Although the principles of drug disposition also apply in neonates, their specific characteristics warrant focussed assessment.
Children display maturation in drug disposition, but this is most prominent in the first year of life. Besides maturational aspects of drug
absorption and distribution, maturation mainly relates to (renal) elimination and (hepatic) metabolic clearance.
Renal elimination clearance in early life is low and almost completely depends on glomerular filtration. Despite the overall low clearance,
interindividual variability is already extensive and can be predicted by covariates like postmenstrual age, postnatal age, co-administration
of a non-selective cyclo-oxygenase inhibitor, growth restriction or peripartal asphyxia. These findings are illustrated by observations on
amikacin and vancomycin.
Variation in phenotypic metabolic clearance is based on constitutional, environmental and genetic characteristics. In early life, it mainly
reflects ontogeny, but other covariates may also become relevant. Almost all phase I and phase II metabolic processes display ontogeny
in a iso-enzyme specific pattern. The impact of covariates like postmenstrual age, postnatal age, disease state characteristics and polymorphisms
are illustrated based or ‘probe’ drugs (paracetamol, tramadol, propofol) administered as part of their medical treatment in
critically ill neonates.
The description of a compound specific pattern is beyond compound specific relevance. The maturational patterns described and the extent
of the impact of covariates can subsequently be applied to predict in vivo time-concentration profiles for compounds that undergo
similar routes of elimination. Through improved predictability, such maturational models can serve to improve both the clinical care and
feasibility and safety of clinical studies in neonates.