Protein Binding Determination of Cefepime, Flucloxacillin, Meropenem, Piperacillin and Tazobactam by Ultrafiltration in Critically Ill Patients: The Choice of Device Matters

Therapeutic drug monitoring (TDM) and pharmacokinetic studies of beta-lactam antibiotics in critically ill patients aim to define optimal dosing and require accurate measurement of unbound drug concentrations. This study compared the performance of two ultrafiltration devices, Centrifree and Amicon, for measuring unbound concentrations of cefepime, meropenem, flucloxacillin, piperacillin and tazobactam. Non-specific binding (NSB) was specifically assessed in phosphate-buffered saline, and unbound concentrations were measured using a chromatographic method in both spiked plasma and plasma samples from critically ill patients, in order to compare the two ultrafiltration devices. Centrifree exhibited no significant NSB for any antibiotics (≤ 15%), while Amicon displayed significant NSB for meropenem (26–61%), flucloxacillin (18–58%) and piperacillin (15–72%), resulting in lower unbound concentrations in both patient and spiked plasma samples compared to Centrifree. Additionally, piperacillin, and to a lesser extent flucloxacillin, demonstrated concentration-dependent NSB with Amicon. The comparative mean ± standard deviation of unbound fractions in intensive care unit (ICU) patients were as follows: cefepime, 0.85 ± 0.12 (Centrifree) versus 0.90 ± 0.10 (Amicon); meropenem, 0.87 ± 0.07 (Centrifree) versus 0.77 ± 0.05 (Amicon); flucloxacillin, 0.25 ± 0.13 (Centrifree) versus 0.18 ± 0.13 (Amicon); piperacillin, 0.91 ± 0.07 (Centrifree) versus 0.67 ± 0.13 (Amicon); and tazobactam, 0.97 ± 0.08 (Centrifree) versus 0.95 ± 0.08 (Amicon). These findings underscore significant differences in results between ultrafiltration devices, depending on the drug, influenced by NSB. Such factors should be considered when developing methods for determining unbound concentrations of beta-lactam antibiotics.