Montelukast was chosen as the model drug; it is an amphoteric compound, with high lipophilicity (clogP 8.79), and classified as a BCS class II compound (14). Montelukast is a potent leukotriene receptor antagonist that has demonstrated efficacy and tolerability in the treatment of patients with chronic asthma (15–17). For approved paediatric use, it is available in two dosage forms (granules and chewable tablets), and is used in very young ages from 1 month old (17). The PK profile of montelukast is dose proportional and not substantially altered by age (18). As shown in different in vivo studies in infant subgroups, montelukast formulations are often mixed with drinks or soft foods to facilitate administration (7,16,17,19).