Pharmacokinetics and pharmacodynamics of 'old' polymyxins: what is new?

Structural basis of undecaprenyl phosphate glycosylation leading to polymyxin resistance in Gram-negative bacteria

In Gram-negative bacteria, the enzymatic modification of Lipid A with aminoarabinose (L-Ara4N) leads to resistance against polymyxin antibiotics and cationic antimicrobial peptides. ArnC, an integral membrane glycosyltransferase, attaches a formylated form of aminoarabinose to the lipid undecaprenyl phosphate, enabling its association with the bacterial inner membrane. Here, we present cryo-electron microscopy structures of ArnC from S. enterica in apo and nucleotide-bound conformations. These structures reveal a conformational transition that takes place upon binding of the partial donor substrate. Using coarse-grained and atomistic simulations, we provide insights into substrate coordination before and during catalysis, and we propose a catalytic mechanism that may operate on all similar metal-dependent polyprenyl phosphate glycosyltransferases. The reported structures provide a new target for drug design aiming to combat polymyxin resistance.

Three methods to optimise polymyxin B dosing using estimated AUC after first dose: validation with the data generated by Monte Carlo simulation

To achieve the AUC-guided dosing, we proposed three methods to estimate polymyxin B AUC across 24 h at steady state (AUCSS,24h) using limited concentrations after its first dose.Monte Carlo simulation based on a well-established population PK model was performed to generate the PK profiles of 1000 patients with normal or abnormal renal function. Polymyxin B AUCSS,24h was estimated for each subject using three methods (two-point PK approach, three-point PK approach, and four-point PK approach) based on limited concentration data in its first dose and compared with the actual AUC at steady state calculated using the linear-trapezoidal formula.In patients with normal renal function, the mean bias of two-point PK approach, three-point PK approach, and four-point PK approach was -8.73%, 1.37%, and -0.48%, respectively. The corresponding value was -11.15%, 1.99%, and -0.28% in patients with renal impairment, respectively. The largest mean bias of two-point PK approach, three-point PK approach, and four-point PK approach was -12.63%, -6.47%, and -0.54% when the sampling time shifted.The Excel calculators designed based on the three methods can be potentially used to optimise the dosing regimen of polymyxin B in the clinic.

The use of combination therapy for the improvement of colistin activity against bacterial biofilm

Colistin is used as a last resort for the management of infections caused by multi-drug resistant (MDR) bacteria. However, the use of this antibiotic could lead to different side effects, such as nephrotoxicity, in most patients, and the high prevalence of colistin-resistant strains restricts the use of colistin in the clinical setting. Additionally, colistin could induce resistance through the increased formation of biofilm; biofilm-embedded cells are highly resistant to antibiotics, and as with other antibiotics, colistin is impaired by bacteria in the biofilm community. In this regard, the researchers used combination therapy for the enhancement of colistin activity against bacterial biofilm, especially MDR bacteria. Different antibacterial agents, such as antimicrobial peptides, bacteriophages, natural compounds, antibiotics from different families, N-acetylcysteine, and quorum-sensing inhibitors, showed promising results when combined with colistin. Additionally, the use of different drug platforms could also boost the efficacy of this antibiotic against biofilm. The mentioned colistin-based combination therapy not only could suppress the formation of biofilm but also could destroy the established biofilm. These kinds of treatments also avoided the emergence of colistin-resistant subpopulations, reduced the required dosage of colistin for inhibition of biofilm, and finally enhanced the dosage of this antibiotic at the site of infection. However, the exact interaction of colistin with other antibacterial agents has not been elucidated yet; therefore, further studies are required to identify the precise mechanism underlying the efficient removal of biofilms by colistin-based combination therapy.

Synergistic interaction of polymyxin B with carvacrol: antimicrobial strategy against polymyxin-resistant Klebsiella pneumoniae

Objective: The antimicrobial activities of the synergistic combination of carvacrol and polymyxin B against polymyxin-resistant Klebsiella pneumoniae were evaluated. Methods: The methods employed checkerboard assays to investigate synergism, biofilm inhibition assessment and membrane integrity assay. In addition, the study included in vivo evaluation using a mouse infection model. Results: The checkerboard method evaluated 48 combinations, with 23 indicating synergistic action. Among these, carvacrol 10 mg/kg plus polymyxin B 2 mg/kg exhibited in vivo antimicrobial activity in a mouse model of infection, resulting in increased survival and a significant decrease in bacterial load in the blood. Conclusion: Polymyxin in synergy with carvacrol represents a promising alternative to be explored in the development of new antimicrobials.

Simple and robust LC-MS/MS method for quantification of colistin methanesulfonate and colistin in human plasma for therapeutic drug monitoring

A rapid, simple, and robust LC-MS/MS method was developed and validated for the quantitation of colistin and colistin methanesulfonate (CMS) in human plasma. The method also prevented overestimation of colistin concentration by establishing the stability of CMS under sample preparation conditions, including blood and plasma storage conditions. Polymyxin B1 was used as an internal standard, and positive-ion electrospray ionization in multiple reaction monitoring mode was used for quantification. Chromatographic separation was achieved using a Zorbax eclipse C18 column (3.5 µm, 2.1 mm i.d. × 100 mm), with a flow rate of 0.5 mL/min, 5 μL injection volume, and gradient elution with a mixture of acetonitrile-water (containing 0.1 % trifluoroacetic acid). The method had a quantifiable range of 0.043-8.61 and 0.057-11.39 μg/mL for colistin A and B in human plasma, respectively, under a total runtime of 6.0 min. Further, it demonstrated appropriate extraction efficiency, no significant interference from co-eluting endogenous compounds, and satisfactory intraday and interday precision and accuracy. The proposed procedure for sample preparation successfully addressed the issue of CMS instability, consequently diminishing the probability of overestimating the concentration of colistin. Therefore, this simple and robust LC-MS/MS method for CMS and colistin quantification in human plasma is a valuable tool for clinicians to accurately monitor colistin treatment in patients with infections caused by multidrug-resistant (MDR) Gram-negative bacteria.

A population pharmacokinetic model of polymyxin B based on prospective clinical data to inform dosing in hospitalized patients

OBJECTIVES

To develop a population pharmacokinetic (PK) model with data from the largest polymyxin B-treated patient population studied to date to optimize its dosing in hospitalized patients.

METHODS

Hospitalized patients receiving intravenous polymyxin B for ≥48 hours were enrolled. Blood samples were collected at steady state and drug concentrations were analysed by liquid chromotography tandem mass spectrometry (LC-MS/MS). Population PK analysis and Monte Carlo simulations were performed to determine the probability of target attainment (PTA).

RESULTS

One hundred and forty-two patients received intravenous polymyxin B (1.33-6 mg/kg/day), providing 681 plasma samples. Twenty-four patients were on renal replacement therapy, including 13 on continuous veno-venous hemodiafiltration (CVVHDF). A 2-compartment model adequately described the PK with body weight as a covariate on the volume of distribution that affected Cmax, but it did not impact clearance or exposure. Creatinine clearance was a statistically significant covariate on clearance, although clinically relevant variations of dose-normalized drug exposure were not observed across a wide creatinine clearance range. The model described higher clearance in CVVHDF patients than in non-CVVHDF patients. Maintenance doses of ≥2.5 mg/kg/day or ≥150 mg/day had a PTA ≥90% (for non-pulmonary infections target) at a steady state for minimum inhibitory concentrations ≤2 mg/L. The PTA at a steady state for CVVHDF patients was lower.

DISCUSSION

Fixed loading and maintenance doses of polymyxin B seemed to be more appropriate than weight-based dosing regimens in patients weighing 45-90 kg. Higher doses may be needed in patients on CVVHDF. Substantial variability in polymyxin B clearance and volume of distribution was found, suggesting that therapeutic drug monitoring may be indicated.

Transcriptomic Responses to Polymyxin B and Analogues in Human Kidney Tubular Cells

Polymyxins are last-line antibiotics for the treatment of Gram-negative 'superbugs'. However, nephrotoxicity can occur in up to 60% of patients administered intravenous polymyxins. The mechanisms underpinning nephrotoxicity remain unclear. To understand polymyxin-induced nephrotoxicity, human renal proximal tubule cells were treated for 24 h with 0.1 mM polymyxin B or two new analogues, FADDI-251 or FADDI-287. Transcriptomic analysis was performed, and differentially expressed genes (DEGs) were identified using ANOVA (FDR < 0.2). Cell viability following treatment with polymyxin B, FADDI-251 or FADDI-287 was 66.0 ± 5.33%, 89.3 ± 3.96% and 90.4 ± 1.18%, respectively. Transcriptomics identified 430, 193 and 150 DEGs with polymyxin B, FADDI-251 and FADDI-287, respectively. Genes involved with metallothioneins and Toll-like receptor pathways were significantly perturbed by all polymyxins. Only polymyxin B induced perturbations in signal transduction, including FGFR2 and MAPK signaling. SIGNOR network analysis showed all treatments affected essential regulators in the immune system, autophagy, cell cycle, oxidative stress and apoptosis. All polymyxins caused significant perturbations of metal homeostasis and TLR signaling, while polymyxin B caused the most dramatic perturbations of the transcriptome. This study reveals the impact of polymyxin structure modifications on transcriptomic responses in human renal tubular cells and provides important information for designing safer new-generation polymyxins.

The Impact of Colistin Resistance on the Activation of Innate Immunity by Lipopolysaccharide Modification

Colistin resistance is acquired by different lipopolysaccharide (LPS) modifications. We proposed to evaluate the of effect in vivo colistin resistance acquisition on the innate immune response. We used a pair of ST11 clone Klebsiella pneumoniae strains: an OXA-48, CTX-M-15 K. pneumoniae strain susceptible to colistin (CS-Kp) isolated from a urinary infection and its colistin-resistant variant (CR-Kp) from the same patient after prolonged treatment with colistin. No mutation of previously described genes for colistin resistance (pmrA, pmrB, mgrB, phoP/Q, arnA, arnC, arnT, ugdH, and crrAB) was found in the CR-Kp genome; however, LPS modifications were characterized by negative-ion matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. The strains were cocultured with human monocytes to determine their survival after phagocytosis and induction to apoptosis. Also, monocytes were stimulated with bacterial LPS to study cytokine and immune checkpoint production. The addition of 4-amino-4-deoxy-l-arabinose (Ara4N) to lipid A of CR-Kp accounted for the colistin resistance. CR-Kp survived significantly longer inside human monocytes after being phagocytosed than did the CS-Kp strain. In addition, LPS from CR-Kp induced both higher apoptosis in monocytes and higher levels of cytokine and immune checkpoint production than LPS from CS-Kp. Our data reveal a variable impact of colistin resistance on the innate immune system, depending on the responsible mechanism. Adding Ara4N to LPS in K. pneumoniae increases bacterial survival after phagocytosis and elicits a higher inflammatory response than its colistin-susceptible counterpart.

The Blood-Brain Barrier and Pharmacokinetic/Pharmacodynamic Optimization of Antibiotics for the Treatment of Central Nervous System Infections in Adults

Bacterial central nervous system (CNS) infections are serious and carry significant morbidity and mortality. They encompass many syndromes, the most common being meningitis, which may occur spontaneously or as a consequence of neurosurgical procedures. Many classes of antimicrobials are in clinical use for therapy of CNS infections, some with established roles and indications, others with experimental reporting based on case studies or small series. This review delves into the specifics of the commonly utilized antibacterial agents, updating their therapeutic use in CNS infections from the pharmacokinetic and pharmacodynamic perspectives, with a focus on the optimization of dosing and route of administration that have been described to achieve good clinical outcomes. We also provide a concise synopsis regarding the most focused, clinically relevant information as pertains to each class and subclass of antimicrobial therapeutics. CNS infection morbidity and mortality remain high, and aggressive management is critical in ensuring favorable patient outcomes while averting toxicity and upholding patient safety.

Next-Generation Polymyxin Class of Antibiotics: A Ray of Hope Illuminating a Dark Road

Although new-generation antimicrobials, in particular β-lactam/β-lactamase inhibitors, have largely replaced polymyxins in carbapenem-resistant Gram-negative bacterial infections, polymyxins are still needed for carbapanem-resistant Acinetobacter baumannii infections and in settings where novel agents are not readily available. Despite their potent in vitro activity, the clinical utility of polymyxins is significantly limited by their pharmacokinetic properties and nephrotoxicity risk. There is significant interest, therefore, in developing next-generation polymyxins with activity against colistin-resistant strains and lower toxicity than existing polymyxins. In this review, we aim to present the antibacterial activity mechanisms, in vitro and in vivo efficacy data, and toxicity profiles of new-generation polymyxins, including SPR206, MRX-8, and QPX9003, as well as the general characteristics of old polymyxins. Considering the emergence of colistin-resistant strains particularly in endemic regions, the restoration of the antimicrobial activity of polymyxins via PBT2 is also described in this review.

The combination effect of meropenem/sulbactam/polymyxin-B on the pharmacodynamic parameters for mutant selection windows against carbapenem-resistant Acinetobacter baumannii

The objective of this study was to evaluate whether combinations of sulbactam, meropenem, and polymyxin-B could reduce or close the gap of mutant selection window (MSW) of individual antibiotics against Acinetobacter baumannii harboring OXA-23. MICs of three antimicrobials used alone and in combination (meropenem/polymyxin-B or meropenem/polymyxin-B/sulbactam) were obtained in 11 clinical isolates and mutant prevention concentrations were determined in 4 of the 11 isolates. All isolates were resistant to meropenem or polymyxin-B. Combining meropenem and polymyxin-B with or without sulbactam resulted in synergistic bactericidal activities. Pharmacokinetic (PK) simulations of drug concentrations in the blood and epithelial lining fluid coupled with pharmacodynamic (PD) evaluations revealed that the fractions of time over the 24-h in terms of free drug concentration within the MSW (fT_MSW) and above the MPC (fT_>MPC) were optimized by combination therapy. The resultant clinical regimens of meropenem, polymyxin-B, and sulbactam evaluated in the PK-PD analysis were 2 g q8h, 2.5 mg/kg loading dose followed by 1.5 mg/kg q12h, and 3 g q8h, respectively, in patients with normal renal function. Subsequent corresponding equivalent exposure regimens would depend on the extent of renal failure. The overall results indicate that combination antibiotics consisting of sulbactam/meropenem/polymyxin-B can confer potential efficacy against A. baumannii harboring OXA-23, and reduce the opportunity for bacteria to develop further resistance. This study provides a framework for pharmacodynamic evaluation of drug-resistant mutant suppression in an antimicrobial co-administration setting. The results thereby lay the groundwork for additional studies and future clinical confirmation is warranted.

Pharmacokinetic Characteristics of Nebulized Colistimethate Sodium Using Two Different Types of Nebulizers in Critically Ill Patients with Ventilator-Associated Respiratory Infections

Background: Rising antimicrobial resistance has led to a revived interest in inhaled colistin treatment in the critically ill patient with ventilator-associated respiratory infection (VARI). Nebulization via vibrating mesh nebulizers (VMNs) is considered the current standard-of-care, yet the use of generic jet nebulizers (JNs) is more widespread. Few data exist on the intrapulmonary pharmacokinetics of colistin when administered through VMNs, while there is a complete paucity regarding the use of JNs. Methods: In this study, 18 VARI patients who received 2 million international units of inhaled colistimethate sodium (CMS) through a VMN were pharmacokinetically compared with six VARI patients who received the same drug dose through a JN, in the absence of systemic CMS administration. Results: Surprisingly, VMN and JN led to comparable formed colistin exposures in the epithelial lining fluid (ELF) (median (IQR) AUC0-24: 86.2 (46.0-185.9) mg/L∙h with VMN and 91.5 (78.1-110.3) mg/L∙h with JN). The maximum ELF concentration was 10.4 (4.7-22.6) mg/L and 7.4 (6.2-10.3) mg/L, respectively. Conclusions: Based on our results, JN might be considered a viable alternative to the theoretically superior VMN. Therapeutic drug monitoring in the ELF can be advised due to the observed low exposure, high variability, and appreciable systemic absorption.

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.

The Effect of Different Colistin Dosing Regimens on Nephrotoxicity: A Cohort Study

(1) Background: It is not known whether different daily dosing schemes have different effects on colistin nephrotoxicity. We examined the effect of once- versus twice- or thrice-daily doses of colistin on renal function. (2) Methods: We performed a multicenter retrospective cohort study of hospitalized patients with a baseline glomerular filtration rate ≥ 50 mL/min who received intravenously the same colistin dose once (regimen A), twice (regimen B) or thrice daily (regimen C). The primary endpoint was acute kidney injury (AKI), defined as fulfilment of any of the RIFLE (Risk-Injury-Failure-Loss-End stage renal disease) criteria. (3) Results: We included 306 patients; 132 (43.1%) received regimen A, 151 (49.3%) regimen B, and 23 (7.5%) regimen C. Ninety-nine (32.4%) patients developed AKI; there was no difference between regimen A vs. B and C [45 (34.1%) vs. 54 (31.0%), p = 0.57]. In a propensity score−matched cohort, AKI was similar in patients receiving Regimen A, Regimen B, and Regimen C (31.6% vs. 33.3%, p = 0.78). On logistic regression analysis, diabetes was an independent predictor of AKI (OR = 4.59, 95% CI 2.03−10.39, p = 0.001) while eGFR > 80 mL/min (OR = 0.50, 95% CI 0.25−0.99, p = 0.048) was inversely associated with AKI. (4) Conclusions: Colistin once daily is not more nephrotoxic than the standard colistin regimens. The only independent predictor of nephrotoxicity was diabetes mellitus, while eGFR > 80 mL/min had a protective effect.

Nanostructure Control of an Antibiotic-based Polyion Complex Using a Series of Polycations with Different Side-chain Modification Rates

Developing nanovehicles for delivering antibiotics is a promising approach to overcome the issue of antibiotic resistance. This study aims to utilize a polyion complex (PICs) system for developing novel nanovehicles for polymyxin-type antibiotics, which are known as last resort drugs. The formation of antibiotic-based PIC nanostructures was investigated using colistimethate sodium (CMS), an anionic cyclic short peptide, and a series of block catiomers bearing different amounts of guanidinium moieties on their side chains. In addition, only the modified catiomer, and not the unmodified catiomer, self-assembles with CMS, implying the importance of the guanidine moieties for enhancing the interaction between the catiomer and CMS via the formation of multivalent hydrogen bonding. Moreover, micellar and vesicular PIC nanostructures are selectively formed depending on the ratio of the guanidine residues. Size-exclusion chromatography revealed that the encapsulation efficiency of CMS is dependent on the guanidinium modification ratio. The antimicrobial activity of the PIC nanostructures is also confirmed, indicating that the complexation of CMS in the PICs and further release from the PICs successfully occurs. This article is protected by copyright. All rights reserved.

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.

The Role of Colistin in the Era of New β-Lactam/β-Lactamase Inhibitor Combinations

With the current crisis related to the emergence of carbapenem-resistant Gram-negative bacteria (CR-GNB), classical treatment approaches with so-called “old-fashion antibiotics” are generally unsatisfactory. Newly approved β-lactam/β-lactamase inhibitors (BLBLIs) should be considered as the first-line treatment options for carbapenem-resistant Enterobacterales (CRE) and carbapenem-resistant Pseudomonas aeruginosa (CRPA) infections. However, colistin can be prescribed for uncomplicated lower urinary tract infections caused by CR-GNB by relying on its pharmacokinetic and pharmacodynamic properties. Similarly, colistin can still be regarded as an alternative therapy for infections caused by carbapenem-resistant Acinetobacter baumannii (CRAB) until new and effective agents are approved. Using colistin in combination regimens (i.e., including at least two in vitro active agents) can be considered in CRAB infections, and CRE infections with high risk of mortality. In conclusion, new BLBLIs have largely replaced colistin for the treatment of CR-GNB infections. Nevertheless, colistin may be needed for the treatment of CRAB infections and in the setting where the new BLBLIs are currently unavailable. In addition, with the advent of rapid diagnostic methods and novel antimicrobials, the application of personalized medicine has gained significant importance in the treatment of CRE infections.

Global metabolic regulation in Vibrio parahaemolyticus under polymyxin B stimulation

Vibrio parahaemolyticus is responsible for infection diseases of people who consume the contaminated seafood, but its metabolic regulation profile in response to colistin, the last treatment option for multidrug-resistant Gram-negative bacteria, remains unclear. In this study, the metabolic regulation profile of V. parahaemolyticus ATCC33846 under polymyxin B stimulation has been investigated. V. parahaemolyticus exposed to polymyxin B resulted in 4597 differentially transcribed genes, including 673 significantly up-regulated genes and 569 significantly down-regulated genes. In V. parahaemolyticus under polymyxin B stimulation, the cellular antioxidant systems to prevent bacteria from oxidant stress was activated, the synthesis of some nonessential macromolecules was reduced, and the assembly and modification of lipopolysaccharide and peptidoglycan to resist the attack from other antibiotics were promoted. These findings provide new insights into polymyxin B-related stress response in V. parahaemolyticus which should be useful for developing novel drugs for infection.

Nebulized Colistin in Ventilator-Associated Pneumonia and Tracheobronchitis: Historical Background, Pharmacokinetics and Perspectives

Clinical evidence suggests that nebulized colistimethate sodium (CMS) has benefits for treating lower respiratory tract infections caused by multidrug-resistant Gram-negative bacteria (GNB). Colistin is positively charged, while CMS is negatively charged, and both have a high molecular mass and are hydrophilic. These physico-chemical characteristics impair crossing of the alveolo-capillary membrane but enable the disruption of the bacterial wall of GNB and the aggregation of the circulating lipopolysaccharide. Intravenous CMS is rapidly cleared by glomerular filtration and tubular excretion, and 20-25% is spontaneously hydrolyzed to colistin. Urine colistin is substantially reabsorbed by tubular cells and eliminated by biliary excretion. Colistin is a concentration-dependent antibiotic with post-antibiotic and inoculum effects. As CMS conversion to colistin is slower than its renal clearance, intravenous administration can lead to low plasma and lung colistin concentrations that risk treatment failure. Following nebulization of high doses, colistin (200,000 international units/24h) lung tissue concentrations are > five times minimum inhibitory concentration (MIC) of GNB in regions with multiple foci of bronchopneumonia and in the range of MIC breakpoints in regions with confluent pneumonia. Future research should include: (1) experimental studies using lung microdialysis to assess the PK/PD in the interstitial fluid of the lung following nebulization of high doses of colistin; (2) superiority multicenter randomized controlled trials comparing nebulized and intravenous CMS in patients with pandrug-resistant GNB ventilator-associated pneumonia and ventilator-associated tracheobronchitis; (3) non-inferiority multicenter randomized controlled trials comparing nebulized CMS to intravenous new cephalosporines/ß-lactamase inhibitors in patients with extensive drug-resistant GNB ventilator-associated pneumonia and ventilator-associated tracheobronchitis.

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.

Steady-state pharmacokinetic and pharmacodynamic profiling of colistin in critically ill patients with multi-drug-resistant gram-negative bacterial infections, along with differences in clinical, microbiological and safety outcome

Limited data are present regarding the steady-state pharmacokinetics and pharmacodynamics of colistin in critically ill patients suffering from multi-drug-resistant gram-negative bacterial (MDR-GNB) infections. We aimed to profile the steady-state pharmacokinetics and pharmacodynamics of colistin in critically ill patients with MDR-GNB infections, along with determining the predictors that could influence the clinical, microbiological and safety outcome. We recruited 30 critically ill patients suffering from MDR-GNB infections in our prospective open-label study. Intravenous colistimethate sodium (CMS) 2 million IU was administered concurrently with inhalational CMS 1 million IU every 8 hours. Steady-state plasma colistin levels were measured. Logistic regression analysis was used to identify various predictors of clinical, microbiological and safety outcome. A large variability was observed in the steady-state colistin pharmacokinetic/pharmacodynamic parameters, along with the factors that influenced the clinical, microbiological and safety outcome. In conclusion, steady-state colistin pharmacokinetic and pharmacodynamic parameters observed in our study were largely consistent with those reported in previous studies. High acute physiology and chronic health evaluation II scores were associated with poor clinical outcome. Log-transformed colistin maximum concentration, area under the plasma concentration curve for 8 hours, apparent total body clearance and apparent volume of distribution were significantly associated with the safety outcome.

Polymyxins and Bacterial Membranes: A Review of Antibacterial Activity and Mechanisms of Resistance

Following their initial discovery in the 1940s, polymyxin antibiotics fell into disfavor due to their potential clinical toxicity, especially nephrotoxicity. However, the dry antibiotic development pipeline, together with the rising global prevalence of infections caused by multidrug-resistant (MDR) Gram-negative bacteria have both rejuvenated clinical interest in these polypeptide antibiotics. Parallel to the revival of their use, investigations into the mechanisms of action and resistance to polymyxins have intensified. With an initial known effect on biological membranes, research has uncovered the detailed molecular and chemical interactions that polymyxins have with Gram-negative outer membranes and lipopolysaccharide structure. In addition, genetic and epidemiological studies have revealed the basis of resistance to these agents. Nowadays, resistance to polymyxins in MDR Gram-negative pathogens is well elucidated, with chromosomal as well as plasmid-encoded, transferrable pathways. The aims of the current review are to highlight the important chemical, microbiological, and pharmacological properties of polymyxins, to discuss their mechanistic effects on bacterial membranes, and to revise the current knowledge about Gram-negative acquired resistance to these agents. Finally, recent research, directed towards new perspectives for improving these old agents utilized in the 21st century, to combat drug-resistant pathogens, is summarized.

Population Pharmacokinetics and Limited Sampling Strategy for Therapeutic Drug Monitoring of Polymyxin B in Chinese Patients With Multidrug-Resistant Gram-Negative Bacterial Infections

Polymyxin B is used as a last therapeutic option for the treatment of multidrug-resistant Gram-negative bacterial infections. This study aimed to develop a population pharmacokinetic model and limited sampling strategy, a method to estimate the area under the concentration curve (AUC) by using a limited number of samples, to assist therapeutic drug monitoring of polymyxin B in Chinese patients. Population pharmacokinetic analysis was performed using Phoenix^® NLME with data obtained from 46 adult patients at steady state. Various demographic variables were investigated as potential covariates for population pharmacokinetic modeling. The limited sampling strategies based on the Bayesian approach and multiple linear regression were validated using the intraclass correlation coefficient and Bland-Altman analysis. As a result, the data was described by a two-compartment population pharmacokinetic model. Through the modeling, creatinine clearance was found to be a statistically significant covariate influencing polymyxin B clearance. The limited sampling strategies showed the two-point model (C_0h and C_2h) could predict polymyxin B exposure with good linear relativity (r² > 0.98), and the four-point model (C_1h, C1_.5h, C_4h, and C_8h) performed best in predicting polymyxin B AUC (r² > 0.99). In conclusion, this study successfully developed a population pharmacokinetic model and limited sampling strategies that could be applied in clinical practice to assist in therapeutic drug monitoring of polymyxin B in Chinese patients.

Comparison of the recommended colistin susceptibility testing methods with colistin gradient strips and semi-automated method for antimicrobial-resistant non-fermenting rods

An increased frequency of multidrug-resistant non-fermenting rods isolation has resulted in the excessive use of colistin - often the last chance antimicrobial. However, determination of colistin susceptibility is difficult, mainly because of its structure and limited diffusion properties. This study was performed to compare colistin susceptibility testing among Pseudomonas aeruginosa (n = 49) and Acinetobacter baumannii (n = 49) strains. Four methods were applied: colistin gradient strips (Liofilchem, Italy), semi-automated method Phoenix BD (Becton Dickinson, USA) and two broth microdilution methods: SensiTest Colistin (Liofilchem, Italy) and MICRONAUT MIC-Strip (MERLIN Diagnostika GmbH, Germany). Data were analyzed by comparison of MIC values and strains susceptibility interpretation criteria (resistant and sensitive, respectively). The same interpretation results were obtained for 46 (93.9%) P. aeruginosa and 37 (75.5%) A. baumannii isolates in all of the applied methods. Using broth microdilution methods, the same interpretation was obtained for 48 (98.0%) P. aeruginosa and 42 (85.7%) A. baumannii isolates. The results obtained by colistin gradient strips usually confirm the results of broth microdilution tests for P. aeruginosa isolates, the automated method is in turn less labor-intensive. However, MIC values, obtained with their use, are less precise because of the antibiotic dilutions limited to only several concentrations. The results underline the importance of choosing of the appropriate type of method, also among those recommended and commercially available.

Extensive Therapeutic Drug Monitoring of Colistin in Critically Ill Patients Reveals Undetected Risks

(1) Background: With the rise of multi-/pan-drug resistant (MDR/PDR) pathogens, the less utilized antibiotic Colistin has made a comeback. Colistin fell out of favor due to its small therapeutic range and high potential for toxicity. Today, it is used again as a last resort substance in treating MDR/PDR pathogens. Although new guidelines with detailed recommendations for Colistin dosing are available, finding the right dose in critically ill patients with renal failure remains difficult. Here, we evaluate the efficiency of the current guidelines' recommendations by using high resolution therapeutic drug monitoring of Colistin. (2) Methods: We analyzed plasma levels of Colistin and its prodrug colisthimethate sodium (CMS) in 779 samples, drawn from eight PDR-infected ICU patients, using a HPLC-MS/MS approach. The impact of renal function on proper Colistin target levels was assessed. (3) Results: CMS levels did not correlate with Colistin levels. Over-/Underdosing occurred regardless of renal function and mode of renal replacement therapy. Colistin elimination half-time appeared to be longer than previously reported. (4) Conclusion: Following dose recommendations from the most current guidelines does not necessarily lead to adequate Colistin plasma levels. Use of Colistin without therapeutic drug monitoring might be unsafe and guideline adherence does not warrant efficient target levels in critically ill patients.

Polymyxin derivatives as broad-spectrum antibiotic agents

We designed a few polymyxin derivatives which exhibit broad-spectrum antimicrobial activity. Lead compound P1 could disrupt bacterial membranes rapidly without developing resistance, inhibit biofilms formed by E. coli, and exhibit excellent in vivo activity in an MRSA-infected thigh burden mouse model.

Reservoirs of resistance: polymyxin resistance in veterinary-associated companion animal isolates of Pseudomonas aeruginosa

BACKGROUND

Pseudomonas aeruginosa is an opportunistic pathogen and a major cause of infections. Widespread resistance in human infections are increasing the use of last resort antimicrobials such as polymyxins. However, these have been used for decades in veterinary medicine. Companion animals are an understudied source of antimicrobial resistant P. aeruginosa isolates. This study evaluated the susceptibility of P. aeruginosa veterinary isolates to polymyxins to determine whether the veterinary niche represents a potential reservoir of resistance genes for pathogenic bacteria in both animals and humans.

METHODS AND RESULTS

Clinical P. aeruginosa isolates (n=24) from UK companion animals were compared for antimicrobial susceptibility to a panel of human-associated isolates (n=37). Minimum inhibitory concentration (MIC) values for polymyxin B and colistin in the companion animals was significantly higher than in human isolates (P=0.033 and P=0.013, respectively). Genotyping revealed that the veterinary isolates were spread throughout the P. aeruginosa population, with shared array types from human infections such as keratitis and respiratory infections, suggesting the potential for zoonotic transmission. Whole genome sequencing revealed mutations in genes associated with polymyxin resistance and other antimicrobial resistance-related genes.

CONCLUSION

The high levels of resistance to polymyxin shown here, along with genetic similarities between some human and animal isolates, together suggest a need for sustained surveillance of this veterinary niche as a potential reservoir for resistant, clinically relevant bacteria in both animals and humans.

Optimizing Antibiotic Administration for Pneumonia

Pneumonia, including community-acquired bacterial pneumonia, hospital-acquired bacterial pneumonia, and ventilator-acquired bacterial pneumonia, carries unacceptably high morbidity and mortality. Despite advances in antimicrobial therapy, emergence of multidrug resistance and high rates of treatment failure have made optimization of antibiotic efficacy a priority. This review focuses on pharmacokinetic and pharmacodynamic approaches to antibacterial optimization within the lung environment and in the setting of critical illness. Strategies for including these approaches in drug development programs as well as clinical practice are described and reviewed.

Evaluation of the interaction between polymyxin B and Pseudomonas aeruginosa biofilm and planktonic cells: reactive oxygen species induction and zeta potential

BACKGROUND

Although the most widely accepted mechanism of action for polymyxins is related to bacterial lysis via disruption, we hypothesized that this antimicrobial drug class could have other effects on Pseudomonas aeruginosa planktonic and sessile cells. Little is known regarding oxidative burst and zeta potential (ZP) data associated with the interaction between polymyxin B and P. aeruginosa cells. The present study evaluated endogenous reactive oxygen species (ROS) production and changes in the net charges of biofilm and planktonic cells in response to polymyxin B.

RESULTS

Polymyxin B induced concentration-dependent killing at all concentrations tested in planktonic and sessile cells from P. aeruginosa strains. Sublethal concentrations of polymyxin B induced oxidative burst. ROS production was higher in resistant planktonic cells than in biofilm cells but this was not observed for susceptible cells. Moreover, no net surface charge alterations were observed in planktonic cells from a susceptible strain treated with polymyxin B, but a significant increase of ZP was noted in planktonic cells from a resistant strain.

CONCLUSION

Oxidative burst generated by planktonic and sessile cells from P. aeruginosa strains against polymyxin B indicates that ROS may have an important role in the mechanism of action of this drug. ZP data revealed that electrostatic interactions of the cationic peptide with the anionic surface of the cells are strain-dependent. Therefore, we suggested that the intracellular effects of polymyxin B should be further investigated to understand polymyxin B-induced stress in P. aeruginosa.

Exploring colistin pharmacodynamics against Klebsiella pneumoniae: a need to revise current susceptibility breakpoints

Objectives

Because the pharmacokinetic/pharmacodynamic (PK/PD) characteristics of colistin against Enterobacteriaceae are not well explored, we studied the activity of colistin against K. pneumoniae in an in vitro PK/PD model simulating different dosing regimens.

Methods

Three clinical isolates of K. pneumoniae with MICs of 0.5, 1 and 4 mg/L were tested in an in vitro PK/PD model following a dose-fractionation design over a period of 24 h. A high and low inoculum of 107 and 104 cfu/mL with and without a heteroresistant subpopulation, respectively, were used. PK/PD indices associated with colistin activity were explored and Monte Carlo analysis was performed in order to determine the PTA for achieving a bactericidal effect (2 log kill).

Results

The fAUC/MIC (R2 = 0.64-0.68) followed by fCmax/MIC (R2 = 0.55-0.63) best described colistin's 24 h log10 cfu/mL reduction for both low and high inocula. Dosing regimens with fCmax/MIC ≥6 were always associated with a bactericidal effect (P = 0.0025). However, at clinically achievable concentrations, usually below fCmax/MIC = 6, an fAUC/MIC ≥25 was more predictive of a bactericidal effect. Using a dosing regimen of 9 MU/day, the PTA for this pharmacodynamic target was 100%, 5%-70% and 0%, for isolates with MICs of ≤0.5, 1 and ≥2 mg/L, respectively. Dosing regimens that aim for a trough level of 1 mg/L achieve coverage of strains up to 0.5 mg/L (target trough/MIC = 2 mg/L).

Conclusions

Characterization of the pharmacodynamics of colistin against Enterobacteriaceae in an in vitro model of infection indicates that a revision of current susceptibility breakpoints is needed. Therapeutic drug monitoring of colistin to achieve pharmacodynamic targets in individual patients is highly recommended.

A murine ex vivo 3D kidney proximal tubule model predicts clinical drug-induced nephrotoxicity

Drug attrition and clinical product withdrawals due to nephrotoxicity remain major challenges for pharmaceutical drug development pipelines. Currently, no reliable high-throughput in vitro screening models are available that provide reliable, predictive toxicology data for clinical nephrotoxicity. Drug screens to predict toxicity and pharmacology assessments are compromised by standard two-dimensional (2D) cell monoculture models. Here we extend a previously reported murine three-dimensional (3D) kidney-derived intact proximal tubule model to provide ex vivo drug toxicity data that reliably compare to clinical experiences and improve nephrotoxicity predictions over current 2D cell assays. Proximal tubule cytotoxicity was monitored by ATP depletion for 12 compounds (acarbose, acetylsalicylic acid, captopril, cimetidine, cidofovir, cisplatin, doxorubicin, gentamicin, polymyxin B, polymyxin B nonapeptide, probenecid and vancomycin) in 3D proximal tubule ex vivo assays. Drug concentration-response curves (1-1000 µM) and IC₅₀, lowest effective concentration (LEC) and AUC values were compared to clinical therapeutic exposure levels (C_max). The 100-fold C_max threshold demonstrated best sensitivity (96.9%) and specificity (87.5%) for this assay, with high positive (93.9%) and negative (93.3%) predictive values for nephrotoxicity. IC₅₀ values were superior to LEC. Results also support the model's capability to predict substrate-inhibitor/competitor interactions, yielding toxicity results similar to those reported in vivo. These 3D proximal tubule-based drug screens provide more reliable nephrotoxicity predictions, and more insight into complex mechanisms implicated in nephrotoxicity than current standard 2D cell assays. This new approach for rapid drug toxicity testing produces more reliable clinical comparisons than current 2D cell culture screening techniques.

The Concept of an Ideal Antibiotic: Implications for Drug Design

The emergence and spread of antibiotic-resistant pathogens is a major public health issue, which requires global action of an intersectoral nature. Multidrug-resistant (MDR) pathogens-especially "ESKAPE" bacteria-can withstand lethal doses of antibiotics with various chemical structures and mechanisms of action. Pharmaceutical companies are increasingly turning away from participating in the development of new antibiotics, due to the regulatory environment and the financial risks. There is an urgent need for innovation in antibiotic research, as classical discovery platforms (e.g., mining soil Streptomycetes) are no longer viable options. In addition to discovery platforms, a concept of an ideal antibiotic should be postulated, to act as a blueprint for future drugs, and to aid researchers, pharmaceutical companies, and relevant stakeholders in selecting lead compounds. Based on 150 references, the aim of this review is to summarize current advances regarding the challenges of antibiotic drug discovery and the specific attributes of an ideal antibacterial drug (a prodrug or generally reactive compound with no specific target, broad-spectrum antibacterial activity, adequate penetration through the Gram-negative cell wall, activity in biofilms and in hard-to-treat infections, accumulation in macrophages, availability for oral administration, and for use in sensitive patient groups).

Selective Interaction of Colistin with Lipid Model Membranes

Although colistin's clinical use is limited due to its nephrotoxicity, colistin is considered to be an antibiotic of last resort because it is used to treat patients infected with multidrug-resistant bacteria. In an effort to provide molecular details about colistin's ability to kill Gram-negative (G(-)) but not Gram-positive (G(+)) bacteria, we investigated the biophysics of the interaction between colistin and lipid mixtures mimicking the cytoplasmic membrane of G(+), G(-) bacteria as well as eukaryotic cells. Two different models of the G(-) outer membrane (OM) were assayed: lipid A with two deoxy-manno-octulosonyl sugar residues, and Escherichia coli lipopolysaccharide mixed with dilaurylphosphatidylglycerol. We used circular dichroism and x-ray diffuse scattering at low and wide angle in stacked multilayered samples, and neutron reflectivity of single, tethered bilayers mixed with colistin. We found no differences in secondary structure when colistin was bound to G(-) versus G(+) membrane mimics, ruling out a protein conformational change as the cause of this difference. However, bending modulus K_C perturbation was quite irregular for the G(-) inner membrane, where colistin produced a softening of the membranes at an intermediate lipid/peptide molar ratio but stiffening at lower and higher peptide concentrations, whereas in G(+) and eukaryotic mimics there was only a slight softening. Acyl chain order in G(-) was perturbed similarly to K_C. In G(+), there was only a slight softening and disordering effect, whereas in OM mimics, there was a slight stiffening and ordering of both membranes with increasing colistin. X-ray and neutron reflectivity structural results reveal colistin partitions deepest to reach the hydrocarbon interior in G(-) membranes, but remains in the headgroup region in G(+), OM, and eukaryotic mimics. It is possible that domain formation is responsible for the erratic response of G(-) inner membranes to colistin and for its deeper penetration, which could increase membrane permeability.

Effects of the antibiotic component on in-vitro bacterial killing, physico-chemical properties, aerosolization and dissolution of a ternary-combinational inhalation powder formulation of antibiotics for pan-drug resistant Gram-negative lung infections

Combinational antibiotic formulations have emerged as an important strategy to combat antibiotic resistance. The main objective of this study was to examine effects of individual components on the antimicrobial activity, physico-chemical properties, aerosolization and dissolution of powder aerosol formulations when three synergistic drugs were co-spray dried. A ternary dry powder formulation consisting of meropenem (75.5 %w/w), colistin (15.1 %w/w) and rifampicin (9.4 %w/w) at the selected ratio was produced by spray drying. The ternary formulation was characterized for in-vitro antibacterial activity, physico-chemical properties, surface composition, aerosol performance and dissolution. All of the formulations demonstrated excellent aerosolization behavior achieving a fine particle fraction of >70%, which was substantially higher than those for the Meropenem-SD and Colistin-Meropenem formulations. The results indicated that rifampicin controlled the surface morphology of the ternary and binary combination formulations resulting in the formation of highly corrugated particles. Advanced characterization of surface composition by XPS supported the hypothesis that rifampicin was enriched on the surface of the combination powder formulations. All spray-dried formulations were amorphous and absorbed substantial amount of water at the elevated humidity. Storage at the elevated humidity caused a substantial decline in aerosolization performance for the Meropenem-SD and Colistin-Meropenem, which was attributed to increased inter-particulate capillary forces or particle fusion. In contrast, the ternary combination and binary Meropenem-Rifampicin formulations showed no change in aerosol performance at the elevated storage humidity conditions; attributable to the enriched hydrophobicity of rifampicin on the particle surface that acted as a barrier against moisture condensation and particle fusion. Interestingly, in the ternary formulation rifampicin enrichment on the surface did not interfere with the dissolution of other two components (i.e. meropenem and colistin). Our study provides an insight on the impact of each component on the performance of co-spray dried combinational formulations.

Clinical Use of Polymyxin B

Polymyxin B is another clinically available polymyxin that has re-emerged in clinical practice to treat infections caused by multi-drug (MDR) or extensively-drug-resistant (XDR) Gram-negative bacteria (GNB). Its chemical structure is very similar to the structure of polymyxin E (colistin). However, since the latter is administered as a prodrug, there are major pharmacokinetic differences between both polymyxins that may potentially determine different clinical and microbiological outcomes. Studies addressing clinical or microbiological outcomes in patients treated with polymyxin B for MDR or XDR GNB are reviewed in this chapter.

Use of Colistin in Critically Ill Patients

Due to lack of better therapeutic options, colistin use for extensively drug-resistant Gram-negative organisms was revived in the past two decades, including in patients in intensive-care units (ICU). There are multiple knowledge gaps pertaining to the clinical use and utility of colistin in critically-ill patients, but due to lack of options, it is used in these high risk patients. In this chapter, we critically review the various topics pertaining to colistin use in critically-ill patients, while highlighting the (lack of) controlled evidence supporting common current practices pertaining to colistin use by clinicians.

The use of polymyxins to treat carbapenem resistant infections in neonates and children

INTRODUCTION

The incidence of healthcare-associated multidrug resistant bacterial infections, particularly due to carbapenem resistant organisms, has been on the rise globally. Among these are the carbapenem resistant Acinetobacter baumannii and Enterobacteriaceae, which have been responsible for numerous outbreaks in neonatal units. The polymyxins (colistin and polymyxin B) are considered to be the last resort antibiotics for treating such infections. However, pharmacokinetic and pharmacodynamic data on the use of polymyxins in neonates and children are very limited, and there are safety concerns.

AREAS COVERED

In this review, the authors summarize the global burden of multidrug resistance, particularly carbapenem resistance, in the neonatal and paediatric population, and the potential wider use of polymyxins in treating these infections.

EXPERT OPINION

Both colistin and polymyxin B have similar efficacy in treating multidrug resistant infections but have safety concerns. However, polymyxin B appears to be a better therapeutic option, with more rapid and higher steady state concentrations achieved compared to colistin and less reported nephrotoxicity. There is virtually no data in neonates and children currently; there is therefore an urgent need for pharmacokinetic and safety trials in these populations to determine the optimal drug and dosing regimens and provide recommendations for their use against carbapenem resistant infections.

Cytochrome C suppresses renal accumulation and nephrotoxicity of polymyxin B

The receptor megalin plays an important role in the accumulation of polymyxin B (PMB) in renal cells in vitro. This study aimed to examine the effects of cytochrome c (cyto c), a typical megalin ligand, on renal accumulation and nephrotoxicity of PMB in vivo. Thirty Sprague-Dawley rats were randomly divided into the vehicle control group, PMB group, PMB + cyto c 50, 100, or 200 mg/kg group, respectively, and were treated with intravenous cyto c 30 min before the administration of PMB 4.0 mg/kg once a day for consecutive 5 days. On the 4th day after administration, 24 h urine was collected to determine N-acetyl-β-D-glucosaminidase excretion. Six hours after the last injection on the 5th day, kidneys were harvested to assay PMB concentration and observe pathological alterations, and blood samples were collected to assay serum creatinine (SCr), blood urea nitrogen (BUN), and blood β₂-microglobulin (β₂-MG) levels. Cyto c 50, 100, and 200 mg/kg decreased the accumulation of PMB in the kidney by 18.5%, 39.1% ( p < 0.01), and 36.8% ( p < 0.01), respectively, and reduced 24 h N-acetyl-β-D- glucosaminidase excretion by 22.5% ( p < 0.05), 40.4% ( p < 0.01), and 40.4% ( p < 0.01), respectively. Kidney pathological damage induced by PMB was markedly reduced by cyto c 100 mg/kg and 200 mg/kg. However, there were no significant differences in SCr, BUN, and blood β₂-MG levels among the groups. These results indicated that cyto c may inhibit the renal accumulation and nephrotoxicity of PMB in a rat model, further proving the role of megalin in the accumulation of PMB.

Identification of a novel transposon-associated phosphoethanolamine transferase gene, mcr-5, conferring colistin resistance in d-tartrate fermenting Salmonella enterica subsp. enterica serovar Paratyphi B

Objectives

Plasmid-mediated mobilized colistin resistance is currently known to be caused by phosphoethanolamine transferases termed MCR-1, MCR-2, MCR-3 and MCR-4. However, this study focuses on the dissection of a novel resistance mechanism in mcr-1-, mcr-2- and mcr-3-negative d-tartrate fermenting Salmonella enterica subsp. enterica serovar Paratyphi B (Salmonella Paratyphi B dTa+) isolates with colistin MIC values >2 mg/L.

Methods

A selected isolate from the strain collection of the German National Reference Laboratory for Salmonella was investigated by WGS and bioinformatical analysis to identify novel phosphoethanolamine transferase genes involved in colistin resistance. Subsequently PCR screening, S1-PFGE and DNA-DNA hybridization were performed to analyse the prevalence and location of the identified mcr-5 gene. Cloning and transformation experiments in Escherichia coli DH5α and Salmonella Paratyphi B dTa+ control strains were carried out and the activity of MCR-5 was determined in vitro by MIC testing.

Results

In this study, we identified a novel phosphoethanolamine transferase in 14 mcr-1-, mcr-2- and mcr-3-negative Salmonella Paratyphi B dTa+ isolates with colistin MIC values >2 mg/L that were received during 2011-13. The respective gene, further termed as mcr-5 (1644 bp), is part of a 7337 bp transposon of the Tn3 family and usually located on related multi-copy ColE-type plasmids. Interestingly, in one isolate an additional subclone with a chromosomal location of the mcr-5 transposon was observed.

Conclusions

Our findings suggest that the transfer of colistin-resistance-mediating phosphoethanolamine transferase genes from bacterial chromosomes to mobile genetic elements has occurred in multiple independent events raising concern regarding their variety, prevalence and impact on public health.

An evaluation of hearing in infants administered with colistin in the premature neonatal intensive care unit

AIM

The aim of this study was to investigate the effects of colistin on hearing by evaluating the otoacoustic emission tests and clinical auditory brainstem responses in preterm infants treated with colistin in the neonatal intensive care unit.

METHOD

The study included 30 neonates (male: n = 16, female: n = 14) born before 37 weeks who were admitted to the Neonatal Intensive Care Unit at the Kahramanmaras Sutcu Imam Medical Faculty between January 2014 and January 2015 and who were treated with colistin during their time in intensive care because of infection. A control group was formed consisting of 30 preterm infants (male: n = 18, female: n = 12) with no additional disease born in the hospital during the same period. Following an ear, nose and throat examination the distortion product otoacoustic emission test, transient evoked otoacoustic emission and clinical auditory brainstem response tests were applied to all 60 patients.

RESULTS

The otoacoustic emission responses obtained from the control group were positive and clinical auditory brainstem responses up to 15 dB were obtained. In the colistin group negative otoacoustic emission responses were obtained in two patients unilaterally and in one patient bilaterally and loss was observed at the thresholds in the clinical auditory brainstem response test. Significantly prolonged fifth wave latency was observed in the colistin group compared to the control group for the clinical auditory brainstem response at 15 dB.

CONCLUSIONS

Given that the study results showed unilateral hearing loss in two patients and bilateral hearing loss in one as well as latency at 15 dB, hearing tests to check for ototoxicity are recommended for patients given colistin.

The Epidemiology, Evolution, and Treatment of KPC-Producing Organisms

PURPOSE OF REVIEW

The purpose of this review is to investigate the evolution and epidemiology of Klebsiella pneumoniae carbapenemase (KPC)-producing organisms and the current and future treatment options for infections caused by KPC-producing isolates.

RECENT FINDINGS

The emergence of resistance in Enterobacteriaceae producing carbapenemases globally has increased the challenges in treating infections caused by these organisms. One of the prominent mechanisms of resistance is the production of KPC enzymes. Infections caused by organisms producing KPCs have limited treatment options and are associated with poor clinical outcomes. The rapid rise of KPC-producing organisms necessitated the use of drugs with pharmacokinetic and toxicity limitations, including polymyxins, tigecycline, fosfomycin, and aminoglycosides. The availability of new beta-lactamase inhibitor combinations that are effective against KPC-producing organisms represent an advance in safety and efficacy. Several agents are currently being studied that have activity against KPC-producing organisms and appear to represent promising additions to our armamentarium. KPC-producing organisms cause infections with high morbidity and mortality. Limited treatment options are available, though new therapies have been developed. Pipeline agents are likely to have a place in therapy for the treatment of infections caused by KPC-producing isolates.

Evaluation of Sensititre Broth Microdilution Plate for determining the susceptibility of carbapenem-resistant Klebsiella pneumoniae to polymyxins

Colistin and polymyxin B MICs were determined for 106 carbapenem-resistant Klebsiella pneumoniae (CR-Kp) isolates using Sensititre Research Use Only GNX2F plates (Thermo Fisher) and compared to CLSI broth macrodilution (BMD) as the reference method. For colistin, EUCAST breakpoints were applied and testing of isolates with very major (VM) errors was repeated in duplicate by both methods to determine a majority result. Essential agreement (MIC ± one dilution) of GNX2F with the reference method was 97.1% for polymyxin B and 92.5% for colistin (7 VM errors, 22.6%). After discrepancy testing, there were 28 colistin resistant isolates by BMD and essential agreement was 94.3% with 4 VM errors (14.3%). Colistin and polymyxin B GNX2F results showed acceptable essential agreement with BMD for MICS without interpretation. Colistin VM errors with EUCAST breakpoints were due to MIC variability in the 2 to 4 μg/mL range that could be addressed by establishing an intermediate category.

Polymyxins: Antibacterial Activity, Susceptibility Testing, and Resistance Mechanisms Encoded by Plasmids or Chromosomes

Polymyxins are well-established antibiotics that have recently regained significant interest as a consequence of the increasing incidence of infections due to multidrug-resistant Gram-negative bacteria. Colistin and polymyxin B are being seriously reconsidered as last-resort antibiotics in many areas where multidrug resistance is observed in clinical medicine. In parallel, the heavy use of polymyxins in veterinary medicine is currently being reconsidered due to increased reports of polymyxin-resistant bacteria. Susceptibility testing is challenging with polymyxins, and currently available techniques are presented here. Genotypic and phenotypic methods that provide relevant information for diagnostic laboratories are presented. This review also presents recent works in relation to recently identified mechanisms of polymyxin resistance, including chromosomally encoded resistance traits as well as the recently identified plasmid-encoded polymyxin resistance determinant MCR-1. Epidemiological features summarizing the current knowledge in that field are presented.

Pharmacokinetic/pharmacodynamic parameters for treatment optimization of infection due to antibiotic resistant bacteria: a summary for practical purposes in children and adults

In the last years, there has been a tremendous increase in the incidence of bacterial infections due to resistant strains, especially multi-drug resistant Gram-negative bacilli. In Europe, a north to south and a west to east gradient was noticed, with more than one third of the K. pneumonia isolates being resistant to carbapenems in few countries. New antibiotics are lacking and, as a consequence, pharmacokinetic/pharmacodynamic parameters, normalized to pathogen minimal inhibitory concentration, are used with increased frequency to treat infections due to difficult-to-treat pathogens. These parameters are available at least for the adult population, but sparse in many different publications. This review wants to provide a comprehensive and 'easy to read' text for everyday practice, briefly summarizing the presently available knowledge on pharmacokinetic/pharmacodynamic parameters (normalized for minimal inhibitory concentration values) of different class drugs, that can be applied for an effective antibacterial treatment infections due to antibiotic-resistant pathogens.