Synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine

A Silent Operon of Photorhabdus luminescens Encodes a Prodrug Mimic of GTP

With the overmining of actinomycetes for compounds acting against Gram-negative pathogens, recent efforts to discover novel antibiotics have been focused on other groups of bacteria. Teixobactin, the first antibiotic without detectable resistance that binds lipid II, comes from an uncultured Eleftheria terra, a betaproteobacterium; odilorhabdins, from Xenorhabdus, are broad-spectrum inhibitors of protein synthesis, and darobactins from Photorhabdus target BamA, the essential chaperone of the outer membrane of Gram-negative bacteria. Xenorhabdus and Photorhabdus are symbionts of the nematode gut microbiome and attractive producers of secondary metabolites. Only small portions of their biosynthetic gene clusters (BGC) are expressed in vitro. To access their silent operons, we first separated extracts from a small library of isolates into fractions, resulting in 200-fold concentrated material, and then screened them for antimicrobial activity. This resulted in a hit with selective activity against Escherichia coli, which we identified as a novel natural product antibiotic, 3′-amino 3′-deoxyguanosine (ADG). Mutants resistant to ADG mapped to gsk and gmk, kinases of guanosine. Biochemical analysis shows that ADG is a prodrug that is converted into an active ADG triphosphate (ADG-TP), a mimic of GTP. ADG incorporates into a growing RNA chain, interrupting transcription, and inhibits cell division, apparently by interfering with the GTPase activity of FtsZ. Gsk of the purine salvage pathway, which is the first kinase in the sequential phosphorylation of ADG, is restricted to E. coli and closely related species, explaining the selectivity of the compound. There are probably numerous targets of ADG-TP among GTP-dependent proteins. The discovery of ADG expands our knowledge of prodrugs, which are rare among natural compounds.

Nonhydrolysable Analogues of (p)ppGpp and (p)ppApp Alarmone Nucleotides as Novel Molecular Tools

While alarmone nucleotides guanosine-3',5'-bisdiphosphate (ppGpp) and guanosine-5'-triphosphate-3'-diphosphate (pppGpp) are archetypical bacterial second messengers, their adenosine analogues ppApp (adenosine-3',5'-bisdiphosphate) and pppApp (adenosine-5'-triphosphate-3'-diphosphate) are toxic effectors that abrogate bacterial growth. The alarmones are both synthesized and degraded by the members of the RelA-SpoT Homologue (RSH) enzyme family. Because of the chemical and enzymatic liability of (p)ppGpp and (p)ppApp, these alarmones are prone to degradation during structural biology experiments. To overcome this limitation, we have established an efficient and straightforward procedure for synthesizing nonhydrolysable (p)ppNu_Npp analogues starting from 3'-azido-3'-deoxyribonucleotides as key intermediates. To demonstrate the utility of (p)ppG_Npp as a molecular tool, we show that (i) as an HD substrate mimic, ppG_Npp competes with ppGpp to inhibit the enzymatic activity of human MESH1 Small Alarmone Hyrolase, SAH; and (ii) mimicking the allosteric effects of (p)ppGpp, (p)ppG_Npp acts as a positive regulator of the synthetase activity of long ribosome-associated RSHs Rel and RelA. Finally, by solving the structure of the N-terminal domain region (NTD) of T. thermophilus Rel complexed with pppG_Npp, we show that as an HD substrate mimic, the analogue serves as a bona fide orthosteric regulator that promotes the same intra-NTD structural rearrangements as the native substrate.

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

BACKGROUND

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

CONCLUSION

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

Molecular self-assembly of amphiphilic cyclic phosphoryl gemcitabine with different N-fatty acyl tails and enhanced anticancer effects of the self-assembled nanostructures

Molecular self-assembly of nucleosides is important, and the self-assembled nanostructures may be used for drug delivery and targeting. Gemcitabine (GEM) is an important anticancer nucleoside analog though deactivation and multi-drug resistance frequently happen. Four N-fatty acyl derivatives of cyclic phosphoryl GEM were prepared based on the theory of self-assembled drug delivery systems and the HepDirect prodrug technique wherein the prodrug could be degraded by the cytochrome P450 isozymes in hepatocytes to release active drugs. They are cyclic phosphoryl N-octyl gemcitabine (CPOG), cyclic phosphoryl N-dodecanoyl gemcitabine (CPDG), cyclic phosphoryl N-hexadecanoyl gemcitabine (CPHG), and cyclic phosphoryl N-octadecanoyl gemcitabine (CPODG). Their amphiphilicity and self-assembling property were explored. The self-assembled nanostructures of them were prepared and simulated. The in vitro anticancer activities of the nanostructures were investigated. The derivatives formed the Langmuir monolayers at the air/water interface, though their surface pressure-molecular area isotherms were different with dependency of the length of lipid chains. The derivatives self-assembled into the vesicular or rice-like nanostructures based on the hydrophobic interaction between lipid chains when injected into water, in accordance with the results of computer simulation. The nanostructures showed higher anticancer effects on HepG2 cells than GEM. The nanostructures are promising anticancer nanomedicines.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.