Synthesis and evaluation of symmetric acyclic nucleoside bisphosphonates as inhibitors of the Plasmodium falciparum, Plasmodium vivax and human 6-oxopurine phosphoribosyltransferases and the antimalarial activity of their prodrugs

Cytotoxicity of phosphoramidate, bis-amidate and cycloSal prodrug metabolites against tumour and normal cells

Phosphonate and phosphate prodrugs are integral to enhancing drug permeability, but the potential toxicity of their metabolites requires careful consideration. This study evaluates the impact of widely used phosphoramidate, bis-amidate, and cycloSal phosph(on)ate prodrug metabolites on BxPC3 pancreatic cancer cells, GL261-Luc glioblastoma cells, and primary cultured mouse astrocytes. 1-Naphthol and 2-naphthol demonstrated the greatest toxicity. Notably, 2-naphthol exhibited an ED50 of 21 μM on BxPC3 cells, surpassing 1-naphthol with an ED50 of 82 μM. Real-time xCELLigence experiments revealed notable activity for both metabolites at a low concentration of 16 μM. On primary cultured mouse astrocyte cells, all prodrugs exhibited reduced viability at 128 to 256 μM after only 4 hours of exposure. A cell-type-dependent sensitivity to phosph(on)ate prodrug metabolites was evident, with normal cells showing greater susceptibility than corresponding tumour cells. The results suggest it is essential to consider the potential cytotoxicity of phosph(on)ate prodrugs in the drug design and evaluation process.

Dihydrofolate reductase, thymidylate synthase, and serine hydroxy methyltransferase: successful targets against some infectious diseases

Parasitic diseases have a serious impact on the world in terms of health and economics and are responsible for worldwide mortality and morbidity. The present review features the hybrid targeting involving three main enzymes for the treatment of different parasitic diseases. The enzymes Dihydrofolate reductase, thymidylate synthase, and Serine hydroxy methyltransferase play an essential role in the folate pathway. The present review focuses on these enzymes, which can be targeted against several diseases. It shed light on the past, present, and future of these targets, and it can be assessed that these targets can play a significant role against several infectious diseases. For combating viral and protozoal infectious diseases, these targets in combination should be addressed.

Stereo-Defined Acyclic Nucleoside Phosphonates are Selective and Potent Inhibitors of Parasite 6-Oxopurine Phosphoribosyltransferases

Pathogens such as Plasmodium and Trypanosoma spp. are unable to synthesize purine nucleobases. They rely on the salvage of these purines and their nucleosides from the host cell to synthesize the purine nucleotides required for DNA/RNA production. The key enzymes in this pathway are purine phosphoribosyltransferases (PRTs). Here, we synthesized 16 novel acyclic nucleoside phosphonates, 12 with a chiral center at C-2', and eight bearing a second chiral center at C-6'. Of these, bisphosphonate (S,S)-48 is the most potent inhibitor of the Plasmodium falciparum and P. vivax 6-oxopurine PRTs and the most potent inhibitor of two Trypanosoma brucei (Tbr) 6-oxopurine PRTs yet discovered, with K_i values as low as 2 nM. Crystal structures of (S,S)-48 in complex with human and Tbr 6-oxopurine PRTs show that the inhibitor binds to the enzymes in different conformations, providing an explanation for its potency and selectivity (i.e., 35-fold in favor of the parasite enzymes).

Dimeric Drugs

This review intends to summarize the structures of an extensive number of symmetrical-dimeric drugs, having two monomers linked via a bridging entity while emphasizing the large versatility of biologically active substances reported to possess dimeric structures. The largest number of classes of these compounds consist of anticancer agents, antibiotics/antimicrobials, and anti-AIDS drugs. Other symmetrical-dimeric drugs include antidiabetics, antidepressants, analgesics, anti-inflammatories, drugs for the treatment of Alzheimer's disease, anticholesterolemics, estrogenics, antioxidants, enzyme inhibitors, anti-Parkisonians, laxatives, antiallergy compounds, cannabinoids, etc. Most of the articles reviewed do not compare the activity/potency of the dimers to that of their corresponding monomers. Only in limited cases, various suggestions have been made to justify unexpected higher activity of the dimers vs. the corresponding monomers. These suggestions include statistical effects, the presence of dimeric receptors, binding of a dimer to two receptors simultaneously, and others. It is virtually impossible to predict which dimers will be preferable to their respective monomers, or which linking bridges will lead to the most active compounds. It is expected that the extensive number of articles summarized, and the large variety of substances mentioned, which display various biological activities, should be of interest to many academic and industrial medicinal chemists.

Acyclic nucleoside phosphonates with adenine nucleobase inhibit Trypanosoma brucei adenine phosphoribosyltransferase in vitro

All medically important unicellular protozoans cannot synthesize purines de novo and they entirely rely on the purine salvage pathway (PSP) for their nucleotide generation. Therefore, purine derivatives have been considered as a promising source of anti-parasitic compounds since they can act as inhibitors of the PSP enzymes or as toxic products upon their activation inside of the cell. Here, we characterized a Trypanosoma brucei enzyme involved in the salvage of adenine, the adenine phosphoribosyl transferase (APRT). We showed that its two isoforms (APRT1 and APRT2) localize partly in the cytosol and partly in the glycosomes of the bloodstream form (BSF) of the parasite. RNAi silencing of both APRT enzymes showed no major effect on the growth of BSF parasites unless grown in artificial medium with adenine as sole purine source. To add into the portfolio of inhibitors for various PSP enzymes, we designed three types of acyclic nucleotide analogs as potential APRT inhibitors. Out of fifteen inhibitors, four compounds inhibited the activity of the recombinant APRT1 with Ki in single µM values. The ANP phosphoramidate membrane-permeable prodrugs showed pronounced anti-trypanosomal activity in a cell-based assay, despite the fact that APRT enzymes are dispensable for T. brucei growth in vitro. While this suggests that the tested ANP prodrugs exert their toxicity by other means in T. brucei, the newly designed inhibitors can be further improved and explored to identify their actual target(s).

Helicobacter pylori Xanthine-Guanine-Hypoxanthine Phosphoribosyltransferase-A Putative Target for Drug Discovery against Gastrointestinal Tract Infections

Helicobacter pylori (Hp) is a human pathogen that lives in the gastric mucosa of approximately 50% of the world's population causing gastritis, peptic ulcers, and gastric cancer. An increase in resistance to current drugs has sparked the search for new Hp drug targets and therapeutics. One target is the disruption of nucleic acid production, which can be achieved by impeding the synthesis of 6-oxopurine nucleoside monophosphates, the precursors of DNA and RNA. These metabolites are synthesized by Hp xanthine-guanine-hypoxanthine phosphoribosyltransferase (XGHPRT). Here, nucleoside phosphonates have been evaluated, which inhibit the activity of this enzyme with K_i values as low as 200 nM. The prodrugs of these compounds arrest the growth of Hp at a concentration of 50 μM in cell-based assays. The kinetic properties of HpXGHPRT have been determined together with its X-ray crystal structure in the absence and presence of 9-[(N-3-phosphonopropyl)-aminomethyl-9-deazahypoxanthine, providing a basis for new antibiotic development.

Nucleotide analogues containing a pyrrolidine, piperidine or piperazine ring: Synthesis and evaluation of inhibition of plasmodial and human 6-oxopurine phosphoribosyltransferases and in vitro antimalarial activity

Parasites of the Plasmodium genus are unable to produce purine nucleotides de novo and depend completely on the salvage pathway. This fact makes plasmodial hypoxanthine-guanine-(xanthine) phosphoribosyltransferase [HG(X)PRT] a valuable target for development of antimalarial agents. A series of nucleotide analogues was designed, synthesized and evaluated as potential inhibitors of Plasmodium falciparum HGXPRT, P. vivax HGPRT and human HGPRT. These novel nucleoside phosphonates have a pyrrolidine, piperidine or piperazine ring incorporated into the linker connecting the purine base to a phosphonate group(s) and exhibited a broad range of K_i values between 0.15 and 72 μM. The corresponding phosphoramidate prodrugs, able to cross cell membranes, have been synthesized and evaluated in a P. falciparum infected human erythrocyte assay. Of the eight prodrugs evaluated seven exhibited in vitro antimalarial activity with IC₅₀ values within the range of 2.5-12.1 μM. The bis-phosphoramidate prodrug 13a with a mean (SD) IC₅₀ of 2.5 ± 0.7 μM against the chloroquine-resistant P. falciparum W2 strain exhibited low cytotoxicity in the human hepatocellular liver carcinoma (HepG2) and normal human dermal fibroblasts (NHDF) cell lines at a concentration of 100 μM suggesting good selectivity for further structure-activity relationship investigations.

β-Hydroxy- and β-Aminophosphonate Acyclonucleosides as Potent Inhibitors of Plasmodium falciparum Growth

Malaria is an infectious disease caused by a parasite of the genus Plasmodium, and the emergence of parasites resistant to all current antimalarial drugs highlights the urgency of having new classes of molecules. We developed an effective method for the synthesis of a series of β-modified acyclonucleoside phosphonate (ANP) derivatives, using commercially available and inexpensive materials (i.e., aspartic acid and purine heterocycles). Their biological evaluation in cell culture experiments and SAR revealed that the compounds' effectiveness depends on the presence of a hydroxyl group, the chain length (four carbons), and the nature of the nucleobase (guanine). The most active derivative inhibits the growth of Plasmodium falciparum in vitro in the nanomolar range (IC₅₀ = 74 nM) with high selectivity index (SI > 1350). This compound also showed remarkable in vivo activity in P. berghei-infected mice (ED₅₀ ∼ 0.5 mg/kg) when administered by the ip route and is, although less efficient, still active via the oral route. It is the first ANP derivative with such potent antimalarial activity and therefore has considerable potential for development as a new antimalarial drug.

Hypoxanthine Guanine Phosphoribosyltransferase expression is negatively correlated with immune activity through its regulation of purine synthesis

INTRODUCTION

The purpose of this study was to examine the effects of elevated Hypoxanthine Guanine Phosphoribosyltransferase (HPRT) on the immune response in the tumor microenvironment.

METHODOLOGY

HPRT expression was evaluated in cancer patients and correlated with cytokine expression, survival, and immune cell infiltration. An HPRT knockdown cell line was created to evaluate HPRT impact on purine expression and subsequent purine treatment was administered to immune cells to determine their influence on cell activation.

RESULTS

HPRT expression was negatively correlated with the general expression of both pro-inflammatory and anti-inflammatory cytokines. Additionally, HPRT expression was also negatively correlated with the infiltration of immune cell subsets: B-cells, CD4 + T cells, macrophages, neutrophils, and dendritic cells (p < 0.001) and CD8 + T-cells (p < 0.01). When HPRT was knocked down in a Raji cell line, the levels of adenosine were reduced significantly compared to the wild type. When examining the level of Ca2+ influx of Raji compared to the HPRT Raji knockdown cell, there was a significant decrease in calcium influx in the knockdown cells when compared to the wild type cells. This demonstrates that HPRT had a significant impact on overall cell activation and the ability of the cells to properly influx calcium needed for their activation.

CONCLUSIONS

We conclude that purine levels significantly reduce immune cell activation in cancer and the upregulation of HPRT in malignant tissue is a contributing factors to the immunosuppressive microenvironment.

Synthesis of Tetrasubstituted Thiophenes via Direct Metalation

Thiophene moiety can be derivatized by various synthetic procedures. The most convenient method seems to be derivatization via direct metalation, but synthesis of polysubstituted thiophenes bearing reactive groups is difficult because of high reactivity of organometallic reagents. This work reports the preparation of complex heterocyclic compounds using direct metalation of thiophenes with various reagents (Knochel-Hauser bases, LDA) as an efficient synthetic tool.

Phosphonate prodrugs: an overview and recent advances

Phosphonates, often used as isosteric replacements for phosphates, can provide important interactions with an enzyme. Due to their high charge at physiological pH, however, permeation into cells can be a challenge. Protecting phosphonates as prodrugs has shown promise in drug delivery. Thus, a variety of structures and cleavage/activation mechanisms exist, enabling release of the active compound. This review describes the structural diversity of these pro-moieties, relevant cleavage mechanisms and recent advances in the design of phosphonate prodrugs.

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.

Expedient synthesis and biological evaluation of alkenyl acyclic nucleoside phosphonate prodrugs

The importance of phosphonoamidate prodrugs (ProTides) of acyclic nucleoside phosphonate (ANPs) is highlighted by the approval of Tenofovir Alafenamide Fumarate for the treatment of HIV and HBV infections. In the present paper we are reporting an expedient, one-pot, two-steps synthesis of allyl phosphonoamidates and diamidates that offers a time saving strategy when compared to literature methods. The use of these substrates in the cross metathesis reactions with alkenyl functionalised thymine and uracil nucleobases is reported. ANPs prodrugs synthesized via this methodology were evaluated for their antiviral activities against DNA and RNA viruses. It is anticipated that the use of 5,6,7,8-tetrahydro-1-napthyl as aryloxy moiety is capable to confer antiviral activity among a series of otherwise inactive uracil ProTides.

Synthesis of α-Branched Acyclic Nucleoside Phosphonates as Potential Inhibitors of Bacterial Adenylate Cyclases

Inhibition of Bordetella pertussis adenylate cyclase toxin (ACT) and Bacillus anthracis edema factor (EF), key virulence factors with adenylate cyclase activity, represents a potential method for treating or preventing toxemia related to whooping cough and anthrax, respectively. Novel α-branched acyclic nucleoside phosphonates (ANPs) having a hemiaminal ether moiety were synthesized as potential inhibitors of bacterial adenylate cyclases. ANPs prepared as bisamidates were not cytotoxic, but did not exhibit any profound activity (IC₅₀ >10 μm) toward ACT in J774A.1 macrophages. The apparent lack of activity of the bisamidates is speculated to be due to the inefficient formation of the biologically active species (ANPpp) in the cells. Conversely, two 5-haloanthraniloyl-substituted ANPs in the form of diphosphates were shown to be potent ACT and EF inhibitors with IC₅₀ values ranging from 55 to 362 nm.

Patent Highlights June-July 2017

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.