A Mechanistic Approach for the Scaling of Clearance in Children

BACKGROUND AND OBJECTIVE

Clearance is an important pharmacokinetic concept for scaling dosage, understanding the risks of drug-drug interactions and environmental risk assessment in children. Accurate clearance scaling to children requires prior knowledge of adult clearance mechanisms and the age-dependence of physiological and enzymatic development. The objective of this research was to develop and evaluate ontogeny models that would provide an assessment of the age-dependence of clearance.

METHODS

Using in vitro data and/or in vivo clearance values for children for eight compounds that are eliminated primarily by one process, models for the ontogeny of renal clearance, cytochrome P450 (CYP) 3A4, CYP2E1, CYP1A2, uridine diphosphate glucuronosyltransferase (UGT) 2B7, UGT1A6, sulfonation and biliary clearance were developed. Resulting ontogeny models were evaluated using six compounds that demonstrated elimination via multiple pathways. The proportion of total clearance attributed to each clearance pathway in adults was delineated. Each pathway was individually scaled to the desired age, inclusive of protein-binding prediction, and summed to generate a total plasma clearance for the child under investigation. The paediatric age range included in the study was premature neonates to sub-adults.

RESULTS

There was excellent correlation between observed and predicted clearances for the model development (R2 = 0.979) and test sets (Q2 = 0.927). Clearance in premature neonates could also be well predicted (development R2 = 0.951; test Q2 = 0.899).

CONCLUSION

Paediatric clinical trial development could greatly benefit from clearance scaling, particularly in guiding dosing regimens. Furthermore, since the proportion of clearance via different elimination pathways is age-dependent, information could be gained on the developmental extent of drug-drug interactions.

Human variability in the renal elimination of foreign compounds and renal excretion-related uncertainty factors for risk assessment

Renal excretion is an important route of elimination for xenobiotics and three processes determine the renal clearance of a compound [glomerular filtration (about 120 ml/min), active renal tubular secretion (>120 ml/min) and passive reabsorption (<120 ml/min)]. Human variability in kinetics has been quantified using a database of 15 compounds excreted extensively by the kidney (>60% of a dose) to develop renal-excretion related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [renal and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was low for both routes of exposure, with coefficients of variation of 21% (oral) and 24% (intravenous) that were largely independent of the renal processes involved. Renal-excretion related uncertainty factors were below the default kinetic uncertainty factor of 3.16 for most subgroups analysed with the exception of the elderly (oral data) and neonates (intravenous data) for whom renal excretion-related factors of 4.2 and 3.2 would be required to cover up to 99% of these subgroups respectively.

Population pharmacokinetics of aminoglycoside antibiotics in patients with cystic fibrosis

The population pharmacokinetics of gentamicin and tobramycin were investigated in a group of 51 young adults with cystic fibrosis. Their ages ranged from 14-35 years, weights from 38-82 kg, and 27 of the patients were female. None of the patients had renal impairment, but 3 patients were treated in the intensive therapy unit (ITU) during one of their courses of therapy. Data comprised 219 courses of therapy and 544 concentrations (mean: 11 per patient). Concentration-time data were analyzed using a nonlinear mixed-effects model package (NONMEM) and were best described by a one-compartment model. Factors identified as potentially influencing aminoglycoside pharmacokinetics were added in a stepwise fashion and the best model found that drug clearance and volume of distribution were related to body surface area and admission to ITU. The mean population estimates were a clearance of 2.89 L/hr/m2 and a volume of distribution of 9.21 L/m2 with a 60% increase in patients who were admitted to ITU. Interpatient variability in clearance and volume were 14% and 8%, respectively. The results suggested that a dose of 120 mg/m2 should achieve an average 1 hour postdose peak of 10 mg/L and trough of <1 mg/L and that higher doses might be required in ITU patients.

Reevaluation of Gentamicin Dosing in the Neonatal Population

Objective:

To determine whether current recommendations for gentamicin dosing in full-term newborns yield a serum peak concentration of 6–7 μg/mL and a trough concentration less than 2 μg/mL in treating suspected neonatal sepsis.

Design:

Two-year retrospective chart review.

Setting:

Community hospital.

Results:

Sample consisted of 175 newborns with a gestational age ranging from 36 to 43 weeks and 188 sets of concentrations. Pharmacokinetic parameters were calculated using a one-compartment, first-order elimination model and were reported as follows: volume of distribution 0.59 kg/L, elimination rate constant (ke) 0.11 h−1 (half-life [t1/2] 6.8 h) at 36–37 weeks of age, with a significant change (p < 0.05) in rate constant occurring at 38–43 weeks of life, and ke 0.12 h−1 (t1/2 6.0 h) when using a two-tailed, two-sample t-test. Extrapolated mean peak concentrations were 5.8 ± 1.2 μg/mL and trough concentrations were 1.5 ± 0.5 μg/mL. Furthermore, 14% of newborns had an extrapolated trough concentration of 2.0 μg/mL or more.

Conclusions:

The current 2.5-mg/kg dosage is appropriate for the neonatal population studied. However, to decrease the number of potentially toxic trough concentrations, the initial dosing interval should be extended to every 18 hours for full-term neonates (>37 weeks gestation) with normal kidney function and for neonates with a gestational age of 36–37 weeks.

A simplified model for adjustment of gentamicin dosage in newborn infants

In a retrospective study of 72 neonates during treatment with gentamicin, poor correlation was found between dosage based on body weight and gentamicin serum concentrations. Calculation of adequate gentamicin dosage regimen during steady-state based on individual pharmacokinetic parameters according to Gibaldi & Perrier was then studied in 35 newborn infants during therapy. Predictions were based on gentamicin serum concentrations taken prior to and 1, 3, and 5 hours after the first (n = 8) or second (n = 12) dose (group A), or only prior to and one hour after dose (group B, n = 15). Half-life (t1/2), apparent volume of distribution (Vd), body clearance (Clbody) and elimination rate constant (beta) were not significantly different when calculated after the first or second dose or during steady-state. The correlation between predicted and observed gentamicin concentration was high in both groups (p less than 0.005) and the slopes congruent with unity. After dose or interval correction, 73% of the observed predose concentrations (mean 2.0 micrograms/ml) were within 1 microgram/ml of predicted values. One hour after dose the predicted (mean 5.7 micrograms/ml) and observed (mean 6.2 micrograms/ml) values were not significantly different. Higher precision was noted when the predictions were based on 4 samples (group A) compared to 2 (group B). Since the calculations may be performed by a simple desk calculator rapid advice may be given to the clinical staff on adequate gentamicin dosage even in small severely ill preterm infants.

Pharmacokinetics of gentamicin in very low birth weight preterm infants

Low birth weight preterm infants with suspected infection were administered gentamicin intramuscularly every 18 h (2.5 mg/kg) or 24 h (3.0 mg/kg). For both dosage regimens plasma gentamicin levels were monitored during a dosage interval on three separate occasions over a 10 day period. Both regimens gave satisfactory plasma concentrations and there was no important statistically significant difference between the two. The body clearance of gentamicin correlated with gestational age (r = 0.76, p less than 0.01). The results indicate either regimen may be useful in the clinical situation but from a practical standpoint administration every 24 h may be easier to comply with then every 18 h.

Dosing implications of altered gentamicin disposition in patients with cystic fibrosis

A pharmacokinetic approach was used for gentamicin dosing in 19 children and young adults with cystic fibrosis. A one-compartment open-model analysis of steady-state gentamicin pharmacokinetics revealed a significantly larger apparent volume of distribution and total plasma clearance for patients with CF as compared to a similar population of children without the disease. The increase in the apparent volume of distribution for patients with CF produced a larger daily gentamicin dose requirement to maintain similar steady-state levels as compared to children without the disease. Significant differences in the elimination rate constant and half-life for gentamicin were not found between these populations. Linear correlations between creatinine clearance and kel for gentamicin, and total body body weight and the apparent volume of distribution were demonstrated for children with varying degrees of stable renal function but not patients with CF. Altered gentamicin disposition peculiar to CF precludes application of currently used dosing nomograms or guidelines derived from normal populations, and emphasizes the need for individualized gentamicin therapy guided by a pharmacokinetic approach in these patients.