acid phosphodiester amidates in cells, which leads to their activation.

Prodrugs of biologically active phosphate esters

Bioactivatable protecting groups represent an enormously powerful tool to increase bioavailability or to generally help deliver drugs to cells. This approach is particularly valuable in the case of biologically active phosphates because of the high intrinsic hydrophilicity and the multitude of biological functions phosphate esters exhibit inside cells. Here, the most prominent masking groups used so far are introduced. The stability and toxicology of the resulting prodrugs is discussed. Finally, this review tries to cover briefly some of the work that describes the usefulness and efficiency of the approach in various application areas.

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.

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.

Parallel solid-phase synthesis of nucleoside phosphoramidate libraries

Combinatorial chemistry is playing an increasingly prominent role in the process of drug discovery. A nucleic acid-based (NAB) scaffold can be engineered to create functional group and topological diversity in a library. Described herein is the parallel solid-phase synthesis of combinatorial libraries of nucleoside phosphoramidates, and the first evaluation of antiviral activity against hepatitis B virus (HBV).

Comparison of the antiviral activity of hydrophobic amino acid phosphoramidate monoesters of 2'3'-dideoxyadenosine (DDA) and 3'-azido-3'-deoxythymidine (AZT)

A series of hydrophobic, water soluble and non-toxic amino acid phosphoramidate monoesters of dideoxyadenosine (ddA) and 3'-azido-3'-deoxythymidine were shown to inhibit the replication of HIV-1 in human peripheral blood mononuclear cells (PBMC) from two donors. The tryptophan methyl ester phosphoramidates of AZT and ddA were equally potent (EC50S = 0.3-0.4 microM), while the phenyl methyl ester of ddA was 40- to 100- fold more potent than the AZT derivatives. The alaninyl methyl ester of AZT was found to be 70- fold more potent than the ddA derivative. The methyl amide derivatives were found to be 5-20 fold less active than the methyl esters for the ddA series, while for AZT the derivatives were found to be of similar potency or 60- to 166- fold more potent than the methylesters.

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

A series of phosphoramidate monoesters of 3'-azido-3'-deoxythymidine (AZT) bearing aliphatic amino acid methyl esters (3a, 3c, 4a, 4c, 5-7) and methyl amides (3b, 3d, 4b, 4d) was prepared and evaluated for anti-HIV-1 activity in peripheral blood mononuclear cells (PBMCs). These compounds, which showed no cytotoxicity at concentrations of 100 microM, were effective at inhibiting HIV-1 replication at concentrations of 0.08-30 microM. Since the D-phenylalanine and D-tryptophan derivatives exhibited equivalent or enhanced antiviral activity compared to their L-counterparts, there appears to be no specific stereochemical requirement for the amino acid side chain. In addition, except for the D-phenylalanine derivatives, the methyl amides had greater antiviral activity than the corresponding methyl esters. On the basis of the observed antiviral activity of AZT phosphoramidate monoesters 3a and 4a in PBMCs and CEM cells, the mechanism of action of these two compounds was investigated. AZT-MP and substantial amounts of either phosphoramidate were detected in PBMCs and CEM cells treated with either 3a or 4a. Biological mechanistic studies demonstrated that 3a and 4a affect viral replication at a stage after virus entry and preceding viral DNA integration. Quantitation of the intracellular levels of AZT-TP in PBMCs and CEM cells treated with 3a and 4a in the presence and absence of exogenous thymidine correlated the intracellular levels of AZT-TP to the antiviral activity and suggested that AZT-TP was responsible for the activity observed. In addition, the reduced toxicity of 3a and 4a toward CEM cells relative to AZT correlated with reduced levels of total phosphorylated AZT and not AZT-TP. Stable carbamate analogues of 3a and 4a were prepared and shown to inhibit the production of AZT-MP from cell-free extracts of CEM cells, further suggesting that a phosphoramidate hydrolase may be responsible for intracellular P-N bond cleavage. Taken together, these results suggest that the biological activity and intracellular metabolism of nucleoside phosphoramidate monoesters are distinct from that of phosphoramidate diesters.

Synthesis and biological activity of novel 5-fluoro-2'-deoxyuridine phosphoramidate prodrugs

A series of novel haloethyl and piperidyl phosphoramidate FdUMP prodrug analogues has been synthesized, and the growth inhibitory activity of these compounds has been evaluated against L1210 mouse leukemia cells. All compounds exhibited potent inhibition of L1210 cell proliferation with IC(50) values in the nanomolar range. Growth inhibition was reversed by the addition of 5 microM thymidine, suggesting a mechanism of action involving the intracellular release of FdUMP. (31)P NMR studies carried out on model haloethyl phosphoramidates confirm the release of nucleotide via cyclization of the phosphoramidate anion to the aziridinium ion intermediate followed by hydrolysis of the P-N bond. The data suggests that <50% of the prodrug is converted to FdUMP intracellularly by this pathway. Piperidyl phosphoramidate analogues are also converted to nucleotide intracellularly, presumably by the action of an endogenous phosphoramidase.

Pronucleotides: toward the in vivo delivery of antiviral and anticancer nucleotides

To overcome the many hurdles preventing the use of antiviral and anticancer nucleosides as therapeutics, the development of a prodrug methodology (i.e., pronucleotide) for the in vivo delivery of nucleotides has been proposed as a solution. The ideal pronucleotide should be non-toxic, stable in plasma and blood, capable of being i. v. and/or orally dosed, and intracellularly convertible to the corresponding nucleotide. Although this goal has yet to be achieved, many clever and imaginative pronucleotide approaches have been developed, which are likely to be important pharmacological tools. This review will discuss the major advances and future directions of the emerging field of antiviral and anticancer pronucleotide design and development.