Design and Synthesis of Novel Candidate CK1δ Proteolysis Targeting Chimeras (PROTACs)

Dysregulation of CK1 isoforms is linked to various types of diseases, including neuro-degeneration and different types of neoplasia such as colon-, pancreatic-, breast-, and ovarian cancer. For CK1 isoforms, a plethora of effective small molecule inhibitors are available. However, only a few degraders of CK1α and, more recently, PROteolysis TArgeting Chimeras (PROTACs) for CK1δ/CK1ε have been reported. In this study, we applied the PROTAC concept by harnessing molecular modeling to design and synthe-size a series of candidate CK1δ-targeting PROTACs based on a highly specific and po-tent benzothiazole-based CK1δ-inhibitor that we previously developed in our lab. In the present study, we established a modular synthetic platform to systematically gen-erate a set of PROTAC degrader candidates consisting of the CK1δ-specific inhibitor scaffold, alkyl- and PEG-linker motifs with various lengths, and Cereblon (CRBN)-engaging pomalidomide and thalidomide derivatives as E3 ligase binders. We demonstrate that several PROTACs degrade CK1δ/ε in various cells. The most potent PROTAC P1d inhibits the phosphorylation of downstream substrates through CK1δ/ε degradation. We establish the requirement of CUL4ACRBN and the proteasome for P1d-mediated degradation of CK1δ/ε.