CAR T cell immunotherapy, illustration
Source: Keith Chambers/Science Photo Library

Scientists led by teams at Dana-Farber Cancer Institute and Mass General Cancer Center have created molecular ON-OFF switches that can regulate the activity of chimeric antigen receptor (CAR) T cells, a form of cell-based immunotherapy that has demonstrated dramatic success in treating some advanced cancers, but which poses a significant risk of toxic side effects. The reported studies demonstrated how the switchable CAR T cells can be turned either on or off, by exposure to the commonly used cancer drug, lenalidomide. In the laboratory, the researchers engineered OFF-switch CAR T cells that could be quickly and reversibly turned off by administering the drug, after which the CAR T cells would again recover their anti-tumor activity. Separately, the researchers reported generating ON-switch CAR T cells that only killed tumor cells during lenalidomide treatment.

“From the start, our goal was to build cancer therapies that are less hard on people,” said Max Jan, MD, PhD, first author of the team’s report in Science Translational Medicine. “Having built these switches using human genetic sequences and an FDA-approved drug, we are excited for the potential to translate this research to clinical use.” Jan is affiliated with the laboratories of Benjamin Ebert, MD, PhD, and Marcela Maus, MD, PhD, who are senior authors of the paper, which is titled, “Reversible ON- and OFF-switch chimeric antigen receptors controlled by lenalidomide.” Co-authors on the study included researchers from the Broad Institute of MIT and Harvard, and Harvard Medical School.

CAR T cells are immune cells that have been genetically modified to recognize and attack tumors cells. They are created by harvesting immune T cells from the patient and engineering them in the laboratory to produce a finely-tuned chimeric antigen receptor, which recognizes a specific protein on the surface of the patient’s cancer cells. The engineered CAR T cells are then returned to the patient, where they circulate through the body and home in on the cancer cells by binding to the distinctive surface protein that they have been engineered to recognize. This binding event stimulates an immune attack, destruction of the cancer cells, and proliferation of the CAR T cells.

Once administered to the patient, these “living drugs” proliferate and kill tumor cells over weeks to months, but in some cases they can also cause life-threatening inflammatory reactions that are difficult to control. Uncontrolled proliferation of the CAR T cells can sometimes trigger cytokine release syndrome (CRS), the release of inflammation-causing signals throughout the body that can cause toxicities ranging from mild fever to life-threatening organ failure. “The profound clinical successes of CAR T cell therapies targeting relapsed or refractory B cell malignancies have been achieved in spite of the risk of toxicity from T cell hyperactivation syndromes,” the authors noted. This differentiates CAR T cell therapies from more established forms of cancer treatment—such as chemotherapy or radiotherapy—for which the dose can be precisely tuned up or down over time. And in some patients, the team continued, “… toxicities associated with CAR T cells have stalled further clinical development, despite early signs of efficacy.”

Current management of toxic reactions relies on intensive care unit support and drugs, including immunosuppressive corticosteroids, while many researchers are trying to develop strategies for controlling the activity of CAR T cells in order to prevent these toxic side effects. “CAR T cells can be fantastically effective therapies, but they can also have serious toxicities and can cause significant morbidity and mortality,” said Ebert who is chair of medical oncology at Dana-Farber. “They are currently difficult to control once administered to the patient.” It’s hoped that in the future, switchable cell therapies might allow patients to take a pill—or not—that will tune the amount of CAR T cell activity from day to day, as an approach to reducing toxic side effects.

CAR T cell therapy has had the most success in blood cancers. Three CAR T agents have been approved. These include Kymriah for children and young adults with B-cell precursor acute lymphoblastic leukemia (ALL), both Kymriah and Yescarta for treating adults with diffuse large B-cell lymphoma, and Tecartus for adults with mantle cell lymphoma. Scientists are investigating an array of different approaches that might extend the reach of CAR T therapies to other blood cancers and to solid tumors, if a number of hurdles can be overcome, including the problem of treatment toxicity.

To create the ON and OFF switch systems for CAR T cells, Jan and colleagues used a relatively new technique known as targeted protein degradation. It exploits a mechanism that cells use to dispose of unwanted or abnormal proteins. The proteins are marked for destruction by a structure within cells that acts like a garbage disposal. A small number of drugs, including lenalidomide, act by targeting specific proteins for degradation using this pathway.

The researchers used this technique to engineer small protein tags that are sent to the cellular garbage disposal by lenalidomide. When the degradation tag was affixed to the CAR, it allowed the tagged CAR to be degraded during drug treatment, thereby stopping T cells from recognizing cancer cells. But because CAR proteins are continually manufactured by these engineered T cells, after the drug treatment the new CAR proteins are able to accumulate and restore the cell’s anti-tumor function. The researchers propose that this OFF-switch system might in the future allow patients to have their CAR T cell treatment temporarily paused to prevent short-term toxicity, but still exhibit long-term therapeutic effects against their cancer.

The scientists also built an ON-switch CAR by further engineering the proteins that physically interact with lenalidomide. This system has the potential to be especially safe, because the T cells only recognize and attack tumor cells during drug treatment. If used to treat cancers such as multiple myeloma that are sensitive to lenalidomide, ON-switch CAR T cells could allow for a coordinated attack by the immune cells and the drug that controls them.

The investigators demonstrated the activity of the ON switch and OFF switch systems in both lab tests, and in experiments in live mice. “ON-switch split CARs demonstrated tunable antitumor activity in vitro and in vivo,” they wrote. “In vivo, ON-switch split CARs demonstrated lenalidomide-dependent antitumor activity, and OFF-switch degradable CARs were depleted by drug treatment to limit inflammatory cytokine production while retaining antitumor efficacy.”

“The long-term goal is to have multiple different drugs that control different on and off switches” so that scientists can develop “ever-more complex cellular therapies,” explained Ebert. “Together, the data showed that these lenalidomide-gated switches are rapid, reversible, and clinically suitable systems to control transgene function in diverse gene and cell-based therapies,” the team concluded.

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