Mechanisms of antimicrobial-induced nephrotoxicity in children.

Drug-induced nephrotoxicity is responsible for 20% to 60% of cases of acute kidney injury in hospitalized patients and is associated with increased morbidity and mortality in both children and adults. Antimicrobials are one of the most common classes of medications prescribed globally and also among the most common causes of nephrotoxicity. A broad range of antimicrobial agents have been associated with nephrotoxicity, but the features of kidney injury vary based on the agent, its mechanism of injury and the site of toxicity within the kidney. Distinguishing nephrotoxicity caused by an antimicrobial agent from other potential inciting factors is important to facilitate both early recognition of drug toxicity and prompt cessation of an offending drug, as well as to avoid unnecessary discontinuation of an innocuous therapy. This review will detail the different types of antimicrobial-induced nephrotoxicity: acute tubular necrosis, acute interstitial nephritis and obstructive nephropathy. It will also describe the mechanism of injury caused by specific antimicrobial agents and classes (vancomycin, aminoglycosides, polymyxins, antivirals, amphotericin B), highlight the toxicodynamics of these drugs and provide guidance on administration or monitoring practices that can mitigate toxicity, when known. Particular attention will be paid to paediatric patients, when applicable, in whom nephrotoxin exposure is an often-underappreciated cause of kidney injury.

Effect of Cystatin C on Vancomycin Clearance Estimation in Critically Ill Children Using a Population Pharmacokinetic Modeling Approach.

BACKGROUND: Vancomycin is eliminated by glomerular filtration, but current approaches to estimate kidney function in children are unreliable. The authors sought to compare the suitability of cystatin C (CysC)-based glomerular filtration rate equations with the most commonly used creatinine-based equation, bedside Schwartz, to estimate vancomycin clearance (CL). METHODS: This prospective observational study enrolled critically ill patients (2-18 years) receiving intravenous vancomycin at the Children's Hospital of Philadelphia during December 2015-November 2017. Vancomycin levels were collected during clinical care and at 3 times during a single dosing interval. Plasma CysC was measured within 24 hours before intravenous vancomycin (baseline) initiation or immediately after enrollment and along with the third pharmacokinetic sample. Nonlinear mixed effects modeling was performed using NONMEM software. Covariate selection was used to test model fit with inclusion of the estimated glomerular filtration rate (eGFR) on CL using bedside Schwartz versus various published CysC-based equations. RESULTS: In total, 83 vancomycin levels were obtained from 20 children. The median age was 12.7 years; 6 patients were women. A 1-compartment model best described the data; CL was allometrically scaled to 0.75. During covariate selection, inclusion of the eGFR calculated using a CysC-based equation significantly improved model fit [reduction in objective function value (OFV) range: -17.191 to -18.704] than bedside Schwartz ([INCREMENT]OFV -12.820). Including the full age spectrum equation, an eGFR equation based on both creatinine and CysC, led to the largest OFV reduction (-22.913); female sex was also a significant covariate of CL in the model. Final model pharmacokinetic indices were CL = 0.29 L/h/kg and volume of distribution = 0.48 L/kg. CONCLUSIONS: CysC-based equations help better estimate vancomycin CL than bedside Schwartz in critically ill children.

Midazolam Dose Optimization in Critically Ill Pediatric Patients With Acute Respiratory Failure: A Population Pharmacokinetic-Pharmacogenomic Study.

OBJECTIVES: To develop a pharmacokinetic-pharmacogenomic population model of midazolam in critically ill children with primary respiratory failure. DESIGN: Prospective pharmacokinetic-pharmacogenomic observational study. SETTING: Thirteen PICUs across the United States. PATIENTS: Pediatric subjects mechanically ventilated for acute respiratory failure, weight greater than or equal to 7 kg, receiving morphine and/or midazolam continuous infusions. INTERVENTIONS: Serial blood sampling for drug quantification and a single blood collection for genomic evaluation. MEASUREMENTS AND MAIN RESULTS: Concentrations of midazolam, the 1' (1`-hydroxymidazolam metabolite) and 4' (4`-hydroxymidazolam metabolite) hydroxyl, and the 1' and 4' glucuronide metabolites were measured. Subjects were genotyped using the Illumina HumanOmniExpress genome-wide single nucleotide polymorphism chip. Nonlinear mixed effects modeling was performed to develop the pharmacokinetic-pharmacogenomic model. Body weight, age, hepatic and renal functions, and the UGT2B7 rs62298861 polymorphism are relevant predictors of midazolam pharmacokinetic variables. The estimated midazolam clearance was 0.61 L/min/70kg. Time to reach 50% complete mature midazolam and 1`-hydroxymidazolam metabolite/4`-hydroxymidazolam metabolite clearances was 1.0 and 0.97 years postmenstrual age. The final model suggested a decrease in midazolam clearance with increase in alanine transaminase and a lower clearance of the glucuronide metabolites with a renal dysfunction. In the pharmacogenomic analysis, rs62298861 and rs28365062 in the UGT2B7 gene were in high linkage disequilibrium. Minor alleles were associated with a higher 1`-hydroxymidazolam metabolite clearance in Caucasians. In the pharmacokinetic-pharmacogenomic model, clearance was expected to increase by 10% in heterozygous and 20% in homozygous for the minor allele with respect to homozygous for the major allele. CONCLUSIONS: This work leveraged available knowledge on nonheritable and heritable factors affecting midazolam pharmacokinetic in pediatric subjects with primary respiratory failure requiring mechanical ventilation, providing the basis for a future implementation of an individual-based approach to sedation.

Morphine Dose Optimization in Critically Ill Pediatric Patients With Acute Respiratory Failure: A Population Pharmacokinetic-Pharmacogenomic Study.

OBJECTIVE: To develop a pharmacokinetic-pharmacogenomic population model of morphine in critically ill children with acute respiratory failure. DESIGN: Prospective pharmacokinetic-pharmacogenomic observational study. SETTING: Thirteen PICUs across the United States. PATIENTS: Pediatric subjects (n = 66) mechanically ventilated for acute respiratory failure, weight greater than or equal to 7 kg, receiving morphine and/or midazolam continuous infusions. INTERVENTIONS: Serial blood sampling for drug quantification and a single blood collection for genomic evaluation. MEASUREMENTS AND MAIN RESULTS: Concentrations of morphine, the two main metabolites, morphine-3-glucuronide and morphine-6-glucuronide, were quantified by high-performance liquid chromatography tandem mass spectrometry/mass spectroscopy. Subjects were genotyped using the Illumina HumanOmniExpress genome-wide single nucleotide polymorphism chip. Nonlinear mixed-effects modeling was performed to develop the pharmacokinetic-pharmacogenomic model. A two-compartment model with linear elimination and two individual compartments for metabolites best describe morphine disposition in this population. Our analysis demonstrates that body weight and postmenstrual age are relevant predictors of pharmacokinetic parameters of morphine and its metabolites. Furthermore, our research shows that a duration of mechanical ventilation greater than or equal to 10 days reduces metabolite formation and elimination upwards of 30%. However, due to the small sample size and relative heterogeneity of the population, no heritable factors associated with uridine diphosphate glucuronyl transferase 2B7 metabolism of morphine were identified. CONCLUSIONS: The results provide a better understanding of the disposition of morphine and its metabolites in critically ill children with acute respiratory failure requiring mechanical ventilation due to nonheritable factors. It also provides the groundwork for developing additional studies to investigate the role of heritable factors.

A Population Pharmacokinetic Analysis to Study the Effect of Extracorporeal Membrane Oxygenation on Cefepime Disposition in Children.

OBJECTIVES: Limited data exist on the effects of extracorporeal membrane oxygenation on pharmacokinetics of cefepime in critically ill pediatric patients. The objective was to describe cefepime disposition in children treated with extracorporeal membrane oxygenation using population pharmacokinetic modeling. DESIGN: Multicenter, prospective observational study. SETTING: The pediatric and cardiac ICUs of six sites of the Collaborative Pediatric Critical Care Research Network. PATIENTS: Seventeen critically ill children (30 d to < 2 yr old) on extracorporeal membrane oxygenation who received cefepime as standard of care between January 4, 2014, and August 24, 2015, were enrolled. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: A pharmacokinetic model was developed to evaluate cefepime disposition differences due to extracorporeal membrane oxygenation. A two-compartment model with linear elimination, weight effects on clearance, intercompartmental clearance (Q), central volume of distribution (V1), and peripheral volume of distribution (V2) adequately described the data. The typical value of clearance in this study was 7.1 mL/min (1.9 mL/min/kg) for a patient weighing 5.8 kg. This value decreased by approximately 40% with the addition of renal replacement therapy. The typical value for V1 was 1,170 mL. In the setting of blood transfusions, V1 increased by over two-fold but was reduced with increasing age of the extracorporeal membrane oxygenation circuit oxygenator. CONCLUSIONS: Cefepime clearance was reduced in pediatric patients treated with extracorporeal membrane oxygenation compared with previously reported values in children not receiving extracorporeal membrane oxygenation. The model demonstrated that the age of the extracorporeal membrane oxygenation circuit oxygenator is inversely correlated to V1. For free cefepime, only 14 of the 19 doses (74%) demonstrated a fT_minimum inhibitory concentration of 16 mg/L, an appropriate target for the treatment of pseudomonal infections, for greater than 70% of the dosing interval. Pediatric patients on extracorporeal membrane oxygenation might benefit from the addition of therapeutic drug monitoring of cefepime to assure appropriate dosing.

Cytochrome P450 and flavin-containing monooxygenase families: age-dependent differences in expression and functional activity.

BackgroundAge-dependent differences in pharmacokinetics exist for metabolically cleared medications. Differential contributions in the cytochrome P450 3A (CYP3A), CYP2C, and flavin-containing monooxygenases (FMOs) families have an important role in the metabolic clearance of a large number of drugs administered to children.MethodsUnlike previous semiquantitative characterization of age-dependent changes in the expression of genes and proteins (western blot analysis), this study quantifies both gene and absolute protein expression in the same fetal, pediatric, and adult hepatic tissue. Expression was then correlated with the corresponding functional activities in the same samples.ResultsCYP3A and FMO families showed a distinct switch from fetal (CYP3A7 and FMO1) to adult isoforms (CYP3A4 and FMO3) at birth, whereas CYP2C9 showed a linear maturation from birth into adulthood. In contrast, analysis of CYP2C19 revealed higher expression and catalytic efficiency in pediatric samples compared with that in fetal and adult samples. Further, CYP3A and FMO enzymes exhibited an unexpectedly higher functional activity in fetal samples not entirely explained by protein expression.ConclusionThese surprising findings suggest that CYP and FMO enzymes may encounter development-related differences in their microenvironments that can influence the enzyme activity in addition to protein expression levels.

A Population Pharmacokinetic Analysis to Study the Effect of Therapeutic Hypothermia on Vancomycin Disposition in Children Resuscitated From Cardiac Arrest.

OBJECTIVES: Limited data exist on the effects of therapeutic hypothermia on renal function and pharmacokinetics in pediatric patients after cardiac arrest. The objective was to describe the differences in vancomycin disposition in pediatric patients following cardiac arrest treated with either therapeutic hypothermia or normothermia using population pharmacokinetic modeling. DESIGN: Single-center, retrospective cohort study. SETTING: A tertiary care hospital pediatric and cardiac ICU. PATIENTS: Fifty-two pediatric patients (30 d to 17 yr old) who experienced a cardiac arrest, received vancomycin, and were treated with therapeutic hypothermia (32-34°C) or normothermia (36.3-37.6°C) between January 1, 2010, and September 30, 2014, were reviewed. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: A two-compartment model with linear elimination, weight effects on clearance, intercompartmental clearance (Q), central volume of distribution (V1), and peripheral volume of distribution (V2) adequately described the data despite high variability due to the small sample size. The typical value of clearance in this study was 4.48 L/hr (0.19 L/hr/kg) for a normothermic patient weighing 70 kg and a glomerular filtration rate of 90 mL/min/1.73 m. Patients treated with normothermia but with reduced or poor renal function (≤ 90 mL/min/1.73 m) had up to an 80% reduction in vancomycin clearance compared to those with normal renal function (90-140 mL/min/1.73 m). Patients with normal renal function but treated with therapeutic hypothermia versus normothermia experienced up to 25% reduction in vancomycin clearance. Patients treated with therapeutic hypothermia and with poor renal function experienced up to an 84% reduction in vancomycin clearance. CONCLUSIONS: Patients receiving hypothermia and/or with decreased renal function had lower vancomycin clearances based on a retrospectively fitted two-compartment model in children who experience cardiac arrest.

Quantification of Flavin-containing Monooxygenases 1, 3, and 5 in Human Liver Microsomes by UPLC-MRM-Based Targeted Quantitative Proteomics and Its Application to the Study of Ontogeny.

Flavin-containing monooxygenases (FMOs) have a significant role in the metabolism of small molecule pharmaceuticals. Among the five human FMOs, FMO1, FMO3, and FMO5 are the most relevant to hepatic drug metabolism. Although age-dependent hepatic protein expression, based on immunoquantification, has been reported previously for FMO1 and FMO3, there is very little information on hepatic FMO5 protein expression. To overcome the limitations of immunoquantification, an ultra-performance liquid chromatography (UPLC)-multiple reaction monitoring (MRM)-based targeted quantitative proteomic method was developed and optimized for the quantification of FMO1, FMO3, and FMO5 in human liver microsomes (HLM). A post-in silico product ion screening process was incorporated to verify LC-MRM detection of potential signature peptides before their synthesis. The developed method was validated by correlating marker substrate activity and protein expression in a panel of adult individual donor HLM (age 39-67 years). The mean (range) protein expression of FMO3 and FMO5 was 46 (26-65) pmol/mg HLM protein and 27 (11.5-49) pmol/mg HLM protein, respectively. To demonstrate quantification of FMO1, a panel of fetal individual donor HLM (gestational age 14-20 weeks) was analyzed. The mean (range) FMO1 protein expression was 7.0 (4.9-9.7) pmol/mg HLM protein. Furthermore, the ontogenetic protein expression of FMO5 was evaluated in fetal, pediatric, and adult HLM. The quantification of FMO proteins also was compared using two different calibration standards, recombinant proteins versus synthetic signature peptides, to assess the ratio between holoprotein versus total protein. In conclusion, a UPLC-MRM-based targeted quantitative proteomic method has been developed for the quantification of FMO enzymes in HLM.

Pharmacometric dose optimization of buprenorphine in neonatal opioid withdrawal syndrome.

Results from Blinded Buprenorphine OR Neonatal morphine solution (BBORN), a previous phase III trial in infants with neonatal opioid withdrawal syndrome (NOWS), demonstrated that sublingual buprenorphine resulted in a shorter duration of treatment and shorter length of hospital stay than the comparator, oral morphine. Objectives of Buprenorphine Pharmacometric Open Label Research study of Drug Exposure (BPHORE), a new trial with buprenorphine in a similar population, were to (1) optimize initial dose, up-titration to achieve symptom control and weaning steps of pharmacologic treatment and (2) investigate safety of the revised regimen. A pharmacodynamic model linked buprenorphine exposure to NOWS symptom scores. Adaptive dose regimens were simulated using BBORN results to compare dosing regimens for times to stabilization, weaning, and cessation. A clinical trial using model informed doses (BPHORE), was conducted. Simulations indicated benefits in time to stabilization and weaning when up-titration rates increased to 30%. Stabilization time was not greatly impacted by the starting dose. Time to wean and time to cessation were dose dependent. A weaning rate of 25% shortened time to cessation. Ten infants were enrolled in BPHORE using buprenorphine starting dose of 24 µg/kg/day, 33% titration, and 15% wean rate. Five subjects required adjuvant therapy. Half-maximal effective concentration (EC50 ) values indicated maximum buprenorphine doses did not generate maximal effect size, suggesting potential efficacy of a further increased dose if a goal was to reduce the use of adjunct agents. Simulations indicated that further benefits can be gained by increasing starting doses of buprenorphine and increasing wean rates. Use of a model-based analysis to provide focused guidelines for care can be used with goals of reducing treatment time and hospital stays in infants with NOWS.

Administration and Dosing of Systemic Antifungal Agents in Pediatric Patients.

Neonates and immunosuppressed/immunocompromised pediatric patients are at high risk of invasive fungal diseases. Appropriate antifungal selection and optimized dosing are imperative to the successful prevention and treatment of these life-threatening infections. Conventional amphotericin B was the mainstay of antifungal therapy for many decades, but dose-limiting nephrotoxicity and infusion-related adverse events impeded its use. Despite the development of several new antifungal classes and agents in the past 20 years, and their now routine use in at-risk pediatric populations, data to guide the optimal dosing of antifungals in children are limited. This paper reviews the spectra of activity for approved antifungal agents and summarizes the current literature specific to pediatric patients regarding pharmacokinetic/pharmacodynamic data, dosing, and therapeutic drug monitoring.

Development and validation of a volumetric absorptive microsampling- liquid chromatography mass spectrometry method for the analysis of cefepime in human whole blood: Application to pediatric pharmacokinetic study.

Cefepime is a fourth-generation cephalosporin antibiotic with an extended spectrum of activity against many Gram-positive and Gram-negative bacteria. There is a growing need to develop sensitive, small volume assays, along with less invasive sample collection to facilitate pediatric pharmacokinetic clinical trials and therapeutic drug monitoring. The volumetric absorptive microsampling (VAMS™) approach provides an accurate and precise collection of a fixed volume of blood (10 µL), reducing or eliminating the volumetric blood hematocrit assay-bias associated with the dried blood spotting technique. We developed a high-performance liquid chromatographic method with tandem mass spectrometry detection for quantification of cefepime. Sample extraction from VAMS™ devices, followed by reversed-phase chromatographic separation and selective detection using tandem mass spectrometry with a 4 min runtime per sample was employed. Standard curves were linear between 0.1-100 µg/mL for cefepime. Intra- and inter-day accuracies were within 95.4-113% and precision (CV) was < 15 % based on a 3-day validation study. Recoveries ranged from 40.8 to 62.1% and the matrix effect was within 89.5-96.7% for cefepime. Cefepime was stable in human whole blood under assay conditions (3 h at room temperature, 24 h in autosampler post-extraction). Cefepime was also stable for at least 1 week (7 days) at 4 °C, 1 month (39 days) at -20 °C and 3 months (91 days) at -78 °C as dried microsamples. This assay provides an efficient quantitation of cefepime and was successfully implemented for the analysis of whole blood microsamples in a pediatric clinical trial.

Development and validation of a volumetric absorptive microsampling assay for analysis of voriconazole and voriconazole N-oxide in human whole blood.

Voriconazole is a broad-spectrum antifungal triazole drug for the treatment of invasive fungal infections. It is extensively metabolized by hepatic drug metabolizing enzymes cytochrome (CYP) 2C19 and CYP3A4. Selective inhibition of intestinal CYP3A4 by grapefruit juice may increase the oral bioavailability of voriconazole in children. To test this hypothesis it is necessary to develop a sensitive assay for measuring voriconazole and its major metabolites in a small volume of blood. Mitra® devices from Neoteryx were employed to develop and validate the assay for the quantitation of voriconazole and voriconazole N-oxide. Mitra® devices utilize volumetric absorptive microsampling (VAMS™) technology that enables accurate and precise collection of a fixed volume (10 µL of blood), reducing or eliminating the volumetric blood hematocrit assay-bias associated with the dried blood spotting technique. We developed an ultra-performance liquid chromatographic method with tandem mass spectrometry detection for quantification of voriconazole and voriconazole N-oxide. Sample extraction of Mitra® devices, followed by reversed-phase chromatographic separation and selective detection using tandem mass spectrometry with a 4.00 minute runtime per sample was employed. Standard curves were linear between 10.0 to 10,000 ng/mL for both voriconazole and voriconazole N-oxide. Intra- and inter-day accuracy were within 87-102% and precision (CV) was <12% based on a 3-day validation study. Recoveries were ≥94 % for voriconazole and ≥87 % for voriconazole N-oxide. Voriconazole and voriconazole N-oxide were stable in human whole blood under assay conditions (19 h at room temperature and 24 h in autosampler). Voriconazole was stable for 1-month in dried microsamples under different conditions (4, -20 and -78 °C). This assay provides an efficient quantitation of voriconazole and voriconazole N-oxide and is ready to be implemented for the analysis of whole blood microsamples in a pediatric clinical trial investigating the impact of intestinal inhibition of CYP3A4 on voriconazole pharmacokinetics.

Physiologically Based Pharmacokinetic Models for Adults and Children Reveal a Role of Intracellular Tubulin Binding in Vincristine Disposition.

Vincristine is a cytotoxic chemotherapeutic agent used as first-line therapy for pediatric acute lymphocytic leukemia. It is cleared by hepatic oxidative metabolism by CYP3A4 and CYP3A5 and via hepatic (biliary) efflux mediated by P-glycoprotein (P-gp) transporter. Bottom-up physiologically based pharmacokinetic (PBPK) models were developed to predict vincristine disposition in pediatric and adult populations. The models incorporated physicochemical properties, metabolism by CYP3A4/5, efflux by P-gp, and intracellular binding to ß-tubulin. The adult and pediatric PBPK models predicted pharmacokinetics (PK) within twofold of the observed PK parameters (area under the curve, terminal half-life, volume of distribution, and clearance). Simulating a higher hypothetical (4.9-fold) pediatric expression of ß-tubulin relative to adult improved predictions of vincristine PKs. To our knowledge, this is the first time that intracellular binding has been incorporated into a pediatric PBPK model. Utilizing this PBPK modeling approach, safe and effective doses of vincristine could be predicted.

Vancomycin Prescribing and Therapeutic Drug Monitoring in Children With and Without Acute Kidney Injury After Cardiac Arrest.

BACKGROUND: Acute kidney injury (AKI) commonly occurs after cardiac arrest. Those subsequently treated with vancomycin are at additional risk for drug-induced kidney injury. OBJECTIVE: We aimed to determine whether opportunities exist for improved drug monitoring after cardiac arrest. METHODS: This was a retrospective cohort study of children aged 30 days-17 years treated after cardiac arrest in an intensive care unit from January 2010 to September 2014 who received vancomycin within 24 h of arrest. Vancomycin dosing and monitoring were compared between those with and without AKI, with AKI defined as pRIFLE (pediatric risk, injury, failure, loss, end-stage renal disease) stage 2-3 AKI at day 5 using Schwartz formula-calculated estimated glomerular filtration rate (eGFR). RESULTS: Of 43 children, 16 (37%) had AKI at day 5. Age, arrest duration, median time to first vancomycin dose, and the number of doses before and time to first vancomycin concentration measurement were similar between groups. Children with AKI had higher initial vancomycin concentrations than those without AKI (median 16 vs. 7 mg/L; p = 0.003). A concentration was not measured before the second dose in 44% of children with AKI. Initial eGFR predicted day 5 AKI. In children with AKI, the initial eGFR was lower in those with than those without a concentration measurement before the second dose (29 mL/min/1.73 m2 [interquartile range (IQR) 23-47] vs. 52 [IQR 50-57]; p = 0.03) but well below normal in both. CONCLUSIONS: In children with AKI after cardiac arrest, decreased vancomycin clearance was evident early, and early monitoring was not performed universally in those with low initial eGFR. Earlier vancomycin therapeutic drug monitoring is indicated in this high-risk population.

Use of Intravenous Acetaminophen in Children for Analgesia After Spinal Fusion Surgery: A Randomized Clinical Trial.

OBJECTIVES: Opioid pharmacotherapy is the cornerstone of postoperative analgesia. Despite its effectiveness, it has a variety of potential adverse effects. Therefore, a multimodal approach with non-opioid analgesics would be optimal. The aim of this study was to determine if intravenous (IV) acetaminophen would reduce opioid requirements and improve clinical outcomes in children after surgery. METHODS: A single-center, randomized, double-blind study was conducted in 57 children (10-18 years old) undergoing posterior spine fusion surgery between July 2011 to May 2014. All subjects received either acetaminophen or placebo at the end of surgery, followed by repeated doses every 6 hours for a total of 8 doses. RESULTS: In the first 24 postoperative hours, the average opioid consumption was lower for the active group compared with the placebo group (p = 0.02). The total unadjusted time to patient controlled analgesia (PCA) discontinuation was also longer in the placebo group than the active group (90 hours vs. 73 hours, p = 0.02); however, this was not statistically significant after normalizing for body weight. Additionally, time to first solid intake was longer without the use of acetaminophen (69 hours vs. 49 hours, p = 0.01). CONCLUSIONS: Postoperative use of IV acetaminophen was associated with earlier time to diet advancement and discontinuation of IV analgesics and may result in lower opioid consumption.

A physiologically based pharmacokinetic model for voriconazole disposition predicts intestinal first-pass metabolism in children.

BACKGROUND AND OBJECTIVES: The effect of ontogeny in drug-metabolizing enzymes on pediatric pharmacokinetics is poorly predicted. Voriconazole, a potent antifungal, is cleared predominantly via oxidative metabolism and exhibits vastly different pharmacokinetics between adults and children. A physiologically based pharmacokinetic (PBPK) model was developed integrating hepatic in vitro metabolism data with physiologic parameters to predict pharmacokinetic parameters of voriconazole in adult and pediatric populations. METHODS: Adult and pediatric PBPK models integrated voriconazole physicochemical properties with hepatic in vitro data into the models. Simulated populations contained 100 patients (10 trials with 10 patients each). Trial design and dosing was based on published clinical trials. Simulations yielded pharmacokinetic parameters that were compared against published values and visual predictive checks were employed to validate models. RESULTS: All adult models and the pediatric intravenous model predicted pharmacokinetic parameters that corresponded with observed values within a 20% prediction error, whereas the pediatric oral model predicted an oral bioavailability twofold higher than observed ranges. After incorporating intestinal first-pass metabolism into the model, the prediction of oral bioavailability improved substantially, suggesting that voriconazole is subject to intestinal first-pass metabolism in children, but not in adults. CONCLUSIONS: The PBPK approach used in this study suggests a mechanistic reason for differences in bioavailability between adults and children. If verified, this would be the first example of differential first-pass metabolism in children and adults.