The IMiD target CRBN determines HSP90 activity toward transmembrane proteins essential in multiple myeloma.

The complex architecture of transmembrane proteins requires quality control (QC) of folding, membrane positioning, and trafficking as prerequisites for cellular homeostasis and intercellular communication. However, it has remained unclear whether transmembrane protein-specific QC hubs exist. Here we identify cereblon (CRBN), the target of immunomodulatory drugs (IMiDs), as a co-chaperone that specifically determines chaperone activity of HSP90 toward transmembrane proteins by means of counteracting AHA1. This function is abrogated by IMiDs, which disrupt the interaction of CRBN with HSP90. Among the multiple transmembrane protein clients of CRBN-AHA1-HSP90 revealed by cell surface proteomics, we identify the amino acid transporter LAT1/CD98hc as a determinant of IMiD activity in multiple myeloma (MM) and present an Anticalin-based CD98hc radiopharmaceutical for MM radio-theranostics. These data establish the CRBN-AHA1-HSP90 axis in the biogenesis of transmembrane proteins, link IMiD activity to tumor metabolism, and nominate CD98hc and LAT1 as attractive diagnostic and therapeutic targets in MM.

From Thalidomide to Rational Molecular Glue Design for Targeted Protein Degradation.

Thalidomide and its derivatives are powerful cancer therapeutics that are among the best-understood molecular glue degraders (MGDs). These drugs selectively reprogram the E3 ubiquitin ligase cereblon (CRBN) to commit target proteins for degradation by the ubiquitin-proteasome system. MGDs create novel recognition interfaces on the surface of the E3 ligase that engage in induced protein-protein interactions with neosubstrates. Molecular insight into their mechanism of action opens exciting opportunities to engage a plethora of targets through a specific recognition motif, the G-loop. Our analysis shows that current CRBN-based MGDs can in principle recognize over 2,500 proteins in the human proteome that contain a G-loop. We review recent advances in tuning the specificity between CRBN and its MGD-induced neosubstrates and deduce a set of simple rules that govern these interactions. We conclude that rational MGD design efforts will enable selective degradation of many more proteins, expanding this therapeutic modality to more disease areas.

Differential Analysis of Cereblon Neosubstrates in Rabbit Embryos Using Targeted Proteomics.

Targeted protein degradation is the selective removal of a protein of interest through hijacking intracellular protein cleanup machinery. This rapidly growing field currently relies heavily on the use of the E3 ligase cereblon (CRBN) to target proteins for degradation, including the immunomodulatory drugs (IMiDs) thalidomide, lenalidomide, and pomalidomide which work through a molecular glue mechanism of action with CRBN. While CRBN recruitment can result in degradation of a specific protein of interest (e.g., efficacy), degradation of other proteins (called CRBN neosubstrates) also occurs. Degradation of one or more of these CRBN neosubstrates is believed to play an important role in thalidomide-related developmental toxicity observed in rabbits and primates. We identified a set of 25 proteins of interest associated with CRBN-related protein homeostasis and/or embryo/fetal development. We developed a targeted assay for these proteins combining peptide immunoaffinity enrichment and high-resolution mass spectrometry and successfully applied this assay to rabbit embryo samples from pregnant rabbits dosed with three IMiDs. We confirmed previously reported in vivo decreases in neosubstrates like SALL4, as well as provided evidence of neosubstrate changes for proteins only examined in vitro previously. While there were many proteins that were similarly decreased by all three IMiDs, no compound had the exact same neosubstrate degradation profile as another. We compared our data to previous literature reports of IMiD-induced degradation and known developmental biology associations. Based on our observations, we recommend monitoring at least a major subset of these neosubstrates in a developmental test system to improve CRBN-binding compound-specific risk assessment. A strength of our assay is that it is configurable, and the target list can be readily adapted to focus on only a subset of proteins of interest or expanded to incorporate new findings as additional information about CRBN biology is discovered.

Thalidomide and its metabolite 5-hydroxythalidomide induce teratogenicity via the cereblon neosubstrate PLZF.

Thalidomide causes teratogenic effects by inducing protein degradation via cereblon (CRBN)-containing ubiquitin ligase and modification of its substrate specificity. Human P450 cytochromes convert thalidomide into two monohydroxylated metabolites that are considered to contribute to thalidomide effects, through mechanisms that remain unclear. Here, we report that promyelocytic leukaemia zinc finger (PLZF)/ZBTB16 is a CRBN target protein whose degradation is involved in thalidomide- and 5-hydroxythalidomide-induced teratogenicity. Using a human transcription factor protein array produced in a wheat cell-free protein synthesis system, PLZF was identified as a thalidomide-dependent CRBN substrate. PLZF is degraded by the ubiquitin ligase CRL4CRBN in complex with thalidomide, its derivatives or 5-hydroxythalidomide in a manner dependent on the conserved first and third zinc finger domains of PLZF. Surprisingly, thalidomide and 5-hydroxythalidomide confer distinctly different substrate specificities to mouse and chicken CRBN, and both compounds cause teratogenic phenotypes in chicken embryos. Consistently, knockdown of Plzf induces short bone formation in chicken limbs. Most importantly, degradation of PLZF protein, but not of the known thalidomide-dependent CRBN substrate SALL4, was induced by thalidomide or 5-hydroxythalidomide treatment in chicken embryos. Furthermore, PLZF overexpression partially rescued the thalidomide-induced phenotypes. Our findings implicate PLZF as an important thalidomide-induced CRBN neosubstrate involved in thalidomide teratogenicity.

BET Bromodomain Proteins Function as Master Transcription Elongation Factors Independent of CDK9 Recruitment.

Processive elongation of RNA Polymerase II from a proximal promoter paused state is a rate-limiting event in human gene control. A small number of regulatory factors influence transcription elongation on a global scale. Prior research using small-molecule BET bromodomain inhibitors, such as JQ1, linked BRD4 to context-specific elongation at a limited number of genes associated with massive enhancer regions. Here, the mechanistic characterization of an optimized chemical degrader of BET bromodomain proteins, dBET6, led to the unexpected identification of BET proteins as master regulators of global transcription elongation. In contrast to the selective effect of bromodomain inhibition on transcription, BET degradation prompts a collapse of global elongation that phenocopies CDK9 inhibition. Notably, BRD4 loss does not directly affect CDK9 localization. These studies, performed in translational models of T cell leukemia, establish a mechanism-based rationale for the development of BET bromodomain degradation as cancer therapy.

Development of Phenyl-substituted Isoindolinone- and Benzimidazole-type Cereblon Ligands for Targeted Protein Degradation.

Thalidomide, pomalidomide and lenalidomide, collectively referred to as immunomodulatory imide drugs (IMiDs), are frequently employed in proteolysis-targeting chimeras (PROTACs) as cereblon (CRBN) E3 ligase-recruiting ligands. However, their molecular glue properties that co-opt the CRL4CRBN to degrade its non-natural substrates may lead to undesired off-target effects for the IMiD-based PROTAC degraders. Herein, we reported a small library of potent and cell-permeable CRBN ligands, which exert high selectivity over the well-known CRBN neo-substrates of IMiDs by structure-based design. They were further utilized to construct bromodomain-containing protein 4 (BRD4) degraders, which successfully depleted BRD4 in the tested cells. Overall, we reported a series of functionalized CRBN recruiters that circumvent the promiscuity from traditional IMiDs, and this study is informative to the development of selective CRBN-recruiting PROTACs for many other therapeutic targets.

Crbn-based molecular Glues: Breakthroughs and perspectives.

CRBN is a substrate receptor for the Cullin Ring E3 ubiquitin ligase 4 (CRL4) complex. It has been observed that CRBN can be exploited by small molecules to facilitate the recruitment and ubiquitination of non-natural CRL4 substrates, resulting in the degradation of neosubstrate through the ubiquitin-proteasome system. This phenomenon, known as molecular glue-induced protein degradation, has emerged as an innovative therapeutic approach in contrast to traditional small-molecule drugs. One key advantage of molecular glues, in comparison to conventional small-molecule drugs adhering to Lipinski's Rule of Five, is their ability to operate without the necessity for specific binding pockets on target proteins. This unique characteristic empowers molecular glues to interact with conventionally intractable protein targets, such as transcription factors and scaffold proteins. The ability to induce the degradation of these previously elusive targets by hijacking the ubiquitin-proteasome system presents a promising avenue for the treatment of recalcitrant diseases. Nevertheless, the rational design of molecular glues remains a formidable challenge due to the limited understanding of their mechanisms and actions. This review offers an overview of recent advances and breakthroughs in the field of CRBN-based molecular glues, while also exploring the prospects for a systematic approach to designing these compounds.

SuFEx-based chemical diversification for the systematic discovery of CRBN molecular glues.

Molecular glues are small molecules that stabilize protein-protein interactions, enabling new molecular pharmacologies, such as targeted protein degradation. They offer advantages over proteolysis targeting chimeras (PROTACs), which present challenges associated with the size and properties of heterobifunctional constructions, but glues lack the rational design principles analogous to PROTACs. One notable exception is the ability to alter the structure of Cereblon (CRBN)-based molecular glues and redirect their activity toward new neo-substrate proteins. We took a focused approach toward modifying the CRBN ligand, 5'-amino lenalidomide, to alter its neo-substrate specificity using high-throughput chemical diversification by parallelized sulfur(VI)-fluoride exchange (SuFEx) transformations. We synthesized over 3,000 analogs of 5'-amino lenalidomide using this approach and screened the crude products using a phenotypic screen for cell viability, identifying dozens of analogs with differentiated activity. We characterized four compounds that degrade G-to-S phase transition 1 (GSPT1) protein, providing a proof-of-concept model for SuFEx-based discovery of CRBN molecular glues.

Expanding the ligand spaces for E3 ligases for the design of protein degraders.

Targeted protein degradation (TPD) has recently emerged as an exciting new drug modality. However, the strategy of developing small molecule-based protein degraders has evolved over the past two decades and has now established molecular tags that are already in clinical use, as well as chimeric molecules, PROteolysis TArgeting Chimeras (PROTACs), based mainly on ligand systems developed for the two E3 ligases CRBN and VHL. The large size of the human E3 ligase family suggests that PROTACs can be developed by targeting a large diversity of E3 ligases, some of which have restricted expression patterns with the potential to design disease- or tissue-specific degraders. Indeed, many new E3 ligands have been published recently, confirming the druggability of E3 ligases. This review summarises recent data on E3 ligases and highlights the challenges in developing these molecules into efficient PROTACs rivalling the established degrader systems.

Discovery of highly efficient CRBN-recruiting HPK1-PROTAC as a potential chemical tool for investigation of scaffolding roles in TCR signaling.

Hematopoietic progenitor kinase 1 (HPK1, MAP4K1) is a promising target for immune-oncology therapy. It has been recently demonstrated that loss of HPK1 kinase activity can enhance T cell receptor (TCR) signaling. However, many essential functions mediated by the HPK1 scaffolding role are still beyond the reach of any kinase inhibitor. Proteolysis targeting chimera (PROTAC) has emerged as a promising strategy for pathogenic proteins degradation with the characteristics of rapid, reversible, and low-cost versus RNA interference or DNA knock-out technology. Herein we first disclosed the design, synthesis, and evaluation of a series of thalidomide-based PROTAC molecules and identified B1 as a highly efficient HPK1 degrader with DC50 value of 1.8 nM. Further mechanism investigation demonstrated that compound B1 inhibits phosphorylation of the SLP76 protein with IC50 value of 496.1 nM, and confirmed that B1 is a bona fide HPK1-PROTAC degrader. Thus, this study provides a basis for HPK1 degraders development and the candidate could be used as a potential chemical tool for further investigation of the kinase-independent signaling of HPK1 in TCR.

Molecular glues targeting GSPT1 in cancers: A potent therapy.

G1 to S phase transition 1 (GSPT1) is a key translation termination factor that significantly overexpressed in various cancer tissues and cells. Molecular glue is a kind of small molecule, which can bind to an E3 ligase such as cereblon (CRBN) and subsequently recruit neosubstrate proteins for ubiquitination-proteasomal degradation. This emerging therapeutic approach shows great potential in treating cancers and other diseases. This review aims to introduce current understanding of antitumor mechanism of molecular glues targeting GSPT1, summarize pharmacology profiles of existing molecular glues, and outline development strategies of novel molecular glues. The insights provided in this review will be valuable for future studies.

USP15 antagonizes CRL4CRBN-mediated ubiquitylation of glutamine synthetase and neosubstrates.

Targeted protein degradation by the ubiquitin-proteasome system represents a new strategy to destroy pathogenic proteins in human diseases, including cancer and neurodegenerative diseases. The immunomodulatory drugs (IMiDs) thalidomide, lenalidomide, and pomalidomide have revolutionized the treatment of patients with multiple myeloma (MM) and other hematologic malignancies, but almost all patients eventually develop resistance to IMiDs. CRBN, a substrate receptor of CUL4-RBX1-DDB1-CRBN (CRL4CRBN) E3 ubiquitin ligase, is a direct target for thalidomide teratogenicity and antitumor activity of IMiDs (now known as Cereblon E3 ligase modulators: CELMoDs). Despite recent advances in developing potent CELMoDs and CRBN-based proteolysis-targeting chimeras (PROTACs), many questions apart from clinical efficacy remain unanswered. CRBN is required for the action of IMiDs, but its protein expression levels do not correlate with intrinsic resistance to IMiDs in MM cells, suggesting other factors involved in regulating resistance to IMiDs. Our recent work revealed that the CRL4CRBN-p97 pathway is required for degradation of natural substrate glutamine synthetase (GS) and neosubstrates. Here, I show that USP15 is a key regulator of the CRL4CRBN-p97 pathway to control stability of GS and neosubstrates IKZF1, IKZF3, CK1-α, RNF166, GSPT1, and BRD4, all of which are crucial drug targets in different types of cancer. USP15 antagonizes ubiquitylation of CRL4CRBN target proteins, thereby preventing their degradation. Notably, USP15 is highly expressed in IMiD-resistant cells, and depletion of USP15 sensitizes these cells to lenalidomide. Inhibition of USP15 represents a valuable therapeutic opportunity to potentiate CELMoD and CRBN-based PROTAC therapies for the treatment of cancer.

The Casein kinase 1α agonist pyrvinium attenuates Wnt-mediated CK1α degradation via interaction with the E3 ubiquitin ligase component Cereblon.

The Cullin-RING ligase 4 E3 ubiquitin ligase component Cereblon (CRBN) is a well-established target for a class of small molecules termed immunomodulatory drugs (IMiDs). These drugs drive CRBN to modulate the degradation of a number of neosubstrates required for the growth of multiple cancers. Whereas the mechanism underlying the activation of CRBN by IMiDs is well described, the normal physiological regulation of CRBN is poorly understood. We recently showed that CRBN is activated following exposure to Wnt ligands and subsequently mediates the degradation of a subset of physiological substrates. Among the Wnt-dependent substrates of CRBN is Casein kinase 1α (CK1α), a known negative regulator of Wnt signaling. Wnt-mediated degradation of CK1α occurs via its association with CRBN at a known IMiD binding pocket. Herein, we demonstrate that a small-molecule CK1α agonist, pyrvinium, directly prevents the Wnt-dependent interaction of CRBN with CK1α, attenuating the consequent CK1α degradation. We further show that pyrvinium disrupts the ability of CRBN to interact with CK1α at the IMiD binding pocket within the CRBN-CK1α complex. Of note, this function of pyrvinium is independent of its previously reported ability to enhance CK1α kinase activity. Furthermore, we also demonstrate that pyrvinium attenuates CRBN-induced Wnt pathway activation in vivo. Collectively, these results reveal a novel dual mechanism through which pyrvinium inhibits Wnt signaling by both attenuating the CRBN-mediated destabilization of CK1α and activating CK1α kinase activity.

Design, synthesis and biological evaluation of the tumor hypoxia-activated PROTACs bearing caged CRBN E3 ligase ligands.

Tumor hypoxia-activated proteolysis targeting chimeras (ha-PROTACs) 9 and 10 were designed and synthesized by incorporating the hypoxia-activated leaving group (1-methyl-2-nitro-1H-imidazol-5-yl)methyl or 4­nitrobenzyl into the structure of the cereblon (CRBN) E3 ligand of an epidermal growth factor receptor 19 deletions (EGFRDel19-based PROTAC 8. The in vitro protein degradation assay demonstrated that 9 and 10 could effectively and selectively degrade EGFRDel19 in tumor hypoxia. Meanwhile, these two compounds showed higher potency in inhibiting cell viability and migration, as well as in promoting cells apoptosis in tumor hypoxia. Moreover, nitroreductase reductive activation assay indicated that prodrugs 9 and 10 could successfully release the active compound 8. This study confirmed the feasibility to develop ha-PROTACs to enhance the selectivity of PROTACs by caging CRBN E3 ligase ligand.

Design, Synthesis, and Biological Evaluation of New Potential Unusual Modified Anticancer Immunomodulators for Possible Non-Teratogenic Quinazoline-Based Thalidomide Analogs.

Sixteen new thalidomide analogs were synthesized. The new candidates showed potent in vitro antiproliferative activities against three human cancer cell lines, namely hepatocellular carcinoma (HepG-2), prostate cancer (PC3), and breast cancer (MCF-7). It was found that compounds XII, XIIIa, XIIIb, XIIIc, XIIId, XIVa, XIVb, and XIVc showed IC50 values ranging from 2.03 to 13.39 µg/mL, exhibiting higher activities than thalidomide against all tested cancer cell lines. Compound XIIIa was the most potent candidate, with an IC50 of 2.03 ± 0.11, 2.51 ± 0.2, and 0.82 ± 0.02 µg/mL compared to 11.26 ± 0.54, 14.58 ± 0.57, and 16.87 ± 0.7 µg/mL for thalidomide against HepG-2, PC3, and MCF-7 cells, respectively. Furthermore, compound XIVc reduced the expression of NFκB P65 levels in HepG-2 cells from 278.1 pg/mL to 63.1 pg/mL compared to 110.5 pg/mL for thalidomide. Moreover, compound XIVc induced an eightfold increase in caspase-8 levels with a simultaneous decrease in TNF-α and VEGF levels in HepG-2 cells. Additionally, compound XIVc induced apoptosis and cell cycle arrest. Our results reveal that the new candidates are potential anticancer candidates, particularly XIIIa and XIVc. Consequently, they should be considered for further evaluation for the development of new anticancer drugs.

N-Boc-α-diazo glutarimide as efficient reagent for assembling N-heterocycle-glutarimide diads via Rh(II)-catalyzed N-H insertion reaction.

A technique has been proposed for incorporating a heterocyclic component into a glutarimide framework employing a Rh2(esp)2-catalyzed N-H insertion with the involvement of N-Boc-α-diazo glutarimide. The new diazo reagent is more stable, soluble and convenient to prepare than the previously suggested one. The approach permits the application of diverse heterocycles, including both aromatic and saturated NH-substrates. This yields structures that are appealing for generating cereblon ubiquitin-ligase ligands and for potential use in crafting PROTAC molecules.

Design, synthesis, and biological evaluation of a novel series of 2-(2,6-dioxopiperidin-3-yl)isoquinoline-1,3(2H,4H)-dione derivatives as cereblon modulators.

In the current study, we designed and synthesised a novel series of 2-(2,6-dioxopiperidin-3-yl)isoquinoline-1,3(2H,4H)-dione derivatives as cereblon (CRBN) modulators. The results of the CCK8 assay revealed potent antiproliferative activity for the selected compound 10a against NCI-H929 (IC50=2.25 µM) and U239 (IC50=5.86 µM) cell lines. Compound 10a also can inhibit the TNF-α level (IC50=0.76 µM) in LPS stimulated PMBC and showed nearly no toxicity to this normal human cell line. The TR-FRET assay showed compound 10a having potent inhibitory activity against CRBN (IC50=4.83 µM), and the docking study confirmed a nice fitting of 10a into the active sites of CRBN. Further biology studies revealed compound 10a can increase the apoptotic events, arrest the NCI-H929 cells at G0/G1 cell cycle, and induce the ubiquitination degradation of IKZF1 and IKZF3 proteins by CRL4CRBN. These preliminary results suggested that compound 10a could serve as a potential antitumor drug and worthy of further investigation.

Design, synthesis, and characterization of PROTACs targeting the androgen receptor in prostate and lung cancer models.

Although the androgen receptor (AR) is a validated target for the treatment of prostate cancer, resistance to antiandrogens necessitates the development of new therapeutic modalities. Exploiting the ubiquitin-proteasome system with proteolysis-targeting chimeras (PROTACs) has become a practical approach to degrade specific proteins and thus to extend the portfolio of small molecules used for the treatment of a broader spectrum of diseases. Herein, we present three subgroups of enzalutamide-based PROTACs in which only the exit vector was modified. By recruiting cereblon, we were able to demonstrate the potent degradation of AR in lung cancer cells. Furthermore, the initial evaluation enabled the design of an optimized PROTAC with a rigid linker that degraded AR with a DC50 value in the nanomolar range. These results provide novel AR-directed PROTACs and a clear rationale for further investigating AR involvement in lung cancer models.

Targeting the E3 ubiquitin ligases DCAF15 and cereblon for cancer therapy.

Small molecule-mediated inhibition of protein function is the rational behind therapeutic efficacy of the majority clinically used drugs. In order for a drug to achieve pharmacologically relevant inhibition, efficient target engagement at high selectivity and specificity is necessary to obtain the desired therapeutic effect minimizing offtarget outcomes. Majority of small molecules approaches developed so far have failed in their attempt to reach clinical efficacy because of low selectivity and low specificity in achieving close to 100 % target inhibition. Recently, approaches that directly control cellular protein levels have opened the potential to accomplish a high grade of efficacy not imaginable with traditional small-molecule inhibitors. Research in this area has just started opening avenues to effectively degrade a cellular target of choice and will soon impact clinical efficacy.

Cereblon: promise and challenges for combating human diseases.

Cereblon (CRBN) is a substrate recognition protein in the E3-ligase ubiquitin complex. The binding target of CRBN varies according to tissues and cells, and the protein regulates various biological functions by regulating tissue-specific targets. As new endogenous targets of CRBN have been identified over the past decade, the physiological and pathological functions of CRBN and its potential as a therapeutic target in various diseases have greatly expanded. For this purpose, in this review article, we introduce the basic principle of the ubiquitin-proteasome system, the regulation of physiological/pathological functions related to the endogenous substrate of CRBN, and the discovery of immunomodulatory imide drug-mediated neo-substrates of CRBN. In addition, the development of CRBN-based proteolysis-targeting chimeras, which has been actively researched recently, and strategies for developing therapeutic agents using them are introduced. These recent updates on CRBN will be useful in the establishment of strategies for disease treatment and utilization of CRBNs in biomedical engineering and clinical medicine.