Discovery and characterization of potent spiro-isoxazole-based cereblon ligands with a novel binding mode.

The vast majority of current cereblon (CRBN) ligands is based on the thalidomide scaffold, relying on glutarimide as the core binding moiety. With this architecture, most of these ligands inherit the overall binding mode, interactions with neo-substrates, and thereby potentially also the cytotoxic and teratogenic properties of the parent thalidomide. In this work, by incorporating a spiro-linker to the glutarimide moiety, we have generated a new chemotype that exhibits an unprecedented binding mode for glutarimide-based CRBN ligands. In total, 16 spirocyclic glutarimide derivatives incorporating an isoxazole moiety were synthesized and tested for different criteria. In particular, all ligands showed a favorable lipophilicity, and several were able to outperform the binding affinity of thalidomide as a reference. In addition, all compounds showed favorable cytotoxicity profiles in myeloma cell lines and human peripheral blood mononuclear cells. The novel binding mode, which we determined in co-crystal structures, provides explanations for these improved properties: The incorporation of the spiro-isoxazole changes both the conformation of the glutarimide moiety within the canonical tri-trp pocket and the orientation of the protruding moiety. In this new orientation it forms additional hydrophobic interactions and is not available for direct interactions with the canonical neo-substrates. We therefore propose this chemotype as an attractive building block for the design of PROTACs.

Computational Prediction of Cyclic Peptide Structural Ensembles and Application to the Design of Keap1 Binders.

The Nrf2 transcription factor is a master regulator of the cellular response to oxidative stress, and Keap1 is its primary negative regulator. Activating Nrf2 by inhibiting the Nrf2-Keap1 protein-protein interaction has shown promise for treating cancer and inflammatory diseases. A loop derived from Nrf2 has been shown to inhibit Keap1 selectively, especially when cyclized, but there are no reliable design methods for predicting an optimal macrocyclization strategy. In this work, we employed all-atom, explicit-solvent molecular dynamics simulations with enhanced sampling methods to predict the relative degree of preorganization for a series of peptides cyclized with a set of bis-thioether "staples". We then correlated these predictions to experimentally measured binding affinities for Keap1 and crystal structures of the cyclic peptides bound to Keap1. This work showcases a computational method for designing cyclic peptides by simulating and comparing their entire solution-phase ensembles, providing key insights into designing cyclic peptides as selective inhibitors of protein-protein interactions.

Leveraging Ligand Affinity and Properties: Discovery of Novel Benzamide-Type Cereblon Binders for the Design of PROTACs.

Immunomodulatory imide drugs (IMiDs) such as thalidomide, pomalidomide, and lenalidomide are the most common cereblon (CRBN) recruiters in proteolysis-targeting chimera (PROTAC) design. However, these CRBN ligands induce the degradation of IMiD neosubstrates and are inherently unstable, degrading hydrolytically under moderate conditions. In this work, we simultaneously optimized physiochemical properties, stability, on-target affinity, and off-target neosubstrate modulation features to develop novel nonphthalimide CRBN binders. These efforts led to the discovery of conformationally locked benzamide-type derivatives that replicate the interactions of the natural CRBN degron, exhibit enhanced chemical stability, and display a favorable selectivity profile in terms of neosubstrate recruitment. The utility of the most potent ligands was demonstrated by their transformation into potent degraders of BRD4 and HDAC6 that outperform previously described reference PROTACs. Together with their significantly decreased neomorphic ligase activity on IKZF1/3 and SALL4, these ligands provide opportunities for the design of highly selective and potent chemically inert proximity-inducing compounds.