STOCKHOLM, SWEDEN – A novel chimeric antigen receptor T-cell construct directed against CD22 was able to rescue children with relapsed or refractory B-cell acute lymphoblastic leukemia for whom CD19-directed CAR T therapy had failed, investigators from China reported.
Thirty days after CAR T cell infusion, 12 of 15 children (80%) treated with the unnamed CD22 CAR T product had a complete response (CR) and one (6.7%) had a partial response (PR), for an overall response rate (ORR) of 86.7%, said Jing Pan, MD, from the Beijing Boren Hospital in China.
Surprisingly for a therapy of its type, the CD22 CAR T was well tolerated, with only mild cases of the cytokine release syndrome (CRS), and it could be delivered safely in children with relapsed disease after hematopoietic cell transplants (HCT), she reported in a briefing and in an oral abstract session at the annual congress of the European Hematology Association.
“CD22 CAR T immunotherapy brings hope for patients with refractory or relapsed B-ALL who failed on CD19 CAR T immunotherapy, and we think it’s quite safe. No children died and no children had severe side effects during the study, even in post-HCT patients,” Dr. Pan said.
She noted that in a previous clinical trial by her group, some patients experienced relapses and were resistant to retreatment with CD19-directed CAR T therapy due to mutations or loss of the CD19 antigen (Leukemia. 2017 Dec;31[12]:2587-93).
Since CD22 is highly expressed on leukemic cells in children with B-ALL, the investigators decided to evaluate the safety and efficacy of a CD22 CAR T as a rescue strategy. They enrolled 15 patients who either experienced relapse or did not have a response to CD19 CAR T immunotherapy. The CAR T construct they used contains an anti-CD22 single-chain variable fragment derived from a humanized CD22 antibody.
Patient conditioning with fludarabine and cyclophosphamide was performed simultaneously with CAR T transfection and expansion. After about 7 days, the expanded and transformed CAR T cells were infused at a dose of 8.2 x 105/kg in patients who had not undergone HCT, and 0.9 x 105 in patients who had received a transplant.
The patients ranged from 2 to 18 years old (median 8 years), had a median disease course of 21 months (range 5-84 months), and had a median of 42% bone marrow blasts (range 5%-95.5%).
Four of the 15 patients had relapses following allogeneic HCT, and the remaining 11 had relapses following chemotherapy. Two of the patients were found to be minimal residual disease (MRD)-positive by flow cytometry. Two patients had extramedullary disease only at relapse.
Ten of 11 patients who had experienced a hematologic relapse had either a CR or CR with incomplete recovery of counts (CRi), and of these 10 patients, nine were determined on follow-up to be MRD-negative by flow cytometry.
One of the two patients with extramedullary disease had a CR, and the other had a partial response.
Although two patients had no response to CD22 CAR T therapy, expression of the antigen was strong on leukemia cells from these patients, Dr. Pan said.
All patients experienced CRS, but none had greater than grade 2, Dr. Pan said, although she did not provide give specific numbers. Two patients had grade 1 neurotoxicity, two patients had grade 2 hypoxemia, and one patient had grade 2 liver enzyme elevation.
At a median follow-up of 108 days, six patients had been bridged to allogeneic HCT, and eleven of 12 patients who had a CR or CRi at 30 days had no evidence of disease progression. The remaining patient with an initial CR or CRi had a relapse at day 50. The 6-month progression-free survival rate was 91.7%.
Anton Hagenbeek, MD, PhD, from the Academic Medical Center at the University of Amsterdam, the Netherlands, who moderated the briefing but was not involved in the study, commented that given the high leukemia burden of the patients and the apparent efficacy of the therapy, one would expect to see higher grades of CRS, and asked Dr. Pan whether she could account for the CRS findings in her study, compared with those of trials for CD19-directed CAR T therapy.
It may have to do with differences in density of CD22 expression, compared with CD19 expression on leukemia cells, or on differences in the antibody used to target the cells, Dr. Pan said.
SOURCE: Pan J et al. EHA Congress, Abstract S832.