NEW YORK – Tweaking anti-CD19 CAR T-cell therapy such that the body is more likely to recognize it as human may be an effective strategy for overcoming treatment resistance in certain pediatric blood cancer patients, according to the results of a small but encouraging pilot study out of the Children's Hospital of Philadelphia (CHOP).
To date, chimeric antigen receptors, or CARs, largely feature single-chain variable fragment (scFv) domains derived from murine monoclonal antibodies. As such, the design of a component of many marketed autologous cell therapies, including tisagenlecleucel (Novartis' Kymriah), originated in mice as opposed to humans. According to the authors of a study recently published in the Journal of Clinical Oncology, however, some patients' immune systems may reject the mouse-derived aspect of currently available CAR T-cell therapies and may be contributing to the development of resistance or relapse to the engineered T cells.
"Humans don't naturally make antibodies to self-proteins that are part of our normal cells, so for research and clinical therapy purposes, creating mouse monoclonal antibodies has been one of the primary modes of developing antibodies specific to a particular protein," explained CHOP's Shannon Maude, lead author of the recently published study, in which CHOP researchers developed a CAR with a humanized anti-CD19 scFv domain and a 4-1BB costimulatory domain to try to overcome resistance.
According to Maude, the process of "humanizing" CAR T-cell therapy involved altering the DNA sequence of the gene inserted into the T cells while engineering patients' T cells to express CD19-targeting CARs. "The way a CAR T-cell product is made is that you have the genetic sequence that would make the protein, which includes that single-chain variable fragment domain, the linker, and the intracellular activation domains," she said. "We modified the DNA sequences so that when they encoded the amino acids that make up the protein, that protein would look more like a human protein."
To clinically evaluate the more human-like treatment, dubbed huCART19, CHOP researchers conducted a pilot trial of 74 pediatric and young adult patients, 72 of whom had B-cell acute lymphoblastic leukemia and two of whom had B-cell lymphoblastic lymphoma. The investigators designed the trial to assess the safety and persistence of huCART19, and to determine through an exploratory analysis whether the more humanized versions of traditional CAR T-cell therapy might do a better job of staving off recurrence than existing therapies with mouse-derived elements.
Among the patients who received infusions of huCART19, 33 had previously received CAR T-cell therapy and 41 were naïve to CAR T-cell therapy but refractory to other treatments such as chemotherapy, stem cell transplants, or other immunotherapies. Patients were stratified into cohorts based on their prior CAR T-cell therapy status. The median patient age was 10.3 years old for those who hadn't received CAR T-cell therapy previously and 12.6 years old for those who had become refractory to CAR T-cell treatment.
Twenty-eight days after infusion, 98 percent of treatment-naïve patients and 100 percent of the B-ALL subpopulation in this cohort experienced either a complete remission or complete remission with incomplete count recovery. Among these patients, the relapse-free survival rates were 84 percent one year after infusion and 74 percent two years after infusion.
Among previously treated patients, the overall response rate was 64 percent at 28 days following infusion. Out of 21 patients with B-cell aplasia who achieved complete remission or complete remission with incomplete count recovery, the relapse-free survival rate was 74 percent one year after infusion and 58 percent two years after infusion.
Overall survival results were not yet available for the treatment-naïve patient cohort, but among previously treated, refractory patients, 76 percent were alive one year after infusion, and 55 percent were alive after two years. The median overall survival was 34 months.
"The retreatment responses were encouraging," Maude said, adding that since other studies had suggested that retreatment with CAR T-cell therapy in a refractory population may not be effective, CHOP researchers weren't sure how effective such a strategy would be in their study.
Even though overall survival data aren't available for the treatment-naïve cohort, Maude and her colleagues were "pleasantly surprised" by the high response rate and the proportion of durable remissions seen in this group to the humanized CAR T-cell therapy.
In terms of side effects, 76 percent of patients in the treatment-naïve cohort and 61 percent of previously treated patients had grade 3 serious adverse events eight weeks after infusion. Grade 4 adverse events occurred in 71 percent for treatment-naïve and 62 percent for previously treated patients. Most of the adverse events were reversible, and none of the patients died due to treatment. According to Maude and colleagues, the toxicity profile of huCART19 was similar to that reported for tisagenlecleucel and other non-humanized CD19 CAR T-cell products.
Cumulatively, after six months, 27 percent of patients in the treatment-naïve cohort and 48 percent in the retreatment cohort had lost huCART19 persistence as detected by flow cytometry. The researchers also measured loss of B cells and subsequent B-cell recovery as a way to evaluate treatment persistence. All of the patients who achieved complete remission or complete remission with incomplete count recovery had loss of B cells, or B-cell aplasia, within 28 days. Then, six months later, the researchers observed that a minority of treatment-naïve patients, 15 percent, had B-cell recovery, suggesting the humanized CAR T-cell therapy was still suppressing B cells in most of these patients, while 58 percent among retreated patients had B-cell recovery.
To evaluate how these results stacked up against traditional, non-humanized, CAR T-cell treatments, the researchers conducted an exploratory analysis comparing the time to B-cell recovery in the CAR T-cell naïve cohort of the huCART19 trial to a historical cohort that had received tisa-cel. While Maude and her co-authors were careful to note the limitations of their exploratory analysis, including small sample sizes and potential cofounders like disease burden and prior therapies, they noted a "trend toward a lower cumulative incidence of B-cell recovery" in the huCART19-treated patients.
Specifically, after six months, the rates of B-cell recovery were 15 percent versus 29 percent among those treated with huCART19 and the historical cohort, respectively. Although the difference in B-cell persistence wasn't statistically significant, possibly due in part to the small cohort sizes, the authors noted that "the potential for improved persistence with huCART19, which may be associated with a decreased risk of relapse, warrants further study."
While Maude acknowledged that a randomized controlled clinical trial comparing huCART19 to CAR T-cell therapies with the mouse-derived monoclonal antibodies would be the best way to pin down the relative benefit, that type of trial may not be feasible, at least in the short term.
"In this population, we really want to get therapies to patients as quickly as possible," she said. "It's difficult to do a randomized trial with enough patients to be able to answer that question definitively. That analysis suggested some difference but was not large enough to truly answer that question."
That said, Maude and researchers have already launched a Phase II study of huCART19 that is expected to enroll more patients. "We may be able to get more information from that [on the difference versus non-humanized CARs]," she said, noting that because that trial also has a single-arm design, the "more information" would likely also come by way of comparisons with other trials.
Although the pilot study was conducted at CHOP and fully supported by academic institutions including University of Pennsylvania, Novartis played a role in the huCART19 development, Maude noted. Novartis, which markets the US Food and Drug Administration-approved anti-CD19 CAR T-cell therapy tisa-cel in B-ALL, initially licensed that product from UPenn and now holds a license for huCART19 as well.
The university has a full cell therapy facility on its campus where researchers manufactured the humanized CAR T-cell therapy. However, the anti-CD19 CAR T-cell "backbone," which the researchers tweaked for their humanized version, is also found in tisagenlecleucel.
"The results of the murine CAR T-cell products [like tisa-cel] are really outstanding, so we were not expecting that we may see even improved numbers [in the huCART19 trial]," Maude said, emphasizing again the caveat that the cross-trial comparisons are imperfect.