ETX2514 is a broad-spectrum β-lactamase inhibitor for the treatment of drug-resistant Gram-negative bacteria including Acinetobacter baumannii

Synergistic antibacterial mechanism of Bi2Te3 nanoparticles combined with the ineffective β-lactam antibiotic cefotaxime against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) infections have become a serious threat to public health because traditional antibiotics are less efficient. Here, we developed a simple and efficient combination of Bi₂Te₃ nanoparticles (NPs) with β-lactam antibiotics cefotaxime (CTX), which presented significant synergistic antibacterial activity against MRSA. The minimal inhibitory concentration of CTX decreased from 256 to 32 μg/mL in the presence of 8 μg/mL Bi₂Te₃ NPs. The results of cell membrane potential and cellular K⁺ content measurements demonstrated that the destruction of membrane functions is a factor in the synergistic mechanism. Furthermore, the induction of cellular reactive oxygen species generation, inhibition of β-lactamases induced by CTX and direct damage to the cell structure constituted other factors of the synergistic mechanism. These observations suggest that reviving the efficacy of ineffective β-lactam antibiotic CTX by Bi₂Te₃ NPs may be a potentially effective therapeutic strategy to overcome refractory MRSA infections.

β-Lactamases and β-Lactamase Inhibitors in the 21st Century

The β-lactams retain a central place in the antibacterial armamentarium. In Gram-negative bacteria, β-lactamase enzymes that hydrolyze the amide bond of the four-membered β-lactam ring are the primary resistance mechanism, with multiple enzymes disseminating on mobile genetic elements across opportunistic pathogens such as Enterobacteriaceae (e.g., Escherichia coli) and non-fermenting organisms (e.g., Pseudomonas aeruginosa). β-Lactamases divide into four classes; the active-site serine β-lactamases (classes A, C and D) and the zinc-dependent or metallo-β-lactamases (MBLs; class B). Here we review recent advances in mechanistic understanding of each class, focusing upon how growing numbers of crystal structures, in particular for β-lactam complexes, and methods such as neutron diffraction and molecular simulations, have improved understanding of the biochemistry of β-lactam breakdown. A second focus is β-lactamase interactions with carbapenems, as carbapenem-resistant bacteria are of grave clinical concern and carbapenem-hydrolyzing enzymes such as KPC (class A) NDM (class B) and OXA-48 (class D) are proliferating worldwide. An overview is provided of the changing landscape of β-lactamase inhibitors, exemplified by the introduction to the clinic of combinations of β-lactams with diazabicyclooctanone and cyclic boronate serine β-lactamase inhibitors, and of progress and strategies toward clinically useful MBL inhibitors. Despite the long history of β-lactamase research, we contend that issues including continuing unresolved questions around mechanism; opportunities afforded by new technologies such as serial femtosecond crystallography; the need for new inhibitors, particularly for MBLs; the likely impact of new β-lactam:inhibitor combinations and the continuing clinical importance of β-lactams mean that this remains a rewarding research area.

Targeting Multidrug-Resistant Acinetobacter spp.: Sulbactam and the Diazabicyclooctenone β-Lactamase Inhibitor ETX2514 as a Novel Therapeutic Agent

Multidrug-resistant (MDR) Acinetobacter spp. poses a significant therapeutic challenge in part due to the presence of chromosomally encoded β-lactamases, including class C Acinetobacter-derived cephalosporinases (ADC) and class D oxacillinases (OXA), as well as plasmid-mediated class A β-lactamases. Importantly, OXA-like β-lactamases represent a gap in the spectrum of inhibition by recently approved β-lactamase inhibitors such as avibactam and vaborbactam. ETX2514 is a novel, rationally designed, diazabicyclooctenone inhibitor that effectively targets class A, C, and D β-lactamases. We show that addition of ETX2514 significantly increased the susceptibility of clinical Acinetobacter baumannii isolates to sulbactam. AdeB and AdeJ were identified to be key efflux constituents for ETX2514 in A. baumannii The combination of sulbactam and ETX2514 was efficacious against A. baumannii carrying bla _TEM-1, bla _ADC-82, bla _OXA-23, and bla _OXA-66 in a neutropenic murine thigh infection model. We also show that, in vitro, ETX2514 inhibited ADC-7 (k ₂/K_i 1.0 ± 0.1 × 10⁶ M^-1 s^-1) and OXA-58 (k ₂/K_i 2.5 ± 0.3 × 10⁵ M^-1 s^-1). Cocrystallization of ETX2514 with OXA-24/40 revealed hydrogen bonding interactions between ETX2514 and residues R261, S219, and S128 of OXA-24/40 in addition to a chloride ion occupied in the active site. Further, the C3 methyl group of ETX2514 shifts the position of M223. In conclusion, the sulbactam-ETX2514 combination possesses a broadened inhibitory range to include class D β-lactamases as well as class A and C β-lactamases and is a promising therapeutic candidate for infections caused by MDR Acinetobacter spp.IMPORTANCE The number and diversity of β-lactamases are steadily increasing. The emergence of β-lactamases that hydrolyze carbapenems poses a significant threat to our antibiotic armamentarium. The explosion of OXA enzymes that are carbapenem hydrolyzers is a major challenge (carbapenem-hydrolyzing class D [CHD]). An urgent need exists to discover β-lactamase inhibitors with class D activity. The sulbactam-ETX2514 combination demonstrates the potential to become a treatment regimen of choice for Acinetobacter spp. producing class D β-lactamases.

Novel β-lactam-β-lactamase inhibitor combinations: expectations for the treatment of carbapenem-resistant Gram-negative pathogens

INTRODUCTION

The burden of antimicrobial resistance among Gram-negative bacteria is increasing and growing into a major threat of public health. Treatment options for carbapenem-resistant Enterobacteriaceae are limited and resistance rates to existing compounds are mounting. The pipeline includes only a small number of novel anti-infective agents in development or in the market with promising results against multidrug-resistant (MDR) Gram-negative. Areas covered: Herein the authors present the modern available knowledge regarding novel β-lactam-β-lactamase inhibitors, i.e. mechanisms of action, in vitro activity, current PK/PDs, clinical trials and clinical efficacy against MDR and XDR Gram-negatives, as well as toxicity issues. Expert opinion: Ceftazidime-avibactam and meropenem-vaborbactam are promising therapeutic options as both are active against Enterobacteriaceae producing ESBL, AmpC, and KPC, whereas only avibactam inhibits certain class D β-lactamases, mainly OXA-48. New drugs active against Gram-negative MDR isolates including imipenem/cilastatin with relebactam and avibactam combined with aztreonam or ceftaroline are in different stages of development. However, the disadvantage to be seriously considered by the clinician is that β-lactam/β-lactamase inhibitors are ineffective against metallo-β-lactamases (with the exception of aztreonam-avibactam) as well as Acinetobacter baumannii.

New Treatment Options against Carbapenem-Resistant Acinetobacter baumannii Infections

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a perilous nosocomial pathogen causing substantial morbidity and mortality. Current treatment options for CRAB are limited and suffer from pharmacokinetic limitations, such as high toxicity and low plasma levels. As a result, CRAB is declared as the top priority pathogen by the World Health Organization for the investment in new drugs. This urgent need for new therapies, in combination with faster FDA approval process, accelerated new drug development and placed several drug candidates in the pipeline. This article reviews available information about the new drugs and other therapeutic options focusing on agents in clinical or late-stage preclinical studies for the treatment of CRAB, and it evaluates their expected benefits and potential shortcomings.

Molecules that Inhibit Bacterial Resistance Enzymes

Antibiotic resistance mediated by bacterial enzymes constitutes an unmet clinical challenge for public health, particularly for those currently used antibiotics that are recognized as "last-resort" defense against multidrug-resistant (MDR) bacteria. Inhibitors of resistance enzymes offer an alternative strategy to counter this threat. The combination of inhibitors and antibiotics could effectively prolong the lifespan of clinically relevant antibiotics and minimize the impact and emergence of resistance. In this review, we first provide a brief overview of antibiotic resistance mechanism by bacterial secreted enzymes. Furthermore, we summarize the potential inhibitors that sabotage these resistance pathways and restore the bactericidal activity of inactive antibiotics. Finally, the faced challenges and an outlook for the development of more effective and safer resistance enzyme inhibitors are discussed.

Orally Absorbed Derivatives of the β-Lactamase Inhibitor Avibactam. Design of Novel Prodrugs of Sulfate Containing Drugs

Only one FDA-approved β-lactamase inhibitor has ever been orally available: clavulanic acid, approved in 1984. Avibactam, approved by FDA in 2015, is the first of a new class of BLIs called diazabicyclooctanes, or "DBOs". This class has much broader coverage than clavulanic acid but can only be administered by intravenous injection. Herein, we describe the synthesis and testing of the first approved BLI to be rendered orally bioavailable since clavulanic acid (1984).

Infections by multidrug-resistant Gram-negative Bacteria: What's new in our arsenal and what's in the pipeline?

The spread of multidrug-resistant bacteria is an ever-growing concern, particularly among Gram-negative bacteria because of their intrinsic resistance and how quickly they acquire and spread new resistance mechanisms. Treating infections caused by Gram-negative bacteria is a challenge for medical practitioners and increases patient mortality and cost of care globally. This vulnerability, along with strategies to tackle antimicrobial resistance development, prompts the development of new antibiotic agents and exploration of alternative treatment options. This article summarises the new antibiotics that have recently been approved for Gram-negative bacterial infections, looks down the pipeline at promising agents currently in phase I, II, or III clinical trials, and introduces new alternative avenues that show potential in combating multidrug-resistant Gram-negative bacteria.

Plasma and Intrapulmonary Concentrations of ETX2514 and Sulbactam following Intravenous Administration of ETX2514SUL to Healthy Adult Subjects

ETX2514 is a novel β-lactamase inhibitor that broadly inhibits Ambler class A, C, and D β-lactamases. ETX2514 combined with sulbactam (SUL) in vitro restores sulbactam activity against Acinetobacter baumannii ETX2514-sulbactam (ETX2514SUL) is under development for the treatment of A. baumannii infections. The objective of this study was to determine and compare plasma, epithelial lining fluid (ELF), and alveolar macrophage (AM) concentrations following intravenous (i.v.) ETX2514 and sulbactam. Plasma, ELF, and AM concentrations of ETX2514 and sulbactam were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in 30 healthy adult subjects following repeated dosing (ETX2514 [1 g] and sulbactam [1 g] every 6 h [q6h], as a 3-h i.v. infusion, for a total of 3 doses). A bronchoalveolar lavage (BAL) was performed once in each subject at either 1, 2.5, 3.25, 4, or 6 h after the start of the last infusion. Penetration ratios were calculated from area under the concentration-time curve from 0 to 6 h (AUC_0-6) values for total plasma and ELF using mean and median concentrations at the BAL fluid sampling times. Respective ELF AUC_0-6 values, based on mean and median concentrations, were 40.1 and 39.4 mg · h/liter for ETX2514 and 34.7 and 34.5 mg · h/liter for sulbactam. Respective penetration ratios of ELF to total/unbound plasma concentrations, based on mean and median AUC_0-6 values, of ETX2514 were 0.37/0.41 and 0.36/0.40, whereas these same ratio values were 0.50/0.81 and 0.50/0.80 for sulbactam. ETX2514 and sulbactam concentrations in AM were measurable and fairly constant throughout the dosing interval (median values of 1.31 and 1.01 mg/liter, respectively). These data support further study of ETX2514SUL for the treatment of pneumonia caused by multidrug-resistant A. baumannii (This study has been registered at ClinicalTrials.gov under identifier NCT03303924.).

Treatment of Infections by OXA-48-Producing Enterobacteriaceae

Carbapenemase-producing Enterobacteriaceae (CPE) contribute significantly to the global public health threat of antimicrobial resistance. OXA-48 and its variants are unique carbapenemases with low-level hydrolytic activity toward carbapenems but no intrinsic activity against expanded-spectrum cephalosporins. bla _OXA-48 is typically located on a plasmid but may also be integrated chromosomally, and this gene has progressively disseminated throughout Europe and the Middle East. Despite the inability of OXA-48-like carbapenemases to hydrolyze expanded-spectrum cephalosporins, pooled isolates demonstrate high variable resistance to ceftazidime and cefepime, likely representing high rates of extended-spectrum beta-lactamase (ESBL) coproduction. In vitro data from pooled studies suggest that avibactam is the most potent beta-lactamase inhibitor when combined with ceftazidime, cefepime, aztreonam, meropenem, or imipenem. Resistance to novel avibactam combinations such as imipenem-avibactam or aztreonam-avibactam has not yet been reported in OXA-48 producers, although only a few clinical isolates have been tested. Although combination therapy is thought to improve the chances of clinical cure and survival in CPE infection, successful outcomes were seen in ∼70% of patients with infections caused by OXA-48-producing Enterobacteriaceae treated with ceftazidime-avibactam monotherapy. A carbapenem in combination with either amikacin or colistin has achieved treatment success in a few case reports. Uncertainty remains regarding the best treatment options and strategies for managing these infections. Newly available antibiotics such as ceftazidime-avibactam show promise; however, recent reports of resistance are concerning. Newer choices of antimicrobial agents will likely be required to combat this problem.

Thinking Outside the Box-Novel Antibacterials To Tackle the Resistance Crisis

The public view on antibiotics as reliable medicines changed when reports about "resistant superbugs" appeared in the news. While reasons for this resistance development are easily spotted, solutions for re-establishing effective antibiotics are still in their infancy. This Review encompasses several aspects of the antibiotic development pipeline from very early strategies to mature drugs. An interdisciplinary overview is given of methods suitable for mining novel antibiotics and strategies discussed to unravel their modes of action. Select examples of antibiotics recently identified by using these platforms not only illustrate the efficiency of these measures, but also highlight promising clinical candidates with therapeutic potential. Furthermore, the concept of molecules that disarm pathogens by addressing gatekeepers of virulence will be covered. The Review concludes with an evaluation of antibacterials currently in clinical development. Overall, this Review aims to connect select innovative antimicrobial approaches to stimulate interdisciplinary partnerships between chemists from academia and industry.

Diversity and Proliferation of Metallo-β-Lactamases: a Clarion Call for Clinically Effective Metallo-β-Lactamase Inhibitors

The worldwide proliferation of life-threatening metallo-β-lactamase (MBL)-producing Gram-negative bacteria is a serious concern to public health. MBLs are compromising the therapeutic efficacies of β-lactams, particularly carbapenems, which are last-resort antibiotics indicated for various multidrug-resistant bacterial infections. Inhibition of enzymes mediating antibiotic resistance in bacteria is one of the major promising means for overcoming bacterial resistance. Compounds having potential MBL-inhibitory activity have been reported, but none are currently under clinical trials. The need for developing safe and efficient MBL inhibitors (MBLIs) is obvious, particularly with the continuous spread of MBLs worldwide. In this review, the emergence and escalation of MBLs in Gram-negative bacteria are discussed. The relationships between different class B β-lactamases identified up to 2017 are represented by a phylogenetic tree and summarized. In addition, approved and/or clinical-phase serine β-lactamase inhibitors are recapitulated to reflect the successful advances made in developing class A β-lactamase inhibitors. Reported MBLIs, their inhibitory properties, and their purported modes of inhibition are delineated. Insights into structural variations of MBLs and the challenges involved in developing potent MBLIs are also elucidated and discussed. Currently, natural products and MBL-resistant β-lactam analogues are the most promising agents that can become clinically efficient MBLIs. A deeper comprehension of the mechanisms of action and activity spectra of the various MBLs and their inhibitors will serve as a bedrock for further investigations that can result in clinically useful MBLIs to curb this global menace.

β-lactam/β-lactamase inhibitor combinations: an update

Antibiotic resistance caused by β-lactamase production continues to present a growing challenge to the efficacy of β-lactams and their role as the most important class of clinically used antibiotics. In response to this threat however, only a handful of β-lactamase inhibitors have been introduced to the market over the past thirty years. The first-generation β-lactamase inhibitors (clavulanic acid, sulbactam and tazobactam) are all β-lactam derivatives and work primarily by inactivating class A and some class C serine β-lactamases. The newer generations of β-lactamase inhibitors including avibactam and vaborbactam are based on non-β-lactam structures and their spectrum of inhibition is extended to KPC as an important class A carbapenemase. Despite these advances several class D and virtually all important class B β-lactamases are resistant to existing inhibitors. The present review provides an overview of recent FDA-approved β-lactam/β-lactamase inhibitor combinations as well as an update on research efforts aimed at the discovery and development of novel β-lactamase inhibitors.

Vancomycin Analogue Restores Meropenem Activity against NDM-1 Gram-Negative Pathogens

New Delhi metallo-β-lactamase-1 (NDM-1) is the major contributor to the emergence of carbapenem resistance in Gram-negative pathogens (GNPs) and has caused many clinically available β-lactam antibiotics to become obsolete. A clinically approved inhibitor of metallo-β-lactamase (MBL) that could restore the activity of carbapenems against resistant GNPs has not yet been found, making NDM-1 a serious threat to human health. Here, we have rationally developed an inhibitor for the NDM-1 enzyme, which has the ability to penetrate the outer membrane of GNPs and inactivate the enzyme by depleting the metal ion (Zn²⁺) from the active site. The inhibitor reinstated the activity of meropenem against NDM-1 producing clinical isolates of GNPs like Klebsiella pneumoniae and Escherichia coli. Further, the inhibitor efficiently restored meropenem activity against NDM-1 producing K. pneumoniae in a murine sepsis infection model. These findings demonstrate that a combination of the present inhibitor and meropenem has high potential to be translated clinically to combat carbapenem-resistant GNPs.

First Penicillin-Binding Protein Occupancy Patterns of β-Lactams and β-Lactamase Inhibitors in Klebsiella pneumoniae

Penicillin-binding proteins (PBPs) are the high-affinity target sites of all β-lactam antibiotics in bacteria. It is well known that each β-lactam covalently binds to and thereby inactivates different PBPs with various affinities. Despite β-lactams serving as the cornerstone of our therapeutic armamentarium against Klebsiella pneumoniae, PBP binding data are missing for this pathogen. We aimed to generate the first PBP binding data on 13 chemically diverse and clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae PBP binding was determined using isolated membrane fractions from K. pneumoniae strains ATCC 43816 and ATCC 13883. Binding reactions were conducted using β-lactam concentrations from 0.0075 to 256 mg/liter (or 128 mg/liter). After β-lactam exposure, unbound PBPs were labeled by Bocillin FL. Binding affinities (50% inhibitory concentrations [IC50]) were reported as the β-lactam concentrations that half-maximally inhibited Bocillin FL binding. PBP occupancy patterns by β-lactams were consistent across both strains. Carbapenems bound to all PBPs, with PBP2 and PBP4 as the highest-affinity targets (IC50, <0.0075 mg/liter). Preferential PBP2 binding was observed by mecillinam (amdinocillin; IC50, <0.0075 mg/liter) and avibactam (IC50, 2 mg/liter). Aztreonam showed high affinity for PBP3 (IC50, 0.06 to 0.12 mg/liter). Ceftazidime bound PBP3 at low concentrations (IC50, 0.06 to 0.25 mg/liter) and PBP1a/b at higher concentrations (4 mg/liter), whereas cefepime bound PBPs 1 to 4 at more even concentrations (IC50, 0.015 to 2 mg/liter). These PBP binding data on a comprehensive set of 13 clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae enable, for the first time, the rational design and optimization of double β-lactam and β-lactam-β-lactamase inhibitor combinations.

Comparison of Septic Shock Due to Multidrug-Resistant Acinetobacter baumannii or Klebsiella pneumoniae Carbapenemase-Producing K. pneumoniae in Intensive Care Unit Patients

A significant cause of mortality in the intensive care unit (ICU) is multidrug-resistant (MDR) Gram-negative bacteria, such as MDR Acinetobacter baumannii (MDR-AB) and Klebsiella pneumoniae carbapenemase-producing K. pneumoniae (KPC-Kp). The aim of the present study was to compare the clinical features, therapy, and outcome of patients who developed septic shock due to either MDR-AB or KPC-Kp. We retrospectively analyzed patients admitted to the ICU of a teaching hospital from November 2010 to December 2015 who developed septic shock due to MDR-AB or KPC-Kp infection. Data from 220 patients were analyzed: 128 patients (58.2%) were diagnosed with septic shock due to KPC-Kp, and 92 patients (41.8%) were diagnosed with septic shock due to MDR-AB. The 30-day mortality rate was significantly higher for the MDR-AB group than the KPC-Kp group (84.8% versus 44.5%, respectively; P < 0.001). Steroid exposure and pneumonia were associated with MDR-AB infection, whereas hospitalization in the previous 90 days, primary bacteremia, and KPC-Kp colonization were associated with KPC-Kp infection. For patients with KPC-Kp infections, the use of ≥2 in vitro-active antibiotics as empirical or definitive therapy was associated with higher 30-day survival, while isolation of colistin-resistant strains was linked to mortality. Patients with MDR-AB infections, age >60 years, and a simplified acute physiology score II (SAPS II) of >45 points were associated with increased mortality rates. We concluded that septic shock due to MDR-AB infection is associated with very high mortality rates compared to those with septic shock due to KPC-Kp. Analysis of the clinical features of these critically ill patients might help physicians in choosing appropriate empirical antimicrobial therapy.

Antibacterial Drug Discovery: Some Assembly Required

Our limited understanding of the molecular basis for compound entry into and efflux out of Gram-negative bacteria is now recognized as a key bottleneck for the rational discovery of novel antibacterial compounds. Traditional, large-scale biochemical or target-agnostic phenotypic antibacterial screening efforts have, as a result, not been very fruitful. A main driver of this knowledge gap has been the historical lack of predictive cellular assays, tools, and models that provide structure-activity relationships to inform optimization of compound accumulation. A variety of recent approaches has recently been described to address this conundrum. This Perspective explores these approaches and considers ways in which their integration could successfully redirect antibacterial drug discovery efforts.

Strategic Approaches to Overcome Resistance against Gram-Negative Pathogens Using β-Lactamase Inhibitors and β-Lactam Enhancers: Activity of Three Novel Diazabicyclooctanes WCK 5153, Zidebactam (WCK 5107), and WCK 4234

Limited treatment options exist to combat infections caused by multidrug-resistant (MDR) Gram-negative bacteria possessing broad-spectrum β-lactamases. The design of novel β-lactamase inhibitors is of paramount importance. Here, three novel diazabicyclooctanes (DBOs), WCK 5153, zidebactam (WCK 5107), and WCK 4234 (compounds 1-3, respectively), were synthesized and biochemically characterized against clinically important bacteria. Compound 3 inhibited class A, C, and D β-lactamases with unprecedented k₂/ K values against OXA carbapenemases. Compounds 1 and 2 acylated class A and C β-lactamses rapidly but not the tested OXAs. Compounds 1-3 formed highly stable acyl-complexes as demonstrated by mass spectrometry. Crystallography revealed that 1-3 complexed with KPC-2 adopted a "chair conformation" with the sulfate occupying the carboxylate binding region. The cefepime-2 and meropenem-3 combinations were effective in murine peritonitis and neutropenic lung infection models caused by MDR Acinetobacter baumannii. Compounds 1-3 are novel β-lactamase inhibitors that demonstate potent cross-class inhibition, and clinical studies targeting MDR infections are warranted.

Exploring Additional Dimensions of Complexity in Inhibitor Design for Serine β-Lactamases: Mechanistic and Intra- and Inter-molecular Chemistry Approaches

As a bacterial resistance strategy, serine β-lactamases have evolved from cell wall synthesizing enzymes known as penicillin-binding proteins (PBP), by not only covalently binding β-lactam antibiotics but, also acquiring mechanisms of deacylating these antibiotics. This critical deacylation step leads to release of hydrolyzed and inactivated β-lactams, thereby providing resistance for the bacteria against these antibiotics targeting the cell wall. To combat β-lactamase-mediated antibiotic resistance, numerous β-lactamase inhibitors were developed that utilize various strategies to inactivate the β-lactamase. Most of these compounds are “mechanism-based” inhibitors that in some manner mimic the β-lactam substrate, having a carbonyl moiety and a negatively charged carboxyl or sulfate group. These compounds form a covalent adduct with the catalytic serine via an initial acylation step. To increase the life-time of the inhibitory covalent adduct intermediates, a remarkable array of different strategies was employed to improve inhibition potency. Such approaches include post-acylation intra- and intermolecular chemical rearrangements as well as affecting the deacylation water. These approaches transform the inhibitor design process from a 3-dimensional problem (i.e., XYZ coordinates) to one with additional dimensions of complexity as the reaction coordinate and time spent at each chemical state need to be taken into consideration. This review highlights the mechanistic intricacies of the design efforts of the β-lactamase inhibitors which so far have resulted in the development of “two generations” and 5 clinically available inhibitors.

Acinetobacter baumannii OmpA Is a Selective Antibiotic Permeant Porin

OmpA_Ab is a conserved, abundantly expressed outer membrane porin in Acinetobacter baumannii whose presumed role in antibiotic permeation has not been clearly demonstrated. In this report, we use a titratable heterologous expression system to express OmpA_Ab in isolation and demonstrate selective passage of small molecule antibiotics through OmpA_Ab. ETX2514, a recently discovered broad-spectrum β-lactamase inhibitor, in combination with sulbactam, is currently in clinical testing for the treatment of drug-resistant A. baumannii infections. We demonstrate that ETX2514 permeates OmpA_Ab and potentiates the activity of sulbactam in an OmpA_Ab-dependent manner. In addition, we show that small modifications in the structure of ETX2514 differentially affect its passage through OmpA_Ab, revealing unique structure-porin-permeation relationships. Finally, we confirm the contribution of OmpA_Ab to bacterial fitness using a murine thigh model of A. baumannii infection. These results, combined with the high sequence homology of OmpA across Acinetobacter spp., suggest that optimization of antibiotic entry through OmpA_Ab may prove to be a feasible medicinal chemistry design strategy for future antibacterial discovery efforts.

Therapies for multidrug resistant and extensively drug-resistant non-fermenting gram-negative bacteria causing nosocomial infections: a perilous journey toward 'molecularly targeted' therapy

INTRODUCTION

Non-fermenting Gram-negative bacilli are at the center of the antimicrobial resistance epidemic. Acinetobacter baumannii and Pseudomonas aeruginosa are both designated with a threat level to human health of 'serious' by the Centers for Disease Control and Prevention. Two other major non-fermenting Gram-negative bacilli, Stenotrophomonas maltophilia and Burkholderia cepacia complex, while not as prevalent, have devastating effects on vulnerable populations, such as those with cystic fibrosis, as well as immunosuppressed or hospitalized patients. Areas covered: In this review, we summarize the clinical impact, presentations, and mechanisms of resistance of these four major groups of non-fermenting Gram-negative bacilli. We also describe available and promising novel therapeutic options and strategies, particularly combination antibiotic strategies, with a focus on multidrug resistant variants. Expert commentary: We finally advocate for a therapeutic approach that incorporates in vitro antibiotic susceptibility testing with molecular and genotypic characterization of mechanisms of resistance, as well as pharmacokinetics and pharmacodynamics (PK/PD) parameters. The goal is to begin to formulate a precision medicine approach to antimicrobial therapy: a clinical-decision making model that integrates bacterial phenotype, genotype and patient's PK/PD to arrive at rationally-optimized combination antibiotic chemotherapy regimens tailored to individual clinical scenarios.

Frequency and Mechanism of Spontaneous Resistance to Sulbactam Combined with the Novel β-Lactamase Inhibitor ETX2514 in Clinical Isolates of Acinetobacter baumannii

The novel diazabicyclooctenone ETX2514 is a potent, broad-spectrum serine β-lactamase inhibitor that restores sulbactam activity against resistant Acinetobacter baumannii The frequency of spontaneous resistance to sulbactam-ETX2514 in clinical isolates was found to be 7.6 × 10^-10 to <9.0 × 10^-10 at 4× MIC and mapped to residues near the active site of penicillin binding protein 3 (PBP3). Purified mutant PBP3 proteins demonstrated reduced affinity for sulbactam. In a sulbactam-sensitive isolate, resistance also mapped to stringent response genes associated with resistance to PBP2 inhibitors, suggesting that in addition to β-lactamase inhibition, ETX2514 may enhance sulbactam activity in A. baumannii via inhibition of PBP2.

Kinetics of Sulbactam Hydrolysis by β-Lactamases, and Kinetics of β-Lactamase Inhibition by Sulbactam

Sulbactam is one of four β-lactamase inhibitors in current clinical use to counteract drug resistance caused by degradation of β-lactam antibiotics by these bacterial enzymes. As a β-lactam itself, sulbactam is susceptible to degradation by β-lactamases. I investigated the Michaelis-Menten kinetics of sulbactam hydrolysis by 14 β-lactamases, representing clinically widespread groups within all four Ambler classes, i.e., CTX-M-15, KPC-2, SHV-5, and TEM-1 for class A; IMP-1, NDM-1, and VIM-1 for class B; Acinetobacter baumannii ADC-7, Pseudomonas aeruginosa AmpC, and Enterobacter cloacae P99 for class C; and OXA-10, OXA-23, OXA-24, and OXA-48 for class D. All of the β-lactamases were able to hydrolyze sulbactam, although they varied widely in their kinetic constants for the reaction, even within each class. I also investigated the inactivation kinetics of the inhibition of these enzymes by sulbactam. The class A β-lactamases varied widely in their susceptibility to inhibition, the class C and D enzymes were very weakly inhibited, and the class B enzymes were essentially or completely unaffected. In addition, we measured the sulbactam turnover number, the sulbactam/enzyme molar ratio required for complete inhibition of each enzyme. Class C enzymes had the lowest turnover numbers, class A enzymes varied widely, and class D enzymes had very high turnover numbers. These results are valuable for understanding which β-lactamases ought to be well inhibited by sulbactam. Moreover, since sulbactam has intrinsic antibacterial activity against Acinetobacter species pathogens, these results contribute to understanding β-lactamase-mediated sulbactam resistance in Acinetobacter, especially due to the action of the widespread class D enzymes.

Reversibility of Covalent, Broad-Spectrum Serine β-Lactamase Inhibition by the Diazabicyclooctenone ETX2514

ETX2514 is a non-β-lactam serine β-lactamase inhibitor in clinical development that has greater potency and broader spectrum of β-lactamase inhibition than the related diazabicyclooctanone avibactam. Despite opening of its cyclic urea ring upon acylation, avibactam can recyclize and dissociate intact from certain β-lactamases. We investigated reversibility of ETX2514 acylation of 10 serine β-lactamases representing Ambler classes A, C, and D. Dissociation rate constants varied widely between enzymes and were lowest for class D. For most enzymes, the covalent adduct mass was that of ETX2514 (277 Da). OXA-10 was acylated with 277 and 197 Da adducts, consistent with loss of the sulfate moiety. KPC-2 showed only the 197 Da adduct. ETX2514 recyclized and dissociated intact from AmpC, CTX-M-15, P99, SHV-5 and TEM-1 but not from KPC-2, OXA-10, OXA-23, OXA-24, or OXA-48. Inactivation partition ratios were 1 for all enzymes except KPC-2, for which it increased to 3.0 after 2 h. This result and mass spectrometry showed that KPC-2 very slowly degraded ETX2514. Nevertheless, ETX2514 restored β-lactam activity to equal potency against isogenic Pseudomonas aeruginosa strains each overexpressing one of the 10 β-lactamases.

An update on β-lactamase inhibitor discovery and development

Antibiotic resistance, and the emergence of pan-resistant clinical isolates, seriously threatens our capability to treat bacterial diseases, including potentially deadly hospital-acquired infections. This growing issue certainly requires multiple adequate responses, including the improvement of both diagnosis methods and use of antibacterial agents, and obviously the development of novel antibacterial drugs, especially active against Gram-negative pathogens, which represent an urgent medical need. Considering the clinical relevance of both β-lactam antibiotics and β-lactamase-mediated resistance, the discovery and development of combinations including a β-lactamase inhibitor seems to be particularly attractive, despite being extremely challenging due to the enormous diversity, both structurally and mechanistically, of the potential β-lactamase targets. This review will cover the evolution of currently available β-lactamase inhibitors along with the most recent research leading to new β-lactamase inhibitors of potential clinical interest or already in the stage of clinical development.

Antibiotic adjuvants - A strategy to unlock bacterial resistance to antibiotics

Resistance to available antibiotics in pathogenic bacteria is currently a global challenge since the number of strains that are resistant to multiple types of antibiotics has increased dramatically each year and has spread worldwide. To unlock this problem, the use of an 'antibiotic adjuvant' in combination with an antibiotic is now being exploited. This approach enables us to prolong the lifespan of these life-saving drugs. This digests review provides an overview of the main types of antibiotic adjuvants, the basis of their operation and the remaining issues to be tackled in this field. Particular emphasis is placed on those compounds that are already in clinical development, namely β-lactamase inhibitors.