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.

A Population Pharmacokinetics and Pharmacodynamic Approach To Optimize Tazobactam Activity in Critically Ill Patients

The percentage of the time that the free drug concentration remains above a concentration threshold (%fT > concentration threshold) has frequently been identified to be the optimal pharmacokinetic (PK)-pharmacodynamic (PD) target of interest for tazobactam using in vitro infection models. Similar in vitro models suggested that an 85% fT > concentration threshold of 2 μg/ml for tazobactam is required to demonstrate a 2-log10-unit decrease in the number of CFU per milliliter from that at the baseline at 24 h for high-level β-lactamase-producing Escherichia coli strains. The objective of this study was to characterize the tazobactam concentrations in a cohort of critically ill patients with Gram-negative bacterial infections, determine if traditional dosing regimens achieve a prespecified PK/PD target of an 80% fT > concentration threshold of 2 μg/ml, and propose alternative dosing regimens. Hospitalized critically ill adult patients receiving piperacillin-tazobactam (TZP) for a culture-positive Gram-negative bacterial infection were eligible to consent for study inclusion. Two blood samples were drawn, one during the midpoint of the dosing interval and one at the time of the trough concentration once the patient achieved PK steady state. A population PK model was developed using Phoenix NLME (v8.1) software to characterize the observed concentration-time profile of tazobactam, explore potential covariates to explain the variability in the clearance and volume parameters, and to simulate potential dosing regimens that would achieve the PK/PD target. The PK of tazobactam were adequately described by a one-compartment model with first-order elimination in 18 patients who provided consent. The final model incorporated creatinine clearance as a covariate on clearance. Simulations demonstrated target attainments of less than 50% for tazobactam using traditional dosing regimens (4/0.5 g over 30 min every 6 h). Target attainments of greater than 75% were achieved when using extended infusion times of 4 to 6 h or when administering TZP as a continuous infusion (16/2 g over 24 h). Traditional tazobactam dosing regimens fail to achieve conservative PK/PD targets in critically ill patients. Increases in the tazobactam dose or prolongation of the infusion rate may be warranted to achieve activity against β-lactamase-producing Gram-negative bacteria.

After standard dosage of piperacillin plasma concentrations of drug are subtherapeutic in burn patients

Infections are a major problem in patients with burn diseases. Mortality is high despite antibiotic therapy as studies are controversial concerning drug underdosing. The aims of this prospective, observational study were to monitor plasma concentrations of piperacillin during standard piperacillin/tazobactam treatment in 20 burn patients and 16 controls from the intensive care unit (ICU) and to optimize doses by in silico analyses. Piperacillin/tazobactam (4/0.5 g, tid) was administered over 0.5 h. Blood samples were taken at 1, 4, and 7.5 h after the end of the infusion. Free piperacillin plasma concentrations were determined. Pharmacokinetic parameters and in silico analysis results were calculated using the freeware TDMx. The primary target was defined as percentage of the day (fT_>1xMIC; fT_>4xMIC) when piperacillin concentrations exceeded 1xMIC/4xMIC (minimum inhibitory concentration), considering a MIC breakpoint of 16 mg/L for Pseudomonas aeruginosa. In an off-label approach, two burn patients were treated with 8/1 g piperacillin/tazobactam, 3 h qid. fT_>1xMIC (55 ± 22% vs. 77 ± 24%) and fT_>4xMIC (17 ± 11% vs. 30 ± 11%) were lower in burn than in ICU patients after 4/0.5 g, 0.5 h, tid. In silico analyses indicated that fT_>1xMIC (93 ± 12% burn, 97 ± 4% ICU) and fT_>4xMIC (62 ± 23% burn, 84 ± 19% ICU) values increase by raising the piperacillin dosage to 8/1 g qid and prolonging the infusion time to 3 h. Off-label treatment results were similar to in silico data for burn patients (84%fT_>1xMIC and 47%fT_>4xMIC). Standard dosage regimens for piperacillin/tazobactam resulted in subtherapeutic piperacillin concentrations in burn and ICU patients. Dose adjustments via in silico analyses can help to optimize antibiotic therapy and to predict respective concentrations in vivo. Trial registration: NCT03335137, registered 07.11.2017, retrospectively.