Target-held-out Models -Like in the subsection "Data Preparation", SMILES representations of all the molecules were inputted into MMPDB, structurally similar molecules were paired, and filtering was performed to exclude rare SMIRKs and randomly sample remaining SMIRKs. After that, and unlike the previously described data preparation, all pairs with at least one molecule active against a given target (COX2, DRD2 or HERG)