Driving forward with CAR-T therapy: what does this mean for MM?

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-Ts treating leukemia and lymphoma patients have produced durable responses and significant overall survival (OS) times. This success has prompted the development of CAR-Ts 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 was initially based on a survey of the literature up to 15 October 2017 and was updated again in October 2018.

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 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.

Table 1. Candidate Antigens explored as the target molecule for MM-specific CAR T-cell development.

What are the CAR-T constructs currently in clinical trials for MM?

Following the success of a number of trials in lymphoma, a number of new clinical trials opened up for the treatment of MM. Table 2 summarizes the current status of ongoing clinical trials for MM.

Table 2. Current CAR-T constructs in clinical trials for MM. Click here.

Please note that this table was populated with available resources for registered open clinical trials to treat MM. This table may not be comprehensive for all trials worldwide where other diseases are included in the treatment group and it does not include planned clinical trials or those that have now closed. 

How effective are CAR T-cells in these early trials?

Most of the clinical trials for MM are still in the early stages and therefore any efficacy data is still preliminary. However, interim analyses have shown highly effective data. So far, it appears that using BCMA as the target antigen appears to be the most promising strategy for MM, and as a consequence, there are currently seven different BCMA targeted CAR-T constructs now in development.

The first study to assess anti-BCMA-targeted CAR T-cells in MM was published in Blood. The study used varying doses of cells on 12 heavily pre-treated patients (median of 7 prior lines of therapy). The two patients that received the higher dose levels (9 × 10⁶ CAR⁺ T cells/kg body weight) showed rapid and large reductions in bone marrow plasma cells, with one entering stringent remission for 17 weeks and the other with ongoing very good partial remission.

The data from two of these early CAR-T trials was released in June 2017 at ASCO and was previously reported by the MM Hub. The astounding results reported by Nanjing Legend Pharma, showing that 33 out of 35 (94%) patients had clinical remission, was also reported by ASCO news itself and many other news wires. A further follow-up reported in July 2017 revealed a 100% response rate, with 14/19 (74%) achieving a stringent complete response. This particular trial now plans to recruit 100 more patients at four different hospitals in China. At the same time, the data from Bluebird bio and Celgene was also released, revealing a 100% overall response rate, and 73% of patients achieving a very good partial response (VGPR).

A summary of published results from further clinical trials can be found in Lekha Mikkilieni and James N. Kochenderfer’s review paper here. In general, treatment of heavily pre-treated patients with relapsed MM has led to extremely positive results, with many patients in almost complete remission. However, the real test will be to monitor these patients over time to see if the responses are long-lasting and without any other long-term complications.

What are the associated toxicities of CAR T-cell treatment?

Unfortunately, CAR T-cells do not come without side effects, the most common being cytokine release syndrome (CRS). This is a consequence of the expansion of infused T-cells – this large volume of cells causes the immune system to go into overdrive generating a cytokine storm, which can have serious consequences if not managed rapidly and effectively. Therefore, optimizing the dose of CAR T-cells is crucial in order to limit CRS, at the same time as maintaining maximum efficacy.

Symptoms of CRS include hypotension, vascular leak, coagulopathy, cytopenia, respiratory/renal insufficiency, myalgia, and fevers, as well as neurological complications, including dysphasia, confusion, delirium, visual hallucinations, and seizure-like activity. Historically, CRS has been managed by the use of general supportive care and specific interventions such as ventilation, with limited success. However, corticosteroids and vasopressors can also be administered, but in clinical trials, the anti‑IL‑6 receptor antibody (tocilizumab) has demonstrated efficacy in controlling CRS. Although tocilizumab has been approved by the FDA in this setting, it is still awaiting approval in Europe. Therefore, researchers conducting clinical trials in Europe need to look at ways to manage the availability of this drug. Treating and primary care physicians also need to be educated in how to recognize the early signs of CRS and to react quickly with appropriate measures. This will become easier as biochemical assays are developed to measure markers that act as warning signs of CRS, and can also be used to track progress as the CAR T-cells take effect.

 What is the future of CAR T-cell therapy in MM?

To date, BCMA-targeted CAR T-cells have shown the most benefit in MM patients, so results from trials with these constructs are eagerly awaited. Much of the ongoing research is geared towards improving the efficacy of CAR-Ts. For example, problems associated with off-target effects can be circumvented by incorporating an off-switch into the construct. An example of this is Cellectis’ UCART19 (now also licensed to Servier and Pfizer), which is only active when rapamycin is present. This also allows the rapid destruction of the target cells after a limited period of time, which could help limit toxicity to normal cells. Likewise, a new technology called GoCAR-T, in development with Bellicum Pharmaceuticals uses Rimiducid for CAR-T cell activation.

Due to the clonal heterogeneity of MM, an ongoing issue with CAR-T cell therapy in MM will be whether new clones lacking the target antigen will arise. Therefore, CAR T-cells with multiple targets could be beneficial and have been termed compound CARs (cCARs). These are generated using two separate CAR constructs, each with a different target antigen. However, this needs to be balanced with ‘off-target’ effects (in this case meaning targets other than the tumor cells).

Going forward, it will also be of interest to look at combining CAR-Ts with standard-of-care regimens for MM and managing the use of CRS preventative drugs alongside these combinations. Researchers are also keen to define the elusive MM cancer stem cell, as well as the mutational drivers that occur early on in MM and have the biggest overall influence on the outcome. This may, in turn, identify new targets with greater specificity.

The biochemical characteristics of the therapy also need to be further defined, both in terms of measuring progress during therapy or markers of success both before and after. Fred Locke and colleagues showed that biochemical markers that appear after conditioning therapy can help predict the success of therapy. In addition, they found that the expansion of CAR T-cells over time corresponded to response rates as well as grade 3 and neurologic AEs. The disease burden prior to treatment in ALL, as assessed by minimal residual disease (MRD), also correlated with outcome.

It is also hoped that ‘off-the-shelf’ allogenic T-cells from donors, largely still in development, will help to improve the turnaround time and help to reduce the cost, so that CAR T-therapy is applicable to a wider group of patients. However, allogenic cell products place patients at greater risk of rejecting the cells, and may not lead to such durable responses.

How much will CAR T-cell therapy cost?

The enormous cost of CAR T-cell therapy has been a topic of huge debate and is possibly the biggest hurdle in terms of driving this therapy forward as a mainstream treatment with global availability. The one-off cost of a round of CAR T-cell therapy comes at an extremely high price – it is currently set at $475,000 by Novartis for tisagenlecleucel-T.  However, some companies are considering a success-based payment scheme – effectively a money back guarantee if the treatment fails to work, which would ensure that any further disease-related costs are minimal. Currently, clinical trials for MM are only recruiting heavily pre-treated patients that have relapsed disease, but if the trials prove to be as successful as hoped, such therapy could become a first-line treatment, replacing transplant plus 10–20 years of maintenance therapy. The companies selling the technology argue that the one-off cost of a single infusion is cheaper when compared to such long-term care.

Will CAR T-cells provide a cure?

Finally, the question that everyone would like answered is whether CAR T-cells will prove to be a cure. With clinical trials still in such early phases, it is certainly still too early to answer, as clinical researchers have yet to assess whether these cells have the capacity for long-term persistence and will retain full activity. To date, most of the early data indicate huge remission rates that are extremely promising, but perhaps less well advertised are the patient deaths, as well as severe graft versus host disease (GvHD) effects from allogenic off-the-shelf therapies. Such effects will need to be better managed going forward. In addition, the effects of a given CAR T-cell therapy may differ from patient to patient, depending on the relative expression of the target antigen, and therefore pre-screening for expression of specific target antigens will almost certainly become part of the process.

Conclusion

CAR T-cells are a hugely exciting and novel therapy making their way towards treatment via clinical trials. Success in lymphoma paved the way for the development of a number of MM-specific CAR T-cells, with several companies and research teams now in the race for their products to be approved. While some challenges remain, emerging data suggests extremely high efficacy, making this once visionary concept a certain future treatment option for patients with MM.

Acknowledgments

This article is an editorial contribution, without industry input and based on the published evidence and a survey of clinical trials as of 15 October 2017 and was updated again in October 2018. The article has been peer-reviewed internally by Anne Villeneuve and Karl Kemp-O’Brien, and externally by Hermann Einsele.

Suggested further reading:

1. Mikkilineni L. and Kochenderfer JN. ChimericAntigen Receptor T-cell Therapies for Multiple Myeloma.  2017 Sep 19. pii: blood-2017-06-793869. DOI:10.1182/blood-2017-06-793869

2. Davila ML. and Sadelain M. Biology and clinical application of CART cells for B cell malignancies. Int J Hematol. 2016 Jul;104(1):6-17. DOI:1007/s12185-016-2039-6

3. Almåsbak et al.CAR T Cell Therapy: A Game Changer in Cancer Treatment. Journal of Immunology Research, vol. 2016, Article ID 5474602, 10 pages, 2016.  DOI: 10.1155/2016/5474602

4. Jackson HJ. et al. DrivingCAR T-cells forward. Nat Rev Clin Oncol. 2016 Jun;13(6):370-83. DOI: 10.1038/nrclinonc.2016.36. Epub 2016 Mar 22.

5. Hartmann J. et al. Clinical development of CAR T cells-challenges and opportunities in translating innovative treatment concepts. EMBO Mol Med. 2017 Sep;9(9):1183-1197. DOI: 10.15252/emmm.201607485.

6. Atanackovic D. et al. Chimeric Antigen Receptor (CAR) therapy for multiple myeloma. British Journal of Haematology. Volume 172, Issue 5, DOI:10.1111/bjh.13889