Challenge for higher colistin dosage in critically ill patients receiving continuous venovenous haemodiafiltration

Disposition of colistin in critically-ill patients on sustained low-efficiency dialysis: a population pharmacokinetic study

OBJECTIVES

Although colistin (administered as colistin methanesulphonate [CMS]) is used to treat infections in critically-ill patients undergoing sustained low-efficiency dialysis (SLED), there is a paucity of information on appropriate dosing regimens. This study aimed to characterize the population pharmacokinetics (popPK) of colistin during SLED and evaluate the likelihood of antibacterial benefit and colistin nephrotoxicity for different regimens.

METHODS

A prospective popPK study included 13 critically-ill patients (six females) treated with CMS and receiving SLED (6-8h). For each subject, the PK of formed colistin was studied on a nonSLED day and a SLED day (n=8 studied during SLED day first). A single intravenous daily dose [150mg colistin base activity (CBA)] was administered on a nonSLED day. On a SLED day, patients received 150mg CBA 12-hourly. Serial blood, urine and dialysate samples were collected over 24h on both days. Colistin plasma concentrations were measured by high-performance liquid chromatography. PopPK modeling and Monte Carlo Simulations were performed.

RESULTS

A linear one-compartment disposition model well-described the data. The population mean apparent colistin body clearance, excluding SLED clearance, was 1.69 L/h (20% interindividual variability [IIV], 42.1% interoccasion variability). The apparent colistin SLED clearance was 3.49 L/h (41.7% IIV), i.e. 67.4% of total colistin clearance on a SLED day. The apparent volume of distribution was 50.2 L (23.0% IIV).

CONCLUSIONS

Colistin clearance was substantially higher during SLED; therefore, SLED should be accounted for in CMS dosing regimens. This project generated clinically applicable regimens, including loading doses, to achieve required probabilities of target attainment in patients undergoing SLED.

A Systematic Review of Pharmacokinetic Studies of Colistin and Polymyxin B in Adult Populations

BACKGROUND AND OBJECTIVE

The pharmacokinetics of polymyxins are highly variable and conventional dosing regimens may likely lead to sub-optimal exposures and outcomes, particularly in critically ill patients with multi-drug-resistant infections. The aim of this systematic review is to describe the published pharmacokinetic data and to investigate variables that have been shown to affect the pharmacokinetics of colistimethate sodium, colistin, and polymyxin B in adult populations.

METHODS

Sixty studies were identified. A total of 27 and 33 studies described the pharmacokinetics of colistin and polymyxin B, respectively.

RESULTS

The most common dosing regimen for colistimethate sodium was a loading dose of 9 MIU, followed by 9 MIU/day in two to three divided doses, while for polymyxin B, a loading dose of 100-200 mg, followed by 50-100 mg every 12 h was given. Studies that used colistin sulfate instead of colistimethate sodium reported lower inter-individual variability, which may be attributed to the formulation of colistin sulfate being an active drug. The volume of distribution for colistin is typically lower in healthy individuals than in critically ill patients, owing to variations in physiological and pathological conditions. The clearance of colistimethate sodium in critically ill patients not undergoing dialysis was higher, around 13 L/h, compared with those receiving continuous renal replacement therapy, where clearance ranged from 2.31 to 8.23 L/h. In patients receiving continuous renal replacement therapy, clearance of colistin was higher compared with colistimethate sodium (2.06-6.63 L/h and 1.57-3.85 L/h, respectively). Colistin protein binding in critically ill patients ranged from 51% to 79%. The volume of distribution of polymyxin B was similar between critically ill and acutely ill patients, with range of 6.3-33.1 L and 6.22-38.6 L, respectively. Clearance of polymyxin B was also almost similar between critically ill and acutely ill patients (range of 1.27-2.32 L/h). There were two studies that reported free drug concentrations instead of the total drug concentrations of polymyxin B. In critically ill patients, protein binding ranged from 48.8% to 92.4% for polymyxin B. Creatinine clearance was the most common patient characteristic associated with altered clearance of colistimethate sodium and/or colistin, and polymyxin B.

CONCLUSIONS

Critically ill patients exhibit complex pharmacokinetics for colistin and polymyxin B, influenced by renal function, body weight, and clinical factors such as acute kidney injury, augmented renal clearance, serum albumin, and liver function. These factors necessitate individualized dosing adjustments to avoid toxicity and achieve therapeutic efficacy. Model-informed precision dosing provides a promising approach to optimize their use by integrating population pharmacokinetic parameters, patient-specific variables, and therapeutic drug monitoring, ensuring a balance between efficacy, safety, and resistance prevention.

Population pharmacokinetics of colistin sulfate in patients on continuous veno-venous hemodiafiltration

OBJECTIVE

The aim of this study is to establish a population pharmacokinetic (PK) model for patients undergoing continuous veno-venous hemodiafiltration (CVVHDF) and optimize the dosing regimen of colistin sulfate.

METHODS

A prospective observational study in a single center was conducted on patients who were administrated with colistin sulfate and CVVHDF for at least 48 h. Blood samples were obtained prior to dosing and four to six blood samples (primarily C0.5h, C1h, C2h, C4h, and C6h) after dosing. The blood concentration of colistin sulfate was determined by ultra-high performance liquid chromatography-tandem mass spectrometry assay. The NONMEM program was used to establish the population PK model and perform Monte Carlo simulations. The predictability and stability of the model were internally evaluated by the goodness of fit plots, visual prediction check, and bootstraps.

RESULTS

A total of 86 plasma concentrations from 20 patients were used for population PK modeling. A two-compartment model with first-order linear elimination best described the population PK characteristics of colistin sulfate. Cystatin C (CysC) and body weight (WT) were identified as covariates for clearance (CL). Internal evaluation results showed that the final model had good stability and prediction performance. Monte Carlo simulations showed that only when the body WT was 50 kg with CysC ≥3.07 mg/l, and when the body WT was 65 kg with CysC = 5.11 mg/l, and minimum inhibitory concentration (MIC) = 0.25 mg/l, the target attainment probability (PTA) of the daily dose of 1.5 million U regimen was ≥90%. All treatment regimens fail to achieve the target PTA when MIC = 1 mg/l.

CONCLUSIONS

With the decrease of CysC levels and the increase of WT, the dose of colistin sulfate may need to be increased. It may be prudent for colistin sulfate to consider an initial dose doubling and subsequent maintenance dosing regimen of 200-225 million unit daily, administered in 2-3 divided doses, to attain PTA standard. This study was registered at the Chinese Clinical Trial Registry (www.chictr.org.cn) (trial registration number ChiCTR2300072191).

Antimicrobial dosing recommendations during continuous renal replacement therapy: different databases, different doses

Meticulous antimicrobial management is essential among critically ill patients with acute kidney injury, particularly if renal replacement therapy is needed. Many factors affect drug removal in patients undergoing continuous renal replacement therapy CRRT. In this study, we aimed to compare current databases that are frequently used to adjust CRRT dosages of antimicrobial drugs with the gold standard. The dosage recommendations from various databases for antimicrobial drugs eliminated by CRRT were investigated. The book 'Renal Pharmacotherapy: Dosage Adjustment of Medications Eliminated by the Kidneys' was chosen as the gold standard. There were variations in the databases. Micromedex, UpToDate, and Sanford had similar rates to the gold standard of 45%, 35%, and 30%, respectively. The Micromedex database shows the most similar results to the gold standard source. In addition, a consensus was reached as a result of the expert panel meetings established to discuss the different antimicrobial dose recommendations of the databases.

High-dose colistin pharmacokinetics in critically ill patients receiving continuous renal replacement therapy

BACKGROUND

Colistin, administered as intravenous colistimethate (CMS), is still used in the critical care setting and current guidelines recommend high dosage CMS in patients undergoing continuous renal replacement therapy (CRRT). Due to the paucity of real-life data, we aimed to describe colistin pharmacokinetic/pharmacodynamic (PK/PD) profile in a cohort of critically ill patients with infections due to carbapenem-resistant (CR) bacteria undergoing CRRT.

RESULTS

All consecutive patients admitted to three Intensive Care Units (ICUs) of a large metropolitan University Hospital, treated with colistin for at least 48 h at the dosage of 6.75 MUI q12, after 9 MIU loading dose, and undergoing CRRT were included. After the seventh dose, patients underwent blood serial sampling during a time frame of 24 h. We included 20 patients, who had CR-Acinetobacter baumannii ventilator-associated pneumonia and were characterized by a median SAPS II and SOFA score of 41 [34.5-59.3] and 9 [6.7-11], respectively. Fifteen patients died during ICU stay and six recovered renal function. Median peak and trough colistin concentrations were 16.6 mcg/mL [14.8-20.6] and 3.9 mcg/mL [3.3-4.4], respectively. Median area under the time-concentration curve (AUC0 - 24) and average steady-state concentration (Css, avg) were 193.9 mcg h/mL [170.6-208.6] and 8.07 mcg/mL [7.1-8.7]. Probability of target attainment of colistin pharmacodynamics according to the fAUC0 - 24/MIC target ≥ 12 was 100% for MIC ≤ 2 mcg/mL and 85% for MIC = 4 mcg/ML, although exceeding the toxicity limit of Css, avg 3-4 mcg/mL.

CONCLUSIONS

In critically ill patients with CR infections undergoing CRRT, recommended CMS dosage resulted in colistin plasmatic levels above bacterial MIC90, but exceeding the safety Css, avg. limit.

TRIAL REGISTRATION

This trial was registered in ClinicalTrials.gov on 23/07/2021 with the ID NCT04995133 (https//clinicaltrials.gov/study/NCT04995133).

Clinical outcomes of colistin methanesulfonate sodium in correlation with pharmacokinetic parameters in critically ill patients with multi-drug resistant bacteria-mediated infection: A systematic review and meta-analysis

BACKGROUND

Colistin is a viable option for multidrug resistant gram-negative bacteria emerged from inappropriate antibiotic use. Nonetheless, suboptimal colistin concentrations and nephrotoxicity risks hinder its clinical use. Thus, the aim of this study is to investigate clinical outcomes in correlation with pharmacokinetic differences and infection types in critically ill patients on intravenous colistin methanesulfornate sodium (CMS).

METHODS

A systematic literature search of Embase, Google Scholars, and PubMed was performed to identify clinical trials evaluating pharmacokinetic parameters along with clinical outcomes of CMS treatment from inception to July 2023. The pooled analyses of clinical impact of CMS on nephrotoxicity, mortality, clinical cure, and colistin concentration at steady state (Css,avg) were performed. This study was registered in the PROSPERO (CRD 42023456120).

RESULTS

Total of 695 critically ill patients from 17 studies were included. The mortality was substantially lower in clinically cured patients (OR 0.05; 95% CI 0.02 - 0.14), whereas the mortality rate was statistically insignificant between nephrotoxic and non-nephrotoxic patients. Inter-patient variability of pharmacokinetic parameters of CMS and colistin was observed in critically ill patients. The standard mean differences of Css,avg were statistically insignificant between clinically cure and clinically failure groups (standard mean difference (SMD) -0.25; 95% CI -0.69 - 0.19) and between nephrotoxic and non-nephrotoxic groups (SMD 0.67; 95% CI -0.27-1.61). The clinical cure rate is substantially lower in pneumonia patients (OR 0.09; 95% CI 0.01 - 0.56), and pharmacokinetic parameters pertaining to microbiological cure were different among strains.

CONCLUSION

The mortality rate was substantially lower in clinically cured patients with CMS. However, no significant differences in Css,avg of colistin were examined to determine the impact of pharmacokinetic differences on clinical outcomes including mortality rate and nephrotoxicity risk. Nevertheless, the clinical cure rate is substantially lower in patients with respiratory infection than patients with urinary tract infection.

Empirical antibiotic therapy for difficult-to-treat Gram-negative infections: when, how, and how long?

PURPOSE OF REVIEW

To discuss empirical therapy for severe infections due to Gram-negative bacteria with difficult-to-treat resistance (GNB-DTR) in current clinical practice, focusing in particular on the positioning of novel therapeutic agents and rapid diagnostic tests.

RECENT FINDINGS

The current era of novel agents active against GNB-DTR and showing differential activity against specific determinants of resistance is an unprecedented scenario, in which the clinical reasoning leading to the choice of the empirical therapy for treating severe GNB-DTR infections is becoming more complex, but it also allows for enhanced treatment precision.

SUMMARY

Novel agents should be used in line with antimicrobial stewardship principles, aimed at reducing selective pressure for antimicrobial resistance. However, this does not mean that they should not be used. Indeed, excesses in restrictive uses may be unethical by precluding access to the most effective and less toxic treatments for patients with severe GNB-DTR infections. Given these premises (the 'how'), empirical treatment with novel agents should be considered in all patients with risk factors for GNB-DTR and severe clinical presentation of acute infection (the 'when'). Furthermore, empirical novel agents should preferably be continued only for a few hours, until de-escalation, modification, or confirmation (as targeted therapy) is made possible by the results of rapid diagnostic tests (the 'how long').

Therapeutic Drug Monitoring of Antibiotic Drugs in Patients Receiving Continuous Renal Replacement Therapy or Intermittent Hemodialysis: A Critical Review

PURPOSE

Antibiotics are frequently used in patients receiving intermittent or continuous renal replacement therapy (RRT). Continuous renal replacement may alter the pharmacokinetics (PK) and the ability to achieve PK/pharmacodynamic (PD) targets. Therapeutic drug monitoring (TDM) could help evaluate drug exposure and guide antibiotic dosage adjustment. The present review describes recent TDM data on antibiotic exposure and PK/PD target attainment (TA) in patients receiving intermittent or continuous RRT, proposing practical guidelines for performing TDM.

METHODS

Studies on antibiotic TDM performed in patients receiving intermittent or continuous RRT published between 2000 and 2020 were searched and assessed. The authors focused on studies that reported data on PK/PD TA. TDM recommendations were based on clinically relevant PK/PD relationships and previously published guidelines.

RESULTS

In total, 2383 reports were retrieved. After excluding nonrelevant publications, 139 articles were selected. Overall, 107 studies reported PK/PD TA for 24 agents. Data were available for various intermittent and continuous RRT techniques. The study design, TDM practice, and definition of PK/PD targets were inconsistent across studies. Drug exposure and TA rates were highly variable. TDM seems to be necessary to control drug exposure in patients receiving intermittent and continuous RRT techniques, especially for antibiotics with narrow therapeutic margins and in critically ill patients. Practical recommendations can provide insights on relevant PK/PD targets, sampling, and timing of TDM for various antibiotic classes.

CONCLUSIONS

Highly variable antibiotic exposure and TA have been reported in patients receiving intermittent or continuous RRT. TDM for aminoglycosides, beta-lactams, glycopeptides, linezolid, and colistin is recommended in patients receiving RRT and suggested for daptomycin, fluoroquinolones, and tigecycline in critically ill patients on RRT.

Agents of Last Resort: An Update on Polymyxin Resistance

Polymyxin resistance is a major public health threat, because the polymyxins represent last-line therapeutics for gram-negative pathogens resistant to essentially all other antibiotics. Minimizing any potential emergence and dissemination of polymyxin resistance relies on an improved understanding of mechanisms of and risk factors for polymyxin resistance, infection prevention and stewardship strategies, together with optimization of dosing of polymyxins (eg, combination regimens).

Recommendation of Antimicrobial Dosing Optimization During Continuous Renal Replacement Therapy

Continuous Renal Replacement Therapy (CRRT) is more and more widely used in patients for various indications recent years. It is still intricate for clinicians to decide a suitable empiric antimicrobial dosing for patients receiving CRRT. Inappropriate doses of antimicrobial agents may lead to treatment failure or drug resistance of pathogens. CRRT factors, patient individual conditions and drug pharmacokinetics/pharmacodynamics are the main elements effecting the antimicrobial dosing adjustment. With the development of CRRT techniques, some antimicrobial dosing recommendations in earlier studies were no longer appropriate for clinical use now. Here, we reviewed the literatures involving in new progresses of antimicrobial dosages, and complied the updated empirical dosing strategies based on CRRT modalities and effluent flow rates. The following antimicrobial agents were included for review: flucloxacillin, piperacillin/tazobactam, ceftriaxone, ceftazidime/avibactam, cefepime, ceftolozane/tazobactam, sulbactam, meropenem, imipenem, panipenem, biapenem, ertapenem, doripenem, amikacin, ciprofloxacin, levofloxacin, moxifloxacin, clindamycin, azithromycin, tigecycline, polymyxin B, colistin, vancomycin, teicoplanin, linezolid, daptomycin, sulfamethoxazole/trimethoprim, fluconazole, voriconazole, posaconzole, caspofungin, micafungin, amphotericin B, acyclovir, ganciclovir, oseltamivir, and peramivir.

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.

International Consensus Guidelines for the Optimal Use of the Polymyxins: Endorsed by the American College of Clinical Pharmacy (ACCP), European Society of Clinical Microbiology and Infectious Diseases (ESCMID), Infectious Diseases Society of America (IDSA), International Society for Anti-infective Pharmacology (ISAP), Society of Critical Care Medicine (SCCM), and Society of Infectious Diseases Pharmacists (SIDP)

The polymyxin antibiotics colistin (polymyxin E) and polymyxin B became available in the 1950s and thus did not undergo contemporary drug development procedures. Their clinical use has recently resurged, assuming an important role as salvage therapy for otherwise untreatable gram-negative infections. Since their reintroduction into the clinic, significant confusion remains due to the existence of several different conventions used to describe doses of the polymyxins, differences in their formulations, outdated product information, and uncertainties about susceptibility testing that has led to lack of clarity on how to optimally utilize and dose colistin and polymyxin B. We report consensus therapeutic guidelines for agent selection and dosing of the polymyxin antibiotics for optimal use in adult patients, as endorsed by the American College of Clinical Pharmacy (ACCP), Infectious Diseases Society of America (IDSA), International Society of Anti-Infective Pharmacology (ISAP), Society for Critical Care Medicine (SCCM), and Society of Infectious Diseases Pharmacists (SIDP). The European Society for Clinical Microbiology and Infectious Diseases (ESCMID) endorses this document as a consensus statement. The overall conclusions in the document are endorsed by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). We established a diverse international expert panel to make therapeutic recommendations regarding the pharmacokinetic and pharmacodynamic properties of the drugs and pharmacokinetic targets, polymyxin agent selection, dosing, dosage adjustment and monitoring of colistin and polymyxin B, use of polymyxin-based combination therapy, intrathecal therapy, inhalation therapy, toxicity, and prevention of renal failure. The treatment guidelines provide the first ever consensus recommendations for colistin and polymyxin B therapy that are intended to guide optimal clinical use.

[Pharmacokinetics and pharmacodynamics in extracorporeal renal replacement therapy]

Intermittent hemodialysis, continuous hemofiltration and prolonged daily dialysis are used for renal replacement therapy in the intensive care units. Independent of the replacement modality, antibiotic therapy must start with a high loading dose. Dose adjustment to the kidneys must follow 48 h later to prevent toxic accumulation. Dose recommendations on product labels are often underdosed. On continuous hemofiltration, meanwhile many intensivists administer a normal standard dose because the high filtration rate corresponds to a half-normal glomerular filtration rate. After intermittent hemodialysis, a dose similar to the loading dose will be needed. On day off dialysis, the maintenance dose must be adjusted to the failing kidney function. Immediately after prolonged daily dialysis, a loading dose should be given; with twice daily dosing the maintenance dose needs to be adjusted to kidney function. Therapeutic drug monitoring is recommended for gentamicin, vancomycin, piperacillin, meropenem and voriconazole. Due to pharmacodynamic reasons, the target concentration corresponds to the concentration producing the half-maximum effect. Accordingly, the target concentration is the normal peak for concentration-dependent action with bolus dosing. The target is the average steady-state concentration for antibiotics with time-dependent action and continuous infusion.

Treatment of Ventilator-associated Pneumonia with High-dose Colistin Under Continuous Veno-venous Hemofiltration

BACKGROUND AND OBJECTIVES

High-dose colistin (COL) ensures adequate treatment of pneumonia caused by multidrug resistant gram-negative bacteria (MDR-GNB) but must be weighed against a higher risk of nephrotoxicity. Continuous veno-venous hemofiltration (CVVH) clears COL by filtering and membrane adsorption that permits to avoid dose accumulation and excessively high peak concentrations. We evaluated clinical/microbiological efficacy of the high-dose COL treatment under CVVH in patients with newly diagnosed MDR-GNB ventilator-associated pneumonia (VAP).

METHODS

Observational cohort study in critically ill adult patients with MDR-GNB VAP. Colistimethate sodium (CMS) was administered as a 9 million international units (MIU) of loading dose followed by 3 × 4.5 MIU daily. CVVH was performed over a highly adsorptive membrane. Clinical and microbiological efficacies were assessed at the end of therapy. In survivors, serum creatinine level was evaluated before and at the end of therapy.

RESULTS

Fourteen patients (8 male patients, aged 57 ± 14 years) were consecutively included. Isolated pathogens were Pseudomonas aeruginosa in 7, Klebsiella pneumoniae in 5, and other Enterobacteriaceae in 2 patients. A favorable clinical response was observed in 9 patients (64%). Full and presumed microbiological eradication was observed in 12 patients (86%). Two patients were diagnosed with Stage 1 acute kidney injury.

CONCLUSIONS

In patients with MDR-GNB VAP, CVVH may represent an interesting option to enable effective high-dose COL treatment.

A Guide to Understanding Antimicrobial Drug Dosing in Critically Ill Patients on Renal Replacement Therapy

A careful management of antimicrobials is essential in the critically ill with acute kidney injury, especially if renal replacement therapy is required. Acute kidney injury may lead per se to clinically significant modifications of drugs' pharmacokinetic parameters, and the need for renal replacement therapy represents a further variable that should be considered to avoid inappropriate antimicrobial therapy. The most important pharmacokinetic parameters, useful to determine the significance of extracorporeal removal of a given drug, are molecular weight, protein binding, and distribution volume. In many cases, the extracorporeal removal of antimicrobials can be relevant, with a consistent risk of underdosing-related treatment failure and/or potential onset of bacterial resistance. It should also be taken into account that renal replacement therapies are often not standardized in critically ill patients, and their impact on plasma drug concentrations may substantially vary in relation to membrane characteristics, treatment modality, and delivered dialysis dose. Thus, in this clinical scenario, the knowledge of the pharmacokinetic and pharmacodynamic properties of different antimicrobial classes is crucial to tailor maintenance dose and/or time interval according to clinical needs. Finally, especially for antimicrobials known for a tight therapeutic range, therapeutic drug monitoring is strongly suggested to guide dosing adjustment in complex clinical settings, such as septic patients with acute kidney injury undergoing renal replacement therapy.

Monitoring of tobramycin serum concentrations in selected critically ill patients receiving selective decontamination of the digestive tract: a retrospective evaluation

INTRODUCTION

Selective decontamination of the digestive tract (SDD) is a strategy in mechanically ventilated patients to reduce mortality. Treatment consists of enterally administered non-absorbable antibiotics, i.e., tobramycin. However, most intensive care unit (ICU) patients with SDD appear to have detectable tobramycin serum concentrations. The Rijnstate Hospital implemented a protocol for therapeutic drug monitoring (TDM) of tobramycin in patients at risk. The aim of this study was to evaluate the necessity of TDM in these patients and to optimize the current protocol.

METHODS

This retrospective observational study included ICU patients with SDD treatment for ≥ 7 days and renal failure. These patients were considered eligible for monitoring of tobramycin. Tobramycin serum concentrations, relevant laboratory parameters (i.e., renal function, lactate), and patient data were extracted from the National Intensive Care Evaluation database and the hospital electronic patient data system.

RESULTS

In 23 subjects, a total of 43 tobramycin serum concentrations was determined. The median tobramycin serum concentration was 0.33 (IQR 0.17-0.49) mg/L of which 12 (27.9%) samples had concentrations < 0.2 mg/L, 30 (69.8%) had concentrations 0.2-1.0 mg/L and 1 (2.3%) had a toxic concentration > 1.0 mg/L. In 3 (7.0%) cases, an intervention was conducted based on the tobramycin serum concentration.

CONCLUSION

The majority (83.7%) of samples had detectable tobramycin serum concentrations. Monitoring of tobramycin serum concentrations can be considered necessary in patients at risk. However, the current protocol should be optimized to intercept patients more precise.

Multicenter Population Pharmacokinetic Study of Colistimethate Sodium and Colistin Dosed as in Normal Renal Function in Patients on Continuous Renal Replacement Therapy

Intravenous colistimethate sodium (CMS) is used to treat infections with multiresistant Gram-negative bacteria. Optimal dosing in patients undergoing continuous renal replacement therapy (CRRT) is unclear. In a prospective study, we determined CMS and colistin pharmacokinetics in 10 critically ill patients requiring CRRT (8 underwent continuous venovenous hemodialysis [CVVHD]; median blood flow, 100 ml/min). Intensive sampling was performed on treatment days 1, 3, and 5 after an intravenous CMS loading dose of 9 million international units (MU) (6 MU if body weight was <60 kg) with a consecutive 3-MU (respectively, 2 MU) maintenance dose at 8 h. CMS and colistin concentrations were determined by liquid chromatography with mass spectroscopy. A model-based population pharmacokinetic analysis incorporating CRRT settings was applied to the observations. Sequential model building indicated a monocompartmental distribution for both CMS and colistin, with interindividual variability in both volume and clearance. Hematocrit was shown to affect the efficacy of drug transfer across the filter. CRRT clearance accounted for, on average, 41% of total CMS and 28% of total colistin clearance, confirming enhanced elimination of colistin compared to normal renal function. Target colistin steady-state trough concentrations of at least 2.5 mg/liter were achieved in all patients receiving 3 MU at 8 h. In conclusion, a loading dose of 9 MU followed after 8 h by a maintenance dose of 3 MU every 8 h independent of body weight is expected to achieve therapeutic colistin concentrations in patients undergoing CVVHD using low blood flows. Colistin therapeutic drug monitoring might help to further ensure optimal dosing in individual patients. (This study has been registered at ClinicalTrials.gov under identifier NCT02081560.).

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.

Dosing guidance for intravenous colistin in critically-ill patients

Background

Intravenous colistin is difficult to use because plasma concentrations for antibacterial effect overlap those causing nephrotoxicity, and there is large inter-patient variability in pharmacokinetics. The aim was to develop dosing algorithms for achievement of a clinically desirable average steady-state plasma colistin concentration (C_ss,avg) of 2mg/L.

Methods

Plasma concentration-time data from 214 adult critically-ill patients (creatinine clearance 0-236mL/min; 29 receiving renal replacement therapy (RRT)) were subjected to population pharmacokinetic analysis. Development of an algorithm for patients not receiving RRT was based upon the relationship between the dose of colistimethate that would be needed to achieve a desired C_ss,avg and creatinine clearance. The increase in colistin clearance when patients were on RRT was determined from the population analysis and guided the supplemental dosing needed. To balance potential antibacterial benefit against risk of nephrotoxicity the algorithms were designed to achieve target attainment rates of >80% for C_ss,avg ≥2 and <30% for C_ss,avg ≥4mg/L.

Results

When algorithm doses were applied back to individual patients not on RRT (including patients prescribed intermittent dialysis on a non-dialysis day), >80% of patients with creatinine clearance <80mL/min achieved C_ss,avg ≥2mg/L; but for patients with creatinine clearance ≥80mL/min target attainment was <40%, even with the maximum allowed daily dose of 360mg colistin base activity. For patients receiving RRT, target attainment rates were >80% with the proposed supplemental dosing. In all categories of patients, <30% of patients attained C_ss,avg ≥4mg/L.

Conclusions

The project has generated clinician-friendly dosing algorithms and pointed to circumstances where intravenous monotherapy may be inadequate.