The Quest for Off-the-Shelf CAR T Cells

One of immuno-oncology's most vibrant fields is that of chimeric antigen receptor (CAR) T-cell therapy, with the first two treatments, tisagenlecleucel (Kymriah; Novartis) and axicabtagene ciloleucel (Yescarta; Gilead), gaining approval in 2017. Even as companies tinker with autologous, or patient-derived, T cells—to shorten the manufacturing process and enhance efficacy—some have begun probing ways and means of engineering an allogeneic version from unrelated donors.

The latter represents “the ultimate possibility, in that one could easily manufacture CAR T cells from a particular source and administer the treatment, off the shelf, to multiple recipients,” says Michel Sadelain, MD, PhD, of Memorial Sloan Kettering Cancer Center in New York, NY.

“It would change market access entirely for CAR T-cell therapy,” adds André Choulika, PhD, of the biotech Cellectis—based in Paris, France, and New York, NY—which paved the way for allogeneic therapy and is the furthest ahead clinically.

Autologous CAR T cells are still a boutique therapy, notes Devon Shedlock, PhD, of San Diego, CA–based Poseida Therapeutics. As well, “the raw material harvested from each patient is typically in pretty bad shape,” he explains, “whereas on the allogeneic side, you could screen donors and select the healthiest ones with easily modifiable, good-quality T cells.”

Steven Rosenberg, MD, PhD, of the NCI, agrees that “an off-the-shelf approach would be a major simplification,” saving both money and time. Ongoing research in this area is “something I completely applaud,” he adds, and, despite the challenges involved, getting to FDA-approved allogeneic CAR T cells is, he thinks, an achievable goal.

There are two inherent issues when it comes to allogeneic CAR T-cell therapy, Sadelain says: Donor cells could attack the recipient, causing graft-versus-host disease (GVHD); conversely, a patient's own T cells could reject the infused product, resulting in a lack of therapeutic impact.

Companies pursuing an off-the-shelf strategy have dealt with the first problem by using gene-editing tools such as TALEN or CRISPR/Cas9 to remove the native T-cell receptor (TCR) from donor cells. “I'd say this mostly resolves the issue, but not completely,” observes Marcela Maus, MD, PhD, of Massachusetts General Hospital Cancer Center in Boston, “because the infused product is usually not 100% pure, and we don't yet know what threshold [of purity] is acceptable.”

Even a tiny percentage of T cells retaining their TCR risks the development of potentially lethal GVHD, explains Eric Ostertag, MD, PhD, also of Poseida. As well, the gene editing necessary for allogeneic CAR T cells “has the FDA quite concerned about the possibility of unwanted, off-target mutations that could be pretty detrimental.”

Laurence Cooper, MD, PhD, of Ziopharm Oncology in Boston, MA, notes that although the focus has mostly been on targeting the αβ TCR—found in 95% of human T cells—the minority of T cells expressing the γδ TCR may emerge as a candidate for off-the-shelf therapy. There are “intriguing hints in preclinical and bone marrow transplant data,” he says. “It's thought to be unlikely that this receptor will trespass and attack something vital like a recipient's heart or lungs.”

Meanwhile, to keep rejection at bay, patients receive lymphodepleting chemotherapy. Choulika says that even with autologous CAR T-cell therapy, “you don't get strong, durable antileukemic activity without prior lymphodepletion. You have to create space for the [therapeutic] T cells to expand.”

However, “I don't really see a future for allogeneic CAR T-cell therapy in which every patient has to be severely immunocompromised, and exposed to the consequences thereof, in order to receive this treatment,” Sadelain remarks. “This problem is still awaiting an effective solution; it will require creative immunology and sophisticated engineering.”

One approach, which both Cellectis and Poseida are evaluating, is to disrupt not only TCR expression, but also that of MHC I, in donor T cells. “We're trying to make a product that's fully rather than semi-allogeneic; the removal of MHC I helps our cells better resist host rejection,” Ostertag says.

Yet, Cooper cautions that “if you eliminate all MHC I molecules, you've now removed the ligand for KIR [killer immunoglobulin-like receptor],” which keeps endogenous natural killer (NK) cells in check. Without this inhibitory signal, the infused therapy would be vulnerable to NK cell–mediated destruction.

These are complex issues, Maus agrees. In the lab, her team is probing MHC I expression levels low enough to mitigate host rejection, yet avoid CAR T-cell elimination by NK cells. The hope is to figure out “a happy medium,” she says.

Essentially, “there's this yin and yang with allogeneic CAR T-cell therapy,” Cooper says. “You need to simultaneously prevent toxicity and improve engraftment; the two coexist on this knife edge.”

“We're still figuring out exactly how our candidate therapies work in patients, to better understand what would be helpful, versus too much, on the gene-editing front,” Choulika adds. “I favor moving stepwise—after learning to crawl, we can walk, then start running.”

Sadelain and his group have used CRISPR/Cas9 to create CD19-recognizing TRAC-CAR T cells that are semi-allogeneic—the native TCR is disrupted in the TRAC locus—and, preclinically, much enhanced in potency (Nature 2017;543:113–7). “We hope to open a clinical trial by the end of this year,” he says. “It would be an important advance to demonstrate that CRISPR/Cas9-edited cells are at least as good as, if not better than, conventionally engineered CAR T cells.”

Beyond Cellectis's medical first with one of their TALEN-edited products, UCART19—two infants with acute lymphoblastic leukemia (ALL) were successfully treated in 2015 and remain cancer-free—this candidate therapy continues to look promising, Choulika says. Data from CALM and PALL, two phase I trials, were presented during the American Society of Hematology (ASH) 2017 Annual Meeting in Atlanta, GA: Among seven adults and seven children with relapsed/refractory ALL who received UCART19, the rate of complete molecular remission, or negative minimal residual disease, was 83%.

Another candidate, UCART123, is the first allogeneic CAR T-cell therapy greenlighted for phase I trials in the United States, in patients with acute myeloid leukemia and blastic plasmacytoid dendritic cell neoplasm. As well, UCART22, aimed at treating ALL and non–Hodgkin lymphoma, is likely to enter clinical studies in the coming months.

“Overall, we're very much technology-agnostic,” Choulika adds. That said, “we've optimized TALEN for gene editing at an industrial scale, and we tend to favor it in terms of simplicity, precision, and therapeutic safety.”

Poseida has set its sights on allogeneic CAR T cells that recognize BCMA, a key target in multiple myeloma. At ASH 2017, Shedlock highlighted preclinical findings for their product, P-BCMA-ALLO1. These CAR T cells were not only greatly reduced in their alloreactivity, but also showed strong efficacy in vitro and are being tested further in a mouse model of aggressive human multiple myeloma.

Alongside their proprietary gene-editing platform, Cas-CLOVER, which Shedlock describes as highly efficient with low to no off-target cutting, the nonviral piggyBac system used for CAR delivery “enables us to do our editing in resting T cells,” he adds. Viral vectors, by contrast, require actively replicating T cells. “The fewer times we have to force cells to divide, the greater our chances are of getting a high proportion of CAR-bearing stem cell memory T cells,” he explains. “We favor this subset because it's long-lived, durable, and multipotent.”

Ostertag says the bar for FDA approval will be higher for allogeneic CAR T-cell therapy. Nonetheless, Rosenberg, who in 2010 first reported the eradication of a patient's advanced lymphoma using autologous CD19-targeted CAR T cells, says that “where there's a will, there's a way” (Blood 2010;116:4099–102). That patient remains cancer-free.

“I had every expectation, even back then, that CAR T-cell therapy would enter modern oncology,” Rosenberg says. “If it prolongs life and has curative potential … we'll figure out how to overcome the complexities.” –Alissa Poh

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