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ACS Chem Biol. 2018 Mar 16;13(3):553-560. doi: 10.1021/acschembio.7b00969. Epub 2018 Jan 29.

Translation Termination Factor GSPT1 Is a Phenotypically Relevant Off-Target of Heterobifunctional Phthalimide Degraders.

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Department of Medical Oncology , Dana-Farber Cancer Institute , Boston , Massachusetts 02115 , United States.
CeMM Research Center for Molecular Medicine of the Austrian Academy of Science , Lazarettgasse 14, AKH Bt. 25.3 , 1090 Vienna , Austria.
University of Vienna , Department of Pharmaceutical Chemistry , Althanstrasse 14 , 1090 Vienna , Austria.
Novartis Institutes for Biomedical Research , Cambridge , Massachusetts 02139 , United States.


Protein degradation is an emerging therapeutic strategy with a unique molecular pharmacology that enables the disruption of all functions associated with a target. This is particularly relevant for proteins depending on molecular scaffolding, such as transcription factors or receptor tyrosine kinases (RTKs). To address tractability of multiple RTKs for chemical degradation by the E3 ligase CUL4-RBX1-DDB1-CRBN (CRL4CRBN), we synthesized a series of phthalimide degraders based on the promiscuous kinase inhibitors sunitinib and PHA665752. While both series failed to induce degradation of their consensus targets, individual molecules displayed pronounced efficacy in leukemia cell lines. Orthogonal target identification supported by molecular docking led us to identify the translation termination factor G1 to S phase transition 1 (GSPT1) as a converging off-target, resulting from inadvertent E3 ligase modulation. This research highlights the importance of monitoring degradation events that are independent of the respective targeting ligand as a unique feature of small-molecule degraders.


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