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Clinical assessment of drug-drug interactions (DDIs) in children is not a common practice in drug development. Therefore, physiologically-based pharmacokinetic (PBPK) modeling can be beneficial for informing drug labeling. Using ivabradine and its metabolite (both cytochrome P450 3A4 enzyme (CYP3A4) substrates), the objectives were (i) to scale ivabradine-metabolite adult PBPK/PD to pediatrics, (ii) to predict the DDIs with a strong CYP3A4 inhibitor, and (iii) to compare the sensitivity of children to DDIs using two CYP3A4 hepatic ontogeny functions: Salem and Upreti. A scaled parent-metabolite PBPK/PD model from adults to children satisfactorily predicted pharmacokinetics (PK) and pharmacodynamics (PD) in 74 children (0.5-18 years) regardless of CYP3A4 hepatic ontogeny function applied. However, using the Salem ontogeny, mean predicted parent and metabolite area under the concentration-time curve over 12 hours (AUC 12h ) and heart rate change from baseline were 2-fold, 1.5-fold, and 1.4-fold higher in young children (0.5-3 years old) compared with Upreti ontogeny, respectively. Despite these differences, choice of appropriate hepatic CYP3A4 ontogeny was challenging due to sparse PK and PD data. Different sensitivity to ivabradine-ketoconazole DDIs was simulated in young children relative to adults depending on the choice of hepatic CYP3A4 ontogeny. Predicted ivabradine and metabolite AUC DDI /AUC control were 2-fold lower in the youngest children (0.5-1 year old) compared with adults (Salem function). In contrast, the Upreti function predicted comparable ivabradine DDIs across all age groups, although predicted metabolite AUC DDI/ AUC control was 1.3-fold higher between the youngest children and adults. In the case of PD, differences in predicted DDIs were minor across age groups and between both functions. Current work highlights the importance of careful consideration of hepatic CYP3A4 ontogeny function and implications on labeling recommendations in the pediatric population., (© 2020 The Authors. Clinical Pharmacology & Therapeutics © 2020 American Society for Clinical Pharmacology and Therapeutics.)

Ivabradine and its metabolite both demonstrate heart rate-reducing effect (I f current inhibitors) and undergo CYP3A4 metabolism. The purpose of this study was to develop a joint parent-metabolite physiologically based pharmacokinetic (PBPK)/pharmacodynamic (PD) model to predict the PK and PD of ivabradine and its metabolite following intravenous (i.v.) or oral administration (alone or co-administered with CYP3A4 inhibitors). Firstly, a parent-metabolite disposition model was developed and optimised using individual plasma concentration-time data following i.v. administration of ivabradine or metabolite within a Bayesian framework. Secondly, the model was extended and combined with a mechanistic intestinal model to account for oral absorption and drug-drug interactions (DDIs) with CYP3A4 inhibitors (ketoconazole, grapefruit juice). Lastly, a PD model was linked to the PBPK model to relate parent and metabolite PK to heart rate (HR) reduction. The disposition model described successfully parent-metabolite PK following i.v. administration. Following integration of a gut model, the PBPK model adequately predicted plasma concentration profiles and the DDI risk (92% and 85% of predicted AUC +inhibitor /AUC control and Cmax +inhibitor /Cmax control for ivabradine and metabolite within the prediction limits). Ivabradine-metabolite PBPK model was linked to PD by using the simulated unbound parent-metabolite concentrations in the heart. This approach successfully predicted the effects of both entities on HR (observed vs predicted - 7.7/- 5.9 bpm and - 15.8/- 14.0 bpm, control and ketoconazole group, respectively). This study provides a framework for PBPK/PD modelling of a parent-metabolite and can be scaled to other populations or used for investigation of untested scenarios (e.g. evaluation of DDI risk in special populations).