An overview of CAR T cell mediated B cell Maturation Antigen therapy

Multiple Myeloma (MM) is one of the incurable types of cancer in plasma cells. While immense progress has been made in the treatment of this malignancy, a large percentage of patients were unable to adapt to such therapy. Additionally, these therapies might be associated with significant diseases and are not always tolerated well in all patients. Since cancer in plasma cells has no cure, patients develop resistance to treatments, resulting in R/R MM. BCMA is primarily produced on mature B cells. Its up-regulation and activation are associated with multiple myeloma in both murine and human models, indicating that this might be an effective therapeutic target for this type of malignancy. Additionally, BCMA's predictive value, association with effective clinical trials, and capacity to be utilized in previously difficult to observe patient populations, imply that it might be used as a biomarker for multiple myeloma. Numerous kinds of BCMA-targeting medicines have demonstrated antimyeloma efficacy in individuals with refractory/relapsed MM, including CAR T-cell treatments, ADCs, bispecific antibody constructs. Among these medications, CART cell-mediated BCMA therapy has shown significant outcomes in multiple myeloma clinical trials. This review article outlines CAR T cell mediated BCMA medicines have the efficiency to change the therapeutic pattern for multiple myeloma significantly.


Introduction:
The advancement of chimeric antigen receptor T cell treatment has prevailed significant promising outcomes in immunotherapy over the past decades. It has risen as one of the most propitious areas of cancer therapy because of its outstanding outcomes in hematological malignancies and intriguing pre-clinical anti-tumor efficacy against a wide variety of solid tumors. The first chimeric antigen receptor T cell immunotherapies; Kymriah and Yescarta were authorized by the FDA due to their effectiveness in treating B cell malignancies [1]. Kymirah and Yescarta can target CD19 to cure adults with large B cell lymphoma and children with B lymphoblastic leukaemia . In 2020 and 2021, the FDA authorized two more CAR-T cell therapies, Tecartus and Breyanzi, to cure highly resistant large B cell lymphoma and mantle cell lymphoma (MCL), respectively [2].
Multiple myeloma is the very often identified hematological malignancy in the USA, accounting for 1% of all cancers and 10% of all hematological tumors. [3]. It is occurred by developing malignant plasma cells in the BM, linked with an aberrant elevated level of monoclonal immunoglobins in the patient's blood and urine. Additionally, individuals with MM develop severe osteolytic bone lesions and suffer from immunosuppression, impairing longevity and quality of life [4,5]. Latest developments in potential treatments including PI and IMiD have given rise to a considerable improvement in outcomes for individuals with multiple myeloma (MM) [3,[6][7][8]. Precision treatment has grown in popularity as a result of the introduction of focused therapy. Several unavoidable side effects remain, such as multidrug resistance, which reduces treatment efficacy [9].
As a result, innovative therapeutic options are needed, particularly for patients with high-risk R/R multiple myeloma [12][13][14][15]. The process of developing a new drug continues to be lengthy, taking roughly ten to twelve years to deliver an effective medicine or prospective therapeutic molecule from the laboratory to the market [16].
BCMA is considered as the leading target for the treatment against multiple myeloma. At the present, the three most often utilized therapeutic approaches for BCMA are bispecific antibody constructs such as antibody-drug conjugates (ADCs), BiTE® (bispecific T-cell engager), immuno-oncology therapies, and CAR-modified T-cell therapy. CAR T cells mediated B cell maturation antigen treatment has demonstrated clinical effectiveness in multiple myeloma. This review article is an overall outline of all available CART cell-mediated BCMA in myeloma treatment.

B cell Maturation Antigen (BCMA):
BCMA is also referred to be CD269 or TNFRSF17 [17,18]. The 2.92-kilobase-long TNFRSF17 gene transcribes it on chromosome 16's short arm (16p13. 13). It is composed of three exons and two introns. BCMA is a 184-amino acid peptide. It possesses a type III transmembrane glycoprotein of 20.2-kDa with a conserved six-cysteine motif at the extracellular N terminus [19][20][21][22]. It belongs to the superfamily of tumour necrosis factor receptors (TNFRs) [7]. Human BCMA has four natural splice variants with varying receptor binding affinities, intracellular domain signalling and membrane anchoring capabilities [20,23]. It contains two ligands: APRIL and BAFF, which are primarily produced in a paracrine manner in the bone marrow by osteoclasts, stromal cells, and macrophages [24][25][26][27]. It is mainly expressed by mature B lymphocytes, with low nonhematopoietic tissue and hematopoietic stem cells levels. It is indispensable in order to function bone marrow plasma cells which were known to be long lived [28][29][30][31]. BCMA, in conjunction with the other two functionally active parts of the TNFR superfamily, BAFF; B cell-activating factor, BAFF-R; B cell-activating factor receptor, and transmembrane activator and calcium modulator and cyclophilin ligand interactor TACI regulates B cell proliferation, maturation, and survival, as well as plasma cell differentiation [32][33][34][35][36][37][38][39]. APRIL is a proliferation-stimulating ligand with a greater affinity for BCMA than BAFF [40] (Figure 1). It binds to TACI [41], while BAFF has a greater affinity for BAFF-R ( Figure 1). BCMA is predominantly expressed in plasma blasts and plasmacytoid cells [42]. Additionally, low amount of the substances are detected on memory B cells that are dedicated towards PC development as well as plasmacytoid dendritic cells [43]. With the exception of some parts of the human body including the trachea, the testis and certain segments of the gastrointestinal tract, the BCMA is not detectable in naive B cells,normal non-hematologic tissues or hematopoietic stem cells. [16]. The function and expression of BCMA BCMA surface protein, commonly termed as CD269 or TNFRSF17, is initially expressed in late memory B cells and is afterwards frequently observed on plasma cells. [4,5]. BAFF and APRIL, two B-cell activation and expnasion -inducing ligands, bind to BCMA. As a result, multiple myeloma cells activate considerable growth and survival signalling cascades, most notably NF-kB, as well as the PI3K-PKB/Akt and RAS/MAPK signalling pathways [44][45][46][47].
APRIL and BAFF serum levels are approximately 5-fold higher in multiple myeloma patients than healthy donors [41]. The more progressed myeloma phase and the elevated concentration of ligands was identified [48]. Multiple myeloma cells have been demonstrated to encourage osteoclasts to release more APRIL, resulting in a reduction of the efficacy of the immune system on BM microenvironment [24,49]. It has been reported that the addition of APRIL-blocking monoclonal antibodies (mAbs) to B cell maturation antigen directed immunotherapies may help alleviate the immunosuppressive BM milieu created by MM cells, hence increasing antibody-dependent cell-mediated cytotoxicity (ADCC) against multiple myeloma cells. BCMA are mandatory for the maturation and survival of long-lived plasma cells but maybe less crucial for the overall humoral B-cell response [29,30]. BCMA is frequently expressed at a wide variety of epitope densities on multiple myeloma cells [24] and has been shown to enhance MM development and reduction of immune system activation in the BM microenvironment [50]. Membrane-bound BCMA can be hydrolyzed by y-secretase, resulting in the reduction of overall BCMA cell surface expression and the formation of a soluble form (sBCMA) that interferes with the binding of therapeutic compounds to BCMA. [50]. Notably, there has been no evidence of many BCMA expressions in nonhematological regions. BCMA overexpression considerably enhances xenografted multiple myeloma cell proliferation in vivo in mouse models [24]. Additionally, BCMA expression is up-regulated during the pathophysiology and progression of multiple myeloma, from average to monoclonal gammopathy of undetermined significance to smoldering MM to active MM [38].
Up-regulated BCMA levels are related to worse outcomes [31], implying that BCMA may be a valuable indicator of disease activity and prognosis in multiple myeloma. BCMA overexpression activates the NF-κβ and MAPK pathways in multiple myeloma cells in vitro where APRIL/BAFF activation is not required. [51]. Additionally, there are numerous interactions between the APRIL or BCMA signalling pathway and other pathways. For instance, APRIL promotes the survival and rapid multiplication of myeloma cells by interacting with HSPG which is also known as CD138/syndecan-1 and heparan sulfate proteoglycans [52]. BCMA expression decrease when FGF-R3 and JAK2 are inhibited concurrently [53]. Additionally, an in vitro investigation demonstrates that BCMA co-immunoprecipitants with a master transcription factor (interferon regulatory factor-4; IRF-4), involved in MM cell survival, underlining BCMA's significance in MM oncogenesis [54].

BCMA: A Potential Target in Cancer Treatment
Given that plasma cells frequently comprise a small subsection of bone marrow cells in multiple malignant myelomas, identifying these cells are essential for accurately characterizing the disease. [55].CD138 is a well-established multiple myeloma biomarker that is highly selective for plasma cells but swiftly fades from the cell surface when samples are delayed or frozen [55].
As a result, additional biomarkers for diagnosing and monitoring multiple myeloma are required. B cell maturation antigen is extremely expressive on malignant PCs isolated from MM patients compared to normal bone marrow mononuclear cells from healthy individuals. Numerous researches have examined its value as a diagnostic, prognostic, and/or predictive marker of clinical outcomes [56][57][58][59][60][61]. In contrast to CD138, BCMA is usually observed in multiple myeloma samples that have been delayed or frozen [55]. The volume of membrane-bound BCMA can be quantified using flow cytometry and immunohistochemistry; however, the measurement of BCMA volume can vary between researches due to methodological variances [56,61]. Interestingly, malignant PCs express BCMA's mRNA at comparable levels in newly analyzed multiple myeloma and refractory/relapsed multiple myeloma patients, implying that BCMA may be a feasible therapeutic target throughout the MM disease stages [57]. BCMA possesses a soluble form which known as soluble BCMA, formed from the membrane BCMA being shed directly via y-secretase activity. The extracellular domain and a portion of the transmembrane region are retained in sBCMA [50].
Soluble BCMA may serve as an identification marker for B cell involvement in autoimmune illnesses including rheumatoid arthritis, multiple sclerosis and systemic lupus erythematosus in humans [50,62]. The serum volume of sBCMA is substantially greater in MM patients than in healthy patients [33].An elevated blood level of soluble BCMA is associated with a more significant disease strain, a less favorable clinical and radiological reaction, and a worse diagnosis [63]. A substantial drop in the volume of soluble BCMA was identified in individuals who responded well to BCMA-targeted immunotherapy, implying that it may be an innovative biomarker to monitor response of multiple myeloma treatment [59].
In contrast to sBCMA, numerous investigations have demonstrated that the amount of BCMA on the cell surface appears not to influence the reaction to BCMA-targeted therapy [64]. It is also feasible that the different volume of BCMA on the cell surface of myeloma patients are simply the product of membrane BCMA shedding differences. An elevated level of soluble BCMA can impair BCMA-targeted therapy by decreasing the overall quantity of BCMA on the cell surface and isolate the circulating ligands or anti-BCMA antibodies, thereby damaging their efficiency of binding to MM cells [65,66]. A y -secretase inhibitor (GSI, LY3039478/JSMD194) was recently reported to lower soluble BCMA concentrations while increasing cell-surface BCMA expression in multiple myeloma cell lines, patient tumour cells, and animal models. This inhibitor dramatically enhanced the recognition of BCMA-specific CAR-T cells in vitro and their anti-tumour activity in vivo.
Additionally, preclinical studies have revealed that short-term GSI treatment of MM patients significantly raised the percentage of BCMA+ tumour cells [67].

CAR T cell targeted BCMA therapy
CAR T cells are engineered T cells that express a CAR directed against a specific tumour-associated antigen that, upon contact, activates T cells in a way independent of human leukocyte antigen [68][69][70][71]. These CAR constructs are composed of TAA-targeted scFvs (usually animal or human) coupled to the CD3ζ intracellular signalling domain via an extracellular spacer and transmembrane region and costimulatory domains (CD28, OX40, 4-1BB etc) [69][70][71].
While the first generation of CARs featured only a CD3ζ signalling domain, the second, third, and fourth generations of CARs contained numerous costimulatory domains to increase the likelihood of CAR T-cell proliferation [69,70]. CAR T cell proliferation in vivo has been demonstrated to correlate with clinical efficacy and are routinely examined in preclinical and clinical investigations [59,61 72]. T cells can express CARs by genetic alteration techniques such as CRISPR. CARs are fusion proteins with an antigen detection site on the surface, often scFv generated from an antibody and a costimulation domain inside the cell. Despite T cells altered with a T cell receptor, CAR T cells are not bound by the major histocompatibility complex. [73]. Since the last five years, CAR T    Five patients (23%) developed NTX (One showed grade 1, Two showed grade 2, One showed grade 3, One showed grade 4) all of which were resolved satisfactorily [75]. Moreover, there is currently no indication that bb21217 is more advanced than bb2121 in clinical response durability.

LCAR B38M
It is a CAR T cell treatment which uses autologous T cells. This CAR T cell treatment is consist of dual BCMA epitope binding which means it can targets two different BCMA epitopes Despite this promising early outcome, additional research is needed to determine the shielding and effectiveness of this BCMA-targeted CT103A.

Descartes-08
Descartes-08 is known as autologous CD8+ T lymphocytes that have been treated with an anti-BCMA CAR produced from mRNA [83]. It produces cytokine IFNy, TNFα, and IL-2, which were used to kill multiple myeloma cell lines, trigger cytotoxic degranulation and also helps to kill primary MM cells from individuals with newly discovered MM and refractory/relapsed MM .It also decreases MM in a rodent model of diffuse human multiple myeloma. Descartes-08 has a time limit inactivity, which may reduce the risk of severe central nervous system toxicities in some cases.
Descartes-08 is now being studied in stage one clinical trial (NCT03448978).

Allogenic BCMA CAR T cells:
BCMA 1 R2 are anti--BCMA allogeneic CAR T cells which are taken from healthy individuals to overcome the lag in treatment caused by the autologous CAR T's cell collection and manufacturing [84]. To decrease the potential of GVHD , the genetic alternation technique TALEN was used.
BCMA 1-R2 CAR T cells were made through TALEN by disrupting the CD52 gene. These cells were resistant to lymphodepletion treatment with alemtuzumab when exposed to the drug.
Moreover, it is engineered to contain an intra-CAR rituximab binding domain that served as an off switch, enabling the eradication of these CAR T-cells by rituximab. BCMA 1-R2 CAR T cells have illustrated anti-tumour efficiency in MM mouse models when treated with human cytokine interleukin 7 and 15.

Bi-specific CAR T cells:
Engineered

CD-19 CAR T Cells
In recent years, researchers in China have developed a bispecific CAR-T product that contains both the CD19 and BCMA. It is effective against BCMA and CD19. In order to produce it, BCMA and There have been no reports of neurotoxicity, dose-limiting toxicity, or fatalities [92].

APRIL and TriPRIL CAR T cell
APRIL-CAR T-cells are a third-generation CAR that target tumors by using a shortened version of the single form of APRIL protein as the tumour-targeting domain. It identifies both the TACI and the BCMA antigens on multiple myeloma cells [92] . In myeloma cells, April CAR T-cells were able to induce considerable cytolytic activity even at very insignificant level of antigen concentrations.
In an intramedullary murine myeloma model, the eradication of MM cells was also seen. April sequences, and it has the potential to address antigen escape from monospecific target therapy.

BCMA-CS1 cCAR T-cells
BCMA-CS1 cCAR T-cells contain a fully functional BCMA CAR connected to a fully functional CS1 CAR by a self-cleaving P2A peptide that express both BCMA and CS1 CAR molecules on the T cell surface [93]. Elotuzumab directed against CS1 has been successfully utilized to treat multiple myeloma, and CS1-CAR therapy has shown promising efficacy in preclinical investigations for multiple myeloma [94,95]. It exhibits potent, consistent, and direct cytotoxicity against either CS1or BCMA myeloma cells as a target antigen. BC1cCAR T cells were found to be more cytotoxic against multiple myeloma communities in animal models when compared with CAR T cells that were only expressed once. The 24-month PFS rate was 49.16 percent, and the overall survival rate was 53.95 percent, respectively. Ten patients had grade 1-2 cytokine reaction syndrome, and three patients had tolerable grade 3 cytokine reaction syndrome occurrences. Thus far, the trial has lasted more than 26 months [96].

MCARH171
A new human-derived chimeric antigen receptor T cell treatment named MCARH171 accompanied by a shortened EGFR safety mechanism is now being explored in a stage 1 dose-escalation study [97]. Among eleven patients, five have received a high dose level ≥450 × 106 cells which obtained an objective response. The overall response rate was 64 percent. The responses lasted between 17 days to 235 days. Three of the five patients who received a high dose maintained a longer response about six months; another two patients maintained a response till 7.5 to 10 months. Cytokine reaction syndrome of grades 1 to 2 occurred among 40% and 20% of patients. NTX was observed in one patient.

FCARH143
It is a completely human derived CAR T-cell treatment directed against BCMA. FCARH143 is produced with same quantity of CD4+ to CD8+ CAR T cells for administration.It expresses a shortened, nonfunctional human EGF receptor to help in the identification of transduced T cells [98]. At 28 days , FCARH143 therapy was linked with a 100 percent overall response rate in six patients, and all six patients exhibited no indentifiable aberrant BM plasma cells by flow cytometry or immunohistochemistry. At an average range of sixteen weeks of follow-up, all individuals were alive. Among all the patients ; 86 % of them has shown cytotoxicity of grade 2 or below, and no NTX or neurotoxicity was observed.

JCARH125
Another fully human-derived CAR T-cell therapy that contains a costimulatory domain of 4-1BB. It has been studied in people with R/R MM in a multicenter clinical trial of level one and two [88].
The ORR was about 82 % in forty four patients who were treated with a dose range of 50, 150, or

NIH CAR-BCMA
A clinical trial of level one of dose-escalation trial using NIH CAR-BCMA was conducted in individuals with detectable multiple myeloma and uniform BCMA expression on PCs [60,61]. The ORR rate was 81 percent in sixteen patients treated with dosages level of 9 ×106 cells/kg or more, and all eleven analyzed patients had minimal residual disease negative status two months following NIH CAR-BCMA drug administration, as determined by BM flow cytometry. The responses lasted between two and fifty-two weeks, and six of the eleven individuals who tested negative for minimal residual disease had a continuing reaction of drug till the final follow-up prior to publication. At lower dosages, treatment-related toxicity was average (no grade ≥3 CRS). Nonetheless, cytokine response syndrome damage was considerable when the dose levels were high (9× 106 cells/kg), particularly in patients with a huge tumor burden.

KITE-585
It is second generation CAR T cell product which is produced by using lentiviral vector and it has a completely human derived scFv (anti-BCMA), a CD28 co-stimulatory domain , and a CD3 activation domain. It exhibits effective in prelinical studies against multiple myeloma cell lines even in the presence of sBCMA. It also exterminated xenografted multiple myeloma tumours in murine model [101,102]. A phase one open-label, clinical trial in human was proposed (NCT03318861) to investigate the shielding and feasibility of KITE-585 in patients with refractory/relapsed multiple myeloma [103]. However, this device's development was eventually terminated.

P BCMA-101
It is an unique second-generation CAR-T cell therapy composed entirely of human anti-BCMA CentyrinTM co-stimulatory motifs, a CD3 activation domain and a 4-1BB co-stimulatory motif.
P-BCMA-101 is unique among all CAR-T lymphocyte products that it is generated using the piggyBacTM (PB) DNA Modification System rather than by a viral vector [104]. Due to the absence of viral transfection, the product may be more cost-effective. Additionally, it comprises a refined population of CAR+ cells that exhibits a significant proportion of the favourable stem cell memory T phenotype (TSCM). Studies indicated that one dose of P-BCMA-101 was highly effective in decreasing tumour burden and avoiding recurrence in a variety of xenograft models [105]. The stage one dose-escalation trial (NCT03288493) enrolled twelve patients with heavily pretreated multiple myeloma who had previously received a proteasome inhibitors and immuno-modulatory drugs or were double-refractory [106][107][108][109][110] trial has been planned using the results from phase 1. The study will enroll 100 people with refractory/relapsed multiple myeloma who undergone at least three lines of treatment before, including a PI, an IMiD, and CD38 targeted therapy. There is no requirement for a minimum level of BCMA expression. Notably, patients who have previously received BCMA-targeted agents or CAR T cell therapy were also eligible. After a standard three-day CTX/FAMP conditioning regimen, a single dose of P-BCMA-101 CAR-T cells 6-15 106 cells/kg will be given intravenously.
Unlike other CAR-T treatments, P-BCMA-101 does not require hospitalization and may be administered in an outpatient environment, based on its phase 1 safety profile [112].

CT053
CT053 is another CAR-T cell immunotherapy that is being studied in China at the moment. It is a  [47,122]. The high rate of cytokine syndrome reactions and neurotoxicity, and also the grade ≥3 hematologic adverse events associated with pre-conditioning lymphodepletion, are significant concerns, despite the fact that BCMA CAR-T cells were successfully used in the treatment of patients with refractory/relapsed multiple myeloma [60].
To mitigate the adverse effects of conditioning chemotherapy, viable options include employing chemotherapy without toxicity, treating patients early in the illness cycle with a low tumor load, and providing increased supportive care are essential [123]. Numerous inflammatory cytokines such as interleukin 7 have been investigated, and predictive models for severe cytokine releasing syndrome have been created [123,124]. Cytokine-targeted treatment with tocilizumab, an anti-IL6 receptor inhibitor, may be utilized to relieve CRS symptoms [124,125]. Other strategies for minimizing side effects include modifying the CAR structure, for example, by incorporating suicide genes into modified T cells [126][127][128], co-expressing an inhibitory chimeric antigen receptor on engineered T cells to inhibit off-target immune responses [129], or controlling CARs with a small molecule system [130,131].