On-target toxicity of GSPT1 molecular glue degraders in mice

Thalidomide is teratogenic in humans but not in rodents due to species-specific differences in the sequence of Cereblon (CRBN), an E3 ubiquitin ligase targeted by thalidomide and its derivative molecular glue degraders (MGDs). This species divergence has hindered the accurate prediction of MGD-induced toxicities in standard laboratory animals. GSPT1 MGDs, such as CC-90009, have shown potent anticancer activities in preclinical models and leukemia patients; however, their clinical development was challenging due to severe adverse effects. This highlights the critical need to characterize on-target toxicities in relevant animal models to exploit the therapeutic safety of this class of MGDs. Here, we generated humanized CrbnV380E and CrbnV380E/I391V knock-in mouse strains, in combination with a degradation-resistant Gspt1G574N strain, to interrogate the in vivo on-target effects of CC-90009 and its analog CC-885. We found that targeted GSPT1 depletion in mice led to rapid mortality, preceded by multiple dysfunctions including intestinal obstruction, liver damage, splenic atrophy, and hematological abnormalities. Remarkably, these toxicities, along with the underlying transcriptional perturbations, were completely rescued by the undegradable Gspt1G574N mutant, establishing a definitive causal link between GSPT1 degradation and systemic injury. Induced proximity and degradation proteomic analyses revealed that GSPT1 loss triggered a secondary downregulation of many proteins, including MYC, PLK1 and CDK4, which were not directly recruited by these MGDs to CRBN. Collectively, our data define the in vivo on-target toxicities associated with endogenous GSPT1 degradation and provide a genetic framework to guide the preclinical safety evaluation of CRBN-based MGD therapeutics.