Pharmacokinetics-Pharmacodynamics of Enmetazobactam Combined with Cefepime in a Neutropenic Murine Thigh Infection Model

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

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

Cefepime/Enmetazobactam: First Approval

Cefepime/enmetazobactam (EXBLIFEP®), an intravenous (IV) antibacterial fixed-dose combination of a 4th generation cephalosporin and an extended-spectrum β-lactamase (ESBL) inhibitor, is being developed by Allecra Therapeutics and ADVANZ PHARMA for the treatment of infections caused by multi-drug-resistant (MDR) Gram-negative bacteria. In February 2024, cefepime/enmetazobactam was approved in the USA for use in adults with complicated urinary tract infections (cUTI) including pyelonephritis, caused by susceptible strains of Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus mirabilis, and Enterobacter cloacae complex. In March 2024, cefepime/enmetazobactam was approved in the EU for use in adults for the treatment of cUTI, including pyelonephritis, and hospital-acquired pneumonia, including ventilator associated pneumonia, and the treatment of patients with bacteraemia occurring in association with or suspected to be associated with any of these infections. This article summarizes the milestones in the development of cefepime/enmetazobactam leading to this first approval for the treatment of adults with infections caused by MDR Gram-negative bacteria.

Evaluation of the activity of cefepime/enmetazobactam against Enterobacterales bacteria collected in Europe from 2019 to 2021, including third-generation cephalosporin-resistant isolates

OBJECTIVES

This study was performed to investigate the activity of the novel ß-lactam/ß-lactamase inhibitor combination cefepime/enmetazobactam, against recently circulating Enterobacterales isolates from Europe from 2019 to 2021.

METHODS

A total of 2627 isolates were collected, and antimicrobial susceptibility was determined according to the European Committee on Antimicrobial Susceptibility Testing guidelines. Isolates with phenotypic resistance to ceftriaxone and ceftazidime (but susceptible to meropenem) and isolates nonsusceptible to meropenem were screened for the presence of ß-lactamases.

RESULTS

Overall, susceptibility to third-generation cephalosporins was 77%, and 97.3% were susceptible to meropenem. Cefepime/enmetazobactam susceptibility was 97.9% (72% of these isolates were Klebsiella pneumoniae from Italy), compared with 80.0% susceptibility to piperacillin/tazobactam and 99.4% to ceftazidime/avibactam. A total of 320 isolates (12.2%) were resistant to third-generation cephalosporins but susceptible to meropenem, and virtually all (96.3%) carried an extended-spectrum ß-lactamase with or without an AmpC and these were all susceptible to cefepime/enmetazobactam. Most meropenem-nonsusceptible isolates carried a KPC (68%), which were not inhibited by cefepime/enmetazobactam but were inhibited by ceftazidime/avibactam. Additionally, most meropenem-nonsusceptible isolates carrying OXA-48 (9/12 isolates) were susceptible to cefepime/enmetazobactam.

CONCLUSIONS

Cefepime/enmetazobactam was highly active against Enterobacterales isolates, especially those resistant to third-generation cephalosporins. These data suggest that cefepime/enmetazobactam could be used as a carbapenem-sparing agent to replace piperacillin/tazobactam.

In vitro pharmacokinetics/pharmacodynamics of FL058 (a novel beta-lactamase inhibitor) combined with meropenem against carbapenemase-producing Enterobacterales

Objective: FL058 is a novel beta-lactamase inhibitor with a broad spectrum of activity and a favorable safety profile. The objective of this study was to evaluate pharmacokinetic/pharmacodynamic (PK/PD) relationships for the combination of FL058 and meropenem in an in vitro infection model. Methods: By simulating human concentration-time profiles in the in vitro model, meropenem combined with FL058 when administered 1 g/0.5 g, 1 g/1 g, 2 g/1 g, and 2 g/2 g q8h by 3-h infusion achieved approximately 2- and 4-log10 kill to KPC/OXA-producing Klebsiella pneumoniae and Escherichia coli; the combination therapy could not inhibit NDM-producing K. pneumoniae but could maintain NDM-producing E. coli around a baseline. Results: The PK/PD indexes that best described the bacterial killing from baseline in log10 CFU/mL at 24 h were the percent time of free drug above the minimal inhibitory concentration (MIC) (%fT > MIC, MIC with FL058 at 4 mg/L) for meropenem and the percent time of free drug above 1 mg/L (%fT > 1 mg/L) for FL058. The targets for achieving a static effect and the 1- and 2-log10 kill were 74, 83, and 99 for %fT > MIC of meropenem and 40, 48, and 64 for %fT > 1 mg/L of FL058, respectively. The PK/PD index of %fT > 1 mg/L can provide a basis for evaluating clinical dosing regimens for FL058 combined with meropenem. Conclusion: FL058 combined with meropenem might be a potential treatment for KPC- and/or OXA-48-producing Enterobacterales infection.

Novel Antimicrobial Agents for Gram-Negative Pathogens

Gram-negative bacterial resistance to antimicrobials has had an exponential increase at a global level during the last decades and represent an everyday challenge, especially for the hospital practice of our era. Concerted efforts from the researchers and the industry have recently provided several novel promising antimicrobials, resilient to various bacterial resistance mechanisms. There are new antimicrobials that became commercially available during the last five years, namely, cefiderocol, imipenem-cilastatin-relebactam, eravacycline, omadacycline, and plazomicin. Furthermore, other agents are in advanced development, having reached phase 3 clinical trials, namely, aztreonam-avibactam, cefepime-enmetazobactam, cefepime-taniborbactam, cefepime-zidebactam, sulopenem, tebipenem, and benapenem. In this present review, we critically discuss the characteristics of the above-mentioned antimicrobials, their pharmacokinetic/pharmacodynamic properties and the current clinical data.

Biochemical exploration of β-lactamase inhibitors

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

The role of adjuvants in overcoming antibacterial resistance due to enzymatic drug modification

Antibacterial resistance is a prominent issue with monotherapy often leading to treatment failure in serious infections. Many mechanisms can lead to antibacterial resistance including deactivation of antibacterial agents by bacterial enzymes. Enzymatic drug modification confers resistance to β-lactams, aminoglycosides, chloramphenicol, macrolides, isoniazid, rifamycins, fosfomycin and lincosamides. Novel enzyme inhibitor adjuvants have been developed in an attempt to overcome resistance to these agents, only a few of which have so far reached the market. This review discusses the different enzymatic processes that lead to deactivation of antibacterial agents and provides an update on the current and potential enzyme inhibitors that may restore bacterial susceptibility.

Influence of tramadol on bacterial burden in the standard neutropenic thigh infection model

The neutropenic thigh infection model is one of the standard models in pharmacokinetic/ pharmacodynamic (PK/PD) characterization of novel antibacterials which are urgently needed due to the rise of antimicrobial resistance. The model enables to investigate PK/PD parameters crucial for translation of animal results towards humans. However, the neutropenic thigh infection model can result in moderate to severe discomfort of the animals, especially when high inocula are used. Tramadol has been proven to reduce pain effectively. This study investigates if tramadol influences the bacterial burden in the primary organ, the thighs, and organs affected by secondary seeding. Therefore, several strains of the ESKAPE pathogens, namely S. aureus, P. aeruginosa, K. pneumoniae, E. coli, A. baumannii and E. faecalis were examined. It was shown that tramadol did not influence the bacterial burden neither in thighs nor in organs affected by secondary seeding for the strains of E. faecalis, S. aureus, P. aeruginosa, K. pneumoniae and E.coli tested here, whereas secondary seeding seemed to be affected by tramadol for the tested strain of A. baumannii. Consequently, it was demonstrated that tramadol is an option to reduce discomfort in the untreated group for the strains of five out of the six tested ESKAPE pathogens and, thereby, contributes to the refinement of one of the standard PK/PD models.

Penicillanic Acid Sulfones Inactivate the Extended-Spectrum β-Lactamase CTX-M-15 through Formation of a Serine-Lysine Cross-Link: an Alternative Mechanism of β-Lactamase Inhibition

β-Lactamases hydrolyze β-lactam antibiotics and are major determinants of antibiotic resistance in Gram-negative pathogens. Enmetazobactam (formerly AAI101) and tazobactam are penicillanic acid sulfone (PAS) β-lactamase inhibitors that differ by an additional methyl group on the triazole ring of enmetazobactam, rendering it zwitterionic. In this study, ultrahigh-resolution X-ray crystal structures and mass spectrometry revealed the mechanism of PAS inhibition of CTX-M-15, an extended-spectrum β-lactamase (ESBL) globally disseminated among Enterobacterales. CTX-M-15 crystals grown in the presence of enmetazobactam or tazobactam revealed loss of the Ser70 hydroxyl group and formation of a lysinoalanine cross-link between Lys73 and Ser70, two residues critical for catalysis. Moreover, the residue at position 70 undergoes epimerization, resulting in formation of a d-amino acid. Cocrystallization of enmetazobactam or tazobactam with CTX-M-15 with a Glu166Gln mutant revealed the same cross-link, indicating that this modification is not dependent on Glu166-catalyzed deacylation of the PAS-acylenzyme. A cocrystal structure of enmetazobactam with CTX-M-15 with a Lys73Ala mutation indicates that epimerization can occur without cross-link formation and positions the Ser70 Cβ closer to Lys73, likely facilitating formation of the Ser70-Lys73 cross-link. A crystal structure of a tazobactam-derived imine intermediate covalently linked to Ser70, obtained after 30 min of exposure of CTX-M-15 crystals to tazobactam, supports formation of an initial acylenzyme by PAS inhibitors on reaction with CTX-M-15. These data rationalize earlier results showing CTX-M-15 deactivation by PAS inhibitors to involve loss of protein mass, and they identify a distinct mechanism of β-lactamase inhibition by these agents.

Studies on enmetazobactam clarify mechanisms of widely used β-lactamase inhibitors

β-Lactams are the most important class of antibacterials, but their use is increasingly compromised by resistance, most importantly via serine β-lactamase (SBL)-catalyzed hydrolysis. The scope of β-lactam antibacterial activity can be substantially extended by coadministration with a penicillin-derived SBL inhibitor (SBLi), i.e., the penam sulfones tazobactam and sulbactam, which are mechanism-based inhibitors working by acylation of the nucleophilic serine. The new SBLi enmetazobactam, an N-methylated tazobactam derivative, has recently completed clinical trials. Biophysical studies on the mechanism of SBL inhibition by enmetazobactam reveal that it inhibits representatives of all SBL classes without undergoing substantial scaffold fragmentation, a finding that contrasts with previous reports on SBL inhibition by tazobactam and sulbactam. We therefore reinvestigated the mechanisms of tazobactam and sulbactam using mass spectrometry under denaturing and nondenaturing conditions, X-ray crystallography, and NMR spectroscopy. The results imply that the reported extensive fragmentation of penam sulfone–derived acyl–enzyme complexes does not substantially contribute to SBL inhibition. In addition to observation of previously identified inhibitor-induced SBL modifications, the results reveal that prolonged reaction of penam sulfones with SBLs can induce dehydration of the nucleophilic serine to give a dehydroalanine residue that undergoes reaction to give a previously unobserved lysinoalanine cross-link. The results clarify the mechanisms of action of widely clinically used SBLi, reveal limitations on the interpretation of mass spectrometry studies concerning mechanisms of SBLi, and will inform the development of new SBLi working by reaction to form hydrolytically stable acyl–enzyme complexes.

Microbiological, Clinical, and PK/PD Features of the New Anti-Gram-Negative Antibiotics: β-Lactam/β-Lactamase Inhibitors in Combination and Cefiderocol—An All-Inclusive Guide for Clinicians

Bacterial resistance mechanisms are continuously and rapidly evolving. This is particularly true for Gram-negative bacteria. Over the last decade, the strategy to develop new β-lactam/β-lactamase inhibitors (BLs/BLIs) combinations has paid off and results from phase 3 and real-world studies are becoming available for several compounds. Cefiderocol warrants a separate discussion for its peculiar mechanism of action. Considering the complexity of summarizing and integrating the emerging literature data of clinical outcomes, microbiological mechanisms, and pharmacokinetic/pharmacodynamic properties of the new BL/BLI and cefiderocol, we aimed to provide an overview of data on the following compounds: aztreonam/avibactam, cefepime/enmetazobactam, cefepime/taniborbactam, cefepime/zidebactam, cefiderocol, ceftaroline/avibactam, ceftolozane/tazobactam, ceftazidime/avibactam, imipenem/relebactam, meropenem/nacubactam and meropenem/vaborbactam. Each compound is described in a dedicated section by experts in infectious diseases, microbiology, and pharmacology, with tables providing at-a-glance information.

Multicenter surveillance of in vitro activities of cefepime-zidebactam, cefepime-enmetazobactam, omadacycline, eravacycline, and comparator antibiotics against Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii complex causing bloodstream infection in Taiwan, 2020

OBJECTIVES

: To determine the in vitro activities of novel and comparator antibiotics against gram-negative bacteria (GNB) in Taiwan.

METHODS

: Isolates of Escherichia coli (n = 335), Klebsiella pneumoniae (n = 316; 144 isolates with hyperviscosity characteristics), Pseudomonas aeruginosa (n = 271), Acinetobacter baumannii complex species (n = 187), and non-typhoidal Salmonella species (n = 226), Shigella species (n = 13) from miscellaneous culture sources were collected in 2020 in Taiwan. The MICs of the isolates to test antibiotics were determined using the broth microdilution method. GeneXpert was used to detect genes encoding carbapenemases among the carbapenem-non-susceptible (NS) Enterobacterales isolates.

RESULTS

: The MIC values of the cefepime-enmetazobactam combination against extended-spectrum β-lactamase-producing E. coli and K. pneumoniae isolates (MIC₉₀ ≤ 0.5 mg/L), bla_KPC-harboring E. coli isolates (0.25 mg/L; n = 2), and 80% of bla_OXA-48-like gene-harboring K. pneumoniae isolates (≤2 mg/L) were low. The MIC ranges of the cefepime-zidebactam against carbapenemase-producing Enterobacterales isolates (irrespective of the carbapenemase type [MIC₉₀ ≤ 4 mg/L]) and carbapenem-NS or ceftolozane-tazobactam-NS P. aeruginosa isolates (MIC₉₀ value, 8 mg/L) were significantly lower than those of the cefepime-enmetazobactam.

CONCLUSIONS

: The efficacy of novel antibiotics against important drug-resistant GNB must be monitored and validated during the clinical treatment of patients.

Sigmoid Emax Modeling To Define the Fixed Concentration of Enmetazobactam for MIC Testing in Combination with Cefepime

The use of carbapenem antibiotics to treat infections caused by Enterobacterales expressing increasingly aggressive extended-spectrum β-lactamases (ESBLs) has contributed to the emergence of carbapenem resistance. Enmetazobactam is a novel ESBL inhibitor being developed in combination with cefepime as a carbapenem-sparing option for infections caused by ESBL-producing Enterobacterales. Cefepime-enmetazobactam checkerboard MIC profiles were obtained for a challenge panel of cefepime-resistant ESBL-producing clinical isolates of Klebsiella pneumoniae. Sigmoid maximum effect (E_max) modeling described cefepime MICs as a function of enmetazobactam concentration with no bias. A concentration of 8 μg/ml enmetazobactam proved sufficient to restore >95% of cefepime antibacterial activity in vitro against >95% of isolates tested. These results support a fixed concentration of 8 μg/ml of enmetazobactam for MIC testing.

An update on cefepime and its future role in combination with novel β-lactamase inhibitors for MDR Enterobacterales and Pseudomonas aeruginosa

Cefepime, a wide-spectrum β-lactam antibiotic, has been in use for the treatment of serious bacterial infections for almost 25 years. Since its clinical development, there has been a dramatic shift in its dosing, with 2 g every 8 hours being preferred for serious infections to optimize pharmacokinetic/pharmacodynamic considerations. The advent of ESBLs has become a threat to its ongoing use, although future coadministration with β-lactamase inhibitors (BLIs) under development is an area of intense study. There are currently four new cefepime/BLI combinations in clinical development. Cefepime/zidebactam is generally active against MBL-producing Enterobacterales and Pseudomonas aeruginosa, in vitro and in animal studies, and cefepime/taniborbactam has activity against KPC and OXA-48 producers. Cefepime/enmetazobactam and cefepime/tazobactam are potential carbapenem-sparing agents with activity against ESBLs. Cefepime/enmetazobactam has completed Phase III and cefepime/taniborbactam is in Phase III clinical studies, where they are being tested against carbapenems or piperacillin/tazobactam for the treatment of complicated urinary tract infections. While these combinations are promising, their role in the treatment of MDR Gram-negative infections can only be determined with further clinical studies.

Pharmacokinetics and pharmacodynamics of enrofloxacin treatment of Escherichia coli in a murine thigh infection modeling

BACKGROUND

Enrofloxacin is an antibacterial drug with broad-spectrum activity that is widely indicated for veterinary use. We aim to develop the clinical applications of Enrofloxacin against colibacillosis by using the neutropenic mice thigh infection model.

RESULTS

The minimum inhibitory concentration (MIC) distribution of 67 isolated E. coli strains to ENR was calculated using CLSI guidelines. Whereas, the MIC₅₀ value calculation was considered as the population PD parameter for ENR against E. coli strains. The MIC values of 15 E. coli strains were found to be nearest to the MIC₅₀ i.e., 0.25 μg/mL. Of all the tested strains, the PK-PD and E. coli disease model was established via selected E. coli strain i.e., Heilong 15. We analyzed the PK characteristics of ENR and its metabolite ciprofloxacin (CIP) following a single subcutaneous (s.c.) injection of ENR (1.25, 2.5, 5, 10 mg/kg). The concentration-time profiling of ENR within the plasma specimens was determined by considering the non-compartmental analysis (NCA). The basic PK parameters of ENR for the peak drug concentration (C_max) and the area under the concentration-time curve (AUC) values were found to be in the range of 0.27-1.97 μg/mL and 0.62-3.14 μg.h/mL, respectively. Multiple s.c. injection over 24 h (1.25, 2.5, 5, 10 mg/kg at various time points i.e., 6, 8, 12, and 24 h respectively) were administered to assess the targeted PD values. The Akaike Information Criterion (AIC) was used to choose PD models, and the model with the lowest AIC was chosen. The inhibitory E_max model was employed to calculate the related PK-PD parameters. The results of our study indicated that there was a strong correlation between the AUC/MIC and various antibacterial activities (R² = 0.9928). The target values of dividing AUC/MIC by 24 h for bacteriostatic action were 1-log10 reduction, 2-log10 reduction, and 3-log10 reduction 0.325, 0.4375, 0.63, and 0.95 accordingly.

CONCLUSION

The identified pharmacodynamics targets for various antibacterial effects will be crucial in enhancing ENR clinical applications and serving as a key step in reducing bacterial resistance.

Antimicrobial Resistance Conferred by OXA-48 β-Lactamases: Towards a Detailed Mechanistic Understanding

OXA-48-type β-lactamases are now routinely encountered in bacterial infections caused by carbapenem-resistant Enterobacterales These enzymes are of high and growing clinical significance due to the importance of carbapenems in treatment of health care-associated infections by Gram-negative bacteria, the wide and increasing dissemination of OXA-48 enzymes on plasmids, and the challenges posed by their detection. OXA-48 confers resistance to penicillin (which is efficiently hydrolyzed) and carbapenem antibiotics (which is more slowly broken down). In addition to the parent enzyme, a growing array of variants of OXA-48 is now emerging. The spectrum of activity of these variants varies, with some hydrolyzing expanded-spectrum oxyimino-cephalosporins. The growth in importance and diversity of the OXA-48 group has motivated increasing numbers of studies that aim to elucidate the relationship between structure and specificity and establish the mechanistic basis for β-lactam turnover in this enzyme family. In this review, we collate recently published structural, kinetic, and mechanistic information on the interactions between clinically relevant β-lactam antibiotics and inhibitors and OXA-48 β-lactamases. Collectively, these studies are starting to form a detailed picture of the underlying bases for the differences in β-lactam specificity between OXA-48 variants and the consequent differences in resistance phenotype. We focus specifically on aspects of carbapenemase and cephalosporinase activities of OXA-48 β-lactamases and discuss β-lactamase inhibitor development in this context. Throughout the review, we also outline key open research questions for future investigation.

Efficacy of a novel lantibiotic, CMB001, against MRSA

OBJECTIVES

To evaluate the efficacy of a novel lantibiotic, CMB001, against MRSA biofilms in vitro and in an in vivo experimental model of bacterial infection.

METHODS

Antibacterial activity of CMB001 was measured in vitro after its exposure to whole blood or to platelet-poor plasma. In vitro efficacy of CMB001 against a Staphylococcus aureus biofilm was studied using scanning electron microscopy. The maximum tolerable dose in mice was determined and a preliminary pharmacokinetic analysis for CMB001 was performed in mice. In vivo efficacy was evaluated in a neutropenic mouse thigh model of infection.

RESULTS

CMB001 maintained its antibacterial activity in the presence of blood or plasma for up to 24 h at 37°C. CMB001 efficiently killed S. aureus within the biofilm by causing significant damage to the bacterial cell wall. The maximum tolerable dose in mice was established to be 10 mg/kg and could be increased to 30 mg/kg in mice pretreated with antihistamines. In neutropenic mice infected with MRSA, treatment with CMB001 reduced the bacterial burden with an efficacy equivalent to that of vancomycin.

CONCLUSIONS

CMB001 offers potential as an alternative treatment option to combat MRSA. It will be of interest to evaluate the in vivo efficacy of CMB001 against infections caused by other pathogens, including Clostridioides difficile and Acinetobacter baumannii, and to expand its pharmacokinetic/pharmacodynamic parameters and safety profile.

Intrapulmonary Pharmacokinetics of Cefepime and Enmetazobactam in Healthy Volunteers: Towards New Treatments for Nosocomial Pneumonia

Cefepime-enmetazobactam is a novel β-lactam-β-lactamase inhibitor combination with broad-spectrum antimicrobial activity against a range of multidrug-resistant Enterobacteriaceae This agent is being developed for a range of serious hospital infections. An understanding of the extent of partitioning of β-lactam-β-lactamase inhibitor combinations into the human lung is required to better understand the potential role of cefepime-enmetazobactam for the treatment of nosocomial pneumonia. A total of 20 healthy volunteers were used to study the intrapulmonary pharmacokinetics of a regimen of 2 g cefepime-1 g enmetazobactam every 8 h intravenously (2 g/1 g q8h i.v.). Each volunteer contributed multiple plasma samples and a single epithelial lining fluid (ELF) sample, obtained by bronchoalveolar lavage. Concentrations of cefepime and enmetazobactam were quantified using liquid chromatography-tandem mass spectrometry. The pharmacokinetic data were modeled using a population methodology, and Monte Carlo simulations were performed to assess the attainment of pharmacodynamic targets defined in preclinical models. The concentration-time profiles of both agents in plasma and ELF were similar. The mean ± standard deviation percentage of partitioning of total drug concentrations of cefepime and enmetazobactam between plasma and ELF was 60.59% ± 28.62% and 53.03% ± 21.05%, respectively. Using pharmacodynamic targets for cefepime of greater than the MIC and free enmetazobactam concentrations of >2 mg/liter in ELF of 20% of the dosing interval, a regimen of cefepime-enmetazobactam of 2 g/0.5 g q8h i.v. infused over 2 h resulted in a probability of target attainment of ≥90% for Enterobacteriaceae with cefepime-enmetazobactam MICs of ≤8 mg/liter. This result provides a rationale to further consider cefepime-enmetazobactam for the treatment of nosocomial pneumonia caused by multidrug-resistant Enterobacteriaceae.