Development of a novel simplified PBPK absorption model to explain the higher relative bioavailability of the OROS® formulation of oxybutynin

A new minimal Segmented Transit and Absorption model (mSAT) model has been recently proposed and combined with intrinsic intestinal effective permeability (Peff,int) to predict the regional gastrointestinal (GI) absorption (fabs) of several drugs. Herein this model was extended and applied for the prediction of oral bioavailability and pharmacokinetics of oxybutynin and its enantiomers to provide a mechanistic explanation of the higher relative bioavailability observed for oxybutynin’s modified-release OROS® formulation compared to its immediate-release (IR) counterpart. The expansion of the model involved the incorporation of mechanistic equations for the prediction of release, transit, dissolution, permeation and first-pass metabolism. The predicted pharmacokinetics of oxybutynin enantiomers after oral administration for both the IR and OROS® formulations were in close agreement with the observed data. The predicted absolute bioavailability for the IR formulation was within 5% of the observed value and the model adequately predicted the higher relative bioavailability observed for the OROS® formulation vs. the IR counterpart. From the model predictions it can be noticed that the higher bioavailability observed for the OROS® formulation was mainly attributable to differences in the intestinal availability (FG) rather than due to a higher colonic fabs, thus confirming previous hypotheses. The predicted fabs was almost 70% lower for the OROS® formulation compared to the IR formulation, whereas the FG was almost eight fold higher than in the IR formulation. These results provide further support to the hypothesis of an increased FG as the main factor responsible for the higher bioavailability of oxybutynin’s OROS® formulation vs the IR.