Training compounds and data processing. Compounds with an annotated activity values (IC50, EC50, Kd, Ki) <1 µM (pActivity ≥6) were retrieved from ChEMBL24 to cover the chemical space of biologically active compounds. Molecular structures were encoded as canonical SMILES strings with the RDKit package (v2018.03, www.rdkit.org), and only SMILES strings with a length of up to 140 tokens (characters) were retained. SMILES strings of were standardized in Python (v3.6.5, www.python.org) by removing stereochemical information, salts and duplicates. This data preparation resulted in a set of 365,063 bioactive molecules encoded as unique SMILES strings (referred to as "ChEMBL24").