Optimisation of first clinical studies in special populations : towards semi-physiological pharmacokinetic models

Special populations are groups of patients that may respond differently to drug treatment due to a variety of factors, such as age or disease. Therefore, in drug development dedicated clinical studies are often required to determine the optimal dose for these (vulnerable) patient groups. Such studies are complicated by ethical and practical barriers that can hinder the objective of the study when not well designed. To optimise the design of such studies, the application of model-based approaches is essential. In this thesis, we aimed to develop a semi-physiological framework that constitutes a scientific basis for optimisation of study designs in special populations. First, we examined the accuracy of existing approaches in paediatric patients. For the "allometric scaling plus maturation function" approach, the accuracy was shown low especially in young children. An alternative approach was found in the physiological well-stirred-model of hepatic clearance. On this basis, the semi-physiological PK models were developed by interfacing descriptive compartmental pharmacokinetic models with the well-stirred-model of hepatic clearance, and a mechanistic description of plasma-protein binding. The performance of these models was evaluated using two paradigm-drugs (solifenacin and tamsulosin) and was shown successful for the prediction of the pharmacokinetics in paediatric, hepatic-impaired and renal-impaired patients.The research was performed within the framework of project number D2-104 of the Dutch Top Institute PharmaUBL - phd migration 201

First dose in children: physiological insights into pharmacokinetic scaling approaches and their implications in paediatric drug development

Dose selection for “first in children” trials often relies on scaling of the pharmacokinetics from adults to children. Commonly used approaches are physiologically-based pharmacokinetic modeling (PBPK) and allometric scaling (AS) in combination with maturation of clearance for early life. In this investigation, a comparison of the two approaches was performed to provide insight into the physiological meaning of AS maturation functions and their interchangeability. The analysis focused on the AS maturation functions established using paracetamol and morphine paediatric data after intravenous administration. First, the estimated AS maturation functions were compared with the maturation functions of the liver enzymes as used in the PBPK models. Second, absolute clearance predictions using AS in combination with maturation functions were compared to PBPK predictions for hypothetical drugs with different pharmacokinetic properties. The results of this investigation showed that AS maturation functions do not solely represent ontogeny of enzyme activity, but aggregate multiple pharmacokinetic properties, as for example extraction ratio and lipophilicity (log P). Especially in children younger than 1 year, predictions using AS in combination with maturation functions and PBPK were not interchangeable. This highlights the necessity of investigating methodological uncertainty to allow a proper estimation of the “first dose in children” and assessment of its risk and benefits