Cereblon modulator iberdomide induces degradation of the transcription factors Ikaros and Aiolos: immunomodulation in healthy volunteers and relevance to systemic lupus erythematosus

Cereblon modulators: Low molecular weight inducers of protein degradation

Targeted protein degradation has become an exciting new paradigm in drug discovery with the potential to target new protein families for therapeutic intervention. In 2010, Hiroshi Handa and colleagues discovered that the drug thalidomide binds to the protein cereblon, a component of the CRL4^CRBN E3 ubiquitin ligase. In contrast to the heterobifunctional small molecule degraders reported in the literature, thalidomide is of very low molecular weight (∼258Da) with molecular properties (solubility, metabolic stability, permeability etc) that readily support pharmaceutical dosing. It was subsequently shown that thalidomide and the analogues lenalidomide and pomalidomide are able to degrade the transcription factors Ikaros and Aiolos. CK1α and GSPT1 were subsequently identified as substrates for specific ligands, indicating that this molecular class could be tuned for selective protein degradation. Structural studies showed that the thalidomide analogues bind to a shallow hydrophobic pocket on the surface of cereblon, and scaffold a protein-protein interaction with target proteins. Target proteins do not need any affinity for the cereblon modulators, and as such undruggable, or even unligandable, proteins can be targeted for degradation. A similar mechanism of action was subsequently identified for the clinical molecule indisulam, indicating that low molecular weight degraders are not unique to cereblon. The groundbreaking work on cereblon represents a case study for the discovery and characterization of low molecular weight protein degraders for other ligases.

Novel Treatments in Lupus

Purpose of Review: The standard treatment options for systemic lupus erythematosus (SLE) are focused on non-specific immunosuppression. Over the past few years, scientific studies and ongoing clinical trials have shifted the paradigm with rapid advances in developing biologics and small molecules. A number of monoclonal antibodies and small molecule inhibitors have been developed to target specific pathways involved in SLE. Many of these novel therapeutic agents are already being tested in clinical trials and they may 1 day reshape the landscape of SLE treatment. Herein we review potential future therapeutic options for SLE.

UBE2G1 governs the destruction of cereblon neomorphic substrates

The cereblon modulating agents (CMs) including lenalidomide, pomalidomide and CC-220 repurpose the Cul4-RBX1-DDB1-CRBN (CRL4^CRBN) E3 ubiquitin ligase complex to induce the degradation of specific neomorphic substrates via polyubiquitination in conjunction with E2 ubiquitin-conjugating enzymes, which have until now remained elusive. Here we show that the ubiquitin-conjugating enzymes UBE2G1 and UBE2D3 cooperatively promote the K48-linked polyubiquitination of CRL4^CRBN neomorphic substrates via a sequential ubiquitination mechanism. Blockade of UBE2G1 diminishes the ubiquitination and degradation of neomorphic substrates, and consequent antitumor activities elicited by all tested CMs. For example, UBE2G1 inactivation significantly attenuated the degradation of myeloma survival factors IKZF1 and IKZF3 induced by lenalidomide and pomalidomide, hence conferring drug resistance. UBE2G1-deficient myeloma cells, however, remained sensitive to a more potent IKZF1/3 degrader CC-220. Collectively, it will be of fundamental interest to explore if loss of UBE2G1 activity is linked to clinical resistance to drugs that hijack the CRL4^CRBN to eliminate disease-driving proteins.