Targeting Resistance: Navigating the Complexity of the Tumor Microenvironment and CAR T-Cell Efficacy
CAR T-cell therapy in multiple myeloma faces challenges from the immunosuppressive cells in the tumor microenvironment.
Multiple myeloma (MM) is a cancer of the bone marrow characterized by clonal proliferation of malignant plasma cells that encourage the overproduction of mature but dysfunctional B lymphocytes. It is an incurable, progressive disease characterized by periods of remission followed by relapse, in which outcomes typically worsen with each successive line of therapy. Over the past 2 decades, significant research findings, advancements in treatments, and promising clinical trials offer renewed hope for controlling the progression of disease and prolonging life in patients with MM. Chimeric antigen receptor (CAR) T-cell therapy has gained significant attention in recent years; however, resistance and relapse continue to be a barrier to efficacious treatment and lasting remission. Because disease progression is substantially regulated by the immunosuppressive tumor microenvironment (TME), understanding the TME and the role immunosuppressive cells is crucial for overcoming resistance to CAR T-cell therapy in patients with MM.
The complexity of tumor microenvironment, including the impact of immunosuppressive cells and the persistence of CAR T cells, highlights the importance of refining therapeutic approaches. Image Credit: © Animager - stock.adobe.com
The TME of MM is highly complex and composed of various overexpressed, overproduced immune cells, including regulatory T-cells (Tregs), B cells, macrophages, and myeloid-derived suppressor cells (MDSCs), as well as proteins that inhibit the functioning of healthy cells. Tregs play a critical role in immune regulation and, although their role has been considered controversial, studies indicate that they are responsible for the suppression of other cytotoxic T-cells. Simultaneously, B cells interfere with immune function through the absorption of homeostatic cytokines necessary for regulating the immune system.1-3
In an interview with Pharmacy Times, Dan Schrum, PharmD, BCOP, inpatient lead for the cellular therapies group at Duke University Hospital, explained that the modulation of homeostatic cytokines, referred to as cytokine sinks, is crucial for effective lymphodepletion and the subsequent response to CAR T-cell treatment.
“The premise here is that there are these endogenous immune cells, including T-cells, B cells, that are soaking up homeostatic cytokines,” he said. “And by removing those with lymphodepletion, and kind of allowing those homeostatic cytokines to sit there, they can be used by CAR T-cells, as opposed to endogenously found immune cells that normally would be using those.”3
Macrophages are one of the main components of the TME in MM and play a key role in the progression of disease due to their impact on tumor biology. They are differentiated into 2 subgroups: M1, which are “classically activated” and act as antitumoral agents through secretion of pro-inflammatory cytokines, reactive oxygen species, and nitric oxide; and M2, which are “alternatively activated” and contain tumor-associated macrophages (TAM) that facilitate progression through immunosuppression. According to clinical studies, increased presence of M2 macrophages in the bone marrow is associated with poorer prognosis and worse response to autologous stem cell transplantation and chemotherapy, and high expression of M1 macrophages has been associated with better outcomes.1
M2 macrophages are involved in the development of drug-resistant tumor cells, contributing to the progression of MM and myeloma cell proliferation through TAMs. Despite positive outcomes and responses to proteasome inhibitor bortezomib (Velcade; Takeda Pharmaceuticals), evidence indicates it may also promote the production of pro-inflammatory macrophages that are linked to MM cell survival and aggressive disease, which may inform the underlying mechanism associated with resistance to bortezomib.1.2
MCDSs play a dynamic role in the progression of disease and proliferation of cancerous cells through the promotion of tumor growth, immune system suppression, and facilitation of bone damage. These immunosuppressive cells influence Treg differentiation, proliferation of T-cells, and osteoclast formation. They also support tumor growth by facilitating angiogenesis, the formation of new blood vessels that supply the tumor with nutrients. MDSCs achieve these effects through mechanisms such as exosome uptake from cancer cells and the secretion of cytokines that further support tumor progression and immune suppression.1,2
“There's some evidence that tumor associated macrophages can inhibit cytotoxic T lymphocyte responses and a few other mechanisms as well,” Kelsea Seago, PharmD, BCOP, clinical pharmacy specialist in hematological malignancies and cellular therapies at West Virginia University, said in an interview with Pharmacy Times. “And the myeloid derived suppressor cells can also suppress the activation and proliferation of T-cells, which can be associated with insufficient product expansion, and that can affect [CAR T-cell therapy] resistance.”4
CAR T-cells are artificial receptors that target specific proteins to induce T-cell activation and cytotoxicity through the fusion of an extracellular binding domain, a cytoplasmic domain, and costimulatory signaling. BCMA and CD19 are among the most prominent and widely studied targets for CAR; and new therapeutic targets continue to emerge, such as GPRC5D which is found in approximately 90% of patients with MM. In the phase 1 CC-95266-MM-001 trial (NCT04674813), participants who received GPRC5D-targeted CAR T-cell therapy in the first line had an overall response rate of 96% and lower incidence of infection, demonstrating its high efficacy and low toxicity.5-7
CARs are infused after lymphodepletion, commonly through the cytotoxic agent bendamustine (Bendeka; Teva Pharmaceuticals), to create an immunologically advantageous environment that facilitates expansion of the CAR T-cells. Effective lymphodepletion plays a substantial role in the success of CAR T and studies find evidence of increased tumor regression with the absence of host lymphocytes. However, the immunosuppressive effects of Treg and B cells can undermine the effectiveness of CAR T-cells by disrupting essential processes. Effective lymphodepletion, which minimizes cytokine competition and enhances CAR T-cell expansion, is crucial for optimizing the overall therapeutic response.3,5
Response to CAR T therapy is also significantly impacted by prior lines of therapy, with continued exposure to prolonged antigen stimulation and lymphotoxic therapies potentially resulting in T-cell exhaustion and diminished functional capacity of CAR T-cells. Evidence suggests that repeated exposure to antigenic targets in BCMA- and CD19-targeted therapies may lead to antigen loss and reduced response rates.3,4,8
The persistence, quality, and functionality of CAR T-cells are crucial for maintaining their activity in the body over an extended period, which is vital for achieving sustained tumor regression and preventing the tumor from recurring or repopulating.3,4,8
“The number of lines or the amount of lymphotoxic therapy that a patient has seen in the past can substantially affect memory T-cells, which when CAR T is being made, a portion of those cells are memory T-cells,” said Schrum. “And so, if there's a lower quantity or more dysfunctional memory T-cells that are being used to manufacture CAR T-cells, the actual CAR T product can be affected and have lower persistence.”3
The use of cytotoxic agents, like bendamustine, has shown success in conditioning regimens to reduce host lymphocytes, thereby facilitating a more favorable environment for CAR T-cells to expand and target malignant cells. However, contradictory evidence suggests that using bendamustine within 9 months prior to apheresis may result in substantially worse outcomes, potentially due to the long-lasting lymphotoxic effects impairing the quality and functionality of the collected T-cells.3,4,8
CAR T-cell therapy resistance is also influenced by patient-specific factors, such as disease state, presence of comorbidities, prior treatment history, and individual immune responses. Individuals with heavier disease burden or relapsed/refractory disease can have lower T-cell fitness overall, increasing the likelihood of T-cell exhaustion and subsequent treatment resistance. These insights underscore the need for developing new strategies to address CAR T-cell therapy resistance, with research exploring advanced methods to enhance treatment effectiveness and patient response.4
Given these challenges, ongoing research is focusing on innovative methods to overcome CAR T-cell therapy resistance, aiming to improve patient outcomes and enhance treatment efficacy. Alongside the continued investigation of new and existing targets, innovative multi-antigen targeting products are being explored. For example, there are bispecific CARs that express 2 different receptors to target multiple antigens simultaneously, as well as older CAR designs that combine mono-specific with tandem CARs.4
Allogeneic, or off-the-shelf, CAR T-cells are also being developed, particularly for patients who struggle with cell collection or manufacturing of autologous products, or who cannot tolerate the extensive washouts or prolonged manufacturing times required for traditional therapies. Further research is needed, but the development holds significant implications for niche patient populations with limited treatment options.4
The complexity of tumor microenvironment, including the impact of immunosuppressive cells and the persistence of CAR T-cells, highlights the importance of refining therapeutic approaches. Resistance mechanisms, such as antigen escape, T-cell exhaustion, and the influence of immunosuppressive cells like Tregs and MDSCs, underscore the need for targeted strategies to address these barriers. As research progresses, the landscape of CAR T-cell therapy continues to evolve, offering new avenues for overcoming existing challenges and optimizing care for patients with MM.
