The combination of colistin and doripenem is synergistic against Klebsiella pneumoniae at multiple inocula and suppresses colistin resistance in an in vitro pharmacokinetic/pharmacodynamic model

Novel nitroxoline derivative combating resistant bacterial infections through outer membrane disruption and competitive NDM-1 inhibition

ABSTRACTNew Delhi metallo-β-lactamase-1 (NDM-1) has rapidly disseminated worldwide, leading to multidrug resistance and worse clinical prognosis. Designing and developing effective NDM-1 inhibitors is a critical and urgent challenge. In this study, we constructed a library of long-lasting nitroxoline derivatives and identified ASN-1733 as a promising dual-functional antibiotic. ASN-1733 can effectively compete for Ca2+ on the bacterial surface, causing the detachment of lipopolysaccharides (LPS), thereby compromising the outer membrane integrity and permeability and exhibiting broad-spectrum bactericidal activity. Moreover, ASN-1733 demonstrated wider therapeutic applications than nitroxoline in mouse sepsis, thigh and mild abdominal infections. Furthermore, ASN-1733 can effectively inhibit the hydrolytic capability of NDM-1 and exhibits synergistic killing effects in combination with meropenem against NDM-1 positive bacteria. Mechanistic studies using enzymatic experiments and computer simulations revealed that ASN-1733 can bind to key residues on Loop10 of NDM-1, hindering substrate entry into the enzyme's active site and achieving potent inhibitory activity (Ki = 0.22 µM), even in the presence of excessive Zn2+. These findings elucidate the antibacterial mechanism of nitroxoline and its derivatives, expand their potential application in the field of antibacterial agents and provide new insights into the development of novel NDM-1 inhibitors.

Treatment Outcomes and Associated Factors of Intravenous Colistin for Nosocomial Infections in Pediatric Patients: A Retrospective Study in a University Hospital in Thailand

BACKGROUND

This study aimed to investigate the efficacy and safety of intravenous colistin in pediatric patients with nosocomial gram-negative bacteria infections and to determine factors associated with treatment outcomes.

METHODS

This retrospective study recruited patients aged <18 years receiving intravenous colistin between January 2014 and December 2018. Clinical data and treatment outcomes were reviewed, and factors associated with treatment outcomes were assessed.

RESULTS

This study included 178 patients with a median age of 3.4 years (range, 0.1-17.8). The mean ± SD dose of colistin prescribed to patients without renal impairment was 5.1 ± 0.6 mg/kg/day. The clinical response rate was 70.8% in patients receiving colistin for specific treatment. Infection-related mortality and crude mortality were 17.5% and 19.7%, respectively. The nephrotoxicity rate was 29.8%; approximately 70% of the episodes occurred between the 3rd and 7th day of treatment. The presence of at least 2 organ dysfunctions [adjusted hazard ratio (aHR): 7.17; 95% CI: 1.64-31.40], septic shock (aHR: 2.69; 95% CI: 1.36-5.32) and receiving chemotherapy/immunosuppressants (aHR: 2.68; 95% CI: 1.36-5.25) were observed to be associated with clinical failure. The factors observed to be associated with nephrotoxicity included hypoalbuminemia (aHR: 2.93; 95% CI: 1.26-6.78), receiving amphotericin B (aHR: 2.29; 95% CI: 1.16-4.52), vancomycin (aHR: 3.36; 95% CI: 1.50-7.56) and vasopressors (aHR: 2.57; 95% CI: 1.27-5.21).

CONCLUSION

Colistin is generally effective in the treatment of nosocomial gram-negative bacteria infections in pediatric patients. Close monitoring of renal function should be considered, especially in high-risk patients. Optimal dosage regimens for pediatric populations to promote more favorable clinical outcomes and minimize nephrotoxicity require further investigation.

Potent activity of polymyxin B is associated with long-lived super-stoichiometric accumulation mediated by weak-affinity binding to lipid A

Polymyxins are gram-negative antibiotics that target lipid A, the conserved membrane anchor of lipopolysaccharide in the outer membrane. Despite their clinical importance, the molecular mechanisms underpinning polymyxin activity remain unresolved. Here, we use surface plasmon resonance to kinetically interrogate interactions between polymyxins and lipid A and derive a phenomenological model. Our analyses suggest a lipid A-catalyzed, three-state mechanism for polymyxins: transient binding, membrane insertion, and super-stoichiometric cluster accumulation with a long residence time. Accumulation also occurs for brevicidine, another lipid A-targeting antibacterial molecule. Lipid A modifications that impart polymyxin resistance and a non-bactericidal polymyxin derivative exhibit binding that does not evolve into long-lived species. We propose that transient binding to lipid A permeabilizes the outer membrane and cluster accumulation enables the bactericidal activity of polymyxins. These findings could establish a blueprint for discovery of lipid A-targeting antibiotics and provide a generalizable approach to study interactions with the gram-negative outer membrane.

Polymyxin combination therapy for multidrug-resistant, extensively-drug resistant, and difficult-to-treat drug-resistant gram-negative infections: is it superior to polymyxin monotherapy?

INTRODUCTION

The increasing prevalence of infections with multidrug-resistant (MDR), extensively-drug resistant (XDR) or difficult-to-treat drug resistant (DTR) Gram-negative bacilli (GNB), including Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Enterobacter species, and Escherichia coli poses a severe challenge.

AREAS COVERED

The rapid growing of multi-resistant GNB as well as the considerable deceleration in development of new anti-infective agents have made polymyxins (e.g. polymyxin B and colistin) a mainstay in clinical practices as either monotherapy or combination therapy. However, whether the polymyxin-based combinations lead to better outcomes remains unknown. This review mainly focuses on the effect of polymyxin combination therapy versus monotherapy on treating GNB-related infections. We also provide several factors in designing studies and their impact on optimizing polymyxin combinations.

EXPERT OPINION

An abundance of recent in vitro and preclinical in vivo data suggest clinical benefit for polymyxin-drug combination therapies, especially colistin plus meropenem and colistin plus rifampicin, with synergistic killing against MDR, XDR, and DTR P. aeruginosa, K. pneumoniae and A. baumannii. The beneficial effects of polymyxin-drug combinations (e.g. colistin or polymyxin B + carbapenem against carbapenem-resistant K. pneumoniae and carbapenem-resistant A. baumannii, polymyxin B + carbapenem + rifampin against carbapenem-resistant K. pneumoniae, and colistin + ceftolozan/tazobactam + rifampin against PDR-P. aeruginosa) have often been shown in clinical setting by retrospective studies. However, high-certainty evidence from large randomized controlled trials is necessary. These clinical trials should incorporate careful attention to patient's sample size, characteristics of patient's groups, PK/PD relationships and dosing, rapid detection of resistance, MIC determinations, and therapeutic drug monitoring.

Fighting Antimicrobial Resistance in Neonatal Intensive Care Units: Rational Use of Antibiotics in Neonatal Sepsis

Antibiotics are the most frequently prescribed drugs in neonatal intensive care units (NICUs) due to the severity of complications accompanying neonatal sepsis. However, antimicrobial drugs are often used inappropriately due to the difficulties in diagnosing sepsis in the neonatal population. The reckless use of antibiotics leads to the development of resistant strains, rendering multidrug-resistant pathogens a serious problem in NICUs and a global threat to public health. The aim of this narrative review is to provide a brief overview of neonatal sepsis and an update on the data regarding indications for antimicrobial therapy initiation, current guidance in the empirical antimicrobial selection and duration of therapy, and indications for early discontinuation.

Carbapenem Resistance Associated with Coliuria among Outpatient and Hospitalized Urology Patients

The World Health Organization in 2017 listed carbapenem resistant Enterobacteriaceae (CRE) with critical priority for research. A research to assess carbapenem resistant Escherichia coli (CREc) in coliuria among the outpatients and inpatients of a tertiary health institution was carried out using conventional methods, polymerase chain reaction, Sanger sequencing, and bioinformatics. There were 39 positive coliuria cases from the urine samples collected from a total of 126 patients with various genitourinary diseases. The E. coli enumeration (log₁₀ CFU/mL) revealed that 82.1% (n = 32) of the samples showed significant coliuria, 12.8% (n = 5) showed non-significant coliuria while 5.1% (n = 2) showed indeterminate coliuria even when repeated. Significantly higher numbers (p > 0.05) of the sampled inpatients yielded positive coliuria (57.9%) than the outpatients. Though there were significantly more (P > 0.05) urology female patients (n = 77) than male (n = 49), coliuria was more prevalent in sampled male patients (34.9%) than female (28.6%). Highest prevalence of coliuria was observed among the age range (18–30) years. Selected CREc that was sequenced and the sequences submitted to GenBank of National Center for Biotechnological Information (NCBI) were Escherichia coli AYO-WINI111 and Escherichia coli AYO-WINI112 with accession number MT735391 and MT735392, respectively. High resistance was observed against ertapenem (53%), imipenem (62%), meropenem (48%), and doripenem (47%), while 7%–22% of the isolates showed phenotypic intermediate carbapenem resistance. Critically dangerous CREc are harboured by large number urology patients in the study area, depicting the need for more attention in the management of the condition, as CREc are close to achieving totally antibiotic resistance.

Risk Factors for the Development of Colistin Resistance during Colistin Treatment of Carbapenem-Resistant Klebsiella pneumoniae Infections

Colistin is one of the last-resort options for carbapenem-resistant Klebsiella pneumoniae (CRKP) infections if novel antibiotics are unavailable, where the development of colistin resistance during treatment represents a major challenge for clinicians. We aimed to investigate the risk factors associated with the development of colistin resistance in patients with CRKP infections following colistin treatment. We conducted a retrospective case-control study of patients with CRKP strains available before and after colistin treatment at a medical center in Taiwan, between October 2016 and November 2020. Cases (n = 35) included patients with an initial colistin-susceptible CRKP (ColS-CRKP) strain and a subsequent colistin-resistant CRKP (ColR-CRKP) strain. Controls (n = 18) included patients with ColS-CRKP as both the initial and subsequent strains. The 30-day mortality rate after the subsequent CRKP isolation was not different between cases and controls (12/35 [34%] versus 5/18 [28%] [P = 0.631]). blaKPC (n = 38) and blaOXA-48 (n = 11) accounted for the major mechanisms of carbapenem resistance. Alterations in mgrB were found in 18/35 (51%) ColR-CRKP strains, and mcr-1 was not detected in any of the strains. More patients received combination therapy in the control group than in the case group (17/18 versus 21/35 [P = 0.008]). The logistic regression model indicated that combination therapy with tigecycline was protective against the acquisition of colistin resistance (odds ratio, 0.17; 95% confidence interval, 0.05 to 0.62 [P = 0.008]). We observed that the inclusion of tigecycline in colistin treatment mitigated the risk of acquiring colistin resistance. These results offer insight into using the combination of tigecycline and colistin for the treatment of CRKP infections in antimicrobial stewardship. IMPORTANCE Treatment of carbapenem-resistant Klebsiella pneumoniae (CRKP) infections is challenging due to the limited options of antibiotics. Colistin is one of the last-resort antibiotics if novel antimicrobial agents are not available. It is crucial to identify modifiable clinical factors associated with the emergence of resistance during colistin treatment. Here, we found that the addition of tigecycline to colistin treatment prevented the acquisition of colistin resistance. Colistin-tigecycline combination therapy is therefore considered a hopeful option in antimicrobial stewardship to treat CRKP infections.

Pharmacokinetics and pharmacodynamics of peptide antibiotics

Antimicrobial resistance is a major global health challenge. As few new efficacious antibiotics will become available in the near future, peptide antibiotics continue to be major therapeutic options for treating infections caused by multidrug-resistant pathogens. Rational use of antibiotics requires optimisation of the pharmacokinetics and pharmacodynamics for the treatment of different types of infections. Toxicodynamics must also be considered to improve the safety of antibiotic use and, where appropriate, to guide therapeutic drug monitoring. This review focuses on the pharmacokinetics/pharmacodynamics/toxicodynamics of peptide antibiotics against multidrug-resistant Gram-negative and Gram-positive pathogens. Optimising antibiotic exposure at the infection site is essential for improving their efficacy and minimising emergence of resistance.

In vitro Bactericidal Activities of Combination Antibiotic Therapies Against Carbapenem-Resistant Klebsiella pneumoniae With Different Carbapenemases and Sequence Types

Carbapenem-resistant Klebsiella pneumoniae (CRKP) is becoming increasingly problematic due to the limited effectiveness of new antimicrobials or other factors such as treatment cost. Thus, combination therapy remains a suitable treatment option. We aimed to evaluate the in vitro bactericidal activity of various antibiotic combinations against CRKP with different carbapenemase genotypes and sequence types (STs). Thirty-seven CRKP with various STs and carbapenemases were exposed to 11 antibiotic combinations (polymyxin B or tigecycline in combination with β-lactams including aztreonam, cefepime, piperacillin/tazobactam, doripenem, meropenem, and polymyxin B with tigecycline) in static time-kill studies (TKS) using clinically achievable concentrations. Out of the 407 isolate-combination pairs, only 146 (35.8%) were bactericidal (≥3 log₁₀CFU/mL decrease from initial inoculum). Polymyxin B in combination with doripenem, meropenem, or cefepime was the most active, each demonstrating bactericidal activity in 27, 24, and 24 out of 37 isolates, respectively. Tigecycline in combination with β-lactams was rarely bactericidal. Aside from the lower frequency of bactericidal activity in the dual-carbapenemase producers, there was no apparent difference in combination activity among the strains with other carbapenemase types. In addition, bactericidal combinations were varied even in strains with similar STs, carbapenemases, and other genomic characteristics. Our findings demonstrate that the bactericidal activity of antibiotic combinations is highly strain-specific likely owing to the complex interplay of carbapenem-resistance mechanisms, i.e., carbapenemase genotype alone cannot predict in vitro bactericidal activity. The availability of WGS information can help rationalize the activity of certain combinations. Further studies should explore the use of genomic markers with phenotypic information to predict combination activity.

Polymyxins, the last-resort antibiotics: Mode of action, resistance emergence, and potential solutions

Infections caused by multi-drug resistant (MDR) bacterial pathogens are a leading cause of mortality and morbidity across the world. Indiscriminate use of broad-spectrum antibiotics has seriously affected this situation. With the diminishing discovery of novel antibiotics, new treatment methods are urgently required to combat MDR pathogens. Polymyxins, the cationic lipopeptide antibiotics, discovered more than half a century ago, are considered to be the last-line of antibiotics available at the moment. This antibiotic shows a great bactericidal effect against Gram-negative bacteria. Polymyxins primarily target the bacterial membrane and disrupt them, causing lethality. Because of their membrane interacting mode of action, polymyxins cause nephrotoxicity and neurotoxicity in humans, limiting their usability. However, recent modifications in their chemical structure have been able to reduce the toxic effects. The development of better dosing regimens has also helped in getting better clinical outcomes in the infections caused by MDR pathogens. Since the mid-1990s the use of polymyxins has increased manifold in clinical settings, resulting in the emergence of polymyxin-resistant strains. The risk posed by the polymyxin-resistant nosocomial pathogens such as the Enterobacteriaceae group, Pseudomonas aeruginosa, and Acinetobacter baumannii, etc. is very serious considering these pathogens are resistant to almost all available antibacterial drugs. In this review article, the mode of action of the polymyxins and the genetic regulatory mechanism responsible for the emergence of resistance are discussed. Specifically, this review aims to update our current understanding in the field and suggest possible solutions that can be pursued for future antibiotic development. As polymyxins primarily target the bacterial membranes, resistance to polymyxins arises primarily by the modification of the lipopolysaccharides (LPS) in the outer membrane (OM). The LPS modification pathways are largely regulated by the bacterial two-component signal transduction (TCS) systems. Therefore, targeting or modulating the TCS signalling mechanisms can be pursued as an alternative to treat the infections caused by polymyxin-resistant MDR pathogens. In this review article, this aspect is also highlighted.

Dose recommendations for intravenous colistin in pediatric patients from a prospective, multicenter, population pharmacokinetic study

OBJECTIVES

The aim of this study was to describe the population pharmacokinetics of intravenous colistin use in children and to propose optimal dosage regimens.

METHODS

A prospective, multicenter, population pharmacokinetic (PPK) study was conducted. Phoenix 64 version 8.3 was used for the PPK analysis. Simulations were performed to estimate the probability of target attainment for patients achieving target plasma colistin average steady-state concentrations (C_ss,avg).

RESULTS

A total of 334 plasma colistin concentrations were obtained from 79 pediatric patients with a median age (interquartile range) of 2.6 years (0.8-6.8 years); 73 (92.4%) were admitted to intensive care units. Colistin pharmacokinetics were adequately described by a one-compartment model with first-order elimination along with serum creatinine (SCr) as a significant covariate in colistin clearance. The simulation demonstrated that the recommended dose of 5 mg of colistin base activity (CBA)/kg/day resulted in 18.2-63.0% probability of achieving a target C_ss,avg of 2 mg/l. With a lower targeted C_ss,avg of 1 mg/l, colistin dosing with 7.5 mg and 5 mg of CBA/kg/day were adequate for children with SCr levels of 0.1-0.3 mg/dl and >0.3 mg/dl, respectively.

CONCLUSIONS

SCr is a significant covariate in colistin clearance in children. Colistin dosing should be selected according to the patient's SCr level and the desired target C_ss,avg.

Synergy of the Polymyxin-Chloramphenicol Combination against New Delhi Metallo-β-Lactamase-Producing Klebsiella pneumoniae Is Predominately Driven by Chloramphenicol

Carbapenem-resistant Klebsiella pneumoniae has been classified as an Urgent Threat by the Centers for Disease Control and Prevention (CDC). The combination of two "old" antibiotics, polymyxin and chloramphenicol, displays synergistic killing against New Delhi metallo-β-lactamase (NDM)-producing K. pneumoniae. However, the mechanism(s) underpinning their synergistic killing are not well studied. We employed an in vitro pharmacokinetic/pharmacodynamic model to mimic the pharmacokinetics of the antibiotics in patients and examined bacterial killing against NDM-producing K. pneumoniae using a metabolomic approach. Metabolomic analysis was integrated with an isolate-specific genome-scale metabolic network (GSMN). Our results show that metabolic responses to polymyxin B and/or chloramphenicol against NDM-producing K. pneumoniae involved the inhibition of cell envelope biogenesis, metabolism of arginine and nucleotides, glycolysis, and pentose phosphate pathways. Our metabolomic and GSMN modeling results highlight the novel mechanisms of a synergistic antibiotic combination at the network level and may have a significant potential in developing precision antimicrobial chemotherapy in patients.

Colistin Resistance Development Following Colistin-Meropenem Combination Therapy Versus Colistin Monotherapy in Patients With Infections Caused by Carbapenem-Resistant Organisms

BACKGROUND

We evaluated whether carbapenem-colistin combination therapy reduces the emergence of colistin resistance, compared to colistin monotherapy, when given to patients with infections due to carbapenem-resistant Gram-negative organisms.

METHODS

This is a pre-planned analysis of a secondary outcome from a randomized, controlled trial comparing colistin monotherapy with colistin-meropenem combination for the treatment of severe infections caused by carbapenem-resistant, colistin-susceptible Gram-negative bacteria. We evaluated rectal swabs taken on Day 7 or later for the presence of new colistin-resistant (ColR) isolates. We evaluated the emergence of any ColR isolate and the emergence of ColR Enterobacteriaceae (ColR-E).

RESULTS

Data were available for 214 patients for the primary analysis; emergent ColR organisms were detected in 22 (10.3%). No difference was observed between patients randomized to treatment with colistin monotherapy (10/106, 9.4%) versus patients randomized to colistin-meropenem combination therapy (12/108, 11.1%; P = .669). ColR-E organisms were detected in 18/249 (7.2%) patients available for analysis. No difference was observed between the 2 treatment arms (colistin monotherapy 6/128 [4.7%] vs combination therapy 12/121 [9.9%]; P = .111). Enterobacteriaceae, as the index isolate, was found to be associated with development of ColR-E (hazard ratio, 3.875; 95% confidence interval, 1.475-10.184; P = .006).

CONCLUSIONS

Carbapenem-colistin combination therapy did not reduce the incidence of colistin resistance emergence in patients with infections due to carbapenem-resistant organisms. Further studies are necessary to elucidate the development of colistin resistance and methods for its prevention.

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.

Rescuing the Last-Line Polymyxins: Achievements and Challenges

Antibiotic resistance is a major global health challenge and, worryingly, several key Gram negative pathogens can become resistant to most currently available antibiotics. Polymyxins have been revived as a last-line therapeutic option for the treatment of infections caused by multidrug-resistant Gram negative bacteria, in particular Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacterales. Polymyxins were first discovered in the late 1940s but were abandoned soon after their approval in the late 1950s as a result of toxicities (e.g., nephrotoxicity) and the availability of "safer" antibiotics approved at that time. Therefore, knowledge on polymyxins had been scarce until recently, when enormous efforts have been made by several research teams around the world to elucidate the chemical, microbiological, pharmacokinetic/pharmacodynamic, and toxicological properties of polymyxins. One of the major achievements is the development of the first scientifically based dosage regimens for colistin that are crucial to ensure its safe and effective use in patients. Although the guideline has not been developed for polymyxin B, a large clinical trial is currently being conducted to optimize its clinical use. Importantly, several novel, safer polymyxin-like lipopeptides are developed to overcome the nephrotoxicity, poor efficacy against pulmonary infections, and narrow therapeutic windows of the currently used polymyxin B and colistin. This review discusses the latest achievements on polymyxins and highlights the major challenges ahead in optimizing their clinical use and discovering new-generation polymyxins. To save lives from the deadly infections caused by Gram negative "superbugs," every effort must be made to improve the clinical utility of the last-line polymyxins. SIGNIFICANCE STATEMENT: Antimicrobial resistance poses a significant threat to global health. The increasing prevalence of multidrug-resistant (MDR) bacterial infections has been highlighted by leading global health organizations and authorities. Polymyxins are a last-line defense against difficult-to-treat MDR Gram negative pathogens. Unfortunately, the pharmacological information on polymyxins was very limited until recently. This review provides a comprehensive overview on the major achievements and challenges in polymyxin pharmacology and clinical use and how the recent findings have been employed to improve clinical practice worldwide.

Polymyxins, the last-resort antibiotics: Mode of action, resistance emergence, and potential solutions

Infections caused by multi-drug resistant (MDR) bacterial pathogens are a leading cause of mortality and morbidity across the world. Indiscriminate use of broad-spectrum antibiotics has seriously affected this situation. With the diminishing discovery of novel antibiotics, new treatment methods are urgently required to combat MDR pathogens. Polymyxins, the cationic lipopeptide antibiotics, discovered more than half a century ago, are considered to be the last-line of antibiotics available at the moment. This antibiotic shows a great bactericidal effect against Gram-negative bacteria. Polymyxins primarily target the bacterial membrane and disrupt them, causing lethality. Because of their membrane interacting mode of action, polymyxins cause nephrotoxicity and neurotoxicity in humans, limiting their usability. However, recent modifications in their chemical structure have been able to reduce the toxic effects. The development of better dosing regimens has also helped in getting better clinical outcomes in the infections caused by MDR pathogens. Since the mid1990s the use of polymyxins has increased manifold in clinical settings, resulting in the emergence of polymyxin-resistant strains. The risk posed by the polymyxin-resistant nosocomial pathogens such as the Enterobacteriaceae group, Pseudomonas aeruginosa, and Acinetobacter baumannii, etc. is very serious considering these pathogens are resistant to almost all available antibacterial drugs. In this review article, the mode of action of the polymyxins and the genetic regulatory mechanism responsible for the emergence of resistance are discussed. Specifically, this review aims to update our current understanding in the field and suggest possible solutions that can be pursued for future antibiotic development. As polymyxins primarily target the bacterial membranes, resistance to polymyxins arises primarily by the modification of the lipopolysaccharides (LPS) in the outer membrane (OM). The LPS modification pathways are largely regulated by the bacterial two-component signal transduction (TCS) systems. Therefore, targeting or modulating the TCS signalling mechanisms can be pursued as an alternative to treat the infections caused by polymyxin-resistant MDR pathogens. In this review article, this aspect is also highlighted.

Polymyxin Triple Combinations against Polymyxin-Resistant, Multidrug-Resistant, KPC-Producing Klebsiella pneumoniae

Resistance to polymyxin antibiotics is increasing. Without new antibiotic classes, combination therapy is often required. We systematically investigated bacterial killing with polymyxin-based combinations against multidrug-resistant (including polymyxin-resistant), carbapenemase-producing Klebsiella pneumoniae Monotherapies and double- and triple-combination therapies were compared to identify the most efficacious treatment using static time-kill studies (24 h, six isolates), an in vitro pharmacokinetic/pharmacodynamic model (IVM; 48 h, two isolates), and the mouse thigh infection model (24 h, six isolates). In static time-kill studies, all monotherapies (polymyxin B, rifampin, amikacin, meropenem, or minocycline) were ineffective. Initial bacterial killing was enhanced with various polymyxin B-containing double combinations; however, substantial regrowth occurred in most cases by 24 h. Most polymyxin B-containing triple combinations provided greater and more sustained killing than double combinations. Standard dosage regimens of polymyxin B (2.5 mg/kg of body weight/day), rifampin (600 mg every 12 h), and amikacin (7.5 mg/kg every 12 h) were simulated in the IVM. Against isolate ATH 16, no viable bacteria were detected across 5 to 25 h with triple therapy, with regrowth to ∼2-log10 CFU/ml occurring at 48 h. Against isolate BD 32, rapid initial killing of ∼3.5-log10 CFU/ml at 5 h was followed by a slow decline to ∼2-log10 CFU/ml at 48 h. In infected mice, polymyxin B monotherapy (60 mg/kg/day) generally was ineffective. With triple therapy (polymyxin B at 60 mg/kg/day, rifampin at 120 mg/kg/day, and amikacin at 300 mg/kg/day), at 24 h there was an ∼1.7-log10 CFU/thigh reduction compared to the starting inoculum for all six isolates. Our results demonstrate that the polymyxin B-rifampin-amikacin combination significantly enhanced in vitro and in vivo bacterial killing, providing important information for the optimization of polymyxin-based combinations in patients.

Transcriptomic responses of a New Delhi metallo-β-lactamase-producing Klebsiella pneumoniae isolate to the combination of polymyxin B and chloramphenicol

The combination of polymyxins and chloramphenicol possesses synergistic killing activity against New Delhi metallo-β-lactamase (NDM)-producing Klebsiella pneumoniae. This systems study examined the transcriptomic responses to the polymyxin/chloramphenicol combination in clinical NDM-producing K. pneumoniae isolate S01. Klebsiella pneumoniae S01 (initial inoculum ~10⁸ CFU/mL) was treated with polymyxin B (1 mg/L, continuous infusion) or chloramphenicol [maximum concentration (C_max) = 8 mg/L, half-life (t_1/2) = 4 h], alone or in combination, using an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model to mimic their pharmacokinetics in patients. Transcriptomic profiles of bacterial samples collected at 0, 0.25, 1, 4 and 24 h were examined using RNA sequencing (RNA-Seq). Chloramphenicol monotherapy significantly increased the expression of genes involved in ribosomal synthesis across the entire 24-h treatment, reflective of chloramphenicol-mediated inhibition of protein synthesis. The effect of polymyxin B was rapid and no major pathways were perturbed at later time points (4 h and 24 h). Combination treatment yielded the highest number of differentially expressed genes, including a large number observed following chloramphenicol monotherapy, in particular carbohydrate, nucleotide, amino acid and cell wall metabolism. Notably, chloramphenicol alone and in combination with polymyxin B significantly inhibited the expression of the arn operon that is responsible for lipid A modification and polymyxin resistance. These results indicate that the polymyxin/chloramphenicol combination displayed persistent transcriptomic responses over 24 h mainly on cell envelope synthesis and metabolism of carbohydrates, nucleotides and amino acids.

International Consensus Guidelines for the Optimal Use of the Polymyxins: Endorsed by the American College of Clinical Pharmacy (ACCP), European Society of Clinical Microbiology and Infectious Diseases (ESCMID), Infectious Diseases Society of America (IDSA), International Society for Anti-infective Pharmacology (ISAP), Society of Critical Care Medicine (SCCM), and Society of Infectious Diseases Pharmacists (SIDP)

The polymyxin antibiotics colistin (polymyxin E) and polymyxin B became available in the 1950s and thus did not undergo contemporary drug development procedures. Their clinical use has recently resurged, assuming an important role as salvage therapy for otherwise untreatable gram-negative infections. Since their reintroduction into the clinic, significant confusion remains due to the existence of several different conventions used to describe doses of the polymyxins, differences in their formulations, outdated product information, and uncertainties about susceptibility testing that has led to lack of clarity on how to optimally utilize and dose colistin and polymyxin B. We report consensus therapeutic guidelines for agent selection and dosing of the polymyxin antibiotics for optimal use in adult patients, as endorsed by the American College of Clinical Pharmacy (ACCP), Infectious Diseases Society of America (IDSA), International Society of Anti-Infective Pharmacology (ISAP), Society for Critical Care Medicine (SCCM), and Society of Infectious Diseases Pharmacists (SIDP). The European Society for Clinical Microbiology and Infectious Diseases (ESCMID) endorses this document as a consensus statement. The overall conclusions in the document are endorsed by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). We established a diverse international expert panel to make therapeutic recommendations regarding the pharmacokinetic and pharmacodynamic properties of the drugs and pharmacokinetic targets, polymyxin agent selection, dosing, dosage adjustment and monitoring of colistin and polymyxin B, use of polymyxin-based combination therapy, intrathecal therapy, inhalation therapy, toxicity, and prevention of renal failure. The treatment guidelines provide the first ever consensus recommendations for colistin and polymyxin B therapy that are intended to guide optimal clinical use.

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.

A review on bacterial resistance to carbapenems: epidemiology, detection and treatment options

Carbapenems are a class of antimicrobial agents reserved for infections caused by multidrug-resistant microorganisms. The emergence of carbapenem resistance has become a serious public health threat. This type of antimicrobial resistance is spreading at an alarming rate, resulting in major outbreaks and treatment failure of community-acquired and nosocomial infections caused by the clinically relevant carbapenem-producing Enterobacteriaceae or carbapenem-resistant Enterobacteriaceae. This review is focused on carbapenem resistance, including mechanisms of resistance, history and epidemiology, phenotypic and genotypic detection in the clinically relevant bacterial pathogens and the possible treatment options available.

Comparative Antibiofilm Efficacy of Meropenem Alone and in Combination with Colistin in an In Vitro Pharmacodynamic Model by Extended-Spectrum-β-Lactamase-Producing Klebsiella pneumoniae

We compared the efficacies of meropenem alone and in combination with colistin against two strains of extended-spectrum-β-lactamase-producing Klebsiella pneumoniae, using an in vitro pharmacodynamic model that mimicked two different biofilm conditions. Meropenem monotherapy achieved remarkable efficacy (even a bactericidal effect) under all conditions, whereas colistin was almost inactive and resistance emerged. The addition of colistin to meropenem produced no relevant benefits, in contrast to experiences with other microorganisms.

In vitro Pharmacodynamics and PK/PD in Animals

In the last decade, considerable advancements have been made to identify the pharmacokinetic/pharmacodynamic (PK/PD) index that defines the antimicrobial activity of polymyxins. Dose-fractionation studies performed in hollow-fiber models found that altering the dosing schedule had little impact on the killing or suppression of resistance emergence, alluding to AUC/MIC as the pharmacodynamic index that best describes polymyxin's activity. For in vivo efficacy, the PK/PD index that was the most predictive of the antibacterial effect of colistin against P. aeruginosa and A. baumannii was ƒAUC/MIC.

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.

Ursolic acid inhibits colistin efflux and curtails colistin resistant Enterobacteriaceae

Colistin resistance in Enterobacteriaceae especially Klebsiella pneumoniae and Escherichia coli is driving the evolution of pan drug resistant strains. Screening a library of 13 plant nutraceuticals led to the identification of acetyl shikonin and ursolic acid, which exhibited synergy with colistin against extremely drug resistant (XDR) clinical strains of E. coli (U3790) and K. pneumoniae (BC936). Ursolic acid caused a significant colistin MIC reversal of 16-fold in U3790 and 4-fold in BC936 strains. Ursolic acid also potentiated the bactericidal effect of colistin against both U3790 and BC936 by causing ~ 4 to 4.5 log fold decline in CFU of both clinical isolates in a time kill assay. At 2× minimum effective concentration, ursolic acid was non-toxic to zebrafish as evidenced by brain and liver enzyme profiles and by histopathology studies. In combination with colistin, ursolic acid reduced bacterial bioburden of U3790/BC936 by 1–1.58 log fold from the infected muscle tissue of zebrafish. Mechanistic explorations via studies on real time efflux, membrane potential and intracellular accumulation of dansyl chloride tagged colistin revealed that colistin efflux is inhibited by ursolic acid. In addition, ursolic acid also enhanced outer membrane permeability which probably facilitates colistin’s attack on outer and inner membranes. Our study shows that ursolic acid synergizes with colistin by inhibiting colistin efflux in Enterobacteriaceae that helps to curtail colistin resistant Enterobacteriaceae.

Rational Combinations of Polymyxins with Other Antibiotics

Combinations of antimicrobial agents are often used in the management of infectious diseases. Antimicrobial agents used as part of combination therapy are often selected empirically. As regrowth and the emergence of polymyxin (either colistin or polymyxin B) resistance has been observed with polymyxin monotherapy, polymyxin combination therapy has been suggested as a possible means by which to increase antimicrobial activity and reduce the development of resistance. This chapter provides an overview of preclinical and clinical investigations of CMS/colistin and polymyxin B combination therapy. In vitro data and animal model data suggests a potential clinical benefit with many drug combinations containing clinically achievable concentrations of polymyxins, even when resistance to one or more of the drugs in combination is present and including antibiotics normally inactive against Gram-negative organisms. The growing body of data on the emergence of polymyxin resistance with monotherapy lends theoretical support to a benefit with combination therapy. Benefits include enhanced bacterial killing and a suppression of polymyxin resistant subpopulations. However, the complexity of the critically ill patient population, and high rates of treatment failure and death irrespective of infection-related outcome make demonstrating a potential benefit for polymyxin combinations extremely challenging. Polymyxin combination therapy in the clinic remains a heavily debated and controversial topic. When combinations are selected, optimizing the dosage regimens for the polymyxin and the combinatorial agent is critical to ensure that the benefits outweigh the risk of the development of toxicity. Importantly, patient characteristics, pharmacokinetics, the site of infection, pathogen and resistance mechanism must be taken into account to define optimal and rational polymyxin combination regimens in the clinic.

An integrative, multi-omics approach towards the prioritization of Klebsiella pneumoniae drug targets

Klebsiella pneumoniae (Kp) is a globally disseminated opportunistic pathogen that can cause life-threatening infections. It has been found as the culprit of many infection outbreaks in hospital environments, being particularly aggressive towards newborns and adults under intensive care. Many Kp strains produce extended-spectrum β-lactamases, enzymes that promote resistance against antibiotics used to fight these infections. The presence of other resistance determinants leading to multidrug-resistance also limit therapeutic options, and the use of 'last-resort' drugs, such as polymyxins, is not uncommon. The global emergence and spread of resistant strains underline the need for novel antimicrobials against Kp and related bacterial pathogens. To tackle this great challenge, we generated multiple layers of 'omics' data related to Kp and prioritized proteins that could serve as attractive targets for antimicrobial development. Genomics, transcriptomics, structuromic and metabolic information were integrated in order to prioritize candidate targets, and this data compendium is freely available as a web server. Twenty-nine proteins with desirable characteristics from a drug development perspective were shortlisted, which participate in important processes such as lipid synthesis, cofactor production, and core metabolism. Collectively, our results point towards novel targets for the control of Kp and related bacterial pathogens.

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.

Synergistic Activity of Colistin/Fosfomycin Combination against Carbapenemase-Producing Klebsiella pneumoniae in an In Vitro Pharmacokinetic/Pharmacodynamic Model

Carbapenemase-producing Klebsiella pneumoniae is globally recognized as one of the greatest threats to public health, and combination therapy may be the chemotherapeutic option. In the present study, we aimed to evaluate the antibacterial effects of colistin/fosfomycin combination against carbapenemase-producing K. pneumoniae. The antibacterial effects were determined in a one-compartment in vitro pharmacokinetic model over a period of 24 h. The initial inoculum was 10⁸ CFU/mL. Low, medium, and high C_max values of colistin at 0.5, 2, and 5 mg/L as well as C_max of fosfomycin at 100 mg/L were simulated in the model. Doses of both colistin and fosfomycin were given every 8 h until 24 h. For the colistin- and fosfomycin-susceptible isolate KP47, three combination regimens showed greater killing effect compared with colistin monotherapy. The greatest killing effect was observed in combination regimen containing 5 mg/L colistin. For colistin-heteroresistant and fosfomycin-susceptible isolate KP79, combination regimen containing low dose colistin (0.5 mg/L) showed no synergistic or additive effects. However, combination regimens containing 2 and 5 mg/L colistin maintained the bactericidal effect until 24 h compared with colistin monotherapy. For colistin-heteroresistant and fosfomycin-resistant isolates KP42 and KP11, bactericidal activity was barely enhanced by combination regimens. Moreover, combination regimen containing 5 mg/L colistin could only prevent the emergence of colistin-resistant subpopulation in colistin and fosfomycin-susceptible isolate. It is necessary to know the resistant patterns of the K. pneumoniae before using combination of colistin and fosfomycin in clinical practice.

Novel Polymyxin Combination With Antineoplastic Mitotane Improved the Bacterial Killing Against Polymyxin-Resistant Multidrug-Resistant Gram-Negative Pathogens

Due to limited new antibiotics, polymyxins are increasingly used to treat multidrug-resistant (MDR) Gram-negative bacteria, in particular carbapenem-resistant Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Unfortunately, polymyxin monotherapy has led to the emergence of resistance. Polymyxin combination therapy has been demonstrated to improve bacterial killing and prevent the emergence of resistance. From a preliminary screening of an FDA drug library, we identified antineoplastic mitotane as a potential candidate for combination therapy with polymyxin B against polymyxin-resistant Gram-negative bacteria. Here, we demonstrated that the combination of polymyxin B with mitotane enhances the in vitro antimicrobial activity of polymyxin B against 10 strains of A. baumannii, P. aeruginosa, and K. pneumoniae, including polymyxin-resistant MDR clinical isolates. Time-kill studies showed that the combination of polymyxin B (2 mg/L) and mitotane (4 mg/L) provided superior bacterial killing against all strains during the first 6 h of treatment, compared to monotherapies, and prevented regrowth and emergence of polymyxin resistance in the polymyxin-susceptible isolates. Electron microscopy imaging revealed that the combination potentially affected cell division in A. baumannii. The enhanced antimicrobial activity of the combination was confirmed in a mouse burn infection model against a polymyxin-resistant A. baumannii isolate. As mitotane is hydrophobic, it was very likely that the synergistic killing of the combination resulted from that polymyxin B permeabilized the outer membrane of the Gram-negative bacteria and allowed mitotane to enter bacterial cells and exert its antimicrobial effect. These results have important implications for repositioning non-antibiotic drugs for antimicrobial purposes, which may expedite the discovery of novel therapies to combat the rapid emergence of antibiotic resistance.

Emergence of polymyxin B resistance in a polymyxin B-susceptible KPC-producing Klebsiella pneumoniae causing bloodstream infection in a neutropenic patient during polymyxin B therapy

The emergence of resistance to polymyxins in KPC-producing Klebsiella pneumoniae isolates has been a major clinical problem. This study evaluated the molecular mechanisms associated with polymyxin B (PMB) resistance that emerged in a previously PMB-susceptible KPC-2-producing K. pneumoniae during PMB therapy for a bloodstream infection in a neutropenic patient. The first isolate (PMB-susceptible) was obtained while the patient was receiving meropenem and other isolates were recovered from 2 sets of blood cultures in different dates while the patient was receiving PMB therapy (4 of 6 blood cultures bottles yielded isolates with full PMB resistance). The population analysis profile of the first isolate revealed the growth of resistant subpopulations with PFGE profile distinct from the parental isolate but undistinguishable from those obtained in subsequent days under PMB exposure. Resistant subpopulations were obtained from all parental PMB-susceptible and in one PMB-resistant isolate recovered from the patient. The molecular mechanism observed in the hetero-resistant subpopulations (IS1-like in mgrB-promoter region, increased rstB transcription with no mutation and non-identified mechanism) differed from those found in the PMB-resistant isolates, in which no mutation or transcriptional alterations were detected. This study showed that the mechanism of resistance to PMB that emerged during PMB therapy was not related to those observed in subpopulations selected in vitro from PMB-susceptible isolates recovered from the patient. The absence of mutations in the former isolates may be due to adaptive resistance occurred because of sub-optimal PMB levels as well as amikacin and meropenem used in combination.

Emergence and clonal spread of colistin resistance due to multiple mutational mechanisms in carbapenemase-producing Klebsiella pneumoniae in London

Carbapenemase-producing Enterobacteriaceae (CPE) are emerging worldwide, limiting therapeutic options. Mutational and plasmid-mediated mechanisms of colistin resistance have both been reported. The emergence and clonal spread of colistin resistance was analysed in 40 epidemiologically-related NDM-1 carbapenemase producing Klebsiella pneumoniae isolates identified during an outbreak in a group of London hospitals. Isolates from July 2014 to October 2015 were tested for colistin susceptibility using agar dilution, and characterised by whole genome sequencing (WGS). Colistin resistance was detected in 25/38 (65.8%) cases for which colistin susceptibility was tested. WGS found that three potential mechanisms of colistin resistance had emerged separately, two due to different mutations in mgrB, and one due to a mutation in phoQ, with onward transmission of two distinct colistin-resistant variants, resulting in two sub-clones associated with transmission at separate hospitals. A high rate of colistin resistance (66%) emerged over a 10 month period. WGS demonstrated that mutational colistin resistance emerged three times during the outbreak, with transmission of two colistin-resistant variants.

Agents of Last Resort: Polymyxin Resistance

Polymyxin resistance is a major public health threat, as the polymyxins represent "last-line" therapeutics for Gram-negative pathogens resistant to essentially all other antibiotics. Improved understanding of mechanisms of, and risk factors for, polymyxin resistance, as well as infection prevention and stewardship strategies, together with optimization of dosing of polymyxins including in combination regimens, can help to limit the emergence and dissemination of polymyxin resistance.

An audit of colistin use in neonatal sepsis from a tertiary care centre of a resource-limited country

Background & objectives:

Sepsis due to multidrug-resistant Gram-negative pathogens is a challenge for clinicians and microbiologists and has led to use of parenteral colistin. There is a paucity of data regarding safety and efficacy of intravenous colistin use in neonates. The objective of this retrospective analysis was to study the efficacy and safety of intravenous colistin in the treatment of neonatal sepsis.

Methods:

An audit of the data from neonates, admitted to a neonatal intensive care unit of a tertiary care hospital during January 2012 to December 2012, and who received intravenous colistin was carried out.

Results:

Sixty two neonates received intravenous colistin (52 preterm and 10 term) for the treatment of pneumonia, bloodstream infections and meningitis. The isolated pathogens in decreasing order of frequency were Acinetobacter baumannii, Klebsiella pneumonia and Pseudomonas aeruginosa. Of the total 62 neonates, 41 (66.12%) survived and 21 (33.87%) died. Significantly higher mortality was observed in neonates with lower body weights (P < 0.05). A significant association of mortality was found in those with sepsis due to Klebsiella species. Only one of seven with this infection survived as against 15 of the 23 who grew other organisms [P = 0.03; crude odds ratio = 11.25 (1.2, 110.5)]. None of the neonates developed neurotoxicity or nephrotoxicity.

Interpretation & conclusions:

This retrospective study in neonates with sepsis showed that intravenous colistin was safe and effective in the treatment of neonatal sepsis. Further, well–controlled, prospective clinical trials need to be done to corroborate these findings.

Untargeted metabolomics analysis reveals key pathways responsible for the synergistic killing of colistin and doripenem combination against Acinetobacter baumannii

Combination therapy is deployed for the treatment of multidrug-resistant Acinetobacter baumannii, as it can rapidly develop resistance to current antibiotics. This is the first study to investigate the synergistic effect of colistin/doripenem combination on the metabolome of A. baumannii. The metabolite levels were measured using LC-MS following treatment with colistin (2 mg/L) or doripenem (25 mg/L) alone, and their combination at 15 min, 1 hr and 4 hr (n = 4). Colistin caused early (15 min and 1 hr) disruption of the bacterial outer membrane and cell wall, as demonstrated by perturbation of glycerophospholipids and fatty acids. Concentrations of peptidoglycan biosynthesis metabolites decreased at 4 hr by doripenem alone, reflecting its mechanism of action. The combination induced significant changes to more key metabolic pathways relative to either monotherapy. Down-regulation of cell wall biosynthesis (via D-sedoheptulose 7-phosphate) and nucleotide metabolism (via D-ribose 5-phosphate) was associated with perturbations in the pentose phosphate pathway induced initially by colistin (15 min and 1 hr) and later by doripenem (4 hr). We discovered that the combination synergistically killed A. baumannii via time-dependent inhibition of different key metabolic pathways. Our study highlights the significant potential of systems pharmacology in elucidating the mechanism of synergy and optimizing antibiotic pharmacokinetics/pharmacodynamics.

Clinical translation of polymyxin-based combination therapy: Facts, challenges and future opportunities

The emergence and spread of multidrug resistant Gram-negative bacteria has led to a resurgence in the clinical use of polymyxin antibiotics. However, the prevalence of polymyxin resistance is on the rise at an alarming rate, motivating the idea of combination therapy to sustain the revival of these "old" antibiotics. Although ample evidence in favor of combination therapy has emerged, it seems impracticable and confusing to find a promising combination from the diverse reports or gain adequate information on the efficacy and safety profile. With a stagnating discovery pipeline of novel antimicrobials, there is a clear need to fill the knowledge gaps in translating these basic research data to beneficial clinical practice. In this review, we examined the factors and ambiguities that stand as major hurdles in bringing polymyxin combination therapy to bedside care, highlighting the importance and urgency of incorporating translational research insights into areas of difficulty. We also discussed future research priorities that are essential to gather the necessary evidence and insights for promoting the best possible use of polymyxins in combination therapy.

Synergistic combinations of polymyxins

The proliferation of extensively drug-resistant Gram-negative pathogens has necessitated the therapeutic use of colistin and polymyxin B. However, treatment failures with polymyxin monotherapies and the emergence of polymyxin resistance have catalysed the search for polymyxin combinations that synergistically kill polymyxin-susceptible and -resistant organisms. This mini-review examines recent (2011-2016) in vitro and in vivo studies that have attempted to identify synergistic polymyxin combinations against Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii. Clinical evidence for the use of combination regimens is also discussed.

Optimizing dosing of antibiotics in critically ill patients

PURPOSE OF REVIEW

Recent studies suggest that contemporary antibiotic dosing is unlikely to achieve best outcomes for critically ill patients because of extensive pharmacokinetic variability and altered pharmacodynamics. Dose adaptation is considered quite challenging because of unpredictable dose-exposure relationships. Consequently, individualization of antibiotic dosing has been advocated. Herein, we describe recent developments in the optimization of antibiotic dosing in the critically ill.

RECENT FINDINGS

Conventional doses of many antibiotics frequently result in sub or supratherapeutic exposures in the critically ill. Clinical studies continue to illustrate that dose-exposure relationships are highly variable in severely ill patients. Dose optimization based on pharmacokinetic/pharmacodynamic principles can effectively improve antibiotic exposure. Therapeutic drug monitoring (TDM) with adaptive feedback is likely to be the most robust approach to optimize dosing for individual patients. This more accurate approach to dosing is made possible with the user-friendly dosing software that is emerging.

SUMMARY

The scope of TDM is broadening from the traditional focus on prevention of toxicity, to include optimization of antibiotic exposure thereby improving patient outcomes. However, the evidence relating TDM practice with improved clinical outcome remains limited. Well designed, multicentre, randomized controlled studies are warranted.

Verification of an Automated, Digital Dispensing Platform for At-Will Broth Microdilution-Based Antimicrobial Susceptibility Testing

With rapid emergence of multidrug-resistant bacteria, there is often a need to perform susceptibility testing for less commonly used or newer antimicrobial agents. Such testing can often be performed only by using labor-intensive, manual dilution methods and lies outside the capacity of most clinical labs, necessitating reference laboratory testing and thereby delaying the availability of susceptibility data. To address the compelling clinical need for microbiology laboratories to perform such testing in-house, we explored a novel, automated, at-will broth microdilution-based susceptibility testing platform. Specifically, we used the modified inkjet printer technology in the HP D300 digital dispensing system to dispense, directly from stock solutions into a 384-well plate, the 2-fold serial dilution series required for broth microdilution testing. This technology was combined with automated absorbance readings and data analysis to determine MICs. Performance was verified by testing members of the Enterobacteriaceae for susceptibility to ampicillin, cefazolin, ciprofloxacin, colistin, gentamicin, meropenem, and tetracycline in comparison to the results obtained with a broth microdilution reference standard. In precision studies, essential and categorical agreement levels were 96.8% and 98.3%, respectively. Furthermore, significantly fewer D300-based measurements were outside ±1 dilution from the modal MIC, suggesting enhanced reproducibility. In accuracy studies performed using a panel of 80 curated clinical isolates, rates of essential and categorical agreement and very major, major, and minor errors were 94%, 96.6%, 0%, 0%, and 3.4%, respectively. Based on these promising initial results, it is anticipated that the D300-based methodology will enable hospital-based clinical microbiology laboratories to perform at-will broth microdilution testing of antimicrobials and to address a critical testing gap.

Colistin and Polymyxin B Dosage Regimens against Acinetobacter baumannii: Differences in Activity and the Emergence of Resistance

Infections caused by multidrug-resistant Acinetobacter baumannii are a major public health problem, and polymyxins are often the last line of therapy for recalcitrant infections by such isolates. The pharmacokinetics of the two clinically used polymyxins, polymyxin B and colistin, differ considerably, since colistin is administered as an inactive prodrug that undergoes slow conversion to colistin. However, the impact of these substantial pharmacokinetic differences on bacterial killing and resistance emergence is poorly understood. We assessed clinically relevant polymyxin B and colistin dosage regimens against one reference and three clinical A. baumannii strains in a dynamic one-compartment in vitro model. A new mechanism-based pharmacodynamic model was developed to describe and predict the drug concentrations and viable counts of the total and resistant populations. Rapid attainment of target concentrations was shown to be critical for polymyxin-induced bacterial killing. All polymyxin B regimens achieved peak concentrations of at least 1 mg/liter within 1 h and caused ≥4 log10 killing at 1 h. In contrast, the slow rise of colistin concentrations to 3 mg/liter over 48 h resulted in markedly reduced bacterial killing. A significant (4 to 6 log10 CFU/ml) amplification of resistant bacterial populations was common to all dosage regimens. The developed mechanism-based model explained the observed bacterial killing, regrowth, and resistance. The model also implicated adaptive polymyxin resistance as a key driver of bacterial regrowth and predicted the amplification of preexisting, highly polymyxin-resistant bacterial populations following polymyxin treatment. Antibiotic combination therapies seem the most promising option for minimizing the emergence of polymyxin resistance.

Colistin combination therapy improves microbiologic cure in critically ill patients with multi-drug resistant gram-negative pneumonia

Currently, in vitro synergy with colistin has not translated into improved clinical outcomes. This study aimed to compare colistin combination therapy to colistin monotherapy in critically ill patients with multi-drug resistant gram-negative (MDR-GN) pneumonia. This was a retrospective analysis of critically ill adult patients receiving intravenous colistin for MDR-GN pneumonia comparing colistin combination therapy to colistin monotherapy with a primary endpoint of clinical cure. Combination therapy was defined by administration of another antibiotic to which the MDR-GN pathogen was reported as susceptible or intermediate. Ninety patients were included for evaluation (41 combination therapy and 49 monotherapy). Baseline characteristics were similar between groups. No difference in clinical cure was observed between combination therapy and monotherapy in univariate analysis, nor when adjusted for APACHE II score and time to appropriate antibiotic therapy (57.1 vs. 63.4 %, adjusted OR 1.15, p = 0.78). Microbiological cure was significantly higher for combination therapy (87 vs. 35.5 %, p < 0.001). Colistin combination therapy was associated with a significant improvement in microbiological cure, without improvement in clinical cure. Based on the in vitro synergy and improvement in microbiological clearance, colistin combination therapy should be prescribed for MDR-GN pneumonia. Further research is warranted to determine if in vitro synergy with colistin translates into improved clinical outcomes.

Polymyxin Resistance in Acinetobacter baumannii: Genetic Mutations and Transcriptomic Changes in Response to Clinically Relevant Dosage Regimens

Polymyxins are often last-line therapeutic agents used to treat infections caused by multidrug-resistant A. baumannii. Recent reports of polymyxin-resistant A. baumannii highlight the urgent need for research into mechanisms of polymyxin resistance. This study employed genomic and transcriptomic analyses to investigate the mechanisms of polymyxin resistance in A. baumannii AB307-0294 using an in vitro dynamic model to mimic four different clinically relevant dosage regimens of polymyxin B and colistin over 96 h. Polymyxin B dosage regimens that achieved peak concentrations above 1 mg/L within 1 h caused significant bacterial killing (~5 log₁₀CFU/mL), while the gradual accumulation of colistin resulted in no bacterial killing. Polymyxin resistance was observed across all dosage regimens; partial reversion to susceptibility was observed in 6 of 8 bacterial samples during drug-free passaging. Stable polymyxin-resistant samples contained a mutation in pmrB. The transcriptomes of stable and non-stable polymyxin-resistant samples were not substantially different and featured altered expression of genes associated with outer membrane structure and biogenesis. These findings were further supported via integrated analysis of previously published transcriptomics data from strain ATCC19606. Our results provide a foundation for understanding the mechanisms of polymyxin resistance following exposure to polymyxins and the need to explore effective combination therapies.

A Novel Chemical Biology Approach for Mapping of Polymyxin Lipopeptide Antibody Binding Epitopes

Polymyxins B and E (i.e., colistin) are a family of naturally occurring lipopeptide antibiotics that are our last line of defense against multidrug resistant (MDR) Gram-negative pathogens. Unfortunately, nephrotoxicity is a dose-limiting factor for polymyxins that limits their clinical utility. Our recent studies demonstrate that polymyxin-induced nephrotoxicity is a result of their extensive accumulation in renal tubular cells. The design and development of safer, novel polymyxin lipopeptides is hampered by our limited understanding of their complex structure-nephrotoxicity relationships. This is the first study to employ a novel targeted chemical biology approach to map the polymyxin recognition epitope of a commercially available polymyxin mAb and demonstrate its utility for mapping the kidney distribution of a novel, less nephrotoxic polymyxin lipopeptide. Eighteen novel polymyxin lipopeptide analogues were synthesized with modifications in the polymyxin core domains, namely, the N-terminal fatty acyl region, tripeptide linear segment, and cyclic heptapeptide. Surface plasmon resonance epitope mapping revealed that the monoclonal antibody (mAb) recognition epitope consisted of the hydrophobic domain (N-terminal fatty acyl and position 6/7) and diaminobutyric acid (Dab) residues at positions 3, 5, 8, and 9 of the polymyxin molecule. Structural diversity within the hydrophobic domains and Dab 3 position are tolerated. Enlightened with an understating of the structure-binding relationships between the polymyxin mAb and the core polymyxin scaffold, we can now rationally employ the mAb to probe the kidney distribution of novel polymyxin lipopeptides. This information will be vital in the design of novel, safer polymyxins through chemical tailoring of the core scaffold and exploration of the elusive/complex polymyxin structure-nephrotoxicity relationships.

Colistin Use in Patients With Reduced Kidney Function

Colistin (polymyxin E) is a mainly concentration-dependent bactericidal antimicrobial active against multidrug-resistant Gram-negative bacteria. After being abandoned over the past 30 years due to its neuro- and nephrotoxicity, colistin has been reintroduced recently as a last-resort drug for the treatment of multidrug-resistant Gram-negative bacteria infections in combination with other antimicrobials. Unfortunately, although renal toxicity is a well-known dose-related adverse effect of colistin, relatively few studies are currently available on its peculiar pharmacodynamic/pharmacokinetic properties in clinical settings at high risk for drug accumulation, such as acute or chronic kidney disease. In these specific contexts, the risk for underdosing is also substantial because colistin can be easily removed by dialysis/hemofiltration, especially when the most efficient modalities of renal replacement therapy (RRT) are used in critically ill patients. For this reason, recent recommendations in patients undergoing RRT have shifted toward higher dosing regimens, and therapeutic drug monitoring is advised. This review aims to summarize the main issues related to chemical structure, pharmacodynamics/pharmacokinetics, and renal toxicity of colistin. Moreover, recent data and current recommendations concerning colistin dosing in patients with reduced kidney function, with special regard to those receiving RRT such as dialysis or hemofiltration, are also discussed.

Insights into Newer Antimicrobial Agents Against Gram-negative Bacteria

Currently, drug resistance, especially against cephalosporins and carbapenems, among gram-negative bacteria is an important challenge, which is further enhanced by the limited availability of drugs against these bugs. There are certain antibiotics (colistin, fosfomycin, temocillin, and rifampicin) that have been revived from the past to tackle the menace of superbugs, including members of Enterobacteriaceae, Acinetobacter species, and Pseudomonas species. Very few newer antibiotics have been added to the pool of existing drugs. There are still many antibiotics that are passing through various phases of clinical trials. The initiative of Infectious Disease Society of America to develop 10 novel antibiotics against gram-negative bacilli by 2020 is a step to fill the gap of limited availability of drugs. This review aims to provide insights into the current and newer drugs in pipeline for the treatment of gram-negative bacteria and also discusses the major challenging issues for their management.

Dynamic interaction of colistin and meropenem on a WT and a resistant strain of Pseudomonas aeruginosa as quantified in a PK/PD model

OBJECTIVES

Combination therapy can be a strategy to ensure effective bacterial killing when treating Pseudomonas aeruginosa, a Gram-negative bacterium with high potential for developing resistance. The aim of this study was to develop a pharmacokinetic/pharmacodynamic (PK/PD) model that describes the in vitro bacterial time-kill curves of colistin and meropenem alone and in combination for one WT and one meropenem-resistant strain of P. aeruginosa.

METHODS

In vitro time-kill curve experiments were conducted with a P. aeruginosa WT (ATCC 27853) (MICs: meropenem 1 mg/L; colistin 1 mg/L) and a meropenem-resistant type (ARU552) (MICs: meropenem 16 mg/L; colistin 1.5 mg/L). PK/PD models characterizing resistance were fitted to the observed bacterial counts in NONMEM. The final model was applied to predict the bacterial killing of ARU552 for different combination dosages of colistin and meropenem.

RESULTS

A model with compartments for growing and resting bacteria, where the bacterial killing by colistin reduced with continued exposure and a small fraction (0.15%) of the start inoculum was resistant to meropenem, characterized the bactericidal effect and resistance development of the two antibiotics. For a typical patient, a loading dose of colistin combined with a high dose of meropenem (2000 mg q8h) was predicted to result in a pronounced kill of the meropenem-resistant strain over 24 h.

CONCLUSIONS

The developed PK/PD model successfully described the time course of bacterial counts following exposures to colistin and meropenem, alone and in combination, for both strains, and identified a dynamic drug interaction. The study illustrates the application of a PK/PD model and supports high-dose combination therapy of colistin and meropenem to overcome meropenem resistance.