Peptide deformylase--a promising therapeutic target for tuberculosis and antibacterial drug discovery

Toward More Selective Antibiotic Inhibitors: A Structural View of the Complexed Binding Pocket of E. coli Peptide Deformylase

Peptide deformylase (PDF) is involved in bacterial protein maturation processes. Originating from the interest in a new antibiotic, tremendous effort was put into the refinement of PDF inhibitors (PDFIs) and their selectivity. We obtained a full NMR backbone assignment the emergent additional protein backbone resonances of ecPDF 1-147 in complex with 2-(5-bromo-1H-indol-3-yl)-N-hydroxyacetamide (2), a potential new structural scaffold for more selective PDFIs. We also determined the complex crystal structures of E. coli PDF (ecPDF fl) and 2. Our structure suggests an alternative ligand conformation within the protein, a possible starting point for further selectivity optimization. The orientation of the second ligand conformation in the crystal structure points toward a small region of the S1' pocket, which differs between bacterial PDFs and human PDF. Moreover, we analyzed the binding mode of 2 via NMR TITAN line shape analysis, revealing an induced fit mechanism.

Structural Insights into Antibacterial Payload Release from Gold Nanoparticles Bound to E. coli Peptide Deformylase

The lack of new antibiotics and the rapidly rising number of pathogens resistant to antibiotics pose a serious problem to mankind. In bacteria, the cell membrane provides the first line of defence to antibiotics by preventing them from reaching their molecular target. To overcome this entrance barrier, it has been suggested[1] that small Gold-Nanoparticles (AuNP) could possibly function as drug delivery systems for antibiotic ligands. Using actinonin-based ligands, we provide here proof-of-principle of AuNP functionalisation, the capability to bind and inhibit the target protein of the ligand, and the possibility to selectively release the antimicrobial payload. To this end, we successfully synthesised AuNP coated with thio-functionalised actinonin and a derivative. Interactions between 15N-enriched His-peptide deformylase 1-147 from E. coli (His-ecPDF 1-147) and compound-coated AuNP were investigated via 2D 1H-15N-HSQC NMR spectra proving the direct binding to His-ecPDF 1-147. More importantly by adding dithiothreitol (DTT), we show that the derivative is successfully released from AuNPs while still bound to His-ecPDF 1-147. Our findings indicate that AuNP-conjugated ligands can address and bind intracellular target proteins. The system introduced here presents a new delivery platform for antibiotics and allows for the easy optimisation of ligand coated AuNPs.

Therapeutic potential of human serum albumin nanoparticles encapsulated actinonin in murine model of lung adenocarcinoma

Non-small cell lung cancer comprises 85% of the global lung cancer cases. Conventional chemotherapeutics possess certain limitations like systemic toxicity and drug resistance that requires the development of new therapeutic agents for successful treatment of lung cancer. Actinonin, a human peptide deformylase inhibitor, has demonstrated anti-cancerous properties in various leukemias and solid cancer types. However, it has limited therapeutic application because of its low bioavailability and systemic toxicity if administered in free form. This limitation can be overcome by using nano-delivery systems that will increase the therapeutic efficacy of actinonin. In the present study, human serum albumin actinonin nanoparticles were prepared using a desolvation technique and folic acid was conjugated to lysine residues of albumin for effective delivery to the lung. The lung adenocarcinoma model was established 24 weeks after intraperitoneal administration of urethane and chemotherapeutic efficacy of free as well as nanoencapsulated actinonin was evaluated. This study demonstrated anti-proliferative potential of folic acid conjugated human serum albumin nanoparticles encapsulating actinonin. The intraperitoneally administered nanoformulation exhibited sustain release profile of actinonin with longer half-life and mean retention time. The reduced dose frequency resulted in therapeutic efficacy comparable to free drug in vivo in terms of 100% survival and reduced tumor burden along with downregulation of epidermal growth factor receptor, folate receptor α and peptide deformylase expression in lung adenocarcinoma mice model. Therefore, actinonin encapsulated albumin nanoparticles-based therapy holds great potential as an alternative strategy to improve its anti-cancerous activity against lung adenocarcinoma.

Design, synthesis and antibacterial activity against pathogenic mycobacteria of conjugated hydroxamic acids, hydrazides and O-alkyl/O-acyl protected hydroxamic derivatives

With the aim to discover new antituberculous molecules, three novel series of 23 hydroxamic acids, 13 hydrazides, and 9O-alkyl/O-acyl protected hydroxamic acid derivatives have been synthesized, and fully characterized by spectral ¹H NMR, ¹³C NMR, HRMS) analysis. These compounds were further biologically screened for their in vitro antibacterial activities against three pathogenic mycobacteria - M. abscessus S and R, M. marinum, and M. tuberculosis - as well as for their toxicity towards murine macrophages by the resazurin microtiter assay (REMA). Among the 45 derivatives, 17 compounds (3 hydroxamic acids, 9 hydrazides, and 5O-alkyl/O-acyl protected hydroxamic acids) were nontoxic against murine macrophages. When tested for their antibacterial activity, hydroxamic acid 9 h was found to be the most potent inhibitor against M. abscessus S and R only. Regarding hydrazide series, only 7h was active against M. abscessus R, M. marinum and M. tuberculosis; while the O-acyl protected hydroxamic acid derivatives 14d and 15d displayed promising antibacterial activity against both M. marinum and M. tuberculosis. Since such hydroxamic- and hydrazide-chelating groups have been reported to impair the activity of the peptide deformylase, in silico molecular docking studies in M. tuberculosis peptide deformylase enzyme active site were further performed with 7h in order to predict the possible interaction mode and binding energy of this molecule at the molecular level.

A Repurposing Approach for Uncovering the Anti-Tubercular Activity of FDA-Approved Drugs with Potential Multi-Targeting Profiles

Tuberculosis (TB) is one of the top 10 causes of death worldwide. This scenario is further complicated by the insurgence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) TB. The identification of appropriate drugs with multi-target affinity profiles is considered to be a widely accepted strategy to overcome the rapid development of resistance. The aim of this study was to discover Food and Drug Administration (FDA)-approved drugs possessing antimycobacterial activity, potentially coupled to an effective multi-target profile. An integrated screening platform was implemented based on computational procedures (high-throughput docking techniques on the target enzymes peptide deformylase and Zmp1) and in vitro phenotypic screening assays using two models to evaluate the activity of the selected drugs against Mycobacterium tuberculosis (Mtb), namely, growth of Mtb H37Rv and of two clinical isolates in axenic media, and infection of peripheral blood mononuclear cells with Mtb. Starting from over 3000 FDA-approved drugs, we selected 29 marketed drugs for submission to biological evaluation. Out of 29 drugs selected, 20 showed antimycobacterial activity. Further characterization suggested that five drugs possessed promising profiles for further studies. Following a repurposing strategy, by combining computational and biological efforts, we identified marketed drugs with relevant antimycobacterial profiles.

Inhibitors of Selected Bacterial Metalloenzymes

The utilization of bacterial metalloenzymes, especially ones not having mammalian (human) counterparts, has drawn attention to develop novel antibacterial agents to overcome drug resistance and especially multidrug resistance. In this review, we focus on the recent achievements on the development of inhibitors of bacterial enzymes peptide deformylase (PDF), metallo-β-lactamase (MBL), methionine aminopeptidase (MetAP) and UDP-3-O-acyl- N-acetylglucosamine deacetylase (LpxC). The state of the art of the design and investigation of inhibitors of bacterial metalloenzymes is presented, and challenges are outlined and discussed.

Lack of formylated methionyl-tRNA has pleiotropic effects on Bacillus subtilis

Bacteria initiate translation using a modified amino acid, N-formylmethionine (fMet), adapted specifically for this function. Most proteins are processed co-translationally by peptide deformylase (PDF) to remove this modification. Although PDF activity is essential in WT cells and is the target of the antibiotic actinonin, bypass mutations in the fmt gene that eliminate the formylation of Met-tRNAMet render PDF dispensable. The extent to which the emergence of fmt bypass mutations might compromise the therapeutic utility of actinonin is determined, in part, by the effects of these bypass mutations on fitness. Here, we characterize the phenotypic consequences of an fmt null mutation in the model organism Bacillus subtilis. An fmt null mutant is defective for several post-exponential phase adaptive programmes including antibiotic resistance, biofilm formation, swarming and swimming motility and sporulation. In addition, a survey of well-characterized stress responses reveals an increased sensitivity to metal ion excess and oxidative stress. These diverse phenotypes presumably reflect altered synthesis or stability of key proteins involved in these processes.

New peptides with metal binding abilities and their use as drug carriers

Many new designed molecules that target efficiently in vitro bacterial metalloproteases were completely inactive in cellulo against Gram negative bacteria. Their activities were limited by the severe restriction of the penetration/diffusion rate through the outer membrane barrier. To bypass this limitation, we have assayed the strategy of metallodrugs, to improve the delivery of hydroxamic acid inhibitors to peptide deformylase. In this metal-chaperone, to facilitate bacterial uptake, the ancillary ligand tris(2-pyridylmethyl)amine (TPA) or di(picolyl)amine (DPA) was functionalized by a tetrapeptide analogue of antimicrobial peptide, RWRW(OBn) (AA08 with TPA) and/or an efflux pump modulator PAβN (AA09 with TPA and AA27 with DPA). We prepared Co(III), Zn(II), and Cu(II) metallodrugs. Using a fluorescent hydroxamic acid, we showed that, in contrast to Cu(II) metallodrugs, Co(III) metallodrugs were stable in the Mueller Hinton (MH) broth during the time required for bacterial assays. The antibacterial activities were determined against E. coli strain wild-type (AG100) and E. coli strain deleted from acrAB efflux pump (AG100A). While none of the PDFinhs used in this study (SMP289 with an indole scaffold, AT015 and AT019 built on a 1,2,4-oxadiazole scaffold) displayed activity higher than 128 μM, all the metallodrugs were active with MICs around 8 μM both against AG100 and AG100A. However, compared to the activities of equimolar combinations of PDFinhs and the free chelating peptides (AA08, AA09, or AA27), they showed similar activities. A synergistic association between AT019 and AA08 or AA09 was determined using the fractional inhibitory concentration with AG100 and AG100A. Combinations of peptides lacking the chelating group with PDFinhs were inefficient. LC-MS analyses showed that the chelating peptides bind Zn(II) cation when incubated in MH broth. These results support the in situ formation of a zinc metallodrug, but we failed to detect it by LC-MS in MH. Nevertheless, this chelating peptides metalated with zinc act as permeabilizers which are more efficient than PAβN to facilitate the uptake of PDFinhs by Gram(-) bacteria.

Synthesis, antibacterial activity, and biological evaluation of formyl hydroxyamino derivatives as novel potent peptide deformylase inhibitors against drug-resistant bacteria

Peptide deformylase (PDF) has been identified as a promising target for novel antibacterial agents. In this study, a series of novel formyl hydroxyamino derivatives were designed and synthesized as PDF inhibitors and their antibacterial activities were evaluated. Among the potent PDF inhibitors (1o, 1q, 1o', 1q', and 1x), in vivo studies showed that compound 1q possesses mild toxicity, a good pharmacokinetic profile and protective effects. The good in vivo efficacy and low toxicity suggest that this class of compounds has potential for development and use in future antibacterial drugs.

Inhibition of human mitochondrial peptide deformylase causes apoptosis in c-myc-overexpressing hematopoietic cancers

Inhibition of human mitochondrial peptide deformylase (HsPDF) depolarizes the mitochondrial membrane, reduces mitochondrial protein translation and causes apoptosis in Burkitt's lymphoma. We showed that HsPDF mRNA and protein levels were overexpressed in cancer cells and primary acute myeloid leukemia samples. Myc regulates mitochondria and metabolism; we also demonstrated c-myc regulated the expression of HsPDF, likely indirectly. Inhibition of HsPDF by actinonin blocked mitochondrial protein translation and caused apoptotic death of myc-positive Burkitt's lymphoma, but not myc-negative B cells. Inhibition of mitochondrial translation by chloramphenicol or tetracycline, structurally different inhibitors of the mitochondrial ribosome, which is upstream of deformylase activity, followed by treatment with actinonin, resulted in reversal of the biochemical events and abrogation of the apoptosis induced by actinonin. This reversal was specific to inhibitors of HsPDF. Inhibition of HsPDF resulted in a mitochondrial unfolded protein response (increased transcription factors CHOP and CEB/P and the mitochondrial protease Lon), which may be a mechanism mediating cell death. Therefore, HsPDF may be a therapeutic target for these hematopoietic cancers, acting via a new mechanism.

Antibiotics in the clinical pipeline in 2013

The continued emergence of multi-drug-resistant bacteria is a major public health concern. The identification and development of new antibiotics, especially those with new modes of action, is imperative to help treat these infections. This review lists the 22 new antibiotics launched since 2000 and details the two first-in-class antibiotics, fidaxomicin (1) and bedaquiline (2), launched in 2011 and 2012, respectively. The development status, mode of action, spectra of activity, historical discovery and origin of the drug pharmacophore (natural product, natural product derived, synthetic or protein/mammalian peptide) of the 49 compounds and 6 β-lactamase/β-lactam combinations in active clinical development are discussed, as well as compounds that have been discontinued from clinical development since 2011. New antibacterial pharmacophore templates are also reviewed and analyzed.

Dual-action inhibitors of HIF prolyl hydroxylases that induce binding of a second iron ion

Inhibition of the hypoxia-inducible factor (HIF) prolyl hydroxylases (PHD or EGLN enzymes) is of interest for the treatment of anemia and ischemia-related diseases. Most PHD inhibitors work by binding to the single ferrous ion and competing with 2-oxoglutarate (2OG) co-substrate for binding at the PHD active site. Non-specific iron chelators also inhibit the PHDs, both in vitro and in cells. We report the identification of dual action PHD inhibitors, which bind to the active site iron and also induce the binding of a second iron ion at the active site. Following analysis of small-molecule iron complexes and application of non-denaturing protein mass spectrometry to assess PHD2·iron·inhibitor stoichiometry, selected diacylhydrazines were identified as PHD2 inhibitors that induce the binding of a second iron ion. Some compounds were shown to inhibit the HIF hydroxylases in human hepatoma and renal carcinoma cell lines.

New peptide deformylase inhibitors and cooperative interaction: a combination to improve antibacterial activity

OBJECTIVES

Bacterial drug resistance is a worrying public health problem and there is an urgent need for research and development to provide new antibacterial molecules. Peptide deformylase (PDF) is now a well-described intracellular target selected for the design of a new antibiotic group, PDF inhibitors (PDFIs). The initial bacterial susceptibility to an inhibitor of a cytoplasmic target is directly associated with the diffusion of the compound through the membrane barrier of Gram-negative bacteria and with its cytosolic accumulation at the required concentration.

METHODS

We have recently demonstrated that the activity of different PDFIs is strongly dependent on the accumulation of the active molecules by using permeabilizing agents, efflux inhibitors or efflux-mutated strains. In this work we assessed various combination protocols using different putative inhibitors (PDFIs, methionine aminopeptidase inhibitors etc.) to improve antibacterial activity against various resistant Gram-negative bacteria.

RESULTS

The maximum effect was observed when combining actinonin with a dual inhibitor of methionine aminopeptidase and PDF, this molecule being also able to interact with the target while actinonin is bound to the PDF active site.

CONCLUSIONS

Such a combination of inhibitors acting on two tightly associated metabolic steps results in a cooperative effect on bacterial cells and opens an original way to combat multidrug-resistant bacteria.

Understanding the origins of time-dependent inhibition by polypeptide deformylase inhibitors

The continual bacterial adaptation to antibiotics creates an ongoing medical need for the development of novel therapeutics. Polypeptide deformylase (PDF) is a highly conserved bacterial enzyme, which is essential for viability. It has previously been shown that PDF inhibitors represent a promising new area for the development of antimicrobial agents, and that many of the best PDF inhibitors demonstrate slow, time-dependent binding. To improve our understanding of the mechanistic origin of this time-dependent inhibition, we examined in detail the kinetics of PDF catalysis and inhibition by several different PDF inhibitors. Varying pH and solvent isotope led to clear changes in time-dependent inhibition parameters, as did inclusion of NaCl, which binds to the active site metal of PDF. Quantitative analysis of these results demonstrated that the observed time dependence arises from slow binding of the inhibitors to the active site metal. However, we also found several metal binding inhibitors that exhibited rapid, non-time-dependent onset of inhibition. By a combination of structural and chemical modification studies, we show that metal binding is only slow when the rest of the inhibitor makes optimal hydrogen bonds within the subsites of PDF. Both of these interactions between the inhibitor and enzyme were found to be necessary to observe time-dependent inhibition, as elimination of either leads to its loss.

High tolerance to mutations in a Chlamydia trachomatis peptide deformylase loop

AIM: To determine if and how a loop region in the peptide deformylase (PDF) of Chlamydia trachomatis regulates enzyme function.

METHODS: Molecular dynamics simulation was used to study a structural model of the chlamydial PDF (cPDF) and predict the temperature factor per residue for the protein backbone atoms. Site-directed mutagenesis was performed to construct cPDF variants. Catalytic properties of the resulting variants were determined by an enzyme assay using formyl-Met-Ala-Ser as a substrate.

RESULTS: In silico analysis predicted a significant increase in atomic motion in the DGELV sequence (residues 68-72) of a loop region in a cPDF mutant, which is resistant to PDF inhibitors due to two amino acid substitutions near the active site, as compared to wild-type cPDF. The D68R and D68R/E70R cPDF variants demonstrated significantly increased catalytic efficiency. The E70R mutant showed only slightly decreased efficiency. Although deletion of residues 68-72 resulted in a nearly threefold loss in substrate binding, this deficiency was compensated for by increased catalytic efficiency.

CONCLUSION: Movement of the DGELV loop region is involved in a rate-limiting conformational change of the enzyme during catalysis. However, there is no stringent sequence requirement for this region for cPDF enzyme activity.

Peptidomimetic inhibitors of bacterial peptide deformylase

A series of N-formyl hydroxylamine peptide deformylase inhibitors containing a cyclic azaamino acid moiety between the P1' and P3' substituents are presented. Selected compounds display antibacterial activity against pathogens associated with respiratory tract infections with representative compounds showing excellent MICs against Haemophilus influenzae.

Challenges of antibacterial discovery

The discovery of novel small-molecule antibacterial drugs has been stalled for many years. The purpose of this review is to underscore and illustrate those scientific problems unique to the discovery and optimization of novel antibacterial agents that have adversely affected the output of the effort. The major challenges fall into two areas: (i) proper target selection, particularly the necessity of pursuing molecular targets that are not prone to rapid resistance development, and (ii) improvement of chemical libraries to overcome limitations of diversity, especially that which is necessary to overcome barriers to bacterial entry and proclivity to be effluxed, especially in Gram-negative organisms. Failure to address these problems has led to a great deal of misdirected effort.

Synthesis and in vitro antibacterial activity of oxazolidine LBM-415 analogs as peptide deformylase inhibitors

The drug resistant bacteria pose a severe threat to human health. The increasing resistance of those pathogens to traditional antibacterial therapy renders the identification of new antibacterial agents with novel antibacterial mechanisms an urgent need. In this study, a series of (2S)-N-substituted-1-[(formyhydroxyamino)methyl]-1-oxohexyl]-2-oxazolidinecarboxamides were designed, synthesized and evaluated for in vitro antibacterial activity. Most of these compounds displayed good activities against Gram-positive organisms comparable to reference agent LBM-415.

Characterization of peptide deformylase homologues from Staphylococcus epidermidis

The emergence of multi-drug-resistant strains of Staphylococcus epidermidis emphasizes the need to develop new antibiotics. The unique and essential role of the peptide deformylase (PDF) in catalysing the removal of the N-terminal formyl group from newly synthesized polypeptides in eubacteria makes it an attractive antibacterial drug target. In the present study, both deformylase homologues from S. epidermidis (SePDF-1 and SePDF-2) were cloned and expressed, and their enzymic activities were characterized. Co(2+)-substituted SePDF-1 exhibited much higher enzymic activity (k(cat)/K(m) 6.3 × 10(4) M(-1) s(-1)) than those of Ni(2+)- and Zn(2+)-substituted SePDF-1, and SePDF-1 showed much weaker binding ability towards Ni(2+) than towards Co(2+) and Zn(2+), which is different from PDF in Staphylococcus aureus (SaPDF), although they share 80 % amino-acid sequence identity. The determined crystal structure of SePDF-1 was similar to that of (SaPDF), except for differences in the metal-binding sites. The other deformylase homologue, SePDF-2, was shown to have no peptide deformylase activity; the function of SePDF-2 needs to be further investigated.