1. Structure-Based Design of a Macrocyclic PROTAC
Xavier Lucas, Alessio Ciulli, Andrea Testa, Jane E Wright, Scott J Hughes Angew Chem Int Ed Engl . 2020 Jan 20;59(4):1727-1734. doi: 10.1002/anie.201914396.
Constraining a molecule in its bioactive conformation via macrocyclization represents an attractive strategy to rationally design functional chemical probes. While this approach has been applied to enzyme inhibitors or receptor antagonists, to date it remains unprecedented for bifunctional molecules that bring proteins together, such as PROTAC degraders. Herein, we report the design and synthesis of a macrocyclic PROTAC by adding a cyclizing linker to the BET degrader MZ1. A co-crystal structure of macroPROTAC-1 bound in a ternary complex with VHL and the second bromodomain of Brd4 validated the rational design. Biophysical studies revealed enhanced discrimination between the second and the first bromodomains of BET proteins. Despite a 12-fold loss of binary binding affinity for Brd4, macroPROTAC-1 exhibited cellular activity comparable to MZ1. Our findings support macrocyclization as an advantageous strategy to enhance PROTAC degradation potency and selectivity between homologous targets.
2. Antibody-PROTAC Conjugates Enable HER2-Dependent Targeted Protein Degradation of BRD4
James Richard Baker, Cyrille S Kounde, Edward W Tate, Marı A Maneiro, Maria M Shchepinova, Vijay Chudasama, Nafsika Forte ACS Chem Biol . 2020 Jun 19;15(6):1306-1312. doi: 10.1021/acschembio.0c00285.
Targeting protein degradation with Proteolysis-Targeting Chimeras (PROTACs) is an area of great current interest in drug discovery. Nevertheless, although the high effectiveness of PROTACs against a wide variety of targets has been established, most degraders reported to date display limited intrinsic tissue selectivity and do not discriminate between cells of different types. Here, we describe a strategy for selective protein degradation in a specific cell type. We report the design and synthesis of a trastuzumab-PROTAC conjugate (Ab-PROTAC3) in which E3 ligase-directed degrader activity is caged with an antibody linker which can be hydrolyzed following antibody-PROTAC internalization, releasing the active PROTAC and inducing catalytic protein degradation. We show that3selectively targets bromodomain-containing protein 4 (BRD4) for degradation only in HER2 positive breast cancer cell lines, while sparing HER2 negative cells. Using live cell confocal microscopy, we show internalization and lysosomal trafficking of the conjugate specifically in HER2 positive cells, leading to the release of active PROTAC in quantities sufficient to induce potent BRD4 degradation. These studies demonstrate proof-of-concept for tissue-specific BRD4 degradation, overcoming limitations of PROTAC selectivity, with significant potential for application to novel targets.
3. Phenyl-Glutarimides: Alternative Cereblon Binders for the Design of PROTACs
Anand Mayasundari, Marcus Fischer, Zoran Rankovic, Brandon Young, Fatemeh Keramatnia, Stephen W White, Sergio Chai, Lei Yang, Patrick Ryan Potts, Martine F Roussel, Gisele Nishiguchi, Stanley Nithianantham, Sourav Das, Seung Wook Yang, Taosheng Chen, Zhenmei Li, Yong Li, Jamie Jarusiewicz, Mi-Kyung Yun, Jaeki Min, Anang Shelat, Jake Slavish, Barbara Jonchere, Marisa Actis, Richard E Lee, Shalandus H Garrett, Xiang Fu Angew Chem Int Ed Engl . 2021 Dec 13;60(51):26663-26670. doi: 10.1002/anie.202108848.
Targeting cereblon (CRBN) is currently one of the most frequently reported proteolysis-targeting chimera (PROTAC) approaches, owing to favorable drug-like properties of CRBN ligands, immunomodulatory imide drugs (IMiDs). However, IMiDs are known to be inherently unstable, readily undergoing hydrolysis in body fluids. Here we show that IMiDs and IMiD-based PROTACs rapidly hydrolyze in commonly utilized cell media, which significantly affects their cell efficacy. We designed novel CRBN binders, phenyl glutarimide (PG) analogues, and showed that they retained affinity for CRBN with high ligand efficiency (LE >0.48) and displayed improved chemical stability. Our efforts led to the discovery of PG PROTAC 4 c (SJ995973), a uniquely potent degrader of bromodomain and extra-terminal (BET) proteins that inhibited the viability of human acute myeloid leukemia MV4-11 cells at low picomolar concentrations (IC50=3 pM; BRD4 DC50=0.87 nM). These findings strongly support the utility of PG derivatives in the design of CRBN-directed PROTACs.
