Bioreversible Maskierung von Nucleosiddiphosphaten

Membrane Permeant Analogs for Independent Cellular Introduction of the Terpene Precursors Isopentenyl- and Dimethylallyl-Pyrophosphate

Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) are the central five-carbon precursors to all terpenes. Despite their significance, exogenous, independent delivery of IPP and DMAPP to cells is impossible as the negatively charged pyrophosphate makes these molecules membrane impermeant. Herein, we demonstrate a facile method to circumvent this challenge through esterification of the β-phosphate with two self-immolative esters (SIEs) that neutralize the negatively charged pyrophosphate to yield membrane-permeant analogs of IPP and DMAPP. Following cellular incorporation, general esterase activity initiates cleavage of the SIEs, resulting in traceless release of IPP and DMAPP for metabolic utilization. Addition of the synthesized IPP and DMAPP precursor analogs rescued cell growth of glioblastoma (U-87MG) cancer cells concurrently treated with the HMG-CoA reductase inhibitor pitavastatin, which otherwise abrogates cell growth via blocking production of IPP and DMAPP. This work demonstrates a new application of a prodrug strategy to incorporate a metabolic intermediate and promises to enable future interrogation of the distinct biological roles of IPP and DMAPP.

Stable Isotope Phosphate Labelling of Diverse Metabolites is Enabled by a Family of 18O-Phosphoramidites

Stable isotope labelling is state-of-the-art in quantitative mass spectrometry, yet often accessing the required standards is cumbersome and very expensive. Here, a unifying synthetic concept for 18O-labelled phosphates is presented, based on a family of modified 18O2-phosphoramidite reagents. This toolbox offers access to major classes of biologically highly relevant phosphorylated metabolites as their isotopologues including nucleotides, inositol phosphates, -pyrophosphates, and inorganic polyphosphates. 18O-enrichment ratios >95 % and good yields are obtained consistently in gram-scale reactions, while enabling late-stage labelling. We demonstrate the utility of the 18O-labelled inositol phosphates and pyrophosphates by assignment of these metabolites from different biological matrices. We demonstrate that phosphate neutral loss is negligible in an analytical setup employing capillary electrophoresis electrospray ionisation triple quadrupole mass spectrometry.

Stable Isotope Phosphate Labelling of Diverse Metabolites is Enabled by a Family of 18 O-Phosphoramidites*

Stable isotope labelling is state‐of‐the‐art in quantitative mass spectrometry, yet often accessing the required standards is cumbersome and very expensive. Here, a unifying synthetic concept for ¹⁸O‐labelled phosphates is presented, based on a family of modified ¹⁸O₂‐phosphoramidite reagents. This toolbox offers access to major classes of biologically highly relevant phosphorylated metabolites as their isotopologues including nucleotides, inositol phosphates, ‐pyrophosphates, and inorganic polyphosphates. ¹⁸O‐enrichment ratios >95 % and good yields are obtained consistently in gram‐scale reactions, while enabling late‐stage labelling. We demonstrate the utility of the ¹⁸O‐labelled inositol phosphates and pyrophosphates by assignment of these metabolites from different biological matrices. We demonstrate that phosphate neutral loss is negligible in an analytical setup employing capillary electrophoresis electrospray ionisation triple quadrupole mass spectrometry.

Prodrugs of γ-Alkyl-Modified Nucleoside Triphosphates: Improved Inhibition of HIV Reverse Transcriptase

The development of nucleoside triphosphate prodrugs is one option to apply nucleoside reverse transcriptase inhibitors. Herein, we report the synthesis and evaluation of d4TTP analogues, in which the γ-phosphate was modified covalently by lipophilic alkyl residues, and acyloxybenzyl prodrugs of these γ-alkyl-modified d4TTPs, with the aim of delivering of γ-alkyl-d4TTP into cells. Selective formation of γ-alkyl-d4TTP was proven with esterase and in CD4+ -cell extracts. In contrast to d4TTP, γ-alkyl-d4TTPs proved highly stable against dephosphorylation. Primer extension assays with HIV reverse transcriptase (RT) and DNA-polymerases α, β or γ showed that γ-alkyl-d4TTPs were substrates for HIV-RT only. In antiviral assays, compounds were highly potent inhibitors of HIV-1 and HIV-2 also in thymidine-kinase-deficient T-cell cultures (CEM/TK- ). Thus, the intracellular delivery of such γ-alkyl-nucleoside triphosphates may potentially lead to nucleoside triphosphates with a higher selectivity towards the viral polymerase that can act in virus-infected cells.

γ-Non-Symmetrically Dimasked TriPPPro Prodrugs as Potential Antiviral Agents against HIV

Nucleoside analogue reverse transcriptase inhibitors (NRTI) and nucleoside analogue monophosphate prodrugs are used in combination antiretroviral therapy (cART). The design of antivirally active nucleoside triphosphate prodrugs is a recent and an important advancement in the field of nucleoside analogue drug development. Here, we report on TriPPPro‐derivatives of nucleoside analogue triphosphates (NTPs) that comprised two different acyloxybenzyl‐masks at the γ‐phosphate of the NTP aiming to achieve the metabolic bypass. Thus, γ‐non‐symmetrically dimasked TriPPPro‐compounds (γ‐(AB,ab)‐d4TTPs) were synthesized and they proved to be active against HIV‐1 and HIV‐2 in cultures of infected wild‐type human CD4⁺ T‐lymphocyte (CEM/0) cells and more importantly also in thymidine kinase‐deficient CD4⁺ T‐cells (CEM/TK‐). From hydrolysis studies both in phosphate buffer (PB, pH 7.3) and CEM cell extracts, there was surprisingly no differentiation in the cleavage of the two acyloxybenzyl prodrug‐masks. However, if within one of the two acyloxybenzyl groups a short PEG‐type methoxytriglycol group was introduced, the “standard” acyloxybenzyl‐mask was cleaved with high preference.

Prodrugs of Nucleoside Triphosphates as a Sound and Challenging Approach: A Pioneering Work That Opens a New Era in the Direct Intracellular Delivery of Nucleoside Triphosphates

Synthetic nucleosides, designed to mimic naturally occurring nucleosides, are important antiviral and anticancer chemotherapeutic agents. However, nucleosides are not active as such and need to be metabolized, step by step, to their corresponding active nucleoside triphosphates (NTPs). This is mediated by phosphorylating enzymes, mainly host cellular kinases with strong specificity for their substrates; in many cases, this specificity prevents efficient conversion into the NTPs. To circumvent this metabolic handicap, successful nucleo(s/t)ide prodrugs have been developed as a valuable concept in the design of effective drugs. The unique concept of the TriPPPro approach, developed by Chris Meier and colleagues, is a powerful tool for the intracellular delivery of active NTPs, bypassing all the phosphorylation steps required by nucleosides to yield the active NTP metabolites. This concept is illustrated herein with general examples.

In Vivo Incorporation of Azide Groups into DNA by Using Membrane-Permeable Nucleotide Triesters

Metabolic incorporation of bioorthogonal functional groups into cellular nucleic acids can be impeded by insufficient phosphorylation of nucleosides. Previous studies found that 5azidomethyl-2'-deoxyuridine (AmdU) was incorporated into the DNA of HeLa cells expressing a low-fidelity thymidine kinase, but not by wild-type HeLa cells. Here we report that membrane-permeable phosphotriester derivatives of AmdU can exhibit enhanced incorporation into the DNA of wild-type cells and animals. AmdU monophosphate derivatives bearing either 5'-bispivaloyloxymethyl (POM), 5'-bis-(4-acetoxybenzyl) (AB), or "Protide" protective groups were used to mask the phosphate group of AmdU prior to its entry into cells. The POM derivative "POM-AmdU" exhibited better chemical stability, greater metabolic incorporation efficiency, and lower toxicity than "AB-AmdU". Remarkably, the addition of POM-AmdU to the water of zebrafish larvae enabled the biosynthesis of azide-modified DNA throughout the body.

Synthesis of a Bioreversibly Masked Lipophilic Adenosine Diphosphate Ribose Derivative

The design of a bioreversibly protected lipophilic sugar nucleotide as a potential membrane-permeable precursor of adenosine diphosphate ribose (ADPR) is described. ADPR is the most potent activator of the transient receptor potential melastatin 2 (TRPM2) ion channel. Membrane-permeable, lipophilic derivatives of ADPR are of great interest as tools for study of the mechanism of TRPM2. The approach described here was based on our recently disclosed "DiPPro" and "TriPPPro" prodrug approaches developed for the intracellular delivery of nucleotides. A lipophilic, bioreversibly masked ADPR analogue containing an enzymatically cleavable 4-pentanoyloxybenzyl (PB) mask at the phosphate moiety next to the 5'-position of adenosine, together with O-acetyl groups, was prepared in high yields. Chemical and enzymatic hydrolysis studies in phosphate buffer (pH 7.3) were performed to assess chemical stability and possible (selective) enzymatic demasking of the ADPR analogue. HPLC-MS revealed that the PB group was readily cleaved enzymatically. In addition, the formation of partially deacetylated ADPR compounds and also of fully unprotected ADPR was observed.

Membrane-permeable Triphosphate Prodrugs of Nucleoside Analogues

The metabolic conversion of nucleoside analogues into their triphosphates often proceeds insufficiently. Rate-limitations can be at the mono-, but also at the di- and triphosphorylation steps. We developed a nucleoside triphosphate (NTP) delivery system (TriPPPro-approach). In this approach, NTPs are masked by two bioreversible units at the γ-phosphate. Using a procedure involving H-phosphonate chemistry, a series of derivatives bearing approved, as well as potentially antivirally active, nucleoside analogues was synthesized. The enzyme-triggered delivery of NTPs was demonstrated by pig liver esterase, in human T-lymphocyte cell extracts and by a polymerase chain reaction using a prodrug of thymidine triphosphate. The TriPPPro-compounds of some HIV-inactive nucleoside analogues showed marked anti-HIV activity. For cellular uptake studies, a fluorescent TriPPPro-compound was prepared that delivered the triphosphorylated metabolite to intact CEM cells.

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

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

A Modular Synthesis of Modified Phosphoanhydrides

Phosphoanhydrides (P-anhydrides) are ubiquitously occurring modifications in nature. Nucleotides and their conjugates, for example, are among the most important building blocks and signaling molecules in cell biology. To study and manipulate their biological functions, a diverse range of analogues have been developed. Phosphate-modified analogues have been successfully applied to study proteins that depend on these abundant cellular building blocks, but very often both the preparation and purification of these molecules are challenging. This study discloses a general access to P-anhydrides, including different nucleotide probes, that greatly facilitates their preparation and isolation. The convenient and scalable synthesis of, for example, (18) O labeled nucleoside triphosphates holds promise for future applications in phosphoproteomics.

Prometabolites of 5-Diphospho-myo-inositol Pentakisphosphate

Diphospho-myo-inositol phosphates (PP-InsP(y)) are an important class of cellular messengers. Thus far, no method for the transport of PP-InsP(y) into living cells is available. Owing to their high negative charge density, PP-InsP(y) will not cross the cell membrane. A strategy to circumvent this issue involves the generation of precursors in which the negative charges are masked with biolabile groups. A PP-InsP(y) prometabolite would require twelve to thirteen biolabile groups, which need to be cleaved by cellular enzymes to release the parent molecules. Such densely modified prometabolites of phosphate esters and anhydrides have never been reported to date. This study discloses the synthesis of such agents and an analysis of their metabolism in tissue homogenates by gel electrophoresis. The acetoxybenzyl-protected system is capable of releasing 5-PP-InsP5 in mammalian cell/tissue homogenates within a few minutes and can be used to release 5-PP-InsP5 inside cells. These molecules will serve as a platform for the development of fundamental tools required to study PP-InsP(y) physiology.

Bis(benzoyloxybenzyl)-DiPPro nucleoside diphosphates of anti-HIV active nucleoside analogues

Nucleoside analogues are extensively used as antiviral and anticancer agents. Their efficiency is dependent on their metabolism into the ultimately active nucleoside triphosphates. Often one step or even more in the metabolism of the nucleoside to the triphosphate is inefficient. To overcome this hurdle, prodrugs of the nucleotides are needed. Bis(acyloxybenzyl)nucleoside diphosphates have been reported by us as a first example of an efficient nucleoside diphosphate prodrug (DiPPro nucleotides). Here, the synthesis and the properties of bis(benzoyloxybenzyl)nucleoside diphosphates of the nucleoside analogues d4T and AZT are disclosed. The synthesis was achieved by using a phosphoramidite/oxidation route. In chemical hydrolysis studies, most of the compounds formed a nucleoside diphosphate. This was confirmed in CEM cell extracts, although the prodrug stability in extracts was lower than in phosphate buffer. Furthermore, the stability and the amount of nucleoside diphosphate formed were dependent on the substituent in the benzoyl moiety. Some of the compounds were more active against HIV in thymidine kinase-deficient CEM/TK(-) cells than were d4T or AZT.

Nucleoside mono- and diphosphate prodrugs of 2',3'-dideoxyuridine and 2',3'-dideoxy-2',3'-didehydrouridine

Despite their close structural similarity to nucleoside analogues such as the anti-HIV drugs AZT and d4T, 2',3'-dideoxyuridine (ddU) and 2',3'-dideoxy-2',3'-didehydrouridine (d4U) are entirely inactive against HIV in their nucleoside form. However, it has been shown that the corresponding triphosphates of these two nucleosides can effectively block HIV reverse transcriptase. Herein we report on two types of nucleotide prodrugs (cycloSal and DiPPro nucleotides) of ddU and d4U to investigate their ability to overcome insufficient intracellular phosphorylation, which may be the reason behind their low anti-HIV activity. The release of the corresponding mono- and diphosphates from these compounds was demonstrated by hydrolysis studies in phosphate buffer (pH 7.3) and human CD4 (+) T-lymphocyte CEM cell extracts. Surprisingly, however, these compounds showed low or no anti-HIV activity in tests with human CD4 (+) T-lymphocyte CEM cells. Studies of the conversion of ddUDP and d4UDP into their triphosphate metabolites by nucleoside diphosphate kinase (NDPK) showed nearly no conversion of either diphosphate, which may be the reason for low intracellular triphosphate levels that result in low antiviral activity.

Synthesis of densely phosphorylated bis-1,5-diphospho-myo-inositol tetrakisphosphate and its enantiomer by bidirectional P-anhydride formation

The ubiquitous mammalian signaling molecule bis-diphosphoinositol tetrakisphosphate (1,5-(PP)2 -myo-InsP4 , or InsP8 ) displays the most congested three-dimensional array of phosphate groups found in nature. The high charge density, the accumulation of unstable P-anhydrides and P-esters, the lack of UV absorbance, and low levels of optical rotation constitute severe obstacles to its synthesis, characterization, and purification. Herein, we describe the first procedure for the synthesis of enantiopure 1,5-(PP)2 -myo-InsP4 and 3,5-(PP)2 -myo-InsP4 utilizing a C2 -symmetric P-amidite for desymmetrization and concomitant phosphitylation followed by a one-pot bidirectional P-anhydride-forming reaction that combines sixteen chemical transformations with high efficiency. The configuration of these materials is unambiguously shown by subsequent X-ray analyses of both enantiomers after being individually soaked into crystals of the kinase domain of human diphosphoinositol pentakisphosphate kinase 2.

Elucidating diphosphoinositol polyphosphate function with nonhydrolyzable analogues

The diphosphoinositol polyphosphates (PP-IPs) represent a novel class of high-energy phosphate-containing messengers which control a wide variety of cellular processes. It is thought that PP-IPs exert their pleiotropic effects as allosteric regulators and through pyrophosphorylation of protein substrates. However, most details of PP-IP signaling have remained elusive because of a paucity of suitable tools. We describe the synthesis of PP-IP bisphosphonate analogues (PCP-IPs), which are resistant to chemical and biochemical degradation. While the two regioisomers 1PCP-IP5 and 5PCP-IP5 inhibited Akt phosphorylation with similar potencies, 1PCP-IP5 was much more effective at inhibiting its cognate phosphatase hDIPP1. Furthermore, the PCP analogues inhibit protein pyrophosphorylation because of their inability to transfer the β-phosphoryl group, and thus enable the distinction between PP-IP signaling mechanisms. As such, the PCP analogues will find widespread applications for the structural and biochemical characterization of PP-IP signaling properties.

The DiPPro approach: synthesis, hydrolysis, and antiviral activity of lipophilic d4T diphosphate prodrugs

Bioreversible protection of the β-phosphate group of nucleoside diphosphates (NDPs) as bis(acyloxybenzyl)phosphate esters is presented. To investigate the structure-activity relationship of these potential NDP prodrugs (DiPPro drugs) a series of DiPPro compounds was synthesized bearing fatty acids of various lengths and d4T as a model nucleoside. For synthesis of the lipophilically modified diphosphate group, preformed phosphoramidites were allowed to react with nucleotides, and the β-P(III) moiety was subsequently oxidized. The chemical and enzymatic stability of these prodrugs was studied in different media such as phosphate buffer (pH 7.3) or CEM cell extracts. In all media, the hydrolysis rate was clearly dependent on the acyl moiety and decreased with increasing alkyl chain length. The compounds showed a markedly lower half-life in cell extracts than in pH 7.3 phosphate buffer due to the presence of enzyme-catalyzed cleavage. In all media, the DiPPro compounds released d4T diphosphate (d4TDP) as the main product beside d4TMP. In antiviral assays, the compounds proved to be at least as potent as d4T against HIV-1 and 2 in wild-type CEM/0 cells. As a proof of concept, compounds with longer acyl residues showed very good anti-HIV activities in thymidine-kinase-deficient cells (CEM/TK(-) ), indicating their ability to penetrate cell membranes and the delivery of phosphorylated metabolites.

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.