Safety and Anti-Leukemic Activity of CD123-CAR T Cells in Pediatric Patients with AML: Preliminary Results from a Phase 1 Trial

Background

The prognosis of pediatric patients with relapsed or refractory (r/r) Acute Myeloid Leukemia (AML) remains dismal. Evaluation of novel therapies such as chimeric antigen receptor (CAR) T cells is therefore urgently needed. CD123 is overexpressed on AML blasts and leukemic stem cells (LSC), making it an attractive target for CAR T-cell directed therapy.

Methods and Results

We designed a ‘bridge to allogeneic hematopoietic cell transplant (HCT)’ Phase 1 study to evaluate the safety and feasibility of treating pediatric patients with r/r AML with CD123-CAR T-cell on four dose levels (DL): DL1: 3x10⁵/kg, DL2: 1x10⁶/kg, DL3: 3x10⁶/kg, DL4: 1x10⁷/kg (NCT04318678). CD123-CAR T-cells were generated from CD4/CD8-selected autologous leukapheresis products using a lentiviral vector, which encoded a CD123-CAR with CD28.z signaling domain and a CD20 safety switch. Patients received lymphodepleting chemotherapy with Fludarabine and Cyclophosphamide followed by a single CAR T cell infusion.

To date we have enrolled 12 patients. The median age of treated patients was 17 years (range 12-21years). Except for the first patient who had primary refractory disease, all other patients had relapsed following HCT (n=1-4 previous HCTs). Despite patients being heavily pre-treated, we successfully manufactured products on all patients. The infused products had a predominantly CD4+ effector memory immunophenotype, with a median CD123-CAR+/CD20+ expression of 60.2% (range 46.1-79.4%, N=6). All infused CAR T cell products exhibited potent antitumor activity in vitro when cultured against CD123+ targets (including autologous blasts for 2 patients).

We infused 2 patients on DL1 and 3 patients on DL2. Two patients were infused on single patient protocols. All infusions were well tolerated, without any adverse infusion events. Post infusion, we observed isolated fevers that resolved within 24 hours (possible Grade 1 Cytokine Release Syndrome (CRS)). No Grade 2 ≥ CRS or neurotoxicity was seen. Multiplex cytokine analysis showed statistically significant increase of IL15 after lymphodepletion without significant increase in Th1 cytokines or IL6 after T cell infusion. While transient cytopenias were observed, patients did not develop persistent marrow aplasia. No dose limiting toxicities were noted.

Disease evaluation was performed at 4-6 weeks following CAR T cell infusion. The 2 patients on DL1 showed no response. In the 3 patients on DL2, we observed: reduction in blast percentage without complete remission (CR) in 1 patient, no response in 1 patient and CR in 1 patient. The patient with CR had isolated extramedullary disease, showing complete resolution of all lesions by week 4 by PET imaging (Figure 1). She recurred 2 months following infusion, coinciding with loss of CAR T-cell detection by qPCR. She received a second infusion off study and again achieved a short-lived CR. One additional patient was infused on dose level 2 off protocol. She achieved morphological CR at day 28 with low level minimal residual disease (0.19%).

Correlative studies revealed CD123-CAR T cell expansion in patients on DL2 (Figure 2), but not on DL1. However, peak expansion on DL2 was low in comparison to published reports on CAR T cells for Acute Lymphoblastic Leukemia. Detailed phenotypic analysis revealed that CD123-CAR T cell products were predominantly effector memory (CD45RO+CCR7-) with <2% naïve-like (CD45RO-CCR7+ or CD45RO-CD62L+) T cells. In addition, PD1, TIM3 and CD39 were expressed at a median of 43, 81 and 16% in CD4+ and 3, 72 and 45% in CD8+ T cells. This phenotype could be completely reversed by inhibiting CAR signaling with dasatinib during the manufacturing process, resulting in a CAR T cell product dominated by naïve-like T cells that did not express TIM3, PD1 or CD39, leading to enhanced T-cell effector function. Mechanistic analysis revealed that the observed phenotype could be explained by the transient, increased cell surface expression of CD123 on T cells upon activation during manufacturing, and not by tonic CAR signaling.

Conclusions

Our initial clinical experience with CD123-CAR T cells for pediatric patients with r/r AML demonstrates feasibility, safety, and evidence of anti-leukemic activity. Upcoming CD123-CAR T cell products will be manufactured in the presence of dasatinib to limit T-cell differentiation and exhaustion as we continue to explore our CD123-CAR T cell therapy approach for pediatric r/r AML.