NEW YORK – Autologous CAR T cells targeting the glypican 3 antigen, when engineered to co-express interleukin-15, could help treat solid tumors including liver cancers, according to early yet encouraging research presented during the American Society of Gene & Cell Therapy's annual meeting on Thursday.
The approach improved anti-tumor activity in a small group of patients, but they also had toxicity on the cell therapy. Fortunately, researchers including David Steffin, an assistant professor of pediatric hematology and oncology at the Baylor College of Medicine, have developed a safety switch that shuts down the CAR T cells if they're causing severe toxicities in patients.
In presenting the Phase I data on Thursday, Steffin explained that he and his team chose to target glypican 3 (GPC3) with the CAR T-cell therapy because the antigen is preferentially expressed on the surface of certain solid cancers including hepatoblastoma and hepatocellular carcinoma. GPC3 is rarely, if ever, expressed on normal, healthy tissue, he said.
One of the main challenges with treating solid tumors with autologous CAR T-cell therapies has been maintaining the T cells' persistence and proliferation once they're infused in the body. This is why Steffin's team chose to design a cell therapy that co-expressed interleukin-15.
"IL-15 is important for homeostatic maintenance of CD8 memory cells [and] can enhance proliferative capacity," Steffin said. "And in a number of preclinical models of leukemia, neuroblastoma, and glioblastoma, it's been shown that IL-15-expressing CAR T cells can enhance persistence and anti-tumor activity."
When Steffin and colleagues translated their research into the clinic, they infused separate groups of patients — all with GPC3-expressing cancers as determined by immunohistochemistry — with either the GPC3-targeting CAR T cells alone or the GPC3-targeting CAR T cells engineered to express IL-15. Of 18 patients total, six received just the GPC3-targeting cells and 12 received the cells with the added IL-15 expression.
The CAR products were all manufactured in Baylor College of Medicine's manufacturing facility, Steffin explained. Patients received standard lymphodepleting chemotherapy before CAR T-cell infusion and were closely monitored for four weeks afterward. Investigators assessed patients' cancer responses after the four-week follow-up.
All but five patients, ranging from 4 years to 72 years old, had liver cancer. The five others had various GPC3-expressing non-liver tumors. All of the patients had extensive metastases and had received other therapies.
Mitigating toxicities with iC9 'safety switch'
Patients who received the IL-15 expressing cells, Steffin underscored, also experienced more adverse events than those who received only GPC3-targeted therapy; the rate of toxicities were nearly double that of the non-IL-15 CAR T-cell group.
The IL-15 group had an increase in cytokine release syndrome as well as impaired liver and kidney function. Three patients developed severe enough toxicities that they required the use of the drug rimiducid, which works with the inducible caspase 9 (iC9) safety switch built into the CAR T-cell therapy design. When the cells expressing the iC9 encounter rimiducid, they begin apoptosis, which is why these constructs are often called "suicide switches." The mechanism is supposed to stop cell proliferation and ensure the treatment doesn't continue to cause and worsen severe side effects in patients.
This mechanism could rapidly curb the IL-15 expansion, Steffin said, and in the trial, rimiducid infusion significantly improved the three patients' clinical symptoms and reduced IL-15 expression within four hours of infusion. There was also a significant drop in inflammatory cytokines that had contributed to the severe toxicities.
The trade-off with the safety switch, of course, is that the CAR levels drop off significantly after it's administered. But it's an important feature given the risk of increased cytokine release syndrome with these treatments. "While we saw responses in all three patients who received the agent, most of them ultimately progressed over time," Steffin explained. "That said, reinfusions are a possibility, but finding a way to balance toxicity with treatment effect remains a big concern."
Preliminary efficacy favors IL-15
"We were able to show that IL-15 CAR T cells could induce fairly promising anti-tumor responses in metastatic sites and in primary lesions," Steffin said. He shared examples of two liver cancer patients whose primary tumors decreased in size and metastatic lesions, including in their lungs, disappeared or also shrunk. "This was quite exciting," he said.
In patients for whom a post-treatment biopsy was available, the investigators found necrotic tumor cells with negative GPC3 expression via IHC testing, indicating that the CAR T cells had been effective in killing the tumor cells.
"What we were able to show is that the IL-15 CAR group induces improved anti-tumor response versus the CAR alone," Steffin said. Four patients had a partial response by RECIST criteria, and eight patients' tumors shrunk compared to their size pre-infusion. "This was quite reassuring," he said.
Among patients on the CAR T cells without IL-15, two had progressive disease and four had stable disease. Three patients who received the CAR T cells with IL-15, meanwhile, had partial responses including resolution of their lung metastases. The response rates were technically zero for those on CAR T-cell therapy without IL-15, and 50 percent for the patients who received the IL-15 CAR T-cell therapy.
Steffin acknowledged that the short follow-up — only about a year for those tracked the longest — left room for uncertainty regarding long-term outcomes. "Durability still remains a pretty significant concern with both our CAR and CAR-IL-15 group," he said.
Exploring response differences
In an effort to account for the differences in anti-tumor activity between patients who received the non-IL-15 and the IL-15 CAR therapy product, Steffin and colleagues analyzed the cell transcriptomes and cell surface phenotypes.
Using single-cell RNA sequencing, the investigators found that, at baseline, the IL-15 CAR group had significantly different gene expression profiles than the non-IL-15 CAR group. They identified 213 differentially expressed genes among the two groups. For example, the IL-15 CAR group was enriched for CD8 A and B cytolytic markers but some chemokines were downregulated.
In the cell phenotypic protein analysis, they found that patients on IL-15 CAR were enriched for CD8 T-cell subsets and more effector cell subsets.
In patients' peripheral blood post-infusion, the investigators also performed single-cell sequencing and found that there was an evolution depending on whether IL-15 was co-expressed or not. In the IL-15 CAR group, they saw an increase in cytolytic markers and expression of CD69, CD38, LAG3, and granzyme components reflecting what Steffin called "fairly activated and differentiated T-cell states."
They also identified gene sets associated with mitochondrial metabolism, which he pointed out is consistent with what the researchers had previously observed with IL-15. "This helps explain some anti-tumor activity," Steffin said.