Nanoparticles Create Effective CAR T Cells in Living Mice

May 5, 2017, by NCI Staff

Researchers have developed a method to genetically engineer cancer-fighting immune cells in living animals using nanoparticles that carry DNA. The new study shows that the resulting immune cells, known as CAR T cells, eliminated leukemia in mice.

For clinical treatment, CAR T cells are usually generated by collecting T cells from a patient , modifying them in the laboratory so they target cancer cells, allowing them to grow in number, and then infusing them back into the patient. This approach—although effective in some small clinical studies—is expensive, can take several weeks to complete, and requires special expertise and equipment. Plus, it has to be performed for each patient receiving CAR T-cell therapy.

To make CAR T-cell therapy accessible to more patients, Matthias Stephan, M.D., Ph.D., of Fred Hutchinson Cancer Research Center, and his colleagues set out to develop a streamlined and inexpensive approach for producing CAR T cells. To do so, they created a nanoparticle that homes in on T cells and contains DNA instructions to make a chimeric antigen receptor (CAR) that recognizes leukemia cells.

“The real novelty is you don’t have to make CAR T cells outside of the patient; you can deliver these nanoparticles directly to the patient,” said Terry Fry, M.D., head of the Hematologic Malignancies Section of NCI’s Center for Cancer Research, who was not involved in the study.

The goal is to make an “off-the-shelf CAR T-cell therapy … and it’s possible that this approach could accomplish that,” he continued.

The results of the study were reported April 17 in Nature Nanotechnology.

Teaching T Cells

T cells have a natural ability to attack cancer cells, but they often need help identifying their target and growing to large enough numbers to launch an effective attack. CAR T-cell therapy provides an extra push, teaching a patient’s T cells how to find cancer cells and producing a massive number of these cancer-killing T cells.

Several ongoing clinical trials are assessing the safety and efficacy of CAR T-cell therapies in patients with leukemia, other blood cancers, and, more recently, some solid tumors.

But scaling up this patient-by-patient process to treat large numbers of people with cancer “is not practical,” Dr. Stephan explained. Scaling up CAR T-cell therapy is a major “hurdle in the field,” agreed Dr. Fry.

So Dr. Stephan and his colleagues turned to nanoparticles, which can deliver DNA to specific cells, are easy to make and store, and are inexpensive to produce.

The nanoparticle they designed has several key elements, such as the gene for a CAR that binds to CD19—a molecule that is present at higher than normal amounts on leukemia cells. It also contains molecules that help the gene insert into T cells’ DNA, where it can be expressed efficiently. And the researchers added proteins on the nanoparticle’s surface that bind selectively to T cells.

The nanoparticle could be stored for a year, the researchers found, making an “off-the-shelf” treatment a potential option.

When they treated T cells with these nanoparticles in lab experiments, they found that the nanoparticles bound to and were taken up by T cells. One day following treatment, T cells began to express the CAR, and, when mixed with leukemia cells, effectively killed these cells.

Dr. Stephan and his colleagues next added a fluorescent signal to their nanoparticle so they could track its activity in mice. The majority of the nanoparticles latched onto and were taken up by T cells in the mice. Separate experiments showed that a few days after being treated with the nanoparticles, T cells began to express the CAR.

The conventional CAR T-cell approach involves an “expansion” step, where CAR T cells are grown in the lab until they number more than a billion cells. In mice treated with the nanoparticles, the researchers found the CAR T-cell population expanded rapidly, increasing 5.5-fold over 6 days.

“Basically what’s happening is the exact same expansion that would usually happen in a lab incubator, but it’s happening inside the mice, where their tumors are,” explained Dr. Stephan.

The researchers then tested the nanoparticle in a xenograft mouse model of acute lymphoblastic leukemia. Mice left untreated or treated with nanoparticles lacking a key element had increased tumor growth and a median survival of about 14 days.

In contrast, mice treated with fully functioning nanoparticles went into remission or had a substantial reduction in tumor number and size. These mice lived an average of 58 days longer than the control mice. The researchers observed similar results in mice treated with conventional lab-grown CAR T cells.

Safety Concerns

The nanoparticles did not cause a widespread immune response in the bloodstreams of mice or lead to any other toxic effects, the researchers reported.

They found that some nanoparticles bound to immune cells other than T cells and a very small proportion of these cells took up the nanoparticles and expressed the CAR, but this expression diminished over time. The authors concluded that “toxicities arising from CAR expression in non-T cells would at most be minimal, and manageable in a clinical setting.”

However, with their current nanoparticle design, the CAR gene inserts randomly into T cell DNA which could potentially result in a harmful mutation, the investigators pointed out. Greater control could be achieved by incorporating into the nanoparticle a genetic element that integrates CAR genes into “safe” spots, they suggested.

Another safety concern is that the dose of CAR T cells can’t be controlled with this approach, noted Dr. Fry.

“If you were to start using this therapy in humans, you’d never quite know how many T cells you’re going to end up with in the patient. And CAR T cells certainly have toxicity,” he explained.

The safety profile of this approach “must still be evaluated, either in a nonhuman primate model or directly in a phase-1 dose-escalation trial,” Dr. Stephan and his colleagues wrote.

Looking Towards Clinic Treatment

The team’s next step is to look for an industry partner to help them manufacture these nanoparticles on a large scale, and then possibly test them in clinical studies.

These nanoparticles potentially could be used to treat patients in a variety of ways, Dr. Stephan believes. For example, he said, “we could easily mix different nanoparticles and reprogram T cells to express multiple CARs so that we target the tumor from various angles.” This could help prevent resistance, he explained.

In addition, the nanoparticles could be combined with other therapies that may potentially enhance their function, such as immune checkpoint inhibitors or cancer vaccines, he said.

The technology and methods for producing conventional CAR T-cell therapy is also evolving rapidly, said Dr. Fry. This nanoparticle potentially could be combined with these advances, he noted.

“With the CAR T-cell therapy field taking off,” said Dr. Stephan, “it’s hopefully a great environment for us to translate our approach into a clinical setting.”