Immunomodulatory drugs (IMiDs), today also known as cereblon (CRBN) binding drugs, are therapeutically important anti-cancer and anti-inflammatory drugs. IMiDs are analogs of their prototype compound thalidomide. IMiDs have immune-modulation, anti-angiogenic, anti-inflammatory and anti-proliferative effects. CRBN is a component and substrate receptor of the Cullin 4 Ring E3 Ubiquitin Protein Ligase complex (CRL4). CRL4 consists of Cullin 4, RING finger protein (Roc1), and DNA damage binding protein 1 (DDB1). CRBN binds to its substrate proteins and it leads to ubiquitination of these substrates by the CRL4. CRBN is also involved in IMiDs-mediated T-cell co-stimulation and cytokine production. CRBN is a primary target of thalidomide teratogenicity. The binding of IMiDs to CRBN is associated with cytotoxicity of IMiDs and is used to treat multiple myeloma (MM), myelodysplastic syndromes (MDS), lymphomas and chronic lymphocytic leukemia. CRBN is composed of an N-terminal ATP-dependent serine protease Lon-like domain, which links to the E3 ubiquitin protein ligase complex CRL4, and a C-terminal domain, which binds IMiDs. CRBN binding is mediated by a glutarimide ring in thalidomide, lenalidomide, pomalidomide, CC-122, CC-220, CC-885 and CC-90009. Development of effector molecules mediating targeted ubiquitination of disease related proteins through cereblon is a new important way in pharmacology.
immunomodulatory drugs, cereblon, cullin 4 E3 ubiquitin protein ligase, Ikaros transcription factors famil, casein kinase 1A1, translation termination factor GSPT1, ubiquitination, proteasome
IMiDs include thalidomide, lenalidomide, pomalidomide (Figure 1), CC-122, CC-220, CC-885 and CC-90009 till now. Both, lenalidomide (Revlimid®, initially known as CC-5013), and pomalidomide (Pomalyst®, initially known as CC-4047, Actimid) are a synthetic derivative of thalidomide (Thalomid®, Inmunoprin, Talidex, Talizer).
Figure 1. Chemical structures of immunomodulatory drugs (IMiDs) including thalidomide, lenalidomide and pomalidomide. Lenalidomide and pomalidomide are synthetic compounds derived by modifying the chemical structure of thalidomide.
Thalidomide [(RS)-2-(2,6-dioxopiperidin-3-yl)-1H-isoindole-1,3(2H)-dione] was synthesized in Germany, in 1954, from α-phtaloylisoglutamine, to be used as a non-barbiturate sedative and antimetic drug (Contergan). In 1957, after a short period of preclinical studies, thalidomide was approved for first trimester gestational sickness in humans. The appearance of malformations such as phocomelia in the newborn banned its use three years later. Thalidomide was responsible for birth defects in more than 10,000 children [1,2]. The US Food and Drug Administration (FDA) approved thalidomide in 1998 for the treatment of erythema nodosum leprosum . Thalidomide exhibits potent antiangiogenic and immunomodulatory effects, and is currently used around the world to treat a range of conditions, mainly multiple myeloma. However, long-term use of thalidomide has detrimental side effects, such as peripheral neuropathy [4,5].
Lenalidomide was developed in order to avoid thalidomide side effects (sedation and neuropathy), and to increase efficacy [6,7]. Lenalidomide shares a number of structural and biological properties with thalidomide but is safer and more potent than thalidomide Lenalidomide [3-(4-amino-1-oxo1,3-dihydro-2H-isoindol-2-yl) piperidine-2,6-dione] is 4-amino-glutarimide analog of thalidomide with potent immunomodulatory, antiangiogenic and direct neoplastic cell inhibitory activity [6-26].
Pomalidomide [4-Amino-2-(2,6-dioxopiperidin-3-yl) isoindole-1,3-dione] is a potent second-generation IMiD [27-34]. Pomalidomide has direct antiproliferative, pro-apoptotic, and antiangiogenic effects, as well as modulatory effects on bone resorption and on the immune system. Chemical structure of the IMiD drugs is shown in Figure 1.
CC-122 hydrochloride is a novel immunomodulatory agent-like thalidomide analog which directly binds to CRBN and promotes ubiquitination and degradation of zinc finger transcription factors Aiolos and Ikaros in difuse large B cell-lymphoma (DLBCL) [35-39]. CC-220 is a further IMiD that binds to CRBN and is currently in phase 1b/2aclinical trials for the treatment of relapsed and refractory multiple myeloma (MM) and in phase 2 clinical trials for systemic lupus erythematosus (SLE) . CC-220 binds tighter to the CRBN in the complex of CRL4 E3 ubiquitin protein ligase than lenalidomide and causes more potent degradation of transcription factors Aiolos and Ikaros. CC-885 binds also to CRL4CRBN but mediates not only degradation of Aiolos but also specifically binds to the eukaryotic translation termination factor 3a (eRF3a), also known as GSPT1 (G1 to S phase transition 1) and mediates degradation of this protein [41,42]. GSPT1 together with the eukaryotic translation termination factor 1 (eRF1) functions in stop codon recognition and nascent protein release from ribosome in the process of mRNA translation. Lenalidomide and pomalidomide do not mediate the degradation of GSPT1. CC-90009 is a further IMiD that binds the E3 protein ubiquitin ligase CRL4CRBN and mediates degradation of certain lymphoid transcription factors, including Ikaros (IKZF1) and Aiolos (IKZF3), which are transcriptional repressors in T-cells. This reduces the levels of these transcription factors, and modulates the activity of the immune system, which may include the activation of T-lymphocytes. In addition, this downregulates the expression of other proteins, including interferon regulatory factor 4 (IRF4) and c-myc, which plays a key role in the proliferation of certain cancer cell types. CC-90009 is now in phase 1 clinical trials (NCT 02848001) in subjects with relapsed or refractory acute myeloid leukemia (AML) in Canada and USA.
Mechanism of immunomodulatory drugs action in the treatment of multiple myeloma
IMiDs target both MM cells and their microenvironment, while also modulating the immune system. The exact molecular mechanisms of the antitumor effects of IMiDs remain uncertain. IMiDs affect various molecular and cellular elements within the tumor microenvironment. IMiDS change the concentration of various cytokines that support tumor cell growth [8-10,27,43,44]. IMiDs disrupt bone marrow stromal support for malignant MM cells, although the exact mechanisms of these actions remain unclear. IMiDs decrease the expression of adhesion molecules that facilitate the interaction between MM cells and bone marrow stromal cells (BMSCs) . Importantly, the downregulation of MM cells adherence to BMSCs can overcome the cellular adhesion-mediated drug resistence by malignant MM cells. This effect of IMiDs is further increased by their ability to downregulate TNFα [9,43,46,47]. Surprisingly, in contrast to the inhibitory effect of lenalidomide in BMSCs, lenalidomide was totally ineffective in inhibiting TNFα mRNA expression in MM cells .
Immunomodulatory activities of IMiDs
IMiDs are a potent co-stimulator of primary human T cells, synergizing with stimulation via T-cell receptor complex to increase IL-2-mediated T-cell proliferation and interferon gamma (IFN-γ) production [12-14,48-50]. Secretion of IL-2 and IFN-γ increases the number of natural killer (NK) cells, improves their function and mediates lysis of MM cells (Figure 2). NKT cells are a heterogeneous group of T-cells that recognize lipids and glycolipids presented by CD1d molecules. NKT cells activated in presence of lenalidomide, have greater ability to secrete IFN-γ. Lenalidomide enhances antigen-specific expansion of NKT cells . Regulatory T-cells (Tregs) are a component of the immune system that suppresses immune response of other cells. Tregs were elevated in MM patients. IMiDs strongly inhibits Tregs proliferation via decreased FOXP3 mRNA expression .
Figure 2. Schematic diagram of ubiquitination and degradation of zinc finger lymphoid transcription factors Ikaros and Aiolos by CRL4cereblon E3 ubiquitin ligase and proteasomes. Binding of cereblon (CRBN) by lenalidomide induces ubiquitination (marking IKZF1 / Ikaros / and IKZF3 / Aiolos /) and degradation of both, Ikaros and Aiolos transcription factors. CRBN functions as a substrate recognition component (substrate receptor) of this E3 ubiquitin ligase enzyme complex. CRL4cereblon complex consists of cullin 4A, RING finger protein regulator of cullins (Roc1), and DNA damage binding protein 1 (DDB1).
Anti-angiogenic properties of IMiDs
All IMiDs have anti-angiogenic activity. Thalidomide has predominant anti-angiogenic activity while lenalidomide and pomalidomide have far greater immune enhancing effects . It appears that anti-angiogenesis occurs via the modulation of chemotactic factors involved in endothelial cells migration including TNFα, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) from BMSCs rather than a direct inhibition of endothelial cells proliferation [53,54]. Inhibition of VEGF and bFGF production by IMiDs is likely to have multiple other biological effects beyond that of anti-angiogenesis, including inhibition of IL-6 production by BMSC. VEGF and bFGF up-regulate IL-6 and other pro-inflammatory cytokines .
Inhibition of cell cycle and induction of apoptosis by IMiDs
IMiDs induce cell cycle arrest via upregulation of tumor suppressor genes (cyclin dependent kinase inhibitors p15INK4b, p16INK4a, p21Cip1,Waf1, p27Kip1; early response transcription factors / Erg1, Erg2 and Erg3/) and apoptosis by caspase activation [56,57]. The activity of the transcription factor NF-κB inhibition by IMiDs resulted in reduced expression of anti-apoptotic proteins including cellular inhibitor of apoptosis protein 2 (cIAP2)  and FLIP [Fas-associated protein with death protein (FADD)-like interleukin-1β-converting enzyme (FLICE) inhibitor protein] . These anti-apoptotic proteins inhibit caspase-8, that is on the other hand stimulated by IMiDs .
Disruption of bone marrow stromal support for malignant MM cells
IMiDs down-regulate the expression of adhesion molecules [leukocyte function-associated antigen 1 (LFA-1, CD11a), intercellular adhesion molecule 1 (ICAM-1, CD54), vascular cell adhesion molecule 1 (VCAM-1, CD106) and very late antigen 4 (VLA-4)] that facilitate the interaction between MM cells and BMSC. As we described, IMiDs inhibit NF-κB, a transription factor that has important growth and anti-apoptotic roles and which is connected with the upregulation of intracellular adhesion molecules and many cytokines .
IMiDs effect on myeloma cell proliferation
The direct anti-MM effect of IMiDs was shown to occur through the induction of a G1 phase of cell cycle growth arrest of MM cells  and was associated with a decrease in interferon regulatory factor 4 (IRF4), a transcription factor that is critical for MM cell growth and survival, (Figure 2), [61,62].
Raje, et al.  showed strong synergism of anti-MM activity of rapamycin (Rapamune), a specific mTOR inhibitor, combined with CC-5013. Importantly, this combination was able to overcome drug resistance when tested against MM cell lines resistant to conventional chemotherapy. Moreover, the combination, but not rapamycin alone, was able to overcome the growth advantage conferred on MM cells by interleukin-6 (IL-6), insulin-like growth factor-1 (IGF-1), or adherence to bone marrow stromal cells (BMSCs). Combining rapamycin and CC-5013 induced apoptosis of MM cells. Differential signaling cascades, including the mitogen-activated protein kinase (MAPK) and the phosphatidylinositol 3'-kinase / Akt kinase (PI3K /Akt) pathways [64-67], were targeted by these drugs individually and in combination, suggesting the molecular mechanism by which they interfere with MM growth and survival. These studies, therefore, provide the framework for clinical evaluation of mTOR inhibitors combined with IMiDs to improve patient outcome in MM.
IMiDs down-regulate CCAAT/enhancer-binding protein-β (C/EBPβ) resulting in abrogation of cell proliferation . Overexpression of C/EBPβ rescued MM cells from IMiD-induced inhibition of proliferation, indicating that C/EBPβ is critical in mediating antiproliferative effects. IMiD-induced decrease of C/EBPβ protein led to impaired transcription of interferon regulatory factor 4 (IRF4).
Down-regulation of IRF4 by lenalidomide was confirmed by longitudinal studies of bone marrow samples from 23 patients obtained before and during lenalidomide treatment using CD138⁺/IRF4⁺ double labeling. In contrast to down-regulation of C/EBPβ protein, IMiD compounds did not alter C/EBPβ mRNA levels or protein stability, suggesting translational regulation of C/EBPβ. We could demonstrate that C/EBPβ protein expression is under eIF4E-translational control in MM. Furthermore, inhibition of the eIF4E-C/EBPβ axis by IMiD compounds was not observed in IMiD-resistant MM cells.
However, targeting translation at a different level by inhibiting eukaryotic translation initiation factor 4E-binding protein 1 phosphorylation overcame resistance, suggesting that this pathway is critical and might be a target to overcome drug resistance. MM cell lines and primary MM cells strongly expressed C/EBPβ, whereas normal B cells and plasma cells had little or no detectable levels of C/EBPβ . Silencing of C/EBPβ led to down-regulation of transcription factors such as IRF4, XBP1, and BLIMP1 accompanied by a strong inhibition of proliferation. Further, silencing of C/EBPβ led to a complete down-regulation of antiapoptotic B-cell lymphoma 2 (BCL2) expression. In chromatin immunoprecipitation assays, C/EBPβ directly bound to the promoter region of IRF4, BLIMP1, and BCL2. C/EBPβ is involved in the regulatory network of transcription factors that are critical for plasma cell differentiation and survival. Targeting C/EBPβ may provide a novel therapeutic strategy in the treatment of multiple myeloma.
Exposure plasma cells to lenalidomide activated the Wnt/β-catenin pathway and its downstream targets such as cyclin D1 and MYC . The accumulation of β-catenin during treatment with lenalidomide might be cause of drug resistance . N-cadherin-based interaction between MM cells and osteoblasts block MM cell growth. Therefore, the high levels of N-cadherin expression in osteoblasts confers strong proliferation block on MM cells. Since β-catenin associates with N-cadherin at the cell membrane, N-cadherin adhesion is disrupted, β-catenin is released and will translocate to the nucleus leading to the transcription of target genes, and ultimately, cell proliferation . Resistance of myeloma to lenalidomide is an emerging clinical problem, and though it has been associated in part with activation of Wnt/β-catenin signaling, the mediators of this phenotype remained undefined. Lenalidomide-resistant models were found to overexpress the hyaluronan (HA)-binding protein CD44, a downstream Wnt/β-catenin transcriptional target . Consistent with a role of CD44 in cell adhesion-mediated drug resistance (CAM-DR), lenalidomide-resistant myeloma cells were more adhesive to bone marrow stroma and HA-coated plates. Blockade of CD44 with monoclonal antibodies, free HA or CD44 knockdown reduced adhesion and sensitized to lenalidomide. Wnt/β-catenin inhibition by FH535, a compound that suppresses both Wnt/β-catenin and peroxisome proliferator-activated receptor (PPAR) signaling, enhanced the activity of lenalidomide, as did interleukin-6 neutralization with siltuximab.
Cereblon as the direct target protein of IMiDs
Ito, et al. [73-76] developed a new affinity bead technology for isolating ligand-binding proteins. Polymer-coated beads were constructed that allow single-step purification of ligand target molecules. These beads include styrene-glycidyl-methacrylate (SG) beads and ferrite-glycidyl-methacrylate (FG) beads. FG beads were used for the purification of thalidomide-binding proteins from various cell extracts. Thalidomide-modified beads were incubated with cell extracts and then washed with buffer. Bound proteins were eluted with free thalidomide and analyzed by gel electrophoresis. Only two specific protein bands were detected (55 kDa and 127 kDa). These proteins were identified as CRBN and DDB1. As we described in Abstract and Introduction, CRBN, like DDB1, is a component of the cullin 4 ring E3 ubiquitin ligase complex (CRL4). Auto-ubiquitination of CRBN was inhibited by thalidomide in vitro, suggesting that thalidomide is an inhibitor of E3 ubiquitin ligase .
Cereblon as a primary target for thalidomide teratogenicity
The zebrafish was adopted as a model animal for in vivo study of thalidomide teratogenicity [73-76]. Zebrafish have a protein zCrbn, which is 70% homologous to human CRBN and possesses thalidomide binding activity. Thalidomide exposure induces fin and ear (otic vesicle) defects in zebrafish. A reduction in in the size of otic vesicle was observed in zebrafish following thalidomide exposure. Zebrafish fins and tetrapod limbs are homologous with respect to early patterning and gene expression, although skeletal structures in adult fish and tetrapods are quite different. Shortening of the pectoral fins along the proximo-distal axis and the inhibition of fibroblast growth factor 8 (Fgf8) gene expression were found in thalidomide-treated zebrafish embryos. Finally, chicks were employed for the investigation of the conserved role of CRBN. Down-regulation of Fgf8 and Fgf10 cause multiple birth defects and limb deformities.
The role of Ikaros family proteins in IMiDs and CRBN mechanism
Using distinct but complementary proteomic techniques and systems, three groups have recently simultaneously reported that IMiDs induced the CRBN-dependent proteasomal degradation of IKZF1 (Ikaros) and IKZF3 (Aiolos) [50,78,79]. Schema is shown in Figure 2. IKZF1, a zinc finger transcription factor initially discovered as a regulator of the T cell receptor, is required for hematopoiesis, particularly lymphocyte development and plasma cell maturation. Loss of function mutations of IKZF1 and IKZF3 are associated with acute lymphoblastic leukemia, consistent with a tumor suppressor function. On the other hand, IKZF1 and IKZF3 are required for the viability of many MM cell lines. IKZF1 and IKZF3 are also involved in the complex process of chromatin remodeling, and the nature of their interactions is poorly understood.
IKZF1 bound and activated the IRF4 gene promoter and loss of IKZF1 led to decreased IRF4 and MYC expression. However, lenalidomide could also inhibit MM cell lines with high basal levels of IRF4 unchanged by drug treatment, suggesting that other IKZF1/3 targets can play a role in the therapeutic response to IMiDs. IKZF1/3 are known repressors of IL-2 gene promoter. The degradation of IKZF1/3 in response to IMiDs explains enhanced T cell IL-2 production. Hence, many of the effects of IMiDs can be explained by a unified mechanism: IMiDs re-target the cullin 4 ring E3 ubiquitin ligase activity toward IKZF1/3 in a change-of-function effect (Figure 2).
An alternative CRL4CRBN substrate in the lenalidomide-responsive myelodysplastic syndrome with del(5q)
Krӧnke, et al  identified a novel target casein kinase1A1 (CSNK1A1) by quantitative proteomics in the myeloid cell line KG-1. CSNK1A1 is encoded in the del(5q) commonly deleted region and the gene is haploinsufficient. Lenalidomide treatment leads to increased ubiquitination of CSNK1A1 and decreased protein abundance (Figure 3). CSNK1A1 was shown as a therapeutic target in a murine model of AML [80,81] and in MDS with del(5q) [82,83]. CSNK1A1 negatively regulates β-catenin which drives stem cell self-renewal and CSNK1A1 haploinsufficiency causes the initial clonal expansion in patients with the del(5q) MDS and contributes to the pathogenesis of del(5q) MDS. Further inhibition of CSNK1A1 in del(5q) MDS is associated with del(5q) cells apoptosis and p53 activation. The inhibition of CSNK1A1 reduced RPS6 phosphorylation, induced p53 expression, and triggered myeloid differentiation program. TP53-null leukemia did not respond to CSNK1A1 inhibition, strongly supporting the importance of the p53 expression for the yield of CSNK1A1 inhibition. CSNK1A1 mutations have been recently found in 5-18% of MDS patients with del(5q) [82,84-87]. These mutations are associated similarly to the effect of TP53 mutations with rise to a poor prognosis in del(5q) MDS .
Figure 3. Schematic diagram of ubiquitination and degradation of casein kinase 1α by CRL4cereblon E3 ubiquitin ligase and proteasomes. Binding of cereblon (CRBN) by lenalidomide induces ubiquitination (marking CK1α) and degradation of casein kinase 1α. CRBN functions as a substrate recognition component (substrate receptor) of this E3 ubiquitin ligase enzyme complex. CRL4cereblon complex consists of cullin 4A, RING finger protein regulator of cullins (Roc1), and DNA damage binding protein 1 (DDB1).
While CSNK1A1 is CRL4CRBN target in del(5q) MDS, CRL4CRBN targets in lower risk non-del(5q) remain to be determined. The mechanism of action of lenalidomide is still unclear in non-del(5q) MDS cells.
Importance of valosin-containing protein/p97 for the degradation of all known CRL4CRBN targets
Valosin-containing protein (VCP)/p97 is ATPase which delivers ubiquitinated proteins for degradation in proteasomes. VCP is required for degradation of all known CRL4CRBN targets (IKZF1, IKZF3, casein kinase 1α, and the translation termination factor GSPT1) . VCP promotes also degradation of glutamine synthetase (GS). GS is important for de novo synthesis of glutamine and functions tn detoxification of glutamate and ammonia. GS is associated with diseases including cancer, Alzheimer´s disease and Huntington disease .
Immunomodulatory drugs disrupt the cereblon-CD147-MCT1 axis to exert anti-cancer activity and teratogenicity in a ubiquitin-independent way
CRBN promotes the activation of the CD147 (basigin; BSG) - MCT1 (solute carrier family 16 members 1; SLC16A1) transmembrane complex. This complex activatesvarious biological functions, including angiogenesis, proliferation, invasion and lactate export. Binding of IMiDs to CRBN leads to destabilization of the CD147-MCT1 complex and to the inhibition of tumor growth .
Measurement of cereblon mRNA and cereblon protein levels as biomarkers for IMiDs response
The measurement of CRBN mRNA by quantitative RT-PCR typically uses commercial assays, such as predesigned TaqMan assays, where primers and probe are optimized with respect to known gene splicing information. For CRBN mRNA, the current “best coverage assay” detects the exon 8-exon 9 junction and exon 10 as a measure of gene expression . This assay (Hs00372271_m1; Applied Biosystems, Life Technologies Corp.) measures all CRBN mRNA variants that are translated to functional protein with exception of the variants with removed exon 10 (part of IMIDs binding region). The presence of multiple CRBN mRNA splice variants complicates the transcript measurement by Affymetrix array.
We have found that MDS patients with isolated del(5q) (the so-called 5q minus syndrome) have higher levels of full-length CRBN mRNA than other patients with lower risk MDS, linking higher levels of a known lenalidomide target CRBN and an MDS subgroup known to be especially sensitive to lenalidomide .
Currently available commercial antibodies are neither sensitive nor specific for reliable detection of CRBN protein levels. Gandhi et al.  characterized a monoclonal antibody CRBN65 and compared its properties with the commonly used, currently available commercial antibodies against CRBN. This antibody is the most sensitive and specific and can detect as little as 200 pg of CRBN protein via Western blot .
Appropriate antibodies and validated assays for cereblon protein detection and CRBN gene expression that account for the known gene splicing information are needed for CRBN measurements in the clinic.
While it has been shown that CRBN and IRF4 levels correlate with lenalidomide responsiveness in MM patients, previous in vitro investigations using cytogenetically discrete human myeloma cell lines have not been able to replicate this phenomenon [93,94]. The implications of this apparent discordance are two-fold. First, these results show that it is important to investigate CRBN and IRF4 gene expression in both CD138- and CD138+ myeloma cells. Second, because there are multiple isoforms of CRBN, it may be important to study expression levels of each one in the context of both in vivo and in vitro settings .
Conclusion and perspectives
The studies showing IMiDs induced the CRBN-dependent proteasomal degradation of IKZF1, IKZF3, and caein kinase 1α [18,50,78,79] have greatly advanced our understanding of the mechanism of action of IMiDs in MM and MDS with del(5q). The small-molecule drug lenalidomide modulates the activity of the CRBN-CRL4 E3 ubiquitin ligase complex to increase ubiquitination of two transcription factors, IKZF1 and IKZF3, or casein kinase 1α. It does so by specific binding to one component of the system, cereblon. Crystal structures of these complexes were studied in detail [83,94,95]. All these studies may have wider implications for the targeting of E3 ubiquitin ligases in drug discovery for other diseases. There are very few approved and experimental drugs that modulate the ubiquitin system like lenalidomide in this case. Other small molecules could be developed to alter the specificity of ubiquitination complexes with the aim of inducing the specific degradation of previously “undruggable” oncoproteins such as Ras and Myc in many different types of cancer [96-99]. Nevertheless, there are still gaps in our understanding of the mechanism of action of IMiDs in MM and other hematological malignancies. For example, the proteasomal inhibitor bortezomib (VelcadeTM, PS-341) is used for the treatment of MM. Bortezomib is used efficiently also in combination with lenalidomide. There is an apparent paradox, because inhibition of proteasomal destruction of IKZF1 and IKZF3 by bortezomib contradicts the preposed mechanism. Lu et al.  hypothesize that since the proteasomal inhibition by bortezomib is incomplete with therapeutic dosing, this might allow sufficient destruction of IKZF1 and IKZF3 while retaining other therapeutic effect of bortezomib. Alternatively, they hypothesize that IKZF1 and IKZF3 once polyubiquitylated, may be inactive or act as dominant-negatives.
Future dissection of CRBN direct down-stream substrates and CRBN indirect down-stream factors will help to identify mechanisms of IMiD action and find new biomarkers for prediction of IMiD response and IMiD resistence as well as developing a new therapy to treat the patients with MM.
It is also possible to speculate whether or not anti-myeloma activity of IMiDs is associated with the interaction between CRBN and AMPK (AMP-activated protein kinase) or KCNT1 (Potassium Sodium-Activated Channel Subfamily T Member 1). Both AMPK and KCNT1 are potential substrates of the CRBN-CRL4 E3 ubiquitin ligase complex, but they may not be associated with downstream signaling that leads to anti-myeloma activity of IMiDs.
This work was supported by the project for conceptual development of research organisation No 00023736 (Institute of Hematology and Blood Transfusion) from the Ministry of Health of the Czech Republic.
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