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.

Bioanalysis and Stability of Polymyxins.

Clinical use of the polymyxin antibiotics began approximately 10 years after their discovery in the late 1940s. Their concentrations in biological fluids were measured using microbiological methods. These methods were reasonably accurate for measuring the active polymyxin base, such as polymyxin B and colistin (polymyxin E), but were used inappropriately for measuring the concentrations of "colistin" in humans or animals following the administration of colistimethate, also known as colistin methanesulphonate (CMS). The use of polymyxins for systemic infections waned in the 1970s because of their toxicity and the preference for other antibiotics, but their value for treating infections caused by several important Gram-negative pathogens becoming resistant to other antibiotics was realized in the mid-1990s. The lack of adequate pharmacokinetic and pharmacodynamic knowledge spurred the development of methods more specific for measuring polymyxin B and colistin after their administrations as sulphate salts, and of colistin and CMS after the administration of CMS sodium. These methods have been based on high-performance liquid chromatography, detection and quantification of fluorescent derivatives of the polymyxin bases, or of the bases themselves with detection and quantification by mass spectrometry.

Pharmacokinetics of Polymyxins in Animals.

All of the small number of studies conducted during the second half of last century to investigate the pharmacokinetics of polymyxins in animals used microbiological methods to quantify the compounds in biological fluids. Those methods generally lacked the accuracy and precision required for such investigations and, in the case of studies involving administration of colistin methanesulfonate (CMS), ongoing conversion to colistin during microbiological incubation of collected samples artifactually elevated the measured concentration of colistin. The pharmacokinetic studies reviewed in this chapter involved use of more accurate, precise and specific methods for the measurement of the relevant compounds in biological matrices. The studies have been conducted in a number of pre-clinical animal species following administration via various routes (e.g. intravenous, intrapulmonary), and have provided important insights into not only the global pharmacokinetics as viewed from plasma but also the tissue distribution and handling by key organs particularly the kidneys.

In vitro Pharmacodynamics and PK/PD in Animals.

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

Clinical Pharmacokinetics, Pharmacodynamics and Toxicodynamics of Polymyxins: Implications for Therapeutic Use.

The availability of sensitive, accurate and specific analytical methods for the measurement of polymyxins in biological fluids has enabled an understanding of the pharmacokinetics of these important antibiotics in healthy humans and patients. Colistin is administered as its inactive prodrug colistin methanesulfonate (CMS) and has especially complex pharmacokinetics. CMS undergoes conversion in vivo to the active entity colistin, but the rate of conversion varies from brand to brand and possibly from batch to batch. The extent of conversion is generally quite low and depends on the relative magnitudes of the conversion clearance and other clearance pathways for CMS of which renal excretion is a major component. Formed colistin in the systemic circulation undergoes very extensive tubular reabsorption; the same mechanism operates for polymyxin B which is administered in its active form. The extensive renal tubular reabsorption undoubtedly contributes to the propensity for the polymyxins to cause nephrotoxicity. While there are some aspects of pharmacokinetic behaviour that are similar between the two clinically used polymyxins, there are also substantial differences. In this chapter, the pharmacokinetics of colistin, administered as CMS, and polymyxin B are reviewed, and the therapeutic implications are discussed.

Resistencia plasmídica a colistin por el gen mcr-1 en Enterobacteriaceae en Paraguay / Plasmid-Mediated Colistin Resistance Gene mcr-1 in Enterobacteriaceae in Paraguay

La resistencia a las polimixinas mediada por plásmidos (gen mcr-1) representa una amenaza para la salud pública, puesto que colistina es utilizada en la práctica médica como una de las últimas alternativas para el tratamiento de gérmenes multiresistentes. Este estudio describe la circulaciónde cepas de Enterobacterias que portan este gen de resistencia, aisladas de pacientes hospitalizados, así como también de la comunidad. Los hallazgos de la Red de Vigilancia de la Resistencia a los Antimicrobianos-Paraguay fueron de casi el 5 % (4,7) en cepas remitidas con criterio de sospecha, siendo las especies involucradas Escherichiacoli, Klebsiella pneumoniae y Salmonella Schwarzengrund. Además, por métodos moleculares se confirmaron en todas ellas la portación de otros genes de resistencia (KPC, CTX-M, Qnr B, Qnr S, aac (6`)-Ib-cr) asociados al mcr-1. Palabras claves: Enterobacterias, resistencia, colistina, mcr-1.

Resistance to polymyxins mediated by plasmids (mcr-1 gene) represents a threat to public health, since colistin is used in medical practice, as one of the last alternatives, for the treatment of multi-resistant germs. This study describes the circulation of strains of Enterobacteria that carry this resistance gene, isolated from hospitalized patients, as well as from the community. The findings of the Red de Vigilancia de la Resistencia a los Antimicrobianos­Paraguay were almost 5% (4.7) in strains submitted with suspicion criteria; the species involved being Escherichia coli, Klebsiella pneumoniae and Salmonella Schwarzengrund. In addition, molecular methods confirmed in all of them the carrying of other resistance genes (KPC, CTX-M, Qnr B, Qnr S, aac (6`)-Ib-cr) associated with mcr-1. Key words: Enterobacteria, resistance, colistin, mcr-1.

Pharmacokinetic/Pharmacodynamics-Optimized Antimicrobial Therapy in Patients with Hospital-Acquired Pneumonia/Ventilator-Associated Pneumonia.

Hospital-acquired pneumonia and ventilator-associated pneumonia continue to cause significant morbidity and mortality. With increasing rates of antimicrobial resistance, the importance of optimizing antibiotic treatment is key to maximize treatment outcomes. This is especially important in critically ill patients in intensive care units, in whom the infection is usually caused by less susceptible organisms. In addition, the marked physiological changes that can occur in these patients can cause serious changes in antibiotic pharmacokinetics which in turn alter the attainment of therapeutic drug exposures. This article reviews the various aspects of the pharmacokinetic changes that can occur in the critically ill patients, the barriers to achieving therapeutic drug exposures in pneumonia for systemically delivered antibiotics, the optimization for commonly used antibiotics in hospital- and ventilator-associated pneumonia, the agents that should be avoided in the treatment regimen, as well as the use of adjunctive therapy in the form of nebulized antibiotics.

Pharmacokinetics of the Individual Major Components of Polymyxin B and Colistin in Rats.

The pharmacokinetics of polymyxin B1, polymyxin B2, colistin A, and colistin B were investigated in a rat model following intravenous administration (0.8 mg/kg) of each individual component. Plasma and urine concentrations were determined by LC-MS/MS, and plasma protein binding was measured by ultracentrifugation. Total and unbound pharmacokinetic parameters for each component were calculated using noncompartmental analysis. All of the polymyxin components had a similar clearance, volume of distribution, elimination half-life, and urinary recovery. The area under the concentration-time curve for polymyxins B1 and B2 was greater than those of colistins A and B. Colistin A (56.6 ± 9.25%) and colistin B (41.7 ± 12.4%) displayed lower plasma protein binding in rat plasma compared to polymyxin B1 (82.3 ± 4.30%) and polymyxin B2 (68.4 ± 3.50%). These differences in plasma protein binding potentially equate to significant differences in unbound pharmacokinetics, highlighting the need for more stringent standardization of the composition of commercial products currently available for clinical use.

Human oligopeptide transporter 2 (PEPT2) mediates cellular uptake of polymyxins.

OBJECTIVES: Polymyxins are a last-line therapy to treat MDR Gram-negative bacterial infections. Nephrotoxicity is the dose-limiting factor for polymyxins and recent studies demonstrated significant accumulation of polymyxins in renal tubular cells. However, little is known about the mechanism of polymyxin uptake into these cells. Oligopeptide transporter 2 (PEPT2) is a solute carrier transporter (SLC) expressed at the apical membrane of renal proximal tubular cells and facilitates drug reabsorption in the kidney. In this study, we examined the role of PEPT2 in polymyxin uptake into renal tubular cells. METHODS: We investigated the inhibitory effects of colistin and polymyxin B on the substrate uptake mediated through 15 essential SLCs in overexpressing HEK293 cells. The inhibitory potency of both polymyxins on PEPT2-mediated substrate uptake was measured. Fluorescence imaging was employed to investigate PEPT2-mediated uptake of the polymyxin fluorescent probe MIPS-9541 and a transport assay was conducted with MIPS-9541 and [(3)H]polymyxin B1. RESULTS: Colistin and polymyxin B potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 11.4 ± 3.1 and 18.3 ± 4.2 µM, respectively). In contrast, they had no or only mild inhibitory effects on the transport activity of the other 14 SLCs evaluated. MIPS-9541 potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 15.9 µM) and is also a substrate of PEPT2 (Km 74.9 µM). [(3)H]polymyxin B1 was also significantly taken up by PEPT2-expressing cells (Km 87.3 µM). CONCLUSIONS: Our study provides the first evidence of PEPT2-mediated uptake of polymyxins and contributes to a better understanding of the accumulation of polymyxins in renal tubular cells.

Investigating nephrotoxicity of polymyxin derivatives by mapping renal distribution using mass spectrometry imaging.

Colistin and polymyxin B are effective treatment options for Gram-negative resistant bacteria but are used as last-line therapy due to their dose-limiting nephrotoxicity. A critical factor in developing safer polymyxin analogues is understanding accumulation of the drugs and their metabolites, which is currently limited due to the lack of effective techniques for analysis of these challenging molecules. Mass spectrometry imaging (MSI) allows direct detection of targets (drugs, metabolites, and endogenous compounds) from tissue sections. The presented study exemplifies the utility of MSI by measuring the distribution of polymyxin B1, colistin, and polymyxin B nonapeptide (PMBN) within dosed rat kidney tissue sections. The label-free MSI analysis revealed that the nephrotoxic compounds (polymyxin B1 and colistin) preferentially accumulated in the renal cortical region. The less nephrotoxic analogue, polymyxin B nonapeptide, was more uniformly distributed throughout the kidney. In addition, metabolites of the dosed compounds were detected by MSI. Kidney homogenates were analyzed using LC/MS/MS to determine total drug exposure and for metabolite identification. To our knowledge, this is the first time such techniques have been utilized to measure the distribution of polymyxin drugs and their metabolites. By simultaneously detecting the distribution of drug and drug metabolites, MSI offers a powerful alternative to tissue homogenization analysis and label or antibody-based imaging.

Polymyxin combinations: pharmacokinetics and pharmacodynamics for rationale use.

Since their reintroduction into the clinic in the 1980s, the polymyxin antibiotics colistin-administered intravenously as an inactive prodrug, colistin methanesulfonate (CMS)-and polymyxin B have assumed an important role as salvage therapy for otherwise untreatable gram-negative infections. However, the emerging pharmacodynamic and pharmacokinetic data on CMS/colistin and polymyxin B indicate that polymyxin monotherapy is unlikely to generate plasma concentrations that are reliably efficacious. Additionally, regrowth and the emergence of resistance with monotherapy are commonly reported even when concentrations exceed those achieved clinically. Given this situation, polymyxin combination therapy, which is increasingly being used clinically, has been suggested as a possible means of increasing antimicrobial activity and reducing the development of resistance. Although considerable in vitro data support this view, investigations of polymyxin combination therapy in patients have only recently commenced. The currently available clinical data for polymyxin combinations are generally limited to retrospective analyses and small, low-powered, prospective studies using traditional dosage regimens that achieve low plasma concentrations. Considering the potential for rapid development of resistance to polymyxins, well-designed clinical trials that include higher-dose polymyxin regimens are urgently required to provide a more definitive answer regarding the role of polymyxin combination therapy compared with monotherapy. In this article, we provide an overview of key in vitro and clinical investigations examining CMS/colistin and polymyxin B combination therapy.

Adverse reactions associated with systemic polymyxin therapy.

The systemic polymyxins, colistin and polymyxin B, are increasingly used for multidrug-resistant bacterial infections and have a long history of dose-limiting toxicity. This review summarizes the most recent available information about the mechanisms, incidence, risk factors, and minimization strategies for polymyxin toxicity. Nephrotoxicity is related to polymyxin exposure with both size of dose and length of therapy associated with frequency. Newer studies have questioned conventional thinking that the relative risk of nephrotoxicity is lower for colistin than polymyxin B, especially in light of evolving dosing practices. Neurotoxicities and hypersensitivity reactions are less common than nephrotoxicity. New techniques to minimize or avoid polymyxin toxicities are now emerging including a growing interest in clinical assays for therapeutic drug monitoring and the development of novel, less toxic agents (e.g., polymyxin derivatives) for the treatment of multidrug-resistant bacterial infections.

Combination therapy for carbapenem-resistant Gram-negative bacteria.

The emergence of resistant to carbapenems Gram-negative bacteria (CR GNB) has severely challenged antimicrobial therapy. Many CR GNB isolates are only susceptible to polymyxins; however, therapy with polymyxins and other potentially active antibiotics presents some drawbacks, which have discouraged their use in monotherapy. In this context, along with strong pre-clinical evidence of benefit in combining antimicrobials against CR GNB, the clinical use of combination therapy has been raised as an interesting strategy to overcome these potential limitations of a single agent. Polymyxins, tigecycline and even carbapenems are usually the cornerstone agents in combination schemes. Optimization of the probability to attain the pharmacokinetic/pharmacodynamic targets by both cornerstone drug and adjuvant drug is of paramount importance to achieve better clinical and microbiological outcomes. Clinical evidence of the major drugs utilized in combination schemes and how they should be prescribed considering pharmacokinetic/pharmacodynamic characteristics against CR GNB will be reviewed in this article.

Discovery of Dap-3 polymyxin analogues for the treatment of multidrug-resistant Gram-negative nosocomial infections.

We report novel polymyxin analogues with improved antibacterial in vitro potency against polymyxin resistant recent clinical isolates of Acinetobacter baumannii and Pseudomonas aeruginosa . In addition, a human renal cell in vitro assay (hRPTEC) was used to inform structure-toxicity relationships and further differentiate analogues. Replacement of the Dab-3 residue with a Dap-3 in combination with a relatively polar 6-oxo-1-phenyl-1,6-dihydropyridine-3-carbonyl side chain as a fatty acyl replacement yielded analogue 5x, which demonstrated an improved in vitro antimicrobial and renal cytotoxicity profiles relative to polymyxin B (PMB). However, in vivo PK/PD comparison of 5x and PMB in a murine neutropenic thigh model against P. aeruginosa strains with matched MICs showed that 5x was inferior to PMB in vivo, suggesting a lack of improved therapeutic index in spite of apparent in vitro advantages.

Pharmacology of polymyxins: new insights into an 'old' class of antibiotics.

Increasing antibiotic resistance in Gram-negative bacteria, particularly in Pseudomonas aeruginosa, Acinetobacter baumannii and Klebsiella pneumoniae, presents a global medical challenge. No new antibiotics will be available for these 'superbugs' in the near future due to the dry antibiotic discovery pipeline. Colistin and polymyxin B are increasingly used as the last-line therapeutic options for treatment of infections caused by multidrug-resistant Gram-negative bacteria. This article surveys the significant progress over the last decade in understanding polymyxin chemistry, mechanisms of antibacterial activity and resistance, structure-activity relationships and pharmacokinetics/pharmacodynamics. In the 'Bad Bugs, No Drugs' era, we must pursue structure-activity relationship-based approaches to develop novel polymyxin-like lipopeptides targeting polymyxin-resistant Gram-negative 'superbugs'. Before new antibiotics become available, we must optimize the clinical use of polymyxins through the application of pharmacokinetic/pharmacodynamic principles, thereby minimizing the development of resistance.

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

'Old' colistin and polymyxin B are increasingly used as last-line therapy against multidrug-resistant Gram-negative bacteria Pseudomonas aeruginosa, Acinetobacter baumannii, and Klebsiella pneumoniae. For intravenous administration, colistin is dosed as its inactive prodrug colistin methanesulfonate (sodium), while polymyxin B is used as its sulfate (active antibacterial). Over the last decade, significant progress has been made in understanding their chemistry, pharmacokinetics (PK), and pharmacodynamics (PD). The first scientifically based dosing suggestions are now available for colistin methanesulfonate to generate a desired target steady-state plasma concentration of formed colistin in various categories of critically ill patients. As simply increasing polymyxin dosage regimens is not an option for optimizing their PK/PD due to nephrotoxicity, combination therapy with other antibiotics has great potential to maximize the efficacy of polymyxins while minimizing emergence of resistance. We must pursue rational approaches to the use of polymyxins and other existing antibiotics through the application of PK/PD principles.

Can Pharmacokinetic and Pharmacodynamic Principles Be Applied to the Treatment of Multidrug-Resistant Acinetobacter?

OBJECTIVE: To discuss treatment options that can be used for treatment of Acinetobac/erinfections. DATA SOURCES: A MEDLINE search (1966-November 2010) was conducted to identify English-language literature on pharmacotherapy of Acinetobacter and the bibliographies of pertinent articles. Programs and abstracts from infectious diseases meetings were also searched. Search terms included Acinetobacter, multidrug resistance, pharmacokinetics, pharmacodynamics, Monte Carlo simulation, nosocomial pneumonia, carbapenems, polymyxins, sulbactam, aminoglycosides, tetracyclines, tigecycline, rifampin, and fluoroquinolones. DATA SELECTION AND DATA EXTRACTION: All articles were critically evaluated and all pertinent information was included in this review. DATA SYNTHESIS: Multidrug resistant (MDR) Acinetobacter, defined as resistance to 3 or more antimicrobial classes, has increased over the past decade. The incidence of carbapenem-resistant Acinetobacter is also increasing, leading to an increased use of dose optimization techniques and/or alternative antimicrobials, which is driven by local susceptibility patterns. However, Acinetobacter infections that are resistant to all commercially available antibiotics have been reported. General principles are available to guide dose optimization of aminoglycosides, ß-lactams, fluoroquinolones, and tigecycline for infections due to gram-negative pathogens. Unfortunately, data specific to patients with Acinetobacter infections are limited. Recent pharmacokinetic-pharmacodynamic information has shed light on colistin dosing. The dilemma with colistin is its concentration-dependent killing, which makes once-daily dosing seem like an attractive option, but its short postantibiotic effect limits a clinician's ability to extend the dosing interval. Localized delivery of antimicrobials is also an attractive option due to the ability to increase drug concentration at the infection site while minimizing systemic adverse events, but more data are needed regarding this approach. CONCLUSIONS: Increased reliance on dosage optimization, combination therapy, and localized delivery of antimicrobials are methods to pursue positive clinical outcomes in MDR Acinetobacter infections since novel antimicrobials will not be available for several years. Well-designed clinical trials with MDR Acinetobacter are needed to define the best treatment options for these patients.

LC-MS/MS method development and validation for the determination of polymyxins and vancomycin in rat plasma.

Simple, sensitive and robust liquid chromatography-tandem mass spectrometer (LC-MS/MS) methods were developed and validated for the determination of lipopeptide polymyxins and glycopeptide vancomycin in rat plasma. The effect of trichloroacetic acid (TCA) concentration on sample recoveries (peak area of sample recovered from plasma/peak area of sample from neat solvent solutions) was studied and an optimized concentration of 30% TCA were determined that gives the best sample recovery for the peptides from rat plasma. The effect of the TCA concentration on the chromatographic behavior of peptides was studied on a Phenomenex Jupiter C18 5µ 300Å 50mm×2mm column using a mobile phase with a pH of 2.8. Other than protein precipitation, TCA also acted as ion pairing reagent and was only present in the samples but not in the mobile phases. The data demonstrated that by increasing the TCA concentration, the analyte retention and sensitivity were improved. The absence of TCA in mobile phase helped to reduce the ion source contamination and to achieve good reproducibility. The plasma method was linearly calibrated from 5 to 5000ng/mL for polymyxins with precisions to be of 2.3-10.8%, and accuracies to be 91.7-107.4% for polymyxin B1, B2, E1, E2, respectively. For vancomycin the calibration is from 1 to 5000ng/mL with precisions to be of 7.8-10.3 and accuracies to be 96.2-102.0%. The LLOQs corresponding with a coefficient of variation less than 20% were 7.5, 18.1, 7.3, 5.0 and 1.0ng/mL for polymyxin B1, B2, E1, E2 and vancomycin, respectively.

Current use for old antibacterial agents: polymyxins, rifamycins, and aminoglycosides.

This article reviews three classes of antibacterial agents that are uncommonly used in bacterial infections and therefore can be thought of as special-use agents. The polymyxins are reserved for gram-negative bacilli that are resistant to virtually all other classes of drugs. Rifampin is used therapeutically, occasionally as a companion drug in treatment of refractory gram-positive coccal infections, especially those involving foreign bodies. Rifaximin is a new rifamycin that is a strict enteric antibiotic approved for treatment of traveler's diarrhea and is showing promise as a possible agent for refractory Clostridium difficile infections. The aminoglycosides are used mainly as companion drugs for the treatment of resistant gram-negative bacillary infections and for gram-positive coccal endocarditis.

Pharmacokinetics of novel antimicrobial cationic peptides NAB 7061 and NAB 739 in rats following intravenous administration.

OBJECTIVES: To determine the disposition of novel antimicrobial cationic peptides NAB 7061 and NAB 739 following intravenous administration in rats. METHODS: Sprague-Dawley rats received a single intravenous bolus of 1.0 mg/kg NAB 7061 or NAB 739. Plasma concentrations of NAB 7061 or NAB 739 were determined by HPLC or liquid chromatography-mass spectrometry. The pharmacokinetic parameters of NAB 7061 and NAB 739 were calculated using non-compartmental analysis. RESULTS: Corresponding total body clearance, volume of distribution at steady state and terminal half-life of NAB 7061 and NAB 739 averaged 3.84 and 2.63 mL/min/kg, 339 and 222 mL/kg, and 66.2 and 69.0 min, respectively. Approximately 7.16% and 19.4% of the dose was eliminated in an unchanged form via the urine in 24 h for NAB 7061 and NAB 739, respectively. CONCLUSIONS: While both compounds had generally similar pharmacokinetics to colistin, even minor alterations in the chemical structures appear to have an impact on their pharmacokinetics, especially on their clearance by the kidney. There are also substantial differences in relation to the relative contributions of renal and non-renal clearance to overall elimination from the body.