Chimeric Antigen Receptor T-cell (CAR T-cell) therapy is the ultimate in precision medicine, as it harvests a patient’s own immune system to fight cancer. Such adoptive cell transfer (ACT) therapies are a promising treatment strategy for a range of hematological malignancies. Early trials with CAR T-cells treating leukemia and lymphoma patients have produced durable responses and significant overall survival (OS) times. This success has prompted the development of CAR T-cells to treat Multiple Myeloma (MM) and several clinical trials for MM are ongoing, with highly promising data emerging.
The first CAR T-cell therapy to be approved by the US Food and Drug Administration (FDA) was tisagenlecleucel-T* (Kymriah®, Novartis), on 31 August 2017, for treatment of children and young adults (age 3–25 years) with relapsed or refractory acute lymphoblastic leukemia (ALL). This was a landmark decision making the concept of a ‘living drug’ a reality. (*Formerly referred to as CTL019). FDA approval of Yescarta™ (Axicabtagene Ciloleucel, Kite) followed in 2018, for the treatment of adult patients with relapsed or refractory Large B-Cell Lymphoma after two or more lines of systemic therapy.
In this review, we take an in-depth look at CAR T-cells and in particular how they are currently being developed for the treatment of MM. This is based on a survey of the literature up to 15 October 2017; further developments will be covered in the form of updates to this review over the coming months.
What exactly are CAR T-cells?
CARs (chimeric antigen receptors) are genetically engineered fusion proteins that consist of an antigen-recognition domain linked via a hinge and transmembrane region, to a T-cell signaling domain. This so-called CAR construct is packaged into either a suitable vector for delivery into T cells (commonly retroviral or lentiviral). These constructs are then transferred into a patient’s own T-cells, enabling expression of the resultant fusion protein and thus conferring the ability to specifically recognize tumor cells. These engineered products are then called CAR T-cells (or CAR-Ts). MM cells can be targeted by choosing an antigen expressed by the target malignant myeloma cells.
This specific recognition offers advantages over other cellular therapies, such as allogenic hematopoietic stem cell transplantation (AHSCT) and marrow infiltrating lymphocytes (MILs), which can lead to complications such as graft versus host disease (GVHD). Unlike engineered T-cell receptor (TCR) constructs, CAR-Ts are not HLA-restricted, so patients of any HLA-type can be treated with a given CAR T-cell.
During the last 15 years, CAR-T constructs have been optimized and the addition of a co-stimulatory region led to a huge improvement in cytolytic capacity, due to enhanced activation and improved cytokine production. First generation CARs used only CD3zeta to activate T-cell responses, but second generation CARs now include a second co-stimulatory molecule such as CD28, 4-1BB, OX40, or ICOS. Building on this, third generation CARs now use CD3zeta plus two co-stimulatory molecules. There are also moves to fully humanize the single chain variable fragment (ScFv) region, as early CAR constructs used murine regions, which can elicit off-target responses.
Figure 1. Schematic representation of CAR-T constructs with specificities being investigated for MM. Click here for a larger version.
How are CAR T-cell therapies produced?
T-cells are harvested from a patient by apheresis. The patient’s own T-cells are then transferred to a laboratory where they are activated and the CAR construct is introduced using transduction. The transduced T cells are then cultured and allowed to proliferate and expand, a process that takes around 22 days. In the meantime, the patient is subjected to lymphocyte-depleting chemotherapy (such as low-dose cyclophosphamide and fludarabine) in order to remove any remaining endogenous lymphocytes, which may impede activity of the CAR T-cells. The genetically engineered CAR T-cells are then re-infused back into the patient where they multiply further. Studies have shown that there is a peak in CAR T-cell expansion within the first few days after re-infusion and that this peak is associated with adverse effects (AEs) and efficacy (See Lymphoma Hub article). In theory, the cells will also persist for a long period of time in order to drive durable responses. However, the role of long-term persistence in terms of measurable cell numbers and the expansion of CAR T-cells in treatment outcome and long-term remission is still not fully understood (see Lymphoma Hub article).
Several trials have established dosing regimens and cells are normally administered at a concentration of approximately 0.2-6 x 106 cells/kg. However, dosage levels may need to be established for each specific CAR construct, as this could change according to the relative expression of the target antigen.
What are the major targets for MM-specific CAR T-cells?
The success of a given CAR T-cell construct is dependent on the choice of antigen, which ideally is both exclusively and uniformly expressed by the malignant cells, but not normal cells. This is a challenge in MM as sub-clones emerge over time leading to tumor heterogeneity, and therefore, choosing more than one target could improve functionality. If targets are not exclusive to MM cells it can lead to the destruction of normal cells, with drastic consequences. Even extremely low expression of the antigen on normal cells can lead to deleterious effects if not managed correctly. For example, the use of CD19 CAR T-cells for lymphoma can lead to hypogammaglobulinemia in some patients, as a result of the destruction of healthy B cells, although this is now managed by γ-globulin replacement therapy (Matthew J. Frigault and Marcela V. Maus, 2016).
Currently, there is no antigen known to be exclusively expressed on MM cells and not normal cells. However, candidate molecules are those that are over-expressed on MM cells and have limited expression in other compartments. Table 1 summarizes the candidate antigens for MM-specific CAR-Ts.