Role of Immunotherapy in Targeting the Bone Marrow Microenvironment in Multiple Myeloma: An Evolving Therapeutic Strategy

Clement Chung

Abstract

Multiple myeloma (referred to henceforth as myeloma) is a B-cell malignancy characterized by unregulated growth of plasma cells in the bone marrow. The treatment paradigm for myeloma underwent significant evolution in the last decade, with an improved understanding of the pathogenesis of the disease as well as the development of therapeutic agents that target not only the tumor cells but also their microenvironment. Despite these therapeutic advances, the prognosis of patients with relapsed or refractory myeloma remains poor. Accordingly, a need exists for new therapeutic avenues that can overcome resistance to current therapies and improve survival outcomes. In addition, myeloma is associated with progressive immune dysregulation, with defects in T-cell immunity, natural killer cell function, and the antigenpresenting capacity of dendritic cells, resulting in a tumor microenvironment that promotes disease tolerance and progression. Together, the immunosuppressive microenvironment and oncogenic mutations activate signaling networks that promote myeloma cell survival. Immunotherapy incorporates novel treatment options (e.g., monoclonal antibodies, antibodydrug conjugates, chimeric antigen receptor T-cell therapy, immune checkpoint inhibitors, bispecific antibodies, and tumor vaccines) either alone or in combination with existing lines of therapies (e.g., immunomodulatory agents, proteasome inhibitors, and histone deacetylase inhibitors) to enhance the host anti-myeloma immunity within the bone marrow microenvironment and improve clinical response. Following the United States Food and Drug Administration approval of daratumumab and elotuzumab in 2015, more immunotherapeutic agents are expected to be become available as valuable treatment options in the near future. This review aims to provide readers with a basic understanding of the role of immunotherapy in modulating the bone marrow tumor microenvironment and its role in the treatment of myeloma. Clinical efficacy and safety of recently approved therapeutic monoclonal antibodies (daratumumab, elotuzumab) are discussed, along with the therapeutic potential of emerging immunotherapies (antibody-drug conjugates, chimeric antigen receptor T cell therapy, tumor vaccines, and immune checkpoint inhibitors).

Keywords: Immunotherapy, Daratumumab, Elotuzumab, Immune checkpoint inhibitors, CAR-T cell therapy, Tumor vaccine, Multiple myeloma, BiTE, Monoclonal antibodies

Introduction

Multiple myeloma (referred to henceforth as myeloma for simplicity) is the second most common hematologic malignancy in the United States, with an estimated 26,850 new cases and 11,240 predicted deaths in 2016. 1 Higher incidences are found in western countries and in individuals of African descent.51Myeloma is characterized by the proliferation of neoplastic clones of plasma cells derived from B lymphocytes (or simply, B cells). These neoplastic clones grow in the bone marrow, frequently invade the adjacent bone, disrupt both bone homeostasis and hematopoiesis, and cause multifocal destructive lesions throughout the skeleton that result in bone pain and fracture. Patients with myeloma are often susceptible to infections due to the lack of functional hematopoietic stem cells in the bone marrow and the inability to mount a strong immune response. Neutrophils are functionally impaired whereas eosinophils promote human myeloma growth. 2, 3 Recurrent infections are common.
Until 2000, the mainstay of therapy for myeloma was melphalan or doxorubicin-based regimens with corticosteroids. Autologous hematopoietic stem cell transplantation, an option in select patients, improves median overall survival by about 12 months. 4 To date, the introduction of proteasome inhibitors (e.g., bortezomib, carfilzomib, and ixazomib), histone deacetylase inhibitors (e.g., panobinostat, vorinostat) and immunomodulatory agents (IMiDs) (e.g., thalidomide, lenalidomide, and pomalidomide) have opened up numerous therapeutic avenues for patients with myeloma. Unfortunately, due to the highly heterogeneous cytogenetic and molecular abnormalities of myeloma, almost all patients eventually experience disease relapse. Decisions on treatment options of relapsed disease are generally based on timing of relapse, efficacy and toxicity of the drugs used in prior therapies, age, bone marrow and renal function, comorbidities, and patient preference. The duration of remission in relapsed disease decreases with each regimen. Patients with disease that is refractory to both proteasome inhibitors and IMiDs often have poor prognoses and limited treatment options. Improved understanding on the cellular pathogenesis of the disease has led to new therapeutic approaches.
This article aims to provide a basic understanding of the role of immunotherapy in modulating the bone marrow or tumor microenvironment and its role in the treatment of myeloma. Clinical efficacy and safety of recently approved therapeutic monoclonal antibodies (MAbs) (daratumumab, elotuzumab) are discussed, along with the therapeutic potential of emerging immunotherapies (antibody-drug conjugates [ADCs], chimeric antigen receptor T-cell therapy [CAR-T], tumor vaccines, and immune checkpoint inhibitors).

Impaired Immune Response in Patients with Myeloma

Infections represent a major cause of death in patients with myeloma due to profound immune dysfunction in these patients. Furthermore, the immunosuppressive nature of drug therapy for myeloma also causes an increased risk of infection. The risk generally decreases when the disease responds to therapy. Additional risk factors for infection include chemotherapy-related neutropenia and prolonged treatment with high-dose corticosteroids. The growth of the tumor within the bone marrow eventually leads to a collapse of normal hematopoiesis. Together, myeloma cells and stromal cells create a cytokine-chemokine microenvironment by favoring malignant cell growth while suppressing local and systemic immunity.
In healthy individuals , natural killer (NK) cells and T cells mediate protective immune responses against foreign antigens, whereas in patients with myeloma, B-cell responses are altered, and patients are often in a state of functional hypogammaglobulinemia.5 T-cell–mediated immunity is impaired due to the negative influence of myeloma cells on immune cells such as effector T cells and antigen-presenting cells (APCs). Of note, dendritic cells are specialized APCs that play a major role in the induction of antigen-specific T-cell responses. Myeloma cells inhibit the function of dendritic cells through the secretion of inhibitory growth factors (e.g., interleukin-6). Furthermore, the T-cell repertoire is characterized by a selective loss of tumor-specific lymphocytes (e.g., CD4 helper cells and CD8 cytolytic cells). In addition to the depletion of immune-reactive T cells, there is a rise in immune suppressor cells (such as regulatory T cells and myeloid-derived suppressor cells) in the bone marrow microenvironment. 6, 7

Treatment Options that Target the Microenvironment

The aberrant signaling pathways that mediate myeloma cell proliferation and survival within the microenvironment can be targeted by proteasome inhibitors, IMiDs, and novel therapies that target components of the immune network (Figure 1). IMiDs target both tumor cells and the microenvironment. Proteasome inhibitors induce apoptosis, reverse drug resistance of myeloma cells, and affect the microenvironment by blocking cytokine circuits, cell adhesion, and angiogenesis.8-11 Whereas malignant cells induce myeloma-promoting inflammation, IMiDs modulate the host immunity myeloma through three main modalities: (1) induction of the activation of NK cells, (2) stimulation of both CD4 and CD8 T cells, and (3) inhibition of regulatory T cells. 9, 10 IMiDs induce epigenetic modification of genes involved in the cytokine signaling of immune effector cells, leading to immune responses against myeloma.11 They inhibit proinflammatory cytokine production and increase T-cell responses. Moreover, they can promote the host immunity by increasing the cytotoxic activity of T cells and NK cells against myeloma cells while abrogating the protection conferred by the bone marrow microenvironment. 10, 11

Rationale of Immunotherapy for Myeloma

Historical observations that myeloma can regress due to the graft-versus-myeloma effect in allogeneic stem cell transplantation provide insight into the role of T-cell–based immunotherapy.12, 13 This immune-mediated graft-versus-myeloma effect is based on the rationale that the immune system can eradicate a malignant clone with immune-reactive T cells. However, clinical success was limited by unpredictable response and treatment-related complications. At present, although the role of allogeneic stem cell transplantation and graftversus-myeloma remains largely investigational and controversial in patients with myeloma, T-cell activity and regulation have provided pivotal mechanisms for “host-versus-myeloma therapy”—that is, immunotherapy through T-cell response that can be modulated by four therapeutic approaches in general: (1) deployment of passive immunotherapy through administration of external agents (e.g., therapeutic MAbs against myeloma-associated antigens); (2) active immunotherapy (e.g., use of tumor vaccines to elicit an active immune response ); (3) adoptive T-cell therapy (e.g., isolation and ex vivo expansion of tumorspecific T cells, which are then administered back into patients); and (4) the use of IMiDs or checkpoint inhibitors in reversing certain critical pathways of the bone marrow microenvironment.14 Successful therapy requires a combination of many different therapeutic approaches, and examples will be discussed in the sections below.
In myeloma, immune escape or evasion of myeloid cells from immune surveillance generally involves two mechanisms: immune editing of tumor cells and suppression of immune functions. 15 Whereas myeloma cells induce myeloma-promoting inflammation, NK cells and T cells mediate protective anti-myeloma responses. NK cells represent an important effector that is not dependent on the major histocompatibility complex (MHC) restriction. They become activated either due to the increased expression of activating ligands or decreased expression of inhibitory ligands. Furthermore, they express a wide range of germline-encoded receptors that allow them to recognize tumor cells (immunosurveillance) and secrete cytokines and chemokines with antitumor activities (e.g., interferon-γ). In addition, ligands on myeloma cells that bind to receptors on NK cells are progressively edited during disease progression, suggesting a role of the NK cell in early disease. 16 Impaired NK cell–mediated immune response may cause progression from precursor myeloma to clinical myeloma. Similarly, defective antigen presentation by dendritic cells may lead to deficient Tcell responses. 17 These defective immune processes provide the therapeutic basis for targeting specific pathways to restore immune functions.

Therapeutic Monoclonal Antibodies

In myeloma, a standard panel of cell surface leukocyte differentiation antigens (cluster of differentiation [CD] markers) may aid in identification and quantification of tumor cells. To be considered for therapeutic use, these molecular targets selected by MAbs and/or ADCs (Table 1) must have high level of expression in myeloma cells and a low level of expression in normal cells.18, 19 MAbs generally promote antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity on tumor recognition. Of note, ADCs do not need to recruit effector cells or activate complement due to direct, internalized release of the cytotoxic agent after binding to the target.
In addition to targeting cell surface antigens, MAbs may be directed against noncellular components of the bone marrow microenvironment, resulting in the neutralization of growth factors, inhibition of angiogenesis, modulation of mediators of bone disease, and enhancement of the host antitumor immune response. When employed as monotherapy, MAbs generally do not produce significant response in patients with myeloma. MAbs act synergistically with corticosteroids, IMiDs, and proteasome inhibitors to overcome drug resistance. 20
Many MAbs have shown promise for treating myeloma (Table 1). However, MAbs acting through the patient’s own immune system against malignant cells often have limited antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. 21 Consequently, attention has shifted to the targeting of cell surface antigens. Among these MAbs, two agents have received regulatory approval by the U.S. Food and Drug Administration (FDA) in November 2015 for relapsed or refractory myeloma, namely, the anti-CD38 MAb, daratumumab (Darzalex; Janssen Biotech, Inc., Horsham, PA), and the antiSLAMF7 MAb, elotuzumab (Empliciti; Bristol-Myers Squibb, Princeton, NJ).

Monoclonal Antibodies Targeting CD38

Originally discovered in 1980, CD38 is a 46-KDa transmembrane protein thought to be involved in the maturation of normal blood cells and leukemic cells, based on the observation that its expression is downregulated in resting B cells and is induced during Bcell activation. 22 Although not a B-cell lineage–specific marker, CD38 expression is tightly regulated during B-cell ontogenesis. Further research now shows that CD38 is a multifunctional receptor that regulates signaling, homing, adhesion, and migration of hematopoietic cells. Overexpression of CD38 is seen in the majority of lymphoid tumors, notably myeloma, making it a valid therapeutic target. 23 It is expressed in low levels in other immune cells (e.g., immature B and T cells, NK cells, activated T cells, and monocytes). 24
Of interest, CD38 is also expressed on human red blood cells. Patient treated with anti-CD38 therapy can develop alloantibody to endogenous CD38 found in reagent red blood cells during transfusion screening, causing interference of blood compatibility testing results. Multiple approaches to negate the interference of CD38 exist. In addition, anti-CD38 therapy interferes with clinical assays of myeloma proteins. In all patients treated with daratumumab, interpretation of treatment responses should be exercised with caution.
In addition, CD38 signaling also occurs via the cross-talk between antigen-receptor complexes on T cells, B cells, and other receptors. Cross-talk between CD38 and MHC is involved in the secretion of immunoglobulin G1 as well as the activation of NK cells. CD38 serves as an enzyme that catalyzes the metabolism of certain key secondary messengers 26, 27 that regulate important cellular processes.
At present, several antibodies or antibody variants to human CD38 have been developed to induce the killing of CD38-positive hematologic malignancies, including myeloma. Three anti-CD38 MAbs, namely, daratumumab, isatuximab, and MOR202 (also known as MOR03087), have been tested in clinical trials. 28, 29 Among them, daratumumb is the leading agent that recently received regulatory approval for the treatment of relapsed or refractory multiple myeloma in patients who have received at least three prior lines of treatment, including a proteasome inhibitor and an IMiD, or who are double refractory to a proteasome inhibitor and an IMiD.

Daratumumab

Daratumumab is a novel fully human anti-CD38 MAb. The binding of the CD38 epitope on myeloma cells to the Fc receptor of the MAb facilitates complement activation and complement-dependent cytotoxicity. 30, 31 Daratumumab exerts its anti-myeloma effects via multiple mechanisms: (1) it directly induces apoptosis of myeloma cells when crosslinked with Fc receptors, which are expressed on effector cells (e.g., NK cells); (2) it inhibits CD38-mediated ribosyl cyclase activity that may contribute to the direct killing of myeloma cells; and (3) preclinical studies showed that daratumumab-induced, antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity are associated with CD38 expression on tumor cells whereas downregulation of CD38 may contribute to the development of acquired resistance. 30-32
In the first clinical study of single-agent daratumumab in 32 patients with relapsed or refractory myeloma, with doses ranging from 0.005–24 mg/kg administered intravenously over 9 weeks, results showed that of the 12 patients who received ≥ 4 mg/kg of daratumumab, 42% of patients responded and another 25% had a minor response. 33 These results showed that single-agent daratumumab exhibited a dose-dependent anti-myeloma effect. Most common adverse effects were infusion-related events, which occurred in 44% of patients, predominantly during the first infusion. Of note, the corticosteroid equivalent to dexamethasone 27 mg per week was given as prophylaxis for infusion-related events to the cohort that received the maximum dose. Treatment-related hematologic toxicities occurred in 10-20% of patients. 33
Recent clinical trials demonstrated that both IMiDs and proteasome inhibitors enhance the therapeutic effect of daratumumab by sensitizing tumor cells to antibodymediated cellular cytotoxicity. 34, 35 A recently completed phase III trial (CASTOR trial) comparing the combination of daratumumab, bortezomib, and dexamethasone versus the combination of bortezomib and dexamethasone in 498 patients with relapsed or refractory myeloma demonstrated improved progression-free survival (not reached vs 7.2 months; hazard ratio 0.39%, 95% confidence interval [CI] 0.28-0.53, p<0.001) in the daratumumab combination group but with increased hematologic toxicities. 36. Similarly, in a study by Plesner et al, daratumumab was shown to augment the clinical response when added to lenalidomide and dexamethasone. 34 The overall response rate (ORR) reached as high as 92.3%. On the other hand, single-agent daratumumab resulted in a 1-year overall survival rate of 65% (95% CI 51.2–75.5%) and an ORR of 29.2% (95% CI 20.8–38.9%) in patients with double refractory and heavily pretreated myeloma.37
Common adverse events associated with single-agent daratumumab included fatigue (39.6%), anemia (33%), nausea (29.2%), and thrombocytopenia (25.5%) (Table 2).38, 49 Grade 1 or 2 infusion-related events (transient bronchospasm, headache, dyspnea, and fever) were seen in 42.5 % of patients, mainly during the first infusion. 37, 38 In addition to corticosteroid prophylaxis, infusion-related events can also be mitigated by a longer infusion, if necessary, especially during the first or second infusions. 39 In clinical trials, the first dose of daratumumab was infused over 6-7 hours. The median infusion duration was able to be reduced to 3.3 hours by the third infusion.40 The overall favorable toxicity profile of daratumumab poises itself to be an effective therapeutic option as part of combination therapy for relapsed or refractory myeloma. Dosing and administration considerations associated with daratumumab are summarized in Table 2.

Monoclonal Antibodies Targeting SLAM

SLAMF7 (also known as CS1 or CD319) stands for family member 7 (F7) of the SLAM (signaling lymphocytic activation molecule) family, a subset of the immunoglobulin superfamily of receptors expressed on several hematopoietic cells, including myeloma cells. The SLAM family consists of transmembrane proteins that share a similar motif of an extracellular domain, a transmembrane domain, and a cytoplasmic domain. The cytoplasmic domain of these members serves to recruit or elicit signal transduction events (except SLAMF7). An unusual property of the SLAM family receptors is that most of them serve as “self-ligands”—that is, they recognize the same receptor on another cell as ligands. Together, the SLAM family members are immunomodulatory receptors with roles in cytotoxicity, humoral immunity, autoimmunity, cell survival, lymphocyte development, and cell adhesion. They are implicated in normal immune regulation, pathogenesis of immunodeficiency (such as X-linked lymphoproliferative syndrome), and autoimmune diseases. The classification of SLAM family members and their immunomodulatory functions are shown in Table 3.41 SLAMF7 is a transmembrane protein expressed in high levels on myeloma cells, with little or no expression in normal hematopoietic stem cells, leading to its suggested role to evade immune recognition in malignant hematopoietic cells.42, 43 As a key regulator of normal immune cell function, SLAMF7 activates NK cells and is thought to have a growthpromoting role in normal B-cell development and an inhibitory role in T-cell development. It may possess a tumor-stimulating effect by promoting myeloma cell adhesion to bone marrow stromal cells in the microenvironment. 42, 43
The high level of SLAMF7 expression on myeloma cells stimulated the interest of drug development. Elotuzumab, a humanized MAb that binds to SLAMF7, received regulatory approval for the treatment of relapsed or refractory myeloma in combination with lenalidomide and dexamethasone in patients who have received one to three prior therapies. When administered as a single agent, elotuzumab has minimal clinical activity. 44

Elotuzumab

Elotuzumab has no direct cytotoxic or cytostatic effect on myeloma cells but exerts antibody-dependent cellular cytotoxicity via SLAMF7 ligation in which the Fc portion of elotuzumab binds to the activating Fc receptor (CD16) on NK cells, whereas the Fab portion of elotuzumab binds to SLAMF7 on myeloma cells (Figure 2). 43, 46 As a result, NK cells are activated, which secrete cytokines such as interferon-γ, further enhancing the anti-myeloma effect. Furthermore, elotuzumab interferes with the function of the immunosuppressive cells in the microenvironment and augments the activity of other effector cells.43, 46
In an early phase I clinical study by Lonial et al, patients were treated with elotuzumab (5, 10, or 20 mg/kg on days 1, 8, 15, and 22 of a 28-day cycle for the first 2 cycles and on days 1 and 15 of subsequent cycles) in combination with lenalidomide and dexamethasone. 45 Twenty-nine patients were evaluable and achieved an 80-90% response that lasted 33 months; even patients with high-risk myeloma (e.g., those with deletion 17p and t(4:14) phenotype) still experienced a 35-57% reduction in risk of disease progression with combination therapy compared with patients who received lenalidomide alone. This initial efficacy subsequently led to two phase III studies, ELOQUENT-1 and -2, that evaluated elotuzumab in the newly diagnosed, previously untreated and relapsed or refractory patients, respectively. 47, 48 Results from the ELOQUENT-2 trial demonstrated that the combination of elotuzumab, lenalidomide, and dexamethasone (triplet arm) extended progression-free survival and increased ORR compared with lenalidomide and dexamethasone (doublet arm) in patients with multiple myeloma who had one to three prior therapies. Median progression-free survival was improved by 4.5 months with the triplet arm versus the doublet arm, and ORR was 79% versus 66%, respectively. 48
Common grade 3 or 4 adverse effects in both the doublet and triplet arms of the ELOQUENT-2 trial were lymphocytopenia, neutropenia, fatigue, and pneumonia. 48, 49 Infusion-related reactions occurred in 33 patients (10%) in the elotuzumab group (triplet arm) and were grade 1 or 2 in 29 of those patients. In general, dosage reduction of MAbs is not recommended; however, dose delay is the primary method for the management of adverse effects. Current data suggest that renal impairment does not affect the pharmacokinetics of MAbs.50 Studies with daratumumab and elotuzumab in patients with severe renal impairment are ongoing, and their dosing and administration considerations are summarized in Table 2. Current National Comprehensive Cancer Network guidelines recommend elotuzumab in combination with lenalidomide and dexamethasone as a preferred option (category 1) for patients with previously treated multiple myeloma. 51

Other Novel Therapeutic Strategies

Antibody-Drug Conjugates

ADCs are MAbs conjugated with antitubulin cytotoxic agents through a chemical linker. After an ADC binds to the target antigen, the conjugate is internalized and the toxin is released, leading to tumor cell death. ADCs can circumvent the need to rely strictly on cellular-mediated processes (e.g., antibody-dependent cellular cytotoxicity and complementdependent cytotoxicity) as mechanisms for tumor cell death. CD 138 (syndecan-1) is a cell surface proteoglycan expressed on normal and malignant plasma cells. It functions as a receptor for collagen and fibronectin in the extracellular matrix within the myeloma microenvironment. 52 Recently, a phase I trial with single-agent indatuximab, an anti-CD138 MAb, demonstrated a response rate of 11% in 27 patients. 53 On the other hand, in another phase I/IIa study of indatuximab ravatansine (BT062), an anti-CD138 ADC, which was studied in combination with lenalidomide and dexamethasone in nine evaluable patients, demonstrated a 78% ORR. 54 Furthermore, another ADC that targets the myeloma cell surface antigen, B-cell maturation antigen (BCMA), is being investigated in a phase I trial (ClinicalTrials.gov identifier NCT02064387) for relapsed or refractory myeloma. Other agents in clinical development for myeloma include lorvotuzumab mertansine and milatuzumab-doxorubicin.

Bispecific Antibodies

Tumor-specific T cells play a pivotal role in the immune surveillance of cancer cells. However, the development of T-cell therapy is often hampered by the immune escape of tumor cells and limiting factors such as the activity of tumor-specific T cells and their priming, survival, and recognition of tumor cells within the tumor tissue. Rather than eliciting specific T-cell response, which relies on the expression of MHC class I molecules by tumor cells and the presentation of specific peptide antigens, recent therapeutic developments focus on targeting cytotoxic T cells with recombinant antibodies. Typically, antibodies cannot engage T cells, which lack receptors to bind to antibodies. Bispecific T-cell engagers (BiTE), also known as bispecific antibodies, represent a novel treatment option in which cytotoxic T cells are directed to tumor cells through dual specificity of a recombinant antibody to two different epitopes: one epitope is the CD3 molecules on tumor-specific T cells, whereas the other epitope is a specific antigen on myeloma cells. Engagement and activation of T cells is achieved through simultaneous binding of these epitopes by BiTE on the cell surface of tumor cells. Thus, unlike regular T-cell mediated cytotoxicity, BiTE-mediated cytotoxicity does not require APCs, the MHC/antigen complex, and costimulatory molecules. By selecting a particular surface antigen on the target cells, T-cell therapy can be applied to a broad range of malignant diseases. 56 BiTE antibodies hold promise in immuno-oncology by redirecting a vast number of existing T-cell clones, which may otherwise have evaded immunosurveillance. Clinical trials involving BiTE antibodies for myeloma are still in early development. One example is the generation of a CD138- and CD3-specific recombinant antibody with antitumor effects. 57 Currently, blinatumumab is the only BiTE approved by the FDA for clinical use.

Chimeric Antigen Receptor T-Cell Therapy

CAR-T therapy is a form of adoptive T-cell therapy in which a patient’s collected T cells are genetically modified to express chimeric antigen receptors (CARs), which are fusion proteins that include domains for antigen recognition and T-cell activation. 58 These T cells are then reinfused to target the tumor-associated antigen. Similar to endogenous T-cell activation, binding of tumor-associated antigen triggers T-cell activation and results in cytokine release and tumor-directed cytotoxicity of T cells. Signal sequences from various costimulatory molecules (e.g., CD28, 41BB) can be incorporated into second- and thirdgenerations of CAR-T to provide a constitutive source for T-cell activation and antitumor response. An important difference is that the target killing by CAR-T is independent of the

MHC restriction since the signaling between antigen-presenting cells and T cells is bypassed.

For an antigen to be considered an appropriate target for CAR-T therapy for myeloma, the antigen must be expressed on the myeloma cell surface but not on the cell surface of normal cells to avoid severe on-target toxicity from potent T-cell activation. 59 Recently, preliminary results of the first phase I CAR-T therapy that targets B-cell maturation antigen (BCMA or CD 269) in patients with myeloma was recently reported by Ali et al. 60 Among 12 patients with advanced disease who had been heavily treated, one patient achieved a stringent complete response that was sustained for more than 3 months, whereas another achieved a very good partial response that lasted for 8 weeks. This was the first clinical trial that demonstrated that CAR-T was efficacious in reducing the large tumor burden associated with patients with myeloma. Although higher doses of the engineered CAR cells correlated with better responses, they were also associated with increased adverse events of cytokine release syndrome (e.g., fever, tachycardia, hypotension, elevated liver enzyme levels, and elevated creatine phosphokinase levels). Other common adverse effects of CAR-T include prolonged cytopenia, loss of normal plasma cells, and hypogammaglobulinemia. Thus, patients required immunoglobulin replacement.

Tumor Vaccines

Tumor vaccines are designed to reeducate the host immunity to recognize myeloma cells as foreign by expanding tumor-specific T cells and creating long-term memory to prevent recurrence. Tumor vaccines for myeloma can be either dendritic cell based or peptide based. Myeloma cells present tumor-associated antigens in the absence of costimulatory molecules. As a result, the immune responses of effector cells are often inadequate. A potent myeloma vaccine must effectively present tumor-associated antigens, with costimulatory molecules, to evoke adequate immune response. Dendritic cells have been extensively studied as prime candidates in vaccination protocols for cancer treatment, due to their role as potent APCs that express costimulatory molecules and secrete cytokines that activate T cells. The role of a dendritic cell-based vaccine was recently studied in a trial with positive outcome. 61
Similar to a dendritic cell-based vaccine, tumor-associated antigens and idiotype proteins (e.g., immunoglobulin protein secreted by myeloma cells) were used as targets to generate effector immune responses in a peptide-based vaccine. Preliminary results demonstrated that single-peptide vaccines can elicit immune responses but had modest effect on disease control. 62 Recently, proteins that demonstrated antigen-specific immune response were identified in patients with precursor myeloma, raising the possibility that vaccines can be developed to target disease progression or even relapse of myeloma. 63 Many clinical trials (ClinicalTrials.gov identifiers NCT00162500, NCT00090493, NCT00186316,
NCT01758328, and NCT01380145) with tumor-specific antigens or idiotype proteins are ongoing. Notably, many of these tumor-specific antigens originate from a family of tumorassociated antigens, known as the cancer-testis antigens (CTAs). Expression of CTAs is increased following disease progression from precursor myeloma (e.g., monoclonal gammopathy) to clinical myeloma.
Results from a recent phase I/II trial that evaluated the immunogenicity and response to a peptide-based vaccine, PVX-410, showed that it resulted in immune response in all 22 patients and was well tolerated. 64 Its role in enhancing myeloma-specific immune response with either PD-1 inhibitor or IMiD is being studied (Clinicaltrials.gov identifiers NCT 01718899 and NCT02886065). Nevertheless, peptide-based vaccines generally demonstrated immune responses against specific myeloma antigens but failed to show significant clinical outcomes or objective responses. 65-68A limiting factor of vaccine-based therapy is the poor response of native effector cells, particularly in patients with advanced disease. 67 The place of tumor vaccines in myeloma therapy remains to be further characterized.

Immune Checkpoint Inhibitors

PD-1 (CD279), a member of the CD28 family of receptors, is a transmembrane protein expressed on the surface of antigen-activated T and B cells. It has two ligands, PD-L1 (B7-H1; CD274) and PD-L2 (B7-DC; CD273). PD-L1 is expressed on APCs (such as dendritic cells) but is also found on a wide range of non-hematopoietic cells. In its normal physiologic state, PD-L1/PD-1 interaction counters T-cell stimulatory signals (such as the binding of B7 on APC to the CD28 molecule on T cells) and maintains T-cell homeostasis and self-tolerance by suppressing activation and proliferation of autoreactive T cells. Since
T-cell activation depends on a second signal (costimulatory) in addition to triggering of the T-cell receptor, the binding of PD-L1 to PD-1 delivers an inhibitory (or negative) costimulatory signal that induces a state of T-cell exhaustion, produces cytokine production, and prevents activation and proliferation of T cells.69 Recent advances in the understanding of the PD-L1/PD-1 pathway led to the regulatory approval and clinical use of immune checkpoint inhibitors in patients with many solid malignancies.69 On the other hand, the role of PD-L1/PD-1 pathway for hematologic malignancies is not well established. To date, the only hematologic malignancy that demonstrated clinical efficacy in the blockade of the PDL1/PD-1pathway is Hodgkin lymphoma, which is characterized by significant tumor-specific T-cell infiltration. 70
On the basis of the broad expression of PD-1 and its ligands in the myeloma microenvironment, the PD-L1/PD-1pathway is postulated to play an important role in immune evasion of myeloma cells as well as disease progression. Within the microenvironment, bone marrow stromal cells induce the expression of PD-L1 on myeloma cells, resulting in increased proliferation of the tumor cells, and dampen susceptibility to antimyeloma chemotherapy.71 Thus, targeting the PD-L1/PD-1pathway with immune checkpoint inhibitors is a feasible strategy for the treatment of myeloma. Recently, San Miguel et al reported the preliminary results from 17 evaluable patients who received pembrolizumab, a PD-1 inhibitor, at fixed dose of 200 mg intravenously, in combination with lenalidomide and dexamethasone in the phase I KEYNOTE-023 trial. 72 The ORR was 76%, and very good partial response (VGPR) rate was 23%. About 75% of patients achieved stable disease. Median patient age was 60 years, and many patients were heavily treated with other lines of therapy. Nearly all of the 17 patients experienced at least one adverse event of any grade. Pancytopenia (anemia, neutropenia, and thrombocytopenia), fatigue, hyperglycemia, and muscle spasms were the common adverse effects. On the contrary, another recent phase I trial with nivolumab failed to demonstrate any objective response, although 18 of 27 patients achieved stable disease. 73 Preliminary results from another nivolumab study also showed stable disease. 74 The researchers from the KEYNOTE-023 trial postulated that antitumor synergy existed when the anti–PD-1 antibody was combined with IMiD. More studies are warranted.
In addition, due to the observation that blockade of the PD-L1/PD-1pathway can augment immune response to fusion cell vaccines (vaccines of dendritic cells/myeloma cells), immune checkpoint inhibitors are hypothesized to be an effective strategy to expand myeloma-reactive T cells. 75 A phase II trial (ClinicalTrials.gov identifier NCT01067287) is ongoing to assess the efficacy of an investigational agent, pidilizumab, in combination with lenalidomide or a dendritic cell/myeloma cell fusion vaccine following autologous hematopoietic stem cell transplantation.

Discussion

Immune strategies for myeloma include IMiDs, corticosteroids, and novel therapeutic MAbs. At present, the use of ADCs, bispecific antibodies, CAR-T, tumor vaccines, and immune checkpoint inhibitors is considered investigational. Regardless, the therapeutic goal is to eliminate tumor cells; alter the malignant cell microenvironment that contributes to their survival, growth, and drug resistance; and improve patient survival. Novel agents such as daratumumab and elotuzumab activate T cells, exhibit enhanced activity when coupled with IMiDs, and have the potential to induce longlasting anti-myeloma immunity, which may help overcome the genetic and epigenetic events that contribute to the progression and relapse of the disease. Daratumumab and elotuzumab are also active in patients with traditionally high-risk myeloma (e.g., 17p deletion). 41, 44-46
In addition, myeloma is a heterogeneous disease, with numerous DNA damages and multiple neoplastic clones that may represent many separate diseases. Therefore, combination therapy, which generally consists of a proteasome inhibitor with corticosteroid and an IMiD, is often used in initial induction therapy to enhance the synergistic antimyeloma effect and immune response by modulating cytokine production, NK cell– and CD8 T-cell–mediated cytotoxicity, and cell adhesion in the microenvironment. In older, transplant-ineligible patients, attenuated doses of the combination may be used. Since frail and elderly patients often have multiple comorbidities and are more susceptible to adverse drug reactions, immunotherapy with MAbs remains a good option for them. This is in contrast to the younger, transplant-eligible populations, where the depth of response is associated with improved long-term outcomes.
Furthermore, as mentioned above, due to the heterogeneity of the disease, patients with myeloma display marked heterogeneity in response to immunotherapy. The majority of the relapsed or refractory patients with myeloma who initially responded to MAbs eventually develop resistance. Tumor cells in patients with myeloma may escape immune recognition via various mechanisms (e.g., lack of costimulatory molecules during T-cell activation, downregulation of MHC class I molecules, or presence of suppressive factors such as regulatory T cells or immunosuppressive cytokines). 5-8 Immunotherapy offers the selectivity, adaptability, and potency of an effective host-versus-myeloma effect, even in the setting of disease relapse due to clonal evolution. It remains an effective treatment strategy and can maintain responses in both transplant-eligible and -ineligible patients.
The advent of MAbs and their combination with other modalities of treatment (proteasome inhibitors, IMiDs, corticosteroids, histone deacetylase inhibitors) represents the wave of the future in the management of myeloma. The rationale of combination therapy is justified by the depth of response achieved through concurrent therapy with different agents, which result in durable response. Although it may be possible to deepen response with combination therapy in double-, triple-, or even quadruple-refractory patients with myeloma, toxicities may also increase, and dose reductions are often necessary. Hence, the optimal and rational combination of different classes of agents and sequencing of different therapies necessitate more studies.
Myeloma may be preceded by either monoclonal gammopathy of undetermined significance (MGUS) or smoldering myeloma, both of which are considered premalignant or precursor conditions that are associated with 1-10% risk of disease progression per year to clinical myeloma.76 Research now shows that immune deregulation in patients with myeloma may be involved in the transition from a premalignant to a malignant stage of the disease.16 Immunotherapy agents are likely to be considered in the treatment of disease relapse to delay disease progression and induce durable response. They hold promise in transforming the natural course of this disease, especially when used in early stages of the disease.

Conclusion

Immunotherapy for myeloma is an evolving area of cancer therapy and incorporates novel treatment options (e.g., MAbs, ADCs, CAR-T, immune checkpoint inhibitors, bispecific antibodies, and tumor vaccines) either alone or in combination with existing lines of therapies (e.g., IMiDs, proteasome inhibitors, or histone deacetylase inhibitors) that can also enhance the host anti-myeloma immunity within the bone marrow microenvironment and clinical response. Immunotherapy may attenuate the systemic toxicity of cytotoxic chemotherapy and/or other systemic therapies and provide new therapeutic options to patients who have relapsed and/or refractory myeloma with a poor prognosis and limited treatment modalities. Immunotherapeutic agents may represent promising new therapeutic strategies in the treatment of early disease. Following the FDA approval of daratumumab and elotuzumab, more MAbs and immunotherapeutic agents are expected to be become available as valuable treatment options in the near future.

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