Proteolysis Targeting Chimeras (PROTACs) are event-driven bifunctional small-molecules that simultaneously engage an E3 ubiquitin ligase and a protein of interest (POI). Ternary complex formation between POI-PROTAC- E3 ligase, results in E3-ligase mediated POI ubiquitination and subsequent degradation of the POI by the 26S proteasome. The next innovation for PROTAC technology is the induction of tumor-specific protein degradation. PROTACs that induce degradation selectively in tumor cells would likely have improved therapeutic utility due to decreased off-target cytotoxicity. Currently, the E3 ligases most commonly hijacked for PROTAC-mediated POI degradation, von Hippel-Lindau, Cereblon, and Mouse double minute 2 homolog, are expressed in both cancerous and untransformed tissues. Therefore, new E3 ligase recruiting elements (E3REs) that engage E3 ligases with tumor-specific expression must be developed to impart tumor-specificity. Type I Melanoma Antigen Gene (MAGE) family proteins are cancer testis antigens, whose expression is restricted to the male germ line, but can be re-expressed in cancers. MAGE-A3 binds TRIM28, a ubiquitously expressed protein with E3 ligase activity, to form an oncogenic tumor-specific E3 ligase complex. A PROTAC harboring a MAGE-A3 E3RE may be able to recruit MAGE-A3/TRIM28 and induce protein degradation in a tumor-specific manner. MAGE proteins bind their cognate E3 ligases and substrates via a conserved MAGE homology domain (MHD). Using Schrödinger Glide docking software, we screened >60,000 compounds against the recently resolved structure of the MAGE-A3 MHD to identify ligands in silico that are predicted to disrupt MAGE-A3- substrate binding. We have identified a subset of lead-like compounds using intrinsic tryptophan fluorescence and are currently corroborating these findings via orthogonal biophysical assays such as isothermal calorimetry, and various NMR-based strategies. A structure-activity relationship study on bona-fide MAGE-A3 binders will then be performed to improve solubility, increase affinity, and identify (a) potential vector(s) for linker attachment in subsequent PROTAC development. Once tight-binding MAGE-A3 ligands have been developed, we will synthesize MAGE-A3-based-HaloPROTACs and test their ability to degrade HaloTag7-GFP in a MAGE-A3- dependent manner. Subsequently, we will further test the utility of recruiting MAGE-A3/TRIM28 E3 ligase complex by targeting Bromodomain-containing protein 4 (BRD4) for MAGE based-PROTAC mediated degradation. Induction of tumor-specific degradation of BRD4 and induction of apoptosis in a tumor-specific manner by our MAGE-A3 based-PROTACs will be evaluated. Overall, this project will determine the MAGE- A3/TRIM28 E3 ligase complex induce tumor-specific protein degradation. Additionally, development of a new E3RE will help spark excitement for identification of novel E3REs for other E3 ligases, thereby greatly expanding the number of E3 ligase amenable to the PRO...