2017 ASCO Annual Meeting: Immunotherapy in MM

The Multiple Myeloma Hub were delighted to attend the 2017 ASCO Annual Meeting in Chicago, Illinois at McCormick Place from June 2^nd to 6^th. The conference boasted record attendance and covered numerous sessions focused on Multiple Myeloma (MM).

Overview of the Immune System in Plasma Cell Disorders

The immunotherapy in MM session was opened by a talk given by Professor Madhav Dhodapkar, aimed at conveying the importance of the immune system in the context of MM pathogenesis, as well as the role it plays in current therapies. Professor Dhodapkar began by outlining the major components of the immune system, drawing attention to components that have pro-tumor effects such as T regulatory cells (T regs) and T Follicular Helper cells (TFH) in the adaptive compartment, and myeloid derived suppressor cells (MDSCs), M2-macrophages and dendritic cells (DCs) in the innate compartment. The growing role of so called ‘unconventional T cells’- cells that no longer fit with the concept of two defined compartments and often live in the tissues, was touched upon, with lipid-reactive natural killer T cells an example of such cells that have been linked to MM.

The features of humoral immune deficiency syndrome, that routinely occurs in MM, was briefly outlined. A reduction in normal plasma cells correlates with disease burden, and patients have a poor serologic response to vaccines, with infection posing a routine problem. IgG levels also appear to be an independent predictor for risk of progression to MM.

The question as to whether T cells recognize the early lesions of the precursor state (monoclonal gammopathy of undetermined significance, MGUS), and whether this early recognition affects progression or correlates with outcome, is unknown. Patients with MGUS were found to have high levels of preneoplasia-specific effector T cells in the bone marrow, although functionality was lost upon progression to MM, possibly as a consequence of immunosuppressive mechanisms.

Targets of T cell immunity in MM were noted to include neo-antigens from expressed oncogenic mutations, cancer-testis antigens, NY-ESO1, MAGE-C1, tumor associated shared antigens, and so-called stem cell antigens – which appear to be important for the bulk of the tumor despite being expressed by only a small number of cells. Particular antigens with a link to progression were highlighted with SOX2 expression linked to a reduced risk of progressing to MM from any precursor state, and expression of PD-L1 on both tumor and tumor-infiltrating cells found to correlate with an increased risk of progression to MM.

Further discussion focused on the balance between tumor promoting and suppressing factors. Adoptive transfer experiments were described, in which there was an altered functional capacity of NKT cells to respond to antigen, which emerged in several models as a Type I versus Type II response. Dendritic cells (DCs) have also been shown to cross-talk with osteoclast cells, and myeloid derived suppressor cells (MDSCs) also have relevance in the microenvironment. The clinical stability of a precursor state is thought to depend at least in part by tumor extrinsic signals. The model proposed by Professor Dhodapkar’s group is a process of clonal evolution, where both growth-restrictive and growth-permissive signals begin even before MGUS, but the microenvironment is considered the ‘final arbiter’.

Finally, the question of how the immune system affects current therapies was briefly addressed. Daratumumab therapy has been shown to activate both complement dependent cytotoxicity (CDC) and antigen dependent cell-mediated cytotoxicity (ADCC). The expression of CD38 on immune regulatory cells is responsible for daratumumab altering suppressive events in the microenvironment and driving enhanced T cell responses in patients. Imuunomodulatory drugs (IMiDs) have been shown to have pleiotropic effects, although a specific example of single agent pomalidomide was presented, and it was found to lead to a depletion of Ikaros within hours after treatment. In conclusion, harnessing tumor immunity is a promising approach to the treatment of MM.

Monoclonal and Bispecific Antibodies for Myeloma: Nipping at a cure 

The second talk was given by Ivan Borrello, from the John Hopkins University. He began by highlighting the therapeutic antibodies currently used for the treatment of MM, which include elotuzumab (anti-SLAMF7), daratumumab (anti-CD38), antibodies directed against CD19, which can also potentially target the precursor-state, anti-BCMA, PDL-1 and FcRH5. He highlighted promising data from the pivotal trial for elotuzumab in which it has potent activity in high-risk disease, but is only active when given in combination with an IMiD. The impressive PFS achieved in the two pivotal trials for daratumumab (CASTOR and POLLUX) was also highlighted, as well as the increased efficacy of triplet versus doublet regimens.

The next part of the talk focused on antibodies that have been engineered for superior functionality. For example, antibody drug conjugates, which use a linker to directly attach a cytotoxic agent for delivery in a targeted manner. A specific example is GSK2857916, currently being developed by GlaxoSmithKline, and for which the mechanism of action relies on multiple pathways – direct ADC, where the drug is internalized and can kill, ADCC, and immunogenic death – where the way in which the cells die is actually immuno-stimulatory rather than immuno-suppressive. Preliminary studies indicate high tolerance and minimal safety issues - some ocular toxicity was reported but was treatable, and thrombocytopenia was transient. Consequently, studies are moving forward with high doses. The importance of BCMA as a target in MM was also highlighted (see related MM Hub article). Doses of anti-BCMA greater than 3.4 mg have given significant clinical responses and therefore further clinical trial data is eagerly awaited.

Next bi-specific antibodies termed BiTEs (Bi-specific T cell engagers) were described. The variable fragment is fused with the variable fragment of another antibody, with the overall aim of bringing together T cells with the target cell. One such BiTE, blimotumumab (anti-CD3/CD19 fusion) is already in clinical trial. Different formats of BiTEs have also been formulated, either with the Fc domain (IgG-like) which has a longer half-life but weak tissue penetration, or a format lacking the Fc- domain, which has a short half-life but high levels of tissue penetration. Both appear to have pros and cons.

A BiTE with specificity for BCMA and CD3 is EM801, and this has a lower affinity for CD3 than it does for BCMA (a kinetic profile that appears to give the best responses in MM). The Fc portion of EM801 is mutated to avoid activation of ADCC and CDC. One of the advantages of BiTEs is that direct engagement of the T cells and activation of the immune response, means that a lower concentration of antibody is required.

One molecule of particular interest as a potential target is FcRH5, which is expressed on pre-B cells through to plasma cells, both healthy and malignant. The FcRH5 gene is located at chromosomal breakpoint 1q21, and shows increased mRNA expression in high-risk MM patients with this amplification. Consequently, the efficacy of killing with this antibody was directly related to overall expression of surface FcRH5, although was also effective at low levels, and may be a useful target for these high-risk patients.

Finally, another novel antibody technology called Y-traps was described. These are complete immunoglobulins that have been engineered to include full specificity against a tumor antigen in the variable domain, but with a linker and additional moiety (such as a receptor extracellular domain, ECD) attached to the full Fc domain, which can bind and disable immunosuppressive signals. Y-traps have been designed with a cassette format, so that a range of antigen specificities and ECDs can be mixed and matched. One such Y-trap that has shown promising results in-vitro is an IL-6 and TGFβRII combination, as both cytokines are over-expressed in MM and contribute to osteolytic bone disease. In a study conducted by Kimberly Noonan, treatment with IL-6 and TGFβ skewed plasma cells to express IL-17. When these cells were co-cultured with autologous plasma from MM patients (also containing high levels of soluble IL-6 and TGFβ) the same effect was observed. Addition of the Y-trap abrogated the Th17 phenotype and reverted cells back to a Th1 phenotype. A similar effect was observed using activated T cells, where a switch from Th1 to Th17 abrogated the formation of osteoclasts. Blocking IL-6 reduced the formation of osteoclasts, but using either IL6/TGFβRII or IL6/RANKL Y-traps, inhibited osteoclast formation altogether. This proved that such antibodies could modulate the immune response, and the advantage of using Y-traps, as opposed to single antibodies with the same specificities, was also shown in an in-vivo setting.

Cellular Immunotherapies in Myeloma

The final talk in this series was given by Edward Allen Stadtmauer from the Abramson Cancer Center, USA, who spoke about cellular immunotherapy, and the use of chimeric antigen receptors (CARs). Rationale for such therapy in MM is extensive, with the main goal to engineer a patient’s own immune cells to avoid graft versus host disease (GvHD). Many of the initial studies have used cellular therapy in addition to other therapies, under the premise that it may only work in the setting of minimal residual disease (MRD).

Bead based artificial dendritic cells (DCs) with anti-CD3 and anti-CD28 were highlighted as a key early development, as they enabled ex-vivo expansion, activation and manipulation of clinically relevant doses of autologous lymphocytes, with capacity for subsequent in-vivo proliferation. The first studies utilizing this therapy were anti-infectious vaccines, to help overcome poor immunity in MM and improve humoral immunity. Anti-tumor vaccines were used in the same way and found to be safe and to generate immune responses, but the anti-myeloma effect of such infusions was unclear. However, potential limitations were thought to be surmountable with the improved engineering of T cells. Several approaches have been used. One approach is to collect infiltrating lymphocytes, expand them ex-vivo, and then re-introduce these as antimyeloma agents, although despite positive data from ex-vivo studies, the incremental efficacy overall was uncertain. The second approach is the collection and expansion of PBMCs, which are then engineered to express chimeric antigen receptors (CARs) and infused back into patients.

The anatomy of a CAR construct was described: an exogenous tumor-specific ectodomain (a fusion of heavy and light chains with a linker) is fused to a membrane region taken from CD3 or CD28, a co-stimulatory molecule (CD27, CD28, ICOS, 4-1BB, OX40) and has a CD3 stimulatory region. The delivery of such CARS into T cells using a lentiviral vector was also explained. It was noted that the first infusions for MM (of CD19 CARs – with rationale that this could target B cells from precursor state to malignancy) were given 3 years ago, and now numerous international trials across all hematological malignancies are underway, with many durable responses and complete remissions reported.

However, the toxicity of such CAR T-cell infusions was emphasized, with on-target toxicities such as tumor lysis syndrome, B cell aplasia and hypogammaglobulinemia, and the main off-target toxicity of cytokine release syndrome (CRS), along with some (mostly) reversible neurotoxicity. Steroids can control CRS, but affect CAR T-cell retention. Targets for CARs in MM were listed as: CD138, CD38, CD56, kappa light chain, CD44v6, MAGE A3/NY-ESO-1 (tumor antigens), CS1/SLAMF7, BCMA and CD19.

Data from a clinical trial with CD19-directed CAR T-cells conducted by Alfred Garfall was presented, and the follow-up of a particular patient in this study with 10 prior lines of therapy, who had never entered complete remission (CR) after 5 years of treatment, was highlighted. This patient entered CR, with negative MRD and no measurable disease by all biochemical parameters. Despite low persistence of CAR T-cells, remission lasted 15 months. Progression occurred with development of a solitary isolated plasmacytoma, which was effectively irradiated, and treatment with single agent daratumumab maintained full biochemical remission. The entire cohort was heavily pre-treated with poor prognostic disease, but 80% had a remission or response, and 2 had remission reversions.

Further discussion centered on BCMA as a prime target candidate for MM (see related MM Hub article) and a clinical trial conducted by Syed Abbas Ali was highlighted. Patients received cyclophosphamide and fludarabine followed by a single infusion of CAR T-cells. Responses were seen in 4 out of 12 patients and were associated with expansion of cells; severe CRS and delirium were reported as side-effects. Further data from Adam Cohen’s study of BCMA CAR T’s was also presented, with treatment of 100% high-risk patients giving encouraging responses. Finally, an impressive responder in a study conducted by Stadtmauer's group had received 11 prior lines of therapy and had 70% MM cells in the bone marrow. By day 28 after an infusion of 2 x 10⁸ CAR T-cells (with no lymphodepletion therapy), a robust expansion and persistence was observed along with a negative BM biopsy. The patient entered strict complete remission and 18 months later is still going well. Further studies of BCMA-CAR T-cells are ongoing, and a clinical trial of a BCMA-directed CAR with a 41BB fusion is also currently underway.

The key question following such studies is what makes the responders different and how can results be improved? One mechanism to improve persistence of CAR T-cells could be administering the cells sooner after transplant (eg. 60 days), and trials are underway to address this using CD19-directed CAR T. In addition, further studies have been designed to address the question of whether prior lympho-depletion affects overall efficacy, as well as the effect of differing numbers of cells in the infusions. 

Excellent responses have also been seen in the bluebird report with doses of 15 x 10⁷ cells. This technology is under constant revision, and CARTyrin technology, which incorporates a non-immunoglobulin alternative scaffold Centyrin molecule (a “CARTyrin”) is manufactured with a novel non-viral piggyBac^TM (PB) transposon-based system, will hopefully circumvent some of the issues with current CAR T constructs. Allogenic CAR T-cells are also being designed as an ‘off-the-shelf’ product, and use TCR alpha knockout in order to limit GvHD, as well as suicide genes as an off-switch.

This session was concluded on a highly positive note following such impressive data, although it was noted that there are still some challenges ahead in order to limit toxicity. However, it is certainly clear that the age of immunotherapy in MM is very much upon us.