From Pluripotent Stem to CAR T Cells

As interest grows in allogeneic chimeric antigen receptor (CAR) T-cell therapy, one company has begun exploring in vitro differentiation of induced pluripotent stem cells (iPSC) as their T-cell source, rather than harvesting T cells in bulk from suitable donors. Preclinical data on an off-the-shelf candidate, FT819, were presented by Bob Valamehr, PhD, vice president of La Jolla, CA–based Fate Therapeutics, during the American Association for Cancer Research Annual Meeting 2018 in Chicago, IL, April 14–18.

“Using a proprietary cocktail of small molecules and genes, we initiate cellular reprogramming of a given cell type, such as human fibroblasts, into iPSCs,” Valamehr explained. After careful screening, single clones are engineered and edited, then expanded into master iPSC lines that can undergo directed differentiation into CAR-bearing T cells.

“We have a continuous source of material, because of the unlimited self-renewal potential of iPSCs,” he observed. “And just one round of manufacturing, starting with a very small quantity of iPSCs, yields 10¹¹ CAR T cells. This would translate to thousands of doses for patients.”

Harnessing CRISPR/Cas9 technology to place a CD19-recognizing CAR gene into the TRAC locus of a single iPSC ensures complete elimination of the native T-cell receptor in the final therapeutic product, Valamehr said. As such, FT819 should not trigger graft-versus-host disease (GVHD) in patients. The company collaborated with Michel Sadelain, MD, PhD, of Memorial Sloan Kettering Cancer Center in New York, NY, to use this strategy, which Sadelain's group developed. They also worked together to fine-tune the production of T cells from iPSCs.

Valamehr reported that FT819 effectively attacked CD19-positive tumor cells in vitro, producing cytokines such as IFNγ and TNFα, along with cell death mediators, including perforin and granzyme B. This cytotoxicity was antigen-specific. Additionally, because FT819 is dual-targeted—these CAR T cells also express the antibody-engaging CD16 Fc receptor—the addition of rituximab (Rituxan; Genentech) elicited antibody-dependent cell-mediated cytotoxicity against CD19-negative tumor cells, enhancing therapeutic efficacy.

Sadelain and the Fate team “should be congratulated for figuring out how to differentiate iPSCs into functional T cells,” said Carl June, MD, of the University of Pennsylvania in Philadelphia. “It's a real challenge that has held the field back, but they persevered, and what they've shown is very convincing.” He noted that although GVHD is unlikely to occur with FT819, host rejection will still need to be addressed.

Another potential issue, prior to human studies, would be providing the FDA with sufficient evidence that FT819 is completely free of undifferentiated iPSCs. “We have several layers of assurance to implement here,” Valamehr said. “For instance, qPCR and other high-resolution strategies can help us identify any residual iPSCs. We'll also test our product in immunocompromised mice—where anything that can grow, will grow.”

“It's not widely known in oncology, but within the field of regenerative medicine, several clinical trials of products derived from iPSCs or embryonic stem cells are already under way,” Sadelain pointed out. “Those protocols would have required first satisfying the FDA; we believe what we provide [to the agency] will be at least as good.”

June looks forward to the clinical evaluation of FT819. Even so, he added, “I don't think it'll be possible—within the next decade, anyway—to make allogeneic CAR T cells that last as long, or are as safe, as an autologous version.”

“Two extreme views are that we stick with the autologous route because this [off-the-shelf strategy] fails entirely, or it works extraordinarily well and renders autologous CAR T-cell therapy obsolete,” Sadelain remarked. “Quite possibly, though, both approaches could coexist, with one or the other being the right thing to do in certain settings.” –Alissa Poh