Development of proteolysis targeting chimeric molecules (PROTACs) and studies on the biological activity of tyroscherin
Proteolysis Target Chimeric Molecules, or PROTACs, are heterobifunctional molecules designed to target a protein of interest for post-translational degradation via the proteasome, by facilitating the formation of a complex between the target protein and an E3 ubiquitin ligase. Previous generations of PROTACs developed in the Crews laboratory incorporated both peptide and small molecule components, and successfully targeted proteins such as the androgen receptor and MetAP-2 for ubiquitination and subsequent proteasomal degradation.
Chapter 2 describes efforts to exploit the post-translational nature of the PROTACs technique to target a protein based on three-dimensional conformation, and the subsequent development of a novel peptide ligand for Ras. Chapter 3 describes attempts to develop a PROTAC that would bind covalently to an E3 ligase, potentially improving the kinetics of degradation and allowing for the development of a PROTAC library. However, during this process an unexpected result was observed, leading to the exploration and development of a novel hydrophobic ligand-based protein degradation technique.
A significant improvement to the previous generation of peptide-based PROTACs is explored in Chapter 4, wherein the development of an all-small molecule PROTAC is described. This PROTAC utilizes the small molecule ligand known as Nutlin, which binds to the E3 ligase MDM2, and was shown to successfully degrade the androgen receptor in vivo.
Finally, Chapter 5 describes the exploration of the biological activity of the natural product tyroscherin, which was reported to inhibit the IGF-dependent growth of breast cancer cells with an IC50 of 29 nM. Though we were unable to reproduce the reported nanomolar activity of the natural product, tyroscherin was found to exhibit a low micromolar cytotoxicity against a variety of cell lines. In addition, a thorough SAR study of all sixteen possible tyroscherin stereoisomers revealed a fascinating stereochemical tolerance for the biological activity.