The Ikaros protein family includes five members named Ikaros, Helios, Aiolos, Eos, and Pegasus. Ikaros, Helios, and Aiolos are principally expressed in some hematopoietic cells and lymphoid cells; notwithstanding, Ikaros is also found in the brain. Eos and Pegasus are widely detected throughout the body, including the brain, liver, skeletal muscle, kidney, and heart (Fan and Lu, 2016) (Table 1). Various IKZF protein TFs emit diverse effects in the maintenance of normal physiological activities or progression of some diseases.

Ikaros proteins are identified to serve as suppressors in diverse types of lymphoma or leukemia (Heizmann et al., 2018; Chan, 2019), whereas they are overexpressed in other malignancies to maintain cancer cell proliferation and survival, such as in malignant plasma cells, monoclonal gammopathy of undetermined significance, and multiple myeloma (MM) (Cippitelli et al., 2021). Detailed functional analyses showed that Ikaros could excite or suppress oncogenes or tumor suppressors genes; consequently, Ikaros-mediated transcriptional expression of target genes regulates survival and proliferation of cancer cells (Gowda et al., 2017a). Specifically, Ikaros was reported to control human skin fibroblast cell migration negatively by GSK3β-Ikaros-ANXA4 signaling (Wang et al., 2020a).

It is assumed that Ikaros family deficiency may lead to a variety of immune-associated diseases, including immune thrombocytopenia (Sriaroon et al., 2019), presumed autoimmune hepatitis (Groth et al., 2020), systemic lupus erythematosus (SLE) (Cunninghame Graham et al., 2011; Jeng et al., 2019; Chen et al., 2020), rheumatoid arthritis (Yang et al., 2019), asthma (Igartua et al., 2015), type 1 diabetes (Davidson and Diamond, 2001; Swafford et al., 2011; Lempainen et al., 2013; Khamechian et al., 2018), Graves’ disease (Li et al., 2018a), Hashimoto thyroiditis (Hu et al., 2019), inflammatory bowel disease (Crohn disease) (Barrett et al., 2008; Eskandarian et al., 2019; Sznurkowska et al., 2020), di George syndrome (Klocperk et al., 2014), antiphospholipid syndrome (Dieudonné et al., 2019), Parkinson disease (Daneshvar Kakhaki et al., 2020), Sjogren’s syndrome, and systemic sclerosis (Gorlova et al., 2011).

It also shows that Ikaros can manage myeloid cell proliferation, and somatic Ikaros mutations are related to myeloproliferative disorders (Theocharides et al., 2015). In addition, the occurrence and maintenance of numerous human cancers, such as pediatric B-cell precursor acute lymphoblastic leukemia (BCP-ALL) (Churchman et al., 2018; Stanulla et al., 2018; Tayel et al., 2019), lung cancer (Li et al., 2014; Zhao et al., 2020), breast cancer (Edgren et al., 2011), nasopharyngeal carcinoma (Verhoeven et al., 2019), ovarian (He et al., 2012), liver (Liu et al., 2017), and colorectal cancer (Javierre et al., 2011), are also correlated with the abnormal expression of Ikaros family proteins. Recently, for some solid tumors, it was shown that a higher level of Ikaros is correlated with poor differentiation and advanced stage of ovarian cancer (He et al., 2012), while it functions as an anticancer character in hepatocellular carcinoma through inhibiting CD133 and ANXA4 expression (Liu et al., 2017). Besides, the hypermethylation of Ikaros levels could be considered as a sign of the progression of colorectal cancer (CRC) and inform adequacy of surgical resection about CRC (Javierre et al., 2011; Symonds et al., 2018; Symonds et al., 2020).

For patients with MM, the Ikaros family proteins served as predictors of prognosis for MM patients treated by Len (Kriegsmann et al., 2019; Tachita et al., 2020). However, a correlation between the expression of IKZF1 or IKZF3 and patients’ reaction to Len from immunohistochemical analysis remains obscure (Dimopoulos et al., 2019). It is considered that the effect of low IKZF1 or IKZF3 levels on the adverse outcome of Len therapy results in shorter progression-free survival and overall survival (Zhu et al., 2014; Pourabdollah et al., 2016; Dimopoulos et al., 2019).

In ALL, approximately 50% of adult patients possess IKZF1 genetic mutations, including beyond 80% of patients with BCR-ABL1-positive (Ph+) ALL. A total of 15% of IKZF1 genetic alteration can be found in childhood B-cell ALL, covering about 70% of Ph + ALL patients (Mullighan et al., 2007; Mullighan et al., 2008). Patients with ALL have Ikaros mutations with characteristic resistance to treatment (Marke et al., 2016), high relapse rate (Kuiper et al., 2010; Berry et al., 2020), and poor prognosis (Mullighan et al., 2009; Aref et al., 2020). Genetic and functional abnormalities of IKZF1, including deletion of a single Ikaros, were regarded as new prognostic indicators for high-risk leukemia in clinical trials (NCT00993538; NCT03709719, NCT01431664) (Mi et al., 2012; Tang et al., 2019; Granados-Zamora et al., 2020).

In chronic myelogenous leukemia (CML), the deletions in IKZF1 and codeletion of other genes are identified in a chronic phase CML diagnostic sample (Klumb et al., 2019). Moreover, the deficiency or reduction of Ikaros is deemed as a common step and potential diagnostic precursor of progressive myeloid disease in patients with CML (Beer et al., 2015).

Immunomodulatory drugs (IMiDs), consist of thalidomide, Len, pomalidomide, and an analog, target a ubiquitous protein called CRBN to induce the degradation of Ikaros. The efficacy and safety of those drugs have been verified in a wide range of clinical trials, and it is increasingly clear that the efficacy of IMiDs in the treatment of MM (Gao et al., 2020), myelodysplastic syndrome (MDS) with deletion of chromosome 5q (Fenaux et al., 2011), mantle cell lymphoma (MCL) (Ruan et al., 2018) and chronic lymphocytic leukemia (CLL) (Vitale et al., 2016; Zhou et al., 2020) is promising. Despite the development of Ikaros-targeted therapy, the incidence of drug resistance is increasing. Herein, we summarized the molecular characteristic of Ikaros, the mechanism of the Ikaros-associated pathway, and recent anti-Ikaros drug development based on clinical trials in our review.

Studies on signaling pathways about Ikaros have attracted some attention (Figure 2). The most fully studied Ikaros-related pathway is the preBCR (B-cell receptor) signal pathway, which has formed a distinct picture of the regulatory network (Alkhatib et al., 2012). Reciprocally, the activated preBCR pathway eventually decreased the activity of Ikaros; Ikaros can counteract this effect by suppressing two sites in the pathway (Yasuda et al., 2000; Nera et al., 2006; Nakayama et al., 2009).

The Notch pathway has also been relatively well studied. Notch3 converts the alternative splicing mode of Ikaros to a dominant-negative isoform by upregulating the expression of RNA binding protein HuD (Bellavia et al., 2007). On the contrary, Ikaros counteracts the Notch effect on CSL activation through competing with the CSL DNA binding site in the promoter region and inhibits the expression of the downstream genes, including the components of preTCR (Dumortier et al., 2006).

It was regarded that ERK1/2-mediated ETS1 phosphorylation decreased the ability of ETS1 to increase Ikaros expression (Joshi et al., 2014). And the activated integrin signaling pathway was reported to be restrained by Ikaros as well. Ikaros is affected by several interferon regulatory factors (IRFs), among which IRF4 and IRF8 were considered to induce the expression of Ikaros and its homologous gene Aiolos (Ma et al., 2008); however, another study showed that IRF8 rather than IRF4 triggers the IKZF1 promoter, and IRF5 could inhibit this activation (Fang et al., 2012).

Ikaros activities are thought to be regulated by post-translational phosphorylation, small ubiquitin-related modifier (SUMOylation), and ubiquitination (Sridharan and Smale, 2007). Besides, the hypomethylated pattern of CpG island in the IKZF1 promoter region may be the basis of abnormal Ikaros expression patterns associated with malignant tumors (Chen et al., 2019; Rahmani et al., 2019). SUMOylation interferes in the interaction of Ikaros with transcriptional co-repressors SIN3A, SIN3B, Mi-2β, and CtBP and weakens the inhibitory activity of Ikaros (Gómez-del Arco et al., 2005). And the participations of Ikaros in HDAC-dependent and HDAC-independent inhibition are disrupted by Ikaros SUMOylation, but the nuclear localization to pericentromeric heterochromatin is not affected (Gómez-del Arco et al., 2005). Ikaros SUMOylation was discovered to exist in B-ALL cells, whereas it did not show up in normal peripheral blood leukocytes, indicating its potential work in leukemia (Chen et al., 2019). The process of deSUMOylation was actively modulated by SUMO-specific protease Senp1, Axam, and yeast Ulp1 (Ihara et al., 2007). Certainly, Ikaros is ubiquitinated by E3 ligase CRBN7 and degraded by proteasome under the induction of IMiDs, while inhibition of Ikaros ubiquitination is correlated to interact with some TFs like runt-related transcription factor families (RUNXs) (Zhou et al., 2019; Liu et al., 2021). Otherwise, the absence of the E120 enhancer led to an evidential decrease in Ikzf1 mRNA. Nevertheless, the epigenetic pattern and 3D topology of this locus are only slightly impacted, emphasizing the complicacy of the regulatory pattern of the Ikzf1 locus (Alomairi et al., 2020).

The effect of carcinogenic casein kinase II (CK2) on the phosphorylation of Ikaros has been widely studied. CK2 is a multipotent serine/threonine kinase, which is overexpressed in various cancers, including leukemia (Cunninghame Graham et al., 2011; Jeng et al., 2019; Chen et al., 2020). Studies have shown that CK2 directly phosphorylates multiple amino acids in the whole Ikaros protein, and hyperphosphorylated Ikaros facilitates self-degradation through the ubiquitin/proteasome pathway (Dovat et al., 2011). The application of phosphomimetic esters and phosphoresistant Ikaros mutants found that the phosphorylation of CK2 phosphate sites seriously decreases the ability of Ikaros to bind to DNA and alters the localization to pericentromeric heterochromatin, resulting in the dysfunction of Ikaros proteins (Gurel et al., 2008). Pharmacological inhibition of CK2 can restore the DNA binding ability and tumor inhibitory activity of Ikaros and cause leukemia cytotoxicity in the high-risk model of xenotransplantation in patients with ALL, highlighting the fact that CK2 inhibitors can be used as potential therapeutic strategies for high-risk pediatric leukemia (Song et al., 2015; Gowda et al., 2017a). Ikaros phosphorylation by CK2 is cell periodicity, indicating that CK2 effects the regulation of Ikaros function during G1/S transition and S phase in human leukemia (Arco et al., 2004; Li et al., 2012). Besides, CK2-mediated phosphorylation of Ikaros was vital to regulate the transcriptionally of the terminal deoxy transferase gene during differentiation of thymocytes (Wang et al., 2014). SYK is able to phosphorylate Ikaros at dissimilar sites, affecting Ikaros’ nuclear localization (Uckun et al., 2012).

CK2-mediated phosphorylation is reversed by PP1 to dephosphorylate Ikaros (Popescu et al., 2009; Song et al., 2011). The mutation of the PP1 interaction site of Ikaros or the pharmacological inhibition of PP1 lead to the hyperphosphorylation of Ikaros, which seriously reduces the DNA binding ability of Ikaros, loses the pericentromeric localization of Ikaros, and increases degradation of Ikaros through the ubiquitous protein pathway (Popescu et al., 2009).

Ikaros has different effects on the growth, reproduction, and differentiation of many kinds of innate or adaptive lymphocytes in vivo (Figure 3). It has been previously reported that Ikaros is vital for the conversion between the large and small pre-B stages (Schwickert et al., 2014). Ikaros also sustains B-cell proliferation and differentiation through initiating kinase-signaling cascades and collaborating with chromatin protein 4 (Ochiai et al., 2020). In addition, the plasma cell mal-differentiation of sub1 deficient B cells can be saved by Ikaros and IRF4 (Ochiai et al., 2020). For innate lymphoid cells, Ikaros, the significant regulator of ILC3 existence and function, represses the transcriptional activity of aryl hydrocarbon receptors in a zinc finger-dependent manner, inhibits ILC3 in a cellular manner, and controls intestinal immune response in steady-state and disease (Li et al., 2016). Ikaros and Aiolos play a critical role in regulating the trans-differentiation of ILC3-ILC1/NK cells (Bald et al., 2019).

IKZF1 was demonstrated to regulate embryonic T lymphopoiesis of zebrafish via chemokine receptor 9 and IRF4 (Huang et al., 2019). Members of the IKZF family are also involved in the differentiation and property of single T helper cell subsets, covering TH1, TH2, TH17, T follicular, and Tregs (Powell et al., 2019). The function of Ikaros protein in T cell differentiation has been revealed, whereas contradictory results were obtained in certain mouse models. Research utilizing T cells possessing germline Ikaros gene mutations demonstrated that Ikaros promotes Th17 and Treg cell differentiation and suppresses the polarization of Th1 (Cippitelli et al., 2021). Nevertheless, in a mouse model in which Ikaros conditioned knockout in mature T cells, the deficiency of Ikaros is related to the acquirement of Th1, Th2, Th17 but not Treg cells (Lyon de Ana et al., 2019).

Ikaros supports signal-induced downregulation of recombination-activating gene 1 (RAG1) and RAG2 gene expression in CD4 ⁺ CD8⁺ positive thymocytes through a non-redundant manner (Naik et al., 2019). Ikaros plays a role in preventing autoimmunity by administering BCR unresponsiveness and repressing TLR signaling transduction (Schwickert et al., 2019). Although lymphocytes need Ikaros to differentiate, the role of the Ikaros protein family in the myeloid cell is not clear (Park et al., 2015). The mouse model has shown that Ikaros participates in the regulation of differentiation of neutrophils by silencing permissible or specific pathways in the ordinary precursors of macrophage-monocyte evolution (Dumortier et al., 2003). Ikaros modulates early phase differentiation of neutrophils but is optional for mature neutrophils (Dumortier et al., 2003).

Ikaros appears to function as a transcriptional inhibitor and activator by binding to different targets, such as some nuclear factors related to epigenetic regulation (Figure 4). Ikaros suppresses target gene transcription by directly binding or recruiting HDAC1 (Ikaros-HDAC1 complex) to induce the formation of inhibitory chromatin: the former way results in the increase of H3K9me3 and the reduction of H3K9ac, while the latter complex can promote the affinity for DNA binding to the promoter of lysine [K]-specific demethylase 5B (KDM5B) by CK2 inhibitors and raise formation of H3K27me3 and reduce H3K9ac (Song et al., 2016; Wang et al., 2016). The CK2-Ikaros axis also exerts beneficial control over Ikaros target gene expression, increasing PHD finger protein 2 (PHF2) expression, which forms a complex with AT-rich interactive domain-containing protein 5B (ARID5B) to activate the target genes’ transcription (Ge et al., 2018a; Ge et al., 2018b). The phosphorylation by CK2 overexpressed in B-ALL depresses Ikaros combining and recruiting HDAC1 to the promoter of BCL2L1, which causes repression of BCL2L1 and increases expression of BCL-XL (Schott et al., 2020). Recent studies have indicated that Ikaros has a negative effect on the development of T cell leukemia via globally regulating the enhancer or super-enhancer landscape and pioneering activity (Ding et al., 2019). Ikaros has a critical role in regulating de novo enhancer formation, super-enhancers formation, depletion of enhancers, and stimulation of poised enhancers, demonstrating that Ikaros direct modulates the expression of numerous genes than hypothesized formerly (Gowda et al., 2020). Furthermore, miR-26b expression is induced by the differential expression of Ikaros isoforms and transcriptional regulators of miR-26b modulated by PTEN (Yuan et al., 2017). The negative prognosis of IKZF1 deletion in BCP-ALL may be strengthened by the activation of Janus kinase signal transducer and activator of transcription (JAK/STAT) signaling and recede by ERG deletion (Stanulla et al., 2018). The mutual control of Notch1 and Ikaros in DN2 subsets of the thymus in tumor-bearing mice promotes the early stagnation of T cell development at the DN2a stage and its transfer to dendritic cells lineage (Guha et al., 2020). In terms of metabolism, the expression of functional paired box 5(PAX5) and Ikaros induces the powerful upregulation of glucose-6-phosphate dehydrogenase (G6PD). Interestingly, in clinical trials of patients with lymphoma, patients with high expression of G6PD were linked to poor overall prognosis (Xiao et al., 2018).

These variants of IKZF1 were distributed across the whole gene, particularly located to the C-terminal zinc finger dimerization domain, whereas in immunodeficient patients, germline IKZF1 variants are restricted to the N-terminal zinc fingers (Churchman et al., 2018). IKZF1 mutations and deletions have been suggested to contribute to the occurrence and poor prognosis of ALL and AML (Mullighan et al., 2008; Mullighan et al., 2009; de Rooij et al., 2015; Zhang et al., 2020). Deletion of IKZF1 was also demonstrated as an acquired convert at the transformation phase from CML to lymphoid blast crisis or ALL (Li et al., 2018b). The IKZF1 deletions lead to haploinsufficiency, dominant-negative Ikaros forms, or complete loss of expression of Ikaros. The principle of this dominant-negative effect may be that the high level of mutant unbinding DNA isomers with less than three N-terminal sequences occurs through the formation of heterodimers with wild-type isomers, resulting in alteration in DNA binding properties and them becoming repressors (Vairy and Tran, 2020). Mutations at the C-terminal zinc fingers could also cause a deep effect on the transcription of target genes (Vairy and Tran, 2020). Even after a comprehensive analysis of DNA binding sites of Ikaros in murine hematopoietic cells, the molecular mechanism of tumor suppressor effects in leukemia regulated by Ikaros-mediated transcription is still not clear (Song et al., 2015).

Overexpression of Ikaros leads to inhibition of transcription of genes supporting the PI3K pathway and generates transcription of genes, like INPP5D, repressing the PI3K pathway (Song et al., 2015). And AKT phosphorylation decreasing by overexpression of Ikaros in leukemia cells is identical to the effect of treatment with imatinib (Gowda et al., 2017a). Nevertheless, the JAK/STAT pathways and PI3K/AKT pathways were proved to activate with Ik6 expression in ALL (Song et al., 2015; Qu et al., 2019). Ikaros overexpression also directly suppresses the promoter activity of these clinically significant cell cycle progression genes, including ANAPC1, ANAPC7, CCND3, CDK2, CDK6, CDC2, CDC7, CDC16, CDC25a, CDC25c, and CCNE2 (Dhanyamraju et al., 2020). In addition, KDM5B expression is downregulated by Ikaros through recruiting HDAC1 to the KDM5B gene promoter, which causes an inhibitory chromatin condition and transcriptional inhibition consequently (Dhanyamraju et al., 2020). Ikaros upregulates the PHF2 expression through chromatin remodeling, as demonstrated by the appearance of increased H3K4me3 in the promoter of the PHF2 gene (Ge et al., 2016a; Ge et al., 2018a; Maciel et al., 2019). The expression of ARID5B is positively adjusted by Ikaros, and the loss of a single copy of IKZF1 is associated with low ARID5B expression (Ge et al., 2018b). Gain- and loss-of-function experiments of Ikaros in ALL shows that Ikaros inhibits c-myc gene transcription but controls transcription of MYC binding protein 2 positively, which suppresses the activity of myc (Ge et al., 2015). STAT5 regulates the super-enhancer of the myc gene by means of competition for binding to target sites or regulating histone acetylation in an opposing way to Ikaros (Katerndahl et al., 2017). It was revealed that the genes related to the BCR signal pathway and the IKZF family are presumed to be upstream of the MYC/IRF4 axis (Tsukamoto et al., 2020). In patients with trisomy 12 CLL, IRF4 by means of inducing Ikaros mediates the overexpression of CD49d, which was considered to identify those patients treated with ibrutinib with the characteristics of inferior nodal responses and shorter clinical outcomes (Fiorcari et al., 2019).

Moreover, Ikaros was also verified to negatively regulate IL7R expression and promote SH2B adaptor protein 3 (SH2B3) transcription (Ge et al., 2016b). Ikaros represses B-cell lymphoma 6 (BCL6) transcription and initiates basic leucine zipper transcription factor 2 (BACH2) gene transcription, suggested by BCL6 and BACH2 functional experiments (Ge et al., 2017). Some data identified that Ikaros inhibits dynamin 2 in leukemia by directly combining with the promoter and inducing heterochromatin formation (Ge et al., 2016). PAX5, RUNX1, and IKZF1 were found to overlap abundant target genes, and a modality with dominant-negative effective of IKZF1 or ETV6-RUNX1 fusion protein cooperates with the loss of heterozygosity of PAX5 to regulate gene expression (Okuyama et al., 2019). Furthermore, RAG1 is considered as an immediate target of Ikaros (Li et al., 2016; Dimopoulos et al., 2019). IKZF1 was defined as a crucial trans-activator of the SLAMF-7 gene, which is mainly expressed in MM cells and deemed as an ideal target for immunotherapy (Kikuchi et al., 2020).

In contrast to the role of IKZF2 as a tumor suppressor gene in hypodiploid B-ALL, it is essential for AML (Park et al., 2019). The deficiency of IKZF2 in AML cells is related to the reduction of colony formation, augment of differentiation and apoptosis, defective leukemic stem cells function, and tardive leukemogenesis (Park et al., 2015; Park et al., 2019). One of the main mechanisms may be that IKZF2 determines a self-renewal gene expression procedure called HOXA9 and represses a C/EBP-driven differentiation program. In addition, this forced reduction of Ikzf2 is accompanied by decreasing HOXA9 and BCL2, but not Mll, Myc, or Meis1.

The deletion of Ikaros and Aiolos maintains the occurrence and progress of MM by contributing to downregulate c-Myc and IRF4 (Bjorklund et al., 2015). There is a positive feedback loop between IRF4 and c-Myc; IRF4 combines with the region of c-myc promoter and promotes c-myc expression, while the C-Myc protein transactivates the IRF4 gene directly (Shaffer et al., 2008). The target genes of C-Myc cover genes that regulate cell metabolism (GLUT1), cell cycle (CDKs, cyclins, and E2FTF), and apoptosis (Jovanović et al., 2018). On the side, Krüppel-like factor 2 (KLF2) and B lymphocyte maturation inducing protein-1 (Blimp-1) exert a significant effect on regulating the growth and survival of MM. In the positive feedback loop, IRF4 and KLF2 transactivate and promote the expression of each other and are also upregulated by KDM3A via removing H3K9me marks at promoters (Ohguchi et al., 2016). Therefore, the lessening of each protein induced by IMiDs represses the expression of integrin alpha-4/beta-7, thus affecting the migration and homing of MM cells to bone marrow. In addition, the expression and activity of Blimp-1 decreased by IMiDs through a variety of mechanisms: 1) Blimp-1 is repressed at the transcriptional level as the target gene of IRF4; 2) Aiolos interacts with Blimp-1 and synergistically inhibits apoptosis-related genes; and 3) ubiquitination and proteasome degradation of Blimp-1 can be enhanced directly (Hung et al., 2016). Moreover, the degradation of Ikaros and Aiolos induced by IMiDs can upregulate the surface expression of CD38 in MM cells, preparing for the antibody-dependent cytotoxicity induced by daratumab in NK cells (Fedele et al., 2018).

IKZF TFs are central to regulate the innate or adaptive immune response in patients with MM. The ILCs consist of lymphoid tissue inducer cells, NK cells, ILC1s, ILC2s, and ILC3s (Vivier et al., 2018; Krabbendam et al., 2021). For NK cells, their activity induced by IMids is due to the fact that T lymphocytes are stimulated to produce IL-2 (Gandhi et al., 2014). Drugs also directly bind to activate kinase ZAP70, downregulate Aiolos, and enhance the expression of cytotoxic molecular granzyme B (Hideshima et al., 2021). Len directly enhances the activity of NK cells by increasing the proportion of synapses that can penetrate IFN-γ vesicles and increasing the production of IFN-γ (Lagrue et al., 2015). Len and pomalidomide increased the MICA expression of NK cell-mediated immune surveillance molecule NKG2D ligand and PVR/CD155 of dNaM-1 ligand on the human MM cell membrane, thus enhancing the sensitivity to the identification and killing of MM cells (Fionda et al., 2015). As for ILCs, it was demonstrated that the activity of ILCs was enhanced by reducing Ikaros and Aiolos protein (Bald et al., 2019). The 2 TFs are significant regulators for the conversion of ILC3 into ILC1/NK cells. Len increased the expression of certain ILC3-related genes (such as rorc, baff, il22, and nrp1) and upregulated the proportion of ILC3 producing IL-22. Concomitantly, a process of transdifferentiation of ILC3 to ILC1 was inhibited by Len with the downregulation of Aiolos and depression of ILC1-associated transcripts (prf1, gzmb, cd244, lef1, and ncr3). After treatment with Len, DC differentiated from peripheral blood and bone marrow monocytes in patients with MM expressed higher standards of cytokines and chemokines (IL-8, TNF, CCL2, and CCL5) and strengthened the function to stimulate the proliferation of allogeneic T cells (Costa et al., 2017).

IMiDs promote specific CD4⁺ and CD8⁺ T cell responses in MM, whereas they inhibit the expansion and function of Tregs. T cells with less expression of Ikaros require less TCR to participate in immune cell activation, show greater proliferation response to IL-2, and are less sensitive to inhibitors of TCR and IL-2R signal transduction, suggesting that the lower the expression of Ikaros, the lower the T cell activation threshold (Avitahl et al., 1999). Besides, the loss of IKZFs is the reason for the increase of IFN-γ, IL-21, and IL-2 production by T cells treated with IMiDs (Gandhi et al., 2014; Brissot et al., 2015). In addition, Len also decreased the levels of Ikaros and Aiolos in chimeric antigen receptor (CAR) T-cells, which helped to enhance the ability of CAR T-cells to restore IL-2 gene transcription and facilitate IL-2 secretion against solid cancer cells (Wang et al., 2020b).

IMiDs, including thalidomide, Len, and pomalidomide, are effective in treating certain hematological malignancies, in combination with steroids, proteasome inhibitors (PIs), or monoclonal antibodies, such as MM (Gao et al., 2020), MDS with deletion of chromosome 5q (Fenaux et al., 2011), MCL (Ruan et al., 2018), and CLL (Vitale et al., 2016). IMiDs exert their antitumor activity directly through different treatment-related effects, such as inhibiting angiogenesis, repressing cancer cell proliferation, and inducing apoptosis (Le Roy et al., 2018). The cellular target of IMiDs is CRBN, a ubiquitous protein that acts as a substrate receptor for Cullin-4-RING E3 ubiquitin ligase complexes, as well as DDB1, ROC1, and CUL4.

CRBN E3 ligase modulating drugs (CELMoDs), such as avadomide (CC-122), iberdomide (CC-220), CC-885, CC-92480, and novel thalidomide analogs, are in current clinical trials as a monotherapy and in combination (NCT01421524; NCT02773030; NCT03374085) (Table 4). The CELMoDs have strong antitumor and immunostimulatory capabilities in patients with hematological malignancies (Matyskiela et al., 2016; Matyskiela et al., 2018; Ito and Handa, 2019; Rasco et al., 2019; Gao et al., 2020; Hansen et al., 2020).

CC-122 (avadomide) was combined with CRL4^CRBN E3 ligase to conduce the degradation of Ikaros and Aiolos in MM cells and DLBCL cells (Hagner et al., 2015). CC-122 has been applied to a variety of clinical trials for diverse diseases, including non-Hodgkin’s lymphoma, MM, HCC, melanoma, and CLL/SLL (Rasco et al., 2019). It was found in research in pre-clinical experiments and a phase I clinical trial that avadomide + obinutuzumab was a tolerable therapeutic regimen for plenty of patients with manageable toxicity. Nevertheless, the predetermined activity threshold was not reached in this trial. There is no denying that cereblon-modulators plus anti-CD20 antibodies should be investigated further as a chemotherapy-free approach for relapsed or refractory non-Hodgkin lymphoma (NCT02417285) (Michot et al., 2020). In another phase I clinical trial, avadomide monotherapy presented manageable toxicity and promising pharmacokinetics in patients with solid tumors, NHL, and multiple myeloma. Of five patients with NHL, one realized CR, and two achieved PR (NCT01421524) (Rasco et al., 2019).

CC-220 (iberdomide) is another analog of thalidomide, which enhances the efficacy to downregulate Ikaros and Aiolos by tightly binding to CRL4^CRBN E3 ligase for the treatment of RRMM and SLE (Matyskiela et al., 2018). The peripheral blood mononuclear cells of patients with SLE showed strikingly higher levels of IKZF1 (2.1-fold) and IKZF3 (4.1-fold) mRNA than a healthy person. And iberdomide significantly decreased levels of Ikaros and Aiolos protein in B cells, T cells, and monocytes, supporting its further clinical development for treating SLE (NCT01733875) (Schafer et al., 2018).

In addition to the pharmacologic function of CC-220, CC-885 can also induce CRL4^CRBN-dependent delegation of translation termination factor GSPT1, which cannot be degraded by neither Len nor pomalidomide, demonstrating the spectrum of the different substrates of CC-885 from pomalidomide and Len (Matyskiela et al., 2016). Hence, CC-885 may have the extra potential for AML therapy. Relevant pre-clinical and clinical trials are ongoing and being analyzed due to their novelty. CC-92480 is another novel protein degrader acting on the refractory/relapsed MM (RRMM) of Len (Hansen et al., 2020).

PIs are a class of important drugs for the treatment of MM and MCL, and are being studied for the treatment of other diseases. BTZ is the first PI to be approved by the Food and Drug Administration (FDA) of the United States. Carfilzomib and ixazomib have been approved successively, and more drugs are in development (Fricker, 2020). The phase III GIMEMA-MMY-3006 trial illuminated the superiority of the BTZ + thalidomide + dexamethasone (VTD) regimen over thalidomide and dexamethasone (TD) in improving CR and prolonging PFS (HR, 0·60; 95% CI, 0·48–0.76; p < 0·0001) as an induction regimen before and intensification therapy after double autologous hematopoietic stem-cell transplantation (ASCT) for patients with NDMM (NCT01134484) (Cavo et al., 2010; Tacchetti et al., 2020). The randomized phase III trial SWOG S0777 and long-term follow-up of SWOG S0777 revealed a similar pattern that the addition of BTZ to Len and Dex for induction therapy led to a statistically and clinically meaningful increase in PFS as well as OS (NCT00644228) (Durie et al., 2017; Durie et al., 2020). Another trial demonstrated that the first single-agent ixazomib option in patients with NDMM not undergoing ASCT promoted mPFS (17.4 vs. 9.4 months; HR, 0.659; 95% CI, 0.542–0.801; p < 0.001; median follow-up, 21.1 months) without unexpected toxicity (NCT02312258) (Dimopoulos et al., 2020a). For combination therapy of ixazomib, the addition of ixazomib to a Len and Dex regimen was correlated to longer PFS (20.6 vs. 14.7 months in Len and Dex; HR, 0.74; p = 0.01) with limited toxic effects (NCT01564537) (Moreau et al., 2016). Besides, in patients with RRMM, carfilzomib in conjunction with Len and Dex (KRd) generated markedly improved mPFS (26.3 months, vs. 17.6 months in the Len and Dex group; HR, 0.69; 95% CI, 0.57–0.83; p = 0.0001) at the interim analysis and presented an improving risk-benefit profile (NCT01080391) (Stewart et al., 2015). However, the KRd scheme cannot effectively improve PFS (34·6 months vs. 34·4 months in the Len and Dex group; HR, 1·04; 95% CI, 0.83–1.31; p = 0·74) compared with a control group in patients with NDMM, and had more toxicity (NCT01863550) (Kumar et al., 2020).

Immunotherapy is one of the most promising treatments for several cancers. In future clinical trials, early identification of high-risk patients with ALL is essential for the best treatment, and to ensure innovative treatments, such as immunotherapy, are introduced in a timely manner (Stanulla et al., 2018). In some studies about certain solid tumors that lack IKZF1 expression, overexpression of Ikaros leads to increased immune recruitment infiltration and tumor sensitivity to cytotoxic T lymphocyte-associated antigen-4 (CTLA4) and programmed cell death protein 1 (PD1) inhibitors (Chen et al., 2015; Chen et al., 2018). Nevertheless, meaningful MM regression was not achieved by single-drug PD-1 blockade in early clinical studies. Besides, the FDA established that the risks of the pembrolizumab plus pomalidomide and dexamethasone combination outweighed its benefits for patients with RRMM and halted the study (NCT02576977) (Mateos et al., 2019). Equally, the interim analysis results presented that the benefit-risk event of pembrolizumab plus lenalidomide and dexamethasone is adverse for patients with NDMM. Survival and long-term safety follow-up are ongoing (NCT02579863) (Usmani et al., 2019). CAR-T immunotherapy faces many problems in the treatment of solid tumors, and T-cell dysfunction or failure is one of them. Len improved CAR-T function by inducing the degradation of Ikaros and Aiolos, promoting the killing of CD133-CAR-T and increasing the secretion of the cytokine and the proliferation of CD133-CAR-T (Wang et al., 2020b).

Adding imatinib to the therapy for patients with BCR-ABL1 fusion to strengthen the treatment of IKZF1-del patients can eliminate the adverse effect of IKZF1-del in contemporary ALL treatment (Yeoh et al., 2018). Imatinib, as an intensifying therapy for childhood B-ALL with IKZF1-del, significantly decreased the risk of recurrence and improved the 5-years OS from 69.6% in MS2003 to 91.6% in MS 2010 (p = 0.007) (NCT0289464) (Yeoh et al., 2018). Besides, ibrutinib also presented enduring single-agent efficacy in patients with relapsed or refractory MCL. A CR rate of 21% and a PR rate of 47% were detected. And the most common adverse events related to treatment were diarrhea, fatigue, and nausea (NCT01236391) (Wang et al., 2013).

DD-03–171 is an optimized Bruton tyrosine kinase (BTK) inhibitor analog, which showed an enhanced anti-proliferation outcome on MCL cells in vitro through the degradation of BTK, Ikaros, and Aiolos (Dobrovolsky et al., 2019). Acalabrutinib, a selective second-generation BTK inhibitor, provided superior durable responses and an acceptable toxicity profile in patients with relapsed or refractory MCL. At the median follow-up of 15.2 months, 81% of patients attained an overall response, and 40% of patients achieved a CR. The most common adverse events were headache (38%), diarrhea (31%), fatigue (27%), and myalgia (21%) (NCT02213926) (Wang et al., 2018).

The CK2 inhibitor CX4945 can restore Ikaros function and exert the antileukemia effect in vitro or in pre-clinical leukemia models (Gowda et al., 2017b; Borgo et al., 2021). There are also many cancer clinical trials currently running (NCT01199718, NCT00891280, NCT03897036, NCT02128282, NCT03904862, and NCT03571438).

Bromodomain-containing protein 4 (BRD4) is a chromatin-binding protein that was considered to immediately regulate a range of genes involved in the BCR signal pathway, covering Aiolos, B-cell linker, PAX5, and several oncogenes, like MYB. Thalidomide derivatives to various Brd4 small molecule inhibitors through different junctions were used to synthesize CRL4^CRBN E3-based PROTAC targeting Brd4. For example, the synthesis of many pre-clinical trials like dBET1, ARV-825, BETd-246, BETd-260, and QCA570. Of course, there are other degradation pathways, such as the CRL2^VHL E3-based pathway (Duan et al., 2018). BRD4 inhibitors were suggested to enhance the anti-MCL effect of Len or ibrutinib synergistically. It works even in BTZ-resistant MCL cells (Tsukamoto et al., 2020).

In breast cancer, the application of ginseng polysaccharide was able to suppress MDA-MB-231 cell proliferation by the activation of IKZF1 (Zhou et al., 2020). The anticancer capability of imatinib in Ik6+ and Ph + ALL can be strengthened by plant elements such as Huaier extract (Qu et al., 2019). The use of caffeic acid phenethyl ester analogs in the so-called Achilles heel of myeloma, aimed at the Ikaros/IRF4 axis, showed positive clinical outcomes (Murugesan et al., 2020).

Daratumumab, a monoclonal antibody targeting CD38, has presented substantial efficacy as a monotherapy in heavily pretreated and refractory patients with MM and in conjunction with bortezomib in patients with NDMM as well as with IMiD in patients with RRMM. Overall responses were noted in 29.2% of refractory patients with MM treated with daratumumab monotherapy (NCT01985126) (Lonial et al., 2016; Usmani et al., 2020). In addition, the combined administration may be an ideal backbone for the future research of anti-CD38 monoclonal antibodies in terms of their good safety and oral administration. The addition of daratumumab with Len-Dex induced a high ORR and PFS, markedly decreasing the risk for progression and associated death in patients with RRMM and NDMM who were ineligible for ASCT in contrast to Len-Dex therapy (Dimopoulos et al., 2016). And after 44.3 months of follow-up, triple therapy significantly prolonged PFS in the treated population (44.5 vs. 17.5 months; HR, 0.44; 95% CI, 0.35–0.55; p < 0.0001). In terms of adverse profiles, a higher frequency of neutropenia and pneumonia was noticed in the daratumumab group (NCT02076009, NCT02252172) (Facon et al., 2019; Bahlis et al., 2020). Moreover, daratumumab plus Pom-Dex could also reduce the risk of disease progression and improve mPFS (12·4 vs. 6·9 months; HR, 0·63; 95% CI, 0·47–0.85) in patients with RRMM compared to Pom-Dex, and can be considered a new therapeutic strategy (NCT03180736) (Chari et al., 2017; Dimopoulos et al., 2021a). Besides, daratumumab applied with carfilzomib and Dex in patients with RRMM was associated with a promising benefit-risk profile and prolonging PFS (NCT03158688) (Dimopoulos et al., 2020b). In quadruple therapy, induction and remarkable consolidation treatment of daratumumab plus Len, BTZ, and Dex improved response depth and PFS with acceptable safety in patients with transplant-eligible NDMM (NCT02874742, NCT02541383) (Moreau et al., 2019; Voorhees et al., 2020). Isatuximab, another monoclonal antibody targeting CD38, has been evaluated as a monotherapy and combined with Dex in patients with RRMM. The combination of isatuximab and Dex promoted ORR (43.6 vs. 23.9% in the Isa arm) and survival outcomes with no adverse effect on safety (NCT01084252) (Dimopoulos et al., 2021b). On the basis of Pom-Dex, the addition of isatuximab was able to importantly improve PFS in patients with RRMM, particularly for patients who are refractory to Len and a PI (NCT02990338) (Attal et al., 2019).

In phase Ib-III clinical studies, elotuzumab, an immunostimulatory antibody against signaling lymphocytic activation molecule F7 (SLAMF7), achieved a reduction of 30% in the risk of disease progression or death in patients with RRMM combined with Len and Dex (NCT01239797) (Lonial et al., 2015; Dimopoulos et al., 2020c). Particularly among patients with MM who failed after treatment with Len and a PI, the addition of elotuzumab to Pom-Dex tremendously lowered the risk of disease progression or death and prolonged mPFS (10.3 vs. 4.7 months in the control group) versus Pom-Dex alone (NCT02654132) (Dimopoulos et al., 2018). Nevertheless, in the SWOG-1211 trial, elotuzumab combined with BTZ, Len, and Dex did not improve clinical outcomes of patients with NDMM as induction and maintenance treatments (Usmani et al., 2021) (NCT01668719).

The majority of newly diagnosed patients respond to therapy of an immunomodulator, and yet most develop resistance to administered therapies eventually. Strategies to surmount resistance have been proposed (Mogollón et al., 2019). In MM, distinguishing mechanisms of resistance to both Len and Pom are apparent, therefore, tumors with acquired resistance to pomalidomide react to Len and vice versa (Ocio et al., 2015). STAT3 and MEK1/2 inhibitors have been testified to overcome IMiDs resistance effectively in pre-clinical studies (Ocio et al., 2015; Zhu et al., 2019). Enhancing degradation of CRBN has also been proved to be more sensitive to IMiDs (Kim et al., 2020). Protecting IKZFs from degradation through RUNXs inhibition resulted in conquering the resistance of IMiDs in MM (Zhou et al., 2019). In addition, a synthesized HDAC6 selective inhibitor, A452, is a strategy to overcome resistance to IMiDs (He et al., 2020). Furthermore, Cys reinforced the sensitivity of MDS/AML cells to Len without affecting the activation of T cells (He et al., 2020). It has been reported that the addition of cyclophosphamide to an IMiD could enhance the efficacy of IMiDs, prolong PFS and OS, and even save patients with Len-refractory disease, regardless of whether they have pre-accepted ASCT (NCT02244125) (Garderet et al., 2018). In patients with Len-refractory MM, the combination of pomalidomide, cyclophosphamide, and Dex confer a superior ORR and PFS than pomalidomide and Dex (NCT01432600) (Baz et al., 2016). In ALL, treatment with retinoid receptor agonists is able to fortify the sensitivity of IKZF1-aberrant BCR-ABL1 ALL patients to tyrosine kinase inhibitor therapy (Churchman et al., 2015).