Residue determination of two co-administered antibacterial agents--cephalexin and colistin--in calf tissues using high-performance liquid chromatography and microbiological methods

Liquid crystalline coatings loaded with colistin for preventing development of biofilms on orthopedic implants

The current antibacterial strategies focus on antibiotic therapy and extensive hygienic measures during orthopedic surgery. However, potential development of implant-associated infections remains a persistent clinical challenge. There is, therefore, a growing interest in introducing innovative safe antibacterial strategies for preventing and combating biofilm development on implants. Antibacterial coatings, particularly, are attractive for local delivery of antibacterial agents. We aim in this proof-of-concept study at introducing a novel and translatable implant coating approach, focusing on directed assembly of inverse non-lamellar lyotropic liquid crystalline (LLC) nanostructures on implants for prevention of initial bacterial attachment and biofilm formation through local delivery of the widely used cationic antibiotic colistin (COL). On exposure of dry lipid films deposited on model implants to aqueous solutions of COL prepared at different COL concentrations, a set of LLC coatings based on a commercial distilled monoglyceride product (or glycerol monooleate) were produced. In addition to small-angle X-ray scattering (SAXS) characterization investigations, in vitro studies were conducted for evaluating the antibacterial and antibiofilm properties of the LLC coatings against the Gram-negative bacteria Pseudomonas aeruginosa. The SAXS analysis indicated that all samples are inverse bicontinuous cubic Pn3m phases. Significant COL's antibacterial activity and efficient protection against bacterial adhesion were demonstrated on coating model implants with LLC surface films produced by using aqueous solutions containing COL at concentrations of 50 and 500 µg/mL. On exposure to serum, the detected structural alterations and changes in COL's antibacterial activity are also discussed. This study also highlights the implications of LLC self-assemblies for designing nanostructural coatings on orthopedic implants, which can prevent implant-associated biofilm infections through local delivery of antibacterial agents.

A Rapid and Simple HPLC-MS/MS Method for the Quantitative Determination of Colistin for Therapeutic Drug Monitoring in Clinical Practice

Colistin is the last-line option for the treatment of multidrug-resistant gram-negative bacterial infections with narrow therapeutic window. It is essential to ensure its efficacy and safety by therapeutic drug monitoring (TDM). Quantitative determination of colistin is difficult due to its complex ingredients. Previous determination methods demand intricate sample pre-treatment which are not only time-consuming but also costly, and is difficult to apply in clinical practice. Therefore, in order to carry out quantitative determination of colistin accurately and quickly, we establish a rapid high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) with simple sample pre-treatment process. The sample was purified by acetonitrile to remove the plasma protein. Then purified colistin was effectively separated from terfenadine, an internal standard (IS) using Phenomenex Kinetex C18 column (50.0×2.1mm, 5µm) with acetonitrile and water mobile phase at a flow rate of 0.5 mL/min and 40°C column temperature. Colistin and IS were monitored in positive ion mode. Our method expressed good linearity in 50.0~6000 ng/mL of colistin B and 28.31~3397.51 ng/mL of colistin A in plasma. Methodology validations, including selectivity, precision, accuracy, recovery, stability, matrix effect, and dilution integrity met acceptance criteria of Bioanalytical Method Validation (M10) of the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH).

Recent Trends in Therapeutic Drug Monitoring of Peptide Antibiotics

Antimicrobial peptides take a specific position in the field of antibiotics (ATBs), however, from a large number of available molecules only a few of them were approved and are used in clinics. These therapeutic modalities play a crucial role in the management of diseases caused by multidrug-resistant bacterial pathogens and represent the last-line therapy for bacterial infections. Therefore, there is a demand for a rationale use of such ATBs based on optimization of the dosing strategy to minimize the risk of resistance and ensure the sustainable efficacy of the drug in real clinical practice. Therapeutic drug monitoring, as a measurement of drug concentration in the body fluids or tissues, results in the optimization of the patient´s medication and therapy outcome. This strategy is beneficial and could result in tailored therapy for different types of infection and the prolongation of the use and efficacy of ATBs in hospitals. This review paper provides an actual overview of approved antimicrobial peptides used in clinical practice and covers current trends in their analysis by convenient and advanced methodologies used for their identification and/or quantitation in biological matrices for therapeutic drug monitoring purposes. Special emphasis is given to the methods with perspective clinical outcomes.

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

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

Pharmacokinetics and Pharmacodynamics of Fungal Defensin NZX Against Staphylococcus aureus-Induced Mouse Peritonitis Model

Staphylococcus aureus (S. aureus) is one of the most common pathogenic bacteria responsible for causing a life-threatening peritonitis disease. NZX, as a variant of fungal defensin plectasin, displayed potent antibacterial activity against S. aureus. In this study, the antibacterial and resistance characteristics, pharmacokinetics, and pharmacodynamics of NZX against the S. aureus E48 and S. aureus E48-induced mouse peritonitis model were studied, respectively. NZX exhibited a more rapid killing activity to S. aureus (minimal inhibitory concentration, 1 μg/ml) compared with linezolid, ampicillin and daptomycin, and serial passaging of S. aureus E48 for 30 days at 1/2 × MIC, NZX had a lower risk of resistance compared with ampicillin and daptomycin. Also, it displayed a high biocompatibility and tolerance to physiological salt, serum environment, and phagolysosome proteinase environment, except for acid environment in phagolysosome. The murine serum protein-binding rate of NZX was 89.25% measured by ultrafiltration method. Based on the free NZX concentration in serum after tail vein administration, the main pharmacokinetic parameters for T_1/2, C_max, V_d, MRT, and AUC ranged from 0.32 to 0.45 h, 2.85 to 20.55 μg/ml, 1469.10 to 2073.90 ml/kg, 0.32 to 0.56 h, and 1.11 to 8.89 μg.h/ml, respectively. Additionally, the in vivo pharmacodynamics against S. aureus demonstrated that NZX administrated two times by tail vein at 20 mg/kg could rescue all infected mice in the lethal mouse peritonitis model. And NZX treatment (20 mg/kg) significantly reduced CFU counts in the liver, lung, and spleen, especially for intracellular bacteria in the peritoneal fluid, which were similar or superior to those of daptomycin. In vivo efficacies of NZX against total bacteria and intracellular bacteria were significantly correlated with three PK/PD indices of ƒAUC/MIC, ƒC_max/MIC, and ƒT% > MIC analyzed by a sigmoid maximum-effect model. These results showed that NZX may be a potential candidate for treating peritonitis disease caused by intracellular S. aureus.

Evaluation of three ELISA kits for the screening of colistin residue in porcine and poultry muscle according to the European guideline for the validation of screening methods

Colistin is a polypeptide antibiotic mainly used in porcine and poultry to treat gastrointestinal infections. It has been included by the World Health Organisation (WHO) in the list of critically important human antibiotics of high priority for antimicrobial resistance since 2017. Therefore, it is necessary to develop specific and sensitive screening methods for this molecule. Screening for colistin with immunoassays is an interesting alternative to LC-MS/MS screening methods. The performance of three commercially available ELISA kits was evaluated in poultry and porcine muscles for the detection of colistin in regards to its European maximum residue limit (MRL) (150 µg/kg). The applicability of the three ELISA kits to the detection of colistin at or below the MRL in porcine and poultry muscles was demonstrated. The detection capabilities (CCβ) of two kits were or lower than or equal to the MRL (150 µg/kg). The lowest detection capability (30 µg/kg) was achieved with the third ELISA kit. The specificity of the three kits was very satisfactory (false positive rates 0%). The three kits are very specific for the detection of colistin (colistin A and B) and polymyxin B.

Polymyxin derivatives as broad-spectrum antibiotic agents

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

Rapid one-step enzyme immunoassay and lateral flow immunochromatographic assay for colistin in animal feed and food

Background

Colistin (polymyxin E) is a kind of peptide antibiotic which has been approved in animal production for the purposes of disease prevention, treatment, and growth promotion. However, the wide use of colistin in animal feed may accelerate the spread of colistin-resistance gene MCR-1 from animal production to human beings, and its residue in animal-origin food may also pose serious health hazards to humans. Thus, it is necessary to develop corresponding analytical methods to monitor the addition of colistin in animal feed and the colistin residue in animal-origin food.

Results

A one-step enzyme-linked immunosorbent assay (ELISA) and a lateral flow immunochromatographic assay (LFIA) for colistin were developed based on a newly developed monoclonal antibody. The ELISA showed a 50% inhibition value (IC₅₀) of 9.7 ng/mL with assay time less than 60 min, while the LFIA had a strip reader-based detection limit of 0.87 ng/mL in phosphate buffer with assay time less than 15 min. For reducing the non-specific adsorption of colistin onto sample vial, the components of sample extraction solution were optimized and proved to greatly improve the assay accuracy. The spiked recovery experiment showed that the recoveries of colistin from feed, milk and meat samples were in the range of 77.83% to 113.38% with coefficient of variations less than 13% by ELISA analysis and less than 18% by LFIA analysis, respectively. Furthermore, actual sample analysis indicated that the two immunoassays can produce results consistent with instrumental analysis.

Conclusions

The developed assays can be used for rapid qualitative or quantitative detection of colistin in animal feed and food.

Electronic supplementary material

The online version of this article (10.1186/s40104-019-0389-7) contains supplementary material, which is available to authorized users.

Rapid, simple, and clinically applicable high-performance liquid chromatography method for clinical determination of plasma colistin concentrations

Background

Since both the antibacterial effects and common adverse effects of colistin are concentration-dependent, determination of the most appropriate dosage regimen and administration method for colistin therapy is essential to ensure its efficacy and safety. We aimed to establish a rapid and simple high-performance liquid chromatography (HPLC)-based system for the clinical determination of colistin serum concentrations.

Methods

Extraction using a solid-phase C18 cartridge, derivatisation with 9-fluorenylmethyl chloroformate, and elution with a short reversed-phase Cl8 column effectively separated colistin from an internal standard. The HPLC apparatus and conditions were as follows: analytical column, Hydrosphere C18; sample injection volume, 50 μL; column temperature, 40 °C; detector, Shimadzu RF-5300 fluorescence spectrophotometer (excitation wavelength, 260 nm; emission wavelength, 315 nm); mobile phase, acetonitrile/tetrahydrofuran/distilled water (50,14,20, v/v/v); flow-rate, 1.6 mL/min.

Results

The calibration curves obtained for colistin were linear in the concentration range of 0.10–8.0 μg/mL. The regression equation was y = 0.6496× − 0.0141 (r² = 0.9999). The limit of detection was ~ 0.025 μg/mL, and the assay intra- and inter-day precisions were 0.87–3.74% and 1.97–6.17%, respectively. The analytical peaks of colistin A, colistin B, and the internal standard were resolved with adequate peak symmetries, and their retention times were approximately 8.2, 6.8, and 5.4 min, respectively. Furthermore, the assay was successfully applied to quantify the plasma colistin levels of a haemodialysis patient.

Conclusion

The assay is a simple, rapid, accurate, selective, clinically applicable HPLC-based method for the quantification of colistin in human plasma.

Development of HPLC with fluorescent detection using NBD-F for the quantification of colistin sulfate in rat plasma and its pharmacokinetic applications

Colistin sulfate, composed of a mixture of colistin A sulfate (CLA) and colistin B sulfate (CLB), is available for treating life-threatening infections caused by multidrug-resistant Gram-negative bacteria. In this study, the CLA and CLB were quantified separately. Colistin sulfate was extracted from rat plasma with a solid-phase extraction C₁₈ cartridge and reacted with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), and the fluorescent derivatives were subjected to reversed-phase high-performance liquid chromatography analysis and used to investigate the pharmacokinetics of CLA and CLB in rat plasma. The recovery rates of CLA and CLB were 41.2 ± 4.4 and 45.5 ± 3.1%, respectively. The recovery rate calculated from the total area of CLA and CLB was 43.9 ± 3.6%. When 2 mm NBD-F and 10 mm boric acid buffer (pH 9.5) were added to colistin sulfate, the highest recovery rate was obtained. The best heating time was 5 min at 60°C. The lower limits of quantification for CLA, CLB and the total area of CLA and CLB were 0.05, 0.05 and 0.1 μg/mL; the coefficients of variations were 13.5, 14.5 and 14.1%, respectively. This method was found to have acceptable linearity, precision and accuracy, and has been successfully applied to a pharmacokinetic study in rat plasma.

Colistin-entrapped liposomes driven by the electrostatic interaction: Mechanism of drug loading and in vivo characterization

The potential in vivo application of liposome for polycationic colistin has been hindered by the poor entrapment efficiency (EE) due to their phospholipid membrane permeability. The objective of this study is to investigate the loading mechanism and validity of applying electrostatic attraction for the colistin entrapment and delivery in liposomes. Anionic lipids with various structures were used for colistin entrapment, and the properties of resulting liposomes (i.e. zeta-potential, EE and release rate) were highly dependent on the structure of anionic lipids. Based on consideration of intermolecular interactions, the retention of electrostatically entrapped colistin is essentially determined by the balance of interfacial hydrophobic attraction and electrostatic repulsion. The liposomal colistin showed the reduced bacterial killing rate, but did not compromise the in vitro antibacterial activity. Specially, the PEGylated liposomal colistin of sodium cholesteryl sulfate (Chol-SO4^-) showed the best drug retention, resulting in the significantly increased maximum-tolerated dose, prolonged blood circulation and decreased colistin distribution in kidney after intravenous administration in mice. These results highlight the potential utility of electrostatically entrapped liposome for polycationic colistin delivery.

Electrostatically entrapped colistin liposomes for the treatment of Pseudomonas aeruginosa infection

The potential use of liposomes for the pulmonary delivery of colistin has been hindered by their phospholipid membrane permeability resulting in a very low entrapment of colistin in the liposomes. To increase the entrapment capacity of colistin in liposomes, the anionic lipid sodium cholesteryl sulfate (Chol-SO4^-) was used to enhance the electrostatic attraction between colistin and the lipid membrane. The resulting colistin-entrapped liposomes of Chol-SO4^- (CCL) showed significantly greater entrapment efficiency in comparison with liposomes without Chol-SO4^-. A time-kill kinetics study showed that colistin could redistribute from the liposomes into a new bacterial cell membrane to exert bactericidal activity. After intratracheal instillation, the CCL exhibited prolonged colistin retention in the lung with less colistin being transferred to the bloodstream and kidney, and the improved biodistribution further resulted in the enhanced therapeutic efficacy in a murine pulmonary Pseudomonas aeruginosa infection model compared with the colistin solution. These results highlight the suitability of applying an electrostatic attraction to entrap colistin in liposomes for pulmonary delivery by increasing colistin retention in the lung, while reducing the systemic exposure.

Determination of Cephalexin Monohydrate in Pharmaceutical Dosage Form by Stability-Indicating RP-UFLC and UV Spectroscopic Methods

An ultra-fast liquid chromatographic method and two UV spectroscopic methods were developed for the determination of cephalexin monohydrate in pharmaceutical dosage forms. Isocratic separation was performed on an Enable C₁₈G column (250 mm × 4.6 mm i.d., 5 μm) using methanol:0.01 M TBAHS (50:50, v/v) as the mobile phase at a flow rate of 1.0 ml/min. The PDA detection wavelength was set at 254 nm. The UV spectroscopic method was performed at 261 nm and at 256–266 nm for the AUC method using a phosphate buffer (pH=5.5). The linearity was observed over a concentration range of 1.0–120 μg/ml for UFLC and both of the UV spectroscopic methods (correlation coefficient=0.999). The developed methods were validated according to ICH guidelines. The relative standard deviation values for the intraday and interday precision studies were < 2%, and the accuracy was > 99% for all of the three methods. The developed methods were used successfully for the determination of cephalexin in dry syrup formulation.

Treatment options for multidrug-resistant bacteria

As a consequence of antibiotic overuse and misuse, nosocomial infections caused by multidrug-resistant bacteria represent a physician's nightmare throughout the world. No newer antimicrobials active against Pseudomonas aeruginosa, the main multidrug-resistant nosocomial pathogen, are available or under investigation. The only exceptions are linezolid, some newer glycopeptides (dalbavancin, oritavancin and telavancin) and daptomycin (a lipopeptide), which are active against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) strains, as well as tigecycline, a potent in vitro glycylcycline against MRSA, VRE, Acinetobacter baumannii and entended-spectrum beta-lactamase (ESBL)+ Enterobacteriaceae. Colistin, an antibiotic of the 1950s has been rediscovered by intensive care unit physicians for use against ESBL+ Enterobacteriaceae, as well as against multidrug-resistant P. aeruginosa and A. baumannii isolates. Although success rates with colistin range between 50 and 73%, almost all studies are retrospective. Immunostimulation efforts against S. aureus are still under development. As antibiotic research and development stagnate, rational policies for prescribing existing antibiotics plus strict infection control are the current mainstay efforts for preventing and combating multidrug-resistant bacterial infections.

In vivo pharmacokinetics/pharmacodynamics of colistin and imipenem in Pseudomonas aeruginosa biofilm infection

Many Pseudomonas aeruginosa isolates from the airways of patients with cystic fibrosis (CF) are sensitive to antibiotics in susceptibility testing, but eradication of the infection is difficult. The main reason is the biofilm formation in the airways of patients with CF. The pharmacokinetics (PKs) and pharmacodynamics (PDs) of antimicrobials can reliably be used to predict whether antimicrobial regimens will achieve the maximum bactericidal effect against infections. Unfortunately, however, most PK/PD studies of antimicrobials have been done on planktonic cells and very few PK/PD studies have been done on biofilms, partly due to the lack of suitable models in vivo. In the present study, a biofilm lung infection model was developed to provide an objective and quantitative evaluation of the PK/PD profile of antimicrobials. Killing curves were set up to detect the antimicrobial kinetics on planktonic and biofilm P. aeruginosa cells in vivo. Colistin showed concentration-dependent killing, while imipenem showed time-dependent killing on both planktonic and biofilm P. aeruginosa cells in vivo. The parameter best correlated to the elimination of bacteria in lung by colistin was the area under the curve (AUC) versus MIC (AUC/MIC) for planktonic cells or the AUC versus minimal biofilm inhibitory concentration (MBIC; AUC/MBIC) for biofilm cells. The best-correlated parameter for imipenem was the time that the drug concentration was above the MIC for planktonic cells (T(MIC)) or time that the drug concentration was above the MBIC (T(MBIC)) for biofilm cells. However, the AUC/MIC of imipenem showed a better correlation with the efficacy of imipenem for biofilm infections (R(2) = 0.89) than planktonic cell infections (R(2) = 0.38). The postantibiotic effect (PAE) of colistin and imipenem was shorter in biofilm infections than planktonic cell infections in this model.

[Shedding light on the use of colistin: still gaps to be filled]

Colistin (polymyxin E), an old antibiotic replaced by other less toxic antibiotics in the 1970s, has been increasingly used over the last decade due to multidrug-resistance in Gram-negative bacteria and lack of new antibiotics. However, there is a dearth of information on the pharmacokinetics (PK), pharmacodynamics (PD) and toxicodynamics (TD) of colistin and its non-active prodrug colistimethate sodium (CMS). Optimised dose regimens have not been established for different types of patients. Additionally, most PK data available in the literature were obtained from concentrations derived from potentially misleading microbiological assays. Therefore, it is urgent to conduct prospective studies to optimise CMS/colistin use in patients, in particular the critically ill. This review summarises recent key clinical studies evaluating the efficacy, toxicity and PK/PD of colistin/CMS.

Pharmacokinetics of a novel amoxicillin/colistin suspension after intramuscular administration in pigs

An amoxicillin (AMO) or colistin (COS) oil suspension was developed and corresponding pharmacokinetics studies were conducted in pigs after i.m. injection. The combination product is a white- to cream-colored oil suspension which is easy to be re-dispersed. Settling volume ratio, syringeability and flowability of the product is well consistent with the technical standards set by the Ministry of Agriculture of People's Republic of China. Two studies were conducted to investigate the pharmacokinetics of the combination product in swine. First, the pharmacokinetics of the combination product was compared with those of the same products merely removing either AMO or COS. No significant change in the major pharmacokinetic parameters (C(max) , T(max) , MRT, t(1/2λ) , AUC and AUMC) was observed when either component was removed from the combination product, indicating that AMO and COS do not interfere with each other in their absorption and distribution in the tissue when used as a combination. Second, the pharmacokinetics of the combination product was compared with that of their respective single products. It was found that the apparent elimination half-lives (t(1/2λ) ) of AMO and COS in combination product were 6.38 and 8.09 h, which were 2.40 and 2.38 times longer than the single products, respectively. Thus, the novel AMO/COS suspension extended significantly the half-life of both drugs to maintain a longer drug residence time in pigs when compared to their single products.

Pharmacokinetics/pharmacodynamics of colistin and imipenem on mucoid and nonmucoid Pseudomonas aeruginosa biofilms

The time course of activity of colistin and imipenem against mucoid and nonmucoid Pseudomonas aeruginosa growing in a biofilm showed that compared with those for planktonic bacteria, the kinetics of colistin and imipenem retained the concentration- and time-dependent killing, respectively, but higher doses of antibiotics and longer dosing periods were required for biofilm eradication. Biofilms of mucoid P. aeruginosa were more difficult to eradicate than nonmucoid biofilms.

Pharmacokinetic evaluation of colistin sodium

IMPORTANCE OF THE FIELD

Although colistin has recently played a key role in the treatment of nosocomial infections due to multidrug resistant Gram-negative pathogens, there is a lack of clinical studies examining colistin pharmacokinetics (PKs) in humans. This refers to all routes of colistin administration in clinical practice. Colistin PK data are also limited in critically ill patients.

AREAS COVERED IN THIS REVIEW

Literature search took into account data dealing with colistin PK obtained from animal studies performed during previous decades (1970s, 1980s and 1990s) and from recent human studies performed during the last decade.

WHAT THE READER WILL GAIN

Valuable information on pharmacodynamics (PD)/PK of colistin used in the treatments of nosocomial infections due to multidrug resistant Gram-negative pathogens, mostly Pseudomonas aeruginosa, Acinetobacter baumannii and Klebsiella pneumoniae. A better understanding of PKs could offer significant improvement of colistin use in humans, especially optimization of colistin doses in different routes of administration in order to maximize clinical efficacy and minimize adverse effects and rate of resistance.

TAKE HOME MESSAGE

There is a lack of human studies on colistin PK and PD. Significant PD parameters best predicting colistin efficacy and their optimal values such as C(max):MIC ratio, AUC/MIC and T > MIC have not yet been clearly defined. It should be noted that further investigation on colistin PK/PD in vitro and in vivo models is required.

Self-assembly behavior of colistin and its prodrug colistin methanesulfonate: implications for solution stability and solubilization

Colistin is an amphiphilic antibiotic that has re-emerged into clinical use due to the increasing prevalence of difficult-to-treat Gram-negative infections. The existence of self-assembling colloids in solutions of colistin and its derivative prodrug, colistin methanesulfonate (CMS), was investigated. Colistin and CMS reduced the air-water interfacial tension, and dynamic light scattering (DLS) studies showed the existence of 2.07 +/- 0.3 nm aggregates above 1.5 mM for colistin and of 1.98 +/- 0.36 nm aggregates for CMS above 3.5 mM (mean +/- SD). Above the respective critical micelle concentrations (CMC) the solubility of azithromycin, a hydrophobic antibiotic, increased approximately linearly with increasing surfactant concentration (5:1 mol ratio colistin:azithromycin), suggestive of hydrophobic domains within the micellar cores. Rapid conversion of CMS to colistin occurred below the CMC (60% over 48 h), while conversion above the CMC was less than 1%. The formation of colistin and CMS micelles demonstrated in this study is the proposed mechanism for solubilization of azithromycin and the concentration-dependent stability of CMS.

Brain penetration of colistin in mice assessed by a novel high-performance liquid chromatographic technique

A sensitive and reliable liquid chromatographic method was developed and validated for the determination of colistin concentrations in mouse brain homogenate. With a mobile phase consisting of acetonitrile-tetrahydrofuran-water (50:25:25 [vol/vol]) at a flow rate of 1 ml/min, a linear correlation between peak area and colistin concentration was observed over the concentration range of 93.8 to 3,000 ng/g in brain tissue (R2 > 0.994). Intra- and interday coefficients of variation were 5.1 to 8.3% and 5.8 to 8.5%, respectively, and the recovery ranged from 85% to 94%. This assay was then utilized to determine the amount of colistin that permeated the blood-brain barrier over a 2-h period following bolus intravenous administration of colistin sulfate to mice. After a single dose of 5 mg/kg of body weight to mice, brain homogenate concentrations of colistin were very low, relative to plasma colistin concentrations, suggesting that colistin permeability across the healthy blood-brain barrier is negligible during this experimental period.

Colistin methanesulfonate is an inactive prodrug of colistin against Pseudomonas aeruginosa

There is a dearth of information on the pharmacodynamics of "colistin," despite its increasing use as a last line of defense for treatment of infections caused by multidrug-resistant gram-negative organisms. The antimicrobial activities of colistin and colistin methanesulfonate (CMS) were investigated by studying the time-kill kinetics of each against a type culture of Pseudomonas aeruginosa in cation-adjusted Mueller-Hinton broth. The appearance of colistin from CMS spiked at 8.0 and 32 mg/liter was measured by high-performance liquid chromatography, which generated colistin concentration-time profiles. These concentration-time profiles were subsequently mimicked in other incubations, independent of CMS, by incrementally spiking colistin. When the cultures were spiked with CMS at either concentration, there was a substantial delay in the onset of the killing effect which was not evident until the concentrations of colistin generated from the hydrolysis of CMS had reached approximately 0.5 to 1 mg/liter (i.e., approximately 0.5 to 1 times the MIC for colistin). The time course of the killing effect was similar when colistin was added incrementally to achieve the same colistin concentration-time course observed from the hydrolysis of CMS. Given that the killing kinetics of CMS can be accounted for by the appearance of colistin, CMS is an inactive prodrug of colistin with activity against P. aeruginosa. This is the first study to demonstrate the formation of colistin in microbiological media containing CMS and to demonstrate that CMS is an inactive prodrug of colistin. These findings have important implications for susceptibility testing involving "colistin," in particular, for MIC measurement and for microbiological assays and pharmacokinetic and pharmacodynamic studies.

Evaluation of colistin as an agent against multi-resistant Gram-negative bacteria

Infections caused by multi-resistant Gram-negative bacteria, particularly Pseudomonas aeruginosa, are increasing worldwide. In patients with cystic fibrosis (CF), resistance in P. aeruginosa to numerous anti-pseudomonal agents is becoming common. The absence since 1995, of new substances active against resistant Gram-negative bacteria, has caused increasing concern. Colistin, an old antibiotic also known as polymyxin E, has attracted more interest recently because of its significant activity against multi-resistant P. aeruginosa, Acinetobacter baumannii and Klebsiella pneumoniae, and the low resistance rates to it. Because its use as an anti-pseudomonal agent was displaced by the potentially less toxic aminoglycosides in 1970s, our knowledge of this drug is limited. However, there has been a significant recent increase in the data gathered on colistin, focussing on its chemistry, antibacterial activity, mechanism of action and resistance, pharmacokinetics, pharmacodynamics and new clinical application. It is likely that colistin will be an important antimicrobial option against multi-resistant Gram-negative bacteria, for some years to come.

Determination of polymyxin E1 in rat plasma by high-performance liquid chromatography

A precise and accurate HPLC assay for polymyxin E(1) in rat and dog plasma has been validated. Samples and standards are extracted from plasma with a 96-well C(8) extraction disk plate. Sample extracts are derivatized with dansyl chloride, and polymyxin E(1) derivative is quantitated on a C(8) column by HPLC with fluorescence detection. The assay is linear in the range of 0.050-5.00 micro g/ml for polymyxin E(1). The precision and accuracy of polymyxin E(1) plasma assay was well within the recommended limits set in the FDA Guidance for Bioanalytical Method Validation. Polymyxin E(1) stability in rat and dog plasma for 24 h at room temperature and through three freeze-thaw cycles was demonstrated.

Stability of colistin and colistin methanesulfonate in aqueous media and plasma as determined by high-performance liquid chromatography

The stabilities of colistin and colistin methanesulfonate (CMS) in different aqueous media were studied by specific high-performance liquid chromatography (HPLC) methods. Colistin was stable in water at 4 and 37 degrees C for up to 60 days and 120 h, respectively. However, degradation was observed when colistin was stored in isotonic phosphate buffer (0.067 M, pH 7.4) and human plasma at 37 degrees C. The stability of CMS from three different sources in water was explored by strong-anion-exchange (SAX) HPLC for CMS and by measuring the concentrations of colistin formed from the hydrolysis of CMS. The peaks of CMS in SAX HPLC disappeared almost completely after 12 h at 37 degrees C, but appeared to remain intact for up to 2 days at 4 degrees C. Over the same period, there was no formation of colistin at 4 degrees C. In water, phosphate buffer, and plasma, there was rapid formation of colistin within 24 to 48 h at 37 degrees C from the three sources of CMS. The hydrolysis products were assumed to be a complex mixture of many different sulfomethyl derivatives, including colistin. The stability of a fourth source of CMS in Mueller-Hinton broth examined during 30 min at 37 degrees C revealed no formation of colistin. Along with previous microbiological studies, this suggested that different sulfomethyl CMSs possess intrinsic antibacterial activity. These results will be helpful for understanding the pharmacokinetics and pharmacodynamics of colistin and CMS in humans and animals.

Simple method for assaying colistin methanesulfonate in plasma and urine using high-performance liquid chromatography

A simple and sensitive high-performance liquid chromatographic method is described for the determination of colistimethate sodium in plasma and urine. The accuracy and reproducibility was within 10.1 and 11.2% with rat plasma and urine, respectively. Several commonly coadministered antibacterial agents do not interfere with the assay.

A simple method for the assay of colistin in human plasma, using pre-column derivatization with 9-fluorenylmethyl chloroformate in solid-phase extraction cartridges and reversed-phase high-performance liquid chromatography

A simple, selective and sensitive high-performance liquid chromatographic (HPLC) method is described for the determination of colistin in human plasma. Derivatization with 9-fluorenylmethyl chloroformate was performed in the same solid-phase extraction C18 cartridge used for sample pre-treatment, followed by reversed-phase HPLC with fluorimetric detection. Quantification was achieved using the ratio of the summed peak areas of colistin A and B derivatives to that of the derivative of netilmicin (internal standard). Linear calibration curves were obtained within the concentrations of colistin sulfate from 0.10 to 4.0 mg/l in plasma. Accuracy was within 10% and reproducibility (RSD) was less than 10%.