1. Background and IntroductionThe balance between cell death and cell proliferation contributes to the maintenance of tissue homeostasis. Cell death evasion is a hallmark of human cancers (including B cell malignancies; Table 1) and might contribute to treatment resistance [1]. A number of molecular pathways underpin cell growth and cell survival in B cell malignancies (Figure 1). For example, anti-apoptotic proteins from the B-cell lymphoma/leukemia 2 (BCL2) family (such as Mcl-1, Bcl-2, Bcl-xL, and Bcl-w) are overexpressed in most B cell malignancies (Table 1) as a result of genetic lesions or changes in signal transduction [2,3]. This expression is associated with a poor prognosis [3,4,5,6]. Anti-apoptotic BCL2 proteins interact with the pro-apoptotic “BH3-only” proteins (such as Bim, Bid, Puma, Bad, and Noxa) or bind and sequester the apoptotic effectors Bax and Bak in an inactive form (Figure 1a) [3,7]. The pro-apoptotic proteins induce apoptosis by activating Bax and Bak either directly or indirectly via inhibition of the anti-apoptotic proteins (Figure 1a) [3,7]. Active B-cell receptor (BCR) signaling also contributes to cell survival, proliferation, and resistance to apoptosis in B cell malignancies [8,9,10,11,12] (Table 1). BCR-mediated proliferation of malignant B cells may be caused by BCR pathway mutations and/or chronic active stimulation of the BCR [8,9,10,11,12]. The mechanism of BCR pathway activation in B cell malignancies is complex and has been well reviewed by Burger and Wiestner [13]; it includes continuous BCR stimulation by microbial antigens or autoantigens, activating mutations within the BCR complex or downstream signaling partners, and antigen-independent BCR signaling [13]. The BCR pathway includes a large array of kinases, adaptor proteins and transcription factors (such as NF-κB); in particular, Bruton tyrosine kinase (BTK) and phosphatidylinositol-3 kinase (PI3K) are critical signaling enzymes involved in uncontrolled B cell proliferation and survival [10,11,12,13] (Figure 1b). Lastly, high levels of expression of certain tumor-associated antigens (TAAs; CD19, CD20, CD22, and CD38) in most B cell malignancies are correlated with a high proliferation rate and disease progression (Table 1) [14,15,16,17]. These TAAs are physically associated with the BCR [18,19,20,21] and are involved in modulating BCR-dependent (Figure 1b) and BCR-independent proliferation/survival signals (Figure 1c) [13,15,16,17,18,20,21,22,23,24,25,26].Over the last 25 years, greater knowledge of the detrimental roles of these survival-associated proteins in malignant B cell pathologies have prompted the development of a large array of inhibitors. These novel anticancer agents include small molecules that target BCL2 members, BTK, or PI3K, and mAbs and their derivatives (mono/bispecific Ab, antibody–drug conjugate/ADC, antibody–radioisotope conjugate/ARC, bispecific T cell engager/BiTE, and chimeric antigen receptor-modified T cells/CAR-T) against TAAs. The use of some of these novel agents have been authorized by the US Food and Drug Administration (FDA) [27]. Although these drugs improve clinical outcomes when used alone or in combination with other treatments, some patients do not respond, and others relapse after an initial response [28,29,30,31,32,33,34]. As such, efforts to develop more effective, selective inhibitors against the same targets or new targets in malignant B cells must be continued. Specifically, novel mAbs (acting on other TAAs) and small-molecule inhibitors (acting on survival-associated proteins, such as signaling proteins, proteases, immune checkpoints, etc.) are in preclinical or clinical development. Here, we first review the current management of B lymphoid tumors with FDA-approved drugs targeting the above-cited TAAs, Bcl-2, or BTK/PI3K. We then focus on the clinical efficacy of newly developed drug candidates for inhibiting current and novel protein targets and overcoming some forms of resistance. Lastly, we review perspectives for therapy, with emphasis on the importance of metabolic reprogramming in tumor B cell biology and opportunities for inhibiting this process.

Table 1. Characteristics of the main B cell malignancies.

Table 1. Characteristics of the main B cell malignancies.

B Cell MalignancyPathophysiologyExpression of BCL2 Prosurvival ProteinsActive (Pre-)BCR Signaling Surface Markers That Are Strongly ExpressedReferencesB-ALLTransformation and expansion of lymphatic B progenitor cellsBcl-2, Mcl-1, Bcl-xLpre +CD19 CD20
CD22 CXCR4[7,35,36,37,38,39,40,41]HCLAccumulation of CD5+ B cells most in the blood, bone marrow and spleenBcl-2 > Mcl-1+CD19 CD20
CD22 CD38
CXCR4[2,11,40,42,43]CLLAccumulation of CD5+ B cells primarily in the blood and bone marrowHigh Bcl-2 > Mcl-1 >> Bcl-xL+CD19 CD20
CD38 (poor prognosis)
BAFF-R ROR1
CXCR4[3,7,36,38,40,41,44,45]SLL
(B-NHL)Accumulation of CD5+ B cells most in the lymph nodesBcl-2, Mcl-1,
Bcl-xL+CD19 CD20
CD38 (poor prognosis) ROR1[40,44,45,46,47]MMAccumulation of clonal, Ig secreting plasma cells in the bone marrow High Bcl-2, Mcl-1, Bcl-xL, Bcl-w+CD19 CD22
CD38 BCMA
CD13 ADAM17
ROR1 CXCR4[3,7,40,41,48,49,50,51]FL
(B-NHL)Extensive proliferation and accumulation of abnormal B cells in lymph nodesHigh Bcl-2, Bcl-w and Bcl-xL > Mcl-1−/+CD19 CD20
CD22 CD38
CXCR4[3,7,40,41,45,48,52]MCL
(B-NHL)Development of abnormal B cells in the mantle zone of lymph nodes, spleen, bone marrow, blood, and gastrointestinal tractHigh Bcl-w and Bcl-2 > Mcl-1+CD19 CD20
CD22 CD38
ROR1 CXCR4[3,7,40,41,47,52,53]MZL
(B-NHL)Development of abnormal B cells in the marginal zones of lymph tissueHigh Bcl-2 and Bcl-w > Bcl-xL+CD19 CD20
CD22 CXCR4[3,10,40,41,52,54,55]DLBCL
(B-NHL)Development of abnormal B cells in germinal centers of secondary lymphoid organsBcl-2 and Bcl-w > Mcl-1, Bcl-xL+CD19 CD20
CD22 CD38
CXCR4[3,7,36,40,41,45,52,56,57]WM
(B-NHL)Proliferation of clonal, IgM-secreting plasma cells in the lymph nodes and bone marrowHigh Bcl-2 > Bcl-xL and Mcl-1+CD19 CD20
CD38
CXCR4 (mutation)[8,40,46,58,59] 5. Concluding Remarks and PerspectivesToday, a key challenge in using FDA-approved drugs in the current treatment of B cell malignancies (Figure 2a) is that these therapies must simultaneously eliminate tumor cells and preserve/reestablish normal hematopoiesis. However, the effectiveness of these anticancer therapies is sometimes limited by primary and acquired resistance [1]. Drug-mediated T cell exhaustion may lead to the possibility of immune escape of hematological malignancies [1]. Residual disease and/or resistance in patients treated with Bcl-2/BTK/PI3K inhibitors (venetoclax, ibritunib, idelalisib, etc.) is associated with genomic resistance (mutations, activation, instability, p53 aberrations.) as well as non-genomic, acquired resistance through (re)activation of signaling survival pathways (PI3K, NF-κB, MYC, etc.), cancer stem cells, and the tumor microenvironment [60,443,444,445,446,447]. The main adverse effects of mAbs therapies include cytokine release syndrome, neurotoxicity, and on-target/off-tumor toxicity resulting from a direct attack on normal tissues that have shared expression of the target antigen [223,448]. To improve clinical outcomes (residual and progressive disease) and immune function in patients with B cell malignancies, while evading the emergence of drug resistance, newer drugs that inhibit the same targets as the current treatments are in continuous clinical development. For instance, ongoing trials focus on overcoming venetoclax resistance by targeting the BCL2 family members Mcl-1 and Bcl-xL, and evaluate more highly selective BTK and PI3Kδ inhibitors with fewer off-target effects. These second-generation drugs, although more potent, are not likely to be used only as single agents. Drug combinations can favor compounds with synergistic antitumor effects and ensure manageable toxicity. Thus, the combinations of BTK inhibitors with BCL2 inhibitors and/or rituximab (anti-CD20 mAb), or PI3K inhibitors with BCL2 inhibitors, are of interest.Importantly, this review outlines the increasing importance of therapeutic mAbs (mono-/bi-specific, ADC, BiTE, CAR-T cells, Bi-CAR-T cells, CAR-NK cells) as the predominant treatment modality for B cell malignancies. To face the resistance mechanism, BiTEs and CAR-T cells redirect T cells to target antigen-expressing tumors; novel BiTEs and CAR-T cells with various costimulatory signals and delivery systems, as well as Bi-CAR-T cells (anti-CD19/CD20 CAR-T, anti-CD19/CD22 CAR-T, and anti-CD20/CD22 CAR-T) may therefore represent an effective solution to the challenge of antigen escape in immunotherapy (Table 3). As already planned for the treatment of solid tumors [449], BiTEs and CAR-T cells could be combined with standard chemotherapy and/or targeted therapies of B cell malignancies to reduce the tumor burden and/or modulate the immune response. Recently, CAR-T cells and BiTEs have been engineered into a single immunotherapy platform (BiTE-secreting CAR-T) for T-cell-directed therapy of solid tumors [450,451]. BiTEs secreted by CAR-T cells exhibit potent antitumor activity in vitro and in vivo with significant sensitivity and specificity, demonstrating a promising strategy in solid tumor therapy [450,451]. This approach could have therapeutic application in hematological malignancies. Lastly, compared with CAR-T cells, CAR-NK cell constructs offer advantages, including better safety, and multiple mechanisms for activating cytotoxic activity [452,453,454]. For therapy of B cell malignancies, equipping anti-CD19 CAR-NK cells with on-board cytokines or chemokines might improve clinical efficacy by enhancing both persistence and cytotoxicity against tumor cells.The consideration of other TAAs (including BCMA, BAFF-R, ROR1, CXCR4, CS1, FcRL5, GPRC5D, etc.) as druggable targets for B cell malignancy immunotherapy, has promoted the development of novel inhibitors, with some of them under single or combined evaluation in clinical trials for a variety of patients with B cell malignancies (Table 4). Most TAAs show their potential for ADC, BiTE, CAR-T, and CAR-NK therapies. Regarding the PD1/PD-L1 axis responsible in part for cancer immune escape, immune checkpoint blockade therapies with anti-PD1/anti-PD-L1 mAbs in combination with BTK (ibrutinib), anti-CD38 (isatuximab) or anti-CD19 CAR-T (axicabtagene ciloleucel) may optimize the landscape of B-NHL therapy.Targeting additional intracellular signaling pathways contributing to B malignant cell dysfunction may also prove efficacious. For example, ruxolitinib, a potent JAK1/2 inhibitor, demonstrates antitumor activity as a single agent in MM (NCT03110822) [455]. Anvatirsen, an antisense oligonucleotide inhibiting STAT3, combined with durvalumab + tremelimumab is well-tolerated in patients with R/R DLBCL (NCT02807454) [311]. The efficacy and safety of a combination of panobinostat (a pan-histone deacetylase inhibitor) with bortezomib + dexamethasone is being evaluated in MM patients having received two or more lines of treatment (NCT02654990). Tazemetostat/EPZ-6348, which inhibits the histone methyl transferase EZH2, shows safety and antitumor activity in patients with DLBCL (NCT02889523) and FL (phase I/II NCT01897571) [456,457,458]. Combination therapy with temsirolimus, an inhibitor of the oncogenic kinase mTOR, and lenalidomide demonstrates encouraging activity in patients with DLBCL and FL (NCT01076543) [459]. Ponatinib is a third-generation tyrosine kinase inhibitor with a wide spectrum of kinase inhibition [460]; it targets BCR-ABL1, an abnormal tyrosine kinase that is expressed in chronic myeloid leukemia and Philadelphia chromosome-positive (Ph+) ALL; ponatinib shows clinical activity in R/R Ph+ (BCR-ABL) ALL [461], and in the first-line setting in combination with standard chemotherapy (NCT01641107, [462]; NCT02776605, [463]) or blinatumomab (NCT03263572) [464]. A Phase III study is comparing the efficacy and the safety of the first in-class selective inhibitor of nuclear export, selinexor, in combination with bortezomib + dexamethasone vs. bortezomib + dexamethasone in patients with R/R MM (NCT03110562) [465]. Pharmacological inhibition of the bromodomain and extra-terminal (BRD/BET) family proteins block downstream components of BCR signaling, downregulate Bcl-2 transcription, and suppress NF-κB signaling in CLL and B-NHL [466]; among novel single-molecules cotargeting BRD4 and other tumor targets recently developed, SRX3177 targeting CDK4/6-PI3K-BRD4 and SRX3305 targeting BTK-PI3K-BRD4, demonstrate preclinical activity against MCL [467], CLL [468], and MCL [469]. These studies underscore the potential effectiveness of these novel multi-action small molecule inhibitors, alone or combined, for potential treatment of B tumors.Last, but not least, metabolic changes in tumor cells represent a novel opportunity for combination therapy approaches [470]. Metabolic reprogramming is linked to oncogenic transformation. One hallmark of BCL2, PI3K, BTK, and mTOR proteins and some TAAs such as CD38, concerns their participation in tumor metabolism. Activation of these oncogenic pathways makes tumors more metabolically active, and conversely, active metabolism upregulates BCL2, PI3K, BTK, and mTOR proteins, inhibiting susceptibility to cell death. In a consistent way, a few clinical trials have started to include metabolic inhibitors (targeting for instance glycolysis, OxPhos, amino acids) with venetoclax, ibrutinib and other drugs, to overcome the limitations of targeted therapeutic strategies in lymphoid malignancies. As metabolic reprogramming is closely linked to tumor B cells’ microenvironment and to immunoevasion, strategies targeting these crosstalks may also open new avenues for overcoming therapeutic resistance.

To conclude, most therapeutic drugs have underscored our advancement in the understanding of the biology of malignant B cells, and has improved outcomes for many patients. In the search for better efficacy and safety, the status of therapies in B cell malignancies is continually advancing with emerging concepts in therapy and evolving results from clinical protocols. A large variety of more effective and selective inhibitors and targeted combinations are being evaluated in these diseases, and it is expected that some of these new compounds will proceed into the clinic in the near future.