As the ligand encoding and selection strategies have been explored, we further illustrate the overall active learning based chemical space exploration cycle using the same experimentally characterized set of PDE2 inhibitors (Fig. ). Here, we relied on the narrowing protocol of ligand selection and performed 6 iterations of model training and the subsequent binding affinity prediction. As the ligand library is analysed retrospectively, we readily have access to the experimentally determined affinities (Fig. ).