Human-Simulated Antimicrobial Regimens in Animal Models: Transparency and Validation Are Imperative

Pharmacokinetics and pharmacodynamics of bacteriophage therapy: a review with a focus on multidrug-resistant Gram-negative bacterial infections

SUMMARYDespite the early recognition of their therapeutic potential and the current escalation of multidrug-resistant (MDR) pathogens, the adoption of bacteriophages into mainstream clinical practice is hindered by unfamiliarity with their basic pharmacokinetic (PK) and pharmacodynamic (PD) properties, among others. Given the self-replicative nature of bacteriophages in the presence of host bacteria, the adsorption rate, and the clearance by the host's immunity, their PK/PD characteristics cannot be estimated by conventional approaches, and thus, the introduction of new considerations is required. Furthermore, the multitude of different bacteriophage types, preparations, and treatment schedules impedes drawing general conclusions on their in vivo PK/PD features. Additionally, the drawback of acquired bacteriophage resistance of MDR pathogens with clinical and environmental implications should be taken into consideration. Here, we provide an overview of the current state of the field of PK and PD of bacteriophage therapy with a focus on its application against MDR Gram-negative infections, highlighting the potential knowledge gaps and the challenges in translation from the bench to the bedside. After reviewing the in vitro PKs and PDs of bacteriophages against the four major MDR Gram-negative pathogens, Klebsiella pneumoniae, Acinetobacter baumannii complex, Pseudomonas aeruginosa, and Escherichia coli, specific data on in vivo PKs (tissue distribution, route of administration, and basic PK parameters in animals and humans) and PDs (survival and reduction of bacterial burden in relation to the route of administration, timing of therapy, dosing regimens, and resistance) are summarized. Currently available data merit close scrutiny, and optimization of bacteriophage therapy in the context of a better understanding of the underlying PK/PD principles is urgent to improve its therapeutic effect and to minimize the occurrence of bacteriophage resistance.

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.

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.

Phenotypes, genotypes and breakpoints: an assessment of β-lactam/β-lactamase inhibitor combinations against OXA-48

BACKGROUND

Two of the three recently approved β-lactam agent (BL)/β-lactamase inhibitor (BLI) combinations have higher CLSI susceptibility breakpoints (ceftazidime/avibactam 8 mg/L; meropenem/vaborbactam 4 mg/L) compared with the BL alone (ceftazidime 4 mg/L; meropenem 1 mg/L). This can lead to a therapeutic grey area on susceptibility reports depending on resistance mechanism. For instance, a meropenem-resistant OXA-48 isolate (MIC 4 mg/L) may appear as meropenem/vaborbactam-susceptible (MIC 4 mg/L) despite vaborbactam's lack of OXA-48 inhibitory activity.

METHODS

OXA-48-positive (n = 51) and OXA-48-negative (KPC, n = 5; Klebsiella pneumoniae wild-type, n = 1) Enterobacterales were utilized. Susceptibility tests (broth microdilution) were conducted with ceftazidime/avibactam, imipenem/relebactam and meropenem/vaborbactam, as well as their respective BL partner. Antimicrobial activity of all six agents was evaluated in the murine neutropenic thigh model using clinically relevant exposures. Efficacy was assessed as the change in bacterial growth at 24 h, compared with 0 h controls.

RESULTS

On average, the three BL/BLI agents resulted in robust bacteria killing among OXA-48-negative isolates. Among OXA-48-positive isolates, poor in vivo activity with imipenem/relebactam was concordant with its resistant phenotypic profile. Variable meropenem/vaborbactam activity was observed among isolates with a 'susceptible' MIC of 4 mg/L. Only 30% (7/23) of isolates at meropenem/vaborbactam MICs of 2 and 4 mg/L met the ≥1-log bacterial reduction threshold predictive of clinical efficacy in serious infections. In contrast, ceftazidime/avibactam resulted in marked bacterial density reduction across the range of MICs, and 96% (49/51) of isolates exceeded the ≥1-log bacterial reduction threshold.

CONCLUSIONS

Data demonstrate that current imipenem/relebactam and ceftazidime/avibactam CLSI breakpoints are appropriate. Data also suggest that higher meropenem/vaborbactam breakpoints relative to meropenem can translate to potentially poor clinical outcomes in patients infected with OXA-48-harbouring isolates.

Phenotypes, genotypes and breakpoints: an assessment of β-lactam/ β-lactamase inhibitor combinations against OXA-48

BACKGROUND

Two out of the three recently approved β-lactam (BL)/β-lactamase inhibitors (BLIs) have higher CLSI susceptibility breakpoints (ceftazidime/avibactam 8 mg/L; meropenem/vaborbactam 4 mg/L) compared with the BL alone (ceftazidime 4 mg/L; meropenem 1 mg/L). This can lead to a therapeutic grey area on susceptibility reports depending on resistance mechanism. For instance, a meropenem-resistant OXA-48 isolate (MIC 4 mg/L) may appear as meropenem/vaborbactam-susceptible (MIC 4 mg/L) despite vaborbactam's lack of OXA-48 inhibitory activity.

METHODS

OXA-48-positive (n = 51) and OXA-48-negative (KPC, n = 5; Klebsiella pneumoniae WT, n = 1) Enterobacterales were utilized. Susceptibility tests (broth microdilution) were conducted with ceftazidime/avibactam, imipenem/relebactam and meropenem/vaborbactam, as well as their respective BL partner. Antimicrobial activity of all six agents was evaluated in the murine neutropenic thigh model using clinically relevant exposures. Efficacy was assessed as the change in bacterial growth at 24 h, compared with 0 h controls.

RESULTS

On average, the three BL/BLI agents resulted in robust bacteria killing among OXA-48-negative isolates. Among OXA-48-positive isolates, poor in vivo activity with imipenem/relebactam was concordant with its resistant phenotypic profile. Variable meropenem/vaborbactam activity was observed among isolates with a 'susceptible' MIC of 4 mg/L. Only 30% (7/23) of isolates at meropenem/vaborbactam MICs of 2 and 4 mg/L met the ≥1 log bacterial reduction threshold predictive of clinical efficacy in serious infections. In contrast, ceftazidime/avibactam resulted in marked bacterial density reduction across the range of MICs and 73% (37/51) of isolates exceeded the ≥1 log bacterial reduction threshold.

CONCLUSIONS

Data demonstrate that current imipenem/relebactam and ceftazidime/avibactam CLSI breakpoints are appropriate. Data also suggest that higher meropenem/vaborbactam breakpoints relative to meropenem can translate to potentially poor clinical outcomes in patients infected with OXA-48-harbouring isolates.

Murine Model for Measuring Effects of Humanized-Dosing of Antibiotics on the Gut Microbiome

There is a current need for enhancing our insight in the effects of antimicrobial treatment on the composition of human microbiota. Also, the spontaneous restoration of the microbiota after antimicrobial treatment requires better understanding. This is best addressed in well-defined animal models. We here present a model in which immune-competent or neutropenic mice were administered piperacillin-tazobactam (TZP) according to human treatment schedules. Before, during and after the TZP treatment, fecal specimens were longitudinally collected at established intervals over several weeks. Gut microbial taxonomic distribution and abundance were assessed through culture and molecular means during all periods. Non-targeted metabolomics analyses of stool samples using Quadrupole Time of Flight mass spectrometry (QTOF MS) were also applied to determine if a metabolic fingerprint correlated with antibiotic use, immune status, and microbial abundance. TZP treatment led to a 5–10-fold decrease in bacterial fecal viability counts which were not fully restored during post-antibiotic follow up. Two distinct, relatively uniform and reproducible restoration scenarios of microbiota changes were seen in post TZP-treatment mice. Post-antibiotic flora could consist of predominantly Firmicutes or, alternatively, a more diverse mix of taxa. In general, the pre-treatment microbial communities were not fully restored within the screening periods applied. A new species, closely related to Eubacterium siraeum, Mageeibacillus indolicus, and Saccharofermentans acetigenes, became predominant post-treatment in a significant proportion of mice, identified by 16S rRNA gene sequencing. Principal component analysis of QTOF MS of mouse feces successfully distinguished treated from non-treated mice as well as immunocompetent from neutropenic mice. We observe dynamic but distinct and reproducible responses in the mouse gut microbiota during and after TZP treatment and propose the current murine model as a useful tool for defining the more general post-antibiotic effects in the gastro-intestinal ecosystem where humanized antibiotic dosing may ultimately facilitate extrapolation to humans.

In search for a synergistic combination against pandrug-resistant A. baumannii; methodological considerations

PURPOSE

Pending approval of new antimicrobials, synergistic combinations are the only treatment option against pandrug-resistant A. baumannii (PDRAB). Considering the lack of a standardized methodology, the aim of this manuscript is to systematically review the methodology and discuss unique considerations for assessing antimicrobial combinations against PDRAB.

METHODS

Post-hoc analysis of a systematic review (conducted in PubMed and Scopus from inception to April 2021) of studies evaluating antimicrobial combination against A. baumannii, based on antimicrobials that are inactive in vitro alone.

RESULTS

Eighty-four publications were reviewed, using a variety of synergy testing methods, including; gradient-based methods (n = 11), disk-based methods (n = 6), agar dilution (n = 2), checkerboard assay (n = 44), time-kill assay (n = 50), dynamic in vitro PK/PD models (n = 6), semi-mechanistic PK/PD models (n = 5), and in vivo animal models (n = 11). Several variations in definitions of synergy and interpretation of each method were observed and are discussed. Challenges related to testing combinations of antimicrobials that are inactive alone (with regards to concentrations at which the combinations are assessed), as well as other considerations (assessment of stasis vs killing, clinical relevance of re-growth in vitro after initial killing, role of in vitro vs in vivo conditions, challenges of clinical testing of antimicrobial combinations against PDRAB infections) are discussed.

CONCLUSION

This review demonstrates the need for consensus on a standardized methodology and clinically relevant definitions for synergy. Modifications in the methodology and definitions of synergy as well as a roadmap for further development of antimicrobial combinations against PDRAB are proposed.

Comparative in vivo activity of human-simulated plasma and epithelial lining fluid exposures of WCK 5222 (cefepime/zidebactam) against KPC- and OXA-48-like-producing Klebsiella pneumoniae in the neutropenic murine pneumonia model

OBJECTIVES

This was a comparative assessment of WCK 5222 (cefepime/zidebactam 2/1 g as a 1 h infusion every 8 h) efficacy using human-simulated plasma and ELF exposures against serine-carbapenemase-producing Klebsiella pneumoniae in the neutropenic murine pneumonia model.

METHODS

Ten clinical isolates were utilized: eight were serine-carbapenemase-producing (KPC, n = 4; OXA-48-like, n = 4) Enterobacterales with WCK 5222 MICs (1:1) ranging from 1 to 4 mg/L; and two were previously studied MDR isolates serving as quality controls. Lungs of mice were inoculated with 50 μL of 107 cfu/mL. Treatment mice received human-simulated regimens of cefepime, zidebactam or WCK 5222 derived from plasma or epithelial lining fluid (ELF) profiles obtained from healthy subjects. Lung bacterial densities resulting from the humanized exposures in plasma and ELF were compared.

RESULTS

Initial lung bacterial densities ranged from 6.06 to 6.87 log10 cfu/lungs, with a mean bacterial burden increase to 9.06 ± 0.42 after 24 h. Human-simulated plasma and ELF exposures of cefepime and zidebactam monotherapy had no activity. Human-simulated WCK 5222 plasma exposures resulted in a >1 log10 cfu/lungs reduction in bacterial burden for all isolates. Humanized WCK 5222 ELF exposures achieved a >1 log10 cfu/lungs reduction for all isolates. While statistically significant differences in bacterial burden reduction were observed between the plasma and ELF exposures for WCK 5222 in 5/8 isolates, all treatments achieved the translational kill target of a >1 log10 cfu reduction.

CONCLUSIONS

Clinically achievable WCK 5222 plasma and ELF exposures produced in vivo killing of carbapenem-resistant Enterobacterales in the neutropenic murine pneumonia model that is predictive of efficacy in humans.

Activity of β-Lactam Antibiotics against Metallo-β-Lactamase-Producing Enterobacterales in Animal Infection Models: a Current State of Affairs

Metallo-β-lactamases (MBLs) result in resistance to nearly all β-lactam antimicrobial agents, as determined by currently employed susceptibility testing methods. However, recently reported data demonstrate that variable and supraphysiologic zinc concentrations in conventional susceptibility testing media compared with physiologic (bioactive) zinc concentrations may be mediating discordant in vitro-in vivo MBL resistance. While treatment outcomes in patients appear suggestive of this discordance, these limited data are confounded by comorbidities and combination therapy. To that end, the goal of this review is to evaluate the extent of β-lactam activity against MBL-harboring Enterobacterales in published animal infection model studies and provide contemporary considerations to facilitate the optimization of current antimicrobials and development of novel therapeutics.