Chimeric Antigen Receptor–Directed Natural Killer Cells for B Cell Malignancies

Reports of remarkable response rates and long-lasting remissions are beginning to emerge from clinical trials of chimeric antigen receptor (CAR) T cells in patients with B cell malignancies. However, the promising therapy is out of reach for patients who lack sufficient T cells for CAR T cell generation or cannot afford to forgo treatment for the time it takes to generate CAR T cells.

To sidestep these barriers, researchers at The University of Texas MD Anderson Cancer Center are turning to natural killer (NK) cells to expand the use of CAR-directed therapy in patients with B cell malignancies. And starting soon is a clinical trial of CAR NK cells in patients with relapsed or refractory B cell malignancies.

Potential limitations of T cells

In CAR T cell therapy, T cells collected from a patient’s blood via apheresis are brought to a laboratory, where they are engineered to express CARs on their surfaces. The CARs increase the T cells’ ability to target certain cancer cells by enabling the T cells to recognize specific antigens the cancer cells express. The resultant CAR T cells are expanded in vitro and then infused back into the patient, where they attack and kill cancer cells that express the target antigen. Because the CAR T cells remain in the body long after they have been infused, they can protect against recurrence and result in durable remissions.

However, CAR-directed therapy with T cells has a significant drawback. “The main problem with CAR T cell therapy is that we have to use autologous T cells, and this means that we have to generate a personalized product for each patient,” said Katy Rezvani, M.D., Ph.D., a professor in the Department of Stem Cell Transplantation and Cellular Therapy, noting that allogeneic T cells cannot be used for CAR T cell therapy because they carry a high risk of life-threatening graft-versus-host disease (GVHD). “Taking the patient’s own T cells and engineering them to express the CAR and then expanding them and giving them back to the patient takes time, and it’s not always feasible. Sometimes the patient’s disease progresses; and sometimes the patient has had a lot of chemotherapy, so there may not be an acceptable number of T cells to generate the product.”

To avoid the potential limitations of CAR T cells, a group led by Dr. Rezvani and Elizabeth Shpall, M.D., a professor in the Department of Stem Cell Transplantation and Cellular Therapy, proposes to use NK cells instead for CAR-directed therapy.

The advantage of NK cells

“The beauty of NK cells is that you can give a patient allogeneic NK cells and they will not cause GVHD, like allogeneic T cells would,” Dr. Rezvani said. “Many, many patients at our center and other centers have received allogeneic NK cells for immunotherapy, and there’s no risk of GVHD.”

This simple beauty could translate into a potentially big benefit in CAR-directed therapy. NK cells from umbilical cord blood can be engineered to express CARs and then stored for use in virtually any patient. This approach would eliminate not only the need for creating a new batch of tumor-targeting cells for each patient but also the weeks-long wait that goes with doing so.

“Our argument is that using NK cells will overcome the limitation of having to make a new product for each patient, like we have to do with T cells,” Dr. Rezvani said. “We can have an off-the-shelf product that is ready to use.”

At MD Anderson, NK cells for CAR-directed therapy are harvested from cord blood specimens maintained in the institution’s Cord Blood Bank, which is led by Dr. Shpall. The cells are modified by stable transduction with a retroviral vector to introduce several new genes with specific functions into the cells’ DNA. CD19, a hallmark of B cell malignancies, increases the CAR NK cells’ specificity for the disease. IL15, which enhances cell proliferation and survival, prolongs the presence of CAR NK cells in the body; without this gene, the cells would not last more than 2 weeks after infusion.

“With the addition of IL15, you get more persistence and hopefully better therapeutic efficacy,” Dr. Rezvani said.

Finally, an inducible CASP9-based “suicide gene,” whose activation by a small-molecule dimerizer induces apoptosis of the CAR NK cells, is included as a means to eliminate the cells if they are found to cause substantial toxicity.

“Non-engineered allogeneic NK cells have minimal if any toxicity, but once you engineer NK cells to express a CAR and have cytokine receptors, they may end up being toxic, which is why we need the suicide gene,” Dr. Rezvani said.

Preclinical studies of the cells have shown promising results. Compared with CAR NK cells expressing only the CD19 receptor, CAR NK cells transduced with CD19, IL15, and the CASP9-based suicide gene had significantly better proliferation in vitro and resulted in significantly greater tumor inhibition and longer survival in a murine model of lymphoma. In addition, pharmacological activation of the suicide gene efficiently eliminated the CAR NK cells both in vitro and in the murine model.

Upcoming clinical trial

On the basis of the promising preclinical studies, Dr. Rezvani, Dr. Shpall, and their colleagues are beginning to move their CAR NK cells into clinical trials. The first such trial will enroll patients with B cell malignancies expressing CD19 because the antigen has been successfully targeted with CAR T cells in these cancers. In the phase I/II trial, patients with relapsed or refractory B cell malignancies—including acute lymphoblastic leukemia, chronic lymphocytic leukemia, and non-Hodgkin lymphoma—will receive basic standard chemotherapy with cyclophosphamide and fludarabine before receiving CAR NK cells.

“These patients do not otherwise have many options for eradicating disease that has relapsed or is not responding to therapy,” Dr. Rezvani said.

Dr. Rezvani anticipates that the limitations of the therapy will be manageable. “I think the main limitations are going to be related to toxicity, based on what we can extrapolate from what’s happened with CAR T cells,” Dr. Rezvani said. CAR T cells can cause cytokine release syndrome, a condition arising from the activated T cells’ mass secretion of cytokines that, in its extreme, causes symptoms akin to those of a severe systemic inflammatory response. CAR T cells can also have neurotoxic effects. “The good news is, we have experience in managing the toxicity of CAR T cells, and we’ve incorporated that experience into our protocol with a very strict algorithm of how to prevent and manage such toxicity if we see it with CAR NK cells.”

The trial protocol has already entered the approval process, Dr. Rezvani said. She and Dr. Shpall hope to open the trial to patients at MD Anderson soon.

A potential game-changer

Dr. Rezvani said that plans are in place to use CAR NK cells targeting other hematological cancers, including multiple myeloma and acute myelogenous leukemia, as well as myelodysplastic syndrome. Using CAR NK cells to target some solid tumors is also a possibility, and preliminary data in this area are beginning to accrue.

“If the CAR NK cell approach works, I think it’s going to be game-changing because for the first time we would have an effective, off-the-shelf therapy that would be readily available to many more patients than what we are doing at the moment with CAR T cells,” Dr. Rezvani said.

For more information, contact Dr. Katy Rezvani at 713-794-4260 or Dr. Elizabeth Shpall at 713-745-2161.

OncoLog, May-June 2017, Volume 62, Issue 5-6