Tissue Penetration of Antibiotics in CriTically Ill Children

Pathophysiological alterations during critical illness, such as severe sepsis and septic shock, frequently alter the pharmacokinetics (PK) and pharmacodynamics (PD) of beta-lactam antibiotics. In pediatric critical care, these changes are further complicated by ongoing physiological growth and developmental changes in drug absorption, distribution, metabolism, and excretion. Although research in critically ill children has demonstrated that standard intravenous dosing regimens often fail to achieve target plasma concentrations, plasma levels alone do not necessarily reflect drug disposition at the actual site of infection. For extracellular pathogens, the target site of infection corresponds to the interstitial fluid (ISF) of tissues. Studies in adult septic populations suggest that subtherapeutic antibiotic concentrations in ISF may contribute to treatment failure, even when total plasma concentrations appear adequate. Data regarding antibiotic tissue disposition in pediatric intensive care unit (PICU) patients remain extremely limited. This monocentric, prospective, observational pharmacokinetic study utilizes in vivo microdialysis to directly quantify unbound tissue concentrations of three hydrophilic beta-lactam antibiotics-piperacillin-tazobactam, meropenem, and amoxicillin-clavulanic acid-in the ISF of skeletal muscle in critically ill pediatric patients.

The study will be conducted as follows:

• Microdialysis Probe Insertion and Placement: Under continuous analgosedation, a 63 Microdialysis Catheter (M Dialysis AB, Solna, Sweden; 20 kDa molecular weight cut-off, 10 mm membrane length) is inserted into the vastus lateralis muscle of the anterolateral thigh using a steel guidance needle.
• Perfusion and Calibration: The probe is continuously perfused with 0.9% physiological saline at a fixed flow rate via microinfusion precision pumps. After an equilibration period, in vivo probe recovery is determined using both retrodialysis and internal standard methods:

• Retrodialysis: Performed prior to drug administration by perfusing the target antibiotic through the probe (2 x 30-minute intervals) to calculate relative recovery based on drug loss across the membrane.
• Internal Standard Method: An internal calibrator (penicillin G for piperacillin-tazobactam and amoxicillin-clavulanic acid; cefuroxime for meropenem) is added to the perfusate solution to continuously monitor recovery throughout the experiment.
• Biological Sampling: Following calibration, dialysate fractions are collected over designated time intervals across one or two antibiotic dosing cycles (representing early-dose and steady-state conditions). Paired blood samples are drawn concurrently from an existing intra-arterial or central venous catheter to quantify total plasma antibiotic levels. Unbound fractions in plasma are separated using ultrafiltration.
• Analytical and Pharmacokinetic Modeling: Microdialysate and plasma drug concentrations are measured using high-performance liquid chromatography-mass spectrometry (HPLC-MS). Population pharmacokinetic modeling (non-linear mixed-effects modeling) and Monte Carlo simulations will be applied to derive PK parameters and evaluate PK/PD target attainment in the ISF in muscle.

Inclusion criteria

• Patient admitted to the Pediatric Intensive Care Unit (PICU).
• Age: 1 month to 15 years.
• Receiving intravenous antibiotic treatment with piperacillin-tazobactam, meropenem, or amoxicillin-clavulanic acid.
• Receiving continuous analgosedation (required for microdialysis catheter insertion; if analgosedation is stopped after insertion, the catheter may remain in place).
• Intra-arterial (preferred) or intravenous access available for blood sampling.

Exclusion criteria

• Presence of coagulopathy and/or thrombocytopenia:

• Platelet count < 50,000/µL
• International Normalized Ratio (INR) > 1.5
• Activated partial thromboplastin time (aPTT) > 1.5 x upper limit of normal value (age-specific thresholds: > 50 sec for 30-90 days; > 42 sec for 90-180 days; > 38 sec for > 180 days)
• Personal or family history of excessive bleeding.
• Current treatment with coumarin derivative anticoagulants.
• Known hypersensitivity to beta-lactam antibiotics.
• Pregnancy.
• Absence of parental (or legal guardian) informed consent.
• Absence of suitable catheter for blood sampling.