The Low-Alkalinity Polymyxin Derivative, AL-6, Shows High Activity Against Multidrug-Resistant Acinetobacter baumannii Clinical Isolates In Vitro and A. baumannii ATCC 19606 In Vivo: Preliminary Analysis of the Antibacterial Mechanism.

Polymyxin B and colistin (polymyxin E) are increasingly used as the last line of therapy for infections caused by multidrug-resistant (MDR) gram-negative pathogens. However, nephrotoxicity is still a limiting factor for the use of polymyxin. Therefore, better tolerated and more effective polymyxin derivatives are urgently needed. In this study, we aimed to evaluate the activity of the low-alkalinity polymyxin derivative, AL-6, against MDR Acinetobacter baumannii (Ab) clinical isolates in vitro and A. baumannii ATCC 19606 in vivo. Additionally, we performed a preliminarily study of the antibacterial mechanism. AL-6 showed much higher activity (0.125-0.25 µg/mL) against MDR A. baumannii clinical isolates than polymyxin E2 (PE2, 0.5-1 µg/mL). AL-6 also showed much higher activity (0.5-256 µg/mL) against polymyxin-resistant strains than PE2 (16-1024 µg/mL). Additionally, AL-6 showed slow resistance against A. baumannii. AL-6 also increased the survival rates of mice by 10% at 48 h compared with PE2 (5 mg/kg). AL-6 could be used at a dose of up to 10 mg/kg, increasing the survival rate to 30% at 72 h after infection. A preliminary study of the antibacterial mechanism showed that AL-6 permeabilized the outer membrane and destroyed cell membrane integrity. Moreover, there was a substantial increase in zeta potential (i.e., less negative) upon AL-6 exposure for A. baumannii. Overall, AL-6 carrying only four positive charges showed high activity against A. baumannii in vitro by disrupting cell membrane integrity. Higher doses of AL-6 could increase survival rates of mice. Thus, AL-6 may have potential applications as a bactericidal agent.

Treatment of infections caused by Gram-negative pathogens: current status on the pharmacokinetics/pharmacodynamics of parenteral and inhaled polymyxins in patients.

Polymyxins are increasingly used as a last resort for the treatment of infections caused by multidrug-resistant Gram-negative bacteria in patients. Over the last decade, significant progress has been made in understanding the pharmacokinetics/pharmacodynamics/toxicodynamics (PK/PD/TD) of parenteral and inhaled polymyxins. This mini-review provides an overview of polymyxin chemistry, different dose definitions, and the latest research on their clinical use, toxicities, and PK/PD after intravenous and inhalation administration. Optimising the PK/PD/TD of polymyxins in patients is critical to maximise their efficacy while minimising toxicities and the emergence of resistance.

Acute and long-term effects of antibiotics commonly used in laboratory animal medicine on the fecal microbiota.

Biomedical research relies on the use of animal models, and the animals used in those models receive medical care, including antibiotics for brief periods of time to treat conditions such as dermatitis, fight wounds, and suspected bacterial pathogens of unknown etiology. As many mouse model phenotypes are sensitive to changes in the gut microbiota, our goal was to examine the effect of antibiotics commonly administered to mice. Therefore, four treatment groups (subcutaneous enrofloxacin for 7 days, oral enrofloxacin for 14 days, oral trimethoprim-sulfamethoxazole for 14 days, and topical triple antibiotic ointment for 14 days) alongside a fifth control group receiving no treatment (n = 12/group) were included in our study. Fecal samples were collected prior to treatment, immediately after two weeks of exposure, and four weeks after cessation of treatment, and subjected to 16S rRNA library sequencing. The entire experimental design was replicated in mice from two different suppliers. As expected, several treatments including enrofloxacin and triple antibiotic ointment substantially decreased the amount of DNA recovered from fecal material, as well as the microbial richness. Notably, many of these effects were long-lasting with diminished gut microbiota (GM) richness four weeks following exposure, in both substrains of mice. Trimethoprim-sulfamethoxazole induced minimal to no discernible changes in the taxonomic composition beyond that seen in control mice. Collectively, these data highlight the need to consider the impact on GM of brief and seemingly routine use of antibiotics in the clinical care of research animals.

Polymyxin Susceptibility Testing and Interpretive Breakpoints: Recommendations from the United States Committee on Antimicrobial Susceptibility Testing (USCAST).

The polymyxins are important agents for carbapenem-resistant Gram-negative bacilli. The United States Committee on Antimicrobial Susceptibility Testing breakpoint recommendations for colistin and polymyxin B are that isolates of Pseudomonas aeruginosa, Acinetobacter baumannii, and Enterobacteriaceae are considered susceptible at MIC values of ≤2 mg/liter. These recommendations are contingent upon dosing and testing strategies that are described in this commentary. Importantly, these recommendations are not applicable to lower respiratory tract infections, for which we recommend no breakpoints. Furthermore, there is no breakpoint recommendation for polymyxin B for lower urinary tract infections.

Polymyxin and Bacitracin in the Irrigation Solution Provide No Benefit for Bacterial Killing in Vitro.

BACKGROUND: Many surgeons add topical antibiotics to irrigation solutions assuming that this has a local effect and eliminates bacteria. However, prior studies have suggested that the addition of antibiotics to irrigation solution confers little benefit, adds cost, may potentiate anaphylactic reactions, and may contribute to antimicrobial resistance. We sought to compare the antimicrobial efficacy and cytotoxicity of an irrigation solution containing polymyxin-bacitracin with other commonly used irrigation solutions. METHODS: Staphylococcus aureus and Escherichia coli were exposed to irrigation solutions containing topical antibiotics (500,000-U/L polymyxin and 50,000-U/L bacitracin; 1-g/L vancomycin; or 80-mg/L gentamicin), as well as commonly used irrigation solutions (saline solution 0.9%; povidone-iodine 0.3%; chlorhexidine 0.05%; Castile soap 0.45%; and sodium hypochlorite 0.125%). Following 1 and 3 minutes of exposure, surviving bacteria were manually counted. Failure to eradicate all bacteria in any of the 3 replicates was considered not effective for that respective solution. Cytotoxicity analysis in human fibroblasts, osteoblasts, and chondrocytes exposed to the irrigation solutions was performed by visualization of cell structure and was quantified by lactate dehydrogenase (LDH) activity. Efficacy and cytotoxicity were assessed in triplicate experiments, with generalized linear mixed models. RESULTS: Polymyxin-bacitracin, saline solution, and Castile soap at both exposure times were not effective at eradicating S. aureus or E. coli. In contrast, povidone-iodine, chlorhexidine, and sodium hypochlorite irrigation were effective against both S. aureus and E. coli (p < 0.001). Vancomycin irrigation was effective against S. aureus but not against E. coli, whereas gentamicin irrigation showed partial efficacy against E. coli but none against S. aureus. Within fibroblasts, the greatest cytotoxicity was seen with chlorhexidine (mean [and standard error], 49.38% ± 0.80%; p < 0.0001), followed by Castile soap (33.57% ± 2.17%; p < 0.0001) and polymyxin-bacitracin (8.90% ± 1.40%). Povidone-iodine showed the least cytotoxicity of the efficacious solutions (5.00% ± 0.86%). Similar trends were seen at both exposure times and across fibroblasts, osteoblasts, and chondrocytes. CONCLUSIONS: Irrigation with polymyxin-bacitracin was ineffective at bacterial eradication, and statistically inferior to povidone-iodine. Chlorhexidine lavage conferred the greatest in vitro cytotoxicity. CLINICAL RELEVANCE: These data suggest that the addition of polymyxin-bacitracin to saline solution irrigation has little value. Given the cost and antimicrobial resistance implications, our findings, combined with prior clinical literature, provide adequate reason to avoid widespread use of antibiotics in irrigation solutions. Povidone-iodine may be a more effective and safer option.

An update on polymyxin susceptibility testing methods for Acinetobacter baumannii.

Introduction: Carbapenem-resistant Acinetobacter baumannii (CRAB) currently comprises >50% of A. baumannii isolates in Europe and the USA and commonly are extensively or pandrug resistant. They cause severe hospital infections, which according to CDC have very high mortality. Polymyxins are universally recognized as the last-line choice against CRAB infections. The role of polymyxin susceptibility testing (ST) for these pathogens remains a subject of debate, although it is critical for selection of appropriate antimicrobial therapy. There are significant discrepancies between in vitro ST methods and important shortcomings for diffusion and automated methods have been repeatedly verified. The joint CLSI-EUCAST Working Group has recently recommended broth microdilution (BMD) without additives as the reference method for polymyxin ST. Areas covered: This review, focused on this threatening pathogen, summarizes the current available ST methods for polymyxins and discusses the challenges encountered by clinical microbiological laboratories in obtaining and interpreting valid susceptibility results. Studies on polymyxin ST methods for Acinetobacter spp., retrieved from Pubmed database until May 2019, were evaluated. Expert opinion: Currently, no single commercial ST method for polymyxins is providing fully accurate results. Further optimization and standardization of the ST methods and international harmonization of guidelines should be achieved to facilitate the accurate detection of polymyxin-resistant A. baumannii.

Clinical Effects of Perioperative Selective Decontamination of the Digestive Tract (SDD) in Cardiac Surgery: A Propensity Score Matched Cohort Analysis.

OBJECTIVES: To determine the clinical effects of perioperative endotoxin reduction in the gut lumen in patients undergoing cardiac surgery with cardiopulmonary bypass. DESIGN: Retrospective cohort analysis with propensity score matching according to treatment group. SETTING: Tertiary center for cardiopulmonary diseases and intensive care medicine. PARTICIPANTS: Included were patients who underwent cardiac surgery with cardiopulmonary bypass between 2008 and 2017. Excluded were readmitted patients. INTERVENTIONS: Endotoxin reduction in the gut lumen by ingestion of oral tobramycin 80 mg and polymyxin B 100 mg 4 times daily (TP) as part of selective digestive tract decontamination, which contains amphotericin B 500 mg as well. MEASUREMENTS AND MAIN RESULTS: A total of 6,394 patients were included, of whom 2,044 patients were in the intervention group. A total of 835 patients received both pre- and postoperative TP (Pre+/Post+), and 1,165 patients received TP only postoperatively (Pre-/Post+). The control group, not treated with TP at any moment, consisted of 4,350 patients (Pre-/Post-). After matching, 652 Pre+/Post+ patients were compared with an equal number of controls (Pre-/Post-). Pre+/Post+ group did not do better for any clinical outcome. A total of 682 Pre+/Post+ patients matched with an equal number of Pre-/Post+ patients. The latter group had a 0.3 points higher mean Sequential Organ Failure Assessment score and in the regression analysis a significantly higher intensive care unit mortality but not hospital mortality. A significant reduction in length of stay and length of mechanical ventilation for the Pre+/Post+ group was shown compared with Pre-/Post+, but these differences can be explained by unbalanced differences in the severity of illness. CONCLUSION: Cardiosurgical patients who receive tobramycin and polymyxin orally preoperatively to reduce the gut endotoxin level do not expose convincing and relevant beneficial effects on clinical outcomes in this retrospective propensity score matching cohort study.

Evaluation of the bacterial ocular surface microbiome in clinically normal horses before and after treatment with topical neomycin-polymyxin-bacitracin.

Next generation sequencing (NGS) studies have demonstrated a rich and diverse ocular surface-associated microbiota in people that was previously undetected by traditional culture-based methods. The ocular surface microbiome of horses has yet to be investigated using NGS techniques. This study aimed to determine the bacterial composition of the ocular surface microbiome in healthy horses, and to identify whether there are microbial community changes over time and following topical antibiotic use. One eye of 12 horses was treated 3 times daily for 1 week with neomycin-polymyxin-bacitracin ophthalmic ointment. Contralateral eyes served as untreated controls. The inferior conjunctival fornix of both eyes was sampled at baseline prior to initiating treatment (day 0), after 1 week of treatment (day 7), and 4 weeks after concluding treatment (day 35). Genomic DNA was extracted from ocular surface swabs and sequenced using primers that target the V4 region of bacterial 16S rRNA. At baseline, the most abundant phyla identified were Proteobacteria (46.1%), Firmicutes (24.6%), Actinobacteria (12.6%), and Bacteroidetes (11.2%). The most abundant families included Pasteurellaceae (13.7%), Sphingomonadaceae (7.9%), an unclassified Order of Cardiobacteriales (7.7%), and Moraxellaceae (4.8%). Alpha and beta diversity measurements were unchanged in both treatment and control eyes over time. Overall, the major bacterial taxa on the equine ocular surface remained stable over time and following topical antibiotic therapy.

PRISM study: Comparison of a nystatin-neomycin-polymyxin B combination with miconazole for the empirical treatment of infectious vaginitis.

OBJECTIVE: An empirical treatment of infectious vaginitis is justified because of its multiple etiologies, the frequent uncertainty of clinical diagnosis and limits of microbiological analysis. Our aim was to comparatively investigate nystatin-neomycin-polymyxin B combination (NNP, Polygynax®) and miconazole. PATIENTS AND METHODS: In this European multicenter, double-blind PRISM trial, participating women presenting with infectious vaginitis were randomized to receive one vaginal capsule containing either NNP for 12 days or miconazole for 3 days followed by 9 days of placebo. RESULTS: The clinical success rate was higher in the NNP group (n=302) than the miconazole group (n=309), with a difference between groups close to statistical significance (91.1% vs. 86.7%, P=0.0906). The risk of treatment failure was 36% lower in the NNP group (odds ratio, 0.64; 95% confidence interval, 0.38-1.07). Vaginal burning on Day 2 and vaginal discharge on Day 4 were significantly less intense in the NNP group than in the miconazole group (39.1 vs. 42.3, P=0.031 and 34.6 vs. 37.6, P=0.031, respectively). Adverse drug reactions were reported by 1.2% and 2.1% of patients in the NNP and miconazole group respectively, with the ratio of adverse drug reactions relative to total adverse events significantly higher in the miconazole group (20.3% vs. 6.9%, P=0.022). CONCLUSION: The widespread use of NNP for several decades and its good efficacy and safety profile, as well as the frequent diagnostic uncertainties due to the various pathogens sustain the initiation of this broad-spectrum empirical treatment in infectious vaginitis.

Neomycin-polymyxin or gentamicin bladder instillations decrease symptomatic urinary tract infections in neurogenic bladder patients on clean intermittent catheterization.

INTRODUCTION: Recurrent urinary tract infections (UTIs) are common in patients with neurogenic bladder (NGB) performing clean intermittent catheterization (CIC) treated with or without oral antibiotic prophylaxis. OBJECTIVE: The authors aim to determine if daily neomycin-polymyxin or gentamicin bladder instillations reduce the rate of symptomatic UTIs, the need for oral antibiotic prophylaxis, emergency department (ED) visits for UTI, and inpatient hospitalizations for UTI in patients with NGB on CIC. The authors also aim to investigate resistance patterns in urine microorganisms in patients treated with antibiotic bladder instillations. STUDY DESIGN: The authors retrospectively reviewed the records of all-age patients cared for in the pediatric urology clinic with NGB on CIC having symptomatic UTIs and on daily intravesical instillations of neomycin-polymyxin or gentamicin between 2013 and 2017. Symptomatic UTIs were defined as a positive urine culture with greater than 10,000 colony forming units/mL associated with one or more of the following patient complaints: cloudy/foul-smelling urine, fevers, chills, increase in bladder spasms, pain, urinary leakage, or physician decision for antibiotic treatment. Multidrug-resistant organisms were resistant to two or more classes of antibiotics. RESULTS: Fifty-two patients with a median age of 14.5 years and 192 distinct urine cultures were identified. 90.4% and 9.6% of patients received neomycin-polymyxin and gentamicin instillations, respectively. After initiation of intravesical antibiotics, the rate of symptomatic UTIs was reduced by 58% (incidence rate ratio [IRR]: 0.42, 95% confidence interval [CI]: 0.31-0.56; P < 0.001), the rate of ED visits was reduced by 54% (IRR: 0.46, 95% CI: 0.30-0.71; P < 0.001), and the rate of inpatient hospitalizations for UTI was reduced by 39% (IRR: 0.61, 95% CI: 0.37-0.98; P = 0.043). Fewer patients received oral antibiotic prophylaxis after initiation of antibiotic instillations (odds ratio: 0.12, 95% CI: 0.02-0.067; P = 0.016). There was a trend toward a decrease in multidrug resistance and no change in gentamicin resistance in urine microorganisms. DISCUSSION: This study describes a feasible alternative treatment for patients with NGB on CIC who have persistent UTIs despite oral antibiotic prophylaxis, and for some patients, it may suggest a possibility of discontinuing oral prophylaxis. Limitations include a retrospective design with a small cohort of patients and varying dosages of neomycin-polymyxin. CONCLUSIONS: Antibiotic bladder instillations appear to decrease frequency of symptomatic UTIs, ED visits for UTI, inpatient hospitalizations for UTI, and the need for oral antibiotic prophylaxis in patients with NGB on CIC. There was no increase in multidrug resistance or gentamicin resistance in UTI organisms with use of intravesical antibiotic instillation.

Incidences of Pseudomonas aeruginosa-Associated Ventilator-Associated Pneumonia within Studies of Respiratory Tract Applications of Polymyxin: Testing the Stoutenbeek Concurrency Postulates.

Regimens containing topical polymyxin appear highly effective at preventing ventilator-associated pneumonia (VAP) overall and, more so, VAP caused by Gram-negative bacteria. However, Stoutenbeek's postulates that VAP incidences within studies of topical antibiotics depend on the context of whether the component (control and intervention) groups of each study were concurrent versus nonconcurrent remain untested. The literature was searched for concurrent control (CC) versus nonconcurrent control (NCC) designed studies of respiratory tract applications of topical polymyxin to mechanically ventilated (MV) patients that reported incidences of Pseudomonas-associated ventilator-associated pneumonia (PsVAP). Studies of various interventions other than topical polymyxin (nonpolymyxin studies) served to provide additional points of reference. The PsVAP incidences within the component groups of all studies were benchmarked against groups from observational studies. This was undertaken by meta-regression using generalized estimating equation methods. Dot plots, caterpillar plots, and funnel plots enable visual benchmarking. The PsVAP benchmark (and 95% confidence interval [CI]) derived from 102 observational groups is 4.6% (4.0 to 5.3%). In contrast, the mean PsVAP within NCC polymyxin intervention groups (1.6%; CI, 1.0 to 4.5%) is lower than that of all other component group categories. The mean PsVAP within CC polymyxin control groups (9.9%; CI, 7.6 to 12.8%) is higher than that of all other component group categories. The PsVAP incidences of control and intervention groups of studies of respiratory tract applications of polymyxin are dependent on whether the groups were within a concurrent versus nonconcurrent study. Stoutenbeek's concurrency postulates are validated.

Paradoxical Acinetobacter-associated ventilator-associated pneumonia incidence rates within prevention studies using respiratory tract applications of topical polymyxin: benchmarking the evidence base.

BACKGROUND: Regimens containing topical polymyxin appear to be more effective in preventing ventilator-associated pneumonia (VAP) than other methods. AIM: To benchmark the incidence rates of Acinetobacter-associated VAP (AAVAP) within component (control and intervention) groups from concurrent controlled studies of polymyxin compared with studies of various VAP prevention methods other than polymyxin (non-polymyxin studies). METHODS: An AAVAP benchmark was derived using data from 77 observational groups without any VAP prevention method under study. Data from 41 non-polymyxin studies provided additional points of reference. The benchmarking was undertaken by meta-regression using generalized estimating equation methods. RESULTS: Within 20 studies of topical polymyxin, the mean AAVAP was 4.6% [95% confidence interval (CI) 3.0-6.9] and 3.7% (95% CI 2.0-5.3) for control and intervention groups, respectively. In contrast, the AAVAP benchmark was 1.5% (95% CI 1.2-2.0). In the AAVAP meta-regression model, group origin from a trauma intensive care unit (+0.55; +0.16 to +0.94, P = 0.006) or membership of a polymyxin control group (+0.64; +0.21 to +1.31, P = 0.023), but not membership of a polymyxin intervention group (+0.24; -0.37 to +0.84, P = 0.45), were significant positive correlates. CONCLUSIONS: The mean incidence of AAVAP within the control groups of studies of topical polymyxin is more than double the benchmark, whereas the incidence rates within the groups of non-polymyxin studies and, paradoxically, polymyxin intervention groups are more similar to the benchmark. These incidence rates, which are paradoxical in the context of an apparent effect against VAP within controlled trials of topical polymyxin-based interventions, force a re-appraisal.

Clinical use of the polymyxins: the tale of the fox and the cat.

BACKGROUND: There is a need to identify practice patterns of polymyxin use, quantify gaps in knowledge, and recognize areas of persistent confusion. METHODS: A structured electronic survey was distributed to physicians, pharmacists and microbiologists. Demographic information was obtained, along with data regarding availability, stewardship principles, therapeutic usage, dosing, microbiological testing, and knowledge, attitudes and beliefs regarding the polymyxins. RESULTS: In total, there were 420 respondents with a median of 8 (interquartile range 4-15) years of experience in infectious diseases (52.5%) and critical care (35%). Of the respondents who reported that only one polymyxin was available for use, 17.1% used polymyxin B. Over half (52.5%) of the respondents utilized a loading dose very often/always, and 66.8% dosed both polymyxins in milligrams, with the most common doses of colistin and polymyxin B being 2.5 mg/kg twice daily (60.3%) and 1.5 mg/kg twice daily (65%), respectively, for patients with normal renal function. Polymyxins were most often used for respiratory infections (63%) in combination with a carbapenem (63.6%). Approximately 85% of respondents reported their knowledge level to be fair, good or very good, although 34.9% answered two of the three knowledge questions incorrectly. More than 70% of respondents agreed that confusion exists in all surveyed areas of polymyxin use. Almost all respondents (91.2%) agreed that a polymyxin guideline would be a helpful resource. CONCLUSIONS: This survey revealed objective and subjective variability in the use and perception of the polymyxins, and identified several areas in which they were being used contrary to the available evidence. The information provided herein lays the framework to harmonize clinical practice, guide future research and shape consensus guidelines.

Polymyxin derivatives NAB739 and NAB815 are more effective than polymyxin B in murine Escherichia coli pyelonephritis.

Objectives: Extremely multiresistant strains of Enterobacteriaceae, such as those of Escherichia coli and Klebsiella pneumoniae, are emerging and spreading at a worrisome speed. Polymyxins (polymyxin B, colistin) are used as last-line therapy against such strains, in spite of their notable nephrotoxicity that may even require discontinuation of the therapy. We have previously developed polymyxin derivatives NAB739 and NAB815 that are better tolerated in cynomolgus monkeys than polymyxin B and are, in contrast to polymyxin B, excreted in the cynomolgus urine to a very significant degree. Here we have compared the efficacy of these NAB compounds and polymyxin B in the therapy of murine pyelonephritis caused by E. coli. Methods: The challenge organism was a uropathogenic E. coli clinical isolate. Mice were inoculated via urethral catheterization with 5 × 108 cfu. All treatment groups consisted of 12 animals. On day 1 and day 2 post-infection, the mice were treated subcutaneously with NAB739, NAB815, polymyxin B or vehicle twice a day and on day 3 post-infection the animals were sacrificed. cfu in the kidney and bladder tissues and in the urine were determined. Results: NAB739 reduced the bacterial burden in the kidney, urine and bladder at doses approximately 10-fold lower than those of polymyxin B. In the kidneys, the half-maximal effective dose (ED50) was 9-fold lower for NAB739 than for polymyxin B (0.24 mg/kg versus 2.1 mg/kg, respectively). NAB815 was as effective as NAB739. Conclusions: NAB739 and NAB815 were unequivocally more effective than polymyxin B in the murine pyelonephritis model.

Triple combination antibiotic therapy for carbapenemase-producing Klebsiella pneumoniae: a systematic review.

BACKGROUND: The spread of carbapenemase-producing K. pneumoniae (CPKP) has become a significant problem worldwide. Combination therapy for CPKP is encouraging, but polymyxin resistance to many antibiotics is hampering effective treatment. Combination therapy with three or more antibiotics is being increasingly reported, therefore we performed a systematic review of triple combination cases in an effort to evaluate their clinical effectiveness for CPKP infections. METHODS: The PubMed database was searched to identify all published clinical outcomes of CPKP infections treated with triple combination therapy. Articles were stratified into two tiers depending on the level of clinical detail provided. A tier 1 study included: antibiotic regimen, regimen-specific outcome, patient status at onset of infection, and source of infection. Articles not reaching these criteria were considered tier 2. RESULTS: Thirty-three studies were eligible, 23 tier 1 and ten tier 2. Among tier 1 studies, 53 cases were included in this analysis. The most common infection was pneumonia (31%) followed by primary or catheter-related bacteremia (21%) and urinary tract infection (17%). Different combinations of antibiotic classes were utilized in triple combinations, the most common being a polymyxin (colistin or polymyxin B, 86.8%), tigecycline (73.6%), aminoglycoside (43.4%), or carbapenem (43.4%). Clinical and microbiological failure occurred in 14/39 patients (35.9%) and 22/42 patients (52.4%), respectively. Overall mortality for patients treated with triple combination therapy was 35.8% (19/53 patients). CONCLUSIONS: Triple combination therapy is being considered as a treatment option for CPKP. Polymyxin-based therapy is the backbone antibiotic in these regimens, but its effectiveness needs establishing in prospective clinical trials.

MCR-1 Inhibition with Peptide-Conjugated Phosphorodiamidate Morpholino Oligomers Restores Sensitivity to Polymyxin in Escherichia coli.

In late 2015, the first example of a transferrable polymyxin resistance mechanism in Gram-negative pathogens, MCR-1, was reported. Since that report, MCR-1 has been described to occur in many Gram-negative pathogens, and the mechanism of MCR-1-mediated resistance was rapidly determined: an ethanolamine is attached to lipid A phosphate groups, rendering the membrane more electropositive and repelling positively charged polymyxins. Acquisition of MCR-1 is clinically significant because polymyxins are frequently last-line antibiotics used to treat extensively resistant organisms, so acquisition of this mechanism might lead to pan-resistant strains. Therefore, the ability to inhibit MCR-1 and restore polymyxin sensitivity would be an important scientific advancement. Peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs) are antisense molecules that were designed to target mRNA, preventing translation. Peptide conjugation enhances cellular entry, but they are positively charged, so we tested our lead antibacterial PPMOs by targeting an essential Escherichia coli gene, acpP, and demonstrated that they were still effective in mcr-1-positive E. coli strains. We then designed and synthesized two PPMOs targeted to mcr-1 mRNA. Five clinical mcr-1-positive E. coli strains were resensitized to polymyxins by MCR-1 inhibition, reducing MICs 2- to 16-fold. Finally, therapeutic dosing of BALB/c mice with MCR-1 PPMO combined with colistin in a sepsis model reduced morbidity and bacterial burden in the spleen at 24 h and offered a survival advantage out to 5 days. This is the first example of a way to modulate colistin resistance with an antisense approach and may be a viable strategy to combat this globally emerging antibiotic resistance threat.IMPORTANCE Polymyxin use has been increasing as a last line of defense against Gram-negative pathogens with high-level resistance mechanisms, such as carbapenemases. The recently described MCR-1 is a plasmid-mediated mechanism of resistance to polymyxins. MCR-1 is currently found in Gram-negative organisms already possessing high-level resistance mechanisms, leaving clinicians few or no antibacterial options for infections caused by these strains. This study utilizes antisense molecules that target mRNA, preventing protein translation. Herein we describe antisense molecules that can be directly antibacterial because they target genes essential to bacterial growth or blockade of MCR-1, restoring polymyxin sensitivity. We also demonstrate that MCR-1 antisense molecules restore the efficacies of polymyxins in mouse models of E. coli septicemia. Considering all things together, we demonstrate that antisense molecules may be effective therapeutics either alone when they target an essential gene or combined with antibiotics when they target specific resistance mechanisms, such as those seen with MCR-1.

Intravenous polymyxins: Revival with puzzle.

With the increasing incidence of multi-drug resistant strains, especially carbapenem resistant strains, polymyxsins (mainly colistin and polymyxin B) based regimens seem to be a revival as an effective treatment of last resort in these infections. Evidence from 47 clinical trials or case series we reviewed showed that polymyxins based regimens are effective and have less toxicity compared with previous trials. When used alone, the mortality of intravenous polymyxsins ranged from 0% to 74.3%, clinical response (cure and improvement) rate was 7-82.1%, and microbiological eradication was 27.3-73.9%. The main reasons for the combination therapy are to get potential synergistic effects and to prevent the selection of heteroresistant strains. Several studies showed combination therapy seemed to be more effective than monotherapy, though a few doubts remain. Clinically, polymyxsins can be used in combination with several antibiotics, such as carberpenem, sulbactam, tigecycline, fosfomycin, glycopeptide, rifampicin and so on, but the optimal combination regimen is yet to be confirmed. The optimal dose of polymyxins is also controversial. With the limited clinical evidence, it's suggested loading dose regimens may be more effective, but more attention should be paid to adverse effects. Although recommended in some studies, high dose polymxins regimens with inconsistent clinical evidence need more trials to confirm. It is important to note that concerning dosing regimens, colistin and polymyxin B are not quite the same. In renal impaired patients polymyxin B should be prescribed without dosing adjustment. Risk of renal failure may increase in the following situations, such as the combination of intravenous colistin plus intravenous vancomycin, higher doses-colistin, and intravenous colistin combined with inhalational colistin. In conclusion, there're still controversies in combination regimens, dosing strategies and so on. Prospective trials of lager sample size are needed.

Clinical considerations for optimal use of the polymyxins: A focus on agent selection and dosing.

Polymyxins have remained the drug of choice for treatment due to carbapenem-resistant Gram-negative bacilli. Unfortunately, the utility of these agents has been limited by a lack of pharmacokinetic understanding, a high toxicity rate, and an extremely narrow therapeutic index. Significant advancements have been achieved in the understanding of the polymyxins over the past decade, and have led to the recognition of several differences between available intravenous formulations. The purpose of this review is to discuss the implications of these differences, assess comparative efficacy and safety of the polymyxins, and provide recommendations for polymyxin dosing and selection.

Topical Polymyxin-Trimethoprim Prophylaxis May Decrease the Incidence of Driveline Infections in Patients With Continuous-Flow Left Ventricular Assist Devices.

This retrospective cohort study evaluated the effect of topical polymyxin-trimethoprim (poly) prophylaxis on the incidence of driveline infections (DLIs) in patients with continuous-flow left ventricular assist devices. All 84 cases implanted 2005-2014 with device support ≥30 days were reviewed; support ranged 1 m-5.2 yrs. Beginning 2008, poly was applied to the exit site with dressing changes. Sixty-five patients received poly (poly group) for duration of follow-up, 19 did not (no-poly); group baseline characteristics were similar. No patient developed side effects from poly. Nineteen DLIs (10 in no-poly) occurred; not using poly was a risk factor. 89% of poly group DLIs were superficial, 4 were culture-negative. DLI-related bacteremia occurred in 11% of no-poly group and 0% of poly group. Compared with no-poly, poly group demonstrated improved freedom from DLI by Kaplan-Meier analysis (P < 0.0001) and a 75% lower overall and 95% lower deep DLI incidence (P ≤ 0.001). Deep DLIs occurred in 31.6% of no-poly vs. 1.5% of poly patients (P = 0.0004), although mean support duration (1 yr) and % support >1 yr (38%) were similar. These findings, which should be confirmed with larger comparative studies, suggest that topical polymyxin-trimethoprim prophylaxis may be effective in preventing DLIs.

Are there any ways around the exposure-limiting nephrotoxicity of the polymyxins?

The polymyxins (colistin and polymyxin B) have emerged over the past 20 years as essential antibacterial agents that often are the only remaining active class against troublesome multidrug-resistant Gram-negative bacilli such as carbapenem-resistant Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacteriaceae. The utility of this class is limited by its dose-dependent nephrotoxicity, which can occur in more than one-half of patients receiving therapy with either agent. Strategies are urgently needed to optimise the use of this class of agents to ensure optimal activity while minimising the treatment-limiting nephrotoxicity. This review will focus on risk factors for polymyxin-associated nephrotoxicity, potential strategies for limiting this exposure-dependent toxicity and, finally, unknowns and future research directions pertinent to this topic.