The safety of polymyxin antibiotics

In Vitro and In Vivo Characterization of NOSO-502, a Novel Inhibitor of Bacterial Translation

Antibacterial activity screening of a collection of Xenorhabdus strains led to the discovery of the odilorhabdins, a new antibiotic class with broad-spectrum activity against Gram-positive and Gram-negative pathogens. Odilorhabdins inhibit bacterial translation by a new mechanism of action on ribosomes.

Antibacterial activity screening of a collection of Xenorhabdus strains led to the discovery of the odilorhabdins, a new antibiotic class with broad-spectrum activity against Gram-positive and Gram-negative pathogens. Odilorhabdins inhibit bacterial translation by a new mechanism of action on ribosomes. A lead optimization program identified NOSO-502 as a promising candidate. NOSO-502 has MIC values ranging from 0.5 to 4 μg/ml against standard Enterobacteriaceae strains and carbapenem-resistant Enterobacteriaceae (CRE) isolates that produce KPC, AmpC, or OXA enzymes and metallo-β-lactamases. In addition, this compound overcomes multiple chromosome-encoded or plasmid-mediated resistance mechanisms of acquired resistance to colistin. It is effective in mouse systemic infection models against Escherichia coli EN122 (extended-spectrum β-lactamase [ESBL]) or E. coli ATCC BAA-2469 (NDM-1), achieving a 50% effective dose (ED₅₀) of 3.5 mg/kg of body weight and 1-, 2-, and 3-log reductions in blood burden at 2.6, 3.8, and 5.9 mg/kg, respectively, in the first model and 100% survival in the second, starting with a dose as low as 4 mg/kg. In a urinary tract infection (UTI) model with E. coli UTI89, urine, bladder, and kidney burdens were reduced by 2.39, 1.96, and 1.36 log₁₀ CFU/ml, respectively, after injection of 24 mg/kg. There was no cytotoxicity against HepG2, HK-2, or human renal proximal tubular epithelial cells (HRPTEpiC), no inhibition of hERG-CHO or Nav 1.5-HEK current, and no increase of micronuclei at 512 μM. NOSO-502, a compound with a new mechanism of action, is active against Enterobacteriaceae, including all classes of CRE, has a low potential for resistance development, shows efficacy in several mouse models, and has a favorable in vitro safety profile.

Hypoalbuminemia as a predictor of acute kidney injury during colistin treatment

This study aimed to assess the predictors of acute kidney injury (AKI) during colistin therapy in a cohort of patients with bloodstream infections (BSI) due to colistin-susceptible Gram-negative bacteria, focusing on the role of serum albumin levels. The study consisted of two parts: (1) a multicentre retrospective clinical study to assess the predictors of AKI during colistin therapy, defined according to the Kidney Disease: Improving Global Outcomes (KDIGO) criteria; and (2) bioinformatic and biochemical characterization of the possible interaction between human serum albumin and colistin. Among the 170 patients included in the study, 71 (42%), 35 (21%), and 11 (6%) developed KDIGO stage 1 (K1-AKI), KDIGO stage 2 (K2-AKI), and KDIGO stage 3 (K3-AKI), respectively. In multivariable analyses, serum albumin <2.5 g/dL was independently associated with K1-AKI (subdistribution hazard ratio [sHR] 1.85, 95% confidence interval [CI] 1.17-2.93, p = 0.009) and K2-AKI (sHR 2.37, 95% CI 1.15-4.87, p = 0.019). Bioinformatic and biochemical analyses provided additional information nurturing the discussion on how hypoalbuminemia favors development of AKI during colistin therapy. In conclusion, severe hypoalbuminemia independently predicted AKI during colistin therapy in a large cohort of patients with BSI due to colistin-susceptible Gram-negative bacteria. Further study is needed to clarify the underlying causal pathways.

Polymyxin B-induced skin hyperpigmentation: a rare case report and literature review

Background

Polymyxin B (PMB), which is regarded as the ultimate antibacterial treatment against some intractable gram-negative bacteria with its outstanding anti-bacterial activities, inflicts several adverse effects on patients. However, skin hyperpigmentaion (SH) induced by PMB is very rare. Here, we report a case of polymyxin B-induced skin hyperpigmentation (PMB-iSH) in a 21-year-old female. To the best of our knowledge, this is the first case of PMB-iSH in China.

Case presentation

A 21-year-old female patient with sepsis received the administration of PMB by intravenous injection for the treatment of multi-drug resistant Klebsiella pneumoniae (MDR-KP) infection. She later suffered from a rare adverse drug reaction (ADR), namely PMB-iSH, after 5-day PMB administration during her treatment. There were multiple red rashes spread on the whole body skin at first. With the rashes fading away, SH with dark round spots appeared, associated with no pain or pruritus. The skin of the head and neck was darkened evidently, and dark brown spots were spread on the skin of trunk and limbs. About a month after her admission, urged by the relatives, the patient was transferred back to the local hospital for further treatment in the end, and her skin color didn’t recover to the previous state at that time.

Conclusion

Both our case and the literature review highlight that PMB can give rise to SH indeed. Clinicians and pharmacists should attach great importance to this rare pigmentary disorder and further investigation is warranted.

Colistin causes profound morphological alteration but minimal cytoplasmic membrane perforation in populations of Escherichia coli and Pseudomonas aeruginosa

Whilst colistin (polymyxin E) represents the last mainstream treatment option for multidrug-resistant Gram-negative pathogens, details of its mechanism of action remain to be fully resolved. In this study, the effects of sub-inhibitory, inhibitory-bactericidal, and supra-bactericidal levels of colistin on the membrane integrity and morphology of Escherichia coli and Pseudomonas aeruginosa were investigated using potassium loss, flow cytometry, and scanning electron microscopy (SEM). Supra-bactericidal colistin concentrations induced just 4-12% intracellular potassium loss from bacteria after 24 h. Flow cytometry data suggested colistin might alter cell arrangement, and SEM confirmed the antibiotic causes bacterial aggregation. Filamentation was not detected in either species at any concentration or time-point up to 24 h. These results argue against the hypotheses that colistin kills bacteria by puncturing the cytoplasmic membrane or disrupting DNA synthesis. The colistin-induced bacterial aggregation detected has implications for the interpretation of MBC, time-kill, and other test results obtained with this antibiotic.

Can octapeptin antibiotics combat extensively drug-resistant (XDR) bacteria?

INTRODUCTION

The octapeptins are a family of cyclic lipopeptides first reported in the 1970s then largely ignored. At the time, their reported antibiotic activity against polymyxin-resistant bacteria was a curiosity. Today, the advent of widespread drug resistance in Gram-negative bacteria has prompted their 'rediscovery.' The paucity of new antibiotics in the clinical pipeline is coupled with a global spread of increasing antibiotic resistance, particularly to meropenem and polymyxins B and E (colistin). Areas covered: We review the original discovery of octapeptins, their recent first chemical syntheses, and their mode of action, then discuss their potential as a new class of antibiotics to treat extensively drug-resistant (XDR) Gram-negative infections, with direct comparisons to the closely related polymyxins. Expert commentary: Cyclic lipopeptides in clinical use (polymyxin antibiotics) have significant dose-limiting nephrotoxicity inherent to their chemotype. This toxicity has prevented improved polymyxin analogs from progressing to the clinic, and tainted the perception of lipopeptide antibiotics in general. We argue that the octapeptins are fundamentally different from the polymyxins, with a disparate mode of action, spectra of action against MDR and XDR bacteria and a superior preclinical safety profile. They represent early-stage candidates that can help prime the antibiotic discovery pipeline.

Novobiocin Enhances Polymyxin Activity by Stimulating Lipopolysaccharide Transport

Gram-negative bacteria are challenging to kill with antibiotics due to their impenetrable outer membrane containing lipopolysaccharide (LPS). The polymyxins, including colistin, are the drugs of last resort for treating Gram-negative infections. These drugs bind LPS and disrupt the outer membrane; however, their toxicity limits their usefulness. Polymyxin has been shown to synergize with many antibiotics including novobiocin, which inhibits DNA gyrase, by facilitating transport of these antibiotics across the outer membrane. Recently, we have shown that novobiocin not only inhibits DNA gyrase but also binds and stimulates LptB, the ATPase that powers LPS transport. Here, we report the synthesis of novobiocin derivatives that separate these two activities. One analog retains LptB-stimulatory activity but is unable to inhibit DNA gyrase. This analog, which is not toxic on its own, nevertheless enhances the lethality of polymyxin by binding LptB and stimulating LPS transport. Therefore, LPS transport agonism contributes substantially to novobiocin-polymyxin synergy. We also report other novobiocin analogs that inhibit DNA gyrase better than or equal to novobiocin, but bind better to LptB and therefore have even greater LptB stimulatory activity. These compounds are more potent than novobiocin when used in combination with polymyxin. Novobiocin analogs optimized for both gyrase inhibition and LPS transport agonism may allow the use of lower doses of polymyxin, increasing its efficacy and safety.

Intravenous colistin combination antimicrobial treatment vs. monotherapy: a systematic review and meta-analysis

BACKGROUND

To evaluate whether intravenous colistin in combination with other antibiotics (IVCC) is associated with lower mortality compared with intravenous colistin monotherapy (IVCM), and to identify factors influencing study outcomes.

METHODS

PubMed and Scopus were searched up to November 2016. Studies were included if they evaluated adult patients with multi-drug-resistant (MDR) or extensively-drug-resistant Gram-negative infections, and reported comparative mortality data (adjusted and unadjusted) for patients receiving IVCC vs. IVCM. Random effects meta-analyses were performed.

FINDINGS

Thirty-two studies (29 observational, three randomized) were included. The overall quality of data was low to very low, and studies were characterized by the lack of adjusted data. The majority of studies were not designed to evaluate the outcome of the meta-analysis, and focused mainly on infections due to Acinetobacter baumannii and Klebsiella pneumoniae. Colistin was administered at variable doses, with or without a loading dose, and in combination with several antibiotics. Overall, IVCC was not associated with lower mortality than IVCM [32 studies, 2328 patients, risk ratio (RR) 0.91, 95% confidence interval (CI) 0.81-1.02, I2 8%]. A significant difference was observed in favour of IVCC when high-dose (>6 million international units) colistin was used (RR 0.80, 95% CI 0.69-0.93), in studies conducted in Asia (RR 0.82, 95% CI 0.71-0.95), in patients with bacteraemia (RR 0.75, 95% CI 0.57-0.98) and in patients with acinetobacter infections (RR 0.88, 95% CI 0.78-1.00).

INTERPRETATION

Overall, low-quality data suggest that IVCC did not lower mortality in patients with MDR Gram-negative infections. However, there is some evidence for a benefit observed with high intravenous doses of colistin.

New polymyxin derivatives that display improved efficacy in animal infection models as compared to polymyxin B and colistin

Polymyxin B and colistin (polymyxin E) are bactericidal pentacationic lipopeptides that act specifically on Gram-negative bacteria, first by disrupting their outermost permeability barrier, the outer membrane (OM), and then damaging the cytoplasmic membrane. The discovery of both polymyxin B and colistin was published independently by three laboratories as early as in 1947. They were subsequently used in intravenous therapy. Unfortunately, they also exhibit significant and dose-limiting nephrotoxicity. Therefore, polymyxins were reserved as agents of last-line defense. The emergence of extremely multiresistant strains has now forced clinicians to reinstate polymyxins in the therapy of severe infections. However, the current dosage regimens lead to insufficient drug concentrations in serum and clinicians have been advised to use larger doses, which further increases the risk of nephrotoxicity. Very recently, the interest in developing better tolerated and more effective polymyxins has grown. This review focuses on describing four development programs that have yielded novel derivatives that are more effective than the old polymyxins in animal infection models. Compounds from three programs are superior to the old polymyxins in the rodent lung infection model with Acinetobacter baumannii and/or Pseudomonas aeruginosa. One of them is also more effective than polymyxin B in A. baumannii mouse thigh infection. The fourth program includes compounds that are approximately tenfold more effective in Escherichia coli murine pyelonephritis than polymyxin B.

Intravenous plus inhaled versus intravenous colistin monotherapy for lower respiratory tract infections: A systematic review and meta-analysis

OBJECTIVE

To evaluate whether intravenous plus inhaled combination (IV/INHCC) compared to intravenous monotherapy (IVCM) was associated with patient outcomes and identify factors influencing study outcomes.

METHODS

PubMed and Scopus were searched till November 2016. Studies were included if they evaluated adult patients with lower respiratory tract infections due to MDR/XDR Gram-negative bacteria and reported comparative mortality data (adjusted and unadjusted) for patients receiving IV/INHCC versus IVCM. Random effects meta-analyses were performed.

RESULTS

Thirteen studies (11 retrospective, 2 prospective) were included. The overall quality of data was low to very low and characterized by the lack of adjusted data. The majority of the studies were designed to evaluate the outcome of the meta-analysis. Both IV and inhaled colistin were administered at variable doses. There was no difference in mortality between IV/INHCC and IVCM when all studies were combined (13 studies, 1115 patients, risk ratio 0.94, 95% confidence interval 0.81-1.08). Only the analysis that included studies with low-dose IV colistin showed significant difference in favor of IV/INHCC versus IVCM (0.65, 0.45-0.94).

CONCLUSIONS

Overall, low quality data suggest that IV/INHCC did not lower mortality in patients with MDR Gram negative infections unless low IV colistin dose was administered.

Substrate analog interaction with MCR-1 offers insight into the rising threat of the plasmid-mediated transferable colistin resistance

Colistin is considered a last-resort antibiotic against most gram-negative bacteria. Recent discoveries of a plasmid-mediated, transferable mobilized colistin-resistance gene ( mcr-1) on all continents have heralded the imminent emergence of pan-drug-resistant superbacteria. The inner-membrane protein MCR-1 can catalyze the transfer of phosphoethanolamine (PEA) to lipid A, resulting in colistin resistance. However, little is known about the mechanism, and few drugs exist to address this issue. We present crystal structures revealing the MCR-1 catalytic domain (cMCR-1) as a monozinc metalloprotein with ethanolamine (ETA) and d-glucose, respectively, thus highlighting 2 possible substrate-binding pockets in the MCR-1-catalyzed PEA transfer reaction. Mutation of the residues involved in ETA and d-glucose binding impairs colistin resistance in recombinant Escherichia coli containing full-length MCR-1. Partial analogs of the substrate are used for cocrystallization with cMCR-1, providing valuable information about the family of PEA transferases. One of the analogs, ETA, causes clear inhibition of polymyxin B resistance, highlighting its potential for drug development. These data demonstrate the crucial role of the PEA- and lipid A-binding pockets and provide novel insights into the structure-based mechanisms, important drug-target hot spots, and a drug template for further drug development to combat the urgent, rising threat of MCR-1-mediated antibiotic resistance.-Wei, P., Song, G., Shi, M., Zhou, Y., Liu, Y., Lei, J., Chen, P., Yin, L. Substrate analog interaction with MCR-1 offers insight into the rising threat of the plasmid-mediated transferable colistin resistance.

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

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

Minocycline attenuates colistin-induced neurotoxicity via suppression of apoptosis, mitochondrial dysfunction and oxidative stress

Background

Neurotoxicity is an adverse effect patients experience during colistin therapy. The development of effective neuroprotective agents that can be co-administered during polymyxin therapy remains a priority area in antimicrobial chemotherapy. The present study investigates the neuroprotective effect of the synergistic tetracycline antibiotic minocycline against colistin-induced neurotoxicity.

Methods

The impact of minocycline pretreatment on colistin-induced apoptosis, caspase activation, oxidative stress and mitochondrial dysfunction were investigated using cultured mouse neuroblastoma-2a (N2a) and primary cortical neuronal cells.

Results

Colistin-induced neurotoxicity in mouse N2a and primary cortical cells gives rise to the generation of reactive oxygen species (ROS) and subsequent cell death via apoptosis. Pretreatment of the neuronal cells with minocycline at 5, 10 and 20 μM for 2 h prior to colistin (200 μM) exposure (24 h), had an neuroprotective effect by significantly decreasing intracellular ROS production and by upregulating the activities of the anti-ROS enzymes superoxide dismutase and catalase. Minocycline pretreatment also protected the cells from colistin-induced mitochondrial dysfunction, caspase activation and subsequent apoptosis. Immunohistochemical imaging studies revealed colistin accumulates within the dendrite projections and cell body of primary cortical neuronal cells.

Conclusions

To our knowledge, this is first study demonstrating the protective effect of minocycline on colistin-induced neurotoxicity by scavenging of ROS and suppression of apoptosis. Our study highlights that co-administration of minocycline kills two birds with one stone: in addition to its synergistic antimicrobial activity, minocycline could potentially ameliorate unwanted neurotoxicity in patients undergoing polymyxin therapy.

Pharmacokinetic/Toxicodynamic Analysis of Colistin-Associated Acute Kidney Injury in Critically Ill Patients

Acute kidney injury (AKI) occurs in a substantial proportion of critically ill patients receiving intravenous colistin. In the pharmacokinetic/toxicodynamic analysis reported here, the relationship of the occurrence of AKI to exposure to colistin and a number of potential patient factors was explored in 153 critically ill patients, none of whom were receiving renal replacement therapy. Tree-based modeling revealed that the rates of AKI were substantially higher when the average steady-state plasma colistin concentration was greater than ∼2 mg/liter.

Macrophage-like nanoparticles concurrently absorbing endotoxins and proinflammatory cytokines for sepsis management

Sepsis, resulting from uncontrolled inflammatory responses to bacterial infections, continues to cause high morbidity and mortality worldwide. Currently, effective sepsis treatments are lacking in the clinic, and care remains primarily supportive. Here we report the development of macrophage biomimetic nanoparticles for the management of sepsis. The nanoparticles, made by wrapping polymeric cores with cell membrane derived from macrophages, possess an antigenic exterior the same as the source cells. By acting as macrophage decoys, these nanoparticles bind and neutralize endotoxins that would otherwise trigger immune activation. In addition, these macrophage-like nanoparticles sequester proinflammatory cytokines and inhibit their ability to potentiate the sepsis cascade. In a mouse Escherichia coli bacteremia model, treatment with macrophage mimicking nanoparticles, termed MΦ-NPs, reduced proinflammatory cytokine levels, inhibited bacterial dissemination, and ultimately conferred a significant survival advantage to infected mice. Employing MΦ-NPs as a biomimetic detoxification strategy shows promise for improving patient outcomes, potentially shifting the current paradigm of sepsis management.

Polymyxin-B and vancomycin-associated acute kidney injury in critically ill patients

BACKGROUND

This study aims to investigate renal toxicities of Polymyxin B and Vancomycin among critically ill patients and risk factors for acute kidney injury (AKI).

METHODS

This is a cross-sectional study conducted with patients admitted to an intensive care unit (ICU) of a tertiary hospital in Brazil. Patients were divided into two groups: those who used association of Polymyxin B + Vancomycin (Group I) and those who used only Polymyxin B (Group II). Risk factors for AKI were also analyzed.

RESULTS

A total of 115 patients were included. Mean age was 59.2 ± 16.1 years, and 52.2% were males. Group I presented higher GFR (117.1 ± 70.5 vs. 91.5 ± 50 ml/min/1.73 m², p = 0.02) as well as lower creatinine (0.9 ± 0.82 vs. 1.0 ± 0.59 mg/dL, p = 0.014) and urea (51.8 ± 23.7 vs. 94.5 ± 4.9 mg/dL, p = 0.006) than group II on admission. Group I also manifested significantly higher incidence of AKI than group II (62.7% vs. 28.5%, p = 0.005), even when stratified according to RIFLE criteria ('Risk' 33.9% vs. 10.7%; 'Injury' 10.2% vs. 8.9%; 'Failure' 18.6% vs. 8.9%; p = 0.03). Accumulated Polymyxin B dose > 10 million IU was an independent predictor for AKI (OR = 2.72, 95% CI = 1.13-6.51, p = 0.024).

CONCLUSIONS

Although patients who received Polymyxin B plus vancomycin had more favorable clinical profile and higher previous GFR, they presented a higher AKI incidence than those patients who received Polymyxin B alone. Cumulative Polymyxin B dose > 10 million IU was independently associated to AKI.

Structure-activity studies on polymyxin derivatives carrying three positive charges only reveal a new class of compounds with strong antibacterial activity

Recent years have brought in an increased interest to develop improved polymyxins. The currently used polymyxins, i.e. polymyxin B and colistin (polymyxin E) are pentacationic lipopeptides that possess a cyclic heptapeptide part with three positive charges, a linear "panhandle" part with two positive charges, and a fatty acyl tail. Unfortunately, their clinical use is shadowed by their notable nephrotoxicity. We have previously developed a polymyxin derivative NAB739 which lacks the positive charges in the linear part. This derivative is better tolerated than polymyxin B in cynomolgus monkeys and is, in contrast to polymyxin B, excreted into urine in monkeys and rats. Here we have conducted further structure-activity relationship (SAR) studies on 17 derivatives with three positive charges only. We discovered a remarkably antibacterial class, as exemplified by NAB815, that carries two positive charges only in the cyclic part.

Nephrotoxicity of Polymyxins: Is There Any Difference between Colistimethate and Polymyxin B?

Nephrotoxicity is a common adverse effect of the clinically used polymyxins, colistin and polymyxin B. This adverse effect is dose limiting for both polymyxins, as the plasma polymyxin concentrations associated with renal damage overlap those required for antibacterial effect. Since development of acute kidney injury (AKI) during therapy is highly undesirable, it is extremely important to know whether there is any difference between the nephrotoxic potential of colistin (administered as its inefficient prodrug, colistimethate) and polymyxin B (administered as the active form). Both polymyxins are cytotoxic to renal tubular cells and are prone to cause nephrotoxicity in vivo because of the renal handling mechanisms that facilitate accumulation of these compounds in these cells, processes that are reviewed in this article. Also reviewed are the emerging data that strongly suggest significantly higher rates of AKI in patients treated with colistimethate compared to patients treated with polymyxin B. This finding may be due to differences in pharmacokinetics and renal handling mechanisms of colistimethate and formed colistin versus polymyxin B, and consequently the relative amount of polymyxin material delivered to tubular cells. A lower risk of AKI with polymyxin B is one of several potential advantages over colistimethate. The relative safety and efficacy of the two agents require closer examination in well-designed clinical studies.

Old antibiotics for multidrug-resistant pathogens: from in vitro activity to clinical outcomes

Antimicrobial resistance is a major and emerging threat worldwide. New antimicrobials have been unable to meet the resistance challenge, and treatment options are limited for a growing number of resistant pathogens. More and more clinicians are relying on older antimicrobials for the treatment of multidrug-resistant (MDR) bacteria. Some older antimicrobials have maintained excellent in vitro activity against highly resistant pathogens. In some instances, use of older agents is limited by unfavourable pharmacokinetic/pharmacodynamic characteristics and/or toxicities. In general, clinical data pertaining to the use of older agents for the treatment of MDR pathogens are scarce. Research efforts should be focused on the evaluation of older agents for the treatment of MDR pathogens as well as evaluating how these agents perform in complex patient populations with various and multiple co-morbid conditions.

Development of new polymyxin derivatives for multi-drug resistant Gram-negative infections

Over the last decade, there has been a resurgence of interest in polymyxins owing to the rapid rise in multi-drug resistant Gram-negative bacteria against which polymyxins offer a last-resort treatment. Although having excellent antibacterial activity, the clinical utility of polymyxins is limited by toxicity, especially renal toxicity. There is much interest therefore in developing polymyxin analogues with an improved therapeutic index. This review describes recent work aimed at improving the activity and/or reducing the toxicity of polymyxins. Consideration to providing activity against emerging strains with reduced susceptibility to polymyxins is also made.