Evolution of Cefiderocol Non-Susceptibility in Pseudomonas aeruginosa in a Patient Without Previous Exposure to the Antibiotic

Cefiderocol heteroresistance associated with mutations in TonB-dependent receptor genes in Pseudomonas aeruginosa of clinical origin

The siderophore-cephalosporin cefiderocol (FDC) presents a promising treatment option for carbapenem-resistant (CR) P. aeruginosa (PA). FDC circumvents traditional porin and efflux-mediated resistance by utilizing TonB-dependent receptors (TBDRs) to access the periplasmic space. Emerging FDC resistance has been associated with loss of function mutations within TBDR genes or the regulatory genes controlling TBDR expression. Further, difficulties with antimicrobial susceptibility testing (AST) and unexpected negative clinical treatment outcomes have prompted concerns for heteroresistance, where a single lineage isolate contains resistant subpopulations not detectable by standard AST. This study aimed to evaluate the prevalence of TBDR mutations among clinical isolates of P. aeruginosa and the phenotypic effect on FDC susceptibility and heteroresistance. We evaluated the sequence of pirR, pirS, pirA, piuA, or piuD from 498 unique isolates collected before the introduction of FDC from four clinical sites in Portland, OR (1), Houston, TX (2), and Santiago, Chile (1). At some clinical sites, TBDR mutations were seen in up to 25% of isolates, and insertion, deletion, or frameshift mutations were predicted to impair protein function were seen in 3% of all isolates (n = 15). Using population analysis profile testing, we found that P. aeruginosa with major TBDR mutations were enriched for a heteroresistant phenotype and undergo a shift in the susceptibility distribution of the population as compared to susceptible strains with wild-type TBDR genes. Our results indicate that mutations in TBDR genes predate the clinical introduction of FDC, and these mutations may predispose to the emergence of FDC resistance.

Resistance to ceftazidime-avibactam and other new β-lactams in Pseudomonas aeruginosa clinical isolates: a multi-center surveillance study

New β-lactam-β-lactamase inhibitor combinations represent last-resort antibiotics to treat infections caused by multidrug-resistant Pseudomonas aeruginosa. Carbapenemase gene acquisition can limit their spectrum of activity, and reports of resistance toward these new molecules are increasing. In this multi-center study, we evaluated the prevalence of resistance to ceftazidime-avibactam (CZA) and comparators among P. aeruginosa clinical isolates from bloodstream infections, hospital-acquired or ventilator-associated pneumonia, and urinary tract infections, circulating in Southern Italy. We also investigated the clonality and content of relevant β-lactam resistance mechanisms of CZA-resistant (CZAR) isolates. A total of 120 P. aeruginosa isolates were collected. CZA was among the most active β-lactams, retaining susceptibility in the 81.7% of cases, preceded by cefiderocol (95.8%) and followed by ceftolozane-tazobactam (79.2%), meropenem-vaborbactam (76.1%), imipenem-relebactam (75%), and aztreonam (69.6%). Among non-β-lactams, colistin and amikacin were active against 100% and 85.8% of isolates respectively. In CZAR strains subjected to whole-genome sequencing (n = 18), resistance was mainly due to the expression of metallo-β-lactamases (66.6% VIM-type and 5.5% FIM-1), followed by PER-1 (16.6%) and GES-1 (5.5%) extended-spectrum β-lactamases, mostly carried by international high-risk clones (ST111 and ST235). Of note, two strains producing the PER-1 enzyme were resistant to all β-lactams, including cefiderocol. In conclusion, the CZA resistance rate among P. aeruginosa clinical isolates in Southern Italy remained low. CZAR isolates were mostly metallo-β-lactamases producers and belonging to ST111 and ST253 epidemic clones. It is important to implement robust surveillance systems to monitor emergence of new resistance mechanisms and to limit the spread of P. aeruginosa high-risk clones.

IMPORTANCE

Multidrug-resistant Pseudomonas aeruginosa infections are a growing threat due to the limited therapeutic options available. Ceftazidime-avibactam (CZA) is among the last-resort antibiotics for the treatment of difficult-to-treat P. aeruginosa infections, although resistance due to the acquisition of transferable β-lactamase genes is increasing. With this work, we report that CZA represents a highly active antipseudomonal β-lactam compound (after cefiderocol), and that metallo-β-lactamases (VIM-type) and extended-spectrum β-lactamases (GES and PER-type) production is the major factor underlying CZA resistance in isolates from Southern Italian hospitals. In addition, we reported that such resistance mechanisms were mainly carried by the international high-risk clones ST111 and ST235.

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.

Siderophores promote cooperative interspecies and intraspecies cross-protection against antibiotics in vitro

The antibiotic cefiderocol hijacks iron transporters to facilitate its uptake and resists β-lactamase degradation. While effective, resistance has been detected clinically with unknown mechanisms. Here, using experimental evolution, we identified cefiderocol resistance mutations in Pseudomonas aeruginosa. Resistance was multifactorial in host-mimicking growth media, led to multidrug resistance and paid fitness costs in cefiderocol-free environments. However, kin selection drove some resistant populations to cross-protect susceptible individuals from killing by increasing pyoverdine secretion via a two-component sensor mutation. While pyochelin sensitized P. aeruginosa to cefiderocol killing, pyoverdine and the enterobacteria siderophore enterobactin displaced iron from cefiderocol, preventing uptake by susceptible cells. Among 113 P. aeruginosa intensive care unit clinical isolates, pyoverdine production directly correlated with cefiderocol tolerance, and high pyoverdine producing isolates cross-protected susceptible P. aeruginosa and other Gram-negative bacteria. These in vitro data show that antibiotic cross-protection can occur via degradation-independent mechanisms and siderophores can serve unexpected protective cooperative roles in polymicrobial communities.

Cefiderocol heteroresistance associated with mutations in TonB-dependent receptor genes in Pseudomonas aeruginosa of clinical origin

The siderophore-cephalosporin cefiderocol(FDC) presents a promising treatment option for carbapenem-resistant (CR) P. aeruginosa (PA). FDC circumvents traditional porin and efflux mediated resistance by utilizing TonB-dependent receptors (TBDRs) to access the periplasmic space. Emerging FDC resistance has been associated with loss of function mutations within TBDR genes or the regulatory genes controlling TBDR expression. Further, difficulties with antimicrobial susceptibility testing (AST) and unexpected negative clinical treatment outcomes have prompted concerns for heteroresistance, where a single lineage isolate contains resistant subpopulations not detectable by standard AST. This study aimed to evaluate the prevalence of TBDR mutations among clinical isolates of P. aeruginosa and the phenotypic effect on FDC susceptibility and heteroresistance. We evaluated the sequence of pirR , pirS , pirA , piuA or piuD from 498 unique isolates collected before the introduction of FDC from 4 clinical sites in Portland, OR (1), Houston, TX (2), and Santiago, Chile (1). At some clinical sites, TBDR mutations were seen in up to 25% of isolates, and insertion, deletion, or frameshift mutations were predicted to impair protein function were seen in 3% of all isolates (n=15). Using population analysis profile testing, we found that P. aeruginosa with major TBDR mutations were enriched for a heteroresistant phenotype and undergo a shift in the susceptibility distribution of the population as compared to susceptible strains with wild type TBDR genes. Our results indicate that mutations in TBDR genes predate the clinical introduction of FDC, and these mutations may predispose to the emergence of FDC resistance.

The emergence of cefiderocol resistance in Pseudomonas aeruginosa from a heteroresistant isolate during prolonged therapy

Cefiderocol is a siderophore cephalosporin designed to target multi-drug-resistant Gram-negative bacteria. Previously, the emergence of cefiderocol non-susceptibility has been associated with mutations in the chromosomal cephalosporinase (PDC) along with mutations in the PirA and PiuA/D TonB-dependent receptor pathways. Here, we report a clinical case of cefiderocol-resistant P. aeruginosa that emerged in a patient during treatment. This resistance was associated with mutations not previously reported, suggesting potential novel pathways to cefiderocol resistance.

Approaches to Testing Novel β-Lactam and β-Lactam Combination Agents in the Clinical Laboratory

The rapid emergence of multi-drug resistant Gram-negative pathogens has driven the introduction of novel β-lactam combination agents (BLCs) to the antibiotic market: ceftolozane-tazobactam, ceftazidime-avibactam, meropenem-vaborbactam, imipenem-relebactam, cefiderocol, and sulbactam-durlobactam. These agents are equipped with innovative mechanisms that confer broad Gram-negative activity, notably against certain challenging carbapenemases. While their introduction offers a beacon of hope, clinical microbiology laboratories must navigate the complexities of susceptibility testing for these agents due to their diverse activity profiles against specific β-lactamases and the possibility of acquired resistance mechanisms in some bacterial isolates. This review explores the complexities of these novel antimicrobial agents detailing the intricacies of their application, providing guidance on the nuances of susceptibility testing, interpretation, and result reporting in clinical microbiology laboratories.

Extreme resistance to the novel siderophore-cephalosporin cefiderocol in an extensively drug-resistant Klebsiella pneumoniae strain causing fatal pneumonia with sepsis: genomic analysis and synergistic combinations for resistance reversal

Cefiderocol (CFDC) is the first-in-class siderophore-cephalosporin. Klebsiella pneumoniae strain that is extremely resistant to CFDC (MIC: 256 µg/ml) was isolated for the first time in the United Arab Emirates from a patient with pneumonia and sepsis. It belonged to sequence-type 14 (ST14), with a novel core genome ST. Resistance was driven by the co-expression of β-lactamases (blaNDM-1, blaOXA-232 and blaCTX-M-15) and a mutation in catecholate-siderophore receptor, utilized by CFDC to enter the bacterial cell. Synergistic combinations (β-lactamase inhibitors, aztreonam plus CFDC) re-sensitized the bacteria to CFDC. Although CFDC resistance is multifactorial, the combination with β-lactamase inhibitors represents a promising approach in resistance reversal for fighting superbugs.

Detection of Carbapenemase Encoding Gene and Resistance to Cefiderocol in Hospital and Community eXtensive Drug Resistance and Carbapenem-Resistant Pseudomonas aeruginosa Strains in Morocco

Pseudomonas aeruginosa (Pa) remains among clinically-significant Gram-negative species. The carbapenems are often the last resort for treating infections due to multidrug resistant isolates such as Pa. The carbapenems' efficacy is increasingly compromised by the emergence and the rapid spread of Pa carrying carbapenemases which represent a serious threat to public health. This study aimed to establish the resistance profile and to identify carbapenemase genes in isolates with imipenem resistant phenotypes. Among 134 Pa isolates collected both in the community (46) and hospital (88) from January 2021 to December 2021 in Morocco, 18 (8 were from the community and 10 from the hospital settings) were carbapenem resistant. The identification of these strains has been confirmed using matrix assisted laser desorption ionization-time of flight (MALDI-TOF). The antibiotic susceptibility testing against 16 antibiotics was carried out and interpreted according to the recommendations of the European Committee on Antimicrobial Susceptibility Testing (2021). The worrying antibiotics resistance profiles, which spread to cefiderocol for two isolates, were obtained for all isolates, which were eXtensive Drug Resistance showing highly resistant to all antibiotic categories tested, even to ceftolozane-tazobactam. Colistin (100% susceptible) and cefiderocol (88.88%) were the most active agents against carbapenem-resistant Pa (CRPa). Phenotypic detection by NP-CARBA and NG-CARBA tests of metallo‑β‑lactamase (MβL) production was confirmed by PCR amplification and sequencing. Three CRPa isolates coharboring blaVIM-2-blaNDM-1 (two isolates) and blaVIM-2-blaIMP-8 (one isolate) genes were detected. In this study, we describe the coexistence of these MβL genes and the cefiderocol resistance in CRPa strains in Morocco. The alarming antibiotic resistance patterns of all these CRPa isolates and their resistance genes emphasize the importance of antimicrobial susceptibility testing in the choice of antibiotics for treating Pa infections.

In vitro activity of cefiderocol against Pseudomonas aeruginosa demonstrating evolved resistance to novel β-lactam/β-lactamase inhibitors

Background

Cefiderocol demonstrates excellent activity against MDR Pseudomonas aeruginosa; however, the activity against isolates from patients previously treated with β-lactam agents is unknown. We aimed to determine the activity of cefiderocol against P. aeruginosa collected before and after treatment with traditional β-lactams and new β-lactam/β-lactamase inhibitors.

Methods

Cefiderocol MICs were determined in triplicate in iron-depleted cation-adjusted Mueller-Hinton broth and compared with β-lactam MICs tested by standard methods. All isolates underwent WGS analysis to identify mutations associated with resistance.

Results

One hundred and seventy-eight P. aeruginosa isolates were evaluated; 48% (86/178) were non-susceptible to ceftazidime/avibactam, ceftolozane/tazobactam and/or imipenem/relebactam. The cefiderocol MIC50 and MIC90 were 0.12 and 1 mg/L, respectively. Median cefiderocol MICs did not vary against isolates classified as MDR, XDR, or those non-susceptible to ceftazidime/avibactam, ceftolozane/tazobactam and/or imipenem/relebactam when compared with non-MDR isolates. Against isolates collected from patients previously treated with ceftolozane/tazobactam, cefiderocol MICs were increased 4-fold compared with baseline. Cross-resistance to cefiderocol was identified in 21% (3/14) of patients who developed treatment-emergent resistance to ceftolozane/tazobactam. Overall, 6% (11/178) of isolates demonstrated cefiderocol MICs ≥2 mg/L, which were disproportionately collected from patients previously treated with ceftolozane/tazobactam (73%; 8/11). Isolates with reduced cefiderocol susceptibility harboured mutations in ampC, tonB-dependent receptors, the response regulator pirR and ftsI.

Conclusions

Cefiderocol demonstrates excellent in vitro activity against P. aeruginosa isolates exposed to other novel β-lactam agents; however, some exceptions were identified. Cross-resistance between cefiderocol and ceftolozane/tazobactam was evident, but not with ceftazidime/avibactam or imipenem/relebactam. Reduced cefiderocol susceptibility was mediated by mutations in ampC and tonB-dependent receptors.

Rapid development of cefiderocol resistance in a carbapenem-resistant Pseudomonas aeruginosa isolate associated with mutations in the pyoverdine biosynthesis pathway

Here we report the in vivo development of cefiderocol resistance within 11 days after therapy initiation in a critically ill patient with bloodstream infection, infection of peri-anal fistula, and pneumonia caused by a VIM-2 harbouring, carbapenem-resistant Pseudomonas aeruginosa. Compared to a cefiderocol-naïve P. aeruginosa blood culture isolate, agar diffusion susceptibility testing found a reduced cefiderocol inhibition zone diameter in a P. aeruginosa recovered from peri-anal abscess tissue cultures after initiation of cefiderocol therapy. Subsequent whole-genome sequencing suggested that both isolates were of clonal origin. Comparison of genomes found an accumulation of missense mutations within pvdP, pvdE, pvdJ, and pvdD (i.e. genes associated with biosynthesis of pyoverdine), the main siderophore produced by P. aeruginosa. Quantification of pyoverdine production under iron-depleted conditions showed a significantly (P = 0.0003) higher pyoverdine production by the cefiderocol-resistant isolate. While pyoverdine quantity alone appears not to be decisive for cefiderocol resistance, the reported case highlights the potentially rapid emergence of cefiderocol resistance in P. aeruginosa and points towards a potential involvement of iron up-take systems in this process.

Metallo-sideromycin as a dual functional complex for combating antimicrobial resistance

The rapid emergence of antimicrobial resistance (AMR) pathogens highlights the urgent need to approach this global burden with alternative strategies. Cefiderocol (Fetroja®) is a clinically-used sideromycin, that is utilized for the treatment of severe drug-resistant infections, caused by Gram-negative bacteria; there is evidence of cefiderocol-resistance occurring in bacterial strains however. To increase the efficacy and extend the life-span of sideromycins, we demonstrate strong synergisms between cefiderocol and metallodrugs (e.g., colloidal bismuth citrate (CBS)), against Pseudomonas aeruginosa and Burkholderia cepacia. Moreover, CBS enhances cefiderocol efficacy against biofilm formation, suppresses the resistance development in P. aeruginosa and resensitizes clinically isolated resistant P. aeruginosa to cefiderocol. Notably, the co-therapy of CBS and cefiderocol significantly increases the survival rate of mice and decreases bacterial loads in the lung in a murine acute pneumonia model. The observed phenomena are partially attributable to the competitive binding of Bi3+ to cefiderocol with Fe3+, leading to enhanced uptake of Bi3+ and reduced levels of Fe3+ in cells. Our studies provide insight into the antimicrobial potential of metallo-sideromycins.

Case Commentary: Successful Use of Cefepime/Zidebactam (WCK 5222) as a Salvage Therapy for the Treatment of Disseminated Extensively Drug-Resistant New Delhi Metallo-β-Lactamase-Producing Pseudomonas aeruginosa Infection in an Adult Patient with Acute T-Cell Leukemia

Multidrug-resistant/extensively drug-resistant (MDR/XDR) Pseudomonas aeruginosa (PA) are critical antimicrobial resistance threats. Despite their increasing prevalence, treatment options for metallo-β-lactamase (MBL)-producing PA are limited, especially for New Delhi metallo-β-lactamase (NDM) producers. Pending further clinical studies, this case provides support for limited-scope use of cefepime-zidebactam for treating disseminated infections secondary to NDM-producing XDR PA. Susceptibilities should be tested and/or alternative regimens considered when treating isolates with alternative MBLs or increased efflux pump expression because some in vitro data suggest associated loss of cefepime-zidebactam susceptibility.

Induced Heteroresistance in Carbapenem-Resistant Acinetobacter baumannii (CRAB) via Exposure to Human Pleural Fluid (HPF) and Its Impact on Cefiderocol Susceptibility

Infections caused by Carbapenem-resistant Acinetobacter baumannii (CRAB) isolates, such as hospital-acquired pneumonia (HAP), bacteremia, and skin and soft tissue infections, among others, are particularly challenging to treat. Cefiderocol, a chlorocatechol-substituted siderophore antibiotic, was approved by the U.S. Food and Drug Administration (FDA) in 2019 and prescribed for the treatment of CRAB infections. Despite the initial positive treatment outcomes with this antimicrobial, recent studies reported a higher-than-average all-cause mortality rate in patients treated with cefiderocol compared to the best available therapy. The cause(s) behind these outcomes remains unconfirmed. A plausible hypothesis is heteroresistance, a phenotype characterized by the survival of a small proportion of cells in a population that is seemingly isogenic. Recent results have demonstrated that the addition of human fluids to CRAB cultures leads to cefiderocol heteroresistance. Here, we describe the molecular and phenotypic analyses of CRAB heteroresistant bacterial subpopulations to better understand the nature of the less-than-expected successful outcomes after cefiderocol treatment. Isolation of heteroresistant variants of the CRAB strain AMA40 was carried out in cultures supplemented with cefiderocol and human pleural fluid (HPF). Two AMA40 variants, AMA40 IHC1 and IHC2, were resistant to cefiderocol. To identify mutations and gene expression changes associated with cefiderocol heteroresistance, we subjected these variants to whole genome sequencing and global transcriptional analysis. We then assessed the impact of these mutations on the pharmacodynamic activity of cefiderocol via susceptibility testing, EDTA and boronic acid inhibition analysis, biofilm formation, and static time-kill assays. Heteroresistant variants AMA40 IHC1 and AMA40 IHC2 have 53 chromosomal mutations, of which 40 are common to both strains. None of the mutations occurred in genes associated with high affinity iron-uptake systems or β-lactam resistance. However, transcriptional analyses demonstrated significant modifications in levels of expression of genes associated with iron-uptake systems or β-lactam resistance. The blaNDM-1 and blaADC-2, as well as various iron-uptake system genes, were expressed at higher levels than the parental strain. On the other hand, the carO and ompA genes' expression was reduced. One of the mutations common to both heteroresistant strains was mapped within ppiA, a gene associated with iron homeostasis in other species. Static time-kill assays demonstrated that supplementing cation-adjusted Mueller-Hinton broth with human serum albumin (HAS), the main protein component of HPF, considerably reduced cefiderocol killing activity for all three strains tested. Notably, collateral resistance to amikacin was observed in both variants. We conclude that exposing CRAB to fluids with high HSA concentrations facilitates the rise of heteroresistance associated with point mutations and transcriptional upregulation of genes coding for β-lactamases and biofilm formation. The findings from this study hold significant implications for understanding the emergence of CRAB resistance mechanisms against cefiderocol treatment. This understanding is vital for the development of treatment guidelines that can effectively address the challenges posed by CRAB infections.

Metallo-β-lactamase-mediated antimicrobial resistance and progress in inhibitor discovery

Resistance to β-lactam antibiotics is rapidly growing, substantially due to the spread of serine-β-lactamases (SBLs) and metallo-β-lactamases (MBLs), which efficiently catalyse β-lactam hydrolysis. Combinations of a β-lactam antibiotic with an SBL inhibitor have been clinically successful; however, no MBL inhibitors have been developed for clinical use. MBLs are a worrying resistance vector because they catalyse hydrolysis of all β-lactam antibiotic classes, except the monobactams, and they are being disseminated across many bacterial species worldwide. Here we review the classification, structures, substrate profiles, and inhibition mechanisms of MBLs, highlighting current clinical problems due to MBL-mediated resistance and progress in understanding and combating MBL-mediated resistance.

An Overview of Cefiderocol's Therapeutic Potential and Underlying Resistance Mechanisms

Antimicrobial resistance continues to increase globally and treatment of difficult-to-treat (DTT) infections, mostly associated with carbapenem-resistant (CR) Pseudomonas aeruginosa, CR Acinetobacter baumannii, and CR- and third-generation-cephalosporins-resistant Enterobacterales remains a challenge for the clinician. The recent approval of cefiderocol has broaden the armamentarium for the treatment of patients with DTT infections. Cefiderocol is a siderophore cephalosporin that has shown excellent antibacterial activity, in part due to its innovative way of cell permeation. It is relatively stable compared to most commonly found carbapenamases. However, some resistant mechanisms to cefiderocol have already been identified and reduced susceptibility has developed during patient treatment, highlighting that the clinical use of cefiderocol must be rational. In this review, we summarize the current available treatments against the former resistant bacteria, and we revise and discuss the mechanism of action of cefiderocol, underlying the biological function of siderophores, the therapeutic potential of cefiderocol, and the mechanisms of resistance reported so far.

Exploring Cefiderocol Resistance Mechanisms in Burkholderia pseudomallei

Cefiderocol is a siderophore cephalosporin designed mainly for treatment of infections caused by β-lactam and multidrug-resistant Gram-negative bacteria. Burkholderia pseudomallei clinical isolates are usually highly cefiderocol susceptible, with in vitro resistance found in a few isolates. Resistance in clinical B. pseudomallei isolates from Australia is caused by a hitherto uncharacterized mechanism. We show that, like in other Gram-negatives, the PiuA outer membrane receptor plays a major role in cefiderocol nonsusceptibility in isolates from Malaysia.

Cefiderocol Treatment for Severe Infections due to Difficult-to-Treat-Resistant Non-Fermentative Gram-Negative Bacilli in ICU Patients: A Case Series and Narrative Literature Review

Cefiderocol (FDC) is a siderophore cephalosporin now recognized as a new weapon in the treatment of difficult-to-treat-resistant (DTR) Gram-negative pathogens, including carbapenemase-producing enterobacterales and non-fermentative Gram-negative bacilli (GNB). This article reports our experience with an FDC-based regimen in the treatment of 16 extremely severe patients (invasive mechanical ventilation, 15/16; extracorporeal membrane oxygenation, 9/16; and renal replacement therapy, 8/16) infected with DTR GNB. Our case series provides detailed insight into the pharmacokinetic profile and the microbiological data in real-life conditions. In the narrative review, we discuss the interest of FDC in the treatment of non-fermentative GNB in critically ill patients. We reviewed the microbiological spectrum, resistance mechanisms, pharmacokinetics/pharmacodynamics, efficacy and safety profiles, and real-world evidence for FDC. On the basis of our experience and the available literature, we discuss the optimal FDC-based regimen, FDC dosage, and duration of therapy in critically ill patients with DTR non-fermentative GNB infections.

In Vitro Activity of Cefiderocol on Multiresistant Bacterial Strains and Genomic Analysis of Two Cefiderocol Resistant Strains

Cefiderocol is a new siderophore cephalosporin that is effective against multidrug-resistant Gram-negative bacteria, including carbapenem-resistant strains. The aim of this study was to evaluate the activity of this new antimicrobial agent against a collection of pathogens using broth microdilution assays and to analyze the possible mechanism of cefiderocol resistance in two resistant Klebsiella pneumoniae isolates. One hundred and ten isolates were tested, comprising 67 Enterobacterales, two Acinetobacter baumannii, one Achromobacter xylosoxidans, 33 Pseudomonas aeruginosa and seven Stenotrophomonas maltophilia. Cefiderocol showed good in vitro activity, with an MIC < 2 μg/mL, and was able to inhibit 94% of the tested isolates. We observed a resistance rate of 6%. The resistant isolates consisted of six Klebsiella pneumoniae and one Escherichia coli, leading to a resistance rate of 10.4% among the Enterobacterales. Whole-genome sequencing analysis was performed on two cefiderocol-resistant Klebsiella pneumoniae isolates to investigate the possible mutations responsible for the observed resistance. Both strains belonged to ST383 and harbored different resistant and virulence genes. The analysis of genes involved in iron uptake and transport showed the presence of different mutations located in fhuA, fepA, iutA, cirA, sitC, apbC, fepG, fepC, fetB, yicI, yicJ, and yicL. Furthermore, for the first time, to the best of our knowledge, we described two Klebsiella pneumoniae isolates that synthesize a truncated fecA protein due to the transition from G to A, leading to a premature stop codon in the amino acid position 569, and a TonB protein carrying a 4-amino acid insertion (PKPK) after Lysine 103. In conclusion, our data show that cefiderocol is an effective drug against multidrug-resistant Gram-negative bacteria. However, the higher resistance rate observed in Enterobacterales underlines the need for active surveillance to limit the spread of these pathogens and to avoid the risks associated with the emergence of resistance to new drugs.

Evaluation of Variability in Interpretation of Disk Diffusion Testing for Cefiderocol Using Different Brands of Mueller-Hinton Agar

BACKGROUND

Cefiderocol is a new antibiotic used to treat infections with antibiotic resistant Gram-negative bacilli. The impact of differences between Mueller-Hinton agar (MHA) brands on susceptibility testing is underexplored. Compounding the implementation of cefiderocol susceptibility testing is a lack of harmonization between different regulatory body breakpoint criteria.

METHODS

We performed Kirby-Bauer disk diffusion using BD, Hardy, and Remel MHA, in addition to broth microdilution for Acinetobacter baumannii (n = 25), Enterobacterales (n = 25), Stenotrophomonas maltophilia (n = 24), and Pseudomonas aeruginosa (n = 23). We analyzed disk diffusion diameters and minimum inhibitory concentrations using interpretive criteria from the Clinical and Laboratory Standards Institute (CLSI), US Food and Drug Administration (FDA), and the European Committee on Antimicrobial Susceptibility Testing (EUCAST).

RESULTS

Breakpoint criteria impacted interpretation of susceptibly testing results, for example with the broth microdilution we found 8% (2/25) of A. baumannii isolates change interpretation between CLSI and EUCAST and 32% (8/25) change between CLSI and FDA, 12% (3/25) of Enterobacterales change between CLSI and EUCAST, 13% (3/23) of P. aeruginosa interpretations change between CLSI and FDA, and 4% (1/25) S. maltophilia change between CLSI and FDA. There was a significant difference between the zone disk diffusion diameters for P. aeruginosa and S. maltophilia between Hardy and BD; which changed interpretation (using CLSI criteria) for 8.7% (2/23) for P. aeruginosa but 0% (0/24) for S. maltophilia.

CONCLUSIONS

Breakpoint criteria impact cefiderocol susceptibility testing interpretation for broth microdilution and disk diffusion. Choice of MHA brand can also affect result interpretation.

Real-world experience with cefiderocol therapy for Pseudomonas aeruginosa and other multidrug resistant gram-negative infections within the Veterans Health Administration, 2019-2022

Objective

Single-center and regional studies have reported outcomes after treatment with cefiderocol, a novel siderophore cephalosporin. We report on real-world use, clinical outcomes, and microbiological outcomes with cefiderocol therapy within the Veterans' Health Administration (VHA).

Design

Prospective, observational descriptive study.

Setting

Veterans' Health Administration, 132 sites across the United States, during 2019-2022.

Patients

This study included patients admitted to any VHA medical center who received cefiderocol for ≥2 days.

Methods

Data were obtained from the VHA Corporate Data Warehouse and through manual chart review. We extracted clinical and microbiologic characteristics and outcomes.

Results

In total, 8,763,652 patients received 1,142,940,842 prescriptions during the study period. Of these, 48 unique individuals received cefiderocol. The median age of this cohort was 70.5 years (IQR, 60.5-74), and the median Charlson comorbidity score was 6 (IQR, 3-9). The most common infectious syndromes were lower respiratory tract infection in 23 patients (47.9%) and urinary tract infection in 14 patients (29.2%). The most common pathogen cultured was P. aeruginosa in 30 patients (62.5%). The clinical failure rate was 35.4% (17 of 48), and 15 (88.2%) of these 17 patients died within 3 days of clinical failure. The 30-day and 90-day all-cause mortality rates were 27.1% (13 of 48) and 45.8% (22 of 48), respectively. The 30-day and 90-day microbiologic failure rates were 29.2% (14 of 48) and 41.7% (20 of 48), respectively.

Conclusions

In this nationwide VHA cohort clinical and microbiologic failure occurred in >30% of patients treated with cefiderocol, and >40% of these died within 90 days. Cefiderocol is not widely used, and many of the patients who received it had substantial comorbidities.

Antibacterial spectrum of cefiderocol

Cefiderocol, a siderophore catechol cephalosporin, recently introduced in the market has been developed to enhance the in vitro activity of extended spectrum cephalosporins and to avoid resistance mechanisms affecting cephalosporins and carbapenems. The in vitro study of cefiderocol in the laboratory requires iron depleted media when MIC values are determined by broth microdilution. Disk diffusion presents good correlation with MIC values. In surveillance studies and in clinical trials it has been demonstrated excellent activity against Gram-negatives, including carbapenemase producers and non-fermenters such as Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia. Few cefiderocol resistant isolates have been found in surveillance studies. Resistance mechanisms are not directly associated with porin deficiency and or efflux pumps. On the contrary, they are related with gene mutations affecting iron transporters, AmpC mutations in the omega loop and with certain beta-lactamases such us KPC-variants determining also ceftazidime-avibactam resistance, certain infrequent extended-spectrum betalactamases (PER, BEL) and metallo-beta-lactamases (certain NDM variants and SPM enzyme).

Treatment of carbapenem-resistant Pseudomonas aeruginosa infections: a case for cefiderocol

INTRODUCTION

Carbapenem-resistant (CR) Pseudomonas aeruginosa infections constitute a serious clinical threat globally. Patients are often critically ill and/or immunocompromised. Antibiotic options are limited and are currently centered on beta-lactam-beta-lactamase inhibitor (BL-BLI) combinations and the siderophore cephalosporin cefiderocol.

AREAS COVERED

This article reviews the mechanisms of P. aeruginosa resistance and their potential impact on the activity of current treatment options, along with evidence for the clinical efficacy of BL-BLI combinations in P. aeruginosa infections, some of which specifically target infections due to CR organisms. The preclinical and clinical evidence supporting cefiderocol as a treatment option for P. aeruginosa involving infections is also reviewed.

EXPERT OPINION

Cefiderocol is active against most known P. aeruginosa mechanisms mediating carbapenem resistance. It is stable against different serine- and metallo-beta-lactamases, and, due to its iron channel-dependent uptake mechanism, is not impacted by porin channel loss. Furthermore, the periplasmic level of cefiderocol is not affected by upregulated efflux pumps. The potential for on-treatment resistance development currently appears to be low, although more clinical data are required. Information from surveillance programs, real-world compassionate use, and clinical studies demonstrate that cefiderocol is an important treatment option for CR P. aeruginosa infections.

Safety and efficacy of cefiderocol for off-label treatment indications: A systematic review

PURPOSE

Cefiderocol is a siderophore cephalosporin recently approved by the United States Food and Drug Administration for the treatment of hospital- and ventilator-acquired bacterial pneumonia and complicated urinary tract infections. However, there is potential for cefiderocol utility for a variety of other infections. The purpose of this systematic review was to identify literature examining the safety and efficacy of cefiderocol for off-label indications.

METHODS

The PRISMA guidelines were utilized for reporting. Databases searched included PubMed, Scopus, and Embase, from inception to September 2021. Manuscripts describing cefiderocol off-label use in clinical settings were included. Exclusion criteria were studies focused on labeled indications, animal studies, pharmacodynamic/pharmacokinetic studies, in vitro or laboratory studies, and manuscripts in languages other than English or Arabic. Each stage of review utilized two independent investigators, with conflicts resolved and critical appraisal performed. Data regarding presentation, clinical course, and infection characteristics were extracted and descriptively analyzed.

RESULTS

The search identified a total of 985 records, narrowed to a final set of 27 studies. Among studies included were 18 (66.7%) case reports, 8 (29.6%) case series, and 1 (3.7%) phase 3 clinical trial. Cefiderocol was most frequently used off-label for bacteremia/sepsis with or without an identified source in 51 (67.1%) out of a total of 76 included patients. Among case series/reports with available data, 43 of 53 patients (81.1%) received combination antibiotic therapy. The most common pathogens identified included multi/extensively drug-resistant Pseudomonas aeruginosa and/or Acinetobacter baumannii. Various clinical end points were reported, while microbiological end points were reported in 18 (66.7%) studies. Cefiderocol-related side effects were uncommon and rarely use-limiting.

CONCLUSIONS

This systematic review depicts relative clinical effectiveness of off-label cefiderocol, most commonly for P. aeruginosa and A. baumannii infections as combination antibiotic therapy. Further study is needed to elucidate the safety and efficacy of cefiderocol across an expanded set of patients and indications.

Pseudomonas aeruginosa Infections in Cancer Patients

Pseudomonas aeruginosa (P. aeruginosa) is one of the most frequent opportunistic microorganisms causing infections in oncological patients, especially those with neutropenia. Through its ability to adapt to difficult environmental conditions and high intrinsic resistance to antibiotics, it successfully adapts and survives in the hospital environment, causing sporadic infections and outbreaks. It produces a variety of virulence factors that damage host cells, evade host immune responses, and permit colonization and infections of hospitalized patients, who usually develop blood stream, respiratory, urinary tract and skin infections. The wide intrinsic and the increasing acquired resistance of P. aeruginosa to antibiotics make the treatment of infections caused by this microorganism a growing challenge. Although novel antibiotics expand the arsenal of antipseudomonal drugs, they do not show activity against all strains, e.g., MBL (metalo-β-lactamase) producers. Moreover, resistance to novel antibiotics has already emerged. Consequently, preventive methods such as limiting the transmission of resistant strains, active surveillance screening for MDR (multidrug-resistant) strains colonization, microbiological diagnostics, antimicrobial stewardship and antibiotic prophylaxis are of particular importance in cancer patients. Unfortunately, surveillance screening in the case of P. aeruginosa is not highly effective, and a fluoroquinolone prophylaxis in the era of increasing resistance to antibiotics is controversial.

Cefiderocol: Systematic Review of Mechanisms of Resistance, Heteroresistance and In Vivo Emergence of Resistance

Cefiderocol appears promising, as it can overcome most β-lactam resistance mechanisms (including β-lactamases, porin mutations, and efflux pumps). Resistance is uncommon according to large multinational cohorts, including against isolates resistant to carbapenems, ceftazidime/avibactam, ceftolozane/tazobactam, and colistin. However, alarming proportions of resistance have been reported in some recent cohorts (up to 50%). A systematic review was conducted in PubMed and Scopus from inception to May 2022 to review mechanisms of resistance, prevalence of heteroresistance, and in vivo emergence of resistance to cefiderocol during treatment. A variety of mechanisms, typically acting in concert, have been reported to confer resistance to cefiderocol: β-lactamases (especially NDM, KPC and AmpC variants conferring resistance to ceftazidime/avibactam, OXA-427, and PER- and SHV-type ESBLs), porin mutations, and mutations affecting siderophore receptors, efflux pumps, and target (PBP-3) modifications. Coexpression of multiple β-lactamases, often in combination with permeability defects, appears to be the main mechanism of resistance. Heteroresistance is highly prevalent (especially in A. baumannii), but its clinical impact is unclear, considering that in vivo emergence of resistance appears to be low in clinical studies. Nevertheless, cases of in vivo emerging cefiderocol resistance are increasingly being reported. Continued surveillance of cefiderocol’s activity is important as this agent is introduced in clinical practice.

New Drugs for the Treatment of Pseudomonas aeruginosa Infections with Limited Treatment Options: A Narrative Review

P. aeruginosa is still one of the most threatening pathogens responsible for serious hospital-acquired infections. It is intrinsically resistant to many antimicrobial agents and additional acquired resistance further complicates the management of such infections. High rates of combined antimicrobial resistance persist in many countries, especially in the eastern and south-eastern parts of Europe. The aim of this narrative review is to provide a comprehensive assessment of the epidemiology, latest data, and clinical evidence on the current and new available drugs active against P. aeruginosa isolates with limited treatment options. The latest evidence and recommendations supporting the use of ceftolozane-tazobactam and ceftazidime-avibactam, characterized by targeted clinical activity against a significant proportion of P. aeruginosa strains with limited treatment options, are described based on a review of the latest microbiological and clinical studies. Cefiderocol, with excellent in vitro activity against P. aeruginosa isolates, good stability to all β-lactamases and against porin and efflux pumps mutations, is also examined. New carbapenem combinations are explored, reviewing the latest experimental and initial clinical evidence. One section is devoted to a review of new anti-pseudomonal antibiotics in the pipeline, such as cefepime-taniborbactam and cefepime-zidebactam. Finally, other “old” antimicrobials, mainly fosfomycin, that can be used as combination strategies, are described.

Infectious Diseases Society of America 2022 Guidance on the Treatment of Extended-Spectrum β-lactamase Producing Enterobacterales (ESBL-E), Carbapenem-Resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with Difficult-to-Treat Resistance (DTR-P. aeruginosa)

BACKGROUND

The Infectious Diseases Society of America (IDSA) is committed to providing up-to-date guidance on the treatment of antimicrobial-resistant infections. The initial guidance document on infections caused by extended-spectrum β-lactamase producing Enterobacterales (ESBL-E), carbapenem-resistant Enterobacterales (CRE), and Pseudomonas aeruginosa with difficult-to-treat resistance (DTR-P. aeruginosa) was published on 17 September 2020. Over the past year, there have been a number of important publications furthering our understanding of the management of ESBL-E, CRE, and DTR-P. aeruginosa infections, prompting a rereview of the literature and this updated guidance document.

METHODS

A panel of 6 infectious diseases specialists with expertise in managing antimicrobial-resistant infections reviewed, updated, and expanded previously developed questions and recommendations about the treatment of ESBL-E, CRE, and DTR-P. aeruginosa infections. Because of differences in the epidemiology of resistance and availability of specific anti-infectives internationally, this document focuses on the treatment of infections in the United States.

RESULTS

Preferred and alternative treatment recommendations are provided with accompanying rationales, assuming the causative organism has been identified and antibiotic susceptibility results are known. Approaches to empiric treatment, duration of therapy, and other management considerations are also discussed briefly. Recommendations apply for both adult and pediatric populations.

CONCLUSIONS

The field of antimicrobial resistance is highly dynamic. Consultation with an infectious diseases specialist is recommended for the treatment of antimicrobial-resistant infections. This document is current as of 24 October 2021. The most current versions of IDSA documents, including dates of publication, are available at www.idsociety.org/practice-guideline/amr-guidance/.

Class C β-Lactamases: Molecular Characteristics

Class C β-lactamases or cephalosporinases can be classified into two functional groups (1, 1e) with considerable molecular variability (≤20% sequence identity). These enzymes are mostly encoded by chromosomal and inducible genes and are widespread among bacteria, including Proteobacteria in particular. Molecular identification is based principally on three catalytic motifs (⁶⁴SXSK, ¹⁵⁰YXN, ³¹⁵KTG), but more than 70 conserved amino-acid residues (≥90%) have been identified, many close to these catalytic motifs. Nevertheless, the identification of a tiny, phylogenetically distant cluster (including enzymes from the genera Legionella, Bradyrhizobium, and Parachlamydia) has raised questions about the possible existence of a C2 subclass of β-lactamases, previously identified as serine hydrolases. In a context of the clinical emergence of extended-spectrum AmpC β-lactamases (ESACs), the genetic modifications observed in vivo and in vitro (point mutations, insertions, or deletions) during the evolution of these enzymes have mostly involved the Ω- and H-10/R2-loops, which vary considerably between genera, and, in some cases, the conserved triplet ¹⁵⁰YXN. Furthermore, the conserved deletion of several amino-acid residues in opportunistic pathogenic species of Acinetobacter, such as A. baumannii, A. calcoaceticus, A. pittii and A. nosocomialis (deletion of residues 304-306), and in Hafnia alvei and H. paralvei (deletion of residues 289-290), provides support for the notion of natural ESACs. The emergence of higher levels of resistance to β-lactams, including carbapenems, and to inhibitors such as avibactam is a reality, as the enzymes responsible are subject to complex regulation encompassing several other genes (ampR, ampD, ampG, etc.). Combinations of resistance mechanisms may therefore be at work, including overproduction or change in permeability, with the loss of porins and/or activation of efflux systems.

Cefiderocol for the Treatment of Multidrug-Resistant Gram-Negative Bacteria: A Systematic Review of Currently Available Evidence

Cefiderocol is a novel synthetic siderophore-conjugated antibiotic that hijacks the bacterial iron transport systems facilitating drug entry into cells, achieving high periplasmic concentrations. This systematic review analyzed the currently available literature on cefiderocol. It summarized in vitro susceptibility data, in vivo antimicrobial activity, pharmacokinetics/pharmacodynamics (PK/PD), clinical efficacy, safety and resistance mechanisms of cefiderocol. Cefiderocol has potent in vitro and in vivo activity against multidrug-resistant (MDR) Gram-negative bacteria, including carbapenem-resistant isolates. But New Delhi Metallo-β-lactamase (NDM)- positive isolates showed significantly higher MICs than other carbapenemase-producing Enterobacterales, with a susceptible rate of 83.4% for cefiderocol. Cefiderocol is well-tolerated, and the PK/PD target values can be achieved using a standard dose regimen or adjusted doses according to renal function. Clinical trials demonstrated that cefiderocol was non-inferiority to the comparator drugs in treating complicated urinary tract infection and nosocomial pneumonia. Case reports and series showed that cefiderocol was a promising therapeutic agent in carbapenem-resistant infections. However, resistant isolates and reduced susceptibility during treatment to cefiderocol have already been reported. In conclusion, cefiderocol is a promising powerful weapon for treating MDR recalcitrant infections.

Pharmacokinetic evaluation of cefiderocol for the treatment of multidrug resistant Gram-negative infections

INTRODUCTION

Cefiderocol is a siderophore cephalosporin antibiotic and first of its kind approved by the Food and Drug Administration for the treatment of complicated urinary tract infections (cUTI) and hospital-acquired and ventilator-associated bacterial pneumonia (HABP/VABP) in patients 18 years or older caused by susceptible organisms. Cefiderocol's unique mechanism of iron chelation improves Gram-negative membrane penetration as the bacteria's iron uptake mechanism recognizes the chelated iron antibiotic and iron for entry. This also allows for the evasion of cefiderocol from cell entry-related resistance mechanisms.

AREAS COVERED

This review covers the mechanism of action, resistance mechanisms, pharmacokinetics in various patient populations, and pharmacodynamics. Relevant literature evaluating efficacy and safety are discussed.

EXPERT OPINION

Limited treatment options are available for the treatment of carbapenem-resistantorganisms. Clinical trials have demonstrated that cefiderocol is no worse than alternative treatment options for cUTIs and HABP/VABP, but more data are currently available to support the use of beta-lactam beta-lactamase inhibitor agents, where susceptible. Mortality differences demonstrated in patients with pneumonia and bloodstream infections must further be explored and logistical and practical considerations regarding susceptibility testing and use as monotherapy vs. combination therapy must be considered prior to confidently recommending cefiderocol for regular use in systemic infections.

Human Serum Proteins and Susceptibility of Acinetobacter baumannii to Cefiderocol: Role of Iron Transport

Cefiderocol, a recently introduced antibiotic, has a chemical structure that includes a cephalosporin that targets cell wall synthesis and a chlorocatechol siderophore moiety that facilitates cell penetration by active iron transporters. Analysis of the effect that human serum, human serum albumin, and human pleural fluid had on growing Acinetobacter baumannii showed that genes related to iron uptake were down-regulated. At the same time, β-lactamase genes were expressed at higher levels. The minimum inhibitory concentrations of this antimicrobial in A. baumannii cells growing in the presence of human serum, human serum albumin, or human pleural fluid were higher than those measured when these fluids were absent from the culture medium. These results correlate with increased expression levels of β-lactamase genes and the down-regulation of iron uptake-related genes in cultures containing human serum, human serum albumin, or human pleural fluid. These modifications in gene expression could explain the less-than-ideal clinical response observed in patients with pulmonary or bloodstream A. baumannii infections. The exposure of the infecting cells to the host’s fluids could cause reduced cefiderocol transport capabilities and increased resistance to β-lactams. The regulation of genes that could impact the A. baumannii susceptibility to cefiderocol, or other antibacterials, is an understudied phenomenon that merits further investigation.

Urgent need for novel antibiotics in Republic of Korea to combat multidrug-resistant bacteria

Multidrug resistance in bacteria is an important issue and is increasing in frequency worldwide because of the limitations of therapeutic agents. From 2010 to 2019, 14 new systemic antibiotics received regulatory approval in the United States. However, few new antibiotics have been introduced in Republic of Korea to combat multidrug-resistant pathogens. Here, we introduce six novel antibiotics for Gram-positive bacteria and five for Gram-negative bacteria approved by the United States Food and Drug Administration and the European Medicines Agency from 2009 to October 2021, and recommend that they be approved for use in Republic of Korea at the earliest possible date.

Therapeutic Strategies for Emerging Multidrug-Resistant Pseudomonas aeruginosa

Multidrug-resistant (MDR) and extensively drug-resistant (XDR) Pseudomonas aeruginosa isolates are frequent causes of serious nosocomial infections that may compromise the selection of antimicrobial therapy. The goal of this review is to summarize recent epidemiologic, microbiologic, and clinical data pertinent to the therapeutic management of patients with infections caused by MDR/XDR-P. aeruginosa. Historically, conventional antipseudomonal β-lactam antibiotics have been used for the empiric treatment of MDR/XDR-P. aeruginosa. Owing to the remarkable capacity of P. aeruginosa to confer resistance via multiple mechanisms, these traditional therapies are often rendered ineffective. To increase the likelihood of administering empiric antipseudomonal therapy with in vitro activity, a second agent from a different antibiotic class is often administered concomitantly with a traditional antipseudomonal β-lactam. However, combination therapy may pose an increased risk of antibiotic toxicity and secondary infection, notably, Clostridioides difficile. Multiple novel agents that demonstrate in vitro activity against MDR-P. aeruginosa (e.g., β-lactam/β-lactamase inhibitor combinations and cefiderocol) have been recently granted US Food and Drug Administration (FDA) approval and are promising additions to the antipseudomonal armamentarium. Even so, comparative clinical data pertaining to these novel agents is sparse, and concerns surrounding the scarcity of antibiotics active against refractory MDR/XDR-P. aeruginosa necessitates continued assessment of alternative therapies. This is particularly important in patients with cystic fibrosis (CF) who may be chronically colonized and suffer from recurrent infections and disease exacerbations due in part to limited efficacious antipseudomonal agents. Bacteriophages represent a promising candidate for combatting recurrent and refractory infections with their ability to target specific host bacteria and circumvent traditional mechanisms of antibiotic resistance seen in MDR/XDR-P. aeruginosa. Future goals for the management of these infections include increased comparator clinical data of novel agents to determine in what scenario certain agents may be preferred over others. Until then, appropriate treatment of these infections requires a thorough evaluation of patient- and infection-specific factors to guide empiric and definitive therapeutic decisions.

β-lactam Resistance in Pseudomonas aeruginosa: Current Status, Future Prospects

Pseudomonas aeruginosa is a major opportunistic pathogen, causing a wide range of acute and chronic infections. β-lactam antibiotics including penicillins, carbapenems, monobactams, and cephalosporins play a key role in the treatment of P. aeruginosa infections. However, a significant number of isolates of these bacteria are resistant to β-lactams, complicating treatment of infections and leading to worse outcomes for patients. In this review, we summarize studies demonstrating the health and economic impacts associated with β-lactam-resistant P. aeruginosa. We then describe how β-lactams bind to and inhibit P. aeruginosa penicillin-binding proteins that are required for synthesis and remodelling of peptidoglycan. Resistance to β-lactams is multifactorial and can involve changes to a key target protein, penicillin-binding protein 3, that is essential for cell division; reduced uptake or increased efflux of β-lactams; degradation of β-lactam antibiotics by increased expression or altered substrate specificity of an AmpC β-lactamase, or by the acquisition of β-lactamases through horizontal gene transfer; and changes to biofilm formation and metabolism. The current understanding of these mechanisms is discussed. Lastly, important knowledge gaps are identified, and possible strategies for enhancing the effectiveness of β-lactam antibiotics in treating P. aeruginosa infections are considered.

Real-life experience with compassionate use of cefiderocol for difficult-to-treat resistant Pseudomonas aeruginosa (DTR-P) infections

Objectives

To describe our real-life experience with cefiderocol in XDR and difficult-to-treat resistant Pseudomonas aeruginosa (DTR-P) infections without any other available treatment options.

Methods

We included patients with a proven infection due to an XDR/DTR-P, who had failed on previous regimens, and were treated with cefiderocol, following them prospectively to day 90 or until hospital discharge or death.

Results

Seventeen patients treated for >72 h with cefiderocol were included: 14 receiving combination regimens (82.4%) and 3 receiving monotherapy (17.6%). Fourteen patients were males (82%) with a median age of 64 years (IQR 58–73). Fifteen patients (88.2%) were admitted to the ICU and five had septic shock (29%). Seven cases (41.2%) were ventilator-associated pneumonia, of which 71% (5/7) occurred in COVID-19 patients. Four were complicated intrabdominal infections, one ecthyma gangrenosum, one nosocomial pneumonia and one empyema, one osteomyelitis, one primary bacteraemia, and one nosocomial external ventricular drainage meningitis. Clinical cure and microbiological cure rates were 70.6% and 76.5%, respectively. There were six deaths (35.3%) after a median of 8 days (IQR 3–10) from the end of treatment, but only two of them (11.7%) were associated with P. aeruginosa infection progression.

Conclusions

Our experience collecting this large case series of DTR-P treated with cefiderocol may help clinicians consider this new option in this hard-to-manage setting. Our results are even more relevant in the current scenario of ceftolozane/tazobactam shortage. Importantly, this is the first study providing real-life data indicating adequate cefiderocol concentrations in CSF.

New Perspectives on Antimicrobial Agents: Cefiderocol

Bacterial resistance to carbapenem agents has reached alarming levels. Accordingly, collaborative efforts between national and international organizations and the pharmaceutical industry have led to an impressive expansion of commercially available β-lactam agents in recent years. No available agent comes close to the broad range of activity afforded by cefiderocol, a novel siderophore-cephalosporin conjugate. The novelty of and need for cefiderocol are clear, but available clinical data are conflicting, leaving infectious diseases specialists puzzled as to when to prescribe this agent in clinical practice. After a brief overview of cefiderocol pharmacokinetics and pharmacodynamics, safety data, cefiderocol susceptibility testing, and putative mechanisms of cefiderocol resistance, this review focuses on determining cefiderocol's role in the management of specific pathogens, including carbapenem-resistant Acinetobacter baumannii complex, carbapenem-resistant Pseudomonas aeruginosa, carbapenem-resistant Enterobacterales, and less commonly identified glucose-nonfermenting organisms such as Stenotrophomonas maltophilia, Burkholderia species, and Achromobacter species. Available preclinical, clinical trial, and postmarketing data are summarized for each organism, and each section concludes with our opinions on where to position cefiderocol as a clinical therapeutic.

Cefiderocol Activity Against Clinical Pseudomonas aeruginosa Isolates Exhibiting Ceftolozane-Tazobactam Resistance

BACKGROUND

Mutations in the AmpC-AmpR region are associated with treatment-emergent ceftolozane-tazobactam (TOL-TAZ) and ceftazidime-avibactam (CAZ-AVI) resistance. We sought to determine if these mutations impact susceptibility to the novel cephalosporin-siderophore compound cefiderocol.

METHODS

Thirty-two paired isolates from 16 patients with index P. aeruginosa isolates susceptible to TOL-TAZ and subsequent P. aeruginosa isolates available after TOL-TAZ exposure from January 2019 to December 2020 were included. TOL-TAZ, CAZ-AVI, imipenem-relebactam (IMI-REL), and cefiderocol minimum inhibitory concentrations (MICs) were determined using broth microdilution. Whole-genome sequencing of paired isolates was used to identify mechanisms of resistance to cefiderocol that emerged, focusing on putative mechanisms of resistance to cefiderocol or earlier siderophore-antibiotic conjugates based on the previously published literature.

RESULTS

Analyzing the 16 pairs of P. aeruginosa isolates, ≥4-fold increases in cefiderocol MICs occurred in 4 of 16 isolates. Cefiderocol nonsusceptibility criteria were met for only 1 of the 4 isolates, using Clinical and Laboratory Standards Institute criteria. Specific mechanisms identified included the following: AmpC E247K (2 isolates), MexR A66V and L57D (1 isolate each), and AmpD G116D (1 isolate) substitutions. For both isolates with AmpC E247K mutations, ≥4-fold MIC increases occurred for both TOL-TAZ and CAZ-AVI, while a ≥4-fold reduction in IMI-REL MICs was observed.

CONCLUSIONS

Our findings suggest that alterations in the target binding sites of P. aeruginosa-derived AmpC β-lactamases have the potential to reduce the activity of 3 of 4 novel β-lactams (ie, ceftolozane-tazobactam, ceftazidime-avibactam, and cefiderocol) and potentially increase susceptibility to imipenem-relebactam. These findings are in need of validation in a larger cohort.