Frontiers in Clinical Drug Research - Anti Infectives: Volume 4 E-Book
32,64 €
Mehr erfahren.
- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Serie: Frontiers in Clinical Drug Research - Anti Infectives
- Sprache: Englisch
Frontiers in Clinical Drug Research – Anti infectives is an eBook series that brings updated reviews to readers interested in learning about advances in the development of pharmaceutical agents for the treatment of infectious diseases. The scope of the eBook series covers a range of topics including the chemistry, pharmacology, molecular biology and biochemistry of natural and synthetic drugs employed in the treatment of infectious diseases. Reviews in this series also include research on multi drug resistance and pre-clinical / clinical findings on novel antibiotics, vaccines, antifungal agents and antitubercular agents. Frontiers in Clinical Drug Research – Anti infectives is a valuable resource for pharmaceutical scientists and postgraduate students seeking updated and critically important information for developing clinical trials and devising research plans in the field of anti infective drug discovery and epidemiology.
The fourth volume of this series features reviews that cover a variety of topics including:
-Strategies to prolong antibiotic life
-Catalytic antibodies for targeting rabies and influenza viruses
-Antiviral therapies for arenaviruses
-The role of steroids in viral infections
-Monoclonal antibody manufacturing processes
-Prevention and eradication of biofilms in medical devices
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 475
Veröffentlichungsjahr: 2017
Ähnliche
Frontiers in Clinical Drug
Research-Anti Infectives
(Volume 4)
Edited by
Atta-ur-Rahman, FRS
BENTHAM SCIENCE PUBLISHERS LTD.
End User License Agreement (for non-institutional, personal use)
This is an agreement between you and Bentham Science Publishers Ltd. Please read this License Agreement carefully before using the ebook/echapter/ejournal (“Work”). Your use of the Work constitutes your agreement to the terms and conditions set forth in this License Agreement. If you do not agree to these terms and conditions then you should not use the Work.
Bentham Science Publishers agrees to grant you a non-exclusive, non-transferable limited license to use the Work subject to and in accordance with the following terms and conditions. This License Agreement is for non-library, personal use only. For a library / institutional / multi user license in respect of the Work, please contact: [email protected].
Usage Rules:
All rights reserved: The Work is the subject of copyright and Bentham Science Publishers either owns the Work (and the copyright in it) or is licensed to distribute the Work. You shall not copy, reproduce, modify, remove, delete, augment, add to, publish, transmit, sell, resell, create derivative works from, or in any way exploit the Work or make the Work available for others to do any of the same, in any form or by any means, in whole or in part, in each case without the prior written permission of Bentham Science Publishers, unless stated otherwise in this License Agreement.You may download a copy of the Work on one occasion to one personal computer (including tablet, laptop, desktop, or other such devices). You may make one back-up copy of the Work to avoid losing it. The following DRM (Digital Rights Management) policy may also be applicable to the Work at Bentham Science Publishers’ election, acting in its sole discretion:25 ‘copy’ commands can be executed every 7 days in respect of the Work. The text selected for copying cannot extend to more than a single page. Each time a text ‘copy’ command is executed, irrespective of whether the text selection is made from within one page or from separate pages, it will be considered as a separate / individual ‘copy’ command.25 pages only from the Work can be printed every 7 days.3. The unauthorised use or distribution of copyrighted or other proprietary content is illegal and could subject you to liability for substantial money damages. You will be liable for any damage resulting from your misuse of the Work or any violation of this License Agreement, including any infringement by you of copyrights or proprietary rights.
Disclaimer:
Bentham Science Publishers does not guarantee that the information in the Work is error-free, or warrant that it will meet your requirements or that access to the Work will be uninterrupted or error-free. The Work is provided "as is" without warranty of any kind, either express or implied or statutory, including, without limitation, implied warranties of merchantability and fitness for a particular purpose. The entire risk as to the results and performance of the Work is assumed by you. No responsibility is assumed by Bentham Science Publishers, its staff, editors and/or authors for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products instruction, advertisements or ideas contained in the Work.
Limitation of Liability:
In no event will Bentham Science Publishers, its staff, editors and/or authors, be liable for any damages, including, without limitation, special, incidental and/or consequential damages and/or damages for lost data and/or profits arising out of (whether directly or indirectly) the use or inability to use the Work. The entire liability of Bentham Science Publishers shall be limited to the amount actually paid by you for the Work.
General:
Any dispute or claim arising out of or in connection with this License Agreement or the Work (including non-contractual disputes or claims) will be governed by and construed in accordance with the laws of the U.A.E. as applied in the Emirate of Dubai. Each party agrees that the courts of the Emirate of Dubai shall have exclusive jurisdiction to settle any dispute or claim arising out of or in connection with this License Agreement or the Work (including non-contractual disputes or claims).Your rights under this License Agreement will automatically terminate without notice and without the need for a court order if at any point you breach any terms of this License Agreement. In no event will any delay or failure by Bentham Science Publishers in enforcing your compliance with this License Agreement constitute a waiver of any of its rights.You acknowledge that you have read this License Agreement, and agree to be bound by its terms and conditions. To the extent that any other terms and conditions presented on any website of Bentham Science Publishers conflict with, or are inconsistent with, the terms and conditions set out in this License Agreement, you acknowledge that the terms and conditions set out in this License Agreement shall prevail.Bentham Science Publishers Ltd. Executive Suite Y - 2 PO Box 7917, Saif Zone Sharjah, U.A.E. Email: [email protected]
PREFACE
The fourth volume of Frontiers in Clinical Drug Research – Anti Infectives comprises six chapters that cover a variety of topics including, prolonging antibiotic life, biofilms in medical devices and various antiviral drugs.
In the first chapter, Tolmasky reviews pharmacological strategies that can be used to improve the efficacy of antibiotics in the body. These strategies are important in the wake of an ongoing crisis of multi-drug resistant organisms. In the second chapter, Uda et al. review the information about catalytic antibodies that can act as antiviral medicines against specific virus targets including the rabies and influenza viruses.
In chapter 3, Castilla et al. review the recent advances in drugs that target arenaviruses, pathogens that are responsible for haemorrhagic fever which is endemic to South America. In the fourth chapter, Ramírez and colleagues discuss the new role of steroids as therapeutic agents in viral infections.
In chapter 5, Freire et al., provide an overview on the production and utilization of monoclonal antibodies as therapeutic agents, including their use for treating infectious diseases. In the last chapter, Azevedo et al. discuss different proposals for combating microfilms that form in medical indwelling devices (MIDs) in the body. Advances on this front can help medical professionals and engineers in keeping such devices longer in the body. Limiting infections caused by such biofilms also has clear benefits to patients.
I hope that the readers will be pleased with the excellent and comprehensive reviews presented in this book in areas of contemporary importance.
Finally, I would like to thank all the editorial staff of Bentham Science Publishers, particularly Dr. Faryal Sami, Mr. Shehzad Naqvi and Mr. Mahmood Alam for their technical support.
Prof. Atta-ur-Rahman, FRS Honorary Life Fellow, Kings College, University of Cambridge, Cambridge, UKList of Contributors
Strategies to Prolong the Useful Life of Existing Antibiotics and Help Overcoming the Antibiotic Resistance Crisis
Marcelo E. TolmaskyAbstract
Antibiotic resistance occurs through several mechanisms that can coexist simultaneously in the same cell. The main ones include modification of the target, reduced permeability, export by active efflux pumps, sequestration of the drug by tight binding to an endogenous molecule, and enzymatic inactivation of the antibiotic molecule. The development of new antibiotics, a path currently being pursued by several laboratories using different approaches, is an obvious answer to the problem of resistance. However, this solution has not yielded the results one would expect; few new antimicrobials have been generated in the past decades. This article illustrates alternative methodologies that are being explored to produce chemicals that, although not having antimicrobial properties, act as potentiators such that combination therapies including the antibiotic and the adjuvant can overcome resistance. These compounds can achieve the expected outcome by inhibition of expression of resistance traits or interference with the function of components of resistance mechanisms.
INTRODUCTION
Evidence of attempts to find means to fight infection goes back more than a thousand years when infections were not remotely understood, but their consequences were commonly felt. However, like with other kinds of conditions, treatments were rudimentary and success elusive. However, the discovery of penicillin, and later other antibiotics, produced one of the greatest triumphs in the history of the fight between humans and disease. Infections that once were a death
warrant became treatable, and as time went by and medicine advanced, antibiotics became essential for procedures to treat other conditions. Surgery, treatment of cancer and other chronic diseases, organ transplants, orthopedic implants, or selected dentistry procedures would be impossible without the ability to control infection with antibiotics [1-7].
Unfortunately, many circumstances, some avoidable and some not, led to development and dissemination of antibiotic resistance [8, 9]. As a consequence, a growing number of pathogens are acquiring multiple resistance traits that make them able to survive and thrive in the presence of several antibiotics. Furthermore, strains resistant to all or nearly all available antibiotics have already developed [10]. The magnitude of the problem has attracted the interest of numerous organizations and governments, and it has been identified as one of the greatest threats to human health [8, 11-14].
There are several mechanisms by which bacteria resist the action of antibiotics [15]. Based on their characteristics these mechanisms are classified into three broad categories: intrinsic, adaptive, and acquired. Intrinsic resistance to an antibacterial agent is due to structural or functional features of the bacterium such as low permeability due to the structure of the cell envelope [16]. In the case of adaptive resistance, there is a temporary decrease in the susceptibility to the drug due to alterations in gene expression. A typical example of adaptive resistance is formation of a biofilm [17, 18]. Acquired resistance occurs when the cells import genetic material through the different gene transfer mechanisms such as conjugation, transduction, or transformation [19-22]. These mechanisms promote active horizontal transfer, which makes them responsible for the active dissemination of antibiotic resistance currently being observed in the clinics. Although the most obvious solution to the problem would be to find or design new classes of antibiotics, the reality is that the number of new antibiotics in development is dangerously low with respect to the speed at which bacteria acquire resistance traits [23, 24]. In addition to the quest for new classes of antibiotics other strategies are being pursued that could add ammunition against multiresistant infections. These strategies include the search for new scaffolds within existing antibiotic families [25], derivatization and modification of existing antibiotics [25-27], generation of hybrid molecules containing more than one antibiotic [28], or design of compounds that act as adjuvant or potentiators of existing antibiotics by interfering with resistance [29-32]. This article presents an overview of research advances in identifying these compounds. The magnitude of the data on the search for compounds that inhibit expression of genes involved in resistance or interfere with the action of components of resistance mechanisms is such that the information included should be considered representative rather than a comprehensive.
INHIBITION OF COMPONENTS OF RESISTANCE MECHANISMS
Efflux Pumps
Efflux pumps are systems that can transport compounds outside the cell. A remarkable characteristic of these systems is their lack of specificity for the compounds they can expel. As a consequence, an efflux pump can detoxify the cells when they are exposed to several antibiotics [33-36]. Overexpression of efflux pumps has been associated with levels of resistance of clinical relevance [36-38]. The promiscuous nature of these antibiotic resistance systems makes them serious candidates to be targeted by inhibitors.
There are five kinds of efflux pumps recognized in prokaryotes, one of them known as Resistance Nodulation Division (RND) is found only in Gram-negative bacteria and is composed of a multi-protein complex that spans the inner and outer membranes [39]. All four other groups, the major facilitator (MFS) superfamily, the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily, the small multidrug resistance (SMR) family [14], and the multidrug and toxic compound extrusion (MATE) family, are found in Gram-negative and Gram-positive bacteria [33-36]. The first report of resistance to antibiotics mediated by extrusion described a plasmid-mediated Escherichia coli system capable of transporting tetracycline outside the cell [40]. Efflux pumps described later were encoded by the chromosome as well as plasmids [41]. Therefore, efflux pump-mediated resistance can be intrinsic or acquired. In the following paragraphs, there is a description of selected advances in the quest to find inhibitors of efflux pumps. For more comprehensive descriptions the reader is referred to some recent excellent reviews [33, 34, 42-48].
Inhibitors of efflux pumps would have the potential to act as adjuvants of several antibiotic substrates of the targeted pump. About two decades ago the natural product reserpine (Fig. 1A) was shown to inhibit NorA, a Staphylococcus aureus member of the MFS family efflux pump, the most relevant in Gram-positive bacteria [37, 49]. In its presence, the minimal inhibitory concentration of fluoroquinolones was significantly reduced, and the emergence of resistant mutants was prevented [37]. Later other plant-derived inhibitors were isolated that were modified to obtain more active inhibitors [50-52]. Analysis of a collection of over hundred chalcone derivatives permitted to isolate two N,N-dimethylaminoethoxyphenyl derivatives (Fig. 1B) that showed higher than 98% inhibition of efflux in S. aureus when measured using ethidium bromide as substrate [53]. Other inhibitors of the NorA efflux pump include N-cinnamoylphenalkylamide and 3,4-dihydronaphth-2-yl-propenoic acid derivatives, the natural alkaloid piperine and synthetic analogs, substituted indols, derivatives of benzyloxybezylamine and benzimidazole, and others [43, 48, 54-56].
Overexpression of RND efflux pumps plays a major role in resistance to antimicrobials in Gram-negative pathogens [42]. Interestingly, the relevance of these efflux pumps in pathogenicity is enhanced by their additional roles in biofilm formation and as virulence factors [36, 57-59]. Therefore, inhibition of RND efflux pumps could overcome the resistance to certain antibiotics and reduce the virulence of the infecting agent. The first known class of RND efflux pump inhibitors in P. aeruginosa, which can host multiple efflux pumps (MexAB-OprM; MexCD-OprJ; MexEF-OprN; MexXY-OprM; MexJK-OprM; MexVW-OprM) [60], were derivatives of a dipeptide amide lead compound that showed 8-fold potentiation of the activity of levofloxacin [61]. The most active derivatives achieved the same level of potentiation at 5 μg/ml, and one of them was validated using a murine neutropenic thigh model of infection (Fig. 1C) [61]. Other peptidomimetics were derived that showed improved stability [62], higher activity [63], and reduced toxicity [64]. Unfortunately, in spite of the improvements achieved on these compounds, the toxicity levels could never be reduced to acceptable levels [44]. Quinoline, arylpiperazine, pyridopyrimidine, arylhydantoin, and pyranopyridine derivatives also showed inhibitory activity of RND efflux pumps. Different quinolone derivatives restored susceptibility to a variety of antibiotics including tetracyclines, chloramphenicol, and norfloxacin by interfering with the AcrAB-TolC efflux transporter in Klebsiella pneumoniae and Enterobacter aerogenes isolates [65-67]. Arylpiperazine derivatives showed inhibitory activity against both E. coli AcrAB and AcrEF efflux pumps [68]. At least one compound, 1-(1-Naphthylmethyl)-piperazine (Fig. 1D), increased susceptibility to antimicrobials with different efficiency in E. coli and other Enterobacteriaceae as well as in Vibrio cholera and Acinetobacter baumannii [69-71]. These studies included a comparison to the effect of another known inhibitor of efflux pumps, phenyl-arginine-β-naphthylamide (Fig. 1E), which showed significant differences in its effect depending on the bacterial species and drugs tested [69-71]. Derivatives of pyridopyrimidine showed inhibitory activity against the P. aeruginosa MexAB-OprM efflux pump and the lead compound, D13-9001 (Fig. 1F), potentiated the activity of levofloxacin and aztreonam [72]. A pyranopyridine derivative, MBX2319 (Fig. 1G), which inhibits the AcrAB-TolC efflux pump in E. coli and other Enterobacteriaceae, was identified through a high-throughput assay that screened 183,400 small molecules [73]. In the presence of this compound, the MICs of several antimicrobials known to be substrates of the efflux pump were significantly decreased. In addition, the compound was also active, albeit with less potency, against P. aeruginosa [73]. Structure activity relationship analyses permitted to design derivatives of MBX2319 such as compound MBX3796 (Fig. 1G), with enhanced or similar potency, but higher solubility and stability as well as tolerable toxicity [42].
Fig. (1)) Chemical structures of representative inhibitors of efflux pumps. A. Reserpine. B. Chalcone derivatives (compounds 9 and 10 in [53]). C. Dipeptide amide derivative (compound 5 in [61]). D. 1-(1-Naphthylmethyl)-piperazine. E. phenyl-arginine-β-naphthylamide. F. Pyridopyrimidine derivative (compound D13-9001 in [72]). G. Pyranopyridine derivatives (compounds MBX2319 and MBX3796 in [42, 73]).Inactivating Enzymes
Enzymatic inactivation of antibiotics by hydrolysis or addition of chemical groups to the molecule is prevalent in the clinical setting. Hundreds of β-lactamases and aminoglycoside modifying enzymes have been described [21, 74, 75]. These mechanisms of resistance are particularly problematic because these enzymes' coding genes are usually found in mobile genetic elements that confer them the capability to reach virtually all bacterial types [21, 75-78]. Per the relevance of these mechanisms, finding enzymatic inhibitors that can circumvent resistance is of critical importance.
Inhibitors of β-lactamases
β-lactam antibiotics are the most prescribed antimicrobials; they are highly effective against Gram-negative and Gram-positive infections and present low toxicity [79]. Since the introduction of these antibiotic in the clinics, bacteria started developing resistance, mainly through β-lactamase enzymes that catalyze the disruption of the β-lactam ring through a hydrolytic reaction. To tackle this problem, researchers looked for related versions of β-lactams (Fig. 2A) that were resistant to known β-lactamases or inhibitors of these enzymes that could be administered in combination with the antibiotic. However, although these strategies prolong the useful life of the antibiotics, bacteria most probably will continue developing new β-lactamase enzymes that will require the introduction of newer β-lactams and inhibitors.
Production of β-lactamases is one of the leading causes of resistance to β-lactams. The first β-lactamases were found several decades ago, and the large number and variety known to date led to different classification schemes [80, 81]. A common classification scheme considers their amino acid sequences as the criterion for grouping into four classes (A – D) [80]. The search for inhibitors of β-lactamases that can be administered in combination with the β-lactam to treat a resistant infection has been the most successful attempt to reverse resistance using this strategy. Clavulanic acid is a natural β-lactamase inhibitor produced by Streptomyces clavuligerus (Fig. 2B) [82]. It was identified several decades ago, it was the first inhibitor introduced in the clinics, and it is still in use in combination with ampicillin or ticarcillin [83]. Currently more combinations are being researched to increase the power and scope of action of clavulanic acid [84]. Other successful inhibitory compounds were penicillanic acid sulfones, two of which were approved for use in the clinics (Fig. 2B) [85, 86]. One of these inhibitors, sulbactam, is used in combination with ampicillin [87, 88], and the other, tazobactam, is combined with piperacillin, cefoperazone or ceftolozane [89-91]. One more β-lactamase inhibitor is approved for human use, the diazabicyclooctane avibactam (Fig. 2B) [92, 93], which is used in combination ceftazidime [91, 94]. Other potential inhibitors are undergoing clinical trials and could soon be introduced in the clinics [79].
Fig. (2)) Chemical structures of representative β-lactams and β-lactamase inhibitors. A. Classes of β-lactams. R, Chemical groups occupy these positions originating different antibiotics of the same group. B. Chemical structures of β-lactamase inhibitors.Inhibitors of Aminoglycoside Modifying Enzymes
Aminoglycosides are a diverse group of antibiotics that share some basic structural characteristics. They include an aminocyclitol nucleus (streptamine, 2- deoxystreptamine, streptidine, or fortamine) attached to amino sugars by glycosidic bonds (Fig. 3A). Also, other compounds that do not strictly conform to this structure are also considered aminoglycosides. An example is spectinomycin, in which the aminocyclitol is bound to the non-amino sugar actinospectose (Fig. 3A) [95]. Aminoglycosides are effective in the treatment of infections caused by aerobic Gram-negative bacteria and, in combination with other antibiotics, some Gram-positive bacteria [96]. They are also used to treat mycobacterial infections including tuberculosis [97-100]. Unfortunately, aminoglycosides present significant toxicity dangers. Nephrotoxicity, which can be reversible, is most commonly presented as nonoliguric acute kidney injury [101, 102]. Ototoxicity manifestations are permanent hearing loss and temporary vestibular hypofunction [103]. On the bright side, various drugs that could protect against these effects are being tested [104-106]. Although resistance to aminoglycoside antibiotics occurs through different mechanisms that can coexist in the same cells [75, 100, 107], phosphorylation, acetylation, or nucleotidylation mediated by aminoglycoside modifying enzymes are the most clinically relevant [30, 75, 76, 100, 108, 109]. These enzymes, aminoglycoside N-acetyltransferases, aminoglycoside O-nucleotidyltransferases, and aminoglycoside O- phosphotransferases, catalyze the addition of the chemical groups at –OH or –NH2 groups of the aminocyclitol or the sugar moieties. Although localization experiments have been limited, by the available information [110, 111], these enzymes are presumed to be cytoplasmic. Known aminoglycoside modifying enzymes are numerous, and the genes are found in a high variety of genetic environments [22]. Their high impact in clinical resistance has led researchers to try to replicate the success achieved against β-lactamases with the use of inhibitors of the resistance enzyme. The first kind of inhibitors of aminoglycoside modifying enzymes to be explored were bisubstrates, i.e., compounds in which the two substrates are covalently linked. Besides their potential as part of combination therapies, these compounds were instrumental in mechanistic and structural analyses [30, 112-120]. In spite of their robustness as inhibitors in vitro, most of these compounds showed poor ability to induce a significant reduction in levels of resistance in cells in culture, most probably due to inability to penetrate the cell envelope and reach the cytosol. Successful approaches to improve penetration and activity in cells in culture include the design of an aminoglycoside-pantetheine prodrug that is converted to a bisubstrate inhibitor in the cytosol [119] and the synthesis of truncated aminoglycoside-coenzyme A bisubstrate analogs [115]. Cationic peptides [121], non-carbohydrate diamine derivatives [122], barbituric acid analogs [123], small molecules identified by computer prediction [123, 124] or designed based on an NMR-guided fragment-based approach [125], as well as other compounds including natural products [126-128], are examples of compounds that inhibit different aminoglycoside modifying enzymes but showed modest activity in cells in culture at best. The structural relations between eukaryotic protein kinases and aminoglycoside O-phosphotransferases permitted to assay already known compounds, many of which turned out to be inhibitors of the phosphorylation of aminoglycosides [129-132]. In a recent work, two pyrazolopyrimidine derivatives (Fig. 3B) in combination with kanamycin inhibited growth of a resistant E. coli that harbored the APH(3′)-Ia aminoglycoside O-phosphotransferase [132]. Computer docking was also successful in identifying two allosteric inhibitors of the APH(3′)-IIIa and APH(2′)-IVa enzymes from a 100,000 compounds library [133]. Eis (enhanced intracellular survival) is a particular case within aminoglycoside acetyltransferases, it catalyzes acetylation at multiple locations of the antibiotics and is found in many kanamycin- and amikacin-resistant Mycobacterium tuberculosis clinical strains [134-136]. Identification of inhibitors of this enzyme was intensely pursued, and as a consequence, several different compounds and scaffolds were selected, some of them effective in vivo [137-139]. An illustrative example is a recent study that described a compound that reduced the resistance levels of a kanamycin-resistant M. tuberculosis strain to susceptibility when combined with kanamycin (Fig. 3B) [139].
Fig. (3)) Chemical structures of representative aminoglycosides and aminoglycoside modifying enzyme inhibitors. A. Three representative aminoglycosides. B. PP, two pyrazolopyrimidine derivatives; 2c, 3-(1,3- dioxolano)-2-indolinone core with an m-fluoro-phenyl substituent.Zn+2 and other metal ions were recently found to inhibit the enzymatic acetylation of aminoglycosides by some aminoglycoside N-acetyltransferases [140-142]. The ability of ZnCl2 to interfere with AAC(6′)-Ib-mediated resistance to amikacin was assessed in clinical A. baumannii, K. pneumoniae, and E. coli strains, In all cases, there was a significant reduction of the resistance levels, but the concentrations needed to observe this effect were 2 mM or higher, probably due to low permeability or the action of efflux pumps. Instead, when a coordination complex between zinc and the ionophore pyrithione (Fig. 3B), which facilitates the internalization of Zn+2 inside cells [143] was used, the concentration required for phenotypic conversion to susceptibility was reduced ~1000-fold [140, 142]. Similar results were obtained when analyzing the inhibitory characteristics of Cu+2 [140-142]. The mechanism of inhibition by metal ions remains to be confirmed. However, since a) Zn+2 and Cu+2 salts inhibit acetylation catalyzed by diverse aminoglycoside modifying enzymes [142] and b) aminoglycosides possess multiple hydroxyl and amine groups on different rings, which results in numerous potential coordination sites for metal ions [144], it is possible that inhibition occurs through protection of the substrate aminoglycoside through formation of a coordination complex with the cation.
INHIBITION OF EXPRESSION OF COMPONENTS OF RESISTANCE MECHANISMS
Antisense Technologies
Antisense inhibition of gene expression as the basis for new therapies was researched for about two decades with limited success. For a long time only one drug, fomivirsen, was approved for clinical use [145]. However, continuing efforts led to the development of mipomersen, a phosphorothioate oligonucleotide in which the sugar residues are deoxyribose in the middle and 2'-O-methoxyethyl-modified ribose at the ends of the molecule, for the treatment of homozygous familial hypercholesterolemia; and eteplirsen, a phosphorodiamidate morpholino antisense oligonucleotide for the treatment of Duchenne muscular dystrophy that works modulating splicing [146-149]. In addition, numerous compounds are presently in clinical trials [150-154]. Different laboratories are making major progress in overcoming the main challenges (development of nuclease resistant analogs and facilitation of penetration into the target cells) for reducing antisense technologies to practice [31, 152, 155-164]. As a consequence, research on antisense technologies is experiencing renewed interest and expectations. Although the advances in antisense technologies applied to prokaryotes still lag in comparison to those that target other human diseases, several strategies have been tried in bacteria with mixed results [155, 156, 165-171]. This section will describe representative examples of the utilization of antisense technologies to inhibit expression of resistance genes.
Inhibition of expression of the blaTEM gene by an antisense was first shown using a photoactivatable oligonucleotide containing psoralen monoadducts, which induce a stable cross-link within the target sequence upon activation. A 9-mer containing a psoralen 4',5'-monoadduct complementary to this gene reduced the levels of resistance to ampicillin of an E. coli strain [172, 173]. Significant levels of inhibition of expression of blaTEM and concomitantly resistance to ampicillin were also achieved using 15-mer peptide nucleic acid oligomers [174]. Peptide nucleic are promising nuclease resistant analogs that have been used to downregulate multiple genes [175]. Cell penetration of these oligomers was mainly achieved by conjugation to cell penetrating peptides but also using other methodologies such as incorporation into liposomes or in association to nanoparticles. The proposed mechanism of inhibition of expression of blaTEM by the 15-mer peptide nucleic acid oligomer was proposed to be translation steric hindrance [174]. Mono- and di-DNAzymes were also successfully used to inhibit expression of blaTEM [176]. One of the initial applications of EGS (external guide sequence) technology was to reduce expression of blaTEM [177]. EGS technology takes advantage of the endogenous RNase P ability to cleave a target RNA molecule if the appropriate structure is formed when interacting with a short antisense RNA or suitable analog [31, 155, 166, 178]. This technology was later used to inhibit expression of diverse genes [31, 165, 179] including cat [177] and aac(6')-Ib [180-183], genes specifying resistance to chloramphenicol [184] and aminoglycosides [76, 185], respectively. It is worth mentioning that some of the work done on EGS technology applied to prokaryotic systems was done transcribing the oligoribonucleotide EGS from recombinant plasmids and the endogenous RNase P-mediated degradation of a target mRNA [165, 181, 186, 187]. This general strategy, while useful to provide proof of concept, is not viable to reduce the concept to practice. For this purpose, the EGSs must be added to the cell’s environment and penetrate the cell envelope without being degraded by environ-mental or cellular nucleases. Recent progress showed that phosphorodiamidate morpholino oligonucleotides covalently bound to permeabilizer peptides and chimeric gapmers composed of locked nucleic acids at the ends and deoxyribonucleotide residues in the middle covalently bound to a permeabilizer peptide were suitable nuclease resistant analogs for EGS technology [179, 188-190]. Inhibition of expression of aac(6')-Ib and concomitantly reversal of resistance to amikacin in a clinical A. baumannii strain was also achieved using a chimeric oligonucleotide composed of 2',4'-bridge nucleic acid-NC and deoxyribonucleotide residues covalently bound to the (RXR)4XB (“X” and “B” stand for 6-aminohexanoic acid and β-alanine, respectively) cell penetrating peptide (Fig. 4) [171]. This antisense compound was designed to target a duplicated region at the N-terminus coding region of an allele of the aac(6')-Ib gene that is found in numerous Gram-negative isolates [76, 171, 191].
Restoration of susceptibility to NDM-1 mediated carbapenem resistant bacteria was achieved using a phosphorodiamidate morpholino oligomer conjugated to a cell penetrating peptide [192]. Overcoming resistance to carbapenem is crucial because of the rise of multidrug resistant pathogens for which treatment options are becoming extremely limited [193-197]. Cell penetrating peptide-conjugated phosphorodiamidate morpholino antisense oligomers were also used to reduce expression of many other genes in bacteria [198].
Fig. (4)) Chemical structure of an oligomer that targets an aac(6')-Ib allele. A. Chemical structure of a cell penetrating peptide bound to a chimeric antisense oligomer. X, 6-aminohexanoic acid; B, β-alanine; bold, 2',4'-bridge nucleic acid-NC residue. B. Chemical structure of a 2',4'-bridge nucleic acid-NC residue. C. Nucleotide and amino acid sequences of the N-terminus coding region of the aac(6')-Ib allele present in A. baumannii A155 and other bacteria [171]. The lines on top identify the regions within direct repeats that are complementary to the antisense oligomer.Resistance to oxacillin in methicillin-resistant Staphylococcus aureus (MRSA) was reversed using a sensitization strategy by two different research groups [199, 200]. MRSA strains harbor the mecA gene, which codes for a modified penicillin binding protein (PBP), known as PBP2a, that has low affinity for β-lactams. As a consequence strains producing both PBP and PBP2a resist the action of these antibiotics that only inhibit PBP [201]. A liposome-encapsulated phosphor-othioate analog [199] or a cell penetrating peptide-bound peptide nucleic acid analog [200] significantly reduced resistance to oxacillin. Similar effect was achieved using an antisense cell penetrating peptide-bound peptide nucleic acid that targets the MRSA ftsZ gene [200, 202]. The inhibition of expression of ftsZ to sensitize cells to antibiotics can be an alternative to the numerous attempts of designing and antibiotic antisense that targets ftsZ [165, 203-206]. Antisense inhibition of expression of the acrA gene, which codes for a component of the AcrAB-TolC efflux system, resulted in increased susceptibility to several antibiotics [170].
CONCLUDING REMARKS
Multidrug resistant bacterial infections are a cause of death and compromised health across the world [207, 208]. Bacterial pathogens are rapidly acquiring resistance to the existing drugs and the number of new antibiotics in development is dangerously low [23, 24]. As a result, the costs of treatment are increasing, and a growing number of patients are succumbing to these infections. Furthermore, the increase in hard to treat or even untreatable bacteria also threatens medical procedures like surgery, treatment of cancer and other chronic diseases, organ transplants, dental work, and care for premature infants [4-7]. Although there will not be replacement for the finding and design of new kinds of antibiotics, extending the life of existing ones is an essential complement in the fight to keep infectious diseases and their consequences in check. This article provided illustrative examples of the efforts by numerous research groups to find compounds that in combination with antibiotics can defeat resistant bacteria.
CONFLICT OF INTEREST
The author (editor) declares no conflict of interest, financial or otherwise.
ACKNOWLEDGEMENTS
Authors’ work cited in this review article was funded by Public Health Service Grant 2R15AI047115 from the National Institutes of Health.
