Accurate prediction of oral pharmacokinetics remains challenging. This study investigated quantitative approaches for the prediction of the area under the plasma concentration-time curve after oral administration (AUC(p,oral)) to rats using the in vitro-in vivo extrapolation (IVIVE), in silico model using machine learning approaches and the combination of the in silico model and in vitro data. A set of 595 structurally diverse compounds with determined AUC(p,oral) at 1 mg/kg, in vitro intrinsic clearance (CLint), an unbound fraction in plasma (f(u,p)) in rats, and kinetic solubility at pH 6.8 was used for this assessment. Prediction models developed by two different types of machine learning techniques (i.e., random forest regression and Gaussian processes) were evaluated using three validation methods implementing the time and cluster-split training and test set and fivefold cross-validation. The developed machine learning models have a square of correlation coefficient (R-2) in the range of 0.381-0.685 with 33-45% of the compounds being predicted within 2-fold of the observed AUC(p,oral) value. The predictivity was improved by incorporating CLint, f(u,p), and solubility as explanatory variables with R-2 = 0.554-0.743. In cases where extraction by the liver is the main elimination pathway and intestinal extraction is negligible, AUC(p,oral) can be expressed by dose, CLint, and f(u,p) based on a well-stirred model. By using this conventional IVIVE approach, only 1.7-5.0% of compounds were predicted within the 2-fold error with R-2 = 0.354-0.487. Two empirical scaling factors (ESFs) determined by linear regression analysis and machine learning approaches improved the predictivity of AUC(p,oral) with 33-44% predicted within twofold variability. The IVIVE using ESF predicted by random forest regression showed better predictivity of AUC(p,oral) with R-2 = 0.471-0.618, while it still showed lower predictivity than machine learning approaches applied directly to AUC(p,oral) prediction. This study demonstrated that the combination of in silico and in vitro parameters is useful to improve the predictivity of the machine learning model for rat AUC(p,oral) and supports consideration for predicting AUC(p,oral) for human and other non-clinical species in a similar manner.
