LAUSR

&NA; In the current study, we investigated the metoprolol absorption kinetics of an in‐house produced oral sustained‐release formulation, matrices manufactured via prilling, and two commercially available formulations, ZOK‐ZID® (reservoir) and Slow‐Lopresor® (matrix) in both New Zealand White rabbits and Beagle dogs, using a population pharmacokinetic analysis approach. The aim of this study was to compare the in vivo pharmacokinetic (PK) profiles of different formulations based on metoprolol, a selective adrenergic &bgr;1‐receptor antagonist, in dogs and rabbits and to contrast the observed differences. To that end, metoprolol (50 to 200 mg) was administered to 6 Beagle dogs and 6 New Zealand White rabbits as a single intravenous (IV) bolus injection and to 8 dogs and 6 rabbits as an oral modified release formulation. To derive pharmacokinetic parameters from the data, a non‐linear mixed‐effects model was developed using NONMEM® where the contribution of observations below the limit of detection (BDL, below detection limit) to the parameter estimates was taken into account in the parameter estimation procedure. In both species and for the three modified release formulations, different absorption models were tested to describe the PK of metoprolol following oral dosing. In Beagle dogs, plasma concentration‐time profiles were best described using a sequential zero‐ and first‐order absorption model. In rabbits though, the absorption phase was best described using a first‐order process only. In both species, the reservoir formulation ZOK‐ZID® was behaving quite similarly. In contrast, the absorption properties of both matrix formulations were rather different between species. This study indicates that the PK of the reservoir formulation is similar in both species, even after accounting for the almost completely missed absorption phase in rabbits. The insights gained further illustrate that rabbits are not very well suited to study the PK of the current matrix formulations in view of their less optimal prolonged release characteristics and the resulting fast decline in metoprolol plasma levels. Graphical abstract Figure. No caption available.