There are a pair of approved CAR T drugs, Yescarta (axicabtagene ciloleucel) from Gilead Sciences Inc. and Kymriah (tisagenlecleucel) from Novartis AG, that have been available since 2017 for a few hematological cancers, including some lymphomas and leukemias. But little is known about how those engineered chimeric antigen receptor T cells that both target CD19, an antigen prevalent in the cells of many B-cell malignancies, move through the body and proliferate after they are first removed, altered, expanded in number and, finally, returned to a patient’s body.

Researchers at the University of Pennsylvania have developed a way to genetically engineer CAR T cells with molecular tags using a bacterial enzyme that enables them to be imaged when they appear in the body in a certain quantity via positron emission tomography (PET) scan in combination with a radiotracer specific to that enzyme. The bacterial enzyme is otherwise not present in humans or mice.

They successfully tested their approach in a mouse model and published those results in the Nov. 5, 2019, issue of Molecular Therapy. The NIH specifically highlighted the research results, which it also funded via several different awards.

Into the clinic

Now, the researchers expect to submit to the FDA for an IND, enabling a first-in-human trial for the PET radiotracer. They are also in the process of getting the preclinical data for a separate IND for a clinical trial that would combine the use of the PET tracer with a specific tagged cell therapy.

Human testing is expected to start in about a year, and a startup, Vellum Biosciences, has been created to support the commercialization of those reagents. Pharma companies have already expressed interest in the research, which could prove to be the basis of treatment biomarkers as well as help to elucidate the workings of CAR T cells in the body.

The approach could be used in conjunction with any cellular or gene therapy to track concentrated pockets of tagged cells. It’s expected to be useful not only for research purposes, but also could enable CAR T drugs that are tagged and traceable by physicians to monitor treatment progress.

“It’s such a high bar to get something into a patient from engineering something in the lab, but most certainly we’re getting interest from anyone that does viral vector therapy,” Mark Sellmyer, who is an assistant professor of Radiology at the Perelman School of Medicine at the University of Pennsylvania, told BioWorld. “Let’s say you’re a cystic fibrosis patient, and someone’s got a viral vector that wants to fix your CF mutation. This will be a way to track the longevity of that viral vector with imaging. You wouldn’t want to do a biopsy on the lung.

“Or a patient that is getting a retinal AAV [adeno-associated virus] that’s replacing a gene for patients that have genetic blindness. You wouldn’t really want to biopsy the retina. So, we’re getting interested in developing the supporting data needed to enable INDs related to those particular fields in gene therapy,” he continued. “Then any cell therapy, as you can imagine, down the line if they need to know whether or not their cells are still present could be a potential use.”

Bacterial manipulation

In the research, CAR T cells were genetically tagged with the bacterial protein E. coli dihydrofolate reductase enzyme (eDHFR) and then inserted into mice. Next, the mice were injected with a radiotracer derived from the antibiotic trimethoprim, which has a high affinity for that bacterial enzyme.

The researchers could then track the tagged CAR T cells in real time with a PET/CT scan. Once the cells started to proliferate in the body, those cells carried the same tag and were also visible. The approach had a higher sensitivity for detecting, about 11,000 cells per cubic millimeter, than researchers had anticipated.

They were able to detect the CAR T cells accumulated in the spleen after about seven days. After about 13 days, the CAR T cells began to accumulate in the antigen-positive tumors. That shows there may be early and late locations that CAR T cells tend to move once in the body.

While CAR T cells have had successes in hematological cancers, solid tumors have remained elusive. As research expands into other targets, as well as into engineering other types of immune cells such as natural killer cells or macrophages, that could open up more potential for engineered immune cells in solid tumors. Being able to monitor their presence in a tumor, as well as to visualize their numerical expansion in the body, could aid in those research efforts.

Sellmyer’s lab is also working on related research that could enable the simultaneous monitoring of two or even three different kinds of engineered cells at once, which could prove useful as genetic engineering of immune cells moves beyond just T cells and beckon toward cellular combination approaches.

But before researchers get there, the immediate next step is to demonstrate clearly the safety of the tagged CAR T and associated radiotracer in the human body. In computer models and testing with in vitro white blood cells, eDHFR has not been found to be very immunogenic, Sellmyer said. When it came to the activity of the CAR T drug, researchers did not see a decrease in CAR T efficacy or a difference in the metabolism or secretion of cytokines.

“We’re in a place where we started a company to help foster the patent interests of the radiotracer,” summed up Sellmyer. “The idea at the end of the day – we’ve already had interest from various pharma companies – but I think this next step will still be to show that we can put it into people and that it will be safe as imaging agent,” he added. “The hope is that once lots of people see that, they’ll be interested in using it in their particular model.”

A diagram of the CAR T imaging process. Credit: Perelman School of Medicine at the University of Pennsylvania

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