1. Kumar Das, Dibash PhD

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Chimeric antigen receptor (CAR) T-cell therapies are transforming clinical care. CAR T-cell therapy involves engineering a patient's T cells with the addition of a CAR to specifically target and destroy cancer cells. This has spurred more than 500 clinical trials analyzing CAR T cells as potential treatments for various cancer types and applications.

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However, there are challenges to CAR T-based therapies. Speaking to Oncology Times, Gabe A. Kwong, PhD, Associate Professor and the Wallace H. Coulter Distinguished Faculty Fellow in the Department of Biomedical Engineering at Georgia Tech and Emory School of Medicine, stated that "the success of treating blood cancers with CAR T cells has largely been difficult to extend to solid tumors.


"A small percentage of the millions of CAR T cells that we infuse actually infiltrate solid tumors, and once they are there, the tumor microenvironment is suppressive and turns off the CAR T cells. Another challenge is that tumors are heterogenous-not all cancerous cells express a particular CAR antigen. What this means is that conventional CAR T treatment will eventually lead to tumor resistance and outgrowth of antigen-negative cells."


Many promising approaches to improve anti-tumor activity of engineered T cells include the use of immunostimulatory agents such as cytokines, checkpoint blockade inhibitor antibodies, and bispecific T-cell engagers (BiTEs). However, they lack specificity and can be toxic to healthy tumors, which narrows their use cases. Thus, there remains an unmet need to develop an approach that targets and locally activates CAR T-cell functions at tumor and disease sites.


As such, Kwong and his collaborators previously published work exploring remotely controlled cell therapies, in which the team could accurately target tumors, located anywhere in the body, with a local deposition of heat (ACS Synth Biol 2018;


Building on that work, Kwong and his team at the Georgia Institute of Technology have now expanded the precision and ability of CAR T-cell therapies by having made cell modifications to improve the cancer-killing capabilities of CAR T cells. The details of the research were explained in a study published recently in the journal Nature Biomedical Engineering (2021;


"We made several upgrades. First, we designed an on/off switch that can be controlled by mild elevations in heat," Kwong noted.


To enable CAR T cells to respond to heat, the team constructed and screened panels of synthetic thermal gene switches containing combinations of heat shock elements and core promoters to recognize an architecture that responds to mild hyperthermia.


"These genetic switches can be turned on by photothermal irradiation-essentially, we took a laser and shined it on the tumor to turn on the CAR T cells," Kwong further added.


The team used plasmonic gold nanorods to convert near-infrared light into heat. In addition to a switch that responds to heat, "these genetic switches were further engineered to produce immune proteins after they are turned on to enhance CAR T-cell activity. We showed that both cytokines and bispecific antibodies can be produced in this way. These molecules are really good at stimulating other immune cells as well and, therefore, they are too toxic to be given systemically. By localizing the production of these molecules in the tumors, they make CAR T cells better at destroying tumors without the negative side effects," Kwong further explained.


The research shows that the activity of intratumoral CAR T cells can be controlled photothermally via synthetic gene switches that trigger the expression of transgenes in response to mild temperature elevations (40-42[degrees] C), while remaining non-responsive to orthogonal cell stresses such as hypoxia. In vitro, short pulses of heat (15-30 min) to primary T cells lead to greater than 60-fold increases in gene expression without affecting key T-cell functions such as proliferation, migration, and cytotoxicity.


Next, they injected mice with both CAR antigen-negative and antigen-positive tumors with the gold nanorods and the engineered CAR T cells containing the gene switches. To generate heat in a mouse's tumor, they shone laser pulses from outside the animal's body, onto the spot where the tumors were located. In vivo, the systemic delivery of CAR T cells followed by intratumoral production, heated via gold nanorods, led to a significant reduction in tumor burden.


"What we found is that, if we localize the production of bispecific antibodies by thermal control, this broadens the different antigens that CAR T cells can recognize and prevents relapse. In mouse models of breast cancer, we were able to completely cure approximately 50 percent of all treated mice without any signs of tumor recurrence," noted Kwong.


For follow-up studies, Kwong and colleagues plan on applying their approach to difficult-to-treat brain tumors.


"This includes glioblastoma and brain metastases. Because of their location in the brain, these sites are difficult to target and usually surgery can only be carried out once. With our approach, we can locally heat brain tumors noninvasively, such as with focused ultrasound. This will allow us to target multifocal disease and locally tune the potency CAR T cells throughout the course of therapy," concluded Kwong.


Dibash Kumar Das is a contributing writer.