Velcrin compounds are small molecules which act as molecular glues to form a PDE3A-SLFN12 complex. This complex formation induces apoptosis in cancer cells expressing elevated levels of PDE3A and SLFN12 (de Waal et al., Nat Chem Biol 2016; Wu et al., J Bio Chem 2020). Unlike traditional targeted therapies that leverage dependencies created by genomic alterations in cancer cells, velcrins instead kill cancer cells by a gain-of-function mechanism dependent on stimulation of SLFN12 RNase activity by complex formation with PDE3A (Garvie et al., Nat Commun 2021). Based on in-vivo tool compounds which were discovered by hit-to-lead optimization from a phenotypic screen (Lewis et al., ACS Med Chem Lett 2019), the team set out to optimize the properties of the tool compounds. While already showing excellent potency and in-vivo efficacy in various tumor models, the compounds were only approximately tenfold selective against PDE3A when compared to their cell killing activity. PDE3A/B activity most notably influences cardiohemodynamics and inhibition of PDE3A/B was to be reduced to generate a larger safety window. Co-crystallization of analogs with PDE3A showed the binding mode of the velcrin class and changes in the solvent exposed region of the PDE3A binders led to steep activity changes in complex formation and cell killing. Careful optimization of the ratio of cell killing to PDE3A activity while optimizing DMPK and safety pharmacology led to the discovery of BAY 2666605. BAY 2666605 has recently entered a First-in-Human study (NCT04809805) in patients with advanced solid tumors that co-express PDE3A and SLFN12, including melanoma, ovarian cancer, and sarcoma. We will discuss the discovery and structure activity relationships of BAY 2666605.