Generating Robust and Informative Nonclinical In Vitro and In Vivo Bacterial Infection Model Efficacy Data To Support Translation to Humans

Site-Specific Antimicrobial Activity of a Dual-Responsive Ciprofloxacin Prodrug

Bacterial infections create distinctive microenvironments with a unique mix of metabolites and enzymes compared with healthy tissues that can be used to trigger the activation of antibiotic prodrugs. Here, a single and dual prodrug masking the C3 carboxylate and C7 piperazine of the fluoroquinolone, ciprofloxacin, responsive to nitroreductase (NTR) and/or hydrogen sulfide (H2S), was developed. Masking both functional groups reduced the activity of the prodrug against Staphylococcus aureus and Escherichia coli, increasing its minimum inhibitory concentration (MIC) by ∼512-fold (S. aureus) and ∼8000-fold (E. coli strains), while masking a single group only increased the MIC by ∼128-fold. Bacteria subjected to prolonged prodrug exposure did not show any increase in resistance. Triggering assays demonstrated the conversion of prodrugs to ciprofloxacin, and in a murine infection model, responsive prodrugs showed antibacterial activity comparable to that of ciprofloxacin, suggesting in vivo activation of prodrugs. Thus, the potential for site-specific antibiotic treatment with reduced threat of resistance is demonstrated.

Impact of continuous-infusion meropenem degradation and infusion bag changes on bacterial killing of Pseudomonas aeruginosa based on model-informed translation

Continuous infusion of meropenem has been proposed to increase target attainment in critically ill patients, although stability might limit its practical use. This study investigated the impact of meropenem degradation and infusion bag changes on the concentration-time profiles and bacterial growth and killing of P. aeruginosa given different continuous-infusion solutions. A semi-mechanistic PK-PD model quantifying meropenem concentrations (CMEM) and bacterial counts of a resistant P. aeruginosa strain (ARU552, MIC=16 mg/L) over 24 hours was used to translate in vitro antibiotic effects to patients with severe infections. Concentration-dependent drug degradation of saline infusion solutions was considered using an additional compartment in the population PK model. CMEM, fT>MIC (time that concentrations exceed the MIC) and total bacterial load (BTOT) after 24 h were simulated for different scenarios (n=144), considering low- and high-dose regimens (3000/6000 mg/day±loading dose), clinically relevant infusion solutions (20/40/50 mg/mL), different intervals of infusion bag changes (every 8/24 hours, q8/24h), and varied renal function (creatinine clearance 40/80/120 mL/min) and MIC values (8/16 mg/L). Highest deviations between changing infusion bags q8h and q24h were observed for 50 mg/mL solutions and scenarios with CMEM_24h close to the MIC, with differences (Δ) in CMEM_24h up to 4.9 mg/L, ΔfT>MIC≤65.7%, and ΔBTOT_24h≤1.1 log10 CFU/mL, thus affecting conclusions on whether bacteriostasis was reached. In summary, this study indicated that for continuous infusion of meropenem, eight-hourly infusion bag changes improved PK/PD target attainment and might be beneficial particularly for high meropenem concentrations of saline infusion solutions and for plasma concentrations in close proximity to the MIC.

Therapeutic Drug Monitoring and Biomarkers; towards Better Dosing of Antimicrobial Therapy

Due to variability in pharmacokinetics and pharmacodynamics, clinical outcomes of antimicrobial drug therapy vary between patients. As such, personalised medication management, considering both pharmacokinetics and pharmacodynamics, is a growing concept of interest in the field of infectious diseases. Therapeutic drug monitoring is used to adjust and individualise drug regimens until predefined pharmacokinetic exposure targets are achieved. Minimum inhibitory concentration (drug susceptibility) is the best available pharmacodynamic parameter but is associated with many limitations. Identification of other pharmacodynamic parameters is necessary. Repurposing diagnostic biomarkers as pharmacodynamic parameters to evaluate treatment response is attractive. When combined with therapeutic drug monitoring, it could facilitate making more informed dosing decisions. We believe the approach has potential and justifies further research.

Model-based learn and confirm: designing effective treatment regimens against multidrug resistant Gram-negative pathogens

Over the last decade, there has been a growing appreciation for the use of in vitro and in vivo infection models to generate robust and informative nonclinical PK/PD data to accelerate the clinical translation of treatment regimens. The objective of this study was to develop a model-based "learn and confirm" approach to help with the design of combination regimens using in vitro infection models to optimise the clinical utility of existing antibiotics. Static concentration time-kill studies were used to evaluate the PD activity of polymyxin B (PMB) and meropenem against two carbapenem-resistant Klebsiella pneumoniae (CRKP) isolates; BAA2146 (PMB-susceptible) and BRKP67 (PMB-resistant). A mechanism-based model (MBM) was developed to quantify the joint activity of PMB and meropenem. In silico simulations were used to predict the time-course of bacterial killing using clinically-relevant PK exposure profiles. The predictive accuracy of the model was further evaluated by validating the model predictions using a one-compartment PK/PD in vitro dynamic infection model (IVDIM). The MBM captured the reduction in bacterial burden and regrowth well in both the BAA2146 and BRKP67 isolate (R2 = 0.900 and 0.940, respectively). The bacterial killing and regrowth predicted by the MBM were consistent with observations in the IVDIM: sustained activity against BAA2146 and complete regrowth of the BRKP67 isolate. Differences observed in PD activity suggest that additional dose optimisation might be beneficial in PMB-resistant isolates. The model-based approach presented here demonstrates the utility of the MBM as a translational tool from static to dynamic in vitro systems to effectively perform model-informed drug optimisation.

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.

Assessing the in vivo impact of novel β-lactamase inhibitors on the efficacy of their partner β-lactams against serine carbapenemase-producing Pseudomonas aeruginosa using human-simulated exposures

OBJECTIVES

To evaluate the efficacy of human-simulated regimens (HSRs) of ceftazidime, ceftazidime/avibactam, imipenem, imipenem/relebactam, meropenem and meropenem/vaborbactam in a murine thigh infection model against serine carbapenemase-producing Pseudomonas aeruginosa.

METHODS

Nine P. aeruginosa clinical isolates harbouring GES-5 (n = 1), GES-20 (n = 1), GES-5/20 (n = 1), GES-19, GES-20 (n = 3) and KPC (n = 3) were evaluated. Six mice were administered HSRs of ceftazidime 2 g q8h (2 h infusion), ceftazidime/avibactam 2.5 g q8h (2 h infusion), meropenem 2 g q8h (3 h infusion), imipenem 0.5 g q6h (0.5 h infusion), imipenem/relebactam 1.25 g q6h (0.5 h infusion) and meropenem/vaborbactam 4 g q8h (3 h infusion). Change in bacterial burden relative to baseline and the percent of isolates meeting the 1 log10 kill endpoint were assessed.

RESULTS

The addition of avibactam to ceftazidime increased the percentage of isolates meeting 1 log10 kill from 33% to 100% of GES- or KPC-harbouring isolates. Imipenem/relebactam HSR produced ≥1 log10 of kill against 83% and 100% of GES- and KPC-harbouring isolates, respectively, while imipenem alone failed to reach 1 log10 kill for any isolates. Vaborbactam resulted in variable restoration of meropenem activity as 1 log10 kill was achieved in only 33% and 66% of GES- and KPC-harbouring isolates, respectively, compared with no isolates for meropenem alone.

CONCLUSIONS

Ceftazidime/avibactam and imipenem/relebactam were active against 100% and 89% of KPC- or GES-harbouring isolates tested in vivo. The activity of meropenem/vaborbactam was variable, suggesting this may be an inferior treatment option in this setting. Further studies to evaluate clinical outcomes in GES- and KPC-producing P. aeruginosa are warranted given their increasing prevalence worldwide.

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

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

Acceleration of infectious disease drug discovery and development using a humanized model of drug metabolism

A key step in drug discovery, common to many disease areas, is preclinical demonstration of efficacy in a mouse model of disease. However, this demonstration and its translation to the clinic can be impeded by mouse-specific pathways of drug metabolism. Here, we show that a mouse line extensively humanized for the cytochrome P450 gene superfamily ("8HUM") can circumvent these problems. The pharmacokinetics, metabolite profiles, and magnitude of drug-drug interactions of a test set of approved medicines were in much closer alignment with clinical observations than in wild-type mice. Infection with Mycobacterium tuberculosis, Leishmania donovani, and Trypanosoma cruzi was well tolerated in 8HUM, permitting efficacy assessment. During such assessments, mouse-specific metabolic liabilities were bypassed while the impact of clinically relevant active metabolites and DDI on efficacy were well captured. Removal of species differences in metabolism by replacement of wild-type mice with 8HUM therefore reduces compound attrition while improving clinical translation, accelerating drug discovery.

Improved characterization of aminoglycoside penetration into human lung epithelial lining fluid via population pharmacokinetics

Aminoglycosides are important treatment options for serious lung infections, but modeling analyses to quantify their human lung epithelial lining fluid (ELF) penetration are lacking. We estimated the extent and rate of penetration for five aminoglycosides via population pharmacokinetics from eight published studies. The area under the curve in ELF vs plasma ranged from 50% to 100% and equilibration half-lives from 0.61 to 5.80 h, indicating extensive system hysteresis. Aminoglycoside ELF peak concentrations were blunted, but overall exposures were moderately high.

Target attainment of intravenous lefamulin for treatment of acute bacterial skin and skin structure infections

OBJECTIVES

Lefamulin is a pleuromutilin antibiotic approved for the treatment of community-acquired bacterial pneumonia (CABP). Its spectrum of activity, good penetration into soft tissues and low rates of cross-resistance also make lefamulin a potentially valuable option for treatment of acute bacterial skin and skin structure infections (ABSSSIs). A Phase 2 trial of lefamulin for ABSSSI indicated similar efficacy of 100 and 150 mg q12h IV dosing regimens. In the present study, the potential of lefamulin for this indication was further evaluated from a translational pharmacokinetic/pharmacodynamic perspective.

METHODS

PTA was determined for various dosages using Monte Carlo simulations of a population pharmacokinetic model of lefamulin in ABSSSI patients and preclinical exposure targets associated with bacteriostasis and a 1-log reduction in bacterial count. Overall target attainment against MSSA and MRSA was calculated using lefamulin MIC distributions.

RESULTS

Overall attainment of the bacteriostasis target was 94% against MSSA and 84% against MRSA for the IV dosage approved for CABP (150 mg q12h). Using the same target, for the 100 mg q12h regimen, overall target attainment dropped to 68% against MSSA and 50% against MRSA. Using the 1-log reduction target, overall target attainment for both regimens was <40%.

CONCLUSIONS

Lefamulin at the currently approved IV dosage covers most Staphylococcus aureus isolates when targeting drug exposure associated with bacteriostasis, suggesting potential of lefamulin for the treatment of ABSSSIs. Lefamulin may not be appropriate in ABSSSI when rapid bactericidal activity is warranted.

Animal Models in Regulatory Breakpoint Determination: Review of New Drug Applications of Approved Antibiotics from 2014-2022

We sought to better understand the utility and role of animal models of infection for Food and Drug Administration (FDA)-approved antibiotics for the indications of community-, hospital-acquired-, and ventilator-associated bacterial pneumonia (CABP, HABP, VABP), complicated urinary tract infection (cUTI), complicated intra-abdominal infection (cIAI), and acute bacterial skin and structural infections (ABSSSIs). We reviewed relevant documents from new drug applications (NDA) of FDA-approved antibiotics from 2014-2019 for the above indications. Murine neutropenic thigh infection models supported the choice of a pharmacokinetic-pharmacodynamic (PKPD) target in 11/12 NDAs reviewed. PKPD targets associated with at least a 1-log bacterial decrease were commonly considered ideal (10/12 NDAs) to support breakpoints. Plasma PK, as opposed to organ specific PK, was generally considered most reliable for PKPD correlation. Breakpoint determination was multi-disciplinary, accounting at minimum for epidemiologic cutoffs, non-clinical PKPD, clinical exposure-response and clinical efficacy. Non-clinical PKPD targets in combination with probability of target attainment (PTA) analyses generated breakpoints that were consistent with epidemiologic cutoffs and clinically derived breakpoints. In 6/12 NDAs, there was limited data to support clinically derived breakpoints, and hence the non-clinical PKPD targets in combination with PTA analyses played a heightened role in the final breakpoint determination. Sponsor and FDA breakpoint decisions were in general agreement. Disagreement may have arisen from differences in the definition of the optimal PKPD index or the ability to extrapolate protein binding from animals to humans. Overall, murine neutropenic thigh infection models supported the reviewed NDAs by providing evidence of pre-clinical efficacy and PKPD target determination, and played, in combination with PTA analysis, a significant role in breakpoint determination for labeling purposes.

Translational PK/PD for the Development of Novel Antibiotics-A Drug Developer's Perspective

Antibiotic development traditionally involved large Phase 3 programs, preceded by Phase 2 studies. Recognizing the high unmet medical need for new antibiotics and, in some cases, challenges to conducting large clinical trials, regulators created a streamlined clinical development pathway in which a lean clinical efficacy dataset is complemented by nonclinical data as supportive evidence of efficacy. In this context, translational Pharmacokinetic/Pharmacodynamic (PK/PD) plays a key role and is a major contributor to a "robust" nonclinical package. The classical PK/PD index approach, proven successful for established classes of antibiotics, is at the core of recent antibiotic approvals and the current antibacterial PK/PD guidelines by regulators. Nevertheless, in the case of novel antibiotics with a novel Mechanism of Action (MoA), there is no prior experience with the PK/PD index approach as the basis for translating nonclinical efficacy to clinical outcome, and additional nonclinical studies and PK/PD analyses might be considered to increase confidence. In this review, we discuss the value and limitations of the classical PK/PD approach and present potential risk mitigation activities, including the introduction of a semi-mechanism-based PK/PD modeling approach. We propose a general nonclinical PK/PD package from which drug developers might choose the studies most relevant for each individual candidate in order to build up a "robust" nonclinical PK/PD understanding.

Distinguishing Inducible and Non-Inducible Resistance to Colistin in Pseudomonas aeruginosa by Quantitative and Systems Pharmacology Modeling at Low and Standard Inocula

Colistin is a polymyxin and peptide antibiotic that can yield rapid bacterial killing, but also leads to resistance emergence. We aimed to develop a novel experimental and Quantitative and Systems Pharmacology approach to distinguish between inducible and non-inducible resistance. Viable count profiles for the total and less susceptible populations of Pseudomonas aeruginosa ATCC 27853 from static and dynamic in vitro infection models were simultaneously modeled. We studied low and normal initial inocula to distinguish between inducible and non-inducible resistance. A novel cutoff filter approach allowed us to describe the eradication and inter-conversion of bacterial populations. At all inocula, 4.84 mg/L of colistin (sulfate) yielded ≥4 log10 killing, followed by >4 log10 regrowth. A pre-existing, less susceptible population was present at standard but not at low inocula. Formation of a non-pre-existing, less susceptible population was most pronounced at intermediate colistin (sulfate) concentrations (0.9 to 5 mg/L). Both less susceptible populations inter-converted with the susceptible population. Simultaneously modeling of the total and less susceptible populations at low and standard inocula enabled us to identify the de novo formation of an inducible, less susceptible population. Inducible resistance at intermediate colistin concentrations highlights the importance of rapidly achieving efficacious polymyxin concentrations by front-loaded dosage regimens.

Effective Antimicrobial Prophylaxis in Surgery: The Relevance and Role of Pharmacokinetics-Pharmacodynamics

Appropriate surgical antimicrobial prophylaxis (SAP) is an important measure in preventing surgical site infections (SSIs). Although antimicrobial pharmacokinetics-pharmacodynamics (PKPD) is integral to optimizing antibiotic dosing for the treatment of infections, there is less research on preventing infections postsurgery. Whereas clinical studies of SAP dose, preincision timing, and redosing are informative, it is difficult to isolate their effect on SSI outcomes. Antimicrobial PKPD aims to explain the complex relationship between antibiotic exposure during surgery and the subsequent development of SSI. It accounts for the many factors that influence the PKs and antibiotic concentrations in patients and considers the susceptibilities of bacteria most likely to contaminate the surgical site. This narrative review examines the relevance and role of PKPD in providing effective SAP. The dose-response relationship i.e., association between lower dose and SSI in cefazolin prophylaxis is discussed. A comprehensive review of the evidence for an antibiotic concentration-response (SSI) relationship in SAP is also presented. Finally, PKPD considerations for improving SAP are explored with a focus on cefazolin prophylaxis in adults and outstanding questions regarding its dose, preincision timing, and redosing during surgery.

Emerging strategies to target virulence in Pseudomonas aeruginosa respiratory infections

Pseudomonas aeruginosa is an opportunistic pathogen that is responsible for infections in people living with chronic respiratory conditions, such as cystic fibrosis (CF) and non-CF bronchiectasis (NCFB). Traditionally, in people with chronic respiratory disorders, P. aeruginosa infection has been managed with a combination of inhaled and intravenous antibiotic therapies. However, due in part to the prolonged use of antibiotics in these people, the emergence of multi-drug resistant P. aeruginosa strains is a growing concern. The development of anti-virulence therapeutics may provide a new means of treating P. aeruginosa lung infections whilst also combatting the AMR crisis, as these agents are presumed to exert reduced pressure for the emergence of drug resistance as compared to antibiotics. However, the pipeline for developing anti-virulence therapeutics is poorly defined, and it is currently unclear as to whether in vivo and in vitro models effectively replicate the complex pulmonary environment sufficiently to enable development and testing of such therapies for future clinical use. Here, we discuss potential targets for P. aeruginosa anti-virulence therapeutics and the effectiveness of the current models used to study them. Focus is given to the difficulty of replicating the virulence gene expression patterns of P. aeruginosa in the CF and NCFB lung under laboratory conditions and to the challenges this poses for anti-virulence therapeutic development.

Optimizing Antimicrobial Treatment Schedules: Some Fundamental Analytical Results

This work studies fundamental questions regarding the optimal design of antimicrobial treatment protocols, using pharmacodynamic and pharmacokinetic mathematical models. We consider the problem of designing an antimicrobial treatment schedule to achieve eradication of a microbial infection, while minimizing the area under the time-concentration curve (AUC), which is equivalent to minimizing the cumulative dosage. We first solve this problem under the assumption that an arbitrary antimicrobial concentration profile may be chosen, and prove that the ideal concentration profile consists of a constant concentration over a finite time duration, where explicit expressions for the optimal concentration and the time duration are given in terms of the pharmacodynamic parameters. Since antimicrobial concentration profiles are induced by a dosing schedule and the antimicrobial pharmacokinetics, the 'ideal' concentration profile is not strictly feasible. We therefore also investigate the possibility of achieving outcomes which are close to those provided by the 'ideal' concentration profile, using a bolus+continuous dosing schedule, which consists of a loading dose followed by infusion of the antimicrobial at a constant rate. We explicitly find the optimal bolus+continuous dosing schedule, and show that, for realistic parameter ranges, this schedule achieves results which are nearly as efficient as those attained by the 'ideal' concentration profile. The optimality results obtained here provide a baseline and reference point for comparison and evaluation of antimicrobial treatment plans.

The Coming of Age of AI/ML in Drug Discovery, Development, Clinical Testing, and Manufacturing: The FDA Perspectives

Artificial intelligence (AI) and machine learning (ML) represent significant advancements in computing, building on technologies that humanity has developed over millions of years-from the abacus to quantum computers. These tools have reached a pivotal moment in their development. In 2021 alone, the U.S. Food and Drug Administration (FDA) received over 100 product registration submissions that heavily relied on AI/ML for applications such as monitoring and improving human performance in compiling dossiers. To ensure the safe and effective use of AI/ML in drug discovery and manufacturing, the FDA and numerous other U.S. federal agencies have issued continuously updated, stringent guidelines. Intriguingly, these guidelines are often generated or updated with the aid of AI/ML tools themselves. The overarching goal is to expedite drug discovery, enhance the safety profiles of existing drugs, introduce novel treatment modalities, and improve manufacturing compliance and robustness. Recent FDA publications offer an encouraging outlook on the potential of these tools, emphasizing the need for their careful deployment. This has expanded market opportunities for retraining personnel handling these technologies and enabled innovative applications in emerging therapies such as gene editing, CRISPR-Cas9, CAR-T cells, mRNA-based treatments, and personalized medicine. In summary, the maturation of AI/ML technologies is a testament to human ingenuity. Far from being autonomous entities, these are tools created by and for humans designed to solve complex problems now and in the future. This paper aims to present the status of these technologies, along with examples of their present and future applications.

Antibacterial effect of seven days exposure to ceftolozane-tazobactam as monotherapy and in combination with fosfomycin or tobramycin against Pseudomonas aeruginosa with ceftolozane-tazobactam MICs at or above 4 mg/l in an in vitro pharmacokinetic model

OBJECTIVES

To use a pre-clinical pharmacokinetic infection model to assess the antibacterial effect of ceftolozane/tazobactam alone or in combination with fosfomycin or tobramycin against Pseudomonas aeruginosa strains with MICs at or higher than the clinical breakpoint (MIC ≥ 4 mg/L).

METHODS

An in vitro model was used to assess changes in bacterial load and population profiles after exposure to mean human serum concentrations of ceftolozane/tazobactam associated with doses of 2 g/1 g q8h, fosfomycin concentrations associated with doses of 8 g q8h or tobramycin at doses of 7 mg/kg q24 h over 168 h.

RESULTS

Simulations of ceftolozane/tazobactam at 2 g/1 g q8h alone produced 3.5-4.5 log reductions in count by 6 h post drug exposure for strains with MIC ≤32 mg/L. The antibacterial effect over the first 24 h was related to ceftolozane/tazobactam MIC. There was subsequent regrowth with most strains to bacterial densities of >106 CFU/mL. Addition of either fosfomycin or tobramycin resulted in suppression of regrowth and in the case of tobramycin more rapid initial bacterial killing up to 6 h. These effects could not be related to either fosfomycin or tobramycin MICs. Changes in population profiles were noted with ceftolozane/tazobactam alone often after 96 h exposure but such changes were suppressed by fosfomycin and almost abolished by the addition of tobramycin.

CONCLUSIONS

The addition of either fosfomycin or tobramycin to ceftolozane/tazobactam at simulated human clinically observed concentrations reduced P. aeruginosa bacterial loads and the risk of resistance to ceftolozane/tazobactam when strains had ceftolozane/tazobactam MIC values at or above the clinical breakpoint.

Neutropenic Rat Thigh Infection Model for Evaluation of the Pharmacokinetics/Pharmacodynamics of Anti-Infectives

The neutropenic mouse infection model is extensively used to characterize the pharmacokinetics/pharmacodynamics (PK/PD) of anti-infective agents. However, it is difficult to evaluate agents following intravenous (i.v.) infusions using this model. Furthermore, in many drug discovery programs, lead identification and optimization is performed in rats, and pharmacology is performed in mice. Alternative models of infection are needed for robust predictions of PK/PD in humans. The rat is an alternative model of infection which can overcome the shortcomings of the mouse model. However, the rat neutropenic thigh infection (NTI) model has not been adequately characterized for evaluation of the PK/PD of anti-infectives. The aim of this study was to characterize the PK/PD of ciprofloxacin against bacterial pathogens in a rat NTI model. We studied the PK/PD relationships of ciprofloxacin against wild-type Escherichia coli, Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae in neutropenic Wistar rats following administration of 10, 30, and 100 mg/kg as single intravenous boluses and 30- and 60-min infusions. The PK/PD of ciprofloxacin against all four pathogens was AUC/MIC dependent and independent of the duration of administration at 10, 30, and 100 mg/kg. At human-equivalent rat doses, the PK/PD targets of ciprofloxacin achieved in rats for microbiological cure were similar to those reported in human patients. The neutropenic rat thigh infection model can be used to evaluate anti-infective agents intended to be administered as infusions in the clinic, and it complements the mouse model, increasing the robustness of PK/PD predictions in humans. IMPORTANCE Many antibiotics are administered as intravenous infusions in the clinic, especially in intensive care units. Anti-infective drug discovery companies develop clinical candidates that are intended to be administered as i.v. infusions in the clinic. However, there are no well-characterized models with which they can evaluate the PK/PD of the candidates following i.v. infusions. The neutropenic rat thigh infection model reported in this study helps in evaluating anti-infective agents that are intended to be administered as i.v. infusions in the clinic. The rat model is useful for simulating the clinical conditions for i.v. infusions for treatment of infections, such as acute bacterial skin and skin structure, lung, and urinary tract infections. This model is predictive of efficacy in humans and can serve as an additional confirmatory model, along with the mouse model, for determining the proof of concept and for making robust predictions of efficacy in humans.

International consensus recommendations for the use of prolonged-infusion beta-lactam antibiotics: Endorsed by the American College of Clinical Pharmacy, British Society for Antimicrobial Chemotherapy, Cystic Fibrosis Foundation, European Society of Clinical Microbiology and Infectious Diseases, Infectious Diseases Society of America, Society of Critical Care Medicine, and Society of Infectious Diseases Pharmacists

Intravenous β-lactam antibiotics remain a cornerstone in the management of bacterial infections due to their broad spectrum of activity and excellent tolerability. β-lactams are well established to display time-dependent bactericidal activity, where reductions in bacterial burden are directly associated with the time that free drug concentrations remain above the minimum inhibitory concentration (MIC) of the pathogen during the dosing interval. In an effort to take advantage of these bactericidal characteristics, prolonged (extended and continuous) infusions (PIs) can be applied during the administration of intravenous β-lactams to increase time above the MIC. PI dosing regimens have been implemented worldwide, but implementation is inconsistent. We report consensus therapeutic recommendations for the use of PI β-lactams developed by an expert international panel with representation from clinical pharmacy and medicine. This consensus guideline provides recommendations regarding pharmacokinetic and pharmacodynamic targets, therapeutic drug-monitoring considerations, and the use of PI β-lactam therapy in the following patient populations: severely ill and nonseverely ill adult patients, pediatric patients, and obese patients. These recommendations provide the first consensus guidance for the use of β-lactam therapy administered as PIs and have been reviewed and endorsed by the American College of Clinical Pharmacy (ACCP), the British Society for Antimicrobial Chemotherapy (BSAC), the Cystic Fibrosis Foundation (CFF), the European Society of Clinical Microbiology and Infectious Diseases (ESCMID), the Infectious Diseases Society of America (IDSA), the Society of Critical Care Medicine (SCCM), and the Society of Infectious Diseases Pharmacists (SIDP).

In vivo Pharmacokinetics/Pharmacodynamics Profiles for Appropriate Doses of Cefditoren pivoxil against S. pneumoniae in Murine Lung-Infection Model

PURPOSE

Cefditoren, the active form of cefditoren pivoxil, is an oral cephalosporin antimicrobial drug. Although cefditoren exhibits high antimicrobial activity against Streptococcus pneumoniae, its pharmacokinetics/pharmacodynamics (PK/PD) characteristics remain unknown. This study aimed to determine its PK/PD parameter with target values for cefditoren against S. pneumoniae in S. pneumoniae lung-infected mice and to simulate MIC range of S. pneumoniae that can be expected to be treated at approved cefditoren doses in human using population pharmacokinetic (PPK) data from patients.

METHODS

Susceptibility testing and time-kill assays against S. pneumoniae ATCC® 49619 were performed for in vitro PD evaluation. Based on the results of a PK study in healthy mice and PD studies in S. pneumoniae lung-infected mice, optimal PK/PD parameters were determined using the correlation curve between the PK/PD parameters and lung bacterial count changes. The target value was calculated to achieve a 2 log10 reduction in the lung bacterial counts.

RESULTS

In vitro PD evaluation showed that cefditoren had a potent antimicrobial effect against S. pneumoniae in a time-dependent manner at concentrations above the MIC. In PK/PD analyses, both fAUC24/MIC and fCmax/MIC were well correlated with bactericidal efficacy, achieving 2 log10-kill with fAUC24/MIC ≥ 63 and fCmax/MIC ≥ 16.

CONCLUSIONS

Cefditoren pivoxil has good therapeutic efficacy against acute pneumonia caused by S. pneumoniae with a MIC ≤ 0.031-0.063 mg/L at approved doses in adults and children.

The nematode worm Caenorhabditis elegans as an animal experiment replacement for assessing the virulence of different Salmonella enterica strains

Caenorhabditis (C.) elegans has become a popular toxicological and biological test organism in the last two decades. Furthermore, the role of C. elegans as an alternative for replacing or reducing animal experiments is continuously discussed and investigated. In the current study, we investigated whether C. elegans survival assays can help in determining differences in the virulence of Salmonella enterica strains and to what extent C. elegans assays could replace animal experiments for this purpose. We focused on three currently discussed examples where we compared the longevity of C. elegans when fed (i) with S. enterica serovar Enteritidis vaccination or wild-type strains, (ii) with lipopolysaccharide (LPS) deficient rough or LPS forming smooth S. enterica serovar Enteritidis, and (iii) with an S. enterica subsp. diarizonae strain in the presence or absence of the typical pSASd plasmid encoding a bundle of putative virulence factors. We found that the C. elegans survival assay could indicate differences in the longevity of C. elegans when fed with the compared strain pairs to a certain extent. Putatively higher virulent S. enterica strains reduced the lifespan of C. elegans to a greater extent than putatively less virulent strains. The C. elegans survival assay is an effective and relatively easy method for classifying the virulence of different bacterial isolates in vivo, but it has some limitations. The assay cannot replace animal experiments designed to determine differences in the virulence of Salmonella enterica strains. Instead, we recommend using the described method for pre-screening bacterial strains of interest to select the most promising candidates for further animal experiments. The C. elegans assay possesses the potential to reduce the number of animal experiments. Further development of the C. elegans assay in conjunction with omics technologies, such as transcriptomics, could refine results relating to the estimation of the virulent potential of test organisms.

Rapid In Vitro Assessment of Antimicrobial Drug Effect Bridging Clinically Relevant Pharmacokinetics: A Comprehensive Methodology

Rapid in vitro assessment of antimicrobial drug efficacy under clinically relevant pharmacokinetic conditions is an essential element of both drug development and clinical use. Here, we present a comprehensive overview of a recently developed novel integrated methodology for rapid assessment of such efficacy, particularly against the emergence of resistant bacterial strains, as jointly researched by the authors in recent years. This methodology enables rapid in vitro assessment of the antimicrobial efficacy of single or multiple drugs in combination, following clinically relevant pharmacokinetics. The proposed methodology entails (a) the automated collection of longitudinal time-kill data in an optical-density instrument; (b) the processing of collected time-kill data with the aid of a mathematical model to determine optimal dosing regimens under clinically relevant pharmacokinetics for single or multiple drugs; and (c) in vitro validation of promising dosing regimens in a hollow fiber system. Proof-of-concept of this methodology through a number of in vitro studies is discussed. Future directions for the refinement of optimal data collection and processing are discussed.

Individual Components of Polymyxin B Modeled via Population Pharmacokinetics to Design Humanized Dosage Regimens for a Bloodstream and Lung Infection Model in Immune-Competent Mice

Polymyxin B is a "last-line-of-defense" antibiotic approved in the 1960s. However, the population pharmacokinetics (PK) of its four main components has not been reported in infected mice. We aimed to determine the PK of polymyxin B1, B1-Ile, B2, and B3 in a murine bloodstream and lung infection model of Acinetobacter baumannii and develop humanized dosage regimens. A linear 1-compartment model, plus an epithelial lining fluid (ELF) compartment for the lung model, best described the PK. Clearance and volume of distribution were similar among the four components. The bioavailability fractions were 72.6% for polymyxin B1, 12.0% for B1-Ile, 11.5% for B2, and 3.81% for B3 for the lung model and were similar for the bloodstream model. While the volume of distribution was comparable between both models (17.3 mL for the lung and ~27 mL for the bloodstream model), clearance was considerably smaller for the lung (2.85 mL/h) compared to that of the bloodstream model (5.59 mL/h). The total drug exposure (AUC) in ELF was high due to the saturable binding of polymyxin B presumably to bacterial lipopolysaccharides. However, the modeled unbound AUC in ELF was ~16.7% compared to the total drug AUC in plasma. The long elimination half-life (~4 h) of polymyxin B enabled humanized dosage regimens with every 12 h dosing in mice. Daily doses that optimally matched the range of drug concentrations observed in patients were 21 mg/kg for the bloodstream and 13 mg/kg for the lung model. These dosage regimens and population PK models support translational studies for polymyxin B at clinically relevant drug exposures.

In Vivo Pharmacodynamic Profile of EVER206, a Novel Polymyxin Antimicrobial, against Gram-Negative Bacteria in the Murine Thigh Infection Model

The objective was to determine the magnitude of the EVER206 free-plasma area under the concentration time curve (fAUC)/MIC target associated with bacteriostasis and 1-log10 kill against clinically relevant Gram-negative bacteria in the murine thigh model. Twenty-seven clinical isolates (Pseudomonas aeruginosa, n = 10; Escherichia coli, n = 9; Klebsiella pneumoniae, n = 5; Enterobacter cloacae, n = 2; and Klebsiella aerogenes, n = 1) were tested. Mice were pretreated with cyclophosphamide (induce neutropenia) and uranyl nitrate (increase the exposure of test compound through predictable renal dysfunction). Two hours postinoculation, five doses of EVER206 were administered subcutaneously. EVER206 pharmacokinetics were determined in infected mice. Data were fit using maximum effect (Emax) models to elucidate the fAUC/MIC targets for stasis and 1-log10 bacterial kill (reported as mean [range] by species). EVER206 MICs (mg/L) ranged from 0.25 to 2 mg/L (P. aeruginosa), 0.06 to 2 mg/L (E. coli), 0.06 to 0.125 mg/L (E. cloacae), 0.06 mg/L (K. aerogenes), and 0.06 to 2 mg/L (K. pneumoniae). In vivo, the mean 0-h baseline bacterial burden was 5.57 ± 0.39 log10 CFU/thigh. Stasis was achieved in 9/10 P. aeruginosa (fAUC/MIC, 88.13 [50.33 to 129.74]), 9/9 E. coli (fAUC/MIC, 112.84 [19.19 to 279.38]), 2/2 E. cloacae (fAUC/MIC, 259.28 [124.08 to 394.47]), 0/1 K. aerogenes, and 4/5 K. pneumoniae (fAUC/MIC, 99.26 [62.3 to 144.43]) isolates tested. 1-log10 kill was achieved in 9/10 for P. aeruginosa (fAUC/MIC, 106.43 [55.22 to 152.08]), 3/9 for E. coli (fAUC/MIC, 258.96 [74.08 to 559.4]), and 1/2 for E. cloacae (fAUC/MIC, 255.33). Using the murine thigh model, the fAUC/MIC targets of EVER206 were assessed across a broad MIC distribution. Integrating these data with microbiologic and clinical exposure data will aid in determining the clinical dose of EVER206.

Pharmacokinetic-pharmacodynamic (PK/PD) modeling to study the hepatoprotective effect of Perilla Folium on the acute hepatic injury rats

ETHNOPHARMACOLOGICAL RELEVANCE

Perilla Folium (PF), is a traditional medicinal material with the homology of medicine and food in China and has been widely used due to its rich nutritional content and medicinal value. The hepatoprotective effects of PF extract include their protection against acute hepatic injury, tert-butylhydroperoxide (t-BHP) induced oxidative damage, and Lipopolysaccharide (LPS) and D-galactosamine (D-GalN) induced hepatic injury have been well studied. However, there are few reports on the pharmacokinetics studies of PF extract in acute hepatic injury model rats, and the anti-hepatic injury activity of PF is still unclear.

AIM OF THE STUDY

The differences in the plasma pharmacokinetic of 21 active compounds between the normal and model groups were compared， and established pharmacokinetics/pharmacodynamics (PK/PD) modeling was to analyze the hepatoprotective effects of PF.

MATERIALS AND METHODS

The acute hepatic injury model was induced with an intraperitoneal injection of lipopolysaccharide (LPS) and D-galactosamine (D-GalN), and the plasma pharmacokinetics of 21 active compounds of PF were analyzed in the normal and model groups using ultra-high performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS). The correlation between plasma components and hepatoprotective effects indicators (the alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactic dehydrogenase (LDH)) in the model group was also investigated and established a Pharmacokinetic/pharmacodynamic (PK/PD) correlation analysis of the hepatoprotective effects of PF.

RESULTS

The results revealed that organic acid compounds possessed the characteristics of faster absorption, shorter peak time and slower metabolism, while the flavonoid compounds had slower absorption and longer peak time, and the pharmacokinetics of various components were significantly affected after modeling. The results of PK/PD modeling analysis demonstrated that the plasma drug concentration of each component existed a good correlation with the three AST, ALT, and LDH, and the lag time of the efficacy of each component is relatively long.

CONCLUSIONS

The plasma drug concentration of each component existed a good correlation with the three AST, ALT, and LDH, and the lag time of the efficacy of each component is relatively long in vivo.

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

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

Emerging issues on Staphylococcus aureus endocarditis and the role in therapy of daptomycin plus fosfomycin

INTRODUCTION

Methicillin-resistant and -susceptible Staphylococcus aureus (MRSA/MSSA) infections are a major global health-care problem. Bacteremia with S. aureus exhibits high rates of morbidity and mortality and can cause complicated infections such as infective endocarditis (IE). The emerging resistance profile of S. aureus is worrisome, and several international agencies have appealed for new treatment approaches to be developed.

AREAS COVERED

Daptomycin presents a rapid bactericidal effect against MRSA and has been considered at least as effective as vancomycin in treating MRSA bacteremia. However, therapy failure is often related to deep-seated infections, e.g. endocarditis, with high bacterial inocula and daptomycin regimens <10 mg/kg/day. Current antibiotic options for treating invasive S. aureus infections have limitations in monotherapy. Daptomycin in combination with other antibiotics, e.g. fosfomycin, may be effective in improving clinical outcomes in patients with MRSA IE.

EXPERT OPINION

Exploring therapeutic combinations has shown fosfomycin to have a unique mechanism of action and to be the most effective option in preventing the onset of resistance to and optimizing the efficacy of daptomycin, suggesting the synergistic combination of fosfomycin with daptomycin is a useful alternative treatment option for MSSA or MRSA IE.

Optimizing antibiotic dosing regimens for nosocomial pneumonia: a window of opportunity for pharmacokinetic and pharmacodynamic modeling

INTRODUCTION

Determining antibiotic exposure in the lung and the threshold(s) needed for effective antibacterial killing is paramount during development of new antibiotics for the treatment of nosocomial pneumonia, as these exposures directly affect clinical outcomes and resistance development. The use of pharmacokinetic and pharmacodynamic modeling is recommended by regulatory agencies to evaluate antibiotic pulmonary exposure and optimize dosage regimen selection. This process has been implemented in newer antibiotic development.

AREAS COVERED

This review will discuss the basis for conducting pharmacokinetic and pharmacodynamic studies to support dosage regimen selection and optimization for the treatment of nosocomial pneumonia. Pharmacokinetic/pharmacodynamic data that supported recent hospital-acquired bacterial pneumonia/ventilator-associated bacterial pneumonia indications for ceftolozane/tazobactam, ceftazidime/avibactam, imipenem/cilastatin/relebactam, and cefiderocol will be reviewed.

EXPERT OPINION

Optimal drug development requires the integration of preclinical pharmacodynamic studies, healthy volunteers and ideally patient bronchoalveolar lavage pharmacokinetic studies, Monte-Carlo simulation, and clinical trials. Currently, plasma exposure has been successfully used as a surrogate for lung exposure threshold. Future studies are needed to identify the value of lung pharmacodynamic thresholds in nosocomial pneumonia antibiotic dosage optimization.

Pharmacokinetics and Pharmacodynamics of a Novel Vancomycin Derivative LYSC98 in a Murine Thigh Infection Model Against Staphylococcus aureus

Introduction

LYSC98 is a novel vancomycin derivative used for gram-positive bacterial infections. Here we compared the antibacterial activity of LYSC98 with vancomycin and linezolid in vitro and in vivo. Besides, we also reported the pharmacokinetic/pharmacodynamic (PK/PD) index and efficacy-target values of LYSC98.

Methods

The MIC values of LYSC98 were identified through broth microdilution method. A mice sepsis model was established to investigate the protective effect of LYSC98 in vivo. Single-dose pharmacokinetics of LYSC98 was studied in thigh-infected mice and liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was used to determine LYSC98 concentration in plasma. Dose fractionation studies were performed to evaluate different PK/PD indices. Two methicillin-resistant Staphylococcus aureus (MRSA) clinical strains were used in the dose ranging studies to determine the efficacy-target values.

Results

LYSC98 showed a universal antibacterial effect in Staphylococcus aureus with a MIC range of 2-4 µg/mL. In vivo, LYSC98 demonstrated distinctive mortality protection in mice sepsis model with an ED₅₀ value of 0.41-1.86 mg/kg. The pharmacokinetics results displayed maximum plasma concentration (C_max) 11,466.67-48,866.67 ng/mL, area under the concentration-time curve from 0 to 24 h (AUC_0-24) 14,788.42-91,885.93 ng/mL·h, and elimination half-life (T_1/2) 1.70-2.64 h, respectively. C_max/MIC (R ² 0.8941) was proved to be the most suitable PK/PD index for LYSC98 to predict its antibacterial efficacy. The magnitude of LYSC98 C_max/MIC associated with net stasis, 1, 2, 3 and 4 - log ₁₀ kill were 5.78, 8.17, 11.14, 15.85 and 30.58, respectively.

Conclusion

Our study demonstrates that LYSC98 is more effective than vancomycin either in killing vancomycin-resistant Staphylococcus aureus (VRSA) in vitro or treating S. aureus infections in vivo, making it a novel and promising antibiotic. The PK/PD analysis will also contribute to the LYSC98 Phase I dose design.

In vivo efficacy & resistance prevention of cefiderocol in combination with ceftazidime/avibactam, ampicillin/sulbactam or meropenem using human-simulated regimens versus Acinetobacter baumannii

OBJECTIVE

Evaluate the in vivo efficacy and resistance prevention of cefiderocol in combination with ceftazidime/avibactam, ampicillin/sulbactam and meropenem using human-simulated regimens (HSR) in the murine infection model.

METHODS

In total, 15 clinical A. baumannii were assessed: cefiderocol MICs, 2 mg/L (previously developed resistance on therapy), n = 3; 8 mg/L, n = 2; ≥32 mg/L, n = 10 (including VEB and PER-harbouring isolates). Mice received inactive control, cefiderocol, cefiderocol + ceftazidime/avibactam (C-CZA), cefiderocol + ampicillin/sulbactam (C-SAM) or cefiderocol + meropenem (C-MEM) HSRs. The mean change in log10 cfu/thigh compared with starting inoculum was assessed. Resistance development on treatment was a >4-fold increase in MIC relative control animals. In vitro activities of combinations were assessed by disc stacking.

RESULTS

Against cefiderocol-non-susceptible isolates, combinations produced significant kill with C-CZA -3.75 ± 0.37 reduction in log10 cfu/thigh, C-SAM produced -3.55 ± 0.50 and C-MEM produced -2.18 ± 1.75 relative to baseline. Elevated MICs in cefiderocol treated animals occurred in three out of three isolates with MICs of 2 mg/L. Of these isolates, one developed elevated MICs with C-MEM compared with none treated with C-CZA or C-SAM. Disc stacking with C-CZA or C-SAM returned all isolates to at least the CLSI intermediate breakpoint, which may correlate with in vivo efficacy.

CONCLUSIONS

Against cefiderocol-non-susceptible isolates, cefiderocol + ceftazidime/avibactam or ampicillin/sulbactam HSR produced in vivo kill against all 12 cefiderocol-non-susceptible isolates. Cefiderocol with ceftazidime/avibactam or ampicillin/sulbactam prevented the development of resistance during treatment against cefiderocol-high-end-susceptible isolates with a propensity for resistance on therapy. These data support the clinical evaluation of cefiderocol with ceftazidime/avibactam or ampicillin/sulbactam against A. baumannii, including multi-drug-resistant isolates.

In vivo pharmacokinetic/pharmacodynamic evaluation of cefepime/taniborbactam combination against cefepime-non-susceptible Enterobacterales and Pseudomonas aeruginosa in a murine pneumonia model

BACKGROUND

Cefepime/taniborbactam is a cephalosporin/bicyclic boronate β-lactamase inhibitor combination in clinical development for nosocomial pneumonia due to MDR Gram-negative bacteria. A murine pneumonia model was used to characterize cefepime/taniborbactam in vivo pharmacodynamics against Enterobacterales and Pseudomonas aeruginosa strains.

METHODS

Clinical cefepime-non-susceptible Enterobacterales and P. aeruginosa strains expressing serine carbapenemases and/or other cefepime-hydrolysing β-lactamases with cefepime/taniborbactam combination MICs of 0.12-16 mg/L were used. Cefepime and taniborbactam human-simulated regimens equivalent to clinical doses (i.e. 2/0.5 g q8h) were established in the pneumonia model. The in vivo activity of the cefepime human-simulated regimen given alone or concomitantly with escalating taniborbactam exposures against eight Enterobacterales and four P. aeruginosa strains was assessed. Taniborbactam pharmacokinetics were evaluated to determine systemic exposures of regimens used; taniborbactam fAUC0-24/MIC values required for efficacy were estimated using the Hill equation. In addition, the in vivo activity of the cefepime/taniborbactam combination human-simulated regimen was assessed against 18 strains.

RESULTS

Among Enterobacterales, median taniborbactam fAUC0-24/MIC values associated with stasis and 1 log kill were 0.96 and 4.03, respectively, while for P. aeruginosa, requirements were 1.35 and 3.02 for stasis and 1 log kill, respectively. The cefepime/taniborbactam human-simulated regimen produced >2 log kill in 14/18 strains and >1 log kill in 18/18 strains.

CONCLUSIONS

Cefepime/taniborbactam produced marked in vivo bactericidal activity against cefepime-non-susceptible Enterobacterales and P. aeruginosa isolates with cefepime/taniborbactam MICs up to and including 16 mg/L in the pneumonia model. Assessments of the probability of clinical attainment of the identified targets should be undertaken to support the selected cefepime/taniborbactam dose for treatment of nosocomial pneumonia.

Full-length whole-genome sequencing analysis of emerged meropenem-resistant mutants during long-term in vitro exposure to meropenem for borderline meropenem-susceptible carbapenemase-producing and non-carbapenemase-producing Enterobacterales

OBJECTIVES

Molecular analysis of meropenem-resistant mechanisms in mutants emerging from long-term in vitro meropenem exposure to borderline meropenem-susceptible carbapenemase-producing Enterobacterales (CPE) and non-CPE.

METHODS

Escherichia coli TUM13867 harbouring both blaIMP-6- and blaCTX-M-2-carrying IncN plasmid and Citrobacter koseri TUM13189 with blaCTX-M-2-carrying chromosome were used. Meropenem MIC was 1 mg/L against both strains. Each strain was cultured in the hollow-fibre infection model (HFIM) to approximately 1 × 106 colony formation unit (cfu)/mL, and meropenem 1 g q8h treatment was initiated. Then, changes in total and meropenem-resistant populations were observed for 124 h. Meropenem resistance mechanisms were analysed using full-length whole-genome sequencing (WGS), reverse-transcription quantitative PCR and digital PCR.

RESULTS

Meropenem reduced TUM13867 and TUM13189 to approximately 5 and 2 log10 cfu/mL, respectively, at 2 h after initiation, but regrowth was observed at 24 h. The meropenem-resistant mutant emergence frequency at 120 and 124 h was 4.4 × 10-4 for TUM13867 and 7.6 × 10-1 for TUM13189. Meropenem MIC of the mutants derived from TUM13867 (TUM20902) and TUM13189 (TUM20903) increased 4- and 16-fold, respectively. TUM20902, which harboured pMTY20902_IncN plasmid with a 27 505-bp deletion that included blaCTX-M-2, and blaIMP-6 showed 4.21-fold higher levels of transcription than the parental strain. TUM20903 had a 49 316-bp deletion that included ompC and a replicative increase of blaCTX-M-2 to three copies.

CONCLUSIONS

Molecular analysis including full-length WGS revealed that the resistance mechanisms of meropenem-resistant mutants that emerged during long-term in vitro meropenem exposure were increased blaIMP-6 transcripts in CPE and increased blaCTX-M-2 transcripts due to gene triplication and OmpC loss resulting from ompC deletion in non-CPE.

Sitafloxacin pharmacokinetics/pharmacodynamics against multidrug-resistant bacteria in a dynamic urinary tract infection in vitro model

OBJECTIVES

Sitafloxacin is one of the newer generation fluoroquinolones with highly active against multidrug-resistant (MDR) bacteria. Our objectives were to identify the sitafloxacin pharmacokinetic/pharmacodynamic (PK/PD) index and breakpoints against MDR isolate in the urinary tract infection model.

METHODS

Forty-eight MDR isolates underwent sitafloxacin and levofloxacin microdilution susceptibility testing. A 24 h in vitro model was established that simulated the healthy subjects urodynamics of sitafloxacin fumarate injection. Ten MDR isolates (four carbapenem-resistant Escherichia coli, three carbapenem-resistant P. aeruginosa and three vancomycin-resistant E. faecium) were selected. The drug efficacy was quantified by the change in log colony counts within 24 h. A sigmoid Emax model was fitted to the killing effect data. Monte Carlo simulations were performed to assess target attainment for the sitafloxacin fumarate doses of 100 and 200 mg q24h.

RESULTS

Analysis indicated that the MICs of sitafloxacin were all significantly lower than that of levofloxacin (P < 0.01). The UAUC0-24h/MIC targets required to achieve stasis, 1-log10 killing and 2-log10 killing were 63.60, 79.49 and 99.45 (carbapenem-resistant E. coli), 60.85, 90.31 and 128.95 (carbapenem-resistant P. aeruginosa), 65.91, 77.81 and 103.11 (vancomycin-resistant E. faecium). Monte Carlo simulation showed the infusion of sitafloxacin fumarate 100 mg q24h was able to achieve 90% probability of target attainment against bacteria with MIC of 8 mg/L for the common complicated urinary tract infections.

CONCLUSIONS

Sitafloxacin fumarate injection is an alternative therapeutic agent for the treatment of UTIs caused by MDR isolates.

Applications of the hollow-fibre infection model (HFIM) in viral infection studies

Conventional cell culture systems involve growing cells in stationary cultures in the presence of growth medium containing various types of supplements. At confluency, the cells are divided and further expanded in new culture dishes. This passage from confluent monolayer to sparse cultures does not reflect normal physiological conditions and represents quite a drastic physiological change that may affect the natural cell physiobiology. Hollow-fibre bioreactors were in part developed to overcome these limitations and since their inception, they have widely been used in production of monoclonal antibodies and recombinant proteins. These bioreactors are increasingly used to study antibacterial drug effects via simulation of in vivo pharmacokinetic profiles. The use of the hollow-fibre infection model (HFIM) in viral infection studies is less well developed and in this review we have analysed and summarized the current available literature on the use of these bioreactors, with an emphasis on viruses. Our work has demonstrated that this system can be applied for viral expansion, studies of drug resistance mechanisms, and studies of pharmacokinetic/pharmacodynamic (PK/PD) of antiviral compounds. These platforms could therefore have great applications in large-scale vaccine development, and in studies of mechanisms driving antiviral resistance, since the HFIM could recapitulate the same resistance mechanisms and mutations observed in vivo in clinic. Furthermore, some dosage and spacing regimens evaluated in the HFIM system, as allowing maximal viral suppression, are in line with clinical practice and highlight this 'in vivo-like' system as a powerful tool for experimental validation of in vitro-predicted antiviral activities.

Assessing Clinical Potential of Old Antibiotics against Severe Infections by Multi-Drug-Resistant Gram-Negative Bacteria Using In Silico Modelling

In the light of increasing antimicrobial resistance among gram-negative bacteria and the lack of new more potent antimicrobial agents, new strategies have been explored. Old antibiotics, such as colistin, temocillin, fosfomycin, mecillinam, nitrofurantoin, minocycline, and chloramphenicol, have attracted the attention since they often exhibit in vitro activity against multi-drug-resistant (MDR) gram-negative bacteria, such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. The current review provides a summary of the in vitro activity, pharmacokinetics and PK/PD characteristics of old antibiotics. In silico modelling was then performed using Monte Carlo simulation in order to combine all preclinical data with human pharmacokinetics and determine the probability of target (1-log kill in thigh/lung infection animal models) attainment (PTA) of different dosing regimens. The potential of clinical efficacy of a drug against severe infections by MDR gram-negative bacteria was considered when PTA was >95% at the epidemiological cutoff values of corresponding species. In vitro potent activity against MDR gram-negative pathogens has been shown for colistin, polymyxin B, temocillin (against E. coli and K. pneumoniae), fosfomycin (against E. coli), mecillinam (against E. coli), minocycline (against E. coli, K. pneumoniae, A. baumannii), and chloramphenicol (against E. coli) with ECOFF or MIC90 ≤ 16 mg/L. When preclinical PK/PD targets were combined with human pharmacokinetics, Monte Carlo analysis showed that among the old antibiotics analyzed, there is clinical potential for polymyxin B against E. coli, K. pneumoniae, and A. baumannii; for temocillin against K. pneumoniae and E. coli; for fosfomycin against E. coli and K. pneumoniae; and for mecillinam against E. coli. Clinical studies are needed to verify the potential of those antibiotics to effectively treat infections by multi-drug resistant gram-negative bacteria.

Cefepime pharmacodynamic targets against Enterobacterales employing neutropenic murine lung infection and in vitro pharmacokinetic models

BACKGROUND

Very limited studies, so far, have been conducted to identify the pharmacodynamic targets of cefepime, a well-established fourth-generation cephalosporin. As a result, conventional targets representing the cephalosporin class are used for cefepime target attainment analysis.

OBJECTIVES

We employed both a neutropenic murine lung infection model and an in vitro pharmacokinetic model (IVPM) to determine cefepime's pharmacodynamic target [percentage of the dosing interval during which unbound drug concentrations remain higher than the MIC (%fT>MIC)] for bacteriostatic and 1 log10 kill effects.

METHODS

Ten strains with cefepime MICs ranging from 0.03 to 16 mg/L were studied in the lung infection. In the IVPM, five cefepime-resistant strains with cefepime/tazobactam (fixed 8 mg/L) MICs ranging from 0.25 to 8 mg/L were included. Through 24 h dose fractionation, both in lung infection and IVPM (in the latter case, tazobactam 8 mg/L continuous infusion was used to protect cefepime), varying cefepime exposures and corresponding pharmacodynamic effect scenarios were generated to identify the pharmacodynamic targets.

RESULTS

Using a non-linear sigmoidal maximum-effect (Emax) model, the cefepime's plasma fT>MIC for 1 log10 kill in lung infection ranged from 17% to 53.7% and a combined exposure-response plot yielded 30%. In the case of IVPM, T>MIC ranged from 6.9% to 75.4% with a mean value of 34.2% for 1 log10 kill.

CONCLUSIONS

Both in vivo and in vitro studies showed that cefepime's pharmacodynamic requirements are lower than generally reported for cephalosporins (50%-70% fT>MIC). The lower requirement for cefepime could be linked with factors such as cefepime's better permeation properties and multiple PBP affinity-driven enhanced bactericidal action.

Assessment of flomoxef combined with amikacin in a hollow-fibre infection model for the treatment of neonatal sepsis in low- and middle-income healthcare settings

BACKGROUND

Annual mortality from neonatal sepsis is an estimated 430 000-680 000 infants globally, most of which occur in low- and middle-income countries (LMICs). The WHO currently recommends a narrow-spectrum β-lactam (e.g. ampicillin) and gentamicin as first-line empirical therapy. However, available epidemiological data demonstrate high rates of resistance to both agents. Alternative empirical regimens are needed. Flomoxef and amikacin are two off-patent antibiotics with potential for use in this setting.

OBJECTIVES

To assess the pharmacodynamics of flomoxef and amikacin in combination.

METHODS

The pharmacodynamic interaction of flomoxef and amikacin was assessed in chequerboard assays and a 16-arm dose-ranged hollow-fibre infection model (HFIM) experiment. The combination was further assessed in HFIM experiments mimicking neonatal plasma exposures of clinically relevant doses of both drugs against five Enterobacterales isolates with a range of flomoxef/amikacin MICs.

RESULTS

Flomoxef and amikacin in combination were synergistic in bacterial killing in both assays and prevention of emergence of amikacin resistance in the HFIM. In the HFIM assessing neonatal-like drug exposures, the combination killed 3/5 strains to sterility, (including 2/5 that monotherapy with either drug failed to kill) and failed to kill the 2/5 strains with flomoxef MICs of 32 mg/L.

CONCLUSIONS

We conclude that the combination of flomoxef and amikacin is synergistic and is a potentially clinically effective regimen for the empirical treatment of neonatal sepsis in LMIC settings and is therefore suitable for further assessment in a clinical trial.

Postexposure Prophylaxis and Treatment of Bacillus anthracis Infections: A Systematic Review and Meta-analyses of Animal Models, 1947-2019

BACKGROUND

Anthrax is endemic to many countries, including the United States. The causative agent, Bacillus anthracis, poses a global bioterrorism threat. Without effective antimicrobial postexposure prophylaxis (PEPAbx) and treatment, the mortality of systemic anthrax is high. To inform clinical guidelines for PEPAbx and treatment of B. anthracis infections in humans, we systematically evaluated animal anthrax treatment model studies.

METHODS

We searched for survival outcome data in 9 scientific search engines for articles describing antimicrobial PEPAbx or treatment of anthrax in animals in any language through February 2019. We performed meta-analyses of efficacy of antimicrobial PEPAbx and treatment for each drug or drug combination using random-effects models. Pharmacokinetic/pharmacodynamic relationships were developed for 5 antimicrobials with available pharmacokinetic data. Monte Carlo simulations were used to predict unbound drug exposures in humans.

RESULTS

We synthesized data from 34 peer-reviewed studies with 3262 animals. For PEPAbx and treatment of infection by susceptible B. anthracis, effective monotherapy can be accomplished with fluoroquinolones, tetracyclines, β-lactams (including penicillin, amoxicillin-clavulanate, and imipenem-cilastatin), and lipopeptides or glycopeptides. For naturally occurring strains, unbound drug exposures in humans were predicted to adequately cover the minimal inhibitory concentrations (MICs; those required to inhibit the growth of 50% or 90% of organisms [MIC50 or MIC90]) for ciprofloxacin, levofloxacin, and doxycycline for both the PEPAbx and treatment targets. Dalbavancin covered its MIC50 for PEPAbx.

CONCLUSIONS

These animal studies show many reviewed antimicrobials are good choices for PEPAbx or treatment of susceptible B. anthracis strains, and some are also promising options for combating resistant strains. Monte Carlo simulations suggest that oral ciprofloxacin, levofloxacin, and doxycycline are particularly robust choices for PEPAbx or treatment.

Research priorities towards precision antibiotic therapy to improve patient care

Antibiotic resistance presents an incessant threat to our drug armamentarium that necessitates novel approaches to therapy. Over the past several decades, investigation of pharmacokinetic and pharmacodynamic (PKPD) principles has substantially improved our understanding of the relationships between the antibiotic, pathogen, and infected patient. However, crucial gaps in our understanding of the pharmacology of antibacterials and their optimal use in the care of patients continue to exist; simply attaining antibiotic exposures that are considered adequate based on traditional targets can still result in treatment being unsuccessful and resistance proliferation for some infections. It is this salient paradox that points to key future directions for research in antibiotic therapeutics. This Personal View discusses six priority areas for antibiotic pharmacology research: (1) antibiotic-pathogen interactions, (2) antibiotic targets for combination therapy, (3) mechanistic models that describe the time-course of treatment response, (4) understanding and modelling of host response to infection, (5) personalised medicine through therapeutic drug management, and (6) application of these principles to support development of novel therapies. Innovative approaches that enhance our understanding of antibiotic pharmacology and facilitate more accurate predictions of treatment success, coupled with traditional pharmacology research, can be applied at the population level and to individual patients to improve outcomes.

Trimethoprim/sulfamethoxazole pharmacodynamics against Stenotrophomonas maltophilia in the in vitro chemostat model

BACKGROUND

Trimethoprim/sulfamethoxazole has historically been the treatment of choice for infection caused by Stenotrophomonas maltophilia. This study sought to define the pharmacodynamic indices and magnitude of exposure required for stasis and 1 log10 cfu reductions.

METHODS

Pharmacodynamic studies were conducted using the in vitro chemostat model over 24 h against three trimethoprim/sulfamethoxazole-susceptible S. maltophilia isolates with MICs from 0.25/4.75 to 2/38 mg/L. The primary endpoint was the change in cfu at 24 h relative to baseline. The log ratio of the area under the cfu curve (LR AUcfu) was a secondary endpoint. Trimethoprim and sulfamethoxazole exposures required for stasis and 1 log10 cfu/mL reduction were determined.

RESULTS

Trimethoprim/sulfamethoxazole exposures achieved stasis and 1 log10 cfu/mL reductions in 9/16 (56%) and 2/16 (13%) of experiments. Both the fAUC/MIC and fCmax/MIC were identified as equivalent pharmacodynamic drivers, with stasis achieved at an fAUC/MIC of 67.4 and 30.0 for trimethoprim and sulfamethoxazole, respectively. Clinically meaningful exposures required to achieve 1 log10 cfu/mL reductions were not quantifiable. The LR AUcfu analysis supported the lack of overall bacterial burden reduction against S. maltophilia.

CONCLUSIONS

In this in vitro chemostat model, trimethoprim/sulfamethoxazole monotherapy, even at higher doses, achieved limited activity against susceptible S. maltophilia.

Variability of murine bacterial pneumonia models used to evaluate antimicrobial agents

Antimicrobial resistance has become one of the greatest threats to human health, and new antibacterial treatments are urgently needed. As a tool to develop novel therapies, animal models are essential to bridge the gap between preclinical and clinical research. However, despite common usage of in vivo models that mimic clinical infection, translational challenges remain high. Standardization of in vivo models is deemed necessary to improve the robustness and reproducibility of preclinical studies and thus translational research. The European Innovative Medicines Initiative (IMI)-funded “Collaboration for prevention and treatment of MDR bacterial infections” (COMBINE) consortium, aims to develop a standardized, quality-controlled murine pneumonia model for preclinical efficacy testing of novel anti-infective candidates and to improve tools for the translation of preclinical data to the clinic. In this review of murine pneumonia model data published in the last 10 years, we present our findings of considerable variability in the protocols employed for testing the efficacy of antimicrobial compounds using this in vivo model. Based on specific inclusion criteria, fifty-three studies focusing on antimicrobial assessment against Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii were reviewed in detail. The data revealed marked differences in the experimental design of the murine pneumonia models employed in the literature. Notably, several differences were observed in variables that are expected to impact the obtained results, such as the immune status of the animals, the age, infection route and sample processing, highlighting the necessity of a standardized model.

Expert workshop summary: Advancing toward a standardized murine model to evaluate treatments for antimicrobial resistance lung infections

The rise in antimicrobial resistance (AMR), and increase in treatment-refractory AMR infections, generates an urgent need to accelerate the discovery and development of novel anti-infectives. Preclinical animal models play a crucial role in assessing the efficacy of novel drugs, informing human dosing regimens and progressing drug candidates into the clinic. The Innovative Medicines Initiative-funded “Collaboration for prevention and treatment of MDR bacterial infections” (COMBINE) consortium is establishing a validated and globally harmonized preclinical model to increase reproducibility and more reliably translate results from animals to humans. Toward this goal, in April 2021, COMBINE organized the expert workshop “Advancing toward a standardized murine model to evaluate treatments for AMR lung infections”. This workshop explored the conduct and interpretation of mouse infection models, with presentations on PK/PD and efficacy studies of small molecule antibiotics, combination treatments (β-lactam/β-lactamase inhibitor), bacteriophage therapy, monoclonal antibodies and iron sequestering molecules, with a focus on the major Gram-negative AMR respiratory pathogens Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii. Here we summarize the factors of variability that we identified in murine lung infection models used for antimicrobial efficacy testing, as well as the workshop presentations, panel discussions and the survey results for the harmonization of key experimental parameters. The resulting recommendations for standard design parameters are presented in this document and will provide the basis for the development of a harmonized and bench-marked efficacy studies in preclinical murine pneumonia model.

In vivo translational assessment of the GES genotype on the killing profile of ceftazidime, ceftazidime/avibactam and meropenem against Pseudomonas aeruginosa

Objectives

To evaluate the in vivo killing profile of human-simulated exposures of ceftazidime, ceftazidime/avibactam and meropenem against GES-harbouring Pseudomonas aeruginosa in the murine thigh infection model.

Methods

Five P. aeruginosa isolates [three isogenic (GES-1, GES-5 and GES-15) and two clinical (GES-5 and GES-15)] were evaluated. MICs were determined using broth microdilution. Human-simulated regimens (HSRs) of ceftazidime 2 g IV q8h as a 2 h infusion, ceftazidime/avibactam 2.5 g IV q8h as a 2 h infusion and meropenem 2 g IV q8h as a 3 h infusion were administered. Change in bacterial burden relative to baseline was assessed.

Results

Modal MICs ranged from 8 to >64 mg/L for ceftazidime, from 1 to 16 mg/L for ceftazidime/avibactam and from 1 to >64 mg/L for meropenem. In vivo, for the isogenic strains, avibactam augmented ceftazidime activity against the GES-1- and GES-15-harbouring isolates. Both ceftazidime and ceftazidime/avibactam resulted in significant kill against the GES-5 isogenic isolate. The meropenem HSR produced >1 log₁₀ kill against each isogenic isolate (MICs of 1–4 mg/L). Against the GES-5 clinical isolate, ceftazidime and ceftazidime/avibactam resulted in >1 log₁₀ kill compared with bacterial growth with the meropenem HSR. In the clinical isolate harbouring GES-15, the elevated MICs of ceftazidime and ceftazidime/avibactam reduced the effectiveness of both compounds, while the observed reduction in meropenem MIC translated into in vivo efficacy of the HSR regimen, predictive of clinical efficacy.

Conclusions

In GES-harbouring P. aeruginosa, quantitative reductions in bacterial density observed with the translational murine model suggest that the phenotypic profile of ceftazidime, ceftazidime/avibactam and meropenem is predictive of clinical efficacy when using the evaluated dosing regimens.

Clinical Pharmacokinetics and Pharmacodynamics of Cefepime

Cefepime is a broad-spectrum fourth-generation cephalosporin with activity against Gram-positive and Gram-negative pathogens. It is generally administered as an infusion over 30-60 min or as a prolonged infusion with infusion times from 3 h to continuous administration. Cefepime is widely distributed in biological fluids and tissues with an average volume of distribution of ~ 0.2 L/kg in healthy adults with normal renal function. Protein binding is relatively low (20%), and elimination is mainly renal. About 85% of the dose is excreted unchanged in the urine, with an elimination half-life of 2-2.3 h. The pharmacokinetics of cefepime is altered under certain pathophysiological conditions, resulting in high inter-individual variability in cefepime volume of distribution and clearance, which poses challenges for population dosing approaches. Consequently, therapeutic drug monitoring of cefepime may be beneficial in certain patients including those who are critically ill, have life-threatening infections, or are infected with more resistant pathogens. Cefepime is generally safe and efficacious, with a goal exposure target of 70% time of the free drug concentration over the minimum inhibitory concentration for clinical efficacy. In recent years, reports of neurotoxicity have increased, specifically in patients with impaired renal function. This review summarizes the pharmacokinetics, pharmacodynamics, and toxicodynamics of cefepime contemporarily in the setting of increasing cefepime exposures. We explore the potential benefits of extended or continuous infusions and therapeutic drug monitoring in special populations.

Comparison of Ceftolozane/Tazobactam Infusion Regimens in a Hollow-Fiber Infection Model against Extensively Drug-Resistant Pseudomonas aeruginosa Isolates

The aim of this study was to compare the efficacy of intermittent (1-h), extended (4-h), and continuous ceftolozane-tazobactam (C/T) infusion against three extensively drug-resistant (XDR) sequence type (ST) 175 P. aeruginosa isolates with different susceptibilities to C/T (MIC = 2 to 16 mg/L) in a 7-day hollow-fiber infection model (HFIM). C/T in continuous infusion achieved the largest reduction in total number of bacterial colonies in the overall treatment arms for both C/T-susceptible and -resistant isolates. It was also the only regimen with bactericidal activity against all three isolates. These data suggest that continuous C/T infusion should be considered a potential treatment for infections caused by XDR P. aeruginosa isolates, including nonsusceptible ones. Proper use of C/T dosing regimens may lead to better clinical management of XDR P. aeruginosa infections.

IMPORTANCE Ceftolozane-tazobactam (C/T) is an antipseudomonal antibiotic with a high clinical impact in treating infection caused by extensively drug-resistant (XDR) Pseudomonas aeruginosa isolates, but resistance is emerging. Given its time-dependent behavior, C/T continuous infusion can improve exposure and therefore the pharmacokinetic/pharmacodynamic target attainment. We compared the efficacy of intermittent, extended, and continuous C/T infusion against three XDR ST175 P. aeruginosa isolates with different C/T MICs by means of an in vitro dynamic hollow-fiber model. We demonstrated that C/T in continuous infusion achieved the largest reduction in bacterial density in the overall treatment arms for both susceptible and resistant isolates. It was also the only regimen with bactericidal activity against all three isolates. Through this study, we want to demonstrate that developing individually tailored antimicrobial treatments is becoming essential. Our results support the role of C/T level monitoring and of dose adjustments for better clinical management and outcomes.

Activity of Omadacycline Alone and in Combination against Carbapenem-Nonsusceptible Acinetobacter baumannii with Varying Minocycline Susceptibility

Tetracycline-based combinations are increasingly used for serious carbapenem-nonsusceptible Acinetobacter baumannii (CNSAb) infections given their potent in vitro activity, synergism with other agents, and acceptable toxicity profile. Omadacycline is a novel aminomethylcycline with activity against minocycline-resistant pathogens, once daily oral dosing, and favorable pharmacokinetic properties. Given these potential advantages, the in vitro potency and antibacterial activity of omadacycline were evaluated alone and in combination against CNSAb with varying minocycline susceptibility. Broth microdilution testing of 41 CNSAb revealed that omadacycline (MIC50/90: 4/8 mg/L) inhibited 68.3% (28/41) of isolates at ≤4 mg/L and its activity was unaffected by minocycline nonsusceptibility (MIC50/90: 4/8 mg/L; 74.2% [23/31] inhibited at ≤4 mg/L). Ten (5 minocycline susceptible and 5 nonsusceptible) of the 41 CNSAb isolates were then evaluated in time-kill analyses against omadacycline and comparator agents alone and in dual- and triple-drug combinations at the free maximum concentration of drug in serum (fCmax). Amikacin, meropenem, and polymyxin B alone were each bactericidal against 4 of 10 (40%) isolates while omadacycline and sulbactam were bactericidal against 0 (0%) and 1 (10%), respectively. In dual-drug combinations with omadacycline, synergy was observed against 80% of isolates with sulbactam followed by 30% with amikacin or polymyxin B and 0% with meropenem or rifampin. The triple-drug combination of omadacycline, sulbactam, and polymyxin B achieved synergy against just one additional strain over the omadacycline-sulbactam dual combination but significantly reduced the time to 99.9% kill by more than 6 h (4.6 ± 2.8 h vs. 11.3 ± 5.9 h, P < 0.01). These results support the continued investigation into tetracycline-based combinations against CNSAb, particularly those including sulbactam, and suggest that omadacycline may have in vitro advantages over existing tetracycline-derivatives. IMPORTANCE Treatment of infections due to Acinetobacter baumannii often involves the use of multiple antibiotics simultaneously as combination therapy, but it is unknown which antibiotics are best used together. Tetracycline agents such as minocycline and tigecycline maintain good activity against A. baumannii and are often used with one or more other agents to achieve better killing of the bacteria. Omadacycline is a new tetracycline that may have a role in the treatment of A. baumannii, but no data are available evaluating its interaction with other commonly used drugs such as polymyxin B and sulbactam. Therefore, the purpose of this study was to investigate the antibacterial activity of omadacycline when combined with one or more other agents against carbapenem-resistant strains of A. baumannii. These findings may then be used to design confirmatory studies that could help decide what drugs work best together and what combination of agents should be used for patients.

Evaluating Mono- and Combination Therapy of Meropenem and Amikacin against Pseudomonas aeruginosa Bacteremia in the Hollow-Fiber Infection Model

Debate continues as to the role of combination antibiotic therapy for the management of Pseudomonas aeruginosa infections. We studied the extent of bacterial killing by and the emergence of resistance to meropenem and amikacin as monotherapies and as a combination therapy against susceptible and resistant P. aeruginosa isolates from bacteremic patients using the dynamic in vitro hollow-fiber infection model. Three P. aeruginosa isolates (meropenem MICs of 0.125, 0.25, and 64 mg/L) were used, simulating bacteremia with an initial inoculum of ~1 × 105 CFU/mL and the expected pharmacokinetics of meropenem and amikacin in critically ill patients. For isolates susceptible to amikacin and meropenem (isolates 1 and 2), the extent of bacterial killing was increased with the combination regimen compared with the killing by monotherapy of either antibiotic. Both the combination and meropenem monotherapy were able to sustain bacterial killing throughout the 7-day treatment course, whereas regrowth of bacteria occurred with amikacin monotherapy after 12 h. For the meropenem-resistant P. aeruginosa isolate (isolate 3), only the combination regimen demonstrated bacterial killing. Given that tailored antibiotic regimens can maximize potential synergy against some isolates, future studies should explore the benefit of combination therapy against resistant P. aeruginosa. IMPORTANCE Current guidelines recommend that aminoglycosides should be used in combination with β-lactam antibiotics as initial empirical therapy for serious infections, and otherwise, patients should receive β-lactam antibiotic monotherapy. Given the challenges associated with studying the clinical effect of different antibiotic strategies on patient outcomes, useful data for subsequent informed clinical testing can be obtained from in vitro models like the hollow-fiber infection model (HFIM). Based on the findings of our HFIM, we propose that the initial use of combination therapy with meropenem and amikacin provides some bacterial killing against carbapenem-resistant P. aeruginosa isolates. For susceptible isolates, combination therapy may only be of benefit in specific patient populations, such as critically ill or immunocompromised patients. Therefore, clinicians may want to consider using the combination therapy for the initial management and ceasing the aminoglycosides once antibiotic susceptibility results have been obtained.