Hui Bai

Antisense inhibition of gene expression and growth in gram-negative bacteria by cell-penetrating peptide conjugates of peptide nucleic acids targeted to rpoD gene.

Gram-negative bacteria (GNB) cause common and severe hospital- and community-acquired infections with a high incidence of multidrug resistance (MDR) and mortality. The emergence and spread of MDR-GNB strains limit therapeutic options and highlight the need to develop new therapeutic strategies. In this study, the peptide (RXR)(4)XB- and (KFF)(3)K-conjugated peptide nucleic acids (PPNAs) were developed to target rpoD, which encodes an RNA polymerase primary σ(70) that is thought to be essential for bacterial growth. Their antimicrobial activities were tested against different clinical isolates of MDR-GNB in vitro and in infection models. The (RXR)(4)XB- and (KFF)(3)K- conjugated PNAs were bactericidal against different strains of MDR-GNB in concentration-dependent and sequence-selective manner, whereas a PPNA with a scrambled base sequence had no effect on growth. Among tested PPNAs, (RXR)(4)XB conjugate PPNA06 showed more potent and broad spectrum inhibition in multidrug-resistant Escherichia coli, Salmonella enterica, Klebsiella pneumoniae, and Shigella flexneri in vitro and in vivo. The results were associated with suppression of rpoD mRNA and σ(70) expression, as well as σ(70) downstream regulated genes including ftsZ, mazF, prfB, rpoS, seqA, turfB and ygjD. The treatment of PPNA06 on mono- or multiple MDR-GBN infected human gastric mucosal epithelial cells demonstrated the complete inhibition on bacterial growth and no influence on morphology and growth of human cells. Also, PPNA06 did not show the induction of antibiotic resistance as compared with classical antibiotics in GNB. These findings firstly demonstrate that rpoD is potential target for developing antisense antibiotics, and indicate that peptide conjugates of anti-rpoD PNA are active against GNBs in vitro and in vivo. Our results offer a feasible strategy for treating MDR-GNB infections.

Targeting RNA Polymerase Primary σ70 as a Therapeutic Strategy against Methicillin-Resistant Staphylococcus aureus by Antisense Peptide Nucleic Acid

Background Methicillin-resistant Staphylococcus aureus (MRSA) causes threatening infection-related mortality worldwide. Currently, spread of multi-drug resistance (MDR) MRSA limits therapeutic options and requires new approaches to “druggable” target discovery, as well as development of novel MRSA-active antibiotics. RNA polymerase primary σ70 (encoded by gene rpoD) is a highly conserved prokaryotic factor essential for transcription initiation in exponentially growing cells of diverse S. aureus, implying potential for antisense inhibition. Methodology/Principal Findings By synthesizing a serial of cell penetrating peptide conjugated peptide nucleic acids (PPNAs) based on software predicted parameters and further design optimization, we identified a target sequence (234 to 243 nt) within rpoD mRNA conserved region 3.0 being more sensitive to antisense inhibition. A (KFF)3K peptide conjugated 10-mer complementary PNA (PPNA2332) was developed for potent micromolar-range growth inhibitory effects against four pathogenic S. aureus strains with different resistance phenotypes, including clinical vancomycin-intermediate resistance S. aureus and MDR-MRSA isolates. PPNA2332 showed bacteriocidal antisense effect at 3.2 fold of MIC value against MRSA/VISA Mu50, and its sequence specificity was demonstrated in that PPNA with scrambled PNA sequence (Scr PPNA2332) exhibited no growth inhibitory effect at higher concentrations. Also, PPNA2332 specifically interferes with rpoD mRNA, inhibiting translation of its protein product σ70 in a concentration-dependent manner. Full decay of mRNA and suppressed expression of σ70 were observed for 40 µM or 12.5 µM PPNA2332 treatment, respectively, but not for 40 µM Scr PPNA2332 treatment in pure culture of MRSA/VISA Mu50 strain. PPNA2332 (≥1 µM) essentially cleared lethal MRSA/VISA Mu50 infection in epithelial cell cultures, and eliminated viable bacterial cells in a time- and concentration- dependent manner, without showing any apparent toxicity at 10 µM. Conclusions The present result suggested that RNAP primary σ70 is a very promising candidate target for developing novel antisense antibiotic to treat severe MRSA infections.

Antisense Antibiotics: A Brief Review of Novel Target Discovery and Delivery

The nightmare of multi-drug resistant bacteria will still haunt if no panacea is ever found. Efforts on seeking desirable natural products with bactericidal property and screening chemically modified derivatives of traditional antibiotics have lagged behind the emergence of new multi-drug resistant bacteria. The concept of using antisense antibiotics, now as revolutionary as is on threshold has experienced ups and downs in the past decade. In the past five years, however, significant technology advances in the fields of microbial genomics, structural modification of oligonucleotides and efficient delivery system have led to fundamental progress in the research and in vivo application of this paradigm. The wealthy information provided in the microbial genomics era has allowed the identification and/or validation of a number of essential genes that may serve as possible targets for antisense inhibition; antisense oligodeoxynucleotides (ODNs) based on the 3rd generation of modified structures, e.g., peptide nucleic acids (PNAs) and phosphorodiamidate morpholino oligomers (PMOs) have shown great potency in gene expression inhibition in a sequence-specific and dosedependent manner at low micromolar concentrations; and cell penetrating peptide mediated delivery system has enabled the effective display of intracellular antisense inhibition of targeted genes both in vitro and in vivo. The new methods show promise in the discovery of novel gene-specific antisense antibiotics that will be useful in the future battle against drug-resistant bacterial infections. This review describes this promising paradigm, the targets that have been identified and the recent technologies on which it is delivered.

Underlying Mechanism of In vivo and In vitro Activity of C-terminal–amidated Thanatin Against Clinical Isolates of Extended-Spectrum β-lactamase–Producing Escherichia coli

BACKGROUND Infections with extended-spectrum β-lactamase-producing Escherichia coli (ESBL-EC) have developed resistance to current therapies. Therefore, the underlying mechanisms of in vivo and in vitro activity of C-terminal-amidated thanatin (A-thanatin) against clinical isolates of ESBL-EC were studied in an attempt to resolve this problem. METHODS A-thanatin was synthesized to determine its minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and kill curve for ESBL-EC. The hemolytic toxicity, stability, and resistance induction of A-thanatin were determined. ESBL-EC-infected mice were used to determine the in vivo activity of A-thanatin. Scanning and transmission electron microscopy and fluorescence microscopy were used to study the underlying mechanism of A-thanatin. RESULTS A-thanatin is highly effective against ESBL-EC in vitro, with MIC values ≤4 μg/mL. It has been confirmed that A-thanatin has little hemolysis and relative high stability in plasma. Excellent in vivo therapeutic effects were also observed in a septicemic animal model, with survival rates of 50.0%, 66.7%, and 91.7% in the low-dose, middle-dose, and high-dose groups, respectively. Membrane permeabilization may be a major biological action of A-thanatin. CONCLUSIONS Because the development of multidrug resistance limits the available therapeutic options, A-thanatin may provide a novel strategy for treating ESBL-EC infection and other infections due to multidrug-resistant bacteria.

oprM as a new target for reversion of multidrug resistance in Pseudomonas aeruginosa by antisense phosphorothioate oligodeoxynucleotides

Multidrug-resistant Pseudomonas aeruginosa (MDR-PA) is one of the leading Gram-negative organisms associated with nosocomial infections. The increasing frequency of MDR-PA has represented a huge challenge in conventional antibacterial therapy. The loss of effectiveness of commonly used antibiotics calls for the immediate need to develop an alternative strategy for combating MDR-PA infections. The multiantibiotic resistance of MDR-PA is largely attributable to the production of multidrug efflux pumps, MexAB-OprM. OprM forms the antibiotic-ejecting duct and plays a crucial role in exporting incoming chemotherapeutic agents across the membranes. Disruption of the OprM expression may inhibit the function of multidrug efflux pumps and lead to restoration of MDR-PA susceptibility to antibiotics. In this study, we developed a novel anion liposome for encapsulating and delivering specific anti-oprM phosphorothioate oligodeoxynucleotide (PS-ODN617) and polycation polyethylenimine (PEI) complexes. The additions of the encapsulated anti-oprM PS-ODN617/PEI to MDR-PA isolates caused a significant reduction of oprM expression and inhibition of MDR-PA growth in the presence of piperacillin in a concentration-dependent manner. The encapsulated PS-ODN617 treatment also reduced minimal inhibitory concentrations of five most commonly used antibiotics to the sensitive margin values on MDR-PA clinical isolates, respectively. The results of present study firstly indicate that PS-ODN targeted to oprM can significantly restore the susceptibility of MDR-PA to existing antibiotics, which appears to be a novel strategy for treating MDR-PA infections.

Promoting effects of chemical permeation enhancers on insulin permeation across TR146 cell model of buccal epithelium in vitro

To find potential enhancers for facilitating the buccal delivery of insulin, a TR146 cell-culture model of buccal epithelium, cultured on commercially available insert plates, was used to evaluate the permeability-enhancing effects of several traditional and new types of chemical enhancers, including N-acetyl-L-cysteine (NAC), sodium deoxycholate (SDC), sodium nitroprusside (SNP), reduced glutathione (GSH), glutamine (Gln), chitosan (CS), L-arginine (Arg), 1-dodecylazacycloheptan-2-one (Azone), and soybean lecithin (SPC) (50 and 10 μg/mL respectively). Permeability studies were performed to determine the enhancing effects of these compounds on insulin permeation across the cell-culture model. The enhancing effects of the enhancers were assessed by calculating the apparent permeability coefficients and enhancement ratio. Cytotoxicity of the permeation enhancers at different concentrations was investigated by using the methylthiazolydiphenyl-tetrazolium bromide (MTT) assay. Results showed that 50 μg/mL of NAC, SDC, GSH, CS, Arg, Azone, SPC, SNP, and 10 μg/mL of SNP had a significant enhancing effect on promoting the transport of insulin across the TR146 cell model. MTT assays showed that 50 μg/mL of Gln, Azone, SDC, SNP, Arg, 10 μg/mL SDC, and Arg had obvious toxic effects on TR146 cells. Therefore, NAC, GSH, CS, SPC, and SNP appear to be safe, effective permeability enhancers that promote the transport of insulin across the TR146 cell-culture model of buccal epithelium and may be potential enhancers for buccal delivery of insulin with both low toxicity and high efficiency.

Co-blockade of mecR1/blaR1 signal pathway to restore antibiotic susceptibility in clinical isolates of methicillin-resistant Staphylococcus aureus.

Introduction Methicillin-resistant Staphylococcus aureus (MRSA) is caused by the production of low-affinity penicillin-binding protein 2a and β-lactamases, which are encoded by mecA and blaZ, respectively. Expressions of the two key genes are mutually regulated by MecI and BlaI. The aim of this study was to design specific anti-mecR1 and anti-blaR1 deoxyribozymes and identify the restoration of susceptibility in MRSA isolates with mecI or blaI or no deletions by interfering with the mutual regulation of mecA and blaZ. Material and methods Specific deoxyribozymes were designed by using the program RNA structure 4.6. RNA substrates were obtained by transcription in vitro and used to assess the target cleavage of DNAzymes. Transcription of mecR1-mecA and blaR1-blaZ was analysed by real time RT-PCR. The susceptibility of MRSA was tested. Results Specific deoxyribozymes showed efficient catalytic activity to each own substrate mecR1 or blaR1 in vitro and caused the reduction of mecR1 and blaR1 transcription in vivo. Furthermore, simultaneous administration of two DNAzymes to knockdown mecR1 and blaR1 resulted in increased susceptibility of all MRSA strains tested in this study. Conclusions These results demonstrated that combined use of the two specific phosphorothioate deoxyribozymes could be a viable and promising strategy to restore the susceptibility of almost all MRSA clinical isolates.

Restoration of antibiotic susceptibility in fluoroquinolone-resistant Escherichia coli by targeting acrB with antisense phosphorothioate oligonucleotide encapsulated in novel anion liposome

Fluoroquinolone-resistant Escherichia coli (FREC) is one of the leading causes of Gram-negative bacterial infections short of effective antibiotics, thus necessitating development of novel antibacterial agents such as antisense resistance inhibitors. Aiming to restore susceptibility of FREC to fluoroquinolones by antisense inhibition of essential resistance mechanism, we designed and synthesized anion liposome encapsulated phosphorothioate oligodeoxynucleotide 831 (PS-ODN831) targeting gene acrB, which encodes the AcrAB–TolC efflux pump responsible for decreasing intercellular antibiotic concentrations. In all encapsulated PS-ODN831-treated groups, the MICs of ciprofloxacin and levofloxacin to FREC were reduced at different degrees, therefore inhibiting growth of FREC in a concentration-dependent manner. Reversion of their bactericidal effects was the result of specific and potent inhibition of acrB mRNA and the activity of efflux pump of AcrAB–TolC in FREC strains by liposome-encapsulated PS-ODN831. The study indicated that antisense targeting of AcrAB–TolC efflux pump system may be a feasible and potential strategy to treat FREC infections.

Glutathione modulation and oxidative stress in human liver slices.

Glutathione (GSH) levels are modulated in human liver slices to evaluate if drug induced liver injury is enhanced by a poor liver GSH status. Liver slice GSH levels were decreased by: 1) BSO (L-buthionine-S-sulfoximine) to inhibit GSH synthesis, and by 2) APAP (acetaminophen) which consumes GSH via conjugation to a metabolite. In this study, methimazole (MMI) liver injury was evaluated in the presence of a poor GSH status. MMI was selected because its structural thione moiety is linked with hepatotoxicity and during metabolism GSH is co-oxidized. MMI (500-1000 µM) affected oxidative stress pathways and mitochondrial function, resulting in lower liver slice GSH and ATP levels. Co-incubation of MMI with BSO or APAP led to further decreases of GSH and ATP levels in some human livers, at time points and concentrations not detected with MMI alone. Variation in human response was evident and demonstrated that some subjects with a poor liver GSH status could be further compromised with high MMI concentrations. MMI induced an up-regulation of gene expression linked with the GSH pathway, mitochondrial GSH and inflammation. Co-treatment of MMI with BSO induced a mixed response of oxidative stress related genes and an up-regulation of heat shock genes. The combination of MMI with APAP increased the expression of genes involved with oxidative stress and anti-oxidant defense, likely to protect the cells from mitochondrial injury. In summary, MMI induces oxidative stress at high concentrations, which can be augmented when liver GSH levels are decreased by the co-administration of some drugs or health status.

Comparison of microplate and macrodilution methods in time–kill study of new antimicrobial drugs

In consideration of high production costs of new antimicrobial drugs, a more convenient and economical method for time–kill study is urgently required. In the present experiment, we attempted to demonstrate the feasibility of microplate method as an alternative measure of macrodilution method for time–kill study. Three conventional antibiotics (ciprofloxacin, ceftazidime, and levofloxacin) and two antimicrobial peptides [A-thanatin and K4-S4(1–16)a] were used to determine time–kill curves against Escherichia coli ATCC 25922 and Staphylococcus epidermidis ATCC 14990. Meanwhile, both methods were also performed with three antisense peptide nucleic acids (PNA3, PNA4, and PNA5) targeting ropD gene of Staphylococcus aureus ATCC 29213 and MRSA WHO-2. In order to study the correlation between the two methods, the growth inhibition rate of PNAs, antimicrobial peptides, and antibiotics for the tested strains were evaluated. A strong agreement between the results obtained from the two methods has been demonstrated. Although microplate method required longer incubation time for a significant result than macrodilution method, the former provides a more convenient, economical, and stable way to perform time–kill test for these agents. Thus, we concluded that microplate method was an available measure for time–kill study of new antimicrobial drugs.