Next, we predicted compound brain concentration-time profiles by inputting these predicted parameters. The prediction accuracy data (scenario II) are shown in Table , Figure , and phase (4 h and 6 h), the predictions were not generally accurate, as evidenced by RMSEs ranging from 0.496 to 0.573 and R2-values ranging from 0.011 to 0.342. This difference could have been related to inaccurate prediction of kdeep_br_in and kdeep_br_out. Hence, we concluded that application of this ML model to the brain PK hybrid model would work for most time points and exhibit practical usefulness and novelty. Although the predictivity of the model was worse in the initial and terminal phases, it was found suitable for predicting the brain concentration-time profiles without in vivo brain concentration data.