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

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.