Investigational Monoclonal Antibodies in the Treatment of Multiple Myeloma: A Systematic Review of Agents under Clinical Development

Background: Immunotherapy for multiple myeloma (MM) has been the focus in recent years due to its myeloma-specific immune responses. We reviewed the literature on non-Food and Drug Administration (FDA) approved monoclonal antibodies (mAbs) to highlight future perspectives. We searched PubMed, EMBASE, Web of Science, Cochrane Library and ClinicalTrials.gov to include phase I/II clinical trials. Data from 39 studies (1906 patients) were included. Of all the agents, Isatuximab (Isa, anti-CD38) and F50067 (anti-CXCR4) were the only mAbs to produce encouraging results as monotherapy with overall response rates (ORRs) of 66.7% and 32% respectively. Isa showed activity when used in combination with lenalidomide (Len) and dexamethasone (Dex), producing a clinical benefit rate (CBR) of 83%. Additionally, Isa used in combination with pomalidomide (Pom) and Dex resulted in a CBR of 73%. Indatuximab Ravtansine (anti-CD138 antibody-drug conjugate) produced an ORR of 78% and 79% when used in combination with Len-Dex and Pom-Dex, respectively. Conclusions: Combination therapy using mAbs such as indatuximab, pembrolizumab, lorvotuzumab, siltuximab or dacetuzumab with chemotherapy agents produced better outcomes as compared to monotherapies. Further clinical trials investigating mAbs targeting CD38 used in combination therapy are warranted.


Results
We identified 2730 studies using PubMed (90), Cochrane (150), EMBASE (725), SCOPUS (1749) and ClinicalTrials.gov (16). After excluding 335 duplicates, we read titles and abstracts of the remaining 2395 articles and excluded an additional 2298 articles that were review articles, expert opinions, preclinical studies, monoclonal antibodies targeting cancers other than MM and studies on daratumumab and Elotuzumab. We read full-length manuscripts of the remaining 97 articles and excluded 58 articles based on the following reasons: no efficacy outcomes reported (15), interim analysis (10), preclinical studies (28), review articles (5). Thirty-Nine (39) clinical trials that met our inclusion criteria included 1906 patients. (Figure 1: PRISMA flow diagram).
We summarized adverse effects related to mAb drugs in Table 3.

CD38
CD38 is highly expressed in MM cells. It plays a key role in the growth of cells and their survival. CD38 acts as a surface receptor to regulate the proliferation of T-cells by means of cytokine recruitment and cell adhesion. It also acts as an ectoenzyme in the synthesis of cyclic adenosine diphosphate ribose (cADPR) and the hydrolysis of cADPR to ADPR and generates NAADP from NADP. NAADP, cADPR and ADPR play important roles in the regulation of calcium influx and other cellular signaling pathways [18,19]. Isatuximab is anti-CD38 mAb that produces its anticancer effects by means of CDC, ADCC or direct toxicity. Isatuximab as monotherapy achieved an ORR of 32% and 24% in patients who had failed 6 and 5 lines of therapy, respectively [20,21]. When isatuximab was combined with lenalidomide and dexamethasone in doses of 3, 5 or 10mg/kg, it produced an ORR of 64.5% and 56% in patients with 6 and 5 prior lines of therapies respectively [22,23]. The use of isatuximab + Rd in doses of 10mg/kg and 20mg/kg achieved a similar ORR of 50% with a clinical benefit response (CBR) significantly higher at 83% with a 10mg/kg dose as compared with CBR of 50% with a 20mg/kg dose [24]. Mikhael et al. studied isatuximab in combination with pomalidomide and dexamethasone in patients with 4 prior lines of therapy and achieved the highest PR of 62%. In that study, 1 patient achieved CR and 8 patients achieved very good partial response (VGPR) with clinical benefit response (CBR) of 73% [25]. Patients tolerated Isatuximab at 10mg/kg dosing while 20mg/kg caused adverse events such as pneumonia, fatigue, hypokalemia, anaphylaxis and febrile neutropenia. A randomized phase III trial to compare isatuximab + Pd versus Pd is currently ongoing [26]. MOR202 is another anti-CD38 antibody under investigation as a single agent and in combination with lenalidomide and pomalidomide separately. Investigators saw long lasting tumor control with MOR202 monotherapy with patients achieving 19% PR and 13% VGPR, MOR202 + lenalidomide resulted in 5 out of 7 patients achieving PR and MOR202 + pomalidomide resulted in achieving CR in 2 out of 5 patients [27].

CD40
CD40 is a surface receptor that is highly expressed on cells with increased potential for proliferation. It has been detected in various B-cell neoplasms including MM [28]. Dacetuzumab is an anti-CD40 mAb that is being tested both as single and combination therapy. The use of Dacetuzumab as monotherapy in 44 RRMM patients with prior 5 lines of therapy achieved no ORR with 20% patients in SD [29]. The use of Dacetuzumab + Rd in 36 RRMM with prior 4 lines of therapy produced OR of 39% with PR of 33% [30]. Lucatumumab is another anti-CD40 mAb with two distinct anti-tumor activities; it stops CD40-CD40L dependent cell growth and causes tumor cell lysis by ADCC [31]. The use of Lucatumumab as monotherapy in 28 patients who relapsed to 3 lines of therapy achieved PR in one patient and 43% of patients achieved SD [31].

KIR
Killer-cell immunoglobulin like receptors (KIR) are transmembrane glycoproteins that express on the surface of nature killer (NK) cells. KIRs are mostly inhibitory as they decrease the cytotoxic potential of NK cells. NK-cells play an important role in protection against cancer development. IPH-2101 is an anti-KIR mAb that enhances the cytotoxic potential of NK cells against cancer cells by ADCC. IPH-2101 did not produce any ORR when used as monotherapy in RRMM [32]. Even in phase I-II trials, IPH-2101 used as monotherapy in smoldering MM did not achieve a clinical response [33]. When IPH-2101 was used in combination therapy with lenalidomide in the phase-I trial, ORR of 33.3% (n = 5) was achieved with two VGPR and three PR while six patients had SD. IPH-2101 in a dose range of 0.2-2mg/kg was also well tolerated [34].

CD74
CD74 is a membrane glycoprotein found on B-cells, MM cells and monocytes. Because of its rapid internalization, association with major histocompatibility complex class II and restricted expression by normal cells, CD74 can deliver cytotoxic drugs inside cancer cells [35]. These properties led to a phase I clinical trial of anti-CD74 humanized mAb, milatuzumab for treating RRMM patients. 25 MM patients who relapsed after 5 prior lines of therapies were treated with milatuzumab and no ORR was achieved. Out of 19 patients who completed treatment, five had SD for 3 months after therapy while one had SD for 17 months. CRS was seen in five patients [36]. There is a need for further trials using milatuzumab with combination chemotherapy.

ICAM-1 (CD54)
Intercellular adhesion molecule (ICAM-1) helps in the adhesion of MM cells to marrow stromal cells with resultant tumor proliferation. The ICAM-1 overexpression is associated with resistance to chemotherapy resulting in advanced disease [37,38]. Targeting ICAM-1 by humanized IgG1 mAb BI-505 as monotherapy in 35 RRMM patients did not achieve an ORR with 24% patients having SD and 65% having PD [39].
3.1.6. Insulin Like Growth Factor-1 (IGF-1) IGF-1 receptor and binding proteins play a critical part in MM pathogenesis by up regulating proliferation, angiogenesis, tumor survival and osteolysis by stimulating osteoclast activity. IGF-1 attracts MM cells to bone marrow, up regulates expression of anti-apoptotic molecules and down regulates pro-apoptotic molecules hence stimulating proliferation of MM cells. The expression of IGF-1 in MM is associated with poor outcomes and causes resistance to therapy [40]. In phase I trials, investigators have tested AVE1642 and figitumumab (Anti-IGF-1R mAb) as monotherapy versus combination with bortezomib or dexamethasone. The use of figitumumab as monotherapy achieved no ORR while its combination with dexamethasone achieved PR in 6 out of 27 patients [41]. When AVE1642 was used as monotherapy, it produced MR in 1 patient while maintaining SD in 7 out of 17 patients. AVE1642 + dexamethasone produced CR and PD in one patient each while SD in 3 out of 11 patients [42]. Despite favorable safety as both monotherapy and in combination, investigators did not consider the response as significant.

GRP78
Glucose regulated protein 78 (GRP78) is a cellular protein that is found mostly in the endoplasmic reticulum (ER) and mitochondria and helps in protein assembly and controls ER stress signaling. GRP78 is an anti-apoptotic protein and can translocate to cytosol and the cell surface in tumor cells where it plays a role in angiogenesis, tumor progression and metastasis. It also results in resistance to proteasome and BRAF inhibitors [43]. PAT-SM6 targets GRP-78 and causes cytotoxicity in MM cells by inducing apoptosis and CDC [44]. Although PAT-SM6 was well tolerated in a phase I trial as a single agent in 12 RRMM patients with 4 lines of therapy it failed to produce OR with SD in 33.3% [44]. The favorable safety profile makes it a candidate for further trials with combination therapies [45].

CXCR-4
CXCR-4 is expressed in many hematological malignancies and is thought to have a role in cancer cell survival [46]. CXCR-4/SDF1 axis plays an important role in the localization of MM cells in bone marrow, regulation of MM cells trafficking by adhesion, invasion and mobilization of MM cells out of the bone marrow [47]. F50067 is a humanized IgG1 anti-CXCR-4 mAb and exerts its ant-tumor effects by decreasing the interaction of MM cells with bone marrow microenvironment and resultant toxicity through ADCC and CDC. Fouquet et al studied 14 RRMM patients as F50067 monotherapy and combination of F50067 + Rd (low-dose dexamethasone) with 66.7% ORR (≥PR) in combination cohort and OR of 33.3% (≥SD). Investigators discontinued the study due to hematological toxicities [48].

Interleukin 6 (IL 6)
IL-6 is a cytokine that mediates B-and T-cell immune function. It is an important regulator of the inflammatory response and mediates differentiation and proliferation of plasma cells. Siltuximab is an anti-IL6 mAb that demonstrated potent activity against MM in preclinical studies in combination with bortezomib and dexamethasone [49,50] [52]. In another trial of siltuximab as monotherapy versus siltuximab and dexamethasone in patients who failed four previous lines of therapy, siltuximab monotherapy did not achieve OR with SD of 62% and PD of 39%. In combination therapy, ORR of 23% was achieved with PFS of 3.7 months, SD 57% and PD 17% [53]. Suzuki et al. evaluated siltuximab + Rd in 9 patients with 1-2 previous lines of therapy and reported CR in 22% and PR in 44% [54].

Vascular Endothelial Growth Factor (VEGF)
Angiogenesis plays a vital role in the growth and spread of neoplasms. MM can secrete VEGF, resulting in an increase in bone marrow micro-vascularity, which is an important prognostic factor in MM [55][56][57]. VEGF stimulates the endothelium of microvasculature and bone marrow stromal cells to secrete IL-6, which is also a strong growth factor for malignant MM cells [58]. Bevacizumab is a mAb, which binds VEGF and blocks its effects on angiogenesis and proliferation [59]. Callender et al. evaluated the role of bevacizumab + Rd in 31 patients who had failed prior median 3 lines of therapy and described OR of 70%. The results were not considered superior to Rd alone where OR was 60% [60]. In an RCT by Somlo et al. using bevacizumab alone versus bevacizumab ± thalidomide in 6 patients each with prior median three lines of therapy, bevacizumab monotherapy did not achieve OR while the addition of thalidomide slightly improved the response with PR of 33% [61]. In another RCT by White et al. for comparing bevacizumab + bortezomib in 49 patients versus bortezomib + placebo in 53 patients with 1-3 previous lines of therapy, there was no statistically significant difference in ORR (51% vs. 43.3%) [62].

B-Cell Activating Factor (BAFF)
BAFF and APRIL (a proliferation-inducing ligand) are members of the TNF-α family and are highly expressed in MM. Both targets activate phosphatidylinositol-3 (PI-3) kinase and mitogen activated protein kinase with resultant up regulation of MCL-1 and BCL-2 anti-apoptotic proteins in MM. They also protect MM cells from dexamethasone-induced apoptosis [63]. Their levels are also associated with disease severity [64]. Atacicept and tabalumab are anti-BAFF mAbs. Atacicept blocks soluble forms of BAFF and APRIL, while tabalumab blocks both soluble and membrane bound forms of BAFF [65]. Rossi et al. evaluated atacicept as monotherapy in 12 patients who had failed 5 prior lines of therapy and no OR was achieved [65]. Shinsuki et al. studied tabalumab in doses of 100-200 mg in combination with Vd in RRMM patients with 1-5 prior lines of therapy and reported a combined OR of 56.3% and tabalumab was well tolerated [66]. Raje et al. studied tabalumab 100mg, 300mg and placebo + Vd each and found no statistically significant difference between OR and PFS. They reported an ORR of 58.1%, 59.5% and 61.1% with tabalumab of 100mg, 300mg and placebo + Vd respectively [67]. RRMM patients who had low BAFF levels achieved a moderately long PFS, which points to its role as an important prognostic indicator in RRMM [67].

TRAIL
Tumor necrosis factor related apoptosis-inducing ligand (TRAIL/Apo2L) is a part of the TNF family and can induce programmed cell death of neoplastic cells while causing no toxicity against normal body tissues. TRAIL mediated apoptosis occurs following its binding to death receptors, TRAIL-R1 (DR4) and/or TRAIL-R2 (DR5). Mapatumumab is a humanized mAb that targets and triggers TRAILR1 receptor. The combination of mapatumumab with bortezomib showed promising results in preclinical studies that led to clinical studies of mapatumumab [68]. Belch et al. [69] studied mapatumumab in combination with bortezomib. They divided patients into three arms: Arm Abortezomib alone, Arm B10 − mapatumumab 10 mg/kg + bortezomib, Arm B20 − mapatumumab 20 mg/kg + bortezomib. They reported ORR of 51.4%, 30.3% and 52.8% while the median duration of response was 8.5, 9.3 and 7.6 months in arms A, B10 and B20, respectively. Although there was no significant toxicity, mapatumumab failed to achieve significant clinical improvement. These results do not favor further studies of mapatumumab + bortezomib in RRMM patients.

PD-1/PD-L1
Investigators saw PD-1 expression on the surface of T-cells and expression of its ligand PDL-1 on tumor cells. PD-1/PDL-1 interaction inhibits the proliferation of T-cells. PD-1 expression on plasma cells is increased in the presence of bone marrow stromal cells and is associated with advanced disease [70]. mAbs against PD-1/PD-L1 block the escape of tumor cells from the immune system by enhancing T-cell function [71]. Nivolumab, a mAb against PD-1, achieved CR and OR in only one patient out of 27 RRMM patients when it was used as monotherapy [72]. The use of nivolumab in combination with ipilimumab (CTLA4 mAb) in 7 RRMM who failed 5 prior lines of therapy failed to achieve ORR and had results similar to nivolumab monotherapy [73]. Ribrag et al. studied pembrolizumab in 30 RRMM patients with 57% SD as the best response [74]. Pembrolizumab in combination with Pd were studies by Badros et al. in 48 patients who had 3 previous lines of therapy. Among 27 response evaluable patients, 4 had a stringent complete response (sCR), 3 had VGPR and 14 PR with mPFS of 17.4 months [75]. Investigators studied Pidilizumab that is another mAb against PD-1 in combination with lenalidomide. Only one patient achieved PR and VGPR was achieved by 3 patients out of 12 RRMM patients [76].
There have been many concerns regarding the safety of combination of mAbs targeting PD-1/PD-L1 with IMiDs. This is supported by data from KEYNOTE-183 [74] (pembrolizumab with pomalidomide and dexamethasone) and KEYNOTE-185 95 (pembrolizumab with lenalidomide) trials that reported increased mortality, increased incidence of grade 3-5 adverse events in patients treated with pembrolizumab as compared to control group.

CD56
CD56 (neuronal cell adhesion molecule) is a cell surface glycoprotein that is particularly expressed on NK cells, neuronal cells and cytotoxic cells [77]. We generally do not see expression of CD56 on benign B-cells and it can be expressed on more than 70% of MM cells [78]. Lorvotuzumab is anti-CD56 mAb that is conjugated to a maytansinoid cytotoxic agent known as DM-1 [79]. Channan et al. evaluated lorvotuzumab mertansine (LM) monotherapy in 37 patients with 6 previous lines of therapy and those patients did not achieve an OR [78]. Berdeja et al. evaluated LM in addition to Rd in 44 patients with median 2 previous lines of therapy. An ORR of 58%, 1 sCR, CR each and 8 VGPR were reported in 32 response evaluable patients. Peripheral neuropathy was the most commonly found an adverse effect. Investigators felt that this toxicity could be explained by CD56 distribution on neuronal and NK cells [80].

CD138
CD138 (syndecan-1) is a heparan-sulfate coated glycoprotein that facilitates MM cell adhesion and its loss from the cell surface may lead to MM migration and metastasis [81]. It also functions as a co-receptor for MM growth receptors [82]. Indatuximab ravtansine (IR, BT-062) is the antibody drug conjugate of anti-CD138 chimerized mAb and cytotoxic maytansinoid DM4.
The binding of BT-062 and CD138 leads to CD138 internalization and release of anti-microtubule agent cytotoxic DM4 that that leads to MM cell death. Heffner et al. studied IR in 29 patients with medians of 2 previous lines of therapy as monotherapy and reported OR in one patient [83]. Kelly et al. evaluated IR + Rd in 64 patients with medians of 3 previous lines of therapy and reported an OR of 78%. Later they did head to head trials of IR + Rd versus IR + Pd. They reported no statistically significant difference in OR (78% vs. 79%) [84].

Bi-Specific Antibodies (BiAb)
Bi-specific antibodies are a novel approach in the treatment of MM. They have a unique potential to target two different antigens at the same time. These targets could be a cancer cell or these antibodies can target body's immune cells and cancer cells at the same time. They involve both adoptive cytolytic and innate immune cells simultaneously against cancer cells. There are different kinds of bi-specific antibodies (BiAb) like Bi-specific T-cell engaging antibody (BiTEs) and bi-specific antigen binding fragments (BiFabs), depending on targets. BiTEs bind tumor cells and immune cells by means of the interaction of CD3 with antigens on tumor cells (Figure 2). This interaction activates T-cells that attack the targeted tumor cells [85]. Various BiTEs that target BCMA and CD3 such as EM801, JNJ-64007957 and BI 836909 have shown favorable results in in vivo and in vitro models of MM [86,87]. Topp et al. studied AMG 420 (previously BI 836909) in 35 RRMM patients with ≥2 prior lines of therapy. At dose of 400ug they reported ORR of 83% (5/6) with 3 patients having MRD-negative CRs. a co-receptor for MM growth receptors [82]. Indatuximab ravtansine (IR, BT-062) is the antibody drug 190 conjugate of anti-CD138 chimerized mAb and cytotoxic maytansinoid DM4.

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The binding of BT-062 and CD138 leads to CD138 internalization and release of anti-microtubule 192 agent cytotoxic DM4 that that leads to MM cell death. Heffner

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Several clinical trials are currently ongoing to study BiTEs. PF-06863135 is an anti-CD3/antiand NCT02514239). There are a few other BiTEs in pre-clinical studies combining CD3 and CD138 217 that showed significant activity against MM in murine and in vitro models [88]. NKG2D is an Several clinical trials are currently ongoing to study BiTEs. PF-06863135 is an anti-CD3/anti-BCMA bispecific monoclonal antibody for MM patients (NCT03269136, NCT03145181, NCT03269136 and NCT02514239). There are a few other BiTEs in pre-clinical studies combining CD3 and CD138 that showed significant activity against MM in murine and in vitro models [88]. NKG2D is an activation receptor present on CD8 + T-cells, NK cells, γδ T-cells and NKT-cells. NKG2D and CS1 tumor related antigens represent a possible target by bi-specific antibodies. A bi-specific antibody targeting NKG2D and CS1 simultaneously has shown an enhanced killing of MM cells in vivo models [89]. There is another novel approach to treat MM using BiFabs that are bi-specific antibodies which target two distinct epitopes on the same antigen. Comparison of BiFabs targeting BCMA and CS1 demonstrated that BiFab-BCMA had greater anti-myeloma potential than BiFab-CS1. BiFab-BCMA specifically directs T-cells for MM cell lysis and produces results similar to CAR-T BCMA in vivo and in vitro studies [90]. Cytokine release syndrome is a form of cytokine storm that can happen as a dangerous complication of extreme T-cell activation.

CD47
TTI-622 is recombinant soluble fusion protein that is produced by linking sequences coding N-terminal CD47 binding domain of human SIRPα with Fc domain of IG4. TTI-622 binds human CD47 and prevents the delivery of inhibitory "do not eat" signal to macrophages. There is an ongoing phase 1a/1b trial of TTI-622 in the treatment of advanced relapsed or refractory multiple myeloma and lymphoma [91].

Discussion
An extensive review of the literature showed that isatuximab (anti-CD38) and F50067 (anti-CXCR4) were the only two mAbs that produced encouraging results as monotherapy with ORR of 66.7% and 32% respectively [32,92]. Isatuximab used in combination with Rd produced CBR of 83% and in combination with Pd produced CBR of 73% [93,94]. However, the clinical trial of F50067 with both monotherapy and combination of Rd was discontinued due to significant hematological toxicities [32]. Our review also shows that trials using indatuximab, pembrolizumab, lorvotuzumab, siltuximab and dacetuzumab in combination therapy produced better outcomes as compared to monotherapies in RRMM.
Immune therapy has been of increasing interest as the immune system is significantly impaired with negative effects on both humoral and cellular immunity [16]. Antibody drug conjugates are an emerging treatment modality for MM because of their targeted nature. Indatuximab ravtansine could not produce favorable outcomes as a monotherapy [68] but produced an ORR 78% and 79% in combination with Rd and Pd, respectively [69,70]. Similarly, Lorvotuzumab mertansine did not achieve an ORR as a monotherapy but achieved an ORR of 59% in combination with Rd [64,65]. These results highlight the possibility of combination therapies being effective where original monotherapy was not. Another possible treatment strategy of immune checkpoint inhibitors couldn't show a beneficial effect in RRMM and was associated with prohibitive toxicities [70,[95][96][97].
Investigators seeking future treatment recommendations should seek guidance from data produced in large well-designed randomized phase II-III trials, but they can formulate preliminary predictions. Due to the wavering escape strategies of myeloma cells, the role of precision medicine utilizing immunotherapies as a cornerstone in combination with approved therapies to treat MM is evolving [98]. It is challenging to target multiple evolving abnormalities at a precise time, in the right sequence and with the correct combination to prevent future relapses. This forms the basis of on-going trials evaluating 4 drug treatment strategies with or without a mAb compared to standard of care triple drug regimens [14].
Many questions remain unanswered, most relevant to immune therapy being the ideal timing of mAb in clinical course and the role of genomic profiling. It seems logical to use targeted immuno-modulation early in the disease when the immune system is more capable of responding but toxicities may be offset some of these approaches. Patient who cannot achieve consolidation with high dose melphalan and autologous transplant may benefit from earlier use of mAbs. Understanding and controlling the adverse effects associated with these drugs will also be important. Despite advances in understanding, different options are deployed in a relatively empiric fashion, relying on disease characteristics clinically rather on genotype, molecular features or mutations [99]. Receptor density studies [100], immuno-phenotyping and emerging understanding about molecular minimal residual disease (MRD) status may be of utility in deciding drugs for induction and maintenance therapy [101,102] and aid in determining when to stop any drug treatment altogether based on MRD negativity in order to ultimately reduce the financial burden and clinical toxicity [15,103]. With further treatment insight about biological drivers, checkpoint inhibitors, tumor microenvironment and interaction with the tumor cells we can hope to approach the threshold of treatment by individualizing regimen for each patient.