Dual beta-lactam therapy for serious Gram-negative infections: is it time to revisit?

Synergistic oral beta-lactam combinations for treating tuberculosis

BACKGROUND

The enormous burden of tuberculosis (TB) worldwide is a major challenge to human health, but the costs and risks associated with novel drug discovery have limited treatment options for patients. Repurposing existing antimicrobial drugs offers a promising avenue to expand TB treatment possibilities. This study aimed to explore the activity and synergy of beta-lactams in combination with a beta-lactamase inhibitor, which have been underutilized in TB treatment to date.

METHODS

Based on inhibitory concentration, oral bioavailability, and commercial availability, seven beta-lactams (cefadroxil, tebipenem, cephradine, cephalexin, cefdinir, penicillin V, and flucloxacillin), two beta-lactamase inhibitors (avibactam and clavulanate), and three second-line TB drugs (moxifloxacin, levofloxacin, and linezolid) were selected for combination in vitro testing against Mycobacterium tuberculosis H37Rv. Resazurin assays and colony forming unit enumeration were used to quantify drug efficacy, Chou-Talalay calculations were performed to identify drug synergy and Chou-Martin calculations were performed to quantify drug dose reduction index.

RESULTS

The order of activity of beta-lactams was cefadroxil > tebipenem > cephradine > cephalexin > cefdinir > penicillin V > flucloxacillin. The addition of clavulanate improved beta-lactam activity to a greater degree than the addition of avibactam. As a result, avibactam was excluded from further investigations, which focused on clavulanate. Synergy was demonstrated for cefdinir/cephradine, cefadroxil/tebipenem, cefadroxil/penicillin V, cefadroxil/cefdinir, cephalexin/tebipenem, cephalexin/penicillin V, cephalexin/cefdinir, cephalexin/cephradine, and cefadroxil/cephalexin, all with clavulanate. However, combining beta-lactams with moxifloxacin, levofloxacin, or linezolid resulted in antagonistic effects, except for the combinations of penicillin V/levofloxacin, penicillin V/moxifloxacin, and cefdinir/moxifloxacin.

CONCLUSIONS

Beta-lactam synergy may provide viable combination therapies for the treatment of TB.

Evaluation of antibiofilm activity of cefiderocol alone and in combination with imipenem against Pseudomonas aeruginosa

OBJECTIVES

The main aim of this study was to evaluate the antibiofilm activity of cefiderocol alone and in combination with imipenem vs. sessile cells of Pseudomonas aeruginosa, assessing a potential synergistic bactericidal effect.

METHODS

Ten P. aeruginosa clinical isolates from infected implants and bloodstream were included in the study. Cefiderocol was tested alone and in combination with imipenem on 24-h-old P. aeruginosa biofilm formed on porous glass beads. For each antibiotic formulation, minimum bactericidal biofilm concentration (MBBC), defined as the lowest concentration that determined a reduction of at least 3 log10 CFU/mL compared with the untreated control, was evaluated. Scanning electron microscopy (SEM) was used to investigate the biofilm of P. aeruginosa treated with cefiderocol, imipenem, or their combination.

RESULTS

Cefiderocol and imipenem were tested alone on P. aeruginosa biofilm and a reasonable reduction in the number of viable cells was observed, especially at high drug concentrations tested. The synergistic effect of cefiderocol in combination with imipenem was evaluated for five selected isolates. Cotreatment with the two drugs led to a remarkable reduction of cell viability by resulting in synergistic bactericidal activity in all tested strains and in synergistic eradicating activity in only one isolate. SEM analysis revealed that, in cefiderocol-treated biofilm, bacterial cells became more elongated than in the untreated control, forming filaments in which bacterial division seems to be inhibited.

CONCLUSIONS

Cefiderocol exhibited an encouraging antibiofilm activity against tested strains, representing a valid option for the treatment of P. aeruginosa biofilm-associated infections, especially when administered in combination with imipenem.

Synergistic effects of ceftazidime/avibactam combined with meropenem in a murine model of infection with KPC-producing Klebsiella pneumoniae

OBJECTIVES

The emergence and expansion of carbapenem-resistant Klebsiella pneumoniae infections is a concern due to the lack of 'first-line' antibiotic treatment options. The ceftazidime/avibactam is an important clinical treatment for carbapenem-resistant K. pneumoniae infections but there is an increasing number of cases of treatment failure and drug resistance. Therefore, a potential solution is combination therapies that result in synergistic activity against K. pneumoniae carbapenemase: producing K. pneumoniae (KPC-Kp) isolates and preventing the emergence of KPC mutants resistant to ceftazidime/avibactam are needed in lieu of novel antibiotics.

METHODS

To evaluate their synergistic activity, antibiotic combinations were tested against 26 KPC-Kp strains. Antibiotic resistance profiles, molecular characteristics and virulence genes were investigated by susceptibility testing and whole-genome sequencing. Antibiotic synergy was evaluated by in vitro chequerboard experiments, time-killing curves and dose-response assays. The mouse thigh model was used to confirm antibiotic combination activities in vivo. Additionally, antibiotic combinations were evaluated for their ability to prevent the emergence of ceftazidime/avibactam resistant mutations of blaKPC.

RESULTS

The combination of ceftazidime/avibactam plus meropenem showed remarkable synergistic activity against 26 strains and restored susceptibility to both the partnering antibiotics. The significant therapeutic effect of ceftazidime/avibactam combined with meropenem was also confirmed in the mouse model and bacterial loads in the thigh muscle of the combination groups were significantly reduced. Furthermore, ceftazidime/avibactam plus meropenem showed significant activity in preventing the occurrence of resistance mutations.

CONCLUSIONS

Our results indicated that the combination of ceftazidime/avibactam plus meropenem offers viable therapeutic alternatives in treating serious infections due to KPC-Kp.

Comprehensive stability analysis of 13 β-lactams and β-lactamase inhibitors in in vitro media, and novel supplement dosing strategy to mitigate thermal drug degradation

Non-clinical antibiotic development relies on in vitro susceptibility and infection model studies. Validating the achievement of the targeted drug concentrations is essential to avoid under-estimation of drug effects and over-estimation of resistance emergence. While certain β-lactams (e.g., imipenem) and β-lactamase inhibitors (BLIs; clavulanic acid) are believed to be relatively unstable, limited tangible data on their stability in commonly used in vitro media are known. We aimed to determine the thermal stability of 10 β-lactams and 3 BLIs via LC-MS/MS in cation-adjusted Mueller Hinton broth at 25 and 36°C as well as agar at 4 and 37°C, and in water at -20, 4, and 25°C. Supplement dosing algorithms were developed to achieve broth concentrations close to their target over 24 h. During incubation in broth (pH 7.25)/agar, degradation half-lives were 16.9/21.8 h for imipenem, 20.7/31.6 h for biapenem, 29.0 h for clavulanic acid (studied in broth only), 23.1/71.6 h for cefsulodin, 40.6/57.9 h for doripenem, 46.5/64.6 h for meropenem, 50.8/97.7 h for cefepime, 61.5/99.5 h for piperacillin, and >120 h for all other compounds. Broth stability decreased at higher pH. All drugs were ≥90% stable for 72 h in agar at 4°C. Degradation half-lives in water at 25°C were >200 h for all drugs except imipenem (14.7 h, at 1,000 mg/L) and doripenem (59.5 h). One imipenem supplement dose allowed concentrations to stay within ±31% of their target concentration. This study provides comprehensive stability data on β-lactams and BLIs in relevant in vitro media using LC-MS/MS. Future studies are warranted applying these data to antimicrobial susceptibility testing and assessing the impact of β-lactamase-related degradation.

Penicillin-Binding Protein 5/6 Acting as a Decoy Target in Pseudomonas aeruginosa Identified by Whole-Cell Receptor Binding and Quantitative Systems Pharmacology

The β-lactam antibiotics have been successfully used for decades to combat susceptible Pseudomonas aeruginosa, which has a notoriously difficult to penetrate outer membrane (OM). However, there is a dearth of data on target site penetration and covalent binding of penicillin-binding proteins (PBP) for β-lactams and β-lactamase inhibitors in intact bacteria. We aimed to determine the time course of PBP binding in intact and lysed cells and estimate the target site penetration and PBP access for 15 compounds in P. aeruginosa PAO1. All β-lactams (at 2 × MIC) considerably bound PBPs 1 to 4 in lysed bacteria. However, PBP binding in intact bacteria was substantially attenuated for slow but not for rapid penetrating β-lactams. Imipenem yielded 1.5 ± 0.11 log10 killing at 1h compared to <0.5 log10 killing for all other drugs. Relative to imipenem, the rate of net influx and PBP access was ~ 2-fold slower for doripenem and meropenem, 7.6-fold for avibactam, 14-fold for ceftazidime, 45-fold for cefepime, 50-fold for sulbactam, 72-fold for ertapenem, ~ 249-fold for piperacillin and aztreonam, 358-fold for tazobactam, ~547-fold for carbenicillin and ticarcillin, and 1,019-fold for cefoxitin. At 2 × MIC, the extent of PBP5/6 binding was highly correlated (r2 = 0.96) with the rate of net influx and PBP access, suggesting that PBP5/6 acted as a decoy target that should be avoided by slowly penetrating, future β-lactams. This first comprehensive assessment of the time course of PBP binding in intact and lysed P. aeruginosa explained why only imipenem killed rapidly. The developed novel covalent binding assay in intact bacteria accounts for all expressed resistance mechanisms.

Pharmacokinetics of Ceftazidime-Avibactam in Combination with Aztreonam (COMBINE) in a Phase 1, Open-Label Study of Healthy Adults

Scant pharmacokinetic (PK) data are available on ceftazidime-avibactam (CZA) and aztreonam (ATM) in combination, and it is unknown if CZA-ATM exacerbates alanine aminotransferase (ALT)/aspartate aminotransferase (AST) elevations relative to ATM alone. This phase 1 study sought to describe the PK of CZA-ATM and assess the associations between ATM exposures and ALT/AST elevations. Subjects (n = 48) were assigned to one of six cohorts (intermittent infusion [II] CZA, continuous infusion [CI] CZA, II ATM, CI ATM [8 g/daily], II CZA with II ATM [6 g/daily], and II CZA with II ATM [8 g/daily]), and study product(s) were administered for 7 days. A total of 19 subjects (40%) had ALT/AST elevations, and most (89%) occurred in the ATM/CZA-ATM cohorts. Two subjects in the CI ATM cohort experienced severe ALT/AST elevations, which halted the study. All subjects with ALT/AST elevations were asymptomatic with no other signs of liver injury, and all ALT/AST elevations resolved without sequalae after cessation of dosing. In the population PK (PopPK) analyses, CZA-ATM administration reduced total ATM clearance by 16%, had a negligible effect on total ceftazidime clearance, and was not a covariate in the avibactam PopPK model. In the exposure-response analyses, coadministration of CZA-ATM was not found to augment ALT/AST elevations. Modest associations were observed between ATM exposure (maximum concentration of drug in serum [Cmax] and area under the concentration-time curve [AUC]) and ALT/AST elevations in the analysis of subjects in the II ATM/CZA-ATM cohorts. The findings suggest that administration of CZA-ATM reduces ATM clearance but does not exacerbate AST/ALT elevations relative to ATM alone. The results also indicate that CI ATM should be used with caution.

Safety of Ceftazidime-Avibactam in Combination with Aztreonam (COMBINE) in a Phase I, Open-Label Study in Healthy Adult Volunteers

This phase I study evaluated the safety of the optimal ceftazidime-avibactam (CZA) with aztreonam (ATM) regimens identified in hollow fiber infection models of MBL-producing Enterobacterales. Eligible healthy subjects aged 18 to 45 years were assigned to one of six cohorts: 2.5 g CZA over 2 h every 8 h (approved dose), CZA continuous infusion (CI) (7.5 g daily), 2 g ATM over 2 h every 6 h, ATM CI (8 g daily), CZA (approved dose) with 1.5 g ATM over 2 h every 6 h, and CZA (approved dose) with 2 g ATM over 2 h every 6 h. Study drug(s) were administered for 7 days. The most frequently observed adverse events (AEs) were hepatic aminotransferase (ALT/AST) elevations (n = 19 subjects). Seventeen of the 19 subjects with ALT/AST elevations received ATM alone or CZA-ATM. The incidence of ALT/AST elevations was comparable between the ATM-alone and CZA-ATM cohorts. Two subjects in the ATM CI cohort experienced severe ALT/AST elevation AEs. All subjects with ALT/AST elevations were asymptomatic with no other findings suggestive of liver injury. Most other AEs were of mild to moderate severity and were similar across cohorts, except for prolonged prothrombin time (more frequent in CZA-ATM cohorts). These results suggest that CZA-ATM administered as 2-h intermittent infusions is safe and that some caution should be exercised with the use of ATM CI at an ATM dose of 8 g daily. If CZA-ATM is prescribed, clinicians are advised to monitor liver function, hematologic, and coagulation parameters. Future controlled studies are required to better define the safety and efficacy of the CZA-ATM regimens evaluated in this phase I study.

Evaluation of In Vitro Activity of Double-Carbapenem Combinations against KPC-2-, OXA-48- and NDM-Producing Escherichia coli and Klebsiella pneumoniae

Double-carbapenem combinations have shown synergistic potential against carbapenemase-producing Enterobacterales, but data remain inconclusive. This study evaluated the activity of double-carbapenem combinations against 51 clinical KPC-2-, OXA-48-, NDM-1, and NDM-5-producing Escherichia coli and Klebsiella pneumoniae and against constructed E. coli strains harboring genes encoding KPC-2, OXA-48, or NDM-1 in an otherwise isogenic background. Two-drug combinations of ertapenem, meropenem, and doripenem were evaluated in 24 h time-lapse microscopy experiments with a subsequent spot assay and in static time-kill experiments. An enhanced effect in time-lapse microscopy experiments at 24 h and synergy in the spot assay was detected with one or more combinations against 4/14 KPC-2-, 17/17 OXA-48-, 2/17 NDM-, and 1/3 NDM-1+OXA-48-producing clinical isolates. Synergy rates were higher against meropenem- and doripenem-susceptible isolates and against OXA-48 producers. NDM production was associated with significantly lower synergy rates in E. coli. In time-kill experiments with constructed KPC-2-, OXA-48- and NDM-1-producing E. coli, 24 h synergy was not observed; however, synergy at earlier time points was found against the KPC-2- and OXA-48-producing constructs. Our findings indicate that the benefit of double-carbapenem combinations against carbapenemase-producing E. coli and K. pneumoniae is limited, especially against isolates that are resistant to the constituent antibiotics and produce NDM.

Carbapenem-Only Combination Therapy against Multi-Drug Resistant Pseudomonas aeruginosa: Assessment of In Vitro and In Vivo Efficacy and Mode of Action

The aim of the study was to determine the efficacy of carbapenem-only combination treatments derived from four approved drugs (meropenem, doripenem, ertapenem and imipenem) against a MDR strain of P. aeruginosa in a Galleria mellonella larvae infection model. G. mellonella larvae were infected with P. aeruginosa NCTC 13437 (carrying the VIM 10 carbapenamase) and the efficacy of the six possible dual, four triple, and one quadruple carbapenem combination(s) were compared to their constituent monotherapies. Four of these combinations showed significantly enhanced survival compared to monotherapies and reduced the bacterial burden inside infected larvae but without complete elimination. Bacteria that survived combination therapy were slower growing, less virulent but with unchanged carbapenem MICs-observations that are consistent with a persister phenotype. In vitro time-kill assays confirmed that the combinations were bactericidal and confirmed that a low number of bacteria survived exposure. Mass spectrometry was used to quantify changes in the concentration of carbapenems in the presence of carbapenemase-carrying P. aeruginosa. The rate of degradation of individual carbapenems was altered, and often significantly reduced, when the drugs were in combinations compared with the drugs alone. These differences may account for the enhanced inhibitory effects of the combinations against carbapenem-resistant P. aeruginosa and are consistent with a 'shielding' hypothesis. In conclusion, carbapenem combinations show promise in combating MDR P. aeruginosa and are worthy of additional study and development.

High potency of sequential therapy with only β-lactam antibiotics

Evolutionary adaptation is a major source of antibiotic resistance in bacterial pathogens. Evolution-informed therapy aims to constrain resistance by accounting for bacterial evolvability. Sequential treatments with antibiotics that target different bacterial processes were previously shown to limit adaptation through genetic resistance trade-offs and negative hysteresis. Treatment with homogeneous sets of antibiotics is generally viewed to be disadvantageous as it should rapidly lead to cross-resistance. We here challenged this assumption by determining the evolutionary response of Pseudomonas aeruginosa to experimental sequential treatments involving both heterogenous and homogeneous antibiotic sets. To our surprise, we found that fast switching between only β-lactam antibiotics resulted in increased extinction of bacterial populations. We demonstrate that extinction is favored by low rates of spontaneous resistance emergence and low levels of spontaneous cross-resistance among the antibiotics in sequence. The uncovered principles may help to guide the optimized use of available antibiotics in highly potent, evolution-informed treatment designs.

Overuse of antibiotic drugs is leading to the appearance of antibiotic-resistant bacteria; this is, bacteria with mutations that allow them to survive treatment with specific antibiotics. This has made some bacterial infections difficult or impossible to treat. Learning more about how bacteria evolve resistance to antibiotics could help scientists find ways to prevent it and develop more effective treatments.

Changing antibiotics frequently may be one way to prevent bacteria from evolving resistance. That way if a bacterium acquires mutations that allow it to escape one antibiotic, another antibiotic will kill it, stopping it from dividing and preventing the appearance of descendants with resistance to several antibiotics. In order to use this approach, testing is needed to find the best sequences of antibiotics to apply and the optimal timings of treatment.

To find out more, Batra, Roemhild et al. grew bacteria in the laboratory and exposed them to different sequences of antibiotics, switching antibiotics at different time intervals. This showed that sequential treatments with different antibiotics can limit bacterial evolution, especially when antibiotics are switched quickly. Unexpectedly, one of the most effective sequences used very similar antibiotics. This was surprising because using similar antibiotics should lead to the evolution of cross-resistance, which is when a drug causes changes that make the bacterium less sensitive to other treatments. However, in the tested case, cross-resistance did not evolve when antibiotics were switched quickly, thereby ensuring efficiency of treatment.

Batra et al. show that alternating sequences of antibiotics may be an effective strategy to prevent drug resistance. Because the experiments were done in a laboratory setting it will be important to verify the results in studies in animals and humans before the approach can be used in medical or veterinary settings. If the results are confirmed, it could reduce the need to develop new antibiotics, which is expensive and time consuming.

Determining the optimal dosing of a novel combination regimen of ceftazidime/avibactam with aztreonam against NDM-1-producing Enterobacteriaceae using a hollow-fibre infection model

BACKGROUND

MBL-producing strains of Enterobacteriaceae are a major public health concern. We sought to define optimal combination regimens of ceftazidime/avibactam with aztreonam in a hollow-fibre infection model (HFIM) of MBL-producing strains of Escherichia coli and Klebsiella pneumoniae.

METHODS

E. coli ARLG-1013 (blaNDM-1, blaCTX-M, blaCMY, blaTEM) and K. pneumoniae ARLG-1002 (blaNDM-1, blaCTXM-15, blaDHA, blaSHV, blaTEM) were studied in the HFIM using simulated human dosing regimens of ceftazidime/avibactam and aztreonam. Experiments were designed to evaluate the effect of staggered versus simultaneous administration, infusion duration and aztreonam daily dose (6 g/day versus 8 g/day) on bacterial killing and resistance suppression. Prospective validation experiments for the most active combination regimens were performed in triplicate to ensure reproducibility.

RESULTS

Staggered administration of the combination (ceftazidime/avibactam followed by aztreonam) was found to be inferior to simultaneous administration. Longer infusion durations (2 h and continuous infusion) also resulted in enhanced bacterial killing relative to 30 min infusions. The rate of killing was more pronounced with 8 g/day versus 6 g/day aztreonam combination regimens for both tested strains. In the prospective validation experiments, ceftazidime/avibactam with aztreonam dosed every 8 and 6 h, respectively (ceftazidime/avibactam 2/0.5 g every 8 h + aztreonam 2 g every 6 h), or ceftazidime/avibactam with aztreonam as continuous infusions resulted in maximal bacterial killing and resistance suppression over 7 days.

CONCLUSIONS

Simultaneous administration of aztreonam 8 g/day given as a continuous or 2 h infusion with ceftazidime/avibactam resulted in complete bacterial eradication and resistance suppression. Further study of this combination is needed with additional MBL-producing Gram-negative pathogens. The safety of this double β-lactam strategy also warrants further study in Phase 1 clinical trials.

Combating Multidrug-Resistant Bacteria by Integrating a Novel Target Site Penetration and Receptor Binding Assay Platform Into Translational Modeling

Multidrug-resistant bacteria are causing a serious global health crisis. A dramatic decline in antibiotic discovery and development investment by pharmaceutical industry over the last decades has slowed the adoption of new technologies. It is imperative that we create new mechanistic insights based on latest technologies, and use translational strategies to optimize patient therapy. Although drug development has relied on minimal inhibitory concentration testing and established in vitro and mouse infection models, the limited understanding of outer membrane permeability in Gram-negative bacteria presents major challenges. Our team has developed a platform using the latest technologies to characterize target site penetration and receptor binding in intact bacteria that inform translational modeling and guide new discovery. Enhanced assays can quantify the outer membrane permeability of β-lactam antibiotics and β-lactamase inhibitors using multiplex liquid chromatography tandem mass spectrometry. While β-lactam antibiotics are known to bind to multiple different penicillin-binding proteins (PBPs), their binding profiles are almost always studied in lysed bacteria. Novel assays for PBP binding in the periplasm of intact bacteria were developed and proteins identified via proteomics. To characterize bacterial morphology changes in response to PBP binding, high-throughput flow cytometry and time-lapse confocal microscopy with fluorescent probes provide unprecedented mechanistic insights. Moreover, novel assays to quantify cytosolic receptor binding and intracellular drug concentrations inform target site occupancy. These mechanistic data are integrated by quantitative and systems pharmacology modeling to maximize bacterial killing and minimize resistance in in vitro and mouse infection models. This translational approach holds promise to identify antibiotic combination dosing strategies for patients with serious infections.

First Penicillin-Binding Protein Occupancy Patterns for 15 β-Lactams and β-Lactamase Inhibitors in Mycobacterium abscessus

Mycobacterium abscessus causes serious infections that often require over 18 months of antibiotic combination therapy. There is no standard regimen for the treatment of M. abscessus infections, and the multitude of combinations that have been used clinically have had low success rates and high rates of toxicities. With β-lactam antibiotics being safe, double β-lactam and β-lactam/β-lactamase inhibitor combinations are of interest for improving the treatment of M. abscessus infections and minimizing toxicity. However, a mechanistic approach for building these combinations is lacking since little is known about which penicillin-binding protein (PBP) target receptors are inactivated by different β-lactams in M. abscessus We determined the preferred PBP targets of 13 β-lactams and 2 β-lactamase inhibitors in two M. abscessus strains and identified PBP sequences by proteomics. The Bocillin FL binding assay was used to determine the β-lactam concentrations that half-maximally inhibited Bocillin binding (50% inhibitory concentrations [IC₅₀s]). Principal component analysis identified four clusters of PBP occupancy patterns. Carbapenems inactivated all PBPs at low concentrations (0.016 to 0.5 mg/liter) (cluster 1). Cephalosporins (cluster 2) inactivated PonA2, PonA1, and PbpA at low (0.031 to 1 mg/liter) (ceftriaxone and cefotaxime) or intermediate (0.35 to 16 mg/liter) (ceftazidime and cefoxitin) concentrations. Sulbactam, aztreonam, carumonam, mecillinam, and avibactam (cluster 3) inactivated the same PBPs as cephalosporins but required higher concentrations. Other penicillins (cluster 4) specifically targeted PbpA at 2 to 16 mg/liter. Carbapenems, ceftriaxone, and cefotaxime were the most promising β-lactams since they inactivated most or all PBPs at clinically relevant concentrations. These first PBP occupancy patterns in M. abscessus provide a mechanistic foundation for selecting and optimizing safe and effective combination therapies with β-lactams.

Clearance of Persistent Staphylococcus aureus Bacteremia in a Preterm Neonate With the Use of Combination Cefazolin and Ertapenem

Late-onset sepsis caused by Staphylococcus aureus is a serious and relatively common complication encountered by preterm neonates in NICUs. Typical treatment regimens for invasive methicillin-sensitive Staphylococcus aureus (MSSA) include semisynthetic beta lactam antibiotics, such as nafcillin. This report describes the first use of a combination of cefazolin and ertapenem to successfully treat persistent MSSA bacteremia in a preterm neonate who failed traditional first-line therapy.

Approach to the Treatment of Patients with Serious Multidrug-Resistant Pseudomonas aeruginosa Infections

Multidrug resistance(MDR) among Pseudomonas aeruginosa (PSA) isolates presents a significant clinical challenge and can substantially complicate the approach to selection of optimal antibiotic therapy. This review addresses major considerations in antibiotic selection for patients with suspected or documented serious MDR-PSA infections. Common mechanisms contributing to MDR among clinical PSA isolates are summarized. Empiric and definitive therapy considerations are addressed including the potential role of combination therapy. Newer agents with in vitro activity against MDR-PSA (e.g., ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, and cefiderocol) and their potential roles in clinical settings are discussed. Although these newer agents are promising options for the treatment of MDR-PSA, clinical data remain generally limited. Future studies are needed to determine optimal agents for the empiric and definitive treatment of MDR-PSA.

Antimicrobial Susceptibility of Mycobacterium abscessus Complex Clinical Isolates from a Chinese Tertiary Hospital

Introduction

Mycobacterium abscessus complex (MABC) is a group of important infectious agents that are highly associated with drug resistance, and antibiotic treatment is usually ineffective. This study investigated the characteristics of antimicrobial susceptibility of MABC isolates and the synergy between certain β-lactam combinations against MABC infection.

Methods

We collected 129 MABC isolates from patients with lower respiratory tract infections and categorized them into three subspecies. The minimum inhibitory concentrations (MICs) of 15 antimicrobials for the MABC isolates were determined using commercial Sensititre RAPMYCOI MIC plates and the broth microdilution method, as recommended in the CLSI (M24-A2). In addition, the MICs of imipenem, alone and with ceftazidime and/or avibactam, were assessed in vitro for all isolates. The erm(41) and rrl genes were also sequenced.

Results

The MABC isolates exhibited >80% resistance to 11 of the 15 antimicrobials. Regarding the remaining four antimicrobials, the isolates were least resistant to tigecycline (12.4%) and amikacin (3.9%), and only partially resistant to two cefoxitin (39.5%) and imipenem (40.3%). Compared with M. massiliense isolates, M. abscessus and M. bolletii isolates were more resistant to amikacin and imipenem, whereas M. abscessus was significantly less resistant to tigecycline relative to M. massiliense and M. bolletii isolates. The clarithromycin inducible resistance rate was 68.4% and 74.3% among M. bolletii and M. abscessus isolates. Furthermore, 88.7% of the M. abscessus isolates carried a T at position 28 of erm(41), which is associated with inducible clarithromycin resistance. In addition, compared to imipenem with avibactam only, the MIC₅₀ and MIC₉₀values of imipenem after adding ceftazidime plus avibactam were decreased fourfold.

Conclusion

The antimicrobial resistance rates and the characteristics of the erm(41) gene associated with inducible clarithromycin resistance were different among the three MABC subspecies. There was also synergy between imipenem and 100μg/mL ceftazidime against MABC isolates.

Dual β-lactam combination therapy for multi-drug resistant Pseudomonas aeruginosa infection: enhanced efficacy in vivo and comparison with monotherapies of penicillin-binding protein inhibition

The aim of the study was to determine the efficacy of dual β-lactam combination treatments derived from eight approved drugs against Galleria mellonella larvae infected with MDR strains of P. aeruginosa. Carbapenem-resistant P. aeruginosa NCTC 13437 and an unrelated clinical isolate were used to infect G. mellonella larvae and the efficacy of twenty-eight dual β-lactam combination therapies were compared to their constituent monotherapies. For the most potent combinations identified, penicillin-binding protein (PBP) inhibition profiles were measured and compared with each constituent antibiotic. Five of the dual β-lactam combinations resulted in greater than 70% survival of infected G. mellonella. Two combinations showed potent, enhanced efficacy versus both strains − ceftazidime + meropenem and aztreonam + meropenem. Comparison of PBP inhibition profiles revealed that the enhanced efficacy of these two dual β-lactam combinations could not be explained by more potent inhibition of PBPs or inhibition of a broader range of PBPs. A possible contribution to the enhanced efficacy of the combinations could be stimulation of innate immunity via increased haemocyte numbers compared to their constituent monotherapies. Combinations of β-lactam antibiotics show promise in overcoming MDR P. aeruginosa and are worthy of additional study and development.

Comparable Efficacy and Better Safety of Double β-Lactam Combination Therapy versus β‑Lactam plus Aminoglycoside in Gram-Negative Bacteria in Randomized, Controlled Trials

There is a great need for efficacious therapies against Gram-negative bacteria. Double β-lactam combination(s) (DBL) are relatively safe, and preclinical data are promising; however, their clinical role has not been well defined. We conducted a metaanalysis of the clinical and microbiological efficacy of DBL compared to β-lactam plus aminoglycoside combinations (BLAG). PubMed, Embase, ISI Web of Knowledge, and Cochrane Controlled Trials Register database were searched through July 2018. We included randomized controlled clinical trials that compared DBL with BLAG combinations. Clinical response was used as the primary outcome and microbiological response in Gram-negative bacteria as the secondary outcome; sensitivity analyses were performed for Pseudomonas aeruginosa, Klebsiella spp., and Escherichia coli Heterogeneity and risk of bias were assessed. Safety results were classified by systems and organs. Thirteen studies evaluated 2,771 cases for clinical response and 665 cases for microbiological response in various Gram-negative species. DBL achieved slightly, but not significantly, better clinical response (risk ratio, 1.05; 95% confidence interval [CI], 0.99 to 1.11) and microbiological response in Gram-negatives (risk ratio, 1.11; 95% CI, 0.99 to 1.25) compared with BLAG. Sensitivity analyses by pathogen showed the same trend. No significant heterogeneity across studies was found. DBL was significantly safer than BLAG regarding renal toxicity (6.6% versus 8.8%, P = 0.0338) and ototoxicity (0.7 versus 3.1%, P = 0.0137). Other adverse events were largely comparable. Overall, empirically designed DBL showed comparable clinical and microbiological responses across different Gram-negative species, and were significantly safer than BLAG. Therefore, DBL should be rationally optimized via the latest translational approaches, leveraging mechanistic insights and newer β-lactams for future evaluation in clinical trials.

Dual β-Lactam Combinations Highly Active against Mycobacterium abscessus Complex In Vitro

As a consequence of a growing population of immunocompromised individuals, including transplant recipients and cystic fibrosis patients, there has been a dramatic increase in chronic infections caused by Mycobacterium abscessus complex (MABC) strains that are usually recalcitrant to effective antibiotic therapy. The recent rise of macrolide resistance in MABC has further complicated this clinical dilemma, dramatizing the need for novel agents. The repurposing of current antibiotics is one rapid path from discovery to patient care. In this study, we have discovered that dual β-lactams, and specifically the combination of ceftazidime with either ceftaroline or imipenem, are synergistic and have clinically relevant activities, with MIC50s of 0.25 (ceftaroline with 100 µg/ml ceftazidime) and 0.5 µg/ml (imipenem with 100 µg/ml ceftazidime) against clinical MABC isolates. Similar synergy was observed in time-kill studies against the M. abscessus ATCC 19977 strain using clinically achievable concentrations of either imipenem (4 µg/ml) or ceftaroline (2 µg/ml), as the addition of ceftazidime at concentrations of ≥50 µg/ml showed a persistent bactericidal effect over 5 days. Treatment of THP-1 human macrophages infected with three different M. abscessus clinical isolates supported the in vitro findings, as the combination of 100 µg/ml ceftazidime and 0.125 µg/ml ceftaroline or 100 µg/ml ceftazidime and 0.25 µg/ml imipenem dramatically reduced the CFU counts to near baseline levels of infection. This study's finding that there is synergy between certain β-lactam combinations against M. abscessus infection provides optimism toward identifying an optimum dual β-lactam treatment regimen.IMPORTANCE The emergence of chronic MABC infections among immunocompromised populations and their inherent and acquired resistance to effective antibiotic therapy have created clinical challenges in advancing patients for transplant surgery and treating those with disease. There is an urgent need for new treatment regimens, and the repurposing of existing antibiotics provides a rapid strategy to advance a laboratory finding to patient care. Our recent discoveries that dual β-lactams, specifically the combination of ceftazidime with ceftaroline or ceftazidime with imipenem, have significant in vitro MIC values and kill curve activities and are effective against infected THP-1 human macrophages provide optimism for a dual β-lactam treatment strategy against MABC infections. The unexpected synergistic activities reported in this study create a new path of discovery to repurpose the large family of β-lactam drugs.

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.

Can Ceftazidime-Avibactam and Aztreonam Overcome β-Lactam Resistance Conferred by Metallo-β-Lactamases in Enterobacteriaceae?

Based upon knowledge of the hydrolytic profile of major β-lactamases found in Gram-negative bacteria, we tested the efficacy of the combination of ceftazidime-avibactam (CAZ-AVI) with aztreonam (ATM) against carbapenem-resistant enteric bacteria possessing metallo-β-lactamases (MBLs). Disk diffusion and agar-based antimicrobial susceptibility testing were initially performed to determine the in vitro efficacy of a unique combination of CAZ-AVI and ATM against 21 representative Enterobacteriaceae isolates with a complex molecular background that included bla_IMP, bla_NDM, bla_OXA-48, bla_CTX-M, bla_AmpC, and combinations thereof. Time-kill assays were conducted, and the in vivo efficacy of this combination was assessed in a murine neutropenic thigh infection model. By disk diffusion assay, all 21 isolates were resistant to CAZ-AVI alone, and 19/21 were resistant to ATM. The in vitro activity of CAZ-AVI in combination with ATM against diverse Enterobacteriaceae possessing MBLs was demonstrated in 17/21 isolates, where the zone of inhibition was ≥21 mm. All isolates demonstrated a reduction in CAZ-AVI agar dilution MICs with the addition of ATM. At 2 h, time-kill assays demonstrated a ≥4-log₁₀-CFU decrease for all groups that had CAZ-AVI with ATM (8 μg/ml) added, compared to the group treated with CAZ-AVI alone. In the murine neutropenic thigh infection model, an almost 4-log₁₀-CFU reduction was noted at 24 h for CAZ-AVI (32 mg/kg every 8 h [q8h]) plus ATM (32 mg/kg q8h) versus CAZ-AVI (32 mg/kg q8h) alone. The data presented herein require us to carefully consider this new therapeutic combination to treat infections caused by MBL-producing Enterobacteriaceae.

Successful Treatment of Bloodstream Infection Due to Metallo-β-Lactamase-Producing Stenotrophomonas maltophilia in a Renal Transplant Patient

Stenotrophomonas maltophilia is an emerging multidrug-resistant (MDR) opportunistic pathogen for which new antibiotic options are urgently needed. We report our clinical experience treating a 19-year-old renal transplant recipient who developed prolonged bacteremia due to metallo-β-lactamase-producing S. maltophilia refractory to conventional treatment. The infection recurred despite a prolonged course of colistimethate sodium (colistin) but resolved with the use of a novel drug combination with clinical efficacy against the patient's S. maltophilia isolate.

Treatment options for infections caused by carbapenem-resistant Enterobacteriaceae: can we apply "precision medicine" to antimicrobial chemotherapy?

INTRODUCTION

For the past three decades, carbapenems played a central role in our antibiotic armamentarium, trusted to effectively treat infections caused by drug-resistant bacteria. The utility of this class of antibiotics has been compromised by the emergence of resistance especially among Enterobacteriaceae.

AREAS COVERED

We review the current mainstays of pharmacotherapy against infections caused by carbapenem-resistant Enterobacteriaceae (CRE) including tigecycline, aminoglycosides, and rediscovered 'old' antibiotics such as fosfomycin and polymyxins, and discuss their efficacy and potential toxicity. We also summarize the contemporary clinical experience treating CRE infections with antibiotic combination therapy. Finally, we discuss ceftazidime/avibactam and imipenem/relebactam, containing a new generation of beta-lactamase inhibitors, which may offer alternatives to treat CRE infections. We critically evaluate the published literature, identify relevant clinical trials and review documents submitted to the United States Food and Drug Administration.

EXPERT OPINION

Defining the molecular mechanisms of resistance and applying insights about pharmacodynamic and pharmacokinetic properties of antibiotics, in order to maximize the impact of old and new therapeutic approaches should be the new paradigm in treating infections caused by CRE. A concerted effort is needed to carry out high-quality clinical trials that: i) establish the superiority of combination therapy vs. monotherapy; ii) confirm the role of novel beta-lactam/beta-lactamase inhibitor combinations as therapy against KPC- and OXA-48 producing Enterobacteriaceae; and, iii) evaluate new antibiotics active against CRE as they are introduced into the clinic.

Glycosyltransferases and Transpeptidases/Penicillin-Binding Proteins: Valuable Targets for New Antibacterials

Peptidoglycan (PG) is an essential macromolecular sacculus surrounding most bacteria. It is assembled by the glycosyltransferase (GT) and transpeptidase (TP) activities of multimodular penicillin-binding proteins (PBPs) within multiprotein complex machineries. Both activities are essential for the synthesis of a functional stress-bearing PG shell. Although good progress has been made in terms of the functional and structural understanding of GT, finding a clinically useful antibiotic against them has been challenging until now. In contrast, the TP/PBP module has been successfully targeted by β-lactam derivatives, but the extensive use of these antibiotics has selected resistant bacterial strains that employ a wide variety of mechanisms to escape the lethal action of these antibiotics. In addition to traditional β-lactams, other classes of molecules (non-β-lactams) that inhibit PBPs are now emerging, opening new perspectives for tackling the resistance problem while taking advantage of these valuable targets, for which a wealth of structural and functional knowledge has been accumulated. The overall evidence shows that PBPs are part of multiprotein machineries whose activities are modulated by cofactors. Perturbation of these systems could lead to lethal effects. Developing screening strategies to take advantage of these mechanisms could lead to new inhibitors of PG assembly. In this paper, we present a general background on the GTs and TPs/PBPs, a survey of recent issues of bacterial resistance and a review of recent works describing new inhibitors of these enzymes.

Automated image analysis for quantification of filamentous bacteria

Background

Antibiotics of the β-lactam group are able to alter the shape of the bacterial cell wall, e.g. filamentation or a spheroplast formation. Early determination of antimicrobial susceptibility may be complicated by filamentation of bacteria as this can be falsely interpreted as growth in systems relying on colorimetry or turbidometry (such as Vitek-2, Phoenix, MicroScan WalkAway). The objective was to examine an automated image analysis algorithm for quantification of filamentous bacteria using the 3D digital microscopy imaging system, oCelloScope.

Results

Three E. coli strains displaying different resistant profiles and differences in filamentation kinetics were used to study a novel image analysis algorithm to quantify length of bacteria and bacterial filamentation. A total of 12 β-lactam antibiotics or β-lactam–β-lactamase inhibitor combinations were analyzed for their ability to induce filamentation. Filamentation peaked at approximately 120 min with an average cell length of 30 μm.

Conclusion

The automated image analysis algorithm showed a clear ability to rapidly detect and quantify β-lactam-induced filamentation in E. coli. This rapid determination of β-lactam-mediated morphological alterations may facilitate future development of fast and accurate AST systems, which in turn will enable early targeted antimicrobial therapy. Therefore, rapid detection of β-lactam-mediated morphological changes may have important clinical implications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0583-5) contains supplementary material, which is available to authorized users.

Amoxicillin plus temocillin as an alternative empiric therapy for the treatment of severe hospital-acquired pneumonia: results from a retrospective audit

A formulary decision was made at a large provider of acute hospital services in Surrey to replace piperacillin/tazobactam with amoxicillin+temocillin for the empiric treatment of severe hospital-acquired pneumonia. This decision was made because the use of broad-spectrum-β-lactam antibiotics is a known risk factor for Clostridium difficile infection (CDI) and for the selection of resistance. After the antibiotic formulary was changed, a retrospective audit was conducted to assess the effect of this change. Data from patients hospitalised between January 2011 and July 2012 for severe hospital-acquired pneumonia and treated empirically with piperacillin/tazobactam or amoxicillin+temocillin were reviewed retrospectively. Clinical characteristics of patients, data related to the episode of pneumonia, clinical success and incidence of significant diarrhoea and CDI were analysed. One hundred ninety-two episodes of severe hospital-acquired pneumonia in 188 patients were identified from hospital records. Ninety-eight patients received piperacillin/tazobactam and 94 amoxicillin+temocillin. At baseline, the two treatment groups were comparable, except that more patients with renal insufficiency were treated with piperacillin/tazobactam. Clinical success was comparable (80 versus 82 %; P = 0.86), but differences were observed between piperacillin/tazobactam and amoxicillin+temocillin for the rates of significant diarrhoea (34 versus 4 %, respectively; P < 0.0001) and for CDI (7 versus 0 %, respectively; P < 0.0028). This preliminary study suggests that the combination amoxicillin+temocillin is a viable alternative to piperacillin/tazobactam for the treatment of severe hospital-acquired pneumonia. This combination appears to be associated with fewer gastrointestinal adverse events. Further studies are needed to evaluate the place of amoxicillin+temocillin as empiric treatment of severe hospital-acquired pneumonia.

Characterization of Escherichia coli NDM isolates with decreased susceptibility to aztreonam/avibactam: role of a novel insertion in PBP3

OBJECTIVES

The spread of NDM-1 amongst Enterobacteriaceae has highlighted a significant threat to the clinical management of serious infections. The combination of aztreonam and avibactam, a non-β-lactam β-lactamase inhibitor, may provide a much-needed therapeutic alternative. This combination was potent against most NDM-containing Enterobacteriaceae, although activity was diminished against many Escherichia coli isolates. These E. coli isolates were characterized to elucidate the mechanism of decreased susceptibility to aztreonam/avibactam.

METHODS

MIC determinations were performed using broth microdilution, and whole-genome sequencing was performed to enable sequence-based analyses.

RESULTS

The decreased susceptibility was not due to avibactam being unable to inhibit the serine β-lactamases found in the E. coli isolates. Rather, it was manifested by a four-amino-acid insertion in PBP3. This same insertion was also found in non-NDM-containing E. coli that had reduced susceptibility to aztreonam/avibactam. Construction of an isogenic mutant confirmed that this insertion resulted in decreased susceptibility to aztreonam and several cephalosporins, but had no impact on carbapenem potency. Structural analysis suggests that this insertion will impact the accessibility of the β-lactam drugs to the transpeptidase pocket of PBP3.

CONCLUSIONS

The acquisition of β-lactamases is the predominant mechanism of β-lactam resistance in Enterobacteriaceae. We have demonstrated that small PBP3 changes will affect the susceptibility to a broad range of β-lactams. These changes were identified in multiple MLST lineages of E. coli, and were enriched in NDM-containing isolates. However, they were not present in other key species of Enterobacteriaceae despite significant conservation among the PBP3 proteins.