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

What is the clinical significance of 'heteroresistance' in nonfermenting Gram-negative strains?

PURPOSE OF REVIEW

The aim of this study was to discuss the potential clinical significance of heteroresistance in nonfermenting Gram-negative bacilli (GNB).

RECENT FINDINGS

Recently, heteroresistance has been considered potentially responsible for clinical failure in Acinetobacter baumannii infections. This raised a scientific debate, still open, about the potential clinical significance of heteroresistance in nonfermenting GNB.

SUMMARY

We reviewed the literature of last 20 years and found a limited number of studies evaluating the relationship between heteroresistance and clinical outcome in nonfermenting GNB. Unlike Gram-positive bacteria, heteroresistance is reported in a significant proportion of nonfermenting GNB with some studies describing it in all tested strains and for several antibiotics (including tigecycline, carbapenems, levofloxacin, cefiderocol, colistin). One important issue is the need for validated detection method since the population analysis profile test, that is considered the gold standard, requires high costs and time. Studies evaluating the correlation between heteroresistance and clinical outcome are contrasting and have several limitations. Although in-vitro detection of heteroresistance in nonfermenting GNB has not been associated with in-vivo treatment failure, its presence may suggest to prefer combination regimens instead monotherapy when treating infections by nonfermenters. Further studies are needed to clarify the clinical significance of heteroresistance.

Ceftolozane/tazobactam plus tobramycin against free-floating and biofilm bacteria of hypermutable Pseudomonas aeruginosa epidemic strains: Resistance mechanisms and synergistic activity

OBJECTIVE

Acute exacerbations of biofilm-associated Pseudomonas aeruginosa infections in cystic fibrosis (CF) have limited treatment options. Ceftolozane/tazobactam (alone and with a second antibiotic) has not yet been investigated against hypermutable clinical P. aeruginosa isolates in biofilm growth. This study aimed to evaluate, using an in vitro dynamic biofilm model, ceftolozane/tazobactam alone and in combination with tobramycin at simulated representative lung fluid pharmacokinetics against free-floating (planktonic) and biofilm states of two hypermutable P. aeruginosa epidemic strains (LES-1 and CC274) from adolescents with CF.

METHODS

Regimens were intravenous ceftolozane/tazobactam 4.5 g/day continuous infusion, inhaled tobramycin 300 mg 12-hourly, intravenous tobramycin 10 mg/kg 24-hourly, and both ceftolozane/tazobactam-tobramycin combinations. The isolates were susceptible to both antibiotics. Total and less-susceptible free-floating and biofilm bacteria were quantified over 120-168 h. Ceftolozane/tazobactam resistance mechanisms were investigated by whole-genome sequencing. Mechanism-based modelling of bacterial viable counts was performed.

RESULTS

Monotherapies of ceftolozane/tazobactam and tobramycin did not sufficiently suppress emergence of less-susceptible subpopulations, although inhaled tobramycin was more effective than intravenous tobramycin. Ceftolozane/tazobactam resistance development was associated with classical (AmpC overexpression plus structural modification) and novel (CpxR mutations) mechanisms depending on the strain. Against both isolates, combination regimens demonstrated synergy and completely suppressed the emergence of ceftolozane/tazobactam and tobramycin less-susceptible free-floating and biofilm bacterial subpopulations.

CONCLUSION

Mechanism-based modelling incorporating subpopulation and mechanistic synergy well described the antibacterial effects of all regimens against free-floating and biofilm bacterial states. These findings support further investigation of ceftolozane/tazobactam in combination with tobramycin against biofilm-associated P. aeruginosa infections in adolescents with CF.

Serendipitous identification of phenylhydrazine derivatives as potent inhibitors of carbapenem-resistant Acinetobacter baumannii

Background: In the authors' previous study, 4-(2-((3-methyl-4-oxo-2-thioxo/dioxothiazolidin-5-ylidene) methyl) hydrazineyl) benzonitriles were found to demonstrate potent antibacterial activity against Acinetobacter baumannii. Interestingly, the aforementioned compounds contain a 4-cyanophenylhydrazine motif. Materials & methods: Intrigued by this observation, the authors focused on preparing a library of 4-cyanophenylhydrazine derivatives and studied their detailed antibacterial potential. Results: This study led to the identification of a 4-cyanophenylhydrazine with potent inhibitory activity against carbapenem-resistant A. baumannii BAA-1605, with minimum inhibitory concentration (MIC) of 0.25 μg/ml and highest selectivity index of 640. The compound also demonstrated potent inhibition against multidrug-resistant A. baumannii isolates (MIC: 0.25-1 μg/ml). Conclusion: The identified 4-cyanophenylhydrazine compound exhibited synergistic activity with amikacin, tobramycin and polymyxin B against carbapenem-resistant A. baumannii BAA-1605.

Effects of amikacin, polymyxin-B, and sulbactam combination on the pharmacodynamic indices of mutant selection against multi-drug resistant Acinetobacter baumannii

Amikacin and polymyxins as monotherapies are ineffective against multidrug-resistant Acinetobacter baumannii at the clinical dose. When polymyxins, aminoglycosides, and sulbactam are co-administered, the combinations exhibit in vitro synergistic activities. The minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) were determined in 11 and 5 clinical resistant isolates of A. baumannii harboring OXA-23, respectively, in order to derive the fraction of time over the 24-h wherein the free drug concentration was within the mutant selection window (fT_MSW) and the fraction of time that the free drug concentration was above the MPC (fT_>MPC) from simulated pharmacokinetic profiles. The combination of these three antibiotics can confer susceptibility in multi-drug resistant A. baumannii and reduce the opportunity for bacteria to develop further resistance. Clinical intravenous dosing regimens of amikacin, polymyxin-B, and sulbactam were predicted to optimize fT_MSW and fT_>MPC from drug exposures in the blood. Mean fT_>MPC were ≥ 60% and ≥ 80% for amikacin and polymyxin-B, whereas mean fT_MSW was reduced to <30% and <15%, respectively, in the triple antibiotic combination. Due to the low free drug concentration of amikacin and polymyxin-B simulated in the epithelial lining fluid, the two predicted pharmacodynamic parameters in the lung after intravenous administration were not optimal even in the combination therapy setting.

Infections Due to Acinetobacter baumannii-calcoaceticus Complex: Escalation of Antimicrobial Resistance and Evolving Treatment Options

Bacteria within the genus Acinetobacter (principally A. baumannii-calcoaceticus complex [ABC]) are gram-negative coccobacilli that most often cause infections in nosocomial settings. Community-acquired infections are rare, but may occur in patients with comorbidities, advanced age, diabetes mellitus, chronic lung or renal disease, malignancy, or impaired immunity. Most common sites of infections include blood stream, skin/soft-tissue/surgical wounds, ventilator-associated pneumonia, orthopaedic or neurosurgical procedures, and urinary tract. Acinetobacter species are intrinsically resistant to multiple antimicrobials, and have a remarkable ability to acquire new resistance determinants via plasmids, transposons, integrons, and resistance islands. Since the 1990s, antimicrobial resistance (AMR) has escalated dramatically among ABC. Global spread of multidrug-resistant (MDR)-ABC strains reflects dissemination of a few clones between hospitals, geographic regions, and continents; excessive antibiotic use amplifies this spread. Many isolates are resistant to all antimicrobials except colistimethate sodium and tetracyclines (minocycline or tigecycline); some infections are untreatable with existing antimicrobial agents. AMR poses a serious threat to effectively treat or prevent ABC infections. Strategies to curtail environmental colonization with MDR-ABC require aggressive infection-control efforts and cohorting of infected patients. Thoughtful antibiotic strategies are essential to limit the spread of MDR-ABC. Optimal therapy will likely require combination antimicrobial therapy with existing antibiotics as well as development of novel antibiotic classes.

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.

A polytherapy based approach to combat antimicrobial resistance using cubosomes

A depleted antimicrobial drug pipeline combined with an increasing prevalence of Gram-negative ‘superbugs’ has increased interest in nano therapies to treat antibiotic resistance. As cubosomes and polymyxins disrupt the outer membrane of Gram-negative bacteria via different mechanisms, we herein examine the antimicrobial activity of polymyxin-loaded cubosomes and explore an alternative strategy via the polytherapy treatment of pathogens with cubosomes in combination with polymyxin. The polytherapy treatment substantially increases antimicrobial activity compared to polymyxin B-loaded cubosomes or polymyxin and cubosomes alone. Confocal microscopy and neutron reflectometry suggest the superior polytherapy activity is achieved via a two-step process. Firstly, electrostatic interactions between polymyxin and lipid A initially destabilize the outer membrane. Subsequently, an influx of cubosomes results in further membrane disruption via a lipid exchange process. These findings demonstrate that nanoparticle-based polytherapy treatments may potentially serve as improved alternatives to the conventional use of drug-loaded lipid nanoparticles for the treatment of “superbugs”.

An increasing prevalence of Gram-negative bacteria increases the interest in nanotherapies to treat antibiotic resistance. Here, the authors examine the antimicrobial activity of polymyxin-loaded cubosomes and explore a polytherapy treatment of pathogens with cubosomes in combination with polymyxin.

Treatment of patients with serious infections due to carbapenem-resistant Acinetobacter baumannii: How viable are the current options?

This review critically appraises the published microbiologic and clinical data on the treatment of patients with carbapenem-resistant Acinetobacter baumannii infections. Despite being recognized as an urgent threat pathogen by the CDC and WHO, optimal treatment of patients with serious CRAB infections remains ill-defined. Few commercially available agents exhibit reliable in vitro activity against CRAB. Historically, polymyxins have been the most active agents in vitro, though interpretations of susceptibility data are difficult given issues surrounding MIC testing methodologies and lack of correlation between MICs and clinical outcomes. Most available preclinical and clinical data involve use of polymyxins, tetracyclines, and sulbactam, alone and in combination. As the number of viable treatment options is limited, combination therapy with a polymyxin is often used for patients with CRAB infections, despite the significant risk of nephrotoxicity. However, no treatment regimen has been found to reduce mortality, which exceeds 40% across most studies, or substantially improve clinical response. While some newer agents, such as eravacycline and cefiderocol, have demonstrated in vitro activity, clinical efficacy has not been fully established. New agents with clinically relevant activity against CRAB isolates and favorable toxicity profiles are sorely needed.

Losing the Battle but Winning the War: Can Defeated Antibacterials Form Alliances to Combat Drug-Resistant Pathogens?

Despite the recent development of antibacterials that are active against multidrug-resistant pathogens, drug combinations are often necessary to optimize the killing of difficult-to-treat organisms. Antimicrobial combinations typically are composed of multiple agents that are active against the target organism; however, many studies have investigated the potential utility of combinations that consist of one or more antibacterials that individually are incapable of killing the relevant pathogen. The current review summarizes in vitro, in vivo, and clinical studies that evaluate combinations that include at least one drug that is not active individually against Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumannii, or Staphylococcus aureus. Polymyxins were often included in combinations against all three of the Gram-negative pathogens, and carbapenems were commonly incorporated into combinations against K. pneumoniae and A. baumannii. Minocycline, sulbactam, and rifampin were also frequently investigated in combinations against A. baumannii, whereas the addition of ceftaroline or another β-lactam to vancomycin or daptomycin showed promise against S. aureus with reduced susceptibility to vancomycin or daptomycin. Although additional clinical studies are needed to define the optimal combination against specific drug-resistant pathogens, the large amount of in vitro and in vivo studies available in the literature may provide some guidance on the rational design of antibacterial combinations.

Antibiotic pharmacokinetic/pharmacodynamic modelling: MIC, pharmacodynamic indices and beyond

The dramatic increase in antimicrobial resistance and the limited pharmacological treatment options highlight the urgent need to optimize therapeutic regimens of new and available anti-infectives. Several in-vitro and in-vivo infection models are employed to understand the relationship between drug exposure profiles in plasma or at the site of infection (pharmacokinetics) and the time course of therapeutic response (pharmacodynamics) to select and optimize dosage regimens for new and approved drugs. Well-designed preclinical studies, combined with mathematical-model-based pharmacokinetic/pharmacodynamic analysis and in-silico simulations, are critical for the effective translation of preclinical data and design of appropriate and successful clinical trials. Integration with population pharmacokinetic modelling and simulations allows for the incorporation of interindividual variability that occurs in both pharmacokinetics and pharmacodynamics, and helps to predict the probability of target attainment and treatment outcome in patients. This article reviews the role of pharmacokinetic/pharmacodynamic approaches in the optimization of dosage regimens to maximize antibacterial efficacy while minimizing toxicity and emergence of resistance, and to achieve a high likelihood of therapeutic success. Polymyxin B, an approved drug with a narrow therapeutic window, serves as an illustrative example to highlight the importance of pharmacokinetic/pharmacodynamic modelling in conjunction with experimentation, employing static time-kill studies followed by dynamic in-vitro or in-vivo models, or both, to learn and confirm mechanistic insights necessary for translation to the bedside.

Systematic review and meta-analysis of in vitro efficacy of antibiotic combination therapy against carbapenem-resistant Gram-negative bacilli

The superiority of combination therapy for carbapenem-resistant Gram-negative bacilli (CR-GNB) infections remains controversial. In vitro models may predict the efficacy of antibiotic regimens against CR-GNB. A systematic review and meta-analysis was performed including pharmacokinetic/pharmacodynamic (PK/PD) and time-kill (TK) studies examining the in vitro efficacy of antibiotic combinations against CR-GNB [PROSPERO registration no. CRD42019128104]. The primary outcome was in vitro synergy based on the effect size (ES): high, ES ≥ 0.75, moderate, 0.35 < ES < 0.75; low, ES ≤ 0.35; and absent, ES = 0). A network meta-analysis assessed the bactericidal effect and re-growth rate (secondary outcomes). An adapted version of the ToxRTool was used for risk-of-bias assessment. Over 180 combination regimens from 136 studies were included. The most frequently analysed classes were polymyxins and carbapenems. Limited data were available for ceftazidime/avibactam, ceftolozane/tazobactam and imipenem/relebactam. High or moderate synergism was shown for polymyxin/rifampicin against Acinetobacter baumannii [ES = 0.91, 95% confidence interval (CI) 0.44-1.00], polymyxin/fosfomycin against Klebsiella pneumoniae (ES = 1.00, 95% CI 0.66-1.00) and imipenem/amikacin against Pseudomonas aeruginosa (ES = 1.00, 95% CI 0.21-1.00). Compared with monotherapy, increased bactericidal activity and lower re-growth rates were reported for colistin/fosfomycin and polymyxin/rifampicin in K. pneumoniae and for imipenem/amikacin or imipenem/tobramycin against P. aeruginosa. High quality was documented for 65% and 53% of PK/PD and TK studies, respectively. Well-designed in vitro studies should be encouraged to guide the selection of combination therapies in clinical trials and to improve the armamentarium against carbapenem-resistant bacteria.

Interaction of Staphylococcus aureus and Acinetobacter baumannii during In Vitro β-Lactam Exposure

We sought to determine if Acinetobacter baumannii is capable of altering the pharmacodynamics of an antistaphylococcal β-lactam. Two strains of methicillin-susceptible Staphylococcus aureus (MSSA) and two A. baumannii isolates were studied in 24-h static time-killing experiments under monoculture or coculture conditions. Bacterial killing of meropenem was described using an empirical pharmacokinetics/pharmacodynamics model that was developed using Hill functions. A mechanism-based pharmacodynamic model was also used to describe the effect of meropenem on each species of bacterium, interspecies interactions, and strain-based covariate effects. Monte Carlo simulations of bacterial killing effects were generated based on the population pharmacokinetics of meropenem in 2,500 simulated critically ill subjects over 48 h. Against one of the two MSSA isolates, the magnitude of bacterial killing (E _Δ) decreased from -4.61 (95% confidence interval [CI], -5.85 to -3.38) to -2.23 (95% CI, -2.85 to -1.61) when cultured in the presence of carbapenem-resistant A. baumannii (CRAB). Similarly, the data were best described by a mechanism-based model where the number of A. baumannii cells produced a systematic increase in the S. aureus concentration for a 50% maximum killing effect (KC₅₀) of 3.53-fold, thereby decreasing MSSA sensitivity to meropenem. A covariate effect by the CRAB isolate resulted in a more pronounced increase in the MSSA KC₅₀ for meropenem (31.8-fold increase). However, Monte Carlo simulations demonstrated that a high-intensity meropenem regimen is capable of sustained killing against both MSSA isolates despite protection from A. baumannii Thus, A. baumannii and MSSA engage in complex interactions during β-lactam exposure, but optimal antimicrobial dosing is likely capable of killing MSSA despite the potentially beneficial interplay with A. baumannii.

Evaluation Strategies for Triple-Drug Combinations against Carbapenemase-Producing Klebsiella Pneumoniae in an In Vitro Hollow-Fiber Infection Model

Mounting antimicrobial resistance to carbapenemase‐producing Klebsiella pneumoniae (CPKP) highlights the need to optimize currently available treatment options. The objective of this study was to explore alternative dosing strategies that limit the emergence of resistance to preserve the utility of last‐line antibiotics by: (i) evaluating the pharmacodynamic (PD) killing activity of simulated humanized exposures to monotherapy and two‐drug and three‐drug combinations against CPKP bacterial isolates with different resistance mechanisms; and (ii) optimizing polymyxin B (PMB) exposure simulated in the three‐drug combination regimens to maximize the killing activity. Two CPKP clinical isolates (BAA2146 (NDM‐1) and BRKP76 (KPC‐2)) were evaluated over 168 hours using a hollow‐fiber infection model simulating clinically relevant PMB, fosfomycin, and meropenem dosing regimens. PMB‐based three‐drug combinations were further optimized by varying the initial exposure (0–24 hours) or maintenance dose received over the duration of treatment. The area under the bacterial load‐versus‐time curve (AUCFU) was used to determine PD activity. Overall reductions in PMB exposure ranged from 2 to 84%. BAA2146 and BRKP76 had median (range) AUCFUs of 11.0 (10.6–11.6) log₁₀ CFU hour/mL and 7.08 (7.04–11.9) log₁₀ CFU hour/mL, respectively. The PMB “front loaded” 2.5 mg/kg/day + 0.5 mg/kg maintenance dose in combination with meropenem and fosfomycin was a promising regimen against BRKP76, with an overall reduction in PMB exposure of 56% while still eradicating the bacteria. Tailored triple‐combination therapy allows for the optimization of dose and treatment duration of last‐line agents like PMB to achieve adequate drug exposure and appropriate PD activity while minimizing the emergence of resistance.

Combatting the Rising Tide of Antimicrobial Resistance: Pharmacokinetic/Pharmacodynamic Dosing Strategies for Maximal Precision

OBJECTIVE

Antimicrobial pharmacokinetics/pharmacodynamics (PK/PD) principles and PK/PD models have been essential in characterizing the mechanism of antibiotic bacterial killing and determining the most optimal dosing regimen that maximizes clinical outcomes. This review summarized the fundamentals of antimicrobial PK/PD and the various types of PK/PD experiments that shaped the utilization and dosing strategies of antibiotics today.

METHODS

Multiple databases - including PubMed, Scopus, and EMBASE - were searched for published articles that involved PK/PD modelling and precision dosing. Data from in vitro, in vivo and mechanistic PK/PD models were reviewed as a basis for compiling studies that guide dosing regimens used in clinical trials.

RESULTS

Literature regarding the utilization of exposure-response analyses, mathematical modelling and simulations that were summarized are able to provide a better understanding of antibiotic pharmacodynamics that influence translational drug development. Optimal pharmacokinetic sampling of antibiotics from patients can lead to personalized dosing regimens that attain target concentrations while minimizing toxicity. Thus the development of a fully integrated mechanistic model based on systems pharmacology can continually adapt to data generated from clinical responses, which can provide the framework for individualized dosing regimens.

CONCLUSIONS

The promise of what PK/PD can provide through precision dosing for antibiotics has not been fully realized in the clinical setting. Antimicrobial resistance, which has emerged as a significant public health threat, has forced clinicians to empirically utilize therapies. Future research focused on implementation and translation of PK/PD-based approaches integrating novel approaches that combine knowledge of combination therapies, systems pharmacology and resistance mechanisms are necessary. To fully realize maximally precise therapeutics, optimal PK/PD strategies are critical to maximize antimicrobial efficacy against extremely-drug-resistant organisms, while minimizing toxicity.

Synergistic antibacterial effect of inhaled aztreonam and tobramycin fixed dose combination to combat multidrug-resistant Gram-negative bacteria

The limited therapeutic option for respiratory infections caused by multi-drug resistant microbial pathogens is a major global health threat. Topical delivery of antibacterial combinations to the lung could dramatically enhance antibacterial activities and provide a means to overcome bacterial resistance development. The aim of the study was to investigate the potential of new inhalable dry powder combinations consisting of a fixed dose of aztreonam (Azt) and tobramycin (Tob) using a spray drying process, against antibiotic resistant Gram-negative respiratory pathogens. The interactions of Azt with Tob on resistant Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii were determined by calculating factional inhibitory concentration indices (FICI). A fixed concentration ratio of Azt and Tob that exhibited a synergistic antimicrobial effect was selected and formulated into inhalable dry powders by co-spray drying with and without L-leucine. The obtained dry powders were characterized with respect to the morphology, particle size distribution, solid state, moisture sorption behaviour, and in vitro dissolution. Storage stability, aerosol performance, and in vitro antibacterial activity were also evaluated. Inhalable dry powders consisting of Azt, Tob and L-leucine could be readily obtained via the spray drying process with a fine particle fraction of above 40% as determined using a next generation impactor. The co-spray drying process resulted in amorphous Azt/Tob dry powders with or without the addition of L-leucine as indicated by X-ray powder diffraction. The dissolution rates of the co-spray dried Azt/Tob dry powders were decreased, and the storage stability was improved with an increase in the proportion of L-leucine in the formulations. The inclusion of L-leucine did not affect the minimum inhibitory concentration and the co-spray dried powders reserved the synergistic antibacterial effects and exhibited enhanced antibacterial activities as compared to the individual antibiotic used alone on multidrug-resistant (Azt and Tob resistant) P. aeruginosa 25756 and A. baumannii K31. This study demonstrates that inhalable Azt/Tob dry powders using L-leucine as a moisture protector as well as a dispersing agent can be readily prepared by the spray drying process. This new inhalable fixed dose combinational dry powders may represent an alternative treatment against multidrug-resistant Gram-negative respiratory pathogens.

Colistin heteroresistance in Acinetobacter spp.: systematic review and meta-analysis of the prevalence and discussion of the mechanisms and potential therapeutic implications

BACKGROUND

Colistin is one of the few remaining options for carbapenem-resistant Acinetobacter baumannii (A. baumannii); however, emergence of resistance from heteroresistant populations is possible. This review aimed to systematically search and consolidate the literature on the prevalence, mechanisms and therapeutic implications of colistin heteroresistance in Acinetobacter spp.

METHODS

A systematic search was conducted in PubMed and Scopus. The pooled prevalence of colistin heteroresistance was calculated using meta-analysis of proportions with the Freeman-Tukey transformation and the random-effects (DerSimonian and Laird) method.

RESULTS

Based on 15 studies the prevalence of colistin heteroresistance was 33% (95% CI 16-53%) but considerable heterogeneity was observed (I² = 96%, P < 0.001). Prior exposure to colistin was associated with a higher proportion of resistant subpopulations. Colistin heteroresistance may result from chromosomal mutations in resistant subpopulations (predominantly in PmrAB and lpx genes) resulting in lipopolysaccharide modification or loss, or overexpression of efflux pumps. No dosage scheme of colistin monotherapy can prevent the emergence of resistant subpopulations in vitro, but few studies have reported in vivo emergence of resistance from heteroresistant A. baumannii during treatment, and studies examining the correlation between heteroresistance and clinical/microbiological outcomes are lacking. Several colistin-based combinations have been shown in vitro to prevent the emergence of the resistant subpopulations but none have been translated so far into clinical benefit. Reasons for this discrepancy are discussed.

CONCLUSIONS

Colistin heteroresistance was common but highly variable between studies. The impact of colistin heteroresistance (frequency of emergent resistance during treatment and correlation with treatment outcomes) requires further study.

Acinetobacter baumannii Resistance: A Real Challenge for Clinicians

Acinetobacter baumannii (named in honor of the American bacteriologists Paul and Linda Baumann) is a Gram-negative, multidrug-resistant (MDR) pathogen that causes nosocomial infections, especially in intensive care units (ICUs) and immunocompromised patients with central venous catheters. A. baumannii has developed a broad spectrum of antimicrobial resistance, associated with a higher mortality rate among infected patients compared with other non-baumannii species. In terms of clinical impact, resistant strains are associated with increases in both in-hospital length of stay and mortality. A. baumannii can cause a variety of infections; most involve the respiratory tract, especially ventilator-associated pneumonia, but bacteremia and skin wound infections have also been reported, the latter of which has been prominently observed in the context of war-related trauma. Cases of meningitis associated with A. baumannii have been documented. The most common risk factor for the acquisition of MDR A baumannii is previous antibiotic use, following by mechanical ventilation, length of ICU/hospital stay, severity of illness, and use of medical devices. Current efforts focus on addressing all the antimicrobial resistance mechanisms described in A. baumannii, with the objective of identifying the most promising therapeutic scheme. Bacteriophage- and artilysin-based therapeutic approaches have been described as effective, but further research into their clinical use is required.

Using machine learning to optimize antibiotic combinations: dosing strategies for meropenem and polymyxin B against carbapenem-resistant Acinetobacter baumannii

OBJECTIVES

Increased rates of carbapenem-resistant strains of Acinetobacter baumannii have forced clinicians to rely upon last-line agents, such as the polymyxins, or empirical, unoptimized combination therapy. Therefore, the objectives of this study were: (a) to evaluate the in vitro pharmacodynamics of meropenem and polymyxin B (PMB) combinations against A. baumannii; (b) to utilize a mechanism-based mathematical model to quantify bacterial killing; and (c) to develop a genetic algorithm (GA) to define optimal dosing strategies for meropenem and PMB.

METHODS

A. baumannii (N16870; MIC_meropenem = 16 mg/L, MIC_PMB = 0.5 mg/L) was studied in the hollow-fibre infection model (initial inoculum 10⁸ cfu/mL) over 14 days against meropenem and PMB combinations. A mechanism-based model of the data and population pharmacokinetics of each drug were used to develop a GA to define the optimal regimen parameters.

RESULTS

Monotherapies resulted in regrowth to ~10¹⁰ cfu/mL by 24 h, while combination regimens employing high-intensity PMB exposure achieved complete bacterial eradication (0 cfu/mL) by 336 h. The mechanism-based model demonstrated an SC₅₀ (PMB concentration for 50% of maximum synergy on meropenem killing) of 0.0927 mg/L for PMB-susceptible subpopulations versus 3.40 mg/L for PMB-resistant subpopulations. The GA had a preference for meropenem regimens that improved the %T > MIC via longer infusion times and shorter dosing intervals. The GA predicted that treating 90% of simulated subjects harbouring a 10⁸ cfu/mL starting inoculum to a point of 10⁰ cfu/mL would require a regimen of meropenem 19.6 g/day 2 h prolonged infusion (2 hPI) q5h + PMB 5.17 mg/kg/day 2 hPI q6h (where the 0 h meropenem and PMB doses should be 'loaded' with 80.5% and 42.2% of the daily dose, respectively).

CONCLUSION

This study provides a methodology leveraging in vitro experimental data, a mathematical pharmacodynamic model, and population pharmacokinetics provide a possible avenue to optimize treatment regimens beyond the use of the 'traditional' indices of antibiotic action.

Individualising Therapy to Minimize Bacterial Multidrug Resistance

The scourge of antibiotic resistance threatens modern healthcare delivery. A contributing factor to this significant issue may be antibiotic dosing, whereby standard antibiotic regimens are unable to suppress the emergence of antibiotic resistance. This article aims to review the role of pharmacokinetic and pharmacodynamic (PK/PD) measures for optimising antibiotic therapy to minimise resistance emergence. It also seeks to describe the utility of combination antibiotic therapy for suppression of resistance and summarise the role of biomarkers in individualising antibiotic therapy. Scientific journals indexed in PubMed and Web of Science were searched to identify relevant articles and summarise existing evidence. Studies suggest that optimising antibiotic dosing to attain defined PK/PD ratios may limit the emergence of resistance. A maximum aminoglycoside concentration to minimum inhibitory concentration (MIC) ratio of > 20, a fluoroquinolone area under the concentration-time curve to MIC ratio of > 285 and a β-lactam trough concentration of > 6 × MIC are likely required for resistance suppression. In vitro studies demonstrate a clear advantage for some antibiotic combinations. However, clinical evidence is limited, suggesting that the use of combination regimens should be assessed on an individual patient basis. Biomarkers, such as procalcitonin, may help to individualise and reduce the duration of antibiotic treatment, which may minimise antibiotic resistance emergence during therapy. Future studies should translate laboratory-based studies into clinical trials and validate the appropriate clinical PK/PD predictors required for resistance suppression in vivo. Other adjunct strategies, such as biomarker-guided therapy or the use of antibiotic combinations require further investigation.

Mechanism-Based Disease Progression Model Describing Host-Pathogen Interactions During the Pathogenesis of Acinetobacter baumannii Pneumonia

The emergence of highly resistant bacteria is a serious threat to global public health. The host immune response is vital for clearing bacteria from the infected host; however, the current drug development paradigm does not take host‐pathogen interactions into consideration. Here, we used a systems‐based approach to develop a quantitative, mechanism‐based disease progression model to describe bacterial dynamics, host immune response, and lung injury in an immunocompetent rat pneumonia model. Previously, Long‐Evans rats were infected with Acinetobacter baumannii (A. baumannii) strain 307‐0294 at five different inocula and total lung bacteria, interleukin‐1beta (IL‐1β), tumor necrosis factor‐α (TNF‐α), cytokine‐induced neutrophil chemoattractant 1 (CINC‐1), neutrophil counts, and albumin were quantified. Model development was conducted in ADAPT5 version 5.0.54 using a pooled approach with maximum likelihood estimation; all data were co‐modeled. The final model characterized host‐pathogen interactions during the natural time course of bacterial pneumonia. Parameters were estimated with good precision. Our expandable model will integrate drug effects to aid in the design of optimized antibiotic regimens.

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 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.

Detection of colistin heteroresistance in Acinetobacter baumannii from blood and respiratory isolates

Polymyxins are one of the last-line antibiotics for multidrug-resistant Acinetobacter baumannii. Reports have demonstrated the emergence of colistin heteroresistance in A. baumannii, which can complicate assessment of minimum inhibitory concentrations and promote resistance to colistin. We aimed to determine the presence of colistin heteroresistance in A. baumannii isolates and correlate the results with clinical and microbiological outcomes via a retrospective study of 24 adult patients: 12 blood and 12 invasive respiratory cultures positive for colistin-susceptible A. baumannii between 1 January 2013 and 31 July 2015. Heteroresistance testing was performed by plating a 100-μL bacterial cell suspension on Mueller-Hinton agar plates containing 0, 1, 2, and 4 μg/mL colistin, and assessing for growth at 24 and 48 h. Colistin heteroresistance was exhibited in 83% of isolates. Median age was 56 [43-65] years, 10 (42%) patients resided at a facility prior to admission, 5 (21%) had a chronic tracheostomy, 18 (75%) were in the intensive care unit at the time of culture collection, and median infection-related length of stay was 12 [7-15] days. Clinical and microbiological cures were achieved in 75% of patients. Overall infection-related mortality was 21%. Our study demonstrated a high rate of colistin heteroresistance in clinical isolates of colistin-susceptible A. baumannii, although this was not associated with suboptimal clinical outcomes due to the use of aggressive colistin dosing and combination therapy. Further studies are needed to establish the association between in vitro colistin heteroresistance and clinical and microbiological outcomes.

Evaluation of Activity and Emergence of Resistance of Polymyxin B and ZTI-01 (Fosfomycin for Injection) against KPC-Producing Klebsiella pneumoniae

ZTI-01 (fosfomycin for injection) is a broad-spectrum antibiotic with a novel mechanism of action and is currently under development in the United States for treatment of complicated urinary tract infections. Globally, fosfomycin and polymyxin B are increasingly being used to treat multidrug-resistant Gram-negative infections. The objectives were to evaluate the pharmacodynamic activity of polymyxin B and fosfomycin alone and in combination against KPC-producing Klebsiella pneumoniae and to assess the rate and extent of emergence of resistance to different antibiotic regimens. Two clinical isolates, BRKP26 (MIC of polymyxin B[MIC_PMB], 0.5 mg/liter; MIC of fosfomycin [MIC_FOF], 32 mg/liter) and BRKP67 (MIC_PMB, 8 mg/liter; MIC_FOF, 32 mg/liter) at an initial inoculum of 10⁷ CFU/ml, were evaluated over 168 h in a hollow-fiber infection model simulating clinically relevant polymyxin B (2.5-mg/kg loading dose as a 2 h-infusion followed by 1.5-mg/kg dose every 12 h [q12h] as a 1-h infusion) and fosfomycin (6 g q6h as a 1-h or 3-h infusion) regimens alone and in combination. Population analysis profiles (PAPs) and MIC testing were performed to assess emergence of resistance. Polymyxin B or fosfomycin monotherapy was ineffective and selected for resistance by 24 h. Polymyxin B plus a fosfomycin 1-h infusion demonstrated sustained bactericidal activity by 4 h, with undetectable colony counts beyond 144 h. Polymyxin B plus a fosfomycin 3-h infusion demonstrated bactericidal activity at 4 h, followed by regrowth similar to that of the control by 144 h. PAPs revealed resistant subpopulations by 120 h. The combination of polymyxin B and a fosfomycin 1-h infusion is a promising treatment option for KPC-producing K. pneumoniae and suppresses the emergence of resistance. Further evaluation of novel dosing strategies is warranted to optimize therapy.

Severe Infusion-Related Adverse Events and Renal Failure in Patients Receiving High-Dose Intravenous Polymyxin B

The use of very high doses of polymyxin B (PMB) against carbapenem-resistant Gram-negative bacilli has been addressed in in vitro experiments as a strategy to improve bacterial killing and suppress resistance emergence. However, the toxicities of very high doses in patients are unknown. We conducted a retrospective cohort study assessing patients receiving PMB at >3 mg/kg of body weight/day or a total dose of ≥250 mg/day. The main outcomes were severe infusion-related adverse events according to the Common Terminology Criteria for Adverse Events and the renal failure category of RIFLE criteria for acute kidney injury (AKI) during treatment. A total of 222 patients were included for analysis of infusion-related events. The mean PMB dose was 3.61 ± 0.97 mg/kg/day (median total dose/day = 268 mg). Severe infusion-related adverse events occurred in two patients, resulting in an incidence of 0.9% (95% confidence interval, 0.2 to 3.2%); one was classified as a life-threatening adverse event, and one was classified as a severe adverse event. Renal failure was analyzed in 115 patients, and 25 (21.7%) patients presented renal failure (54 [47.0%] developed any degree of AKI, categorized as risk [27.8%], injury [25.9%], and failure [46.3%]). Treatment with a vasoactive drug, concomitant treatment with nephrotoxic drugs, and baseline creatinine clearance were independent risk factors for renal failure. Neither the PMB daily dose scaled by body weight nor the total daily dose was associated with renal failure. The in-hospital mortality rate was 60% (134 patients): 26% of deaths (57 patients) occurred during treatment, and none occurred during infusion. Our data suggest that high-dose schemes have an acceptable safety profile and could be further tested in clinical trials assessing strategies to improve patient outcomes and minimize the emergence of PMB resistance.

Semi-mechanistic pharmacokinetic-pharmacodynamic modelling of antibiotic drug combinations

BACKGROUND

Deriving suitable dosing regimens for antibiotic combination therapy poses several challenges as the drug interaction can be highly complex, the traditional pharmacokinetic-pharmacodynamic (PKPD) index methodology cannot be applied straightforwardly, and exploring all possible dose combinations is unfeasible. Therefore, semi-mechanistic PKPD models developed based on in vitro single and combination experiments can be valuable to suggest suitable combination dosing regimens.

AIMS

To outline how the interaction between two antibiotics has been characterized in semi-mechanistic PKPD models. We also explain how such models can be applied to support dosing regimens and design future studies.

SOURCES

PubMed search for published semi-mechanistic PKPD models of antibiotic drug combinations.

CONTENT

Thirteen publications were identified where ten had applied subpopulation synergy to characterize the combined effect, i.e. independent killing rates for each drug and bacterial subpopulation. We report the various types of interaction functions that have been used to describe the combined drug effects and that characterized potential deviations from additivity under the PKPD model. Simulations from the models had commonly been performed to compare single versus combined dosing regimens and/or to propose improved dosing regimens.

IMPLICATIONS

Semi-mechanistic PKPD models allow for integration of knowledge on the interaction between antibiotics for various PK and PD profiles, and can account for associated variability within the population as well as parameter uncertainty. Decisions on suitable combination regimens can thereby be facilitated. We find the application of semi-mechanistic PKPD models to be essential for efficient development of antibiotic combination regimens that optimize bacterial killing and/or suppress resistance development.

Biology of Acinetobacter baumannii: Pathogenesis, Antibiotic Resistance Mechanisms, and Prospective Treatment Options

Acinetobacter baumannii is undoubtedly one of the most successful pathogens responsible for hospital-acquired nosocomial infections in the modern healthcare system. Due to the prevalence of infections and outbreaks caused by multi-drug resistant A. baumannii, few antibiotics are effective for treating infections caused by this pathogen. To overcome this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. In this review, we summarize current studies on the virulence factors that contribute to A. baumannii pathogenesis, including porins, capsular polysaccharides, lipopolysaccharides, phospholipases, outer membrane vesicles, metal acquisition systems, and protein secretion systems. Mechanisms of antibiotic resistance of this organism, including acquirement of β-lactamases, up-regulation of multidrug efflux pumps, modification of aminoglycosides, permeability defects, and alteration of target sites, are also discussed. Lastly, novel prospective treatment options for infections caused by multi-drug resistant A. baumannii are summarized.

Anticipating the Unpredictable: A Review of Antimicrobial Stewardship and Acinetobacter Infections

Acinetobacter remains one of the most challenging pathogens in the field of infectious diseases owing primarily to the uniqueness and multiplicity of its resistance mechanisms. This resistance often leads to devastatingly long delays in time to appropriate therapy and increased mortality for patients afflicted with Acinetobacter infections. Selecting appropriate empiric and definitive antibacterial therapy for Acinetobacter is further complicated by the lack of reliability in commercial antimicrobial susceptibility testing devices and limited breakpoint interpretations for available agents. Existing treatment options for infections due to Acinetobacter are limited by a lack of robust efficacy and safety data along with concerns regarding appropriate dosing, pharmacokinetic/pharmacodynamic targets, and toxicity. Antimicrobial stewardship programs are essential to combat this unpredictable pathogen through use of infection prevention, rapid diagnostics, antibiogram-optimized treatment regimens, and avoidance of overuse of antimicrobials. The drug development pipeline includes several agents with encouraging in vitro activity against Acinetobacter, but their place in therapy and contribution to the armamentarium against this pathogen remain to be defined. The objective of this review is to highlight the unique challenge of treating infections due to Acinetobacter and summarize recent literature regarding optimal antimicrobial treatment for this pathogen. The drug development pipeline is also explored for future potentially effective treatment options.