Immune Therapies in Multiple Myeloma

The past decade has witnessed an unprecedented increase in treatment options for multiple myeloma (MM), leading to near tripling of the median survival in this disease (1). This has been paralleled by our understanding of the disease biology, and the improvements in diagnosis, and prognostication as discussed elsewhere in this CCR Focus (2, 3). In particular, the introduction of the proteasome inhibitor (PI) bortezomib and immunomodulatory drugs (IMiD), such as thalidomide and lenalidomide, has dramatically altered the treatment paradigm for this disease. This was followed by newer drugs within the same classes, such as carfilzomib and ixazomib (PIs) and pomalidomide (IMiD), further increasing efficacy and improving the toxicity profile. Another class of drug new to myeloma has been the histone deacetylate inhibitor (panobinostat). Despite many of these advances, myeloma remains a chronic disease, and novel approaches continue to be a priority for this disease (4). In this context, advances in immune therapies and their application for the treatment of myeloma have taken center stage, and immune-based approaches represent the most exciting area for new myeloma therapeutics. Given the lack of positive results from the early attempts at immunotherapy in myeloma, the immune system is often considered to be significantly compromised by the disease state. Multiple mechanisms of immune evasion by MM cells have been described, including reduced expression of tumor antigens and HLA molecules by the malignant plasma cell, enhanced expression of inhibitory ligands such as programmed cell death ligand 1 (PD-L1) by the plasma cells, and recruitment of regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSC), both of which can contribute to immune suppression. The potential opportunities for intervention are illustrated in Fig. 1. However, the recent successes with immune approaches in this area have again stimulated intense interest in harnessing the immune system to eliminate the tumor cells. This review focuses on monoclonal antibodies, checkpoint inhibitors, vaccination strategies, and cellular therapy approaches, with major attention on the currently approved treatments and those with significant demonstrated promise. The IMiDs (thalidomide, lenalidomide, and pomalidomide) are considered to have immunomodulatory activity, and various mechanisms have been proposed, but are not included in this review. The current approaches can be broadly grouped into those that use the existing immune system to facilitate their antimyeloma efficacy [e.g., monoclonal antibodies that utilize antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody–drug conjugates (ADC)] and those that enhance tumor-specific immunity [e.g., checkpoint inhibitors and chimeric antigen receptor (CAR) T cells]. Clearly, there is evidence for the former, and the latter group is going through trials, with early evidence pointing toward efficacy for checkpoint inhibition combined with IMiDs, and a single patient response observed after CAR T cells.

This class of drugs has enjoyed spectacular success in the management of a wide array of hematologic and solid tumors, but until recently, attempts have not been successful in myeloma. The targets for monoclonal antibody development in MM have been either surface proteins or cytokines considered to be relevant for disease biology.

Elotuzumab is a humanized IgG1 monoclonal antibody that targets signaling lymphocyte activation molecule family member 7 (SLAMF7, also known as CS-1), a glycoprotein highly expressed on plasma cells but with little expression outside of plasma cells except for natural killer (NK) cells and activated monocytes. SLAMF7 is a member of the lymphocyte-activating molecule–related receptor family made up of an extracellular immunoglobulin domain and an intracellular signaling domain. Its function on the plasma cell is not clearly known. Multiple mechanisms have been proposed for the antimyeloma activity of elotuzumab, but the major component of its activity appears to be related to the ADCC via complex with CD16 and activation of EAT-2 on the surface of NK cells (5). Additional mechanisms include stimulation of the NK cells as well as cytotoxicity related to crosslinking of SLAMF7. In vitro studies also suggest interference between the myeloma cell and the bone marrow stromal cells (5). In preclinical studies, elotuzumab alone and in combination with lenalidomide as well as bortezomib showed significant activity, leading to its evaluation in the clinic (5, 6). The potential mechanisms of action of elotuzumab are summarized in Fig. 2. 

The clinical efficacy of elotuzumab has been examined in several phase I to III trials (Table 1). Initial studies examined the antimyeloma activity of elotuzumab as a single agent as well as in combination with lenalidomide or bortezomib in patients with relapsed myeloma (6–9). In the phase I study of single-agent elotuzumab, no responses were seen among 35 patients with relapsed/refractory MM treated at doses ranging from 0.5 to 20 mg/kg every 2 weeks (9). No maximum tolerated dose was identified up to the maximum planned dose of 20 mg/kg. Elotuzumab was then studied in combination with standard doses of lenalidomide at 3 doses—given 5, 10, and 20 mg/kg—given on days 1, 8, 15, and 22 of a 28-day cycle in the first two cycles, and days 1 and 15 of each subsequent cycle (8). The overall response rate was 82%, with durable responses lasting beyond a year. In another phase I study, elotuzumab at 4 different doses—2.5, 5.0, 10, or 20 mg/kg—was administered on days 1 and 11 and bortezomib (1.3 mg/m² i.v.) on days 1, 4, 8, and 11 of a 21-day cycle, with a response rate of 48% and no dose-limiting toxicities. In a pivotal phase III trial (ELOQUENT-2), elotuzumab was added to standard doses of lenalidomide at 10 mg/kg on days 1, 8, 15, and 22 of a 28-day cycle in the first two cycles, and days 1 and 15 of each subsequent cycle. Median progression-free survival (PFS) in the elotuzumab group was 19.4 months versus 14.9 months in the control group, a significant improvement. The overall response rate in the elotuzumab group was 79% versus 66% in the control group. The treatment was well tolerated with lymphocytopenia, neutropenia, fatigue, and pneumonia being the most common side effects other than infusion reactions in 10% patients. A smaller, randomized phase II study examined the combination of elotuzumab with bortezomib and dexamethasone, which showed improved response rates and PFS.

Daratumumab is an IgG1 kappa human monoclonal antibody directed against CD38, a type II transmembrane glycoprotein that is expressed in relatively high density on the plasma cells, as well as other lymphoid and myeloid cells and some nonhematopoietic tissues (10–12). Although the exact role of CD38 in plasma cell biology is not clear, multiple functions, including ectoenzymatic activity as well as receptor-mediated regulation of cell adhesion and signal transduction, have been attributed to it (13–15). In preclinical studies, daratumumab demonstrated potent activity against human myeloma cells, triggering complement-dependent cytotoxicity (CDC) as well as ADCC in patient MM cells in the presence of both autologous and allogeneic effector cells (11). In vivo experiments in mouse models confirmed the efficacy observed with cell lines and primary tumor cells. The CDC and ADCC observed against the primary myeloma cells were independent of the disease status, suggesting that this approach may overcome resistance to conventional therapies (16). Daratumumab also appears to recruit additional aspects of the immune system, contributing to the overall observed efficacy. This includes antibody-dependent cellular phagocytosis (ADCP) mediated by macrophages as well as FcR-mediated crosslinking of the CD38 molecules on the tumor cells leading to apoptosis (17, 18). In addition, daratumumab can lead to depletion of regulatory B cells, MDSCs, and immunosuppressive Tregs expressing CD38, eliminating some of the brakes on the immune system (19). Daratumumab also induces an increase in CD8⁺:CD4⁺ and CD8⁺:Treg ratios and increases memory T cells while decreasing naïve T cells, further enhancing the overall immune response to the tumor cell. The potential mechanisms of action of daratumumab are summarized in Fig. 2. Preclinical studies have also provided crucial data that have informed its clinical development. In vitro studies in combination with lenalidomide demonstrated an increased level of NK cell–mediated ADCC, which was also evident in animal models, leading to its clinical evaluation in combination with IMiDs (20). Finally, in vitro studies as well as studies done as part of the clinical trials suggest that the CD38 expression intensity on the MM cells may correlate with drug efficacy (16).

Daratumumab has been studied as monotherapy as well as in combination with other myeloma therapies in phase I to III trials, eventually leading to FDA approval in late 2015 (21). Data from the different clinical trials of daratumumab are summarized in Table 1. The initial phase I study examined increasing doses and different schedules of daratumumab in relapsed MM, identifying 16 mg/kg given weekly for 8 weeks, followed by every other week for 16 weeks and then monthly, as an effective dose and schedule. At the recommended phase II dosing, daratumumab, as a single agent, resulted in a partial response or better in nearly a third of the heavily pretreated patients included in two separate trials (22–24). These included patients with a deep response, including complete responses. The responses were durable, with the median response duration exceeding a year. Two large phase III trials have been reported combining daratumumab with lenalidomide and dexamethasone as well as bortezomib and dexamethasone (25). Both trials demonstrated an impressive improvement in PFS with the addition of daratumumab to an IMiD or a PI (Table 1).

Daratumumab is very well tolerated, with the main adverse event reported in the trials being infusion reactions. Over half of the patients had infusion reaction of some severity, with over 90% of the reactions being limited to the first infusion. Premedication with steroids and antihistamines mitigates the severity of the reactions, and delayed reactions are generally uncommon. Other common toxicities included fatigue, nausea, anemia, thrombocytopenia and infections. Treatment with monoclonal antibodies in general poses some issues with respect to disease assessment using the myeloma M protein when the patient has an IgG kappa monoclonal protein. To confirm a complete response by immunofixation, it is important to show that any detectable monoclonal protein represent the therapeutic antibody rather than the disease-related protein (26). Specific interference assays are being developed to address this. Another consideration, specifically for daratumumab, is the interference with blood type and cross-match resulting in false-positive antibody screens due to the daratumumab binding to the red cells (27, 28).

Isatuximab (SAR650984) is a humanized IgG1 monoclonal antibody that binds selectively to a unique epitope on the human CD38 receptor. Multiple mechanisms of action have been proposed, including ADCC, ADCP, CDC; direct cytotoxicity without crosslinking; and inhibition of the CD38 enzymatic activity (29, 30). Early clinical trials with this antibody are underway, with initial results demonstrating activity similar to that of daratumumab, which also targets CD38 (Table 1). Studies continue to explore different doses and schedules of isatuximab as single agent as well as in combination with lenalidomide. As a monotherapy, it has been studied at 3 mg/kg, 10 mg/kg, and 20 mg/kg, with infusions every other week without weekly infusions for cycle 1. The overall response rate has ranged from 18% at the lower doses to 29% at the higher doses in a group of heavily pretreated patients. Isatuximab has been combined with standard doses of lenalidomide and dexamethasone, and overall response rates of 50% to 60% have been observed in patients with relapsed myeloma. Toxicities were similar to those of daratumumab, with nearly half of the patients having an infusion reaction, mostly during first infusion, and hematologic, gastrointestinal, and infectious complications. Isatuximab has a shorter infusion time compared with daratumumab, but trials with subcutaneous administration of daratumumab are ongoing and may significantly improve the treatment duration.

Several other targets have been identified as having therapeutic potential in myeloma. IL6 and VEGF are major cytokines implicated in the disease biology and appear to play a major role in cell survival and disease progression. However, antibody-based approaches targeting either IL6 (siltuximab) or VEGF (avastin) have not demonstrated any meaningful activity against myeloma in the initial studies. Other targets that have been studied are summarized in Table 2. 

The application of immune checkpoint inhibitors has opened up a new and exciting area in the treatment of various malignancies, with striking efficacy seen in multiple cancers, especially melanoma and Hodgkin lymphoma. The PD-1/PD-L1 pathway is a negative costimulatory pathway that leads to a T-cell exhaustion phenotype, preventing appropriate cellular response to antigens. In vitro studies in myeloma have clearly suggested a role for the PD-1/PD-L1 axis in the immune defects observed in MM and open up the possibility of using this therapeutic approach (31). Early in vitro and in vivo work have suggested enhanced expression of PDL-1 on myeloma cells and a potential role for PD-1/PD-L1 inhibitors in the treatment of myeloma (32–34). It has been shown that NK cells from MM patients express PD-1, and engagement by PD-L1 on primary MM cells downmodulate the NK-cell versus MM effect. The plasmacytoid dendritic cells in myeloma have been shown to exhibit PDL-1 likely contributing to the T-cell exhaustion phenotype observed in this disease. CT-011, a novel anti–PD-1 antibody, especially in combination with lenalidomide, enhanced human NK-cell function against autologous, primary MM cells, likely by reversing this phenomenon (21, 32, 35).

Despite strong evidence supporting a role for immune checkpoint blockade in MM, the initial trial of the PD-1 inhibitor nivolumab in myeloma was disappointing. In a phase I study of advanced hematologic malignancies, which included 27 patients with relapsed refractory MM, no objective responses were noted with single-agent nivolumab (36). In contrast, significant efficacy was observed when another PD-1 inhibitor, pembrolizumab, was added to an IMiD (lenalidomide or pomalidomide) for the treatment of patients with relapsed and refractory myeloma (Table 1). In a phase I/II study that included patients with relapsed/refractory MM with >2 prior therapies, including a PI and an IMiD, patients were treated with increasing doses of lenalidomide combined with pembrolizumab. Patients were then enrolled on an expansion phase at pembrolizumab 200 mg/kg given every 2 weeks in combination with standard dose of lenalidomide and dexamethasone. The overall response rate was 50%, including 38% responses in lenalidomide-refractory patients. Responses were relatively durable, with a median duration of response of 11.3 months. The toxicity profile was similar to that observed with other diseases states, and immune-mediated reactions were relatively uncommon. In a phase II trial, pembrolizumab was combined with pomalidomide in patients with relapsed MM, with a median of 3 prior lines of therapy, including 70% of patients who were double refractory to an IMiD and a PI. The overall response rate was 60%, including a 55% response rate for the double-refractory population. Toxicities were manageable, and immune-related reactions were limited. Based on these promising data, two phase III trials are evaluating the combination of lenalidomide (NCT02579863) and pomalidomide (NCT02576977) with pembrolizumab in patients with newly diagnosed and relapsed myeloma, respectively. Success of these phase III trials will demonstrate the efficacy of these drugs and lead to their approval for use in the clinic. Other drugs belonging to this class of drugs, such as the previously mentioned nivolumab, are currently in clinical trials. In addition, combinations of checkpoint inhibitors with monoclonal antibodies such as daratumumab and elotuzumab are all being explored.

The key to developing vaccination strategies in any disease is the identification of antigens that are uniquely and highly expressed by the tumor cells, thus limiting off-target effects, which should ideally be highly immunogenic as well as critical for tumor cell survival so that its expression will not be downregulated in the face of immunologic pressure. Several such antigens have been identified in myeloma cells, such as the cancer testis antigens NY-ESO, WT1, RHAMM, HSP96, MUC1, MAGE, DKK1, and HM1.24 (37–40). The innate failure of myeloma cells to elicit an immune response despite the presence of these unique antigens can be traced to the lack of necessary costimulatory molecules. To overcome this, dendritic cell–based vaccination approaches, which rely on leading the cells with the antigen of interest, have been evaluated. Initial studies using idiotype-pulsed dendritic cells demonstrated effective T-cell responses, but overall clinical results based on several studies have been disappointing. Given the immune dysregulation in MM and data suggesting some normalization following autologous stem cell transplantation, many of the trials have used high-dose therapy as a platform for examining the efficacy of vaccination strategies (41). One study did demonstrate an improved (42–44) overall survival compared with historical controls, but with no effect on PFS, suggesting that the approach might have a resetting effect on the immune system (45). This has redirected the focus to other antigens, and peptide-based vaccination strategies with WT1 and RHAMM have been evaluated in clinical trials with immune responses and mixed clinical responses (38, 46, 47). Preclinical studies have shown benefit to adding IMiDs, such as lenalidomide, to the vaccination approaches (48). In a phase I/II study of ImMucin (a 21-mer cancer vaccine encoding the signal peptide domain of the MUC1 tumor–associated antigen), 6 or 12 biweekly intradermal ImMucin vaccine was coadministered with GM-CSF to 15 MUC1-positive MM patients following autologous stem cell transplantation (49). A notable enhancement of antigen-specific cellular and antibody response, along with stable disease or improvement, persisting for 17.5 to 41.3 months was seen in 11 of 15 patients. Prolonged responses were observed. Several ongoing clinical trials are evaluating the role of different vaccination approaches at various disease stages.

The most significant drawback of vaccination approaches is the immune milieu that is already compromised by the disease, limiting the immune response to the vaccines in vivo. As a result, several investigators have tried ex vivo expansion of activated T cells directed against myeloma cells. In a phase II study, ex vivo–expanded autologous T cells were primed in vivo using a MAGE-A3 multipeptide combined with Poly-ICLC and GM-CSF. Twenty-seven patients received anti–CD3/anti–CD28-costimulated autologous T cells, accompanied by MAGE-A3 peptide immunizations before T-cell collection and five times after ASCT (40). MAGE-A3–specific CD8 T cells were observed in 7 of 8 evaluable patients and vaccine-specific cytokine-producing T cells in 19 of 25 patients. More recently, tumor-infiltrating lymphocytes (TIL) have been explored as a way to target tumor cells using a polyclonal T-cell population of both CD4⁺ and CD8⁺ T cells targeting multiple tumor-associated antigens. This can potentially overcome the limitation of relying on a single antigenic target that may be lost with tumor evolution or suppressed in the face of immune pressure. Studies have shown that TILs can recognize myeloma cells after activation with anti-CD3/CD28 beads with higher frequency than activated peripheral blood lymphocytes from the same patients (50). Ongoing trials are examining the possibility of ex vivo activation of TILs and reinfusion after autologous stem cell transplantation.

Chimeric antigen receptors (CARs) are chimeric proteins that bring together the signaling moieties of the T-cell receptor (TCR) complex and the variable domains of an antibody targeted to an antigen of interest. T cells from a given patient can then be genetically modified to express the chimeric protein, expanded ex vivo, and reinfused into the patient. This allows the CAR T cells to recognize the target the tumor antigen in an MHC-independent manner, leading to T-cell activation and tumor cell cytotoxicity. The first-generation CARs induce T-cell catalytic activity as in the case of the endogenous TCR activation without any T-cell expansion. Second- and third-generation CARs include moieties of costimulatory molecules such as CD28 and 4-1BB to enhance T-cell proliferation and survival. However, the key to success remains the identification of antigens that are unique to the tumor cells and expressed at high levels. A variety of antigen targets are being studied at this time and include BCMA, SLAMF7, CD138, NKG2DA ligands, kappa light chain, and CD19. The first clinically reported success with the CAR-T approach in myeloma was in a patient with refractory MM who had previously received lenalidomide, bortezomib, carfilzomib, pomalidomide, vorinostat, clarithromycin, and elotuzumab, as well as a prior autologous stem cell transplantation (51). Following a second autologous stem cell transplantation, the patient received autologous T cells transduced with an anti-CD19 CAR, which led to a complete response that was sustained at least 12 months after treatment. This response was achieved despite the absence of CD19 expression in the neoplastic plasma cells. In vivo work has shown promise with several other antigens, and these are being evaluated in clinical trials (52–54). CAR T-cell approaches continue to evolve, particularly with the use of targets that are more specific to the myeloma cell, such as BCMA and SLAMF7. Ongoing clinical trials will define their role in the coming years.

Immune approaches for treatment of myeloma have finally come of age in terms of clinical translation, with active treatments available in the clinic. The challenge for the future remains the integration of these approaches into the clinic, especially with several novel agents having been introduced in the clinic in recent years. Elsewhere in this CCR Focus, Orlowski and Lonial discuss how to integrate some of the novel drugs into the treatment paradigm in myeloma (4). One of the critical aspects remains the timing of immune approaches during the disease course. Early intervention at a stage where the immune system is more capable of responding, such as in patients with newly diagnosed myeloma or even high-risk smoldering myeloma, appears attractive and logical but has to be balanced against the toxicity of some of these approaches. As the clinical experience and toxicity management improve, we will see these agents being incorporated more and more into the early stages. Meanwhile, many of the trials will continue to explore immune therapy options in patients for whom treatment with currently available active drugs and combinations has failed. The concept of using immunotherapy for eradication of minimal residual disease remaining after treatment with the current combinations or transplantation may offer an approach with durable disease control. The revised response criteria incorporating minimal residual disease testing, discussed elsewhere in this CCR Focus, will allow design of these trials (2).

In the current scenario, with the approval of elotuzumab and daratumumab, the question of sequencing these drugs has become important. In general, as with all drugs we use for this disease, the practice should be guided by the results of clinical trials. Also, given the chronic nature of myeloma, it is likely that all drugs will be used in some sequence or other. Currently, elotuzumab is approved in combination with lenalidomide and dexamethasone and should be considered in patients early on in the initial disease relapse. Daratumumab, in contrast, is approved as single agent, and recent trials have indicated its activity in combination with lenalidomide, pomalidomide, and bortezomib. It is anticipated that both of these antibodies will move into the upfront setting for initial treatment of myeloma. Approaches such as CAR T cells, with the greater potential for severe toxicity, at least for time being, should be reserved for patients who are relapsing after currently available therapies.

S.K. Kumar is a consultant/advisory board member for AbbVie (uncompensated), Amgen (uncompensated), Bristol-Myers Squibb (uncompensated), Janssen (uncompensated), Kesios Therapeutics, NOXXON Pharma, SkylineDx, and Takeda (uncompensated). K.C. Anderson has ownership interest (including patents) in Acetylene Pharmaceuticals, C4 Therapeutics, and OncoPep and is a consultant/advisory board member for Bristol-Myers Squibb, Celgene, Gilead, and Millennium Pharmaceuticals. No other potential conflicts of interest were disclosed.

Conception and design: S.K. Kumar, K.C. Anderson

Development of methodology: S.K. Kumar

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): S.K. Kumar

Writing, review, and/or revision of the manuscript: S.K. Kumar, K.C. Anderson

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): S.K. Kumar