Determination of glomerular filtration rate "en passant" after high doses of iohexol for computed tomography in intensive care medicine-a proof of concept

Accurate assessment of renal function is of great clinical and scientific importance, as it is an important pharmacokinetic covariate of pivotal drugs. The iohexol clearance is nearly identical to the glomerular filtration rate, but its determination usually requires an intravenous injection and therefore bears intrinsic risks. This motivates to showcase an "en passant" approach to quantification of renal function without additional risk or blood sampling beyond routine care using real-world data. We enrolled 37 intensive care patients who received high doses of iohexol for computed tomography imaging, and quantified series of iohexol plasma concentrations by high-performance liquid chromatography (HPLC-UV). Iohexol clearance was derived by both log-linear regression and nonlinear least squares fitting and compared to glomerular filtration rate estimated by the CKD-EPI-2021 formulas. Nonlinear fitting not only turned out to be more accurate but also more robust in handling the irregularly timed data points. Concordance of iohexol clearance against estimations based on both creatinine and cystatin C showed a slightly higher bias (-3.44 mL/min/1.73 m2) compared to estimations based on creatinine alone (-0.76 mL/min/1.73 m2), but considerably narrower limits of agreement (±42.8 vs. 56 mL/min/1.73 m2) and higher Lin's correlation (0.84 vs. 0.72). In summary, we have demonstrated the feasibility and performance of the "en passant" variant of the iohexol method in intensive care medicine and described a working protocol for its application in clinical practice and pharmacologic studies.

Perioperative administration of cefazolin and metronidazole in obese and non-obese patients: a pharmacokinetic study in plasma and interstitial fluid

OBJECTIVES

To assess plasma and tissue pharmacokinetics of cefazolin and metronidazole in obese patients undergoing bariatric surgery and non-obese patients undergoing intra-abdominal surgery.

PATIENTS AND METHODS

Fifteen obese and 15 non-obese patients received an IV short infusion of 2 g cefazolin and 0.5 g metronidazole for perioperative prophylaxis. Plasma and microdialysate from subcutaneous tissue were sampled until 8 h after dosing. Drug concentrations were determined by HPLC-UV. Pharmacokinetic parameters were calculated non-compartmentally.

RESULTS

In obese patients (BMI 39.5-69.3 kg/m2) compared with non-obese patients (BMI 18.7-29.8 kg/m2), mean Cmax of total cefazolin in plasma was lower (115 versus 174 mg/L) and Vss was higher (19.4 versus 14.2 L). The mean differences in t½ (2.7 versus 2.4 h), CL (5.14 versus 4.63 L/h) and AUC∞ (402 versus 450 mg·h/L) were not significant. The influence of obesity on the pharmacokinetics of metronidazole was similar (Cmax 8.99 versus 14.7 mg/L, Vss 73.9 versus 51.8 L, t½ 11.9 versus 9.1 h, CL 4.62 versus 4.13 L/h, AUC∞ 116 versus 127 mg·h/L). Regarding interstitial fluid (ISF), mean concentrations of cefazolin remained >4 mg/L until 6 h in both groups, and those of metronidazole up to 8 h in the non-obese group. In obese patients, the mean ISF concentrations of metronidazole were between 3 and 3.5 mg/L throughout the measuring interval.

CONCLUSIONS

During the time of surgery, cefazolin concentrations in plasma and ISF of subcutaneous tissue were lower in obese patients, but not clinically relevant. Regarding metronidazole, the respective differences were higher, and may influence dosing of metronidazole for perioperative prophylaxis in obese patients.

Measurement of Free Plasma Concentrations of Beta-Lactam Antibiotics: An Applicability Study in Intensive Care Unit Patients

BACKGROUND

The antibacterial effect of antibiotics is linked to the free drug concentration. This study investigated the applicability of an ultrafiltration method to determine free plasma concentrations of beta-lactam antibiotics in ICU patients.

METHODS

Eligible patients included adult ICU patients treated with ceftazidime (CAZ), meropenem (MEM), piperacillin (PIP)/tazobactam (TAZ), or flucloxacillin (FXN) by continuous infusion. Up to 2 arterial blood samples were drawn at steady state. Patients could be included more than once if they received another antibiotic. Free drug concentrations were determined by high-performance liquid chromatography with ultraviolet detection after ultrafiltration, using a method that maintained physiological conditions (pH 7.4/37°C). Total drug concentrations were determined to calculate the unbound fraction. In a post-hoc analysis, free concentrations were compared with the target value of 4× the epidemiological cut-off value (ECOFF) for Pseudomonas aeruginosa as a worst-case scenario for empirical therapy with CAZ, MEM or PIP/tazobactam and against methicillin-sensitive Staphylococcus aureus for targeted therapy with FXN.

RESULTS

Fifty different antibiotic treatment periods in 38 patients were evaluated. The concentrations of the antibiotics showed a wide range because of the fixed dosing regimen in a mixed population with variable kidney function. The mean unbound fractions (fu) of CAZ, MEM, and PIP were 102.5%, 98.4%, and 95.7%, with interpatient variability of <6%. The mean fu of FXN was 11.6%, with interpatient variability of 39%. It was observed that 2 of 12 free concentrations of CAZ, 1 of 40 concentrations of MEM, and 11 of 23 concentrations of PIP were below the applied target concentration of 4 × ECOFF for P. aeruginosa. All concentrations of FXN (9 samples from 6 patients) were >8 × ECOFF for methicillin-sensitive Staphylococcus aureus.

CONCLUSIONS

For therapeutic drug monitoring purposes, measuring total or free concentrations of CAZ, MEM, or PIP is seemingly adequate. For highly protein-bound beta-lactams such as FXN, free concentrations should be favored in ICU patients with prevalent hypoalbuminemia.

Neurocognitive Impairment After Propofol With Relevance for Neurosurgical Patients and Awake Craniotomies-A Prospective Observational Study

Background: Short-acting anesthetics are used for rapid recovery, especially for neurological testing during awake craniotomy. Extent and duration of neurocognitive impairment are ambiguous.

Methods: Prospective evaluation of patients undergoing craniotomy for tumor resection during general anesthesia with propofol (N of craniotomies = 35). Lexical word fluency, digit span and trail making were tested preoperatively and up to 24 h after extubation. Results were stratified for age, tumor localization and hemisphere of surgery. Results in digit span test were compared to 21 patients during awake craniotomies.

Results: Word fluency was reduced to 30, 33, 47, and 87% of preoperative values 10, 30, 60 min and 24 h after extubation, respectively. Digit span was decreased to 41, 47, 55, and 86%. Performances were still significantly impaired 24 h after extubation, especially in elderly. Results of digit span test were not worse in patients with left hemisphere surgery. Significance of difference to baseline remained, when patients with left or frontal lesions, i.e., brain areas essential for these tests, were excluded from analysis. Time for trail making was increased by 87% at 1 h after extubation, and recovered within 24 h. In 21 patients undergoing awake craniotomies without pharmacological sedation, digit span was unaffected during intraoperative testing.

Conclusion: Selected aspects of higher cognitive functions are compromised for up to 24 h after propofol anesthesia for craniotomy. Propofol and the direct effects of surgical resection on brain networks may be two major factors contributing (possibly jointly) to the observed deficits. Neurocognitive testing was unimpaired in patients undergoing awake craniotomies without sedation.

The association of the anesthesiologist’s academic and educational status with self-confidence, self-rated knowledge and objective knowledge in rational antibiotic application

Objective

This study aimed to investigate the association of anesthetists’ academic and educational status with self-confidence, self-rated knowledge and objective knowledge about rational antibiotic application. Therefore, anesthetists in Germany were asked about their self-confidence, self-rated knowledge and objective knowledge on antibiotic therapy via the Multiinstitutional Reconnaissance of practice with Multiresistant bacteria (MR2) survey. Other analysis from the survey have been published elsewhere, before.

Results

361 (52.8%) questionnaires were completed by specialists and built the study group. In overall analysis the Certification in Intensive Care (CIC) was significantly associated with self-confidence (p < 0.001), self-rated knowledge (p < 0.001) and objective knowledge (p = 0.029) about antibiotic prescription. Senior consultant status was linked to self-confidence (p < 0.001) and self-rated knowledge (p = 0.005) but not objective knowledge. Likewise, working on Intensive Care Unit (ICU) during the last 12 months was significantly associated with self-rated knowledge and self-confidence (all p < 0.001). In a logistic regression model, senior consultant status was not associated with any tested influence factor. This analysis unveiled that CIC and working on ICU were more associated with anesthesiologists’ self-confidence and self-rated knowledge than senior consultant status. However, neither of the characteristics was thoroughly associated with objective knowledge.

Determination of free clindamycin, flucloxacillin or tedizolid in plasma: Pay attention to physiological conditions when using ultrafiltration

Pharmacokinetic/pharmacodynamic indices of anti-infective drugs should be referenced to free drug concentrations. In the present study, clindamycin, flucloxacillin and tedizolid have been determined in human plasma by HPLC-UV. The drugs were separated isocratically within 3-6 min on a C₁₈ column using mixtures of phosphate buffer-acetonitrile of pH 7.1-7.2. Sample treatment for the determination of total drug concentrations in plasma included extraction/back-extraction (clindamycin) or protein precipitation (flucloxacillin, tedizolid). The free drug concentrations were determined after ultrafiltration. An ultrafiltration device with a membrane consisting of regenerated cellulose proved to be suitable for all drugs. Maintaining a physiological pH was crucial for clindamycin, whereas maintaining body temperature was essential for tedizolid. The methods were applied to the analysis of total and free drug concentrations in clinical samples and were sufficiently sensitive for pharmacokinetic studies and therapeutic drug monitoring.

[Is the discipline associated with self-confidence in handling rational antibiotic prescription? : Results from the MR2 study in German hospitals]

BACKGROUND

Besides public awareness and specialist knowledge and training of physicians, their self-confidence plays a key role for clinical decision-making in the respective area.

OBJECTIVE

This exploratory study investigated the influence of the discipline on differences in self-confidence in dealing with antibiotics and in the self-rated knowledge.

METHODS

In 2015 the multi-institutional reconnaissance of practice with multiresistant bacteria (MR2) questionnaire containing items on antibiotic prescription and multiresistant pathogens was sent out to 1061 physicians working in departments for internal medicine, general surgery, gynecology and obstetrics and urology. In 2017 a similar MR2 survey was sent to 1268 specialist and assistant physicians in anesthesiology in Germany. Besides demographic data 4 items on self-confidence in the use of antibiotic treatment and 11 items concerning self-rated knowledge about rational antibiotic therapy and multiresistant pathogens were included in the present analysis. Logistic regression analysis, the χ²-test and the Kruskal-Wallis test were used for statistical analysis of the influence of the discipline on these items.

RESULTS

The response rates were 43% (456 out of 1061) from the non-anesthetists and 56% (705 out of 1268) from the anesthetists. Of the non-anesthetists 44% and 57% of the anesthetists had had no advanced training on antibiotic stewardship during the year before the study. In the overall analysis anesthetists (mean±SD: 2.53±0.54) were significantly less self-confident about antibiotics than colleagues from other departments (internal medicine: 3.10±0.50, general surgery: 2.97±0.44, gynecology and obstetrics: 3.12±0.42 and urology: 3.15±0.44) in the unadjusted (all p<0.001) and adjusted comparison. The analysis of self-rated knowledge about rational antibiotic prescription showed similar results. Senior consultant status and advanced training in infectiology were significantly associated with self-confidence and self-rated knowledge about antibiotics.

CONCLUSION

Anesthetists showed significantly less self-confidence in dealing with antibiotics than colleagues from other disciplines. Advanced training on a rational prescription of antibiotics was associated with a greater self-confidence, so that the implementation of compulsory courses on rational antibiotic stewardship in the respective residency curriculum needs to be considered.

Tigecycline in critically ill patients on continuous renal replacement therapy: a population pharmacokinetic study

Background

Tigecycline is a vital antibiotic treatment option for infections caused by multiresistant bacteria in the intensive care unit (ICU). Acute kidney injury (AKI) is a common complication in the ICU requiring continuous renal replacement therapy (CRRT), but pharmacokinetic data for tigecycline in patients receiving CRRT are lacking.

Methods

Eleven patients mainly with intra-abdominal infections receiving either continuous veno-venous hemodialysis (CVVHD, n = 8) or hemodiafiltration (CVVHDF, n = 3) were enrolled, and plasma as well as effluent samples were collected according to a rich sampling schedule. Total and free tigecycline was determined by ultrafiltration and high-performance liquid chromatography (HPLC)-UV. Population pharmacokinetic modeling using NONMEM® 7.4 was used to determine the pharmacokinetic parameters as well as the clearance of CVVHD and CVVHDF. Pharmacokinetic/pharmacodynamic target attainment analyses were performed to explore the potential need for dose adjustments of tigecycline in CRRT.

Results

A two-compartment population pharmacokinetic (PK) model was suitable to simultaneously describe the plasma PK and effluent measurements of tigecycline. Tigecycline dialysability was high, as indicated by the high mean saturation coefficients of 0.79 and 0.90 for CVVHD and CVVHDF, respectively, and in range of the concentration-dependent unbound fraction of tigecycline (45–94%). However, the contribution of CRRT to tigecycline clearance (CL) was only moderate (CL_CVVHD: 1.69 L/h, CL_CVVHDF: 2.71 L/h) in comparison with CL_body (physiological part of the total clearance) of 18.3 L/h. Bilirubin was identified as a covariate on CL_body in our collective, reducing the observed interindividual variability on CL_body from 58.6% to 43.6%. The probability of target attainment under CRRT for abdominal infections was ≥ 0.88 for minimal inhibitory concentration (MIC) values ≤ 0.5 mg/L and similar to patients without AKI.

Conclusions

Despite high dialysability, dialysis clearance displayed only a minor contribution to tigecycline elimination, being in the range of renal elimination in patients without AKI. No dose adjustment of tigecycline seems necessary in CRRT.

Trial registration

EudraCT, 2012–005617-39. Registered on 7 August 2013.

Electronic supplementary material

The online version of this article (10.1186/s13054-018-2278-4) contains supplementary material, which is available to authorized users.

Impact of Experimental Variables on the Protein Binding of Tigecycline in Human Plasma as Determined by Ultrafiltration

Tigecycline, a tetracycline derivative, shows atypical plasma protein binding behavior. The unbound fraction decreases with increasing concentration at therapeutic concentrations. Moreover, uncertainty exists about the magnitude of tigecyline's protein binding in man. Unbound fractions between 2.5% and 35% have been reported in plasma from healthy volunteers, and between 25% and 100% in patients, respectively. In the present study, the protein binding of tigecycline has been investigated by ultrafiltration using different experimental conditions. Whereas temperature had only a marginal influence, the unbound fraction at 0.3/3.0 mg/L was low at pH 8.2 (9.4%/1.9%) or in unbuffered pooled plasma (6.3%/1.2%), compared with plasma buffered with HEPES to pH 7.4 (65.9%/39.7%). In experiments with phosphate buffer and EDTA, the concentration dependency was markedly attenuated or abolished, which is compatible with a cooperative binding mechanism involving divalent cations such as calcium. The unbound fraction in clinical plasma samples from patients treated with tigecycline was determined to 66.3 ± 13.7% at concentrations <0.3 mg/L compared with 41.3 ± 16.0% at >1 to <5 mg/L. To summarize, tigecycline appears to be only moderately bound to plasma proteins as determined by ultrafiltration, when a physiological pH is maintained.

Antimicrobials: a global alliance for optimizing their rational use in intra-abdominal infections (AGORA)

Intra-abdominal infections (IAI) are an important cause of morbidity and are frequently associated with poor prognosis, particularly in high-risk patients. The cornerstones in the management of complicated IAIs are timely effective source control with appropriate antimicrobial therapy. Empiric antimicrobial therapy is important in the management of intra-abdominal infections and must be broad enough to cover all likely organisms because inappropriate initial antimicrobial therapy is associated with poor patient outcomes and the development of bacterial resistance. The overuse of antimicrobials is widely accepted as a major driver of some emerging infections (such as C. difficile), the selection of resistant pathogens in individual patients, and for the continued development of antimicrobial resistance globally. The growing emergence of multi-drug resistant organisms and the limited development of new agents available to counteract them have caused an impending crisis with alarming implications, especially with regards to Gram-negative bacteria. An international task force from 79 different countries has joined this project by sharing a document on the rational use of antimicrobials for patients with IAIs. The project has been termed AGORA (Antimicrobials: A Global Alliance for Optimizing their Rational Use in Intra-Abdominal Infections). The authors hope that AGORA, involving many of the world's leading experts, can actively raise awareness in health workers and can improve prescribing behavior in treating IAIs.

Spectrum adequacy of antibiotic regimens for secondary peritonitis: a retrospective analysis in intermediate and intensive care unit patients

Background

Secondary peritonitis requires surgical source control and adequate antimicrobial treatment. Antimicrobial regimens are usually selected according to local susceptibility data of individual pathogens against single agents, but this neglects both the polymicrobial nature of the infection and the use of combination therapy. We analysed the probability of common regimens to cover all relevant pathogens isolated in one patient (“spectrum adequacy rate”, SAR) in a real-life data set.

Methods

Data from 242 patients with secondary peritonitis (88 community acquired, 154 postoperative cases) treated in our IMCU/ICU were obtained retrospectively. The relative frequency of pathogens, resistance rates and the SAR were analysed using the free software R.

Results

Enterococci were isolated in 47.1 % of all patients, followed by Escherichia coli (42.6 %), other enterobacteriaceae (33.1 %), anaerobes (29.8 %) and Candida spp. (28.9 %). Resistance patterns were consistent with general surveillance data from our hospital. The susceptibility rates and SAR were lower in postoperative than in community acquired cases. The following regimens yielded a SAR > 95 % when enterobacteriaceae only were considered: piperacillin/tazobactam + gentamicin, cefotaxim (only for community acquired cases), cefotaxim + gentamicin, meropenem, tigecycline + gentamicin or tigecycline + ciprofloxaxin. When enterococci were also considered, all betalactam based regimens required combination with vancomycin or linezolid for a SAR > 95 %, whereas TGC based regimens were not compromised. As for Candida spp., the SAR of fluconazole was 81.9–87.5 %.

Conclusions

This study demonstrates a rational approach to assess the adequacy of antimicrobial regimens in secondary peritonitis, which may help to adjust local guidelines or to select candidate regimens for clinical studies.

Population pharmacokinetics of meropenem during continuous infusion in surgical ICU patients

Continuous infusion of meropenem is a candidate strategy for optimization of its pharmacokinetic/pharmacodynamic profile. However, plasma concentrations are difficult to predict in critically ill patients. Steady-state concentrations of meropenem were determined prospectively during continuous infusion in 32 surgical ICU patients (aged 21-85 years, body weight 55-125 kg, APACHE II 5-29, measured creatinine clearance 22.7-297 mL/min). Urine was collected for the quantification of renal clearance of meropenem and creatinine. Cystatin C was measured as an additional marker of renal function. Population pharmacokinetic models were developed using NONMEM(®) , which described total meropenem clearance and its relationship with several estimates of renal function (measured creatinine clearance CLCR , Cockcroft-Gault formula CLCG , Hoek formula, 1/plasma creatinine, 1/plasma cystatin C) and other patient characteristics. Any estimate of renal function improved the model performance. The strongest association of clearance was found with CLCR (typical clearance = 11.3 L/h × [1 + 0.00932 × (CLCR  - 80 mL/min)]), followed by 1/plasma cystatin C; CLCG was the least predictive covariate. Neither age, weight, nor sex was found to be significant. These models can be used to predict dosing requirements or meropenem concentrations during continuous infusion. The covariate CLCR offers the best predictive performance; if not available, cystatin C may provide a promising alternative to plasma creatinine.

Protein binding characteristics and pharmacokinetics of ceftriaxone in intensive care unit patients

AIMS

The aim of the present study was to assess the pharmacokinetics of total and unbound ceftriaxone in intensive care unit (ICU) patients and its protein binding characteristics.

METHODS

Twenty patients (m/f 15/5, age 25-86 years, body weight 60-121 kg, APACHE II 7-40, estimated glomerular filtration rate 19-157 ml min(-1) , albumin 11.7-30.1 g l(-1) , total bilirubin <0.1-36.1 mg dl(-1) ) treated with intravenous ceftriaxone were recruited from two ICUs. Timed plasma samples were obtained using an opportunistic study protocol. Ceftriaxone concentrations were determined by high-performance liquid chromatography; unbound concentrations were determined after ultrafiltration using a new method which maintains physiological pH and temperature. The pharmacokinetics was described by a one-compartment model, the protein-binding characteristics by Michaelis-Menten kinetics.

RESULTS

For total drug, the volume of distribution was 20.2 l (median; interquartile range 15.6-24.5 l), the half-life 14.5 h (10.0-25.5 h) and the clearance 0.96 l h(-1) (0.55-1.28 l h(-1) ). The clearance of unbound drug was 1.91 l h(-1) (1.46-6.20 l h(-1) ) and linearly correlated with estimated glomerular filtration rate (slope 0.85, y-intercept 0.24 l h(-1) , r(2)  = 0.70). The unbound fraction was higher in ICU patients (33.0%; 20.2-44.5%) than reported in healthy volunteers, particularly when renal impairment or severe hyperbilirubinaemia was present. In all patients, unbound concentrations during treatment with ceftriaxone 2 g once daily remained above the EUCAST susceptibility breakpoint (≤1 mg l(-1) ) throughout the whole dosing interval.

CONCLUSIONS

Protein binding of ceftriaxone is reduced and variable in ICU patients due to hypoalbuminaemia, but also to altered binding characteristics. Despite these changes, the pharmacokinetics of unbound ceftriaxone is governed by renal function. For patients with normal or reduced renal function, standard doses are sufficient.

Estimation of creatinine clearance using plasma creatinine or cystatin C: a secondary analysis of two pharmacokinetic studies in surgical ICU patients

Background

In ICU patients, glomerular filtration is often impaired, but also supraphysiological values are observed (“augmented renal clearance”, >130 mL/min/1.73 m²). Renally eliminated drugs (e.g. many antibiotics) must be adjusted accordingly, which requires a quantitative measure of renal function throughout all the range of clinically encountered values. Estimation from plasma creatinine is standard, but cystatin C may be a valuable alternative.

Methods

This was a secondary analysis of renal function parameters in 100 ICU patients from two pharmacokinetic studies on vancomycin and betalactam antibiotics. Estimated clearance values obtained by the Cockcroft-Gault formula (eCL_CG), the CKD-EPI formula (eCL_CKD-EPI) or the cystatin C based Hoek formula (eCL_Hoek) were compared with the measured endogenous creatinine clearance (CL_CR). Agreement of values was assessed by modified Bland-Altman plots and by calculating bias (median error) and precision (median absolute error). Sensitivity and specificity of estimates to identify patients with reduced (<60 mL/min/1.73 m²) or augmented (>130 mL/min/1.73 m²) CL_CR were calculated.

Results

The CL_CR was well distributed from highly compromised to supraphysiological values (median 73.2, range 16.8-234 mL/min/1.73 m²), even when plasma creatinine was not elevated (≤0.8 mg/dL for women, ≤1.1 mg/dL for men). Bias and precision were +13.5 mL/min/1.73 m² and ±18.5 mL/min/1.73 m² for eCL_CG, +7.59 and ±16.8 mL/min/1.73 m² for eCL_CKD-EPI, and -4.15 and ±12.9 mL/min/1.73 m² for eCL_Hoek, respectively, with eCL_Hoek being more precise than the other two (p < 0.05). The central 95% of observed errors fell between -59.8 and +250 mL/min/1.73 m² for eCL_CG, -83.9 and +79.8 mL/min/1.73 m² for eCL_CKD-EPI, and -103 and +27.9 mL/min/1.73 m² for eCL_Hoek. Augmented renal clearance was underestimated by eCL_CKD-EPI and eCL_Hoek. Patients with reduced CL_CR were identified with good specificity by eCL_CG, eCL_CKD-EPI and eCL_Hoek (0.95, 0.97 and 0.91, respectively), but with less sensitivity (0.55, 0.55 and 0.83). For augmented renal clearance, specificity was 0.81, 0.96 and 0.96, but sensitivity only 0.69, 0.25 and 0.38.

Conclusions

Normal plasma creatinine concentrations can be highly misleading in ICU patients. Agreement of the cystatin C based eCL_Hoek with CL_CR is better than that of the creatinine based eCL_CG or eCL_CKD-EPI. Detection and quantification of augmented renal clearance by estimates is problematic, and should rather rely on CL_CR.

Electronic supplementary material

The online version of this article (doi:10.1186/s12871-015-0043-7) contains supplementary material, which is available to authorized users.

Unbound fraction of ertapenem in intensive care unit patients

OBJECTIVES

To determine unbound ertapenem concentrations in plasma and to describe the pharmacokinetics of unbound ertapenem in intensive care unit (ICU) patients.

PATIENTS AND METHODS

For assessing the influence of experimental conditions and for development of the ultrafiltration protocol, plasma from healthy volunteers was used. Concentrations of total and unbound ertapenem were determined by HPLC in 29 plasma samples from six ICU patients treated with 1 g of ertapenem once daily. The concentration-time courses were described by a one-compartment model. Ertapenem binding to albumin was assessed by Michaelis-Menten kinetics in solutions of human serum albumin, in plasma from healthy volunteers and in plasma from ICU patients.

RESULTS

The unbound fraction (fu) of ertapenem was highly susceptible to pH and temperature during ultrafiltration and was ∼20% in plasma from healthy volunteers at clinically relevant concentrations. In ICU patients, fu was substantially higher (range 30.9%-53.6%). The unbound concentrations of ertapenem exceeded 2 mg/L for 72% (median; range 39%-100%) of the 24 h dosing interval and 0.25 mg/L for 100% (range 79%-100%). The number of binding sites per albumin molecule was 1.22 (95% CI 1.07-1.38) in plasma from healthy volunteers and 0.404 (95% CI 0.158-0.650) in samples from ICU patients.

CONCLUSIONS

Determination of unbound ertapenem by ultrafiltration is susceptible to experimental conditions. When determined at physiological pH and temperature, fu of ertapenem is 2- to 4-fold higher than previously reported and even higher in ICU patients. Binding studies indicate that hypoalbuminaemia alone does not explain these differences. This issue should be further investigated for its clinical relevance.

Unbound fraction of vancomycin in intensive care unit patients

Published data on the unbound fraction of vancomycin in patient samples exhibit high variability. In the present study, a robust ultrafiltration method was developed and applied to 102 clinical samples from 22 intensive care unit patients who were treated with continuous infusion of vancomycin. A validated HPLC method was used for determination of total and unbound concentrations. The mean unbound fraction was 67.2% (standard deviation 7.5%, range 47.2-92.1%) and independent of total concentration of vancomycin or of albumin. The unbound fraction was significantly correlated (r = +0.67, P = .0009) with the renally filtered fraction (drug clearance/creatinine clearance), providing functional evidence for the validity of the measurements. Ultrafiltration proved to be susceptible to variations in the experimental conditions such as pH, temperature and centrifugal force. The measured unbound fraction increased from 60% at pH 6 to 100% at pH 9, from 57% at 4°C to 80% at 37°C, and was 76% at 1,000 g compared with 45% at 10,000 g. Lack of standardization may therefore partly explain the variable results reported in the literature.

[Strategies to avoid antibiotic resistance]

Antibiotics are used very frequently in critically ill patients as a causal and often life-saving treatment; however, the high density of use of broad spectrum antibiotics contributes to a further deterioration in resistance trends, which makes a rational prescription behavior mandatory. This particularly includes measures which lead to the reduction of antibiotic use, i.e. rigorous indications, targeted de-escalation and limited duration. For optimal efficacy of a necessary treatment the integration of pharmacokinetic and pharmacodynamic principles can be helpful.

Omeprazole impairs the absorption of mycophenolate mofetil but not of enteric-coated mycophenolate sodium in healthy volunteers

In 2 crossover studies, 12 healthy volunteers (6 male/6 female) received a single oral dose of mycophenolate mofetil (MMF) 1000 mg or an equimolar dose of enteric-coated mycophenolate sodium (EC-MPS) 720 mg fasting with and without coadministered omeprazole 20 mg bid. The plasma concentrations of mycophenolic acid (MPA) and of the inactive metabolite mycophenolic acid glucuronide (MPA-G) were measured by high-performance liquid chromatography (HPLC). In addition, dissolution of MMF 500 mg or EC-MPS 360 mg tablets was determined using an USP paddle apparatus in aqueous buffer of pH 1 to 7. The bioavailability of MPA following administration of MMF or EC-MPS was similar except for the time to peak concentration, which was longer in the EC-MPS group. Concomitant treatment with omeprazole lowered significantly C(max) and AUC(12h) of MPA following administration of MMF. The pharmacokinetics of EC-MPS was not affected. Dissolution of MMF in aqueous buffer decreased dramatically at pH above 4.5. The EC-MPS tablet was stable up to pH 5. Above, EC-MPS was quantitatively disintegrated and MPS quantitatively dissolved. There is strong evidence that impaired absorption of MMF with concomitant proton pump inhibitors is due to incomplete dissolution of MMF in the stomach at elevated pH.

Clearance of vancomycin during continuous infusion in Intensive Care Unit patients: correlation with measured and estimated creatinine clearance and serum cystatin C

Vancomycin (VAN) dosing requires adjustment to renal function, which is often estimated using the Cockcroft-Gault formula; however, its precision is poor in Intensive Care Unit (ICU) patients. VAN clearance (CL(Van)) during continuous infusion was prospectively determined in 25 ICU patients [14 male, 11 female; age range 31-82 years; body mass index (BMI) 16.5-41.5 kg/m²; Acute Physiology and Chronic Health Evaluation (APACHE) II score at admission 8-36; creatinine clearance 25-195 mL/min] and its correlation with measured creatinine clearance (CL(Crea)), estimated creatinine clearance using the Cockcroft-Gault formula (CL(CG)) and estimated glomerular filtration rate according to Hoek's formula based on serum cystatin C (GFR(Hoek)) was investigated. The correlation between CL(Van) and CL(Crea) was very good (r²=0.88), but it was rather poor with CL(CG) (r² = 0.37) and was acceptable with GFR(Hoek) (r² = 0.70). For VAN dose adjustments in ICU patients, determination of cystatin C may be an interesting and practical alternative to measured CL(Crea), whereas the Cockcroft-Gault formula should be used with caution.

[Ringer solution: osmolarity and composition revisited]

The composition of Ringer solution, a crystalloid fluid that is often used in anaesthesia and intensive care, varies depending on the manufacturer. The knowledge of the actual content in electrolytes and of the characteristics of this fluid is necessary before it is used. We call attention to a certain Ringer solution (Ringer Maco Pharma, Maco Pharma), for which the manufacturer's information about the tonicity and the osmolarity was incorrect. Contrary to what is written on the bag and in the product description (isotonicity, osmolarity of 276.8 mOsm/l), the theoretical osmolarity was 221.4 mOsm/l and the measured osmolality was about 208 mmol/kg, exposing the hypotonic characteristics of this fluid. The use of this product is potentially dangerous in patients with pathologies where the infusion of free water is especially badly supported.

Via β-adrenoceptors, stimulation of extrasplenic sympathetic nerve fibers inhibits lipopolysaccharide-induced TNF secretion in perfused rat spleen

Using a spleen slice microsuperfusion technique in mice, we have previously characterized the role of norepinephrine (NE) and other neurotransmitters for sympathetic modulation of IL-6 and TNF secretion of splenic macrophages. Since experiments in spleen slices do not reflect the situation of an entire perfused organ, we investigated sympathetic modulation of lipopolysaccharide (LPS)-induced secretion of IL-6 and TNF in perfusion experiments of rat spleen. In an organ bath, perfusion was performed in explanted whole spleens, and catecholamines and cytokines were measured by HPLC and ELISA, respectively. Release of NE depended on stimulation frequency (maximum at 10 Hz). Apart from NE, perfusates also contained significant amounts of dopamine and epinephrine. Furthermore, perfusate epinephrine levels correlated positively with perfusate NE levels (RRank=0.750, p<0.001) but not with plasma epinephrine concentrations. This indicates that epinephrine is a conversion product of sympathetically released NE. Early electrical stimulation of extrasplenic splenic nerves, 20 min after administration of LPS, significantly inhibited TNF secretion. This electrically induced effect was abrogated by simultaneous administration of propranolol (10(-6) M) but it was not influenced by administration of either an alpha1- or alpha2-adrenergic antagonist. Late electrical stimulation of splenic nerves, 2.5 h after administration of LPS, did not change TNF secretion. Interestingly, no influence of early or late sympathetic nerve fiber stimulation on IL-6 secretion was observed. In conclusion, this is the first perfusion study of the entire spleen that demonstrates that early electrical stimulation of sympathetic splenic nerve fibers directly inhibits LPS-induced TNF secretion. This study corroborates the idea that splenic sympathetic nerves are able to inhibit important activators of the innate immune system when stimulation happens very early or even prior to their induction by LPS.