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

Analysis of Co-localized Biosynthetic Gene Clusters Identifies a Membrane-Permeabilizing Natural Product

Combination therapy is an effective strategy to combat antibiotic resistance. Multiple synergistic antimicrobial combinations are produced by enzymes encoded in biosynthetic gene clusters (BGCs) that co-localize on the bacterial genome. This phenomenon led to the hypothesis that mining co-localized BGCs will reveal new synergistic combinations of natural products. Here, we bioinformatically identified 38 pairs of co-localized BGCs, which we predict to produce natural products that are related to known compounds, including polycyclic tetramate macrolactams (PoTeMs). We further showed that ikarugamycin, a PoTeM, increases the membrane permeability of Acinetobacter baumannii and Staphylococcus aureus, which suggests that ikarugamycin might be an adjuvant that facilitates the entry of other natural products. Our work outlines a promising avenue to discover synergistic combinations of natural products by mining bacterial genomes.

An Update on the Nitrogen Heterocycle Compositions and Properties of U.S. FDA-Approved Pharmaceuticals (2013-2023)

This Perspective is a continuation of our analysis of U.S. FDA-approved small-molecule drugs (1938-2012) containing nitrogen heterocycles. In this study we report drug structure and property analyses of 321 unique new small-molecule drugs approved from January 2013 to December 2023 as well as information about frequency of important heteroatoms such as sulfur and fluorine and key small nitrogen substituents (CN and NO2). The most notable change is an incredible increase in drugs containing at least one nitrogen heterocycle─82%, compared to 59% from preceding decades─as well as a significant increase in the number of nitrogen heterocycles per drug. Pyridine has claimed the #1 high-frequency nitrogen heterocycle occurrence spot from piperidine (#2), with pyrimidine (#5), pyrazole (#6), and morpholine (#9) being the big top 10 climbers. Also notable is high number of fused nitrogen heterocycles, apparently driven largely by newly approved cancer drugs.

New Agents Are Coming, and So Is the Resistance

Antimicrobial resistance is a global threat that requires urgent attention to slow the spread of resistant pathogens. The United States Centers for Disease Control and Prevention (CDC) has emphasized clinician-driven antimicrobial stewardship approaches including the reporting and proper documentation of antimicrobial usage and resistance. Additional efforts have targeted the development of new antimicrobial agents, but narrow profit margins have hindered manufacturers from investing in novel antimicrobials for clinical use and therefore the production of new antibiotics has decreased. In order to combat this, both antimicrobial drug discovery processes and healthcare reimbursement programs must be improved. Without action, this poses a high probability to culminate in a deadly post-antibiotic era. This review will highlight some of the global health challenges faced both today and in the future. Furthermore, the new Infectious Diseases Society of America (IDSA) guidelines for resistant Gram-negative pathogens will be discussed. This includes new antimicrobial agents which have gained or are likely to gain FDA approval. Emphasis will be placed on which human pathogens each of these agents cover, as well as how these new agents could be utilized in clinical practice.

Sulbactam-Durlobactam in the Treatment of Carbapenem-Resistant Acinetobacter baumannii Infections

OBJECTIVE

To review the pharmacology, efficacy, and safety of intravenous sulbactam-durlobactam (SUL-DUR) in the treatment of carbapenem-resistant Acinetobacter baumannii (CRAB) infections.

DATA SOURCES

PubMed databases and ClinicalTrials.gov were searched using the following terms: Sulbactam Durlobactam, ETX2514, Xacduro, Sulbactam-ETX2514, ETX2514SUL.

STUDY SELECTION AND DATA EXTRACTION

Articles published in English between January 1985 and September 13, 2023, related to pharmacology, safety, efficacy, and clinical trials were reviewed.

DATA SYNTHESIS

A phase II trial compared SUL-DUR with placebo with imipenem and cilastatin in both groups. Overall treatment success in the microbiological intention-to-treat analysis was reported in 76.6% of patients in the SUL-DUR group compared with 81% patients in the placebo group. A phase III trial compared SUL-DUR with colistin in adults with confirmed CRAB infections. Patients received either SUL-DUR or colistin and background therapy with imipenem-cilastatin. SUL-DUR was noninferior to colistin for 28-day all-cause mortality (19% vs 32.3%, treatment difference -13.2%; 95% CI [-30.0 to 3.5]).

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE IN COMPARISON TO EXISTING DRUGS

Clinicians have limited options to treat CRAB infections. SUL-DUR has demonstrated efficacy against CRAB in patients with pneumonia and may be considered a viable treatment option. Nonetheless, potential impact of concomitant imipenem-cilastatin as background therapy on clinical trial findings is unclear. Further studies are needed to elucidate the role of SUL-DUR alone or in combination with other active antimicrobials for the treatment of CRAB infections.

CONCLUSIONS

SUL-DUR has shown to be predominantly noninferior to alternative antibiotics in the treatment of pneumonias caused by CRAB, making it a viable treatment option. Further postmarketing data is needed to ascertain its role in other infections.

In vitro antibacterial activity of sulbactam-durlobactam in combination with other antimicrobial agents against Acinetobacter baumannii-calcoaceticus complex

Combinations of the β-lactam/β-lactamase inhibitor sulbactam-durlobactam and seventeen antimicrobial agents were tested against strains of Acinetobacter baumannii in checkerboard assays. Most combinations resulted in indifference with no instances of antagonism. These results suggest sulbactam-durlobactam antibacterial activity against A. baumannii is unlikely to be affected if co-dosed with other antimicrobial agents.

Evolving resistance landscape in gram-negative pathogens: An update on β-lactam and β-lactam-inhibitor treatment combinations for carbapenem-resistant organisms

Antibiotic resistance has become a global threat as it is continuously growing due to the evolution of β-lactamases diminishing the activity of classic β-lactam (BL) antibiotics. Recent antibiotic discovery and development efforts have led to the availability of β-lactamase inhibitors (BLIs) with activity against extended-spectrum β-lactamases as well as Klebsiella pneumoniae carbapenemase (KPC)-producing carbapenem-resistant organisms (CRO). Nevertheless, there is still a lack of drugs that target metallo-β-lactamases (MBL), which hydrolyze carbapenems efficiently, and oxacillinases (OXA) often present in carbapenem-resistant Acinetobacter baumannii. This review aims to provide a snapshot of microbiology, pharmacology, and clinical data for currently available BL/BLI treatment options as well as agents in late stage development for CRO harboring various β-lactamases including MBL and OXA-enzymes.

ESKAPE pathogens: antimicrobial resistance, epidemiology, clinical impact and therapeutics

The rise of antibiotic resistance and a dwindling antimicrobial pipeline have been recognized as emerging threats to public health. The ESKAPE pathogens - Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp. - were initially identified as critical multidrug-resistant bacteria for which effective therapies were rapidly needed. Now, entering the third decade of the twenty-first century, and despite the introduction of several new antibiotics and antibiotic adjuvants, such as novel β-lactamase inhibitors, these organisms continue to represent major therapeutic challenges. These bacteria share several key biological features, including adaptations for survival in the modern health-care setting, diverse methods for acquiring resistance determinants and the dissemination of successful high-risk clones around the world. With the advent of next-generation sequencing, novel tools to track and combat the spread of these organisms have rapidly evolved, as well as renewed interest in non-traditional antibiotic approaches. In this Review, we explore the current epidemiology and clinical impact of this important group of bacterial pathogens and discuss relevant mechanisms of resistance to recently introduced antibiotics that affect their use in clinical settings. Furthermore, we discuss emerging therapeutic strategies needed for effective patient care in the era of widespread antimicrobial resistance.

Optimizing Treatment for Carbapenem-Resistant Acinetobacter baumannii Complex Infections: A Review of Current Evidence

Carbapenem-resistant Acinetobacter baumannii complex (CRAB) poses a significant global health challenge owing to its resistance to multiple antibiotics and limited treatment options. Polymyxin-based therapies have been widely used to treat CRAB infections; however, they are associated with high mortality rates and common adverse events such as nephrotoxicity. Recent developments include numerous observational studies and randomized clinical trials investigating antibiotic combinations, repurposing existing antibiotics, and the development of novel agents. Consequently, recommendations for treating CRAB are undergoing significant changes. The importance of colistin is decreasing, and the role of sulbactam, which exhibits direct antibacterial activity against A. baumannii complex, is being reassessed. High-dose ampicillin-sulbactam-based combination therapies, as well as combinations of sulbactam and durlobactam, which prevent the hydrolysis of sulbactam and binds to penicillin-binding protein 2, have shown promising results. This review introduces recent advancements in CRAB infection treatment based on clinical trial data, highlighting the need for optimized treatment protocols and comprehensive clinical trials to combat the evolving threat of CRAB effectively.

Antibiotic Treatment of Carbapenem-Resistant Acinetobacter baumannii Infections in View of the Newly Developed β-Lactams: A Narrative Review of the Existing Evidence

It is estimated that antimicrobial resistance (AMR) is responsible for nearly 5 million human deaths worldwide each year and will reach 10 million by 2050. Carbapenem-resistant Acinetobacter baumannii (CRAB) infections represent the fourth-leading cause of death attributable to antimicrobial resistance globally, but a standardized therapy is still lacking. Among the antibiotics under consideration, Sulbactam/durlobactam seems to be the best candidate to replace current back-bone agents. Cefiderocol could play a pivotal role within combination therapy regimens. Due to toxicity and the pharmacokinetics/pharmacodynamics (PK/PD) limitations, colistin (or polymyxin B) should be used as an alternative agent (when no other options are available). Tigecycline (or minocycline) and fosfomycin could represent suitable partners for both NBLs. Randomized clinical trials (RCTs) are needed to better evaluate the role of NBLs in CRAB infection treatment and to compare the efficacy of tigecycline and fosfomycin as partner antibiotics. Synergism should be tested between NBLs and "old" drugs (rifampicin and trimethoprim/sulfamethoxazole). Huge efforts should be made to accelerate pre-clinical and clinical studies on safer polymyxin candidates with improved lung activity, as well as on the iv rifabutin formulation. In this narrative review, we focused the antibiotic treatment of CRAB infections in view of newly developed β-lactam agents (NBLs).

Durlobactam, a Diazabicyclooctane β-Lactamase Inhibitor, Inhibits BlaC and Peptidoglycan Transpeptidases of Mycobacterium tuberculosis

Peptidoglycan synthesis is an underutilized drug target in Mycobacterium tuberculosis (Mtb). Diazabicyclooctanes (DBOs) are a class of broad-spectrum β-lactamase inhibitors that also inhibit certain peptidoglycan transpeptidases that are important in mycobacterial cell wall synthesis. We evaluated the DBO durlobactam as an inhibitor of BlaC, the Mtb β-lactamase, and multiple Mtb peptidoglycan transpeptidases (PonA1, LdtMt1, LdtMt2, LdtMt3, and LdtMt5). Timed electrospray ionization mass spectrometry (ESI-MS) captured acyl-enzyme complexes with BlaC and all transpeptidases except LdtMt5. Inhibition kinetics demonstrated durlobactam was a potent and efficient DBO inhibitor of BlaC (KI app 9.2 ± 0.9 μM, k2/K 5600 ± 560 M-1 s-1) and similar to clavulanate (KI app 3.3 ± 0.6 μM, k2/K 8400 ± 840 M-1 s-1); however, durlobactam had a lower turnover number (tn = kcat/kinact) than clavulanate (1 and 8, respectively). KI app values with durlobactam and clavulanate were similar for peptidoglycan transpeptidases, but ESI-MS captured durlobactam complexes at more time points. Molecular docking and simulation demonstrated several productive interactions of durlobactam in the active sites of BlaC, PonA1, and LdtMt2. Antibiotic susceptibility testing was conducted on 11 Mtb isolates with amoxicillin, ceftriaxone, meropenem, imipenem, clavulanate, and durlobactam. Durlobactam had a minimum inhibitory concentration (MIC) range of 0.5-16 μg/mL, similar to the ranges for meropenem (1-32 μg/mL) and imipenem (0.5-64 μg/mL). In β-lactam + durlobactam combinations (1:1 mass/volume), MICs were lowered 4- to 64-fold for all isolates except one with meropenem-durlobactam. This work supports further exploration of novel β-lactamase inhibitors that target BlaC and Mtb peptidoglycan transpeptidases.

Characterization of Acinetobacter baumannii-calcoaceticus complex isolates and microbiological outcome for patients treated with sulbactam-durlobactam in a phase 3 trial (ATTACK)

Acinetobacter baumannii-calcoaceticus complex (ABC) causes severe, difficult-to-treat infections that are frequently antibiotic resistant. Sulbactam-durlobactam (SUL-DUR) is a targeted β-lactam/β-lactamase inhibitor combination antibiotic designed to treat ABC infections, including those caused by multidrug-resistant strains. In a global, pathogen-specific, randomized, controlled phase 3 trial (ATTACK), the efficacy and safety of SUL-DUR were compared to colistin, both dosed with imipenem-cilastatin as background therapy, in patients with serious infections caused by carbapenem-resistant ABC. Results from ATTACK showed that SUL-DUR met the criteria for non-inferiority to colistin for the primary efficacy endpoint of 28-day all-cause mortality with improved clinical and microbiological outcomes compared to colistin. This report describes the characterization of the baseline ABC isolates from patients enrolled in ATTACK, including an analysis of the correlation of microbiological outcomes with SUL-DUR MIC values and the molecular drivers of SUL-DUR resistance.

Clinical Outcomes for Patients With Monomicrobial vs Polymicrobial Acinetobacter baumannii-calcoaceticus Complex Infections Treated With Sulbactam-Durlobactam or Colistin: A Subset Analysis From a Phase 3 Clinical Trial

Background

In a previous study, the efficacy and safety of sulbactam-durlobactam vs colistin for the treatment of patients with carbapenem-resistant Acinetobacter baumannii-calcoaceticus complex (CRABC) infections were evaluated in a randomized controlled phase 3 trial. Both arms were dosed on a background of imipenem-cilastatin to treat coinfecting gram-negative pathogens. Thirty-six percent of infections in the primary efficacy population were polymicrobial.

Methods

A subset analysis was performed to compare clinical and microbiological outcomes at test of cure (7 ± 2 days after the last dose) for patients with monomicrobial and polymicrobial CRABC infections. Minimal inhibitory concentrations of antibiotics against baseline isolates were determined by broth microdilution according to Clinical and Laboratory Standards Institute methodology.

Results

Clinical cure, 28-day all-cause mortality, and microbiological outcomes were similar for patients in the sulbactam-durlobactam treatment arm with monomicrobial or polymicrobial A baumannii-calcoaceticus infections. Patients in the colistin arm with monomicrobial CRABC infections had higher mortality rates with worse clinical and microbiological outcomes as compared with those with polymicrobial infections. For patients who received sulbactam-durlobactam, imipenem susceptibility of coinfecting gram-negative pathogens trended with clinical benefit for patients with polymicrobial A baumannii-calcoaceticus infections. When tested in vitro, durlobactam restored imipenem susceptibility to the majority of coinfecting gram-negative pathogens from the sulbactam-durlobactam arm. This phenotype appeared to be related to the clinical outcome in 13 of 15 evaluable cases.

Conclusions

These results suggest that the use of sulbactam-durlobactam plus a carbapenem could be an effective approach to treat polymicrobial infections that include CRABC, but additional clinical data are needed to demonstrate efficacy.

Current treatment options for pneumonia caused by carbapenem-resistant Acinetobacter baumannii

PURPOSE OF REVIEW

The purpose of this review is to briefly summarize the challenges associated with the treatment of pneumonia caused by carbapenem-resistant Acinetobacter baumannii (CRAB), discuss its carbapenem-resistance, and review the literature supporting the current treatment paradigm and therapeutic options.

RECENT FINDINGS

In a multicenter, randomized, and controlled trial the novel β-lactam-β-lactamase inhibitor sulbactam-durlobactam was compared to colistin, both in addition to imipenem-cilastatin. The drug met the prespecified criteria for noninferiority for 28-day all-cause mortality while demonstrating higher clinical cure rates in the treatment of CRAB pneumonia. In an international, randomized, double-blind, placebo controlled trial colistin monotherapy was compared to colistin combined with meropenem. In this trial, combination therapy was not superior to monotherapy in the treatment of drug-resistant gram-negative organisms including CRAB pneumonia.

SUMMARY

CRAB pneumonia is a preeminent public health threat without an agreed upon first line treatment strategy. Historically, there have been drawbacks to available treatment modalities without a clear consensus on the first-line treatment regimen. CRAB pneumonia is a top priority for the continued development of antimicrobials, adjuvant therapies and refinement of current treatment strategies.

Antibacterial agents active against Gram Negative Bacilli in phase I, II, or III clinical trials

INTRODUCTION

Antimicrobial resistance is a major threat to modern healthcare, and it is often regarded that the antibiotic pipeline is 'dry.'

AREAS COVERED

Antimicrobial agents active against Gram negative bacilli in Phase I, II, or III clinical trials were reviewed.

EXPERT OPINION

Nearly 50 antimicrobial agents (28 small molecules and 21 non-traditional antimicrobial agents) active against Gram-negative bacilli are currently in clinical trials. These have the potential to provide substantial improvements to the antimicrobial armamentarium, although it is known that 'leakage' from the pipeline occurs due to findings of toxicity during clinical trials. Significantly, a lack of funding for large phase III clinical trials is likely to prevent trials occurring for the indications most relevant to loss of life attributed to antimicrobial resistance such as ventilator-associated pneumonia. Non-traditional antimicrobial agents face issues in clinical development such as a lack of readily available and reliable susceptibility tests, and the potential need for superiority trials rather than non-inferiority trials. Most importantly, concrete plans must be made during clinical development for access of new antimicrobial agents to areas of the world where resistance to Gram negative bacilli is most frequent.

Sulbactam-durlobactam: a β-lactam/β-lactamase inhibitor combination targeting Acinetobacter baumannii

Sulbactam-durlobactam is a pathogen-targeted β-lactam/β-lactamase inhibitor combination that has been approved by the US FDA for the treatment of hospital-acquired and ventilator-associated bacterial pneumonia caused by susceptible isolates of Acinetobacter baumannii-calcoaceticus complex (ABC) in patients 18 years of age and older. Sulbactam is a penicillin derivative with antibacterial activity against Acinetobacter but is prone to hydrolysis by β-lactamases encoded by contemporary isolates. Durlobactam is a diazabicyclooctane β-lactamase inhibitor with activity against Ambler classes A, C and D serine β-lactamases that restores sulbactam activity both in vitro and in vivo against multidrug-resistant ABC. Sulbactam-durlobactam is a promising alternative therapy for the treatment of serious Acinetobacter infections, which can have high rates of mortality.

Sulbactam-durlobactam susceptibility test method development and quality control ranges for MIC and disk diffusion tests

Sulbactam-durlobactam is a β-lactam/β-lactamase inhibitor combination developed to treat hospital-acquired and ventilator-associated bacterial pneumonia caused by Acinetobacter baumannii-calcoaceticus complex (ABC). Durlobactam is a diazabicyclooctane β-lactamase inhibitor with potent activity against Ambler classes A, C, and D serine β-lactamases and restores sulbactam activity against multidrug-resistant ABC. Studies were conducted to establish sulbactam-durlobactam antimicrobial susceptibility testing methods for both broth microdilution minimal inhibitory concentration (MIC) and disk diffusion tests as well as quality control (QC) ranges. To establish the MIC test method, combinations of sulbactam and durlobactam were evaluated using a panel of genetically characterized A. baumannii isolates which were categorized as predicted to be susceptible or resistant based on the spectrum of β-lactamase inhibition by durlobactam. MIC testing with doubling dilutions of sulbactam with a fixed concentration of 4 µg/mL of durlobactam resulted in the greatest discrimination of the pre-defined susceptible and resistant strains. Similarly, the sulbactam/durlobactam 10/10 µg disk concentration showed the best discrimination as well as correlation with the MIC test. A. baumannii NCTC 13304 was selected for QC purposes because it assesses the activity of both sulbactam and durlobactam with clear endpoints. Multi-laboratory QC studies were conducted according to CLSI M23 Tier 2 criteria. A sulbactam-durlobactam broth MIC QC range of 0.5/4-2/4 µg/mL and a zone diameter QC range of 24-30 mm were determined for A. baumannii NCTC 13304 and have been approved by CLSI. These studies will enable clinical laboratories to perform susceptibility tests with accurate and reproducible methods.

n vitro pharmacokinetics/pharmacodynamics of the β-lactamase inhibitor, durlobactam, in combination with sulbactam against Acinetobacter baumannii-calcoaceticus complex

Infections caused by Acinetobacter baumannii are increasingly multidrug resistant and associated with high rates of morbidity and mortality. Sulbactam is a β-lactamase inhibitor with intrinsic antibacterial activity against A. baumannii. Durlobactam is a non-β-lactam β-lactamase inhibitor with an extended spectrum of activity compared to other inhibitors of its class. In vitro pharmacodynamic infection models were undertaken to establish the pharmacokinetic/pharmacodynamic (PK/PD) index and magnitudes associated with sulbactam and durlobactam efficacy and to simulate epithelial lining fluid (ELF) exposures at clinical doses to understand sulbactam-durlobactam activity with and without co-administration of a carbapenem. Hollow fiber infection models (HFIMs) and one-compartment systems were used to identify the PK/PD indices and exposure magnitudes associated of 1-log10 and 2-log10 colony-forming unit (CFU)/mL reductions. Sulbactam and durlobactam demonstrated PK/PD drivers of % time above the minimum inhibition concentration (%T > MIC) and area under the plasma concentration-time curve from time 0 to 24 h (AUC0-24)/MIC, respectively. Against a sulbactam-susceptible strain, sulbactam %T > MIC of 71.5 and 82.0 were associated with 1-log10 and 2-log10 CFU/mL reductions, respectively, in the HFIM. Against a non-susceptible strain, durlobactam restored the activity of sulbactam with an AUC0-24/MICs of 34.0 and 46.8 using a polysulfone cartridge to achieve a 1-log10 and 2-log10 CFU/mL reduction. These magnitudes were reduced to 13.8 and 24.2, respectively, using a polyvinylidene fluoride cartridge with a membrane pore size of 0.1 μm. In the one-compartment model, durlobactam AUC0-24/MIC to achieve 1-log10 and 2-log10 CFU/mL reduction were 7.6 and 33.4, respectively. Simulations of clinical ELF exposures in the HFIM showed cidal activity at MICs ≤4 µg/mL. Penicillin binding protein 3 mutant strains with MICs of 8 μg/mL may benefit from the addition of a carbapenem at clinical exposures.

In vivo dose response and efficacy of the β-lactamase inhibitor, durlobactam, in combination with sulbactam against the Acinetobacter baumannii-calcoaceticus complex

Multi-drug resistant (MDR) Acinetobacter baumannii is emerging as a pathogen of increasing prevalence and concern. Infections associated with this Gram-negative pathogen are often associated with increased morbidity and mortality and few therapeutic options. The β-lactamase inhibitor sulbactam used commonly in combination with ampicillin demonstrates intrinsic antibacterial activity against A. baumannii acting as an inhibitor of PBP1 and PBP3, which participate in cell wall biosynthesis. The production of β-lactamases, particularly class D oxacillinases, however, has limited the utility of sulbactam resorting to increased doses and the need for alternate therapies. Durlobactam is a non-β-lactam β-lactamase inhibitor that demonstrates broad β-lactamase inhibition including class D enzymes produced by A. baumannii and has shown potent in vitro activity against MDR A. baumannii, particularly carbapenem-resistant isolates in susceptibility and pharmacodynamic model systems. The objective of this study is to evaluate the exposure-response relationship of sulbactam and durlobactam in combination using in vivo neutropenic thigh and lung models to establish PK/PD exposure magnitudes to project clinically effective doses. Utilizing established PK/PD determinants of %T>MIC and AUC/MIC for sulbactam and durlobactam, respectively, non-linear regressional analysis of drug exposure was evaluated relative to the 24-hour change in bacterial burden (log10 CFU/g). Co-modeling of the data across multiple strains exhibiting a broad range of MIC susceptibility suggested net 1-log10 CFU/g0 reduction can be achieved when sulbactam T>MIC exceeds 50% of the dosing interval and durlobactam AUC/MIC is 10. These data were ultimately used to support sulbactam-durlobactam dose selection for Phase 3 clinical trials.

Drug Discovery in the Field of β-Lactams: An Academic Perspective

β-Lactams are the most widely prescribed class of antibiotics that inhibit penicillin-binding proteins (PBPs), particularly transpeptidases that function in peptidoglycan synthesis. A major mechanism of antibiotic resistance is the production of β-lactamase enzymes, which are capable of hydrolyzing β-lactam antibiotics. There have been many efforts to counter increasing bacterial resistance against β-lactams. These studies have mainly focused on three areas: discovering novel inhibitors against β-lactamases, developing new β-lactams less susceptible to existing resistance mechanisms, and identifying non-β-lactam inhibitors against cell wall transpeptidases. Drug discovery in the β-lactam field has afforded a range of research opportunities for academia. In this review, we summarize the recent new findings on both β-lactamases and cell wall transpeptidases because these two groups of enzymes are evolutionarily and functionally connected. Many efforts to develop new β-lactams have aimed to inhibit both transpeptidases and β-lactamases, while several promising novel β-lactamase inhibitors have shown the potential to be further developed into transpeptidase inhibitors. In addition, the drug discovery progress against each group of enzymes is presented in three aspects: understanding the targets, screening methodology, and new inhibitor chemotypes. This is to offer insights into not only the advancement in this field but also the challenges, opportunities, and resources for future research. In particular, cyclic boronate compounds are now capable of inhibiting all classes of β-lactamases, while the diazabicyclooctane (DBO) series of small molecules has led to not only new β-lactamase inhibitors but potentially a new class of antibiotics by directly targeting PBPs. With the cautiously optimistic successes of a number of new β-lactamase inhibitor chemotypes and many questions remaining to be answered about the structure and function of cell wall transpeptidases, non-β-lactam transpeptidase inhibitors may usher in the next exciting phase of drug discovery in this field.

Physical compatibility of sulbactam/durlobactam with select intravenous drugs during simulated Y-site administration

PURPOSE

Sulbactam/durlobactam is a combination antibiotic designed to target Acinetobacter baumannii, including carbapenem-resistant and multidrug-resistant strains. The objective of this study was to determine the physical compatibility of sulbactam/durlobactam solution during simulated Y-site administration with 95 intravenous (IV) drugs.

METHODS

Vials of sulbactam/durlobactam solution were diluted in 0.9% sodium chloride injection to a volume of 100 mL (the final concentration of both drugs was 15 mg/mL). All other IV drugs were reconstituted according to the manufacturer's recommendations and diluted with 0.9% sodium chloride injection to the upper range of concentrations used clinically or tested undiluted as intended for administration. Y-site conditions were simulated by mixing 5 mL of sulbactam/durlobactam with 5 mL of the tested drug solutions in a 1:1 ratio. Solutions were inspected for physical characteristics (clarity, color, and Tyndall effect), turbidity, and pH changes before admixture, immediately post admixture, and over 4 hours. Incompatibility was defined as any observed precipitation, significant color change, positive Tyndall test, or turbidity change of ≥0.5 nephelometric turbidity unit during the observation period.

RESULTS

Sulbactam/durlobactam was physically compatible with 38 out of 42 antimicrobials tested (90.5%) and compatible overall with 86 of 95 drugs tested (90.5%). Incompatibility was observed with albumin, amiodarone hydrochloride, ceftaroline fosamil, ciprofloxacin, daptomycin, levofloxacin, phenytoin sodium, vecuronium, and propofol.

CONCLUSION

The Y-site compatibility of sulbactam/durlobactam with 95 IV drugs was described. These compatibility data will assist pharmacists and nurses to safely coordinate administration of IV medications with sulbactam/durlobactam.

Sulbactam-Durlobactam, A Novel Drug for the Treatment of Multidrug Resistant Acinetobacter baumannii Infections - A Systematic Review

BACKGROUND

Sulbactam-durlobactam (SUL-DUR) has been tested in vitro for its ability to generate resistance in clinical isolates of Acinetobacter species. According to prior studies, combining durlobactam with sulbactam causes sulbactam-resistant isolates to become more active and revert to susceptibility. We aimed to conduct a systematic review of the in vitro activity of SUL-DUR on A. baumannii (Ab) isolates, including carbapenem-resistant A. baumannii (CRAb), to provide an overview for physicians dealing with Ab infections.

METHODS

The following keywords were searched in the PubMed, Google Scholar, and EMBASE databases to look for eligible original works that have been published without restrictions till June 30, 2023: A. baumannii and sulbactam-durlobactam, SUL-DUR, durlobactam, and sulbactam-ETX2514. We also searched clinicaltrials.gov and the Clinical Trials Registry of India (CTRI) for clinical trials involving sulbactamdurlobactam and Acinetobacter.

RESULTS

There were a total of 852 abstracts found. Among them, 633 articles with titles, abstracts, and keywords were reviewed, and 574 articles were removed after the initial screening. A total of 59 full-text eligible articles were evaluated, and 51 of them were eliminated because they did not satisfy the criteria set for inclusion. The full texts of the final 8 in vitro studies on A. baumanii and sulbactam/durlobactam were further evaluated. There were 5 trials on A. baumanii and sulbactam/durlobactam found on clinicaltrials.gov and the Clinical Trial Registry of India (CTRI).

CONCLUSION

The findings from the studies show that SUL-DUR might be a successful therapeutic option for multidrug-resistant-Ab infections. Future clinical trials will be required to validate the possibility of using this combination to treat multidrug-resistant A. baumannii infections.

Cefiderocol and Sulbactam-Durlobactam against Carbapenem-Resistant Acinetobacter baumannii

Infections caused by carbapenem-resistant Acinetobacter baumannii (CRAB) remain a clinical challenge due to limited treatment options. Recently, cefiderocol, a novel siderophore cephalosporin, and sulbactam-durlobactam, a bactericidal β-lactam-β-lactamase inhibitor combination, have been approved by the Food and Drug Administration for the treatment of A. baumannii infections. In this review, we discuss the mechanisms of action of and resistance to cefiderocol and sulbactam-durlobactam, the antimicrobial susceptibility of A. baumannii isolates to these drugs, as well as the clinical effectiveness of cefiderocol and sulbactam/durlobactam-based regimens against CRAB. Overall, cefiderocol and sulbactam-durlobactam show an excellent antimicrobial activity against CRAB. The review of clinical studies evaluating the efficacy of cefiderocol therapy against CRAB indicates it is non-inferior to colistin/other treatments for CRAB infections, with a better safety profile. Combination treatment is not associated with improved outcomes compared to monotherapy. Higher mortality rates are often associated with prior patient comorbidities and the severity of the underlying infection. Regarding sulbactam-durlobactam, current data from the pivotal clinical trial and case reports suggest this antibiotic combination could be a valuable option in critically ill patients affected by CRAB infections, in particular where no other antibiotic appears to be effective.

Antibiotic therapy for nonfermenting Gram-negative bacilli infections: future perspectives

PURPOSE OF REVIEW

Serious infections caused by nonfermenting Gram-negative bacteria (NF-GNB) pose a significant challenge for clinicians due to the limited treatment options available, which are frequently associated with issues of toxicity and unfavourable pharmacokinetic profiles. The aim of this review is to provide a brief overview of the existing data concerning the ongoing development of antiinfective agents targeting NF-GNB.

RECENT FINDINGS

Several agents exhibiting efficacy against NF-GNB are under clinical investigation. Durlobactam-sulbactam and cefepime-taniborbactam emerge as promising therapeutic avenues against carbapenem-resistant Acinetobacter baumanii . Cefepime-zidebactam may serve as a suitable treatment option for urinary tract infections caused by a wide range of NF-GNB. Cefepime-enmetazobactam demonstrates potent in vitro activity against various NF-GNB strains; however, its role as an anti- Pseudomonal agent is inadequately substantiated by available data. Xeruborbactam is a wide β-lactamase inhibitor that can be associated with a range of agents, enhancing in-vitro activity of these against many NF-GNB, including those resistant to newer, broader spectrum options. Lastly, murepavadin appears to be a potential pathogen-specific solution for severe Pseudomonas infections; however, additional investigation is necessary to establish the safety profile of this compound.

SUMMARY

Each of the novel molecules reviewed possesses an interesting range of in-vitro activity against NF-GNB. In addition, some of them have already been proved effective in vivo, underscoring their potential as future treatment options.

In vitro synergy of the combination of sulbactam-durlobactam and cefepime at clinically relevant concentrations against A. baumannii, P. aeruginosa and Enterobacterales

BACKGROUND

Sulbactam-durlobactam is a potent combination active against Acinetobacter baumannii; however, it lacks activity against other nosocomial pathogens. Cefepime is a common first-line therapy for hospital/ventilator-associated pneumonia caused by Gram-negative pathogens including Pseudomonas aeruginosa and Enterobacterales. With increasing resistance to cefepime, and the significant proportion of polymicrobial nosocomial infections, effective therapy for infections caused by Acinetobacter baumannii, P. aeruginosa and Enterobacterales is needed. This study investigated the in vitro synergy of sulbactam-durlobactam plus cefepime against relevant pathogens.

METHODS

Static time-kills assays were performed in duplicate against 14 cefepime-resistant isolates (A. baumannii, n = 4; P. aeruginosa, n = 4; Escherichia coli, n = 3; Klebsiella pneumoniae, n = 3). One WT K. pneumoniae isolate was included. Antibiotic concentrations simulated the free-steady state average concentration of clinically administered doses in patients.

RESULTS

Sulbactam-durlobactam alone showed significant activity against A. baumannii consistent with the MIC values. Sulbactam-durlobactam plus cefepime showed synergy against one A. baumannii isolate with an elevated MIC to sulbactam-durlobactam (32 mg/L). Against all P. aeruginosa isolates, synergy was observed with sulbactam-durlobactam plus cefepime. For the Enterobacterales, one E. coli isolate demonstrated synergy while the others were indifferent due to significant kill from sulbactam-durlobactam alone. The combination of sulbactam-durlobactam plus cefepime showed synergy against one of the K. pneumoniae and additive effects against the other two K. pneumoniae tested. No antagonism was observed in any isolates including the WT strain.

CONCLUSIONS

Synergy and no antagonism was observed with a combination of sulbactam-durlobactam and cefepime; further in vivo pharmacokinetic/pharmacodynamics data and clinical correlation are necessary to support our findings.

Molecular drivers of resistance to sulbactam-durlobactam in contemporary clinical isolates of Acinetobacter baumannii

Acinetobacter baumannii-calcoaceticus complex (ABC) causes severe infections that are difficult to treat due to pre-existing antibiotic resistance. Sulbactam-durlobactam (SUL-DUR) is a targeted β-lactam/β-lactamase inhibitor combination antibiotic designed to treat serious infections caused by Acinetobacter, including multidrug- and carbapenem-resistant strains. In a recent global surveillance study of 5,032 ABC clinical isolates collected from 2016 to 2021, less than 2% of ABC isolates had SUL-DUR MIC values >4 µg/mL. Molecular characterization of these isolates confirmed the primary drivers of resistance are metallo-β-lactamases or penicillin-binding protein 3 (PBP3) mutations, as previously described. In addition, this study shows that certain common PBP3 variants, such as A515V, are insufficient to confer sulbactam resistance and that the efflux of durlobactam by AdeIJK is likely to play a role in a subset of strains.

In vitro and in vivo evaluation of two combined β-lactamase inhibitors against carbapenem-resistant Acinetobacter baumannii

The objective of this study was to evaluate the in vitro and in vivo efficacy of clavulanic acid (C/A) in combination with tazobactam against clinical strains of carbapenem-resistant Acinetobacter baumannii. The MIC of 24 clinical strains of A. baumannii was determined, and a checkerboard assay and time-kill curve analysis were performed in selected strains to determine the synergy between C/A and tazobactam. The efficacy of C/A in monotherapy and in combination with tazobactam was evaluated in vitro in cell culture experiments and in a murine peritoneal sepsis model. The C/A and C/A plus tazobactam MIC50 were 128 and <1 mg/L, respectively. The checkerboard assay showed that tazobactam (4 and 8 mg/L) demonstrated synergy with C/A against A. baumannii Ab40, an OXA-24 producer strain, and Ab293, a lacking OXA β-lactamase strain. The time-kill curve assay showed both bactericidal and synergistic effects against Ab40 and Ab293, with C/A 1xMIC and tazobactam (4 and 8 mg/L) at 24 h. In the murine peritoneal sepsis model with Ab293 strain, the combination of C/A and tazobactam reduced bacterial loads in tissues and blood by 2 and 4 log10 CFU/g or mL compared with C/A alone. Combining C/A with tazobactam could be considered as a potential alternative strategy to treat A. baumannii in some cases, and future work with more strains is needed to confirm this possibility.

Aromatic Diboronic Acids as Effective KPC/AmpC Inhibitors

Over 30 compounds, including para-, meta-, and ortho-phenylenediboronic acids, ortho-substituted phenylboronic acids, benzenetriboronic acids, di- and triboronated thiophenes, and pyridine derivatives were investigated as potential β-lactamase inhibitors. The highest activity against KPC-type carbapenemases was found for ortho-phenylenediboronic acid 3a, which at the concentration of 8/4 mg/L reduced carbapenems' MICs up to 16/8-fold, respectively. Checkerboard assays revealed strong synergy between carbapenems and 3a with the fractional inhibitory concentrations indices of 0.1-0.32. The nitrocefin hydrolysis test and the whole cell assay with E. coli DH5α transformant carrying blaKPC-3 proved KPC enzyme being its molecular target. para-Phenylenediboronic acids efficiently potentiated carbapenems against KPC-producers and ceftazidime against AmpC-producers, whereas meta-phenylenediboronic acids enhanced only ceftazidime activity against the latter ones. Finally, the statistical analysis confirmed that ortho-phenylenediboronic acids act synergistically with carbapenems significantly stronger than other groups. Since the obtained phenylenediboronic compounds are not toxic to MRC-5 human fibroblasts at the tested concentrations, they can be considered promising scaffolds for the future development of novel KPC/AmpC inhibitors. The complexation of KPC-2 with the most representative isomeric phenylenediboronic acids 1a, 2a, and 3a was modeled by quantum mechanics/molecular mechanics calculations. Compound 3a reached the most effective configuration enabling covalent binding to the catalytic Ser70 residue.

The Combination of Antibiotic and Non-Antibiotic Compounds Improves Antibiotic Efficacy against Multidrug-Resistant Bacteria

Bacterial antibiotic resistance, especially the emergence of multidrug-resistant (MDR) strains, urgently requires the development of effective treatment strategies. It is always of interest to delve into the mechanisms of resistance to current antibiotics and target them to promote the efficacy of existing antibiotics. In recent years, non-antibiotic compounds have played an important auxiliary role in improving the efficacy of antibiotics and promoting the treatment of drug-resistant bacteria. The combination of non-antibiotic compounds with antibiotics is considered a promising strategy against MDR bacteria. In this review, we first briefly summarize the main resistance mechanisms of current antibiotics. In addition, we propose several strategies to enhance antibiotic action based on resistance mechanisms. Then, the research progress of non-antibiotic compounds that can promote antibiotic-resistant bacteria through different mechanisms in recent years is also summarized. Finally, the development prospects and challenges of these non-antibiotic compounds in combination with antibiotics are discussed.

Penicillin-binding protein (PBP) inhibitor development: A 10-year chemical perspective

Bacterial cell wall formation is essential for cellular survival and morphogenesis. The peptidoglycan (PG), a heteropolymer that surrounds the bacterial membrane, is a key component of the cell wall, and its multistep biosynthetic process is an attractive antibacterial development target. Penicillin-binding proteins (PBPs) are responsible for cross-linking PG stem peptides, and their central role in bacterial cell wall synthesis has made them the target of successful antibiotics, including β-lactams, that have been used worldwide for decades. Following the discovery of penicillin, several other compounds with antibiotic activity have been discovered and, since then, have saved millions of lives. However, since pathogens inevitably become resistant to antibiotics, the search for new active compounds is continuous. The present review highlights the ongoing development of inhibitors acting mainly in the transpeptidase domain of PBPs with potential therapeutic applications for the development of new antibiotic agents. Both the critical aspects of the strategy, design, and structure-activity relationships (SAR) are discussed, covering the main published articles over the last 10 years. Some of the molecules described display activities against main bacterial pathogens and could open avenues toward the development of new, efficient antibacterial drugs.

In Vivo Emergence of Pandrug-Resistant Acinetobacter baumannii Strain: Comprehensive Resistance Characterization and Compassionate Use of Sulbactam-Durlobactam

The treatment of patients with infection secondary to carbapenem-resistant Acinetobacter baumannii with emerging cefiderocol resistance remains challenging and unclear. We present a case of in vivo emergence of pandrug-resistant A baumannii that was successfully treated with the compassionate use of investigational sulbactam-durlobactam-based antibiotic regimen. We also performed a longitudinal genomic analysis of the bacterial isolates and showed the development of resistance and genetic mutations over time.

Enzyme Patterns and Factors Associated with Mortality among Patients with Carbapenem Resistant AcinetobacterBaumannii (CRAB) Bacteremia: Real World Evidence from a Tertiary Center in India

Introduction

In the Indian setting, antimicrobial resistance in A. baumannii is a considerable problem, especially in intensive care units (ICUs). Due to the limited data, clinicians are left with very few choices except polymyxins for treating serious infections caused by A. baumannii. There is sparse data regarding the local mechanisms of resistance. Given the current therapeutic challenges, it is critical to know the local enzymatic patterns and antibiograms.

Materials and methods

A retrospective analysis of 50 episodes of bacteremia caused by CRAB. We analyzed the enzyme patterns and the susceptibility rates to various antibiotics.

Results

The resistance rates for amikacin, tigecycline, minocycline, and fluoroquinolones were 88, 82, 50, and 88% respectively. OXA-23 was the most commonly isolated enzyme (86% of the isolates produced OXA-23) followed by OXA-51 and NDM. The overall mortality was high (58%). On univariate analysis, pneumonia, and higher Pitt's bacteremia score were significantly associated with mortality (p = 0.04 and p = 0.001 respectively). Of the total patients who received combination therapy, a majority (58%) received polymyxin plus meropenem. Combination therapy using polymyxins as a backbone was not associated with reduced mortality (p = 0.1).

Conclusion

A. baumannii is associated with significant morbidity and mortality, as shown in our study. The rates of resistance for aminoglycosides were very high, and minocycline showed better susceptibility rates in comparison with tigecycline. In our study, OXA-23 and NDM remained the most important enzymes. The routine use of the combination of polymyxin and meropenem may not offer a significant advantage over monotherapy.

How to cite this article

Prayag PS, Patwardhan SA, Joshi RS, Panchakshari SP, Rane T, Prayag AP. Enzyme Patterns and Factors Associated with Mortality among Patients with Carbapenem Resistant Acinetobacter Baumannii (CRAB) Bacteremia: Real World Evidence from a Tertiary Center in India. Indian J Crit Care Med 2023;27(9):663-668.

Sulbactam/Durlobactam: First Approval

Sulbactam/durlobactam (XACDURO®), is a co-packaged antibacterial product that has been developed by Entasis Therapeutics Inc. for the treatment of infections caused by Acinetobacter baumannii-calcoaceticus complex (ABC). Coadministration of durlobactam (a β-lactamase inhibitor with potent activity against a broad range of serine β-lactamases) with sulbactam (an established class A β-lactamase inhibitor with antibacterial activity against A. baumannii) prevents sulbactam degradation by ABC-produced β-lactamases. In May 2023, sulbactam/durlobactam was approved in the USA for use in patients 18 years of age and older for the treatment of hospital-acquired bacterial pneumonia and ventilator-associated bacterial pneumonia (HABP/VABP) caused by susceptible isolates of ABC. This article summarizes the milestones in the development of sulbactam/durlobactam leading to this first approval for the treatment of infections caused by ABC.

CpxA/R-Controlled Nitroreductase Expression as Target for Combinatorial Therapy against Uropathogens by Promoting Reactive Oxygen Species Generation

The antibiotic resistances emerged in uropathogens lead to accumulative treatment failure and recurrent episodes of urinary tract infection (UTI), necessitating more innovative therapeutics to curb UTI before systematic infection. In the current study, the combination of amikacin and nitrofurantoin is found to synergistically eradicate Gram-negative uropathogens in vitro and in vivo. The mechanistic analysis demonstrates that the amikacin, as an aminoglycoside, induced bacterial envelope stress by introducing mistranslated proteins, thereby constitutively activating the cpxA/R two-component system (Cpx signaling). The activation of Cpx signaling stimulates the expression of bacterial major nitroreductases (nfsA/nfsB) through soxS/marA regulons. As a result, the CpxA/R-dependent nitroreductases overexpression generates considerable quantity of lethal reactive intermediates via nitroreduction and promotes the prodrug activation of nitrofurantoin. As such, these actions together disrupt the bacterial cellular redox balance with excessively-produced reactive oxygen species (ROS) as "Domino effect", accelerating the clearance of uropathogens. Although aminoglycosides are used as proof-of-principle to elucidate the mechanism, the synergy between nitrofurantoin is generally applicable to other Cpx stimuli. To summarize, this study highlights the potential of aminoglycoside-nitrofurantoin combination to replenish the arsenal against recurrent Gram-negative uropathogens and shed light on the Cpx signaling-controlled nitroreductase as a potential target to manipulate the antibiotic susceptibility.

Efficacy and safety of sulbactam-durlobactam versus colistin for the treatment of patients with serious infections caused by Acinetobacter baumannii-calcoaceticus complex: a multicentre, randomised, active-controlled, phase 3, non-inferiority clinical trial (ATTACK)

BACKGROUND

An urgent need exists for antibiotics to treat infections caused by carbapenem-resistant Acinetobacter baumannii-calcoaceticus complex (ABC). Sulbactam-durlobactam is a β-lactam-β-lactamase inhibitor combination with activity against Acinetobacter, including multidrug-resistant strains. In a phase 3, pathogen-specific, randomised controlled trial, we compared the efficacy and safety of sulbactam-durlobactam versus colistin, both in combination with imipenem-cilastatin as background therapy, in patients with serious infections caused by carbapenem-resistant ABC.

METHODS

The ATTACK trial was done at 59 clinical sites in 16 countries. Adults aged 18 years or older with ABC-confirmed hospital-acquired bacterial pneumonia, ventilator-associated bacterial pneumonia, ventilated pneumonia, or bloodstream infections were randomised 1:1 using a block size of four to sulbactam-durlobactam (1·0 g of each drug in combination over 3 h every 6 h) or colistin (2·5 mg/kg over 30 min every 12 h) for 7-14 days. All patients received imipenem-cilastatin (1·0 g of each drug in combination over 1 h every 6 h) as background therapy. The primary efficacy endpoint was 28-day all-cause mortality in patients with laboratory-confirmed carbapenem-resistant ABC (the carbapenem-resistant ABC microbiologically modified intention-to-treat population). Non-inferiority was concluded if the upper bound of the 95% CI for the treatment difference was less than +20%. The primary safety endpoint was incidence of nephrotoxicity assessed using modified Risk, Injury, Failure, Loss, End-stage renal disease criteria measured by creatinine level or glomerular filtration rate through day 42. This trial is registered at ClinicalTrials.gov, NCT03894046.

FINDINGS

Between Sep 5, 2019, and July 26, 2021, 181 patients were randomly assigned to sulbactam-durlobactam or colistin (176 hospital-acquired bacterial pneumonia, ventilator-associated bacterial pneumonia, or ventilated pneumonia; and five bloodstream infections); 125 patients with laboratory-confirmed carbapenem-resistant ABC isolates were included in the primary efficacy analysis. 28-day all-cause mortality was 12 (19%) of 63 in the sulbactam-durlobactam group and 20 (32%) of 62 in the colistin group, a difference of -13·2% (95% CI -30·0 to 3·5), which met criteria for non-inferiority. Incidence of nephrotoxicity was significantly (p<0·001) lower with sulbactam-durlobactam than colistin (12 [13%] of 91 vs 32 [38%] of 85). Serious adverse events were reported in 36 (40%) of 91 patients in the sulbactam-durlobactam group and 42 (49%) of 86 patients in the colistin group. Treatment-related adverse events leading to study drug discontinuation were reported in ten (11%) of 91 patients in the sulbactam-durlobactam group and 14 (16%) of 86 patients in the colistin group.

INTERPRETATION

Our data show that sulbactam-durlobactam was non-inferior to colistin, both agents given in combination with imipenem-cilastatin, for the primary endpoint of 28-day all-cause mortality. Sulbactam-durlobactam was well tolerated and could be an effective intervention to reduce mortality from serious infections caused by carbapenem-resistant ABC, including multidrug-resistant strains.

FUNDING

Entasis Therapeutics and Zai Lab.

Antibiotics in the clinical pipeline as of December 2022

The need for new antibacterial drugs to treat the increasing global prevalence of drug-resistant bacterial infections has clearly attracted global attention, with a range of existing and upcoming funding, policy, and legislative initiatives designed to revive antibacterial R&D. It is essential to assess whether these programs are having any real-world impact and this review continues our systematic analyses that began in 2011. Direct-acting antibacterials (47), non-traditional small molecule antibacterials (5), and β-lactam/β-lactamase inhibitor combinations (10) under clinical development as of December 2022 are described, as are the three antibacterial drugs launched since 2020. Encouragingly, the increased number of early-stage clinical candidates observed in the 2019 review increased in 2022, although the number of first-time drug approvals from 2020 to 2022 was disappointingly low. It will be critical to monitor how many Phase-I and -II candidates move into Phase-III and beyond in the next few years. There was also an enhanced presence of novel antibacterial pharmacophores in early-stage trials, and at least 18 of the 26 phase-I candidates were targeted to treat Gram-negative bacteria infections. Despite the promising early-stage antibacterial pipeline, it is essential to maintain funding for antibacterial R&D and to ensure that plans to address late-stage pipeline issues succeed.

Antibiotic Therapy Strategies for Treating Gram-Negative Severe Infections in the Critically Ill: A Narrative Review

INTRODUCTION

Not enough data exist to inform the optimal duration and type of antimicrobial therapy against GN infections in critically ill patients.

METHODS

Narrative review based on a literature search through PubMed and Cochrane using the following keywords: "multi-drug resistant (MDR)", "extensively drug resistant (XDR)", "pan-drug-resistant (PDR)", "difficult-to-treat (DTR) Gram-negative infection," "antibiotic duration therapy", "antibiotic combination therapy" "antibiotic monotherapy" "Gram-negative bacteremia", "Gram-negative pneumonia", and "Gram-negative intra-abdominal infection".

RESULTS

Current literature data suggest adopting longer (≥10-14 days) courses of synergistic combination therapy due to the high global prevalence of ESBL-producing (45-50%), MDR (35%), XDR (15-20%), PDR (5.9-6.2%), and carbapenemases (CP)/metallo-β-lactamases (MBL)-producing (12.5-20%) Gram-negative (GN) microorganisms (i.e., Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumanii). On the other hand, shorter courses (≤5-7 days) of monotherapy should be limited to treating infections caused by GN with higher (≥3 antibiotic classes) antibiotic susceptibility. A general approach should be based on (i) third or further generation cephalosporins ± quinolones/aminoglycosides in the case of MDR-GN; (ii) carbapenems ± fosfomycin/aminoglycosides for extended-spectrum β-lactamases (ESBLs); and (iii) the association of old drugs with new expanded-spectrum β-lactamase inhibitors for XDR, PDR, and CP microorganisms. Therapeutic drug monitoring (TDM) in combination with minimum inhibitory concentration (MIC), bactericidal vs. bacteriostatic antibiotics, and the presence of resistance risk predictors (linked to patient, antibiotic, and microorganism) should represent variables affecting the antimicrobial strategies for treating GN infections.

CONCLUSIONS

Despite the strategies of therapy described in the results, clinicians must remember that all treatment decisions are dynamic, requiring frequent reassessments depending on both the clinical and microbiological responses of the patient.

The C5α-Methyl-Substituted Carbapenem NA-1-157 Exhibits Potent Activity against Klebsiella spp. Isolates Producing OXA-48-Type Carbapenemases

The wide spread of carbapenem-hydrolyzing β-lactamases in Gram-negative bacteria has diminished the utility of the last-resort carbapenem antibiotics, significantly narrowing the available therapeutic options. In the Enterobacteriaceae family, which includes many important clinical pathogens such as Klebsiella pneumoniae and Escherichia coli, production of class D β-lactamases from the OXA-48-type family constitutes the major mechanism of resistance to carbapenems. To address the public health threat posed by these enzymes, novel, effective therapeutics are urgently needed. Here, we report evaluation of a novel, C5α-methyl-substituted carbapenem, NA-1-157, and show that its MICs against bacteria producing OXA-48-type enzymes were reduced by 4- to 32-fold when compared to meropenem. When combined with commercial carbapenems, the potency of NA-1-157 was further enhanced, resulting in target potentiation concentrations ranging from 0.125 to 2 μg/mL. Kinetic studies demonstrated that the compound is poorly hydrolyzed by OXA-48, with a catalytic efficiency 30- to 50-fold lower than those of imipenem and meropenem. Acylation of OXA-48 by NA-1-157 was severely impaired, with a rate 10,000- to 36,000-fold slower when compared to the commercial carbapenems. Docking, molecular dynamics, and structural studies demonstrated that the presence of the C5α-methyl group in NA-1-157 creates steric clashes within the active site, leading to differences in the position and the hydrogen-bonding pattern of the compound, which are incompatible with efficient acylation. This study demonstrates that NA-1-157 is a promising novel carbapenem for treatment of infections caused by OXA-48-producing bacterial pathogens.

In vitro activities of omadacycline, eravacycline, cefiderocol, apramycin, and comparator antibiotics against Acinetobacter baumannii causing bloodstream infections in Greece, 2020-2021: a multicenter study

Resistance of Acinetobacter baumannii to multiple clinically important antimicrobials has increased to very high rates in Greece, rendering most of them obsolete. The aim of this study was to determine the molecular epidemiology and susceptibilities of A. baumannii isolates collected from different hospitals across Greece. Single-patient A. baumannii strains isolated from blood cultures (n = 271), from 19 hospitals, in a 6-month period (November 2020-April 2021) were subjected to minimum inhibitory concentration determination and molecular testing for carbapenemase, 16S rRNA methyltransferase and mcr gene detection and epidemiological evaluation. 98.9% of all isolates produced carbapenemase OXA-23. The vast majority (91.8%) of OXA-23 producers harbored the armA and were assigned mainly (94.3%) to sequence group G1, corresponding to IC II. Apramycin (EBL-1003) was the most active agent inhibiting 100% of the isolates at ≤16 mg/L, followed by cefiderocol which was active against at least 86% of them. Minocycline, colistin and ampicillin-sulbactam exhibited only sparse activity (S <19%), while eravacycline was 8- and 2-fold more active than minocycline and tigecycline respectively, by comparison of their MIC₅₀/₉₀ values. OXA-23-ArmA producing A. baumannii of international clone II appears to be the prevailing epidemiological type of this organism in Greece. Cefiderocol could provide a useful alternative for difficult to treat Gram-negative infections, while apramycin (EBL-1003), the structurally unique aminoglycoside currently in clinical development, may represent a highly promising agent against multi-drug resistant A. baumanni infections, due to its high susceptibility rates and low toxicity.

Sulbactam-durlobactam: A Step Forward in Treating Carbapenem-Resistant Acinetobacter baumannii (CRAB) Infections

Antimicrobial resistance in gram-negative pathogens, such as Acinetobacter baumannii, is a serious threat to human health. Sulbactam-durlobactam, a unique β-lactam and a β-lactamase inhibitor combination, is a novel agent targeted against carbapenem-resistant A. baumannii. This supplement provides a summary of the development of SUL-DUR, discussing its unique features and role in treating infections caused by CRAB pathogens.

The Pharmacokinetics/Pharmacodynamic Relationship of Durlobactam in Combination With Sulbactam in In Vitro and In Vivo Infection Model Systems Versus Acinetobacter baumannii-calcoaceticus Complex

Sulbactam-durlobactam is a β-lactam/β-lactamase inhibitor combination currently in development for the treatment of infections caused by Acinetobacter, including multidrug-resistant (MDR) isolates. Although sulbactam is a β-lactamase inhibitor of a subset of Ambler class A enzymes, it also demonstrates intrinsic antibacterial activity against a limited number of bacterial species, including Acinetobacter, and has been used effectively in the treatment of susceptible Acinetobacter-associated infections. Increasing prevalence of β-lactamase-mediated resistance, however, has eroded the effectiveness of sulbactam in the treatment of this pathogen. Durlobactam is a rationally designed β-lactamase inhibitor within the diazabicyclooctane (DBO) class. The compound demonstrates a broad spectrum of inhibition of serine β-lactamase activity with particularly potent activity against class D enzymes, an attribute which differentiates it from other DBO inhibitors. When combined with sulbactam, durlobactam effectively restores the susceptibility of resistant isolates through β-lactamase inhibition. The present review describes the pharmacokinetic/pharmacodynamic (PK/PD) relationship associated with the activity of sulbactam and durlobactam established in nonclinical infection models with MDR Acinetobacter baumannii isolates. This information aids in the determination of PK/PD targets for efficacy, which can be used to forecast efficacious dose regimens of the combination in humans.

Pathogen-Targeted Clinical Development to Address Unmet Medical Need: Design, Safety, and Efficacy of the ATTACK Trial

There is a crucial need for novel antibiotics to stem the tide of antimicrobial resistance, particularly against difficult to treat gram-negative pathogens like Acinetobacter baumannii-calcoaceticus complex (ABC). An innovative approach to addressing antimicrobial resistance may be pathogen-targeted development programs. Sulbactam-durlobactam (SUL-DUR) is a β-lactam/β-lactamase inhibitor combination antibiotic that is being developed to specifically target drug-resistant ABC. The development of SUL-DUR culminated with the Acinetobacter Treatment Trial Against Colistin (ATTACK) trial, a global, randomized, active-controlled phase 3 clinical trial that compared SUL-DUR with colistin for treating serious infections due to carbapenem-resistant ABC. SUL-DUR met the primary noninferiority endpoint of 28-day all-cause mortality. Furthermore, SUL-DUR had a favorable safety profile with a statistically significant lower incidence of nephrotoxicity compared with colistin. If approved, SUL-DUR could be an important treatment option for infections caused by ABC, including carbapenem-resistant and multidrug-resistant strains. The development program and the ATTACK trial highlight the potential for pathogen-targeted development programs to address the challenge of antimicrobial resistance.

Durlobactam, a Broad-Spectrum Serine β-lactamase Inhibitor, Restores Sulbactam Activity Against Acinetobacter Species

Sulbactam-durlobactam is a pathogen-targeted β-lactam/β-lactamase inhibitor combination in late-stage development for the treatment of Acinetobacter infections, including those caused by multidrug-resistant strains. Durlobactam is a member of the diazabicyclooctane class of β-lactamase inhibitors with broad-spectrum serine β-lactamase activity. Sulbactam is a first-generation, narrow-spectrum β-lactamase inhibitor that also has intrinsic antibacterial activity against Acinetobacter spp. due to its ability to inhibit penicillin-binding proteins 1 and 3. The clinical utility of sulbactam for the treatment of contemporary Acinetobacter infections has been eroded over the last decades due to its susceptibility to cleavage by numerous β-lactamases present in this species. However, when combined with durlobactam, the activity of sulbactam is restored against this problematic pathogen. The following summary describes what is known about the molecular drivers of activity and resistance as well as results from surveillance and in vivo efficacy studies for this novel combination.

Synthesis of Novel Ciprofloxacin-Avibactam Conjugates for the Development of Second-Generation Non-β-Lactam-β-Lactamase Inhibitors

To overcome the antibiotic resistance challenge, we synthesized a novel class of conjugates based on ciprofloxacin and avibactam, covalently linked by diverse amino acids. In vitro studies of these conjugates have shown improved antibacterial efficacy of avibactam when used alone against some ESKAPE pathogens, i.e., S. aureus, E. coli, and A. baumannii. Further, ceftazidime was screened in combination with all conjugates and found to be less synergistically effective than avibactam-ceftazidime co-dosing against K. pneumoniae and E. coli bacterial strains. Subsequently, the top-ranked active conjugates were investigated against the commercially available β-lactamase-II (Penicillinase from Bacillus cereus) through in vitro studies. These studies elucidated two conjugates i.e, 9 (IC₅₀ = 1.69 ± 0.35 nM) and 24b (IC₅₀ = 57.37 ± 5.39 nM), which have higher inhibition profile than avibactam (IC₅₀ = 141.08 ± 12.20 nM). These outcomes allude to avibactam integration with ciprofloxacin is a novel and fruitful approach to discovering clinically valuable next-generation non-β-lactam-β-lactamase inhibitors.

Antibiotic Adjuvants: A Versatile Approach to Combat Antibiotic Resistance

The problem of antibiotic resistance is on the rise, with multidrug-resistant strains emerging even to the last resort antibiotics. The drug discovery process is often stalled by stringent cut-offs required for effective drug design. In such a scenario, it is prudent to delve into the varying mechanisms of resistance to existing antibiotics and target them to improve antibiotic efficacy. Nonantibiotic compounds called antibiotic adjuvants which target bacterial resistance can be used in combination with obsolete drugs for an improved therapeutic regime. The field of "antibiotic adjuvants" has gained significant traction in recent years where mechanisms other than β-lactamase inhibition have been explored. This review discusses the multitude of acquired and inherent resistance mechanisms employed by bacteria to resist antibiotic action. The major focus of this review is how to target these resistance mechanisms by the use of antibiotic adjuvants. Different types of direct acting and indirect resistance breakers are discussed including enzyme inhibitors, efflux pump inhibitors, inhibitors of teichoic acid synthesis, and other cellular processes. The multifaceted class of membrane-targeting compounds with poly pharmacological effects and the potential of host immune-modulating compounds have also been reviewed. We conclude with providing insights about the existing challenges preventing clinical translation of different classes of adjuvants, especially membrane-perturbing compounds, and a framework about the possible directions which can be pursued to fill this gap. Antibiotic-adjuvant combinatorial therapy indeed has immense potential to be used as an upcoming orthogonal strategy to conventional antibiotic discovery.

Chemical Basis of Combination Therapy to Combat Antibiotic Resistance

The antimicrobial resistance crisis is a global health issue requiring discovery and development of novel therapeutics. However, conventional screening of natural products or synthetic chemical libraries is uncertain. Combination therapy using approved antibiotics with inhibitors targeting innate resistance mechanisms provides an alternative strategy to develop potent therapeutics. This review discusses the chemical structures of effective β-lactamase inhibitors, outer membrane permeabilizers, and efflux pump inhibitors that act as adjuvant molecules of classical antibiotics. Rational design of the chemical structures of adjuvants will provide methods to impart or restore efficacy to classical antibiotics for inherently antibiotic-resistant bacteria. As many bacteria have multiple resistance pathways, adjuvant molecules simultaneously targeting multiple pathways are promising approaches to combat multidrug-resistant bacterial infections.

Biochemical exploration of β-lactamase inhibitors

The alarming rise of microbial resistance to antibiotics has severely limited the efficacy of current treatment options. The prevalence of β-lactamase enzymes is a significant contributor to the emergence of antibiotic resistance. There are four classes of β-lactamases: A, B, C, and D. Class B is the metallo-β-lactamase, while the rest are serine β-lactamases. The clinical use of β-lactamase inhibitors began as an attempt to combat β-lactamase-mediated resistance. Although β-lactamase inhibitors alone are ineffective against bacteria, research has shown that combining inhibitors with antibiotics is a safe and effective treatment that not only prevents β-lactamase formation but also broadens the range of activity. These inhibitors may cause either temporary or permanent inhibition. The development of new β-lactamase inhibitors will be a primary focus of future research. This study discusses recent advances in our knowledge of the biochemistry behind β-lactam breakdown, with special emphasis on the mechanism of inhibitors for β-lactam complexes with β-lactamase. The study also focuses on the pharmacokinetic and pharmacodynamic properties of all inhibitors and then applies them in clinical settings. Our analysis and discussion of the challenges that exist in designing inhibitors might help pharmaceutical researchers address root issues and develop more effective inhibitors.

Fighting antibiotic resistance-strategies and (pre)clinical developments to find new antibacterials

Antibacterial resistance is one of the greatest threats to human health. The development of new therapeutics against bacterial pathogens has slowed drastically since the approvals of the first antibiotics in the early and mid-20^th century. Most of the currently investigated drug leads are modifications of approved antibacterials, many of which are derived from natural products. In this review, we highlight the challenges, advancements and current standing of the clinical and preclinical antibacterial research pipeline. Additionally, we present novel strategies for rejuvenating the discovery process and advocate for renewed and enthusiastic investment in the antibacterial discovery pipeline.