Discovery of a novel molecular glue degrader targeting GSPT1/2 with a non-IMiD-based CRBN binder

Targeting undruggable proteins by inducing proximity between E3 ligase and their substrates has emerged as an innovative strategy for tackling challenging diseases. In this study, we identified a novel GSPT1 degrader, 4a (KMG-1068), through screening of our in-house small molecule library. Treatment with 4a demonstrated significant anti-proliferative activity across multiple cell lines, which was diminished by co-treatment with MLN4924, suggesting the involvement of the Cullin-containing complex. Quantitative proteomic analysis indicated that 4a predominantly induces the degradation of GSPT1/2. We further validated that 4a-mediated GSPT1/2 degradation is dependent on both CUL4 and CRBN. Moreover, 4a forms a ternary complex with CRBN and GSPT1/2, albeit with weaker binding affinity compared to reported GSPT1 molecular glues. BRET assays and competition assays with pomalidomide demonstrated that 4a binds to the C-terminal IMiD binding site of CRBN, leading to the degradation of GSPT1. Despite lacking the characteristic glutarimide moiety present in other CRBN-based molecular glue degraders, 4a interacts effectively with the IMiD binding site of CRBN. Structural characterization through analog synthesis further underscored the importance of specific structural features for CRBN engagement, GSPT1/2 degradation, and anti-proliferative effects, establishing 4a as a promising novel GSPT1/2 degrader with significant therapeutic potential.