First-in-Man CD123-Specific Chimeric Antigen Receptor-Modified T Cells for the Treatment of Refractory Acute Myeloid Leukemia

Introduction: Adoptive immunotherapy using T-cells endowed with chimeric antigen receptors (CARs) has emerged as a promising new approach to treating CD19+ acute lymphoblastic leukemia (ALL). However,treatments for relapse/refractory acute myeloid leukemia (AML) have remained largely unchanged for nearly 50 years, and some AML patients have very poor prognosis. The interleukin-3 receptor alpha chain (CD123) has been identified as a potential immunotherapeutic target due to its overexpression in AML cells compared with normal hematopoietic stem cells. Antibodies targeting CD123 for the treatment of AML have demonstrated promising anti-leukemic activity in murine models but showed limited efficacy in clinical trials, suggesting that alternative and more potent therapies targeting CD123 are required.

Methods: We have generated a 4^th generation, apoptosis-inducible lentiviral CAR targeting CD123: CD123-scFv/CD28/CD137/CD27/CD3ζ-iCasp9 (4SCAR123), and demonstrated its high AML killing and AP1903-inducible apoptosis functions in ex vivo analyses. In a pilot trial of 4SCAR123, we enrolled a 47-year-old male patient with AML-M2 (FLT3/ITD+). The patient underwent allogeneic hematopoietic stem cell transplantation and relapsed. After 3 chemotherapies combined with sorafenib, his AML cells kept at 59% in bone marrow. He received CTX 250mg/kg/day for 3 days as conditioning regimen followed by 1.8x10⁶/kg 4SCAR123 T cell infusion. Serum cytokine levels were measured by flow cytometric bead assay.

Results: At day 1 after 4SCAR123 T infusion, the patient experienced rigorous chills and fevers, low blood pressure and hypoxemia. We detected elevated serum cytokine levels including interleukin-6 (2,500pg/ml) and tumor necrosis factor-α (33.8 pg/ml) at day 8, and the patient suffered from severe cytokine release syndrome (CRS) on day 4, which was controlled by one dose of Tocilizumab. BM examination detected a decrease of blasts from 59% to 45% 20 days after CAR-T therapy.

Conclusion: Here we report a first-in-man pilot safety study of CD123 CAR-T therapy for AML patients. The 4SCAR123 exhibited potent cytotoxicity against AML in vitro, and in this pilot trial, the patient developed a rapid response consistent with CRS and achieved partial remission within 20 days. Importantly, although CD123 is universally expressed in myeloid and endothelial cells, we did not observe overt off-target cytotoxicity from the 4SCAR123 T cells, except for a controllable CRS. Our pilot study warrants further exploration of CD123 CAR for the management of refractory AML.