There’s no arguing that Chimeric Antigen Receptor (CAR) T cells have revolutionized the field of cancer immunotherapy, bringing real hope to thousands of patients with previously untreatable disease and constituting an entirely new way to think about cancer treatment. CAR T cells—that is, ‘synthetic’ T cells which have been engineered to recognize and respond to antigens present on tumor cells—differ from traditional cancer therapies because once they’re administered to a patient, they take on a life of their own: they proliferate upon recognizing their targets, dynamically respond to their environments, and may become a lasting constituent of their host’s immune system. For this reason, CAR T cells are often called a ‘living drug.’ While this characteristic may in part be responsible for their cancer-killing prowess, it comes with a notable downside: a fundamental lack of control on the clinician’s end. If administering conventional chemotherapies is like fighting a fire with a water hose, then using CAR T cells is more like training a pack of rabid, fire-fighting dogs (yes, with little hats), letting them loose, and hoping for the best.
But don’t take my word for it—take it from Dr. Jacob Appelbaum, a clinician-scholar at Fred Hutch who led a recent study that pushes CAR T cell technology into new and exciting territory. “If a clinician wants to treat a patient with CAR T cells, all they can really control is how many cells they administer,” notes Appelbaum. “After that, the CAR T cells expand their population by orders of magnitude and essentially function on their own; they fight the tumor, sure, but they can also recognize and kill non-tumor cells or over-activate and cause adverse effects like cytokine release syndrome, which may end up harming or even killing the patient. What we are really working towards is a system with the potency of CAR T cells but the control of a more traditional therapy like radiation or chemo, where the physician can precisely control the dose and timing of the therapeutic being administered.”
To accomplish this goal, Appelbaum and his multidisciplinary team engineered what is essentially a CAR T cell with an ‘on/off’ switch. Dubbed DARIC33, these CAR T cells have a receptor consisting of two halves: an extracellular antigen targeting domain (which the T cells use to bind their targets) and an intracellular signaling domain (which is responsible for activating the T cell after it binds its target). But here’s the trick: the team fused these two receptor halves to two other protein domains which won’t interact without a drug called rapamycin. Once these T cells encounter rapamycin, the drug acts as a ‘molecular glue” to link the antigen targeting and signaling domains, forming a functional T cell receptor and licensing the T cells to sense and respond to immune stimuli. “Scientists have long known about rapamycin’s role as an incredibly potent glue between these two proteins as part of the growth-signaling mTOR pathway—we basically took this knowledge from this unrelated field and engineered these protein domains into our split CAR T cell receptor system,” notes Appelbaum.
What followed was a truly extensive series of experiments in which Appelbaum and colleagues verified that their DARIC33 product worked just as they’d hoped. Both in vitro and in vivo, the team showed that the T cells were nearly silent until supplied with low nanomolar concentrations of rapamycin, which dimerized their T cell receptors and licensed them to bind and respond to CD33 (the particular tumor antigen which they designed DARIC33 to target) in a dose-dependent manner. Even more exciting, the team found that withdrawing rapamycin rapidly and reproducibly reverted the cells back into an ‘off’ state, fitting with their original intention of using rapamycin to bi-directionally modulate T cell activity. After verifying that they could manufacture these cells at an appropriate quantity and purity for clinical use, they tested them in a mouse xenograft model of leukemia. Here, the team elegantly demonstrated the precise control which their system offers—give a mouse DARIC33 along with rapamycin and tumors stayed small, withdraw rapamycin and the tumors begin growing, but re-administer rapamycin and tumor growth stalls again!
Although these results would already make for an impressive study, Appelbaum and colleagues weren’t satisfied. Using pharmacodynamic modeling to design optimized rapamycin dosing schedules, they launched a phase 1 clinical trial to evaluate DARIC33 in several patients with relapsed or refractory acute myeloid leukemia (AML)—the first trial of drug-controllable CAR T cell technology ever undertaken in patients. While relatively small and still ongoing, this trial has so far revealed that DARIC33 is functional and rapamycin-responsive in patients, paving the way for further development of the technology.
“Overall, we’re really excited to provide evidence that this approach could work for CAR T cell therapy—and not just as a theoretical concept, but in practice as well,” notes Appelbaum. “The ability to control CAR T cell activity opens up a whole new world of clinical possibilities: titrating T cell activity to control on-target, off-tumor side effects, modulating T cell activation to prevent or reduce overactivation or exhaustion, or even using this system as a ‘brake pedal’ to improve the safety of other CAR T cell-supercharging strategies being developed by other investigators at the Hutch [see one example of this here]. Of course, there are many new challenges to tackle as well—for example, how to design and optimize rapamycin dosing schedules to best control these CAR T cells—and we’re excited to keep working on these problems and others as we continue building on this already transformative cancer therapy.”
The spotlighted work was supported by a Paul Calabresi Career Development Award for Clinical Oncology, the American Society of Hematology, and the European Hematology Association.
Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium members Drs. Jacob Appelbaum and Adam Lamble contributed to this study.
Appelbaum, J., Price, A. E., Oda, K., Zhang, J., Leung, W.-H., Tampella, G., Xia, D., So, P. P. L., Hilton, S. K., Evandy, C., Sarkar, S., Martin, U., Krostag, A.-R., Leonardi, M., Zak, D. E., Logan, R., Lewis, P., Franke-Welch, S., Ngwenyama, N., … Jensen, M. C. (2024). Drug-regulated CD33-targeted CAR T cells control AML using clinically optimized rapamycin dosing. Journal of Clinical Investigation, 134(9), e162593.
Science Spotlight writer David Sokolov is a graduate student in the Sullivan Lab at Fred Hutch. He studies how cancer cells modify their metabolism to facilitate rapid proliferation and accomodate tumor-driving mitochondrial defects. He's originally from the east coast and has bachelors' and masters' degrees from West Virginia University. Outside of the lab, you'll find him enjoying the outdoors, playing music, or raising composting worms in his front yard.