Development and qualification of a pharmacodynamic model for the pronounced inoculum effect of ceftazidime against Pseudomonas aeruginosa

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.

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.

Ceftolozane-Tazobactam against Pseudomonas aeruginosa Cystic Fibrosis Clinical Isolates in the Hollow-Fiber Infection Model: Challenges Imposed by Hypermutability and Heteroresistance

Pseudomonas aeruginosa remains a challenge in chronic respiratory infections in cystic fibrosis (CF). Ceftolozane-tazobactam has not yet been evaluated against multidrug-resistant hypermutable P. aeruginosa isolates in the hollow-fiber infection model (HFIM). Isolates CW41, CW35, and CW44 (ceftolozane-tazobactam MICs of 4, 4, and 2 mg/L, respectively) from adults with CF were exposed to simulated representative epithelial lining fluid pharmacokinetics of ceftolozane-tazobactam in the HFIM. Regimens were continuous infusion (CI; 4.5 g/day to 9 g/day, all isolates) and 1-h infusions (1.5 g every 8 hours and 3 g every 8 hours, CW41). Whole-genome sequencing and mechanism-based modeling were performed for CW41. CW41 (in four of five biological replicates) and CW44 harbored preexisting resistant subpopulations; CW35 did not. For replicates 1 to 4 of CW41 and CW44, 9 g/day CI decreased bacterial counts to <3 log10 CFU/mL for 24 to 48 h, followed by regrowth and resistance amplification. Replicate 5 of CW41 had no preexisting subpopulations and was suppressed below ~3 log10 CFU/mL for 120 h by 9 g/day CI, followed by resistant regrowth. Both CI regimens reduced CW35 bacterial counts to <1 log10 CFU/mL by 120 h without regrowth. These results corresponded with the presence or absence of preexisting resistant subpopulations and resistance-associated mutations at baseline. Mutations in ampC, algO, and mexY were identified following CW41 exposure to ceftolozane-tazobactam at 167 to 215 h. Mechanism-based modeling well described total and resistant bacterial counts. The findings highlight the impact of heteroresistance and baseline mutations on the effect of ceftolozane-tazobactam and limitations of MIC to predict bacterial outcomes. The resistance amplification in two of three isolates supports current guidelines that ceftolozane-tazobactam should be utilized together with another antibiotic against P. aeruginosa in CF.

Pharmacokinetic-pharmacodynamic models for time courses of antibiotic effects: VSI: Antimicrobial Pharmacometrics

Pharmacokinetic-pharmacodynamic (PKPD) models have emerged as valuable tools for the characterisation and translation of antibiotic effects, and consequently for drug development and therapy. In contrast to traditional PKPD concepts for antibiotics like MIC and PKPD indices, PKPD models enable to describe the continuous, often species- or population-dependent time course of antimicrobial effects, commonly considering mechanistic pathogen- and drug-related knowledge. This review presents a comprehensive overview of previously published PKPD models describing repeated measurements of antibiotic effects. We conducted a literature review to identify PKPD models based on (i) antibiotic compounds, (ii) Gram-positive or Gram-negative pathogens, and (iii) in vitro or in vivo longitudinal colony forming unit data. We identified 132 publications released between 1963 and 2021, including models based on exposure with single antibiotics (n=92) and drug combinations (n=40), as well as different experimental settings (e.g., static/traditional dynamic/hollow-fibre/animal time-kill models, n=90/27/32/11). An interactive, fully searchable table summarises the details of each model, i.e. variants and mechanistic elements of PKPD submodels capturing observed bacterial growth, regrowth, drug effects, and interactions. Furthermore, the review highlights main purposes of PKPD model development, including the translation of preclinical PKPD to clinical settings and the assessment of varied dosing regimens and patient characteristics for their impact on clinical antibiotic effects. In summary, this comprehensive overview of PKPD models shall assist in identifying PKPD modelling strategies to describe growth, killing, regrowth and interaction patterns for pathogen-antibiotic combinations over time and ultimately facilitate model-informed antibiotic translation, dosing and drug development.

Simulated intravenous versus inhaled tobramycin with and without intravenous ceftazidime evaluated against hypermutable Pseudomonas aeruginosa via a dynamic biofilm model and mechanism-based modeling

Acute exacerbations of chronic respiratory infections in patients with cystic fibrosis are highly challenging due to hypermutable Pseudomonas aeruginosa, biofilm formation and resistance emergence. We aimed to systematically evaluate the effects of intravenous versus inhaled tobramycin with and without intravenous ceftazidime. Two hypermutable P. aeruginosa isolates, CW30 (MIC_CAZ 0.5mg/L, MIC_TOB 2mg/L) and CW8 (MIC_CAZ 2mg/L, MIC_TOB 8mg/L), were investigated for 120h in dynamic in vitro biofilm studies. Treatments were: intravenous ceftazidime 9g/day (33% lung fluid penetration); intravenous tobramycin 10mg/kg 24-hourly (50% lung fluid penetration); inhaled tobramycin 300mg 12-hourly, and both ceftazidime-tobramycin combinations. Total and less-susceptible planktonic and biofilm bacteria were quantified over 120h. Mechanism-based modeling was performed. All monotherapies were ineffective for both isolates, with regrowth of planktonic (≥4.7log₁₀ CFU/mL) and biofilm (>3.8log₁₀ CFU/cm²) bacteria, and resistance amplification by 120h. Both combination treatments demonstrated synergistic or enhanced bacterial killing of planktonic and biofilm bacteria. With the combination simulating tobramycin inhalation, planktonic bacterial counts of the two isolates at 120h were 0.47% and 36% of those for the combination with intravenous tobramycin; for biofilm bacteria the corresponding values were 8.2% and 13%. Combination regimens achieved substantial suppression of resistance of planktonic and biofilm bacteria compared to each antibiotic in monotherapy for both isolates. Mechanism-based modeling well described all planktonic and biofilm counts, and indicated synergy of the combination regimens despite reduced activity of tobramycin in biofilm. Combination regimens of inhaled tobramycin with ceftazidime hold promise to treat acute exacerbations caused by hypermutable P. aeruginosa strains and warrant further investigation.

In Vitro Activities of Ceftazidime-Avibactam and Aztreonam-Avibactam at Different Inoculum Sizes of Extended-Spectrum β-Lactam-Resistant Enterobacterales Blood Isolates

β-lactam–avibactam combinations have been proposed as carbapenem-sparing therapies, but little data exist on their in vitro activities in infections with high bacterial inocula. We investigated the in vitro efficacies and the inoculum effects of ceftazidime–avibactam and aztreonam–avibactam against extended-spectrum β-lactam-resistant Enterobacterales blood isolates. A total of 228 non-repetitive extended-spectrum β-lactam-resistant Escherichia coli and Klebsiella pneumoniae blood isolates were prospectively collected in a tertiary center. In vitro susceptibilities to ceftazidime, aztreonam, meropenem, ceftazidime–avibactam, and aztreonam–avibactam were evaluated by broth microdilution method using standard and high inocula. An inoculum effect was defined as an eightfold or greater increase in MIC when tested with the high inoculum. Of the 228 isolates, 99% were susceptible to ceftazidime–avibactam and 99% had low aztreonam–avibactam MICs (≤8 mg/L). Ceftazidime–avibactam and aztreonam–avibactam exhibited good in vitro activities; MIC₅₀/MIC₉₀ values were 0.5/2 mg/L, 0.125/0.5 mg/L, and ≤0.03/0.25 mg/L, respectively, and aztreonam–avibactam was more active than ceftazidime–avibactam. The frequencies of the inoculum effect with ceftazidime–avibactam and aztreonam–avibactam were lower than with meropenem (14% vs. 38%, p < 0.001 and 30% vs. 38%, p = 0.03, respectively). The β-lactam-avibactam combinations could be useful as carbapenem-sparing strategies, and aztreonam–avibactam has the better in vitro activity but is more subject to the inoculum effect than ceftazidime–avibactam.

A new PKPD model to characterize the inoculum effect of Acinetobacter baumannii on polymyxin B in vitro

The inoculum effect (i.e., reduction in antimicrobial activity at large starting inoculum) is a phenomenon described for various pathogens. Since limited data exist regarding inoculum effect of Acinetobacter baumannii, we evaluated killing of A. baumannii by polymyxin B, a last-resort antibiotic, at several starting inocula and developed a PKPD model to capture this phenomenon. In vitro static time-kill experiments were performed using polymyxin B at concentrations ranging from 0.125 to 128 mg/L against a clinical A. baumannii isolate at four starting inocula from 10⁵ to 10⁸ CFU/mL. Samples were collected up to 30 h to quantify the viable bacterial burden and were simultaneously modeled in the NONMEM software program. The expression of polymyxin B resistance genes (lpxACD, pmrCAB and wzc), and genetic modifications were studied by RT-qPCR and DNA sequencing experiments, respectively. The PKPD model included a single homogeneous bacterial population with adaptive resistance. Polymyxin B effect was modelled as a sigmoidal E_max model and the inoculum effect as an increase of polymyxin B EC₅₀ with increasing starting inoculum using a power function. Polymyxin B displayed a reduced activity as the starting inoculum increased: a 20-fold increase of polymyxin B EC₅₀ was observed between the lowest and the highest inoculum. No effects of polymyxin B and inoculum size were observed on the studied genes. The proposed PKPD model successfully described and predicted the pronounced in vitro inoculum effect of A. baumannii on polymyxin B activity. These results should be further validated using other bacteria/antibiotic combinations and in vivo models.

A personalised approach to antibiotic pharmacokinetics and pharmacodynamics in critically ill patients

Critically ill patients admitted to intensive care unit (ICU) with severe infections, or those who develop nosocomial infections, have poor outcomes with substantial morbidity and mortality. Such patients commonly have suboptimal antibiotic exposures at routinely used antibiotic doses related to an increased volume of distribution and altered clearance due to their underlying altered physiology. Furthermore, the use of extracorporeal devices such as renal replacement therapy and extracorporeal membrane oxygenation in these group of patients also has the potential to alter in vivo drug concentrations. Moreover, ICU patients are likely to be infected with less-susceptible pathogens. Therefore, one potential contributing cause to the poor outcomes observed in critically ill patients may be related to subtherapeutic antibiotic exposures. Newer concepts include the clinician considering optimised dosing based on a blood antibiotic exposure defined by pharmacokinetic modelling and therapeutic drug monitoring, combined with a knowledge of the antibiotic penetration into the site of infection, thereby achieving optimal bacterial killing. Such optimised dosing is likely to improve patient outcomes. The aim of this review is to highlight key aspects of antibiotic pharmacokinetics and pharmacodynamics (PK/PD) in critically ill patients and provide a PK/PD approach to tailor antibiotic dosing to the individual patient.

Pharmacodynamics of ceftazidime plus tobramycin combination dosage regimens against hypermutable Pseudomonas aeruginosa isolates at simulated epithelial lining fluid concentrations in a dynamic in vitro infection model

OBJECTIVES

Hypermutable Pseudomonas aeruginosa strains are a major challenge in cystic fibrosis. We investigated bacterial killing and resistance emergence for approved ceftazidime and tobramycin regimens, alone and in combination.

METHODS

Pseudomonas aeruginosa PAOΔmutS and six hypermutable clinical isolates were examined using 48-h static concentration time-kill (SCTK) studies (inoculum ~10^7.5 CFU/mL); four strains were also studied in a dynamic in vitro model (IVM) (inoculum ~10⁸ CFU/mL). The IVM simulated concentration-time profiles in epithelial lining fluid following intravenous administration of ceftazidime (3 g/day and 9 g/day continuous infusion), tobramycin (5 mg/kg and 10 mg/kg via 30-min infusion 24-hourly; half-life 3.5 h), and their combinations. Time courses of total and less-susceptible populations were determined.

RESULTS

Ceftazidime plus tobramycin demonstrated synergistic killing in SCTK studies for all strains, although to a lesser extent for ceftazidime-resistant strains. In the IVM, ceftazidime and tobramycin monotherapies provided ≤5.4 and ≤3.4 log₁₀ initial killing, respectively; however, re-growth with resistance occurred by 72 h. Against strains susceptible to one or both antibiotics, high-dose combination regimens provided >6 log₁₀ initial killing, which was generally synergistic from 8-24 h, and marked suppression of re-growth and resistance at 72 h. The time course of bacterial density in the IVM was well described by mechanism-based models, enabling Monte Carlo simulations (MCSs) to predict likely effectiveness of the combination in patients.

CONCLUSION

Results of the IVM and MCS suggested antibacterial effect depends both on the strain's susceptibility and hypermutability. Further investigation of the combination against hypermutable P. aeruginosa strains is warranted.

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

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

Model-Informed Drug Development for Anti-Infectives: State of the Art and Future

Model-informed drug development (MIDD) has a long and rich history in infectious diseases. This review describes foundational principles of translational anti-infective pharmacology, including choice of appropriate measures of exposure and pharmacodynamic (PD) measures, patient subpopulations, and drug-drug interactions. Examples are presented for state-of-the-art, empiric, mechanistic, interdisciplinary, and real-world evidence MIDD applications in the development of antibacterials (review of minimum inhibitory concentration-based models, mechanism-based pharmacokinetic/PD (PK/PD) models, PK/PD models of resistance, and immune response), antifungals, antivirals, drugs for the treatment of global health infectious diseases, and medical countermeasures. The degree of adoption of MIDD practices across the infectious diseases field is also summarized. The future application of MIDD in infectious diseases will progress along two planes; "depth" and "breadth" of MIDD methods. "MIDD depth" refers to deeper incorporation of the specific pathogen biology and intrinsic and acquired-resistance mechanisms; host factors, such as immunologic response and infection site, to enable deeper interrogation of pharmacological impact on pathogen clearance; clinical outcome and emergence of resistance from a pathogen; and patient and population perspective. In particular, improved early assessment of the emergence of resistance potential will become a greater focus in MIDD, as this is poorly mitigated by current development approaches. "MIDD breadth" refers to greater adoption of model-centered approaches to anti-infective development. Specifically, this means how various MIDD approaches and translational tools can be integrated or connected in a systematic way that supports decision making by key stakeholders (sponsors, regulators, and payers) across the entire development pathway.

The Spectrum of Antimicrobial Activity of Cyadox against Pathogens Collected from Pigs, Chicken, and Fish in China

Cyadox has potential use as an antimicrobial agent in animals. However, its pharmacodynamic properties have not been systematically studied yet. In this study, the in vitro antibacterial activities of cyadox were assayed, and the antibacterial efficacy of cyadox against facultative anaerobes was also determined under anaerobic conditions. It was shown that Clostridium perfringens and Pasteurella multocida (MIC = 0.25 and 1 μg/mL) from pigs, Campylobacter jejuni and Pasteurella multocida from poultry, E. coli, Streptococcus spp., and Flavobacterium columnare from fish were highly susceptible to cyadox (MIC= 1 and 8 μg/mL). However, F. columnare has no killing effect for drug tolerance. Under in vitro anaerobic conditions, the antibacterial activity of cyadox against most facultative anaerobes was considerably enhanced Under anaerobic conditions for the facultative anaerobes, susceptible bacteria were P. multocida, Aeromonas spp. (including A. hydrophila, A. veronii, A. jandaei, A. caviae, and A. sobria, excluding A. punctata), E. coli, Salmonella spp. (including S. choleraesui, S. typhimurium, and S. pullorum), Proteus mirabilis, Vibrio fluvialis, Yersinia ruckeri, Erysipelothrix, Acinetobacter baumannii, and Streptococcus agalactiae (MICs were 0.25~8 μg/mL, MBCs were 1–64 μg/mL). Intermediate bacteria were Enterococcus spp. (including E. faecalis and E. faecium), Yersinia enterocolitica, and Streptococcus spp. (MICs mainly were 8~32 μg/mL, MBCs were 16~128 μg/mL). This study firstly showed that cyadox had strong antibacterial activity and had the potential to be used as a single drug in the treatment of bacterial infectious diseases.

Inoculum effect of β-lactam antibiotics

The phenomenon of attenuated antibacterial activity at inocula above those utilized for susceptibility testing is referred to as the inoculum effect. Although the inoculum effect has been reported for several decades, it is currently debatable whether the inoculum effect is clinically significant. The aim of the present review was to consolidate currently available evidence to summarize which β-lactam drug classes demonstrate an inoculum effect against specific bacterial pathogens. Review of the literature showed that the majority of studies that evaluated the inoculum effect of β-lactams were in vitro investigations of Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae and Staphylococcus aureus. Across all five pathogens, cephalosporins consistently displayed observable inoculum effects in vitro, whereas carbapenems were less susceptible to an inoculum effect. A handful of animal studies were available that validated that the in vitro inoculum effect translates into attenuated pharmacodynamics of β-lactams in vivo. Only a few clinical investigations were available and suggested that an in vitro inoculum effect of cefazolin against MSSA may correspond to an increased likeliness of adverse clinical outcomes in patients receiving cefazolin for bacteraemia. The presence of β-lactamase enzymes was the primary mechanism responsible for an inoculum effect, but the observation of an inoculum effect in multiple pathogens lacking β-lactamase enzymes indicates that there are likely multiple mechanisms that may result in an inoculum effect. Further clinical studies are needed to better define whether interventions made in the clinic in response to organisms displaying an in vitro inoculum effect will optimize clinical outcomes.

In pursuit of the triple crown: mechanism-based pharmacodynamic modelling for the optimization of three-drug combinations against KPC-producing Klebsiella pneumoniae

OBJECTIVES

Optimal combination therapy for Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae (KPC-Kp) is unknown. The present study sought to characterize the pharmacodynamics (PD) of polymyxin B (PMB), meropenem (MEM) and rifampin (RIF) alone and in combination using a hollow fibre infection model (HFIM) coupled with mechanism-based modelling (MBM).

METHODS

A 10-day HFIM was utilized to simulate human pharmacokinetics (PK) of various PMB, MEM and RIF dosing regimens against a clinical KPC-Kp isolate, with total and resistant subpopulations quantified to capture PD response. A MBM was developed to characterize bacterial subpopulations and synergy between agents. Simulations using the MBM and published population PK models were employed to forecast the bacterial time course and the extent of its variability in infected patients for three-drug regimens.

RESULTS

In the HFIM, a PMB single-dose ('burst') regimen of 5.53 mg/kg combined with MEM 8 g using a 3-hr prolonged infusion every 8 hr and RIF 600 mg every 24 hr resulted in bacterial counts below the quantitative limit within 24 hr and remained undetectable throughout the 10-day experiment. The final MBM consisted of two bacterial subpopulations of differing PMB and MEM joint susceptibility and the ability to form a non-replicating, tolerant subpopulation. Synergistic interactions between PMB, MEM and RIF were well quantified, with the MBM providing adequate capture of the observed data.

DISCUSSION

An in vitro-in silico approach answers questions related to PD optimization as well as overall feasibility of combination therapy against KPC-Kp, offering crucial insights in the absence of clinical trials.

Recommendations for Systemic Antimicrobial Therapy in Fracture-Related Infection: A Consensus From an International Expert Group

Fracture-related infection (FRI) is a major complication in musculoskeletal trauma and one of the leading causes of morbidity. Standardization of general treatment strategies for FRI has been poor. One of the reasons is the heterogeneity in this patient population, including various anatomical locations, multiple fracture patterns, different degrees of soft-tissue injury, and different patient conditions. This variability makes treatment complex and hard to standardize. As these infections are biofilm-related, surgery remains the cornerstone of treatment, and this entails multiple key aspects (eg, fracture fixation, tissue sampling, debridement, and soft-tissue management). Another important aspect, which is sometimes less familiar to the orthopaedic trauma surgeon, is systemic antimicrobial therapy. The aim of this article is to summarize the available evidence and provide recommendations for systemic antimicrobial therapy with respect to FRI, based on the most recent literature combined with expert opinion. LEVEL OF EVIDENCE:: Therapeutic Level V. See Instructions for Authors for a complete description of levels of evidence.

Meropenem-Tobramycin Combination Regimens Combat Carbapenem-Resistant Pseudomonas aeruginosa in the Hollow-Fiber Infection Model Simulating Augmented Renal Clearance in Critically Ill Patients

Augmented renal clearance (ARC) is common in critically ill patients and is associated with subtherapeutic concentrations of renally eliminated antibiotics. We investigated the impact of ARC on bacterial killing and resistance amplification for meropenem and tobramycin regimens in monotherapy and combination. Two carbapenem-resistant Pseudomonas aeruginosa isolates were studied in static-concentration time-kill studies. One isolate was examined comprehensively in a 7-day hollow-fiber infection model (HFIM). Pharmacokinetic profiles representing substantial ARC (creatinine clearance of 250 ml/min) were generated in the HFIM for meropenem (1 g or 2 g administered every 8 h as 30-min infusion and 3 g/day or 6 g/day as continuous infusion [CI]) and tobramycin (7 mg/kg of body weight every 24 h as 30-min infusion) regimens. The time courses of total and less-susceptible bacterial populations and MICs were determined for the monotherapies and all four combination regimens. Mechanism-based mathematical modeling (MBM) was performed. In the HFIM, maximum bacterial killing with any meropenem monotherapy was ∼3 log₁₀ CFU/ml at 7 h, followed by rapid regrowth with increases in resistant populations by 24 h (meropenem MIC of up to 128 mg/liter). Tobramycin monotherapy produced extensive initial killing (∼7 log₁₀ at 4 h) with rapid regrowth by 24 h, including substantial increases in resistant populations (tobramycin MIC of 32 mg/liter). Combination regimens containing meropenem administered intermittently or as a 3-g/day CI suppressed regrowth for ∼1 to 3 days, with rapid regrowth of resistant bacteria. Only a 6-g/day CI of meropenem combined with tobramycin suppressed regrowth and resistance over 7 days. MBM described bacterial killing and regrowth for all regimens well. The mode of meropenem administration was critical for the combination to be maximally effective against carbapenem-resistant P. aeruginosa.

Evaluation of Tobramycin and Ciprofloxacin as a Synergistic Combination Against Hypermutable Pseudomonas Aeruginosa Strains via Mechanism-Based Modelling

Hypermutable Pseudomonas aeruginosa strains have a greatly increased mutation rate and are prevalent in chronic respiratory infections. Initially, we systematically evaluated the time-course of total and resistant populations of hypermutable (PAO∆mutS) and non-hypermutable (PAO1) P. aeruginosa strains in 48-h static concentration time-kill studies with two inocula. Both strains were exposed to clinically relevant concentrations of important antibiotics (aztreonam, ceftazidime, imipenem, meropenem, tobramycin, and ciprofloxacin) in monotherapy. The combination of tobramycin and ciprofloxacin was subsequently assessed in 48-h static concentration time-kill studies against PAO1, PAO∆mutS, and two hypermutable clinical P. aeruginosa strains. Mechanism-based mathematical modelling was conducted to describe the time-course of total and resistant bacteria for all four strains exposed to the combination. With all monotherapies, bacterial regrowth and resistant populations were overall more pronounced for PAO∆mutS compared to PAO1. The combination of tobramycin and ciprofloxacin was synergistic, with up to 10^6.1 colony forming units (CFU)/mL more bacterial killing than the most active monotherapy for all strains, and largely suppressed less-susceptible populations. This work indicates that monotherapies against hypermutable P. aeruginosa strains are not a viable option. Tobramycin with ciprofloxacin was identified as a promising and tangible option to combat hypermutable P. aeruginosa strains.

Pathogenesis and management of fracture-related infection

BACKGROUND

Both fracture-related infections (FRIs) and periprosthetic joint infections (PJIs) include orthopaedic implant-associated infections. However, key aspects of management differ due to the bone and soft tissue damage in FRIs and the option of removing the implant after fracture healing. In contrast to PJIs, research and guidelines for diagnosis and treatment in FRIs are scarce.

OBJECTIVES

This narrative review aims to update clinical microbiologists, infectious disease specialists and surgeons on the management of FRIs.

SOURCES

A computerized search of PubMed was performed to identify relevant studies. Search terms included 'Fracture' and 'Infection'. The reference lists of all retrieved articles were checked for additional relevant references. In addition, when scientific evidence was lacking, recommendations are based on expert opinion.

CONTENT

Pathogenesis, prevention, diagnosis and treatment of FRIs are presented. Whenever available, specific data of patients with FRI are discussed.

IMPLICATIONS

Management of patients with FRI should take into account FRI-specific features. Treatment pathways should implement a multidisciplinary approach to achieve a good outcome. Recently, international consensus guidelines were developed to improve the quality of care for patients suffering from this severe complication, which are highlighted in this review.

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

In June 2017, the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, organized a workshop entitled “Pharmacokinetics-Pharmacodynamics (PK/PD) for Development of Therapeutics against Bacterial Pathogens.” The aims were to discuss details of various PK/PD models and identify sound practices for deriving and utilizing PK/PD relationships to design optimal dosage regimens for patients. Workshop participants encompassed individuals from academia, industry, and government, including the United States Food and Drug Administration.

In June 2017, the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, organized a workshop entitled “Pharmacokinetics-Pharmacodynamics (PK/PD) for Development of Therapeutics against Bacterial Pathogens.” The aims were to discuss details of various PK/PD models and identify sound practices for deriving and utilizing PK/PD relationships to design optimal dosage regimens for patients. Workshop participants encompassed individuals from academia, industry, and government, including the United States Food and Drug Administration. This and the accompanying review on clinical PK/PD summarize the workshop discussions and recommendations. Nonclinical PK/PD models play a critical role in designing human dosage regimens and are essential tools for drug development. These include in vitro and in vivo efficacy models that provide valuable and complementary information for dose selection and translation from the laboratory to human. It is crucial that studies be designed, conducted, and interpreted appropriately. For antibacterial PK/PD, extensive published data and expertise are available. These have been leveraged to develop recommendations, identify common pitfalls, and describe the applications, strengths, and limitations of various nonclinical infection models and translational approaches. Despite these robust tools and published guidance, characterizing nonclinical PK/PD relationships may not be straightforward, especially for a new drug or new class. Antimicrobial PK/PD is an evolving discipline that needs to adapt to future research and development needs. Open communication between academia, pharmaceutical industry, government, and regulatory bodies is essential to share perspectives and collectively solve future challenges.

Novel Polymyxin Combination with the Antiretroviral Zidovudine Exerts Synergistic Killing against NDM-Producing Multidrug-Resistant Klebsiella pneumoniae

Polymyxins are used as a last-line therapy against multidrug-resistant (MDR) New Delhi metallo-β-lactamase (NDM)-producing Klebsiella pneumoniae However, polymyxin resistance can emerge with monotherapy; therefore, novel strategies are urgently needed to minimize the resistance and maintain their clinical utility. This study aimed to investigate the pharmacodynamics of polymyxin B in combination with the antiretroviral drug zidovudine against K. pneumoniae Three isolates were evaluated in static time-kill studies (0 to 64 mg/liter) over 48 h. An in vitro one-compartment pharmacokinetic/pharmacodynamic (PK/PD) model (IVM) was used to simulate humanized dosage regimens of polymyxin B (4 mg/liter as continuous infusion) and zidovudine (as bolus dose thrice daily to achieve maximum concentration of drug in broth [C _max] of 6 mg/liter) against K. pneumoniae BM1 over 72 h. The antimicrobial synergy of the combination was further evaluated in a murine thigh infection model against K. pneumoniae 02. In the static time-kill studies, polymyxin B monotherapy produced rapid and extensive killing against all three isolates followed by extensive regrowth, whereas zidovudine produced modest killing followed by significant regrowth at 24 h. Polymyxin B in combination with zidovudine significantly enhanced the antimicrobial activity (≥4 log₁₀ CFU/ml) and minimized bacterial regrowth. In the IVM, the combination was synergistic and the total bacterial loads were below the limit of detection for up to 72 h. In the murine thigh infection model, the bacterial burden at 24 h in the combination group was ≥3 log₁₀ CFU/thigh lower than each monotherapy against K. pneumoniae 02. Overall, the polymyxin B-zidovudine combination demonstrates superior antimicrobial efficacy and minimized emergence of resistance to polymyxins.

Successful cure of daptomycin-non-susceptible, vancomycin-intermediate Staphylococcus aureus prosthetic aortic valve endocarditis directed by synergistic in vitro time-kill study

Infectious complications following surgical valve replacements are extremely difficult to treat, often requiring prolonged antimicrobials therapy with or without surgery. Vancomycin-intermediate Staphylococcus aureus is an infrequent pathogen, with an estimated prevalence of less than 0.3%, but presents even greater challenges. We report a case of successful cure of daptomycin-non-susceptible and vancomycin-intermediate Staphylococcus aureus prosthetic valve endocarditis using an eight-week course of combination antimicrobial therapy. Using time-kill study, the combination of daptomycin plus ceftaroline and rifampin resulted in a greater than 4 log reduction of bacterial growth at 24 hours. This antimicrobial combination was used for a total of eight weeks with a successful outcome.

Meropenem Combined with Ciprofloxacin Combats Hypermutable Pseudomonas aeruginosa from Respiratory Infections of Cystic Fibrosis Patients

Hypermutable Pseudomonas aeruginosa organisms are prevalent in chronic respiratory infections and have been associated with reduced lung function in cystic fibrosis (CF); these isolates can become resistant to all antibiotics in monotherapy. This study aimed to evaluate the time course of bacterial killing and resistance of meropenem and ciprofloxacin in combination against hypermutable and nonhypermutable P. aeruginosa Static concentration time-kill experiments over 72 h assessed meropenem and ciprofloxacin in mono- and combination therapies against PAO1 (nonhypermutable), PAOΔmutS (hypermutable), and hypermutable isolates CW8, CW35, and CW44 obtained from CF patients with chronic respiratory infections. Meropenem (1 or 2 g every 8 h [q8h] as 3-h infusions and 3 g/day as a continuous infusion) and ciprofloxacin (400 mg q8h as 1-h infusions) in monotherapies and combinations were further evaluated in an 8-day hollow-fiber infection model study (HFIM) against CW44. Concentration-time profiles in lung epithelial lining fluid reflecting the pharmacokinetics in CF patients were simulated and counts of total and resistant bacteria determined. All data were analyzed by mechanism-based modeling (MBM). In the HFIM, all monotherapies resulted in rapid regrowth with resistance at 48 h. The maximum daily doses of 6 g meropenem (T_>MIC of 80% to 88%) and 1.2 g ciprofloxacin (area under the concentration-time curve over 24 h in the steady state divided by the MIC [AUC/MIC], 176), both given intermittently, in monotherapy failed to suppress regrowth and resulted in substantial emergence of resistance (≥7.6 log₁₀ CFU/ml resistant populations). The combination of these regimens achieved synergistic killing and suppressed resistance. MBM with subpopulation and mechanistic synergy yielded unbiased and precise curve fits. Thus, the combination of 6 g/day meropenem plus ciprofloxacin holds promise for future clinical evaluation against infections by susceptible hypermutable P. aeruginosa.

Compliance with perioperative prophylaxis guidelines and the use of novel outcome measures

Postoperative wound infections represent an important source of morbidity and mortality in children. Perioperative antibiotic prophylaxis has been shown to decrease the risk of developing infections and hospital guidelines surrounding antibiotic use exist to standardize patient care. Despite supporting evidence, rates of compliance with guidelines vary. Quality improvement initiatives have been introduced to improve compliance with intraoperative antibiotic guidelines. Thorough infection surveillance, including antibiotic provision in presurgical checklists, computerized voice antibiotic administration prompts, and national feedback systems are now increasingly common. Few studies have been conducted investigating the effectiveness of prophylactic antibiotics in children. Outcome measures such as morbidity and mortality and return to the operating room can be used to examine the relationship between antibiotic use and patient outcome but these measures are limited in that they occur infrequently or are subjective and difficult to measure. Metrics such as days alive out of hospital and length of hospital stay may be useful alternatives for ongoing monitoring of infections and identifying improvements in patient outcomes. Guidelines on antibiotic prophylaxis have facilitated an increase in the correct provision of perioperative antibiotics and a reduction in the incidence of postoperative infection. Measures of patient outcome such as days alive out of hospital and length of hospital stay are easy to collect and calculate but further work is needed to confirm the utility of these measures for monitoring infection rates.

First Penicillin-Binding Protein Occupancy Patterns of β-Lactams and β-Lactamase Inhibitors in Klebsiella pneumoniae

Penicillin-binding proteins (PBPs) are the high-affinity target sites of all β-lactam antibiotics in bacteria. It is well known that each β-lactam covalently binds to and thereby inactivates different PBPs with various affinities. Despite β-lactams serving as the cornerstone of our therapeutic armamentarium against Klebsiella pneumoniae, PBP binding data are missing for this pathogen. We aimed to generate the first PBP binding data on 13 chemically diverse and clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae PBP binding was determined using isolated membrane fractions from K. pneumoniae strains ATCC 43816 and ATCC 13883. Binding reactions were conducted using β-lactam concentrations from 0.0075 to 256 mg/liter (or 128 mg/liter). After β-lactam exposure, unbound PBPs were labeled by Bocillin FL. Binding affinities (50% inhibitory concentrations [IC50]) were reported as the β-lactam concentrations that half-maximally inhibited Bocillin FL binding. PBP occupancy patterns by β-lactams were consistent across both strains. Carbapenems bound to all PBPs, with PBP2 and PBP4 as the highest-affinity targets (IC50, <0.0075 mg/liter). Preferential PBP2 binding was observed by mecillinam (amdinocillin; IC50, <0.0075 mg/liter) and avibactam (IC50, 2 mg/liter). Aztreonam showed high affinity for PBP3 (IC50, 0.06 to 0.12 mg/liter). Ceftazidime bound PBP3 at low concentrations (IC50, 0.06 to 0.25 mg/liter) and PBP1a/b at higher concentrations (4 mg/liter), whereas cefepime bound PBPs 1 to 4 at more even concentrations (IC50, 0.015 to 2 mg/liter). These PBP binding data on a comprehensive set of 13 clinically relevant β-lactams and β-lactamase inhibitors in K. pneumoniae enable, for the first time, the rational design and optimization of double β-lactam and β-lactam-β-lactamase inhibitor combinations.

Polymyxin B in Combination with Enrofloxacin Exerts Synergistic Killing against Extensively Drug-Resistant Pseudomonas aeruginosa

Polymyxins are increasingly used as a last-resort class of antibiotics against extensively drug-resistant (XDR) Gram-negative bacteria. However, resistance to polymyxins can emerge with monotherapy. As nephrotoxicity is the major dose-limiting factor for polymyxin monotherapy, dose escalation to suppress the emergence of polymyxin resistance is not a viable option. Therefore, novel approaches are needed to preserve this last-line class of antibiotics. This study aimed to investigate the antimicrobial synergy of polymyxin B combined with enrofloxacin against Pseudomonas aeruginosa Static time-kill studies were conducted over 24 h with polymyxin B (1 to 4 mg/liter) and enrofloxacin (1 to 4 mg/liter) alone or in combination. Additionally, in vitro one-compartment model (IVM) and hollow-fiber infection model (HFIM) experiments were performed against P. aeruginosa 12196. Polymyxin B and enrofloxacin in monotherapy were ineffective against all of the P. aeruginosa isolates examined, whereas polymyxin B-enrofloxacin in combination was synergistic against P. aeruginosa, with ≥2 to 4 log₁₀ kill at 24 h in the static time-kill studies. In both IVM and HFIM, the combination was synergistic, and the bacterial counting values were below the limit of quantification on day 5 in the HFIM. A population analysis profile indicated that the combination inhibited the emergence of polymyxin resistance in P. aeruginosa 12196. The mechanism-based modeling suggests that the synergistic killing is a result of the combination of mechanistic and subpopulation synergy. Overall, this is the first preclinical study to demonstrate that the polymyxin-enrofloxacin combination is of considerable utility for the treatment of XDR P. aeruginosa infections and warrants future clinical evaluations.

Optimization and Evaluation of Piperacillin-Tobramycin Combination Dosage Regimens against Pseudomonas aeruginosa for Patients with Altered Pharmacokinetics via the Hollow-Fiber Infection Model and Mechanism-Based Modeling

Augmented renal clearance (ARC) in critically ill patients can result in suboptimal drug exposures and treatment failure. Combination dosage regimens accounting for ARC have never been optimized and evaluated against Pseudomonas aeruginosa by use of the hollow-fiber infection model (HFIM). Using a P. aeruginosa isolate from a critically ill patient and static-concentration time-kill experiments (SCTKs), we studied clinically relevant piperacillin and tobramycin concentrations, alone and in combinations, against two inocula (10^5.8 and 10^7.6 CFU/ml) over 72 h. We subsequently evaluated the effects of optimized piperacillin (4 g every 4 h [q4h], given as 0.5-h infusions) plus tobramycin (5 mg/kg of body weight q24h, 7 mg/kg q24h, or 10 mg/kg q48h, given as 0.5-h infusions) regimens on killing and regrowth in the HFIM, simulating a creatinine clearance of 250 ml/min. Mechanism-based modeling was performed in S-ADAPT. In SCTKs, piperacillin plus tobramycin (except combinations with 8 mg/liter tobramycin and against the low inoculum) achieved synergistic killing (≥2 log₁₀ versus the most active monotherapy at 48 h and 72 h) and prevented regrowth. Piperacillin monotherapy (4 g q4h) in the HFIM provided 2.4-log₁₀ initial killing followed by regrowth at 24 h and resistance emergence. Tobramycin monotherapies displayed rapid initial killing (≥5 log₁₀ at 13 h) followed by extensive regrowth. As predicted by mechanism-based modeling, the piperacillin plus tobramycin dosage regimens were synergistic and provided ≥5-log₁₀ killing with resistance suppression over 8 days in the HFIM. Optimized piperacillin-tobramycin regimens provided significant bacterial killing and suppressed resistance emergence. These regimens appear to be highly promising for effective and early treatment, even in the near-worst-case scenario of ARC.

Optimization of a Meropenem-Tobramycin Combination Dosage Regimen against Hypermutable and Nonhypermutable Pseudomonas aeruginosa via Mechanism-Based Modeling and the Hollow-Fiber Infection Model

Hypermutable Pseudomonas aeruginosa strains are prevalent in patients with cystic fibrosis and rapidly become resistant to antibiotic monotherapies. Combination dosage regimens have not been optimized against such strains using mechanism-based modeling (MBM) and the hollow-fiber infection model (HFIM). The PAO1 wild-type strain and its isogenic hypermutable PAOΔmutS strain (MIC_meropenem of 1.0 mg/liter and MIC_tobramycin of 0.5 mg/liter for both) were assessed using 96-h static-concentration time-kill studies (SCTK) and 10-day HFIM studies (inoculum, ∼10^8.4 CFU/ml). MBM of SCTK data were performed to predict expected HFIM outcomes. Regimens studied in the HFIM were meropenem at 1 g every 8 h (0.5-h infusion), meropenem at 3 g/day with continuous infusion, tobramycin at 10 mg/kg of body weight every 24 h (1-h infusion), and both combinations. Meropenem regimens delivered the same total daily dose. Time courses of total and less susceptible populations and MICs were determined. For the PAOΔmutS strain in the HFIM, all monotherapies resulted in rapid regrowth to >10^8.7 CFU/ml with near-complete replacement by less susceptible bacteria by day 3. Meropenem every 8 h with tobramycin caused >7-log₁₀ bacterial killing followed by regrowth to >6 log₁₀ CFU/ml by day 5 and high-level resistance (MIC_meropenem, 32 mg/liter; MIC_tobramycin, 8 mg/liter). Continuous infusion of meropenem with tobramycin achieved >8-log₁₀ bacterial killing without regrowth. For PAO1, meropenem monotherapies suppressed bacterial growth to <4 log₁₀ over 7 to 9 days, with both combination regimens achieving near eradication. An MBM-optimized meropenem plus tobramycin regimen achieved synergistic killing and resistance suppression against a difficult-to-treat hypermutable P. aeruginosa strain. For the combination to be maximally effective, it was critical to achieve the optimal shape of the concentration-time profile for meropenem.

Mechanism-Based Pharmacokinetic/Pharmacodynamic Modeling of Aerosolized Colistin in a Mouse Lung Infection Model

Optimized dosage regimens of aerosolized colistin (as colistin methanesulfonate [CMS]) are urgently required to maximize bacterial killing against multidrug-resistant Gram-negative bacteria while minimizing toxicity. This study aimed to develop a mechanism-based pharmacokinetic (PK)/pharmacodynamic (PD) model (MBM) for aerosolized colistin based upon PK/PD data in neutropenic infected mice and to perform a deterministic simulation with the PK of aerosolized colistin (as CMS) in critically ill patients. In vivo time-kill experiments were carried out with three different strains of Pseudomonas aeruginosa An MBM was developed in S-ADAPT and evaluated by assessing its ability to predict the PK/PD index associated with efficacy in mice. A deterministic simulation with human PK data was undertaken to predict the efficacy of current dosage regimens of aerosolized colistin in critically ill patients. In the final MBM, the total bacterial population for each isolate consisted of colistin-susceptible and -resistant subpopulations. The antimicrobial efficacy of aerosolized colistin was best described by a sigmoidal E_max model whereby colistin enhanced the rate of bacterial death. Deterministic simulation with human PK data predicted that an inhalational dosage regimen of 60 mg colistin base activity (CBA) every 12 h is needed to achieve a ≥2-log₁₀ bacterial reduction (as the number of CFU per lung) in critically ill patients at 24 h after commencement of inhaled therapy. In conclusion, the developed MBM is a useful tool for optimizing inhalational dosage regimens of colistin. Clinical studies are warranted to validate and refine our MBM for aerosolized colistin.

Evaluation of Pharmacokinetic/Pharmacodynamic Model-Based Optimized Combination Regimens against Multidrug-Resistant Pseudomonas aeruginosa in a Murine Thigh Infection Model by Using Humanized Dosing Schemes

We previously optimized imipenem and tobramycin combination regimens against a double-resistant clinical Pseudomonas aeruginosa isolate by using in vitro infection models, mechanism-based pharmacokinetic/pharmacodynamic modeling (MBM), and Monte Carlo simulations. The current study aimed to evaluate these regimens in a neutropenic murine thigh infection model and to characterize the time course of bacterial killing and regrowth via MBM. We studied monotherapies and combinations of imipenem with tobramycin in vivo against the double-resistant clinical P. aeruginosa isolate by using humanized dosing schemes. Viable count profiles of total and resistant populations were quantified over 24 h. Tobramycin monotherapy (7 mg/kg every 24 h [q24h] as a 0.5-h infusion) was ineffective. Imipenem monotherapies (continuous infusion of 4 or 5 g/day with a 1-g loading dose) yielded 2.47 or 2.57 log₁₀ CFU/thigh killing at 6 h. At 24 h, imipenem at 4 g/day led to regrowth up to the initial inoculum (4.79 ± 0.26 log₁₀ CFU/thigh), whereas imipenem at 5 g/day displayed 1.75 log₁₀ killing versus the initial inoculum. The combinations (i.e., imipenem at 4 or 5 g/day plus tobramycin) provided a clear benefit, with bacterial killing of ≥2.51 or ≥1.50 log₁₀ CFU/thigh compared to the respective most active monotherapy at 24 h. No colonies were detected on 3×MIC agar plates for combinations, whereas increased resistance (at 3×MIC) emerged for monotherapies (except imipenem at 5 g/day). MBM suggested that tobramycin considerably enhanced the imipenem target site concentration up to 2.6-fold. The combination regimens, rationally optimized via a translational modeling approach, demonstrated substantially enhanced bacterial killing and suppression of regrowth in vivo against a double-resistant isolate and are therefore promising for future clinical evaluation.

Aminoglycoside Concentrations Required for Synergy with Carbapenems against Pseudomonas aeruginosa Determined via Mechanistic Studies and Modeling

This study aimed to systematically identify the aminoglycoside concentrations required for synergy with a carbapenem and characterize the permeabilizing effect of aminoglycosides on the outer membrane of Pseudomonas aeruginosa Monotherapies and combinations of four aminoglycosides and three carbapenems were studied for activity against P. aeruginosa strain AH298-GFP in 48-h static-concentration time-kill studies (SCTK) (inoculum: 10^7.6 CFU/ml). The outer membrane-permeabilizing effect of tobramycin alone and in combination with imipenem was characterized via electron microscopy, confocal imaging, and the nitrocefin assay. A mechanism-based model (MBM) was developed to simultaneously describe the time course of bacterial killing and prevention of regrowth by imipenem combined with each of the four aminoglycosides. Notably, 0.25 mg/liter of tobramycin, which was inactive in monotherapy, achieved synergy (i.e., ≥2-log₁₀ more killing than the most active monotherapy at 24 h) combined with imipenem. Electron micrographs, confocal image analyses, and the nitrocefin uptake data showed distinct outer membrane damage by tobramycin, which was more extensive for the combination with imipenem. The MBM indicated that aminoglycosides enhanced the imipenem target site concentration up to 4.27-fold. Tobramycin was the most potent aminoglycoside to permeabilize the outer membrane; tobramycin (0.216 mg/liter), gentamicin (0.739 mg/liter), amikacin (1.70 mg/liter), or streptomycin (5.19 mg/liter) was required for half-maximal permeabilization. In summary, our SCTK, mechanistic studies and MBM indicated that tobramycin was highly synergistic and displayed the maximum outer membrane disruption potential among the tested aminoglycosides. These findings support the optimization of highly promising antibiotic combination dosage regimens for critically ill patients.

Potentiation of ceftazidime by avibactam against β-lactam-resistant Pseudomonas aeruginosa in an in vitro infection model

Objectives

This study evaluated the in vitro pharmacodynamics of combinations of ceftazidime and the non-β-lactam β-lactamase inhibitor, avibactam, against ceftazidime-, piperacillin/tazobactam- and meropenem-multiresistant Pseudomonas aeruginosa by a quantitative time-kill method.

Methods

MICs of ceftazidime plus 0-16 mg/L avibactam were determined against eight isolates of P. aeruginosa . Single-compartment, 24 h time-kill kinetics were investigated for three isolates at 0-16 mg/L avibactam with ceftazidime at 0.25-4-fold the MIC as measured at the respective avibactam concentration. Ceftazidime and avibactam concentrations were measured by LC-MS/MS during the time-kill kinetic studies to evaluate drug degradation.

Results

Avibactam alone displayed no antimicrobial activity. MICs of ceftazidime decreased by 8-16-fold in the presence of avibactam at 4 mg/L. The changes in log 10 cfu/mL at both the 10 h and 24 h timepoints (versus 0 h) revealed bacterial killing at ≥1-fold MIC. Significantly higher concentrations of ceftazidime alone, as compared with those of ceftazidime in combination, were required to produce any given kill. Without avibactam, ceftazidime degradation was significant (defined as degradation t 1/2  <   24 h), with as little as 19%   ±   18% of the original concentration remaining at 8 h for the most resistant strain. In combination with avibactam, ceftazidime degradation at ≥ 1-fold MIC was negligible.

Conclusion

The addition of avibactam protected ceftazidime from degradation in a dose-dependent manner and restored its cidal and static activity at concentrations in combination well below the MIC of ceftazidime alone.

Translational Pharmacometric Evaluation of Typical Antibiotic Broad-Spectrum Combination Therapies Against Staphylococcus Aureus Exploiting In Vitro Information

Broad‐spectrum antibiotic combination therapy is frequently applied due to increasing resistance development of infective pathogens. The objective of the present study was to evaluate two common empiric broad‐spectrum combination therapies consisting of either linezolid (LZD) or vancomycin (VAN) combined with meropenem (MER) against Staphylococcus aureus (S. aureus) as the most frequent causative pathogen of severe infections. A semimechanistic pharmacokinetic‐pharmacodynamic (PK‐PD) model mimicking a simplified bacterial life‐cycle of S. aureus was developed upon time‐kill curve data to describe the effects of LZD, VAN, and MER alone and in dual combinations. The PK‐PD model was successfully (i) evaluated with external data from two clinical S. aureus isolates and further drug combinations and (ii) challenged to predict common clinical PK‐PD indices and breakpoints. Finally, clinical trial simulations were performed that revealed that the combination of VAN‐MER might be favorable over LZD‐MER due to an unfavorable antagonistic interaction between LZD and MER.

Current mathematical models for cancer drug discovery

INTRODUCTION

Pharmacometric models represent the most comprehensive approaches for extracting, summarizing and integrating information obtained in the often sparse, limited, and less-than-optimally designed experiments performed in the early phases of oncology drug discovery. Whilst empirical methodologies may be enough for screening and ranking candidate drugs, modeling approaches are needed for optimizing and making economically viable the learn-confirm cycles within an oncology research program and anticipating the dose regimens to be investigated in the subsequent clinical development. Areas covered: Papers appearing in the literature of approximately the last decade reporting modeling approaches applicable to anticancer drug discovery have been listed and commented. Papers were selected based on the interest in the proposed methodology or in its application. Expert opinion: The number of modeling approaches used in the discovery of anticancer drugs is consistently increasing and new models are developed based on the current directions of research of new candidate drugs. These approaches have contributed to a better understanding of new oncological targets and have allowed for the exploitation of the relatively sparse information generated by preclinical experiments. In addition, they are used in translational approaches for guiding and supporting the choice of dosing regimens in early clinical development.

Antimicrobial Effects of β-Lactams on Imipenem-Resistant Ceftazidime-Susceptible Pseudomonas aeruginosa

We studied the resistance mechanism and antimicrobial effects of β-lactams on imipenem-resistant Pseudomonas aeruginosa isolates that were susceptible to ceftazidime as detected by time-kill curve methods. Among 215 P. aeruginosa isolates from hospitalized patients in eight hospitals in the Republic of Korea, 18 isolates (23.4% of 77 imipenem-resistant isolates) were imipenem resistant and ceftazidime susceptible. Multilocus sequence typing revealed diverse genotypes, which indicated independent emergence. These 18 isolates were negative for carbapenemase genes. All 18 imipenem-resistant ceftazidime-susceptible isolates showed decreased mRNA expression of oprD, and overexpression of mexB was observed in 13 isolates. In contrast, overexpression of ampC, mexD, mexF, or mexY was rarely found. Time-kill curve methods were applied to three selected imipenem-resistant ceftazidime-susceptible isolates at a standard inoculum (5 × 10⁵ CFU/ml) or at a high inoculum (5 × 10⁷ CFU/ml) to evaluate the antimicrobial effects of β-lactams. Inoculum effects were detected for all three β-lactam antibiotics, ceftazidime, cefepime, and piperacillin-tazobactam, against all three isolates. The antibiotics had significant killing effects in the standard inoculum, but no effects in the high inoculum were observed. Our results suggest that β-lactam antibiotics should be used with caution in patients with imipenem-resistant ceftazidime-susceptible P. aeruginosa infection, especially in high-inoculum infections such as endocarditis and osteomyelitis.

Substantial Impact of Altered Pharmacokinetics in Critically Ill Patients on the Antibacterial Effects of Meropenem Evaluated via the Dynamic Hollow-Fiber Infection Model

Critically ill patients frequently have substantially altered pharmacokinetics compared to non-critically ill patients. We investigated the impact of pharmacokinetic alterations on bacterial killing and resistance for commonly used meropenem dosing regimens. A Pseudomonas aeruginosa isolate (MIC_meropenem 0.25 mg/liter) was studied in the hollow-fiber infection model (inoculum ∼10^7.5 CFU/ml; 10 days). Pharmacokinetic profiles representing critically ill patients with augmented renal clearance (ARC), normal, or impaired renal function (creatinine clearances of 285, 120, or ∼10 ml/min, respectively) were generated for three meropenem regimens (2, 1, and 0.5 g administered as 8-hourly 30-min infusions), plus 1 g given 12 hourly with impaired renal function. The time course of total and less-susceptible populations and MICs were determined. Mechanism-based modeling (MBM) was performed using S-ADAPT. All dosing regimens across all renal functions produced similar initial bacterial killing (≤∼2.5 log₁₀). For all regimens subjected to ARC, regrowth occurred after 7 h. For normal and impaired renal function, bacterial killing continued until 23 to 47 h; regrowth then occurred with 0.5- and 1-g regimens with normal renal function (fT_>5×MIC = 56 and 69%, fC_min/MIC < 2); the emergence of less-susceptible populations (≥32-fold increases in MIC) accompanied all regrowth. Bacterial counts remained suppressed across 10 days with normal (2-g 8-hourly regimen) and impaired (all regimens) renal function (fT_>5×MIC ≥ 82%, fC_min/MIC ≥ 2). The MBM successfully described bacterial killing and regrowth for all renal functions and regimens simultaneously. Optimized dosing regimens, including extended infusions and/or combinations, supported by MBM and Monte Carlo simulations, should be evaluated in the context of ARC to maximize bacterial killing and suppress resistance emergence.

Influence of rhlR and lasR on Polymyxin Pharmacodynamics in Pseudomonas aeruginosa and Implications for Quorum Sensing Inhibition with Azithromycin

The impact of quorum sensing on polymyxin and azithromycin pharmacodynamics was assessed in Pseudomonas aeruginosa PAO1 and an isogenic rhlR/lasR double knockout. For polymyxin B, greater killing against the rhlR/lasR knockout than against PAO1 was observed at 10⁸ CFU/ml (polymyxin B half-maximal effective concentration [EC₅₀], 5.61 versus 12.5 mg/liter, respectively; P < 0.005). Polymyxin B combined with azithromycin (256 mg/liter) was synergistic against each strain, significantly reducing the respective polymyxin B EC₅₀ compared to those with monotherapy (P < 0.005), and is a promising strategy by which to combat P. aeruginosa.

Optimization of Synergistic Combination Regimens against Carbapenem- and Aminoglycoside-Resistant Clinical Pseudomonas aeruginosa Isolates via Mechanism-Based Pharmacokinetic/Pharmacodynamic Modeling

Optimizing antibiotic combinations is promising to combat multidrug-resistant Pseudomonas aeruginosa This study aimed to systematically evaluate synergistic bacterial killing and prevention of resistance by carbapenem and aminoglycoside combinations and to rationally optimize combination dosage regimens via a mechanism-based mathematical model (MBM). We studied monotherapies and combinations of imipenem with tobramycin or amikacin against three difficult-to-treat double-resistant clinical P. aeruginosa isolates. Viable-count profiles of total and resistant populations were quantified in 48-h static-concentration time-kill studies (inoculum, 107.5 CFU/ml). We rationally optimized combination dosage regimens via MBM and Monte Carlo simulations against isolate FADDI-PA088 (MIC of imipenem [MICimipenem] of 16 mg/liter and MICtobramycin of 32 mg/liter, i.e., both 98th percentiles according to the EUCAST database). Against this isolate, imipenem (1.5× MIC) combined with 1 to 2 mg/liter tobramycin (MIC, 32 mg/liter) or amikacin (MIC, 4 mg/liter) yielded ≥2-log10 more killing than the most active monotherapy at 48 h and prevented resistance. For all three strains, synergistic killing without resistance was achieved by ≥0.88× MICimipenem in combination with a median of 0.75× MICtobramycin (range, 0.032× to 2.0× MICtobramycin) or 0.50× MICamikacin (range, 0.25× to 0.50× MICamikacin). The MBM indicated that aminoglycosides significantly enhanced the imipenem target site concentration up to 3-fold; achieving 50% of this synergistic effect required aminoglycoside concentrations of 1.34 mg/liter (if the aminoglycoside MIC was 4 mg/liter) and 4.88 mg/liter (for MICs of 8 to 32 mg/liter). An optimized combination regimen (continuous infusion of imipenem at 5 g/day plus a 0.5-h infusion with 7 mg/kg of body weight tobramycin) was predicted to achieve >2.0-log10 killing and prevent regrowth at 48 h in 90.3% of patients (median bacterial killing, >4.0 log10 CFU/ml) against double-resistant isolate FADDI-PA088 and therefore was highly promising.

Aggregation Effects and Population-Based Dynamics as a Source of Therapy Resistance in Cancer

OBJECTIVE

Evolution of resistance allows cancer cells to adapt and continue proliferating even when therapy is initially very effective. Most investigations of treatment resistance focus on the adaptive phenotypic properties of individual cells. We propose that the resistance of a single cell to therapy may extend beyond its own phenotypic and molecular properties and be influenced by the phenotypic properties of surrounding cells and variations in cell density. Similar variation exists in population densities of animals living in groups and can significantly affect the outcome of an external threat.

METHODS

We investigate aggregation effects in cancer therapy using Darwinian models that integrate phenotypic properties of individual cells and common population effects found in nature to simulate the dynamics of resistance and sensitivity in the diverse cellular environments within cancers.

RESULTS

We demonstrate that the density of cancer cell populations can profoundly influence response to chemotherapy independent of the properties of individual cells. Most commonly, these aggregation effects benefit the tumor allowing cells to survive even with phenotypic properties that would render them highly vulnerable to therapy in the absence of population effects.

CONCLUSION

We demonstrate aggregation effects likely play a significant role in conferring resistance to therapy on tumor cells that would otherwise be sensitive to treatment.

SIGNIFICANCE

The potential role of aggregation in outcomes from cancer therapy has not been previously investigated. Our results demonstrate these dynamics may play a key role in resistance to therapy and could be used to design evolutionarily-enlightened therapies that exploit aggregation effects to improve treatment outcomes.

Translational PK/PD of anti-infective therapeutics

Translational PK/PD modeling has emerged as a critical technique for quantitative analysis of the relationship between dose, exposure and response of antibiotics. By combining model components for pharmacokinetics, bacterial growth kinetics and concentration-dependent drug effects, these models are able to quantitatively capture and simulate the complex interplay between antibiotic, bacterium and host organism. Fine-tuning of these basic model structures allows to further account for complicating factors such as resistance development, combination therapy, or host responses. With this tool set at hand, mechanism-based PK/PD modeling and simulation allows to develop optimal dosing regimens for novel and established antibiotics for maximum efficacy and minimal resistance development.

Native valve Proteus mirabilis endocarditis: successful treatment of a rare entity formulated by in vitro synergy antibiotic testing

Infective endocarditis caused by Proteus mirabilis is a rare and poorly reported disease, with no well-defined effective antibiotic regimen. Here, we present a case of P. mirabilis aortic valve endocarditis. We reviewed prior cases and treatment regimens, and devised effective treatment, which was guided by in vitro sensitivity and synergy testing on the pathogen. Our patient survived without complications or the need for a surgical intervention.

High-intensity meropenem combinations with polymyxin B: new strategies to overcome carbapenem resistance in Acinetobacter baumannii

OBJECTIVES

The pharmacodynamics of polymyxin/carbapenem combinations against carbapenem-resistant Acinetobacter baumannii (CRAB) are largely unknown. Our objective was to determine whether intensified meropenem regimens in combination with polymyxin B enhance killing and resistance suppression of CRAB.

METHODS

Time-kill experiments for meropenem and polymyxin B combinations were conducted against three polymyxin B-susceptible (MIC of polymyxin B = 0.5 mg/L) CRAB strains with varying meropenem MICs (ATCC 19606, N16870 and 03-149-1; MIC of meropenem = 4, 16 and 64 mg/L, respectively) at 10⁸ cfu/mL. A hollow-fibre infection model was then used to simulate humanized regimens of polymyxin B and meropenem (2, 4, 6 and 8 g prolonged infusions every 8 h) versus N16870 at 10⁸ cfu/mL over 14 days. New mathematical mechanism-based models were developed using S-ADAPT.

RESULTS

Time-kill experiments were well described by the mathematical mechanism-based models, with the presence of polymyxin B drastically decreasing the meropenem concentration needed for half-maximal activity against meropenem-resistant populations from 438 to 82.1 (ATCC 19606), 158 to 93.6 (N16870) and 433 to 76.0 mg/L (03-149-1). The maximum killing effect of combination treatment was similar among all three strains despite divergent meropenem MIC values (E_max = 2.13, 2.08 and 2.15; MIC of meropenem = 4, 16 and 64 mg/L, respectively). Escalating the dose of meropenem in hollow-fibre combination regimens from 2 g every 8 h to 8 g every 8 h resulted in killing that progressed from a >2.5 log₁₀ cfu/mL reduction with regrowth by 72 h (2 g every 8 h) to complete eradication by 336 h (8 g every 8 h).

CONCLUSION

Intensified meropenem dosing in combination with polymyxin B may offer a unique strategy to kill CRAB irrespective of the meropenem MIC.

Resistance suppression by high-intensity, short-duration aminoglycoside exposure against hypermutable and non-hypermutable Pseudomonas aeruginosa

OBJECTIVES

Hypermutable bacteria are causing a drastic problem via their enhanced ability to become resistant. Our objectives were to compare bacterial killing and resistance emergence between differently shaped tobramycin concentration-time profiles at a given fAUC/MIC, and determine the tobramycin exposure durations that prevent resistance.

METHODS

Static concentration time-kill studies over 24 h used Pseudomonas aeruginosa WT strains (ATCC 27853 and PAO1) and hypermutable PAOΔmutS. fAUC/MIC values of 36, 72 and 168 were assessed at initial inocula of 10⁶ and 10⁴ cfu/mL (all strains) and 10^1.2 cfu/mL (PAOΔmutS only) in duplicate. Tobramycin was added at 0 h and removed at 1, 4, 10 or 24 h. Proportions of resistant bacteria and MICs were determined at 24 h. Mechanism-based modelling was conducted.

RESULTS

For all strains, high tobramycin concentrations over 1 and 4 h resulted in more rapid and extensive initial killing compared with 10 and 24 h exposures at a given fAUC/MIC. No resistance emerged for 1 and 4 h durations of exposure, although extensive regrowth of susceptible bacteria occurred. The 24 h duration of exposure revealed less regrowth, but tobramycin-resistant populations had completely replaced susceptible bacteria by 24 h for the 10⁶ cfu/mL inoculum. The hypermutable PAOΔmutS showed the highest numbers of resistant bacteria. Total and resistant bacterial counts were described well by novel mechanism-based modelling.

CONCLUSIONS

Extensive resistance emerged for 10 and 24 h durations of exposure, but not for shorter durations. The tobramycin concentration-time profile shape is vital for resistance prevention and should aid the introduction of optimized combination regimens.

Polymyxin B in combination with doripenem against heteroresistant Acinetobacter baumannii: pharmacodynamics of new dosing strategies

OBJECTIVES

Polymyxin B is being increasingly utilized as a last resort against resistant Gram-negative bacteria. We examined the pharmacodynamics of novel dosing strategies for polymyxin B combinations to maximize efficacy and minimize the emergence of resistance and drug exposure against Acinetobacter baumannii.

METHODS

The pharmacodynamics of polymyxin B together with doripenem were evaluated in time-kill experiments over 48 h against 10⁸ cfu/mL of two polymyxin-heteroresistant A. baumannii isolates (ATCC 19606 and N16870). Pharmacokinetic/pharmacodynamic relationships were mathematically modelled using S-ADAPT. A hollow-fibre infection model (HFIM) was also used to simulate clinically relevant polymyxin B dosing strategies (traditional, augmented 'front-loaded' and 'burst' regimens), together with doripenem, against an initial inoculum of 10⁹ cfu/mL of ATCC 19606.

RESULTS

In static time-kill studies, polymyxin B concentrations >4 mg/L in combination with doripenem 25 mg/L resulted in rapid bactericidal activity against both strains with undetectable bacterial counts by 24 h. The mathematical model described the rapid, concentration-dependent killing as subpopulation and mechanistic synergy. In the HFIM, the traditional polymyxin B combination regimen was synergistic, with a >7.5 log₁₀ reduction by 48 h. The polymyxin B 'front-loaded' combination resulted in more rapid and extensive initial killing (>8 log₁₀) within 24 h, which was sustained over 10 days. With only 25% of the cumulative drug exposure, the polymyxin B 'burst' combination demonstrated antibacterial activity similar to traditional and 'front-loaded' combination strategies. The polymyxin B 'front-loaded' and 'burst' combination regimens suppressed the emergence of resistance.

CONCLUSIONS

Early aggressive dosing regimens for polymyxin combinations demonstrate promise for treatment of heteroresistant A. baumannii infections.

A pharmacokinetic-pharmacodynamic model characterizing the emergence of resistant Escherichia coli subpopulations during ertapenem exposure

OBJECTIVES

Resistant subpopulations with reduced expression of outer membrane porins have been observed in ESBL-producing Escherichia coli during exposure to ertapenem. The aim of this work was to develop a pharmacokinetic-pharmacodynamic (PKPD) model to characterize the emergence of resistant E. coli during exposure to ertapenem and to predict bacterial killing following different dosing regimens of ertapenem.

METHODS

Data from in vitro time-kill experiments were used to develop a mechanism-based PKPD model for three E. coli strains: a native strain, an ESBL-producing strain, and an ESBL-producing strain with reduced expression of porins OmpF and OmpC. Each strain was exposed to static ertapenem concentrations (1-512 × MIC) for 24 h using starting inocula of ∼10(6) and 10(8) cfu/mL.

RESULTS

The developed PKPD model consisted of three bacterial states: susceptible growing, less susceptible non-growing, and non-susceptible non-growing bacteria. A pre-existing bacterial subpopulation was used to describe the emergence of resistance. The PKPD model adequately characterized the data of the three E. coli strains investigated. Results from predictions suggest that the conventional dosage (1 g intravenously once daily) might result in regrowth of resistant subpopulations when used to treat infection caused by ESBL-producing strains.

CONCLUSIONS

Resistant subpopulations frequently emerged in E. coli when exposed to ertapenem, supporting that the time course of emergence of resistance should be taken into consideration when selecting dosing regimens.