Maturation of the glomerular filtration rate in neonates, as reflected by amikacin clearance

During the newborn period and early infancy, renal function matures, resulting in changes in the glomerular filtration rate (GFR). This study was performed to quantify developmental changes in the GFR in (pre)term neonates by use of amikacin clearance as proof of concept. The model was used to derive a rational dosing regimen in comparison with currently used dosing regimens for amikacin.

Population pharmacokinetic modelling was performed in nonlinear mixed-effect modelling software (NONMEM version 6.2) using data from 874 neonates obtained from two previously published datasets (gestational age 24-43 weeks; postnatal age 1-30 days; birthweight 385-4650 g). The influence of different age-related, weight-related and other covariates was investigated. The model was validated both internally and externally.

Postmenstrual age was identified as the most significant covariate on clearance. However, the combination of birthweight and postnatal age proved to be superior to postmenstrual age alone. Birthweight was best described using an allometric function with an exponent of 1.34. Postnatal age was identified using a linear function with a slope of 0.2, while co-administration of ibuprofen proved to be a third covariate. Current bodyweight was the most important covariate for the volume of distribution, using an allometric function. The external evaluation supported the prediction of the final pharmacokinetic model. This analysis illustrated clearly that the currently used dosing regimens for amikacin in reference handbooks may possibly increase the risk of toxicities and should be revised. Consequently, a new model-based dosing regimen based on current bodyweight and postnatal age was derived.

Amikacin clearance, reflecting the GFR in neonates, can be predicted by birthweight representing the antenatal state of maturation of the kidney, postnatal age representing postnatal maturation, and co-administration of ibuprofen. Finally, the model reflects maturation of the GFR, allowing for adjustments of dosing regimens for other renally excreted drugs in preterm and term neonates.