Fusion Molecules Between the STAT5b Inhibitor Stafib-2-CR and a Cereblon Ligand

Selective inhibition of the transcription factor STAT5b is challenging owing to the high degree of similarity to the family member STAT5a. We recently reported catechol bisphosphates as selective inhibitors of STAT5b, with Stafib-2-CR as the most potent selective STAT5b inhibitor reported to date. Here, we describe the design and synthesis of fusion molecules between Stafib-2-CR and a ligand of the E3 ligase cereblon in an effort toward STAT5b-selective proteolysis-targeting chimeras (PROTACs). The fusion molecules retain their activity against STAT5b in competitive fluorescence polarization assays, and the most potent compound is equally active against STAT5b as Stafib-2-CR, indicating that the choice of exit vector and linker is suitable for binding to STAT5b. However, conversion of the most potent fusion molecule into a cell-permeable prodrug turned out to be difficult, pointing toward the challenges of combining prodrug strategies with PROTAC technology.