Genetic predisposition and high exposure to colistin in the early treatment period as independent risk factors for colistin-induced nephrotoxicity.

Colistin is known to cause nephrotoxicity due to its extensive reabsorption and accumulation in renal tubules. In vitro studies have identified the functional role of colistin transporters such as OCTN2, PEPT2, megalin, and P-glycoprotein. However, the role of these transporter gene variants in colistin-induced nephrotoxicity has not been studied. Utilizing targeted next-generation sequencing, we screened for genetic polymorphisms covering the colistin transporters (SLC15A1, SLC15A2, SLC22A5, LRP2, and ABCB1) in 42 critically ill patients who received colistimethate sodium. The genetic variants rs2257212 ((NM_021082.4):c.1048C>G) and rs13397109 ((NM_004525.3):C.7626C > T) were identified as being associated with an increased incidence of acute kidney injury (AKI) on Day 7. Colistin area under the curve (AUC) was predicted using a previously published pharmacokinetic model of colistin. Using logistic regression analysis, the predicted 24-h AUC of colistin was identified as an important contributor for increased odds of AKI on Day 7. Among 42 patients, 4 (9.5%) were identified as having high predisposition to colistin-induced AKI based on the presence of predisposing genetic variants. Determination of the presence of the abovementioned genetic variants and early therapeutic drug monitoring may reduce or prevent colistin-induced nephrotoxicity and facilitate dose optimization of colistimethate sodium.

Individualized optimization of colistin loading doses.

Colistin remains one of the few available options for the treatment of infections caused by resistant bacteria. Pharmacokinetic (PK) studies have been successful in estimating the appropriate colistin methanesulfonate (CMS) dose to achieve a target colistin concentration. Currently, there is a consensus that the dose of CMS should vary according to the patient renal function since CMS is mainly eliminated by renal route. For this same reason, the loading dose should vary according to the patient's renal capacity; however, this is not the current clinical practice. In this study we develop a framework to determine two key parameters for the loading dose regimen: (1) the optimal dose according to the characteristics (renal function and weight) of the patient; (2) the waiting time before the maintenance dose. Based on a previous PK model, our framework allows a fast parameter sweep so as to select optimal loading dose and waiting time minimizing the deviation between the plasma concentration and a target value. The results showed that patients presenting low creatinine clearance (CrCL) should receive a lower CMS loading dose with longer interval to start maintenance treatment to avoid nephrotoxic colistin concentrations. In cases of high CrCL, the dose should be higher and the interval to the next dose shorter to avoid subtherapeutic concentrations. Optimization of the loading dose should considerably improve colistin therapy, as the target concentration is reached more quickly, without reaching toxic values.

Model-Informed Rationale for Early Therapeutic Drug Monitoring of Colistin in Critically Ill Patients.

Adequate colistin exposure is important for microbiological clearance. This study was performed in critically ill patients >18 years old to develop a simplified nonparametric pharmacokinetic (PK) model of colistin for routine clinical use and to determine the role of dose optimization. The Non-Parametric Adaptive Grid algorithm within the Pmetrics software package for R was used to develop a PK model from 47 patients, and external validation of the final model was performed in 13 patients. A 1-compartment multiplicative gamma error model with 0-order input and first-order elimination of colistin was developed with creatinine clearance and serum albumin as covariates on elimination rate constant. An R2 for observed vs individual predicted colistin concentrations of 0.92 was obtained in the validation cohort. High interindividual variability in colistin steady-state area under the plasma concentration-time curve (AUC) from from 120 hours to 144 hours (coefficient of variation = 80.1%) and a high interoccasion variability (median coefficient of variation of AUC from time 0 to hours predicted every 8 hours for initial 96 hours after starting colistin = 23.8) was predicted in patients who received this antibiotic for a period of over 152 hours (n = 22). With the model-suggested dose regimen, only 20% of simulated profiles achieved AUC from time 0 to 24 hours in the range of 50 to 60 mg â€¢ h/L due to high variability in population PK. In this group of patients, steady-state colistin concentrations were predicted to be achieved >96 hours after initiation of colistimethate sodium. This study advocates the need for early and repeated therapeutic drug monitoring and dose optimization in critically ill patients to achieve adequate therapeutic concentration of colistin.

Efficacy of tigecycline alone or in combination for experimental infections by KPC carbapenemase-producing Klebsiella pneumoniae.

Although in vitro data suggest that tigecycline is active against Klebsiella pneumoniae carbapenemase-producing K. pneumoniae (KPC-Kp), experimental and clinical data are limited. We studied the effect of tigecycline alone or in combination for experimental infections by KPC-Kp. A total of 540 male C57BL/6 mice were infected with three genetically diverse KPC-Kp isolates susceptible to tigecycline with meropenem minimum inhibitory concentrations (MICs) of 4, 16 and 256 µg/mL, respectively. Mice were randomly treated with water for injection, tigecycline, meropenem and colistin alone, and double or triple combinations of tigecycline, colistin and meropenem. Mouse survival was recorded for 14 days. In separate experiments, mice were sacrificed 6 h and 24 h after bacterial challenge for quantitative culture of tissues and serological analysis. Time-kill curves were performed. Tigecycline, colistin and meropenem concentrations were measured in tissues and serum by high-performance liquid chromatography (HPLC). Survival was significantly prolonged when mice were treated with tigecycline alone and tigecycline-containing regimens compared with control mice and mice treated with tigecycline-sparing regimens. Tigecycline-sparing regimens were active only against the isolate with a meropenem MIC of 4 µg/mL. Mortality was associated with progression to multiple organ failure. Tigecycline and tigecycline-containing regimens achieved a rapid decrease of bacterial loads both in tissues and in vitro. Tigecycline concentrations in tissues were negatively correlated with tissue bacterial load. Tigecycline alone or in combination with meropenem and/or colistin achieves effective treatment of experimental KPC-Kp infections irrespective of the meropenem MIC.

Promoter variations associated with expression of mcr-1 gene and level of colistin resistance.

OBJECTIVES: Colistin resistance mediated by plasmids for their rapid dissemination in Enterobacteriaceae is alarming. We aimed to characterize the genetic features of mcr-1 gene as well as the role of promoters in gene expression and levels of colistin resistance among clinical isolates of Enterobacteriaceae. METHODS: Clinical isolates of Enterobacteriaceae were collected in thirteen cities in China and screened for mcr-1 gene using polymerase chain reaction (PCR) amplification and sequencing. Antimicrobial susceptibility testing, transformation assay and plasmid sequencing, quantitative real-time PCR were performed for mcr-1-positive isolates. Promoter-probe vector pKK232-8 was utilized to assess the activity of the mcr-1 promoters. RESULTS: This study identified the mcr-1 gene in 15 clinical isolates of Enterobacteriaceae, among which 14 were resistant to colistin, with MICs of 4-8 mg/L, while one mcr-1-bearing isolate EC09 was susceptible to colistin, with an MIC of 0.5 mg/L. Moreover, mcr-1-harbouring plasmids from 10 clinical isolates were transferrable via transformation and belonged to different incompatibility groups (IncI2 and IncX4). Plasmid pEC09 failed to transform and belonged to IncP1. A genetic structure containing the mcr-1-pap2 element was detected in these plasmids. EC09 demonstrated the lowest transcription level of mcr-1 gene, as determined by quantitative real-time PCR, which was in accordance with its susceptibility to colistin. Furthermore, the promoter activity of mcr-1 in pEC09 was the lowest, as determined by promoter-probe vector pKK232-8. CONCLUSION: Promoter variations were associated with expression of the mcr-1 gene and ultimately the levels of colistin resistance.

Optimal empiric treatment for KPC-2-producing Klebsiella pneumoniae infections in critically ill patients with normal or decreased renal function using Monte Carlo simulation.

BACKGROUND: Limited clinical studies describe the pharmacodynamics of fosfomycin (FOS), tigecycline (TGC) and colistin methanesulfonate (CMS) in combination against KPC-producing Klebsiella pneumoniae (KPC-Kp). Population pharmacokinetic models were used in our study. Monte Carlo simulation was conducted to calculate probability of target attainment (PTA) and cumulative fraction of response (CFR) of each agent alone and in combination against KPC-Kp in patients with normal or decreased renal function. RESULTS: The simulated regimen of FOS 6 g q8h reached ≥90% PTA against a MIC of 64 mg/L in patients with normal renal function. For patients with renal impairment, FOS 4 g q8h could provide sufficient antimicrobial coverage against a MIC of 128 mg/L. And increasing the daily dose could result to the cut-off value to 256 mg/L in decreased renal function. For TGC, conventional dosing regimens failed to reach 90% PTA against a MIC of 2 mg/L. Higher loading and daily doses (TGC 200/400 mg loading doses followed by 100 mg q12h/200 mg q24h) were needed. For CMS, none achieved 90% PTA against a MIC of 2 mg/L in normal renal function. Against KPC-Kp, the regimens of 200/400 mg loading dose followed by 100 q12h /200 mg q24h achieved > 80% CFRs regardless of renal function, followed by CMS 9 million IU loading dose followed by 4.5/3 million IU q12h in combination with FOS 8 g q8h (CFR 75-91%). CONCLUSIONS: The use of a loading dose and high daily dose of TGC and CMS in combination with FOS can provide sufficient antimicrobial coverage against critically ill patients infected with KPC-Kp.

Effects of Different Component Contents of Colistin Methanesulfonate on the Pharmacokinetics of Prodrug and Formed Colistin in Human.

PURPOSES: To evaluate the effects of component contents in different colistin methanesulfonate (CMS) formulas on their clinical pharmacokinetics of the prodrug CMS and the formed colistin. METHODS: Two CMS formulas (CTTQ and Parkedale) were investigated in a single dose, randomized, open-label, crossover study conducted in 18 healthy Chinese subjects. Both CMS formulas met the requirements of European Pharmacopoeia 9.2 with 12.1% difference in the two major active components (CMS A and CMS B). The PK parameters after a single intravenous infusion of CMS at 2.5 mg/kg were calculated and the steady-state plasma colistin concentrations (Css,avg) following multiple dosing, once every 12 h for 7 days, were simulated with the non-compartment model. RESULTS: The systemic exposure (AUC0-inf) of CMS were 59.49 ± 5.90 h·µg/mL and 51.09 ± 4.70 h·µg/mL, and the AUC0-inf of colistin were 15.39 ± 2.63 h·µg/mL and 12.36 ± 2.10 h·µg/mL for CTTQ and Parkedale, respectively. The ratios (90% CI) of geometric mean of AUC0-inf of CTTQ to Parkedale were 116.38% (112.95%, 119.91%) and 124.49% (120.76%, 128.35%) for CMS and colistin, respectively. The predicted Css,avg (95% CI) were 0.92 (0.85, 0.99) µg/mL and 0.74 (0.69, 0.79) µg/mL for CTTQ and Parkedale, respectively. CONCLUSION: The difference in component content in the two CMS formulas had a significant (P < 0.001) impact on the systemic exposure of colistin in human, thus, warranted essential considerations in clinical applications.

Colistimethate Acidic Hydrolysis Revisited: Arrhenius Equation Modeling Using UPLC-QToF MS.

Colistimethate (CMS), the prodrug of polymyxin E (colistin), is an antibiotic widely used as a last-line therapy against multidrug resistant Gram-negative bacteria, but little is known about its pharmacokinetics as its administration has stopped as a result of high neuro- and nephro-toxicity. The measurement of CMS levels in patients' biological fluids is of great importance in order to find the optimal dose regimen reducing the drug toxicity. Until now, CMS assay methods are based on the indirect determination after its hydrolysis to colistin (CS). Herein, the aim is to find the optimal conditions for the complete hydrolysis of CMS to CS. The reaction was studied at accelerated conditions: 40 °C, 50 °C, and 60 °C, and the results were evaluated by assessing the Arrhenius equation and computation employing the Tenua software. A validated analytical methodology based on ultra-performance liquid chromatography (UPLC) coupled to a hybrid quadrupole time of flight (QToF) instrument is developed for the simultaneous measurement of CMS and CS. The current methodology resulted in complete hydrolysis, in contrast with the previously reported one.

Pharmacokinetics of colistin in cerebrospinal fluid after intraventricular administration alone in intracranial infections.

The aim of this study was to investigate the pharmacokinetics of colistin in cerebrospinal fluid (CSF) after intraventricular (IVT) administration of colistin methanesulfonate (CMS) for central nervous system (CNS) infections caused by multidrug-resistant Gram-negative bacteria. Ten patients with CNS infection were treated with CMS (active substance colistin equivalent to 100 000 units, every 24 h) by IVT administration. After 3 days of treatment, the concentration of colistin in the CSF was determined by selective ultra-performance liquid chromatography (UPLC) at 2, 4, 6, 8, 12 and 24 h after CMS administration. A pharmacokinetic analysis was performed using Phoenix WinNonlin. Following IVT administration of CMS, the estimated colistin apparent CSF half-life (t1/2) was 10.46 ± 6.98 h, the average peak colistin concentration (Cmax) was 16.95 ± 7.39 µg/mL and the average time to peak concentration (Tmax) was 4.6 ± 0.97 h. The measured trough concentration (Cmin; colistin concentration in CSF at 24 h after administration of CMS) was 1.12-8.33 µg/mL and the average Cmin was 2.91 ± 2.11 µg/mL. CSF concentrations of colistin were above the minimum inhibitory concentration (MIC) of 0.5 µg/mL at 24 h after IVT administration in all patients. Microbiological cure was observed in all patients. In conclusion, this is the first study of colistin pharmacokinetics in CSF after IVT administration alone in patients with CNS infection. It provides essential data for designing relatively safe and effective CMS dosing regimens.

In vitro pharmacokinetics/pharmacodynamics of continuous ceftazidime infusion alone and in combination with colistin against Pseudomonas aeruginosa biofilm.

OBJECTIVES: The pharmacokinetics/pharmacodynamics of continuous infusion (CI) beta-lactams for Pseudomonas aeruginosa biofilm infections has not been defined. This study evaluated the efficacy of several dosage regimens of CI ceftazidime, with or without colistin, an antibiotic with a potential antibiofilm effect, against biofilm-embedded P. aeruginosa. METHODS: Mature biofilms of the reference strain PAO1 and the clinical isolate HUB8 (both ceftazidime- and colistin-susceptible) were investigated over 54h using a dynamic CDC biofilm reactor. CI dosage regimens were ceftazidime monotherapy (4, 10, 20 and 40 mg/L), colistin monotherapy (3.50 mg/L), and combinations of colistin and ceftazidime (4 or 40 mg/L). Efficacy was evaluated by changes in log10colony-forming units (cfu)/mL and confocal microscopy. RESULTS: At 54 h, the antibiofilm activity of ceftazidime monotherapies was slightly higher for ceftazidime 20 mg/L (-2.84 log10cfu/mL) and 40 mg/L (-3.05) against PAO1, but no differences were seen against HUB8. Ceftazidime-resistant colonies emerged with 4 mg/L regimens in both strains and with other regimens in PAO1. Colistin monotherapy had significant antibiofilm activity against HUB8 (-3.07), but lower activity against PAO1 (-1.12), and colistin-resistant strains emerged. Combinations of ceftazidime and colistin had higher antibiofilm activity at 54 h compared with each monotherapy, and prevented the emergence of resistance to both antibiotics; higher antibiofilm activity was observed with ceftazidime 40 mg/L plus colistin compared with ceftazidime 4 mg/L plus colistin (-4.19 vs. -3.10 PAO1; -4.71 vs. -3.44 HUB8). CONCLUSIONS: This study demonstrated that, with %T>MIC=100%, CI ceftazidime displayed concentration-dependent antibiofilm activity against P. aeruginosa biofilm, particularly in combination with colistin. These results support the use of high-dosage regimens of CI ceftazidime with colistin against biofilm-associated infections with ceftazidime-susceptible P. aeruginosa.

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

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

Enhanced bacterial killing with colistin/sulbactam combination against carbapenem-resistant Acinetobacter baumannii.

AIMS: Polymyxin-based combination therapy is often used to treat carbapenem-resistant Acinetobacter baumannii (A. baumannii) infections. Although sulbactam is intrinsically active against A. baumannii, few studies have investigated colistin/sulbactam combinations against carbapenem-resistant A. baumannii. METHODS: Whole genome sequencing was undertaken on eight carbapenem-resistant (colistin-susceptible) isolates of A. baumannii from Chinese patients. Bacterial killing of colistin and sulbactam, alone and in combination, was examined with checkerboard (all isolates) and static and dynamic time-kill studies (three isolates). In the dynamic studies, antibiotics were administered in various clinically-relevant dosing regimens that mimicked patient pharmacokinetics. RESULTS: The eight isolates consisted of ST195, ST191 and ST208 belonging to clonal complex 208, which is the most epidemic clonal type of A. baumannii globally. All isolates possessed Acinetobacter-derived cephalosporinase (ADC-61 or ADC-78) and seven of eight isolates contained the carbapenem-resistance gene blaOXA-23. The colistin/sulbactam combination was synergistic against two of eight isolates in checkerboard studies. In time-kill studies, rapid bacterial killing of ca. 3-6 log10 CFU/mL was observed with colistin monotherapy, followed by steady regrowth. Sulbactam monotherapy was generally ineffective. Substantially enhanced bacterial killing was observed with colistin/sulbactam combinations in both static and dynamic models, especially with the higher sulbactam concentration (2 g) and/or longer sulbactam infusion time (2 hours) in the dynamic model. CONCLUSIONS: This study was the first to use a pharmacokinetics/pharmacodynamics model to investigate synergistic activity of colistin/sulbactam combinations against A. baumannii. It showed that clinically-relevant dosing regimens of colistin combined with sulbactam may substantially improve bacterial killing of multidrug-resistant and carbapenem-resistant A. baumannii.

Clinical efficacy and pharmacokinetics of colistimethate sodium and colistin in critically ill patients in an Indian hospital with high endemic rates of multidrug-resistant Gram-negative bacterial infections: A prospective observational study.

BACKGROUND: Safe and effective use of colistin requires robust pharmacokinetic (PK) and pharmacodynamic (PD) data to guide dosing. AIM: To evaluate the pharmacokinetics of colistimethate sodium and colistin in critically ill patients and correlate with clinical efficacy and renal function. MATERIALS AND METHODS: Twenty critically ill adult patients with colistin-susceptible multidrug-resistant (MDR) infections and normal renal function treated with intravenous colistimethate sodium - at a 9 million units (270 mg CBA) loading dose followed by maintenance (MD) of 3 million units t.i.d, 24 hours later - were evaluated for clinical cure (CC) at the end of therapy. Patient characteristics and plasma colistin levels at 0, 0.5, 1, 2, 4, 8 and 12 hours after the loading dose and at 1, 2 and 8 hours after the eighth and ninth infusion of MD were evaluated. Colistimethate sodium and colistin levels were measured by high-performance liquid chromatography and tandem mass spectrometry (HPLC-MS/MS). RESULTS: Among the 20 patients who were evaluated, 60% had pneumonia. Predominant pathogens were Klebsiella pneumoniae and Acinetobacter spp. Clinical cure was 50% (10/20). Mean peak loading dose concentrations were 3 ± 1.1 mg/L (1.75-5.14) and 2.37 ± 1.2 mg/L (1.52-5.54) for 'cure' and 'failure' groups, respectively (p = 0.13), while mean steady-state (Cssavg) concentrations were 2.25 ± 1.3 mg/L and 1.78 ± 1.1 mg/L in 'cure' and 'failure' groups, respectively (p = 0.19). Nephrotoxicity was 5% on day 7 of therapy. However, bacteriological cure could not be correlated with PK/PD. CONCLUSIONS: Subtherapeutic Cssavg with clinical failure and lower efficacy without significant nephrotoxicity highlights the need for therapeutic drug monitoring to guide colistin dosing.

Optimization of inhalable liposomal powder formulations and evaluation of their in vitro drug delivery behavior in Calu-3 human lung epithelial cells.

Inhalation therapy has advantages for the treatment of multidrug resistant bacterial lung infections with high drug concentrations at the infection sites in the airways and reduced systemic exposure. We have developed liposomal formulations for pulmonary delivery of synergistic ciprofloxacin (Cipro) and colistin (Col) as the potential candidate for treatment of lung infections caused by multidrug resistant Gram-negative bacteria. This study aims to: (1) further optimize the powder formulation by adding drying stabilizers (polyvinyl pyrrolidone or poloxamer) to protect the liposomes during spray-freeze-drying; (2) evaluate the transport and cellular uptake of drugs in a human lung epithelial Calu-3 cell model. The liposomal powder formulations were produced using the ultrasonic spray-freeze-drying technique. The optimal formulation (F5) used mannitol (8% w/v) and sucrose (2% w/v) as the internal lyoprotectants. Adding external lyoprotectants/aerosolization enhancers (i.e. 8% w/v mannitol, 2% w/v sucrose and 1%, w/w PVP 10) produced the superior rehydrated EE values of ciprofloxacin and colistin (50.2 ± 0.9% for Cipro and 37.8 ± 1.2% for Col) as well as satisfactory aerosol performance (FPF: 34.2 ± 0.8% for Cipro and 33.6 ± 0.9% for Col). The cytotoxicity study indicated that F5 with the colistin concentration at 50 µg/mL and ciprofloxacin at 200 µg/mL was not cytotoxic to human lung epithelial Calu-3 cells. The intracellular uptake of ciprofloxacin was concentration-dependent in Calu-3 cells and the uptake of A-B was more than that of B-A for all samples (p < 0.05). This study demonstrates that co-delivery of ciprofloxacin and colistin in a single liposome can lower the transport capability of both drugs across the Calu-3 cell monolayer and their accumulation in the cells. These findings indicate that co-loaded liposomal powder of ciprofloxacin and colistin is a promising potential treatment for respiratory infections caused by multidrug resistant Gram-negative bacteria.

Sequential Time-Kill, a Simple Experimental Trick To Discriminate between Pharmacokinetics/Pharmacodynamics Models with Distinct Heterogeneous Subpopulations versus Homogenous Population with Adaptive Resistance.

Experiments were conducted with polymyxin B and two Klebsiella pneumonia isogenic strains (the wild type, KP_WT, and its transconjugant carrying the mobile colistin resistance gene, KP_MCR-1) to demonstrate that conducting two consecutive time-kill experiments (sequential TK) represents a simple approach to discriminate between pharmacokinetics/pharmacodynamics models with two heterogeneous subpopulations or adaptive resistance.

Quantification of Colistin in Plasma by Liquid Chromatography-Tandem Mass Spectrometry: Application to a Pharmacokinetic Study.

Colistin is a polymixin antibiotic (polymixin E) that is produced by Bacillus colistinus bacteria. The aim of the present study was to develop and validate a method to quantify colistin levels in plasma using high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) technique and then apply it in experimental animals (rats) to investigate the pharmacokinetic profile of colistin in this species. Polymyxin B was used as an internal standard (IS) and the quantitation was carried out using ESI + interface and employing multiple reaction monitoring (MRM) mode. A mobile phase consisting of acetonitrile:water:formic acid (30:70:0.1%; v/v/v) was employed and Zorbax eclipse plus C18 (1.8 µm, 2.1 mm i.d. x 50 mm) was the optimal column for this method and utilized at a flow rate of 0.2 mL/min. The full scan mass spectra of precursor/product ions of colistin A were at m/z 585.5 > 100.8, for colistin B at m/z 578.8 > 101 and for the IS at m/z 602.8 > 101. The lower limit of quantification (LLOQ) was 0.5 µg/mL. The method demonstrated acceptable intra-run and inter-run precision and accuracy for both colistin A and colistin B. Colistin was stable when assessed for long-term stability, freeze-thaw stability and autosampler stability. However, it was not stable when stored at room temperature. The matrix effect evaluation showed minimal or no effect. Incurred sample reanalysis findings were within acceptable ranges (<20% of the nominal concentration). The pharmacokinetic parameters of colistin were investigated in rats using the present method. The developed method for colistin demonstrates that it is rapid, sensitive, specific, accurate, precise, and reliable.

Quantitative determination of colistin A/B and colistin methanesulfonate in biological samples using hydrophilic interaction chromatography tandem mass spectrometry.

Colistin (polymyxin E) is a polycation antibiotic which is increasingly used (administered as colistin methanesulfonate, CMS) as a salvage therapy in critically ill patients with multidrug resistant Gram-negative infections. Even though colistin has been used for more than 50 years, its metabolic fate is poorly understood. One of the current challenges for studying the pharmacokinetics (PK) is the precise and accurate determination of colistin in in vitro and in vivo studies. In the present study, we developed and validated a series of sensitive and robust liquid chromatography tandem mass spectrometry (LC-MS/MS) methods for analysing biological samples obtained from in vitro and in vivo disposition assays. After a zinc acetate-mediated precipitation, hydrophilic-lipophilic-balanced solid phase extraction (HLB-SPE) was used for the extraction of colistin. The compounds were retained on a hydrophilic interaction liquid chromatography (HILIC) column and were detected by MS/MS. CMS was quantified by determining the produced amount of colistin during acidic hydrolysis. The developed methods are sensitive with lower limits of quantification varying between 0.009 µg/mL and 0.071 µg/mL for colistin A, and 0.002 µg/mL to 0.013 µg/mL for colistin B. The intra- and inter-day precision and accuracy were within ±15%. Calibration curves of colistin were linear (0.063 µg/mL to 8.00 µg/mL) within clinically relevant concentration ranges. Zinc acetate-mediated precipitation and the use of a HILIC column were found to be essential. The developed methods are sensitive, accurate, precise, highly efficient and allow monitoring colistin and CMS in biological samples without the need for an internal standard.

Population pharmacokinetics of colistin and the relation to survival in critically ill patients infected with colistin susceptible and carbapenem-resistant bacteria.

OBJECTIVES: The aim was to analyse the population pharmacokinetics of colistin and to explore the relationship between colistin exposure and time to death. METHODS: Patients included in the AIDA randomized controlled trial were treated with colistin for severe infections caused by carbapenem-resistant Gram-negative bacteria. All subjects received a 9 million units (MU) loading dose, followed by a 4.5 MU twice daily maintenance dose, with dose reduction if creatinine clearance (CrCL) < 50 mL/min. Individual colistin exposures were estimated from the developed population pharmacokinetic model and an optimized two-sample per patient sampling design. Time to death was evaluated in a parametric survival analysis. RESULTS: Out of 406 randomized patients, 349 contributed pharmacokinetic data. The median (90% range) colistin plasma concentration was 0.44 (0.14-1.59) mg/L at 15 minutes after the end of first infusion. In samples drawn 10 hr after a maintenance dose, concentrations were >2 mg/L in 94% (195/208) and 44% (38/87) of patients with CrCL ≤120 mL/min, and >120 mL/min, respectively. Colistin methanesulfonate sodium (CMS) and colistin clearances were strongly dependent on CrCL. High colistin exposure to MIC ratio was associated with increased hazard of death in the multivariate analysis (adjusted hazard ratio (95% CI): 1.07 (1.03-1.12)). Other significant predictors included SOFA score at baseline (HR 1.24 (1.19-1.30) per score increase), age and Acinetobacter or Pseudomonas as index pathogen. DISCUSSION: The population pharmacokinetic model predicted that >90% of the patients had colistin concentrations >2 mg/L at steady state, but only 66% at 4 hr after start of treatment. High colistin exposure was associated with poor kidney function, and was not related to a prolonged survival.

Greater optimisation of pharmacokinetic/pharmacodynamic parameters through a loading dose of intravenous colistin in paediatric patients.

Use of colistin in children is rising in line with the increase of multidrug-resistant Gram-negative bacteria (MDR-GNB). In adults, a colistin loading dose is recommended to achieve therapeutic concentrations within 12-24 h. Here we aimed to describe the pharmacokinetic (PK) parameters of a loading dose versus a recommended initial dose of intravenous colistimethate sodium (CMS) in paediatric patients. A prospective, open-label, PK study was conducted in paediatric patients (age 2-18 years) with normal renal function. Patients (n = 20) were randomly assigned to receive either a CMS loading dose (LD group) of 4 mg of colistin base activity (CBA)/kg/dose or a standard initial dose (NLD group) of 2.5 mg (12-h interval) or 1.7 mg (8-h interval) of CBA/kg/dose. Serial blood samples were collected. Plasma concentrations of formed colistin were measured by LC-MS/MS. PK parameters were reported. Acute kidney injury (AKI) was monitored by serum creatinine and urine NGAL. The median (interquartile range) age and body weight were 8.5 (3.5-11.3) years and 21.5 (13.5-20.0) kg. The mean (standard deviation) of first-dose PK parameters of the LD group versus the NLD group were: Cmax, 6.1 (2.4) vs. 4.1 (1.3) mg/L; AUC0-t, 26.5 (12.5) vs. 13.5 (3.6) mg/L·h; Vd, 0.7 (0.4) vs. 0.6 (0.3) L/kg; and t1/2, 2.9 (0.6) vs. 2.6 (0.4) h. No patient developed AKI by serum creatinine criteria. A CMS loading dose is beneficial for improvement of colistin exposure without increased AKI. A higher daily dose of CMS should be considered, especially for MDR-GNB treatment.

Efficacy of Ceftolozane-Tazobactam in Combination with Colistin against Extensively Drug-Resistant Pseudomonas aeruginosa, Including High-Risk Clones, in an In Vitro Pharmacodynamic Model.

Combination therapy is an attractive therapeutic option for extensively drug-resistant (XDR) Pseudomonas aeruginosa infections. Colistin has been the only treatment available for these infections for many years, but its results are suboptimal. Ceftolozane-tazobactam (C/T) is a newly available therapeutic option that has shown good antipseudomonal activity, even against a number of XDR P. aeruginosa strains. However, data about combinations containing C/T are scarce. The aim of this study was to analyze the activity of C/T and colistin alone and in combination against a collection of XDR P. aeruginosa strains containing 24 representative clinical isolates from a multicentre Spanish study. Twenty-four time-kill experiments performed over 24 h were conducted in duplicate to determine the effects of colistin and C/T alone and combined. An in vitro pharmacodynamic chemostat model then was used to validate this combination against three selected XDR P. aeruginosa ST175 isolates with different susceptibility levels to C/T. Static time-kill assays demonstrated superior synergistic or additive effect for C/T plus colistin against 21 of the 24 isolates studied. In the in vitro dynamic pharmacokinetic/pharmacodynamic (PK/PD) model, the C/T regimen of 2/1 g every 8 h with a steady-state concentration of 2 mg/liter colistin effectively suppressed the bacterial growth at 24 h. Additive or synergistic interactions were observed for C/T plus colistin against XDR P. aeruginosa strains and particularly against C/T-resistant strains. C/T plus colistin may be a useful treatment for XDR P. aeruginosa infections, including those caused by high risk-clones resistant to C/T.