• Heather E Vezina, Chee M Ng, Delia M Vazquez, John D Barks, and Varsha Bhatt-Mehta.
• 1Department of Clinical Pharmacology and Therapeutics, The Children's Hospital of Philadelphia, Philadelphia, PA. 2Department of Pediatrics, School of Medicine, University of Pennsylvania, Philadelphia, PA. 3Department of Pediatrics and Communicable Diseases, School of Medicine, University of Michigan, Ann Arbor, MI. 4Department of Clinical, Social and Administrative Sciences, College of Pharmacy, University of Michigan, Ann Arbor, MI.
• Pediatr Crit Care Me. 2014 Jul 1; 15 (6): 546-53.

ObjectivesTo determine the population pharmacokinetics of unbound hydrocortisone in critically ill neonates and infants receiving IV hydrocortisone for treatment of vasopressor-resistant hypotension and to identify patient-specific sources of pharmacokinetic variability.DesignProspective observational cohort study.SettingLevel 3 neonatal ICU.PatientsSixty-two critically ill neonates and infants receiving IV hydrocortisone as part of standard of care for the treatment of vasopressor-resistant hypotension: median gestational age 28 weeks (range, 23-41), median weight 1.2 kg (range, 0.5-4.4), and 29 females.InterventionsNone.Measurements And Main ResultsUnbound baseline cortisol and postdose hydrocortisone concentrations measured from blood samples being drawn for routine laboratory tests. A one-compartment model best described the data. Allometric weight and postmenstrual age were significant covariates on unbound hydrocortisone clearance and volume of distribution. Final population estimates for clearance, volume of distribution, and baseline cortisol concentration were 20.2 L/hr, 244 L, and 1.37 ng/mL, respectively. Using the median weight and postmenstrual age of our subjects (i.e., 1.2 kg and 28 wk) in the final model, the typical unbound hydrocortisone clearance and volume of distribution were 1.0 L/hr and 4.2 L, respectively. The typical half-life for unbound hydrocortisone was 2.9 hours. A sharp and continuous increase in unbound hydrocortisone clearance was observed at 35 weeks postmenstrual age.ConclusionsWe report the first pharmacokinetic data for unbound hydrocortisone, the pharmacologically active moiety, in critically ill neonates and infants with vasopressor-resistant hypotension. Unbound hydrocortisone clearance increased with body weight and was faster in children with an older postmenstrual age. Unbound hydrocortisone clearance increased sharply at 35 weeks postmenstrual age and continued to mature thereafter. This study lays the groundwork for evaluating unbound hydrocortisone exposure-response relationships and drawing definitive conclusions about the dosing of IV hydrocortisone in critically ill neonates and infants with vasopressor-resistant hypotension.

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