Immune-based therapies such as immune checkpoint Tim3 inhibitors, BiTE immunotherapy, ADC immunotherapy, and CAR-T (Table 2) immunotherapy have been becoming therapeutic hot-researches in MM.

There are currently several small molecule inhibitors targeting chromosomal instability such as poly(ADP-ribose) polymerase (PARP), Akt, Aurora kinase, and spindle kinase inhibitors which have been tested in mouse models and in early phase I/II trials [39]. Thyroid hormone receptor-interacting protein (Trip) 13 has been identified as a chromosomal instability gene and is associated with drug resistance, disease recurrence, and adverse prognoses in MM patients [40]. In animal experiments, a small molecule inhibitor (DCZ0415) of Trip13 can inhibit MM cell proliferation. It acts as a toxic agent of the spindle, which causes spindle multipole, destroys the repair of non-homologous end joining (NHEJ), inhibits the synthesis of DNA, and inhibits NF-κB activity in MM cells. More importantly, DCZ0415 works in conjunction with the melphalan and HDAC inhibitor panobinostat to suppress MM cell proliferation [41]. DCZ0415 also inhibits the protective effect of the BM microenvironment on myeloma cells [42]. Therefore, the discovery of more and new targets and new methods by targeting chromosomal instability such as Trip3 for the treatment of MM is important.

ATR inhibition, combined with piperlongumine, triggers synergistic MM cell cytotoxicity by enhancing oxidative stress while concomitantly blocking replicative stress response [43]. This novel combination targeted therapy, that is the synthetic lethal approach, provides an unmet medical need for a poor-prognosis subset of MM patients with extensive chromosomal instability and replicative stress [43].

IRF4 can promote proliferation and survival of both MM stem and tumor cells, and the high level of IRF4 is directly related to the decline of overall survival rate in patients with MM [44]. The ASO of IRF4 inhibits the proliferation and survival of MM cells by reducing the expression of IRF4. IRF4 ASO monotherapy can prevent the formation of a xenograft tumor model and MM cell proliferation and lead to improvement in animal survival rates. In addition, IRF4 ASO presented more obvious advantages for MM treatment than drugs targeting other pathogenic molecules in MM in that it can eliminate the progenitor cells and malignant PCs of MM and retain normal hematopoietic stem cell development [45]. That is, if IRF4 ASO is successful, it may target disease cells more accurately and not cause much damage to other cells. Thus, side effects in MM patients may be greatly reduced. A study has shown that lenalidomide also causes down-regulation of IRF4 expression [46]. However, the use of selective IRF4 ASO may be superior to lenalidomide. ASO can be combined with MM drugs for treatment and has synergistic effects. Therefore, it is possible to study IRF4 ASO since IRF4 represents a potential therapeutic target of MM, especially with respect to the survival of patients who are difficult to treat with a standard of care drugs. Myelocytomatosis oncogene (MYC) is a direct target of IRF4 in activated B cells and myeloma. Critically, IRF4 was itself a direct target of MYC transactivation, generating an autoregulatory circuit in myeloma cells [47]. A novel c-Myc inhibitor, compound 7594-0037, can be used as a novel c-Myc inhibitor and is a potential candidate therapeutic drug for MM [48].

Epigenetic aberrations are the pathogenic events that play important roles in leading to MM. HDACs influence the activity of several transcription factors disrupted in MM through histone modifications [49, 50]. Clinical trials have failed to show a clinical benefit with HDAC is as single-agent therapy. ACY-1215, a selective HDAC6 inhibitor, is a promising epigenetic targeted therapy for myeloma that has shown in vitro and in vivo effectiveness [51] and is now tested in two ongoing phases 1–2 clinical trials (ClinicalTrials.gov. NCT01323751 and NCT01583283) [52].

CELMoDs are larger molecules and work by quickly degrading a specific set of proteins (called Ikaros and Aiolos) [53–55]. They are potent modulators of the cereblon E3 ligase complex. They also stimulate the immune system and kill myeloma cells directly, even for myeloma that has become resistant to certain therapies. A new type of myeloma treatment called CC-92480 (a CELMoD) showed benefit in phase I international clinical trials [53–55]. It is synergistic with other therapies and has “excellent tumor penetration”. The other CELMoD currently in development is iberdomide [53–55].

The mutational spectrum of MM with only a limited number of common recurrent mutations is now observed [56, 57]. Mutations in Kirsten rat sarcoma 2 viral oncogene homolog (KRAS), neuroblastoma RAS viral oncogene homolog (NRAS), and BRAF account for 50% of all cases at presentation and these numbers increase at relapse MM [58]. Inhibition of MEK, located downstream of the cellular homolog of viral raf gene (RAF) in the mitogen-activated protein kinases (MAPK) pathway, has emerged as a potential strategy to treat these patients [58]. Trametinib shows promise as a myeloma therapeutics based on responses seen in a heavily pretreated population [59]. BRAF-targeting vemurafenib has also been shown to result in responses. Targeting of the rat sarcoma (RAS)/MAPK pathway clearly could work in MM. However, it is compromised by intra-clonal and spatiotemporal heterogeneity. This will require changes to the way response is assessed, with markers directed to the subclone being treated becoming important [58].

BM microenvironment, composed of stromal cells, osteoblasts, osteoclasts, endothelial cells, immune cells, and a non-cellular compartment, provides the supportive role of malignant MM cell differentiation, migration, proliferation, survival, and drug resistance. Therefore, it is very interesting to target the BM microenvironment. Targeting specific niches within BM will have the potential for the treatment in MM and may facilitate earlier intervention to disrupt an environment that is permissive for myeloma progression [60]. PD-1 plays a critical role in the suppressive microenvironment of MM. Immunotherapies using anti-PD-1/PD-L1 strategies are promising treatment options for patients with MM [61]. The MM-niche-derived IL-18 drove the generation of myeloid-derived suppressor cells (MDSCs), leading to accelerated disease progression [62]. A preclinical study suggested that IL-18 could be a potential therapeutic target in MM [62].

Circ_0007841 has been shown to be up-regulated in BM biopsy cells from MM patients [63]. Circ_0007841 targets miR-129-5p and inhibit the effect of miR-129-5p, while miR-129-5p can inhibit the expression of jagged canonical Notch ligand 1 (JAG1) mRNA. Thus, circ_0007841 leads to MM cell proliferation, transformation, and drug resistance by promoting JAG1 overexpression. On the other hand, circ_0007841 gene knockout can inhibit the proliferation, metastasis, and drug resistance of MM cells by regulating the miR-129-5p/JAG1 axis. Another study found that circ_0007841 could also directly target miR-338-3p and cause acceleration of the progression of MM [64]. Circ_0007841 can up-regulate the expression of bromodomain-containing protein 4 (BRD4) by binding with miR-338-3p to promote the activation of PI3K/Akt signaling, thus promoting the proliferation and migration of MM cells. Therefore, circ_0007841 MM may become an important target for MM treatment in the future.

NF-κB was previously one of the important targets for the treatment of MM. Five members in the NF-κB family have been found: (1) NF-κB1 (p50 and its precursor p105), (2) NF-κB2 (p52 and its precursor p100), (3) RelA (p65), (4) RelB and (5) c-Rel [65].

Two activation pathways of NF-κB have been demonstrated: 1) classical activation pathway, which is the main activation pathway of some receptors and cytokines. The extracellular signal factor binds to the receptor on the cell membrane to activate IkappaB (IκB) kinase (IKK). IKK phosphorylates the serine at the regulatory site of IκB subunit of NF-κB·IκB complex, which facilitates IκB subunit ubiquitination, which is then degraded by a protease and releases an NF-κB dimer. Free NF-κB will enter the nucleus and bind to the genes with NF-κB binding sites to initiate the transcription process and 2) non-classical activation pathway, mainly consisting of the p52/RelB dimer. The non-classical NF-κB kinase of the IKK complex selectively phosphorylates the p52 precursor, p100. Phosphorylation of p100 results in the removal of the C-terminal part of the protein (also known as IκBδ) and its degradation in the proteasome. This process produces a functional p52 protein, which can then dimerize with RelB and accumulate in the nucleus in which regulates its target gene. At least 17% of primary MM cells and 42% of MM cell lines were found to have mutations in several major components of the NF-κB pathway. Although the NF-κB activation pathway is well known and has led to the production of corresponding drugs, the change of new genes and the activation of new sub-pathways in this pathway will become a new target for MM treatment.

MM characterized by cancerous proliferation of PBs and PCs remains incurable in many patients. Differentially-expressed molecules in MM PCs versus healthy PCs have been explored in order to identify novel targets for treating MM. Some molecules such as BCMA, are highly expressed in cancerous PBs and PCs and have been used as the key targets in the treatment of MM. Apart from positive regulators, negative regulators or tumor-suppressors have also been implicated to be potential biomarkers and therapeutic targets in some cancers [66, 67]. Some tumor suppressors such as down-regulation of candidate 2 (CECR2), contribute to tumor growth in laryngeal squamous cell carcinoma [68]. We searched for novel MM therapeutic targets by comparing mRNA expression patterns between the Mus musculus (mouse) myeloma PB-like myeloma SP 2/0 cell line and lipopolysaccharide (LPS)-induced PB/PC. We found that some molecules, such as Gm6377 and BC094916, were decreased in both normal PCs/PBs and SP 2/0 cells [69, 70]. Specifically, some molecules, such as Loc108167440 and Gm40600, were down-regulated in SP 2/0 cells but not in normal PCs and PBs [71, 72]. Of note, the overexpression of reduced forms of molecules, such as Gm6377, BC094916, Loc108167440, and Gm40600 suppressed SP 2/0 cell proliferation and suppressed tumor progression in the SP 2/0 xenograft mouse model [69–72]. These results suggest that modulation of negative regulators or tumor-suppressors may provide a potential therapeutic method for treating MM.

Recent advances in the treatment of MM have pointed to accurate diagnosis of MM. It is possible that a better understanding of the genetic and epigenetic interactions in MM may reveal a new understanding of MM pathogenesis, new disease biomarkers, and hopefully the development of novel and a more effective diagnosis method. Present studies are evaluating a combined use of “imaging markers”, such as SUVs, MATV, TLG, ADC from positron emission tomography (PET), and magnetic resonance imaging (MRI) techniques respectively, and several “biomarkers” spanning from chemokine immune-modulators, such as PD-1, RANK/RANKL, CXCR4/CXCL12 to transcription factors, such as TP53, RB1, MDM2, runt-related transcription factor (RUNX) family, EZH2, YY1, mitotic arrest deficient 2 (MAD2) [73]. Imaging markers are used to characterize both tumor heterogeneity and metabolic data, whereas biomarkers may improve diagnosis and prognosis leading to precision medicine [73]. Although imaging and molecular integration could allow both early diagnosis and stratification of cancer prognosis, large-scale clinical trials will be necessary to translate pilot studies in the current clinical setting [73].

The presence of del(17p), t(4;14), t(14;16), t(14;20), gain 1q, or p53 mutation is considered high-risk in MM [74]. The presence of any two high-risk factors is considered double-hit myeloma; three or more high-risk factors are triple-hit myeloma [74]. Standard risk patients can opt for delayed autologous stem cell transplantation (ASCT) at first relapse. Patients not candidates for transplant are treated with lenalidomide (Revlimid) plus dexamethasone (DEX, RD) until progression, or alternatively, a triplet regimen such as VRD for approximately 12–18 months. After ASCT, lenalidomide maintenance is considered for standard-risk patients especially in those who are not in VGPR or better, while maintenance with a bortezomib-based regimen is needed for patients with the intermediate or high-risk disease [75].

A better understanding of diagnosis and risk stratification in MM will further inspire the development of precision therapy for MM. To date, the choice of therapy for an individual MM patient has been based on clinical factors such as age and comorbidities [76]. The widespread evolution, validation, and clinical utilization of molecular technologies, such as fluorescence in situ hybridization and next-generation sequencing has enabled the identification of a number of prognostic and predictive biomarkers for PFS, overall survival, and treatment response [76]. This will improve myeloma patient outcomes incorporating such biomarkers into the routine diagnostic workup of patients will allow for the use of personalized, biologically-based treatments [76].