Linezolid brain penetration in neurointensive care patients.

BACKGROUND: Linezolid exposure in critically ill patients is associated with high inter-individual variability, potentially resulting in subtherapeutic antibiotic exposure. Linezolid exhibits good penetration into the CSF, but its penetration into cerebral interstitial fluid (ISF) is unknown. OBJECTIVES: To determine linezolid penetration into CSF and cerebral ISF of neurointensive care patients. PATIENTS AND METHODS: Five neurocritical care patients received 600 mg of linezolid IV twice daily for treatment of extracerebral infections. At steady state, blood and CSF samples were collected from arterial and ventricular catheters, and microdialysate was obtained from a cerebral intraparenchymal probe. RESULTS: The median fAUC0-24 was 57.6 (24.9-365) mg·h/L in plasma, 64.1 (43.5-306.1) mg·h/L in CSF, and 27.0 (10.7-217.6) mg·h/L in cerebral ISF. The median penetration ratio (fAUCbrain_or_CSF/fAUCplasma) was 0.5 (0.25-0.81) for cerebral ISF and 0.92 (0.79-1) for CSF. Cerebral ISF concentrations correlated well with plasma (R = 0.93, P < 0.001) and CSF levels (R = 0.93, P < 0.001).The median fAUC0-24/MIC ratio was ≥100 in plasma and CSF for MICs of ≤0.5 mg/L, and in cerebral ISF for MICs of ≤0.25 mg/L. The median fT>MIC was ≥80% of the dosing interval in CSF for MICs of ≤0.5 mg/L, and in plasma and cerebral ISF for MICs of ≤0.25 mg/L. CONCLUSIONS: Linezolid demonstrates a high degree of cerebral penetration, and brain concentrations correlate well with plasma and CSF levels. However, substantial variability in plasma levels, and thus cerebral concentrations, may result in subtherapeutic tissue concentrations in critically ill patients with standard dosing, necessitating therapeutic drug monitoring.

Tigecycline Soft Tissue Penetration in Obese and Non-obese Surgical Patients Determined by Using In Vivo Microdialysis.

BACKGROUND AND OBJECTIVE: Tigecycline, a broad-spectrum glycylcycline antibiotic, is approved for use at a fixed dose irrespective of body weight. However, its pharmacokinetics may be altered in obesity, which would impact on the antibiotic's effectiveness. The objective of this study was to investigate the plasma and subcutaneous tissue concentrations of tigecycline in obese patients compared with those in a non-obese control group. METHODS: Fifteen obese patients (one class II and 14 class III) undergoing bariatric surgery and 15 non-obese patients undergoing intra-abdominal surgery (mainly tumour resection) received a single dose of 50 or 100 mg tigecycline as an intravenous short infusion. Tigecycline concentrations were measured up to 8 h after dosing in plasma (total concentration), in ultrafiltrate of plasma (free concentration), and in microdialysate from subcutaneous tissue, respectively. RESULTS: In obese patients, total peak plasma concentration (1.31 ± 0.50 vs 2.27 ± 1.40 mg/L) and the area under the concentration-time curve from 0 to 8 h (AUC8h,plasma: 2.15 ± 0.42 vs 2.74 ± 0.73 h⋅mg/L), as normalized to a 100 mg dose, were significantly lower compared with those of non-obese patients. No significant differences were observed regarding the free plasma concentration, as determined by ultrafiltration, or the corresponding AUC8h (fAUC8h,plasma). Concentrations in interstitial fluid (ISF) of subcutaneous tissue were lower than the free plasma concentrations in both groups, and they were lower in obese compared to non-obese patients: the AUC8h in ISF (AUC8h,ISF) was 0.51 ± 0.22 h⋅mg/L in obese and 0.79 ± 0.23 h⋅mg/L in non-obese patients, resulting in a relative tissue drug exposure (AUC8h,ISF/fAUC8h,plasma) of 0.38 ± 0.19 and 0.63 ± 0.24, respectively. CONCLUSION: Following a single dose of tigecycline, concentrations in the ISF of subcutaneous adipose tissue are decreased in heavily obese subjects, calling for an increased loading dose. EU CLINICAL TRIALS REGISTRATION NUMBER: EudraCT No. 2012-004383-22.

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.

Meropenem Plasma and Interstitial Soft Tissue Concentrations in Obese and Nonobese Patients-A Controlled Clinical Trial.

BACKGROUND: This controlled clinical study aimed to investigate the impact of obesity on plasma and tissue pharmacokinetics of meropenem. METHODS: Obese (body mass index (BMI) ≥ 35 kg/m2) and age-/sex-matched nonobese (18.5 kg/m2 ≥ BMI ≤ 30 kg/m2) surgical patients received a short-term infusion of 1000-mg meropenem. Concentrations were determined via high performance liquid chromatography-ultraviolet (HPLC-UV) in the plasma and microdialysate from the interstitial fluid (ISF) of subcutaneous tissue up to eight h after dosing. An analysis was performed in the plasma and ISF by noncompartmental methods. RESULTS: The maximum plasma concentrations in 15 obese (BMI 49 ± 11 kg/m2) and 15 nonobese (BMI 24 ± 2 kg/m2) patients were 54.0 vs. 63.9 mg/L (95% CI for difference: -18.3 to -3.5). The volume of distribution was 22.4 vs. 17.6 L, (2.6-9.1), but the clearance was comparable (12.5 vs. 11.1 L/h, -1.4 to 3.1), leading to a longer half-life (1.52 vs. 1.31 h, 0.05-0.37) and fairly similar area under the curve (AUC)8h (78.7 vs. 89.2 mg*h/L, -21.4 to 8.6). In the ISF, the maximum concentrations differed significantly (12.6 vs. 18.6 L, -16.8 to -0.8) but not the AUC8h (28.5 vs. 42.0 mg*h/L, -33.9 to 5.4). Time above the MIC (T > MIC) in the plasma and ISF did not differ significantly for MICs of 0.25-8 mg/L. CONCLUSIONS: In morbidly obese patients, meropenem has lower maximum concentrations and higher volumes of distribution. However, due to the slightly longer half-life, obesity has no influence on the T > MIC, so dose adjustments for obesity seem unnecessary.

Linezolid Concentrations in Plasma and Subcutaneous Tissue are Reduced in Obese Patients, Resulting in a Higher Risk of Underdosing in Critically Ill Patients: A Controlled Clinical Pharmacokinetic Study.

Background: Linezolid is used for the treatment of soft tissue infections in critically ill patients. However, data for characterizing the pharmacokinetics (PK) and assessing whether effective concentrations are reached at the target site are lacking. We hypothesized that current dosing regimens do not lead to effective concentrations in the plasma and interstitial fluid (ISF) of subcutaneous tissue in obese patients. Methods: As a controlled clinical model, critically ill obese and non-obese patients undergoing intra-abdominal surgery received 600 mg linezolid as a single infusion. Concentrations in the plasma and microdialysate from the ISF of subcutaneous tissue were determined up to 8 h after dosing. Pharmacokinetic analysis was performed by non-compartmental methods. As a therapeutic target, we used fAUC/MIC > 80. Results: Fifteen obese (BMI: 48.7 ± 11.2 kg/m2) and 15 non-obese (23.9 ± 2.1 kg/m2) patients were analyzed. AUC0-8 in ISF decreased by -1.69 mg*h/L (95% CI: -2.59 to -0.79, p < 0.001) for every 10 kg increase in weight. PK in obese patients were characterized by lower maximal plasma concentrations (median 3.8 vs. 8.3 mg/L, p < 0.001) and a higher volume of distribution (41.0 vs. 30.8 L, p < 0.001), and the therapeutic target was not reached for MIC ≥ 1 mg/L in ISF and ≥ 2 mg/L in plasma. Conclusions: Increasing the weight led to a decrease of linezolid concentrations in the plasma and subcutaneous tissue. The current dosing regimen does not seem to produce sufficient concentrations to kill bacteria with MIC ≥ 2 mg/L, especially as empirical antimicrobial therapy in critically ill obese patients.

Repeated determination of moxifloxacin concentrations in interstitial space fluid of muscle and subcutis in septic patients.

BACKGROUND: For an effective antimicrobial treatment, it is crucial that antibiotics reach sufficient concentrations in plasma and tissue. Currently no data exist regarding moxifloxacin plasma concentrations and exposure levels in tissue under septic conditions. OBJECTIVES: To determine the pharmacokinetics of moxifloxacin in plasma and interstitial space fluid over a prolonged period. PATIENTS AND METHODS: Ten septic patients were treated with 400 mg of moxifloxacin once a day; on days 1, 3 and 5 of treatment plasma sampling and microdialysis in the subcutis and muscle of the upper thigh were performed to determine concentrations of moxifloxacin in different compartments. This trial was registered in the German Clinical Trials Register (DRKS, register number DRKS00012985). RESULTS: Mean unbound fraction of moxifloxacin in plasma was 85.5±3.4%. On day 1, Cmax in subcutis and muscle was 2.8±1.8 and 2.5±1.3 mg/L, respectively, AUC was 24.8±15.1 and 21.3±10.5 mg·h/L, respectively, and fAUC0-24/MIC was 100.9±62.9 and 86.5±38.3 h, respectively. Cmax for unbound moxifloxacin in plasma was 3.5±0.9 mg/L, AUC was 23.5±7.5 mg·h/L and fAUC0-24/MIC was 91.6±24.8 h. Key pharmacokinetic parameters on days 3 and 5 showed no significant differences. Clearance was higher than in healthy adults, but tissue concentrations were comparable, most likely due to a lower protein binding. CONCLUSIONS: Surprisingly, the first dose already achieved exposure comparable to steady-state conditions. The approved daily dose of 400 mg was adequate in our patient population. Thus, it seems that in septic patients a loading dose on the first day of treatment with moxifloxacin is not required.

Novel Population Pharmacokinetic Approach to Explain the Differences between Cystic Fibrosis Patients and Healthy Volunteers via Protein Binding.

The pharmacokinetics in patients with cystic fibrosis (CF) has long been thought to differ considerably from that in healthy volunteers. For highly protein bound ß-lactams, profound pharmacokinetic differences were observed between comparatively morbid patients with CF and healthy volunteers. These differences could be explained by body weight and body composition for ß-lactams with low protein binding. This study aimed to develop a novel population modeling approach to describe the pharmacokinetic differences between both subject groups by estimating protein binding. Eight patients with CF (lean body mass [LBM]: 39.8 ± 5.4kg) and six healthy volunteers (LBM: 53.1 ± 9.5kg) received 1027.5 mg cefotiam intravenously. Plasma concentrations and amounts in urine were simultaneously modelled. Unscaled total clearance and volume of distribution were 3% smaller in patients with CF compared to those in healthy volunteers. After allometric scaling by LBM to account for body size and composition, the remaining pharmacokinetic differences were explained by estimating the unbound fraction of cefotiam in plasma. The latter was fixed to 50% in male and estimated as 54.5% in female healthy volunteers as well as 56.3% in male and 74.4% in female patients with CF. This novel approach holds promise for characterizing the pharmacokinetics in special patient populations with altered protein binding.

Plasma and tissue pharmacokinetics of fosfomycin in morbidly obese and non-obese surgical patients: a controlled clinical trial.

OBJECTIVES: To assess the pharmacokinetics and tissue penetration of fosfomycin in obese and non-obese surgical patients. METHODS: Fifteen obese patients undergoing bariatric surgery and 15 non-obese patients undergoing major intra-abdominal surgery received an intravenous single short infusion of 8 g of fosfomycin. Fosfomycin concentrations were determined by LC-MS/MS in plasma and microdialysate from subcutaneous tissue up to 8 h after dosing. The pharmacokinetic analysis was performed in plasma and interstitial fluid (ISF) by non-compartmental methods. RESULTS: Thirteen obese patients (BMI 38-50 kg/m2) and 14 non-obese patients (BMI 0-29 kg/m2) were evaluable. The pharmacokinetics of fosfomycin in obese versus non-obese patients were characterized by lower peak plasma concentrations (468 ±âŸ139 versus 594 ±âŸ149 mg/L, P = 0.040) and higher V (24.4 ±âŸ6.4 versus 19.0 ±âŸ3.1 L, P = 0.010). The differences in AUC∞ were not significant (1275 ±âŸ477 versus 1515 ±âŸ352 mg·h/L, P = 0.16). The peak concentrations in subcutaneous tissue were reached rapidly and declined in parallel with the plasma concentrations. The drug exposure in tissue was nearly halved in obese compared with non-obese patients (AUC∞ 1052 ±âŸ394 versus 1929 ±âŸ725 mg·h/L, P = 0.0010). The tissue/plasma ratio (AUCISF/AUCplasma) was 0.86 ±âŸ0.32 versus 1.27 ±âŸ0.34 (P = 0.0047). CONCLUSIONS: Whereas the pharmacokinetics of fosfomycin in plasma of surgical patients were only marginally different between obese and non-obese patients, the drug exposure in subcutaneous tissue was significantly lower in the obese patients.

Determination of total or free cefazolin and metronidazole in human plasma or interstitial fluid by HPLC-UV for pharmacokinetic studies in man.

Cefazolin (CFZ) plus metronidazole (MTZ) is commonly used for perioperative antibiotic prophylaxis. An HPLC-UV method is described for the simultaneous determination of total or free cefazolin and metronidazole in human plasma or in microdialysate of subcutaneous tissue. Separation was performed isocratically using a reversed phase column and phosphate buffer/acetonitrile as mobile phase. The validation characteristics were similar for both drugs. Linearity has been shown down to 0.1 mg/L (R > 0.9990). Intra- and inter-assay precision (CV) and in-accuracy were < 5%. The method was applied to the determination of cefazolin and metronidazole in plasma and microdialysate of surgical patients following 30-min intravenous infusion of cefazolin/metronidazole 2.0/0.5 g.

Can systemically administered antibiotics be detected in wound tissues and surfaces under negative pressure wound therapy?

In this study, we evaluated a new aspect of negative pressure wound therapy (NPWT) as an analytical tool for pharmacokinetic studies. Twenty-one patients with soft tissue defects scheduled to receive NPWT were included in this study. Concomitant to NPWT, all patients received intravenous moxifloxacin (MX). At different time intervals, blood plasma levels of MX were sampled and compared with synchronous concentrations of MX in the exudate obtained from the NPWT drainage system. Serial measurements were performed upon initiation of the therapy as well as in the steady state (after 5 days). At steady state, wound tissue was obtained intraoperatively. High-performance liquid-chromatography (HPLC) was used for analysis. At 1 hour post-administration, the exudate/plasma levels (mg/L) were 1.92/3.07; at 12 hours, 0.80/1.14; at 24 hours, 0.26/0.43; and at 120 hours (steady state), 0.42/0.47. There was a correlation between exudate and plasma levels reaching approximately 0.75. Until now, methods for pharmacokinetic studies concerning interstitial fluid are difficult to apply in the clinical context. The presented method showed limitations, but we believe that, after methodological improvements, measurements of substances in the interstitial fluid by means of NPWT are feasible.

Determination of total and free ceftolozane and tazobactam in human plasma and interstitial fluid by HPLC-UV.

Ceftolozane/tazobactam is a new cephalosporin/beta-lactamase inhibitor combination. An HPLC-UV method is described for the determination of total and free ceftolozane and tazobactam in human plasma and in microdialysate of subcutaneous tissue, respectively. Separation was performed using a reversed-phase column with phosphate buffer/acetonitrile as eluent and photometric detection at 260 nm (ceftolozane) or 220 nm (tazobactam). Linearity has been shown down to ceftolozane/tazobactam 0.1/0.05 mg/L in plasma and 0.03/0.015 mg/L in saline, respectively. The plasma protein binding of both drugs as determined by ultrafiltration was less than 10%. Temperature, pH or relative centrifugation force (up to 3000 x g) had no significant impact on the protein binding. The method was applied to the determination of ceftolozane and tazobactam in plasma and interstitial fluid of healthy volunteers following intravenous infusion of ceftolozane/tazobactam 1.0/0.5 g.

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.

Differences in the renal antifibrotic cGMP/cGKI-dependent signaling of serelaxin, zaprinast, and their combination.

Renal fibrosis is an important factor for end-stage renal failure. However, only few therapeutic options for its treatment are established. Zaprinast, a phosphodiesterase 5 inhibitor, and serelaxin, the recombinant form of the naturally occurring hormone relaxin, are differently acting modulators of cyclic guanosine monophosphate (cGMP) signaling. Both agents enhance cGMP availability in kidney tissue. These substances alone or in combination might interfere with the development of kidney fibrosis. Therefore, we compared the effects of combination therapy with the effects of monotherapy on renal fibrosis. Renal fibrosis was induced by unilateral ureteral obstruction (UUO) for 7 days in wild-type (WT) and cGKI knockout (KO) mice. Renal antifibrotic effects were assessed after 7 days. In WT, zaprinast and the combination of zaprinast and serelaxin significantly reduced renal interstitial fibrosis assessed by α-SMA, fibronectin, collagen1A1, and gelatinases (MMP2 and MMP9). Intriguingly in cGKI-KO, mRNA and protein expression of fibronectin and collagen1A1 were reduced by zaprinast, in contrast to serelaxin. Gelatinases are not regulated by zaprinast. Although both substances showed similar antifibrotic properties in WT, they distinguished in their effect mechanisms. In contrast to serelaxin which acts both on Smad2 and Erk1, zaprinast did not significantly diminish Erk1/2 phosphorylation. Interestingly, the combination of serelaxin/zaprinast achieved no additive antifibrotic effects compared to the monotherapy. Due to antifibrotic effects of zaprinast in cGKI-KO, we hypothesize that additional cGKI-independent mechanisms are supposed for antifibrotic signaling of zaprinast.

Inhibition of the TGFß signalling pathway by cGMP and cGMP-dependent kinase I in renal fibrosis.

Agents that enhance production of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) ameliorate the progression of renal fibrosis. However, the molecular mechanism of this process is not fully understood. We hypothesize that the antifibrotic effects of cGMP and cGMP-dependent kinase I (cGKI) are mediated via regulation of the TGFß signalling pathway, both via ERK and the Smad-dependent route. Kidney fibrosis was induced by unilateral ureter obstruction (UUO) in wild-type and cGKI-deficient (cGKI-KO) mice. The cGMP/cGKI signalling pathway was activated by application of the soluble guanylate cyclase (sGC) stimulator BAY 41-8543 (BAY), beginning 1 day after UUO. After 7 days, the antifibrotic effects of BAY were analysed by measuring mRNA and protein expression of characteristic fibrotic biomarkers. The effects of cGMP/TGFß on cultured fibroblasts were also analysed in vitro. BAY application influenced the activity of the extracellular matrix (ECM)-degrading matrix metalloproteases (MMP2 and MMP9) and their inhibitor tissue inhibitors of metalloproteinase-1, the secretion of cytokines (e.g. IL-6) and the expression pattern of ECM proteins (e.g. collagen, fibronectin) and profibrotic mediators (e.g. connective tissue growth factors and plasminogen-activator inhibitor-1). Activation of the cGMP/cGKI signalling pathway showed protective effects against fibrosis which were mediated by inhibition of P-Erk1/2 and translocation of P-smad3. The elucidation of these signalling mechanisms might support the development of new therapeutic options regarding cGMP/cGKI-mediated antifibrotic actions.

Decreased protein binding of moxifloxacin in patients with sepsis?

The mean (SD) unbound fraction of moxifloxacin in plasma from patients with severe sepsis or septic shock was determined by ultrafiltration to 85.5±3.0% (range 81.9 and 91.6%) indicating a decreased protein binding of moxifloxacin in this population compared with the value of 58-60% provided in the Summary of Product Characteristics. However, previous investigations neglected the influence of pH and temperature on the protein binding of moxifloxacin. Maintaining physiological conditions (pH 7.4, 37°C) - as in the present study - the unbound fraction of moxifloxacin in plasma from healthy volunteers was 84%. In contrast, the unbound fraction of moxifloxacin was 77% at 4°C and 66-68% in unbuffered plasma or at pH 8.5 in fair agreement with previously published data. PK/PD parameters e.g. fAUC/MIC or ratios between interstitial fluid and free plasma concentrations, which were obtained assuming a protein binding rate of moxifloxacin of 40% or more, should be revised.

Unbound fraction of fluconazole and linezolid in human plasma as determined by ultrafiltration: Impact of membrane type.

Ultrafiltration is a rapid and convenient method to determine the free concentrations of drugs in plasma. Several ultrafiltration devices based on Eppendorf cups are commercially available, but are not validated for such use by the manufacturer. Plasma pH, temperature and relative centrifugal force as well as membrane type can influence the results. In the present work, we developed an ultrafiltration method in order to determine the free concentrations of linezolid or fluconazole, both neutral and moderately lipophilic antiinfective drugs for parenteral as well as oral administration, in plasma of patients. Whereas both substances behaved relatively insensitive in human plasma regarding variations in pH (7.0-8.5), temperature (5-37°C) or relative centrifugal force (1000-10.000xg), losses of linezolid were observed with the Nanosep Omega device due to adsorption onto the polyethersulfone membrane (unbound fraction 75% at 100mg/L and 45% at 0.1mg/L, respectively). No losses were observed with Vivacon which is equipped with a membrane of regenerated cellulose. With fluconazole no differences between Nanosep and Vivacon were observed. Applying standard conditions (pH 7.4/37°C/1000xg/20min), the mean unbound fraction of linezolid in pooled plasma from healthy volunteers was 81.5±2.8% using Vivacon, that of fluconazole was 87.9±3.5% using Nanosep or 89.4±3.3% using Vivacon. The unbound fraction of linezolid was 85.4±3.7% in plasma samples from surgical patients and 92.1±6.2% in ICU patients, respectively. The unbound fraction of fluconazole was 93.9±3.3% in plasma samples from ICU patients.

Variable Linezolid Exposure in Intensive Care Unit Patients-Possible Role of Drug-Drug Interactions.

BACKGROUND: Standard doses of linezolid may not be suitable for all patient groups. Intensive care unit (ICU) patients in particular may be at risk of inadequate concentrations. This study investigated variability of drug exposure and its potential sources in this population. METHODS: Plasma concentrations of linezolid were determined by high-performance liquid chromatography in a convenience sample of 20 ICU patients treated with intravenous linezolid 600 mg twice daily. Ultrafiltration applying physiological conditions (pH 7.4/37°C) was used to determine the unbound fraction. Individual pharmacokinetic (PK) parameters were estimated by population PK modeling. As measures of exposure to linezolid, area under the concentration-time curve (AUC) and trough concentrations (Cmin) were calculated and compared with published therapeutic ranges (AUC 200-400 mg*h/L, Cmin 2-10 mg/L). Coadministered inhibitors or inducers of cytochrome P450 and/or P-glycoprotein were noted. RESULTS: Data from 18 patients were included into the PK evaluation. Drug exposure was highly variable (median, range: AUC 185, 48-618 mg*h/L, calculated Cmin 2.92, 0.0062-18.9 mg/L), and only a minority of patients had values within the target ranges (6 and 7, respectively). AUC and Cmin were linearly correlated (R = 0.98), and classification of patients (underexposed/within therapeutic range/overexposed) according to AUC or Cmin was concordant in 15 cases. Coadministration of inhibitors was associated with a trend to higher drug exposure, whereas 3 patients treated with levothyroxine showed exceedingly low drug exposure (AUC ∼60 mg*h/L, Cmin <0.4 mg/L). The median unbound fraction in all 20 patients was 90.9%. CONCLUSIONS: Drug exposure after standard doses of linezolid is highly variable and difficult to predict in ICU patients, and therapeutic drug monitoring seems advisable. PK drug-drug interactions might partly be responsible and should be further investigated; protein binding appears to be stable and irrelevant.